8Failures with Bearings 8 8

(1)

Chapter 8 · Failures with Bearings – K.J. Bozic

8 Failures with Bearings

K. J. Bozic

Summary

Despite the long-term clinical and radiographic success that has been reported with total knee arthroplasty (TKA), failures related to the articular bearing surface continue to be one of the most significant factors that limit survivorship of TKA implants. Many variables influence wear and bearing surface failure,including factors related to the patient,the surgeon,and the implant.Future efforts should be directed at changes in the materials, implant design, sterilization methods, and surgical technique that could potentially lead to improvements in wear properties and an overall reduction in the incidence of bearing surface failures in TKA.

Introduction

Total knee arthroplasty (TKA) has emerged as one of the most successful and cost-effective interventions in or- thopedic surgery. Clinical and radiographic success rates of greater than 90% at 10- to 15-year follow-up have been reported [7, 11]. Despite the high rates of success, howev- er, problems related to the articular bearing surface have been one of the most common causes of failure. Fatigue and delamination are the most frequently reported mechanisms of bearing surface damage in TKA [19,26].Factors that influence wear rates and bearing surface damage include factors under the control of the surgeon, including

patient selection, implant selection, and surgical technique, and factors under the control of the manufacturer, including material composition, manufacturing technique, sterilization technique, shelf life, thickness, conformity, and issues related to modularity. The purpose of this chapter is to review the literature regarding the most common causes of bearing surface failure associated with TKA.

Mechanisms of Bearing Surface Failure

Bearing surface failures have been associated with articular wear, osteolysis, and problems related to modularity, including locking mechanism failure and backside wear.

Mechanisms of Wear Debris Generation

McKellop and colleagues have defined four modes of wear debris generation [18] (⊡ Table 8-1).Mode 1 describes wear that occurs between the two primary bearing surfaces, as intended by the designers of the implant. This type of wear occurs when the femoral implant articulates with the polyethylene articular insert.Mode 2 refers to the condition of a primary bearing surface rubbing against a secondary surface in a manner not intended by the designer, such as when the femoral component wears through the articular insert and wears against the tibial

⊡ Table 8-1.Modes of wear in prosthetic joints (from [18])

Wear mode Description Example

1 Wear that occurs between the two primary bearing surfaces, Femoral component articulating with the tibial insert as intended by the designers of the implant

2 Primary bearing surface rubbing against a secondary surface Femoral component wears through articular insert in a manner not intended by the designer and articulates against the metal base plate

3 Third body wear that occurs between the primary bearing PMMA, metallic debris, or bone chips in joint space between surfaces due to particulate debris femoral component and tibial insert

4 Wear that occurs between two secondary (nonbearing) Backsided wear that occurs between the UHMWPE articular

surfaces insert and the tibial base plate

(2)

8

debris such as polymethylmethacrylate (PMMA) frag- ments, bone chips, or metallic debris from porous coat- ings. Mode 4 refers to wear that occurs between two sec- ondary (nonbearing) surfaces. This would include ‘backsided’ wear that occurs between the UHMWPE articular insert and the tibial base plate [17, 20, 32].

Wear mechanisms associated with polyethylene articular inserts have been studied extensively in hip and knee replacement [1, 2, 4, 26, 31]. Wear mechanisms that have been associated with hip and knee arthroplasty in- clude abrasive wear, adhesive wear, third-body wear, de- lamination, and fatigue. Abrasive wear results if particles are generated when imperfections in a harder surface (e.g., metal femoral component) plow grooves into a soft- er material (e.g.,UHMWPE articular insert) [15].This can occur either at the primary articulation or at other sec- ondary surfaces. Adhesive wear occurs when small sub- micron particles adhere to the metallic counterface and are pulled off by the passing of the adjacent articulation surface [15]. Third-body wear occurs when third-body particles, such as cement, hydroxyapetite (HA), metal, and/or polyethylene, become embedded in the articulating bearing [26]. Adhesive and abrasive wear are most commonly associated with total hip arthroplasty, leading to biologically active sub-micron particulate wear debris that can result in osteolysis and peri-prosthetic bone loss [18, 28].

