Mechanical wear analysis helps understand a mechanism of failure in retrieved magnetically controlled growing rods: a retrieval study.

BACKGROUND: To assess the relationship between mechanical wear and the failure of the internal lengthening mechanism in retrieved MAGnetic Expansion Control (MAGEC) growing rods. METHODS: This study included 34 MAGEC rods retrieved from 20 patients. The state of the internal mechanism and mechanical wear were assessed in all the rods using plain radiographs and visual inspection. Metrology was then performed to assess the topography and mechanical wear of the telescopic bars, using a Talyrond 365 (Taylor Hobson, Leicester, UK) roundness measuring machine. RESULTS: Plain radiographs showed evidence of a broken internal mechanism in 29% of retrieved rods. Single-side wear marks were found in 97% of retrieved rods. Material loss was found to significantly increase in rods with a damaged internal mechanism (p < 0.05) and rods with longer time in situ (r = 0.692, p < 0.05). CONCLUSIONS: We found an association between damage to the internal mechanism of the rods and (1) patterns of single-side longitudinal wear marks and (2) increased material loss. As the material loss was also found to increase over time of rod in situ, we emphasise the importance of early detection and revision of failed MAGEC rods in clinical practice.

Effect of Bearing Type on Taper Material Loss in Hips From 1 Manufacturer.

BACKGROUND: Numerous studies have reported on clinical significant volumes of material loss and corrosion at the head-stem junction of metal-on-metal (MOM) hips; less is understood about metal-on-polyethylene (MOP) hips. We compared the effect of bearing type (MOM vs MOP) on taper material loss for a hip system of a single design. METHODS: In this cohort study, we recruited retrieved MOM (n = 30) and MOP (n = 22) bearing hips that were consecutively received at our center. We prospectively collected associated clinical and imaging data. We measured the severity of corrosion and volumes of material loss at each head taper surface and used multivariate statistical analysis to investigate differences between the 2 bearing types. RESULTS: The median rate of material loss for the MOM and MOP groups was 0.81 mm3/y (0.01-3.45) and 0.03 mm3/y (0-1.07), respectively (P < .001). Twenty-nine of 30 MOM hips were revised for adverse metal reactions, compared with 1 of 22 MOP hips. CONCLUSION: MOP hips lost significantly less material from their taper junctions than MOM hips. Our results can reassure patients with MOP Pinnacle hips that they are unlikely to experience clinically significant problems related to material loss from the taper junction.

Computed Tomography Techniques Help Understand Wear Patterns in Retrieved Total Knee Arthroplasty.

BACKGROUND: Suboptimal total knee arthroplasty (TKA) position of both femoral and tibial components is thought to be linked with poor clinical outcomes, polyethylene wear and the "unexplained" painful knee arthroplasty. The aim of this study was to better understand the effect of implant orientation on knee implant performance. METHODS: We analyzed 30 retrieved contemporary TKA implants. Implant positioning measurements in the coronal plane were made prior to revision using a diagnostic algorithm, based on 3D computed tomography (CT) images. Each retrieved polyethylene component was imaged using a micro-CT scanner and a high resolution computational 3D model of each component was digitally reconstructed. The difference in thickness between medial and lateral components was calculated. Statistical analysis was performed to investigate the association between component positioning and damage patterns. RESULTS: We found a significant correlation between both the tibiofemoral and femoral angles and difference in thickness between polyethylene compartments: varus angulations were strongly associated with thinner medial compartments, whilst valgus angulations were associated with thinner lateral compartments. Moreover, suboptimal tibiofemoral orientations and tibial component angulations were associated to greater differences in thickness between polyethylene compartments. CONCLUSION: Our study is the first to compare accurate 3D CT measurements of prerevision TKA positioning in the coronal plane with postrevision retrieval analysis from innovative, accurate and highly reliable micro-CT-based method. Our results demonstrate the impact of component positioning on polyethylene damage and helps understanding of the in vivo performance of these implants. LEVEL OF EVIDENCE: III.

