Rod Fracture in Magnetically Controlled Growing Spine Rods.

BACKGROUND: The mechanisms of fracture in magnetically controlled growing rods (MCGRs) and the risk factors associated with this are poorly understood. This retrospective analysis of explanted MCGRs aimed to add understanding to this subject. METHODS: From our cohort of over 120 retrieved MCGRs, we identified 7 rods that had fractured; all were single-rod constructs, retrieved from 6 patients. These were examined and compared with 15 intact single-rod constructs. Retrieval and fractographic analyses were used to determine the failure mode at the fracture site and the implant's functionality. Cobb angle, degree of rod contouring, and the distance between anchoring points were computed on anteroposterior and lateral radiographs. RESULTS: 5/7 versus 3/15 rods had been inserted after the removal of a previously inserted rod, in the fractured versus control groups. All fractured rods failed due to bending fatigue. Fractured rods had greater rod contouring angles in the frontal plane ( P = 0.0407) and lateral plane ( P = 0.0306), and greater distances between anchoring points in both anteroposterior and lateral planes ( P = 0.0061 and P = 0.0074, respectively). CONCLUSIONS: We found all failed due to a fatigue fracture and were virtually all single rod configurations. Fracture initiation points corresponded with mechanical indentation marks induced by the intraoperative rod contouring tool. Fractured rods had undergone greater rod contouring and had greater distances between anchoring points, suggesting that it is preferable to implant double rod constructs in patients with sufficient spinal maturity to avoid this complication. CLINICAL RELEVANCE: Level III.

Pin Fracture in Magnetically Controlled Growing Rods: Influence of the Year of Manufacture.

BACKGROUND: Magnetically controlled growing rods (MCGRs) have a known issue with fracture of the internal locking pin resulting in early revisions. The manufacturer reported that rods manufactured before March 26, 2015, had a 5% risk of locking pin fracture. Locking pins made after this date are thicker in diameter and of a tougher alloy; their rate of pin fracture is not known. The aim of this study was to better understand the impact of the design changes on the performance of MCGRs. METHODS: This study involves 46 patients with 76 removed MCGRs. Forty-six rods were manufactured before March 26, 2015, and 30 rods after that date. Clinical and implant data were collected for all MCGRs. Retrieval analysis comprised plain radiographs evaluations, force and elongation testing, and disassembly. RESULTS: The 2 patient groups were statistically comparable. We found that 14 of 27 patients implanted with rods manufactured before March 26, 2015 (group I) had a fracture of their locking pins. Three of the 17 patients with rods manufactured after this date (group II) were also found to have a fractured pin. CONCLUSIONS: Retrieved rods collected at our center and made after March 26, 2015, had far fewer locking pin fractures than those made before this date; this may be due to the change in pin design.

Comparative retrieval analysis of a novel anatomic tibial tray backside: alterations in tibial component design and surface coating can increase cement adhesions and surface roughness.

BACKGROUND: With the Persona® knee system a novel anatomic total knee design was developed, which has no pre-coating, whereas the predecessor knee system is pre-coated with polymethylmethacrylate (PMMA). Joint registry data have shown no decrease in risk of aseptic revision of PMMA pre-coated tibial components compared with non-pre-coated implants. The aim of this retrieval study was to compare the amount of cement adhesions, geometry and surface features between the two knee designs and to correlate them with the underlying reason for revision surgery. METHODS: Retrieval analysis was performed of 15 NexGen® and 8 Persona® fixed-bearing knee implants from the same manufacturer retrieved from two knee revision centres. A photogrammetric method was used to grade the amount of cement attached to the tibial tray backside. The geometry and dimensions of the tibial trays, tray projections and peripheral lips were measured using digital callipers and compared between the two different designs. To measure the surface roughness on the backside of the tibial tray, a contact profilometer was used. To investigate differences between the two designs statistical analyses (t-test) were performed. RESULTS: All Persona® trays showed evidence of cement adhesion with a % area of 75.4%; half of the NexGen® trays had cement adhesions, with a mean value of 20%. There was a significant difference in the percentage of area covered by cement between the two designs (p < 0.001). Results from the contact profilometer revealed that Persona® and NexGen® tray backsides showed a similar lateral (1.36 µm and 1.10 µm) and medial (1.39 µm and 1.12 µm) mean surface roughness with significant differentiation (p < 0.05) of the lateral and medial roughness values between the two designs. Persona® stems showed a significantly higher mean surface roughness (1.26) compared to NexGen® stems (0.89; p < 0.05). CONCLUSION: The novel anatomic knee system showed significantly more cements adhesions and a higher surface roughness which was most likely attributed to the most obvious design and coating alteration of the tibial tray. This study provides first retrieval findings of a novel TKA design recently introduced to the market.

