Dual-mobility bearings increase the stable range of motion of total hip arthroplasty (THA) but are limited by the mechanical effects of a large diameter metal on polyethylene bearing which may cause high rates of wear from the surfaces of the polyethylene bearing and the head-stem taper. Improved polyethylene (PE) has reduced concern over bearing wear but the effects on the taper junction are unknown. We aimed to better understand the effect of dual mobility bearings on fretting-corrosion damage to the taper junction by comparison to standard bearings. We collected and analysed retrieved hips of one design with either dual mobility (n= 39) or standard bearings (n=30). The bearing size in the dual mobility group was 42mm whereas in the standard bearing group it had a median of 36mm. Stem trunnions had V40 tapers. Time of implantation and body mass index were comparable between the two groups. Fretting and corrosion at the stem trunnions was quantified by: 1) visual scoring and 2) surface profilometry.Introduction
Materials and Methods
Arthroplasty registries are important for the
surveillance of joint replacements and the evaluation of outcome. Independent
validation of registry data ensures high quality. The ability for
orthopaedic implant retrieval centres to validate registry data
is not known. We analysed data from the National Joint Registry
for England, Wales and Northern Ireland (NJR) for primary metal-on-metal
hip arthroplasties performed between 2003 and 2013. Records were
linked to the London Implant Retrieval Centre (RC) for validation.
A total of 67 045 procedures on the NJR and 782 revised pairs of
components from the RC were included. We were able to link 476 procedures
(60.9%) recorded with the RC to the NJR successfully. However, 306
procedures (39.1%) could not be linked. The outcome recorded by the
NJR (as either revised, unrevised or death) for a primary procedure
was incorrect in 79 linked cases (16.6%). The rate of registry-retrieval
linkage and correct assignment of outcome code improved over time.
The rates of error for component reference numbers on the NJR were
as follows: femoral head category number 14/229 (5.0%); femoral head
batch number 13/232 (5.3%); acetabular component category number
2/293 (0.7%) and acetabular component batch number 24/347 (6.5%). Registry-retrieval linkage provided a novel means for the validation
of data, particularly for component fields. This study suggests
that NJR reports may underestimate rates of revision for many types
of metal-on-metal hip replacement. This is topical given the increasing
scope for NJR data. We recommend a system for continuous independent
evaluation of the quality and validity of NJR data. Cite this article:
It has been suggested that corrosion and fretting at the taper junctions of stemmed metal-on-metal hip replacements may contribute to their high failure rates. A peer-reviewed semi-quantitative scoring system [Goldberg et al., 2002] has been used to visually assess the severity of corrosion and fretting of the taper junction but has not been validated using multiple examiners. The aim of this study was to assess the inter-observer variability of this method. Macroscopic and stereomicroscopic examinations of the femoral head and stem tapers of 100 retrieved large diameter metal on metal (MOM) hip components were performed by two independent observers using the methods defined by Goldberg et al. [2002] to quantify corrosion and fretting. Scores ranging from 1 (none) to 4 (severe) were assigned to the medial, lateral, posterior and anterior quadrants of the neck taper and the distal and proximal regions of the head taper. An overall score was then assigned to each surface as a whole. Cohen's weighted Kappa statistic (κ) was used to measure the inter-observer agreement. A quadratic weighting scheme, that allocated weights to the importance of disagreements that are proportional to the square of the number of categories apart, was used to take account of scaled disagreement. Kappa values were assessed using previously established criteria where κ ≤ 0 = poor, 0.01 to 0.20 = slight, 0.21 to 0.40 = fair, 0.41 to 0.60 = moderate, 0.61 to 0.80 = substantial, 0.81 to 1 = almost perfect. A sample size of 100 was used in order to detect a coefficient of 0.60 to within 0.25 with 95% confidence with two experienced observers. Statistical analysis was performed using Stata/IC version 12.1 (StataCorp, College Station, TC, USA) and a p value < 0.05 was considered statistically significant.Introduction
Method
