Several studies have reported that tibial component in varus alignment can worsen the survivorship of medial unicompartmental knee arthroplasty (UKA). On the other hand, Varus/valgus inclination of the tibial component can affect the location of the contact point between femoral and tibial component especially in round on flat bearing surface design. Along with the tibial component inclination, changes in the contact point may also alter the tibial condylar bone stress, which would affect the longevity or complications after UKA. We constructed a validated three-dimensional finite element model of the tibia with a medial component and assessed stress concentrations by changing the tibial component coronal inclinations (squale inclination, 3° and 6° varus, 3° and 6° valgus inclination). We evaluated the Von Mises stress on the medial tibial metaphyseal cortex and the proximal resected surface when a load of 900N was applied on the tibial component surface by two conditions in each inclination models; one is that the loading site is fixed at the mediolateral center of the tibial component (fixed model), and the other is that the loading site is variable depending on the tibial component inclination (variable model) (Fig.1).Background
Method
Restoration of knee function after total knee arthroplasty (TKA) often entails a balance between normal kinematics and normal knee stability, especially in performing demanding physical activities. The ultra-congruent (UC) knee design prioritizes stability over kinematics through close conformity between the femoral component and the tibial insert in extension. This configuration is intended to provide AP stability in the absence of the posterior cruciate ligament during activities that would otherwise cause anterior femoral subluxation. In this study we examine the kinematics of an ultra-congruent knee design in comparison with the intact knee and with conventional articulations used in PCL-retaining (CR) and PCL-substituting (PS) TKR designs. The 3D tibio-femoral kinematics of 6 fresh frozen cadaveric human knees were tested during loaded simulation of squatting in a computer-controlled knee testing rig. Muscle forces were simulated by loading rectus femoris and vastus intermedius (150N), vastus lateralis (100N), vastus medialis (75N), and the hamstring muscles (60N) (total: 385N). Testing was performed on the intact knee, and after implanting a standard design of total knee prosthesis with the posterior cruciate ligament intact (CR-TKA), resected (PCL-substituting insert; PS-TKA), and a UC insert (UC-TKA group). The 3D positions of the tibia and femur were tracked with a high resolution 12 camera motion analysis system (Motion Analysis Inc.) and used to position 3D CT reconstructions of each bone. The translation and rotation of the femur with respect to the tibia were calculated by projecting the femoral transcondylar axis onto a plane normal to the longitudinal anatomical axis of the tibia coincident with the transverse axis of the tibial plateau.Introduction
Materials and Methods
Tibia vara seen in Japanese patients reportedly influences the tibial component alignment when performing TKA. However, it is unclear whether tibia vara affects the component position and size selection. We therefore determined (1) the amount of medial tibial bow, (2) whether the tibia vara influences the aspect ratio of the tibial resected surface in aligning the tibial component with the tibial shaft axis (TSA), and (3) whether currently available tibial components fit the shapes of resected proximal tibias in terms of aspect ratio. The study was performed using CT data from 90 lower limbs in 74 Japanese female patients with primary varus knee OA, scheduled for primary TKAs between January 2010 and March 2012. We measured the tibia vara angle (TVA; the angle between the TSA and the tibial mechanical axis), proximal varus angle (PVA; angle between the TSA and the line connecting the center of the tibial eminence and the center of the proximal 1/3 of the tibia) using three-dimensional preoperative planning software [Fig.1]. Then the mediolateral and middle AP dimensions of the resected surface when the tibial component was set so that its center aligned with the TSA was measured. We determined the correlations of the aspect ratio (the ML dimension divided by the AP dimension) of the resected surface with TVA or PVA and compared the aspect ratios to those of five prosthesis designs.Objective
Material and Methods
The alignment of the knee following total knee arthroplasty (TKA), especially tibial alignment, is a major factor determining the long-term survival of the prosthesis. A disadvantage of using extramedullary alignment guides of the tibia for TKA is the difficulty in correctly identifying the ankle center, and surgeons cannot construct the tibial mechanical axis correctly without the correct location of the ankle center. Although numerous studies have reported bony and soft tissue landmarks for determining the ankle center, a consensus has yet to be reached regarding this matter. This problem is complicated by rotational mismatch between the knee and ankle joint. Because it is difficult to frontalize the knee and ankle joints simultaneously on the same frontal plane. When using extramedullary alignment guides of the tibia, the guides should be applied to the tibia while keeping the knees frontal. The purpose of this study was to determine the position of the ankle center, which is useful for setting extramedullary alignment guides, by using CT data of osteoarthritic knees. CT data of fifty patients (fifty knees) with varus osteoarthritic knees for primary TKA were retrospectively analyzed. Tibial anteroposterior (AP) axis and transmalleolar axis (TMA) were used as reference axes of the knee and ankle joint, respectively. When using above these reference axes, the offset distance from the bimalleolar center was measured as the position of the ankle center. The angular errors were defined as the varus angle of the proximal tibial cut caused by this offset distance when the position of the ankle center was regarded as the bimalleolar center. The position of the ankle center was 1.5 ± 1.2 mm and 2.3 ± 1.5 mm medial to the bimalleolar center with reference to tibial AP axis and TMA, respectively. There was a significant difference between the position of the ankle center with respect to the tibial AP axis and with respect to the TMA (p < 0.01). The mean angular error with respect to the tibial AP axis was 0.3 ± 0.2 °, and the value with respect to TMA was 0.4 ± 0.2 °. The maximum varus angular error along tibial AP axis was less than 0.7 degrees. The positions of the ankle center differed according to the reference axis. Since the angular error was small enough, the bimalleolar center along the tibial AP axis could be used as the ankle center in TKA patients with osteoarthritis knees.
