Hip Replacement Devices

Metal Poisoning (Metallosis): One way in which hip replacement failure occurs is through metallosis. Because metal-on-metal hip implants are made of metal materials, metallic ions, and particles, often from the cobalt-chromium components, these devices can cause major medical issues for the patient. The shedding of cobalt and chromium in the bloodstream can be extremely harmful to the body. Even if you have metal poisoning (metallosis), it is not always painful, so you may not immediately feel it or know that it is happening. This is why it is important to ask your doctor to test your blood for cobalt and chromium levels if you have a metal-on-metal hip implant.

Damage of the Bone and Soft Tissue: Metal particles contaminate the tissue and bone surrounding the hip implant. The body can have a negative reaction to the metal particles. This reaction is known as adverse local tissue reaction (ALTR). When this happens the bone and soft tissue become damaged leading to other problems like implant loosening, device failure, and the need for revision surgery. In revision surgery, the surgeon replaces the existing hip implant with a new one. Revision surgery is an invasive process where the device is chiseled and hammered out of the surrounding bone and soft tissue.

Often surgeons open the patient’s hip to find that the surrounding soft tissue and/or bone has died and turned black due to necrosis caused by metallosis. Necrosis is defined as the premature death of cells in an organ or tissue due to disease, injury, or poor blood supply to the organ. One can only imagine the physical therapy and rehabilitation it would take to ever stand or walk again after needing to replace not only the hip joint, but also the bone and tissue surrounding it.

Pseudotumors: When soft tissue surrounding the hip implant reacts adversely, legions called pseudotumors can begin to form, but they are not always noticeable to the patient. A study on the prevalence of pseudotumors in metal-on-metal hip recipients published in The Journal of Arthroplasty showed that “68.6% [of patients evaluated] developed pseudotumor with 60.9% of the asymptomatic group developing pseudotumor.” The study also showed that elevated cobalt levels in the bloodstream were associated with larger pseudotumors.

Taper Lock Failure: The advantage of modular metal-on-metal hip implants is that they are customizable. Because there are a variety of neck sizes and angles, the surgeon has the flexibility to implant a neck that is the best fit for the patient. The disadvantage of modular metal-on-metal hip implants is that there is now one more place for metal to rub against metal, leading to corrosion of the metal components. Corrosion of this neck/stem or neck/head juncture also causes further release of metallic particles, negative reactions with the tissue, and often ultimately requires revision surgery. A May 2016 study in the Journal of Arthroplasty recently concluded that the high prevalence of severe taper corrosion may be related to elevated cobalt and chromium levels in the patient before revision surgery takes place.

Dissociation: Dissociation of the hip implant occurs when the components fall apart or disconnect. This type of complication occurs most in hip implants with large metal modular femoral heads. A large femoral head can place a large amount of stress on the taper-neck junction where the ball connects to the neck of the femoral stem. As a result, fretting and corrosion of the taper become more likely due to the design of the implant. If there is too much fretting and corrosion between the head and taper, the system falls apart, setting the patient up for a less than pleasant experience.

Acetabular Component Failure: An artificial acetabular system includes the cup and sometimes liner and ball of the artificial hip joint. The acetabular components fail for similar reasons as the taper lock. When adjacent pieces of the acetabular system are metal, the metals can rub together with movement, resulting in metal wear, corrosion, and metallosis. This metal-on-metal wear can damage the tissue and muscle around the hip device, causing the metal to be released into the body, including the bloodstream, and can cause loosening, or device repositioning, severe pain, and often a required removal surgery. This can also lead to a painful dislocation of the hip from its socket which is an experience most people would not wish on their worst enemy.