Friday, February 13, 2009

Twist, Turns and Tibial Torsion

Twists and turns occur in nature, sometimes with great beauty, as in this nautilus shell:

The nautilus shell is an example of a logarithmic spiral, a spiral curve whose shape is unaltered with each successive curve. The logarithmic spiral was first described by Descartes and later extensively investigated by Jakob Bernoulli, who called it Spira mirabilis, "the marvelous spiral". Possibly as a result of this unique property, the spira mirabilis has evolved in nature, appearing in certain growing forms such as nautilus shells and sunflower heads. (Wikipedia).

Twists and turns can also occur in the human body. The cochlea of the ear is spiral-shaped. Spiral shapes can also be pathologic— spiral electrical waves have been linked to cardiac arrhythmias, and pathologic twists of the intestine can be life-threatening.

Pathologic twists are sometimes referred to as "torsion", as in intestinal torsion, testicular torsion, and ovarian torsion. The word "torsion" comes from the Latin and old French, meaning "wringing pain in the bowels, and "to twist".

In the musculoskeletal realm, one can encounter tibial torsion, defined as the degree of twisting between the proximal and distal articular surfaces of the tibia. There is a normal, physiologic amount of tibial torsion, but when it is excessive, tibial torsion is harmful.

Excessive tibial torsion is usually a disease of childhood, but on occasion one can encounter excessive tibial torsion in adulthood as well.

Tibial torsion can be measured using the cross-sectional imaging techniques of MRI and CT. Conventional radiographs are less reliable and reproducible than MRI and CT. There is no general consensus on where exactly to draw the reference lines used to calculate the tibial torsion angle, but one reasonable method is illustrated here:

(A) Transverse CT section through proximal tibia, just below proximal articular surface. A line is drawn bisecting the tibia. (B) Transverse CT section through distal tibia, just above distal articular surface. A line is drawn bisecting the tibia. (C) The angle between these two lines is calculated.

There are ethnic and gender differences in the tibial torsion angle, with reports studying Caucasians reporting values centered at 30-35 degrees, while studies of Japanese and Indian subjects reporting values centered at 22-24 degrees.

In the patient depicted above, the tibial torsion angle in the right leg was 56 degrees. The contralateral limb had a tibial torsion angle of 31 degrees (normal).

The abnormally high angle in the symptomatic limb was due to a tibial fracture which healed with rotation:

Although tibial torsion is typically measured on CT, one can obtain this angle using MRI as well, as long as the proximal and distal tibia are imaged in the axial plane on the same study.

Further information on the measurement of tibial torsion can be found in a nice paper by Mullaji et al. (IJO 43:309-313, 2008).

Vic David MD

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