Effects of clustered drill holes on the breaking strength of the equine third metacarpal bone

The breaking strength (stress at failure) of equine third metacarpal bones, with and without clustered drill holes, was determined in vitro. Paired ossa metacarpalia II-IV of 39 horses (n = 39) between 2 and 7 years old were tested in palmarodorsal 3-point bending. Four treatments were compared. Clustered 2.7- or 3.5-mm drill holes, in a 4- or 7-hole pattern, were made in the dorsal cortex of the distal diaphysis of the left third metacarpal bone. Undrilled right third metacarpi were used as controls. Bones with clustered drill holes failed by an oblique fracture through 1 or more drill holes, whereas undrilled bones failed with a middiaphyseal transverse fracture. Clustered drill holes acted as a stress concentrator and significantly (P less than 0.05) decreased the stress required for failure. However, differences in breaking strength between treatment groups were not significant (P greater than 0.05).