Physical examination of the knee

INTRODUCTION — Knee pain and other knee-related complaints are a common reason for visits to primary care clinics and emergency departments. An effective and efficient evaluation of the patient with knee-related complaints depends upon an understanding of the knee's anatomy and function, and the proper performance of an appropriately focused physical examination.

The examination of the knee is reviewed here. A brief review of knee anatomy and biomechanics is also provided. The work-up of patients with knee-related complaints and specific knee injuries and conditions are discussed separately. (See "Approach to the adult with knee pain likely of musculoskeletal origin" and "Approach to the adult with unspecified knee pain".)

ANATOMY

Bones and articulations — The knee joint contains four bones—femur, tibia, patella, and fibula—and consists of three compartments—the medial tibiofemoral, lateral tibiofemoral, and patellofemoral—all sharing a common synovial cavity (figure 1 and figure 2 and picture 1 and picture 2 and figure 3). The knee has three articulations: medial and lateral tibiofemoral and patellofemoral. The tibiofemoral articulations connect the distal femur, which broadens to form the medial and lateral femoral condyles, and the tibia. The tibia is relatively flat, but the sloped menisci permit a tight articulation with the convex femoral condyles. The femoral condyles are separated by an intercondylar fossa, also called the femoral groove or femoral trochlea.

The patella is a sesamoid bone embedded in the quadriceps tendon that articulates with the trochlear groove of the femur (picture 3 and figure 4). Its function is to increase the mechanical advantage of the quadriceps. The fibular head lies within the capsule of the knee but is not normally involved as a weight-bearing surface. The joint lines are formed by the femoral condyles and the tibial plateaus.

Structures providing support and stability — Several soft tissues contribute to knee stability and provide cushioning within the joint (figure 1). The parts of the tibia and femur contained within the knee joint are lined with shock absorbing hyaline cartilage. Disc-shaped lateral and medial menisci provide additional shock absorption and distribute forces across the joint. The anterior and posterior cruciate ligaments provide stability with anterior and posterior movements and with flexion and extension; the medial and lateral collateral ligaments provide support in their respective planes (picture 1). Other structures that contribute to knee stability include the iliotibial band (figure 5) and parts of the posterolateral corner (figure 6).

Each femoral condyle articulates with a crescent-shaped meniscus that deepens the fossa in which the condyles articulate with the tibia (picture 4 and figure 7). The menisci are thickest at their external margins, where they attach to the rim of the tibia, and taper to become much thinner at their unattached edges in the knee's interior. The lateral meniscus is more circular and has greater mobility anteriorly and posteriorly than the medial meniscus, which is attached to the posterior oblique ligaments and the medial collateral ligament. The relative immobility of the medial meniscus may make it more susceptible to shearing and rotational forces, and it is injured more often than the lateral meniscus. The peripheral one-third of each meniscus is nourished directly by blood vessels, whereas the inner two-thirds are nourished by diffusion from the synovial fluid, reducing the capacity of the inner portion to heal following injury. In addition to their structural role, the menisci absorb compressive forces and play a role in joint lubrication. (See "Meniscal injury of the knee".)

The anterior cruciate ligament (ACL) originates at the posteromedial aspect of the lateral femoral condyle. It courses distally in an anterior and medial fashion to the anteromedial aspect of the tibia between the condyles (figure 2). The ACL is the principal structure providing restraint to anterior translation of the tibia relative to the femur and serves as a secondary restraint against tibial rotation and varus and valgus stress. The ACL contains two bundles: an anteromedial bundle that is tight in flexion and a posterolateral bundle that is tight in extension. (See "Anterior cruciate ligament injury".)

The posterior cruciate ligament (PCL) originates at the anterolateral aspect of the medial femoral condyle (picture 1). It courses distally in a posterior and lateral fashion and inserts behind the intercondylar eminence, medial to the ACL. The PCL is the thickest and strongest of the knee ligaments. The PCL provides the primary restraint to posterior translation, especially with the knee flexed greater than 30 degrees, and to external rotation, especially with the knee flexed 90 degrees or more. The PCL exerts maximum control of external rotation at 90 degrees of knee flexion, where it compensates nearly completely for a damaged posterolateral corner [1-3].

The medial (tibial) collateral ligament (MCL) has a superficial and a deep component (picture 5 and figure 3). The superficial MCL originates slightly proximal and posterior to the medial femoral epicondyle. It inserts in two locations: proximally on the tibia about 1 cm distal to the joint line (attached mainly to soft tissue) and distally about 6 cm below the joint line. The superficial MCL serves as the primary static stabilizer of the medial knee. The superficial MCL provides stability against valgus or external rotation forces. The deep MCL is a thickening of the joint capsule with its origin about 1 cm distal to the origin of the superficial MCL on the femur. It inserts into the medial meniscus and on the medial tibia just distal to the joint line. The deep MCL provides secondary valgus stability when the superficial MCL is injured [4,5]. (See "Medial (tibial) collateral ligament injury of the knee".)

The lateral (fibular) collateral ligament (LCL) originates at the lateral femoral epicondyle and inserts on the lateral head of the fibula (picture 2). The LCL provides the principal restraint to varus stress at the knee [3]. The biceps femoris splits and extends along each side of the LCL to its attachment at the fibular head. The popliteus muscle originates just inferior to the lateral femoral condyle, passes deep to the LCL, and is juxtaposed to the biceps femoris. It is reinforced in the posterolateral component of the joint by the arcuate ligament, with contributing fibers from the posterior capsule and fibular head. Fibers from the lateral meniscus and PCL contribute to the popliteus muscle. The muscle inserts into the posterosuperior portion of the medial tibial condyle and functions as an internal rotator of the tibia or lateral (external) rotator of the distal femur. It also protects the posterior horn of the lateral meniscus during flexion by pulling the horn posteriorly, and reinforces the PCL in preventing anterior translation of the femur during deceleration and downhill running.

The iliotibial band (ITB) is a broad, thick fascia that extends from the tensor fascia lata, gluteus maximus, and other muscles proximally and attaches to the vastus lateralis fascia anteriorly (figure 5). The ITB crosses the prominence of the lateral femoral condyle to attach at Gerdy's tubercle at the anterolateral portion of the proximal tibia, and blends with the lateral patellar retinaculum near the lateral joint line. (See "Iliotibial band syndrome".)

The posterolateral corner (PLC) or posterolateral "complex" consists of several structures. The major structures include the iliotibial tract, lateral collateral ligament, popliteus tendon, popliteofibular ligament, popliteotibial and popliteomeniscal fascicles, middle third of the lateral capsular ligament, fabellofibular ligament, arcuate ligament, posterior horn of the lateral meniscus, lateral coronary ligament, and posterolateral portion of the joint capsule [6]. These structures, in conjunction with the posterior cruciate ligament (PCL), stabilize the knee against external rotation and posterior translation across a wide range of knee positions [2,6]. A detailed discussion of the anatomy and function of each component of the PLC is beyond the scope of this topic.

Plica are embryologic remnants of tissue related to the synovium that appear as thin, narrow veils of tissue when normal, but become thick and fibrotic when abnormal. Of the several plica that can develop, the medial plica most often becomes symptomatic. It arises from the medial wall of the knee joint, passes inferiorly around the medial femoral epicondyle, and inserts into the synovium surrounding the infrapatellar fat pad. (See "Plica syndrome of the knee".)