The most common mechanisms of wear and poly- ethylene damage in total knee arthroplasty are delami- nation and fatigue, whereby the formation of subsurface cracks leads to the generation of particles that are shed from the bearing surface [15]. Delamination can result in the loss of conformity of the articular bearing, leading to an altered pattern of load distribution and ultimately to failure of the articular bearing surface [19] (⊡ Fig. 8-1).

Ultra-high molecular weight polyethylene (UHMWPE) has been the articular bearing surface of choice for total knee replacement for the past 30 years [5]. Polyethylene wear can occur at both the articular surface and the undersurface of the articular insert at the interface of the tibial base plate (so-called backside wear). Many factors affect bearing surface wear, including factors associated with manufacturing, sterilization and shelf life of the implant, implant design factors, patient factors, surgical technique, and duration of time in vivo.

Manufacturing Technique

Several investigators have examined the effect of specific resins on wear rates associated with articular bearing surfaces in TKA. In a retrieval study,Won et al. examined the effect of resin type and manufacturing method on wear rates of Miller-Galante (M-G) I and II tibial inserts that were gamma irradiated in air [36]. Their analysis revealed that M-G I retrieved inserts,which were made by direct compression molding of Hi-fax 1900 resin,had significantly more wear damage in the form of scratching and embedded metallic debris,whereas M-G II retrievals, which were manufactured by machining from a ram- extruded rod of GUR 415 resin, had significantly more wear damage in the form of delamination. Light-micro- scopic examination of thin sections of the retrieved implants revealed that delamination of M-G II components occurred through a subsurface region of severely oxida- tively degraded UHMWPE, while no such subsurface- degraded region was observed in M-G I retrievals. Based on their findings, the authors concluded that wear of UHMWPE tibial inserts is influenced by both resin type and manufacturing technique.

Sterilization Technique

Numerous in vitro and in vivo studies have been performed to evaluate the impact of sterilization techniques, such as gamma irradiation in air and ethylene oxide (EtO) gas,on wear rates in total knee arthroplasty [5,8,16,22,33, 35]. White et al. used qualitative and quantitative techniques to evaluate the effects of sterilization methods on the wear and physical and mechanical properties of 29 retrieved UHMWPE tibial inserts [33]. They reported higher rates of delamination, lower toughness, and lower per- cent elongation in inserts that were sterilized with gamma radiation in air compared with tibial inserts that were sterilized with ethylene oxide. Based on these findings, the authors concluded that EtO sterilization caused less

⊡Fig. 8-1.Explanted tibial insert demonstrating extensive delamination of the articular surface

(3)

microstructural damage to polyethylene and resulted in significantly less wear than sterilization with gamma radiation in air.Reeves and colleagues used Fourier transform infrared analysis and finite element modeling to compare wear rates between gamma-irradiated in air and gas-plasma sterilized UHMWPE tibial inserts [22]. They found that under high cyclic stresses, delamination occurred in the majority of the inserts gamma-irradiated in air but in none of the gas-plasma-sterilized inserts.

In a retrieval study of 1635 UHMWPE tibial inserts, Williams et al. found that those gamma irradiated in air had a high incidence of delamination and cracking, leading at times to complete wear through of the bearing, while inserts that were sterilized with EtO showed no evidence of fatigue damage, even after in vivo clinical use longer than 15 years [35]. Collier and colleagues used mechanical testing and Fourier transform infrared spec- troscopy to study how gamma sterilization leads to degradation of the physical and mechanical properties of UHMWPE tibial inserts [8]. Their results indicated that gamma sterilization in air resulted in elevated oxidation of polyethylene, which reduced static strength and elongation properties and significantly decreased fatigue resistance of polyethylene bearings.

The overwhelming conclusion from most of the in vitro and in vivo studies regarding sterilization technique in total knee arthroplasty has been that gamma irradiation in air has an adverse effect on the physical and mechanical properties, thus leading to higher wear rates of UHMWPE articular bearing surfaces. In fact, in a review article on the subject that was published in 2002, Blunn and colleagues indicated that “delamination of loaded UHMWPE…will not occur in normal use over ten years if the UHMWPE has been well compacted and has not been sterilized by gamma irradiation in air” [5].

