Cobalt Chromium Toxicity Resource Center

Modularity and Total Hip Arthroplasty





Total hip arthroplasty (THA), also known as a hip replacement, involves replacing the hip joint with a prosthetic implant. The surgical procedure is supposed to improve a patient’s quality of life and function. The outdated method of hip replacements was called monoblock. Modularity, implants with at least one modular junction, is employed for total hip arthroplasty. Modularity involves prosthetic hip implant components available in multiple segments, or parts, rather than a single piece (monoblock implants). The popularity of modular implants results from their variations in leg length and size, degree of offset, and version. Modular implants are meant to accommodate a patient’s unique anatomy.

Unfortunately, corrosion, fretting, and fatigue failure of the implants are progressively occurring in the dual modular implants. Evidence suggests a revision rate of 8% to 15% among metal on metal total hip arthroplasty. As a result, modular implants are now under close supervision.

Modularity evolved with the development of the modular femoral head-neck junction. The modular head-neck junction utilizes metallic alloys head alternatives, leg length and offset adjustment, and bearing replacement resulting from wear. Modularity of the head-neck junction occurs at a taper functioning by joining two rotating components in the hip implant. The two components of the taper are a trunnion and bore. The trunnion compresses the bore when it expands, interlocking the two parts, creating stability. A larger head-neck taper may lead to an increased dislocation rate because the prosthesis is impacted faster by different range of motions. However, smaller tapers may lead to increased junction fretting and corrosion.

The article differentiates between two design features: metaphyseal neck-stem modularity (DePuy), and modular neck or proximal modular stems (Stryker). A metaphyseal modular stem has an implant with a distal junction placed distal to the femoral neck. Whereas the proximal modular stem has the distal junction proximal to the femoral neck. The taper connections undergo various physiological stresses depending on the location of the modular junction. Both tapers have the potential to cause neck-body dissociation, elevated levels of metal debris, fretting, and corrosion. Corrosion is the main source for modular implant failure.

Many modular femoral stems are composed of a cobalt-chromium or titanium alloy. The metal implant can corrode from stresses and disruptions. Metal ions are released secondary to the corrosion resulting in elevated serum metal ion levels. Corrosion and fretting cause loss of mechanical integrity of the implant, local tissue infiltration, and adverse local tissue reactions (ALTR). There are multiple sources for head-neck taper corrosion including crevice, fretting, and galvanic. Fluid can enter a gap between the trunnion and bore causing crevice taper corrosion. Fretting corrosion can occur from movement of the head relative to the neck; this is the main source for failure at the modular junctions. Crevice corrosion is reported in 35% to 40% of mixed-metal tapers, and 9% to 28% in single- alloy tapers.

Corrosion is time-sensitive and accelerated with mechanical stresses. Increased local and systemic metal particle exposure is connected to increased corrosion at the taper. Even in properly working metal on metal hip implants, cobalt chromium levels are roughly five fold higher than in patients with other hips. Head-neck interface corrosion results in inflammatory responses, local osteolysis and synovitis. Subsequently, metal particles were found in various organs. Corrosion byproducts stem from adversely affected metals at the taper junctions. Chromium phosphate is one such byproduct associated with femoral head-neck junction corrosion and substantial inflammation. This inflammation causes bone resorption and osteolytic reactions (bone loss). Corrosion and metallosis results in elevated cobalt levels in synovial fluid; this is linked to thyroiditis, auditory disturbances, and granulomatous lesions. Arthroprosthetic cobaltism, increased cobalt levels, is connected to systemic symptoms from malfunctioning hip implants.

Source: Modularity of the Femoral Component in Total Hip Arthroplasty 


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