Abstract
Introduction
Each year, a large number of total hip arthroplasties (THA) are performed, of which 60 % use cementless fixation. The initial fixation is one of the most important factors for a long lasting fixation [Gheduzzi 2007]. The point of optimal initial fixation, the endpoint of insertion, is not easy to achieve, as the margin between optimal fixation and a femoral fracture is small. Femoral fractures are caused by peak stresses induced during broaching or by the hammer blows when the implant is excessively press-fitted in the femur. In order to reduce the peak stresses during broaching, IMT Integral Medizintechnik (Luzern, Switzerland) designed the Woodpecker, a pneumatic broach that generates impulses at a frequency of 70 Hz. This study explores the feasibility of using the Woodpecker for implant insertion by measuring both the strain in the cortical bone and the vibrational response. An in vitro study is presented.
Material and Methods
A Profemur Gladiator modular stem (MicroPort Orthopedics Inc. Arlington, TN, USA) and two artificial femora (composite bone 4th generation #3403, Sawbones Europe AB, Malmö, Sweden) were used. One artificial femur was instrumented with three rectangular strain gauge rosettes (Micro-Measurements, Raleigh, NC, USA). The rosettes were placed medially, posteriorly and anteriorly proximally on the cortical bone. Five paired implant insertions were repeated on both artificial bones, alternating between standard hammering and Woodpecker insertions. During the insertion processes the vibrational response was measured at the implant and Woodpecker side (fig. 1) using two shock accelerometers (PCB Piezotronics, Depew, NY, USA). Frequency spectra were derived from the vibrational responses. The endpoint of insertion was defined as the point when the static strain stopped increasing during the insertion.
Results
Peak stress values calculated out of the strain measurement during the insertion showed to be significantly (p < 0.05) lower at two locations using the Woodpecker compared to the hammer blows at the same level of static strain. However, the final static strain at the endpoint of insertion was approximately a factor two lower using the Woodpecker compared to the hammer. During the last hammer insertion a fracture occurred, which was clearly visible in the frequency spectra. Figure 2 shows the sudden change between the spectra of the hit prior and after the fracture.
Discussion/Conclusion
Peak stresses showed to be lower using the Woodpecker compared to hammer insertion, which is a promising result concerning fracture prevention. However it needs to be taken into account that it was not possible to reach the same level of static strain using the Woodpecker as with the hammer insertion. It is expected that the Woodpecker in its actual design is not able to reach a similar level of press-fit as hammer blows. Using vibrational data showed to be promising for fracture detection, as fractures are not always visible due to the soft tissue.
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