Journal of Refractive Surgery Vol. 23 No. 6 June 2007
Richard M. Davis, MD; Jason A. Evangelista, MD
PURPOSE: To evaluate whether the vacuum of a microkeratome suction ring induces ocular structure changes.
METHODS: A prospective case series using A-scan ultrasonography to measure anterior chamber depth, lens thickness, vitreous body, and axial length was performed. Measurements before and during application of a Hansatome microkeratome suction ring were performed on 69 eyes of 39 consecutive patients scheduled to undergo a first-time LASIK procedure with mechanical creation of a corneal flap.
RESULTS: Mean patient age was 43±12.1 years. Of the 69 eyes, 63 (91.3%) had refractive myopia with a mean spherical equivalent refraction of –2.93±1.56 diopters (D) and 6 (8.7%) had refractive hyperopia with a mean spherical equivalent refraction of 1.37±0.31 D. Overall, the mean spherical equivalent refraction of all eyes was –2.56±1.94 D. Ultrasound measurements during suction revealed a decrease in the anterior chamber depth of –0.06±0.36 mm (P<.05) and lens thickness by –0.14±0.45 mm (P<.05) whereas the vitreous body increased 0.25±0.36 mm (P<.05). Although insignificant, a trend toward increasing axial length was noted. No measurements changed over time during the application of vacuum.
CONCLUSIONS: Vacuum by a microkeratome suction ring induced a compression of the anterior chamber and lens with commensurate expansion of the vitreous body. The assessment of vacuum effects during LASIK suggests that measurements of intraocular compartments are more informative than axial length. [J Refract Surg. 2007;23:563-566.]
From the full text:
Quote: Before the application of the excimer laser in LASIK, vacuum is applied to the external surface of the eye by a suction ring in the form of a microkeratome or a femtosecond laser to assist with the creation of a lamellar corneal fl ap. The vacuum may increase the intraocular pressure (IOP) to >90 mmHg in less than 5 seconds, which could induce vitreoretinal changes.
Quote: Although the limiting factor of using A-scan ultrasonography for lens thickness and using mathematical formulations to calculate vitreous body measurements are acknowledged, our study has conclusively shown compression of the anterior structures with elongation of the vitreous body and a trend of increasing axial length (P=.057). We have illustrated the changes to be more complex than previously reported as the anterior and posterior structures respond differently to high vacuum making the axial length measurement less illuminating. Although the clinical significance has yet to be determined, biomechanical deformation by a rapid sequence of compression and decompression associated with LASIK theoretically may increase the risk of vitreoretinal pathology.