Imaging diagnosis
Case 222
【Discussion】
Dens so called odontoid process of C2 (axis) protrudes upward from body, forming pivot. Atlas and skull move around dens together. It functions to rotate neck and skull together such as “No” movement. Atlas is needed to play a role of buffer between skull and spine. Dens functions as pivot in atlas but not contact to skull. The spaces between occipital bone and atlas, and between atlas and axis are embedded by supporting ligaments. There are three main ligaments between atlas and axis: apical ligament, alar ligament and transverse ligament. Of these, transverse ligament is most tight and thick. It is situated at the dorsal side of dens and at the ventral side of posterior longitudinal ligament. The injury of transverse ligament is related to the degree of dislocation of C2. Alar ligament and apical ligament are attached from dens toward the border of foramen magnum.
Atlas-axis position is preserved with facet joints. Atlas-axis dislocation is largely distinct horizontal from vertical. Horizontal atlas-axis dislocation is caused by apart facet joints and categorized into four types dependent on its rotating degree (1): Type A, atlas-dens distance with less 3mm (indicating within normal limits); Type B, atlas-dens distance with 3 to 5mm; Type C, atlas-dens distance more than 5mm: Type D, dens descends below atlas. Type B dislocation implies injury of transverse ligament. Type C dislocation implies injuries of transverse ligament, alar ligament and posterior longitudinal ligament. Type D dislocation indicates rupture of these three ligaments and implies both horizontal and vertical. Vertical atlas-axis dislocation can occur not only downward but also upward. Upward atlas-axis dislocation does not always imply ruptures of these ligament. It can occur without injuries of these ligaments. Actually, in Case 3, sagittal spine MRI showed upward vertical atlas-axis dislocation with no proof of injuries of these three ligaments.
Severe dens fracture is life-threatening. Judicial hangings indicate the dens break and compress or hit the medulla oblongata, causing sudden death. Dens fracture is categorized into three types (2-7); Type I avulsion fracture of dens tip; Type II dens base fracture: Type III dens fracture extends into C2 body. Treatment strategy is cervical collar in Type I, cervical halo and/or surgical fixation in Type II and Type III. Of these, Type II fracture is the most common, unstable and non-union. Dens is a watershed area of branches of internal carotid artery and vertebral artery. Then, the fracture is thought to be hard to unite. Jefferson fracture which indicates atlas arch fracture irrespective of anterior or posterior, can be associated with dens fracture.
Osteoarthritis is caused by less volume of cartilage at bone joint which gives stress to the adjacent bone, meniscus and disk, inducing to form osteosclerosis and osteophytes. Meanwhile, rheumatoid arthritis is caused by immune action; First, synovia is first damaged by inflammatory cells: macrophages, lymphocytes and fibroblasts (9, 10). These cells accumulate synovia, inducing edematous flame thickened synovia. They erode articular space, cartilage, replacing fibrous tissue and eventually ossifies. Rheumatoid arthritis occur at C2 most in cervical spine probably because synovium is most on C2.
【Summary】
We present two cases with dens fracture of Type II and Type III, and one case with dens dislocation due to rheumatoid arthritis.
It is borne in mind that dens is mainly supported by alar ligament, apical ligament and transverse ligament. Dens functions a pivot to rotate skull and atlas together. Facet joints function extension, flexion and rotation.
Atlas axis dislocation is categorized into 4 types: Type A, atlas-dens distance with less 3mm (indicating within normal limits); Type B, atlas-dens distance with 3 to 5mm; Type C, atlas-dens distance more than 5mm: Type D, dens descends below atlas. Dens fracture is categorized into three types; Type I avulsion fracture of dens tip; Type II dens base fracture: Type III dens fracture extends into C2 body. Rheumatoid arthritis arises from infiltration of inflammatory cells to synovia, inducing flame thickened synovia, implying pannus formation. Pannus is composed of inflammatory cells, granulation tissue, osteoclasts and osteoblasts. Pannus erodes articular space and breaks cartilage, replacing fibrous tissue and eventually ossifies.
【References】
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2021.3.3