Do cobalt and chromium blood metal ion levels normalize after revision of failed metal-on-metal total hip replacements?

INTRODUCTION: Increased cobalt (Co) and chromium (Cr) serum levels are one reason for revision surgery in metal on metal (MoM) total hip arthroplasty (THA) patients. Dual mobility liners are a simple revision option; however, they preserve the metal shell and stem and it is therefore not clear if metal ion levels will fully normalize after revision surgery. MATERIAL AND METHODS: Between April 2013 and December 2017 25 hips (24 patients) underwent revision from a MoM THA to an off-label dual mobility liner. Five patients were lost to follow-up and one patient refused leaving 18 patients (11 men, 7 female, average age 63.9 years) for pre- and postoperative metal ion level blood tests at a minimum follow-up of 2 years. RESULTS: Fourteen patients were revised for osteolysis, two for elevated metal ion levels and two for fluid or cysts around the femoral or acetabular component. The average preoperative Co and Cr levels were 8.3 µg/l and 5.0 µg/l, respectively. At a minimum follow-up of 2 years (30-95 months), metal ions dropped to 0.8 µg/l. Harris Hip Score (HHS), Hip Osteoarthritis Outcome Score (HOOS), Visual Analog Scale (VAS) and the UCLA activity score improved non-significantly from pre- to postoperative. There was one postoperative complication in the study cohort. One patient with persisting pain required revision surgery to a standard acetabular component during the follow-up period. CONCLUSION: The off-label use of a dual-mobility liner in the current study resulted in normalization of the metal ion levels suggesting that preserving the cobalt-chromium acetabular component has little impact on ion levels. In addition, dual mobility liners have a low complication and revision rate.

Quantitative Ultrasound Assessment of Early Osteoarthritis in Human Articular Cartilage Using a High-Frequency Linear Array Transducer.

Quantitative ultrasound (QUS) assessment of osteoarthritis (OA) using high-frequency, research-grade single-element ultrasound systems has been reported. The objective of this ex vivo study was to assess the performance of QUS in detecting early OA using a high-frequency linear array transducer. Osteochondral plugs (n = 26) of human articular cartilage were scanned with ExactVu Micro-Ultrasound using an EV29L side-fire transducer. For comparison, the samples were also imaged with SAM200Ex, a custom 40-MHz scanning acoustic microscope with a single-element, focused transducer. Thirteen QUS parameters were derived from the ultrasound data. Magnetic resonance imaging (MRI) data, with T1 and T2 extracted as the quantitative parameters, were also acquired for comparison. Cartilage degeneration was graded from histology and correlated to all quantitative parameters. A maximum Spearman rank correlation coefficient (ρ) of 0.75 was achieved using a combination of ExactVu QUS parameters, while a maximum ρ of 0.62 was achieved using a combination of parameters from SAM200Ex. A maximum ρ of 0.75 was achieved using the T1 and T2 MRI parameters. This study illustrates the potential of a high-frequency linear array transducer to provide a convenient method for early OA screening with results comparable to those of research-grade single-element ultrasound and MRI.

Ceramic-on-Ceramic Bearing in Total Hip Arthroplasty Reduces the Risk for Revision for Periprosthetic Joint Infection Compared to Ceramic-on-Polyethylene: A Matched Analysis of 118,753 Cementless THA Based on the German Arthroplasty Registry.

Periprosthetic joint infection (PJI) is one of the most common complications in total hip arthroplasty (THA). The influence of bearing material on the risk of PJI remains unclear to date. This registry-based matched study investigates the role of bearing partners in primary cementless THA. Primary cementless THAs recorded in the German Arthroplasty Registry since 2012 with either a ceramic-on-ceramic (CoC) or ceramic-on-polyethylene (CoP) bearings were included in the analysis. Using propensity score matching (PSM) for age, sex, obesity, diabetes mellitus, Elixhauser comorbidity index, year of surgery and head size, we compared the risk for revision for PJI for CoC and CoP. Within the 115,538 THAs (87.1% CoP; 12.9% CoC), 977 revisions were performed due to PJI. There was a significantly higher risk for revision for PJI for CoP compared with CoC over the whole study period (p < 0.01) after 2:1 matching (CoP:CoC) with a hazard ratio of 1.41 (95% confidence interval (CI), 1.09 to 1.80) After 3 years, the risk for revision for PJI was 0.7% (CI 0.5-0.9%) for CoC and 0.9% (CI 0.8-1.1%) for CoP. The risk for revision for all other reasons except PJI did not significantly differ between the two groups over the whole study period (p = 0.4). Cementless THAs with CoC bearings were less likely to be revised because of infection in mid-term follow-up. In the future, registry-embedded studies focusing on long-term follow-up, including clinical data, as well as basic science studies, may give a deeper insight into the influence of the bearing partners.

