Introduction

Reconstruction of the medial patellofemoral ligament (MPFL) is the most common approach for surgical patellar stabilization following a lateral patellar dislocation. Continued dislocations following MPFL reconstruction also has been noted for approximately 5% of patients. The current study focuses on graft function while preventing a dislocation episode by characterizing tension within the graft and patellar tracking during a simulated, unstable pivoting maneuver, in comparison to normal, stable function.

Methods

Knee function was simulated with thirteen dynamic simulation models constructed from MRI scans of subjects being treated for recurrent patellar instability. Dynamic simulation of knee function was performed to represent a pivot landing that induces a noncontact patellar dislocation. Simplified Hertzian contact governed reaction forces developed at the patellofemoral and tibiofemoral joints. The pivoting maneuver was represented without an MPFL graft in place to confirm a patellar dislocation, and then simulated with an MPFL graft in place. A dual limb knee squat was also simulated with the MPFL graft applied to represent stable motion. Output data was compared between the squatting and pivoting maneuvers with the MPFL graft in place.

Results

All thirteen knee models experienced a lateral patellar dislocation for the pivoting maneuver without an MPFL graft. Two of the models also experienced a dislocation for the pivoting maneuver with a graft in place. A bisect offset index of approximately 1.0 was recorded for two other knees with a graft in place, indicating the entire patella was lateral to the deepest point of the trochlear groove. No dislocations were noted for the squatting motion with the graft in place.
Statistical analysis focused on the flexion range prior to any dislocations. Bisect offset index and graft tension were larger for the pivoting motion than squatting. The differences were statistically significant (p < 0.05, paired t-tests) from 5º to 45º of flexion for bisect offset index and from 5º to 40º for graft tension.

Discussion

The results show the interaction of MPFL graft tension and patellar tracking for effective stabilization of the patella. For knee squatting, the passive resistance from an MPFL graft and the articular constraints of the trochlear groove can stabilize the patella. For a more dynamic activity including higher muscle forces and tibial external rotation, MPFL reconstruction does not sufficiently stabilize the patella for all knees. Computational simulation of knee function has the potential to identify the characteristics of knees that cannot be sufficiently stabilized with MPFL reconstruction.

Significance

MPFL reconstruction is the preferred surgical approach for patellar stabilization to minimize surgical complications and recovery time. An improved understanding of how MPFL grafts influence patellar tracking during potentially unstable activities is needed to expand criteria for MPFL reconstruction and minimize post-operative instability.

ACKNOWLEDGEMENT: Department of Defense, Peer Reviewed Medical Research Program Discovery Award W81XWH2010040.