Dynamic Simulation of the Effects of Graft Fixation Errors During Medial Patellofemoral Ligament Reconstruction

Abstract

Background: Medial patellofemoral ligament (MPFL) reconstruction is performed to prevent recurrent instability, but errors in femoral fixation can elevate graft tension.

Hypothesis: Errors related to femoral fixation will overconstrain the patella and increase medial patellofemoral pressures.

Study design: Controlled laboratory study.

Methods: Five knees with patellar instability were represented with computational models. Kinematics during knee extension were characterized from computational reconstruction of motion performed within a dynamic computed tomography (CT) scanner. Multibody dynamic simulation of knee extension, with discrete element analysis used to quantify contact pressures, was performed for the preoperative condition and after MPFL reconstruction. A standard femoral attachment and graft resting length were set for each knee. The resting length was decreased by 2 mm, and the femoral attachment was shifted 5 mm posteriorly. The simulated errors were also combined. Root-mean-square errors were quantified for the comparison of preoperative patellar lateral shift and tilt between computationally reconstructed motion and dynamic simulation. Simulation output was compared between the preoperative and MPFL reconstruction conditions with repeated-measures Friedman tests and Dunnett comparisons against a control, which was the standard MPFL condition, with statistical significance set at P < .05.

Results: Root-mean-square errors for simulated patellar tilt and shift were 5.8° and 3.3 mm, respectively. Patellar lateral tracking for the preoperative condition was significantly larger near full extension compared with the standard MPFL reconstruction (mean differences of 8 mm and 13° for shift and tilt, respectively, at 0°), and lateral tracking was significantly smaller for a posterior femoral attachment (mean differences of 3 mm and 4° for shift and tilt, respectively, at 0°). The maximum medial pressure was also larger for the short graft with a posterior femoral attachment than for standard MPFL reconstruction, with a significant increase in the mean value of 1.6 MPa at 30°.

Conclusion: MPFL reconstruction reduces lateral tracking, but nonanatomic femoral fixation and overtensioning the graft overcorrect patellar tracking and increase pressure applied to medial patellar cartilage.

Clinical relevance: Errors in femoral fixation and graft tensioning can lead to postoperative loss of flexion and overloading of medial cartilage.

Keywords: MPFL reconstruction; cartilage pressure; computational simulation; patellar instability; patellar tracking.

Conflict of interest statement

One or more of the authors has declared the following potential conflict of interest or source of funding: Funding for the study was provided by MedShape. K.E.S. is employed by and owns stock in MedShape. K.G. owns stock in MedShape. J.F. receives consulting fees from and has stock options in MedShape.

Figures

Figure 1.

Multibody dynamic simulation model of the knee. Patellar tracking is shown as a left knee extends from (A) a flexed position to (B) full extension in the preoperative condition and (C) for the knee extended with an MPFL graft wrapping around the femoral condyle. (D) The standard attachment of the MPFL graft based on the depth of the medial condyle is also shown. MPFL, medial patellofemoral ligament.

Figure 2.

Mean (±SD) patellar lateral shift for the preoperative condition, standard MPFL reconstruction, and MPFL reconstruction with a short graft and a posterior femoral attachment. Data points are also shown representing motion of the subjects within the dynamic scanner. MPFL, medial patellofemoral ligament.

Figure 3.

Mean (±SD) patellar lateral tilt for the preoperative condition, standard MPFL reconstruction, and MPFL reconstruction with a short graft and a posterior femoral attachment. Data points are also shown representing motion of the subjects within the dynamic scanner. MPFL, medial patellofemoral ligament.

Figure 4.

Mean (±SD) graft force for a standard medial patellofemoral ligament (MPFL) reconstruction and MPFL reconstruction with a short graft and a posterior femoral attachment.

Figure 5.

Mean (±SD) maximum medial pressure for the preoperative condition, standard medial patellofemoral ligament (MPFL) reconstruction, and MPFL reconstruction with a short graft and a posterior femoral attachment.

Figure 6.

Contact pressure patterns for 1 patella at 30° of flexion for a standard medial patellofemoral ligament (MPFL) reconstruction and reconstruction with a short graft and a posterior femoral attachment.

Figure 7.

Mean (±SD) maximum lateral pressure for the preoperative condition, standard medial patellofemoral ligament (MPFL) reconstruction, and MPFL reconstruction with a short graft and a posterior femoral attachment.

Figure 8.

Mean (±SD) lateral center of pressure for the preoperative condition, standard medial patellofemoral ligament (MPFL) reconstruction, and MPFL reconstruction with a short graft and a posterior femoral attachment.