Method to Estimate the Complete and Two-Turn Cochlear Duct Length

HYPOTHESIS: Using a linear measurement of the cochlea on a single radiographic image can reliably estimate the complete and two-turn cochlear duct length (CDL) in a normal human temporal bone. BACKGROUND: CDL is measured from the middle of the round window to the helicotrema. Histologic studies have shown the length of the organ of Corti (OC) to range from 25 to 35 mm. CDL measurements, performed either radiographically or histologically, are quite tedious and time-consuming. We propose equations that can reliably estimate both two-turn and complete CDL using a single computed tomography (CT) image. METHODS: Prior studies of CDL, measured either histologically or radiographically, were reviewed, which yielded distributions of CDL measured at the OC and the lateral wall of the cochlea. Using Escudé's third equation as a basis, we were able to extrapolate complete and two-turn CDL based on a CT scan measurement of the diameter of the basal turn (A). RESULTS: Using measurement A, the relationship of two-turn CDL measured at the OC is 2TL(oc) = 3.65(A-1) and for 2TL(i) = 3.65(A-0.7). The equation for estimation of complete CDL is CDL(oc) = 4.16A - 4 and for CDL(i) = 4.16A - 2.7. CONCLUSION: Using a single linear measurement from a CT scan image can reliably estimate the two-turn and complete CDLs in human temporal bones. The two-turn length represents the best compromise of cochlear coverage while minimizing intracochlear trauma for electrode insertions.