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Multicenter RSA Evaluation of In Vivo Wear of Vitamin E Stabilized Highly Cross-linked Polyethylene

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Multicenter RSA Evaluation of In Vivo Wear of Vitamin E Stabilized Highly Cross-linked Polyethylene
POSTER NO. P059
Meridith E. Greene, Boston, MA
Charles R. Bragdon, PhD, Boston, MA
Young-Min Kwon, MD, PhD, Boston, MA
Harry E. Rubash, MD, Boston, MA
Annie Gam-Pedersen, RN, Odense, Denmark
Henrik Malchau, MD, Boston, MA
Soeren Overgaard, Odense, Denmark


INTRODUCTION: Vitamin E doping of highly cross-linked polyethylene is a method for enhancing long-term oxidative stability of highly cross-linked ultra-high molecular weight polyethylene for use in total joint arthroplasty. In vitro research and development studies have shown that this material has improved wear performance, better retention of mechanical properties, and high resistance to oxidation due to the anti-oxidative properties of vitamin E. The purpose of this study was to evaluate in vivo properties of vitamin-E doped highly cross-linked polyethylene (VEPE), evaluate the stability of two acetabular cup surface coatings and femoral stem stability using radiostereometric analysis (RSA).

METHODS: One-hundred-and-three hips in 99 patients were recruited into an IRB approved, prospective five year RSA study at two centers. Tantalum beads placed in the VEPE liner, the pelvic bone and the femur at the time of surgery, allow the measurement of femoral head displacement into the polyethylene as well as acetabular and femoral component stability. Fifty-one hips received porous-titanium coated cups while 52 hips received plasma-sprayed cups. Either 32 or 36mm femoral heads were used. RSA radiographs were scheduled immediately post-operatively, at three or six months, one, two, three and five years post-operatively.

RESULTS: Ninety-one hips were followed for three or six months, 64 for one year, 21 for two years, and four for three years. Data for patients with the plasma-sprayed cup were available only at one year follow up. No significant difference was found in femoral head penetration into the VEPE liner between the head sizes. The median head penetration was 0.03±0.01mm at one year, 0.04±0.01mm at two years, and 0.03±0.02mm at three years. The median proximal migration of plasma-sprayed cups was 0.22±0.05mm at one year while the median proximal migration of porous-titanium coated cups was 0.09±0.03mm at one year, 0.11±0.04mm at two years, and 0.17±0.13mm at three years. Both cup types were stable by one year, and the total penetration was not significantly different between the different coating types. The median femoral stem subsidence was 0.13±0.23mm at six months, 0.16±0.27mm at one year, 0.06±0.14mm at two years, and 0.02±2.39mm at three years.

DISCUSSION AND CONCLUSION: This study provides the first in vivo wear measurement of VEPE liners using RSA. The amount of penetration into the liner observed during the early period (creep) is low (0.04mm). Creep in other highly cross-linked polyethylene liners without vitamin E was reported as 0.1mm. There was no significant change in femoral head penetration in vivo over time, indicating that little true wear occurred at the articulation. While not statistically significant, the plasma-sprayed cups tended to migrate more than the porous-titanium coated cups in the early months of follow up. Cups with both types of surface coatings were stable by one year.

Prior work has demonstrated that movement of the acetabular cup greater than 1.2mm in the first two years has a greater than 50% probability of revision. The relatively low amount of early movement seen in both groups is encouraging, indicating that the components were stable at one year follow up. While most stems were stable throughout, the high standard error at three years results from one stem that migrated substantially by six months (9.4mm).

At one year, the stem was stable and at three years remains stable. The 9.4mm of distal migration was visible in plain radiographs. The patient is doing clinically well with no symptoms. The early results of this multicenter RSA study indicate that the VEPE has excellent wear performance; that the plasma-sprayed and porous-titanium coated components both stabilize during the first year in vivo; and that the femoral components were stable.
 

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