Torsion of the human semicircular canals and its influence on their angular relationships.

We used a computer-aided three-dimensional reconstruction and measurement method to measure torsion of the the semicircular canals (SCCs) in 10 normal human temporal bones from 10 individuals (3 months to 76 years old). Torsion of each SCC was measured as the angle between the "standard line" (the line connecting the ampullated and nonampullated ends of the SCC) and the plane of greatest SCC torsion. Torsion was greatest for the anterior SCC (16.4 +/- 3.6 degrees), followed by the lateral SCC (9.5 +/- 6.7 degrees) and the posterior SCC (8.5 +/- 4.2 degrees). The angles between the overall plane described by each SCC and the other SCCs between the anterior and lateral, the anterior and posterior, and the posterior and lateral SCCs were also measured and were found to be 78.7 +/- 10.0, 107.9 +/- 5.5, and 87.3 +/- 6.9 degrees, respectively. Finally the angles between the torsion plane of each SCC and the others were measured and found to be 92.6 +/- 15.4, 95.8 +/- 11.2, and 90.9 +/- 8.3 degrees, respectively. These results indicate that on average torsion in the SCCs tend to bring the angle between pairs of SCCs close to a right angle. This tendency to have a right angle between SCCs is thought to promote optimal vestibular function. Speculation is offered regarding the etiology of the greater torsion of the anterior SCC.