Although loosening of hip and knee implants may not be totally eliminated, fixation of clinically successful implants has demonstrated durability of at least 20 years in many series [4, 11, 37]. Unfortunately, bearing surface wear and tribocorrosion from unexpected sources such as modular connections in THAs and TKAs continue to lead to further surgery in many patients.
Clinical patterns related to implant wear and tribocorrosion are a function of the type of wear product produced. Likewise, the products produced by implant wear or tribocorrosion are a consequence of material and design as well as surgical technique. From the standpoint of current clinical importance, there are four main categories of wear and tribocorrosion: polyethylene wear, ceramic-on-ceramic (CoC) bearing wear, metal-on-metal (MoM) bearing wear, and taper tribocorrosion.
Polyethylene wear creates submicron particles of polyethylene, which produce a macrophage-dominated histologic response around the joint. Clinical findings typically are synovitis and osteolysis, and the effects are local without systemic effect [21, 31]. Metal-on-metal bearing surface wear produces very small metal particles, which liberate metal ions and create a lymphocyte-dominant response. The typical clinical pattern of MoM bearing wear for cobalt-chromium bearings is one of synovitis and local soft tissue reactions [21, 31]. Local soft tissue reactions can be primarily fluid-filled cystic structures, which communicate with the joint and have a relatively thin membrane or they may have mixed fluid and solid components. These local soft tissue reactions have been called by a number of terms including adverse local soft tissue reactions (ALTRs), adverse local reaction to metal debris, and pseudotumor . Metal wear is typically associated with elevated systemic levels of metal ions, and the most important ions clinically have been cobalt and chromium. Taper tribocorrosion liberates or creates metal products from the taper including chromium orthophosphates . The clinical pattern of taper tribocorrosion is synovitis and local soft tissue reactions, which are often similar to those seen with MoM bearings. Systemic metal ion levels typically are elevated. Because taper tribocorrosion damages the metal substrate, implant fractures also can occur.
One way to identify the prevalence of clinical problems related to wear and tribocorrosion is by the revision burden related to wear. The most recent reports of the British National Joint Registry demonstrated 44% of revision THAs were for loosening, 15% for ALTR, 14% for osteolysis, and 13% for wear . Bozic et al.  demonstrated revision for loosening in 19.7% of cases, osteolysis in 6.6%, and bearing surface wear in 5.0% from a large administrative database in the United States, similar to the British National Joint Registry.
Regarding knee revisions, the British National Joint Registry reported that 38% were revised for aseptic loosening, 12% for wear, and 9% for osteolysis . Using the same database for revision knees as was reported for hips, Bozic et al.  reported that 16.1% of knees were revised for loosening, 4.9% for bearing surface wear, and 3.2% for osteolysis.
Each of these four common patterns of bearing surface wear and tribocorrosion have clinical issues related to (1) diagnosis; (2) patient followup and monitoring for the problem in the patient population at risk; (3) treatment; (4) management of specific complications related to the wear or tribocorrosion; and (5) prevention of the wear or tribocorrosion problem. These concerns are examined separately in this article.
Where Are We Now?
The diagnosis of polyethylene wear is usually made without difficulty. Polyethylene wear, even when associated with osteolysis, is associated with few symptoms or with symptoms of synovitis. Hip radiographs typically demonstrate femoral head eccentricity (which can be measured) and osteolysis, which can be diagnosed with plain radiographs [7, 8, 19, 22, 25, 36]. In the knee, clinical findings typically include an effusion (which often develops late), radiographic asymmetry of the polyethylene insert thickness, and osteolysis.
For CoC and MoM bearings, excessive wear may be associated with pain related to synovitis or local soft tissue reaction [10, 32, 42]. For many MoM bearings, a large femoral head is used. Plain films are rarely informative although infrequently may show osteolysis. Metal artifact reduction sequences (MARS) MRI may be used to identify local soft tissue reactions [15-18, 29]. Systemic cobalt and chromium levels are often elevated [9, 12, 24, 40]. Patients with taper tribocorrosion often present with pain. Imaging with MARS MRI and laboratory testing for cobalt and chromium levels can help make the diagnosis [9, 12, 24, 40].
