Cortical Tunneling

Cortical tunneling is the appearance of lucent striations within the cortex along the long axis of the bone. It can be seen as a normal variant (two or fewer in the metacarpal cortex) or in states of rapid bone turnover. Differential considerations include:

• Normal variant in children: High bone turnover.
• Hyperparathyroidism: Almost always associated with subperiosteal resorption.
• Hyperthyroidism:
• Reflex sympathetic dystrophy:
• Acromegaly: High bone turnover due to growth hormone
• Osteoporosis: Commonly seen in the endosteal zone of cortex with sparing of the periosteal cortex. The image shows a cases of cortical tunneling in the distal radius and ulna in a severely osteoporotic patient. When seen en face, the cortical lucencies can mimic medullary lesions.

A grading system for cortical tunneling was initially developed for the metacarpals and metatarsals by Meema and Schatz (1970). I've never heard anyone use this system, but here it is:

• Grade 0: No striation or only single marginal striation.
• Grade +: More marked marginal striation than that of grade 0 and/or minimal intracortical striation occupying less than half of the area under observation.
• Grade ++: Intracortical striae wider than that of grade + and occupying more than half of the area under observation.
• Grade +++: Marked intracortical striation throughout the entire bone.

References

• Genant HK, Kozin F, Bekerman C, McCarty DJ, Sims J. The reflex sympathetic dystrophy syndrome. A comprehensive analysis using fine-detail radiography, photon absorptiometry, and bone and joint scintigraphy. Radiology. 1975 Oct;117(1):21-32.
• Keats TE and Anderson MW. Atlas of Normal Roentgen Variants That May Simulate Disease. 8th edition, page 559; Mosby (2004).
• Meema HE, Schatz DL. Simple radiologic demonstration of cortical bone loss in thyrotoxicosis. Radiology. 1970 Oct;97(1):9-15.
• Weiss C. Normal roentgen variant: cortical tunneling of the distal ulna. Radiology. 1980 Aug;136(2):294.