A Novel Technique for Basilar Invagination Treatment in a Patient with Klippel-Feil Syndrome: A Clinical Example and Brief Literature Review.

Objectives and Background: To present a novel technique of treatment for a patient with basilar invagination. Basilar invagination (BI) is a congenital condition that can compress the cervicomedullary junction, leading to neurological deficits. Severe cases require surgical intervention, but there is debate over the choice of approach. The anterior approach allows direct decompression but carries high complication rates, while the posterior approach provides indirect decompression and offers good stability with fewer complications. Materials and Methods: A 15-year-old boy with severe myelopathy presented to our hospital with neck pain, bilateral upper limb muscle weakness, and hand numbness persisting for 4 years. Additionally, he experienced increased numbness and gait disturbance three months before his visit. On examination, he exhibited hyperreflexia in both upper and lower limbs, muscle weakness in the bilateral upper limbs (MMT 4), bilateral hypoesthesia below the elbow and in both legs, mild urinary and bowel incontinence, and a spastic gait. Radiographs revealed severe basilar invagination (BI). Preoperative images showed severe BI and that the spinal cord was severely compressed with odontoid process. Results: The patient underwent posterior surgery with the C-arm free technique. All screws including occipital screws were inserted into the adequate position under navigation guidance. Reduction was achieved with skull rotation and distraction. A follow-up at one year showed the following results: Manual muscle testing results and sensory function tests showed almost full recovery, with bilateral arm recovery (MMT 5) and smooth walking. The cervical Japanese Orthopedic Association score of the patient improved from 9/17 to 16/17. Postoperative images showed excellent spinal cord decompression, and no major or severe complications had occurred. Conclusions: Basilar invagination alongside Klippel-Feil syndrome represents a relatively uncommon condition. Utilizing a posterior approach for treating reducible BI with a C-arm-free technique proved to be a safe method in addressing severe myelopathy. This novel navigation technique yields excellent outcomes for patients with BI.

Dysphagia as a primary manifestation of basilar impression: A case report.

Secondary basilar invagination or basilar impression is an anomaly at the craniovertebral junction where the odontoid process prolapses into the foramen magnum with the risk of compressing adjacent structures and obstructing the proper flow of cerebrospinal fluid (CSF). The incidence is less than 1% in the general population and occurs mainly in the first three decades of life when it is associated with malformations of the neuroaxis. In older age, the main aetiologies are diseases that alter bone mineral density. The clinical course is usually progressive and the most common symptoms are asthenia, cervical pain and restricted movement, but also dysphonia, dyspnoea and dysphagia. It is a progressive disease which, if left untreated, can cause severe neurological damage and death. We report the case of a 79-year-old woman with osteoporosis and progressive dysphagia leading to severe malnutrition, which conditioned the decision not to intervene due to the high perioperative risk.

A novel classification and management scheme for craniocervical junction disorders with ventral neural element compression.

