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EVALUATION OF FEMORAL IMPACTION GRAFTING FORCES

Biomedical Engineering, Institute of Orthopaedics, UCL, RNOH, Stanmore, Middlesex, HA7 4LP

Introduction: Impaction allografting is a technique that is used at revision where the bone stock in the femur is poor. Femoral heads are ground to create morsellised bone, which is impacted down the femur prior to the cementing a new stem into the canal. The results of this technique are variable and there is a high incidence of stem migration. This variation in results may be due to the degree of bone loss or the techniques used to impact the graft. The aim of this study was to quantify the forces currently used in revision hip surgery with impaction allografting.

Methods: To enable these measurements the Exeter slap hammer (Stryker Howmedica) was altered to include a load washer. The load washer had a special cable welded to it so that sterilisation could still be conducted in an autoclave. During surgery the end of the load washer cable is passed to the operator, who is able to connect it to a laptop computer. A specially written Labview program is then used to store the data and determine the impaction forces. The load washer is mounted within the hammer at the point of impact between the sliding mass and the hammer, consequently it is reading the force transmitted to the hammer, not that transmitted to the graft chips. Calibration was performed in an in vitro experiment with a second load washer, which found that the force in the hammer is three times that in the impactor. The force is so much less because it is taken up in the hammer’s inertia.

Conclusions: The impaction forces have been measured during eight operations performed by three different surgeons. The study shows variability between surgeons, and variability between patients operated on by the same surgeon. These readings show that the forces travelling through the impactor range between three to eleven time body weight.

Correspondence should be addressed to Mr Carlos Wigderowitz, Honorary Secretary BORS, University Dept of Orthopaedic & Trauma Surgery, Ninewells Hospital & Medical School, Dundee DD1 9SY.