Shelf Life

Shelf life is defined as the period of time between when the implant is sterilized and packaged and when it is in- serted into the patient [6]. Kurtz et al. examined the effects of shelf aging following gamma irradiation in air on the mechanical properties of UHMWPE tibial and acetabular inserts [16]. Using finite element modeling techniques, they reported that post-irradiation aging during shelf storage of UHMWPE inserts is likely to worsen long- term wear. Similarly, Bohl et al. investigated the effects of shelf aging on the in vivo performance of 188 gamma- sterilized UHMWPE Synatomic tibial inserts [6]. They found clinical failure (defined as component retrieval be- cause of polyethylene degradation) rates ranging from 20.8% for prostheses that had shelf lives before implanta- tion of 8–11 years to 0% for prostheses that had shelf lives of less than 4 years. The authors concluded that longer

shelf life had an adverse effect on the mechanical properties of UHMWPE tibial inserts. Based on these findings, they recommended component expiration dates should be placed on implant packages and that implants that are shelf aged beyond this date should be discarded.

Other Design Factors (Implant Thickness, Conformity, and Type of Polyethylene)

Bartel et al.investigated the effect of conformity and plastic thickness on contact stresses in metal-backed hip and knee implants [4]. Using both analytical and finite element methods, the investigators found that for metal- backed components, minimum thickness of less than 6 mm for non-conforming surfaces and less than 4 mm for nearly conforming surfaces may result in excessively large contact stresses. Furthermore, their results demonstrated that bonding of the plastic to the metal backing decreased the tensile stresses at the edge of the contact zone and the maximum shear stress immediately under the load, suggesting a potential benefit of non-modular metal-backed tibial components.

In a follow-up study from the same research lab, published in 1986, Bartel and colleagues used elasticity and finite element solutions to estimate the effect of conformity, thickness, and material on contact stresses and surface damage in UHMWPE hip and knee implants [3].

They found that stresses associated with surface damage in the tibial component of the total knee replacement were much greater than those in the acetabular component of the total hip replacement. Also, their analysis of contact stresses of the polyethylene insert for tibial components suggested that a thickness of more than 8–10 mm should be maintained whenever possible. Other interest- ing findings were that contact stresses in the tibial component were reduced most when the articular surfaces conformed in the medial-lateral direction, and were much less sensitive to changes in geometry in the anteri- or-posterior direction. Finally, they found that the use of carbon-fiber reinforced PE resulted in stresses that were greater by as much as 40%.

In a separate study, Wright and Bartel analyzed surface damage in retrieved TKA tibial inserts and compared these findings with analytical studies to assess the influence of thickness, articular conformity, and type of polyethylene on wear patterns [38]. With respect to these design parameters, they found greater surface damage in thin components (less than 4–6 mm), components with relatively flat tibial articulating surfaces, and carbon-reinforced polyethylene components.

Sathasivam and Walker used computer modeling techniques to simulate static and dynamic loading of tibial inserts in total knee arthroplasty [25]. Their results indicated that much lower contact stresses, and therefore

(4)

8

Patient Factors, Length of Time In Vivo

Several authors have suggested that wear of tibial articular inserts is affected by patient factors, including weight and duration of time in vivo [37, 38]. In 1986,Wright and Bartel observed patterns of surface damage from retrieved total knee polyethylene components and compared their findings with analytical studies of contact stresses in articular inserts [38].They reported that the amount and severity of surface damage to the tibial insert increased with patient weight and with length of time in vivo.

In a separate study, Wright and colleagues used light microscopy to evaluate surface damage to carbon fiber- reinforced and non-reinforced UHMWPE tibial inserts that had been retrieved from failed posterior stabilized total knee prostheses [37].They reported that the amount of surface damage was directly proportional to the duration of time in vivo for both types of implants.

Surgical Technique

Wasielewski et al. examined 55 unconstrained polyethylene tibial inserts that were retrieved at revision TKA for evidence of wear [31]. They found the most severe wear patterns in inserts with third-body wear from metal- backed patellar failures and cement debris. Furthermore,

the varus knee and lateral in the valgus knee).The authors concluded that wear associated with unconstrained TKA is affected by clinical and mechanical factors under the surgeon’s control,including component size and position, knee alignment, and ligament balance (⊡ Fig. 8-2).

