An experimental sheep model was used for impaction allografting of 12 hemiarthroplasty femoral components placed into two equal-sized groups. In group 1, a 50:50 mixture of ApaPore hydroxyapatite bone-graft substitute and allograft was used. In group 2, ApaPore and allograft were mixed in a 90:10 ratio. Both groups were killed at six months. Ground reaction force results demonstrated no significant differences (p > 0.05) between the two groups at 8, 16 and 24 weeks post-operatively, and all animals remained active. The mean bone turnover rates were significantly greater in group 1, at 0.00206 mm/day, compared to group 2 at 0.0013 mm/day (p < 0.05). The results for the area of new bone formation demonstrated no significant differences (p > 0.05) between the two groups. No significant differences were found between the two groups in thickness of the cement mantle (p > 0.05) and percentage ApaPore-bone contact (p > 0.05).

The results of this animal study demonstrated that a mixture of ApaPore allograft in a 90:10 ratio was comparable to using a 50:50 mixture.

This study investigates the use of porous biphasic ceramics as graft extenders in impaction grafting of the femur during revision hip surgery. Impaction grafting of the femur was performed in four groups of sheep. Group one received pure allograft, group two 50% allograft and 50% BoneSave, group three 50% allograft and 50% BoneSave type 2 and group four 10% allograft and 90% BoneSave as the graft material. Function was assessed using an index of pre- and post-operative peak vertical ground reaction force ratios. Changes in bone mineral density were measured by dual energy X ray absorptiometry (DEXA) scanning. Loosening and subsidence were assessed radiographically and by histological examination of the explanted specimens. There was no statistically significant difference between the four groups after 18 months of unrestricted functional loading for all outcome measures

In vitro femoral studies have demonstrated the addition of hydroxyapatite (HA), to morcellised bone graft (MBG) decreases femoral prosthesis subsidence. However, with an increased risk of femoral fracture during the impaction of a MBG:HA mixture, possibly due to greater force transmission to the femoral cortex via the HA. The aim was to compare the hoop strains and subsidence of a 1:1 mixture of MBG:HA with pure bone allograft during impaction and subsequent endurance testing in a revision hip arthroplasty model. Materials and methods Large Sawbone femurs were prepared to represent a femur with bone loss (Sawbones, Sweden). 12 uniaxial strain gauges were attached to each femur at 0, 90, 180 and 270 degrees, at distal, midshaft, proximal points to measure hoop strain. Impaction grafting was performed using X-Change 2 instruments and an Instron servohydaulic machine for 2 distal impactions and 4 proximal impactions for 60 impactions each. Study groups. The study consisted of four experimental groups: 1)Pure MBG, force of 1.98 kN 2)Pure MBG, force 3.63kN. 3)1:1 mixture of MBG: porous HA (pHA), 4)1:1 mixture MBG: non porous HA (npHA). 6 samples of each group were performed. Endurance testing. The potted femur was loaded in a manner representing the walking cycle (1.98kN) at 1 Hz for 50 000 cycles. The displacement of the femoral head during loading was measured by two displacement transducers (LVDT) were mounted on aluminum brackets to measure vertical displacement and rotation. Statistical analysis. Statistical analysis was performed using a Mann – Whitney U test for total subsidence and prosthesis cyclical movement at 6 hours between the control MBG1.98 and the other experimental groups. Subsequent analysis compared pHA and npHA mixes with and HA to MBG3. 65kN. Level of significance was taken at p<0.05. Results. Distal strain gauges:. All experimental groups had a significantly greater distal hoop strain than MBG1.98 kN (p=0.004). Middle strain gauges:. No significant differences between the HA groups and MBG1.98. MBG3.65 was significantly greater (p=0.02). Top strain gauges:. No significant differences, p=0.9. Endurance testing There was a non significant trend towards decreased subsidence in the HA groups compared to the bone groups. (p=0.7) but significantly less cyclical subsidence between HA groups and MBG 1.98 (p=0.02). No difference between the MBG 3.65 kN and HA groups for the cyclical subsidence. Statistically significant difference in cyclical rotation between both the MBG1.98 and the pHA (p=0.02) and npHA(p=0.02) and the bone 3.65 and pHA and npHA groups(p=0.04). Conclusion. The addition of HA to MBG decreases total and cyclical subsidence. There were significantly greater hoop strains in the distal and midshaft strain gauges with greater impaction force and in the distal gauges with the use of HA. Endurance testing demonstrated a significant decrease in cyclical motion and cyclical rotational stability of the prosthesis with a trend to a decrease in total subsidence during endurance testing with HA groups. The addition of HA might have longer term benefits in terms of prosthesis stability and subsequent graft healing but caution is needed during impaction

In order to investigate the osteoinductive properties of allograft used in impaction grafting and the effect of strain during impaction on these properties, we designed an in vitro experiment to measure strain-related release of bone morphogenetic protein-7 (BMP-7) from fresh-frozen femoral head allograft. A total of 40 10 mm cubes of cancellous bone were cut from ten samples of fresh-frozen femoral head. The marrow was removed from the cubes and the baseline concentrations of BMP-7 were measured. Specimens from each femoral head were allocated to four groups and subjected to different compressive strains with a material testing machine, after which BMP-7 activity was reassessed. It was present in all groups. There was a linear increase of 102.1 pg/g (95% confidence interval 68.6 to 135.6) BMP-7 for each 10% increase in strain. At 80% strain the mean concentration of BMP-7 released (830.3 pg/g bone) was approximately four times that released at 20% strain. Activity of BMP-7 in fresh-frozen allograft has not previously been demonstrated. This study shows that the freezing and storage of femoral heads allows some maintenance of biological activity, and that impaction grafting provides a source of osteoinductive bone for remodelling.

We have shown that BMP-7 is released from fresh-frozen femoral head cancellous bone in proportion to the strain applied to the bone. This suggests that the impaction process itself may contribute to the biological process of remodelling and bony incorporation.