The anterolateral ligament (ALL) has been identified as a distinct structure originating at the lateral femoral epicondyle, just anterior to LCL, and inserting on the anterolateral aspect of the proximal tibia, midway between Gerdy's tubercle and the fibular head [7]. Some hypothesize that the ALL may play a role in controlling internal tibial rotation, and thus affect the pivot shift phenomenon. However, studies assessing the function of the ALL are predominantly biomechanical and anatomical; studies pertaining to the clinical importance of the ALL, including the need for repair when injured, are lacking [8,9].

Structures involved in knee extension — The extensor mechanism of the knee consists of the quadriceps muscles, the quadriceps tendon, the patellofemoral joint, the patellar tendon, and the tibial tubercle (picture 6 and picture 7). The confluence of the vastus intermedius, vastus lateralis, vastus medialis, and the rectus femoris forms the quadriceps tendon. The patella is a sesamoid bone lying within the quadriceps and patellar tendons. The patella articulates with the trochlear groove of the femur through a substantial lateral facet and a smaller medial facet. This groove is shallow proximally and deeper distally. The patella moves from the proximal narrow part of the groove when the knee is extended, to the distal deeper part of the groove when the knee is flexed. The patella is supported by complex medial and lateral retinacula. (See "Quadriceps muscle and tendon injuries".)

The medial patellofemoral ligament, originating at the medial margin of the patella and inserting into the medial femoral epicondyle, is the most important stabilizer of the patella and is part of the medial retinaculum. Deep to the retinacula are medial and lateral synovial recesses, which communicate with the suprapatellar pouch. The strap-like patellar tendon originates at the inferior patellar pole, runs obliquely to insert on the relatively lateral tibial tubercle.

Popliteal fossa and structures involved in knee flexion — The popliteal fossa defines, in part, the posterior surface anatomy of the knee (picture 8 and picture 9). The distal portion of the hamstring muscles (the semitendinosus, semimembranosus, and the biceps femoris), which are the primary flexors of the knee and play an important role in hip extension, form the superior margin of the popliteal fossa. From there, they diverge: the biceps femoris forms the lateral margin, and the semitendinosus and semimembranosus form the medial margin. The long head of the biceps femoris originates at the ischial tuberosity, while the short head originates from the femoral shaft; both insert at the fibular head. The semitendinosus and semimembranosus both originate from the ischial tuberosity and insert at the medial tibia in the pes anserine region (picture 10 and figure 3). (See "Hamstring muscle and tendon injuries".)

The inferior border of the popliteal fossa is defined by the medial and lateral heads of the gastrocnemius muscle. With the knee extended, the gastrocnemius muscle functions as a plantar-flexor of the ankle; during knee extension, the gastrocnemius helps to prevent anterior translation of the femur on the tibia.

Bursa and cystic structures — There are several cystic structures that are typically found around the knee joint including ganglia and synovial bursae. Ganglion cysts are abnormal, benign, lined by dense fibrous connective tissue, and contain viscous fluid. Bursae are lined by synovial cells, also contain viscous fluid, and are located between structures where there is friction or movement. They are generally found in predictable locations. Ganglia and bursae have variable communication with the intraarticular knee joint. Para-articular cysts are frequently associated with abnormalities, such as meniscal tears or other internal derangements of the knee. (See "Knee bursitis".)

Popliteal (ie, Baker's) cysts are the most common synovial cyst in the body. They arise from the bursa that lies between the medial head of the gastrocnemius muscle and semimembranosus tendon, and typically lie posterior to the superior portion of the medial femoral condyle in the medial popliteal fossa, but can extend medially, laterally, or superiorly, and may by superficial or deep. Although often present in asymptomatic knees, popliteal cysts are frequently associated with internal derangement of the knee.

The anterior knee has four common bursae. The suprapatellar bursa is located proximal to the knee joint capsule between the rectus femoris tendon and the femur. It communicates with the knee joint in most adults. The prepatellar bursa is located directly anterior to the patella. The superficial infrapatellar bursa lies superficial to the tibial tubercle and distal patellar tendon, while the deep infrapatellar bursa lies deep between the distal aspect of the patellar tendon and the anterior tibia. The superficial bursa can become inflamed from overuse or trauma, as during prolonged kneeling, whereas swelling of the deep infrapatellar bursa results from overuse of the knee extensor mechanism, as can occur with repeated jumping or running.

Three bursae are commonly found at the medial aspect of the knee. The pes anserine bursa is located between the tibial insertion of the MCL and the conjoined distal tendon of the sartorius, gracilis, and semitendinosus muscles. Acute bursitis can develop from overuse, as may occur with distance running, while chronic swelling can occur with obesity or degenerative joint disease.

The semimembranosus-tibial collateral ligament is located posterior and superior to the pes anserine bursa [10]. It forms a "U" around the semimembranosus with the superficial part between the tendon and the MCL and the deep part between the tendon and the medial tibial condyle. A bursal space lies just below this ligament and rarely swells and becomes symptomatic. Focal pain at the posterior medial joint line from inflammation of this bursa may mimic a meniscal or other internal injury.

The MCL bursa is present in the large majority of adults and lies between the superficial and deep divisions of the MCL. MCL bursitis can also mimic the symptoms of a meniscal tear.

There are no true or primary bursae at the lateral knee, but chronic friction between the iliotibial band and the lateral femoral condyle can cause the formation of a secondary, or adventitial, bursa.

Neurovascular structures — A neurovascular bundle that includes the popliteal artery, popliteal vein, and tibial nerve (continuation of sciatic nerve) travels directly posterior to the knee joint (picture 8). Vascular injury can occur with severe trauma, such as a tibiofemoral (knee) dislocation. (See "Knee (tibiofemoral) dislocation and reduction".)

The peroneal nerve is the lateral division of the sciatic nerve (figure 8). After it branches off the sciatic nerve, the peroneal nerve briefly courses between the medial border of the biceps femoris muscle and the lateral head of the gastrocnemius muscle, then moves laterally around the biceps femoris muscle, dives between the peroneus longus and the fibular head (peroneal tunnel), and trifurcates as it exits the tunnel and becomes the deep peroneal, superficial peroneal, and recurrent peroneal nerves. With a course that is superficial and in places, constrained by narrow spaces, the peroneal nerve is susceptible to trauma or compression injuries [11]. (See "Overview of lower extremity peripheral nerve syndromes", section on 'Compression at the fibular neck'.)

BIOMECHANICS — Although commonly thought of as a simple hinge between the femur and tibia, the knee's actual motion is more complex. The contours of the medial and lateral femoral condyles are oblique (oval), not round (circular). Because of this condylar obliquity, the center of knee rotation changes as the knee is flexed and extended. The axis around which the knee rotates moves in a J-like shape, following the contour of the condyles from anterior in extension, to a posterior location with the knee in full flexion. In addition, the femur slides anteriorly on the tibia during extension and posteriorly on the tibia during flexion. Thus, with the knee in full flexion, the posterior portions of the femoral condyles lie adjacent to the posterior portions of the tibial plateaus, while the converse is true in extension. The ratio of rotation to sliding varies somewhat among individuals and changes over the course of knee flexion and extension. Overall, these two aspects of knee movement are analogous to the motion of a rocking chair as its long, curved rockers both rotate on and slide along the floor.