[Importance of the tibial slope in knee arthroplasty]. / Bedeutung des tibialen Slopes in der Knieendoprothetik.

Notwithstanding the contributions of soft tissue restraints on postoperative kinematics and long-term survival after total knee arthroplasty (TKA), there is an emerging consensus that the underlying anatomy, especially the posterior inclination of the tibial plateau in the sagittal plane (tibial slope), might just have a comparable impact. However, this has not been fully elucidated as yet. Therefore, a thorough literature search, analysis and presentation of current scientific data was conducted. The tibial slope has been shown to relate linearly to the postoperative range of motion and function of the extensor mechanism. Furthermore, it impacts wear of the tibial insert and loosening, as well as instability of the TKA. As no consensus has been reached on the ideal tibial slope, recommendations range from 0° to 10°. Notably, more recent studies favor reconstructing the native, preoperative tibial slope, and the majority of authors advocate that knowledge of this is crucial for optimal TKA surgery.

head-bent resistant Hsc70 variants show reduced Hsp40 affinity and altered protein folding activity.

The molecular chaperone Hsc70 performs essential tasks by folding proteins. Hsc70 is driven by the hydrolysis of ATP and tuned by the association with various co-chaperones. One such cofactor is the nematode nucleotide exchange factor UNC-23, whose mutation disrupts muscle attachment and induces a severe head-bent phenotype in C.elegans. Interestingly, four mutations in Hsc70 can suppress this phenotype, but the molecular mechanism underlying this suppression is unknown. Here we characterize these four suppressor variants, Hsc70 D233N, S321F, A379V and D384N. In vitro only Hsc70 S321F shows reduced stability and altered nucleotide interaction, but all mutations affect the ATPase stimulation. In particular, Hsc70 D233N and Hsc70 A379V show strongly reduced interactions with DNJ-12 and DNJ-13. Nucleotide exchange factor binding instead is barely influenced in Hsc70 D233N, A379V and D384N and their chaperone activity is preserved. Molecular dynamics simulations suggest that effects in Hsc70 S321F and Hsc70 A379V originate from steric clashes in the vicinity of the mutation site, while D233N disrupts a salt bridge that contributes to Hsc70's nucleotide-induced conformational changes. In summary, the analyzed mutants show altered ATPase and refolding activity caused by changes in Hsp40 binding.

Weight-bearing radiography depends on limb loading.

PURPOSE: The mechanical axis of the lower limb has shown to vary between different weight-bearing conditions and change after total knee arthroplasty (TKA). The purpose of this study was to investigate the correlation between mechanical axis alignment in standing long-leg radiographs and limb loading after TKA. METHODS: Mechanical axis of the lower limb and limb loading have been prospectively evaluated in 115 patients 10 days and 3 months after TKA. By the moment of standing long-leg radiography for analysis of the mechanical leg axis, two digital scales separately captured the load of each limb. RESULTS: Mechanical axis changed from an initial - 1° ± 2° valgus alignment to a varus axis of + 1° ± 2° (p < 0.01). This change in alignment was associated with an increase of limb loading from 89.9 ± 10.7 to 93.0 ± 7.0% (p < 0.01). The mechanical axis strongly correlated with relative limb loading at the first and second measurements (r = 0.804, p < 0.001, respectively, r = 0.562, p < 0.001). A significant change in the rate of outliers was registered within the observation period. These alterations and distinctions were much more pronounced in patients with postoperative incomplete extension (n = 15). CONCLUSIONS: The postoperative mechanical axis correlates with limb loading. A clinical relevant change in frontal alignment of the lower limb is associated with increased limb loading after TKA. The actual mechanical axis can only be assessed at physiological limb loading in long-leg radiographs with complete extension at full weight bearing. LEVEL OF EVIDENCE: Diagnostic study, Level II.

Salvage of a monoblock metal-on-metal cup using a dual mobility liner: a two-year MRI follow-up study.

