Author and graduation date: Arnel L. AGUINALDO, May, 1999

Committee members: Peter R. Francis (Chair), Bob Moore, Paul T. Kolen

Thesis title: Joint Center of Pressure with Valgus Bracing for Knee Osteoarthritis Using a Mechanical Knee Surrogate

During varus loading of the knee, the joint center of pressure (COP) shifts towards the medial compartment, increasing the likelihood and severity of unicompartmental osteoarthritis (OA) of the knee. In treating medial knee OA, a valgus brace decreases associated symptoms by reducing the medial shift in the joint COP. In order to quantify this shift reduction, it was the purpose of this study to evaluate how two commercially available OA braces shifted the COP in a mechanical knee surrogate during varus loading. COP analysis was done with each brace at 0, 10, 20, and 30 degrees knee flexion angles (KFA).

The braces evaluated in this study were the Patient-Ready Monarch (Smith & Nephew Bracing & Support Systems, Vista, CA) and the Unloader Select (Generation 11 USA, Bothell, WA). For direct measurements of joint compartmental loads with and without these braces, a mechanical knee surrogate was employed equipped with compression load cells embedded in the surrogate's analog menisci. Compartment load data was collected from the braced and unbraced surrogate while it was subjected to varus loading applied by a material testing system. Using the load data, COP locations and shifts in COP were calculated and compared between the two braces.

It was found that the shifts in COP caused by the Monarch brace were significantly more lateral than those of the Unloader Select at 0 degrees KFA (p = .001) and at 20 degrees KFA (p = .02) during 12 N-m of applied varus loading. A significant statistical interaction between brace and KFA (F(3,15) = 6.67, p = .004) was found, and thus, main effects testing with Scheffe post hoc analysis was performed across KFAs for each brace. The results revealed that the greatest shift away from the medial compartment occurred during 0 degrees KFA for both braces with the Monarch eliciting the greatest shift of -1.0 cm displaced from geometric center (cm-DFGC) of the joint. COP shifts at the other angles were significantly less (p <.02) than the shift at 0 degrees KFA in both braces.

The findings of this study indicated that valgus bracing shifts the COP laterally, marking a reduction in the varus moment about the knee joint. Brace testing at increasing, quasi-static knee flexion angle revealed that valgus bracing loses some of its ability to shift the COP away from the medial compartment. But whether this reduction in shift has significant clinical relevance has not yet been determined. From a mechanical standpoint, however, this study has shown that valgus bracing is effective in relieving the compressive load on the medial compartment, thereby providing implications for clinical use as treatment for medial knee OA.

Click here to return to the Completed Master Degree Theses in Biomechanics Table of Contents