Comparison between locking and non-locking plates for fixation of metacarpal fractures in an animal model

The use of locking plates increases the primary load to failure, thereby reducing the rate of implant-related failure. The good clinical and biomechanical results of locking plates in long bones might be applicable to treatment of metacarpal fractures. The purpose of this study was to determine strength and stiffness of locking plates in a metacarpal fracture model with mono- and bicortical screw fixation in comparison to non-locking plate mono- and bicortical screw fixation, with both types of plates placed at the dorsal side of the bone.

Fresh second metacarpals from domestic pigs (n=40) were randomized in 4 equal groups. Short, oblique, mid-shaft fractures were generated, using a standardized 3-point bending method. Fractures were plated with non-locking, titanium, 1-mm-thick monocortical (group 1, n=10) or bicortical (group 2, n =10) plates (Leibinger-Stryker; Stryker Corp, Freiburg, Germany). Newly designed locking titanium plates with the same width and thickness (Leibinger-Stryker) were used in the same manner for groups 3 (monocortical) and 4 (bicortical). The metacarpals were then tested to load to failure in a cantilever bending mode.

Bicortical, non-locking fixation (group 2, 359 +/- 90 N) had a higher load to failure than monocortical non-locking fixation (group 1, 250 +/- 56 N) in testing the maximum load to failure (p < .01). There was no significant difference in stiffness between group 1 (46 +/- 12 N/mm) and group 2 (56 +/- 21 N/mm). The difference in maximum load to failure between monocortical (group 3, 440 +/- 85N) and bicortical (group 4, 378 +/- 116 N) locking plate stabilization was not significant. Also, there was no significant difference in stiffness between monocortical (group 3, 83 +/- 35 N/mm) and bicortical locking plates (group 4, 70 +/- 31 N/mm). Comparing non-locking (group 1) and locking plates in a monocortical fixation technique (group 3) demonstrated significant differences in maximum load to failure (group 1, 250 +/- 56 N; group 3, 440 +/- 85 N) and stiffness (group 1, 46 +/- 12 N/mm; group 3, 83 +/- 35 N/mm). The stability of monocortical locking plates was stronger, although not statistically significant, than the non-locking bicortical plates (load to failure, 440 +/- 85 N vs 359 +/- 90 N; stiffness, 83 +/- 35 N/mm vs 56 +/- 21 N/mm).

The new generation of locking plates can be used to achieve a higher stability for fixation of metacarpal fractures. Monocortical, stable fixation can minimize flexor tendon interference and probably reduce bone and soft tissue trauma.