Mechanical comparison of 3 different clamp and 2 different rod types of a new veterinary internal fixation system,...

To compare the gripping force of a standard clamp with click-on and T clamps of the 4.5/5.5-mm VetFix (AO Research Institute, Davos, Switzerland), mounted on smooth and roughened rods.

In vitro mechanical study.

Rods were mounted on a clamp-holding device. Each clamp type was tested on a materials-testing machine in groups of 6 specimens for torsional and axial load stability on smooth and roughened rods (total, 72 specimens). The variable evaluated was the discontinuity point (DP) as a measure for gliding resistance of the clamp on the rod. Means (+/-SD) were calculated for all groups separately. For statistical comparison, a multiple regression model was fitted with level of significance set at P =.05.

The direction of torsion had a significant effect on DP for both asymmetric clamps (standard, click-on). In torsion and axial load, the roughened rod had significantly higher DP levels than the smooth rod for the standard clamp. Rods differed only in axial load for the T clamp, again favoring the roughened rod. The click-on clamp did not reach 10% of the values achieved by the other clamps, independent from the rods and loading conditions. Compared with the T clamp, standard clamp DP values were significantly higher in torsion on the roughened rod as well as in axial load on both rod types.

The roughened rod with a standard clamp was mechanically superior. For the T clamp, the small loss of gripping force may be compensated for by the possibility of inserting 2 more screws.

The roughened rod may enhance the bending, compressive, and torsional properties of the 4.5/5.5-mm VetFix system by reducing the risk of clamp gliding on the rod. The standard clamp is the clamp of choice, except for the metaphyseal or epiphyseal region, at which use of a T clamp may be indicated to increase the number of screws inserted at the end of the rod.