Abstract
Purpose: :
Our goal was to further characterize the role of the PVZ at rest and during accommodation.
Methods: :
The eyes of 12 rhesus monkeys, aged 6-27 yrs, were studied. Accommodation, stimulated via the Edinger-Westphal (E-W) nucleus, was measured by coincidence refractometry. The distance between the ciliary processes and the posterior insertion point of the PVZ was measured by ultrasound biomicroscopy (UBM; 50 MHz, 35 MHz). Ciliary body movement was measured by UBM before and after the posterior zonule was surgically severed in a 25-year-old presbyopic eye.
Results: :
In the resting eye, the distance between the ciliary processes and the posterior insertion point of the PVZ did not change significantly with age (n=4 young, n=4 older). During accommodation, the distance between the ciliary processes and the posterior insertion point of the posterior zonule did not diminish significantly (accommodated minus unaccommodated = 0.043 ± 0.049 mm, n=10; p<0.41) in either the young eyes (0.083 ± 0.109, n=4) or the older eyes (-0.019 ± 0.018, n=4). Qualitative examination of the UBM images showed the tissues posterior to the PVZ insertion point moving forward/stretching as the ciliary body contracted and moved forward in all eyes. In the 25-year-old presbyopic eye, severing the posterior zonule increased forward ciliary body movement from 37º (pre-surgery) to 70º (post-surgery), a magnitude similar to that seen in the young eye.
Conclusions: :
The PVZ’s posterior insertion point moves forward during accommodation in relation to the scleral spur [CROFT et al, ARVO 2008] but not in relation to the ciliary processes or the apex of the ciliary muscle. This demonstrates that the PVZ does not stretch during the accommodative response. The forward movement of the vitreous zonule posterior insertion zone reported last year [CROFT et al, ARVO 2008] was presumably allowed by the forward movement/stretch of the vitreous membrane and the posterior elastic tendons (i.e., choroid, retina), which extend further posteriorly to the vitreous zonule. Removal of the posterior zonule can result in increased forward muscle movement in the presbyopic eye and could enhance the function of accommodating IOLs.
Keywords: accomodation • choroid • aging