OBJECTIVE: Craniocervical junction (CCJ) pathologies with ventral neural element compression are poorly understood, and appropriate management requires accurate understanding, description, and a more uniform nomenclature. The aim of this study was to evaluate patients to identify anatomical clusters and better classify CCJ disorders with ventral compression and guide treatment. METHODS: A retrospective review of adult and pediatric patients with ventral CCJ compression from 2008 to 2022 at a single center was performed. The incidence of anatomical abnormalities and compressive etiologies was assessed. Surgical approach, radiographic data, and outcomes were recorded. Association rules analysis (ARA) was used to assess variable clustering. RESULTS: Among 51 patients, the main causes of compression were either purely bony (retroflexed dens [n = 18]; basilar invagination [BI; n = 13]) or soft tissue (degenerative pannus [n = 16]; inflammatory pannus [n = 2]). The primary cluster in ARA was a retroflexed dens, platybasia, and Chiari malformation (CM), and the secondary cluster was BI, C1-2 subluxation, and reducibility. These, along with degenerative pannus, formed the three major classes. In assessing the optimal treatment strategy, reducibility was evaluated. Of the BI cases, 12 of the 13 patients had anterolisthesis of C1 that was potentially reducible, compared with 2 of the 18 patients with a retroflexed dens (both with concomitant BI), and no pannus cases. The mean C1-2 facet angle was significantly higher in BI at 32.4°, compared with -2.3° in retroflexed dens and 8.1° in degenerative pannus (p < 0.05). Endonasal decompression with posterior fixation was performed in 48 (94.0%) of the 51 patients, whereas posterior reduction/fixation alone was performed in 3 patients (6.0%). Of 16 reducible cases, open posterior reduction alone was successful in 3 (60.0%) of 5 cases, with all successes containing isolated BI. Reduction was not attempted if vertebral anatomy was unfavorable (n = 9) or the C1 lateral mass was absent (n = 5). The mean follow-up was 28 months. Symptoms improved in 88.9% of patients and were stable in the remaining 11.1%. Tracheostomy and percutaneous G-tube placement occurred in 7.8% and 11.8% of patients, respectively. Reoperation for an endonasal CSF leak repair or posterior cervical wound revision both occurred in 3.9% of patients. CONCLUSIONS: In classifying, one cluster caused decreased posterior fossa volume due to an anatomical triad of retroflexed dens, platybasia, and CM. The second cluster caused pannus formation due to degenerative hypertrophy. For both, endonasal decompression with posterior fixation was ideal. The third group contained C1 anterolisthesis characterized by a steep C1-2 facet angle causing reducible BI. Posterior reduction/fixation is the first-line treatment when anatomically feasible or endonasal decompression with in situ posterior fixation when anatomical constraints exist.

Modified interfacet technique using shaped autologous occipital bone mass for basilar invagination.

PURPOSE: In economically undeveloped areas, surgery for basilar invagination (BI) is still a serious economic burden for people. This study introduces a modified interfacet technique for the treatment of BI using shaped autologous occipital bone mass to reduce BI and to save economical expenditure. METHODS: The data of 6 patients with BI who underwent modified interfacet technique using shaped autologous occipital bone mass in our hospital from April 2020 to February 2021 were retrospectively analyzed. During the operation, osteotomy at the external occipital protuberance was performed using ultrasonic osteotome, followed by interfacet release and implantation of shaped autologous occipital bone mass to complete vertical reduction. The atlantodental interval (ADI), Chamberlain's line violation (CLV), clivo-axial angle (CXA) and cervico-medullary angle (CMA) were compared before and after surgery. Additionally, we observed implant stability during the follow-up period to assess the long-term success of the modified interfacet technique. RESULTS: The surgical procedure was successful in all six patients, with no reported incidents of vascular injury, spinal cord injury, or dural tear. Following the operation, improvements were observed in the ADI, CLV, CXA, and CMA. Throughout the follow-up period, the implants remained stable, demonstrating no complications such as bone resorption of the autologous occipital bone mass, implant fracture, or displacement. CONCLUSION: The utilization of shaped autologous occipital bone mass in atlantoaxial interfacet bone grafting has demonstrated effectiveness and feasibility. This technique offers simplicity, ease of preparation, and cost-effectiveness, making it a viable option for treating BI.

Individualized surgical treatment of Chiari 1 malformation: A single-center experience.

OBJECTIVE: To present our experience with an individualized surgical approach to treat Chiari malformation type 1. METHODS: Based on (1) neurological symptoms, (2) the existence and extent of a syrinx and (3) the degree of the tonsillar descent we performed four types of approaches on a case-by-case basis in 81 patients: (1) foramen magnum decompression (FMD) with dura splitting (FMDds); (2) FMD with duraplasty (FMDdp); (3) FMD with duraplasty and tonsillar manipulation (FMDao); and (4) tonsillar resection/reduction (TR). Patient characteristics, Chiari Severity Index (CSI), fourth ventricular roof angle (FVRA) and Chicago Chiari Outcome Scale (CCOS) were analyzed. RESULTS: CCOS was between 13 and 16 points in 8/11 (73 %) patients after FMDds, 38/45 (84 %) patients after FMDdp, and 24/24 (100 %, one patient lost to follow-up) patients after TR. We experienced an overall complication rate of 13.6 % (11/81) in this series, whereas seven of these eleven complications (64 %) occurred in the FMDao group and the complication rate increased with the invasiveness of the approach (0 % FMDds; 4 % FMDdp; 12 % TR). CONCLUSION: Given the clear correlation between the extend of the approach and the complication rate the least invasive approach necessary to achieve clinical improvement should be selected. Due to the high complication rates, FMDao should not be used as a treatment option. The severity of the tonsillar descent, basilar invagination and current CM1 scores could be used to aid in the approach selection.

