Apatite deposition disease. A new arthropathy

Area of Science:

• Rheumatology
• Crystallography
• Biochemistry

Background:

• Crystal deposition diseases are a group of arthropathies characterized by the deposition of crystals in joints.
• Calcium pyrophosphate dihydrate (CPPD) crystal deposition disease and gout (monosodium urate crystals) are well-established.
• The role of other crystalline substances, such as calcium hydroxyapatite, in joint inflammation remains less understood.

Purpose of the Study:

• To develop and validate a method for identifying calcium hydroxyapatite crystals in synovial fluid and biopsy samples.
• To investigate the presence of calcium hydroxyapatite crystals in patients with osteoarthritis and other arthropathies.
• To explore the potential role of calcium hydroxyapatite crystals in causing acute inflammatory arthritis.

Main Methods:

• High-resolution scanning electron microscopy (SEM) was employed for particle identification.
• An energy-dispersive X-ray micro-analysis system was utilized for elemental composition analysis.
• Synovial fluid and biopsy samples from patients with various joint conditions were analyzed.

Main Results:

• Calcium hydroxyapatite particles were identified in the synovial fluid and/or biopsy material of six osteoarthritis patients.
• Three of these osteoarthritis patients experienced acute inflammatory episodes with joint effusions.
• Apatite crystals were not detected in joints affected by rheumatoid arthritis or other arthropathies.
• Animal studies and experimental human studies confirmed that hydroxyapatite crystals can induce acute inflammatory reactions.

Conclusions:

• A reliable method for identifying calcium hydroxyapatite crystals in joint tissues has been established.
• Calcium hydroxyapatite crystal deposition may represent a distinct clinical entity, potentially a third type of crystal-deposition disease.
• The findings suggest that calcium hydroxyapatite crystal deposition disease should be recognized as a specific form of inflammatory arthropathy.