Andrew J. Lozynsky

Improved resistance to wear and fatigue fracture in high pressure crystallized vitamin E-containing ultra-high molecular weight polyethylene

Higher crystallinity and extended chain morphology are induced in ultra-high molecular weight polyethylene (UHMWPE) in the hexagonal phase at temperatures and pressures above the triple point, resulting in improved mechanical properties. In this study, we report the effects of the presence of a plasticizing agent, namely vitamin E (alpha-tocopherol), in UHMWPE during high pressure crystallization. We found that this new vitamin E-blended and high pressure crystallized UHMWPE (VEHPE) has improved fatigue strength and wear resistance compared to virgin high pressure crystallized (HP) UHMWPE. This suggested different mechanisms of wear reduction and fatigue crack propagation resistance in UHMWPE.

A surface crosslinked UHMWPE stabilized by vitamin E with low wear and high fatigue strength

Wear particle-induced periprosthetic osteolysis has been a clinical problem driving the development of wear resistant ultrahigh molecular weight polyethylene (UHMWPE) for total joint replacement. Radiation crosslinking has been used to decrease wear through decreased plastic deformation; but crosslinking also reduces mechanical properties including fatigue resistance, a major factor limiting the longevity of joint implants. Reducing UHMWPE wear with minimal detriment to mechanical properties is an unaddressed need for articular bearing surface development. Here we report a novel approach to achieve this by limiting crosslinking to the articular surface. The antioxidant vitamin E reduces crosslinking efficiency in UHMWPE during irradiation with increasing concentration, thus we propose to spatially control the crosslink density distribution by controlling the vitamin E concentration profile. Surface crosslinking UHMWPE prepared using this approach had high wear resistance and decreased crosslinking in the bulk resulting in high fatigue crack propagation resistance. The interface region did not represent a weakness in the material due to the gradual change in the crosslink density. Such an implant has the potential of decreasing risk of fatigue fracture of total joint implants as well as expanding the use of UHMWPE to younger and more active patients.

Knee simulator wear of polyethylene tibias articulating against explanted rough femoral components.

Highly cross-linked and melted polyethylene tibial inserts have recently been introduced for clinical use to reduce fatigue damage and adhesive wear in tibial inserts. Other authors have studied the effect of counterface roughness on the wear behavior of polyethylene tibial inserts in knee simulators using femoral components that were roughened artificially. They reported a higher wear rate with highly cross-linked polyethylene than with unirradiated polyethylene tibial inserts. Artificial roughening of femoral components may not be clinically relevant. To evaluate this concern, we studied the wear behavior of highly cross-linked and conventional polyethylene tibial inserts articulating in vitro against surgically retrieved femoral components that had become roughened in vivo. The wear rate of the highly cross-linked polyethylene (5.9 and 6.8 mg/1 million cycles with 100 and 50% serum) was 80% lower than the wear rate of the conventional polyethylene (33.5 and 32.2 mg/1 million cycles with 100 and 50% serum) tibial inserts after 2 million cycles of simulated gait. This study suggests that during in vivo use, scratches that are generated on the femoral components are likely to produce a higher wear rate with both cross-linked and conventional polyethylene than a smooth femoral component, but that this wear rate is likely to be higher with conventional polyethylene than with highly cross-linked polyethylene tibial inserts.

In vitro comparison of frictional torque and torsional resistance of aged conventional gamma-in-nitrogen sterilized polyethylene versus aged highly crosslinked polyethylene articulating against head sizes larger than 32 mm.

Background The advent of highly crosslinked polyethylene has allowed the re-evaluation of the use of femoral heads larger than 32 mm for metal-on-polyethylene total hip arthroplasties. However, the effect of larger heads on the frictional torque of highly crosslinked polyethylene is unknown. Methods We performed an in vitro examination of the effect of larger chrome cobalt femoral heads (40 mm diameter) on the frictional torque and torsional resistance of hip articulations on aged liners of polyethylene that were sterilized by gamma rays while in nitrogen, and aged highly crosslinked polyethylene. The frictional torque at the femoral head articulation was usually higher for the highly crosslinked polyethylene than for the conventional polyethylene. The aged conventional liners oxidized considerably, which led to gross failure of the polyethylene at the anti-rotation portion of the rim. The aged crosslinked polyethylene showed no such failures despite the higher frictional torque. Interpretation Our findings suggest that in terms of torsional resistance to fatigue when studied as a device, rather than as an isolated material, under these conditions, aged highly crosslinked polyethylene is preferable to aged conventional polyethylene.

Knee Simulator Wear of Vitamin E Stabilized Irradiated Ultrahigh Molecular Weight Polyethylene

Wear and damage of ultrahigh molecular weight polyethylene (UHMWPE) tibial inserts used in total knee arthroplasty are accelerated by oxidation. Radiation crosslinking reduces wear but produces residual free radicals adversely affecting stability. One alternative to stabilize radiation-crosslinked UHMWPE is to infuse the material with vitamin E (vit E). We investigated the properties of 100-kGy e-beam-irradiated UHMWPE that was subsequently doped with vitamin E in comparison with conventional UHMWPE. Both polymers were sterilized with gamma irradiation in vacuum packaging. Vitamin E-doped UHMWPE showed lower wear before and after aging (2.4 ± 0.5 and 2.5 ± 0.8 mg/million cycle, respectively, vs 26.9 ± 3.5 and 40.8 ± 3.0 mg/million cycle for conventional UHMWPE). Conventional UHMWPE showed oxidation after accelerated aging, and its mechanical properties were adversely affected, whereas vit E-doped UHMWPE showed no oxidation or changes in its mechanical properties. Vitamin E stabilization of radiation-crosslinked UHMWPE resulted in low wear and high oxidation resistance; it is an alternative load-bearing material for total knee applications.

