Aims: Cobalt (Co) and chromium (Cr) ion associated carcinogenesis and chromosomal damage in animals have raised concerns that metal-on-metal (MOM) total hip replacement (THR) in humans may produce the same effects over time. Considering that the risks may be related to the level of these ions in the body, this study compared the serum Co and Cr levels in patients with unilateral versus bilateral 28 mm diameter MOM THR.

Methods: All patients having THR at our institution were prospectively registered on a computerised database. From our database, 108 Ultima MOM THR with 28 mm CoCrMo bearing were identified. After patient review in clinic and before blood results were known, patient matching was performed by date after surgery, activity level and weight. Using these stringent criteria, 11 unilateral THR could be adequately matched with 11 bilateral THR. Blood serum was taken with full anti-contamination protocols and serum analysed via inductively coupled plasma mass spectrometry (ICP-MS) Statistical analysis used the Mann-Whitney U test.

Results: The median serum Co level after unilateral MOM THR was 22 nmol (range 15 to 37 nmol) compared to 42 nmol (range 19 to 221 nmol) for bilateral MOM THR (p=0.001). The median serum Cr level after unilateral MOM THR was 19 nmol (range 2 to 35 nmol) compared to 52 nmol (range 19 to 287 nmol) for bilateral MOM THR (p=0.04).

Conclusions: This study has shown that the serum Co and Cr levels in patients with bilateral MOM THR are significantly higher than those with unilateral MOM THR. With levels of up to 50 times the upper of limit of normal, this finding may be of relevance for the development of potential long-term side effects.

Aims: Metal-on-Metal (MOM) bearings for Total Hip Arthroplasty (THA) are known to elevate the serum concentrations of metal ions, raising concern about possible long-term side-effects. One potential modifier of ion release is the bearing diameter used. Resurfacing MOM bearings have a large surface area available for corrosion, but may benefit from improved lubrication and reduced production of corrodible wear debris. The net effect of these two variables on metal ion release is unknown.

Methods: In this study, we measured the serum cobalt and chromium levels from 22 large diameter MOM resurfacing arthroplasties (Cormet2000 & Birmingham Hip Resurfacing) and compared them to 22 THA (Ultima) with a bearing diameter of 28 mm. Patients were prospectively matched for activity level, weight and date after surgery. All were at least 6 months after surgery.

Results: At a median of 16 months (range 7 to 56) after resurfacing arthroplasty, we found the median serum cobalt and chromium levels to be 38 nmol/l (14 to 44) and 53 nmol/l (23 to 165) respectively. Both these figures were significantly greater than the levels after 28 mm MOM THA, which were 22 nmol/l (15 to 87, p=0.021) and 19 nmol/l (2 to 58, p< 0.001) for cobalt and chromium respectively.

Conclusions: As the upper limit of normal in patients without implants is typically 5 nmol/l, both groups had significantly raised metal ion levels, albeit at a relatively short median follow-up period. Large diameter MOM bearings resulted in a greater systemic release of cobalt and chromium ions than did small diameter bearings. This may be of relevance for potential long-term side-effects. It is not known to what extent this difference is due to corrosion of the component surfaces or of the wear particles produced.

Aims: After Total Hip Replacement (THR), bearing surface pistoning during the gait cycle can affect wear rates. This ‘micro-separation’ has been shown clinically by video-fluoroscopy to be greater with a Metal-on-Polyethylene (MOP) bearing than a Metal-on-Metal (MOM) one. In this study, we quantified the suction forces that these bearings generate during the swing phase of the gait cycle as a result of interfacial tension from the thin fluid film present at the bearing surface. Methods: We used a servo-hydraulic universal testing machine with 250N load cell and programmed a sinusoidal waveform that could vary the loads and frequencies applied to MOP or MOM bearings submerged in 25% serum. We measured the bearing separation (±1μm) at tensile loads of 10N to 100N lasting 0.1s to 0.5s per 1Hz cycle.

Results: MOM bearings resisted tensile loads of up to 35N when applied for 0.1s to 0.5s of the simulated gait cycle. Bearing separation was measured at a maximum of 198 microns. Above 50N, the MOM bearing was unable to prevent separation occurring even when applied for only 0.1s of the simulated gait cycle (p< 0.001). The MOP bearing could not resist separation at any of the applied tensile loads (p< 0.0001).

Conclusions: The suction-fit of the MOM bearing used in this study is insufficient to prevent bearing separation due to gravity (110N). However, it may reduce the total bearing separation distance by delaying the time point at which separation occurs during the finite period of the swing phase (< 0.5s) during the gait cycle. This effect is crucially dependent upon the bearing clearance, bearing diameter, weight of the leg, speed of walking and soft tissue tension around the hip. This ultimately relies upon prosthetic design, patient selection and surgical technique.

Background: Metal-on-metal (MOM) bearing surfaces with low diametric clearance possess a surface tension that prevents easy separation of the surfaces when lubricated. Potentially this ‘suction-fit’ may increase the torque required for dislocation. This study assessed the protective role of a MOM bearing surface as a single risk factor for dislocation.

Method: Prospective data was recorded on a series of 229 patients undergoing 249 primary THR for osteoarthritis. From 1993–8, patients under 70 years old were routinely given a 28mm ceramic-on-polyethylene (COP) bearing surface. Due to a high dislocation rate (see results below), an alternative was sought (1998–2001) and a 28mm metal-on-metal (MOM) bearing system chosen. For all cases in both groups, the acetabulum was uncemented with a modular 10° posterior lip insert allowing the same primary arc range (Duraloc/PFC/ Ultima, Johnson & Johnson). The cemented femoral component was the same in all cases (Ultima). All operations were performed by the same surgeon using the posterior approach. Variables in patient and prosthesis factors were compared. Statistical analysis was performed by the Chi-square and student’s t-test where appropriate.

Results: We identified 140 THR in 129 patients who received a COP bearing and 109 THR in 100 patients who received a MOM bearing. Nine of 140 (6.4%) COP bearings dislocated within 3 months of surgery compared to 1 of 109 (0.9%) in the MOM group (p=0.028). No significant differences were identified between groups when comparing factors relating to the patient or prosthesis.

Discussion: This study has shown a high dislocation rate for a COP bearing that was reduced to a low dislocation rate by changing the bearing surface to a MOM design. A potential mechanism for this may be the ‘suction fit’ from the surface tension of the low clearance, high tolerance that the metal-on-metal bearing possesses, requiring increased torque to dislocate during impingement.