An in vitro biotribological assessment of NUBAC, a polyetheretherketone-on-polyetheretherketone articulating nucleus replacement device: methodology and results from a series of wear tests using different motion profiles, test frequencies, and environmental conditions.

STUDY

DESIGN

In vitro biotribological investigation.

OBJECTIVE

To evaluate the wear resistance and long-term biodurability of NUBAC, a PEEK-on-PEEK articulating nucleus replacement device, using a series of wear tests with different motion profiles, test frequencies, and environmental conditions.

SUMMARY OF BACKGROUND DATA: Wear resistance is critical for any disc arthroplasty device, and osteolysis remains a clinical concern. The use of PEEK for an articulating load bearing nucleus replacement device represents a unique application of this material. NUBAC has an inner ball/socket articulation for motion, similar to total disc replacements. American Society for Testing and Materials and International Organization for Standardization have recommended wear testing methodologies for total disc replacements, however, they have not been clinically validated. Therefore, a series of wear tests were performed to characterize the wear properties of the device.

METHODS

Four groups of devices were evaluated. Group 1 consisted of +/-7.5 degrees flexion/extension to 10 million cycles (Mc) followed by +/-7.5 degrees lateral bending to 10 Mc, alternated to 40 Mc. Groups 2 to 4 consisted of International Organization for Standardization motion and load profiles to 10 Mc, except Group 3 incorporated frequency shifting to ensure a nonrepetitive load and motion profile. Group 4 underwent simulated aging. All studies incorporated a load magnitude of 225 to 1024 N. The average wear rates were determined using linear regression analysis with significant differences between groups determined (analysis of variance).

RESULTS

A wear-in period was observed from 0 to 1 Mc. Wear rates were therefore calculated from 1 Mc. The wear rate for Group 1 was significantly less than Groups 2 to 4 through 10 Mc. From 1 to 5 Mc, the wear rate for Group 1 was significantly less than all groups, with Groups 2 to 4 not significantly different from each other. The wear rates for Groups 2 to 4 were seen to decrease after 5 Mc with only Group 3 significantly different than Group 1. The Group 1 wear rate was consistent throughout the test duration of 40 Mc.

CONCLUSION

The experimental wear rates compare well with the reported wear rates of other material combinations used in nucleus replacement and total disc arthroplasty. Overall, wear rates were relatively low and consistent, suggesting long-term durability, a critical requirement of disc arthroplasty devices.