Blood titanium levels in patients with large and sliding titanium implants.

BACKGROUND: Titanium, which is known to be a highly biologically inert element, is one of the most commonly used metals in orthopaedic implants. While cobalt and chromium blood metal ion testing is routinely used in the clinical monitoring of patients with metal-on-metal hip implants, much less is known about the levels of titanium in patients with other implant types. The aim of this study was to better understand the normal ranges of blood titanium levels in patients implanted with large and sliding titanium constructs by comparison with reference levels from conventional titanium hips. METHODS: This study examined data collected from 136 patients. Over a period of 24 months, whole blood samples were collected from 41 patients implanted with large titanium implants: long (range 15 to 30 cm) spine rods with a sliding mechanism ("spine rods", n = 18), long bone tumour implants ("tumour implants", n = 13) and 3D-printed customised massive acetabular defect implants ("massive acetabular implants", n = 10). This data was compared with standard, uncemented primary titanium hip implants ("standard hips", 15 cm long) (n = 95). Clinical, imaging and blood titanium levels data were collected for all patients and compared statistically between the different groups. RESULTS: The median (range) of blood titanium levels of the standard hip, spine rods, femoral tumour implants and massive acetabular implants were 1.2 ppb (0.6-4.9), 9.7 ppb (4.0-25.4), 2.6 ppb (0.4-104.4) and 5.7 ppb (1.6-31.5) respectively. Spine rods and massive acetabular implants had significantly greater blood titanium levels compared to the standard hips group (p < 0.001). CONCLUSION: This study showed that titanium orthopaedic implants that are large and/or have a sliding mechanism have higher blood titanium levels compared to well-functioning, conventionally sized titanium hips. Reassuringly, the increased levels did not appear to induce adverse metal reactions. This study provides useful baseline data for future studies aimed at assessing blood titanium levels as a biomarker for implant function.

Understanding the implant performance of magnetically controlled growing spine rods: a review article.

PURPOSE: Early-Onset Scoliosis (EOS) (defined as a curvature of the spine ≥ 10° with onset before 10 years of age) if not properly treated, can lead to increased morbidity and mortality. Traditionally Growing Rods (TGRs), implants fixated to the spine and extended every 6-8 months by surgery, are considered the gold standard, but Magnetically Controlled Growing Rods (MCGRs) avoid multiple surgeries. While the potential benefit of outpatient distraction procedure with MCGR is huge, concerns still remain about its risks, up to the release of a Medical Device Alert (MDA) by the Medicines and Healthcare Regulatory Agency (MHRA) advising not to implant MCGRs until further notice. The aim of this literature review is to (1) give an overview on the use of MCGRs and (2) identify what is currently understood about the surgical, implant and patient factors associated with the use of MCGRs. METHODS: Systematic literature review. RESULTS: Surgical factors such as use of single rod configuration or incorrect rod contouring might affect early failure of MCGRs. Patient's older age and higher BMI are correlated with rod slippage. Wear debris and distraction mechanism failure may result from implant design and iteration. CONCLUSION: Despite the complications reported, this technology still offers one of the best solutions to spine surgeons dealing with severe EOS. Lowering the complication rate by identifying risk factors for failure is possible and further studies in this direction are required. Once the risk factors are well described, some of these can be addressed enabling a safer use of MCGRs.

