Abstract
Background: Impaction bone-grafting in revision hip surgery generates high forces that may be transmitted through the graft to the femoral cortex, generating high surface strains and a concomitant risk of femoral fracture. Concern of inducing fracture may lead to under-compaction of the graft, with subsequent risk of implant migration. Vibration is commonly used in civil engineering applications to increase aggregate compressive and shear strengths. We have therefore examined the hypotheses that vibration-assisted graft compaction would (a) increase graft compaction compared with the standard femoral impaction grafting technique and subsequently reduce prosthesis migration and (b) reduce femoral hoop strains in the production of graft of a given density and mechanical properties.
Method: Physiological composite femurs were adapted to represent femurs encountered in revision hip surgery by widening of the internal diameter and thinning of the outer shell. In the control group, revision with the standard Exeter technique was simulated using highly washed morcellised bone graft from fresh-frozen human femoral heads. In the study group, vibration-assisted graft compaction was used. The femurs were mounted on a 5kN capacity load cell to measure the total force imparted during graft impaction. Strain gauges placed at the medial calcar and midshaft, measured hoop strains generated during the impaction process. On completion of graft impaction, an Exeter stem was cemented in place. Implant subsidence under physiological cyclic loading (5x 105 cycles) and graft density using micro CT were measured after compaction.
Results: There were no significant differences between the two groups in the peak forces (3.8-4.1kN) imparted during the impaction process. Similar peak hoop strains were observed in the both groups (1.2-1.4%). However a greater graft density was seen in the vibration group with minimal implant subsidence under cyclic loading.
Conclusion: The use of vibration during the impaction process allowed improved graft compaction to be achieved without increasing hoop strains in the femoral cortex. This has implications in preventing failure from under impaction without increasing the risk of fracture. Furthermore, this analysis is applicable to the study of novel synthetic grafts / mixtures in the impaction process for orthopaedic application.
Correspondence should be addressed to Mr Carlos Wigderowitz, Senior Lecturer, University Department of Orthopaedic and Trauma Surgery, Ninewells Hospital and Medical School, Dundee DD1 9SY.
