Curved “K” wires: The covert villain in circular external fixators

Abstract

Context: Complications associated with “K” wires used in circular external fixators include intraoperative nerve and vessel damage, loss of tension, high rates of pin tract infection, and wire breakage. Aims: To investigate if insertion of “K” wire with a curvature is responsible, in part, for these complications. Settings and Design: This paper is a theoretical analysis of the forces and consequences of a tensioned curved wire on the bone, experimental measurement of the heat produced on inserting a curved compared to a straight (with a guide) wire into a bovine rib bone. Subjects and Methods: 1.8 mm K wires were drilled into two bovine rib bones using a battery-powered hand drill. The temperatures reached were recorded with a thermal camera. Statistical Analysis Used: Student's t-test. Results: (1) A tensioned curved wire would cut through the bone. (2) higher temperatures were reached when inserting a K wire in a curved manner than with a guide. (3) Drilling a curved wire initiates metal fatigue. It is theoretically possible to reach the forces needed to correct the curvature on the wire by “pinching” the wire near its insertion into the bone with the index and thumb of the non-drilling hand, but these forces far exceed the pressure pain threshold. In addition, the constantly changing force and direction would make the necessary proprioceptive changes almost impossible to follow. Conclusions: Theoretically and experimentally a curve on a K wire will increase vessel/nerve damage, thermal necrosis, pin tract infection, loosening, and loss of wire tension. Present-day method of “pinching” the end of the K wire to prevent curvature is probably inefficient and K wires should be inserted using a guide.