Patient Monitoring and Followup
Monitoring methods and frequency of followup for bearing surface wear or tribocorrosion are an important clinical issue. For polyethylene wear and CoC, plain films are the primary imaging method [7, 8, 19, 22, 25, 36]. For MoM bearings, cobalt and chromium levels and MARS MRI or ultrasound are the main monitoring methods [9, 12, 15-18, 24, 29, 40]. For taper tribocorrosion, the main monitoring methods are cobalt and chromium ion levels and imaging with MRI or ultrasound [9, 12, 15-18, 24, 29, 40].
For polyethylene wear, there is no effective nonoperative treatment. Revision of the polyethylene component and bone grafting of periprosthetic osteolysis and exchange of implants as indicated—based on implant stability and other biomechanical requirements of the joint—are standard forms of treatment . For CoC and MoM bearing failures, there is no effective nonoperative treatment, and revision is the standard form of management. However, it is important to note that with CoC and MoM articulations, a more narrow “landing zone” is required for placement of the acetabular component in regard to both inclination angle (abduction) and anteversion . For problematic taper tribocorrosion, there is no effective nonoperative treatment and treatment involves changing the taper. However, it is unknown which taper is best and whether a ceramic femoral head with a titanium sleeve—an approach gaining some traction of late—will solve the problem. Finally, it is known that surgical technique, including the influence of the assembly procedure as well as blood and fat on the taper, may influence risk of taper tribocorrosion [23, 35].
Management of Complications
Complications related to polyethylene wear included periprosthetic fractures, implant loosening, and severe bone loss. With CoC articulations, it is known that edge-loading can contribute to stripe wear and subsequently squeaking [38, 41]. For MoM bearings, the main problems that lead to complications are severe soft tissue reaction or bone necrosis. Dislocation related to soft tissue damage is not an uncommon complication of revision for MoM bearing failure and loosening of new implants may occur if they are placed on necrotic bone. For taper tribocorrosion, the main complications relate to severe soft tissue damage or implant fracture.
Where Do We Need to Go?
Clinical questions of diagnosing polyethylene and CoC wear relate to when three-dimensional (3-D) imaging can be useful to enhance the diagnosis of osteolysis and what the best 3-D imaging methodology (CT versus MRI) is in the future. Clinical questions about imaging MoM bearings include can MRI be further advanced to improve implant-bone interface evaluation? Even with MARS MRI, troublesome artifacts usually are present, which limit their use. In addition, the questions surround the role of ultrasound for diagnosis. This technology has use, is relatively inexpensive, and is evolving. The second main diagnostic clinical question about MoM bearings relates to cobalt and chromium levels. Thresholds for action and significance of increasing ion levels both remain important unknowns. Clinical questions with respect to diagnosis of taper tribocorrosion relate to (as is the case for MoM bearings) improving 3-D imaging techniques, threshold values for cobalt and chromium levels, and also the use of measuring titanium levels.
Patient Monitoring and Followup
The clinical questions for polyethylene wear and CoC monitoring include frequency of radiographs in each population and the role of 3-D imaging. For MoM bearings and taper tribocorrosion, clinical questions relate to which patients should be monitored (symptomatic versus asymptomatic), with what tests, and how often. Should only very high-risk patients be monitored or should monitoring be extended to other populations?
The most important clinical questions with respect to treating polyethylene wear are when to retain implants and when to remove well-fixed cemented and uncemented implants. Other clinical questions relate to management of periprosthetic osteolytic lesions with bone grafting, bone graft substitutes, or neither. An important clinical question with MoM implants is how aggressively the soft tissue lesions should be débrided. There exists a recognized tradeoff between benefits of thorough débridement to remove the inflamed, metal-loaded tissue versus risk of removal of extra soft tissues, which may increase risk of neurovascular injury or joint instability. Optimizing joint stability may be problematic in patients with soft tissue deficiencies. Optimizing implant fixation is also a relevant question in these patients in whom local metal toxicity may reduce bone viability in the region.
