RA frequently affects the upper cervical structures (C1 and C2, with atlantoaxial, atlantoodontoid and atlantooccipital joints). In RA the inflammatory process is characterized by the presence of synovial proliferation with subsequent bone erosions (Fig.1,2,3) and weakening/rupture of the supporting ligaments. These events may lead to instability with possible atlantoaxial subluxation, compression of the cervicomedullary junction and subaxial subluxation.
Anterior atlantoaxial subluxation (AAS) is more common than posterior and lateral AAS. Anterior AAS can be diagnosed when the anterior atlantoodontoid interval, i.e. the space between the posterior aspect of the anterior arch of C1 and the anterior aspect of the odontoid process, is 3 mm greater. Posterior AAS is due to destruction of the dens with posterior slippage of C1. Lateral subluxations are considered as such when the lateral masses of C1 are shifted laterally by more than 2 mm with respect to C2, resulting in rotational deformities.
Atlantoaxial instability can progress in the superior migration of the odontoid process into the foramen magnum (vertical subluxation). The subluxations may result in a canal atlantoaxial stenosis, which is identified when the posterior atlo-dental interval (PADI), measured from the posterior aspect of the dens to the anterior aspect of the C1 lamina, is less than 14 mm. The definition of the cervico-medullary angle is very important. It is defined as the angle determined by the line drawn along the anterior aspect of the cervico-medullary cord and the line drawn along the anterior aspect of the medulla oblongata. The normal angle is between 135° and 175°. Angles < 135° correlate with the presence of paralysis and myelopathy. CT is the modality of choice for the detection of osseous involvement (bone erosions, spinal canal stenosis and fractures) and for the pre-surgical planning.
MRI can visualize the retroodontoid synovial tissues and their impact on neural structures.
Spondyloarthritis mainly involve the spine, including the cervical segment. The two most frequent pathologies of this group are ankylosing spondylitis (AS) and psoriatic arthritis (PsA). In term of involvement of craniocervical junction, anterior AAS is the most common lesion, usually observed in patients with severe and long-lasting disease. AAS is similar to that observed in RA. The inflammatory process can lead to bony-cartilage destruction and ligamentous laxity. In spondyloarthritis, enthesitis of ligaments can also be associated, with reduction of ligament elasticity and instability of the overall structure.
In AS, the typical radiographic lesions are represented by vertebral squaring, syndesmophytes, verterbral erosions, discites, and involvement of the joint facets. The typical features of a long-standing disease are the appearance of a 'bamboo spine' and the calcification of the interspinous and supraspinous ligaments (“dagger sign “).
At the craniocervical level, lesions such as dislocation, zigoapophyseal joint fusion, atlanto-occipital ossification, and vertebral canal stenosis can be observed in the advanced phase.
In PsA, cervical spine abnormalities are generally less common and severe than in RA. Characteristics of PsA are erosive damage, ankylosis, non-marginal syndesmophytes and ligamentous ossification.
DISH is a systemic bone-forming condition. The typical radiographic characteristic is the ossification of ligaments and entheses at prevertebral and prediscal level. Hyperostosis mainly concerns the anterolateral ligaments of the thoracic vertebrae, even if cervical and lumbar vertebrae may be involved. Bone proliferation can sometimes be so massive that it compresses the pharyngoesophageal structures or the trachea. The ossification of the posterior longitudinal ligament or the ossification of the flavum ligament may predispose to myelopathic complications.
In crystals-associated arthropathies there is a pathological deposition of various substances in and around the joint structures, leading to inflammatory and destructive lesions. The three most common conditions are calcium pyrophosphate dihydrate deposition disease (CPPD), hydroxyapatite crystal deposition disease (HADD) and monosodium urate crystal disease (gout).
In CPPD different soft tissue structures, including cartilage, joint capsules, synovial tissue, bursa, tendons and ligaments may be involved. Among the ligaments involved at the cervical level, the transverse ligament of the atlas is particularly important and calcium deposits may appear curvilinear, point or mixed (Fig.4). The clinical entity, characterized by recurrent acute cervical-occipital pain with severe cervical stiffness and biological inflammatory syndrome with fever, is called "crowned dens syndrome" (CDS). CT is the modality of choice for the detection of CPPD crystals deposits surrounding the odontoid process.
Cervical HADD covers three different clinical and radiological entities, respectively longus colli calcific tendonitis, CDS and disc calcinosis. In longus colli calcific tendonitis, imaging shows calcific deposits in prevertebral soft tissues, typically at the C1-C2 level, with associated soft-tissue swelling. CT scans confirm the presence of fine calcifications in the context of the upper tendon fibers of the longus colli.
MRI is the best imaging technique for the detection of retropharyngeal effusion and inflammatory vertebral lesions, such as bone marrow oedema and erosions. Like CPPD, in HADD deposits can cause CDS. In the intervertebral disc, calcifications affect the pulposus nucleus and spare the annular ligament.
Bone erosions and reactive new bone proliferation are the main features of gout at the spinal level. CT shows the presence of intra-articular and iuxta-articular erosions with sclerotic margins.
On MRI, the deposits appear as images with low signal intensity on T1w sequences and variable signal intensity on T2w sequences. Imaging findings are so important in gout that the new classification criteria of the American College of Rheumatology/European League Against Rheumatism focus on the diagnostic power of dual energy CT (DECT). DECT can identify and differentiate monosodium urate crystals within tissues, based on their relative X-ray absorption.
In infectious diseases destructive processes and abscess collection can be observed. CT is the modality of choice to delineate the bone destructive process and in documenting the presence of sequestra. CT is also useful for the differential diagnosis with crystal-associated arthropathies.
MRI is the gold standard technique to assess soft tissue and compression marks on neural structures.