Congenital Atlanto-Occipital Fusion and its Effect on the Myodural Bridge: A Case Report Utilizing the P45 Plastination Technique

Abstract

The atlanto-occipital joint is composed of the superior fossa of the lateral masses of the atlas (C1) and the occipital condyles. Congenital Atlanto-occipital fusion (AOF) involves the osseous union of the base of the occiput (C0) and the atlas (C1). AOF or atlas occipitalization/assimilation represents a craniovertebral junction malformation (CVJM) which can be accompanied by other cranial or spinal malformations. AOF may be asymptomatic or patients may experience symptoms from neural compression as well as limited neck movement. The myodural bridge (MDB) complex is a dense fibrous structure that connects the suboccipital muscular and its related facia to the cervical spinal dura mater, passing through both the posterior atlanto-occipital and atlanto-axial interspaces. It is not known if atlas occipitilization can induce structural changes in the MDB complex and its associated suboccipital musculature. The suboccipital region of a cadaveric head and neck specimen from an 87-year-old Chinese male having a congenital AOF malformation with resultant changes to the MDB complex was observed. After being treated with the P45 plastination method, multiple slices obtained from the cadaveric head and neck specimen were examined with special attention paid to the suboccipital region and the CVJM. Congenital atlanto-occipital fusion malformations are defined as partial or complete fusion of the base of the occiput (C0) with the atlas (C1). In the present case of CVJM, unilateral fusion of the left occipital condyle with the left lateral mass of C1 was observed, as well as posterior central fusion of the posterior margin of the foramen magnum with the posterior arch of C1. Also noted was a unilateral variation of the course of the vertebral artery due to the narrowed posterior atlanto-occipital interspace. Surprisingly, complete agenesis of the rectus capitis posterior minor (RCPmi) and the obliques capitis superior (OCS) muscles was also observed in the plastinated slices. Interestingly, the MDB, which normally originates in part from the RCPmi muscle, was observed to originate from a superior bifurcation within an aspect of the nuchal ligament. Therefore, the observed changes involving the MDB complex appear to be an effective compensation to the suboccipital malformations.

Atlanto-occipital fusion: Case report

“…In isolation, this anatomical variation can be asymptomatic and undetected, only being discovered in radiological research in contexts of trauma or other neurological and orthopedic conditions (Joaquim et al, 2021;Kim, 2015). However, this condition can often be investigated early in the physical examination by testing hyperflexion and hyperextension of the neck, which will present a small limitation of movement (Chi et al, 2022). Greater stress on the C1 -C2 junction is reported in the literature due to the impossibility of movement between the skull and the Atlas, the first mobile segment of the spine, with consequent stretching of the transverse ligament in relation to the odontoid process and gradual loosening of thisevolving into a progressive subluxation of the atlanto-axial joint, favoring the displacement of the odontoid process and resulting in the perpetuation of synostosis through an inflammatory process.…”