Time- and depth-dependent changes in crosslinking and oxidation of shelf-aged polyethylene acetabular liners.

Since crosslinking and oxidation of ultrahigh-molecular-weight polyethylene (UHMWPE) have important roles in determining the wear resistance of UHMWPE total joint components, the time and depth dependence of crosslinking and oxidation of new shelf-aged (2-11 years), ready-to-implant acetabular liners were studied by using solvent extraction and Fourier transform infrared spectroscopy. The ultrastructure of these materials also was examined by using low-voltage scanning electron microscopy in an oil-free vacuum. Oxidation levels increased with time and with depth (p < 0.0001) from the surface of the older liners to a maximum value at about 1-2 mm below the surface, then decreased. They were minimal at the midsection of the liners. The crosslinking of these liners decreased with time and depth (p < 0.0001) and was inversely proportional to the level of oxidation. High and depth-dependent oxidation levels were observed in all older liners made from GUR 415 and 412 resins but were distinctly absent from a comparably aged (i.e., 9 years) liner made from 1900 CM-resin. Some liners showed varying degrees of inhomogeneous and discontinuous morphologic ultrastructure in addition to varying amounts of porosity while others had a more homogeneous ultrastructure. Oxidation and crosslinking of polyethylene are time- and depth-dependent processes that are mutually competitive. We suggest that resin choice and perhaps consolidation-related variables lead to differences in polyethylene's ultrastructure. These ultrastructural differences in polyethylene's inhomogeneities, that is, the type (interconnected or closed-cell) or extent may affect the oxidation resistance of polyethylene. While oxygen diffusion to free radicals in polyethylene already is known to explain some of these time- and depth-dependent effects, perhaps such ultrastructural variations also may facilitate or retard oxygen diffusion in this material. Resin-based ultrastructural variability partially may explain the variability in the clinical performance of polyethylene total joint implant components. Thus resin choice or processing modifications related to polyethylene's ultrastructure may increase its oxidation resistance and ultimately improve the clinical wear performance of polyethylene total joint orthopedic implants.

Validity and reliability of the 6-minute walk test in a cardiac rehabilitation population.

PURPOSE: Although the 6-minute walk test is commonly used to assess the functional status of patients with severe cardiopulmonary disease, few studies have tested its value in a cardiac rehabilitation (CR) population with milder disease status. The purpose of this study was to examine the validity and reliability of the 6-minute walk in a Phase II/III CR program. METHODS: Ninety-four patients (61 men, 33 women) aged 63 +/- 10 years completed three 6-minute walks on nonconsecutive days. Patients also completed the Duke Activity Status Index (DASI) and the Short Form 36 Health Survey (SF-36). In addition, maximum metabolic equivalents (METs) from a symptom-limited graded exercise test were obtained from files. RESULTS: The 6-minute walk was linearly related to maximum METs (r = 0.687, P < 0.001), supporting the validity of the test. Patients walked significantly farther in each 6-minute walk (F = 19.83, P < 0.001), and strong test-retest reliability was demonstrated (intraclass correlation = 0.97). Distance walked decreased with older age (F = 19.49, P < 0.001), with men walking farther than women (F = 7.19, P < 0.01). The 6-minute walk was moderately correlated with scores from the DASI (r = 0.502, P < 0.001), and the Physical Function subscale of the SF-36 (r = 0.624, P < 0.001). CONCLUSIONS: The 6-minute walk is a valid and reliable method of assessing functional ability in a Phase II/III CR population. A learning effect of 6% was observed over the three walks; however, it is unknown if this learning effect will be retained over time. This test may be particularly valuable to smaller CR centers that want to document functional improvements but do not have access to conventional treadmill tests.

Solubility changes in shelf-aged ultra-high molecular weight polyethylene acetabular liners.

Crosslinking of extruded, air-packaged, irradiated, shelf-aged (10 consecutive years) polyethylene acetabular liners was measured versus time and material location by using a hot xylene extraction protocol. Insolubility (crosslinking) of new polyethylene liners was 87%, but decreased to 45% after 10 years of shelf-aging. This degradation is similar to that observed from retrieved (aged in vivo) acetabular cups. Crosslinking varied with depth in the aged liners and with radial location in the unaged liners. Given that crosslinking improves polyethylene's wear resistance, crosslinking degradation of polyethylene orthopaedic components must be controlled to improve the long-term clinical performance of total joint implants.

Multicycle mechanical performance of titanium and stainless steel transpedicular spine implants.

STUDY DESIGN: This was a prospective in vitro study comparing titanium alloy and stainless steel alloy in transpedicular spine implants from two different manufactures. OBJECTIVE: To compare the multicycle mechanical performance of these two alloys, used in each of two different implant designs. SUMMARY OF BACKGROUND DATA: Transpedicular spine implants primarily have been manufactured from stainless steel, but titanium alloy offers imaging advantages. However, the notch sensitivity of titanium alloy has caused concern regarding how implants made from this material will compare in stiffness and fatigue life with implants made from stainless steel. METHODS: Twenty-four implants (two alloys, two designs, six implants per group) were mounted in machined polyethylene wafers and repetitively loaded (up to 1 million cycles) from 80 N to 800 N using a 5-Hertz sinusoidal waveform. Load and displacement data were automatically and periodically sampled throughout the entire test. RESULTS: Implant stiffness increased with cycle load number, reached a steady state, then declined just before fatigue failure. Stiffness varied less in titanium transpedicular spine implants than in their stainless counterparts. All stainless steel implant types were stiffer (steady-state value, P < 0.0001) than their titanium alloy counterparts. One titanium implant design failed with fewer (P < 0.05) load cycles than its stainless steel counterpart, whereas a stainless steel implant of another design failed with fewer (P < 0.002) load cycles than its titanium counterpart. Overall, fatigue life, i.e., the total number of load cycles until failure, was related to implant type (P < 0.0001), but not to implant material. CONCLUSIONS: A transpedicular spine implant's fatigue lifetime depends on both the design and the material and cannot be judged on material alone. Stainless steel implants are stiffer than titanium alloy implants of equal design and size; however, for those designs in which the fatigue life of the titanium alloy version is superior, enlargement of the implant's components can compensate for titanium's lower modulus of elasticity and result in an implant equally stiff as its stainless steel counterpart. Such an implant made from titanium alloy would then be clinically preferable because of titanium's previously reported imaging advantages.