No Correlation Exists Between Tibial- and Femoral-Based Measurements of Patella Alta in a Population With Chronic Patellofemoral Pain or Instability Undergoing Patella Distalization

Abstract

PurposeTo investigate if Patellotrochlear index (PTI) predicts patella alta as determined by tibial based methods of Insall Salvati (IS) and Caton Deschamp (CD) in a pathological population (with patellofemoral pain and/or instability). In addition to determining if PTI and Sagittal Patellofemoral engagement (SPE) correlate with trochlea length as determined by Lateral condyle index (LCI). MethodsPatients with confirmed patella alta (IS/CD ratio > 1.2) undergoing tibial tubercle osteotomy for patellofemoral pain/instability with an available MRI were included. Patients who had undergone previous soft tissue realignment, prior surgery or trauma to the extensor mechanism were excluded. Two raters measured the IS, CD, PTI, SPE, LCI and Knee flexion angle (KFA) on MRI. Inter-observer reliability and correlation between measurements were calculated. Results71 knees were included. PTI(0.73), SPE(0.836), LCI(0.701), KFA(0.8) demonstrated good to near excellent inter-observer reliability. IS(0.65) and CD(0.66) demonstrated moderate inter-observer reliability. PTI and SPE showed the strongest significant correlation (0.8112, p = 2.2x10-16). IS and CD (0.39, p = 0.0007) showed a moderate significant correlation. PTI and KFA (0.53, p = 1.685x10-6), SPE and KFA (0.61, p = 1.991x10-8) had a significant moderate correlation. LCI and KFA (-0.37, p = 0.0017), showed a significant moderate negative correlation. All other measurement indices correlated poorly and were insignificant. A total of 94.4% of the knees were defined as having patella alta using IS with the remaining 5.6% having a raised CDI. Only 14% of cases had an IS of >1.2, a CDI >1.2 and a PTI <0.125, which increased to 39% (28/71) when the threshold for PTI was increased to <0.28. ConclusionThere was no correlation between tibial (IS and CD) and femoral methods (PTI and SPE) of quantifying patella alta. PTI and SPE did not correlate with trochlea length as measured by LCI. PTI, SPE and LCI are significantly affected by the KFA during MRI Level of Evidencelevel IV, retrospective diagnostic radiographic investigation