Summary Statement. Femorotibial constraint is a key property of a total knee arthroplasty (TKA) prosthesis and should reflect the intended function of the device. With a validated simulation methodology, this study evaluated the constraint of two TKA prostheses designed for different intentions. Introduction. TKA prostheses are semi-constrained artificial joints. Femorotibial constraint level is a major property of a prosthesis and should be designed to match the device's intended function. Cruciate Retaining (CR) prostheses are usually indicated for patients with a functioning posterior cruciate ligament (PCL). For patients without a fully functioning PCL, CR-Constrained (CRC) prostheses with additional built-in constraint may be indicated. A CRC prosthesis usually consists of a CR femoral component and a tibial insert which has a more conforming sagittal profile to offer an increased femorotibial constraint. This study evaluated the anterior-posterior (AP) constraint behavior of two lines of prostheses (CR and CRC) from a same TKA product family. Using a validated computer simulation approach, multiple sizes of each product line were evaluated. Methods. Both the CR and CRC prostheses are from the same TKA product family (Optetrak Logic, Exactech, FL, USA) and share identical femoral components and tibial baseplates. The CRC tibial inserts have a more conforming sagittal profile than the CR tibial inserts, especially in the anterior aspect. Three sizes (sizes 1, 3, and 5) from each product line were included in this study. Computer simulations using finite element analysis (FEA) were performed to evaluate the femorotibial constraint of each prosthesis per ASTM F1223 standard [1]. The simulation has been validated by comparison with physical testing (more details submitted in a separate paper to CORS 2013). Briefly, FEA models were created using 10-node tetrahedral elements with all materials considered linear elastic. The tibial baseplate was distally fixed and a constant compressive force (710 N) was applied to the femoral component. Nonlinear Surface-Surface-Contact was established at the articulating surfaces, as well as between the tibial insert and the tibial baseplate. A coefficient of friction of 0.1 was assumed for all articulations [2]. The femoral component was driven under a displacement-controlled scheme to slide along AP direction on the tibial insert. Constraint force occurring at the articulation was derived from the reaction force at the distal fixation; thus, the force-displacement curve can be plotted to characterise the constraint behavior of the prosthesis. A nonlinear FEA solver (NX Nastran SOL601, Siemens, TX, USA) was used to solve the simulations. Results. The force-displacement curves predicted by the simulation exhibited the hysteresis loop appearance for both CR and CRC prostheses. The profile of the curves was generally consistent across different sizes for both product lines. The anterior constraint of the CRC prosthesis was significantly greater than the CR prosthesis. The posterior constraint of the CRC prosthesis was also slightly greater. Larger sizes exhibited reduced constraint compared to smaller sizes. Discussion/Conclusion. The increased constraint of the CRC prosthesis revealed in the study is consistent with the geometrical characteristics and the functional intent of the device. The CRC tibial insert is expected to provide significantly greater anterior constraint than the CR prosthesis to prevent paradoxical femoral translation when the patient's PCL is not fully functioning. The CRC tibial insert is also expected to provide slightly increased posterior constraint due to its elevated
We describe a lateral approach to the distal humerus based on initial location of the superficial branches of the radial nerve, the inferior lateral cutaneous nerve of the arm and the posterior cutaneous nerve of the forearm. In 18 upper limbs the superficial branches of the radial nerve were located in the subcutaneous tissue between the triceps and brachioradialis muscles and dissected proximally to their origin from the radial nerve, exposing the shaft of the humerus. The inferior lateral cutaneous nerve of the arm arose from the radial nerve at the lower part of the spiral groove, at a mean of 14.2 cm proximal to the lateral epicondyle. The posterior cutaneous nerve of the forearm arose from the inferior lateral cutaneous nerve at a mean of 6.9 cm (6.0 to 8.1) proximal to the lateral epicondyle and descended vertically along the dorsal aspect of the forearm. The size and constant site of emergence between the triceps and brachioradialis muscles constitute a readily identifiable landmark to explore the radial nerve and expose the humeral shaft.
