Defining the Morphometrics of the First Metacarpal for the Development of an Osseointegrated Prosthesis Authors: JJ Vaux OMS-IV1, RR Hugate, M.D.2, JW Hills3, RF Grzybowski, D.O.4, CK Funk. Ph.D.1 Affiliations: 1Rocky Vista University College of Osteopathic Medicine, 2Colorado Limb Consultants, 3Dept of Materials and Mechanical Engineering, Denver University, 4Diversified Radiology Objective: Amputation of the thumb presents a serious insult to the hand and can result in up to a 22% loss of functionality in that limb (2,3). To date, several different techniques have been explored for reconstruction of the thumb, however none seem to be incredibly successful (1,4). We believe the answer lies in an osseointegrated prosthesis within the first metacarpal. In order to successfully create an osseointegrated prosthesis, the morphometrics of the first metacarpal are needed. The aim of this study was to define the geometry of the first metacarpal in order to help create a standardized set of stems and prostheses to treat patients who have suffered amputation of the thumb at the level of the first metacarpal phalangeal joint (MCPJ). Methods: A total of eighty first metacarpals from forty-one cadavers were studied. All soft tissues were removed and the first metacarpals were imaged by computed tomography (CT). Three-dimensional models were constructed using cuts from the coronal, sagittal, and axial planes. Using a HyperMesh software, the individual first metacarpals were analyzed and measurements were taken for overall length, radius of curvature, medullary canal diameter, cortical thickness, and distance from the distal end to the center of the isthmus. Data was then analyzed for use in development of standardized stems to fit the contour of an average first metacarpal bone. Results: The average length of the first metacarpal was 47.60mm (± 3.32mm, 39.17–56.92mm). The average radius of curvature was 55.45mm (± 10.74mm, 32.96–78.86mm). Inner bone diameter, measured in two axes, was 10.48mm (± 1.25mm, 5.41–18.68mm) for the major axis and 7.72mm (± 0.89mm, 4.34–17.77mm) for the minor axis. The average cortical thickness was 1.40mm (± 0.32mm, 0.73–3.07mm). The distance to the center of the isthmus from the distal end had an average length of 21.31mm (± 1.93mm, 17–25mm). From the data that we obtained, there was no statistically significant difference between male and female or left and right metacarpals. Discussion: The loss of a thumb presents a significant impairment to the hand and diminished quality of life for the patient physically, vocationally, and possibly psychologically. In order to develop a standardized set of stems and design a percutaneous prosthesis for patients who have suffered amputation of the thumb at the level of the first MCPJ, the morphometrics of the first metacarpal must be better defined. Here, we have measured several essential dimensions in order to provide an accurate analysis of the size and shape of the first metacarpal. This data will be invaluable for future standardization of the first metacarpal geometry and development of a set of stems and prostheses. References: 1. Lundborg G. et al. Osseointegrated Thumb Prostheses: A Concept for Fixation of Digit Prosthetic Devices. J Hand Surg 1996;21A:216-221. 2. Dillingham T.R. et al. Limp Amputation and Limb Deficiency: Epidemiology and Recent Trends in the United States. South Med J. 2002 Aug;95(8):875-83. 3. Guides to the Evaluation of Permanent Impairment, Sixth Ed. American Medical Association, 2007. 4. Lin P.Y. et al. A systematic review of outcomes of toe-to-thumb transfers for isolated traumatic thumb amputation. Hand (NY) 2011 Sep;6(3):235-43. .