Basic knowledge of these flexion-extension biomechanics helps us to understand some knee injury patterns. As an example, hyperextension of the knee with the foot planted (ie, closed chain) causes compression injuries of the anterior articular cartilage and bone of the femur and tibia's joint surfaces. If the knee were a simple hinge, the expected pattern of hyperextension injury would involve pinching of the soft tissues anteriorly and tearing of the supporting tissues posteriorly. However, due to the knee's polycentric rotation and anterior-posterior sliding, hyperextension actually increases weight-bearing, compressive forces across the anterior joint surfaces, which explains the observed pattern of injury.

In addition to polycentric rotation and anterior-posterior sliding in the sagittal plane, knee motion involves rotation in the transverse plane. Both static and dynamic stabilizing structures contribute to this rotation. In full active extension, the smaller and more curved lateral femoral condyle reaches terminal extension earlier than the medial condyle, and further lateral extension is checked by the lateral condyle and by tightening of the anterior cruciate ligament (ACL). The less curved medial femoral condyle allows for continued extension as well as some posterior tibial sliding before medial extension is checked by the condyle and reinforced by posterior cruciate ligament tightening. This increased motion of the medial compartment during extension causes the femur to rotate medially on the tibia, which tightens the collateral ligaments as the knee "screws home" into terminal extension.

While the locking of the knee in extension and medial rotation occurs spontaneously with quadriceps extension and the action of static stabilizers, flexing the knee requires an active unlocking (lateral rotation) of the femur before flexion can begin. This lateral rotation or "unlocking" is performed by the popliteal muscle, and releases tension in the collateral ligaments to allow smooth tibiofemoral rotation and gliding under the influence of the hamstrings, which serve as the primary knee flexors.

Knowledge of knee rotation also helps us to understand certain knee injury mechanisms. As described above, the ACL provides an important restraint to terminal knee extension in the lateral compartment, as well as restraining medial tibial rotation during extension. If the ACL is torn, these restraints are lost and the normal coupling of extension with anterior gliding is disrupted, allowing the tibia to rotate medially. This combination of unrestrained extension and medial rotation allows the tibia to move anteriorly and forcefully strike the lateral femoral condyle. These mechanics explain the classic "kissing bone bruises" seen on sagittal magnetic resonance imaging (MRI) images following an acute ACL tear. These uncontrolled rotatory forces in the lateral compartment also explain why concomitant injury to the lateral meniscus is extremely common in acute ACL injury, rather than the more widely known but much less common "unhappy triad" of medical meniscus, medial collateral ligament, and ACL injury. (See "Anterior cruciate ligament injury".)

TIPS FOR A PRODUCTIVE EXAMINATION — The following tips may be helpful for performing the knee examination effectively and efficiently:

●Observe the patient, noting their posture, ability to bear weight and gait; watch how they move the affected lower extremity.

●Make the patient comfortable. This helps to ensure motion that is as natural as possible and reduces the likelihood of guarding during the examination.

●Perform the examination systematically.

●Compare knees.

●Use patient demographics and the history to guide the functional examination.

Help the patient to relax as much as possible during the examination. A relaxed patient is more likely to display authentic movement patterns and to allow the clinician to perform functional tests with little or no voluntary guarding. Minimize the patient's discomfort by having them take appropriate analgesics (typically over-the-counter medication, such as acetaminophen) prior to the examination.

Use a systematic approach to examine the knee. Perform the essential elements of the examination (inspection, palpation, tests of motion and strength) in the same order and manner each time. This will make you more facile with the examination and prevent you from missing things. Some examiners begin with the patient standing and then proceed to assess gait, followed by a seated and finally a supine examination. This may improve the flow of the exam. Comparing knees and the rest of the lower extremities is often important to performing an accurate examination.

When deciding what examination techniques to perform, pay attention to patient demographics and history. The patient's age, occupation, athletic activities, and history of present illness determine, to a large extent, the likelihood of different knee ailments.

Remember that knee pain may be referred from the hip, lumbar spine, or distal lower extremity.

TELEMEDICINE EXAMINATION — An article with extensive video clips and photographs describing in detail how to perform the musculoskeletal examination of the knee, hip, spine, and shoulder remotely using telemedicine is provided in the following reference [12]. Other discussions of the telemedicine knee evaluation with supplemental materials to assist in the examination are available [13,14].

The COVID-19 pandemic has led to a dramatic increase in the study and use of telemedicine for musculoskeletal assessment and physiotherapy. Overall, telemedicine assessment of musculoskeletal complaints has been shown to be feasible in a physical therapy setting [15]. One study of telemedicine versus in-person assessment of the knee reported "exact" primary diagnosis agreement in 67 and "similar" diagnosis agreement in 89 percent of cases [16]. Intra-rater reliability was 89 percent (high), and inter-rater reliability was 67 percent (moderate). Little has been published about on the efficacy of telemedicine assessment of knee pain in the orthopedic or sports medicine specialty clinic setting.  

INSPECTION — To ensure adequate inspection, make sure your patient is appropriately covered but that the lower extremities are completely exposed. Compare findings with the unaffected lower extremity to detect asymmetries. Look at the following elements:

●Gait – Normal, limping (antalgic), shuffling, or cannot walk

●Swelling – Effusion versus other soft tissue swelling (eg, bursitis)

●Ecchymosis and other signs of injury (eg, abrasions)

●Muscle atrophy

●Alignment – Varus (knee bends outward) or valgus (knee bends inward)

●Skin changes – Scars (surgical or traumatic), rash, lymphangitis

Observe gait — Inspection begins when the patient enters the exam room. Changes in gait, arm swing, seating, squatting, and stooping motions may provide clues to injury patterns. For overuse injuries of the knee, such as like patellofemoral pain or iliotibial band syndrome, observation of dynamic genu varum or valgum and patellar tracking and motion during gait are key components of the clinical assessment.

Swelling and ecchymosis — Look for swelling. Effusion, a collection of fluid within the joint capsule, is a common cause. Some joint effusions are large and easy to see, while others are subtle and only detectable with careful palpation, if they can be identified at all on examination. Swelling within bursae or within the soft tissue or skin can also cause a visibly swollen knee. Ecchymosis suggests blunt trauma. Abrasions or other signs of trauma may be present. (See 'Detection of an effusion' below.)

Atrophy — Atrophy of the quadriceps (vastus medialis atrophy is common) or gastrocnemius/soleus muscle complex indicates disuse or lack of neural stimulation to the atrophied muscle. The cause may be neurologic disease or musculoskeletal injury, although the latter is more common. Injury to the lower extremity can limit full motion or cause motor neuron inhibition, either of which promotes muscle atrophy. A knee effusion can cause visible quadriceps atrophy within days. Comparison with the uninjured knee helps with recognition.

Alignment — Alignment in anterior (or frontal) plane can be determined by having the patient stand barefoot with their knees exposed. With neutral alignment, the patient's knee falls on a straight line drawn from the anterior superior iliac spine to a point midway between the medial and lateral malleolus of the ankle. A varus knee lies lateral to this line, while a valgus knee lies medial.

