Martijn Wessels

Influence of haptic guidance in learning a novel visuomotor task

In (re)learning of movements, haptic guidance can be used to direct the needed adaptations in motor control. Haptic guidance influences the main driving factors of motor adaptation, execution error, and control effort in different ways. Human-control effort is dissipated in the interactions that occur during haptic guidance. Minimizing the control effort would reduce the interaction forces and result in adaptation. However, guidance also decreases the magnitude of the execution errors, which could inhibit motor adaptation. The aim of this study was to assess how different types of haptic guidance affect kinematic adaptation in a novel visuomotor task. Five groups of subjects adapted to a reaching task in which the visual representation of the hand was rotated 30 degrees. Each group was guided by a different force field. The force fields differed in magnitude and direction in order to discern the adaptation based on execution errors and control effort. The results demonstrated that the execution error did indeed play a key role in adaptation. The more the guiding forces restricted the occurrence of execution errors, the smaller the amount and rate of adaptation. However, the force field that enlarged the execution errors did not result in an increased rate of adaptation. The presence of a small amount of adaptation in the groups who did not experience execution errors during training suggested that adaptation could be driven on a much slower rate and on the basis of minimization of control effort as was evidenced by a gradual decrease of the interaction forces during training. Remarkably, also in the group in which the subjects were passive and completely guided, a small but significant adaptation occurred. The conclusion is that both minimization of execution errors and control effort drives kinematic adaptation in a novel visuomotor task, but the latter at a much slower rate.

A novel anchoring system for use in a nonfusion scoliosis correction device

BACKGROUND CONTEXT Insertion of a pedicle screw in the mid- and high thoracic regions has a serious risk of facet joint damage. Because flexible implant systems require intact facet joints, we developed an enhanced fixation that is less destructive to spinal structures. The XSFIX is a posterior fixation system that uses cables that are attached to the transverse processes of a vertebra. PURPOSE To determine whether a fixation to the transverse process using the XSFIX is strong enough to withstand the loads applied by the XSLATOR (a novel, highly flexible nonfusion implant system) and thus, whether it is a suitable alternative for pedicle screw fixation. STUDY DESIGN The strength of a novel fixation system using transverse process cables was determined and compared with the strength of a similar fixation using polyaxial pedicle screws on different vertebral levels. METHODS Each of the 58 vertebrae, isolated from four adult human cadavers, was instrumented with either a pedicle screw anchor (PSA) system or a prototype of the XSFIX. The PSA consisted of two polyaxial pedicle screws and a 5 mm diameter rod. The XSFIX prototype consisted of two bodies that were fixed to the transverse processes, interconnected with a similar rod. Each fixation system was subjected to a lateral or an axial torque. RESULTS The PSA demonstrated fixation strength in lateral loading and torsion higher than required for use in the XSLATOR. The XSFIX demonstrated high enough fixation strength (in both lateral loading and torsion), only in the high and midthoracic regions (T10-T12). CONCLUSIONS This experiment showed that the fixation strength of XSFIX is sufficient for use with the XSLATOR only in mid- and high thoracic regions. For the low thoracic and lumbar region, the PSA is a more rigid fixation. Because the performance of the new fixation system appears to be favorable in the high and midthoracic regions, a clinical study is the next challenge.

Influence of Prestrain, Heat Treatment and Surface Treatment on Bending Fatigue of NiTi Rods

NiTi alloys are extensively used in medical applications such as implants. In long-term implantation, fatigue life becomes crucial. Stressand temperature-induced phase transformations in make NiTi fatigue behaviour complex and unpredictable since it dependents on type of loading, dimensions, test temperature, environment and process parameters such as the amount of cold work and heat and surface treatment [10, Polinski 2006]. The fatigue behaviour in non-zero mean strain loading conditions that apply in for instance non-fusion correction of scoliosis is unknown. Goal of this study was to find this behaviour as function of different heat and surface treatments.

Development of a non-fusion scoliosis correction device : designing and testing

If for some reason spinal development during adolescence malfunctions, a deformity such as scoliosis may develop. Scoliosis is a deformity of the spine, characterised by a lateral curvature and an axial rotation. Treatment of scoliosis includes non-surgical and surgical interventions. Current surgical treatment of scoliosis is unsatisfactory because it embraces the permanent immobilisation of several vertebrae, which results in a fused spine that is unable to flex and to develop normally in immature patients. Therefore, an improved method is required. This dissertation addresses the development of a new implant system that delivers a revolutionary surgical solution to a problem classified as ‘adolescent idiopathic scoliosis’ (AIS). This non-fusion correction system (XS LATOR) comprises two (separate) implants. One implant (XS TOR) is a torsion-generating element that applies a torque on the spine to correct the axial rotation of the vertebrae. The other implant (XS LAT) is a lateral bending element that applies a bending moment to correct the lateral curvature. The implants can be used together in such a way that an optimal configuration for each patient can be implemented. The system will be implanted in the adolescent during the growth phase and removed when the spine is fully grown, which means that the implants must be functional for a period of ten years. Unlike current fusion systems, the system is highly flexible thus fusion of the vertebrae will be avoided. Consequently, the imposed corrective moment and torque on the spine are small but remain present all the time and thus generate a gradual shape change as observed in orthodontic braces. Axial motion is possible due to U-loops at both ends of the devices The U-loops can slide into plain bearings. With axial motion three aspects are covered: spinal growth, variations in spinal lengths of the patients and axial motion due to daily activities.