D’Lima and colleagues used finite element modeling techniques to assess the effect of malalignment on polyethylene contact stresses in total knee arthroplasty [10].

They found that increased conformity significantly reduced contact stresses in neutral alignment, liftoff significantly increased contact stresses under both low- and high-conformity conditions, and malalignment in rota- tion was especially detrimental with the high-conformity insert design.Based on these findings,the authors concluded that both implant design and surgical technique can play a critical role in wear rates in TKA, and that mo- bile bearing designs could result in lower contact stresses by minimizing the effect of rotational malalignment.

Problems Related to Modularity

Another issue that has been debated in the literature is the benefits and drawbacks associated with modular tibial implants [24, 27, 29]. Proponents of modularity cite the ability to alter soft-tissue balancing intra-operatively and the potential for less traumatic revision surgery as benefits of modularity [27,29].Opponents of modularity point to potential problems associated with backside wear and incompetent locking mechanism designs as arguments against the use of modular tibial inserts [21, 24].

Backside Wear

Many investigators have demonstrated that in addition to articular surface damage and wear, backside wear between the articular insert and the metal tibial base plate is a major contributor to wear debris in TKA [17, 20, 24, 30] (⊡ Fig. 8-3). Li et al. performed an in vitro analysis of 55 retrieved tibial inserts from four different TKA designs [17]. Their results suggested that backside wear of tibial inserts can be a significant contributor to polyethylene wear in TKA, and as a result, surgeons and manufactur- ers should pay close attention to the fixation of tibial inserts to metal trays.

In a review article on the subject published in 2002, Wasielewski reported that the forces at the undersurface articulation created during physiological loading are influenced by insert type, articular design, and surgical technique [30]. He noted that increasing articular insert constraint, while reducing contact stresses at the point of contact on the articular surface, can actually increase

⊡Fig. 8-2.Anteroposterior radiograph of a TKA demonstrating medial compartment wear secondary to incomplete ligament balancing of a varus knee. Note the extensive osteolysis in the proximal tibia

(5)

backside wear,due to transfer of forces to the interface between the articular insert and the tibial base plate. Also, designs with a cam-post mechanism, such as posterior cruciate substituting designs, create a significant shear force at this interface (⊡ Fig. 8-4). Furthermore, he reported that factors under the control of the surgeon, including component alignment and position and ligament balance, may also influence backside wear.

Rao et al. analyzed 29 retrieved modular tibial components from 12 fixed bearing designs with regard to backside wear and relative motion between the polyethylene insert and the metal base plate [20]. Their results suggested that backside wear was correlated with the relative motion between the polyethylene insert and the metal base plate.Based on their findings,the authors concluded that new locking mechanism designs directed toward better securing of the polyethylene insert to the tibial tray are needed to minimize the generation of particular wear debris at the modular interface.

Dissociation of Tibial Insert

Although rare,several case reports have been published regarding patients who suffered dissociation of the modular tibial insert from the metal base plate in fixed bearing knees [9, 23]. In a report of two cases of dissociation of a modular PCA insert,Davis et al.postulated that anterior liftoff of the insert during knee flexion was the cause of the dissociation [9]. Ries recently reported a case of a constrained condylar insert-base plate dissociation that occurred as a result of anterior cam-post impingement,leading to failure of the posterior locking mechanism and posterior liftoff [23] (⊡ Fig. 8-5).These authors suggested that use of a modular constrained condylar knee may not be appropriate for patients with a deficient extensor mechanism.

Osteolysis

Osteolysis, although less common in TKA than in total hip arthroplasty (THA), is a major cause of TKA failure leading to the need for revision surgery. The mechanism of periprosthetic bone loss associated with particulate debris has been critically evaluated in both in vitro and in vivo studies [12–14]. Willert and Semlitsch first pro- posed that wear-particle generation and migration into the joint cavity and periprosthetic space may stimulate macrophage recruitment and phagocytosis [34]. Since that time, further research has revealed that sub-micron wear particles are phagocytosed by macrophages, resulting in the release of various pro-inflammatory factors and

⊡Fig. 8-4. Explanted tibial insert from a posterior stabilized TKA demonstrating fracture of the tibial post. (Courtesy of Steve Haas)

