Arthrodesis of the equine proximal interphalangeal joint: a mechanical comparison of 2 parallel 5.5 mm cortical screws and 3 parallel 5.5 mm cortical screws.

OBJECTIVE

To compare the biomechanical characteristics and mode of failure of 2 techniques using parallel 5.5 mm screws for pastern joint arthrodesis in horses.

STUDY DESIGN

Randomized block design, for horse (1-5), method of fixation (two 5.5 mm screws versus three 5.5 mm screws), side (right, left), and end (front, hind). Constructs were tested to failure in 3-point bending.

SAMPLE POPULATION

Twenty limbs (5 cadavers).

METHODS

A combined aiming device was used to facilitate screw placement. Two parallel 5.5 mm screws were inserted in lag fashion in 1 limb of a pair, and three 5.5 mm screws were inserted in the contralateral limb. Constructs were then tested in 3-point bending in a dorsal-to-palmar (plantar) direction using a materials testing machine at a loading rate of 19 mm/s. Maximal bending moment at failure and stiffness were obtained from bending moment-angular deformation curves.

RESULTS

There was no significant difference between two and three 5.5 mm screw constructs for bending moment and stiffness (P<.05). All constructs ultimately failed by bone fracture or screw bending. For proximal interphalangeal (PIP) joint arthrodesis constructs loaded in 3-point bending, no significant effect of treatment, side, or end on maximal bending moment or stiffness was detected.

CONCLUSIONS

Two 5.5 mm cortical screws inserted in parallel should provide a surgically simpler and equally strong PIP joint arthrodesis compared with three 5.5 mm cortical screws.

CLINICAL RELEVANCE

Two 5.5 mm cortical screws inserted in parallel for PIP joint arthrodesis should perform similarly under conditions used in this study, as three 5.5 mm screws inserted in a similar manner, when loaded under bending.