Fretting and Corrosion Damage in Retrieved Metal-on-Polyethylene ...

The Journal of Arthroplasty 33 (2018) 931e938

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Basic Science

Fretting and Corrosion Damage in Retrieved Metal-on-Polyethylene Modular Total Hip Arthroplasty Systems: What Is the Importance of Femoral Head Size? Matthew P. Siljander, MD a, Erin A. Baker, PhD b, *, Kevin C. Baker, PhD b, Meagan R. Salisbury, MS b, Clayton C. Thor, MD a, James J. Verner, MD a a b

Department of Orthopaedic Surgery, Beaumont Health System, Royal Oak, Michigan Department of Orthopaedic Research, Beaumont Health System, Royal Oak, Michigan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 July 2017 Received in revised form 15 September 2017 Accepted 5 October 2017 Available online 16 October 2017

Background: Fretting and corrosion at the modular femoral head-femoral neck (taper) interface have been reported in retrieved total hip arthroplasty (THA) prostheses. This study investigated associations among implant design, radiographic factors, and patient factors with corrosion and fretting at the taper interface in retrieved metal-on-polyethylene modular THA prostheses. Methods: Ninety-two retrieved primary metal-on-polyethylene THA implants were evaluated and graded for fretting, corrosion, and damage at the taper interface, including the femoral stem trunnion and femoral head. Preoperative radiographs were assessed for osteolysis and femoral stem alignment; and medical records were reviewed for demographic data. Results: Male patients had greater head corrosion (P ¼ .037), patient age at revision had a weak, negative correlation with trunnion corrosion (r ¼ 0.20, P ¼ .04), and both body mass index and duration of implantation had weak, positive correlations with head fretting (r ¼ 0.26, P ¼ .01 and r ¼ 0.33, P ¼ .001, respectively). A weak, negative correlation was found between femoral head size and both head fretting and head corrosion (r ¼ 0.26, P ¼ .007 and r ¼ 0.21, P ¼ .028, respectively), and a weak, positive correlation was found between head offset and trunnion fretting (r ¼ 0.23, P ¼ .030). Varus femoral stem alignment was associated with greater head fretting (P ¼ .038). Conclusion: Larger femoral head sizes were correlated with less severe head corrosion and head fretting, with 28-mm heads exhibiting more moderate-to-severe damage. Other factors, such as head-taper engagement and geometry, rather than head size, may affect rates of corrosion and fretting damage at the taper interface. © 2017 Elsevier Inc. All rights reserved.

Keywords: corrosion fretting trunnion damage femoral head size

Source of support: The Retrieved Orthopaedic Implant Registry at Beaumont Health is underwritten by generous donations to the Dorothy and Byron Gerson Implant Analysis Fund. No external funding was received for this study. One or more of the authors of this paper have disclosed potential or pertinent conflicts of interest, which may include receipt of payment, either direct or indirect, institutional support, or association with an entity in the biomedical field which may be perceived to have potential conflict of interest with this work. For full disclosure statements refer to https://doi.org/10.1016/j.arth.2017.10.010. IRB approval: This study was approved by the Institutional Review Board at Beaumont Health Research Institute, under the study protocol #2012-127. * Reprint requests: Erin A. Baker, PhD, Department of Orthopaedic Research, Beaumont Health System, 3811 West 13 Mile Road, Suite 404, Royal Oak, MI 48073. https://doi.org/10.1016/j.arth.2017.10.010 0883-5403/© 2017 Elsevier Inc. All rights reserved.

Because modular total hip arthroplasty (THA) systems were introduced more than 3 decades ago, the modular femoral head-femoral neck (taper) interface design modification provided the surgeon with additional intraoperative options for improving stability as well as modifying offset, leg length, and bearing type [1]. Modularity also allows revision of the articulating surfaces without revision of a wellfixed stem. However, modular designs are susceptible to corrosion and fretting, secondary to micromotion at the taper interface, due to various factors, including chemical and electrochemical environment, component geometry, and component materials. In addition to mechanically assisted corrosion, spaces between modular components may facilitate crevice corrosion [2e4]. Taper corrosion may develop in THA systems with metal-on-metal (MoM) articulation as well as metal-on-polyethylene (MoP) implants, in which adverse tissue reactions have been reported [5e12]. Gilbert

