Approach to acute knee pain and injury in children and skeletally immature adolescents

INTRODUCTION — 

The evaluation of acute knee pain or injury in the child or skeletally immature adolescent is reviewed here. The evaluation of nontraumatic joint pain and swelling in children, chronic knee pain in children and adolescents, and knee pain in athletes or active adults, including skeletally mature adolescents, are all discussed separately:

●Child with swollen nontraumatic joint: (See "Evaluation of the child with joint pain and/or swelling".)

●Child with chronic knee pain: (See "Approach to chronic knee pain or injury in children or skeletally immature adolescents".)

●Skeletally mature adolescent and adult with knee pain: (See "Approach to the adult with knee pain likely of musculoskeletal origin" and "Approach to the adult with unspecified knee pain".)

ANATOMIC CONSIDERATIONS IN CHILDREN — 

The anatomy of the knee (figure 1 and picture 1 and picture 2 and figure 2 and picture 3 and figure 3 and picture 4) with an emphasis on biomechanics is discussed in detail separately. (See "Physical examination of the knee", section on 'Anatomy' and "Physical examination of the knee", section on 'Biomechanics'.)

The physis (ie, growth plate) represents a major anatomical difference between adult and pediatric bone. Growing long bones in children and younger adolescents are composed of the following segments: diaphysis (shaft), metaphysis (where the bone flares), physis, and epiphysis (region between physis and joint; starts as ossification center) (figure 4).

Physes mark a weak point in pediatric bone and are susceptible to fracture (figure 5). Because the tensile strength of pediatric bone is less than that of the ligaments, the same injury mechanism that causes a ligamentous injury in adults (sprain or strain) is more likely to cause a bone injury in children. In other words, forces exerted on the knee will cause the relatively weaker physis to fracture before causing disruption (ie, sprain) of any relatively stronger and more flexible adjacent ligament. Once the physis closes, the tensile strength of the bone exceeds that of the ligaments and ligamentous injury (without bone involvement) becomes the more common injury type. (See "General principles of fracture management: Fracture patterns and description in children", section on 'Physeal (growth plate)'.)

In general, physes around the knee close in late adolescence. However, the timing of physis closure varies among individual patients and by bone and sex. The typical age of physis closure of the distal femur, proximal tibia, proximal fibula, and tibial tuberosity are provided in the figure (figure 6).

DEFINITIONS

Acute injury — In this topic, acute injury refers to sudden trauma (eg, collision between players, fall while riding a bicycle, motor vehicle collision). Of note, some chronic conditions may become apparent during evaluation for a traumatic injury, such as a slipped capital femoral epiphysis (SCFE). (See 'Referred pain' below and 'Systemic disease' below.)

Acute pain — By convention, pain from most musculoskeletal conditions that lasts less than six weeks is described as acute or subacute, while pain lasting longer than six weeks is characterized as chronic. Acute pain typically has no progressive pattern. In response to a particular trauma, acute pain flares, lasts a predictable period, and subsides as healing occurs.

Chronic knee pain in children and skeletally immature adolescents is discussed separately. (See "Approach to chronic knee pain or injury in children or skeletally immature adolescents".)

EPIDEMIOLOGY — 

In large registry studies of emergency department visits, the incidence in children and adolescents ranges from 3 to 12 knee injuries per 1000 patients seen [1,2]. Sports, recreation (eg, bicycle riding and trampoline use), and falls (from stairs, furniture, trees, etc) are the most frequent activities causing injury [1,2]. In the United States, American football, basketball, and soccer account for the largest portion of sports-related knee injuries [1]. Outside of the United States, football (soccer) and basketball pose the greatest risk [3].

Among 5 to 14 year olds, the most common knee injuries are contusions, lacerations, sprains or strains, fractures, and patellar dislocations [1,2]. Although fractures occur more frequently in children and skeletally immature adolescents than in skeletally mature adolescents and adults, sprains or strains are still the most common injury in these patients. Approximately 2 percent of knee injuries result in hospitalization [1], and up to 17 percent of knee injuries warrant surgical repair [3]. Adolescent girls have a higher risk of a major knee injury from sports compared with boys [1,4-6]. (See "Anterior cruciate ligament injury", section on 'Epidemiology'.)

EVALUATION — 

The initial evaluation of the young child or adolescent athlete with acute knee injury requires rapid identification and management of limb-threatening injuries with gross deformity or neurovascular compromise (algorithm 1).

In patients without limb-threatening injuries, the history and physical examination can establish a working diagnosis. In children and skeletally immature adolescents, radiographic evaluation is necessary to identify fractures and hip conditions with referred knee pain, and to fully characterize internal knee abnormalities. (See 'Imaging' below and 'Specific conditions' below.)

The evaluation of chronic knee or joint complaints is discussed separately. (See "Approach to chronic knee pain or injury in children or skeletally immature adolescents" and "Evaluation of the child with joint pain and/or swelling".)

Limb-threatening injury — When assessing an acute knee injury in a child or skeletally immature adolescent, the clinician should first identify any potentially limb-threatening injuries or conditions that require immediate treatment by a surgeon. Key findings suggestive of severe injury include any one of the following (algorithm 1) [7]:

●Gross deformity.

●Neurovascular compromise (eg, foot drop is indicative of peroneal nerve injury).

●Inability to bear weight.

●Large knee effusion. Notable knee swelling associated with significant pain, particularly if it is unresponsive to icing, compression, and analgesics, is associated with significant injury.

●Penetrating injury into knee joint.

●Findings of popliteal artery injury (eg, active hemorrhage, expanding or pulsatile hematoma located in the popliteal fossa, absent distal pulses, distal extremity ischemia). (See "Severe lower extremity injury in the adult patient", section on 'Vascular assessment'.)

Many, albeit not all, limb-threatening injuries can be identified by close inspection and physical examination, with special attention to neurovascular status. Knee (tibiofemoral) dislocations require emergency plain radiography followed by immediate reduction by a knowledgeable clinician. For patients with displaced fractures, the knee should be immobilized in a position of comfort that does not compromise neurovascular function, followed by emergency management of the specific injury. (See 'Limb-threatening injuries' below.)

History — The following questions are helpful in the evaluation of a child or skeletally immature adolescent with acute knee pain or injury [8]:

●Acute trauma – Is there a single event with a definable mechanism of injury? If so, are there videos (eg, smartphone video clips) or eyewitnesses who can describe what happened?

In such cases, the author asks the patient to replicate the mechanism of injury, if possible. If they are non-weight-bearing, the patient may be able to approximate the position of injury using the contralateral knee.

Knowing the mechanism of injury can help to narrow the differential diagnosis (table 1). However, the precise mechanism of injury can be difficult to ascertain, and many mechanisms can cause more than one injury. Thus, the mechanism should be correlated with the rest of the history and physical findings.

●Pain location – Where is the pain? Ask the patient to point with one finger.

This helps to identify the most likely structures that were injured.

●Mechanical symptoms – Are there mechanical symptoms, such as locking, popping, or catching? Are they associated with pain?

Symptoms of locking or catching suggest meniscal tear, intra-articular loose body, or damage to articular cartilage.

●Knee motion – Can you bend and straighten the knee all the way?

This allows the clinician to assess active range of motion and, if loss of flexion is present, suggests intra-articular pathology (eg, effusion, meniscal tear). However, if the patient has been in a straight leg immobilizer for days or weeks, nonspecific stiffness and pain are likely, making range-of-motion testing less accurate.

●Swelling or effusion – If there is swelling, when did it begin after the injury (eg, immediately following)?

Any swelling of the knee joint (effusion) that develops within 24 hours of the injury indicates intraarticular pathology (eg, meniscus tear, ligament tear, chondral injury) until proven otherwise.