PURPOSE: Revision of failed modular metal-on-metal total hip replacement (MoM-THA) can be technically difficult. A dual mobility liner can help to salvage a well-fixed acetabular component. The present paper reports the clinical and radiographic outcome of revision of failed Birmingham modular MoM-THA using a dual mobility liner. METHODS: The present study reports on ten patients (3 female and 7 male) with 11 revision THAs. Patients underwent revision an average of 51 months (range 40-73 months) after index procedure. Mean follow-up after the revision was 31 months (range 24-37 months) and all patients underwent an MRI with metal artifact reduction software (MARS) at least two years after revision to assess for local polyethylene wear and osteolysis. RESULTS: The Harris Hip score improved from 92.2 (range 63.0-100.0) to 100.0 (p = 0.072). One patient had a one-time dislocation within seven days of surgery. No patient required additional surgeries. Radiographs showed no signs of component loosening and osteolysis and MRI imaging revealed no evidence of polyethylene wear or osteolysis. CONCLUSION: A dual mobility liner in an existing Birmingham cup can provide excellent clinical and radiological short-term results without MRI evidence of increased polyethylene wear. Post-operative hip precautions should be enforced.

Predictive factors for metal ion levels in metal-on-metal total hip arthroplasty.

INTRODUCTION: Although metal-on-metal (MoM) total hip arthroplasty (THA) and hip resurfacings (HR) have similar bearing surfaces and comparable wear rates, metal ion levels and risk of failure are higher for MoM-THA. The mechanism behind the increased metal ion levels in large head MoM-THA is not completely understood. The current study aims to identify predictive factors for increased metal ion levels in unilateral and bilateral large head MoM-THA. MATERIALS AND METHODS: 99 Birmingham modular MoM-THA in 87 patients with metal ion levels at least 36 months after index procedure were analyzed. Mean follow-up time was 61.3 months (range 37-108) and the relationship of the following variables (gender, age, BMI, follow-up time, UCLA Activity Score, cup inclination, femoral head size, bilateral surgery) on metal ion levels were analyzed with multivariate regression models. RESULTS: Multivariate regression analysis revealed that bilateral MoM-THA surgery (p < 0.001) had a positive predictive effect on cobalt serum levels, while BMI had a negative (p = 0.018). Female gender (p = 0.012), activity (p = 0.001) and bilateral MoM-THA (p = 0.004) were positively correlated with chromium levels. Positive independent predictors for the cobalt-chromium ratio in the multivariate analysis were overall follow-up time (p = 0.004), bilateral MoM-THA (p < 0.001) and femoral head size (p = 0.007). CONCLUSIONS: The data of the current study suggest that bilateral MoM-THA, increased patient activity levels and female gender are associated with increased chromium levels. Patients with larger component size, longer follow-up time and bilateral MoM-THAs have an increased cobalt-chromium ratio. These patients might be at increased risk for adverse local soft tissue reactions secondary to corrosion. Continuous close monitoring is recommended and bearing-surface change should be discussed if local tissue reactions occur.

Allergy in Total Knee Replacement. Does It Exist?: Review Article.

BACKGROUND: There is little data on whether preexisting allergies to implant materials and bone cement have an impact on the outcome of TKA. QUESTIONS/PURPOSES: This review article analyzes the current literature to evaluate the prevalence and importance of metal and cement allergies for patients undergoing total knee arthroplasty. METHODS: A review of the literature was performed using the following search criteria: "knee," "arthroplasty," and "allergy" as well as "knee," "arthroplasty," and "hypersensitivity." RESULTS: One hundred sixteen articles were identified on PubMed, Seventy articles could be excluded by reviewing the title and abstract leaving 46 articles to be included for this review. The majority of the studies cited patch testing as the gold standard for screening and diagnosis of hypersensitivity following TKA. There is consensus that patients with self-reported allergies against metals or bone cement and positive patch test should be treated with hypoallergenic materials or cementless TKA. Treatment options include the following: coated titanium or cobalt-chromium implants, ceramic, or zirconium oxide implants. CONCLUSION: Allergies against implant materials and bone cement are rare. Patch testing is recommended for patients with self-reported allergies. The use of special implants is recommended for patients with a confirmed allergy.

Do Changes in the Production Process Affect the Outcome of Ceramic Liners: A 3-Year Follow-Up Study.