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M.P. Siljander et al. / The Journal of Arthroplasty 33 (2018) 931e938

Table 1 Series of 92 Retrieved Implants. Manufacturera

Model

DePuy Synthes

S-ROM AML Summit HPS II PCA E-Series PCA Textured Citation TMZF PCA Secur-Fit Max Restoration HA Accolade HNR Omnifit Restoration Modular Definition V40 Textured Meridian VerSys Cemented VerSys Fiber Metal Taper M/L Taper Harris Precoat Harris-Galante Natural ZMR Bi-Metric Porous Taperloc Reach Image Porous PSL

Stryker (including Howmedica and Osteonics)

Zimmer Biomet

Smith & Nephew (including Richards) BioPro

Numberb 15 3 1 1 13 6 6 3 2 2 2 2 1 1 1 1 1 4 4 3 2 2 2 2 1 1 1 1 2

a DePuy Synthes (Warsaw, IN); Stryker (Mahwah, NJ); Zimmer Biomet (Warsaw, IN); Smith & Nephew (Andover, MA); BioPro (Port Huron, MI). b Six systems were unknown.

et al [13] described mechanical corrosion from micromotion, which disrupts the passive oxide layer, resulting in destabilization of the oxide layer and, ultimately, an increased rate of corrosion. Numerous studies have evaluated wear and corrosion at the taper interface as a potential failure mode in THA, but all causative factors of fretting and corrosion at the taper interface have not been defined [14e16]. Increased femoral head size, duration of implantation, femoral head-femoral stem offset, surface roughness, as well as femoral head-femoral stem metal alloy mismatch, decreased contact area, and reduced flexural rigidity have been associated with greater corrosion and/or fretting scores [16e24]. Results from different studies, however, have been inconsistent, as femoral head size and duration of implantation have also been shown to have no effect on corrosion and/or fretting scores [25,26]. Using data from the United States Department of Health & Human Services Agency for Healthcare Research and Quality and Healthcare Cost and Utilization Project State Inpatient Databases,

Kremers et al [27] estimated in 2010 that 2.5 million people in the United States were living with a THA, a prevalence rate of 0.83% of the entire country's population. Projections indicate that the rate of primary THA procedures will increase by 174% by 2030, totaling 542,000 procedures annually; subsequently, revision THA procedures will double by 2026 [27e29]. Understanding the factors leading to implant failure may increase the duration of implantation of primary THA and improve the outcomes following revision THA. The objective of this study is to assess the associations among implant design characteristics, radiographic outcomes, and patient factors with fretting and corrosion damage of the femoral stem trunnion and femoral head, in a series of 92 retrieved MoP modular THA systems from primary procedures; in particular, the association between femoral head size and fretting and corrosion at the head-taper interface was investigated. Materials and Methods Following Institutional Review Board approval, retrieved THA implants were obtained from the orthopedic implant retrieval program at our institution between 2002 and 2012. Retrieved devices were catalogued within 1 week of surgery, and subsequently sonicated in a diluted Micro-90 solution for 5 minutes, rinsed in ethanol, followed by another sonication in 90% ethanol for 5 minutes. All nonpolymer components were also subjected to a 5-minute ultrasonic acetone bath. Explants were deidentified through the assignment of an explant identification number, allowed to air dry, vacuum-sealed to prevent oxidation, placed in storage, and electronically catalogued. Querying the electronic database, 92 retrieved primary, MoP THA systems were identified (Table 1). Of the 92 THA systems, 5 different head sizes were represented, including 26 (n ¼ 4, 4%), 28 (n ¼ 38, 41%), 32 (n ¼ 31, 34%), 36 (n ¼ 15, 16%), 38 (n ¼ 1, 1%), and 40 mm (n ¼ 6, 7%). The case with the 38-mm head size was excluded from statistical analyses. Twenty-six (28%) femoral stems were cemented, and 66 (72%) were uncemented. With respect to the femoral head-femoral next interface, 46 (50%) were mixed metal alloy (titanium and cobalt chromium) combinations and 46 (50%) were matched metal alloy combinations; all femoral heads were cobalt chromium alloy. Femoral head offset ranged from 5 to þ15, including þ0 (n ¼ 41, 45%), þ10 (n ¼ 10, 11%), þ5 (n ¼ 4, 4%), and þ12 (n ¼ 4, 4%). Medical Records Review and Radiographic Analysis Medical records were reviewed to collect patient demographic data, including age at revision, gender, body mass index (BMI), limb, reasons for revision, and duration of implantation (Table 2). Operative