●Instability – Is there a feeling of instability or abnormal movement or a functional limitation (eg, is the athlete unable to bear weight without pain)?

Ask the patient to demonstrate, if they can, with one fist on the other, what the instability feels like. As an example, the author will slide one fist over the other and ask the patient if they ever feel this type of instability, which could be consistent with anterior cruciate ligament (ACL) insufficiency.

●Prior knee injury – Is there a history of previous knee injury? If so, how was it evaluated and treated? See if specific records can be obtained. Did the injury heal? How long did you do rehabilitation before returning to sports?

●Acute-on-chronic pain – Is there a history of chronic pain or illness prior to the acute injury? Was there any fever or limp prior to the injury?

The presence of chronic pain, preceding fever, or limp prior to the acute injury or any acute increase in pain suggests an underlying chronic condition not related to knee trauma.

It is important to consider causes of knee pain unrelated to sports activity. In some children, serious systemic illness may present with a history of incidental trauma. Past medical history, family history, and review of systems should focus on symptoms or signs of other causes of unilateral knee pain or swelling. These are summarized in the following table and may include any of the following (table 2):

●Septic arthritis (see "Bacterial arthritis: Clinical features and diagnosis in infants and children")

●Osteomyelitis (see "Hematogenous osteomyelitis in children: Evaluation and diagnosis")

●Lyme disease (in endemic areas) (see "Lyme disease: Clinical manifestations in children")

●Juvenile idiopathic arthritis (see "Systemic juvenile idiopathic arthritis: Clinical manifestations and diagnosis")

●Hemoglobinopathy or bleeding disorder

●Malignancy (eg, leukemia, osteosarcoma, or Ewing sarcoma)

●Slipped capital femoral epiphysis (SCFE) (see "Evaluation and management of slipped capital femoral epiphysis (SCFE)")

●Legg-Calve-Perthes disease (see "Evaluation of the child with joint pain and/or swelling", section on 'Initial evaluation')

Physical examination — The physical examination of a joint includes inspection, palpation, range of motion assessment, and strength and neurovascular testing. If fracture and joint instability have been excluded, gait should be evaluated. Special tests are selected based upon the most likely diagnostic category, which is based in turn upon the history, including the mechanism of any injury, and physical findings.

Guidance about which examination techniques are most useful for diagnosing specific knee injuries in children and skeletally immature athletes is provided in the text above and a topic devoted to the knee examination. (See 'Specific conditions' below and "Physical examination of the knee".)

Because of the greater likelihood of fractures in children and young adolescents, the clinician should first determine the need for plain radiographs, as suggested by a gross deformity, neurovascular deficit, large knee effusion, bony tenderness, joint-penetrating injury, or signs of pain referred from the hip. If indicated, radiographs should be obtained prior to performing provocative maneuvers or having the patient bear weight or walk (algorithm 1). (See 'Anatomic considerations in children' above and 'Imaging' below.)

●Inspection – The legs should be fully exposed. The overall position of the leg and the presence of gross deformities should be noted; comparison with the opposite knee is often helpful [8]. Gross deformity should alert the clinician to a potentially limb-threatening injury. (See 'Limb-threatening injury' above.)

The skin and soft tissues should be inspected for erythema, swelling, abrasions, laceration, puncture wound, bruising, and foreign body. Swelling within 24 hours of an acute knee injury suggests internal derangement [9].

●Neurovascular assessment – The neurovascular status should be evaluated in all patients with acute knee injury and is of utmost importance in patients with gross deformities. Emergency (ie, immediate) involvement of an orthopedic surgeon with pediatric expertise and a vascular surgeon is warranted for all patients in whom popliteal artery compromise is suspected.

The neurovascular examination includes:

•Distal pulses – Palpation of the dorsalis pedis (figure 7) and posterior tibial pulses and comparison with the unaffected leg to evaluate vascular integrity.

•Popliteal artery and fossa – Examination of the popliteal fossa for an expanding hematoma (picture 5).

•Deep peroneal nerve – Assessment of sensation in the web space between the first and second toes, and ability to actively extend the great toe to evaluate deep peroneal nerve function (figure 8).

•Posterior tibial nerve – Assessment of sensation of the plantar surface of the foot and ability to actively plantarflex the ankle to evaluate posterior tibial nerve function (figure 9).

•Superficial peroneal nerve – Ability to evert the ankle to evaluate superficial peroneal nerve function (figure 8).

Neurovascular injury occurs most commonly in knee (tibiofemoral) dislocations and displaced fractures. It is uncommon after ligamentous injury, even if two ligaments are injured. However, the varus stress caused by simultaneous injury to the lateral collateral ligament (LCL) and one of the cruciate ligaments can be sufficient to injure the superficial peroneal nerve. (See 'Limb-threatening injury' above.)

●Palpation – For patients without gross deformity and whose neurovascular integrity is intact, the physical examination should proceed by asking the patient to indicate, with one finger, the most painful spot. Paying close attention to this area, the entire region of the knee should be palpated, including assessment for an effusion.

Bony structures to palpate include (picture 2 and figure 1):

•Distal femur

•Femoral condyles

•Proximal tibia including tibial tuberosity

•Patella

Patients who have bony point tenderness or tenderness proximal to the joint lines (suggesting fracture) should have the extremity immobilized and undergo plain radiography. (See 'Imaging' below.)

Soft tissues and other areas to be palpated include:

•Anteromedial and anterolateral joint lines including meniscus (picture 2 and picture 3 and picture 4)

•LCLs/medial collateral ligaments (MCLs)

•Quadriceps and patella tendon (picture 2 and figure 10)

•Distal hamstring tendons

Sizeable knee effusions or hemarthroses may be obvious on inspection. With smaller effusions, the medial or lateral dimple adjacent to the patella may be absent compared with the uninvolved knee. To identify small effusions, the author uses a milking maneuver. If an effusion is demonstrated, causes of intra-articular pathology should be investigated. A substantial percentage of effusions that develop following trauma are associated with significant injury, including patellar dislocation, fracture, and ACL tear [10]. Techniques for detecting a knee effusion are described separately. (See "Physical examination of the knee", section on 'Detection of an effusion'.)

●Range of motion – Once a fracture has been excluded based upon physical findings and plain radiographs as indicated, knee motion should be evaluated by asking the patient to flex and extend the knee fully. For patients without full active range of motion, the clinician should perform passive assessment of flexion (picture 6) and extension. Normal, full knee flexion is approximately 135 degrees. (See "Physical examination of the knee", section on 'Range of motion and muscle flexibility'.)

The bounce home test is another test of knee extension. It is performed with the patient supine. The knee is passively flexed to approximately 15 degrees and then permitted to fall passively into extension with gravity (figure 11). The test is positive if the knee does not fully extend (known as "true locking"). True locking can be caused by a displaced torn meniscal flap, a loose intraarticular body, articular surface damage, or a torn ACL.

●Ligaments – Tenderness or joint laxity suggests a ligamentous injury (sprain), the severity of which is graded by the degree of laxity. The grades of sprains are easiest to determine in the immediate post-injury period before the onset of swelling and secondary muscle spasm. Performing the stress portion of the ligament examination gently helps to avoid voluntary guarding. (See "Physical examination of the knee", section on 'Assessing joint stability'.)

●Meniscus – Meniscal tear is suggested by joint line tenderness, joint effusion, loss of smooth passive motion, inability to fully extend the knee, inability to kneel or squat, palpable catching at the joint line as detected by the McMurray maneuver (picture 7), or pain elicited by provocative testing (eg, Thessaly test (movie 1)). (See "Meniscus injury of the knee", section on 'Provocative testing'.)

