Many structures that help stabilize the knee are located mainly outside the joint; they include muscles (eg, quadriceps, hamstrings), their insertions (eg, pes anserinus), and extracapsular ligaments. The lateral collateral ligament is extracapsular; the medial (tibial) collateral ligament has a superficial extracapsular portion and a deep portion that is part of the joint capsule.
Inside the knee, the joint capsule and the posterior and highly vascular anterior cruciate ligaments help stabilize the joint. The medial and lateral menisci are intra-articular cartilaginous structures that act mainly as shock absorbers but provide some stabilization (see figure Ligaments of the knee).
The most commonly injured knee structures are the
The mechanism of injury predicts the type of injury:
Inward (valgus) force: Usually, the medial collateral ligament, followed by the anterior cruciate ligament, then the medial meniscus (this mechanism is the most common and is usually accompanied by some external rotation and flexion, as when being tackled in football)
Outward (varus) force: Often, the lateral collateral ligament, anterior cruciate ligament, or both (this mechanism is the 2nd most common)
Anterior or posterior forces and hyperextension: Typically, the cruciate ligaments
Weight bearing and rotation at the time of injury: Usually, menisci
Swelling and muscle spasm progress over the first few hours. With 2nd-degree sprains, pain is typically moderate or severe. With 3rd-degree sprains, pain may be mild, and surprisingly, some patients can walk unaided.
Some patients hear or feel a pop when the injury occurs. This finding suggests an anterior cruciate ligament tear but is not a reliable indicator.
Location of the tenderness and pain depends on the injury:
Sprained medial or lateral ligaments: Tenderness over the damaged ligament
Medial meniscal injuries: Tenderness in the joint plane (joint line tenderness) medially
Lateral meniscal injuries: Tenderness in the joint plane laterally
Medial and lateral meniscal injuries: Pain made worse by extreme flexion or extension and restricted passive knee motion (locking)
Injuries of any of the knee ligaments or menisci cause a visible and palpable joint effusion.
Diagnosis of knee sprains and meniscal injuries is primarily clinical. Stress testing is usually delayed because the pain is so great initially.
A spontaneously reduced knee dislocation should be suspected in patients with a large hemarthrosis, gross instability, or both; detailed vascular evaluation, including ankle-brachial index (ABI), and CT angiography should be done immediately because popliteal artery injury is possible. Next, the knee is fully examined. Active knee extension is assessed in all patients with knee pain and effusion to check for disruption of the knee extensor mechanism (eg, tears of the quadriceps or patellar tendon, fracture of the patella or tibial tubercle).
Stress testing to evaluate ligament integrity helps distinguish partial from complete tears. However, if patients have significant pain and swelling or muscle spasm, testing is typically delayed until x-rays exclude fractures. Also, significant swelling and spasm may make joint stability difficult to evaluate. Such patients should be examined 2 to 3 days later (after swelling and spasm have subsided). A delayed physical examination of the knee is more sensitive than MRI of the knee (86% vs 76% ) for diagnosis of meniscal and anterior cruciate ligament injuries.
Bedside stress testing is done to check for specific injuries, although most of these tests are not highly accurate or reliable. For bedside stress testing, practitioners move the joint in a direction in which the ligament being tested normally prevents excessive joint movement.
For the Apley test, the patient is prone, and the examiner stabilizes the patient’s thigh. The examiner flexes the patient’s knee 90° and rotates the lower leg while pressing the lower leg downward toward the knee (compression), then rotates the lower leg while pulling it away from the knee (distraction). Pain during compression and rotation suggests a meniscal injury; pain during distraction and rotation suggests a ligamentous or joint capsule injury.
For evaluation of the medial and lateral collateral ligaments, the patient is supine, with the knee flexed about 20° and the hamstring muscles relaxed. The examiner puts one hand over the side of the knee opposite the ligament being tested. With the other hand, the examiner cups the heel and pulls the lower leg outward to test the medial collateral ligament or inward to test the lateral collateral ligament. Moderate instability after acute injury suggests that a meniscus or cruciate ligament is torn as well as the collateral ligament.
The Lachman test is the most sensitive physical test for acute anterior cruciate ligament tears (2). With the patient supine, the examiner supports the patient’s thigh and calf, and the patient’s knee is flexed 20°.The lower leg is moved anteriorly. Excessive passive anterior motion of the lower leg from the femur suggests a significant tear.
Not every patient requires x-rays. However, anteroposterior, lateral, and oblique x-rays are often taken to exclude fractures. The Ottawa knee rules are used to limit x-rays to patients more likely to have a fracture that requires specific treatments. X-rays should be taken only if one of the following is present:
MRI is usually not needed at the initial evaluation. A reasonable approach is to do MRI if symptoms do not resolve after a few weeks of conservative management. However, MRI is often done when severe injury or significant intra-articular injury is suspected or cannot be otherwise excluded.
Other tests may be done to check for related injuries:
1. Rayan F, Bhonsle S, Shukla DD: Clinical, MRI, and arthroscopic correlation in meniscal and anterior cruciate ligament injuries. Int Orthop 2009 33 (1):129–132, 2009. doi: 10.1007/s00264-008-0520-4.
2. Benjaminse A, Gokeler A, van der Schans CP: Clinical diagnosis of an anterior cruciate ligament rupture: A meta-analysis. J Orthop Sports Phys Ther 36 (5):267–288, 2006.
Draining large effusions (see figure Arthrocentesis of the knee) may decrease pain and spasm. Contraindications to knee arthrocentesis include anticoagulation and cellulitis overlying the affected knee.
Most 1st-degree and moderate 2nd-degree injuries can be treated initially with PRICE, including immobilization of the knee at 20° of flexion with a commercially available knee immobilizer or splint. Early range of motion exercises are usually encouraged.
Severe 2nd-degree and most 3rd-degree sprains require casting for ≥ 6 weeks.
Some 3rd-degree injuries of the medial collateral ligament and anterior cruciate ligament require arthroscopic surgical repair. Patients with severe injuries are referred to an orthopedic surgeon for surgical repair.
Meniscal injuries vary widely in their characteristics and treatments. Large, complex, or vertical tears and injuries that result in persistent effusions or disabling symptoms are more likely to require surgery. Patient preference can influence choice of treatment.
Physical therapy may be helpful, depending on the patient and the type of injury.
Stress testing (usually done days after the injury) is necessary for differentiation of partial from complete ligamentous tears and is more sensitive than MRI.
Consider injury to the anterior cruciate ligament and other intra-articular structures if patients have an effusion after an injury.
Consider knee dislocation and popliteal artery injury if patients have a large hemarthrosis, gross instability, or both.
If patients have knee pain and effusion, test active knee extension to check for disruption of the extensor mechanism (eg, tears of the quadriceps or patellar tendon, fracture of the patella or tibial tubercle).
Do MRI if symptoms do not resolve after a few weeks of conservative management or possibly when severe injury or significant intra-articular injury is suspected or cannot be otherwise excluded.