Skin changes — Inspect the skin of the affected lower extremity looking for rashes, bruising, swelling, redness, foreign bodies, and scars, both surgical and traumatic.

PALPATION

General approach — Palpation is best performed with the knee flexed, which allows greater access to the anterior joint line. Palpation should proceed in a systematic fashion and include key structures along the lateral and medial joint lines, key structures off the joint line, including the posterior knee, variations in skin temperature, and joint effusion (picture 6 and picture 11 and picture 12 and picture 13).

We prefer to begin palpation with the patient seated on the exam table with their knee flexed to 90 degrees. If this cannot be done, the patient may lie supine with a pillow placed under their thigh, causing the knee to flex about 20 to 30 degrees. The posterior knee and popliteal fossa can be palpated with the patient sitting or standing, but in some older patients and those with significant injuries or swelling, it is often easier to have them lie prone to allow for more thorough inspection and palpation. If necessary, a pillow may be placed under the ankle of the prone patient to maintain some knee flexion.

Another useful technique is to palpate along the anterior joint line (medial and lateral) while the standing patient repeatedly flexes their knee about 20 degrees and straightens (extends) it using a gradual motion. This maneuver may help the clinician detect clicks or focal tenderness, or may reproduce symptoms, possibly related to a meniscal injury or thickened plica.

Joint line palpation — The joint lines are palpated to assess the medial and lateral knee compartments. Find the joint line by placing your thumbs in the recesses inferolateral and inferomedial to the patella (picture 11). Focal tenderness at a specific site usually indicates damage to a specific structure in that location. Diffuse tenderness along the joint line is most commonly due to irritation of the synovial membrane caused by a degenerative, inflammatory, or infectious process, but localized injuries such as meniscal and collateral ligament tears may also cause diffuse tenderness. (See "Clinical manifestations and diagnosis of gout" and "Septic arthritis in adults" and "Meniscal injury of the knee" and "Medial (tibial) collateral ligament injury of the knee" and "Lateral collateral ligament injury and related posterolateral corner injuries of the knee" and "Clinical manifestations and diagnosis of osteoarthritis", section on 'Knee'.)

Lateral joint line — From the starting position described above, gradually move your thumb posteriorly along the lateral joint line towards the popliteal fossa. You should palpate the following structures in the order listed (picture 12):

●Anterior horn of lateral meniscus

●Lateral Collateral Ligament (origin and insertion are respectively superior and inferior to the joint line)

●Posterior horn of lateral meniscus

●Distal portion of biceps femoris muscle and tendon (crosses joint line)

An alternative approach to palpating the lateral joint line is to have the seated patient place the ankle of the affected extremity atop the knee of the opposite leg. This position widens the lateral joint line allowing for easier palpation of some structures, particularly the LCL. However, not all patients with an injured knee can achieve this position.

Medial joint line — From the starting position shown in the picture above, gradually move your thumb posteriorly along the medial joint line towards popliteal fossa. You should palpate the following structures in the order listed (picture 13):

●Anterior horn of medial meniscus

●Medial collateral ligament (origin and insertion are respectively superior and inferior to the joint line)

●Posterior horn of medial meniscus (meniscus tears most commonly occur in the posterior horns; medial meniscus tears are more common than lateral)

●Distal portion of medial hamstring muscles and tendons (cross joint line):

•Semitendinosus – is round like a tendon

•Semimembranosus – is broad and flat like a membrane

Anterior knee off the joint line — Several structures that may contribute to knee pain are palpated off the joint-line at the anterior and lateral areas of the knee. In the right clinical setting, tenderness of these structures helps to confirm a specific diagnosis. The following structures are encountered with palpation of the anterior knee, moving proximally from the region just distal to the knee joint (picture 11):

●Tibial tuberosity

●Patellar tendon

●Patella

●Quadriceps tendon

Medial to the patellar tendon and below the medial tibial plateau lies the insertion of the medial hamstring muscle the semitendinosus, which joins with the tendons of the sartorius and gracilis to form a common pes anserine tendon (along with its underlying bursa), which should be palpated (picture 10). The pes feels slightly gristly and may be mildly tender without injury. Palpating the opposite, unaffected knee allows the clinician to distinguish normal mild tenderness from pathologic tenderness.

Additional structures to palpate along the lateral knee include:

●Iliotibial band (ITB)

●Lateral femoral condyle

●Fibular head and tibiofibular syndesmosis

The lateral femoral condyle is palpated to assess the integrity of the ITB. ITB syndrome is characterized by localized pain, a palpable snapping, or both where the band crosses the condyle, before inserting on the proximal tibia. The proximal fibula (head and neck) and the proximal tibiofibular syndesmosis are palpated to check for a fibular collateral ligament injury, high syndesmotic tear, or fibular fracture.

Posterior knee — In the normal knee, few structures are palpable in the posterior fossa. In this space, which lies between the medial and lateral hamstring tendons, the only routinely palpable item is the pulse of the popliteal artery (figure 9 and picture 9).

In addition to assessing the popliteal pulse, palpation of the popliteal fossa is performed to determine the cause of any popliteal fullness or discomfort described by the patient. A sensation of pressure or fullness is most often associated with a joint effusion, however, masses too can develop. Most commonly, these masses are non-pulsatile and represent simple popliteal cysts. Popliteal cysts are fluid filled masses originating from the knee joint capsule ("Baker's cyst"), bursae of surrounding muscles (semimembranosus bursitis or gastrocnemius bursitis), or synovial tissue surrounding the numerous tendons and nerves in the region. (See "Popliteal (Baker's) cyst".)

The differential diagnosis of a popliteal mass also includes large posterior fat pads, vascular lesions, and a swollen popliteal tendon. Palpation of the much rarer pulsatile mass in the posterior fossa suggests a fistula or aneurysm of the popliteal artery and warrants urgent investigation. The tendon of the popliteus muscle (popliteal tendon) is only palpable when swollen, but provides an important diagnostic clue to popliteal muscle or posterior-lateral corner injury. When swollen, the popliteal tendon is palpable and tender as it crosses the joint line superficial to the posterior horn of the lateral meniscus. The increased use of musculoskeletal ultrasound has substantially improved the identification and characterization of popliteal masses. (See "Popliteal artery aneurysm".)

Bursae — A number of bursae are found around the knee, including the prepatellar and pes anserine (figure 1). Either of these can become inflamed causing pain and swelling.

Palpate the prepatellar bursa for warmth, focal tenderness, and swelling. Squeeze the walls between your fingertips to assess for bursal wall thickening (picture 14). Acute bursitis is characterized by cystic swelling and variable degrees of tenderness, warmth, and redness. Chronic bursitis is characterized by palpable thickening of the bursal walls.

Palpate the anserine bursa to check for local tenderness. Be sure to palpate the entire tibial plateau so you can distinguish the localized tenderness of anserine bursitis from tenderness associated with the medial collateral ligament (picture 10). An area of tenderness approximately 25 mm in diameter at the level of the tibial tubercle but medial to it is the classic location of anserine bursitis. Tenderness that extends from the anserine bursal area to the joint line is more likely due to irritation or injury of the medial collateral ligament.

