The effect of 4 mm bicortical drill hole defect on bone strength in a pig femur model.

Introduction: In orthopaedic surgery, small bicortical circular bone defects are often produced as a result of internal fixation of fractures. The aim of this study was to determine the amount of torsional strength reduction in animal bone with a bicortical bone defect and how much residual strength remains if the bicortical bone defect was occluded.

Method: Forty pig femurs were divided into four groups. Group 1 femurs were left intact. Group 2 femurs were given a 4 mm bicortical bone defect. Group 3 were prepared as in Group 2, but occluded with a 4.5 mm cortical screw. Group 4 were prepared as in Group 2, but occluded with plaster of paris. Measurements including the length of the bone, working length of the bone, mid-diaphyseal diameter and cortical thickness were recorded. All specimens were tested until failure under torsional loading. Peak torque at failure and angular deformation were recorded. One-way analysis of variance was used to test the sample groups, with a value of P < 0.05 considered to be statistically significant.

Results: When compared with Group 1, all of the other groups showed a reduction in peak torque at failure point. Only the difference in peak torque between Groups 1 and 2 was statistically significant (P = 0.007). Group 2 showed the most reduction with 23.11% reduction in peak torque and 38.19% reduction in total energy absorption. No significant difference was found comparing the bone length, bone diameter and the cortical thickness.

Conclusion: The presence of the defect remains the major contributing factor in long bone strength reduction. It has been shown that a 10% bicortical defect was sufficient to produce a reduction in peak torque and energy absorption under torsional loading. By occluding this defect using a screw or plaster of paris, an improvement in bone strength was achieved. These results may translate clinically to an increased vulnerability to functional loads immediately following screw removal and prior to the residual screw holes healing.