Factors Influencing Graft Function following MPFL Reconstruction: A Dynamic Simulation Study

Abstract

Medial patellofemoral ligament (MPFL) reconstruction is currently the primary surgical procedure for treating recurrent lateral patellar instability. The understanding of graft function has largely been based on studies performed with normal knees. The current study was performed to characterize graft function following MPFL reconstruction, focusing on the influence of pathologic anatomy on graft tension, variations with knee flexion, and the influence on patellar tracking. Knee squatting was simulated with 15 multibody dynamic simulation models representing knees being treated for recurrent lateral patellar instability. Squatting was simulated in a preoperative condition and following MPFL reconstruction with a hamstrings tendon graft set to allow 0.5 quadrants of lateral patellar translation with the knee at 30 degrees of flexion. Linear regressions were performed to relate maximum tension in the graft to parameters of knee anatomy. Repeated measures comparisons evaluated variations in patellar tracking at 5-degree increments of knee flexion. Maximum graft tension was significantly correlated with a parameter characterizing lateral position of the tibial tuberosity (maximum lateral tibial tuberosity to posterior cruciate ligament attachment distance, r 2 = 0.73, p < 0.001). No significant correlations were identified for parameters related to trochlear dysplasia (lateral trochlear inclination) or patella alta (Caton–Deschamps index and patellotrochlear index). Graft tension peaked at low flexion angles and was minimal by 30 degrees of flexion. MPFL reconstruction decreased lateral patellar shift (bisect offset index) compared with preoperative tracking at all flexion angles from 0 to 50 degrees of flexion, except 45 degrees. At 0 degrees, the average bisect offset index decreased from 0.81 for the preoperative condition to 0.71. The results indicate that tension within an MPFL graft increases with the lateral position of the tibial tuberosity. The graft tension peaks at low flexion angles and decreases lateral patellar maltracking. The factors that influence graft function following MPFL reconstruction need to be understood to limit patellar maltracking without overloading the graft or over constraining the patella.

Groove‐deepening trochleoplasty reduces lateral patellar maltracking and increases patellofemoral contact pressures: Dynamic simulation

“…Simulated patterns of patellar tracking were similar to those determined from the subjects, with peak lateral tracking near full extension 25 . One study showed root mean square errors for simulated lateral patellar shift and tilt of 2.5 mm and 4.0°, respectively 19 . Simulations also produced correlations between trochlear groove depth and patellar tracking similar to those determined from the subjects 26 .…”

“…25 One study showed root mean square errors for simulated lateral patellar shift and tilt of 2.5 mm and 4.0°, respectively. 19 Simulations also produced correlations between trochlear groove depth and patellar tracking similar to those determined from the subjects. 26 The simulated contact pressure patterns were also similar to those determined from reconstruction of the diagnostic imaging.…”

“…The multibody dynamic simulation model of each knee (RecurDyn; FunctionBay) was validated by simulating a functional activity performed by the corresponding subject during diagnostic imaging, as described at various stages of model development. [19][20][21] Depending on where each subject was enrolled, the subject participated in either knee extension against gravity within a dynamic computed tomography scanner 22,23 or isometric knee extension against resistance within an MRI scanner 24 to compare in vivo versus simulated function at four or five positions of knee flexion. Models of the femur, patella, and tibia reconstructed from the imaging were used to characterize knee kinematics with respect to an anatomical coordinate system.…”

“…38,39 The loading and boundary conditions maintained patellofemoral stability for the preoperative condition. 14,19,20 The ankle joint was represented with three rotational degrees of freedom.…”

“…Patellar tracking was expressed by bisect offset index (portion of patellar width lateral to the deepest point of the trochlear groove) and patellar lateral tilt at 5°intervals of knee flexion. Automated algorithms quantified both patellar tracking parameters within an axial plane perpendicular to the long axis of the patella at each flexion angle, based on a medial-lateral axis fixed to the patella and a mediallateral direction determined by the posterior condylar axis of the femur14,19,40 (Figure3). The deepest point of the trochlear groove was computationally identified from an axial slice through the femur that also included the most anterior point on the lateral ridge of the groove 23,24.…”