Background Synovitis and osteitis are distinct pathologic findings in patients with rheumatoid arthritis (RA). Their frequent simultaneous occurrence led to the theory of increased communication between the bone marrow and synovial compartments. A widening of pre-existing cortical bone channels, which carry microvessels and penetrate the cortical barrier, has been previously described in animal models as a result of increased osteoclast-mediated bone resorption. To our knowledge these communication channels have not been clearly visualized in humans in vivo.
Objectives We hypothesised that it is possible to detect cortical channels in human RA patients by high-resolution peripheral quantitative computed tomography (HR-pQCT).
Methods We used HR-pQCT (XtremeCT, Scanco Medical, Switzerland) with a resolution as low as 82 microns in voxel size to scan the metacarpophalangeal (MCP) joint 2 of 10 patients with early RA (ACPA positive, mean age 36.2±8.2 years, mean disease duration 1.2±0.8 years, 50% female) without concurrent bone erosions and 10 healthy controls of comparable age (mean age 37.9±7.6 years, 70% female). Images were downsized to the minimum of one slice with the 3D evaluation program provided by the manufacturer. Transversal (Figure 1), coronal and sagittal images were obtained using the subdim feature of the 3D evaluation program. Transversal planes were projected at the proximal insertion of the capsule of MCP2 head. Sagittal and coronal planes were set exactly into the middle of the MCP2 head.
Results Cortical bone channels can be clearly visualized with this technique, which mimics a virtual histopathologic slide of the joint (Figure 1). In the transversal view cortical discontinuations (arrows) were observed more frequently (p=0.05) in RA patients (Fig. 1B) compared to healthy controls (Fig. 1A). Due to the small sample size, statistical analysis of the coronal and sagittal view were not performed, though especially in the sagittal view a trend of more cortical bone channels in RA was detected (4.6±3.5 vs.2.3±1.9).
Conclusions In summary, we show that in vivo visualization of cortical bone channels in humans is feasible and that early RA patients exhibit a significantly higher number of such channels in the area of the proximal insertion site of the capsule compared to controls. These changes may represent key communication pathways between the bone marrow and the joint and the place where inflammation starts in RA patients. These changes cannot be seen by conventional X-ray or MRI. Systematic evaluation of cortical bone channels will be necessary to define the extent of these lesions in inflammatory joint diseases as well as their longitudinal dynamics in larger cohorts.
Disclosure of Interest None declared