Testing for Knee Instability
When it comes to knee injuries, orthopedic surgeons depend on the patient's history and physical exam to find out what's wrong. Specific tests of structure and function are performed to look for joint instability. In this review article, tests for assessing tibiofemoral and patellofemoral instability are described and explained in detail.
These are two joints in the knee. Tibiofemoral refers to the joint formed by the tibia (lower leg bone) and the femur (thigh bone). The patellofemoral joint is where the patella (knee cap) moves up and down over the femur.
Successful treatment depends on an accurate diagnosis. First the orthopedic surgeon pays attention to the mechanism of injury. Knowing how the injury occurred helps point to the structures involved that might have been torn or ruptured.
The diagnosis continues with various physical tests for anterior (forward) or posterior (backward) joint instability. The tests aren't always foolproof, so further testing with diagnostic imaging may be needed.
Anterior instability is tested using well-known tests such as the Lachman or anterior drawer test. Injury to the anterior cruciate ligament (ACL) is the main problem when either test is positive. Studies show the Lachman test is more accurate because it puts more strain on the ACL compared with the anterior drawer test.
Other tests for anterior instability include the pivot-shift test and KT-1000 or KT-2000 arthrometer tests. Arthrometer tests are performed with a special device that places three different amounts of anterior force on the tibia.
Posterior instability occurs when the posterior cruciate ligament (PCL) has been ruptured or damaged. Two other ligaments that can be involved with posterior instability include the medial collateral ligament (MCL) and the lateral collateral ligament (LCL). The posterior drawer, posterior Lachman, and sag tests are used to help diagnose a PCL tear.
Besides anterior and posterior instability, there can also be rotatory, valgus, and varus instabilities. Rotation occurs when the knee is bent or flexed. Rotatory instability occurs as a result of injury while pivoting, cutting, or rapid deceleration (slowing down suddenly). Numerous tests for rotatory instability in all directions are presented.
Varus and valgus movements refer to gapping or rocking between the tibia and the femur. Stress tests can be applied when looking for damage to the collateral ligaments. The knee is placed in full extension during the first test. The test is repeated with the knee bent 30 degrees. Each position tests for specific ligament injury.
The authors also discuss the angle of the patella called the Q-angle, movement of the patella over the joint called patellar tracking, and tests for both.
With all of these tests, false-negative results can be avoided with patient relaxation, proper positioning, and by applying the correct direction of force. Following the recommended technique for using each test is important for accurate results.
James H. Lubowitz, MD, et al. Current Concepts Review. Comprehensive Physical Examination for Instability of the Knee. In The American Journal of Sports Medicine. March 2008. Vol. 36. No. 3. Pp. 577-594.