These planes were utilized to generate coronal and sagittal projections (RaySum) of each metacarpal individually based on the geometric axis of each bone. Using a digital 3D image reconstruction program (TeraRecon, Foster City, CA), the raw data point files from the CT scans were reconstructed in three planes . Doubling the required sample size allowed for the discrimination of nearly 3° of difference with statistical significance. To achieve this power, 15 scans were required. We aimed to power our study to discriminate 5° of change (paralleling the error range in measurement of conventional radiographs). Thirty-five scans of the small metacarpal and 30 scans of the ring metacarpal were included in this study. Scans which were deemed inadequate based on these properties, or those in which there were any acute or chronic pathologic changes involving the small and ring metacarpals were excluded from study analysis. De-identified raw images from each scan were evaluated for adequate resolution, proper sequences, complete visualization of the metacarpals and no obvious prior injury, severe arthritic changes, or other anomalies affecting the fourth and fifth metacarpals. We used a novel method of image manipulation of computer tomography (CT) images to eliminate difficulties inherent in measuring the anatomy in the lateral plane using radiograph, such as the overlap phenomenon.Īfter the institutional review board approval of the project, 50 consecutive hand CT scans were identified retrospectively using a radiographic database. The purpose of this study is to describe the precise anatomy of normal small and ring metacarpals to serve as reference. Due to bony overlap in the lateral radiographic projection, our understanding of the metacarpal anatomy in this plane is significantly limited. There is a paucity of data detailing the normal radiographic anatomy, with only one report using plain radiographs to measure morphometric parameters. As such, a good understanding of the normal anatomy is important in repairing or reconstructing metacarpal fractures. Although some degree of deformity can be tolerated with little functional consequence, the goal of surgical treatment of metacarpal fractures is restoration of normal anatomy. Accurate measurement of deformity is important in assessing the extent of injury and determining an appropriate course of treatment. Recent studies underscore the difficulty in reliably measuring the magnitude of displacement and angulation. There is no consensus regarding a standard method of radiographic measurement of these fractures. Indications for surgical treatment are based on the degree of fracture angulation and rotational alignment. In addition, these fracture types account for 18 % of all hand injuries. These injuries are common, with an incidence of 13.6 per 100,000 person-years for acute hospital care in the USA. Metacarpal neck fractures of the small and ring fingers typically occur as a result of a direct trauma. This novel method of image reconstruction eliminates metacarpal overlap and defines precise anatomical reference for metacarpals. Normal anatomic parameters of metacarpals are based primarily on radiographic data, and as such are limited due to bony overlap in the lateral plane, as well as imperfect radiographic projections that are known to distort anatomical relationships. We also demonstrated that much of the metacarpal apex dorsal bend is in the shaft itself. In contrast to prior reported values, we found the CAA to be less acute and the metacarpal curvature less pronounced on the lateral projection. On the PA images, the shafts are nearly straight. Apex dorsal SBA averaged 12 and 10° in the ring and small metacarpals, respectively. Our results show that CAA averaged 14 and 12° in the ring and small metacarpals, respectively. Using a customized image measurement program, shaft lengths, shaft-bending angle (SBA), and capital-axis angle (CAA) were measured. The 3D images were converted to sagittal and coronal weighted projections to represent lateral and posteroanterior (PA) 2D images that are equivalent to “perfect orthogonal” radiographs. Using a custom digital 3D image reformatting software, CT sections were reconstructed in the plane of each studied metacarpal. Thirty-five scans of the small and 30 scans of the ring metacarpals form the basis for this study. We hypothesize that current plain radiographic data incorrectly describes normal metacarpal anatomy in the lateral plane. Our study aims to describe normal anatomic measurements of small and ring metacarpals using a novel digital reconstruction technique based on raw CT image data. To date, only plain radiographic definitions of normal anatomical parameters have been described.
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