Scanning and motion capturing of vertebral kinematics
Thesis (MScEng (Industrial Engineering))--Stellenbosch University, 2008.
In the context of intervertebral disc replacement and customized implants, human simulation studies are of great importance. Simulation models need input data. This study investigated different in vivo motion capturing methods to capture spinal kinematics that will serve as input for simulation models. Available scanning and motion capturing techniques for capturing cervical kinematics range from simple clinical methods, to expensive specialized equipment and software. With a variety of technologies comes a variety of applications. In this study the focus is on capturing the kinematics of the cervical spine. An important distinction was made between two types of motion capturing technologies: external motion capturing and internal imaging technologies. The available external motion capturing technologies pose many advantages in terms of cost, safety, simplicity, portability and producing accurate three dimensional position and orientation. However, the ability for external motion capturing technologies to give accurate information on the movements at each vertebral level is doubted by critics reasoning that the true vertebral motion is concealed by the skin and soft tissue. Although it would be ideal to use external motion capturing systems, one needs to be confident that these surface markers or sensors truly reflect the vertebral motion at each vertebral level. An empirical study was conducted to evaluate the relationship between motion captured on the skin surface and motion of the vertebrae. Twenty-one subjects received low dosage X-rays, while radio opaque markers were attached to the skin at each respective vertebral level. The motion of external markers and that of the vertebrae could be seen simultaneously on one medium. In the empirical study, two outputs were achieved. Firstly, intervertebral kinematic data, for use in further simulation studies was obtained. Secondly, the relationship between surface markers and vertebrae in different motion instances was investigated. Distance and angle parameters were constructed for vertebral prediction from skin surface markers. The causes of variation in these parameters were identified by investigating the correlations of these parameters with anthropometrical variables. Strong correlations of the parameters were observed in flexion, but in extension, especially full extension, the correlations were poor to insignificant. It was concluded that in neutral, half flexion and full flexion it is possible to predict the vertebral position from surface markers by using the parameters and anthropometrical variables. In half extension this prediction would be less accurate and in full extension alternative methods should be investigated for external motion capturing.