Unilateral Accessory Articular Facet at C1 to C2

Discussion

Although the craniovertebral junction is a common site of congenital anomalies, accessory atlantoaxial facet joints are rare. Accessory articulations may be observed in the occipitocervical region6; however, the etiology of accessory C1 to C2 facet joints remains uncertain. One of the main proposed mechanisms is genetic, with joint formation regulated by HOX genes. Alterations in these genes may promote the formation of “interzones,” representing potential sites for accessory articulation. Abnormal micromotion between C1 and C2 may further perpetuate cavitation in this region, contributing to the development of an accessory joint and disruption of normal embryological processes.7

Authors such as Riesenburger et al and Salunke et al have proposed a congenital origin, resulting from alterations in embryological development, including abnormal fusion of the C1 and C2 sclerotomes or defective segmentation.1,2 Another plausible etiology, as suggested by Boucher et al, is analogous to the mechanism described in Bertolotti syndrome.8 However, 1 limitation of the genetic/embryological hypothesis is the wide variation in the age at diagnosis of accessory facet joints. As a complementary explanation, atlantoaxial instability has been proposed. Salunke et al, in a series of 35 patients with atlantoaxial instability, identified unusual contact points between the inferior aspect of the C1 arch and the C2 isthmus. These were termed “pseudofacets” and were thought to arise from congenital incompetence of the native C1 to C2 facet joints, with the accessory articulation representing a compensatory mechanism to enhance stability.5

Histologically, the characteristics of the accessory articulation may vary depending on the stage of embryonic development at which the process is interrupted. Early interruption may result in bone remodeling without cartilaginous tissue, whereas later interruption may lead to osseous changes associated with cartilaginous tissue, but without a synovial membrane or capsule, as observed in the present case. In theory, the formation of a “true articular facet,” with a capsule and synovium, could occur if the genetic and inductive processes were fully completed.7

One of the earliest clinical manifestations of accessory atlantoaxial articulations is neck pain, often associated with occipital pain, which may be refractory to conventional analgesia. Over time, the condition may progress to neurological dysfunction in cases of neural compression, including features of cervical myelopathy, as observed in the present case.8 From a diagnostic perspective, lateral cervical radiographs may show an apparently “hypertrophied” C1 to C2 joint due to superimposition of the accessory articulation. Computed tomography with bone reconstruction is the primary imaging modality for detailed evaluation of the facet anatomy and assessment of potential C1 to C2 instability. Magnetic resonance imaging is essential for demonstrating spinal cord compression.9 In the present case, both computed tomography and magnetic resonance imaging were performed.

From a clinical perspective, accessory C1 to C2 facet joints may contribute to spinal cord compression through both static and dynamic mechanisms. Direct posterior compression can occur due to hypertrophic bony overgrowth of the accessory articulation, while abnormal micromotion and segmental instability at the craniovertebral junction may further exacerbate neural compromise over time.1,5,8 These features have important implications for surgical planning, as detailed preoperative assessment with computed tomography and magnetic resonance imaging is essential to delineate the anatomical relationships, evaluate potential instability, and guide the choice of surgical approach, including decompression with or without stabilization.7,9 In addition, careful recognition of this entity is necessary to differentiate it from degenerative pseudoarticulations, which are more commonly observed in older patients and are typically associated with age-related changes rather than congenital or developmental anomalies.4,6 The distinction is clinically relevant, as accessory atlantoaxial articulations may represent either a primary developmental abnormality or a compensatory biomechanical adaptation, requiring individualized management strategies based on the underlying pathophysiology.3,10

In addition, Eaton and Viljoen4 described a similar case of unilateral atlantoaxial pseudoarticulation and proposed a biomechanical mechanism analogous to Bertolotti syndrome, supporting the concept that abnormal segmental motion and compensatory changes may contribute to the formation of accessory joints in the upper cervical spine. This comparison is clinically relevant, as it suggests that such pseudoarticulations may represent not only a developmental anomaly but also an adaptive response to altered spinal alignment or biomechanics, including subaxial kyphosis. Recognition of this entity is important, as it may lead to spinal cord compression and neurological deficits, whereas surgical resection has been associated with symptom improvement in reported cases.4

As with most spinal pathologies, initial management of accessory cervical facet joints is typically conservative, consisting of analgesia and physical rehabilitation. Surgical intervention is indicated in the presence of motor deficits or other warning signs, or in cases of failure of conservative treatment. Surgical options include simple laminectomy, laminectomy with posterior fixation, or decompression with laminoplasty.7 In the present case, a standalone laminectomy was performed.

Importantly, postoperative stability should be carefully considered when planning surgical management of accessory atlantoaxial articulations. Although isolated decompression with arthrectomy may be sufficient in selected cases, particularly when preoperative imaging does not demonstrate clear atlantoaxial instability, the removal of accessory or native stabilizing structures may theoretically alter segmental biomechanics. Therefore, thorough preoperative assessment and close postoperative follow-up are essential to identify any signs of delayed instability, and stabilization procedures should be considered on a case-by-case basis depending on intraoperative findings and individual biomechanical factors.