The goal of this thesis is: ''To create a sterilizable instrument that measures the compressive force in the axial direction of the neck of the hip implant during surgery''. This measured axial force in combination with a standardized ROM test could be used to objectively assess the soft tissue tension during surgery. It is expected that this instrument will be a helpful tool for inexperienced surgeons, because literature shows that errors made by the surgeon are the biggest cause of early failures and inexperienced surgeons registered twice as much dislocations as experienced surgeons. Therefore, the instrument could improve the success rate and quality of total hip arthroplasty by reducing the number of early failures caused by dislocation resulting from the inadequate soft tissue tension created by inexperienced surgeons. Furthermore, it could improve the patients' quality of living by reducing their limping and pain after the surgery.
The current techniques to obtain the optimal length and angle of the implant are: preoperative radiography, intraoperative leg length assessment and subjective soft tissue tension assessment methods. Factors that influence soft tissue tension, apart from the implant, are either caused by the patient (inter- or intra-personal), the surgeon or the environment.
Although a 3 DOF sensory system integrated in the test stem is preferable, it is likely that measuring only the axial force in a so called 'the neck' of the prosthesis, in combination with a standardized range of motion (ROM) test, provides valuable inside information about the relation between variation in force and success of the procedure. This assessment is not based measuring absolute forces, but on pattern recognition while moving the hip through its range of motion. The validity of this hypothesis should be investigated in future research.
Different sensors have been tested to see if they survive the standard sterilization program of an autoclave, which uses hot steam at 134 degree Celsius, at a pressure of 2 bar for 20 minutes. The linear Hall sensor was chosen based on this test and a list of pros and cons. After that, a design was made in SolidWorks. Then it was manufactured and calibrated. The final prototype can be implanted in a few seconds when the stem is in place, by sliding it onto the stem and plugging it into a monitor. The device is intuitive to use, because the monitor produces real time graphs of the axial force that are easy to understand. Immediate adjustments to the soft tissue tension can be made by switching the head of the implant, while keeping the force sensing neck in place, in order to provide the greatest chance for surgical success. Furthermore, the device is highly reusable, because it can be sterilized with the standard autoclave procedure. Finally, the prototype was tested in a cadaver at the Erasmus hospital in Rotterdam. The axial hip force was measured at rest, when pulling the leg and when moving the leg through its range of motion. Although there were inaccuracies due to the elastic hysteresis of the rubber spring and friction of between axis and cylinder, consistent patterns in the measured axial hip force were observed. Recommendations are proposed in chapter 5 to reduce the errors caused by elastic hysteresis, friction and ROM tracking.
Based on what was learned from the cadaver test and from literature, a standardized test proposal with hypothesis was made. This test should serve as a guideline to get consistent and useful results in future research. This future research should investigate if the axial force measured during these ROM tests provides enough information to objectively assess the soft tissue tension, or if more information is required.