Acetabular Wear in Total Hip Arthroplasty


Background

Since the introduction of the low-friction total hip arthroplasty (THA) by Sir John Charnley, [1234wear has been a primary issue in hip arthroplasty. Charnley's original choice for bearing surfaces was a stainless-steel head on polytetrafluoroethylene (PTFE). This choice was complicated by a wear rate of 7-10 mm within a 3-year period. This led him to search for other bearing materials, namely high-molecular-weight polyethylene. Although wear remains a problematic issue in THA, its consequences—namely, osteolysis and prosthetic loosening—loom as larger issues.

Wear results when surfaces produce local mechanical damage and unwanted loss of material and the resultant generation of wear particles. Conventional wear includes fatigue and interfacial (bearing surface) wear. Fatigue wear occurs as a result of repetitive stressing of a bearing material. Interfacial wear is divided into abrasive and adhesive wear.

Abrasive wear occurs when a surface asperity cuts or plows into the opposing surface. This may be especially true when the two surface materials have different hardnesses and the harder material cuts into the softer material. Adhesive wear occurs when bonding of microcontacts exceeds the inherent strength of either material. The weaker material may then be torn off and adhere to the stronger material. Other factors in wear include surface roughness, material hardness, contact areas, and loads applied. [567891011121314]

For patient education resources, see Total Hip Replacement.

Anatomy

The relevant anatomy is that of the hip, acetabulum, pelvis, and proximal femur. Surgeons dealing with the complications of acetabular wear in THA must be familiar with the anatomy of the region. In addition, they should be comfortable with a variety of extensile surgical approaches. The armamentarium of surgical approaches includes the following:

  • Anterolateral
  • Direct lateral
  • Transtrochanteric
  • Posterior
  • Posterolateral
  • Ilioinguinal
  • Extended iliofemoral
  • Combined approaches

The most frequently used surgical approaches are the direct lateral, anterolateral, and posterior approaches. Commonly used modifications to the most frequently used approaches include the trochanteric slide, the trochanteric osteotomy, and the extended trochanteric osteotomy.

Pathophysiology

Willert found that capsular tissue has some capacity to transport wear particles through the lymphatic system by way of perivascular lymph spaces. [15If this system is overwhelmed, then these particles accumulate in periarticular tissues and are subsequently phagocytosed by macrophages in the pseudocapsule. This process results in foreign body granuloma formation with areas of necrosis and fibrosis. Extension of this foreign body response may infiltrate into the cement-bone or bone-implant interface and may result in loosening.

Polyethylene particles disperse into the joint fluid surrounding a THA. The effective joint space includes all periprosthetic regions accessible to joint fluid and thereby accessible to particulate wear debris. The quality of contact within the implant-bone, cement-bone, or implant-cement interface determines the limits of the effective joint space. Much variability exists in these contacts for each reconstruction. Joint fluid flows according to pressure gradients and follows the path of least resistance. These patterns shape the extent of osteolysis. Finally, the effective joint space can also expand into soft tissues as well as bone.

Etiology

Periprosthetic bone loss

Stress shielding may result in periprosthetic bone loss from a reduction in the load transmitted to bone. Periprosthetic bone loss may also occur as a result of an inflammatory reaction due to particulate wear debris as generated in the various wear modes (see Presentation, Modes of Wear).

Tissues adjacent to hip replacements consist of synovial and fibrous tissue, lymphocytes, and foreign body inflammatory cells. The number of foreign body inflammatory cells (macrophages and giant cells) present correlates with the number of polyethylene wear particles. Wear particles produce a series of responses on both cellular and tissue levels.

The macrophage appears to be central in the biologic response to wear debris. Macrophages phagocytose small wear particles and may fuse to form foreign body giant cells. Osteoclasts are responsible for most periprosthetic bone resorption. Some evidence also suggests that macrophages and foreign body giant cells may also be capable of direct low-grade resorption.