Comparative retrieval analysis of antioxidant polyethylene: bonding of vitamin-E does not reduce in-vivo surface damage.

BACKGROUND: With the Persona® knee system a new polyethylene formulation incorporating vitamin-E which aims to reduce oxidation and maintain wear resistance was introduced. Although in-vitro studies have demonstrated positive effects of the vitamin-E antioxidants on UHMWPE, no retrieval study has looked at polyethylene damage of this system yet. It was the aim to investigate the in-vivo performance of this new design, by comparing it with its predecessor in retrieval analysis. METHODS: 15 NexGen® and 8 Persona® fixed-bearing implants from the same manufacturer (Zimmer Biomet) were retrieved from two knee revision centres. For retrieval analysis, a macroscopic analysis of polyethylene using a peer-reviewed damage grading method was used (Hood-score). The roughness of all articulating metal components was measured using a contact profilometer. The reason(s) for TKA revision were recorded. Statistical analyses (t-test) were performed to investigate differences between the two designs. RESULTS: The mean Hood score for Persona® inserts was 109.3 and for NexGen® 115.1 without significant differences between the two designs. Results from the profilometer revealed that Persona® and NexGen® femoral implants showed an identical mean surface roughness of 0.14 µm. The Persona® tibial tray showed a significantly smoother surface (0.06 µm) compared to the NexGen® (0.2 µm; p < 0.001). Both Hood score and surface roughness were influenced by the reasons for revision (p < 0.01). CONCLUSIONS: The bonding of the antioxidant vitamin-E to the PE chain used in the novel Persona® knee system does not reduce in-vivo surface damage compared to highly crosslinked PE without supplemented vitamin-E used in its predecessor knee system NexGen®. However, the Persona® titanium alloy tibial tray showed a significantly smoother surface in comparison to the NexGen® titanium alloy tibial tray. This study provides first retrieval findings of a novel TKA design and may help to understand how the new Persona® anatomic knee system performs in vivo.

Management of patients with magnetically controlled growth rods amidst the global COVID-19 pandemic.

INTRODUCTION: At the time of writing, we are all coping with the global COVID-19 pandemic. Amongst other things, this has had a significant impact on postponing virtually all routine clinic visits and elective surgeries. Concurrently, the Magnetic Expansion Control (MAGEC) rod has been issued with a number of field safety notices and UK regulator medical device alerts. METHODS: This document serves to provide an overview of the current situation regarding the use of MAGEC rods, primarily in the UK, and the impact that the pandemic has had on the management of patients with these rods. RESULTS AND CONCLUSION: The care of each patient must of course be determined on an individual basis; however, the experience of the authors is that a short delay in scheduled distractions and clinic visits will not adversely impact patient treatment. The authors caution against a gap in distractions of longer than 6 months and emphasise the importance of continued remote patient monitoring to identify those who may need to be seen more urgently.

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.

Host-specific factors affect the pathogenesis of adverse reaction to metal debris.