To palpate the pes anserine tendon and bursa, find the anteromedial joint line and then move your fingers inferiorly along the anteromedial shaft of the tibia about 6 cm (2 inch) (picture 10). Rub your fingers back and forth over this region and you will feel the gristly pes anserine tendon beneath the skin. The pes anserine region is often slightly tender even when uninjured and thus comparison with the unaffected side is important. Significant differences in tenderness between affected and unaffected sides and confirmation that palpation reproduces the patient's pain indicate a positive examination and suggest pes anserine bursitis or tendon injury.

Infrapatellar bursal swelling is more difficult to palpate but occurs below the insertion of the distal patellar tendon into the tibial tubercle. Warmth and tenderness in this region suggest the presence of infrapatellar bursal swelling; experience with ultrasound confirms that the condition is fairly common.

What is sometimes called "suprapatellar bursitis" commonly refers to swelling in the superior extension of the knee capsule above the patella. Palpation may reveal the area to be tender and puffy. This extension lies under the quadriceps tendon attachment to the superior patella. Swelling here may be the only indication of a joint effusion in many patients.

Skin temperature — Joints are normally cooler than surrounding tissues because they are relatively avascular. As an example, place the back of your hand on your thigh several centimeters above the knee. The thigh feels slightly warm. Now place the back of your hand over the patella or just medial to it. This region will normally feel cooler. Finally, place the back of your hand on the lateral calf and it will feel warm. This normal pattern of "warm-cold-warm" is reassuring that there is no inflammation or irritation inside the joint.

A pattern of "warm-warm-warm" indicates knee irritation. This could be from an acute or chronic injury, and the joint may or may not appear swollen. Regardless of the cause, a "warm-warm-warm" pattern is not normal. Additional tests for joint effusion and internal derangement (eg, meniscus injury, ligament tear, or cartilage/bone fracture) should be performed.

A "warm-hot-warm" pattern almost always indicates a joint infection, which is usually accompanied by swelling, redness around the knee, and possibly other signs of inflammation. A "warm-hot-warm" knee requires urgent investigation, usually including aspiration and analysis of synovial fluid.

Although this approach to monitoring skin temperature around the knee has not been formally studied, the authors and others have found it to be helpful in clinical practice.

DETECTION OF AN EFFUSION — Suspect a joint effusion if the affected knee is enlarged or lacks full flexion. Even small effusions (5 mL) can be detected with careful examination. There are several ways to detect an effusion:

●Have the patient extend their knees and relax their quadriceps muscles, then compare the size and shape of both knees and inspect the medial and lateral peripatellar dimples. Small effusions (5 to 10 mL) will fill these normal anatomic landmarks and give the knee an appearance of general fullness.

●"Milking the fluid" to the lateral aspect of the knee and then compressing this region can cause the sudden appearance of a fluid bulge medial to the patella (bulge, bubble, or wave sign). Press (or "milk") the synovial fluid proximally from the medial side of the patella into the suprapatellar pouch and then with the opposite hand milk the fluid distally from the suprapatellar pouch along the lateral patella. Maintain pressure over the medial dimple to force the synovial fluid into the lateral compartment; when pressure is released and a milking motion is applied to the lateral dimple the fluid will reappear medially. You may be able to feel a fluid wave by placing your index finger just below the distal portion of the lateral border of the patella and feeling for the impulse on the medial side.

This technique requires practice, but is capable of detecting small effusions. It is easier to perform in patients without a great deal of muscle or adipose around the knee.

●The ballottement sign is positive when there is at least 10 to 15 mL of intraarticular fluid. Using both hands, milk the synovial fluid into the center of the knee from all four quadrants. Next, push the patella firmly into the trochlear groove and release. A moderate effusion is associated with a clicking or tapping sensation. If an effusion is present, pressing on the patella may also give the sensation that it is sitting on fluid.

●Large effusions (20 to 30 mL) often obliterate the normal recesses around the medial and lateral borders of the patella and fill the suprapatellar space. The area just above the superior pole of the patella is usually flat or slightly concave. Large effusions cause a convexity above the patella and a bulging under the distal vastus lateralis muscle and fascia.

It can be difficult to detect of an effusion using the physical examination alone in patients who are obese or have extreme quadriceps hypertrophy (eg, weightlifters) or pathologic changes of the knee joint from osteoarthritis or rheumatoid arthritis. Several small observational studies suggest that ultrasound can serve as a useful adjunct to the physical examination for detecting small to moderate effusions [17-19]. (See "Musculoskeletal ultrasound of the knee".)

Traditionally, joint aspiration is the definitive test for determining the presence of a knee effusion. However, if the effusion is too small to be visualized, palpation-guided aspiration is unlikely to be successful. In modern clinical practice, the gold standard for detecting a knee effusion is musculoskeletal ultrasound.

RANGE OF MOTION AND MUSCLE FLEXIBILITY — Range of motion is described as "active" (performed under the patient's voluntary muscle control) or "passive" (the physician moves the joint while the patient exerts no voluntary control). Common reasons for diminished active but intact passive motion include motor nerve damage, excessive pain, and structural disruption of the muscle tendon unit.

If the patient has full, active range of motion it is not usually necessary to assess passive motion (table 1). However, if active motion is limited, an assessment of passive motion is essential to ensure that the patient does not have a mechanical block. Such blocks may be caused by a torn meniscus or ligament or by a loose body (eg, articular cartilage fragment). Mechanically locked knees are a surgical urgency and should be referred immediately for definitive orthopedic care. Similarly, loss of the ability to extend or flex the knee actively could indicate a tendon rupture, which requires urgent orthopedic evaluation.

The heel-to-buttock distance provides an objective, easy to perform, and reproducible estimation of knee flexion (picture 15). Active knee flexion is assessed by having the prone patient flex the knee maximally, bringing the heel as close as possible to the gluteal fold. Flexion can be measured in degrees using a goniometer or by the minimum distance between the heels and the closest gluteal surface. Normal flexion is approximately 130 degrees. Knee flexibility may be reduced by bulky musculature or tightness or contracture of the quadriceps. Intrinsic knee conditions that impair flexion include knee effusion, popliteal cyst, osteoarthritic changes such as large bony osteophytes, tight collateral ligaments, and previous knee surgery.

Knee extension is assessed by having the seated patient extend the knee maximally. Extension beyond a straight leg, or neutral position (0 degrees), is normal in some patients, but is referred to as hyperextension. Up to 3 to 5 degrees of hyperextension is a normal finding. Hyperextension beyond this is referred to as genu recurvatum, an abnormal finding.

Although not well studied, the Thomas Test provides a measure of the flexibility of the quadriceps and hip flexor muscles [20]. To perform the Thomas test, the patient lies supine, flexes the hip and knee of one leg and holds this leg against their chest, while allowing the other lower extremity to dangle off the end of the examination table (picture 16). If a hip flexion contracture is present, the thigh of the hanging extremity will be angled towards the ceiling rather than level with the table or angled downward. The angle formed by the intersection of the dangling thigh and the examination table represents the degree of flexion contracture. If quadriceps tightness is present, the leg of the hanging extremity will be angled away from the examining table. The angle formed by the intersection of the hanging leg and a line perpendicular to the ground represents the degree of quadriceps tightness.

