Cellular Response and Degradation Products In Failed Metal-On-Metal Disc Replacements

Introduction: Metal-on-metal bearings for disc replacement have been introduced as an alternative to polyethylene bearings to reduce the generation of wear particles. These disc replacements utilize CoCrMo alloys but otherwise differ from metal-on-metal hip replacement bearings in geometry, size and kinematics. There is a paucity of data on the histology and degradation products generated in vivo by disc replacement devices.

Methods: Periprosthetic tissues were studied from 5 cases of metal-on-metal lumbar or cervical disc replacement revised for pseudo-tumor formation. Histological sections stained with hematoxylin and eosin and in selected cases with immunohistochemical reagents were studied using light microscopy. Unstained sections were analyzed using backscattered electron imaging and energy-dispersive x-ray analysis.

Results: The masses adjacent to disc replacements revealed large areas with coagulation necrosis of fibro-connective and adipose tissue. Peripheral areas of viable tissue demonstrated fibrosis and chronic inflammation. The cellular infiltrate was dominated by lymphocytes and included a moderate number of macrophages, focal accumulations of degranulating eosinophils and, in one case, numerous plasma cells. Submicron metallic particles similar in composition to wear particles generated by metal-on-metal hip bearings were identified in 3 of the 5 spinal masses. The particles were rich in chromium, oxygen and phosphorous and deficient in cobalt relative to the base alloy. Particles in which cobalt is deficient or absent suggests substantial local metal ion release.

Discussion: The histopathology of pseudo-tumors associated with cervical and lumbar disc replacement was similar to reactions previously characterized as metal hypersensitivity in patients hosting metal-on-metal hip replacement, except for the large number of eosinophils observed in several cases of disc replacement. Degranulating eosinophils may be related to a local allergic reaction in these patients. The prevalence of lymphocyte dominated reactions in patients with metal-on-metal bearings is thought to be <3% in hip replacement and presently is unknown in disc replacement.