Table 2 List of Data Elements Captured. Implant Damage

Clinical Data

Radiographic Evaluation

Implant Characteristics

Femoral head fretting Femoral head corrosion Femoral stem trunnion fretting

Age at revision Body mass index Gender

Osteolysis (location) Implant alignment Implant subsidence

Femoral stem trunnion corrosion

Limb (right, left)

Femoral stem abrasion, pitting, burnishing, scratching

Reason for implantation

Manufacturer Model Component materials (mixed vs same metal femoral stem-femoral head combination) Component fixation (cemented/ uncemented femoral stem) Component features (femoral head offset, femoral head size)

Reason for revision Duration of implantation Infection (organism) Relevant past medical and surgical history (including number of lower limb arthroplasties)


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Fig. 1. Representative digital images from grading of fretting and/or corrosion damage of trunnions, including none (grade ¼ 1) (A), mild (grade ¼ 2) (B), moderate (grade ¼ 3) (C), and severe (grade ¼ 4) (D). Differences in fretting and corrosion damage are illustrated. Severe corrosion, in the form of an adherent black oxide, as well as focal discoloration without gross evidence of mechanical damage, at the base of the taper (E), and fretting, in the form of mild mechanical damage in the middle of the taper (F), are shown.

reports were also reviewed, to record implant fixation and intraoperative observations. Pathology reports were reviewed in cases of suspected infection to confirm infection status. There were 48 men and 44 women, with an average BMI of 30.5 (range 16.2-60.4) and average age of 69 (range 30-93) at revision procedure, and an average duration of implantation of 6.7 years (range 0.1-24). Retrieved components were from the left hip in 47 (51%) cases and right hip in 45 (49%) cases. Reasons for revision included confirmed or suspected infection (n ¼ 32), aseptic loosening (n ¼ 27), unspecified clinical failure (n ¼ 18), instability (n ¼ 6), and periprosthetic fracture (n ¼ 5); 4 cases had unknown reasons for revision. Each case was evaluated for number and type of additional lower extremity arthroplasties; 40 (43%) had no other arthroplasties, 39 (42%) with 1 additional arthroplasty, 10 (11%) with 2 additional arthroplasties, and 3 (3%) with 3 additional arthroplasties. Radiographs from the index procedure and immediate prerevision were reviewed to assess implant positioning as well as the presence or absence of periprosthetic osteolysis. When present, osteolysis was defined by zone, using the method defined by Gruen et al [30] Adequate prerevision radiographs were available for 71 cases, with 34 cases showing evidence of osteolysis. Zonal analysis indicated greatest frequency of osteolysis in zones 7 (n ¼ 22, 65%)

and 1 (n ¼ 21, 62%), respectively; zones 2, 6, and 3 demonstrated lesser frequency of n ¼ 5, n ¼ 4, and n ¼ 1, respectively, while osteolysis in zones 4 and 5 was not exhibited. An analysis of stem alignment indicated acceptable alignment in 48 cases, varus alignment in 18, and valgus alignment in 2 cases. Six cases of subsidence were also identified.

Fretting, Corrosion, and Damage Mode Analyses of Retrieved Implants Both the trunnion surface and femoral head taper were wiped with dehydration alcohol-soaked gauze, before undergoing evaluation and grading by an orthopedic surgery resident and a graduate-trained engineer. Trunnion surfaces were divided into 4 anatomic quadrants (medial, lateral, anterior, posterior), while the femoral head taper was divided into proximal and distal regions. Fretting damage and corrosion were delineated according to Goldberg et al [17]. Briefly, fretting was defined as mechanical damage to surfaces, including burnishing, resulting in focal regions of increased reflectivity, and damage resulting in material removal or plastic deformation. Corrosion was defined as regions with

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Fig. 2. Representative scanning electron micrographs confirming fretting and corrosion damage on surfaces of femoral stem trunnions.

