Impaction in hip arthroplasty

Abstract

Surgical technique is a critical factor in the success of Total Hip Arthroplasty (THA). Cementless implants must be seated by the surgeon through the process of impaction – using a surgical mallet to generate enough force to push an oversized implant into an undersized bone cavity. A careful balance of adequate hoop strain in the bone to ensure implant fixation, but not so much that the bone is fractured, is required. The overarching aim of this PhD thesis was to better understand the process of impaction during THA. An ex-vivo cadaveric study of THA was used to design an in-vitro rig that can replicate impaction in the surgical environment. Patient compliance was identified as an important factor in impact dynamics. Using the in-vitro model, a range of traditional impaction techniques were tested in high and low density bone. It was found that large impaction energies (15J) showed little fixation advantage over strikes of half the energy, but could increase the risk of fracture of bone. In addition a mechanism of strain deterioration, and therefore fixation deterioration, was identified if too many high energy impaction strikes were used. Therefore a medium energy strike, using a heavy surgical mallet but low velocity, is recommended. As well as traditional mallet strikes, emerging oscillatory seating technologies were evaluated. A pneumatic oscillatory device was found to produce similar peak bone strain (and therefore fracture risk) as traditional methods, but less implant fixation. Therefore more research is required to prove the efficacy of these devices over traditional techniques.