Metal-on-metal hip resurfacing arthroplasty: a review of periprosthetic biological reactions.
Mabilleau G, Kwon YM, Pandit H, Murray DW, Sabokbar A.

Acta Orthop. 2008 Dec;79(6):734-47.

Metal- Induced Immune Response Section (in my words and in short posts as this is rather complicated to follow but interesting and will give you a sense of how this metal causes issues in the body.) It has been documented that the size, shape , number and nature of the particles released from the surfaces of the hip prosthesis are responsible for the type and extent of the biological response.

• macrophages -white blood cells. Their role is to phagocytose (engulf and then digest) cellular debris and pathogens.  They also stimulate lymphocytes and other immune cells to respond to pathogens.
• fibroblasts- A fibroblast is a type of cell that synthesizes the extracellular matrix and collagen,^[1]  and plays a critical role in wound healing
• occasionally, lymphocytes-A lymphocyte is a type of white blood cell. White blood cells, or leukocytes (also spelled “leucocytes,” “leuco-” being Greek for white), are cells of the immune system involved in defending the body against both infectious disease and foreign materials.

Chromium and cobalt (contained in the metal on metal hips) have an effect on cells that is different to that of polyethylene and most other biomaterial.

The metal particles are often phagocytosed (engulfed and then digested) by macrophages (white cells that digest debris-like metal.)

Once phagocytosed, Chromium and cobalt particles can be toxic and rapidly kill the cells.  This is likely because they corrode quickly within the cells due to the acidic environment in the cell and then release ions in high concentrations within the cells leading to toxicity.

The cells then lyse (breaking down of the cell), releasing the particles and cell contents to cause further damage.

There is recent evidence that the metal particles, which are orders of magnitude smaller than polyethylene particles, induce an acquired or antigen-specific immune response (involves production of an antibody by the immune system) driven by T lymphocytes.

(T cells (Thymus cells) and B cells (bone cells) are the major cellular components of the adaptive immune response. T cells are involved in cell-mediated immunity whereas B cells are primarily responsible for humoral immunity (relating to antibodies). The function of T cells and B cells is to recognize specific “non-self” antigens, during a process known as antigen presentation.

Once they have identified an invader, the cells generate specific responses that are tailored to maximally eliminate specific pathogens or pathogen infected cells. B cells respond to pathogens by producing large quantities of antibodies which then neutralize foreign objects like bacteria and viruses.

In response to pathogens some T cells, called T helper cells, produce cytokines that direct the immune response while other T cells, called cytotoxic T cells, produce toxic granules that contain powerful enzymes which induce the death of pathogen infected cells. Following activation, B cells and T cells leave a lasting legacy of the antigens they have encountered, in the form of memory cells. Throughout the lifetime of an animal these memory cells will “remember” each specific pathogen encountered, and are able to mount a strong and rapid response if the pathogen is detected again.)

It is likely that the continuous release of metal particles from the meal-on-metal hips and their local accumulation within the joint space, facilitates sensitization with a consequent cell-mediated immunological response.

That response in some patients may lead to two things:

• local reaction around the joint (aka osteolysis)
• [Systemic reaction?] whereby the metal is transported to other areas of the body other than the hip joint.

The article reference is provided for your reference as I am not a doctor, nor a scientist.    In order to follow this stuff, best to read this in sequence.  If you jump in on the 4th part without having read the 3rd part, you are likely to get lost! Many more recaps to come.