BACKGROUND: In 2011, the current liner was withdrawn from the market because of the potential risk for liner fracture secondary to increased pressures used to assemble the metal locking ring. The present study provides a short-term follow-up of patients with this implant. METHODS: We retrospectively evaluated 63 consecutive hips in 53 patients operated by a single surgeon using a recalled ceramic-on-ceramic bearing. There were 30 women and 23 men with an average age of 50.6 years (range 20.3-63.5 years). The mean follow-up was 36.8 months. RESULTS: Six hips in 6 patients were revised (9.5%) because of a liner-fracture during the follow-up period. All liner fractures were identified on computer tomography imaging. Nine patients had self-reported episodes of squeaking (14.3%). All 6 patients that underwent revision surgery for liner fracture described squeaking before revision. There were no revisions for other causes. Two of the revised patients had a subsequent dislocation (33%). CONCLUSION: The recalled ceramic liner lots have an increased liner fracture rate. Patients with mechanical symptoms or squeaking should undergo computer tomography to rule out liner facture.

Reduced rates of non-union with modified periacetabular osteotomy using peracetic-acid sterilized cancellous allografts.

The objective of the present study was to analyze the clinical and radiological results of periacetabular osteotomies (PAO) using Kirschner wire fixation and an allogeneic cancellous bone graft. This retrospective cohort study included 73 patients (85 PAOs). The allografts were processed from distal femur of cadaveric donors, defatted, sterilized with a peracetic-acid ethanol solution and freeze-dried. The clinical outcome, as measured by the Harris Hip Scores (HHS), the complication rate and the acetabular correction, as measured by radiological parameters, were compared. The postoperative femoral head coverage and HSS were significantly improved. Major complications occurred in five cases (6 %), but in no case did we observe a non-union or a graft-associated adverse effect. Fixation of the acetabular fragment with Kirschner wires in combination with an allogeneic cancellous bone graft is a safe method, with a low complication rate, no loss of correction and can prevent the occurrence of non-union with a high degree of probability.

Do cobalt and chromium levels predict osteolysis in metal-on-metal total hip arthroplasty?

INTRODUCTION: Serum metal ions are part of the regular follow-up routine of patients with metal-on-metal total hip arthroplasties (MoM-THA). Increased cobalt levels have been suggested to indicate implant failure and corrosion. QUESTIONS: (1) Is there a correlation between the size of the osteolysis measured on a CT scan and metal ion levels? (2) Can metal ion levels predict the presence of osteolysis in MoM-THA? (3) Are cobalt and chromium serum levels or the cobalt-chromium-ratio diagnostic for osteolysis? MATERIALS AND METHODS: CT scans of patients (n = 75) with a unilateral MoM-THA (Birmingham Hip System, Smith & Nephew, TN, USA) implanted by a single surgeon were reviewed to determine the presence of osteolysis. Statistical analysis was performed to detect its association with metal ion levels at the time of the imaging exam. RESULTS: The incidence of osteolysis was the same in men and women (35.6 vs 35.7 %). The cobalt-chromium-ratio correlates with the size of the osteolysis on the CT scan and the femoral component size in the overall study population (p = 0.050, p = 0.001) and in men (p = 0.002, p = 0.001) but not in women (p = 0.312, p = 0.344). The AUC for the cobalt-chromium-ratio to detect osteolysis was 0.613 (p = 0.112) for the overall population, 0.710 for men (p = 0.021) and 0.453 (p = 0.684) for women. The data suggest that a cut off level of 1.71 for the cobalt-chromium-ratio has a sensitivity of 62.5 % and specificity of 72.4 % to identify male patients with osteolysis. CONCLUSIONS: The disproportional increase of cobalt over chromium, especially in male patients with large component sizes can not be explained by wear alone and suggests that other processes (corrosion) might contribute to metal ion levels and might be more pronounced in patients with larger component sizes.

Cobalt serum levels differ in well functioning Birmingham resurfacing and Birmingham modular THA.