Correlation Among Syrinx Resolution, Cervical Sagittal Realignment, and Surgical Outcome After Posterior Reduction for Basilar Invagination, Atlantoaxial Dislocation, and Syringomyelia.

BACKGROUND: The correlation among syrinx resolution, occipitoaxial sagittal alignment, and surgical outcome in long-term follow-up seems to have not been clarified. OBJECTIVE: To further explore the relationship between the syrinx resolution and occipitoaxial realignment after posterior reduction and fixation in basilar invagination (BI)-atlantoaxial dislocation (AAD) patients with syringomyelia. METHODS: A continuous series of 32 patients with BI-AAD and syringomyelia who received direct posterior reduction met the inclusion criteria of this study. Their clinical and imaging data were analyzed retrospectively. Before surgery and at the last follow-up, we used the Japanese Orthopedic Association (JOA) score and the Neck Disability Index (NDI) to assess the neurological status, respectively. The Pearson correlation coefficient and multiple stepwise regression analysis were used to explore the relevant factors that may affect surgical outcomes. RESULTS: There were significant differences in atlantodental interval, clivus-axial angle, occiput-C2 angle (Oc-C2A), cervicomedullary angle (CMA), subarachnoid space (SAS) at the foramen magnum (FM), syrinx size, NDI, and JOA score after surgery compared with those before surgery. ΔCMA and the resolution rate of syrinx/cord as relevant factors were correlated with the recovery rate of JOA (R 2 = 0.578, P < .001) and NDI (R 2 = 0.369, P < .01). What's more, ΔSAS/FMD (SAS/FM diameter) and ΔOc-C2A were positively correlated with the resolution rate of syrinx/cord (R 2 = 0.643, P < .001). CONCLUSION: With medulla decompression and occipital-cervical sagittal realignment after posterior reduction and fusion for BI-AAD patients with syringomyelia, the structural remodeling of the craniovertebral junction and occipitoaxial realignment could contribute to syringomyelia resolution.

C1-2 Fusion in Atlantoaxial Dislocation and Basilar Invagination with and without Chiari Malformation: Clinical/Radiological and Craniometric Results.

OBJECTIVE: Patients with Chiari malformation (CM) associated with atlantoaxial dislocation (AAD) and basilar invagination (BI) may present with a small posterior cranial fossa, but data on the volumetric analysis are lacking. Additionally, whether additional foramen magnum decompression (FMD) is needed together with atlantoaxial fusion remains controversial. This study evaluated the volumetric alterations of the posterior cranial fossa in these patients and analyzed the radiological and clinical outcomes after posterior C1-C2 reduction and fixation plus C1 posterior arch resection. METHODS: Thirty-two adult CM patients with AAD and BI (CM-AAD/BI group) and 21 AAD and BI patients without CM (AAD/BI-only group) who received posterior atlantoaxial fusion plus C1 posterior arch resection were retrospectively studied. The clinical and radiological outcomes and volumetric measurements of the posterior cranial fossa were evaluated. RESULTS: The majority of CM-AAD/BI patients (94%) improved clinically and radiologically at 12 mo postoperatively, and none required additional FMD. Morphological analysis revealed a significant reduction in the bony posterior cranial fossa volumes of the CM-AAD/BI group (P < 0.01) and the AAD/BI-only group (P < 0.01) relative to those of the CM group. No significant differences were observed between the CM-AAD/BI and AAD/BI groups. CONCLUSIONS: Compared with patients with simple CM, patients with AAD/BI with or without CM demonstrated a considerably and equally reduced bony posterior cranial fossa volume. No additional FMD is needed in the treatment of CM-AAD/BI patients after posterior reduction and fusion plus C1 posterior arch resection.

Advances, Challenges, and Future Directions in the Management of Craniovertebral Junction Pathologies.