Activated macrophages release cytokines (interleukins [ILs] and prostaglandins) that are responsible for the recruitment and differentiation of cells and may also stimulate bone resorption. Macrophages release both IL-1β and tumor necrosis factor (TNF). Matrix metalloproteinases (MMPs) are produced by interfacial membrane tissues around hip replacements and can also affect bone resorption.

The in-vitro response of macrophages to wear debris is a function of the size, shape, and composition of particles. The macrophage response is also dose-dependent. The size of the particles is also important in their ability to stimulate an inflammatory response. Particles larger than 7 µm and smaller than 0.2 µm have less stimulatory effect than those within that range.

Osteolysis

Osteolysis has typically been described as nonlinear, scalloped, or erosive femoral endosteal bone resorption associated with cemented hip prostheses. Osteolysis clearly is bone resorption in association with foreign body response to wear particles.

Charnley reported nonlinear endosteal erosions in association with a cemented Charnley THA. He incorrectly suspected that this bone loss was due to infection. He also noted cystic erosions in the femoral diaphysis in association with stem fracture and related this finding to a deficient cement mantle. Periprosthetic tissues were noted to contain cement particles, though polyethylene particles were not found. [16]

Harris noted a similar pattern of localized bone resorption around a loose THA. [17These tissues were found to have a high degree of osteoclastic bone resorption, a high concentration of macrophages, and occasional foreign body giant cells with phagocytosed cement particles, leading to what was called cement disease.

Jasty also noted osteolysis in association with well-fixed components with adjacent cement particles. [18Polyethylene particles were not implicated as the cause of osteolysis until cementless components were inserted and subsequently shown to have endosteal osteolysis.

Anthony demonstrated a communication between the bearing surface articulation and the endosteal surface of the femur. [19This communication was through a space between the stem and the cement and then through a defect in the cement mantle. Particles of cement, metal, and polyethylene were found in macrophages in these osteolytic lesions. Arthrography confirmed the transfer of contrast material from the articulation to the areas of osteolysis. A common finding in patients with osteolysis around a cemented component is a defect in the cement mantle. The risk of osteolysis may be decreased if technical errors are minimized.

Cementless femoral components with limited proximal porous coatings have been associated with localized bone resorption in the diaphysis. This appears to be due to the limited proximal porous coating, especially if it is not circumferential, which allows egress of joint fluid laden with wear debris and access to the diaphyseal endosteum. Circumferential extensively coated stems have not been associated with diaphyseal osteolysis, even in the presence of proximal stress shielding. (See the images below.)

Acetabular wear in total hip arthroplasty. CementlAcetabular wear in total hip arthroplasty. Cementless total hip arthroplasty in a 60-year-old woman with osteoarthritis.
Acetabular wear in total hip arthroplasty. Woman wAcetabular 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.
Acetabular wear in total hip arthroplasty. LateralAcetabular 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.

Osteolysis can occur in a more linear pattern and progress along a bone-cement interface and contribute to implant loosening. This is especially true for cemented acetabular components. Once stability is lost, further motion can only be detrimental. A highly significant association has been demonstrated between the rate of polyethylene wear and loosening of an acetabular component. Cementless acetabular components may also be subjected to a similar process of linear osteolysis. The integrity of the implant bone interface governs the ingress of joint fluid and wear particles. Acetabular components with tight peripheral press fit reduce the prevalence of progressive peripheral interface radiolucencies.

Cementless acetabular components appear to have a lower prevalence of interface radiolucencies than cemented acetabular components do. Bone resorption associated with cemented acetabular components occurs predominantly along the interface and follows the edge of the cement mantle. Bone resorption in cementless acetabular components progresses away from the interface and into the cancellous bone of the pelvis, resulting in nonlinear osteolysis or expansile osteolysis.

Pelvic osteolysis is associated with younger patients, vertical component positioning, and high volumetric wear of the polyethylene. Other concerns regarding cementless acetabular components are wear of the convex surface of the modular polyethylene insert (backside wear) and fretting of the fixation screws placed into the shell. Backside wear may be especially prominent in acetabular shell designs with poor locking mechanisms that allow significant motion between the polyethylene liner and the concave surface of the acetabular shell.

