Histological characteristics of periprosthetic tissue around metal on metal hip prostheses

The ultimate challenge for orthopedic surgeons who implant hip prosthesis is providing long-term function of the joint without pain. The most common cause of total hip arthroplasty failure is aseptic loosening mostly initiated by wear particles.1 The current paradigm explains aseptic loosening as an inflammatory response to the wear debris particles produced by prosthetic implants. Wear particles are phagocytosed by macrophages and induce a variety of cytokines that mediate inflammatory response leading to the recruitment and activation of osteoclasts, and eventual implant loosening.2 For over 40 years, the metal-on-polyethylene (MoPE) bearing has dominated the field of total hip replacement (THR). The second generation of metal on metal (MoM) THRs made of CoCrMo alloy was introduced into the market in early 1990s with the aim to eliminate the wear of polyethylene (PE) and therefore, to achieve better long-term clinical results compared to conventional MoPE THRs. MoM artificial hip prostheses showed an extremely low rate of wear of the articulation surfaces. In addition, metal particles that are produced are smaller than PE particles and hence, they may induce less tissue reaction.3–5 However, patients with M-M articulations have increased levels of Co and Cr in the serum and urine, and this has raised concerns about toxicity, mutagenesis and hypersensitivity.6 There are reports from orthopedic registers about increase complications and potential problems of early failure of MoM, often require revision surgery.7,8 Several studies reported soft tissue damages with adverse reaction to metal debris.9