BI is often associated with AAD, atlantooccipital fusion, C2-3 fusion, Chiari malformations, and syringomyelia. After ventral compression is relieved and the atlantoaxial stability maintained, Chiari malformation is restored, and the syringomyelia is significantly reduced [8, 9]. This suggests that the primary lesions of these patients often result from the dentate process pressing the medulla oblongata, while cerebellar tonsillar hernia and syringomyelia are secondary changes. Anterior ventral decompression combined with posterior fixation has been proven effective in treating BI with or without ADD [10, 11]. With the development of this treatment strategy and continuous improvement of internal fixation devices, BI treatment has recorded breakthroughs in recent years. In group A, most cases can achieve one-stage reduction and fixation by posterior interarticular distraction combined with internal fixation. The reduction of the odontoid process is known as decompression, and few patients require complex transoral odontoidectomy. Group B had more patients who underwent the posterior fixation technique (the same technique as that of group A) and achieved excellent results. Available evidence shows that posterior surgery is effective in the treatment of BI because the expansion and fixation of the joints relieve the compression of the ventral surface of the brainstem. In summary, the BI treatment strategy gradually changed from the anterior-posterior composite approach to the single posterior approach, and decompression was performed through reduction and fixation technology without additional bone decompression.
The posterior lateral atlantoaxial joint distraction and fixation technique is uncomplicated; however, achieving a sufficient reduction of the dentate process in BIs is difficult. In group A cases, horizontal and vertical displacement of the odontoid process is often combined with angular displacement, which manifests as CCA reduction. Cervical traction after general anesthesia cannot correct all vertical dislocations. Cage insertion between joints can help correct vertical dislocations; however, it is not a suitable method for CCA correction because the two surfaces of the lateral atlantoaxial joint in ideal reduction are not parallel but present a wedge shape [8, 12]. In group B cases, changing the position of the odontoid process via interarticular distraction proves difficult. Because the central atlantoaxial joint is strong and there is no dislocation, the downward movement of the odontoid process is often not observed after cervical traction under general anesthesia. Separating the lateral atlantoaxial joints by a reasonable distance remains challenging even when using a distraction device. In this group of cases, reduction of the CCA is present and ventral compression is significant. Therefore, it is crucial to relieve ventral compression by correcting the CCA.
When vertical dislocation cannot be reduced or the CCA needs to be corrected after cervical traction under general anesthesia, the lateral atlantoaxial joint remodeling technology can be considered and the lateral joint surface can be partially removed during the operation. There are few studies on lateral atlantoaxial joint remodeling technology. Chandra's technology provides us with a new idea. He adopts the joint surface remodeling technology when the lateral atlantoaxial joint is tilted forward and excises the posterior surface of the C2 articular process and the anterior surface of the C1 articular process, which has achieved good results in some complicated cases[13]. Salunke et al. used the comprehensive drilling technique of the lateral mass joint to perform facet osteotomy and applied it to the irreducible atlantoaxial dislocation [14]. Their research has allowed more AAD cases to be treated with the one-stage posterior surgery, avoiding the anterior odontoidectomy. However, they did not pay particular attention to the correction of the CCA nor applied this technique in BI without AAD. Our technology is slightly different. We believe that the joint remodeling technology has a wider application. The range of articular process resection is also concentrated as anteriorly as possible, allowing cage placement to assist in reduction. It can be used in cases where traction alone cannot completely reduce the atlantoaxial joint or in cases where the CCA needs to be corrected. In addition, we also used this technique in BI without AAD. During the operation, an ultrasonic osteotome was used to remove part of the bones on the dorsal side of the C1 and C2 articular processes to form an inclined surface between the joints. After cage placement, the odontoid process could be further moved down and the reduction of angular displacement could be improved. The use of this technique can effectively reduce the compression of the ventral brainstem. In this group, CCA and CMA were significantly increased postoperatively, and pB-C2 was decreased compared with the preoperative value. In addition, AAD was achieved in this group. The odontoid process of BI without AAD was slightly lower than before the operation. The postoperative ADI was within the normal range, and the CLV was lower than its preoperative value.
Of course, this technology also has potential risks. Joint remodeling inevitably leads to the loss of cortical bone structure. During cage implantation, there is a possibility of fracture of the articular surface, especially the risk of vertebral artery injury when combined with the high-riding vertebral artery. Therefore, it is important to pay attention to the shape of the vertebral artery before the operation. When the vertebral artery is close to the upper articular surface of C2, excessive resection of the upper articular process of C2 should be avoided to prevent an injury to the vertebral artery. In our study, the C1 inferior articular process collapsed in two cases, and one case had an avulsion fracture of the C1 anterior arch due to downward movement of the dentate process after cage placement in type B skull base depression. Although no nerve and blood vessel injuries occurred, this technique should be carefully applied in patients with osteoporosis.