"Rescue" Screws for Stripped 3.5-mm Screw Holes: A Biomechanical Analysis of Pullout Strength and Maximum Insertional Torque of 4.0-mm and 4.5-mm Cortical Screws.

Abstract

This study aimed to determine the degree of maximum insertional torque (MIT) and pullout strength (POS) restored when 4.0 or 4.5-mm cortical screws were used as rescue screws through a stripped 3.5-mm screw hole in both normal and osteoporotic bone models. Self-tapping stainless steel screws were tested in 2 synthetic bone models simulating normal and osteoporotic bone. Screw specifications were as follows: 3.5-mm (major/minor diameter 3.5/2.4-mm, 1.25-mm pitch), 4.0-mm (4.0/2.9-mm, 1.25-mm pitch), and 4.5-mm (4.5/3.0-mm, 1.75-mm pitch). Baseline MIT and POS were measured using 3.5-mm screws inserted into native 2.5-mm pilot holes. Stripped holes were created by over-tightening 3.5-mm screws beyond MIT, defined as free screw rotation under constant torque. Rescue screws (4.0 and 4.5 mm) were then inserted into stripped holes. MIT and POS were measured using a torque screwdriver and an Instron 3369 system at 5 mm/min. Restoration of MIT and POS was expressed as a percentage of the baseline 3.5-mm values. Statistical significance was assessed using t tests. In the synthetic normal density bone model, 4.0-mm screws restored 37% of MIT and 62% of POS, while 4.5-mm screws restored 107% of MIT and 98% of POS (P = 0.33, P = 0.34), indicating similarity to native fixation. In the synthetic osteoporotic bone model, 4.0-mm screws restored 79% of MIT and 75% of POS, while 4.5-mm screws restored 229% of MIT and 120% of POS (P < 0.001), showing significant enhancement. The 4.5-mm rescue screws fully restored MIT and POS in synthetic normal and osteoporotic bone models, while 4.0-mm rescue screws provided partial restoration, with better performance in synthetic osteoporotic bone. Therapeutic Level V. See Instructions for Authors for a complete description of levels of evidence.