Hamstring flexibility is most commonly assessed by measuring the popliteal angle (picture 17). With the patient laying supine and the hip flexed to 90 degrees, extend the knee and measure the angle between the leg and a vertical line extending straight up from the hip through the knee. A normal popliteal angle is 0 to 10 degrees for women and 20 to 30 degrees for men.

NEUROVASCULAR ASSESSMENT — Vascular assessment of patients whose primary complaint is knee pain generally consists of palpating lower extremity pulses, namely the dorsalis pedis, posterior tibial, and popliteal. Vascular lesions can present with pulse deficits distal to the site of the lesion. While it may be possible to palpate the popliteal artery in the popliteal fossa of the knee, it is easier and more practical to palpate the dorsalis pedis and posterior tibial pulses to ensure that vascular compromise has not occurred from a knee injury. If deficits are identified that suggest a new or worsening vascular problem, additional testing, and likely referral to a vascular specialist, is needed. (See "Examination of the arterial pulse" and "Clinical features and diagnosis of lower extremity peripheral artery disease", section on 'Diagnosis of lower extremity PAD'.)

Motor testing for patients with knee complaints is usually performed as part of the motor and strength assessments, which are described below. (See 'Range of motion and muscle flexibility' above.)

Basic testing of lower extremity sensation should be performed using objective feedback (eg, sharp vs dull). As neurologic deficits present distal to the site of the lesion, an examination of the L4, L5, and S1 dermatomes is usually sufficient to detect any nerve compression at the knee (figure 10). The sensory examination is described in detail separately. (See "The detailed neurologic examination in adults".)

MOTOR FUNCTION AND STRENGTH — Basic motor function is typically evaluated as part of strength testing using manual resistance provided by the examiner. However, this approach is problematic with the muscles that control knee movement as healthy quadriceps are substantially stronger than the typical examiner's arms and the hamstrings usually can overcome any manual resistance provided. Therefore, the tests described here should be viewed as gross screening tests.

Quadriceps strength — 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 18). Knee extension requires an intact patellar tendon, patella, quadriceps tendon, and quadriceps. With the patient in a seated position and the knee flexed to 45 degrees, firmly grasp the patient's ankle or leg and forcefully attempt to flex the knee while the patient resists, attempting to extend their knee.

Knee pain that is severe enough to cause a gait disturbance leads to weakness and atrophy of the quadriceps muscle within days. An inability to extend the leg against light resistance is consistent with tendon rupture, fracture of the patella, advanced arthritis, or a tense joint effusion. Relative weakness of the quadriceps mechanism is seen with partial tear of the tendons, quadriceps atrophy, and severe pain, and may also be associated with patellofemoral pain or arthritis. Neurologic injury involving the femoral nerve or lumbar nerve roots from L2 to L4 may cause varying degrees of quadriceps weakness.

Hamstring strength — The integrity of the hamstring mechanism and hamstring strength are assessed by asking the patient to flex the leg against resistance placed at the lower leg. Knee flexion requires intact hamstring tendons and muscles. (See "Hamstring muscle and tendon injuries".)

A quick screen for hamstring weakness is performed with the patient in a seated position and the knee flexed to 45 degrees. Firmly grasp the patient's ankle or leg and forcefully attempt to extend the knee while the patient resists, attempting to flex their knee.

Hamstring weakness often persists during recovery from a complete or partial tear of a hamstring tendon or the muscle itself, or from a significant tendinopathy, unless a full course of rehabilitation has been completed. In addition, patients with lumbar radiculopathy, a significant gait disturbance (eg, due to hip osteoarthritis), or sacroiliac joint dysfunction may lead to hamstring weakness on the affected side.

Functional strength tests — When weakness or asymmetries in lower extremity strength may exist, specialized functional strength testing should be performed. Commonly used functional tests include the single leg squat, step-up, step-down, and single leg wall sit. These tests are not useful in every patient with knee pain, but when subtle weakness is suspected or when the general examination does not clarify the diagnosis, these functional strength tests may provide insight into the functional status of the knee and surrounding musculature. The results of these tests also provide a functional baseline that can be used to track a patient's progress as they proceed through a rehabilitation or strengthening program.

●Single leg squat – The simplest functional strength assessment is performed by having the patient stand on their uninjured leg while placing their hands on their hips. The patient then squats as low as possible on one leg and returns to the upright position (picture 19). In a correctly performed single leg squat, the hip, knee, and ankle should form a straight line or incline very slightly outward to form a mild varus angle, and the elbows and hips should be level. The trunk should remain straight and upright to limit assistance from the iliopsoas and quadriceps, making the maneuver a more accurate assessment of hip abductor and external rotator strength. After several practice repetitions, use a goniometer to measure the maximum angle of knee flexion obtained during the squat. Repeat the squat on the affected leg. Differences of more than 5 to 10 degrees may indicate a clinically significant muscle weakness [21].

When performing a single leg squat, collapse of the supporting leg inward (valgus collapse) indicates weakness of the gluteal and/or quadriceps muscles. Perhaps the most common sign of valgus collapse is when the plant leg and free leg come together. Dipping of the unsupported side of the pelvis toward the ground (Trendelenberg position) indicates weakness of the gluteus medius and minimus of the supporting leg.

●Step-up exercise – Instruct the patient to place the foot of their unaffected leg on an exam room stool (be sure it does not have wheels) or sturdy chair and place their hands on their upraised thigh. The foot remaining on the floor is placed in dorsiflexion so that only the heel remains in contact with the ground. Then, have the patient step up onto the stool or chair using only the raised leg, while avoiding pushing off with the other foot. Raise or lower the step until the height is challenging but not excruciating for the patient. Switch legs and have the patient repeat the step-up using the affected leg. Look for differences between sides in exertion or the need to push off with the back foot [21].

●Step-down exercise – Instruct the patient to stand at the front edge of a standard exam room step stool with their hands on their hips. Have them step off the front or side of the stool with their affected leg, touch the heel of the affected leg lightly to the ground and return to the starting position. After several practice repetitions, have the patient repeat the maneuver on the opposite side with the affected leg providing support and the uninjured leg touching down. Indications of muscle weakness include sagging of the shoulders, elbows, or hips; "crashing down" instead of touching the heel lightly to the ground; or an inability to maintain distance between the knees during the step-down.

●Single-leg wall sit – Instruct the patient to place their back against a wall and then walk their feet a distance approximately the length of their thigh away from the wall. Next, ask the patient to lift the foot of the affected leg off the floor and extend the affected knee so the foot remains off the ground. Using the unaffected leg for support, the patient then slides their back down the wall until the unaffected thigh is parallel to the ground. The patient holds this position, supporting their entire body weight with the unaffected leg for as long as possible. Record the time and then repeat the maneuver using affected leg for support. (See 'Neurovascular assessment' above.)

ASSESSING JOINT STABILITY

Valgus stress test for medial instability — The valgus stress test is used to determine the integrity of the medial collateral ligament (MCL). (See "Medial (tibial) collateral ligament injury of the knee".)

To perform the MCL valgus stress test, brace the knee by placing one hand along the lateral aspect of the joint (picture 20). With the other hand, hold the ankle and apply a valgus force to the knee, while keeping the ankle in a neutral position.

