Abstract
There has been much discussion and controversy in the media recently regarding metal toxicity following large head metal on metal (MoM) total hip replacement (THR). Patients have been reported as having hugely elevated levels of metal ions with, at times, devastating systemic, neurolgical and/or orthopaedic sequelae.
However, no direct correlation between metal ion level and severity of metallosis has yet been defined. Normative levels of metal ions in well functioning, non Cobalt-Chrome hips have also not been defined to date.
The Exeter total hip replacement contains no Cobalt-Chrome (Co-Cr) as it is made entirely from stainless steel. However, small levels of these metals may be present in the modular head of the prosthesis, and their effect on metal ion levels in the well functioning patient has not been investigated.
We proposed to define the “normal” levels of metal ions detected by blood test in 20 well functioning patients at a minimum 1 year post primary Exeter total hip replacement, where the patient had had only one joint replaced.
Presently, accepted normal levels of blood Chromium are 10–100 nmol/L and plasma Cobalt are 0–20 nmol/L. The UK Modern Humanities Research Association (MHRA) has suggested that levels of either Cobalt or Chromium above 7 ppb (equivalent to 135 nmol/L for Chromium and 120 nmol/L for Cobalt) may be significant. Below this level it is indicated that significant soft tissue reaction and tissue damage is less likely and the risk of implant failure is reduced.
Hips were a mixture of cemented and hybrid procedures performed by two experienced orthopaedic consultants. Seventy percent were female, with a mixture of head sizes used.
In our cohort, there were no cases where the blood Chromium levels were above the normal range, and in more than 70% of cases, levels were below recordable levels. There were also no cases of elevated plasma Cobalt levels, and in 35% of cases, levels were negligible.
We conclude that the implantation with an Exeter total hip replacement does not lead to elevation of blood metal ion levels.
