Sprains of the external (medial and lateral collateral) or internal (anterior and posterior cruciate) ligaments or injuries of the menisci commonly result from knee trauma. Symptoms include pain, effusion, instability (with severe sprains), and locking (with some meniscal injuries). Diagnosis is by physical examination and sometimes MRI or arthroscopy. Treatment is RICE (rest, ice, compression, elevation) and, for severe injuries, casting or surgical repair.
Many structures that help stabilize the knee are located mainly outside the joint muscles (eg, quadriceps, semimembranosus), 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 (Fig. 9: Fractures, Dislocations, and Sprains: Ligaments of the knee.).
The most commonly injured knee structures are the medial collateral and anterior cruciate ligaments. The most common mechanism for ligamentous knee injuries is an inward, medial force usually accompanied by some external rotation and flexion (as when being tackled in football). In such cases, the medial collateral ligament is usually injured first, followed by the anterior cruciate ligament, then the medial meniscus. The next most common mechanism is an outward force, often injuring the lateral collateral ligament, anterior cruciate ligament, or both. Anterior or posterior forces and hyperextension typically injure the cruciate ligaments. Weight bearing and rotation at the time of injury tend to cause meniscal injuries.
Symptoms and Signs
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. An audible pop suggests an anterior cruciate tear but is uncommon. An effusion suggests injury to the anterior cruciate and possibly other intra-articular structures. However, with severe 3rd-degree tears of the medial collateral ligament or anterior cruciate, no effusion may be apparent because these tears can result in an open joint capsule, allowing blood to exit the joint. Tenderness is often maximal over the injured structure: Medial meniscal injuries cause tenderness in the joint plane (joint line tenderness) medially, and lateral meniscal injuries cause tenderness in the joint plane laterally. These injuries also cause swelling and sometimes restrict passive motion (called locking).
A spontaneously reduced knee dislocation should be suspected in patients with a large hemarthrosis, gross instability, or both; serial evaluation of distal pulses or immediate angiography should be considered. Otherwise, the knee is fully examined first. Knee extension is assessed to check for disruption of the extensor mechanism (eg, tears of the quadriceps muscle or patellar tendon, which can be missed on x-rays; fracture of the patella or tibial tubercle). Knee pain and effusion may indicate disruption of the extensor mechanism.
Other bedside testing is done to check for specific injuries. For the Apley compression 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 knee rotation suggests a meniscal injury; pain during distraction and knee rotation suggests a ligamentous or joint capsule injury.
For assessment of the medial and lateral collateral ligaments, the patient is examined 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.
Lachman's test is the most sensitive physical test for acute anterior cruciate ligament tears. 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.
If patients cannot tolerate stress testing (eg, because of pain or muscle spasm), they should be reexamined after injection of a local anesthetic, after use of systemic analgesia and sedation, or at a follow-up examination 2 to 3 days later (after swelling and spasm have subsided); alternatively, MRI or arthroscopy can be done. MRI or arthroscopy is also done when severe injury cannot be excluded clinically.
Draining large effusions (see Fig. 3: Approach to the Patient With Joint Disease: Arthrocentesis of the knee.) may decrease pain and spasm. Most 1st-degree and mild or moderate 2nd-degree injuries can be treated initially with RICE and immobilization of the knee at 20° of flexion with a commercially available knee immobilizer or splint. Severe 2nd-degree and most 3rd-degree sprains and most meniscal injuries require casting for ≥ 6 wk. However, some 3rd-degree injuries of the medial collateral ligament, anterior cruciate ligament, and menisci require arthroscopic surgical repair.
Last full review/revision October 2007 by James R. Roberts, MD