The SKY is the limit – a novel system for interpreting radiolucent lines around CORAIL uncemented total hip arthroplasty stems: a proof-of-concept study

Background:

The presence of radiolucent lines (RLLs) around total hip replacement (THR) stems correlates with stem failure. However, the zone this occurs in is significant. The widely adopted Gruen Zone classification has disadvantages and is unlikely to be reflective of uncemented biomechanics. We propose a simpler system for describing these changes and introduce its relevance clinically.

Methods:

In a single-centre retrospective study CORAIL THR stems implanted between 2010 and 2013 were analysed. On postoperative radiographs 2 parallel “SKY” lines divided the stem bone interface into 3 zones A, B and C. 1 year and 5 years postoperative radiographs were reviewed by 5 surgeons for the presence and location of RLLs. The revision rates of the UK’s National Joint Registry were correlated with the presence of RLLs.

Results:

1113 stems were included.331 (29.7%) had RLLs, 273 (24.5%) in zone A only, 50 (4.5%) in zones A and B and 8 (0.7%) in all 3 zones A, B and C. In absence of RRLs in zone A, no RLLs were identified in the other zones. 73% RRLs appeared in the first postoperative year, the remaining at 5 years follow-up.

1.8% (20 of 1113) stems were revised. The revision rate for stems with RLLs in zone A alone was 2.9%, with RLLs in zone A and B it was 12% and with RLLs in all 3 zones it was 12.5%.

The odds of revision were 4.7 times higher (p = 0.007) with RLLs in Zone A and 6.6 times higher (p < 0.001) with RLLs in Zones B and C compared to those without.

Conclusions:

The SKY lines can be used to classify RLLs and predict the risk of revision for the CORAIL stem. Stems with RLLs progressing beyond the first SKY line from zone A into Zone B–C should be considered higher risk for revision and hence followed-up closely for signs of failure. Absence of RLLs or RLLs in Zone A only can be discharged to patient-initiated follow-up.