⊡Fig. 8-5.Explanted tibial insert and base plate demonstrating dissociation and displacement of the tibial insert secondary to failure of the base-plate locking mechanism. (Courtesy of Michael Ries)

⊡Fig. 8-3. Explanted tibial insert demonstrating backside wear secondary to motion between the tibial insert and the metal base plate

(6)

play a significant role in osteolysis are interleukin-1 (IL-1), interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α) and prostaglandin E₂(PGE₂) [13]. Proliferation of these cellular mediators then leads to stimulation and differentiation of osteoclasts and inhibition of os- teoblasts. These factors work synergistically, ultimately leading to the dissolution of bone at the prosthetic interface, allowing for prosthetic micro motion that leads to further generation of wear debris [14].

In vitro studies have demonstrated that many factors influence the biological response to wear debris, including the size,volume,surface chemistry,and material composition of the particles [2]. Many investigators have noted that osteolysis is less common in TKA than in THA [12, 15, 26]. In THA, adhesive and abrasive wear mechanisms dominate, resulting in the formation of high volumes of sub-micron particulate debris. Conversely, in TKA, fatigue and delamination are the most common mechanisms of bearing surface damage. These modes of failure produce wear debris particles that are larger than the wear particles observed around total hip replacements. It has been hypothesized by Ayers and others that the more aggressive biological response seen with THA could be explained by the fact that sub-micron particles provide a greater stimulus to the macrophage to produce inflammatory mediators that result in osteolysis [2].

References

1. Ayers DC (2001) Maximizing ultra high molecular weight polyethylene performance in total knee replacement. Instr Course Lect 50:421–429 2. Ayers DC (1997) Polyethylene wear and osteolysis following total knee

replacement. Instr Course Lect 46:205–213

3. Bartel DL, Bicknell VL, Wright TM (1986) The effect of conformity, thickness, and material on stresses in ultra-high molecular weight components for total joint replacement. J Bone Joint Surg [Am] 68:1041–1051 4. Bartel DL, Burstein AH, Toda MD, Edwards DL (1985) The effect of confor-

mity and plastic thickness on contact stresses in metal-backed plastic implants. J Biomech Eng 107:193–199

5. Blunn G, Brach del Preva EM, Costa L, Fisher J, Freeman MA (2002) Ultra high molecular-weight polyethylene (UHMWPE) in total knee replacement: fabrication, sterilisation and wear. J Bone Joint Surg [Br] 84:946–949 6. Bohl JR, Bohl WR, Postak PD, Greenwald AS (1999) The Coventry Award.

The effects of shelf life on clinical outcome for gamma sterilized polyethylene tibial components. Clin Orthop 376:28–38

7. Colizza WA, Insall JN, Scuderi GR (1995) The posterior stabilized total knee prosthesis. Assessment of polyethylene damage and osteolysis after a ten-year-minimum follow-up. J Bone Joint Surg [Am] 77:1713–1720 8. Collier JP, Sperling DK, Currier JH, et al (1996) Impact of gamma steriliza-

tion on clinical performance of polyethylene in the knee. J Arthroplasty 11:377–389

9. Davis P, Bocell J, Tullos H (1999) Dissociation of the tibial component in total knee replacements. Clin Orthop 272:199–204

10. D'Lima DD, Chen PC, Colwell CW Jr (2001) Polyethylene contact stresses, articular congruity, and knee alignment. Clin Orthop 392:232–238 11. Faris PM, Ritter MA, Keating EM, Meding JB, Harty LD (2003) The AGC all-

polyethylene tibial component: a ten-year clinical evaluation. J Bone Joint Surg [Am] 85:489–493

pathogenesis. Instr Course Lect 49:71–82

14. Jacobs J, Rosebuck K, Archibeck M, Hallab M, Glant T (2001) Osteolysis:

basic science. Clin Orthop 393:71–77

15. Jacobs J, Shanbag A, Glant T, Black J, Galante J (1994) Wear debris in total joint replacements. J Am Acad Orthop Surgeons 2: 212–220

