Background: Early failure of morselized impaction bone allograft is usually due to shear forces. Soil mechanics tells us that in aggregates such as bone allograft, the resistance to these shear forces can be increased by altering the fluid concentration, varying the particle size, and improving the morphology of the graft particle. Finding an idealized concentration of fat and water in bone graft could improve the resistance to interparticle shear and therefore decrease the failure rate of impaction bone graft. Ensuring the quality of the bone source is adequate can also improve the initial strength the bone graft. Furthermore optimizing the graft can be achieved by screening methods and simple intra-operative techniques.

Methods: Human femoral heads were retrieved from both total hip arthroplasty and hemiarthroplasty procedures. Bone mineral density was determined by DEXA scanning. The fat and water content of the graft was varied by combinations of squeezing and drying the graft and also by washing the graft using pulse lavaged water and 1:1 mixture of chloroform: methanol. The amount and characteristics of the fat and water in human morselized cancellous bone was quantified by the Karl Fischer extraction techniques, and gas chromatography. The overall shear strength of each graft preparation was determined by the direct shear test, adapted from an accepted protocol in soil mechanics and the optimum mixture which would resist shear forces was determined.

Results: An optimum level of fat and water was determined which was 50% stronger than unaltered bone graft. This is most closely approximated in an operating theatre situation by washing the graft with pulse lavaged normal saline and subsequently squeezing the bone graft in a vice with a force of 335kPa for 5 minutes. Whereas osteoarthritic and osteoporotic bone were similar in their fat and water content and initial resistance to shear forces, after processing, the resistance to shear forces of osteoarthritic bone improved by 147% and that of osteoporotic bone only improved by 12% (p< 0.001) Conclusions: Optimizing the fat and water content of bone graft and closely choosing the source of graft produces a stronger graft which is more resistance to shear stresses, protecting the surgical construct until bone growth can occur.