Objective: (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. Design: In vitro biomechanical study. Specimens: Ten above-the-knee amputation specimens. Main Outcome Measures: Force-displacement curves with direct lateral patellar displacement were compared with curves with distal lateral patellar displacement before and after sectioning the MPFL. Results: 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 = 8.67 × 10-5). The terminal slope declined after ligament division with lateral (P = 0.07) and distal lateral patellar displacement (P = 0.09). Conclusion: 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° 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.