The ACW is the most predominantly involved area of SAPHO syndrome in adults. MRI enables the detection of both active (inflammatory) and chronic (structural) changes in this region with a high sensitivity and level of detail [4, 20, 21].
Any areas of the ACW can be affected, but the sternoclavicular, manubriosternal, and sternocostal joints are the most commonly involved sites according to previous studies [4, 9, 11, 22]. In our study, the most frequently involved sites were the SClJ and the first rib area. Such findings were consistent with a study by Cao et al. based on 99mTc-MDP WBBS [5].
The pathogenesis and evolvement of ACW lesions in SAPHO syndrome remains unclear. The osteoarticular changes in the ACW were subdivided into three stages based on conventional radiography [23]. Stage 1 is mild ossification localized to the costoclavicular ligament. Stage 2 is generalized ossification of the sternocostoclavicular region. Stage 3 is a further progression of the hyperostotic changes, involving the superior margin of the clavicle. This development suggests a disease primarily involving entheses, particularly at the costoclavicular ligament, that spreads to the adjacent joints and bones. However, changes may also begin in the bones, often in the manubrium sterni, and spread to the joints and surrounding capsular and ligamentous structures [6]. Two theories (inflammatory enthesitis and reactive osteitis elicited by slow microorganism infection) regarding the pathology of the disease have also been proposed [24].
Our study identified several characteristics of ACW lesions on MRI that might unveil the pathogenic process of SAPHO syndrome. First, a triad of enthesitis, synovitis, and osteitis was demonstrated. Ligamentous involvement was directly evidenced by bone bridge formations along the ligaments. Second, all of the BMEs were distributed under the articular surface or the cortex, consistent with the distribution of ligaments and joint capsules. The BMEs seem to involve any area attached to ligaments and synovial joints, not necessarily initiating from the costoclavicular ligament as proposed by Sonozaki et al. [20]. Given that all the BMEs were related to the insertion sites of the ligaments and joint capsules, we speculate that enthesitis might be the primary disorder. Similar findings were observed in studies on spinal MRI and CT, which indicate that vertebral involvement in SAPHO syndrome may be triggered by vertebral corner lesions originating from the enthesitis at the junction of the outer fibers of the annulus fibrosis and vertebral epiphyseal ring [24, 25].
Why were the first rib area and the SClJs the most commonly affected sites in the ACW? Anatomically, the first costal cartilage can ossify in adolescence, so the internal environment is relatively complex [26, 27]. In addition, the costoclavicular ligaments and the radiate sternocostal ligaments are related to the first ribs [28]. The SClJ, a true diarthrodial synovial joint, has attachments to three complex ligaments (the interclavicular ligament, anterior sternoclavicular ligament, and posterior sternoclavicular ligament) [28]. However, synovitis of the ACW has been rarely reported [29]. In our study, 90.1% of patients showed effusion in the joint cavities, although some of the manifestation may be physiological effusions, which should be proved by enhanced MRI [30, 31]. In addition, the sternal angle, which is in the vicinity of the second SCoJ (containing an interarticular ligament and two synovial membranes) and contains the manubriosternal joint (containing dorsal and ventral ligaments and classified as a synovial joint in ~ 30% patients [20]), was the third-most frequently involved site.
In summary, the complex anatomical structure in the ACW region may contribute to the patterns of lesions in SAPHO syndrome on MRI. BME can affect any site related to ligaments and synovial joints in solitary or mixed patterns (Fig. 2 a–g). We observed four common patterns of involvement (Fig. 2 h): the first rib area, the sternoclavicular area, the sternal angle area, and the areas of the second to sixth SCoJ. These patterns can occur alone or in combinations. Further longitudinal follow-up studies are needed to investigate the evolvement of these lesions.
Structural changes may occur during periods of remission, when bone erosion tends to heal with sclerosis and periosteal new bone formation [6, 11]. Such changes are best revealed by CT imaging. In our study, we preliminarily evaluated hyperostosis and bone bridges using MRI. Further validation of the results on CT is needed. We also found that fat infiltration was widely distributed with various shapes in the ACW region, which was considered to develop from BME during remission periods [7].
Inflammatory involvement of adjacent soft tissue was often present. Studies have emphasized that voluminous soft tissue may compress or obstruct the subclavian vein, causing thoracic outlet syndrome [8, 32, 33]. Our analysis found that 13 patients had stenosis of the brachiocephalic veins, all of whom were asymptomatic. As non-enhanced images were not suitable for vascular evaluation, the severity of the stenosis was not assessed. Furthermore, the pectoralis major showed obvious edema, which may mimic an aggressive process such as lymphoma or other malignancies. However, there was no delineated soft tissue mass or signs suggesting abscess formation.
There were some limitations to our study. First, there was no control group. Some other diseases may involve the ACW, such as osteoarthritis, septic arthritis, and other inflammatory conditions [20, 34]. Inclusion of control groups may help with better interpretation of the findings in SAPHO syndrome. Second, the non-contrast-enhanced MRI could not show all the involved veins, such as subclavian veins. The sensitivity and specificity for identifying synovitis and tenosynovitis are also lower using T2-weighted sequences alone compared with contrast-enhanced images [30]. Third, there was no computed tomography serving as a reference to accurately assess ossification of the costal cartilage and ligaments. Fourth, the patients were heterogeneous in terms of disease duration and treatment. Most patients had a complex medical history and treatment process, which may lead to heterogeneous manifestations [15]. Last, future studies should incorporate patient reported outcomes (such as magnitude of pain and upper extremity function) to better understand the clinical relevance of the MRI findings.
