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Computer Aided Design Modeling for Evaluation of Safe Zones with Volar Plating of Distal Radius Fractures
Chaitanya Mudgal, MD; Eric Fu, MD; Lydia Helliwell, MD
Massachusetts General Hospital, Boston, MA
Introduction: Open reduction and internal fixation of distal radius fractures is being performed on up to 25% of patients with these fractures. Extensor tendon synovitis and attritional tendon ruptures are devastating complications which have been reported to occur as a result of prominent hardware in 3 to 23% of cases. The purpose of this paper is to establish normative values for safe screw lengths and angles for the variable angle screws used with standard volar distal radius plates using 3D CT models and a novel solid modeling software technique.
Materials and methods: 90 patients with CT scans at our institution for treaetment of carpal injuries from 2001 – 2009 with intact radii were identified. 10 patients (10 men) were selected at random for inclusion. Baseline demographic data for each patient were recorded. 3D models were created using a solid modeling software (3DSlicer¨, Boston, MA) (Figure 1). Variable angle volar distal radius locking plates from Synthes and Hand Innovations were scanned using a structured light scanner (M3DI SLS-SE¨, Middletown, CT). Models of the intact radii and volar distal radius plates were imported into solid modeling software (Rhinoceros¨, Robert McNeel& Associates, Seattle, WA). Virtual matching was performed. Variable angle cones were created about the central screw axes of the DVR distal locking screws (Figure 2). Intercepts to the dorsal cortex were recorded. The primary outcomes for this study were depth and length of safe screw. eFor all statistical tests, differences where p<0.05 (two-tailed) were considered significant.
Results: Average lengths are reported in Figure 3. There was no correlation between screw lengths and height, weight, or BMI. Neutral screws were generally safe: DVR had no neutral screw intra-articular and Synthes had 2 neutral screws penetrate the joint (2.9%). With variable angle screws, there was significantly more potential articular involvement with Synthes plates compared to DVR – 22.9% vs. 2.5% (p < 0.05).
Discussion: We found no screw length correlation with patient variables such as height, weight, or BMI. Neutral screw placement was usually safe in avoiding the articular surface. We recommend great care when placing variable angle screws especially with subchondral and ulnar screws. Methodology of virtual placement of orthopaedic hardware has potential for wide application in future studies. This method has the benefit of relying on existing imaging and thereby allowing for a large catchment of patient and no additional radiation or study costs.
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