BI without AAD is now considered to be a stable type with no evidence of atlantoaxial instability on dynamic X-ray and CT scans. The literature review showed no definitive treatment strategy. The common surgical approach is posterior fossa decompression, including bony and subdural decompression; however, many patients with BI, treated by this procedure, fail to show improvement [10, 11]. Anterior decompression is still considered the classic surgical method; however, anterior oral surgery is complex, has many complications, and additionally requires posterior fixation to maintain stability [12, 13]. Goel et al. believed that the patients’ symptoms were the result of atlantoaxial instability; he achieved good results with direct posterior fixation . Although posterior fixation alone has shown better results, it is not effective in all cases, and instances of re-aggravation have been reported .
Ventral brainstem compression is a common pathogenic feature, the evaluation of which is crucial in the treatment of BI. pB-C2 is an important evaluation index that many researchers have used to assess the degree of ventral brainstem compression in BI [13, 15]. In our case, pB-C2 decreased from a mean of 7.47 ± 2.21 mm preoperatively to 5.68 ± 3.13 mm postoperatively, suggesting that direct posterior surgery in patients with BI without AAD can reduce ventral brainstem compression. The CCA has been used in many studies to measure the degree of the ventral cervical spinal cord compression at the craniovertebral junction, which is a very important indicator of the reduction of AAD [8, 16]. The CCA at a neutral position in normal people measured by some scholars was 145-160° . Botelho and Ferreira measured the CCA in 25 patients with BI, and the average angle in this group was 120° (79–145°) . There are no targeted studies in cases of BI without AAD, and in this group, the CCA tends to be significantly reduced compared to normal.
Goel et al. suggest that the shortening of the neck and height of the cranial fossa is not related to the shortening of the length of the brainstem and spinal cord. The former may be a naturally occurring protective measure that facilitates the inelastic traversal of the spinal cord at the tip of the dentate . Owing to these compensatory mechanisms, the ventral compression of the brainstem in this group of cases is often not serious. Appropriate ventral decompression may improve symptoms. Previous studies that used biomechanical principles similar to those of our study to improve the CCA in the treatment of BI showed promising results in some cases [4, 19]. Henderson’s finite element analysis shows that the normalization of CCA and the stability of fusion are related to clinical improvement .
The correction of the CCA is very important. We have found that if internal fixation leads to a reduction of the CCA, in some cases, it will aggravate the ventral compression of the brainstem, resulting in no improvement or even aggravation of symptoms. This phenomenon is explained in the literature by Tachibana et al. , who found that cervical movements were important in the formation of spinal cavities. They performed the Queckenstedt test on patients with Chiari malformation combined with syringomyelia and found that none had obstruction of cerebrospinal fluid flow when the neck was extended or in a neutral position, and all had obstruction of cerebrospinal fluid flow when the neck was flexed. Wang et al. suggest that a satisfactory reduction can be achieved by repositioning the CCA by 150° or more . Nagashima and Kubota believed that if the CCA was less than 130°, there might be ventral compression of the brainstem, and it should be corrected to a greater angle during surgery . Botelho’s case report suggests that enlarging the CCA in patients with BI can reduce the compression of the ventral brainstem .
Given this situation, we have improved the interfacet distraction and fixation technique to allow for ventral decompression and preservation of atlantoaxial stability through a one-stage procedure. We have attempted to treat BI by expanding the CCA with internal fixation, thereby reducing the ventral compression of the brainstem. In our group of 19 patients, symptoms improved after posterior surgery was performed to increase the CCA. Intraoperatively, we performed microsurgical interfacet release and placed a bone block or cage of approximately 6 mm in height filled with bone debris ventral to the articular surface. The head frame was subsequently adjusted to place the head in a slightly hyperextended position, which is equivalent to a rotational movement of C1 using the bone block or cage as a fulcrum, increasing the CCA, before fixation was applied. The improvement in ventral compression is because of two factors. Firstly, the odontoid process drops to a lower level relative to the C1 vertebra after the interarticular placement of the bone block or cage. This directly reduces the ventral compression. Secondly, with C1 and clivus tilted slightly upwards and backwards after fixation, the inferior articular surface of C1 is not parallel to the superior articular surface of the axis but is rather trapezoidal in the sagittal position (Fig. 1). Therefore, the CCA is increased, which in turn reduces the ventral compression.
The advantage of this technique is that ventral decompression and stabilization of C1 and C2 can be achieved through a posterior one-stage procedure, avoiding the more invasive transoral release followed by posterior fixation. Although this technique requires interarticular manipulation of the lateral atlantoaxial articulation, it tends to be horizontal or only slightly tilted anteriorly and posteriorly in this type of case, unlike in cases of AAD where the articulation tends to be severely anteriorly tilted, and thus the intraoperative interarticular release and manipulation are not complicated.
Some details need attention during the operation: Firstly, the position of the bone block or cage is very important, as it should be placed on the anterior side of the articular surface to facilitate a slight posterior rotation of the atlas. Secondly, the adjustment of the head frame is also noteworthy, as the lateral articulation is deeper in patients with BI, and sometimes the head needs to be adjusted to a slightly flexed position to facilitate exposure, with subsequent slight hyperextension before tightening of the screws after fixation. If necessary, cantilever technology can be used to assist in increasing the CCA. Thirdly, the inter-articular release should be done with gentle and gradual extension. It is often difficult in patients with BI without AAD to hold the lateral block joint open for a greater distance after release, and it is significantly more difficult to place the cage between joints when the cage height exceeds 6 mm, even if the articular surfaces have been treated with an ultrasonic osteotome. In this group, the placement of a cage in the interarticular space on one side resulted in a fracture of the inferior C1 articular surface, but fortunately, there was no resultant neurological or vascular damage.
BI often occurs with syringomyelia , as was seen in 13 cases in our study. Recent studies suggest that syringomyelia is secondary to atlantoaxial instability, a view that is confirmed in many cases of BI . The technique of improving ventral compression through surgical fixation is effective. In our group of 13 cases with syringomyelia, improvement of the CCA without posterior fossa and subdural decompression was achieved, and there was significant relief of syringomyelia in 10 cases at the postoperative follow-up. The improvement of syringomyelia may be because of the relief of the ventral brainstem compression, which subsequently improves the cerebrospinal fluid flow patterns at the craniovertebral junction . The absence of relief of the cavity in individual cases may be explained by insufficient ventral decompression.
There were no serious complications in this group, and most of the patients’ symptoms were significantly relieved.
The limitations of this study need to be noted. As this is a retrospective analysis of a series by the same surgeon, there may be an inherent bias in case selection and treatment. Patients were only followed up for 2.1 years, and the long-term outcomes and complications need to be observed further. The increase in the CCA of type B BI can reduce the compression of the ventral brainstem, but the best degree of CCA correction in such cases still needs further study. Furthermore, in some patients with combined severe platybasia, exposure of the lateral atlantoaxial joint via the posterior approach is difficult and this procedure is inappropriate in such cases.