Measurement system to determining the joint centered leg length during total hip replacement surgery

Introduction: Total hip replacement is one of the most-often performed surgeries in Germany, accounting for 214,000 interventions annually. The information about the leg length and the position of the hip rotation center is an important quality criterion to reconstruct healthy hip joint biomechanics. Leg length differences caused by the surgery can lead to functional restrictions and to the necessity for revision surgeries. The distance between the center of the femoral head and the foot standing surface (hip joint centered leg length) can only be estimated with a low accuracy (inaccuracy > 20,0mm) without using computed tomography, until now. Objectives: The aim of the study was to develop a measurement system to calculate objectively leg length as well as the position of the rotation center of the hip joint with an accuracy of less than 5 mm, allowing to position the hip implant with at a high accuracy. Methods: The measurement system included two LEDs being located in a plastic box as well as an inertial sensor for the measurement of angles. An external camera detects the light spots of the LED. The system is connected to the computer via a USB port. Furthermore, a Labview -based software was created to calculate the leg length and the hip rotation center. For future measurements in the OR, the system will be positioned with a nonslip seating on the upper side of the patient's tibia. During the surgery the leg is lifted hyper extended by the surgeon at the heel. On basis of the sensor movement the hip joint centered leg length and the hip rotation center is calculated. The first measurement hast to be performed before surgery, the second one intraoperatively after hip joint test implantation. If differences are recognized between the measured values, the hip implant can be readjusted with the help of a modular implant until the reconstruction of the original biomechanics. In order to evaluate the technical accuracy a technical system was developed to simulate the movement of the human leg in all directions. For that, an artificial leg was mounted with a ball joint on a metal plate (leg length=395,0mm). The technical accuracy was evaluated repeatedly with a sample number of 30. Results: A measurement system was developed to demonstrate the feasibility of measuring leg length and the position of the hip rotation center objectively. The result of the technical accuracy of the leg length measurements in the test setup was 393.6 ± 2.0 mm (setpoint value=395.0 mm), with minimum and maximum values of =396.9 mm and =389.9 mm, respectively. Conclusions: The technical accuracy of the system allows precise measurements of leg length and can therefore be a support in total hip arthroplasty, providing an objective tool for quality assurance. Until now the distance between the center of the femoral head and the center of the sensor case could already be calculated within a technical test setup. A follow-up evaluation of the clinical accuracy will be performed in clinical trials. The next step will be the development of a device for positioning the sensor case at a defined location at the patient's tibia in relation to the foot standing surface. Doing so would allow obtaining information about the total distance between the hip joint head center and the foot standing surface. Additionally, a second camera will have to be included to calculate the three-dimensional coordinates of the hip rotation center.