INTRODUCTION: Metal-on-metal (MoM) bearings are known to release metal ions secondary to wear and corrosion. This may cause local reactions (adverse soft tissue reactions and osteolysis) and systemic effects. Little is known about the exact pattern and the differences between large head MoM total hip replacements (THA) and resurfacings (HR). QUESTIONS: (1) Is there a difference in metal ion concentrations between HR and MoM-THR using the same bearing design (Birmingham Hip Resurfacing System, Smith & Nephew, Inc. Memphis, TN, USA)? (2) Are metal ion levels changing over time in MoM-THA or HR? (3) Do acetabular inclination angle and femoral component size influence cobalt and chromium levels? Is there a correlation between clinical outcome and metal ion levels? MATERIALS AND METHODS: A retrospective analysis was conducted in 77 well functioning unilateral Birmingham HR and 42 well functioning unilateral modular Birmingham MoM-THA (Smith & Nephew, Inc. Memphis, TN, USA) operated on between 2007 and 2012. Blood samples were taken at a minimum of 13 months and subsequent during annual follow-ups. RESULTS: (1) Cobalt levels were significantly higher in MoM-THA compared to HR (p < 0.001). There was no significant difference in chromium levels (p = 0.313). (2) Cobalt is increasing over time in MoM-THA (p = 0.030) whereas metal ions remain stable in HR. (3) Metal ion levels were not affected by acetabular inclination angle and femoral component size in MoM-THA. Chromium levels correlate with the femoral component size (r = -0.240; p = 0.037), the UCLA activity score (r = -0.344; p = 0.003) and the VAS (r = 0.263; p = 0.38) in HR. CONCLUSION: Considering that HR and MoM-THA used the same MoM bearing design, increased cobalt levels may be related to trunnion wear or corrosion. Elevated cobalt levels should raise concern for corrosion related failure in MoM-THA.

A novel method to measure femoral component migration by computed tomography: a cadaver study.

INTRODUCTION: Radiostereometric analysis (RSA) is the most accurate technique to measure implant migration. However, it requires special equipment, technical expertise and analysis software and has not gained wide acceptance. The current paper analyzes a novel method to measure implant migration utilizing widely available computer tomography (CT). METHODS: Three uncemented total hip replacements were performed in three human cadavers and six tantalum beads were inserted into the femoral bone similar to RSA. Six different 28 mm heads (-3, 0, 2.5, 5.0, 7.5 and 10 mm) were added to simulate five reproducible translations (maximum total point migration) of the center of the head. Implant migration was measured in a 3-D analysis software (Geomagic Studio 7). Repeat manual reconstructions of the center of the head were performed by two investigators to determine repeatability and accuracy. RESULTS: The accuracy of measurements between the centers of two head sizes was 0.11 mm with a CI 95 % of 0.22 mm. The intra-observer repeatability was 0.13 mm (CI 95 % 0.25 mm). The interrater-reliability was 0.943. CONCLUSION: CT based measurement of head displacement in a cadaver model were highly accurate and reproducible.

Total knee arthroplasty for valgus osteoarthritis: the results of a standardized soft-tissue release technique.

PURPOSE: The study reports the 2-year follow-up results of patients with valgus osteoarthritis undergoing total knee arthroplasty (TKA) with a standardized soft-tissue release. METHODS: Between 2008 and 2013, 222 TKAs were performed for valgus osteoarthritis by a single surgeon. A total of 181 TKAs in 164 patients were available for a minimum 2-year follow-up (range 24-87 months). Preoperative and postoperative range of motion (ROM), mechanical alignment, the postoperative medial proximal tibial angle (MPTA), Western Ontario and McMaster Universities Arthritis Index (WOMAC), VF-12 score, visual analogue pain scale (VAS), and the actual UCLA activity score, desired UCLA score, ligamentous stability (medial collateral ligament) and complications and revision rates were recorded. RESULTS: The ROM increased from a preoperative flexion contracture of 4.7° (range 0-40) and flexion of 110° (range 35-135) to a postoperative mean flexion contracture of 0.1° (range -5 to 10) and flexion of 128° (range 100-140). The mean hip-knee-ankle alignment was changed from 8.4° of mechanical valgus (range 5.3-25.4) to 0.02° of varus alignment (range -2.9 to 4.1). Tibia component angle (MPTA) was 90.4° (range 86.1-93.7). The WOMAC score, VF-12, UCLA and VAS significantly improved after surgery (p < 0.05). Two patients (1.1 %) underwent revision surgery for instability. CONCLUSION: The described standardized soft-tissue release (release of the iliotibial band and posterolateral corner) provided excellent clinical results at a minimum 2-year follow-up and can be used safely for a "cook-book" approach to the valgus knee with up to 25° mechanical valgus alignment. LEVEL OF EVIDENCE: IV.

The effect of a low radiation CT protocol on accuracy of CT guided implant migration measurement: A cadaver study.

