Mechanical effects of defect closure following BPTB graft harvest for ACL reconstruction.

Anterior cruciate ligament injury affects roughly 120,000 athletes in the United States every year. One of the most common techniques is the use of a bone-patellar tendon-bone graft. Graft harvest creates a sizeable defect in the remaining patellar tendon. Closure of this defect is based on surgeon preference. To date there has been no study on the effects of defect closure on the mechanical properties of remaining donor patellar tendon. The goal of this study was to investigate the effect of closure on both the strength and stiffness of the remaining patellar tendon. 7 pairs of fresh frozen cadaver patellar tendons were matched by tendon dimensions. Bone-patellar tendon-bone grafts were harvested from all of the specimens and then half of the paired tendons underwent defect closure. All of the donor tendons were then tested in a servohydraulic load frame to failure at a constant displacement rate at room temperature. This study found no differences in the load at failure, the engineering failure stress, stiffness or in the engineering modulus between the donor tendons that underwent defect closure versus those that did not.

Ultra high molecular weight polyethylene: mechanics, morphology, and clinical behavior.

Ultra high molecular weight polyethylene (UHMWPE) is a semicrystalline polymer that has been used for over four decades as a bearing surface in total joint replacements. The mechanical properties and wear properties of UHMWPE are of interest with respect to the in vivo performance of UHMWPE joint replacement components. The mechanical properties of the polymer are dependent on both its crystalline and amorphous phases. Altering either phase (i.e., changing overall crystallinity, crystalline morphology, or crosslinking the amorphous phase) can affect the mechanical behavior of the material. There is also evidence that the morphology of UHMWPE, and, hence, its mechanical properties evolve with loading. UHMWPE has also been shown to be susceptible to oxidative degradation following gamma radiation sterilization with subsequent loss of mechanical properties. Contemporary UHMWPE sterilization methods have been developed to reduce or eliminate oxidative degradation. Also, crosslinking of UHMWPE has been pursued to improve the wear resistance of UHMWPE joint components. The 1st generation of highly crosslinked UHMWPEs have resulted in clinically reduced wear; however, the mechanical properties of these materials, such as ductility and fracture toughness, are reduced when compared with the virgin material. Therefore, a 2nd generation of highly crosslinked UHMWPEs are being introduced to preserve the wear resistance of the 1st generation while also seeking to provide oxidative stability and improved mechanical properties.

Notch strengthening and hardening behavior of conventional and highly crosslinked UHMWPE under applied tensile loading.

This study examined the engineering and true axial stress-strain behavior of smooth cylindrical and shallow and deep notched cylindrical test specimens, under applied axial tensile loading using non-contacting methods, of both conventional and highly crosslinked ultra-high molecular weight polyethylenes (UHMWPEs). The smooth specimens experienced a uniaxial stress state, while the notched specimens experienced a triaxial stress state in the vicinity of the notch. Materials were all prepared from a single batch of medical grade GUR 1050 resin (Ticona, Bayport, TX). The two conventional UHMWPEs were as-received (virgin) and gamma radiation sterilized at 30 kGy in a nitrogen atmosphere (radiation sterilized). The two highly crosslinked UHMWPEs were each irradiated at 100 kGy and then post-processed with one of either of the two thermal treatments: annealing, which was done below the melt transition temperature (T(m)), at 110 degrees C for two hours (110 degrees C-annealed), and remelting, which was done above T(m), at 150 degrees C (150 degrees C-remelted). All of the materials showed notch strengthening; that is, a significant elevation of axial yield properties (both engineering and true) for the shallow and deep notched conditions. Axial ultimate properties (engineering and true) were significantly decreased for the notched conditions compared with the smooth condition. Hardening ratios (both true and engineering), which are defined in this work as the ratio of ultimate stress or strain to yield stress or strain, were also found to significantly decrease with notching. The extent of change was dependent on the UHMWPE material. The micromechanism of fracture differed between the smooth and notched conditions. This study suggests that notches inherent in the design of UHMWPE joint replacement components (posts, undercuts, grooves) will have different notch sensitivities depending on the UHMWPE formulation.