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

Purpose

To investigate whether the patellotrochlear index (PTI) predicts patella alta as determined by tibial-based methods of Insall–Salvati (IS) and Caton–Deschamp (CDI) indexes in a pathological population (with patellofemoral pain and/or instability), in addition to determining whether PTI and sagittal patellofemoral engagement (SPE) correlate with trochlea length as determined by lateral condyle index (LCI).

Methods

Patients with confirmed patella alta (IS/CDI ratio >1.2) undergoing tibial tubercle osteotomy for patellofemoral pain/instability with an available magnetic resonance imaging (MRI) scans were included. Patients who had undergone previous soft-tissue realignment, previous surgery, or trauma to the extensor mechanism were excluded. Two raters measured the IS, CDI, PTI, SPE, LCI, and knee flexion angle (KFA) on MRI. Interobserver reliability and correlation between measurements were calculated.

Results

In total, 71 knees were included. PTI (0.73), SPE (0.836), LCI (0.701), and KFA (0.8) demonstrated good- to near-excellent interobserver reliability. IS (0.65) and CDI (0.66) demonstrated moderate interobserver reliability. PTI and SPE showed the strongest significant correlation (0.8112, P = 2.2 × 10⁻¹⁶). IS and CD (0.39, P = .0007) showed a moderate significant correlation. PTI and KFA (0.53, P = 1.685 × 10⁻⁶) and SPE and KFA (0.61, P = 1.991 × 10⁻⁸) had a significant moderate correlation. LCI and KFA (–0.37, P = .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.

Conclusions

There 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 Evidence

Level IV, retrospective diagnostic radiographic investigation.