Vitamin E–diffused Crosslinked Polyethylene Liners Perform Well in Total Hip Arthroplasty
Key findings
- In a prospective study of 193 total hip arthroplasties, both vitamin E–diffused, highly crosslinked polyethylene (VEPE) and mechanically annealed ultrahigh-molecular-weight polyethylene (ModXLPE) liners exhibited low wear five years postoperatively
- Compared with ModXLPE liners, though, VEPE was associated with significantly less total wear at five years and a significantly lower rate of wear between two- and five-year assessments
- A ceramic femoral head was independently predictive of lower wear at five-years, compared with a metal head, but the difference may not be clinically significant because total five-year wear was low for all head-liner combinations
Over the past decade, various formulations of crosslinked polyethylene (PE) have been used for most total hip arthroplasties (THAs). Crosslinked PE has covalent bonds between its polymer chains, which makes it more resistant to wear. However, its high content of free radicals leaves it susceptible to oxidation, which can make it brittle.
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Chemical stabilization of crosslinked PE using vitamin E, a free radical scavenger, is thought to enhance its long-term oxidative resistance while retaining its mechanical properties. Vitamin E can be either blended into the PE before irradiation or diffused into a radiation-crosslinked PE.
Henrik Malchau, MD, PhD, director emeritus of the Harris Orthopaedics Laboratory, and colleagues recently compared the in vivo wear performance of vitamin E–diffused crosslinked PE (VEPE) with that of moderately crosslinked and mechanically annealed ultrahigh-molecular-weight PE (ModXLPE). During manufacture, the radiation dose is higher for VEPE than for ModXLPE, resulting in higher-density crosslinking of VEPE.
In The Bone and Joint Journal, the research team reports that the results were as they expected: VEPE was independently associated with lower total wear five years after THA, compared with ModXLPE.
Study Design and Patients
A cohort of 208 osteoarthritis patients from four international centers in the U.S., Denmark and Sweden participated in a prospective study. The average age at the time of surgery was 60 (range, 20–75).
All THAs (221 hips) were performed with implants from a single manufacturer. In total, 136 (70%) hips received VEPE liners and the others received ModXLPE. Two centers randomized patients to the type of liner (145 hips); the other two implanted only VEPE liners (76 hips). Choices about the femoral head and the stem design were at the surgeon's discretion.
The primary outcome of the study was wear properties five years after THA, assessed by radiostereometric analysis. Six hips had to be revised prior to that time: four for recurrent dislocation, one for periprosthetic fracture and one for infection.
Four patients died before the five-year follow-up, and eight withdrew consent. All told, 193 hips (87%) were reviewed at five years: 74 VEPE from centers that randomized liners, 62 VEPE from centers that did not randomize and 57 ModXLPE.
Rates of Wear
Between one and two years postoperatively, there was no significant increase in wear in either the VEPE or ModXLPE groups, the researchers determined by consulting the database of their coordinating research organization. Neither was there any difference between the groups in total wear at two years.
At the five-year follow-up, total femoral head penetration was significantly less into VEPE liners than into ModXLPE liners. Still, the amounts of wear were low, a median of 0.06 versus 0.13 mm.
In the VEPE group, median penetration of the femoral head did not increase between the two- and five-year follow-up visits. In the ModXLPE group, however, there was a significant increase in penetration (median, 0.04 mm/year; IQR, 0.00–0.06; P < .001).
Predictors of Wear
On multivariable regression analysis, the factors that independently predicted increased wear were ModXLPE (versus VEPE) liner and metal (versus ceramic) femoral head.
The effect of a metal femoral head (average 0.21 mm more total wear than ceramic) was similar to the effect of ModXLPE. The investigators note that although these effects were statistically significant, they may not be clinically important, because total five-year wear was low for all head-liner combinations.
Patient-reported Outcomes
There were no differences between liner groups in patient-reported outcomes five years postoperatively: satisfaction with surgery as rated on a visual analog scale, pain rated on a visual analog scale, Harris Hip Score, 36-item Short Form Health Survey (Physical Function domain) and EQ-5D, a measure of quality of life.
Neither was there any correlation between PE wear and patient-reported outcomes. Clinical outcomes were excellent, with all patients reporting excellent improvement from the preoperative status.
The investigators plan an additional follow-up to better understand associations between the rate of wear and clinical outcomes.
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