The current study compared the impact of low radiation CT protocols on the accuracy, repeatability, and inter- and intra-observer variability of implant migration studies in total hip arthroplasty. Two total hip replacements were performed in two human cadavers and six tantalum beads were inserted into the femur similar to radiostereometric analysis. Six different 28 mm heads (-3 mm, 0 mm, 2.5 mm, 5.0 mm, 7.5 mm, and 10 mm) were added to simulate five reproducible translations (maximum total point migration) of the center of the head. Three CT scans with varying levels of radiation were performed for each head position. The effective dose (mSv) was 3.8 mSv for Protocol A (standard protocol), 0.7 mSv for Protocol B and 1.6 mSv for Protocol C. Implant migration was measured in a 3-D analysis software (Geomagic Studio 7). The accuracy was 0.16 mm for CT Protocol A, 0.13 mm for Protocol B and 0.14 mm for Protocol C; The repeatability was 0.22 mm for CT Protocol A, 0.18 mm for Protocol B and 0.20 mm for Protocol C; ICC for inter observer reliability was 0.89, intra observer reliability was 0.95. The difference in accuracy between standard protocol A and the two low radiation protocols (B, C) was less than 0.05 mm. The accuracy, inter- and intra-observer reliability of all three CT protocols is comparable to radiostereometric analysis. Reducing the CT radiation exposure to numbers similar to an AP Pelvis radiograph (0.7 mSv protocol B) does not affect the accuracy of implant migration measurements.

What do we get from navigation in primary THA?

Navigation in primary total hip arthroplasty has a history of over 20 years. During this process, imageless computer navigation can be particularly helpful in optimally restoring the hip's biomechanics. This involves the accurate placement of the acetabular component with the determination of the anteversion and abduction, whereby the navigated femur-first technique also allows for a calculation of the combined anteversion. Additional critical parameters such as the reconstruction of the rotation centre, as well as the femoral and acetabular offset, can also be optimally adjusted. Last but not least, an intra-operative evaluation and equalisation of the leg length is possible.Nonetheless, the disadvantages of this surgical technique in terms of the high costs in the acquisition and preservation of the necessary devices, as well as the longer operation time, must be taken into account. However, economic aspects are not the only thing preventing widespread use of the navigation technique. Determining the plane of reference (APP) for the optimal orientation of the implants is based on palpation of the bony landmarks - and this is influenced by the thickness of the soft tissue layer. Furthermore, the experience of the surgeon constitutes a variable that influences the accuracy of navigation.In summary, hip navigation certainly offers an interesting technique for the optimisation of total hip arthroplasty with reconstruction of proper biomechanics. At the same time, there is currently a lack of high-quality randomised controlled long-term trials that evaluate the clinical advantage for the patients, together with cost utility and survival rates. Cite this article: Renner L, Janz V, Perka C, Wassilew GI. What do we get from navigation in primary THA? EFORT Open Rev 2016;1:205-210. 10.1302/2058-5241.1.000034.

Is there a rationale to use a dual mobility poly insert for failed Birmingham metal-on-metal hip replacements? A retrieval analysis.

INTRODUCTION: Previous studies showed poor outcomes for patients undergoing revision of failed metal-on-metal total hip arthroplasty (MoM-THA) and resurfacing (RS) with an increased risk of dislocation. Dual mobility inserts are an option to retain the acetabular component and change the metal-on-metal bearing to plastic-on-metal. The current study analyzes the rationale for the off-label use of a dual mobility poly insert (MDM X3, Stryker, Mahwah, NJ) in a Birmingham metal shell (Smith & Nephew, Memphis, TN). MATERIALS AND METHODS: Based on retrievals from the implant database the study compared the clearance between 20 BHR shells, 31 MDM poly inserts and 24 ADM acetabular components of different sizes. The radial clearance was calculated for each possible combination of implants [n = 81 (MDM/BHR) and n = 119 (MDM/ADM)]. RESULTS: An MDM mobile bearing poly insert in an ADM shell has an average clearance of 0.314 mm (SD 0.031) compared to 0.234 mm (SD 0.030) in a BHR shell (p < 0.01). The minimal clearance is 0.246 and 0.163 mm, respectively. 30.9 % of the MDM/BHR clearances were within the range of the MDM/ADM bearing and 88.9 % had a clearance of more than 0.2 mm. CONCLUSION: Clearances of the MDM poly insert in a BHR shell are reduced, and although the majority of combinations appear safe, the indication needs to be made on an individual base carefully considering alternative treatment options.

Imaging of ceramic liner fractures in total hip arthroplasty: the value of CT.

Fracture of a ceramic liner of a total hip arthroplasty is rare and is radiographically occult if not displaced. We report on two patients in whom ceramic liner fracture was radiographically occult but was diagnosed on subsequent CT scan through appropriate windowing.