●Muscles – The muscles around the knee should be inspected for swelling and ecchymosis and palpated for signs of tenderness. The integrity of the quadriceps mechanism and quadriceps strength are assessed by asking the patient to extend the leg against resistance placed at the lower leg (picture 8).

Within a few days of disuse, quadriceps tone becomes notably reduced. Limited muscle contraction may be caused by effusion, pain, or mechanical obstruction. In such cases, internal derangement of the knee should be suspected. (See "Physical examination of the knee", section on 'Quadriceps strength'.)

●Gait – If the initial evaluation does not suggest fracture or dislocation, the clinician should see if the patient is able to bear weight and, if so, observe their gait. Movement of the knee is painful and guarded in patients with internal derangement (ie, fracture, chondral lesion, injury to the cruciate or collateral ligaments, and/or menisci). Patients with these injuries usually have little spontaneous knee movement and are unable to bear weight, particularly if more than one structure is involved (eg, the medial meniscus and ACL). (See "Clinical assessment of walking and running gait".)

Imaging

Plain radiographs

General indications and approach — The Ottawa knee rules (described below) help determine the need for radiographs. Plain radiographs are indicated for children and skeletally immature adolescents who have one or more of the following findings [8,9,11,12]:

●Gross deformity.

●Large knee effusion or any effusion associated with trauma.

●Focal tenderness over the femoral or tibial physis, fibular head, or patella, or other significant point tenderness. Minimal tenderness not associated with swelling in a patient with normal gait generally does not warrant imaging.

●Inability to bear weight or to walk four steps.

●Concern for laceration or penetrating injury extending into the knee joint.

In addition, the author obtains radiographs in the following circumstances:

●Tenderness at the distal femur that is distinct from the patella and other structures of the extensor mechanism and that is not consistent with conditions affecting the iliotibial band (ITB; eg, bursitis).

●The patient is experiencing significant pain that limits their participation in sport or exercise and complains of mechanical symptoms (eg, catching, locking), but hip motion is normal and knee symptoms cannot be reproduced on examination despite several attempts.

Specific views may be needed depending upon the site of injury. Standard views for the knee, distal femur, and patella are described in the table (table 3). Prior to obtaining radiographs, the clinician should closely examine the hip and ankle. If abnormal findings are present, radiographs of the hip or ankle should be obtained. Hip pathology can frequently cause pain that is referred to the knee. (See 'Referred pain' below.)

Patients with complicated fractures of the patella, distal femur, or proximal tibia may require additional imaging (eg, computed tomography [CT] or magnetic resonance imaging [MRI]) to fully characterize the injury [9]. The choice of study varies according to the injury. Consultation with an orthopedic surgeon and radiologist is advised. (See "Proximal tibial fractures in children", section on 'Radiographic findings' and "Distal femoral fractures in children", section on 'Other imaging'.)

Ottawa knee rules — Clinicians should use the Ottawa knee rules to help determine the need for radiographs. According to the Ottawa knee rules, plain radiographs should be obtained in children with any of the following features after acute knee trauma (see "Approach to the adult with knee pain likely of musculoskeletal origin", section on 'Imaging in the evaluation of acute knee pain'):

●Isolated tenderness of patella (with no other bony tenderness of the knee)

●Tenderness at the head of the fibula

●Inability to flex the knee to 90 degrees

●Inability to bear weight both immediately and in the emergency department or clinic for four steps, regardless of limping (ie, unable to transfer weight onto each lower limb two times)

Although derived in adults, the Ottawa knee rules were validated in a multicenter trial of 750 children aged 2 to 16 years who presented to the emergency department with a knee injury and demonstrated 100 percent sensitivity (95% CI 95-100 percent) for identifying fractures, although 89 percent of patients underwent plain radiography [13]. In a subsequent meta-analysis that included this study and two other prospective, observational studies (1130 children, most >5 years of age), the pooled sensitivity and specificity were 99 and 46 percent, respectively, with a pooled negative predictive value of 99.8 percent (fracture prevalence 8.7 percent) [14].

Based on these findings, application of the Ottawa knee rules in children older than five years of age could result in an estimated reduction in radiography as high as 40 percent, depending on local practice. However, large validation and implementation studies of the Ottawa knee rules in children are lacking.

Bedside ultrasound — The extra-articular structures of the knee are readily accessible for detailed sonographic evaluation. Such structures include the MCLs and LCLs, hamstring and quadriceps muscles and tendons, and structures within the popliteal fossa. Clinicians facile with the musculoskeletal ultrasound examination often include it as part of their focused bedside assessment. Ultrasound examination of the knee is reviewed in detail separately (see "Musculoskeletal ultrasound of the knee"). The author does not use bedside ultrasound in his clinic.

Magnetic resonance imaging — Magnetic resonance imaging (MRI) is the best imaging technique for diagnosing tears of the knee ligaments, menisci, tendons, and muscles [8,9,15]. In addition, it can show evidence of patellar dislocation in patients with spontaneous reduction. For clinicians with limited experience evaluating knee injuries in children, we suggest consulting a physician with the necessary experience before ordering an MRI. MRI is most effective when used to refine preliminary diagnoses as determined by the history and physical examination.

MRI is not necessary in all children and skeletally immature adolescents with acute internal derangement, but is useful in the following circumstances [16]:

●Diagnosis remains in question after initial workup.

●Child does not improve with conservative treatment.

●Management plan is to perform diagnostic arthroscopy only if MRI is abnormal.

Examples of conditions for which MRI would be indicated include patellar dislocation with postreduction locking that suggests an intraarticular foreign body or a clinical meniscal injury that does not improve with nonoperative management.

MRI without contrast has a sensitivity approaching 95 percent for surgically confirmed ACL and meniscal injuries in children [12,17]. However, it is no more accurate, and is sometimes less accurate, than the physical examination of an experienced clinician [18-20]. Therefore, clinicians who are unskilled in the knee examination should consult with a sports medicine physician or orthopedic surgeon with training in sports medicine before obtaining an MRI for these indications.

Imaging of the popliteal artery — In patients with concern for popliteal artery disruption (eg, knee [tibiofemoral] dislocation, displaced distal femoral physeal fracture), options for imaging include Doppler flow ultrasound, CT angiography, arteriography, or magnetic resonance angiography (MRA). Selection of the imaging modality should be performed in consultation with an orthopedic or vascular surgeon and radiologist. (See "Knee (tibiofemoral) dislocation and reduction", section on 'Imaging'.)

Arthroscopy — Based upon observational studies, the most common indications for arthroscopy in children with acute knee injuries include ACL tears, meniscal lesions, osteochondral fractures, and tibial intercondylar eminence fractures [21]. Arthroscopy is the most accurate method of diagnosing the cause of internal derangement in patients with a knee effusion. False-positive results are rare and, apart from posterior meniscus injuries, false-negative results are also rare. Arthroscopy is used selectively as an adjunct to the history, physical examination, and MRI when there is a diagnostic dilemma. The only absolute indication for arthroscopy is mechanical disruption of normal knee function.

APPROACH — 

The clinician should rapidly identify and manage knee injuries that are limb-threatening as follows (algorithm 1):

●Gross deformity – Patients with a gross deformity may have a tibiofemoral dislocation, displaced fracture, or a patellar dislocation. Patients with neurovascular compromise require emergency plain radiography to distinguish between a displaced fracture and tibiofemoral dislocation, emergency consultation with an orthopedic surgeon with pediatric expertise and a vascular surgeon, and emergency imaging of the popliteal artery with the choice of modality determined by the consultants. Tibiofemoral dislocations should be reduced immediately by a knowledgeable clinician. (See 'Limb-threatening injuries' below and "Knee (tibiofemoral) dislocation and reduction", section on 'Procedure'.)