BACKGROUND: Adverse Reaction to Metal Debris (ARMD) is a major reason for revision surgeries in patients with metal-on-metal (MoM) hip replacements. Most failures are related to excessively wearing implant producing harmful metal debris (extrinsic factor). As ARMD may also occur in patients with low-wearing implants, it has been suggested that there are differences in host-specific intrinsic factors contributing to the development of ARMD. However, there are no studies that have directly assessed whether the development of ARMD is actually affected by these intrinsic factors. METHODS: We included all 29 patients (out of 33 patients) with sufficient data who had undergone bilateral revision of ASR MoM hips (58 hips) at our institution. Samples of the inflamed synovia and/or pseudotumour were obtained perioperatively and sent to histopathological analysis. Total wear volumes of the implants were assessed. Patients underwent MARS-MRI imaging of the hips preoperatively. Histological findings, imaging findings and total wear volumes between the hips of each patient were compared. RESULTS: The difference in wear volume between the hips was clinically and statistically significant (median difference 15.35 mm3, range 1 to 39 mm3, IQR 6 to 23 mm3) (p < 0.001). The median ratio of total wear volume between the hips was 2.0 (range 1.09 to 10.0, IQR 1.67 to 3.72). In majority of the histological features and in presence of pseudotumour, there were no differences between the left and right hip of each patient (p > 0.05 for all comparisons). These features included macrophage sheet thickness, perivascular lymphocyte cuff thickness, presence of plasma cells, presence of diffuse lymphocytic infiltration and presence of germinal centers. CONCLUSIONS: Despite the significantly differing amounts of wear (extrinsic factor) seen between the sides, majority of the histological findings were similar in both hips and the presence of pseudotumour was symmetrical in most hips. As a direct consequence, it follows that there must be intrinsic factors which contribute to the symmetry of the findings, ie. the pathogenesis of ARMD, on individual level. This has been hypothesized in the literature but no studies have been conducted to confirm the hypothesis. Further, as the threshold of metal debris needed to develop ARMD appears to be largely variable based on the previous literature, it is likely that there are between-patient differences in these intrinsic factors, ie. the host response to metal debris is individual.

Retrieval analysis of contemporary antioxidant polyethylene: multiple material and design changes may decrease implant performance.

PURPOSE: With the introduction of the Attune Knee System (DePuy) in March 2013, a new polyethylene formulation incorporating anti-oxidants was used. Although several in vitro studies have demonstrated the positive effects of antioxidants on UHMWPE, no retrieval study has looked at polyethylene damage of this system yet. It was the aim of this study to investigate the in vivo performance of this new design, by comparing it with its predecessors in retrieval analysis. METHODS: 24 PFC (18 fixed bearing and 6 rotating platform designs) and 17 Attune (8 fixed bearing and 9 rotating platform designs) implants were retrieved. For retrieval analysis, a macroscopic analysis of polyethylene components, using a peer-reviewed damage grading method was used. Medio-lateral polyethylene thickness difference was measured with a peer-reviewed micro-CT based method. The roughness of metal components was measured. All findings were compared between the two designs. RESULTS: Attune tibial inserts with fixed bearings showed significantly higher hood scores on the backside surface when compared with their PFC counterparts (p = 0.01), no other significant differences were found in the polyethylene damage of all the other surfaces analysed, in the surface roughness of metal components and in medio-lateral linear deformations. CONCLUSION: A significant difference between PFC and Attune fixed bearing designs was found in terms of backside surface damage: multiple changes in material and design features could lead to a potential decrease of implant performance. Results from the present study may help to understand how the new Attune Knee System performs in vivo, impacting over 600,000 patients.

3D patient imaging and retrieval analysis help understand the clinical importance of rotation in knee replacements.

PURPOSE: The purpose of the present study was to correlate highly accurate CT measurements of pre-revision total knee arthroplasty (TKA) implant position with findings of retrieval analysis post-revision, to understand the clinical relevance of TKA orientation. METHODS: This study involved 53 retrieved TKA implants with pre-revision 3D-CT scans used to determine coronal (varus-valgus), sagittal (tibial slope) and rotational (internal rotation-external rotation) TKA orientation as well as tibiofemoral leg axis. Differences between femoral and tibial angles to describe the "relative rotational mismatch" were also calculated. All tibial inserts were forensically analyzed using the Hood score. Statistical analysis was performed to investigate correlations between TKA component orientation and surface damage (p < 0.05). RESULTS: Femoral components were found to have axial rotations mainly within ± 3° (68%), whilst 45% of the tibial components and 66% of the relative rotational mismatches were > 3° and < - 3°, respectively. The majority of femoral and tibial components (87% in both cases), as well as the femorotibial angle (70%), showed coronal orientations within ± 3°. The 64% of the tibial components showed posterior tibial slopes out of both the 0°-3° and 5°-7° ranges. There was a significant correlation between tibial slope and damage score on polyethylene tibial inserts (r = 0.2856; p = 0.0382) as well as a significant correlation between implants' position in the axial plane and damage score on polyethylene tibial inserts (r = 0.6537, p = 0.0240). CONCLUSIONS: This is the first study to use accurate measurements from pre-revision 3DCT to compare tibial and femoral orientation in all three planes with retrieval findings in total knee replacements. A significant correlation between implant position and polyethylene surface damage was found. These results showed the importance of optimizing component position to minimize polyethylene damage. Further analysis involving more accurate polyethylene wear measurements are fundamental to fully understand the role of components' orientation in TKAs.

