Abstract
Introduction: At the time of revision hip surgery, large bony defects are often encountered. The traditional method of replacing this lost bone is by the impaction bone grafting technique. Vibration is commonly used in civil engineering to improve compaction of aggregate particles and to increase the compressive and shear strengths of the aggregate. Studies on soil mechanics have established that vibration applied to an aggregate results in more efficient alignment of particles and reduces the energy required to impact the aggregate. In this in-vitro study we have developed a novel method of applying vibration to the bone impaction process.
Methods: 60 Bovine femoral heads were cut into quarters and then milled using the Noviomagnus manual bone mill. Fat and blood were then removed using a pulsed lavage normal saline system over a sieve tower. A vibration impaction device was developed which housed two 15V DC motors with eccentric weights attached inside a metal cylinder. A weight was dropped onto this from a set height 72 times so as to replicate the bone impaction process. The bone graft underlying this was thus impacted into a pellet, with or without the aid of vibration. A range of frequencies of vibration were tested, as measured using an accelerometer housed in the vibration chamber.
Each shear test was then repeated at four different normal loads so as to generate a family of stress-strain graphs. The Mohr-Coulomb failure envelope from which the shear strength and interlocking vales are derived was plotted for each test.
Results: Graft impacted with the addition of vibration at 60Hz was significantly more resistant to shearing force than graft impacted without vibration (p< 0.03). Testing at 20 and 40 Hz showed no statistical difference (p=0.62, p=0.42).
Conclusion: Civil engineering principles hold true for the impaction bone grafting procedure. The best frequency of vibration to enhance the mechanical properties of the aggregate is in the region of 60Hz.
Correspondence should be addressed to Mr Richard Wallace at Musgrave Park Hospital, 20 Stockman’s Lane, Belfast BT9 7JB, Northern Ireland.