altered optical properties, including discoloration, or loss of reflectivity. For each quadrant (trunnion) or region (head taper), fretting and corrosion were graded on a scale of 1-4, with 1 ¼ none, 2 ¼ mild, 3 ¼ moderate, and 4 ¼ severe, based on criteria described by Goldberg et al [17], using a stereomicroscope (MZ-16; Leica Microsystems, Wetzlar, Germany); also, the differences in the appearance of fretting and corrosion damage were illustrated (Fig. 1). Reviewer scores were compared; in the instance of a discrepancy, a third reviewer (second graduate-trained engineer) examined the implants and determined the final grade. Scanning electron microscopy (VEGA3; Tescan, Brno, Czech Republic) and energy dispersive X-ray spectroscopy were performed on select samples to further resolve mechanical and electrochemical damage. Scanning electron microscopy illustrated areas of corrosion characterized by excessive oxide scale as well as characteristic pitting and scalloping (Fig. 2). Femoral stem components were also visually and microscopically inspected for the presence or absence of the following damage modes: burnishing, pitting, abrasion, and scratching/grooving [31].

Regions with greater reflectivity and polishing, where machining marks were worn away, were defined as burnished. Areas of abrasion damage showed macroscopic and microscopic roughening of the surface. Pitting was identified by small voids, typically grouped, in the surface of the component, while scratching was indicative of areas of noniatrogenic superficial or deep linear cuts [32,33]. Statistical Analysis Study data were statistically analyzed via several software packages (SAS 9.3, SAS Institute, Cary, NC; StatXact 11, Cytel Inc, Cambridge, MA) by a graduate-trained biostatistician to examine the associations among implant damage (fretting and corrosion; specifically, summed trunnion and head grades), implant characteristics, clinical data, and radiographic data (Table 2). Trunnion and head fretting and corrosion grades were compared to femoral head size, using a Wilcoxon rank-sum test; similarly, trunnion and head fretting and corrosion grades of each of the 4 quadrants of the

Fig. 3. Average summed grades of fretting and corrosion damage of the retrieved trunnion (A), with a maximum score of 16, and femoral heads (B), with a maximum score of 8, as categorized by femoral head size.


Fig. 4. Percentage of retrieved trunnion or femoral head components graded as moderate or severe, as categorized by femoral head size.

trunnion and the 2 regions of the femoral head were compared to one another other, to evaluate differences in severity of damage based on zone, using Friedman's test with correction for ties and the sign test, respectively. To assess relationships among clinical, implant damage (summed fretting and corrosion grades; referred to as trunnion fretting, trunnion corrosion, head fretting, and head corrosion in-text), radiographic evaluation, and implant characteristic data, Spearman rank-order correlation, Wilcoxon rank-sum, or Kruskal-Wallis tests were performed as appropriate based on data element characteristics. For all analyses, statistical significance was defined as a ¼ 0.05; however, as a power study was not performed (infeasible due to observational study design and resultant unstandardized distribution), both P-values and magnitude of the difference have been reported. Correlation coefficients (r) of 0.200.40, 0.40-0.60, 0.60-0.80, and 0.80-1.00 were considered weak, moderate, strong, and very strong, respectively [34,35]. Results Fretting, Corrosion, and Damage Mode Analyses of Retrieved Implants Grades were summed for the 4 quadrants of the trunnion, for a maximum possible score of 16, and the 2 femoral head regions, for a maximum score of 8. For all femoral head sizes, the average summed scores of trunnion fretting and corrosion damage were 6.8 (range 4-16) and 6.6 (range 4-16), respectively; similarly, the average summed scores of head fretting and corrosion damage were 3.5 (range 2-8) and 3.5 (range 2-8), respectively. Average summed scores of trunnion fretting and corrosion as well as head fretting and corrosion were grouped by femoral head size (Fig. 3); in addition, trunnion and femoral heads with grades of moderate or severe (grades 3 or 4, respectively) were grouped by 28-mm, 32-mm, and 36-mm femoral head sizes, the most common head sizes in the series (Fig. 4). All the defined damage modesdabrasion, scratching, burnishing, and pittingdwere observed on the components (Fig. 5). For all femoral stem components, scratching was the most common damage mode, observed on 42% of components. Pitting (27%), abrasion (22%), and burnishing (11%) were also documented.

