Current expert views on metal-on-metal hip resurfacing arthroplasty. Consensus of the 6th advanced Hip resurfacing course, Ghent, Belgium, May 2014.

This paper reports the consensus of an international faculty of expert metal-on-metal (MoM) hip resurfacing surgeons, with a combined experience of over 40,000 cases, on the current status of hip resurfacing arthroplasty. Indications, design and metallurgy issues, release of metal ions and adverse soft tissue reactions to particles, management of problematic cases and revisions, as well as required experience and training are covered. The overall consensus is that MoM hip resurfacing should not be banned and should be viewed separately from MoM total hip arthroplasty (THA) with a large diameter head because of the different design and wear behaviour related to the taper/trunnion connection. The use of hip resurfacing has decreased worldwide but specialist centres continue to advocate hip resurfacing in young and active male patients. Regarding age the general recommendation is to avoid hip resurfacing in men older than 65 and in women older than 55, depending on the patient activity and bone quality. Female gender is considered a relative contraindication. Most surgeons would not implant a MoM hip in women who would still like a child. Regardless of gender, there is a consensus not to perform hip resurfacing in case of a femoral head size smaller than 46 mm and in patients with renal insufficiency or with a known metal allergy. Regarding follow-up of hip resurfacing and detection of adverse local tissue reactions, metal ion measurements, MRI and ultrasound are advocated depending on the local expertise. The consensus is that hip resurfacing should be limited to high volume hip surgeons, who are experienced in hip resurfacing or trained to perform hip resurfacing in a specialist centre.

The 2012 Otto Aufranc Award: The interpretation of metal ion levels in unilateral and bilateral hip resurfacing.

BACKGROUND: The interpretation of metal ion concentrations and their role in clinical management of patients with metal-on-metal implants is still controversial. QUESTIONS/PURPOSES: We questioned whether patients undergoing hip resurfacing with no clinical problems could be differentiated from those with clinical (pain, loss of function) and/or radiographic (component malpositioning, migration, bone loss), problems based on metal ion levels, and if there was a threshold metal level that predicted the need for clinical intervention. Furthermore, we asked if patient and implant factors differed between these functional groups. METHODS: We retrospectively identified 453 unilateral and 139 bilateral patients with ion measurements at minimum followup of 12 months (mean, 4.3 years; range, 1-12.9 years). Patients were designated as well functioning or poorly functioning based on strict criteria. The acceptable upper levels within the well-functioning group were determined from the 75th percentile plus 1.5× interquartile range. The sensitivity and specificity of these levels to predict clinical problems were calculated. RESULTS: Well-functioning group ions were lower than the poorly functioning group ion levels. The acceptable upper levels were: chromium (Cr) 4.6 µg/L, cobalt (Co) 4.0 µg/L unilateral and Cr 7.4 µg/L, Co 5.0 µg/L bilateral. The specificity of these levels in predicting poor function was high (95%) and sensitivity was low (25%). There were more males in the well-functioning group and more females and smaller femoral components in the poorly functioning group. CONCLUSIONS: Metal levels higher than these proposed safe upper limits can predict problems with metal-on-metal resurfacings and are important parameters in the management of at-risk patients. LEVEL OF EVIDENCE: Level II, diagnostic study. See Guidelines for Authors for a complete description of levels of evidence.

Failure modes of 433 metal-on-metal hip implants: how, why, and wear.

Metal-on-metal total hip replacements (THRs) and hip resurfacings are coming under increasing scrutiny in light of concerns that they fail because of high wear and elevated metal ions. The aim of this study was to investigate the modes of failure in a collection of 433 metal-on-metal THRs and hip resurfacings and to examine the correlations between the reasons for revision and a range of patient and implant variables considered relevant to implant wear.

Revisions of metal-on-metal hip resurfacing: lessons learned and improved outcome.

This retrospective, consecutive case series of a single surgeon performed between 2001 and 2010 assesses the outcome following revision of metal-on-metal hip resurfacing arthroplasties (N = 113). Mean time to revision was 31 months (0-101) after primary hip resurfacing. Malpositioning of the components with associated wear-induced soft tissue fluid collections was the most frequent factor leading to failure of a hip resurfacing arthroplasty. The mid-term outcome of the revisions was satisfactory; complications occurred in 11 patients (9.7%). Six of these patients underwent a re-revision.

