Registration-Based Morphometry for Shape Analysis of the Bones of the Human Wrist

We present a method that quantifies point-wise changes in surface morphology of the bones of the human wrist. The proposed method, referred to as Registration-based Bone Morphometry (RBM), consists of two steps: an atlas selection step and an atlas warping step. The atlas for individual wrist bones was selected based on the shortest l² distance to the ensemble of wrist bones from a database of a healthy population of subjects. The selected atlas was then warped to the corresponding bones of individuals in the population using a non-linear registration method based on regularized l² distance minimization. The displacement field thus calculated showed local differences in bone shape that then were used for the analysis of group differences. Our results indicate that RBM has potential to provide a standardized approach to shape analysis of bones of the human wrist. We demonstrate the performance of RBM for examining group differences in wrist bone shapes based on sex and between those of the right and left wrists in healthy individuals. We also present data to show the application of RBM for tracking bone erosion status in rheumatoid arthritis.

I. Introduction

The human wrist consists of eight carpal bones, the distal aspects of the bones of the forearm (radius and ulna), and the proximal aspects of the five metacarpal bones. The skeletal maturation process involves changes in wrist bone morphometry [1], [2], and may influence bone fragility in adulthood [3]. The shape of specific carpal bones, such as those of the trapeziometacarpal joint of the thumb (the trapezium and the first metacarpal) and the scapholunate joint (scaphoid and lunate), may lead to altered biomechanics, and predispose to debilitating conditions such as wrist instability, or osteoarthritis of those joints [4]–[7]. There is therefore medical rationale to better understand the implications of bone shape and its variation in disease. Further, there are numerous factors such as age [8], [9], sex [10], [11], metabolic status [12], and genetic and environmental factors [13], [14] that may influence bone shape and there is interest in documenting the influence of these factors individually and collectively. Information regarding bone shape is important in surgical repair [15], as well as in the design and choice of implants or prosthetics [16], [17]. In Rheumatoid Arthritis (RA), carpal bone erosion is a common pathology and erosive changes must be tracked over time to monitor disease progression [18], [19] or to assess response to therapy [20]. In paleoanthropology, carpal bone morphology is considered an excellent indicator of phylogenetic relationships among mammals, and there is interest in being able to quantify changes in bone shapes due to evolution [21]. In summary there is demand for developing standardized quantitative methods that have a high sensitivity to changes in bone shape and enable the aforementioned analyses in a reproducible manner. The development of such a method is the goal of this work.