DYNAMIC AND CYCLIC COMPACTON MECHANICS OF MORSELLISED CANCELLOUS BONE GRAFT

Abstract

Introduction Impaction bone grafting is an established technique in revision surgery to compensate bone loss. The technique involves “dynamic compaction” (compaction using repeated impacts from a moving weight) of cancellous bone particles into a defect until the material is strong enough to carry the patient’s load. The technique has two widely documented complications, per-operative bone fracture and subsidence of the prosthesis, both related to mechanical factors. Lack of bone compaction is the main cause for subsidence of the prosthesis and the large levels of impaction energy needed to ensure sufficient bone compaction are the main cause of fractures. No work exists that relates the number and energy of impacts to the degree of compaction obtained, or the degree of compaction obtained to the amount of subsidence during cyclic loading. The aim of the current study was to determine these relations.

Methods For each sample, six grams of freshly frozen morsellised porcine bone was placed in a 15 mm diameter by 40 mm high cylinder. Samples were compacted dynamically with a range of energies by releasing a weight of 0.702 kg 20 times from a height of either 10, 20, 25 or 50 mm on an impactor. Resulting force on and deformation of the bone column during each impact were sampled at a rate of 3000 Hz. The data was summarized by collecting peak load and concurrent displacement from each consecutive impact. Compacted and non-compacted samples were placed in a testing machine (ESH Testing Ltd.) and cyclically loaded with a peak load of 50, 90 or 180 N (corresponding to 0.28, 0.51 and 1.0 MPa) while collecting applied force and displacement.

Results Peak stresses during dynamic compaction proved an exponential function of concurrent strain. Curves for all four levels of applied energy coincided on a single path in the stress-strain plane although, for an equal number of impacts, higher energy levels generated higher stresses. Permanent strain proved a logarithmic function and peak stress a power function of (impact number × energy1.5). The higher the impaction energy used for compacting the graft the lesser was the displacement and hence the subsidence under cyclical loading at a given force. With virgin bone graft under cyclical loading, the displacement was maximum in the first cycle. With subsequent cycles the subsidence was minimal and was independent of the force of cyclical loading.

Conclusions Stress was a function of (blow number × energy1.5), suggesting that halving the energy level per impact would require three times as many blows to give a comparable stability. This can potentially reduce the incidence of intra-operative fractures. Higher impaction energies might reduce the subsidence of femoral component during the first steps that the patient takes and though subsidence is dependent on the force of cyclical during initial cycles, further subsidence is independent of the same.