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women), but no associations were observed between patient gender and trunnion fretting (P ¼ .36; median scores: 7.0 for men vs 5.0 for women), trunnion corrosion (P ¼ .18; median scores: 5.5 for men vs 5.0 for women), or head fretting (P ¼ .06; median scores: 4.0 for men vs 3.0 for women). There was a weak, negative correlation between patient age and trunnion corrosion (r ¼ 0.20, P ¼ .04) as well as a weak, positive correlation between BMI and head fretting (r ¼ 0.26, P ¼ .01). A weak, positive correlation was found between duration of implantation and both head fretting (r ¼ 0.33, P < .01) and head corrosion (r ¼ 0.22, P ¼ .04), but not trunnion fretting (r ¼ 0.08, P ¼ .48) or trunnion corrosion (r ¼ 0.08, P ¼ .47). An association was found between trunnion fretting and implants retrieved due to aseptic loosening, infection, and clinical failure, the 3 most common reasons for revision in the study population (P ¼ .04). On average, lower trunnion fretting and corrosion scores were recorded for the 32 implants removed due to infection (6.1 and 6.3, respectively; median scores: 4.5 and 4.5, respectively), compared to the 45 implants revised due to clinically failed or loose prostheses (7.2 and 7.6, respectively; median scores: 5 and 6, respectively) and 6 implants revised for instability (7.8 and 9.2, respectively; median scores: 7 and 10, respectively). Greater trunnion fretting was associated with retrieved implants demonstrating trunnion abrasion (P < .01; median scores: 5.0 for implants without abrasion vs 13.5 for implants with abrasion), burnishing (P < .01; median scores: 6.0 for implants without burnishing vs 8.5 for implants with burnishing), and scratching (P < .01; 5.0 for implants without scratching vs 8.0 for implants with scratching).

Implant Damage and Implant Characteristics A weak, negative association was found between femoral head size and both head fretting (r ¼ 0.26, P < .01) and head corrosion (r¼ 0.21, P ¼ .03). However, there was no association found between femoral head size and trunnion fretting (r ¼ 0.11, P ¼ .30) or trunnion corrosion (r ¼ 0.10, P ¼ .33), while a weak, positive correlation was found between femoral head offset and trunnion fretting (r ¼ 0.23, P ¼ .03). No associations were found between femoral head offset and trunnion corrosion (r ¼ 0.17, P ¼ .09), head fretting (r ¼ 0.18, P ¼ .10), or head corrosion (r ¼ 0.15, P ¼ .16). The 28-mm femoral heads exhibited the more moderate-to-severe damage, as compared with 32-mm or 36-mm femoral heads, with 40%-50% of components having a region of moderate or severe damage (grades 3

Implant Damage and Clinical Data The head corrosion grades were substantially greater for men than for women (P ¼ .04; median scores: 4.0 for men vs 2.0 for

Fig. 5. Percentage of retrieved trunnion or femoral head components with abrasion, scratching, burnishing, or pitting damage, as categorized by femoral head size.

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Table 3 Associations Between Trunnion Fretting and Trunnion Regions, According to Goodman-Kruskal Gamma Values.