Analysing a mechanism of failure in retrieved magnetically controlled spinal rods.

PURPOSE: We aim to describe a mechanism of failure in magnetically controlled growth rods which are used for the correction of the early onset scoliosis. METHODS: This retrieval study involved nine magnetically controlled growth rods, of a single design, revised from five patients for metal staining, progression of scoliosis, swelling, fractured actuator pin, and final fusion. All the retrieved rods were radiographed and assessed macroscopically and microscopically for material loss. Two implants were further analysed using micro-CT scanning and then sectioned to allow examination of the internal mechanism. No funding was obtained to analyse these implants. There were no potential conflicts interests. RESULTS: Plain radiographs revealed that three out of nine retrieved rods had a fractured pin. All had evidence of surface degradation on the extendable telescopic rod. There was considerable corrosion along the internal mechanism. CONCLUSIONS: We found that a third of the retrieved magnetically controlled growth rods had failed due to pin fracture secondary to corrosion of the internal mechanism. We recommend that surgeons consider that any inability of magnetically controlled growth rods to distract may be due to corrosive debris building up inside the mechanism, thereby preventing normal function.

Clinical Cold Welding of the Modular Total Hip Arthroplasty Prosthesis.

BACKGROUND: A head that is "clinically cold welded" to a stem is one of the commonest reasons for unplanned removal of the stem. It is not clear which hip designs are at greatest risk of clinical cold welding. METHODS: This was a case-control study of consecutively received hip implant retrievals; we chose the design of hip that had the greatest number of truly cold-welded heads (n = 11). For our controls, we chose retrieved hips of the same design but without cold welding of the head (n = 35). We compared the clinical variables between these 2 groups using nonparametric Mann-Whitney tests to investigate the significance of differences between the cold-welded and non-cold-welded groups. RESULTS: The design that most commonly caused cold welding was a combination of a Ti stem and Ti taper: 11 out of 48 (23%) were truly cold welded. Comparison of the clinical data showed that no individual factor could be used to predict this preoperatively with none of the 4 predictors tested showing any significance: (1) time to revision (P = .687), (2) head size (P = .067), (3) patient age at primary (P = .380), and (4) gender (P = .054). CONCLUSION: We have shown that clinical cold welding is most prevalent in Ti-Ti combinations of the stem and taper; approximately 25% of cases received at our center were cold welded. Analysis of clinical variables showed that it is not possible to predict which will be cold welded preoperatively. Surgeons should be aware of this potential complication when revising a Ti-Ti stem/head junction.

Factors Associated With Trunnionosis in the Metal-on-Metal Pinnacle Hip.

BACKGROUND: Trunnionosis of the tapered head-stem junction of total hip arthroplasties, either through corrosion or mechanical wear, has been implicated in early implant failure. Retrieval analysis of large numbers of failed implants can help us better understand the factors that influence damage at this interface. METHODS: In this study, we examined 120 retrieved total hip arthroplasties of one bearing design, the 36-mm diameter metal-on-metal, DePuy Pinnacle, that had been paired with 3 different stems. We measured material loss of the bearing and head-trunnion taper surfaces and collected clinical and component data for each case. We then used multiple linear regression analysis to determine which factors influenced the rate of taper material loss. RESULTS: We found 4 significant variables: (1) longer time to revision (P = .004), (2) the use of a 12/14 taper for the head-trunnion junction (P < .001), (3) decreased bearing surface wear (P = .003), and (4) vertical femoral offset (P = .05). These together explained 29% of the variability in taper material loss. CONCLUSION: Our most important finding is the effect of trunnion design. Of the 3 types studied, we found that S-ROM design was the most successful at minimizing trunnionosis.

Damage Patterns at the Head-Stem Taper Junction Helps Understand the Mechanisms of Material Loss.