A randomized controlled trial evaluated the results of component fixation with and without cement. Using data from 250 patients with osteoarthritis (mean age, 64 years) who were treated with THA, the authors found significantly lower survival rates for cemented implants compared with cementless implants after a minimum of 17 years of follow-up. [20]

Epidemiology

Polyethylene wear particles

In the 1990s, submicron polyethylene wear particles were recognized as being produced in very large numbers, even by well-functioning prostheses. The concentration of wear particles can extend into the billions per gram of tissue in periprosthetic areas. Wear particles are a function of the type of wear that produces them.

In-vivo wear assessment

In-vivo wear assessments have traditionally been based on radiography. The degree of penetration of the femoral component into the polyethylene on sequential radiography has been noted as linear wear. The method of radiographic assessment most commonly used is a technique described by Livermore. [21]

On sequential radiography, the distance from the center of the femoral head to a particular reference point on the acetabular cup is measured and corrected for magnification. The difference from the initial postoperative radiograph to the most recent radiograph represents linear wear as measured in millimeters. The linear wear rate is that measurement over the period of implantation. The difficulty with this technique for evaluating linear wear is that it cannot distinguish linear wear from creep or plastic deformation.

Perhaps linear wear would more accurately be termed linear penetration. Creep is most notable early in the postoperative period and becomes negligible after 12-18 months. Additionally, backside or mode 4 wear could also contribute to higher rates of linear penetration.

Volumetric wear is a measure of the volume of material removed from a bearing surface. Computer-assisted techniques with digitized radiography have been used with reasonable reliability. Other techniques have included the shadowgraph technique and fluid-displacement methods for retrieved specimens.

Many variables affect in-vivo wear; as a result, reports regarding wear rates have shown great variability. Patient variables include age, sex, weight, general health, and activity level. Variables related to the hip reconstruction include the choice of bearing material, the design and manufacturing of the prosthesis, and characteristics of implantation (ie, operative technique, biomechanical considerations, initial and long-term implant fixation). Multiple assessments of wear over time are more valuable than a single measurement, and comparing rates of linear penetration after different durations of implantation may be difficult.

Both theoretical models and retrieval analyses have shown that the rates of volumetric wear in polyethylene components increases with increases in the diameter of the femoral head. Using a simple cylindrical formula in which volume equals π multiplied by the radius squared, for any given amount of linear wear, the volumetric wear increases exponentially with increases in the femoral head. Findings of a retrieval study showed that for each 1-mm increase in head diameter, volumetric wear increased by 6.3 mm3/year. Similarly, the rate of volumetric wear increased from 7.5% to 10% for each 1-mm increase in head diameter.

In-vivo studies comparing 32-mm heads with smaller heads have shown similar or greater wear rates. Larger heads have also been associated with bone resorption and loosening. Because of their large diameters, surface replacement components have rates of volumetric polyethylene wear four to 10 times higher than conventional THA with 28-mm heads. One potential benefit of larger heads was thought to be a reduction in polyethylene stresses because of large contact areas and thereby a reduction in linear wear. This, however, has not been the case. Thin polyethylene in some 32-mm bearings and in surface replacements has also confounded the relation between wear and the diameter of the femoral head.

The association between volumetric wear and periprosthetic bone resorption appears to be related to the volume of polyethylene wear particles created. Studies of wear particles from retrieved periprosthetic tissues and worn polyethylene surfaces are consistent with an average particle size in the range of 0.5 µm in diameter. A 28-mm head with linear wear of 0.05 mm/year corresponds to a volumetric wear rate of 30 mm3/year. This would also correlate with 500 billion particles, given an average particle diameter of 0.5 µm.

Schmalzried noted a 45-fold difference in the range of gait cycles from the least active to the most active individual. [7Variation in an individual's activity contributes to the variability in wear rates that are commonly observed in in-vivo studies.

