In vivo kinematic behavior of the radio-capitate joint during wrist flexion-extension and radio-ulnar deviation.

The capitate is often considered the "keystone" of the carpus, not simply because of its central and prominent position in the wrist, but also because of its mechanical interactions with neighboring bones. The purpose of this study was to determine in vivo three-dimensional capitate kinematics. Twenty uninjured wrists were investigated using a recently developed, non-invasive markerless bone registration (MBR) technique. Surface contours of the capitate, third metacarpal and radius were extracted from computed tomography images of seven wrist positions and the three-dimensional motions of the capitate and third metacarpal were calculated with respect to the radius in wrist flexion-extension and radio-ulnar deviation. We found that in vivo capitate motion does not simply occur about a single pivot point like a universal joint, as demonstrated by non-intersecting rotation axes for different capitate motions. The distance between flexion and ulnar deviation axes was 3.9+/-2.0 mm, and the distance between extension and ulnar deviation axes was 3.9+/-1.4 mm. Furthermore, capitate axes for males tended to be located more distally than axes for females. However, we believe that this result is related to subject size and not to gender. We also found that there is minimal relative motion between the capitate and third metacarpal during these in vivo wrist motions. These findings demonstrate the complexity of capitate kinematics, as well as the different mechanisms through which wrist flexion, extension, radial deviation and ulnar deviation occur.     

Fitting manifold surfaces to three-dimensional point clouds

We present a technique for fitting a smooth, locally parameterized surface model (called the manifold surface model) to unevenly scattered data describing an anatomical structure. These data are acquired from medical imaging modalities such as CT scans or MRI. The manifold surface is useful for problems which require analyzable or parametric surfaces fitted to data acquired from surfaces of arbitrary topology (e.g., entire bones). This surface modeling work is part of a larger project to model and analyze skeletal joints, in particular the complex of small bones within the wrist and hand. To demonstrate the suitability of this model we fit to several different bones in the hand, and to the same bone from multiple people.     

Strain-rate sensitivity of the rabbit MCL diminishes at traumatic loading rates

This study was performed to determine whether the viscoelastic behavior of ligaments persists at high rates of loading, such as those associated with sports-related trauma or motor vehicle accidents. Medial collateral ligaments (MCLs) from 22 skeletally mature New Zealand White rabbits were tensile tested quasi-statically and via two impact conditions at displacement rates of 0.17 mm/s (n=22), 640+/-160 mm/s (n=10) and 2500+/-270 mm/s (n=12) (corresponding to strain rates of approximately 1.0%/s, 3660%/s and 14,000%/s, respectively). Despite dramatic increases in displacement rate, only a modest strain-rate effect was observed when the specimens tested quasi-statically were compared to those tested via impact (24% and 37% increases in stiffness and failure load, respectively). There were no differences in the structural (e.g. 145+/-30 and 136+/-29 N/mm stiffness values, respectively) or failure properties (e.g. 434+/-91 and 443+/-154 N failure load values, respectively) of the two impact-tested groups. Our findings suggest that the rabbit MCL is not viscoelastic at loading rates approximating those associated with high-energy trauma.