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3D - Computed Tomography Analyses of Intramedullary Headless Screw Fixation of Metacarpal Neck Fractures
Paul Willem Louis Ten Berg, MSc1; Chaitanya S. Mudgal, MD2; Matthew Leibman, MD3; Mark Belsky, MD4; David Ruchelsman, MD4; (1)Academic Medical Centre (AMC), (2)Mass General Hospital / Harvard Medical School, (3)Newton-Wellesley Hospital/Tufts University School of Medicine, (4)Newton Wellesley Hospital
Hand and Upper Extremity Surgery Service, MGH, Boston, MA, USA
Fixation countersunk beneath the articular surface is well-accepted for peri-articular fractures.Limited open intramedullary headless screw fixation offers advantages over Kirschner wire and open techniques. Quantitative 3-dimensional computed tomography (3D-CT) was used to test the hypothesis that articular surface area (SA) and subchondral volume (SCV) utilized with intramedullary headless compression screw (HCS) fixation of metacarpal neck fractures is acceptable and that the countersunk entry site (ES) does not engage the articular surface of the proximal phalanx during a clinically relevant arc of motion.
Retrograde insertion of 2.4mm, 3.0mm HCS and 1.1mm Kirschner wires through the ring/small finger metacarpal heads was simulated in 3D models from 16 adults. Quantitative 3D-CT analyses were performed to calculatemetacarpal head articular SA(mm2)and SCV(mm3)utilized with this technique.The coronal and sagittal plane arcs during which the center and rim of the articular base of the proximal phalanx engage the ES of the countersunk screw were calculated.
•Mean magnitude of metacarpal head SA mated to the proximal phalangeal base varies by arc of motion modeled: 1.neutral position SA=93 mm2; 2.maximal coronal arc (45°), SA=129 mm2; 3.maximal sagittal arc (120°), SA=265 mm2.
•The countersunk portion of the 2.4mm screw occupies 10%, 7.5% and 3.6% of the articular SA in each of these arcs, respectively. For the 3.0mm screw, these values are 13%, 9.4% and 4.5%, respectively. In contrast, the 1.1mm Kirschner wire occupies 1.2%, 0.89%, and 0.43%, respectively.
•Mean volume of the metacarpal head of both the ring and small finger is 927 mm3. The countersunk trailing portion of the 2.4mm and 3.0mm screws occupy 4.0% and 4.8% of head volume, respectively.
•The rim and the center of the phalangeal base does not overlap the countersunk entry site (ES) in 64% and 87% of the 120°sagittal plane arc of motion modeled, respectively.
•During coronal plane motionin neutral metacarpalphalangeal extension, the rim of the phalangeal base overlies the countersunk ES during the entire 45°arc. However, the center of the base never engages the dorsally located countersunk ES
Discussion and Conclusion:
•Metacarpal head SCV occupied is minimal.
• Articular SA violation is least during the more clinically relevant sagittal plane arc.
•The dorsal ES is in-line with the medullary canal and avoids engagement of the center of the articular base through most of the sagittal plane arc.
•This data supports use of an articular ES for metacarpal neck/subcapittal fractures.
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