effect of the diameter of metal-on-metal bearings on systemic exposure to cobalt and chromium, The

The new resurgence in the use of metal-on-metal bearings has l to new concerns over metal wear and elevated systemic horizontals of metal ions.

In order to establish if bearing diameter influences the release of metal ions, we compared the whole life-current levels of cobalt and chromium (at individual year) and the urinary cobalt and chromium output (at individual to three and four to six years) following either a 50 mm or 54 mm Birmingham hip resurfacing or a 28 mm Metasul total hip replacement. The whole vital current concentrations and daily output of cobalt and chromium in these time periods for the two bearings were in the same range and without significant difference.

The understanding that osteolysis, induced by the agency of particulate wear-debris is a limiting factor in the long-term succes of an arthroplasty has l to the resurgence of metal-on-metal bearings. Wear rates for these second-generation devices are a great deal of lower compared with conventional metal-on-polyethylene designs.1

Metal wear and corrosion generate insoluble metal particles and soluble metal ions, the latter passing into the systemic circulation. Because they are essential uncompounded bodys in the body, there is an effective renal clearance mechanism for cobalt, chromium and molybdenum the constituent ultimate parts of most modern all-metal hip devices. life-current levels represent a balance between the release of metal ions from the device and renal clearance. The systemic release rate can be estimated from a 12- or 24-hour urine collection. There are belong tos about the long-term effects of high systemic horizontals

Several factors affect bearing wear. In vitro and in vivo studies have shown that wear and metal ion release are higher in low-carbon cobalt-chrome alloys2-4 and heat-treated alloys.5-11 If all other factors are equal, there is no difference in the wear properties of cast constituents compared with wrought components.4,12,13

In vitro studies upon the effect of bearing diameter upon wear reveal a two-phase effect1 Smaller bearings similar as those of 22 mm diameter have higher wear rates with increasing diameters, as wait fored in a bearing in boundary lubrication way In larger bearings of 28 mm and above, the wear rate decreases with increasing diameter, as awaited from mixed or fluid-film lubrication.

However, the in vitro regimes used in controll wear studies upon hip simulators cannot be awaited to replicate the wide and varied loading patterns of hips in vivo. Therefore, in vivo findings oftentimes do not correlate with in vitro arises It is not known whether larger diameter bearings generate les systemic metal ion release in vivo. Our application of mind looked at this issue through comparing both the daily urinary horizontals (two and five years post-operatively) and whole life-blood levels (one year post-operatively) of metal ions in patients after larger diameter (50 mm and 54 mm) metal-on-metal hip resurfacing with smaller diameter (28 mm) metal-on-metal total hip replacement (THR) patients.

Patients and way s

The group of patients in our application of mind had either a well-functioning 50 mm or 54 mm bearing diameter Birmingham hip resurfacing (BHR; Midland Medical Technologies, Bromsgrove United Kingdom), or a well-functioning 28 mm Metasul total hip replacement (THR Sulzer Orthopaedics, Winterthur, Switzerland). the pair bearings are made of high-carbon cobalt-chrome alloy and are not controled to heat treatment processes in the later stages of manufacturing, although the BHR is made of cast alloy and the Metasul THR is of the forged variety. The BHR has a hydroxyapatite-coated, porous, uncement acetabular constituent and a cemented femoral constituting both made of as-cast high-carbon cobalt-chrome alloy. The Metasul THR has a cementles porous-ingrowth titanium acetabular composing and either a cemented, polished, tapered stainless carburet of iron femoral component or a cementles porousingrowth titanium alloy femoral constituting (Fig. 1).

Fig. 1a

Fig. 1b

A radiograph, taken at five-year follow-up of a) a patient with a 54 mm Birmingham hip resurfacing and b) a patient with a 28 mm Metasul total hip replacement.

We recruited 26 consecutive patients, who were at the two-year post-operative stage after unilateral BHR alone those with a femoral constituent of 50 mm or 54 mm were included. We chose sum of two units femoral sizes to keep the assemblage uniform. The mean age of these patients at operation was 529 years (287 to 671)

We compared these patients with a further 28 who had undergone a unilateral Metasul THR a mean of sum of two units years previously. Each had a 28 mm high-carbon done cobalt-chrome bearing. Their mean age at operation was 566 years (390 to 628)

There were a further sum of two units groups of patients, one of 58 patients with a mean age of 52 years (283 to 744) at operation who had undergone unilateral BHR five years previously and individual of 23 patients with a mean age of 597 years (45 to 67) who had undergone unilateral Metasul THR using a 28 mm bearing at a mean of five years previously.

Patients who were operated upon for diagnoses other than osteoarthritis and those with renal impairment or with other metal devices in the material substance were excluded. All patients gave informed concurrence We obtained 12-hour urine collections from all patients in each assemblage