Studying the effects of shape variation in a parametrically designed lunate implant on wrist kinematics

Abstract

The wrist is one of the most complex joints in the human body, comprised of multiple bones and ligaments, and capable of performing a variety of movements. Proper wrist motion relies heavily on the interplay between the carpal bones of the hand and its ligamentous connections, and when that interplay is compromised hand function can be severely affected. Kienböck disease affects one of said carpal bones, causing the avascular necrosis of the lunate bone, located on the proximal carpal row (PCR). This disease ultimately leads to severe pain, carpal collapse of the lunate, and with it, loss of wrist function and quality of life. However, current treatment options do not meet the desired outcomes, lunate arthroplasty looking like a promising alternative.

This thesis presents the development of a 3D-printable parametric lunate implant and the study of the effects of shape variations on wrist kinematics, using a handheld motion guide device prototype and a CT Scanner. Kienböck disease is firstly introduced and the problems it causes to the lunate bone and surrounding bones are investigated, leading to the definition of the goals of this thesis. The development of a parametric lunate implant is then described, as well as the study of wrist kinematics, finishing with a discussion of the results. A statistical shape model (SSM) approach is used in the implant design, with a purpose to parameterize the design method and have the ability to apply it to various lunate shapes.

After an introduction, the implant requirements were firstly stated and the design approach was described. The parametrization of the implant design is then described, consisting of multiple Matlab codes and Solidworks macros to automatize the process. The resulting lunate implant design can be applied on any lunate implant shape variation described by the SSM. The necessary updates to the handheld motion guide device prototype are then specified, followed by a description of the experiment, looking into the embalmed human specimens used, the manufacturing of the lunate implants, and selection of shape variations to test. The experimental protocol was also clarified and the method used to post-process the experimental results. In the experiment, 3 modes of variation were tested and the effect of shape variations on wrist kinematics were assessed by measuring the scapholunate, radiolunate, and capitolunate angles.

The accuracy of the resulting lunate implant was tested on the SSM mean shape, deviations ranging between 0.00 and 0.13 millimeters. When it comes to the handheld motion guide device, the updates proved to improve the overall functioning of the device, albeit some issues were still encountered. The experiment showed one of the modes of lunate shape variation to have a significantly significant effect on wrist kinematics, t-tests for this mode showing p-values lower than 0.05. That could not be confirmed for the remaining modes tested.

To sum up, this thesis described the design of a lunate implant, showing that a statistical shape modeling approach is feasible in designing a parametric lunate implant. It also showed that one of the lunate shape variations have an effect on wrist kinematics. The implant design method shows promise for implementation in future designs and the knowledge gained from the experiment could be valuable to help us better understand the kinematics of the wrist.