Abstract
Introduction
Modular-neck hips have twice the rate of revision compared to fixed stems. Metal related pathology is the second most common reason for revision of implants featuring titanium stems with cobalt chrome necks. We aimed to understand the in-vivo performance of current designs and explore the rationale for their continued use.
Methods
This study involved the examination of 200 retrieved modular-neck hips grouped according to the material used for their neck and stem. Groups A, B and C had neck/stems featuring CoCr/beta Ti-alloy (TMZF), CoCr/Ti6Al4V-alloy, and Ti6Al4V/Ti6Al4V respectively. Reasons for revision included pain, elevated metal ion levels and fluid collection. The stem-neck interface was assessed for severity of fretting/corrosion using metrology methods to compute linear wear penetration rate.
Results
All retrieved implants with a CoCr/Ti combination showed evidence of moderate-severe fretting corrosion at the neck-stem junction. Maximum penetration depth rate in group A (median of 17.9μm/year) was higher than that in group B (median=5.8μm/year); p=0.0012 and group C (median=1.55μm/year), p=0.0095. Implant failure occurred 2-fold earlier in modular-neck hips with TMZF-alloy stems than Ti6Al4V. There was a strong correlation between severity of damage and time to revision in the TMZF group A (p < 0.0001) and between taper damage and Co levels (p < 0.0001) and Cr levels (p < 0.0012). Patient and implant data did not correlate with the amount of material loss observed, (p>0.05).
Discussion
The findings are in agreement with registry data as designs with CoCr/Ti at neck/stem interfaces had the highest levels of fretting and corrosion and lower threshold for revision.
Conclusion
Corrosion of metal orthopaedic implants remains of clinical concern to patients, surgeons and industry. In light of the findings, the continued use of modular-neck with a metal mismatch at the neck/stem junction is unfavourable and should be avoided.
