Abstract
Purpose
To determine whether there is a difference in the metal ion levels among three different metal-on-metal total hip systems: two monoblock large heads and one modular metal-on-metal total hip replacement system in patients who received these implants in our region.
Method
A group of 56 patients were recruited that had either undergone total hip replacement (THR) with a Birmingham resurfacing socket, the Durom resurfacing socket, or a Pinnacle metal-on-metal bearing surface. All patients recruited were at least one year following their surgery in order that their ion levels had reached a steady state.
We reviewed every patient clinically, radiographically as well as biochemically. Blood was obtained for cobalt (Co) and chromium (Cr) levels. Current radiographs were arranged to assess the stability and mechanics of the total hip systems. All patients signed an informed consent and completed three questionnaires, The Western Ontario and McMaster Universities (WOMAC) index, the Short Form 36 (SF36) and UCLA activity score. A Harris Hip score was completed in order to assess individual hip function. Statistical analysis was performed on the collected data to assess whether there were any other potential influence on the mean levels of Co and Cr.
Results
The blood metal ion levels in the larger non modular acetabular sockets were significantly raised compared to the Pinnacle group. For Co, 1.95 parts per billion (ppb) and 2.70 ppb in the Durom and Birmingham groups respectively compared to only 0.52 ppb in the Pinnacle group (P< 0.001). Cr levels were the same in the two monoblock systems, 1.9 ppb compared to the Pinnacle sockets 1.2 ppb (P<0.001). In all groups however these levels were within an acceptable safe range.
The mean head size used in the Birmingham group was 53.2mm (Range 44mm to 56mm), and in the Durom group, 47.1 (Range 42 to 54mm). The mean head size used in the modular group was 37.3mm (Range 36–44mm).
There was no difference between the three groups in terms of functional outcome and patient demographics were similar in all three groups.
There was no statistical difference between the groups in terms of anteversion and abduction angles. There was also a smaller spread in this group in terms of range of angles. There was also no relationship between these values and the metal ion levels.
Conclusion
All three total hip systems demonstrated average metal ion whole blood levels in a safe range. Larger diameter metal on metal bearings had higher ion levels.
