Introduction: During femoral impaction bone grafting high forces and hoop strains may be generated with subsequent risk of fracture. Vibration is commonly used in civil engineering applications to increase aggregate compressive and shear strengths. We hypothesized that the use of vibration during impaction bone grafting, reduces the maximum hoop strains, and hence risk of fracture, and improves particle interlocking, producing a stronger aggregate.

Method: A series of femoral impaction bone graftings on physiological composite femurs, using morsellised graft from fresh frozen human femoral heads were performed. The standard Exeter impaction technique was used in the control group and vibration assisted compaction used in the study group. Total force imparted, hoop strains and subsidence rate were measured.

Results: Significantly more allograft was used in the vibration group than in the control group (73.1g, 79.5g, p=0.01). Higher mean peak loads were produced during proximal compaction in the control group (3.28kN) than in the vibration group (1.71kN, p=0.005). Higher mean peak and mid proximal hoop strains were generated in the control group (13.2%, 5.6%) compared to the vibration group (4.2%, 2.7% p=0.009, p=0.006). The mean total axial subsidence after 50,000 cycles was significantly less in the control group (2.47mm, SD 0.55) compared to the vibration group (1.79mm, SD 0.30, p=0.03).

Discussion: The use of vibration leads to reduced peak loads and hoop strains in the femur during graft compaction which may reduce the risk of femoral fracture. Additionally the resulting graft is better able to resist subsidence thus conferring improved mechanical stability. A safer, more flexible method to compact bone graft could lead to the more widespread use of IBG in revision hip surgery.

The complications of impaction bone grafting in revision hip replacement includes fracture of the femur and subsidence of the prosthesis. In this in vitro study we aimed to investigate whether the use of vibration, combined with a perforated tamp during the compaction of morsellised allograft would reduce peak loads and hoop strains in the femur as a surrogate marker of the risk of fracture and whether it would also improve graft compaction and prosthetic stability.

We found that the peak loads and hoop strains transmitted to the femoral cortex during graft compaction and subsidence of the stem in subsequent mechanical testing were reduced. This innovative technique has the potential to reduce the risk of intra-operative fracture and to improve graft compaction and therefore prosthetic stability.

Nomograms derived from mathematical analysis indicate that the level of malunion is the most important determinant of changes in the moment arm of the knee, the plane of the ankle and alterations in limb length. Testing in five patients undergoing reconstruction showed a mean error of postoperative limb length of 2.2 mm (sd 0.8 mm), knee moment arm of 4.7 mm (sd 3.3 mm) and ankle angle of 2.6° (sd 2.3°). These nomograms provide the information required when assessing whether a particular degree of angulation may be accepted.