Prognosis

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.



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:

  • 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)
  • 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
  • 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
  • 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. IntraopAcetabular 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. IntraopAcetabular 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. [6Therefore, 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. [22Gamma 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.


Laboratory Studies

Laboratory evaluation prior to any surgical intervention should include a complete blood count (CBC), coagulation studies, and routine chemistries. Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) are useful, especially in excluding indolent infection. In cases where infection is a serious consideration, preoperative aspiration may also prove helpful in further evaluation.

Imaging Studies

 

 

 

Radiography

Standard anteroposterior (AP) hip and pelvis views and frog-leg lateral views are central to the evaluation of acetabular wear in total hip arthroplasty (THA). Most information can be gathered from these views. Serial radiography over time confers additional information regarding wear and prosthetic loosening.

In the setting of significant osteolysis, additional views may be helpful in defining the extent of these lesions and the extent of any bony deficiencies. These include Judet oblique views to evaluate the integrity of both the anterior and posterior columns and the false profile view and sitting lateral view.

Derbyshire et al described a two-dimensional (2D) radiographic wear measurement system that enables the low wear of highly cross-linked polyethylene cups to be accurately measured from standard pelvis radiographs. [23]

Computed Tomography

Computed tomography (CT) may provide supplemental information regarding the pelvis and help differentiate contained bony deficiencies from significant pelvic discontinuity.

Devane et al described a computer-assisted method that uses models of the patient's bones and components built from a single CT scan to measure migration and polyethylene wear. [24]

Bone Scanning

Although bone scintigraphy has been available for more than 30 years, it plays a limited role in the evaluation of a painful hip prosthesis. The main determinants of technetium (Tc)-99m methylene diphosphate (MDP) tracer uptake in bone are blood flow and metabolic bone activity. Increased tracer uptake may be observed for 12-24 months after implantation of a total hip prosthesis because of bone remodeling.

The appearance of abnormal uptake surrounding a painful prosthesis has been used to differentiate loosening from infection. Loose prostheses tend to show increased uptake in the trochanters, at the stem tip, and possibly the acetabular shell. Bone scintigraphy is sensitive for infection but lacks specificity. [25]

Gallium scanning may be useful in the assessment of infection. Gallium binds to serum transferrin and carries an accuracy of 75% in detecting a periprosthetic infection. It has more intense uptake than Tc-99m MDP, and combining these tests may be effective in the evaluation of a painful prosthesis.

Indium-labeled white blood cells (WBCs) accumulate in the region of infection because of chemotaxis. Mixed WBC lines are used in the preparation and may account for the similar sensitivity for acute and chronic infections of 82%.


Approach Considerations

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. [26Many 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 show 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 Therapy

The 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 when 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

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). [27282930 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. [31]

Prevention

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/year. 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/year. Ceramic articulations with alumina on alumina wear at a rate of 0.5-1.5 µm/year.

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. [323334Early 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. [35In 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. [36However, Visuri showed no significant increased risk of malignancies at 15 years following metal-on-metal articulations. [37]

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. [34]

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. [33]

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. [383930]

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, gain 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 years; 90 hips) who underwent THA and were followed for 10-12 years. [40No 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. [414243Cross-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. [9However, 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/year. [2244These studies had small numbers of patients and used all-polyethylene cemented cups.

Subsequent clinical studies have been promising, but additional long-term data are needed. [45461047Other 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.

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/year and 0.016 mm/year, respectively, at an average follow-up of 10 years, [48as well as mean and median two-dimensional volumetric wear rates of 12.79 mm3/year and 5.834 mm3/year, respectively. The authors noted no evidence of radiographic osteolysis and no wear-related revisions.

A retrospective study by Min et al evaluated 85 consecutive THAs (in 67 patients < 50 years with osteonecrosis of the femoral head) performed with highly cross-linked polyethylene liners. [49 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/year (range, 0-0.099 mm/year).

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 [50; however, long-term analysis will be required to determine whether this trend ever becomes significant.

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.



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