Fracture Gap Reduction with Variable Pitch Headless Screws
Austin J Roebke, B.S.; The Ohio State University College of Medicine, Columbus, OH; Logan J Roebke, B.S.; University of Dayton, Dayton, OH; Kanu S Goyal, MD; The Ohio State University, Columbus, OH
Introduction: Fully-threaded variable-pitch headless screws are used in many settings in surgery and have been extensively studied in this context, especially in regard to scaphoid fractures. However, it is not well understood how screw parameters such as diameter, length, and pitch variation, as well as technique parameters such as depth of drilling, affect fracture gap closure.
Material & Methods: Acutrak 2 fully-threaded variable-pitch headless screws of various diameters (Standard, Mini and Micro) and lengths (16-28 mm) were inserted into polyurethane blocks of normal and osteoporotic densities using a custom jig. Three drilling techniques (drill only through first block, 4 mm into second block, or completely through both blocks) were used. During screw insertion, fluoroscopic images were taken and later analyzed to measure fracture gap reduction. The effect of backing the screw out after compression was evaluated. ANOVA and post-hoc student's t-test were performed to evaluate statistical significance (p=0.05).
Results: Drilling at least 4 mm past the fracture site reduces distal fragment push off compared to drilling only through the proximal fragment (Figure 1 and 2). There were no statistically significant differences in fracture gap closure in normal versus osteoporotic bone. The Micro screw had a smaller fracture gap closure than both the Standard and Mini screws (Table 1). Longer screws can achieve a greater fracture gap reduction. The overall fracture gap reduction achieved correlated with the number of threads in the far fragment. After fragment contact and compression with two subsequent full forward turns, backing the screw out by only one full turn resulted in gapping between the fragment blocks (Figure 3).
Conclusions: Fully-threaded headless variable-pitch screws can only obtain compression between bone fragments if the initial bone gap is less than the fracture gap closed. Final closure may be affected by drilling technique, screw size, and screw length. Fragment compression may be immediately lost if the screw is reversed. In summary, we describe characteristics of variable pitch headless screws that may assist the surgeon in screw choice and method of use.
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