The awareness of the impact of chronic inflammation around orthopedic implants in the medical community and the public has grown due to the increase in the incidence of hip and knee osteoarthritis after World War II, the use of orthopedic implants worldwide, types of implant materials, manufacturing methods and post-processing techniques, patients’ and providers’ desire for the best possible and longest-lasting outcomes after implant surgery, and use of digital media technology to rapidly and widely disseminate the issues involving implant surgery. 

Many different materials are used in orthopaedic implants: metal alloys, metal coatings, polymers, ceramics, 3D printed titanium alloy, bone inductive materials, and anti-bacterial agents. It is no surprise that the continuous release of an implant derived material causes periprosthetic chronic inflammation; however, it is of great importance to understand the mechanisms of the complex relationship between particulate material and host response causing chronic inflammation to prevent the occurrence of adverse events.

The expansion of implant science as a field has increased the workload for regulators. In Europe, this is particularly difficult because the new MDR regulations have already strained the relationship between implant manufacturers and regulators. These groups benefit from improved outcome assessment of implants because of mature joint registries in search of long-term true success and early failure signals. However, registries are susceptible to data quality issues for linking primary to revision operations, use failure as their only endpoint, and for most registries the reason for revision is often poorly understood because culture and histopathology results are not known, and closed reduction of dislocations are not recorded.

This series of articles provides an update on the state-of-the-art in the fields of chronic inflammation, the human inflammatory reaction to implant-derived material, and the terminology and definitions used to describe peri-implant inflammation on bone and soft tissues. 

Dr. Ina Lackner reviews and explains the latest implant science on the human mechanical and biological and worn bearings. The unravelling of the biological response to polyethylene is a long and fascinating story. The loosening of cemented femoral stems in the 1970s was wrongly attributed to “cement disease” rather than polyethylene particle-induced inflammation. However, this helped advance uncemented implant technology. In the 1990s, the use of metal-on metal (MoM) bearings was based on the need to avoid polyethylene induced osteolysis. However, this resulted in adverse reaction to metal debris which incorporates both soft tissue and bone inflammation and sometimes greater osteolysis than produced by polyethylene. In the 2000s there were many articles on the problem of taper junction corrosion of large diameter MoM hips which led to their prohibition in 2012. This was followed in the 2010s with tribocorrosion problems with dual modular neck THA implants with CoCrMo neck and Ti alloy femoral stems.

Prof. Catherine van der Straeten explains how our understanding of the risk factors involved in chronic inflammation were accelerated by the COVID-19 pandemic because morbidity and mortality was driven by the host response to the virus through cytokine release. We now better understand how chronic inflammation affects the musculoskeletal system, its role in joint inflammation and osteoarthritis, and risk factors for inflammation in general, including peri-implant inflammation.

For example, we now understand that adipose tissue acts as an endocrine organ with a source of cytokines such as adipokines that can increase inflammation anywhere in the body, including synovial joints. This may explain why synovial inflammation is often found in the early stages of osteoarthritis, challenging the long-held belief that the cartilage degradation precedes joint inflammation.

Adding to this rich discussion, I present three case reports focusing on adverse reactions in hip replacements involving different material pairings: metal-on-metal (MoM), metal-on-polyethylene (MoP), and ceramic-on-ceramic (CoC). These cases illustrate the diagnostic complexities and highlight the necessity for accurate assessment and management of adverse reactions. The third case, involving ceramic-on-ceramic implants, particularly emphasizes the importance of differential diagnosis in cases initially suspected of ARMD but ultimately attributed to other causes. Each case contributes to our understanding of adverse reaction, reinforcing the importance of precise definitions and thorough clinical evaluations. The terms pseudotumour, ALTR, and metallosis often overlap in clinical presentations, making clear and accurate communication essential for managing patient expectations and treatment outcomes.

Through these insights, we aim to enhance the orthopedic community’s understanding of implant related reactions, fostering better patient outcomes through improved knowledge and collaborative regulatory practices.

Happy Reading!

Alister Hart