Metallosis is a syndrome associated with metal joint implants, where friction between two metal surfaces releases metal ions (electrically charged particles) into the joint, and these trigger an inflammatory response.
Within the knee this condition is usually related to a previous knee replacement (usually a total knee replacement, but also sometimes in a partial/unicompartmental1 knee replacement) in which the polyethylene components have worn down and inadvertently allowed metal components to come into contact where they normally would not. In particular it is often the polyethylene part of the metal-backed patella that is at fault, where the polyethylene breaks down and allows the back of the patellar component to make friction contact with the metal femoral component2. Metal-backed patellar components are no longer in frequent use but of course there are still many patients who had had this type of implant done in previous years. Mechanisms of failure of the polyethylene include polyethylene wear, fracture of the polyethylene or the underlying bone, and dissociation of the polyethylene from its metal backing.
The condition is more frequently recognised in hip replacements. Although rare in the knee, metallosis is a diagnosis must be thought of when investigating a patient whose knee is not settling after a knee replacement
Components of a knee replacement
Knee replacements are not all the same. Different manufacturers have their own designs, materials and components, and the surgeon will make his choice among their products based on his/her assessment of the patient's needs and his/her own clinical experience. However, most knee replacements contain -
- a metal femoral component
- a metal tibial component
- a 'bearing' between them of high density polyethylene, which may be mobile or attached to the tibial component
- a patellar component of polyethylene or metal-backed polyethylene
In uncemented components there may be coatings of various kinds at the back of the components to encourage bone adhesion.
The metals from which the implant is manufactured may include3 -
- titanium and titanium alloys
- cobalt and cobalt-chrome alloys. Cobalt chrome alloys contain a nickel component, which is very relevant in people who are allergic to nickel.
- zirconium and zirconium alloys
- ceramics including oxidised zirconium
Hip implants may be metal-on-polyethylene, metal-on-metal, metal-on-ceramic or ceramic-on ceramic, but most knee implants are metal-on-polyethylene. The issue of metal-on-metal designs is current and controversial4. Several hip implant manufacturers have recalled their metal-on-metal implants5, and patients who have already received such implants are carefully monitored and may be subjected to additional surgery to replace them
Local and systemic effects of metal toxicity
There seems to be two types of problems related to the situation we are discussing. On the one hand the polyethylene wear leaves real particulate debris of polyethylene, which is taken up by the lymph cells and transported to the lymph glands. As the body tries to rid itself of these particles a process called 'osteolysis' may occur, where bone is broken down and weakened in areas around the implant. This can be seen on X-ray or CT imaging.
On the other hand, the breakdown of the polyethylene may distort the mechanics of the implant and allow metal to come into contact with metal. The resultant friction may release charged metal ions, which are not the same as the particulate matter of the polyethylene, although sometimes, but rarely, there may be actual metal particles. The presence of metal ions can be inferred from X-ray changes which we will discuss later.
The breakdown of the polyethylene may affect the mechanics of the implant, the osteolysis may allow loosening of the implant, the debris may bring other inflammatory-response cells into the area which will be releasing chemicals of their own. The metal ions are taken up by the synovium (the joint lining), which may become abnormal - bloated in patches and filled up with grey-black staining. Uptake by histiocytes1 may affect regional lymph glands, and metal ions in the bloodstream may cause metal toxicity.
Some patients may have a frank allergy to certain metals, eg nickel,cobalt and chrome while in other patients it may be more a matter of metal toxicity. Skin patch testing may differentiate between the metals, although it may be necessary to do blood tests and histology on the lymph nodes. Potentially the most harmful components are cobalt from cobalt-chromium alloy, nickel from stainless steel, and vanadium from titanium alloy7.
X-rays may show excessive fluid within the joint, osteolysis around the implant. The joint lining and suprapatellar pouch may show a fine linear radio-opacity called a 'metal-line. Other radiographic signs called 'clouds' and 'bubbles'2 may be indicative of the metal within the synovium as well as the florid synovial outgrowth, seen during surgery as areas of joint lining (synovium) heaped up into what look like soft black lumps.
Romesburg JW, Wasserman PL, Schoppe CH. Metallosis and Metal-Induced Synovitis Following Total Knee Arthroplasty: Review of Radiographic and CT Findings. J Radiol Case Rep. 2010;4(9):7-17. doi: 10.3941/jrcr.v4i9.423. Epub 2010 Sep 1.
Knee Replacement Implant Materials.
Metal Ions Questions & Answers
Rae T. The toxicity of metals used in orthopaedic prostheses. An experimental study using cultured human synovial fibroblasts. J Bone Joint Surg Br. 1981;63-B(3):435-40.
Metal Toxicology Expert Part 5: What studies have been conducted regarding cobalt exposure?
Thakur RR, Ast MP, McGraw M, Bostrom MP, Rodriguez JA, Parks ML. Severe persistent synovitis after cobalt-chromium total knee arthroplasty requiring revision. Orthopedics. 2013 Apr;36(4):e520-4. doi: 10.3928/01477447-20130327-34.
Bergschmidt P, Bader R, Ganzer D, Hauzeur C, Lohmann C, Rüther W, Tigani D, Rani N, Prats FL, Zorzi C, Madonna V, Rigotti S, Benazzo F, Rossi SM, Kundt G, Bloch HR, Mittelmeier W. Ceramic femoral components in total knee arthroplasty - two year follow-up results of an international prospective multi-centre study. Open Orthop J. 2012;6:172-8. doi: 10.2174/1874325001206010172. Epub 2012 Apr 20.