Katy L Townsend, Michael P Kowaleski, Paivi Rajala-Schultz, Kenneth A Johnson.
The Ohio State University, Columbus, Ohio, United States

ACVS Abstract 2007 

Cementless femoral THA implants must retain their initial stability to allow for long-term fixation by bone in-growth. A previous study noted that the femoral stem should fill at least 85% of the medullary canal to prevent subsidence.

The purpose of this study of BFXTM implants was to determine the correlation between femoral canal fill, femoral morphology (canal flare index) and age at the time of surgery with the degree of axial stem migration (subsidence) on three-month follow-up radiographs. The medical records and radiographs of dogs undergoing cementless THA using BFXTM implants were studied retrospectively. Canal fill and canal flare were measured on the immediate post-operative radiographs and subsidence measurements were taken on the post-operative and follow up radiographs.

The two methods of measuring subsidence were compared by calculation of concordance correlation coefficients, and the correlations between subsidence and canal fill, canal flare index, and age were analyzed by Pearson’s correlation. The most common breeds were the mixed breed and Labrador retriever and mean age at time of surgery was 2.7 years. Mean body weight was 37.3 kg. Complications occurred in 9 of the 37 cases, including THA luxation (5), intraoperative femoral fissure fracture (2) and post-operative femoral fracture (2). The medio-lateral and cranio-caudal subsidence measures were found not to be significantly different.

All statistical methods employed found no correlation between canal fill, canal flare index, age and degree of subsidence. Canal fill, canal flare index and age were not found to be associated with subsidence. This suggests that other variables such as surgical preparation and quality of the surrounding bone bed may play an important role.