Retro-Odontoid Pseudotumor in Atlantoaxial Instability: Insights Into Presence, Subtypes, and Postoperative Regression.

Objective: Retro-odontoid pseudotumor (ROP) is a nonneoplastic mass associated with atlantoaxial instability (AAI). This study compared ROP-positive and ROP-negative AAI patients and evaluated cystic versus granulation-type ROP regarding regression patterns and surgical outcomes.

Methods: We retrospectively analyzed 112 AAI patients who underwent pre- and postoperative imaging and clinical evaluations. Patients were classified as ROP-positive or ROP-negative, with ROP-positive cases further categorized as cystic or granulation-type. Imaging parameters-including atlantodental interval (ADI), ΔADI, and cervical range of motion (ROM) were compared along with regression time and postoperative outcomes.

Results: Among 112 patients, 57 (50.9%) had ROP. The ROP-positive group was older (67.37±13.13 years vs. 56.90±15.15 years, p<0.001) and had lower ADI (5.63±2.77 mm vs. 6.99±2.33 mm, p=0.034), ΔADI (3.01±2.27 mm vs. 3.89±2.07 mm, p=0.006), and C2-7 ROM (30.78°±15.45° vs. 41.73°±16.58°, p<0.001). In ROP subgroups, the cystic group had greater C1-2 ROM (15.69°±6.34° vs. 10.00°±7.72°, p=0.013) and ADI (6.98±2.68 mm vs. 5.14±2.66 mm, p=0.042). Immediate postoperative ROP thickness remained greater in the cystic group (6.85±2.49 vs. 5.21±1.82 mm, p=0.042), while ROP thickness at 3 months and 1 year showed no significant differences. JOA recovery rates were similar.

Conclusion: This study demonstrates that ROP-positive AAI patients exhibit distinct radiological characteristics, with reduced cervical mobility. Furthermore, cystic ROP shows delayed regression following posterior fusion. These findings underscore the importance of ROP subtypes in surgical planning, requiring closer monitoring and possibly earlier intervention.