BACKGROUND: Material loss at the taper junction of metal-on-metal total hip arthroplasties has been implicated in their early failure. The mechanisms of material loss are not fully understood; analysis of the patterns of damage at the taper can help us better understand why material loss occurs at this junction. METHODS: We mapped the patterns of material loss in a series of 155 metal-on-metal total hip arthroplasties received at our center by scanning the taper surface using a roundness-measuring machine. We examined these material loss maps to develop a 5-tier classification system based on visual differences between different patterns. We correlated these patterns to surgical, implant, and patient factors known to be important for head-stem taper damage. RESULTS: We found that 63 implants had "minimal damage" at the taper (material loss <1 mm3), and the remaining 92 implants could be categorized by 4 distinct patterns of taper material loss. We found that (1) head diameter and (2) time to revision were key significant variables separating the groups. CONCLUSION: These material loss maps allow us to suggest different mechanisms that dominate the cause of the material loss in each pattern: (1) corrosion, (2) mechanically assisted corrosion, or (3) intraoperative damage or poor size tolerances leading to toggling of trunnion in taper.

Retrieval evidence of impingement at the third articulation in contemporary dual mobility cups for total hip arthroplasty.

PURPOSE: We aimed to assess polyethylene liners of retrieved hips of one design of a dual mobility (DM) cup liner and two designs of femoral stems to better understand the role of femoral stem design on polyethylene impingement. METHODS: This was a case-control study involving 70 retrieved highly cross-linked polyethylene (X3) liners used with ABGII (n = 35) and Rejuvenate (n = 35) stems (Stryker). All polyethylene liners were assessed for evidence of rim deformation and the damage quantified using metrology methods. RESULTS: A total of 80% of polyethylene liners paired with ABGII necks had macroscopic evidence of neck impingement resulting in a raised lip whilst 23% of liners paired with Rejuvenate necks had evidence of a raised lip (p < 0.0001). The height of the raised rims of the DM cups paired with ABGII necks had a median (range) of 139 µm (72-255). The height of the raised rims of the DM cups paired with Rejuvenate necks had a median (range) of 52 µm (45-90) (p < 0.0001). CONCLUSION: Our new findings from retrieved dual mobility bearings showed that polyethylene liner rim deformation resulting from impingement with the femoral neck occurs in early in-human function, is circumferential in distribution, and is affected by the stem neck design. We recommend the use of highly polished and non-edged neck designs when used in conjunction with DM cups.

The Relationship Between Cobalt/Chromium Ratios and the High Prevalence of Head-Stem Junction Corrosion in Metal-on-Metal Total Hip Arthroplasty.

BACKGROUND: The size of the clinical impact of corrosion of the taper junction of metal-on-metal total hip arthroplasties (MOM-THAs) is unclear. Examination of a large number of retrieved MOM resurfacings and total hip arthroplasties can help us understand the role of taper corrosion in metal ion release. METHODS: We graded the severity of corrosion at the taper junction of 395 MOM-THAs and compared the prerevision whole blood metal ion levels of these hips with 529 failed MOM hip resurfacings. RESULTS: Virtually all MOM-THA hips (n = 388) had evidence of corrosion of the head-stem taper junction and graded as severe in 31% (n = 124). The median cobalt/chromium (Co/Cr) ratio was 1.58 (0.01-13.82) and 1.08 (0-4.86) for MOM-THA and MOM hip resurfacing, respectively; this difference was significant (P < .001). THA hips with severely corroded tapers had the highest median Co/Cr ratio of 1.86 (0.01-10). CONCLUSIONS: This study demonstrates the high prevalence of severe taper corrosion, which may be related to an elevated Co/Cr ratio before revision.

Clinical significance of corrosion of cemented femoral stems in metal-on-metal hips: a retrieval study.

