To determine the effect of a drill hole on the strength of long bones and on the fracture resulting from rapidly applied torsional loads, paired canine femora were tested, and the following results were obtained:
1. The presence of a 2.8-millimeter or a 3.6-millimeter drill hole in the femoral mid-shaft significantly weakens the bone as measured by a mean reduction in energy-absorbing capacity of 55.2 per cent.
2. The drill hole produces a significant increase in local stresses as demonstrated by a calculated stress concentration factor of 1.6.
3. Changes in the ratio of drill hole area to bone area from .12 to .28 are not accompanied by significant changes in the bone strength or in the stress concentration factor.
4. The resulting spiral fractures occur along planes of maximum tensile stress, and the drill hole does not alter their orientation.
5. In drilled specimens the fracture is localized to the region of the drill hole and is less comminuted.