Stiffness of trabecular bone of the tibial plateau in patients with rheumatoid arthritis of the knee

Abstract

Stiffness of subchondral proximal tibial trabecular bone is a factor in the stability of prostheses implanted into that bone. The stiffness of trabecular bone in osteoarthritis (OA) has been documented. Trabecular bone in rheumatoid arthritis (RA) is osteopenic in numerous sites and morphologically abnormal in the proximal tibia. Reliable data on proximal tibial bone in RA are lacking, although 1 study failed to identify abnormalities. The purposes of this study were 1) to document the stiffness of the proximal tibial cancellous bone in patients with RA, (2) to determine the effect of angular deformity on bone stiffness in rheumatoid patients, and (3) to compare RA stiffness values with those in published reports for OA. Fifteen tibial plateau were obtained from patients with RA during surgery. Each plateau was horizontally seated in a mold and covered with cement. The plateau was divided into 6 regions, which were used to facilitate comparison between specimens and the existing literature. Indentation tests were conducted with a 4-mm-diameter cylindrical indentor controlled by an MTS machine. The indentor descended at a rate of 2 mm/min to a maximum depth of 1.0 mm; load and displacement data were digitally recorded. Stiffness was calculated from the slope of the linear region of the curve using best-fit linear regression. Where varus deformity was present, stiffness in the medial plateau was higher overall than for the other compartment; whereas in the case of valgus deformity, stiffness of the lateral side was significantly higher ( P < .05 for each observation). In comparison to older normal specimens, both the medial compartment of the varus RA specimens ( P < .01) and the posterolateral compartment of the valgus RA specimens ( P < .01) had significantly lower stiffness. Comparison with OA specimens showed that in varus RA, the posteromedial region had significantly lower stiffness than in varus OA at the same site ( P < .01). In valgus RA, the lateral region had significantly lower stiffness than in valgus OA at the same site ( P < .01). The mean stiffness ratio of the valgus RA was significantly ( P < .01) altered from normal, and for the varus RA, it was significantly ( P < .01) different from normal posteriorly. The stiffness ratios for the varus RA were significantly ( P < .01) different from those for varus OA; there was no difference between valgus RA and valgus OA. It is concluded that RA affected bone has significantly lower stiffness than normal and osteoarthritic bone. The loaded plateau is stiffer than the unloaded plateau in angular deformity, but is still less stiff than normal bone and osteoarthritic plateaus with corresponding deformities.