Lateral trunnion Medial trunnion Anterior trunnion Posterior trunnion

Lateral Trunnion

Medial Trunnion

Anterior Trunnion

Posterior Trunnion

1.00

0.79 1.00

0.76 0.89 1.00

0.82 0.88 0.91 1.00

or 4, respectively) (Fig. 4). Assessing mixed and matched alloy combinations, Wilcoxon rank-sum tests showed no differences in means of head fretting (P ¼ .33; median scores: 4.0 for matched vs 3.0 for mixed), head corrosion (P ¼ .65; median scores: 3.0 for matched vs 3.0 for mixed), trunnion fretting (P ¼ .23; median scores: 6.0 for matched vs 7.0 for mixed), or trunnion corrosion (P ¼ .13; median scores: 5.0 for matched vs 6.0 for mixed). Similarly, Wilcoxon rank-sum tests of component fixation (cemented vs uncemented) status also showed no differences in means of head fretting (P ¼ .34; median scores: 4.0 for cemented vs 4.0 for uncemented), head corrosion (P ¼ .95; median scores: 3.0 for cemented vs 3.0 for uncemented), trunnion fretting (P ¼ .44; median scores: 5.0 for cemented vs 7.0 for uncemented), or trunnion corrosion (P ¼ .41; median scores: 5.0 for cemented vs 5.0 for uncemented). Implant Damage and Radiographic Analysis Evaluating fretting and corrosions scores with respect to the presence/absence of osteolysis, no associations were found among head fretting (P ¼ .64; median scores: 4.0 with osteolysis vs 3.0 without osteolysis), head corrosion (P ¼ .59; median scores: 3.0 with osteolysis vs 3.0 without osteolysis), trunnion fretting (P ¼ .57; median scores: 7.0 with osteolysis vs 7.0 without osteolysis), or trunnion corrosion (P ¼ .97; median scores: 5.0 with osteolysis vs 5.0 without osteolysis) and presence/absence of osteolysis; however, when assessing femoral stem alignment, patients with varus alignment had greater head fretting (P ¼ .04; median scores: 3.0 for acceptable, 4.5 for varus, 2.0 for valgus). No associations were found among femoral stem alignment and head corrosion (P ¼ .44; median scores: 3.0 for acceptable, 4.0 for varus, 2.0 for valgus), trunnion fretting (P ¼ .85; median scores: 7.0 for acceptable, 6.5 for varus, 4.5 for valgus), or trunnion corrosion (P ¼ .78; median scores: 5.0 for acceptable, 5.5 for varus, 4.5 for valgus). Fretting and Corrosion Damage of Graded Implant Regions Evaluating the regions of the trunnion of the study population, there were 34 retrieved implants with identical trunnion fretting scores across the 4 regions (median scores for all implants: 2.0 for lateral, 2.0 for medial, 2.0 for anterior, 1.0 for posterior) and 50 retrieved implants with identical trunnion corrosion scores (median scores for all implants: 1.0 for all regionsdlateral, medial, anterior, posterior). A greater difference in trunnion damage scores based on region was found for trunnion fretting (P ¼ .047), but not trunnion corrosion (P ¼ .07), while Goodman-Kruskal gamma associations between trunnion fretting and each region of the trunnion were assessed (Table 3). The strongest associations, indicating regions with similar trunnion fretting grades, were found between the posterior and anterior regions (g ¼ 0.91), medial and anterior regions (g ¼ 0.89), and posterior and medial regions (g ¼ 0.88). Fifty-five retrieved implants demonstrated the same head fretting scores for the proximal and distal regions, respectively. In 35 retrieved implants, head fretting was greater in the distal region than proximal region (median scores: 3.0 for distal vs 1.0 for

proximal, respectively), while head fretting was greater in the proximal region in 2 retrieved implants (median scores: 2.0 for distal vs 3.0 for proximal). Similarly, 62 retrieved implants exhibited the same head corrosion scores in the proximal and distal regions. Greater head corrosion was associated with the distal region in 28 retrieved implants (median scores: 3.0 for distal vs 1.5 for proximal) and proximal region in 2 retrieved implants (median scores: 1.5 for distal vs 2.5 for proximal). A sign test showed that the distal region had greater head fretting and head corrosion than the proximal region (P < .01; median head fretting scores for all implants: 3.0 for distal vs 2.0 for proximal; median head corrosion scores for all implants: 2.0 for distal vs 1.0 for proximal). Discussion Fretting and corrosion at the modular junction of retrieved THA implants have been previously evaluated and reported, and various factors correlated with these implant damage modes, including duration of implantation, head size and offset, flexural rigidity, metal alloy mismatch, varus stem alignment, and taper design [16e18,24,25,36e43]. Conflicting results have been presented regarding the effect of head size, with studies assessing both MoM and MoP systems; specifically, fretting without corrosion, corrosion without fretting, fretting with corrosion, and no effect of femoral head size have all been reported [24,43,44]. We studied 92 retrieved MoP THAs to evaluate the associations between femoral head size and corrosion and fretting at the femoral head-femoral neck interface. The associations among demographic, radiographic, and implant characteristics and head and trunnion fretting and corrosion were also investigated. In our study population, smaller head sizes were correlated with greater head corrosion and fretting. Smaller (28 mm) femoral heads were associated with at least one region with moderate or severe corrosion and fretting damage on the trunnion and head, followed by 36-mm and 32-mm head sizes (Fig. 4). Studies evaluating MoM THA prostheses have reported increased damage in larger head sizes, differing from results in our series of MoP THA prostheses; specifically, Bolland et al and Higgs et al both showed a positive association between damage and femoral head size in MoM implants [43,44]. Other studies have reported either no effect or greater damage with increased femoral head size in MoP implants [24,26,45,46]. Del Balso et al evaluated 28-mm and 32-mm heads with identical taper interfaces and found that larger femoral head sizes were associated with increased fretting, but not corrosion, while Dyrkacz et al showed that 36-mm heads exhibited greater corrosion than 28-mm heads, with no significant difference in fretting damage [24,46] To our knowledge, this is the first study to show an inverse relationship between femoral head size and head corrosion and fretting. These results may indicate that the geometry of the taper interface or other factors (eg, implant alignment, patient factors) is more influential than femoral head size in the setting of corrosion and fretting, particularly with smaller head sizes. A weak, positive correlation was found between increased femoral head offset and trunnion fretting, consistent with other retrieval studies, likely due to increased flexural stresses associated with increased offset, as femoral neck stress is dependent on both length and offset and related to fretting and corrosion [37,40,42,47,48]. Metal alloy mismatch at the taper interface is also known as a factor contributing to corrosion damage [17,43,49]. Triantafyllopoulos et al [26] showed a significant association between metal alloy mismatch and trunnion fretting only, with no associations with head corrosion, head fretting, or trunnion corrosion. In our study population, the median damage scores for both trunnion and head taper were greater for mixed metal alloy combinations compared to matched metal alloys; however, these results were not statistically


