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.
Keywords
patellar instability - MPFL reconstruction - patellar tracking - computational simulation
