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

Abstract

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.

Figure 1

A diagram of the experimental set-up. Letters are used to represent the knee (K), the sensor and handle (S), and the tissue clamps (C). The cables applying the forces representing the VMO, VL, and VI/VML/RF (VI) are also labeled.

Figure 2

The average (± standard deviation) lateral force percentage at all three flexion angles for all three loading conditions, for both intact cartilage and cartilage with a lateral lesion. Statistically significant differences between the loading conditions for each cartilage condition at each flexion angle are indicated (a > b > c at p < 0.01). Similar values of the lateral force percentage and similar trends with the applied loading condition were recorded for the cartilage with silicone and intact medial cartilage and for the cartilage with a medial lesion.

Figure 3

The average (± standard deviation) maximum lateral pressure at all three flexion angles for all three loading conditions, for both intact cartilage and cartilage with a lateral lesion. Statistically significant differences between the loading conditions for each cartilage condition at each flexion angle are indicated (a > b > c at p < 0.05, no letter means no significant differences for a data point, repeated b’s mean no significant difference between two data points). Statistically significant differences between intact cartilage and cartilage with a lateral lesion over the three loading conditions at each flexion angle are also indicated (A > B at p < 0.02).

Figure 4

The average (± standard deviation) maximum medial pressure at all three flexion angles for all three loading conditions, for both intact medial cartilage and cartilage with a medial lesion. Statistically significant differences between the loading conditions for each cartilage condition at each flexion angle are indicated (a > b > c at p < 0.05, repeated b’s mean no significant difference between two data points).

Figure 5

The pressure sensor output at 40° and 80° of flexion for two knees with a lateral lesion for the VMO off and a normal VMO force. The position of the patella ridge and the lesion with respect to each pressure plot is indicated. The highest pressure is recorded in the darkest areas. The pressure is concentrated distally on the patella at 40° and proximally at 80°. Increasing the force applied by the VMO decreased the lateral pressure and increased the medial pressure.