Acetabular Wear in Total Hip Arthroplasty Treatment & Management

Complete preoperative medical evaluation and diagnostic workup should precede any surgical intervention.

Experimental animal models show early promise toward future considerations of medical therapy in treating and preventing osteolysis. However, at the current time, surgical intervention by means of revision hip arthroplasty remains the mainstay of treatment.

The surgical treatment of the consequences of acetabular wear in total hip arthroplasty revolves around the tenets of revision hip arthroplasty. Early surgical options may include polyethylene liner exchange with possible bone grafting of osteolytic lesions in the setting of well-fixed components. However, when more significant osteolysis occurs, especially when associated with implant loosening, revision hip arthroplasty may be warranted.

Preoperative planning is essential in dealing with wear-related issues. Know the specifics regarding the previous implants, ie, manufacturer and sizes. This is especially helpful in cases of liner exchange with bone grafting of osteolytic lesions. Preoperative templating is a necessary step in the development of a reconstructive plan. Component selection is typically a large, hemispherical, cementless acetabular component with adjunctive screw fixation and an extensively coated cementless stem for femoral reconstruction. Familiarity with bulk grafting techniques, acetabular cage reconstruction, and impaction grafting may be necessary.

For excellent patient education resources, visit eMedicine's Foot, Ankle, Knee, and Hip Center and Arthritis Center. Also, see eMedicine's patient education article Total Hip Replacement.

Complications related to revision hip arthroplasty may include infection, dislocation, leg-length inequality, neurovascular injuries, thromboembolism, intraoperative fractures, and wound healing problems.

The squeaking hip

Recent studies have focused on the occurrence of audible squeaking following hip arthroplasty. In one study, squeaking (ie, squeaking, clicking, or grating sound) occurred in 9 of 43 (20.9%) ceramic-on-ceramic noncemented total hip arthroplasties. In these cases, a short neck length of the head seemed to be a risk factor for squeaking. Other studies have also focused on the occurrence and potential causes of squeaking hip, such as prosthetic design.^{[22, 23, 24, 25, 26]} Transient squeaking was noted in patients who received metal-on-metal total hip arthroplasties, with the highest incidence occurring in hip replacements with large-clearance bearings. The friction factor was also found to be highest with these bearings. The lubricating film was lowest in these bearings.^[27]

The outcome and prognosis are related to the surgical procedure performed and mirror experiences with revision hip arthroplasty. Prognosis may be further influenced by implant materials and designs and by potential surgical complications.

The development of wear debris and the biologic response to this debris have fueled the search of alternative bearing surfaces with the hope of reducing the amount of wear debris produced and to reduce the immunogenicity of the particles. If linear wear is less than 0.10 mm/y, then osteolysis is rare. The converse is also true; if linear wear is greater than 0.20 mm/y, then osteolysis is common.

In general, for standard total hip articulations with cobalt chrome on conventional polyethylene, the wear rate is 75-150 µm/y. Although some authors have reported 50-75% less wear with ceramic on conventional polyethylene compared with cobalt chrome-on-polyethylene bearings, others have reported similar or greater in vivo wear rates with ceramic on polyethylene compared with cobalt chrome on polyethylene, especially in the presence of third bodies. Metal-on-metal articulations have a wear rate averaging 2.5 µm/y. Ceramic articulations with alumina on alumina wear at a rate of 0.5-1.5 µm/y.

Metal on metal

Metal-on-metal bearings were employed early in the development of total hip arthroplasty but were abandoned largely because of the success of the Charnley hip and the high frictional torque encountered in early metal-on-metal designs.^{[28, 29, 30]} Early designs were complicated by high frictional torque from inadequate head-cup clearances, which limited lubrication and contributed to implant seizing and subsequent implant loosening. Early designs such as the McKee-Farrar design were also flawed, with fixation problems and failures primarily from acetabular loosening.

Despite this, Jacobsson reported the 20-year survival rate of metal-on-metal McKee-Farrar THA to be 77%, which was comparable to the 73% 20-year survival rate of the Charnley hip.^[31] In addition, fewer osteolytic lesions occurred in patients with metal-on-metal THAs than in those with Charnley THAs. Furthermore, at the time of revision, the metal-on-metal articulation appeared to have a more benign tissue reaction.