Increasing irradiation temperature maximizes vitamin E grafting and wear resistance of ultrahigh molecular weight polyethylene

Vitamin E stabilization of radiation crosslinked ultrahigh molecular weight polyethylene (UHMWPE) for total joint implants can be done by blending of UHMWPE resin powder with vitamin E, followed by consolidation and irradiation of the blend. It is well known that vitamin E prevents crosslinking in UHMWPE during ionizing radiation. We hypothesized that there would also be a significant amount of grafting of vitamin E onto UHMWPE during irradiation. Spectroscopic analysis of radiation crosslinked vitamin E-blended UHMWPE before and after extraction with boiling hexane showed vitamin E grafting in up to 30% of the blended vitamin E. Grafting increased with irradiation temperature. We also discovered that increasing irradiation temperature resulted in better preservation of active vitamin E in the polymer and increased crosslinking efficiency of UHMWPE. As a result, warm-irradiated vitamin E-blended UHMWPEs had significantly less wear than those irradiated at ambient temperature. It may be desirable to graft vitamin E on UHMWPE to decrease the possibility of elution and increase long-term stability. Warm irradiation of vitamin E blends may present an advantage in increasing vitamin E potency, as well as decreasing the wear of UHMWPE, which is crucial in decreasing the incidence of periprosthetic osteolysis in total joint replacement patients.

Effects of femoral neck length, stem size, and body weight on strains in the proximal cement mantle.

Background: Several studies have shown that certain cemented total hip replacement femoral stems have been associated with the complications of early debonding, loosening, and osteolysis. Some authors have suggested that these failures may be related to the surface finish of the stems. We developed an in vitro biomechanical experiment characterized by simulated stair-climbing to investigate the multiple factors involved in loosening of cemented femoral stems. In this study, we measured the effects of stem neck length, body weight, stem size, and calcar-collar contact on the torsional stability, as reflected by the strains in the proximal cement mantle, of one design of cemented femoral stem. Methods: Eight Centralign femoral stems (Zimmer, Warsaw, Indiana) were cemented into eight cadaver femora with use of contemporary cementing techniques. Prior to insertion, fifteen strain-gauge rosettes were mounted around the proximal portion of the stem. The stems were loaded on a jig that simulated static peak loading during stair-climbing. Loading was repeated for each stem with three different joint reaction forces and for three different neck lengths. Calcar loading by the collar was then eliminated by removing a 0.5-mm slice of bone beneath the collar, and all loadings were then repeated. Results: The peak principal tensile strains in the proximal cement increased linearly with both body weight (r2 > 0.95) and neck length (r2 > 0.75). Increasing body weight affected the peak cement strains far more than did increasing neck length. During simulated stair-climbing, calcar-collar contact reduced peak strains in the proximal cement by a factor of 1.5 to two. Peak principal tensile strains in the proximal cement often exceeded 1000 me when the smaller stems were used. Conclusions: In this stair-climbing test model, the peak proximal cement strains were increased more by changes in body weight than they were by changes in neck length. Even during stair-climbing, calcar-collar contact reduced peak cement strains. Clinical Relevance: Many cemented femoral stems that become loose do so by rotating into retroversion. In this study of one design of a cemented femoral component during simulated stair-climbing, the peak strain magnitudes in the proximal cement mantle were increased more by changes in body weight than by changes in the length of the neck of the stem. The strong effect of stem size on the cement strains suggests that cemented femoral stems should not be used in heavy patients with small medullary canals that require a small cemented stem.

Effect of cross-link density on the high pressure crystallization of UHMWPE

Ultrahigh molecular weight polyethylene (UHMWPE) is a bearing surface material for total joint implants. It is radiation cross-linked for high wear resistance and is melted or treated with vitamin E for oxidative stability. We investigated high pressure crystallization (HPC) of irradiated UHMWPE as an alternative method to improve the mechanical strength while stabilizing the residual free radicals from radiation cross-linking. HPC of uncross-linked UHMWPE has resulted in the formation of extended chain crystals and increased crystallinity, leading to improved strength. We hypothesized that increased cross-link density would hinder crystallization during HPC due to decreased chain mobility. Therefore, we investigated the crystalline structure and tensile mechanical properties of high pressure crystallized 25-, 65- and 100-kGy irradiated UHMWPE. We also determined free radical content and wear. The strength of 25- and 65-kGy irradiated UHMWPEs was improved by HPC with increased crystallinity and crystal size. 100-kGy irradiated UHMWPE did not show improved strength, supporting our hypothesis that decreased chain mobility would hinder crystal formation and strength improvement. None of the HPC irradiated UHMWPEs contained detectable free radicals and their wear properties were maintained, suggesting oxidative and mechanical stability in the long term. Therefore, HPC can be used effectively for imparting oxidative stability while strength improvement can be achieved for irradiated UHMWPE with low to moderate cross-link density.

Assessment of the symmetry of bone strains in the proximal femoral medial cortex under load in bilateral pairs of cadaver femurs

In past studies, it has been assumed that contralateral femurs from the same patient have sufficiently symmetric mechanical properties that one femur could reliably serve as representative of the other in mechanical testing experiments dealing with surface bone strains. To assess the accuracy of this assumption, 10 pairs of cadaveric femurs without evidence of osseous abnormalities were instrumented with strain gauges on the periosteal and endosteal surfaces of the proximal medial cortex and strain measurements were made under simulated conditions of single-leg stance and stairclimbing. Although the strains in femurs from different individuals varied greatly, the degree of symmetry in the strains of contralateral femurs was high. These findings add support to the use of contralateral femurs without discernible abnormalities as suitable controls for each other in such situations.