Geometry of Torsional Malalignment Syndrome: Trochlear Dysplasia but Not Torsion Predicts Lateral Patellar Instability

BACKGROUND: The clinical impact of increased torsion on patellar instability and patellofemoral pain syndrome (PFPS) has been suggested by several studies. HYPOTHESIS: The hypotheses of this study were that (1) torsional malalignment (TM) is characterized by a positive correlation between different malalignment parameters that represent an overall picture of the malalignment syndrome and (2) an increase in overall torsion is the underlying difference between patellar instability and isolated patellofemoral pain. STUDY DESIGN: Cohort study; Level of evidence, 3. METHODS: Between April 2015 and July 2017, a total of 428 patients were treated for lateral patellar dislocation (LPD), and 333 patients were treated for PFPS. Sixty-two patients (14.5%) with patellar instability (LPD group) and 29 patients (8.7%) with patellofemoral pain (PFPS group) had additional TM and were included in this study. All patients underwent magnetic resonance imaging for torsional alignment and patellar tracking, including femoral antetorsion, tibial torsion, knee rotation, tibial tuberosity–trochlear groove (TT-TG) distance, tibial tuberosity–posterior cruciate ligament (TT-PCL) distance, Dejour classification of trochlear dysplasia, lateral trochlear inclination (LTI) angle, and patellar height. RESULTS: The LPD and PFPS groups differed significantly in terms of trochlear dysplasia (P < .001), LTI angle (P < .001), and TT-TG distance (P = .0167) but did not differ in terms of femoral antetorsion (20.02° ± 8.80° vs 20.03° ± 7.91°, respectively; P = .8545), tibial torsion (39.53° ± 9.23° vs 41.24° ± 7.28°, respectively; P = .3616), or knee rotation (10.42° ± 5.16° vs 8.48° ± 7.81°, respectively; P = .0163). Only measures of TT-TG distance and TT-PCL distance and measures of TT-TG distance and knee rotation were positively correlated. Trochlear dysplasia (type B-D) was identified as the only significant predictor of patellar instability. CONCLUSION: TM in patients with either PFPS or LPD does not appear to be characterized by a fixed constellation of different malalignment parameters. Between groups, the parameters differed significantly only in terms of trochlear dysplasia and the TT-TG distance, and trochlear dysplasia (type B-D) (but not torsion) was identified as a predictor of lateral patellar instability.