Cup coverage and dislocation risk in THA

This is one of our recent work from Coxa Hospital for Joint Replacement. This relates to post-marketing surveillance which is always very important topic. In short, main primary THR system changed and anecdotal evidence started to accumulate that maybe dislocations had become more common with the new cup system.

We decided to take a deeper look and analyze the incidence of dislocations with the old cup system (Pinnacle) and with the new system (Continuum). This study was just recently published in the Acta Orthopaedica: Increased risk for dislocation after introduction of the Continuum cup system: lessons learnt from a cohort of 1,381 THRs after 1-year follow-up.

We tried to aim high and used DAGs because after all, this study was about explaining.

The directed acyclic graph. Factors indicated in red were included in multivariable logistic regression analyses.

Results were quite clear. The adjusted estimates for different cup types can be found in the text.

The incidence of dislocation was 4/299 with the Pinnacle cup system (1.3%, CI 0.5–3.4) and 37/927 with the Continuum cup system (4.0%, CI 2.9–5.5). 

It is important that only the cup system was changed. Same surgeons were operating the same patients and putting the cups in the same position before and after the tendering process and change of the cup system. So it can be quite safely concluded that the main cause for higher incidence of dislocations is the different cup system.

Why? Due to simple mechanism: the cup coverage is lower in the Continuum cups thus reducing the jumping distance predisposing to dislocation. This is shown quite nicely in our study.

Difference in coverage of the Pinnacle’s and Continuum’s neutral liners. The same ceramic head was first placed in the Pinnacle’s neutral liner (left) and the line at the rim was marked with a pen. Then, the head was moved into the same-sized Continuum’s neutral liner (right).

As always, small things matter. Is this a failure of the clearing process? I don´t know. What can be said is that post-marketing surveillance is important and we need more studies like this if clinical practice gives a reason suspect that something is going on.

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