The MCL valgus test should be performed with the knee at 0 and at approximately 30 degrees of flexion [22]. This is because the MCL functions as the primary restraint, providing more than 50 percent of the stabilizing force, at both flexion angles, but the posterior capsule and the anterior and posterior cruciate ligaments provide significant secondary restraints to valgus opening in full knee extension. However, as the knee flexes to 30 degrees, the capsule and cruciate ligaments provide no secondary stabilization, leaving the MCL as the only restraint to valgus stress once 25 degrees of flexion are achieved [3]. Hence, a positive valgus test at 0 degrees suggests injury to both the MCL and one (or both) of the cruciates, while a positive test at 30 degrees of flexion and a negative test at 0 degrees suggest an isolated MCL injury.

MCL injuries are graded from 1 to 3:

●Grade 1: stretch injury without dissociation (0 to 5 mm opening on valgus stress)

●Grade 2: partial ligament disruption with 6 to 10 mm opening on valgus stress

●Grade 3: complete ligament disruption with >10 mm opening on valgus stress or lack of any firm endpoint

The combination of a history of injury, focal tenderness at the collateral ligament, and opening of the joint line with stress testing compared to the unaffected knee suggests disruption of the ligament. Pain without abnormal movement of the joint is consistent with a first degree sprain. Pain with opening of the joint but a rapid return to a normal position is consistent with a second degree sprain. Pain and persistent looseness of the joint is consistent with a third degree sprain or complete tear. A false positive test can be seen with wear of the medial articular cartilage.

Given its widespread use, the valgus stress test is not well studied. Two older studies reported sensitivities of 86 [23] and 96 percent [24], respectively, as compared to operative findings. However, neither study used the standard examination technique described above, and in the latter study, many of the examinations were performed under anesthesia. Despite the dearth of supporting literature, our clinical experience suggests that the valgus stress test is among the more useful, easily mastered, and clinically reproducible tests for the knee.

Varus stress test for lateral instability — The varus stress test is used to determine the integrity of the lateral collateral ligament (LCL). To perform the varus stress test, brace the knee by placing one hand along the medial aspect of the joint (picture 20). With the other hand, hold the ankle and apply a varus force to the knee while keeping the ankle in a neutral position. (See "Lateral collateral ligament injury and related posterolateral corner injuries of the knee".)

The LCL varus stress test should be performed with the knee at 0 and at 30 degrees of flexion [22]. This is because the LCL functions as the primary restraint, providing more than 50 percent of the stabilizing force, at both flexion angles, but the posterior capsule and anterior cruciate ligament provide significant secondary restraints to varus opening in full extension. However, as the knee flexes to 30 degrees, the capsule and cruciate slacken and provide no secondary stabilization, leaving the LCL as the only restraint to varus stress once 25 degrees of flexion are achieved [3]. Hence, a positive varus test at 0 degrees indicates injury to both the anterior cruciate ligament (ACL) and LCL, while a positive test at 30 degrees of flexion and a negative test at 0 degrees indicate an isolated LCL injury.

LCL injuries are graded from 1 to 3:

●Grade 1: stretch injury without dissociation (0 to 5 mm opening on varus stress)

●Grade 2: partial disruption with 6 to 10 mm opening on varus stress

●Grade 3: complete LCL disruption with >10 mm opening on varus stress or lack of a good endpoint

Few studies have evaluated the sensitivity and specificity of the varus and valgus stress tests, and their interexaminer reliability [22]. One observational study of 67 patients with arthroscopically confirmed MCL injuries reported that 62 were diagnosed by valgus stress test, for a sensitivity of 93 percent. The few studies comparing examination of the collateral ligaments using the varus and valgus stress testing to magnetic resonance imaging (MRI) images of the ligaments report corresponding findings in 65 to 96 percent of cases [22].

The combination of a history of injury, focal tenderness at the collateral ligament, and opening of the joint line with stress testing compared to the unaffected knee suggests disruption of the ligament. Pain without abnormal movement of the joint is consistent with a first degree sprain. Pain with opening of the joint but a rapid return to a normal position is consistent with a second degree sprain. Pain and persistent looseness of the joint is consistent with a third degree sprain or complete tear.

Tests for ACL injury and anterior stability — The Lachman, pivot shift, and anterior drawer tests are the most useful for assessing anterior knee stability and the integrity of the anterior cruciate ligament (ACL). The performance of these maneuvers and a full discussion of ACL injury are found separately. (See "Anterior cruciate ligament injury", section on 'Physical examination'.)

Tests for PCL injury and posterior stability — The posterior drawer test and the posterior sag sign are the two examination tests used most often to detect a posterior cruciate ligament (PCL) injury. Other tests of posterior knee integrity include the quadriceps active test and dial test. Performance of these four tests is described below.

Posterior drawer test — The posterior drawer test is performed with the subject supine, the hip of the affected leg flexed to 45 degrees, the knee flexed to 90 degrees, and the foot in neutral position (figure 11). While sitting on the foot of the patient's affected leg, wrap both hands around the patient's proximal tibia with your thumbs placed in the region of the tibial tuberosity. Then apply a posterior force to the proximal tibia. Increased posterior tibial displacement compared with the uninvolved lower extremity suggests a partial or complete tear of the PCL [22].

Prior to performing the posterior drawer test, the clinician must ascertain the position of the tibia relative to the femur. Posterior subluxation of the tibia due to a loss of PCL integrity can compromise results. If subluxation has occurred, the examiner must first reduce the tibia by pulling it anteriorly into proper alignment. Failure to do this may result in a false positive anterior drawer or Lachman test, leading the clinician to conclude incorrectly that the ACL has been injured, rather than the PCL [25,26].

Evidence pertaining to the test characteristics of the posterior drawer test and other tests of posterior knee stability are limited, but the posterior drawer test is generally considered the most accurate physical examination maneuver for determining the integrity of the PCL [27-29]. One small, blinded randomized trial found the test to have sensitivity and specificity in the 90 percent range [30].

Posterior sag sign — The posterior sag sign uses the same starting position as the posterior drawer test. If the PCL is intact, the medial tibial plateau lies at least 1 cm anterior to the femoral condyle when viewed directly from the side. If this anterior positioning is diminished or lost, the posterior sag sign is considered positive (picture 21) [22]. Comparison with the uninjured side is helpful. In a grade one PCL injury, the tibia continues to lie anterior to the femoral condyles but the difference is slightly diminished (corresponds to 0 to 5 mm joint laxity). In a grade II injury, the tibia is flush with the femoral condyles (5 to 10 mm laxity). In a grade III injury, the tibia lies posterior to the femoral condyles (>10 mm laxity). Although test characteristics have not been studied in the acute setting, in a small, blinded controlled trial, the posterior sag sign was found to have high specificity but limited sensitivity [30].

Quadriceps active test — To perform the quadriceps active test, the patient assumes the same starting position as for the posterior drawer test. Stabilize the foot (usually by sitting on it) and ask the patient to try to slide their foot forward down the table (against the resistance of your hand, or thigh if you are sitting on their foot) (figure 12). This motion causes the quadriceps muscles to contract, which in the PCL-deficient knee results in an anterior shift of the tibia of ≥2 mm (movie 1). When performing the test, focus your attention on the tibial plateau and tibial tuberosity while the quadriceps contract in order to observe the anterior translation of the tibia. In addition, we often place our thumbs or thumb and index finger along the anterior joint line of the patient's knee to palpate for anterior translation.

