Acetabular Wear in Total Hip Arthroplasty Treatment & Management

The surgical indications for management of acetabular wear in total hip arthroplasty (THA) are basically the same as those for revision THA. Issues related to wear include its consequences—namely, osteolysis and implant loosening. ^[27] Many patients presenting with these findings have pain and disability that may warrant surgical intervention. Other symptoms that may warrant surgery include recurrent dislocations and leg-length discrepancies associated with wear and loosening.

Deciding when to intervene is a more difficult issue in an asymptomatic patient with findings of significant osteolysis in whom large bony defects may be present and in whom impending pathologic fracture may occur. Surgical indications may be further influenced by the mode of wear; for example, mode 2 wear may create instability, leg-length discrepancies, and metal wear debris.

Contraindications for revision hip surgery are those medical conditions that would prohibit elective surgery. Patients in whom surgery is contraindicated have severe medical comorbidities in which the risk-to-benefit ratio of revision THA would preclude surgical intervention.

Experimental animal models have shown early promise toward future considerations of medical therapy in treating and preventing osteolysis. At present, however, surgical intervention by means of revision THA remains the mainstay of treatment.

Surgical treatment of the consequences of acetabular wear in THA 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 if it is associated with implant loosening, revision hip arthroplasty may be warranted.

Preoperative planning is essential in dealing with wear-related issues. Complete preoperative medical evaluation and diagnostic workup should precede any surgical intervention.

It is important to 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.

Complications related to revision hip arthroplasty may include the following:

• Infection
• Dislocation
• Leg-length inequality
• Neurovascular injuries
• Thromboembolism
• Intraoperative fractures
• Wound healing problems

Various studies have focused on the occurrence of audible squeaking after hip arthroplasty. In one study, squeaking (ie, squeaking, clicking, or grating sound) occurred in nine of 43 (20.9%) ceramic-on-ceramic noncemented THAs. In these cases, a short neck length of the head seemed to be a risk factor for squeaking.

Other studies also focused on the occurrence and potential causes of squeaking hip (eg, prosthetic design). ^{[28, 29, 30, 31]}  Transient squeaking was noted in patients who received metal-on-metal THAs, 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. ^[32]

The development of wear debris and the biologic response to this debris have fueled a search for alternative bearing surfaces with the hope of reducing the amount of wear debris produced, as well as reducing the immunogenicity of the particles. If linear wear is less than 0.10 mm/year, then osteolysis is rare. The converse is also true; if linear wear is greater than 0.20 mm/year, 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 reported 50-75% less wear with ceramic on conventional polyethylene than with cobalt chrome on polyethylene, others reported similar or greater in-vivo wear rates with ceramic on polyethylene than 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 THA but were abandoned, largely because of the success of the Charnley hip and the high frictional torque encountered in early metal-on-metal designs. ^{[33, 34, 35]}  These 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 associated with fixation problems and failures, primarily from acetabular loosening.

Nevertheless, 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. ^[36] 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 was largely reflected by a report from Dorr et al, which demonstrated no osteolysis at 9 years of follow-up. ^[8]

Metal ion production has been a continued concern with metal-on-metal articulations. Jacobs et al 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. ^[37] However, Visuri showed no significant increased risk of malignancies at 15 years after implantation of metal-on-metal articulations. ^[38]

Passuti and Trevor studied 2614 metal-on-metal THAs with a mean follow-up of 7 years and identified only five cases of unusual osteolysis and 10 of impingement, with no specific severe complications resulting from cobalt or chromium release. ^[35]

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 two groups despite the different size diameters of the bearing surfaces. ^[34]

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. ^{[39, 40, 31]}

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 were unrelated to the bearing surface. The incidence of ceramic fracture during this period was 3.5%, primarily due to manufacturing flaws (eg, large grain size, inclusions/grain boundaries, lack of testing standards, and poor tolerances for taper designs).

Subsequently developed processing techniques eliminated these problems, including hot isostatic pressing, dense fine-grain alumina, grain size less than 2 µm, fewer grain boundaries, and fewer inclusions. Decreased grain size increases burst strength.

