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American Association for Hand Surgery

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Metacarpal Morphology: Radial Curvature of the Metacarpals and Evaluation of Inner and Outer Diameter
Deana Mercer, MD1, Jennifer Weaver, MD2, Ann L Wells, PhD2, Mihra Taljanovic, MD, PhD2, Mahmoud Taha, PhD3, Natasha G Dark, BS2 and Gary Mlady, MD2, (1)Department of Orthopaedic Surgery, University of New Mexico, Albuquerque, NM, (2)University of New Mexico School of Medicine, Albuquerque, NM, (3)Civil Engineering, University of New Mexico, Albuquerque, NM

Introduction
The use of intramedullary implants for fixation of metacarpal shaft fractures has emerged as a promising surgical technique in increasing mobility and stability post-operatively (Hoang et al, 2021). The anatomical mechanism of the five metacarpal bones that form the palmar aspect demonstrates concavity on the palmar aspect (Kollitz et al, 2013). However, we hypothesize that the intermedullary canal of the metacarpal bone is straight, allowing for fixation with intramedullary implants without causing fracture malalignment or distraction. The aim of this study is to define the radius of curvature of metacarpals using measurements of both inner and outer diameter by analyzing CT scans. This study allows us to gain a better understanding of the osteology of the metacarpal bones that in turn contributes to better fracture fixation.
Materials & Methods
Two hundred CT scans from the Office of the Medical Investigator were analyzed. Subjects with significant upper extremity trauma were excluded, as were children. The scans were analyzed using Checkpoint software from Stratovan Corporation (2020). Using the software, a 3-dimensional (3D) model of the metacarpals was created. This model was then sliced along different axes to make measurements (Figure 1).
Fig. 1: 3D views of the five metacarpals for one subject.
Results
The metacarpals have a consistent curvature across the first through fifth metacarpals. The inner diameter through the isthmus is parallel. However, as the canal extends into the proximal and distal metaphyseal flare, the canal is curved. The volar cortex is concave with respect to the dorsal cortex.
Conclusions
When utilizing intramedullary devices for metacarpal fixation, if the fracture extends into the metaphyseal flare, the proximal and distal metacarpal may need additional fixation for rigid stabilization.


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