Correlation of radiographic patellofemoral indices with tibial tubercle transfer distance in Fulkerson osteotomy procedures.

BACKGROUND

A laterally tracking patella is commonly seen in patients with chronic recurrent lateral patellar dislocations. Clinical appearance of the J-sign occurs when the patella is congruent with the trochlear groove in flexion and moves over the lateral border of the femoral condyle as the lower leg reaches complete extension. A Fulkerson osteotomy procedure corrects this maltracking of the patella by medially transferring the tibial tubercle. There are many radiographic patellofemoral indices that can be used describe this incongruence about the patelloformal joint. The current literature supports the use of the tibial tubercle-trochlear groove (TT-TG) index in determining the appropriate amount medialization of the extensor mechanism. However there is little agreement in how far to transfer the tibial tubercle to best achieve maximum patello-femoral congruency. It is the senior author's belief that lateral patellar edge (LPE) measure on voluntary quadriceps active hyperextension MRI scan has the strongest correlation with final operative tibial tubercle transfer distance needed to achieve maximum patellofemoral congruency.

PURPOSE

The purpose of this study was to show that the voluntary quadriceps active hyperextension MRI measurement of lateral patellar edge (LPE) has the strongest correlation with tibial tubercle transfer distance required to achieve maxium patellofemoral congruency intraoperatively in the terminal 30 degrees of active knee extension compared to all other patellofemoral indices measured on axial MRI scans with the knee in voluntary active knee extension to 30 degrees of flexion, passive full extension, and voluntary quadriceps active hyperextension.

STUDY DESIGN

Retrospective case series via review of the electronic medical record.

METHODS

Forty-three Fulkerson osteotomy patient charts were reviewed retrospectively. Three different pre-operative axial MRI views were then examined and measured for Tibial Tubercle-Trochlear Groove (tt-tg), lateral patellar edge (LPE), bisect offset (BSO), and lateral patellar displacement (LPD). Each patient had three MRIs: one with the knee resting in extension, one in voluntary quadriceps active hyperextension, and one in voluntary quadriceps active 30 degree flexion. Statistics were then calculated using Statistical Package for the Social Sciences (SPSS) (IBM corp).

RESULTS

Tibial tubercle transfer distances required to achieve congruency intraoperatively correlated moderately (0.500-0.300) and were statistically significant (alpha .050) for passive extension MRI measurement of TT-TG (Pearson--0.403, alpha 0.010) and LPD (Pearson .362, alpha 0.022); voluntary quadriceps active hyperextension TT-TG (Pearson 0.487, alpha, 0.001); voluntary quadriceps active flexion TT-TG (Pearson .548, alpha< 0.001), LPE (Pearson .332, alpha 0.029), and LPD (Pearson 0.446 alpha .003).

CONCLUSION

The hypothesis that voluntary quadriceps active hyperextension MRI LPE measurement best correlated with tibial tubercle transfer distance was incorrect. The data collected showed correlation and statistical significance for voluntary quadriceps active flexion LPE with required tibal tubercle transfer distance (Pearson 0.34, alpha 0.026). The MRI measurement that best correlated with tibial tubercle transfer distance was voluntary quadriceps active flexion measure of TT-TG (Pearson .556, alpha< 0.001).