Wearing out of Total Hip Replacement
The hip replacement components may wear out due to a number of different reasons besides natural wear and tear. The implants are more likely to wear out in men and in patients aged 50 or less. The implants wearing out in younger patients and men, in general, may be linked to higher activity levels.
Similarly, patients who may be obese or overweight, or more physically active also are at an increased risk of hip implants being worn out. Other factors such as implant positioning during the surgery may also be linked with increased wear and tear of the implant.
Hip replacement implants consist of a prosthetic head of the femur made of metal alloy/or ceramic placed over a metal alloy femoral stem. The stem is usually press-fitted inside the femoral canal so there may be bone in-growth at the implant-bone interface.
The acetabulum prosthetic shell is similarly made of metal alloy parts and is press-fitted in the acetabulum shell. The shell is additionally secured with screws so the implant stays in place as the bone in-growth occurs.
The shell is covered inside with a highly cross-linked polyethylene plastic that allows smooth gliding of the prosthetic head inside the socket. The fixation of the prosthetic stem may also be done by the use of bone cement. However, the majority of the hip replacements done today are uncemented.
The aseptic loosening means loosening of the implants inside the bone without any evidence of infection. The loosening occurs as a result of the formation of particle debris. The debris may form as a result of:
- Adhesion wear occurs as the implant surface microscopically attaches to the polyethylene spacer and pulls off the debris.
- Microscopic abrasion of the metal prosthesis with the polyethylene spacer may lead to the formation of debris.
- As the debris collects in the space between the polyethylene and the metal surface, the debris acts as a third body that may lead to the formation of even more debris.
The debris mainly consists of polyethylene (PE) but may also consist of cobalt-chromium metal ions and bone cement. While metal on polyethylene has the maximum rates of formation of debris, ceramic on ceramic has the least amount of debris formation. However, ceramic on ceramic and metal on metal implants are associated with other side effects, therefore, are not commonly used in hip replacement surgery.
The debris attracts inflammatory cells such as macrophages that try to clear the debris. The macrophages in turn get activated and may lead to the release of inflammatory mediators. The inflammation may cause local swelling due to an increased influx of fluids.
The inflammation leads to activation of cells known as osteoclast that result in cutting of the bone surrounding the implant. The cutting/decrease of the bone surrounding the implant leads to micromotion of the implant. The micromotion of the implant leads to more particulate debris formation. Ultimately, the implant loosens and may present with symptoms suggestive of aseptic loosening.
Patients with aseptic loosening of hip replacement often complain of instability and pain. The pain is usually located in the groin region and is aggravated with weight-bearing. If the prosthetic femoral stem subsides inside the canal, patients may complain of instability with motion which may lead to the partial popping of the head out of the socket (subluxation) or a complete popping of the head out of the socket (dislocation).
The diagnosis of aseptic loosening is made after a physical examination and radiological studies that suggest the aseptic loosening of the implants. The physical examination may reveal a clunking motion, instability, and may elicit tenderness.
The radiological studies in the form of an X-ray may reveal thinning of the bone surrounding the implant and in severe cases, the implants may move out of place. Blood investigations and joint fluid aspiration studies may be done to rule out any infection of the prosthetic joint.
Aseptic loosening of the hip joint is managed surgically by a revision hip replacement. The revision surgery may involve extracting and replacing all the components of a total hip or may involve the replacement of only the loose stem or the acetabulum shell. The revision hip replacement surgery is more complex than primary surgery and is best performed by a surgeon with experience in revision surgeries.