PURPOSE: The clinical significance of corrosion of cemented femoral stems is unclear. The purpose of this retrieval study was to: (1) report on corrosion at the stem-cement interface and (2) correlate these findings with clinical data. METHODS: We analysed cemented stems (n = 36) composed of cobalt-chromium (CoCr) and stainless steel (SS) in a series of revised metal-on-metal hips. We performed detailed inspection of each stem to assess the severity of corrosion at the stem-cement interface using a scale of 1 (low) to 5 (severe). We assessed the severity of corrosion at each stem trunnion and measured wear rates at the head taper and bearing surfaces. We used non-parametric tests to determine the significance of differences between the CoCr and SS stems in relation to: (1) pre-revision whole blood Co and Cr metal ion levels, (2) trunnion corrosion, (3) bearing surface wear and (4) taper material loss. RESULTS: The corrosion scores of CoCr stems were significantly greater than SS stems (p < 0.01). Virtually all stem trunnions in both alloy groups had minimal evidence of corrosion. The median pre-revision Co levels of implants with CoCr stems were significantly greater than the SS stems (p < 0.01). There was no significant difference in relation to pre-revision Cr levels (p = 0.521). There was no significant difference between the two stem types in relation to bearing wear (p = 0.926) or taper wear (p = 0.148). CONCLUSIONS: Severe corrosion of cemented femoral stems is a common finding at our retrieval centre; surgeons should consider corrosion of CoCr stems as a potential source of metal ions when revising a hip.

Corrosion of Metal Modular Cup Liners.

Numerous studies have reported on corrosion at the modular head taper, however less is known about the interface between the metal shell and liner of modular cups. This study examined the backside of a series of metal modular cup liners of two designs (DePuy Pinnacle and Smith & Nephew R3), retrieved from 67 patients. Visual inspection found evidence of corrosion in virtually all liners, with the engaging rim surface significantly more corroded than the polar regions (P<0.001). EDX confirmed that black surface deposits were chromium rich corrosion debris, while SEM analysis revealed considerable pitting in the vicinity of the black debris. The R3 liners were significantly more corroded that the Pinnacles (P<0.001); this may help to explain the higher revision rates of this design.

The reliability of a scoring system for corrosion and fretting, and its relationship to material loss of tapered, modular junctions of retrieved hip implants.

It has been suggested that corrosion and fretting at the tapered, modular junctions of hip arthroplasties may contribute to implant failure. In this study the reliability of a commonly used peer-reviewed scoring system for visual assessment of corrosion and fretting at these junctions was evaluated. Volumetric material loss at the tapered head surface was measured and associations with the visual scores were investigated. We found that the inter-observer reproducibility and single-observer repeatability of the corrosion scores were substantial using Cohen's weighted Kappa statistic (k = 0.64-0.71). The reproducibility and repeatability of the fretting scores however were slight to fair (k = 0.18-0.31). Taper corrosion scores were significantly and moderately correlated with the volume of material loss measured (Spearman's r = 0.59; P < 0.001). We recommend the continued use of this scoring system but it should not be a substitute for measurement of material loss.

Component size mismatch of metal on metal hip arthroplasty: an avoidable never event.

Size mismatch of components used in total hip arthroplasty is a serious, preventable patient safety incident of unknown prevalence as many cases are not detected. Component size mismatch was found in 11 cases (0.9%) at our retrieval centre. All cases of mismatch were not detected on plain radiograph during routine clinical follow up and blood metal ion levels were elevated above the MHRA action level of 7 ppb. Root cause analysis identified manufacturer, hospital and surgeon factors that need to be addressed to reduce the incidence of this avoidable clinical problem. Retrieval analysis is the only method of confirming size mismatch and is likely to be under-represented in National Joint Registries that record the indication for revision at the time of revision.

Metal-on-metal total hip arthroplasty: does increasing modularity affect clinical outcome?

