Severe Impingement of Lumbar Disc Replacements Increases the Functional Biological Activity of Polyethylene Wear Debris

Total disc replacement surgery is intended to both preserve motion and reduce pain in patients with severe lumbar disc degeneration1,2. Two lumbar total disc replacement devices have been approved for use in the United States: the Charite III (DePuy Spine, Raynham, Massachusetts) and the ProDisc (Synthes, West Chester, Pennsylvania). The Charite artificial disc is no longer used, but it has the longest clinical history, having been implanted in Europe since the 1980s; it consists of two CoCr end plates fixed to the adjacent vertebral bodies1. Between the end plates, a mobile, biconvex conventional ultra-high molecular weight polyethylene (hereafter referred to as polyethylene) core articulates against the concave bearing surfaces. Similar to hip and knee joint replacements, motion-preserving total disc replacements are prone to generate wear debris during a patient’s daily activities3-9. The Charite, like the ProDisc, consists of two metallic end plates and a polyethylene core. However, the polyethylene core is firmly attached to the inferior end plate with the use of a locking mechanism. The superior surface of the core is dome-shaped and articulates against a concave superior metallic plate10. Until recently, published data on the role of polyethylene wear debris as a factor limiting the longevity of total disc replacements consisted of only a few case studies11-13. Additionally, the clinical implications of polyethylene debris in spine tissue are still poorly understood6,14,15. Polyethylene implant wear is affected by many variables, including surface roughness, cross-linking, wear path (distance, direction), and applied load16-20; however, the generation of wear debris can be attributed to four predominant wear modes18. Mode-1 wear occurs with articulation of intended bearing surfaces. Mode 2 occurs during articulation of a bearing and nonbearing surface, and Mode 3 occurs when abrasive third-body particles become entrapped between articulating surfaces. Finally, Mode 4 is the result of unintended articulation between nonbearing surfaces18. Impingement, typically ascribed to Mode-4 wear, is a source of concern in total hip and knee replacement. The effects of impingement have been addressed by numerous studies of revised artificial hip, knee, and shoulder components21-26. Specifically, malfunctioning metal-on-metal27-31 and ceramic-on-ceramic32-37 hip bearings as well as rim fracture of highly cross-linked polyethylene liners38,39 have been considered in the context of impingement. Recently, total disc replacement retrieval studies have also identified impingement patterns on the polyethylene core of Charite and ProDisc implants13,40-43. In the Charite implant, the kinematics of the superior bearing surface cause locking of the core, resulting in an accumulation of rim damage, radial cracking, and rim fracture12,40,44. While impingement remains a clinical concern in total disc replacement, the extent to which impingement affects polyethylene particle formation remains uncertain. Thus, we sought to determine (1) differences in polyethylene particle size, shape, number, or biological activity that correspond to mild or severe rim impingement and (2) in an analysis of all total disc replacements, regardless of impingement classification, whether there are correlations between the extent of regional damage and the characteristics of polyethylene wear debris.