A simulator for the accuracy evaluation of the rotational hip centre using CT-less navigation in TKA concerning reduced range of motion and movements of the pelvis

摘要

Objectives: In total knee replacement (TKA), CT-less navigation systems compute the mechanical axis of the leg by pivoting in order to locate the centre of the hip joint. The hypothesis is that a reduction of the range of motion (ROM) and pelvic movements during the data acquisition influence the accuracy of CT-less navigation systems. The purpose of the study was to construct a mechanical model to assess a CT-less navigation system by simulating a certain reduction of the ROM of the hip joint, including defined pelvic movements. Design/Methods: We used the Navitrack~(TM) navigation system (Navitrack~(TM), Centerpulse AG, Zurich, Switzerland). This CT-less navigation system uses rigid bodies equipped with passive markers, fixed to the bone and the tools. The model was constructed concerning the anatomic situation, respecting landmarks and geometry of the hip joint and the femur. The mechanical model of the hip joint (inclination 45掳, ante version 15掳) with a rigid body fixed on the femur, is based on a solid ground plate and allows a ROM maximum 90掳 flexion, 80掳 abduction and 40掳 adduction. The length of the femur is 450mm with a CCD-angle of 120掳 and a transepicondylar line with 60mm distance. The model allows placing the tip of the tracked pointer exactly in the real rotational centre of the ball joint. Certain positions of the femur, organized in different groups of reduced ROM, can be adjusted reproducibly. The system demands 14 positions for registration. Simulations with stepwise reduced ROM of the hip joint have been assessed in a first trail computing the rotational centre of the model's ideal ball joint. This trial excluded pelvic movements. A second trail will examine the influence of pelvic movements. The movement of the pelvis is realized by changing the position of the ground plate with the model fixed on top. Poly-prismatic pedestals can be revolved and cause reproducible translational movements in three axes. The adjustment differs stepwise: minimum 5mm to a maximum extension of +- 15mm. The adjustable variation of the ground plate, simulating the movements of the pelvis achieves reproducible positions. With constant ROM it will be possible to evaluate the quality of the detection of the hip centre depending on pelvic movement. The real rotational centre can be scanned and visualized on the screen with the tip of the tracked pointer. This allows the comparison of the real hip centre with the virtual centre. The virtual centre is graphically presented by three concentric circles in a frontal, sagittal and axial sight. The following score is attached to the circles: tip of the pointer in the inner circle (score 1), score 2 for the middle circle and 3 in the outer circle. This valuation is done in every sight.