Calcium Pyrophosphate Dihydrate Deposition around the Dens

The transverse ligament of the atlas can be involved by calcium pyrophosphate dihydrate (CPPD) deposits in about 6% of the general population and in as many as 2/3 of patients with articular chondrocalcinosis. These deposits can be associated with aging, degenerative disease, or metabolic disorders.

Patients are typically older women, with isolated involvement of the atlanto-axial joints. Crowned dens syndrome refers to acute neck pain due to calcium pyrophosphate dihydrate deposits and calcification surrounding the odontoid process on CT. The neck pain may be accompanied by neck stiffness and fever, and can mimic meningitis. They can be treated with non-steroidal anti-inflammatory medications, and the calcifications usually resorb in about 1-2 weeks.

The deposition can range from linear or stippled calcifications to massive crystal deposition with bone erosion involving the dens. Depending on the extent of CPPD deposition and associated erosions, patients can also be at increased risk for pathologic fracture of the dens.

Radiographs are usually not very sensitive for detection of periodontoid mineralization, and CT is usually needed for characterization. The appearance of the calcifications ranges from curvilinear to stippled, or a mixture of the two. The curvilinear pattern, although less common, is strongly suggestive of calcium pyrophosphate dihydrate deposition. When masslike deposits are present, CT can demonstrate the bony erosions and possible malalignment from associated ligamentous damage and any pathologic fracture of the dens.

MRI, while not as sensitive as CT for the detection of calcification, is better for evaluation of the mass and its effect on the spinal cord, as well as assessment of cartilage, bone, or ligament abnormalities. The retro-odontoid mass is typically hypointense on T2-weighted images and enhances on post-contrast images.

Differential considerations for an extradural mass posterior to the odontoid process include:

• Pannus: Seen with rheumatoid arthritis, ankylosing spondylitis, and psoriatic arthritis. Does not have calcifications.
• Os odontoideum:
• Neoplasm: Epidural metastases, clivus chordoma, foramen magnum meningioma, aneurysmal bone cyst, osteoblastoma. Neoplasms will typically be T2-hyperintense, while the retro-odontoid mass of CPPD will be T2-hypointense.

References

• Baysal T, Baysal O, Kutlu R, Karaman I, Mizrak B. The crowned dens syndrome: a rare form of calcium pyrophosphate deposition disease. Eur Radiol. 2000;10(6):1003-5.
• Kakitsubata Y, Boutin RD, Theodorou DJ, Kerr RM, Steinbach LS, Chan KK, Pathria MN, Haghighi P, Resnick D. Calcium pyrophosphate dihydrate crystal deposition in and around the atlantoaxial joint: association with type 2 odontoid fractures in nine patients. Radiology. 2000 Jul;216(1):213-9.