Primarily observational studies suggest that this test has a sensitivity ranging from 54 to 98 percent and specificity ranging from 97 to 100 percent [22,29]. In one such study, contraction of the quadriceps resulted in anterior translation of the tibia in 41 of 42 knees with a known PCL disruption, but no anterior translation occurred in the 50 subjects with normal knees (n = 25) or isolated ACL disruptions (n = 25) [31].

Dial test — The dial test is used to detect posterolateral corner injury and the presence or absence of concomitant PCL injury. With the patient prone, hold their feet and passively rotate the tibia externally, first with the knees at 30 degrees of flexion and then at 90 degrees of flexion (picture 22). A difference of more than 10 to 15 degrees of external rotation compared with the uninjured side constitutes a positive test. The combination of a positive dial test at 30 degrees of flexion and a negative dial test at 90 degrees of flexion suggests an isolated posterior lateral corner (PLC) injury, whereas a positive dial test at both 30 and 90 degrees of flexion suggests injury to both the PCL and PLC. Few studies of the dial test have been performed, and the neither the sensitivity nor specificity are known [1,32-34].

SPECIAL TESTS FOR SPECIFIC CONDITIONS — A number of provocative tests are used to detect specific knee pathology [35,36]. The role of these tests in diagnosing such conditions is discussed in the UpToDate topics devoted to them. However, as discussed immediately below, it is prudent to keep in mind the limitations of such maneuvers.

A word of caution — Special tests for the knee offer the clinician the allure of making a definitive diagnosis in a single, deft maneuver. Sadly, studies show this notion to be false [37,38]. A comprehensive knee examination, including history, inspection, palpation, range of motion testing, strength and neurovascular testing, and the performance of special tests yields a correct diagnosis in only about 50 percent of patients [37]. Performing special knee tests in isolation, without the context provided by the history and findings from a general knee examination, causes further declines in diagnostic accuracy. Thus, we suggest performing only those special tests that are most likely to be relevant, as determined by the history, initial examination findings, and the test characteristics (if known) of the special test in question.

Several meta-analysis and reviews have attempted to assess the validity of many commonly used special knee tests. The most frequently studied are maneuvers to detect anterior cruciate ligament (ACL) and meniscal injuries. However, even well-performed reviews of the more commonly used tests are limited by methodologic inconsistencies and variable quality among the primary studies. As an example, many study populations include only patients with known injuries or acute injuries or chronic injuries, making it difficult to know how the test would perform in a broader patient population. Other common limitations include small sample size, variations in examiner training or expertise [39], and the use of magnetic resonance imaging (MRI) findings as the "gold standard" rather than arthroscopy or operative findings [40]. Especially for meniscus tears, where studies of MRI report high false positive rates, the use of MRI findings is problematic. Given these limitations, the relatively poor sensitivity and specificity of most special tests, and the abundance and time-consuming nature of special maneuvers, we believe that a positive result from a single special test should never be used in isolation to make a definitive diagnosis.

Tests for patellofemoral pain — A number of tests are used to help diagnose patellofemoral pain, including the patellar compression test. These tests are discussed separately. (See "Patellofemoral pain", section on 'Physical examination'.)

Tests for meniscal tear — In addition to anterior joint line palpation, which is described above, several provocative tests are used to assess for meniscal injury. These tests are described in detail separately. (See "Meniscal injury of the knee", section on 'Physical examination'.)

Tests for iliotibial band syndrome — The iliotibial band syndrome (ITBS), which occurs primarily in runners, is characterized by an aching or burning pain at the site where the ITB courses over the lateral femoral condyle. (See "Iliotibial band syndrome", section on 'Clinical features' and "Running injuries of the lower extremities: Patient evaluation and common conditions", section on 'Iliotibial band syndrome'.)

SUMMARY AND RECOMMENDATIONS

●Anatomy – The knee joint contains four bones—femur, tibia, patella, and fibula—and consists of three compartments—the medial tibiofemoral, lateral tibiofemoral, and patellofemoral—all sharing a common synovial cavity (figure 1 and figure 2 and picture 2 and figure 3). The knee has three articulations: medial and lateral tibiofemoral and patellofemoral. Multiple soft tissues contribute to knee stability (eg, cruciate and collateral ligaments) and provide cushioning within the joint (eg, menisci). Basic knee anatomy, including descriptions of the structures most susceptible to injury, is reviewed in the text. (See 'Anatomy' above.)

●Biomechanics – Although commonly thought of as a simple hinge between the femur and tibia, the knee's actual motion is more complex, involving multiple movements in both the sagittal and transverse planes. Knowledge of knee movement helps us to understand certain knee injury mechanisms. Basic knee biomechanics are described in the text. (See 'Biomechanics' above.)

●Elements and approach to the physical examination – Examination of the knee involves inspection, palpation, assessment for a joint effusion, testing of motion, testing of motor function and strength, assessment of joint stability, and possibly special tests to detect specific conditions. The examination should be performed systematically. It is often useful to compare affected and unaffected joints; it is essential to use patient demographics and the history to focus the functional examination. (See 'Tips for a productive examination' above.)

●Inspection – While inspecting the knee and lower extremity, the clinician should assess the following: gait, swelling, ecchymosis and other signs of injury, muscle atrophy, alignment, and skin changes (eg, scars, rash). (See 'Inspection' above.)

●Palpation – Palpation of the knee should include the anterior joint line (including lateral and medial aspects), anterior knee off the joint line, posterior knee, bursae, and skin temperature. Focal tenderness at a specific site usually indicates damage to a specific structure in that location. Diffuse tenderness along the joint line is most commonly due to irritation of the synovial membrane caused by a degenerative, inflammatory, or infectious process, but localized injuries such as meniscal and collateral ligament tears may also cause diffuse tenderness. The clinician must determine whether a joint effusion is present. (See 'Palpation' above and 'Detection of an effusion' above.)

●Range of motion – If the patient has full, active range of motion, it is not usually necessary to assess passive motion. Common reasons for diminished active but intact passive motion include motor nerve damage, excessive pain, and structural disruption of the muscle tendon unit. Diminished passive motion is often due to a mechanical block (eg, torn meniscus). (See 'Range of motion and muscle flexibility' above.)

●Neurovascular assessment, motor function, and joint stability – Assessments of neurovascular and motor function and of joint stability are fundamental parts of the knee examination. (See 'Neurovascular assessment' above and 'Motor function and strength' above and 'Assessing joint stability' above.)

●Special tests – Provocative tests are used to detect specific knee pathology, but the sensitivity and specificity of such maneuvers is often limited. We suggest performing only those special tests most likely to be relevant, as determined by the history, initial examination findings, and the test characteristics (if known) of the special test in question. The role of these tests in diagnosing specific conditions is discussed in the UpToDate topics devoted to these conditions (eg, anterior cruciate ligament [ACL] injury). (See 'Special tests for specific conditions' above.)