Influence of kinematic alignment on femorotibial kinematics in medial stabilized TKA design compared to mechanical alignment.

INTRODUCTION: Worldwide more and more primary knee replacements are being performed. Kinematic alignment (KA) as one of many methods of surgical alignment has been shown to have a significant impact on kinematics and function. The aim of the present study was to compare KA and mechanical alignment (MA) with regard to femorotibial kinematics. MATERIALS AND METHODS: Eight fresh frozen human specimens were tested on a knee rig during active knee flexion from 30 to 130°. Within the same specimen a medial stabilized (MS) implant design was used first with KA and then with MA. RESULTS: The femorotibial kinematics showed more internal rotation of the tibia in KA compared to MA. At the same time, there was a larger medial rotation point in KA. Both alignment methods showed femoral rollback over the knee bend. CONCLUSION: Relating to an increased internal rotation and a more precise medial pivot point, it can be concluded that KA combined with a MS implant design may partially support the reproduction of physiological knee joint mechanics.

[Partial exchange in total hip arthroplasty : What can we combine?] / Gelenkteilwechsel in der Hüftarthroplastik : Was dürfen wir kombinieren?

BACKGROUND: In case of hip revision arthroplasty, one component (cup/stem) is often well fixed and does not need to be exchanged. The newly implanted component needs to be compatible with the well-fixed implant. The combination of implants from different companies leads to "mix and match" or even mismatch between the implants. OBJECTIVES AND METHODS: The objective of this work was to describe possible combinations including their specifications that need to be considered in partial exchange of hip prostheses. For this purpose the literature, surgical techniques of companies and judgements concerning this topic were analysed and our own results and experiences were included. RESULTS AND CONCLUSIONS: Partial revision arthroplasty can be challenging and needs to be planned in detail. In case of isolated cup or inlay revision with exchange of a modular head the cone of the stem needs to be identified. A ceramic head may be used in revision with a titanium sleeve even from a different company as long as they are compatible. Patients however need to give their informed consent for this mix and match procedure. This procedure is done frequently and good study results support this, however from a juristic point of view a definite recommendation cannot be given. If the inlay of a cup is replaced, the original inlay should be used. If this is not available anymore, it can be manufactured as a special product in many cases. If this is also not possible, an inlay can also be cemented into a well-fixed cup. Biomechanical and clinical studies support this off-label technique. In case of an isolated exchange of the stem with a ceramic inlay that is retained in a well-fixed cup, the revision stem and ceramic head need to be from the same company as the cup. In case of ceramic fracture, a ceramic head with a titanium sleeve should be combined with a PE or ceramic inlay, a metal head or inlay should never be used.

[The biomechanics of screws, cerclage wire and cerclage cable]. / Die Biomechanik von Schrauben, Draht- und Kabelcerclagen.

In contrast to fracture fixation, when performing an osteotomy the surgeon is able to plan preoperatively. The resulting fixation and compression of the bone fragments are the most important points. A stable osteosynthesis should prevent dislocation of bone fragments and improve bone healing. Beside plates, cerclages can be used for tension band or diaphysis bone fixation. Moreover, cortical or cancellous screws can be used for osteotomy fixation. This work describes biomechanical principles for fixation after an osteotomy with cerclages and cortical or cancellous screws. It also summarizes the materials and geometries used, as well as their influence on the stability of the osteosynthesis.

[Anatomy and biomechanics of the patellofemoral joint: physiological conditions and changes after total knee arthroplasty]. / Anatomie und Biomechanik des Patellofemoralgelenks : Physiologische Gegebenheiten und Veränderungen nach Implantation einer Knietotalendoprothese.

The patellofemoral joint constitutes a complex anatomical and functional entity. The tensile force of the quadriceps femoris muscle is transmitted through the patella and patellar ligament onto the tibial tuberosity. This particular three-dimensional arrangement increases the torsional moment acting on the knee joint. Dynamic alignment of the patella is determined by trochlear geometry and is supported by active muscular and passive connective tissue stabilizers. In addition to the retinaculum of the patella, the medial patellofemoral ligament is attracting increasing clinical attention. Multidirectional motion of the patella is closely connected to retropatellar pressure distribution which can be modulated by moving the patellar ligament insertion. Implantation of a knee endoprosthesis changes the joint surface geometry and consequently patella kinematics and retropatellar pressure distribution. Finite element analysis provides the possibility to assess retropatellar pressure distribution before and after implantation of prostheses.

Development and validation of a weight-bearing finite element model for total knee replacement.

Total knee arthroplasty (TKA) is a successful procedure for osteoarthritis. However, some patients (19%) do have pain after surgery. A finite element model was developed based on boundary conditions of a knee rig. A 3D-model of an anatomical full leg was generated from magnetic resonance image data and a total knee prosthesis was implanted without patella resurfacing. In the finite element model, a restarting procedure was programmed in order to hold the ground reaction force constant with an adapted quadriceps muscle force during a squat from 20° to 105° of flexion. Knee rig experimental data were used to validate the numerical model in the patellofemoral and femorotibial joint. Furthermore, sensitivity analyses of Young's modulus of the patella cartilage, posterior cruciate ligament (PCL) stiffness, and patella tendon origin were performed. Pearson's correlations for retropatellar contact area, pressure, patella flexion, and femorotibial ap-movement were near to 1. Lowest root mean square error for retropatellar pressure, patella flexion, and femorotibial ap-movement were found for the baseline model setup with Young's modulus of 5 MPa for patella cartilage, a downscaled PCL stiffness of 25% compared to the literature given value and an anatomical origin of the patella tendon. The results of the conducted finite element model are comparable with the experimental results. Therefore, the finite element model developed in this study can be used for further clinical investigations and will help to better understand the clinical aspects after TKA with an unresurfaced patella.

Finite element analysis of the ovine hip: development, results and comparison with the human hip.

OBJECTIVES: The ovine hip is often used as an experimental research model to simulate the human hip. However, little is known about the contact pressures on the femoral and acetabular cartilage in the ovine hip, and if those are representative for the human hip. METHODS: A model of the ovine hip, including the pelvis, femur, acetabular cartilage, femoral cartilage and ligamentum transversum, was built using computed tomography and micro-computed tomography. Using the finite element method, the peak forces were analysed during simulated walking. RESULTS: The evaluation revealed that the contact pressure distribution on the femoral cartilage is horseshoe-shaped and reaches a maximum value of approximately 6 MPa. The maximum contact pressure is located on the dorsal acetabular side and is predominantly aligned in the cranial-to-caudal direction. The surface stresses acting on the pelvic bone reach an average value of approximately 2 MPa. CONCLUSIONS: The contact pressure distribution, magnitude, and the mean surface stress in the ovine hip are similar to those described in the current literature for the human hip. This suggests that in terms of load distribution, the ovine hip is well suited for the preclinical testing of medical devices designed for the human hip.