Summary

A case-control anatomical MRI study showing that patella instability is associated with tibial rotational asymmetry due to lateralization of the tibial tubercle and extensor mechanism abnormalities producing patella alta and reduced trochlear groove depth, but no difference in the rotational alignment of the posterior condyles or trochlear groove.

Abstract

Purpose

Recurrent patella dislocation is associated with a range of anatomical abnormalities affecting the trochlear, extensor mechanism and tibia. The relationship between patellofemoral instability (PFI) and rotational abnormalities of the posterior condyles, trochlear groove and proximal tibia has not been adequately determined. The aims of this study are to determine whether there are additional anatomical risk factors for PFI and to identify combinations of anatomical abnormality which form subgroups within the group of patella dislocaters.

Methods

A retrospective morphological study was undertaken comparing multiple anatomical measurements in MRI scans of fifty patients with patella instability to a control group of fifty patients. Several techniques were assessed to measure both femoral and tibial axial asymmetry. A new measurement, tibial rotational asymmetry (TRA), comparing a line between the midpoints of the collateral ligaments to the axis between the patella tendon and posterior cruciate ligament was assessed for its association with PFI.

Results

PFI was associated with increased tibial axial asymmetry as measured by the tibial tubercle posterior cruciate ligament distance (mean 22.6mm (SD 0.5) vs 20.6mm (SD 0.5)), the tibial tubercle trochlear distance (mean 15.5mm (SD 0.8) vs 11.2mm (SD 0.5)) and the TRA 2.9 degrees (SD 0.5) externally rotated compared to -1.6 degrees (SD 0.3). The posterior slope of the lateral tibial plateau was decreased by 2.5 degrees (SD 0.4) relative to the medial tibial slope in the PFI group compared to controls. There was no difference in measures of femoral axial shape, including the posterior condylar angle and rotation of the trochlear groove, apart from an increased sulcus angle 154.8 degrees (SD 11.4) vs 137.9 degrees (SD 8.1). The Insall-Salvati ratio was increased 1.51 (SD 0.04) vs 1.22 (SD 0.02). Analysis of the PFI group revealed a subgroup of males with normal anatomy (7/50) and a subgroup of females with isolated patella alta (7/50).

Conclusion

Patella instability is associated with tibial rotational asymmetry due to lateralization of the tibial tubercle and extensor mechanism abnormalities producing patella alta and reduced trochlear groove depth. The femoral axial shape is otherwise unchanged.