Patellar dislocations do not cause neurovascular compromise and can be reduced prior to radiography. (See "Recognition and initial management of patellar dislocations", section on 'Reduction of lateral dislocations'.)

●Neurovascular compromise – Neurovascular compromise may occur with or without gross deformity and may be caused by fractures, knee (tibiofemoral) dislocation, or multiple ligamentous tears. Patients with neurovascular compromise warrant emergency plain radiographs, special imaging of the popliteal artery (eg, Doppler flow study), and consultation with an orthopedic surgeon with pediatric expertise and a vascular surgeon. (See 'Knee (tibiofemoral) dislocation' below.)

●Penetrating joint wound – Open wounds that extend into the joint based upon direct inspection, radiographic features (eg, air in the joint on plain radiograph), or as determined by the saline load test (arthrocentesis with injection of a large volume of normal saline into the joint space and observation of the saline exiting the wound site) warrant prompt care by an orthopedic surgeon with pediatric expertise. (See 'Penetrating joint wound' below.)

In patients without limb-threatening injuries, the clinician should obtain radiographs if any one of the following findings is present or alternatively, according to the Ottawa knee rule (see 'Plain radiographs' above):

●Gross deformity

●Large knee effusion

●Focal tenderness over the femoral or tibial physis, fibular head, or patella

●Inability to flex knee to 90 degrees

●Inability to bear weight or to walk four steps

In addition, if there is concern for laceration or penetrating injury extending into the knee joint, radiographs are indicated.

Close examination of the hip and ankle should be performed. If abnormal findings are identified, plain radiographs of the hip and/or ankle should be obtained. Hip pathology can frequently cause pain that is referred to the knee. (See 'Referred pain' below.)

Patients with fractures receive orthopedic treatment based upon the specific injury and further evaluation to identify associated soft tissue injuries (eg, anterior cruciate ligament [ACL] tear with meniscal injury) and/or an osteochondral fracture. (See 'Fracture' below.)

In patients in whom fractures are not present on plain radiographs but who have a knee effusion, the location of pain, along with key historical features and a careful physical examination can often identify the most likely cause (table 4A-B). If the diagnosis is unclear, the patient should be made non-weight-bearing and prompt follow-up (within 72 hours) should be arranged with a clinician experienced in the evaluation of children and adolescents with knee injuries. (See 'Ligamentous injury' below and 'Meniscal tear' below and 'Quadriceps and patellar tendon injury' below.)

Patients with isolated cutaneous or muscle injury do not usually have a knee effusion. Pain is localized to the wound, cutaneous bruise, or affected muscle. (See 'Cutaneous injury' below and 'Muscle strain or tear' below and 'Popliteus tendinitis' below.)

Systemic disease with unilateral knee pain and/or swelling often has a more indolent and chronic history and is accompanied by findings of inflammation, including fever, warmth, and erythema (table 2). (See 'Systemic disease' below.)

SPECIFIC CONDITIONS

Limb-threatening injuries — A significant risk of neurovascular compromise or infection exists with these injuries and rapid recognition and treatment are required (algorithm 1).

Displaced proximal tibial fracture — Displaced fractures involving the proximal tibial physis are most common in young adolescents who sustain high-energy hyperextension forces. Ligamentous injury and internal knee derangement are possible associated injuries. (See "Proximal tibial fractures in children".)

Specific fractures include:

●Tibial tubercle avulsion fractures – These are rare causes of proximal tibial fracture and occur most commonly in adolescents as the epiphyseal ossification center begins to fuse with the tibial tubercle ossification center. These injuries are sustained most frequently by young athletes during forceful contraction of the quadriceps against resistance or during rapid knee flexion, as occurs in jumping or running sports (eg, planting the foot to dunk a basketball) (figure 12 and image 1). Acute compartment syndrome is a rare yet recognized complication thought to be secondary to disruption of the anterior recurrent tibial artery.

Acute tibial tubercle apophyseal fractures are distinct from tibial tubercle apophysitis (Osgood-Schlatter disease), a common condition of gradual onset that is not limb-threatening. (See "Osgood-Schlatter disease (tibial tuberosity avulsion)".)

●Proximal tibial physis fractures – Proximal physis fractures associated with posterior displacement of the metaphyseal fragment may cause laceration of the popliteal artery (figure 13).

Physical findings of proximal tibial fractures include a large hemarthrosis, pain, and deformity. Anterior-posterior (AP) and lateral plain radiographs of the lower leg, including the knee and ankle, should be obtained. The presentation and management of proximal tibial physeal and apophyseal fractures are discussed separately. (See "Proximal tibial fractures in children".)

Displaced femoral physeal fracture — A hyperextension injury of the distal femur can cause anterior displacement of the epiphysis. Common mechanisms for such injury include a direct blow (eg, bicycle or vehicular trauma) or indirect force (eg, landing from jump out of a tree or off a "jungle gym"). In this situation, the proximal fracture fragment can be driven posterior into the popliteal fossa where it can damage popliteal vessels (picture 5 and picture 9) and the common peroneal or posterior tibial nerves.

Alternatively, a valgus force applied directly to the lateral side of the distal femur can cause a Salter-Harris type II or III fracture (figure 14). These fractures typically have an associated anterior cruciate ligament (ACL) tear associated with knee instability. Less commonly, they may be open fractures. (See "Distal femoral fractures in children", section on 'Mechanism of injury'.)

The presence of a distal femur fracture is usually suspected based upon a history of high-energy trauma and examination findings of swelling and tenderness around the distal femur (picture 10). Patients with distal femur fractures are in severe pain and unable to bear weight. Associated hamstring muscle spasms cause the knee to be flexed with resultant knee deformity.

Distal femur fractures may injure vascular structures behind the knee, and a careful neurovascular examination is imperative. The soft tissues around the knee must be examined for possible acute compartment syndrome (a limb-threatening emergency) at the initial presentation and subsequently. (See "Acute compartment syndrome of the extremities", section on 'Clinical features'.)

Initial evaluation of a suspected distal femur fracture should include standard AP and lateral radiographs of the knee and entire femur, including the hip, to determine the extent of the fracture and degree of displacement and to rule out an associated knee dislocation. The Ottawa Knee Rules provide guidance for determining when radiographs are indicated and are discussed below. (See 'Plain radiographs' above.)

When significant concern for fracture persists despite negative radiographs, CT or MRI can be used to detect occult fractures [22]. Plain radiographs may significantly underestimate fracture displacement, for example with Salter-Harris III injuries, compared with CT or MRI [23]. However, the degree of fracture displacement seen on initial radiographs is significantly associated with the incidence of physeal injury and the arrest of growth [24]. Management of physeal fractures of the distal femur is discussed in greater detail separately. (See "Distal femoral fractures in children".)

Knee (tibiofemoral) dislocation — Knee (tibiofemoral) dislocations are much less common than physeal fractures in skeletally immature children and adolescents. Suspected dislocations warrant emergency imaging with plain radiographs and immediate reduction, followed by close neurovascular assessment including arterial imaging (eg, Doppler flow study, CT angiogram, or arteriography of the popliteal artery). If emergency imaging is not available, reduction should be performed and the patient transferred to a facility with vascular surgery, vascular radiology, and orthopedic surgery expertise. (See "Knee (tibiofemoral) dislocation and reduction", section on 'Procedure' and "Knee (tibiofemoral) dislocation and reduction", section on 'Postreduction management'.)

