Accuracy of CT in the measurement of stapes prostheses using a temporal bone model.

Objectives. To evaluate whether it would be possible to measure accurately a range of stapes prostheses of different lengths and makes using spiral CT imaging and a radiologist who would report the images in a blinded fashion. To establish the distance from the tip of each prosthesis to the metallic component in metal – plastic prostheses. Method. Eight stapes prostheses (2 fluoroplastic-stainless steel; 2 fluoroplastic-platinum; 2 fluoroplastic-nitinol; 2 fluoroplastic only) were selected and placed onto the long process of the incus and over the oval window of a dried temporal bone preparation. CT scans were then performed and multiplanar reformatting techniques used to allow measurement of the length of the prostheses. The prostheses were then photographed and measured using high definition digital prints after slicing them in half to expose the central metallic component. Results. The median difference between the actual and observed length of the 6 metallic/fluoroplastic stapes prostheses was 0.25 mm (0.05–0.55 mm). In contrast the 2 fluoroplastic prostheses were accurately measured as 4 and 5 mm respectively. The median distance of the metallic component from the end of the prosthesis was found to be 1.75 mm (1.5–1.9 mm). Conclusion. Modern spiral CT scanners can visualise the fluoroplastic component of stapes prostheses to assess its position. However, our series shows that the length of mixed metallic/fluoroplastic prostheses may be still be inaccurately measured by up to 0.55 mm. Reliance on measuring the metallic component of the prosthesis may also overestimate the proportion of prostheses that are short of the footplate. References. 1 Kosling S. & Bootz F. (2001) CT and MR imaging after middle ear surgery. Eur. J. Radiol. 40, 113–118 2 Pickuth D., Brandt S., Berghaus A., Spielmann R.P. & Heywang-Kobrunner S.H. (2000) Vertigo after stapes surgery: the role of high resolution CT. Br. J. Radiol. 73, 1021–1023 [ABSTRACT FROM AUTHOR]