Newer taper designs included 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 US Food and Drug Administration (FDA)–sponsored trial, and to date, no ceramic failures have occurred in this group.

Yoo et al evaluated the use of a 36-mm hybrid ceramic bearing on a ceramic liner in 75 patients (43 men, 32 women; mean age, 58 y; 90 hips) who underwent THA and were followed for 10-12 years. ^[41] No fractures of the ceramic liner or head occurred, there was no measurable ceramic wear, and no pelvic or femoral osteolysis was noted. All acetabular and femoral components were bone-ingrown.

Highly cross-linked polyethylene

Most polyethylene used in the past few decades has been partially cross-linked. ^{[42, 43, 44]} Cross-linking arises as an inadvertent byproduct of sterilization with gamma irradiation. The usual dose for sterilization is 25-40 μGy. McKellop showed that wear resistance increases with increasing radiation doses. ^[9] However, the resistance is optimized at 95-100 μGy 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 showed virtually no wear even despite use of a 46-mm head. In-vivo studies by Oonishi and by Wroblewski reported very low wear rates, in the range of 0.02-0.06 mm/y. ^{[22, 45]} These studies had small numbers of patients and used all-polyethylene cemented cups.

Subsequent clinical studies were promising but highlighted the need for additional long-term data. ^{[46, 47, 10, 48]} Other concerns have revolved around how this dose of radiation might affect 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.

Highly cross-linked polyethylene typically goes through free-radical stabilization via remelting, single annealing, or sequential annealing to improve in-vivo oxidation and wear properties. A study by Kump et al found that with long-term in-vivo exposure, such techniques could lead to decreases in mechanical integrity near the surface, which lessened with increasing depths. ^[49]

In a study that included 72 patients aged 50 years or younger who had a 28-millimeter cobalt-chromium femoral head on a highly cross-linked polyethylene acetabular liner, Stambough et al reported mean and median true linear wear rates of 0.0104 mm/y and 0.016 mm/y, respectively, at an average follow-up of 10 years, ^[50] as well as mean and median two-dimensional volumetric wear rates of 12.79 mm³/y and 5.834 mm³/y, respectively. The authors noted no evidence of radiographic osteolysis and no wear-related revisions.

In a retrospective study that included 67 patients younger than 50 years who had osteonecrosis of the femoral head, Min et al evaluated 85 consecutive THAs performed with highly cross-linked polyethylene liners. ^[51]  No mechanical loosening was noted in either femoral nor acetabular components, and none of the components had been revised by final follow-up. No osteolysis was apparent on imaging. Mean liner wear was 0.037 mm/y (range, 0-0.099 mm/y).

In a study of the long-term results (mean follow-up, 16.9 y; range, 15.0-20.1) of metal–on–highly cross-linked polyethylene articulations in 55 THAs (N = 44; mean age at operation, 51.2 y; range, 29-73), Cheung et found no osteolysis in the latest follow-up radiographs. ^[52] ​ The median linear wear rate was 0.038 mm/y, and the median volumetric wear rate was 7.115 mm³/y.

Oxidized zirconium has been investigated as an alternative to cobalt chrome for use with highly cross-linked polyethylene liners. A 5-year comparison by Jassim et al observed a nonsignificant trend towards lower wear with oxidized zirconium than with cobalt chrome in this setting ^[53] ; however, long-term analysis will be required to determine whether this trend becomes significant.

Vitamin E–diffused highly cross-linked polyethylene liners are a variant that showed early promise in cohort studies. An international multicenter (17 centers; eight countries) prospective study (N = 977) by Collins et al compared these liners with conventional highly cross-linked polyethylene liners. ^[54]  They found that the two liner types were not significantly different with respect to 7-year outcomes as measured by acetabular liner wear rate, patient-reported outcome measures, and revision rate. The vitamin E–diffused liners showed less wear, but for both liner types, the wear rate was below the threshold for osteolysis.

The fundamental tenets for a durable, long-term, successful THA 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.