Knee dislocations are most common after high-velocity trauma (eg, motor vehicle collision or fall from height) but can occur from lower-velocity trauma in contact sports. Mechanisms of injury include hyperextension, valgus rotary stress, and varus rotary stress. Knee dislocations result from the rupture of at least two, but usually three, of the four major ligaments (figure 15), resulting in multidirectional instability. The ACL and posterior cruciate ligament (PCL) are involved in most cases (image 2), although not always. Most knee dislocations are anterior or posterior, with the direction of dislocation indicating the movement of the tibia relative to the femur. (See "Knee (tibiofemoral) dislocation and reduction", section on 'Clinical presentation and mechanism of injury'.)

Dislocations are obvious when there is gross knee deformity. However, in children and adolescents with an obvious deformity, displaced fractures may look like a knee dislocation. In addition, gross deformity may be absent when dislocations reduce spontaneously, making diagnosis difficult. Thus, whenever three ligaments have been torn (any combination of the ACL, medial collateral ligament [MCL], PCL, or lateral collateral ligament [LCL]), the possibility of a spontaneously reduced knee dislocation must be considered and the neurovascular status of the lower extremity thoroughly assessed.

It is important to inquire about the mechanism and the position of the leg immediately following the injury, and whether it changed subsequently. Complications of knee dislocation include popliteal artery and peroneal nerve injury (picture 5 and picture 9), which occur primarily after AP and lateral dislocations, respectively. In one observational study of 2267 children 19 years of age or younger with tibiofemoral (knee) dislocations, vascular injury occurred in 2.4 percent of patients [25]. (See "Knee (tibiofemoral) dislocation and reduction", section on 'Complications'.)

Penetrating joint wound — Lacerations that extend through the knee capsule may have associated ligamentous or tendon lacerations and pose a significant risk of joint infection. These wounds can be identified as follows:

●Direct visualization of joint capsule disruption.

●Air or a radiopaque foreign body seen within the joint on plain radiographs or advanced imaging.

●Arthrocentesis with injection of a large volume of normal saline into the joint space and observation of saline exiting the wound site (ie, saline load test). Such injection is used when suspicion is high that the laceration extends into the joint but this is not confirmed by examination or radiograph. (See "Severe lower extremity injury in the adult patient", section on 'Soft tissue and bone assessment'.)

Colored liquids have been added to the injected saline with intention of improving detection. However, in an arthroscopy study using surgical incision wounds, the addition of methylene blue did not improve sensitivity or decrease the fluid volume needed for a positive test [26]. CT has greater sensitivity and specificity for detecting traumatic arthrotomy than the saline load test [27].

Penetrating joint injuries warrant immediate orthopedic consultation for debridement and repair. Tetanus prophylaxis should be provided as needed (table 5). Prophylactic antibiotics may be warranted depending on the nature of the injury (eg, contaminated wound) and patient (eg, immunocompromised). The indications for antibiotic prophylaxis are discussed separately. (See "Infectious complications of puncture wounds", section on 'Antibiotic prophylaxis'.)

Common injuries — The most common knee injuries in children and skeletally immature adolescents consist of cutaneous injuries, ligamentous tears, fractures, and patellar dislocations. ACL tears are the most common ligamentous injury. (See 'Epidemiology' above.)

Cutaneous injury — Abrasions, lacerations, and ecchymoses commonly accompany knee injuries in children and skeletally immature adolescents [1,2]. The management of minor skin wounds and lacerations is reviewed separately. (See "Minor wound evaluation and preparation for closure" and "Skin laceration repair with sutures".)

Patients without internal derangement (eg, fractures, chondral lesions, and injuries to ligaments or menisci) have a normal knee examination, can bear weight, and demonstrate normal or near-normal gait. Oral analgesia with acetaminophen or ibuprofen and local measures (eg, cleaning and covering an abrasion or laceration, providing cold therapy for contusions) can ease apprehension and facilitate cooperation with the knee examination.

Lacerations and puncture wounds near the knee joint should be assessed for disruption of the joint capsule. Wounds that penetrate the joint warrant immediate consultation with an orthopedic surgeon. (See 'Penetrating joint wound' above.)

Prepatellar bursitis — Direct repetitive trauma (eg, from wrestling or other sports or in a patient who spends a lot of time on their knees, possibly due to impaired ambulation) may cause inflammation of the prepatellar bursa, located between the patella and overlying skin. Patients typically have swelling and tenderness of the anterior knee (picture 11 and figure 16). The diagnosis can usually be made by inspection and palpation of the anterior patella. (See "Knee bursitis".)

If the onset of swelling is acute and the bursa is red and tender, a septic bursitis may be present and aspiration of the bursal sac performed. Treatment includes short-term immobilization and antibiotics. (See "Septic bursitis".)

Ligamentous injury — Examination of the ligaments includes palpation for tenderness and assessment of laxity with appropriate maneuvers. These maneuvers can be performed gently to help determine a preliminary diagnosis without causing great discomfort. Guarding frequently limits the sensitivity of the physical examination in patients with acute injuries.

●Anterior cruciate ligament (ACL) – ACL tears are the most common ligamentous knee injury in children and skeletally immature adolescents. They are typically sustained through a noncontact injury but may also occur from a direct blow to the knee causing hyperextension or valgus deformation. Contact injuries that cause ACL tears can also cause fractures (eg, tibial eminence fracture) or meniscus tears.

Ligament tears are suspected based on mechanism and clinical signs. Typical activity- or sport-related mechanisms include sudden change of direction while running (ie, "cutting") or landing from a jump, causing the knee to "pop" or give out. ACL tears typically cause an acute knee effusion. Provocative maneuvers are often positive and can include:

•Lachman test (picture 12)

•Anterior drawer test (picture 13)

•Pivot shift test (figure 17))

•Lever test (figure 18)

The performance and interpretation of provocative maneuvers to identify ACL injuries are discussed in greater detail separately. (See "Anterior cruciate ligament injury", section on 'Physical examination'.)

Definitive diagnosis can be made by MRI or knee arthroscopy. In many instances, the clinical presentation can establish the diagnosis and MRI is not needed emergently. It is generally performed after the acute evaluation to confirm the diagnosis and rule in or out other intraarticular injuries that influence decisions about surgery. (See "Anterior cruciate ligament injury", section on 'Diagnosis'.)

The treatment of ACL injuries, including pediatric considerations, is discussed separately. (See "Anterior cruciate ligament injury", section on 'Treatment'.)

●Medial collateral ligament (MCL) – The MCL may be injured via a direct valgus stress from a blow to the lateral aspect of the knee or via an indirect stress through abduction or rotation of the lower leg. Direct blows typically cause more severe injury. MCL injuries are discussed in detail separately. (See "Medial (tibial) collateral ligament injury of the knee".)

Clinical findings of MCL tears consist of focal tenderness over the ligament and ligamentous laxity (picture 3). Swelling is present in most cases but there is no joint effusion if the injury is isolated to the MCL. An effusion suggests injury to an intra-articular structure (eg, meniscus, ACL) in addition to the MCL.

Laxity of the MCL is evaluated with the valgus stress test (picture 14 and movie 2). Laxity with the knee flexed to 30 degrees suggests an isolated MCL sprain. Laxity with valgus stress at 0 degrees of knee flexion (ie, full extension) may indicate any of the following:

•Fracture (eg, involving distal femoral epiphysis or physis)

•Involvement of posteromedial compartment with a tear of the capsular component of the MCL

•Complete ACL tear in addition to MCL tear (as the ACL contributes to medial stability in full extension). If examination suggests that both the ACL and MCL are torn, possible knee dislocation must be evaluated. (See 'Knee (tibiofemoral) dislocation' above.)

Radiographs should be obtained if the patient is unable to bear weight or if focal bony tenderness, gross deformity, or an effusion is present. In skeletally immature patients, a minority of isolated MCL injuries are due to bone avulsion rather than ligament tears [28]. (See 'Plain radiographs' above.)

