Objective
Our previous study categorized atlanto-occipital joint (AOJ) morphology into 3 types, with types II and III-AOJ associated with Chiari malformation (CM) with and without type II basilar invagination (II-BI), respectively. This study aimed to assess the feasibility of tailoring surgical strategies for patients with CM based on AOJ morphological types.
Methods
We retrospectively studied 212 CM patients who underwent foramen magnum decompression (FMD) or combined occipitocervical fusion (OCF). Patients were divided into 4 groups: (1) pure CM with II-AOJ who underwent FMD (CM-II-FMD); (2) pure CM with III-AOJ who underwent FMD+OCF (CM-III-OCF); (3) CM-III-FMD; and (4) CM+ II-BI with III-AOJ who underwent FMD+OCF (BI-III-OCF). Clinical data, including manifestations, imaging findings, surgical details, and neurological assessments, were analyzed at the final follow-up to assess surgical efficacy.
Results
Patients in the BI-III-OCF, CM-III-OCF, and CM-II-FMD groups exhibited a significant improvement in clinical symptoms (pain, sensory disturbances, motor weakness, gait ataxia, and bladder and bowel dysfunction) compared to preoperative levels (p<0.05). Results from the Japanese Orthopaedic Association scale and Neck Disability Index indicated a significant reduction in the degree of neurological impairment within these groups (p<0.05). Furthermore, the Chicago Chiari Outcome Scale scores indicated superior surgical outcomes for patients in these groups. Imaging analyses demonstrated significant reductions in the syringomyelic segment, syringomyelia width, and tonsillar herniation distance among these patients (p<0.05). However, the CM-III-FMD group did not significantly improve in these areas (p>0.05). Postoperative complications occurred in 4.3% of FMD+OCF patients and 3.3% of FMD-only patients.
Conclusion
AOJ morphological types can guide surgical treatment strategies for CM with or without II-BI. FMD alone is suitable for II-AOJ cases, whereas III-AOJ cases should be treated with FMD combined with OCF.

Objective
Recent studies indicate that 3 morphological types of atlanto-occipital joint (AOJ) exist in the craniovertebral junction and are associated with type II basilar invagination (BI) and atlanto-occipital instability. However, the actual biomechanical effects remain unclear. This study aims to investigate biomechanical differences among AOJ types I, II, and III, and provide further evidence of atlanto-occipital instability in type II BI.
Methods
Models of bilateral AOJ containing various AOJ types were created, including I-I, I-II, II-II, II-III, and III-III models, with increasing AOJ dysplasia across models. Then, 1.5 Nm torque simulated cervical motions. The range of motion (ROM), ligament and joint stress, and basion-dental interval (BDI) were analyzed.
Results
The C0–1 ROM and accompanying rotational ROM increased progressively from model I-I to model III-III, with the ROM of model III-III showing increases between 27.3% and 123.8% indicating ultra-mobility and instability. In contrast, the C1–2 ROM changes were minimal. Meanwhile, the stress distribution pattern was disrupted; in particular, the C1 superior facet stress was concentrated centrally and decreased substantially across the models. The stress on the C0–1 capsule ligament decreased during cervical flexion and increased during bending and rotating loading. In addition, BDI gradually decreased across the models. Further analysis revealed that the dens showed an increase of 110.1% superiorly and 11.4% posteriorly, indicating an increased risk of spinal cord impingement.
Conclusion
Progressive AOJ incongruity critically disrupts supportive tissue loading, enabling incremental atlanto-occipital instability. AOJ dysplasia plays a key biomechanical role in the pathogenesis of type II BI.