A modified posterior approach preserves femoral head oxgenation during hip resurfacing.

In 11 patients, the oxygenation was measured in the superolateral quadrant of the femoral head during resurfacing with a modified posterior approach, designed to preserve the blood supply, using a gas-sensitive electrode. These were compared with measures from 10 patients in whom the standard posterior approach was used. The modified approach patients maintained a significantly (P < .005) higher amount of relative oxygenation after the approach, 78% (standard deviation [SD], 45%) vs 38% (SD, 26%), and acetabular component implantation, 74% (SD, 56%) vs 20% (SD, 28%). The modified posterior approach, unlike the standard extended approach, does not significantly compromise the blood supply to the head; and we recommend this approach be considered for hip resurfacing.

Does intraoperative cell salvage remove cobalt and chromium from reinfused blood?

In 12 patients undergoing a revision hip arthroplasty after a failed metal-on-metal primary hip arthroplasty, the effectiveness of intraoperative cell salvage (ICS) in removing metal ions was investigated. Samples of blood collected during surgery were filtered using 2 ICS devices. The samples had the concentrations of cobalt (Co) and chromium (Cr) measured before and after filtration. There was an average reduction of 76.3% for Cr concentration and 78.6% for Co concentration after ICS filtering. The Co-to-Cr ratio before and after filtration was similar. At the present time, these salvage systems should be used with caution in the patient undergoing revision of metal-on-metal bearing surfaces.

Belgium experience with metal-on-metal surface arthroplasty.

Hybrid metal-on-metal surface arthroplasty of the hip has recently been introduced, with a vast number of implants used in European countries including Belgium. This article presents results in 252 hips with a mean follow-up of 2.8 years. Using a tight press-fit with minimal cement mantle as the technique of femoral fixation, there have been only three failures. The main complications have been avascular necrosis of the femoral head and femoral neck fracture. In most cases, patients returned to a high functional level with no restrictions in their physical activity and were highly satisfied. Future refinements in surgical technique and instruments will make this procedure more accessible and reproducible for the surgeon.

Three pathways for trehalose biosynthesis in mycobacteria.

Trehalose is present as a free disaccharide in the cytoplasm of mycobacteria and as a component of cell-wall glycolipids implicated in tissue damage associated with mycobacterial infection. To obtain an overview of trehalose metabolism, we analysed data from the Mycobacterium tuberculosis genome project and identified ORFs with homology to genes encoding enzymes from three trehalose biosynthesis pathways previously characterized in other bacteria. Functional assays using mycobacterial extracts and recombinant enzymes derived from these ORFs demonstrated that mycobacteria can produce trehalose from glucose 6-phosphate and UDP-glucose (the OtsA-OtsB pathway) from glycogen-like alpha(1-->4)-linked glucose polymers (the TreY-TreZ pathway) and from maltose (the TreS pathway). Each of the pathways was found to be active in both rapid-growing Mycobacterium smegmatis and slow-growing Mycobacterium bovis BCG. The presence of a disrupted treZ gene in Mycobacterium leprae suggests that this pathway is not functional in this organism. The presence of multiple biosynthetic pathways indicates that trehalose plays an important role in mycobacterial physiology.

Alteration of a single amino acid residue reverses fosfomycin resistance of recombinant MurA from Mycobacterium tuberculosis.

Mycobacterium tuberculosis has innate resistance to a range of broad-spectrum antimicrobial agents. This may in part reflect the relative impermeability of the mycobacterial cell wall, but additional specific mechanisms may also be important. In the case of fosfomycin, it has been suggested that a key difference in the active site of the M. tuberculosis MurA enzyme might confer resistance. In Escherichia coli, fosfomycin covalently binds to a cysteine normally involved in the enzymic activity, while protein alignments predict an aspartate at this position in the M. tuberculosis MurA. In the present study, it is demonstrated that the wild-type M. tuberculosis MurA is indeed resistant to fosfomycin, and that it becomes sensitive following replacement of the aspartate residue in position 117 by a cysteine. In addition, the study illustrates the use of an inducible expression system in mycobacteria to allow functional characterization of an M. tuberculosis enzyme that is unstable during constitutive expression.