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

This study explored the relationship between the initial stability of the femoral component and penetration of cement into the graft bed following impaction allografting.

Impaction allografting was carried out in human cadaveric femurs. In one group the cement was pressurised conventionally but in the other it was not pressurised. Migration and micromotion of the implant were measured under simulated walking loads. The specimens were then cross-sectioned and penetration of the cement measured.

Around the distal half of the implant we found approximately 70% and 40% of contact of the cement with the endosteum in the pressure and no-pressure groups, respectively. The distal migration/micromotion, and valgus/varus migration were significantly higher in the no-pressure group than in that subjected to pressure. These motion components correlated negatively with the mean area of cement and its contact with the endosteum.

The presence of cement at the endosteum appears to play an important role in the initial stability of the implant following impaction allografting.