significant. Regarding implant regions most impacted by damage, Munir et al [21] reported that the most severe wear was located on the medial aspect of the trunnion. Our results correspond with these findings, as our series also showed differences in trunnion fretting, head corrosion, and head fretting scores of trunnion and head regions, with medial and lateral trunnion quadrants and the distal portion of the head exhibiting greater damage. In our study population, male patients had greater head corrosion than female patients, patients with greater BMI were associated with greater head fretting, and older patients demonstrated less trunnion corrosion. Tan et al [25] showed no difference based on age or BMI, but hypothesized that these results may be secondary to a decreased activity level in older patients. Previous studies reported relationships between duration of implantation and damage [17,36,43,49,50]. We found a weak, positive correlation between duration of implantation and both head fretting and head corrosion as well as an association between reasons for revision and trunnion fretting, with implants removed for infection having lower trunnion fretting scores and greater scores in patients revised for clinically failed or loose prostheses. Triantafyllopoulos et al [26] found no relationship between damage and reason for revision. Radiographic analysis demonstrated that patients with varus femoral stem positioning had greater head fretting, yet there was no difference in trunnion damage, regardless of presence or absence of osteolysis on prerevision radiographs. No implants in the series were revised for adverse local tissue reaction. Our study is limited by study design, as this is a retrospective implant retrieval study of a heterogeneous group of implant designs, with various manufacturers and models, evaluated via a subjective grading system to assess implant damage, relying on grader consistency; however, these grading systems have been well-established in literature [17,41,51]. The study population also represents only patients with failed implants, not a true representation of all implants, including well-functioning implants. In addition, our implants were retrieved from a single institution, which may result in regional bias (eg, patient factors, implant designs). This series also had small quantities of smaller and larger sizes, therefore some subset analyses were limited to the most common head sizes (ie, 28, 32, and 36 mm). The analysis of this series of retrieved MoP THA implants showed that smaller femoral head sizes were associated with greater head corrosion and head fretting than larger sizes, with 28mm head sizes exhibiting the most moderate-to-severe damage. To our knowledge, this is the first report of a negative association between head size and head-taper interface damage. These results indicate that other factors, such as head-taper engagement and geometry, or interaction between multiple factors, may have a greater effect on head-taper interface corrosion and fretting, particularly in THA systems with smaller femoral head sizes.

Acknowledgments A generous gift provided by Byron and Dorothy Gerson to establish the Byron and Dorothy Gerson Implant Analysis Fund has supported this research. The authors would like to thank Mary Coffey, PhD, for statistical analysis and report preparation. We acknowledge the work of Orthopaedic Research Laboratory interns, Amie Chaloupka and Makenzie Larson, who assisted with preparing implants for evaluation and grading.

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