Pullout strength of connected pins in hand surgery: Effect of pin diameter and insertion angle.

Objective: The purpose of this study was to examine the effect of insertion angles on the pullout strength of connected pins using a synthetic model simulating a hand bone.

Material And Methods: The material consisted of Sawbones® (20 mm×20 mm × 60 mm), fixation pins secured to a connecting rod much like an external fixator, an electric drill (speed 1,290 rpm) and a tensile testing machine. The Sawbones® were drilled with different pin diameters (1.2 mm, 1.5 mm, and 1.8 mm) and insertion angles (100°, 110° and 120°). A vertical displacement of 1 mm/min was applied until the pins were extracted (maximum force).

Results: The pullout strength increased with the insertion angle of the connected pins. It also increased with their diameter. Regardless of the pin diameter, the load-displacement curve during the pullout test had 4 sections (peak 1, ascending slope, peak 2, descending slope) that corresponded to the combined frictional force and contact force between the pins and Sawbones®.

Discussion: Our study findings showed that, theoretically, for wrist or hand fractures treated with connected pins, the larger the diameter and insertion angle, the better the mechanical holding power of the pins.

Level Of Evidence: I, experimental study.