The American experience with metal-on-metal articulations is largely reflected by a report by Dorr, demonstrating no osteolysis at 9 years of follow-up.^[7]

Metal ion production has been a continued concern with metal-on-metal articulations. Jacobs and colleagues demonstrated increased cobalt and chromium levels in both blood and urine in patients with metal-on-metal articulations, raising concerns about potential toxicity and carcinogenicity.^[32] However, Visuri showed no significant increased risk of malignancies 15 years following metal-on-metal articulations.^[33]

Passuti and Trevor studied 2614 metal-on-metal THAs with a mean follow-up of 7 years and identified only 5 cases of unusual osteolysis and 10 of impingement, with no specific severe complications resulting from cobalt or chromium release.^[30]

Moroni et al studied patients who received metal-on-metal hip resurfacing (average head diameter 48 mm) and patients who received 28-mm metal-on-metal THA and found no metal ion level differences between the 2 groups despite the different size diameters of the bearing surfaces.^[29]

Ceramic on ceramic

Ceramic-on-ceramic or alumina-on-alumina bearings have several advantages. Ceramics are quite hard and are scratch resistant. These bearings have a very low coefficient of friction and are hydrophilic, with improved lubrication. Ceramics are estimated to have 150-300 times less linear wear and 1700 times less volumetric wear than conventional metal-on-polyethylene articulations. Ceramic wear debris also appears to be relatively inert compared with polyethylene wear debris.^[34]

Disadvantages of ceramics include a history of problems and expense. Previous experiences with ceramics from the 1970s and 1980s were complicated by neck-socket impingement, ceramic fractures, isolated accelerated wear from chipping, and implant loosening. Both impingement and implant loosening were largely design problems and unrelated to the bearing surface. The incidence of ceramic fracture during this period was 3.5%, primarily due to manufacturing flaws including large grain size, inclusions/grain boundaries, lack of testing standards, and poor tolerances for taper designs. More recent processing techniques have eliminated these problems, including hot isostatic pressing, dense fine grain alumina, gain size less than 2 µm, fewer grain boundaries, and fewer inclusions. Decreased grain size increases burst strength.

Newer taper designs include high tolerances for tapers, eliminating stress concentrations. The ceramic fracture incidence with modern designs in fully seated inserts is 1 in 25,000. In the United States, ceramic-on-ceramic implants are governed by a Food and Drug Administration (FDA)–sponsored trial, and to date, no ceramic failures have occurred in this group.

Highly cross-linked polyethylene

Most polyethylene used for the last 20 years has been partially cross-linked.^{[35, 36, 37]} Cross-linking arises as an inadvertent byproduct of sterilization with gamma irradiation. The usual dose for sterilization is 2.5-4 millirad (mrad). McKellop showed that wear resistance increases with increasing radiation doses.^[8] However, the resistance is optimized at 9.5-10 mrad of irradiation. Cross-linking occurs when a carbon-carbon bond forms between forms between carbon molecules in adjacent chains on parts of the same chain. In conventional polyethylene, the surface polyethylene molecules become oriented in the path of primary motion. However, when cross motion occurs, these molecules may be fractured off. Cross-linking inhibits chain slippage and makes the polyethylene resistant.

In vitro studies using hip stimulators have shown virtually no wear even despite use of a 46-mm head. In vivo studies by Oonishi and by Wroblewski have shown very low wear rates 0.02-0.06 mm/y.^{[14, 38]} These studies had small numbers of patients and used all-polyethylene cemented cups. Subsequent clinical studies have been promising, but long-term follow-up is required.^{[39, 40, 9]} Other concerns revolve around how this dose of radiation affects the material properties of polyethylene. Irradiation at these doses appears to decrease the tensile strength and increase the stiffness or make the polyethylene more brittle.

The fundamental tenets for a durable, long-term, successful total hip arthroplasty include the development and maintenance of a solid bone–implant interface, which provides both mechanical stability and a barrier to joint fluid and particulate debris, and a low rate of biologically active particulate production.