Purpose: In-vitro mechanical tests are often used to pre-clinically assess the primary stability of hip endopros-theses. There is no standard protocol for these tests and the test conditions used vary greatly. This study examined the effect of the abductor muscle and the anterior-posterior component of the hip contact force (Fap) on the primary stability of cementless stems.

Methods: Cementless stems were implanted in 12 composite femurs which were divided into two groups: group 1 (N=6) was loaded with the hip contact force only, whereas group 2 (N=6) was additionally subjected to an abductor force. The cranial-caudal component of the hip contact force was the same in both groups, i.e. 2.3BW at 13° from the femur long axis. Each specimen was subjected to three Fap levels: 0, 0.3BW (walking), and 0.6BW (stair climbing). The implant translation relative to the femur was measured using a custom-built system comprised of 6 LVDT sensors. The resultant migration and micromotion were analyzed using an ANOVA with the abductor a between-group factor and Fap a within-group factor, followed by SNK post-hoc analysis with a significance level of 95%.

Results: Implant motion was not significantly affected when the Fap was increased from 0 to 0.3BW. However, without abductor, increasing Fap from 0.3 to 0.6BW increased migration and micromotion by an average of 291& #956;m (285% increase), and 15& #956;m (75%) respectively. With abductor, increasing Fap to 0.6BW increased migration by 87& #956;m (79%) but did not affect micromotion. The abductor did not significantly affect stem motion at lower Fap, but at Fap = 0.6BW motion was 50% lower compared with hip contact forces only.

Conclusions: Based on these results, inclusion of either abductor and/or Fap has little effect on implant motion when simulating walking. However, stair climbing (higher Fap) generates greater bone-implant motion compared to walking loads, and this effect is greatest in the absence of an abductor force. Funding: Other Education Grant Funding Parties: The Michael Smith Foundation for Health Research