Calcium pyrophosphate dihydrate crystal deposition in and around the atlantoaxial joint: association with type 2 odontoid fractures in nine patients

PURPOSE

To investigate the histopathologic anatomy of calcium pyrophosphate dihydrate (CPPD) crystal deposition in and around the atlantoaxial joint and the association between CPPD crystal deposition and subchondral cysts, erosions, and fracture involving the odontoid process of the axis.

MATERIALS AND METHODS

One adult cadaver demonstrating calcification in the retro-odontoid area at computed tomography (CT) was selected for further radiography, CT, and magnetic resonance (MR) imaging at the C1-2 level. Anatomic sectioning and histologic evaluations were performed in the specimen. For clinical study, radiographs (n = 5), CT scans (n = 8), and MR images (n = 6) in nine patients (mean age, 74.4 years) with odontoid process fractures and CPPD crystal deposits in and around the atlantoaxial joint were reviewed.

RESULTS

In the cadaveric specimen, radiography and CT demonstrated calcifications in the transverse ligament; histologic evaluation confirmed that these calcifications were CPPD crystal deposits. In all nine patients, radiography (n = 5) and CT (n = 8) also showed calcification in areas adjacent to the odontoid process, which included the transverse ligament. T1- and T2-weighted MR imaging showed a retro-odontoid mass of low signal intensity that compressed the cervical cord in six patients. CT, MR imaging, or both demonstrated subchondral cysts, osseous erosions, or a type 2 odontoid fracture in all patients.

CONCLUSION

CPPD crystal deposition disease involving the C1-C2 articulation can be a clinically important entity that may place affected patients at increased risk of pathologic fracture of the odontoid process.

Genetically predicted C-reactive protein mediates the association between rheumatoid arthritis and atlantoaxial subluxation

OBJECTIVE

Investigating the causal relationship between rheumatoid arthritis (RA) and atlantoaxial subluxation (AAS) and identifying and quantifying the role of C-reactive protein (CRP) as a potential mediator.

METHODS

Using summary-level data from a genome-wide association study (GWAS), a two-sample Mendelian randomization (MR) analysis of genetically predicted rheumatoid arthritis (14,361 cases, and 43,923 controls) and AAS (141 cases, 227,388 controls) was performed. Furthermore, we used two-step MR to quantitate the proportion of the effect of c-reactive protein-mediated RA on AAS.

RESULTS

MR analysis identified higher genetically predicted rheumatoid arthritis (primary MR analysis odds ratio (OR) 0.61/SD increase, 95% confidence interval (CI) 1.36-1.90) increased risk of AAS. There was no strong evidence that genetically predicted AAS had an effect on rheumatoid arthritis risk (OR 1.001, 95% CI 0.97-1.03). The proportion of genetically predicted rheumatoid arthritis mediated by C-reactive protein was 3.7% (95%CI 0.1%-7.3%).

CONCLUSION

In conclusion, our study identified a causal relationship between RA and AAS, with a small proportion of the effect mediated by CRP, but a majority of the effect of RA on AAS remains unclear. Further research is needed on additional risk factors as potential mediators. In clinical practice, lesions of the upper cervical spine in RA patients need to be given more attention.

Expression of a wide range of fibrocartilage molecules at the entheses of the alar ligaments - possible antigenic targets for rheumatoid arthritis?

OBJECTIVE

To provide evidence for the existence of a wide range of molecules characteristic of fibrocartilage at normal alar ligament entheses, that may have a bearing on the pathogenesis of rheumatoid arthritis. The alar ligaments provide the mechanical restraint for head rotation and their integrity can be compromised in patients with RA and occasionally in those with spondyloarthropathy.

METHODS

Both alar ligaments from 6 cadavers were fixed in 90% methanol and cryosectioned longitudinally. Sections were immunolabelled with antibodies against collagens, glycosaminoglycans, and proteoglycans. The immunohistochemical data were related to estimates of insertional angle change at the entheses that were calculated from measurements of ligament length and dens diameter.

RESULTS

Molecules typical of fibrocartilage (including type II collagen, link protein, and aggrecan) were found at both entheses, but labelling was more prominent at the odontoid end. This correlated with a 3-fold greater insertional angle change during 40 degrees of head rotation at the attachment of the ligament to the dens. At the odontoid enthesis, fibrocartilage differentiation was most conspicuous posteriorly.

CONCLUSION

The changes in insertional angle mean that compressive forces are prominent at both entheses, but particularly at the odontoid end, where the ligament partly wraps around the dens during head rotation. Thus, mechanical conditions are created that lead to pronounced fibrocartilage development, hence the expression of type II collagen, link protein, and aggrecan. The prominence of these molecules at the entheses raises the possibility that they could be antigenic targets for an autoimmune response in rheumatic diseases.