The modified Dunn procedure can be performed safely in stable slipped capital femoral epiphysis but does not alter avascular necrosis rates in unstable cases: a large single-centre cohort study

PURPOSE: The modified Dunn procedure for slipped capital femoral epiphysis (SCFE) remains controversial. We reviewed our series over ten years to report our learning curve, experience with intraoperative monitoring of femoral head perfusion and its correlation with postoperative Single-photon emission computed tomography (SPECT-CT) bone scan and femoral head collapse in stable and unstable SCFE. METHODS: We retrospectively assessed 217 consecutive modified Dunn procedures performed between 2008 and 2018. In all, 178 had a minimum of one-year follow-up (mean 2.7 years (1 to 9.2)) including 107 stable and 71 unstable SCFE. Postoperative viability was assessed with a three-phase Tc99 bone scan and SPECT-CT. From 2011, femoral head perfusion monitoring was performed intraoperatively using a Codman Intracranial Pressure transducer and the capsulotomy was modified. RESULTS: With intraoperative monitoring, the rate of non-viable femoral heads in stable SCFE decreased from 21.1% to 0% (p < 0.001). In unstable SCFE, the rate remained unchanged from 35.7% to 29.8% (p = 0.669). The positive predictive value (PPV) of pulsatile monitoring for no collapse was 100% in stable and 89.1% in unstable SCFE. Pulsatile monitoring and viable SPECT-CT bone scan gave a 100% PPV for all cases. A non-viable scan defines those hips at risk of collapse since 100% of stable and 68.2% of unstable hips with non-viable bone scans went on to collapse. CONCLUSION: Our protocol enables safe performance of this complex procedure in stable SCFE with intraoperative monitoring being a reliable asset. The avascular necrosis rate for unstable SCFE remained unchanged and further research into its best management is required. LEVEL OF EVIDENCE: Level III