In Vitro Wear of a Novel Vitamin E Crosslinked Polyethylene Lumbar Total Joint Replacement

A novel, lumbar total joint replacement (TJR) design has been developed to treat degeneration across all three columns of the lumbar spine (anterior, middle, and posterior columns). Thus far, there has been no in vitro studies that establish the preclinical safety profile of the vitamin E-stabilized highly crosslinked polyethylene (VE-HXLPE) lumbar TJR relative to historical lumbar anterior disc replacement for the known risks of wear and impingement faced by all motion preserving designs for the lumbar spine.

In this study we asked, (1) what is the wear performance of the VE-HXLPE lumbar TJR under ideal, clean conditions? (2) Is the wear performance of VE-HXLPE in lumbar TJR sensitive to more aggressive, abrasive conditions? (3) How does the VE-HXLPE lumbar TJR perform under impingement conditions?

A lumbar TJR with bilateral VE-HXLPE superior bearings and CoCr inferior bearings was evaluated under clean, impingement, and abrasive conditions. Clean and abrasive testing were guided by ISO 18192-1 and impingement was assessed as per ASTM F3295. For abrasive testing, CoCr components were scratched to simulate in vivo abrasion. The devices were tested for 10 million cycles (MC) under clean conditions, 5 MC under abrasion, and 1 MC under impingement.

Wear rates under clean and abrasive conditions were 1.2 ± 0.5 and 1.1 ± 0.6 mg/MC, respectively. The VE-HXLPE components demonstrated evidence of burnishing and multidirectional microscratching consistent with microabrasive conditions with the cobalt chromium spherical counterfaces. Under impingement, the wear rates ranged between 1.7 ± 1.1 (smallest size) and 3.9 ± 1.1 mg/MC (largest size). No functional or mechanical failure was observed across any of the wear modes.

Overall, we found that that a VE-HXLPE-on-CoCr lumbar total joint replacement design met or exceeded the benchmarks established by traditional anterior disc replacements, with wear rates previously reported in the literature ranging between 1 and 15 mg/MC.

The potential clinical benefits of this novel TJR design, which avoids long-term facet complications through facet removal with a posterior approach, were found to be balanced by the in vitro tribological performance of the VE-HXLPE bearings. Our encouraging in vitro findings have supported initiating an FDA-regulated clinical trial for the design which is currently under way.