Lateral position of tibial tuberosity correlates to maximum medial patellofemoral ligament graft tension
Tension within a medial patellofemoral ligament graft increases with the lateral position of the tibial tuberosity, leading to better graft function and decreased patellar maltracking, according to published results.
Miho J. Tanaka, MD, and colleagues simulated flexion angles in knees with lateral patellar instability using 15 multibody dynamic models. The models were simulated in both a preoperative state and following medial patellofemoral ligament (MPFL) reconstruction, according to the study abstract.
“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<.001),” Tanaka and colleagues wrote in their abstract.
Tanaka and colleagues also found that low flexion angles allowed for optimal graft tension and decreased lateral patellar shift, as measured by the bisect offset index (0.81 after MPFL reconstruction, 0.71 in the preoperative condition).
“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 researchers wrote in the abstract. “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.” – by Max R. Wursta
Perspective
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This study by Tanaka and colleagues was done using dynamic CT and quadriceps active MRI imaging data from a total of 15 patients to create a computer simulation model. The structural data obtained from these individual patients were used to evaluate the load present in the simulated MPFL graft for these patients.
As expected, graft tension is greatest in early flexion as the patella is guided into the trochlear groove. Average graft tension in early flexion was less than 10 N (2.2 lbs.). It is very important to not over tension MPFL grafts and the authors’ model reflects this fact. Patella alta and trochlear dysplasia in these particular knees did not show a significant effect on graft tension, but lateral displacement of the tibial tuberosity did. Patellar tracking was improved after MPFL reconstruction was applied. It should be noted, however, that clinical use of MPFL reconstruction is not recommended clinically to improve tracking but rather to provide a static check rein to lateral displacement (lateral instability).
It is exciting to see the beginning of patient-specific biomechanical computer simulations, which might eventually lead to the ability to model-specific operations in specific patients. I look forward to the further development of this model to include more complex and clinically relevant movements and different surgical interventions.
International Patellofemoral Study Group
Morgantown, West Virginia
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