Retro-odontoid mass resolution analysis and timing following posterior cervical spinal fixation: 16-year paediatric neurosurgery experience in a single UK institute

A retro-odontoid mass (R-OM) is a soft tissue mass that develops posterior to the odontoid process of the C2 vertebra. The instability leads to non-physiologic motion producing an inflammatory process with fibro cartilaginous mass that lead to spinal cord compression. Cervical fusion has been previously shown to reduce it in rheumatoid pannus. (1) Investigate impact of cervical fixation on R-OM in a paediatrics, (2) assess which group have the greatest reduction, and (3) assess the timing of reduction. Between 2005 and 2021, 52 children underwent craniocervical junction (CCJ) fixation. Of these, 35 (67%) children had measurable R-OM at the time of presentation. These children underwent either occipito-cervical or atlanto-axial fixation for atlantoaxial instability. All cases were treated in a single tertiary paediatric neurosurgical centre by a multidisciplinary team. All procedures were performed by the same neurosurgical team. Demographic data, including age, gender, diagnosis, type of surgery, and measurements of pre- and post-operative R-OM were recorded. Serial post-operative scans provided evidence of regression of R-OM. R-OM measurements were performed using T2 W mid-sagittal and axial views of MRI to calculate anterio-posterior (AP), craniocaudal (CC), and mediolateral/width (LL) dimensions. Twenty-four children underwent atlantoaxial (69%), and 11 children had occipito-cervical fixation (31%). The mean age was 8.9 years (range 2-18) with M:F ratio of 1:1.3. Metabolic causes were the most common group undergoing surgery, with MPS IV being the most prevalent subgroup. Pooled evaluation of all cases revealed significant reduction in R-OM following fixation in all measured dimensions. Subgroup analysis of underlying pathology revealed that the metabolic group showed the most significant reduction. Analysis of timing of regression of the R-OM revealed that the maximal reduction occurred at the mean of 3 years (35.54 months) following fixation with the range of 2 weeks to 10 years (0.13 to 120.77 months). In our experience, R-OM is a common feature in atlantoaxial instability in the paediatric population. Cervical fixation leads to regression in R-OM, supporting instability as the driving force for formation of R-OM. Hence, resection of R-OM directly is not recommended and could expose the child to unnecessary risk.