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.

K-wire pullout strength in hand surgery: Impact of diameter, threading length and drilling speed.

INTRODUCTION: The aim of the present study was to assess the impact, combined and in interaction, of diameter, threading length and drilling speed on K-wire pullout strength in a synthetic model of a hand bone. MATERIAL AND METHODS: The material comprised Sawbones® (20 ×20×50mm), K-wires (diameter 1.2mm, 1.5mm, 1.8mm; threading 0mm, 5mm, 10mm, 15mm), a universal chuck with T handle and a drill (speed 0, 320, 500, 830, 1,290rpm), and tensile testing machine and a digital decision aid. The Sawbones® were drilled, varying diameter, threading and speed. The Statistical Design of Experiments (SDOE) methodology enabled the number of trials to be reduced from 300 to 70. Tensile tests at 1mm/s was imposed on the K-wire up to pullout (pullout strength). RESULTS: There was no interaction between threading length and diameter effects or between drilling speed and diameter effects, but a strong interaction between drilling speed and threading length effects. CONCLUSION: Before using K-wires for internal fixation in wrist or hand fracture, the surgeon has to select their characteristics, optimal holding power being theoretically ensured by large diameter wires with long threading inserted by a high-speed drill. LEVEL OF EVIDENCE: I, experimental study.