In this third article in a 3-article series on the craniocervical junction, we define the terms "basilar impression," "cranial settling," "basilar invagination," and "platybasia," noting that these terms are often used interchangeably but represent distinct entities. We then provide examples that represent these pathologies and treatment paradigms. Finally, we discuss the challenges and future direction in the craniovertebral junction surgery space.

Interfacet Distraction for Type B-Basilar Invagination Assisted by a Novel Sliding-Traction Head Holder: 2-Dimensional Operative Video.

Type B basilar invagination (BI) refers to odontoid process prolapse into skull base without dislocation, which is often associated with Chiari malformation and syringomyelia.1 Its pathogenesis and treatment are in controversy. Posterior fossa decompression has been introduced to treat the associated Chiari malformation; however, it may not be effective for symptoms with severe ventral nerve compression.2 C1-2 in situ fixation has been reported to treat minor instability in type B BI,3 yet the assessment of minor instability has not been widely accepted. We believe that the pathogenesis is chronic ventral compression of the brainstem. Transoral odontoidectomy is a classic surgical procedure to treat the pathogenesis, but it is associated with high risks of infection, velopharyngeal incompetence, and airway obstruction and often requires posterior surgery for fusion.4 Thus we use an interfacet distraction technique to move the odontoid process down and relieve ventral brainstem compression.5 Therefore interfacet structural support is necessary to maintain the distance of the interfacet. The reason for fixation is that interfacet distraction disrupts atlantoaxial stability. We designed a novel sliding-traction head holder, which can quantitatively tract and stabilize the head without support for the head or face, avoiding the risk of eyeball compression. It adapts to a variety of surgical methods and allows intraoperative changes in the patient's position. Assisted by the device, a satisfied interfacet distraction and a correction of BI were obtained. Video 1 describes how the interfacet distraction technique assisted by a sliding-traction head holder was performed in an illustrative case. The patient consented to the procedure.

Delayed post-operative aggravation of sleep related disturbances in patients of basilar invagination with Chiari malformation: case report and review of the literature.

Sleep apnoea is common in patients with Basilar Invagination with Arnorld Chiari Malformation (ACM). Various studies have shown its incidence in the range of 60-70% among such patients. Most of the studies have shown improvement in sleep disturbances after decompressive surgeries for Chiari Malformations. There is no report of postoperative deterioration due to sleep apnoea in these patients. Authors report two cases of basilar invagination associated with ACM and Platybasia, who deteriorated probably due to worsening of pre-existing sleep disorders on 3rd and 7th postoperative days after their surgeries, despite clinico-radiological improvements during their early post-operative courses. Authors discuss literature related to sleep apnoea in basilar invagination associated with Chiari Malformations and share precautions, which are relevant and should be undertaken in such patients especially during early post-operative periods to avoid alarming complication which may occur even in experienced hands.

[Hot topic of cranial vertebral surgery: the surgical plan of type A basilar invagination].

Regarding the treatment of type A basilar invagination, there is a debate between the combination of anterior and posterior surgery and posterior fixation. The author's personal opinion on the hot topic of cranial vertebral surgery: the surgical plan of type A basilar invagination is expressed in this article. Overall, the combined anterior-posterior approach is applicable in almost all cases of type A basilar invagination, regardless of how severe atlantoaxial dislocation, and simple posterior surgery is only in cases where atlantoaxial dislocation is not very severe. The anterior-posterior combined surgery has two incisions, and the risk of postoperative infection is higher than for posterior surgery alone. If the patient has no spinal cord symptoms, only posterior surgery is acceptable, even if no anatomical reduction can be achieved. If there is a more severe spinal cord dysfunction, or a large syringomyelia, anterior atlantoaxial release is necessary to ensure achieving anatomical reduction and restoring the normal morphology of the cervical spinal cord.

[Direct intraoperative two-step distraction and reduction for basilar invagination with atlantoaxial dislocation].

