The goal was to evaluate tibiofemoral knee joint kinematics during stair descent, by simulating the full stair descent motion in vitro. The knee joint kinematics were evaluated for two types of knee implants: bi-cruciate retaining and bi-cruciate stabilized. It was hypothesized that the bi-cruciate retaining implant better approximates native kinematics. The in vitro study included 20 specimens which were tested during a full stair descent with physiological muscle forces in a dynamic knee rig. Laxity envelopes were measured by applying external loading conditions in varus/valgus and internal/external direction.Aims
Methods
Robot systems have been successfully introduced to improve the accuracy and reduce severe iatrogenic soft tissue damage in knee arthroplasty. Unfortunately to perform complete a complete bone cut, the cutting tool has to slightly pass the edge of the bone. In the posterior zones were retractor protection is impossible this will lead to contact between the cutting tool and the soft tissue envelope. Therefore, complete soft tissue preservation cannot be guaranteed with the current commercial systems. This study presents an alternative robotic controlled cutting technique to perform the bone resections during TKA by milling a slot with a long slender high-speed milling tool. The system is composed by a long milling tool driven by a high-speed motor and a protector covering the end of the cutter. The protector is rigidly connected to the motor by the support structure next to the mill, which moves behind the mill in the slot created by the cutter. The protector at the end of the cutter has four functions: providing mechanical support for the mill, preventing soft tissue to come into contact with the cutter, sensing the edge of the bone to accurately follow the shape of the bone and releasing the attached soft tissue. The edge of the bone is sensed by force feedback and with the help of a probing motion the adaptive algorithm enables the protector to follow the edge of the bone closely by compensating for small segmentation and registration errors. A pilot test to evaluate the concept was performed on three fresh frozen knees. The flatness of the resection, the iatrogenic soft tissue damage, the cutting time and the efficiency of the bone contour following algorithm was measured.Introduction
Methods
Biomechanical joint contact pressure distribution measurements have proven to be a very valuable tool in orthopaedic research to investigate the influence of surgical techniques such as total knee arthroplasty (TKA) on the human knee joint. Quantification of the in vitro tibiofemoral and patellofemoral contact pressure distribution before and after the intervention are an important measure to evaluate the impact of the surgery. The K scan pressure sensor from Tekscan (South Boston USA) is a commonly reported device for these in vitro pressure measurements. Despite the large interest in the sensor, the effective measurement accuracy for in vitro biomechanical joint contact measurement still remains a big question and therefore the reliability of these measurements should be questioned. Reliable contact pressure measurements can only be done if the sensor behaviour is fully understood. Therefore, a tailored multi-axial testing machine has been designed to profoundly investigate and characterise the sensor behaviour. This test setup is unique through its ability to apply a predefined tangential force or sliding velocity to the sensor's interface next to a normal force. Dynamic effects occurring in knee joint motion can thus be simulated while evaluating the effect on the contact pressure measurements.Background
Methods
In-vitro testing of knee joints remains vital in the understanding of knee surgery and arthroplasty. However, based on the design philosophy of the original Oxford knee simulator, this in-vitro testing has mainly focused on squatting motion. As the activities of daily living might drastically differ from this type of motion, both from a kinematic and kinetic point of view, a new knee simulator is required that allows studying more random motion patterns. This paper describes a novel knee simulator that overcomes the limitations of traditional Oxford simulators, providing both kinematic and kinetic freedom with respect to the applied boundary conditions. This novel test simulator keeps the hip at a fixed position, only providing a single rotational degree of freedom (DOF) in the sagittal plane. In addition, the ankle holds four DOF, including all rotational DOF and the translation along the medio-lateral axis. Combining these boundary conditions leaves five independent DOF to the knee; the knee flexion angle is actively controlled through the positioning of the ankle joint in the antero-posterior and proximal-distal direction. The specimens' quadriceps muscle is actively controlled, the medial and lateral hamstrings are passively loaded. To validate the performance of this simulator, two fresh frozen specimens have been tested during normal squatting and cycling. Their kinematic patterns have been compared to relevant literature data.Background
Methods
For the evaluation of new orthopaedic implants, cadaveric testing remains an attractive solution. However, prior to cadaveric testing, the performance of an implant can be evaluated using numerical simulations. These simulations can provide insight in the kinematics and contact forces associated with a specific implant design and/or positioning. Both a two and three dimensional simulation model have been created using the AnyBody Modelling System (AMS). In the two dimensional model, the knee joint is represented by a hinge. Similarly, the ankle and hip joint are represented by a hinge joint and a variable amplitude quadriceps force is applied to a rigid bar connected to the tibia (Figure 1a). In line with this simulation model, a hinge model was created that could be mounted in the UGent knee simulator to evaluate the performance of the simulated model. The hinge model thereby performs a cyclic motion under varying quadriceps load while recording the ankle reaction forces. In addition to the two dimensional model, a three dimensional model has been developed (Figure 1b). More specifically, a model is built of a sawbone leg holding a posterior stabilized single radius total knee implant. The physical sawbone model contains simplified medial and lateral collateral ligaments. In line with the boundary conditions of the UGent knee simulator, the simulated hip contains a single rotational degree of freedom and the ankle holds four degrees of freedom (three rotations, single translation). In the simulations, the knee is modelled using the force-dependent kinematics (FDK) method built in the AMS. This leaves the knee with six degrees of freedom that are controlled by the ligament tension in combination with the applied quadriceps load and shape of the implant. The physical sawbone model goes through five cycles in the UGent simulator using while recording the kinematics of the femur and tibia using a set of markers rigidly attached to the femur and tibia bone. The position of the implant with respect to the markers was evaluated by CT-scanning the sawbone model.Introduction
Methods
