It is commonly known that under in vivo conditions, metal implants release ions into the surrounding tissues from where they can penetrate other organs via the bloodstream (1, 2). High levels of metal ions cause numerous adverse physiological effects, including cytotoxicity, genotoxicity, carcinogenicity and hypersensitivity to metals(3-8). Previous studies have shown that tissues surrounding hip implants contain elevated levels of cobalt, chromium, nickel, titanium, molybdenum, aluminium and vanadium ions (9-11), and that these metal ions influence the bone metabolism, immune system, development of delayed hypersensitivity and the pathophysiology of aseptic loosening of implants in patients after hip replacement (12). The metal parts of endoprostheses are subject to direct biocorrosion caused by osteoclast activity, which leads to the release of substantial amounts of wear particles and metal ions into the surrounding environment (13-16). The released metal ions, in return, stimulate the immune system and bone metabolism by means of direct and indirect reactions, leading to increased osteolytic activity within the bone around the implant, and thus to its aseptic loosening (17-21). According to Wang (22), metallic particles from titanium alloy spinal implants activated a macrophage cellular response in the spinal tissues similar to that observed in joint prostheses. Inducted an inflammatory, foreign-body reaction in the soft tissue structures adjacent to pain. Cook (23) reported that late operative site pain is most likely caused by local soft tissue reaction to the implants, and late operative site pain of no apparent cause after posterior instrumentation of scoliosis is a distinct clinical entity and is relieved by implant removal in most patients.
In our study no signs suggesting an allergic reaction or infection were found on histological examination. Evaluating the concentrations of metal ions released into the synovial membrane and synovial fluid from stainless steel, cobalt-chromium alloy and titanium alloy endoprostheses fixed to the bone by PMMA, Brien notes that in all the groups of stable hip implants, the concentrations of metal ions in synovial fluid were similar. Tissues from stable and aseptically loose implants were examined. However, the phenomenon of aseptic loosening of the hip implant coexisted with an elevated concentration of metal ions in the synovial fluid and membrane, and this increase was disproportionally high in case of implants made of titanium alloys compared to implants made from steel and Co-Cr alloy(24). Similarly, Huo reports that in a group of patients suffering from aseptic loosening of a cemented hip implant, the concentrations of Co and Cr were four times higher, and the concentration of Ti was on average forty-two times higher, in the direct vicinity of a loose implant than in the joint capsule (25).
Our own evaluation revealed that metal ions, the construction materials of the studied uncemented hip implants, were released into the surrounding environment in significant amounts ranging from several hundred to several hundred thousand µg / kg.
- In the tissues around the cup, the highest average concentrations, reaching several hundred thousand µg / kg, were observed for Ti and Mo ions; lower average concentrations of several thousand µg / kg were observed for Al and Ni ions, whereas the lowest concentrations, being several hundred µg / kg, were observed for Cr and Co ions.
- In the tissues around the stem, Ti and Al ions were found in the highest concentrations, ranging from several to several hundred thousand µg / kg, while the lowest were found for Mo, Ni, Cr and Co, reaching levels of several hundred µg / kg.
The tissues surrounding the acetabular cups contained significantly higher concentrations of some metal ions than those surrounding the stems. Titanium concentration was typically a hundredfold higher, nickel concentration was six times higher and chromium concentration was four times higher, while the levels of aluminum, cobalt and molybdenum were only slightly higher. To decrease possibility of contamination of tissue with metal debris detached for implants during revisions, the samples were taken before implant removing, however we are aware of such possibility. Unfortunately, no reference or limit values referring to concentration of metal ions in tissues surrounding a hip implant can be found. Available reports which deal with similar cases are dedicated exclusively to cemented hip endoprostheses.
Limitations of this study include moderately limited cases and the lack of a control group. However, it was assumed that metal particles were continuously generated from micromotion or macromotion between the implant junctions for the duration of having endoprosthesis.