Objective: To assess the clinical impact of direct two-step distraction reduction (TSDR) for basilar invagination (BI) with atlantoaxial dislocation (AAD). Methods: Retrospective analysis was conducted on the clinical data of patients who underwent TSDR and occipitocervical fusion in West China Hospital between October 2013 and March 2021. Depending on whether the preoperative decrease was greater than 50% on preoperative hyperextension X-rays, the patients were split into two groups. The neurological function [Japanese Orthopedic Association (JOA) score], atlantodens interval (ADI), the distance of odontoid process beyond McRae Line (ML) and Wackenheim Line (WL), cervicomedullary angle (CMA), O-C2 angle (OC2A), and complications incidence were compared between two groups preoperatively and postoperatively. Results: There were 12 men and 23 women among the 35 patients with BI and AAD, and the age ranged from 28 to 71 years, with an mean age of (52.0±13.4) years. In the preoperative reduction ≥50% group, there were 4 males and 9 females with an average age of (54.0±13.8) years; in the preoperative reduction <50% group, there were 8 males and 14 females with a mean age of (50.9±13.4) years. All the patients were followed-up for a mean time of (23.3±13.4) months. There was no significant difference in age, gender, bleeding, length of hospital stay and follow-up time between the two groups (all P>0.05). The JOA score, ADI, WL, ML and CMA of 35 patients were significantly improved when compared with those before operation (all P<0.05). The reduction degree of ADI, ML and WL was more than 80% in 31 cases (88.57%), 30 cases (85.71%) and 31 cases (88.57%), respectively. There was no significant difference in postoperative ADI, ML and WL between the two groups (all P>0.05). All patients had no incision infection, no loosening or breakage of the internal fixators. Dysphagia occurred in 3 patients, non-fusion happened in 1 patient, but no instability in X-ray of cervical dynamic position was found, no loosening or displacement occurred in internal fixators, and partial spontaneous fusion occurred between atlantoaxial lateral mass joints. Conclusions: For BI with AAD without atlantoaxial bony connection or serious atlantoaxial facet joint inclination, TSDR could obtain satisfactory reduction degree. The reduction degree on preoperative hyperextension X-ray doesn't affect the degree of intraoperative reduction.

[Posterior atlanto-axial intraarticular distraction technique as revision surgery to treat atlanto-axial dislocation associated with basilar invagination].

Objective: To examine the effect of posterior atlanto-axial intraarticular distraction technique as revision surgery for failed posterior fossa decompression in patients with basilar invagination(BI) and atlanto-axial dislocation(AAD). Methods: The clinical data of 13 cases of AAD accompanied with BI treated at Department of Neurosurgery, Xuanwu Hospital, Capital Medical University were retrospectively analyzed. There were 3 males and 10 females,aged (42.6±9.5) years (range:30 to 63 years). All cases had assimilation of atlas and once underwent posterior fossa decompression. Anterior tissue was released through posterior approach followed by cage implantation into facet joint and occipital-cervical fixation with cantilever technique. The clinical results were evaluated using Japanese Orthopedic Association scale(JOA) and the main radiological measurements including atlantodental interval (ADI), the distance of odontoid tip above Chamberlain line(DCL),clivus-canal angle(CCA) and the length of syrinx were collected. Paired sample t test was used to compared the data before and after operation. Results: All patients underwent surgery successfully, the mean surgical time was (187.7±47.4) minutes (range from 116 to 261 minutes). Twenty occipital condyle screws, 26 C2 pedicle screws and 3 occipital plates were implanted. Clinical symptoms improved in all patients. Twelve patients had complete reduction of basilar invagination and atlanto-axial dislocation, 1 achieved near completely reduction of basilar invagination. The postoperative ADI, DCL and CCA significantly improved((4.3±1.1) mm vs. (1.8±0.8) mm, (11.7±5.0) mm vs. (6.4±2.8) mm, (142.4±7.9)° vs. (133.3±7.9)°, all P<0.01).There were 5 cases with syringomyelia before surgery, and shrinkage of syrinx was observed 1 week after surgery in all cases. Eight patients achieved bone fusion 3 months after surgery, all patients achieved bone fusion 6 months after surgery. The JOA score increased from 12.8±2.3 before surgery to 14.8±1.3 one year after surgery, with statistically significant difference (t=4.416, P<0.01).No implant failure, spacer subsidence and infection were observed. Conclusion: In cases of failure posterior fossa decompression of basilar invagination and atlanto-axial dislocation, using posterior atlanto-axial intraarticular distraction and cantilever technique with cage implantation could achieve complete reduction and symptomatic relief.