BACKGROUND: Modularity of metal-on-metal (MoM) implants has come under scrutiny due to concerns regarding additional sources of metal debris. This study is a retrieval analysis of implants from the same manufacturer with the same MoM bearing surface. The difference between the implants was presence or absence of modular junctions. METHODS: This is a retrospective study of 31 retrieved implants from 31 patients who received a Conserve Wright Medical MoM hip prosthesis. The 31 implants consisted of 16 resurfacings and 15 implants with modular junctions; 4 conventional THAs and 11 modular-neck THAs. RESULTS: 43% of pre-revision MRI scans performed on resurfacing implants and 91% performed on the modular implants illustrated evidence of an adverse local tissue reaction. There was no difference in pre-revision blood metal ion levels or bearing surface wear between the resurfacings and modular implants. The neck-head tapers of the modular group showed low levels of material loss. However, the neck-stem tapers showed increased severity of corrosion and material loss. CONCLUSIONS: The modular implants had an increased incidence of adverse local tissue reaction. This could be related to the presence of modular junctions, particular the neck-stem junction which showed increased susceptibly to corrosion.

Does modularity of metal-on-metal hip implants increase cobalt: chromium ratio?

BACKGROUND: Blood metal ion levels are used in the surveillance of metal-on-metal (MoM) hip implants. Modular implants contain an extra source of metal debris that may affect the ratio of metal ions in the blood. METHODS: This was a retrospective study of 503 patients with hip replacements made by a single manufacturer (Smith & Nephew, Warwick, UK) with the same bearing surface. There were 54 total hip arthroplasties, 35 Birmingham Mid-Head Resections and 414 hip resurfacings. Whole blood metal ion levels and their ratios were analysed to investigate the effect of a modular junction. RESULTS: The cobalt:chromium ratios were greater in the total hip arthroplasty group (mean 2.3:1) when compared to the resurfacings group (mean 1.3:1, p = <0.05) and Birmingham Mid-Head Resection group (mean 1.1:1, p = 0.11). CONCLUSIONS: This study demonstrated a trend for a higher cobalt:chromium ratio in patients with MoM total hip replacement that may be due to metal debris from the modular stem-head junction. Further work is required to correlate clinical data with retrieval analysis to confirm the effect of taper material loss on the cobalt:chromium ratio.

Retrieval Findings of Recalled Dual-Taper Hips.

BACKGROUND: The recent high-profile recalls of several dual-taper hip designs pose questions regarding why those designs perform poorly. We aimed to characterize taper damage in 1 recalled design to understand failure mechanisms to inform surgeons on which patients should be considered at risk of revision and when to revise. METHODS: High-precision measurement equipment was used to characterize the metal loss from the neck-stem interface of 116 retrieved Rejuvenate femoral stems (Stryker Howmedica Osteonics) revised because of an adverse reaction to metal debris. Head-neck taper surfaces were also investigated, and clinical and laboratory data were examined. RESULTS: The neck-stem junction of each implant was moderately to severely corroded and showed a characteristic wear pattern on both male and female taper surfaces. The severity of taper damage was positively correlated with time to revision (coefficient, 0.040 [95% confidence interval (CI), 0.028 to 0.051]; p < 0.0001) and with serum cobalt concentration (coefficient, 0.02 [95% CI, 0.01 to 0.02]; p < 0.0001) and serum chromium concentration (coefficient, 0.04 [95% CI, 0.009 to 0.070]; p = 0.0142). CONCLUSIONS: A forensic examination of the retrieved components that failed secondary to an adverse reaction to metal debris showed, in all cases, visible corrosion. Of the implant and patient factors investigated, we did not identify any predictors of corrosion. The severity of damage was found to increase with time; cobalt was significantly elevated over chromium. CLINICAL RELEVANCE: Surgeons should scrupulously follow and consider revision for patients with this implant design to avoid extensive tissue excision. Blood metal ion tests may aid in identifying the mechanisms of taper corrosion.

Assessment of material loss of retrieved magnetically controlled implants for limb lengthening.