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.

Analysis of bearing wear, whole blood and synovial fluid metal ion concentrations and histopathological findings in patients with failed ASR hip resurfacings.

BACKGROUND: Adverse Reaction to Metal Debris (ARMD) is still a major reason for revision surgeries in patients with metal-on-metal (MoM) hip replacements. ARMD consists of a wide range of alterations in periprosthetic tissues, most important of which are metallosis, inflammation, pseudotumors and necrosis. Studies investigating histopathological findings and their association to implant wear or indirect measures of wear have yielded inconsistent results. Therefore, we aimed to investigate bearing surface wear volume, whole blood and synovial fluid metal ion concentrations, histopathological findings in periprosthetic tissues and their associations. METHODS: Seventy-eight patients with 85 hips revised for ARMD were included in the study. Prior to revision surgery, all patients had whole blood chromium and cobalt ion levels assessed. In revision surgery, a synovial fluid sample was taken and analyzed for chromium and cobalt. Periprosthetic tissue samples were taken and analyzed for histopathological findings. Explanted implants were analyzed for bearing wear volume of both acetabular cup and femoral head components. RESULTS: Volumetric wear of the failed components was highly variable. The total wear volume of the head and cup had a strong correlation with whole blood chromium and cobalt ion concentrations (Cr: ρ = 0.80, p < 0.001 and Co: ρ = 0.84, p < 0.001) and a bit weaker correlation with fluid chromium and cobalt ion concentrations (Cr: ρ = 0.50, p < 0.01 and Co: ρ = 0.41, p = 0.027). Most tissues displayed only low-to-moderate amounts of macrophages and lymphocytes. Total wear volume correlated with macrophage sheet thickness (ρ = 0.25, p = 0.020) and necrosis (ρ = 0.35, p < 0.01). Whole blood chromium and cobalt ion concentrations had similar correlations. Lymphocyte cuff thickness did not correlate with either total wear volume or whole blood metal ion concentrations, but correlated with the grade of necrosis. CONCLUSIONS: Bearing wear volume correlated with blood metal ion levels and the degree of necrosis and macrophage infiltration in periprosthetic tissues suggesting a dose-response relationship. Whole blood metal ion levels are a useful tool for clinician to estimate bearing wear and subsequent tissue response.

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.

Wear of dual-mobility cups: a review article.

Dual-mobility (DM) cups have been clinically used in hip surgery in Europe for more than 35 years and continue to gain popularity worldwide due to promising results at reducing instability. Concerns related to polyethylene wear apply as in conventional standard bearings but are accentuated by the larger-diameter articulations with multiple surfaces. We critically reviewed the reported literature regarding the in vivo and in vitro wear occurring on all surfaces involved. We looked for patterns to create a rational classification of sites of wear and to identify areas for future research. Wear was a significant problem for first-generation designs and appeared to be design related. Improved polyethylene, thinner and smoother trunnions, chamfered rims and eccentric configuration of insert and shell seem to enhance outcome performance; however, long-term clinical evidence and retrieval studies are needed to better understand the balance of benefit and risk when opting for DM bearings.