Basilar decompression via a far lateral transcondylar approach: technical note.

BACKGROUND: Treatments for symptomatic or unstable basilar invagination (BI) include posterior decompression, distraction/fusion, trans-nasal or trans-oral anterior decompression, and combined techniques, with the need for occipitocervical fusion based on the degree of craniocervical instability. Variations of the far lateral transcondylar approach are described in limited case series for BI, but have not been widely applied. METHODS: A single-institution, retrospective review of consecutive patients undergoing a far lateral transcondylar approach for odontoidectomy (± resection of the inferior clivus) followed by occipitocervical fusion over a 6-year period (1/1/2016 to 12/31/2021) is performed. Detailed technical notes are combined with images from cadaveric dissections and patient surgeries to illustrate our technique using a lateral retroauricular incision. RESULTS: Nine patients were identified (3 males, 6 females; mean age 40.2 ± 19.6 years). All patients had congenital or acquired BI causing neurologic deficits. There were no major neurologic or wound-healing complications. 9/9 patients (100%) experienced improvement in preoperative symptoms. CONCLUSIONS: The far lateral transcondylar approach provides a direct corridor for ventral brainstem decompression in patients with symptomatic BI. A comprehensive knowledge of craniovertebral junction anatomy is critical to the safe performance of this surgery, especially when using a lateral retroauricular incision.

Transoral Odontoidectomy - Our Experience in A Limited-Resource Setup.

BACKGROUND: Although posterior procedures are used for most cases of basilar invagination, transoral decompression is an important tool in complex deformities with severe degeneration and neural compression. METHODS: Consecutive patients who underwent transoral odontoidectomy for basilar invagination and atlantoaxial dislocation at the neurosurgical unit of Lady Reading Hospital Peshawar between June 2016 and January 2022 were retrospectively included. Preoperative and postoperative neurological, clinical, and radiological parameters were recorded and compared. RESULTS: Of the 33 patients included in this study, 22 were men and 11 were women. The mean age was 23.4 years ±8.4 years (mean ± standard deviation). The most common presenting symptoms were gait abnormalities and neck pain. A total of 28 patients were treated for the first time, while 5 patients had prior posterior fusion. The mean distance of the tip of the dens above the chamberlain line was 8.9 ± 4.2 mm, while the mean anterior atlantodental interval was 6.5 ± 2.1 mm. Seven patients had an anomalous course of vertebral artery on at least one side. A total of 28 patients improved significantly, while 5 patients did not improve at the follow-up. The mean follow-up was 8.5 months ±6.3 (mean ± standard deviation). The mean Nurick and Modified Japanese Orthopedic Association scores improved postoperatively (P < 0.05). Three patients underwent reoperation including one with vertebral artery injury. Three patients underwent tracheostomy. CONCLUSIONS: With proper patient selection, transoral odontoidectomy can be safely performed in a limited-resource setting.

Evaluating Craniovertebral Stability in Chiari Malformation Coexisting with Type II Basilar Invagination: An Observational Study Based on Kinematic Computed Tomography and Its Clinical Application.

BACKGROUND: Treatment of Chiari malformation (CM) is controversial, especially when it coexists with "stable" or Type II basilar invagination (CM + II-BI). Precise evaluation of craniovertebral junction (CVJ) stability is crucial in such patients; however, this has never been validated. This study aimed to dynamically evaluate atlanto-condyle and atlantoaxial stability by kinematic computed tomography (CT) and report its surgical treatment. METHODS: The study recruited 101 patients (control, CM, and CM + II-BI groups: 48, 34, and 19 patients, respectively). During kinematic CT, the CVJ stability-related parameters were measured and compared between the 3 groups. The surgical strategy for treating CM + II-BI was based on these results. Preoperative and postoperative images were acquired, and functional scores were used to assess the outcome. RESULTS: Among the 3 groups, the length of the clivus and the height of the condyle were the shortest in the CM + II-BI group, which was accompanied by the greatest rotation of the atlas and atlanto-condyle facet movement on cervical flexion and extension. Moreover, in such patients, increased Chamberlain's baseline violation indicated the aggregate invagination of the odontoid in the flexed position, and asymmetric displacement of atlantoaxial facets was observed. Seventeen CM + II-BI patients underwent surgical treatment with atlantoaxial distraction and occipitocervical fusion. The syringomyelia width and tonsillar herniation decreased significantly, and functional scores indicated symptom relief and good outcomes. CONCLUSIONS: CVJ instability, especially the ultramovement of atlanto-condyle facets, commonly exists in II-BI as evaluated using kinematic CT. The surgical strategy of atlantoaxial distraction and occipitocervical fusion should be considered to treat such patients.

