The most common bearing couple used in total knee arthroplasty (TKA) is ultra-high molecular weight polyethylene (UHMWPE) articulating against a CoCrMo alloy femoral component. Although this couple has demonstrated good clinical results, UHMWPE wear has been identified as one of the principal causes for long-term failure of total knee joint replacements1 indicating a need for improvements in TKA bearings technology. The wear resistance of UHMWPE can be improved by radiation crosslinking; however, in order to get the full benefit of this improved wear resistance, an abrasion resistant ceramic counterface is necessary.2 Since the radiation crosslinking degrades mechanical properties, it is also important to have an optimized radiation dose and subsequent processing. The purpose of this study was to evaluate the long-term wear performance of VERILAST Technology comprising two advanced bearing technologies, abrasion resistant OXINIUM femoral components (OxZr)3-4 and wear/strength optimized 7.5 Mrad crosslinked polyethylene (7.5-XLPE).5 Three component assemblies of LEGION(tm) cruciate retaining (CR) OxZr femoral components, 7.5-XLPE tibial inserts were tested on an AMTI knee simulator under displacement control at 1 Hz frequency as described previously.2 The tibial inserts were manufactured from compression molded GUR 1020 UHMWPE, radiation crosslinked to 7.5 Mrad dose, remelted to extinguish free radicals, and sterilized by EtO. The wear test was conducted for 45 Mcycle, which was considered to be a conservative estimate for the amount of cycles that would occur during 30 years of typical in-vivo use based on the relationship between patient age and the number of loading cycles as reported in the literature.6-8Introduction
Materials and Methods
A new hinge knee system (LEGION HINGE, Smith &
Nephew, Memphis, TN) was designed to treat gross knee instability resulting from loss of collateral ligament function, femoral and/or tibial bone loss, or from comminuted fractures of the proximal tibia or distal femur. The knee system is offered with an insert that guides the motion of the implant for kinematic improvement. The purpose of this study was to evaluate kinematic and wear performance of this novel hinge knee replacement system. The kinematics and kinetics of the Guided Motion (GM) hinge knee were assessed for a deep knee bend using a numerical lower leg simulator. Measurements of A/P translation and I/E rotation were compared to 3D MRI data of healthy weight bearing knees and measurements of M/L patella shear forces were compared to a standard primary knee implant. Three GM knee systems were tested for wear performance. All metal components were fabricated from cobalt chrome except for the Ti-6Al-4V insert locking screw. All plastic components were fabricated from UHMWPE. Wear testing was conducted on an AMTI 6-station force controlled knee simulator for approximately 5 million cycles under ISO 14243-1 load/motion profiles and soft tissue constraints. Simulation results showed that up to 130° of flexion the anterior/posteror (A/P) translation and internal/external (I/E) rotation followed a similar path over the flexion range compared to the MRI data. The magnitude of A/P translation at 130° was 9.5 mm for the GM design compared to 15.7 mm for the MRI data. The magnitude of I/E rotation at 130° was 18° for the GM design compared to 20.8° for the MRI data. The GM design showed a maximum M/L patella shear force of 456.8 N compared to 1152.4 N for a standard primary knee design over the flexion range. All constructs successfully completed wear testing and were fully functional with no issues for binding of the mating parts. All polyethylene components showed only burnishing on the articulating surfaces. The volumetric wear rate of polyethylene components was 17.54±1.24 mm3/Mcycle. The volumetric wear rate of the metal components (excluding femoral and tibial tray) was 0.045±0.01 mm3/Mcycle. Testing showed the GM design has A/P and I/E kinematics that are similar to those seen in a normal healthy knee and good patella tracking as evidenced by the low M/L patella shear forces. The wear rate of the polyethylene parts was within the range of wear rates published in the literature for primary knee designs (up to 35.8 mm3/Mcycle). The low metal wear rate indicates that fretting and corrosion of the components was minimal. We conclude the GM design more closely replicates the kinematics of the natural knee without compromising the wear characteristics. This could lead to better outcomes for the patient population that requires a hinge knee implant.