Metal-on-metal (MOM) total hip arthroplasty and hip resurfacing using large diameter femoral heads offer clinical advantages, however the failure rates of these hips are unacceptably high. Retrieved MOM hips have a wide range of wear rates of their bearing surfaces and there is ongoing research to identify the causes of failure. Detailed visual inspection is the first step in the forensic examination of failed hip components, but there is no universally accepted description or process. Visible features may help explain the mechanism of failure of MOM hips. During our analysis of 2000 MOM hip components, we have developed protocols to undertake detailed, non-destructive macroscopic and stereomicroscopic examinations of each component, using quantitative assessment to document the presence of types of damage. We have established a systematic terminology to describe the types of damage that are observed, allowing for consistency and clarity in the vocabulary used. These include (but are not limited to):
Scratching – when there is an increase in the number and/or magnitude of scratches present on the surface, typically increasing measured roughness parameters. The severity of scratching is determined by rubbing a 0.18 mm thickness acetate gage or fingernail over the surface:
Light –visible but not detectable with gage. Moderate – visible and detectable with gage. Heavy – depth clearly visible and will catch a fingernail or gage. Pitting – indentations in the surface for which the dimensions are similar in all directions. Embedded Particles – hard particles that have become embedded in the bearing surface. Discolouration – observed as a change in the appearance of the surface, often as colour diffraction patterns. Polishing, gouges, etching (from corrosion), surface films, surface deposits and tribochemical reaction layers. To assess the distribution of these types of damage on the components, each surface is considered in terms of zones defined by quadrants (cup, head, stem and taper) and sub-quadrants (cup and head only), Figure 1. Each zone is scored on a scale of 0 to 3 by determining the percentage of the surface area of the quadrant that exhibits the feature in question:
a score of 0 indicates 0%
indicates greater than 0% but less than 25% indicates between 25% and 75% inclusive indicates greater than 75%. The use of zones to differentiate between the polar and equatorial regions of the cup and head surfaces can offer important information relating to the articulation of the bearing surfaces. For example in a cohort of 90 inspections, a score of 3 for light scratching was recorded in all areas of the bearing surface in 40% of cups, whilst approximately 5% had evidence of pitting, occurring near the rim. Current research at our retrieval centre involves correlating the results of detailed inspections with a range of variables, including implant design, size and surgical position.
Our purpose was to study the functional outcome and electrophysiologically to assess the axially nerve function in patients who have undergone surgery using a deltoid-splitting approach to treat complex proximal humeral fractures. This was a prospective observational study and was carried out in the Shoulder injury clinic at a university teaching hospital. Over a one-year period we treated fourteen locally-resident patients (median age 59 years) who presented with a three- or four-part proximal humeral fracture. All patients were treated using the extended deltoid-splitting approach, with open reduction, bone grafting and plate osteosynthesis. All patients were prospectively reviewed and underwent functional testing using the DASH, Constant and SF-36 scores as well as spring balance testing of deltoid power, and dynamic muscle function testing. At one year after surgery, all patients underwent EMG and nerve latency studies to assess axillary nerve function. Thirteen of the fourteen patients united their fractures without complications, and had DASH and Constant score that were good, with comparatively minor residual deficits on assessment of muscle power. Of these thirteen patients, only one had evidence of slight neurogenic change in the anterior deltoid. This patient had no evidence of anterior deltoid paralysis and her functional scores, spring balance and dynamic muscle function test results were indistinguishable from the patients with normal electrophysiological findings. One of the fourteen patients developed osteonecrosis of the humeral head nine months after surgery and had poor functional scores, without evidence of nerve injury on electrophysiological testing. Reconstruction through an extended deltoid-splitting approach provides a useful alternative in the treatment of complex proximal humeral fractures. The approach provides good access for reduction and implant placement and does not appear to be associated with clinically-significant adverse effects.