Basilar Invagination: A Tilt of the Foramen Magnum.

OBJECTIVE: Congenital basilar invagination (BI) is a craniocervical deformity marked by odontoid prolapse into the skull base. The foramen magnum angle (FMA), which is formed by the Chamberlain's line and McRae's line, has not been fully studied. The study aimed to investigate the FMA and its relationship with other craniocervical parameters. METHODS: Participants were divided into control, type A BI, and type B BI groups. Parameters included Chamberlain line violation, atlantodental interval, clivus height, clivus anteroposterior dimension, FMA, basal angle, clivo-axial angle, head and neck flexion angle, Boogard's angle, and subaxial cervical spine lordosis angle. A comparison of these parameters among the 3 groups and correlation analysis between FMA and other parameters were performed. The significance level was set at P < 0.05. RESULTS: A total of 111 controls, 111 type A BI patients, and 62 type B BI patients were enrolled. The FMAs in the control, type A BI, and type B BI groups were 6.21° (3.67°, 8.71°), 22.16° ± 6.61°, and 22.39° (17.27°, 31.08°), respectively. Correlation analysis revealed correlations between the FMA and other variables. In the 2 BI subgroups, FMA was significantly correlated with Chamberlain line violation, clivus height, clivus anteroposterior dimension, basal angle, clivo-axial angle, and Boogard's angle. CONCLUSIONS: The FMA in patients with BI was approximately 22° and approximately 6° in controls, indicating that the foramen magnum in BI had a greater tilt. As a pathological condition, FMA can reflect the degree of BI. Clivus hypogenesis is a reason for the excessive tilt of the FM.

Relevance of C1/2 Facet Configurations and Clivus-Canal-Angles for Adult Patients with Chiari I Malformation with and without Basilar Invagination.

OBJECTIVE: C1/2 facet configurations and clivus-canal-angles (CXAs) have been proposed as criteria for posterior fusion in Chiari I malformation (CMI). METHODS: Three-hundred and forty adults with CMI without basilar invagination (BI), 111 with CMI with BI, and 100 age- and sex-matched controls were studied using sagittal T2-weighted magnetic resonance imaging scans analyzing preoperative and postoperative values with their impact on progression-free survival rates. RESULTS: For CMI without BI, C1/2 facet configurations and CXA were similar to controls (142 ± 11 degrees and 144 ± 10 degrees, respectively) with low rates for posterior C1 displacements (7.1% and 10%, respectively). In CMI with BI, C1 facet displacements were common (54.9%) with lower CXA (120 ± 15 degrees). After foramen magnum decompression (FMD) in CMI without BI (n = 169), 1.8% developed posterior C1 facet displacements without CXA changes and a 97% progression-free survival rate for 10 years. In CMI with BI, patients without ventral compression or instability underwent FMD without fusion (n = 19). Among them, 5.3% developed a posterior C1 facet displacement without CXA changes and a 94% progression-free survival rate for 10 years. The remainder of CMI with BI underwent FMD with C1/2 fusion (n = 48). Among these, CXA values increased with 10-year progression-free survival rates of 74% and 93% with and without ventral compression, respectively. CONCLUSIONS: For adult CMI without BI, C1/2 facet configurations and CXA are irrelevant. FMD alone provides excellent long-term outcomes. In CMI with BI, anterior C1 facet displacements indicate C1/2 instability. Posterior fusions can be reserved for patients with ventral compression or C1/2 instability.