Mechanical performance of pin clamps in external fixators.

Insufficient holding strength on a pin within a clamp may result in the diminution of the overall fixation rigidity as well as pin movement at the pin-bone interface. In this study the holding strength of pin fixation clamps in two representative external fixators (Hoffmann and Orthofix) was evaluated by determination of the torque resistance of pins within a clamp. In the standard Hoffman clamp, the pin-clamping effect was satisfactory in the symmetric two-, three-, and four-pin configurations, whereas the standard Orthofix clamp provided higher holding strength in the symmetric two- and three-pin configurations. All other pin configurations in both clamps resulted in a high variation of pin torsional resistance, and sometimes one of the pins registered low or had no resistance to torsion. The results indicated that the holding power of the clamps was adequate only if certain guidelines were followed at the time of external fixator application. An instrumented torque wrench may be helpful to assess the pin-fastening strength within the clamp. This wrench should also be used to introduce appropriate and uniform tightening torques to the pin clamp screws. However, these results do not apply to those fixators in which each pin will be tightened individually.