Tibial External Rotation Test for Patellar Instability

There is no dynamic physical examination test that reproduces patellar instability or the effect of tibial rotation on patellar tracking.

This article describes a provocative physical examination test that can reproduce lateral patellar subluxation: the tibial external rotation test (TERT). The principle behind the TERT is to vary the quadriceps angle (Q-angle) by changing the rotation of the tibia with the knee at 90° of flexion and asking the patient to forcefully extend the knee against resistance. If the patella subluxates at about 25° of flexion when the tibia is held in the externally rotated position and the knee is extended against resistance (i.e., with an increased Q-angle) but is more stable when the same is done when the tibia is internally rotated, the test is positive.

The TERT confirmed the diagnosis of patellar instability and helped determine if medialization of the tibial tubercle should be performed.

The TERT is a provocative physical examination test that can cause the patella to subluxate laterally. The TERT can simulate the effect of medializing the tibial tubercle on patellar tracking.

Diagnostic Level V. See Instructions for Authors for a complete description of levels of evidence.