To simulate a dynamic weight-bearing knee motion, an upright knee simulator was used. This model has previously been described in detail [16
]. The knee simulator consists of a vertical frame with a linear actuator (termed main actuator: linear electrical servo motor; Parker Hannifin, Offenburg, Germany), five smaller linear actuators (termed muscle actuators; Parker Hannifin, Offenburg, Germany) that generate the forces and motions of the five simulated muscles (Rectus femoris, vastus lateralis, vastus medialis, semitendinosus, and biceps femoris), a three degree-of-freedom (flexion-extension, adduction-abduction and vertical translation guided by a ball rail system) hip joint assembly, and a three degree-of-freedom (Internal-External rotation, flexion-extension, and adduction-abduction) ankle joint assembly (figure ). With the designed degrees of freedom in the hip and ankle joints, the knee simulator allows unconstrained tibiofemoral movements in all six degrees of freedom [17
]. Each of the six actuators can be controlled independently. The movement of the hip joint assembly is guided by a vertically-aligned ball rail bearing. In both of the hip and ankle assemblies, uni-axial (vertical) load sensors (Velomat, Kamenz, Germany) are mounted to measure the vertical reaction forces in the corresponding bearings (ankle and hip forces, respectively). Muscle forces were simulated by linear electrical servo motors via steel cables attached to the tendon of each muscle, and their forces were measured by uni-axial load cells.
Figure 1 Picture of the knee simulator. The system is able to simulate a continuous weight-bearing knee flexion while moving the vertical frame and adjusting the muscle loads according to a predefined constant force measured by an uni-axial load cell in the ankle (more ...)
Twelve fresh-frozen human cadaveric knee specimens with an age at the time of death of 75 ± 13 years (mean ± standard deviation) were studied. The femur and tibia were cut 15 cm from the joint line, and, while keeping the joint capsule and the collateral ligaments intact and the five aforementioned muscle tendons exposed, all other skin and soft tissues were removed. The fibula was secured to the tibia with cortical screws to prevent its motion during the test. Each of the femur and tibia was mounted onto a thick-walled steel cylinder using a bone cement compound (PMMA: Technovit 2060, Heraeus Kulzer, Hanau, Germany) and multiple accurately-positioned set screws. To achieve a secure grip of the tendon, a custom-manufactured metal tendon clamp was used to connect the tendon to the muscle actuator with a cable whose line of action is parallel to the femoral shaft.
To generate the weight-bearing knee flexions, the main actuator produced a continuous, descending motion of the hip assembly from 15 to 90 degrees of knee flexion with a constant rate of 1 deg/s. During this movement, the control system dynamically adjusted the muscle cable tension by varying their lengths so as to maintain a constant resulting ground reaction force on the ankle joint. The reaction force on the ankle joint, which holds equilibrium with the applied muscle forces quasi-statically, was assumed to characterize the amount of body weight. In order to prevent the tendon ruptures caused by excessive muscle forces [16
], we selected a conservative ankle force of 50 Newton to simulate a portion of the body weight, which required the quadriceps-actuators to pull with a linearly rising force and a maximum of approximately 600N at 90 degrees of flexion. During knee flexion, the three quadriceps forces were always maintained identical to one another, while the hamstrings forces were kept constant at 10 N. A constant hamstring force was used to simplify the control algorithm and to reveal the effects of other factors of research interests. This knee flexion with a 50 N simulated partial body weight was repeated twice for each of the four different parameters.
To study the kinematics of the joint, the movements of the femur and the tibia were measured with a marker-based ultrasonic measuring system for 3D motion analysis (ZEBRIS®
CMS100, Isny, Germany) at a sampling rate of 1 Hz and spatial resolution < 1 mm [18
]. A triad of ultrasound markers was attached at each of the femur and tibia fixation cylinders. To define the two body-fixed coordinate systems for the femur and tibia, we first recorded the positions of the medial and lateral prominences of the tibia plateau as reference points using a ZEBRIS®
stylus pointer when the knee was fully extended. Both of tibial and femoral coordinate systems were assumed to be identical at full extension, and their common origins at full extension were defined as the midpoint of the two digitized reference points. For each of the tibial and femoral coordinate systems, the flexion axis (z-axis) was defined along the line between the two reference points. The y-axis was defined as a vector normal to a plane constructed by the z-axis and the longitudinal axis of the respective segment shafts, which were recorded by the ultrasound sensor triads at the corresponding segments. The x-axis was then defined by the cross product of the y and z axes (figure ). During the flexion/extension of the knee, the segment fixed coordinate systems at each instant were calculated according to the positions of tibial and femoral ZEBRIS®
marker triads. The tibial translation with respect to the femur was defined as the position difference between the centers of the two moving coordinates, and the relative orientation of the tibia with respect to the femur was calculated in terms of Euler angles (rotation sequence: flexion-extension, abduction-adduction, internal-external rotation).
To determine the effect of different KA-systems on the kinematics, the aforementioned protocol was performed on the intact knee specimen (termed "Intact" case). The same specimen was retested after a bicruciate-retaining BKA with intact and resected ACL (termed "BKA+"/"BKA-" case: JOURNEY DEUCE, Smith&Nephew, Memphis, Tennessee) and after ACL-sacrificing (PCL-retaining) TKA (termed "TKA" case: GENESIS II, Smith&Nephew, Memphis, Tennessee). The surgeries were performed by an experienced orthopaedic surgeon with the same surgical approach and instruments used for routine clinical care. In the BKA, a longitudinal incision was made over anterior aspect of the knee along the medial border of the patella. The bicruciate retaining prosthesis (figure ) was then implanted using press-fit fixation. The femoral and tibial components were installed using standard intra- and extra-medullary alignment guides. Appropriate soft tissue balancing was performed in flexion and extension to achieve valgus-varus stability before closing the incision with sutures. After resecting the ACL through the partly reopened incision of the operated knee, a second KA with an ACL-sacrificing design was performed in the same knee specimen. The BKA was removed; tibia and femur were properly resurfaced and the ACL-sacrificing prosthesis was then installed. Soft tissue balancing was performed in flexion and extension to ensure valgus-varus stability before closing the incision.
Figure 2 Intraoperative situs after implanting the BKA. Note the resurfaced medial and retropatellar compartment and the intact lateral compartment, patella as well as the cruciate ligaments. The little hole in the lateral condyle origins from a referencing-pin (more ...)
To evaluate the effect of the KA design on the kinematics, at each of the tested knee flexion angles, we conducted a one-way repeated-measure analysis of variance (ANOVA) with knee states as the independent factors. A post hoc test using Tukey-Kramer method was also conducted to investigate the individual effect. A p-value less than 0.05 was considered to be statistically significant. The statistical analysis was conducted with a statistics computer software, SAS® (SAS Institute Inc., Cary, NC).
The research presented in this work conforms to the Helsinki Declaration and to local legislation. It has been approved by the ethical committee of the medical faculty of the University of Tübingen.