Total hip replacement (THR) surgery is one of the most successful orthopedic procedures available today, with most implants functioning well for decades. When complications arise, the common fear of the body “rejecting” the implant is often mischaracterized. True immunological rejection is rare with modern materials. Issues that lead to failure are almost always related to mechanical breakdown, biological reaction to wear debris, or infection.
Defining Device Failure
The term “rejection” suggests the immune system is attacking the metallic or plastic components, similar to an organ transplant. Device failure instead falls into three pathological categories: aseptic, infectious, and biologic.
Aseptic failure, or mechanical loosening, is the most common reason for revision surgery. This occurs when the bond between the implant and the surrounding bone is compromised over time.
Infectious failure, known as periprosthetic joint infection (PJI), is caused by bacteria colonizing the implant surface. The third category is biologic failure, which involves an adverse local tissue reaction (ALTR) to microscopic wear particles from the implant’s bearing surfaces.
Identifying Symptoms of a Failing Hip Replacement
The primary complaint of a failing hip replacement is typically the return of pain, often felt deep in the groin, thigh, or buttock area. Pain related to loosening occurs during weight-bearing activities like walking or when moving from sitting to standing, sometimes radiating down the leg.
Mechanical instability may be noticed as a feeling of the joint “giving way” or a noticeable difference in leg length. Patients may also report new, audible mechanical sounds such as clicking, popping, or grinding within the joint. When infection is the cause, systemic symptoms may arise, including persistent swelling, warmth, redness, drainage from the surgical site, or the onset of fever and chills.
Understanding the Causes of Failure
Aseptic Loosening
Aseptic loosening is largely driven by a process called osteolysis, which is bone loss caused by the body’s response to wear particles. The constant motion of the joint causes tiny fragments of polyethylene plastic or metal to shed into the surrounding tissue. Macrophages, a type of immune cell, attempt to clear these foreign particles, but this process triggers a chronic inflammatory response that dissolves the surrounding bone. This loss of bone stock eventually causes the implant components to become loose, leading to pain and instability.
Periprosthetic Joint Infection (PJI)
Periprosthetic joint infection (PJI) is a complication where bacteria adhere to the surface of the implant components. Once attached, the bacteria secrete a protective layer of extracellular matrix, forming a biofilm. This biofilm shields the bacteria from both the body’s natural immune defenses and systemic antibiotics, making eradication difficult without removing the infected hardware. PJI can occur early after surgery from contamination or years later from bacteria traveling through the bloodstream from a distant infection site.
Biologic Failure (ALTR)
Biologic failure, specifically the adverse local tissue reaction (ALTR), is most commonly associated with older metal-on-metal hip implants. The friction between the two metal surfaces releases cobalt and chromium ions into the surrounding soft tissue. This causes a hypersensitivity or inflammatory reaction, leading to the formation of a solid or cystic mass called a pseudotumor. The tissue damage from this reaction can cause significant destruction of the muscle and bone around the implant, necessitating revision.
Medical Solutions for a Failing Implant
The diagnostic process begins with a clinical examination and laboratory tests to distinguish between the causes of failure. Blood tests are performed to measure inflammatory markers such as the Erythrocyte Sedimentation Rate (ESR) and C-Reactive Protein (CRP). Elevated levels of these markers suggest an active inflammatory process, strongly indicating a possible infection.
Imaging studies start with standard radiographs to look for signs of mechanical loosening, such as radiolucent lines or component migration. If an adverse tissue reaction is suspected, specialized imaging like a Metal Artifact Reduction Sequence (MARS) MRI is often used to visualize the soft tissues and detect any pseudotumor formation. The most definitive step for diagnosing PJI is a hip joint aspiration, where a needle guided by imaging extracts fluid for culture and cell count analysis.
The definitive treatment for a failing hip replacement is revision surgery, which involves removing the failed components and implanting new ones. For mechanical loosening or ALTR, a single-stage revision is typically performed, where the surgeon replaces the old implant in one operation. If a periprosthetic joint infection is confirmed, a two-stage revision is often required, involving a temporary antibiotic-loaded spacer placed during the first surgery, followed by the insertion of the new, permanent implant months later.