History and Physical Examination
The clinical presentation of significant wear in total hip arthroplasty (THA) is related more to the sequelae of wear debris than to wear itself. Osteolysis is the culprit and is often asymptomatic. Patients may present late with implant loosening secondary to osteolysis or even periprosthetic fracture. Pain related to loosening may be a presenting sign, especially groin or thigh pain stemming from acetabular or femoral loosening, respectively.
That wear and osteolysis may be asymptomatic underscores the importance of follow-up radiographic evaluation for wear analysis and screening for osteolysis. Furthermore, radiography does not fully depict the osteolysis revealed operatively, which again underscores the importance of early detection.
Modes of Wear
Four distinct wear modes have been applied to prosthetic joint wear, as follows:
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Mode 1 wear results from motion that occurs between one primary bearing surface and another—for example, the wear from the femoral prosthetic head against the acetabular liner [21] (see the images below)
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Mode 2 wear occurs when a primary bearing surface articulates with a nonbearing surface that is not intended—for example, a prosthetic femoral head penetrating through a polyethylene bearing and articulating with the metallic acetabular shell
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Mode 3 wear occurs from entrapped abrasive particles between primary bearing surfaces; these particles may include fragments of polymethylmethacrylate (PMMA) cement or bone or polyethylene or metallic particulates; this is also known as third-body wear
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Mode 4 wear occurs from motion at two secondary or nonbearing surfaces—for example, impingement of the prosthetic femoral neck onto the rim of the acetabular component or fretting at a Morse taper between the prosthetic femoral neck and head; an emerging type of mode 4 wear occurs between the acetabular shell and the backside of a polyethylene liner insert, also referred to as backside wear
Acetabular wear in total hip arthroplasty. Intraoperative photograph of retrieved specimens at the time of revision arthroplasty. The specimens demonstrate both mode 1 and mode 2 wear.
Acetabular wear in total hip arthroplasty. Intraoperative photograph of retrieved specimens at the time of revision arthroplasty. The specimens demonstrate both mode 1 and mode 2 wear.
The greatest source of wear debris continues to be from the bearing surface (mode 1 wear). The wear of the hard surface of the femoral head is negligible in this wear mode. [6] Therefore, the continued source of the wear and debris problem is polyethylene in the standard metal-on-polyethylene articulation. [6]
Plastic deformation or creep should be distinguished from wear. Creep is plastic deformation of the acetabular liner due to loading without the production of wear debris or particles. This has been termed bedding in or running in by several authors. The rate of creep decreases over time and becomes negligible after 12-18 months.
The wear resistance of polyethylene is affected by sterilization techniques. Until relatively recently, the industry standard for sterilization was gamma irradiation in air. [22] Gamma irradiation breaks molecular bonds in the long polyethylene chains, giving rise to free radicals. In an oxygen environment, oxygen combines with these free radicals, leading to subsurface oxidation. As oxidation increases, so does fatigue cracking and delamination.
Components that have been on the shelf for less than 1 year before implantation have shown decreased in-vivo oxidation and better in-vivo performance than those with longer shelf lives. Laboratory wear studies have shown increased wear rates in polyethylene gamma irradiated in air compared with nonirradiated material.
When free radicals are formed, competing mechanisms exist between oxidation and cross-linking. Cross-linking appears to improve resistance to wear. In general, greater oxidation leads to less cross-linking; the reverse is also true. Techniques for controlled cross-linking have included the administration of chemicals (peroxide), variable gamma irradiation, and electron beam irradiation. Clinical and laboratory studies have shown substantial reduction in wear with cross-linked polyethylene. Polyethylene contact stresses are a function of the thickness, load, and contact area. A minimum thickness of 6 mm is recommended in conforming articulations such those found in hip replacements.
In an articulation that includes a polyethylene bearing surface, the other bearing surface is commonly referred to as the countersurface. The surface characteristics of the countersurface are determined by the material properties and the manufacturing process. The microtopography determines the surface roughness. Increased roughness of the countersurface may rapidly accelerate abrasive wear of the polyethylene. Experimental models of three times increased roughness have led to 10 times greater polyethylene wear. Third-body wear may further exacerbate this problem by increasing countersurface roughness with scratches.
In experimental wear studies, scratches only 2 µm deep increased polyethylene wear by 30- to 70-fold. The susceptibility to scratching is dependent on the hardness of the countersurface. Commonly used materials, in order of hardness from least to greatest, are titanium, stainless steel, cobalt chrome, and ceramics.
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Acetabular wear in total hip arthroplasty. Cementless total hip arthroplasty in a 60-year-old woman with osteoarthritis.
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Acetabular wear in total hip arthroplasty. Woman who underwent cementless total hip arthroplasty for osteoarthritis at age 60 years, 6 years following the index arthroplasty. This image demonstrates eccentric polyethylene wear, osteolysis, and a protrusio defect. The stem appears to be well fixed.
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Acetabular wear in total hip arthroplasty. Lateral radiograph of the left hip of woman who underwent cementless total hip arthroplasty for osteoarthritis at age 60 years, 6 years following the index arthroplasty. She has eccentric polyethylene wear, osteolysis, and a protrusio defect. The stem is well fixed.
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Acetabular wear in total hip arthroplasty. Intraoperative photograph of retrieved specimens at the time of revision arthroplasty. The specimens demonstrate both mode 1 and mode 2 wear.
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Acetabular wear in total hip arthroplasty. Intraoperative photograph of retrieved specimens at the time of revision arthroplasty. The specimens demonstrate both mode 1 and mode 2 wear.
