Osteoimmunology and osteoporosis

Abstract

The concept of osteoimmunology is based on growing insight into the links between the immune system and bone at the anatomical, vascular, cellular, and molecular levels. In both rheumatoid arthritis (RA) and ankylosing spondylitis (AS), bone is a target of inflammation. Activated immune cells at sites of inflammation produce a wide spectrum of cytokines in favor of increased bone resorption in RA and AS, resulting in bone erosions, osteitis, and peri-inflammatory and systemic bone loss. Peri-inflammatory bone formation is impaired in RA, resulting in non-healing of erosions, and this allows a local vicious circle of inflammation between synovitis, osteitis, and local bone loss. In contrast, peri-inflammatory bone formation is increased in AS, resulting in healing of erosions, ossifying enthesitis, and potential ankylosis of sacroiliac joints and intervertebral connections, and this changes the biomechanical competence of the spine. These changes in bone remodeling and structure contribute to the increased risk of vertebral fractures (in RA and AS) and non-vertebral fractures (in RA), and this risk is related to severity of disease and is independent of and superimposed on background fracture risk. Identifying patients who have RA and AS and are at high fracture risk and considering fracture prevention are, therefore, advocated in guidelines. Local peri-inflammatory bone loss and osteitis occur early and precede and predict erosive bone destruction in RA and AS and syndesmophytes in AS, which can occur despite clinically detectable inflammation (the so-called 'disconnection'). With the availability of new techniques to evaluate peri-inflammatory bone loss, osteitis, and erosions, peri-inflammatory bone changes are an exciting field for further research in the context of osteoimmunology.

Figure 1

Osteitis in rheumatoid arthritis (RA) (in metacarpophalangeal joint) and in ankylosing spondylitis (AS) (in the sacroiliac joint and in vertebra).

Figure 2

Methods to quantify bone changes in the hands and vertebrae. (a) Methods to quantify periarticular bone changes. (b) Methods to quantify vertebral bone changes. μCT, micro-computed tomography; DXA, dual-energy x-ray absorptiometry; DXR, digitalized radiogrammetry; HRDR, high-resolution digital radiology; MRI, magnetic resonance imaging; QCT, quantitative computer tomography; QUS, quantitative ultrasound; VFA, vertebral fracture assessment.

Figure 3

Cortical bone changes in rheumatoid arthritis on classical radiography showing striation and lamellation of cortical bone of the phalanx.

Figure 4

Effect of inflammation on bone and fracture risk in rheumatoid arthritis.

Figure 5

Changes in vertebral shapes in ankylosing spondylitis. (a) Vertebral deformation in ankylosing spondylitis. Ha, anterior height; Hp, posterior height. (b) Vertebral deformation due to extensive erosive discitis with osteitis in ankylosing spondylitis (Andersson lesion).

Figure 6

Fracture of dorsal arch and pedicle in a patient with ankylosing spondylitis and bamboo spine and persisting back pain after minimal trauma.

Figure 7

Effect of inflammation on bone and fracture risk in ankylosing spondylitis.