A modified test for patellar instability: the biomechanical basis

(1) Determine if displacement of the patella in a distal lateral direction results in a more sensitive method to show deficiency of the medial patellofemoral ligament (MPFL), the primary restraint to lateral patellar dislocation, than the traditional patellar apprehension test in a direct lateral direction. (2) Determine objective criteria for defining a positive patellar instability test rather than subjective evaluation of apprehension.

In vitro biomechanical study.

Ten above-the-knee amputation specimens.

Force-displacement curves with direct lateral patellar displacement were compared with curves with distal lateral patellar displacement before and after sectioning the MPFL.

After dividing the MPFL, average terminal restraining force to distal lateral patellar displacement declined by 53% (P=0.024), but force declined only by 30% (P=0.09) with lateral displacement. The greatest difference in terminal slope (eg, end point) was with the MPFL intact with lateral displacement compared with distal lateral displacement with the ligament divided (P=8.67x10(-5)). The terminal slope declined after ligament division with lateral (P=0.07) and distal lateral patellar displacement (P=0.09).

Displacement of the patella in a distal lateral direction is a more sensitive maneuver to detect disruption of the MPFL, the primary soft tissue restraint, than with traditional lateral displacement. With the knee flexed 30 degrees and patella displaced 2 cm, objective criteria for a positive patellar instability test include greater ease of patellar translation and a softer end point compared with a normal, contralateral knee.