Changes in crystalline lens radii of curvature and lens tilt and decentration during dynamic accommodation in rhesus monkeys

Abstract

Dynamic changes in crystalline lens radii of curvature and lens tilt and decentration were measured during centrally stimulated accommodation in four iridectomized eyes of two adolescent rhesus monkeys. Phakometry measurements were performed dynamically using a custom-built, video-based, Purkinje-image instrument. Lens anterior and posterior radii were calculated from reflections of paired light sources from the ocular surfaces (Purkinje images PI, PIII, and PIV). Lens tilt and decentration were calculated assuming linearity between Purkinje image positions, eye rotation, lens tilt, and decentration. Because the monkey eyes were iridectomized, Purkinje images were referred to the mid-point of the double first Purkinje image (PI). Mean unaccommodated values of anterior and posterior lens radii of curvature were 11.11 +/- 1.58 mm and -6.64 +/- 0.62 mm, respectively, and these decreased relatively linearly with accommodation in all eyes, at a rate of 0.48 +/- 0.14 mm/D and 0.17 +/- 0.03 mm/D for anterior and posterior lens surfaces, respectively. Tilt and decentration did not change significantly with accommodation except for tilt around the horizontal axis, which changed at a rate of 0.147 +/- 0.25 deg/D. These results are important to fully characterize accommodation in rhesus monkeys.

Figure 1

Accommodative response for each current stimulus for the four eyes of the two monkeys used in this experiment. Each point is the average of three measurements. Error bars are standard deviations.

Figure 2

Dynamic recordings of accommodation from photo-refraction and lens thickness (LT) and anterior chamber depth (ACD) from continuous ultrasound biometry (CUB) from monkey #54 OS, for an accommodation response of 7.75D. Photorefraction was measured first, followed by the CUB measurements for the same stimulus amplitude. This example corresponds to a single response (the 5th response to five, 4-s long stimuli).

Figure 3

Dynamic biometric changes (anterior chamber depth and lens thickness) with accommodation for all eyes. Data are from the last stimulus producing the maximum response with accommodation recorded first with photorefraction and ACD and LT recorded subsequently with the CUB.

Figure 4

Change in anterior and posterior lens radii of curvature as a function of accommodation, for a phakometry sequence (5th stimulus). Phakometry data were calculated using individual biometry data shown in Figure 2, for monkey #54 OS.

Figure 5

Anterior and posterior lens radii of curvature as a function of accommodation for both eyes each of monkey #54 and monkey #58. Data are from the 5th stimulus for the maximum accommodative response in the phakometry sequence. Biometry and photorefraction data from each individual eye, corresponding to that shown in Figure 2, were used in the data processing.

Figure 6

Average changes in anterior and posterior lens curvatures relative to the un-accommodated state from the two monkeys. Data for each eye were fitted using a spline function and were then averaged across eyes for each monkey at different accommodation levels. Dashed lines represent ±1 standard deviation.

Figure 7

Nomenclature and sign conventions for lens tilt and decentration, looking into the monkeys eyes, as seen by the observer.

Movie 1

Phakometry video sequence of changes in the “height” of double Purkinje images PIII (bottom) and PIV (top), with accommodation. The “height” of PI (middle) does not change as the corneal curvature does not change with accommodation. Note that the lens diameter can be seen to decrease with accommodation in the iridectomized eye as the lens surface curvatures become steeper. This sequence is from monkey #54 OS, Stimulus 4, 0.55 mA (accommodation 7.75 D). Phakometry was performed with a fluid interface in front of the eye to neutralize the cornea.