Diagnosis is made clinically based on the history and examination findings. MRI may be used to establish a definitive diagnosis but is not mandatory unless other ligamentous or meniscal injuries that would require surgery are suspected or the patient has sustained severe trauma. Sprains involving the distal MCL near the tibial insertion are less likely to heal than proximal sprains closer to the femoral origin, and MRI may help to estimate recovery time. (See "Medial (tibial) collateral ligament injury of the knee", section on 'Diagnosis'.)

●Lateral collateral ligament (LCL) – Isolated injuries of the LCL are relatively uncommon. Most occur in association with injuries to the posterolateral corner (PLC), posterior cruciate ligament (PCL), lateral meniscus, or ACL. LCL tears are discussed in detail separately. (See "Lateral collateral ligament injury and related posterolateral corner injuries of the knee".)

Patients who have sustained an LCL injury typically present following a blow to the medial or anteromedial aspect of their knee while it was fully extended and report lateral or posterolateral knee pain. Alternatively, patients can sustain a noncontact LCL injury from a sudden varus moment (knee bending laterally) while the knee is hyperextending.

The most common examination finding is tenderness along the lateral knee (picture 4). Localized soft tissue swelling may be appreciated at the site of injury. In the presence of a grade 3 LCL sprain (complete tear), the normally taut LCL cannot be palpated when the patient sits with the involved knee in a figure-of-four position (figure 19).

Laxity of the LCL is evaluated with the varus stress test (picture 14) as follows:

•Laxity with varus testing at 30 degrees of flexion indicates an isolated LCL sprain.

•Gross lateral instability with varus testing of the fully extended knee suggests disruption of the capsular component of the LCL and the midportion of the PCL, and PLC instability, including the arcuate popliteal complex. Other tests for posterior lateral corner injury include the dial test (picture 15) and posterolateral drawer test (picture 16).

Examination maneuvers to assess injuries of the LCL and related structures are discussed separately. (See "Lateral collateral ligament injury and related posterolateral corner injuries of the knee", section on 'Examination findings'.)

A preliminary diagnosis of LCL tear is made based upon clinical findings. Plain radiographs and MRI are indicated if instability is noted with the knee fully extended. (See 'Imaging' above.)

●Posterior cruciate ligament (PCL) – Acute PCL injuries are rare in children and skeletally immature adolescents, but when present are often associated with knee instability and increase the risk for neurovascular compromise. PCL injury can present with symptoms similar to those associated with LCL tears. Children with an isolated PCL tear have posterior knee pain but do not complain of instability (as they may with an ACL tear), unless an injury to the posterolateral corner (PLC) is also present.

The PCL is evaluated with the posterior sag test and the posterior drawer test. In the posterior sag test, the patient is supine, with hips flexed to 45 degrees, knees flexed to 90 degrees, and the feet flat on the table. In a positive test, the involved leg sags backward relative to the uninvolved leg (picture 17). A positive posterior drawer test (figure 20) suggests PCL injury. (See "Physical examination of the knee", section on 'Tests for PCL injury and posterior stability'.)

Associated injury to the PLC is suggested by positive external rotation recurvatum, posterolateral drawer (picture 16), reverse shift, and dial (picture 15) tests. These are described separately. (See "Lateral collateral ligament injury and related posterolateral corner injuries of the knee", section on 'Examination findings'.)

Knee injuries involving the PCL and PLC are typically severe and both plain radiographs to identify associated fractures and MRI should be obtained. Patients with these injuries warrant immediate consultation with an orthopedic surgeon with pediatric expertise. (See "Posterior cruciate ligament injury".)

Fracture — Children and skeletally immature adolescents are at risk for the following fractures after acute knee trauma (see 'Anatomic considerations in children' above):

●Physeal fractures – Fractures of the physes (growth plates) of the distal femur and proximal tibia are important causes of knee pain in the young athlete. Patients with these fractures typically have pain, swelling, and possibly deformity at the site of injury and are unable to walk. Diagnosis is usually made by plain radiograph. (See 'Plain radiographs' above.)

Patients with displaced physeal fractures are at risk for neurovascular compromise or acute compartment syndrome, depending upon the injury. These more concerning fractures are discussed above. (See 'Displaced proximal tibial fracture' above and 'Displaced femoral physeal fracture' above.)

Management of physeal fractures is discussed separately. (See "Proximal tibial fractures in children" and "Distal femoral fractures in children".)

●Tibial spine (intercondylar eminence) avulsion fractures – The tibial spine is an important bony prominence to which the ACL attaches. Tibial spine avulsion fractures result from hyperextension of the knee with concurrent internal rotation of the tibia. This is the same mechanism that causes ACL injury in skeletally mature patients. Patients with tibial spine fractures have hemarthrosis, knee joint tenderness, and a markedly decreased range of motion. Diagnosis is confirmed with plain radiographs. (See "Proximal tibial fractures in children", section on 'Mechanism of injury' and "Proximal tibial fractures in children", section on 'Physical findings'.)

Management of tibial spine fractures is discussed separately. (See "Proximal tibial fractures in children", section on 'Tibial spine avulsion'.)

●Patellar sleeve fractures – Patellar sleeve fractures are caused by an indirect force applied to the patella through sudden, forceful contraction of the quadriceps. These forces remove a "sleeve" of periosteum from the remaining ossified patella. This might occur when someone lands on their feet after jumping from a moderate height or comes to a sudden stop from a full sprint. (See "Patella fractures", section on 'Mechanism of injury and fracture pattern'.)

Examination typically reveals a knee joint effusion or hemarthrosis and focal tenderness of the patella. The patient may not be able to extend the knee against gravity, and a gap in the extensor mechanism may be palpable. (See "Patella fractures", section on 'Clinical presentation and examination'.)

Diagnosis of a patellar sleeve fracture is based upon clinical findings and imaging. Radiographic findings include a high-riding patella or a distal fracture fragment. However, an unremarkable plain radiograph does not rule out a fracture because bone fragments can be difficult to see. Other imaging (eg, ultrasound, CT, or MRI) may be necessary to confirm the diagnosis. The management of avulsion fractures is similar to that for quadriceps or patellar tendon injuries; surgery is typically required. (See "Patella fractures", section on 'Pediatric considerations'.)

●Osteochondral fractures – Osteochondral fractures involving the intraarticular portions of the femoral condyles or tibial plateau can accompany patellar dislocation, ligamentous tears, or meniscal injuries in children and skeletally immature adolescents [29,30]. Osteochondral fragments may arise from these fractures causing mechanical symptoms (eg, catching or locking) [31]. If untreated, these fractures may develop into osteochondral defects and, ultimately, may cause osteoarthritis. Patients typically complain of continued pain, swelling, and mechanical symptoms. These fractures may be detected by MRI [32]. Patients with these injuries warrant referral to an orthopedic surgeon with pediatric expertise.

Patellar dislocation — Patellar dislocation typically occurs when the lower leg is externally rotated and the quadriceps muscles forcefully contract to extend the knee. Patellar dislocation also can follow collision with another person, particularly if the knee sustains a valgus (inward) force (eg, tackle during American football or rugby play). Patients with first-time dislocations may report hearing a loud pop or tear and feeling a sensation of tearing and dislocation. (See "Recognition and initial management of patellar dislocations", section on 'Mechanisms of injury'.)