PURPOSE: We aimed to understand wear from the telescopic component of PRECICE nails, which are used for distraction osteogenesis of the femur or tibia. We also aimed to identify any correlation between implant performance and patient factors. METHODS: This retrieval study involved 11 magnetically controlled intramedullary nails from nine patients who had achieved the targeted leg length. All the nails were assessed macroscopically and microscopically for wear. All implants were radiographed to assess the internal mechanism. A Talyrond 365 (Taylor Hobson, Leicester, UK) roundness measuring machine was used to generate three-dimensional surface maps of the telescopic components to allow for measurement of wear. RESULTS: Visual assessment of all the nails showed evidence of wear from the telescopic component. The radiographs revealed that all the nails had intact internal mechanism and no evidence of fractured pins. The roundness measuring machine showed that the quantity of wear was lowest in the latest design of the PRECICE nail. There was no significant correlation between wear and the two patient factors (duration of the lengthening phase, the time of implantation) included in this study. CONCLUSION: This study is the first to investigate the performance of the three different designs of the PRECICE system with a focus on wear. We found that the latest design had the best implant performance. We are confident of the continued success of the PRECICE system and reassure surgeons and patients that they are unlikely to encounter problems with the implant related to wear.

Inflammatory cell-induced corrosion in total knee arthroplasty: A retrieval study.

Metal release in patients with joint replacements is associated with local tissue reactions, pain, and ultimately revision of implants. One of the causes of this metal loss is speculated to be due to a mechanism of inflammatory cell-induced corrosion (ICIC). In this knee retrieval study, we aimed to: (1) identify the extent and location of ICI corrosion patterns on our femoral and tibial components and (2) correlate our findings with implant and clinical information. We investigated 28 femoral and 9 tibial components made of polished CoCr for presence of ICIC, using macroscopic and microscopic screening and statistical analyses to identify any significant correlations between our results and clinical information. We found that 71% of femoral and 100% of tibial components showed evidence of ICIC and significantly more was present on non-contacting regions (p < 0.0001). We found a significant correlation between the presence of ICIC and instability (p = 0.0113) and a significant difference between poster stabilized and cruciate retaining designs in the amount of ICIC on internal edges (p = 0.0375). This corrosion pattern was prevalent in our series of knee retrievals and may help explain some of the mechanisms of material loss that may occur in vivo. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 460-467, 2018.

Variation in taper surface roughness for a single design effects the wear rate in total hip arthroplasty.

Material loss from the head-stem taper junction of total hip arthroplasty (THA) is implicated in adverse reactions to metal debris (ARMD); the mechanisms for this are multi-factorial. We investigated the relationship between the roughness of the "as manufactured" taper surface and the wear rate from this junction. Fifty retrieved Pinnacle metal-on-metal (MOM) bearings paired with a Corail stem were included in the study. Multivariable statistical analysis was performed to determine the influence of taper roughness on material loss rate after controlling for other confounding surgical, implant, and patient factors. The surface roughness of the "as manufactured" head taper surface was associated with the rate of material loss from this surface. Four of eighteen roughness variables taken from ISO 4,287 and ISO 13,565-2 were significant: The Reduced Peak Height (Rpk, the protruding peaks above the core) (p = 0.004), Material Ratio 1 (Mr1, the ratio of the protruding peaks above the core) (p = 0.002), Area of the Peak Region (A1, the area of the Abbott-Curve that contains the peaks from the profile) (p = 0.003) and the Skewness (Rsk, the asymmetry of the height distribution corresponding to the height or depth of surface features) (p = 0.03). We found a large variability in the measured values with a median (range) of 0.50 (0.05-2.98), 11.98 (0.46-39.98), 30.89 (0.15-581.00), and 0.04 (-0.73-0.84), respectively. A 1-unit increase in Rpk was associated with a 73% increase in the taper wear rate. The variability of "as manufactured" surface roughness has a significant effect on taper material loss. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1784-1792, 2017.