Computational assessment of the influence of vastus medialis obliquus function on patellofemoral pressures: model evaluation.

A study was performed to evaluate a computational model used to characterize the influence of vastus medialis obliquus (VMO) function on the patellofemoral pressure distribution. Ten knees were tested in vitro at 40 degrees, 60 degrees and 80 degrees of knee flexion with quadriceps loads applied to represent a normal VMO, and with the VMO force decreased by approximately 50% to represent a weak VMO. The tests were performed with the cartilage intact and with a full thickness cartilage lesion centered on the lateral facet of the patella. The experimental tests were replicated computationally by applying discrete element analysis to a model of each knee constructed from MRI images. Repeated measures statistical comparisons were used to compare computational to experimental data and identify significant (p<0.05) differences due to the lesion and the applied VMO force. Neither the lateral force percentage nor the maximum lateral pressure varied significantly between the computational and experimental data. Creating a lesion significantly increased the maximum lateral pressure for all comparisons, except for the experimental data at 40 degrees. Both computationally and experimentally, decrease in the VMO force increased the lateral force percentage by approximately 10% for all cases, and each increase was statistically significant. The maximum lateral pressure increase was typically less than 10% but was still significant for the majority of comparisons focused on the VMO strength. The results indicate that computational modeling can be used to characterize how varying quadriceps loading influences the patellofemoral force and pressure distributions while varying the condition of cartilage.

Tension level during preconditioning influences hamstring tendon graft properties.

BACKGROUND: Hamstring tendon grafts used for anterior cruciate ligament reconstruction are preconditioned to reduce the influence of viscoelasticity on the stiffness and tension. HYPOTHESIS: Increasing the tension applied during preconditioning will decrease the loss of tension and stiffness due to viscoelasticity. STUDY DESIGN: Controlled laboratory study. METHODS: Six quadruple-strand hamstring tendon grafts were tested twice in vitro with 2 tension levels applied during preconditioning. The grafts were preconditioned at 80 N or 160 N, followed by 15 minutes of relaxation, and subsequently loaded to 80 N to represent application of initial tension. After 5 minutes of relaxation, the tension was recorded and the stiffness was measured over 0.06 mm of displacement. The tension and stiffness measurements were repeated 3 hours later. The data were compared between the 2 preconditioning levels with a 2-way repeated-measures analysis of variance. RESULTS: Graft tension and stiffness were significantly larger (P < .05) for 160 N during preconditioning. For 80 N, the average tension at 5 minutes and 3 hours was 40 N and 21 N, respectively, compared with 50 N and 30 N, respectively, for 160 N. For 80 N, the average stiffness at 5 minutes and 3 hours was 152 N/mm and 124 N/mm, respectively, compared with 173 N/mm and 146 N/mm, respectively, for 160 N. CONCLUSION: Increasing the tension applied to hamstring tendon grafts during preconditioning can decrease the postoperative loss of tension and stiffness due to viscoelasticity. CLINICAL RELEVANCE: The increased graft tension and stiffness could reduce postoperative knee laxity.

Improving vastus medialis obliquus function reduces pressure applied to lateral patellofemoral cartilage.

The current study was performed to characterize how improving vastus medialis obliquus (VMO) function influences the pressure applied to patellofemoral cartilage. An additional focus was characterizing how lateral and medial cartilage lesions influence cartilage pressures. Ten knees were flexed to 40 degrees, 60 degrees, and 80 degrees in vitro, and forces were applied to represent the VMO and other muscles of the quadriceps group while a thin film sensor measured joint pressures. The knees were loaded with a normal VMO force, with the VMO force decreased by approximately 50%, and with the VMO unloaded. After tests were performed with the cartilage intact, all tests were repeated with a 12-mm-diameter lesion created within the lateral cartilage, with the lateral lesion repaired with silicone, and with a medial lesion created. Based on a two-way repeated measures ANOVA and post-hoc tests, increasing the force applied by the VMO significantly (p < 0.05) decreased the maximum lateral pressure and significantly increased the maximum medial pressure at each flexion angle. A lateral cartilage lesion significantly increased the maximum lateral pressure, while a medial lesion did not significantly influence the maximum medial pressure. Improving VMO function can reduce the pressure applied to lateral cartilage when lateral lesions are present.