PURPOSE. The accommodative structures of the human eye have been studied extensively with histological preparations. Dissection and fixation for histology necessarily disrupts the normal anatomical arrangement. We have used ultrasound biomicroscopy (UBM) to study the anatomy of the accommodative apparatus in situ, in intact, enucleated human eyes. METHOD. Three pairs of normal, phakic human eyes enucleated from donors aged 79, 67, and 64 by eye banks within 16 hours of death were used. After removal of extraocular tissue, the eyes were placed in saline. A Humphrey Instruments model 840 UBM was used to examine the ciliary region of the eyes. The eyes and UBM transducer were carefully oriented as necessary to obtain the clearest images of the anatomical structures. The live UBM image was videotaped and individual images subsequently were captured with image analysis software.
RESULTS. The two main subgroups of zonular fibers were observed; the anterior fibers inserting along the equatorial region of the lens and the posterior or pars plana fibers. Posterior zonular fibers were observed to pass straight from the posterior attachment of the ciliary muscle to the ciliary processes rather than following the curved pars plana of the ciliary body as observed from histology. Additional posterior zonular fibers appear to pass from the posterior attachment of the ciliary body directly to the lens equator. As seen in primate eyes, some posterior zonular fiber bundles bifurcate mid-way between the pars plana and the lens.
CONCLUSIONS. The zonular fiber arrangement observed with UBM in enucleated human eyes is similar to that described from in vivo studies of human and rhesus monkey eyes and differs from that observed histologically. Observations of the undisturbed anatomical arrangement of the accommodative apparatus are necessary to fully understand the accommodative mechanism. While not all anatomical structures are clearly visible with UBM, it provides a useful tool for observing the functional organization of the accommodative apparatus in intact eyes. This may lead to an improved understanding of accommodation.