The clinical questions of treating taper tribocorrosion revolve around when can part of the taper unit be retained; and if part of the taper unit is retained, which new material or implant is best to place against the previously damaged surface? Ceramic femoral heads with titanium sleeves are being widely used, but there are sparse clinical data as yet in terms of the durability of this construct . A further question relates to whether there is any effective method of “refurbishing” damaged tapers in situ such as by manually cleaning the taper. Finally, there remain questions on the appropriate impaction method and force for various materials onto the taper. Rehmer et al.  showed that the taper strength linearly increased with assembly forces and that cobalt-chrome heads combined with cobalt-chrome tapers showed significantly lower pull-off forces and turn-off moments than the combination cobalt-chrome heads with titanium tapers.
Management of Complications
The main clinical questions revolve around optimizing the new implant's fixation in the setting of polyethylene wear during revision surgery. With CoC articulations, questions remain on how to prevent noises, chipping, and impingement and if various designs can contribute to improved results. The main clinical questions with management of complications secondary to MoM articulations revolve around soft tissue management and reconstruction (particularly the abductors and hip capsule); how to maintain, regain, or improve joint stability at the time of revision surgery; and how to achieve implant fixation at revision when bone necrosis is present. The main clinical questions with management of complications secondary to taper tribocorrosion revolve around soft tissue management (again, the destroyed abductors and hip capsule) and reconstruction to enhance hip stability.
How Do We Get There?
In the hip, multiple papers have highlighted excellent clinical results, radiographic evidence of implant fixation, and survivorship at approximately 10 years [3, 26, 33, 39]. Babovic and Trousdale  investigated 50 patients younger than 50 years of age and found 100% survivorship and no radiographic evidence of osteolysis or component loosening at a mean followup of 10 years. Prevention of problems with polyethylene bearings now is focused on improving highly crosslinked polyethylene (HXLPE). Also, efforts are directed toward improved locking mechanisms to minimize backside wear. However, both goals must also maintain polyethylene mechanical strength and thickness.
With CoC bearings, the clinical questions surround whether newer ceramics will be developed and if enhanced designs and techniques will minimize noises, chipping, and impingement. Abdel et al.  recently published a series of ceramic liner fractures with a newer-generation CoC bearing with a low-profile design, indicating that additional work is required.
For MoM bearings, the main method to prevent problems has been to discontinue use of this bearing couple in most THAs. In the future—if these bearings continue to be used—it may include optimizing joint kinematics, metallurgy, and implant design. A current clinical question is the role, if any, of MoM hip resurfacing arthroplasty. For instance, a recent study by Nawabi et al.  revealed that an adverse synovial reaction was detected on MRI in both symptomatic and asymptomatic patients undergoing MoM hip resurfacing.
Taper tribocorrosion remains an important, unsolved clinical problem [27, 28, 30]. Avoidance of unnecessary modularity and careful preclinical testing of new modular junction are strategies that may mitigate these problems. In the hip, ceramic femoral heads may be used in lieu of cobalt-chrome heads, but there is an economic cost associated with this and also there are neck length limitations. Moreover, titanium sleeves are required, and the consequences of such use are unknown. Taper modifications by our engineer colleagues may also be possible. The clinical questions revolve around understanding the history of tapers and how they have changed and which tapers have been most effective over the years. There will be important questions to resolve regarding taper mechanics, metallurgy, and the chemistry of taper tribocorrosion. There will also be important material to learn regarding the biologic response to taper tribocorrosion products. Clinical questions revolve around improving tapers for metal heads and ceramic heads and enhancement of other types of junctions where tapers are used. Finally, the introduction of modular femoral necks has also further created a potential for both accelerated taper wear and tribocorrosion.
Although many aspects of bearing surface wear had been well studied, the widespread use of hard-on-hard bearings has led surgeons to better understand the interactions between wear and component impingement. From a clinical standpoint, this understanding has helped the surgeon recognize the need for more accurate component position. The use of hard-on-hard bearings also allowed for the use of larger femoral heads given the ability to use thinner acetabular composite bearings. However, the use of these larger femoral heads has been associated with the important clinical problem of accelerated trunnion and taper wear and production of corrosion products. In summary, before innovative implants are brought to the general market, rigorous laboratory testing replicating in vivo environments must be completed by those who are and are not involved in the design creation. Moreover, the rationale and clinical needs for such technology must be appropriately identified and documented a priori. Finally, we must continue to promote and analyze joint replacement registries to identify early failures and analyze long-term successes because joint registries are a venue that can serve as a postsurveillance vehicle.
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