16. Kurtz SM, Bartel DL, Rimnac CM (1998) Postirradiation aging affects stress and strain in polyethylene components. Clin Orthop 350:209–220 17. Li S, Scuderi G, Furman BD, et al (2002) Assessment of backside wear from

the analysis of 55 retrieved tibial inserts. Clin Orthop 404:75–82 18. McKellop H, Campbell P, Park S-H, et al (1995) The origin of submicron

polyethylene wear debris in total hip arthroplasty. Clin Orthop 311:3–20 19. Muratoglu OK, Mark A, Vittetoe DA, Harris WH, Rubash HE (2003) Poly- ethylene damage in total knees and use of highly cross-linked polyethylene. J Bone Joint Surg [Am] 85 [Suppl 1]:S7–S13

20. Rao A, Engh G, Collier M, Lounici S (2002) Tibial interface wear in retrieved total knee components and correlations with modular insert motion. J Bone Joint Surg [Am] 84:1849–1855

21. Rao KS, Siddalinga Swamy MK (1989) Sensory recovery in the plantar as- pect of the foot after surgical decompression of posterior tibial nerve.

Possible role of steroids along with decompression. Lepr Rev 60: 283–287 22. Reeves EA, Barton DC, FitzPatrick DP, Fisher J (2000) Comparison of gas plasma and gamma irradiation in air sterilization on the delamination wear of the ultra-high molecular weight polyethylene used in knee replacements. Proc Inst Mech Eng [H] 214:249–255

23. Ries MD (2004) Dissociation of the UHMWPE Insert from tibial baseplate in total knee arthroplasty. A case report. J Bone Joint Surg [Am]

86:1522–1524

24. Rodriguez J, Baez N, Rasquinha V, Ranawat C (2001) Metal-backed and all- polyethylene tibial components in total knee replacement. Clin Orthop 392:174–183

25. Sathasivam S, Walker PS (1998) Computer model to predict subsurface damage in tibial inserts of total knees. J Orthop Res 16: 564–571 26. Schmalzried T, Callaghan J (1999) Wear in total hip and knee replace-

ments. J Bone Joint Surg [Am] 81:115–136

27. Shaw J (1992) Angled bearing inserts in total knee arthroplasty. A brief technical note. J Arthroplasty 7:211–216

28. Sinha R, Shanbhag A, Maloney W, Hasselman C, Rubash H (1998) Os- teoylsis: cause and effect. Instr Course Lect 47:307–320

29. Surace M, Berzins A, Urban R, et al (2002) Backsurface wear and def- moration in polyethylene tibial inserts retrieved postmortem. Clin Orthop 40:14–23

30. Wasielewski RC (2002) The causes of insert backside wear in total knee arthroplasty. Clin Orthop 404:232–246

31. Wasielewski RC, Galante JO, Leighty RM, Natarajan RN, Rosenberg AG (1994) Wear patterns on retrieved polyethylene tibial inserts and their relationship to technical considerations during total knee arthroplasty.

Clin Orthop 299:31–43

32. Wasielewski RC, Parks N, Williams I, et al (1997) Tibial insert undersurface as a contributing source of polyethylene wear debris. Clin Orthop 345:53–59

33. White SE, Paxson RD, Tanner MG, Whiteside LA (1996) Effects of sterilization on wear in total knee arthroplasty. Clin Orthop 331:164–171 34. Willert H, Semlitsch M (1977) Reactions of the articular capsule to wear

products of artificial joint prostheses. J Biomed Mater Res 11:157–164 35. Williams IR, Mayor MB, Collier JP (1998) The impact of sterilization method

on wear in knee arthroplasty. Clin Orthop 356:170–180

36. Won CH, Rohatgi S, Kraay MJ, Goldberg VM, Rimnac CM (2000) Effect of resin type and manufacturing method on wear of polyethylene tibial components. Clin Orthop 376:161–171

37. Wright T, Rimnac C, Faris P, Bansal M (1988) Analysis of surface damage in retrieved carbon fiber-reinforced and plain polyethylene tibial components from posterior stabilized total knee replacements. J Bone Joint Surg [Am] 70:1312–1319

38. Wright TM, Bartel DL (1986) The problem of surface damage in polyethylene total knee components. Clin Orthop 205:67–74

8