An unreduced acute dislocation manifests as an obvious lateral patellar displacement (picture 18). Displacement in any other direction is rare in children and skeletally immature adolescents. In a patient with normal tissue elasticity who dislocates the patella for the first time, lateral dislocation tears the medial patellar stabilizers, including the medial retinaculum, and the medial patellofemoral ligament, which causes a rapidly enlarging medial hemarthrosis. Most often, there is tenderness at the medial patella and medial femoral epicondyle. By contrast, patients who have chronic subluxation of the patella, or in whom the patellofemoral joint is hypermobile, bleeding and swelling after an episode may be minimal.

Acute patellar dislocation can be complicated by bone and chondral fracture. Treatment consists of reduction of the lateral dislocation (figure 21). Imaging is generally obtained following reduction. Dislocations in any other direction warrant emergency imaging and prompt consultation with an orthopedic surgeon with pediatric expertise. (See "Recognition and initial management of patellar dislocations", section on 'Clinical features and diagnosis' and "Recognition and initial management of patellar dislocations", section on 'Reduction of lateral dislocations'.)

Patients with a spontaneously reduced patellar dislocation, and to a lesser extent with patellar subluxation, experience pain in the area of the medial retinaculum and feel apprehensive when the examiner tries to push the patella laterally (apprehension test (figure 22); dynamic apprehension test (figure 23)). While medial joint line tenderness (picture 19) and pain with valgus testing (picture 14) are associated with both MCL sprain and patellar subluxation or dislocation, the apprehension test is typically negative in patients with MCL sprain. (See "Medial (tibial) collateral ligament injury of the knee", section on 'Diagnosis'.)

Muscle strain or tear — Injuries to the muscles surrounding the knee may present acutely or as an acute exacerbation of a chronic problem. Acute injuries typically involve sudden, forceful eccentric contraction of the involved muscle. The muscle groups most commonly injured include the hamstring, quadriceps, and gastrocnemius. Symptoms include local pain, which can be severe and limit motion. Significant swelling, tenderness, and ecchymosis may be present. The diagnosis of a muscle strain or tear is based upon clinical findings. Advanced imaging (eg, MRI) is only employed if there is diagnostic uncertainty. Clinical features and management of muscle strains and tears, organized by muscle group, are discussed separately. (See "Hamstring muscle and tendon injuries" and "Quadriceps muscle and tendon injuries" and "Calf injuries not involving the Achilles tendon".)

Meniscal tear — Meniscal tears occur when the patient twists the leg while bearing weight (eg, landing from a jump). Meniscal injuries are sometimes described as "torn cartilage" or a "locked knee." The mechanisms for meniscal injuries are similar to those of ACL injuries, and these injuries often occur together in children and skeletally immature adolescents [33,34].

Younger children (under 10 years) who present with pain, intermittent snapping of the knee, and reduced knee extension may have a discoid meniscus, a congenital variant in which meniscal tissue extends over the tibial plateau. Most cases involve the lateral meniscus. The discoid meniscus is more prone to tearing than a normal meniscus. Examination findings are similar to meniscal tears generally. Diagnosis can be confirmed with MRI or arthroscopy. (See "Approach to chronic knee pain or injury in children or skeletally immature adolescents", section on 'Discoid meniscus and lateral meniscus injury'.)

Weight-bearing is usually limited by pain in patients with acute meniscal tears, and joint swelling typically develops within 24 hours. Mechanical symptoms may be more common than pain because only the periphery of the menisci contains nerve fibers. Mechanical symptoms include locking, popping, and catching. Other symptoms include swelling, pain with rotation or flexion (particularly at the extremes of those motions), pain along the joint line, weakness or giving way, and generalized aching. (See "Meniscus injury of the knee", section on 'Mechanism and presentation'.)

Signs of meniscal injuries, which are similar to those in children with chondral fractures or injuries to the articular cartilage [35], include effusion, joint line tenderness (picture 19), positive Thessaly test (movie 1), positive McMurray or modified McMurray test (picture 7), positive bounce-home test (figure 11), and restricted joint motion. These tests are discussed in detail separately. (See "Physical examination of the knee", section on 'Special tests for specific conditions'.)

A working diagnosis is based upon clinical findings. Both clinical examination and MRI have a low positive predictive value compared with arthroscopy [36]. The positive predictive value of the clinical examination for medial and lateral meniscal injuries in children and adolescents is 15 and 34 percent, respectively, compared with arthroscopic examination; the negative predictive value for each is over 95 percent [20]. Thus, if the experienced clinician thinks there is no meniscal tear based on clinical assessment, they are justified in not getting an MRI; if they think one is present, performing an MRI is a reasonable next step. (see "Meniscus injury of the knee", section on 'Diagnosis')

Acute-on-chronic knee pain — Sometimes pain due to chronic conditions increases acutely leading patients to seek medical attention. These circumstances are usually clarified by the history. Causes of chronic knee pain in children and skeletally immature adolescents are reviewed in detail separately; a few notable causes that may present because of acute exacerbations in pain are described briefly below. (See "Approach to chronic knee pain or injury in children or skeletally immature adolescents".)

●Osgood-Schlatter disease (tibial tubercle apophysitis) – Osgood-Schlatter is an overuse injury caused by repetitive strain and chronic traction on the secondary ossification center (apophysis) of the tibial tubercle (picture 20). It occurs in active adolescents with a recent growth spurt. The condition causes anterior knee pain that increases gradually over time, but may be exacerbated acutely by direct trauma, running, jumping, or kneeling. The characteristic physical finding is exquisite tenderness over the tibial tubercle, focal swelling and warmth directly over the apophysis, and an otherwise normal knee examination. (See "Osgood-Schlatter disease (tibial tuberosity avulsion)".)

●Sinding-Larsen-Johansson disease (patellar apophysitis) – Sinding-Larsen-Johansson (SLJ) is another overuse injury caused by activities that involve jumping, which causes repetitive traction by the patellar tendon on the patella and avulsion of the secondary ossification center (apophysis) of the inferior pole of the patella. SLJ typically affects children between 10 and 13 years. Physical examination demonstrates pain and swelling at the lower pole of the patella. Patellar tendon thickening and infrapatellar bursitis may be present. Otherwise, the knee examination is usually unremarkable. (See "Approach to chronic knee pain or injury in children or skeletally immature adolescents", section on 'Sinding-Larsen-Johansson disease (patellar apophysitis)'.)

●Hoffa disease (fat pad impingement) – The infrapatellar fat pad is deep to the patellar tendon and extends from the inferior pole of the patella to the upper tibia (figure 24). Impingement of the fat pad causes vague achy pain deep to the patella that can be abrupt and sharp at a specific angle of flexion or extension. Fat pad impingement is characterized by pain at terminal extension of the knee and is seen most often in dancers, gymnasts, or swimmers whose activities require kicking or maximum knee extension. Examination reveals diffuse tenderness over the patellar tendon and infrapatellar fat pad. Extending the knee against resistance and the bounce home test (figure 11) can reproduce pain. (See "Approach to chronic knee pain or injury in children or skeletally immature adolescents", section on 'Fat pad impingement (Hoffa disease)'.)

●Osteochondritis dissecans (OCD) – OCD is a localized osteonecrosis of subchondral bone. OCD of the knee typically presents with pain that may be associated with a specific injury or develops over several months in highly active patients. Pain increases with exercise and may be poorly localized. Knee stiffness and intermittent swelling may occur. With larger lesions, mechanical symptoms (eg, catching or locking) may be present, but motion deficits are uncommon. There may be tenderness over the medial femoral condyle in the flexed knee. Plain radiographs establish the diagnosis (image 3). (See "Osteochondritis dissecans (OCD): Clinical manifestations, evaluation, and diagnosis".)

