Identifying Patients With Patella Alta and/or Severe Trochlear Dysplasia Through the Presence of Patellar Apprehension in Higher Degrees of Flexion.BACKGROUND
 Recurrent patellar instability is frequently treated surgically with reconstruction of the medial patellofemoral ligament (MPFL).  Patients with significant patella alta, trochlear dysplasia, and/or an elevated tibial tubercle-trochlear groove (TT-TG) distance may benefit from a concurrent bony procedure such as tibial tubercle osteotomy or trochleoplasty.  The indications to perform such procedures are traditionally based on imaging criteria but remain controversial.  Patellar apprehension is common in patients with patellar instability but typically resolves in higher degrees of knee flexion.


 The persistence of patellar apprehension at greater than 60° of knee flexion is associated with patella alta, an increased TT-TG distance, and trochlear dysplasia.


 Cross-sectional study; Level of evidence, 3.


 A total of 76 patients with recurrent patellar instability were prospectively identified in a sports medicine clinic.  Patellar apprehension was evaluated in each patient.  The degree of flexion at which patellar apprehension disappeared was recorded.  Plain radiographs and magnetic resonance imaging (MRI) scans were obtained for all patients.  Patellar height was assessed with the Caton-Deschamps (CD) index, and trochlear morphology was assessed through measurements of the sulcus angle and depth on MRI and classified using the Dejour classification system.  Imaging measurements of patients in whom apprehension resolved by 60° of knee flexion were compared with measurements for those with apprehension that persisted deeper into flexion.


 The patients with a delayed resolution of apprehension demonstrated a higher CD index; elevated TT-TG distance; increased sulcus angle; decreased sulcus depth; and higher incidence of Dejour type B, C, or D dysplasia (all P < .05).  Of the 20 patients with a delayed resolution of apprehension, 18 had either Dejour type B, C, or D dysplasia or a CD index of at least 1.30.  A delayed resolution of apprehension was present in 11 of the 16 patients with Dejour type B, C, or D dysplasia.


 CONCLUSION
 Overall, 90% of patients with significant patella alta and the majority of patients with high-grade trochlear dysplasia demonstrated patellar apprehension that persisted beyond 60° of knee flexion.  Additionally, 90% of patients with persistent apprehension had significant patella alta and/or trochlear dysplasia.  Further work is needed to evaluate the utility of these findings to inform surgical decision-making in this population.
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