Cis-cortical over-drilling compromises torsional stability of angle-stable intramedullary nail constructs

To assess the effect of over-drilling the distal, cis-cortical drill holes on the angle stability of a veterinary angle-stable intramedullary nail construct (I-Loc).

Partial, angle-stable, intramedullary nail constructs using 6-mm-diameter I-Loc implants and 19-mm-diameter synthetic bone analog were assembled with cis-cortex holes drilled to 3.2 mm and trans-cortex bolt holes drilled to 2.5 mm to match bolt dimensions. Constructs were subjected to nondestructive torsional loading to ± 1 Nm of torque. Constructs were experimentally tested with 0, 0.1, 0.7, 1.3, 1.9, and 2.3 mm of cis-cortical over-drilling, and torsional load deformation curves were generated. A mathematical formula was generated to compare theoretical torsional toggle to observed torsional toggle.

Angle stability was achieved when cis-cortical drilling matched the interlocking bolt diameter (n = 4). Median torsional toggles of 0.5°, 2.2°, 4.0°, 5.8°, and 6.7° were measured with 0.1, 0.7, 1.3, 1.9, and 2.3 mm of cis-cortical over-drilling, respectively. The mathematical formula predicted the measured torsional toggle to within a mean of 0.17°.

Uniform and symmetric cis-cortical over-drilling of both distal bolt holes for angle-stable intramedullary nailing compromises construct torsional stability.

Cis-cortical over-drilling, related to inaccurate interlocking bolt hole drilling, may compromise the torsional stability of angle-stable intramedullary nail constructs utilized for long bone fracture repair. The derived mathematic formula may provide insight into expected torsional instability when clinical over-drilling is encountered.