Congenital Absence of the C1 Laminae

Ossification of the posterior arch of C1 usually proceeds from ossification centers around the lateral masses, forming two half arches that fuse between the ages of 3 and 5. In about 2% of cases, there is an additional ossification center in the region of the posterior tubercle (rudimentary form of the spinous process) that unites with the lateral processes of the C1 arch.

Problems with the development of the posterior arch can cause median, uni-, and bilateral clefts. In the extreme form, there is complete lack of ossification of the posterior arch, which is the case shown here.

Isolated absence of the posterior arch of C1 may be asymptomatic; however, intermittent quadriparesis has been reported and absence of the posterior arch may be associated with atlantoaxial instability and may predispose the patient to osteoarthritis later in life.

Special thanks to Dr. Jason Tsai for the case.

References

• Logan WW, Stuard ID. Absent posterior arch of the atlas. Am J Roentgenol Radium Ther Nucl Med. 1973 Jun;118(2):431-4.
• Schulze PJ, Buurman R. Absence of the posterior arch of the atlas. AJR Am J Roentgenol. 1980 Jan;134(1):178-80.