Studies on soil mechanics have established that when vibration is applied to an aggregate, it results in more efficient alignment of particles and reduces the energy required to impact the aggregate. Our aim was to develop a method of applying vibration to the bone impaction process and assess its effect on the mechanical properties of the impacted graft.

Phase 1. Eighty bovine femoral heads were milled using the Noviomagus bone mill. The graft was then washed 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. 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 curves. The Mohr-Coulomb failure envelope from which the shear strength and interlocking values are derived was plotted for each test.

Phase 2. Experiments were repeated with the addition of blood so as to replicate a saturated environment as is encountered during operative conditions.

Relatively dry graft impacted with the addition of vibration showed improved shear strength at all frequencies of vibration when compared to impaction without vibration. In our system the optimal frequency of vibration was 60 Hz. Under saturated conditions the addition of vibration is detrimental the shear strength of the aggregate. This is secondary to decreased interlocking between particles and may be explained by the process of liquefaction.

Background

Unicompartmental knee replacement (UKR) is an established treatment for single compartment end-stage knee arthrosis with good recorded survivorship. Although often used in more active younger patients, patient selection remains controversial. To identify risk factors for early failure we compared patients with UKR failure requiring revision to total knee replacement (TKR) with a control group.