The reversed dynamic patellar apprehension test mimics anatomical complexity in lateral patellar instability

Abstract

Purpose: To develop a dynamic physical examination test that functionally simulates actual patellar instability events and that mimics the range of patellar stabilizer insufficiency in an individual patient. Methods: Seventy-eight consecutive patients (male/female 35/43; mean age 22 ± 7 years) with recurrent lateral patellar instability and 35 controls (male/female 16/19; mean age 31 ± 14 years) were prospectively evaluated using the reversed dynamic patellar apprehension test (ReDPAT). Anatomical predisposition was assessed according to Dejour’s classification of trochlear dysplasia, tibial tuberosity–trochlear groove distance, tibial tuberosity–posterior cruciate ligament distance, patellar height, and varus/valgus malalignment. Results: The study group had an average of 3.4 ± 1.0 (1–6) anatomical risk factors for lateral patellar dislocation. Severe trochlear dysplasia (84%) and patella alta (49%) were the most common. Test sensitivity and specificity was 93.7% (95% CI 0.8584–0.9791) and 88.2% (95% CI 0.7255–0.9670), respectively. The positive predictive value reached 94.9% (95% CI 0.8739–0.9859) and the negative predictive value was 85.7% (95% CI 0.6974–0.9519). The ReDPAT results became positive at a mean knee flexion angle of 58° ± 17° (20°–90°). Knee flexion angle correlated significantly with the severity of trochlear dysplasia (p = 0.018), valgus deformity (p = 0.011), and the total number of anatomical risk factors (p = 0.02). Conclusion: This study introduced the reversed dynamic patellar apprehension test as a reliable clinical examination tool in the assessment of lateral patellar instability. The results of this study indicate that the degree of knee joint flexion at which the provocative sense of apprehension becomes positive correlates with severity of trochlear dysplasia, valgus deformity and the total number of anatomical risk factors for patellar instability. This test indicates the patient-specific end of stable patellar tracking and the beginning of patellar stabilizer insufficiency. Therefore, this test might be helpful in deciding for or against a bony procedure in the treatment of patellar dislocation. Level of evidence: II.