●Plica syndrome – Plica syndrome is caused by a redundant synovial thickening called a plica. Patients may complain of anterior or medial pain that is worse with activity, such as running, squatting, going up or down stairs, or kneeling, and after prolonged sitting. The pain is often accompanied by a popping sensation with knee flexion or other mechanical symptoms (eg, clicking, catching). Typically, only one knee is involved. Physical examination is nonspecific but a thickened tender plica may be palpable in the medial superior portion of the knee and there may be swelling medial to the patellar border. The diagnosis is made only after other causes of knee pain are excluded. (See "Plica syndrome of the knee".)

Uncommon causes — Less commonly, acute knee pain in children and skeletally immature adolescents arises from tendon injury, pain referred from hip pathology, or as a local manifestation of systemic disease in a patient with incidental knee trauma.

Quadriceps and patellar tendon injury — Quadriceps and patellar tendon injuries occur uncommonly in children and skeletally immature adolescents. Patients with these injuries may have a secondary cause of tendon weakness, such as anabolic steroid abuse, kidney disease, metabolic bone disease, or treatment with medications associated with tendon rupture (eg, quinolone antibiotics). Tendon tears and complete rupture occur following a sudden, strong contraction of the quadriceps muscles, as happens when landing from a high jump or making a sudden change in direction at high speed. The movement is followed immediately by sharp pain at the knee. (See "Quadriceps muscle and tendon injuries", section on 'Quadriceps and patellar tendon tears'.)

With complete tendon tears, the patient may experience a sudden "pop" or tearing sensation at the patella followed immediately by swelling and difficulty bearing weight. An acute hemarthrosis may develop. Focal tenderness along the anterior knee is present just above the patella or at the superior border for quadriceps ruptures and at the distal pole or anywhere along the course of the patellar tendon for a patellar tendon tear. Typically, knee extension is limited with partial tears and absent with complete tears. Patellar fractures frequently accompany tendon tears.

Diagnosis is made based upon clinical findings and may be confirmed with bedside ultrasound. Plain radiographs are indicated to evaluate for an associated patellar fracture. Quadriceps and patellar tendon injuries are discussed in detail separately. (See "Quadriceps muscle and tendon injuries".)

Popliteus tendinitis — Patients complain of pain localized to the posterolateral knee of relatively acute onset. Pain may occur during prolonged descents during hiking or hill running and is exacerbated by excessive forefoot pronation. The popliteus muscle is a secondary restraint to anterior femoral translation during descent, and demands on the muscle increase as the quadriceps, hamstrings, and iliotibial band (ITB) become exhausted with prolonged exercise. Chronic popliteus tendinopathy may be confused with ITB syndrome. Pain from popliteus tendon injury can often be elicited by placing the affected leg in a figure-of-four position and palpating the origin of the popliteal tendon beside the lateral femoral condyle (figure 25). (See "Calf injuries not involving the Achilles tendon", section on 'Popliteus tendinopathy'.)

Referred pain — Acute knee pain can be referred from primary pathology in the hip, such as septic arthritis, osteomyelitis, slipped capital femoral epiphysis (SCFE), or Legg-Calvé-Perthes disease (avascular necrosis). Typically, the pain is anterior, dull, nonradiating, and does not localize to bone or soft tissues of the knee. In such patients, the knee examination is usually normal and the hip examination demonstrates a limited range of motion and/or pain. Patients with septic arthritis have significant pain on any movement of the hip. The clinician should urgently involve an orthopedic surgeon for joint drainage in patients with a septic hip. (See "Bacterial arthritis in infants and children: Treatment and outcome" and "Approach to hip pain in childhood", section on 'Examination'.)

Adolescents with SCFE can present with knee or thigh pain. Range of motion of the hips, particularly internal rotation, is often limited. Urgent referral is necessary because a delay in diagnosis can lead to worsening of the condition and an increased risk of severe complications (eg, osteonecrosis of the femoral head or narrowing of the joint space). (See "Evaluation and management of slipped capital femoral epiphysis (SCFE)", section on 'Clinical manifestations'.)

Legg-Calvé-Perthes disease (idiopathic avascular necrosis of the femoral head) generally presents between 3 and 12 years of age with a peak incidence at 5 to 7 years of age. While complaints of hip pain are most common, some patients complain of acute or chronic knee pain with activity.

Systemic disease — In some pediatric patients, serious systemic illness may present with a history of incidental trauma. Fever, local findings of inflammation, rash, a history of chronic knee pain, or a past medical history that identifies an underlying bleeding disorder can help identify these patients. Systemic diseases that may cause unilateral knee pain and/or swelling include septic arthritis, juvenile idiopathic arthritis, osteomyelitis, Lyme disease (endemic areas), hemoglobinopathy, bleeding disorder, benign bone tumors, or malignancy (eg, leukemia, osteosarcoma, or Ewing sarcoma) (table 2). (See "Evaluation of the child with joint pain and/or swelling", section on 'Initial evaluation'.)

SOCIETY GUIDELINE LINKS — 

Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Anterior cruciate ligament injury".)

SUMMARY AND RECOMMENDATIONS

●Limb-threatening injuries – Initial evaluation of children with an acute knee injury requires rapid identification of any limb-threatening injuries, which are suggested by gross deformity, neurovascular compromise, or penetration of the joint capsule (algorithm 1). These patients warrant plain radiographs of the knee and emergency (ie, immediate) consultation with an orthopedic surgeon. Such injuries include:

•Displaced proximal tibia fracture

•Displaced femoral physis fracture

•Knee (tibiofemoral) dislocation

•Penetrating joint wound

Knee (tibiofemoral) dislocations should be reduced immediately. Patients with displaced fractures should have the knee immobilized or splinted in a position of comfort without worsening neurovascular function. Patients with neurovascular compromise warrant specialized imaging as determined by orthopedic and vascular surgery consultants. (See 'Limb-threatening injuries' above and 'Limb-threatening injury' above and 'Approach' above.)

●Non-limb-threatening injuries – Once limb-threatening injury is excluded, the history and physical examination provide a working diagnosis. The differential diagnosis can often be narrowed based upon the mechanism of injury and location of pain (table 1).

●Indications for imaging for non-limb-threatening conditions – The clinician should obtain plain radiographs if any of the following findings is present or if indicated by the Ottawa knee rule (see 'History' above and 'Physical examination' above and 'Plain radiographs' above):

•Gross deformity

•Large knee effusion

•Palpable tenderness over the femoral or tibial physis, fibular head, or patella

•Inability to bear weight or to walk four steps

•Concern for laceration or penetrating injury extending into the knee joint

Range of motion and palpation of the hip and ankle should be performed. Patients with abnormal findings should have hip or ankle radiographs in addition to knee radiographs. Hip pathology can frequently refer pain to the knee. (See 'Referred pain' above.)

●Knee effusion without fracture – If no fracture is seen on plain radiograph but an effusion is present, the location of pain, along with key historical features and physical examination findings are often sufficient to determine the most likely cause (table 4A-B). Physical findings associated with specific acute knee conditions are described in the text. (See 'Specific conditions' above.)

Patients with isolated cutaneous or muscle injury do not usually have a knee effusion. Pain is localized to the wound, cutaneous bruise, or affected muscle. (See 'Cutaneous injury' above and 'Muscle strain or tear' above and 'Popliteus tendinitis' above.)

●Diagnostic imaging – MRI is the best technique for the diagnosis of soft tissue injuries including tears of the ligaments, menisci, tendons, and muscles. It can also reveal signs of patellar dislocation following spontaneous reduction.

MRI is not necessary in all children and skeletally immature adolescents with clinical findings of these injuries but can be used if the diagnosis is in question. Clinicians with limited experience evaluating knee conditions should consult with a sports medicine physician or orthopedic surgeon with training in sports medicine before obtaining an MRI. (See 'Magnetic resonance imaging' above.)