A total of 25 patients 25 eyes; 14 males and 11 females; age, 1 month to 6 years [2. A scatterplot of AL and age is shown in Figure 3 A. Figure 3. The average SER was 0. The average crystalline lens dimensions mm were as follows: D , 7. A scatterplot of the crystalline lens dimensions and age is shown in Figure 4. Figure 4. Scatterplot of crystalline lens dimensions and age.
Age-related changes in D solid circle , T open diamond , Ta open triangle , and Tp solid square. The average R and Q values of the anterior lens surface were 6. The mean RMS errors mm of the anterior and posterior fit were 4. The correlation between the crystalline lens shape and other elements is shown in Table 1. A scatterplot of age and the PC1 of the crystalline lens is shown in Figure 5 A. A model of the average eye calculated on the basis of correlations between age and the principal component of the total variance in shape is shown in Figure 5 C.
Multiple linear regression analysis revealed a coefficient of determination of 0. Figure 5. A Scatterplot of the PC1 of crystalline lens shape and age. The dotted line indicates the average value of PC1, and the average value is always set at 0 in the PCA. C The average eye model calculated on the basis of correlations between age and the principal component of the total variance of shape. The average axial length, anterior chamber depth, and lens thickness were allocated for each eye model.
Figure 6. Table 1. View Table. Table 2. The results presented here clarify the relationship between changes in the crystalline lens shape and axial elongation in young children.
Our results show that the crystalline lens shape changes dramatically during development Fig. Interestingly, axial elongation was related to the entire contour of the crystalline lens and not to a particular segment Table 2. These results demonstrate that axial elongation progresses in parallel to change in the crystalline lens shape. In a previous study, Lotmar 33 reported a Gullstrand model for the eye of newborn infants.
That study was based on ocular component dimensions obtained by ultrasonography as well as by the radii of curvature of the crystalline lens measured using deep-frozen sections. The data mm were as follows: ACD, 2. Recently, Borja et al. The calculated anterior and posterior radii of curvature mm of newborn infants were 4. On the other hand, Mutti et al.
Data mm applying to 3-month-olds were as follows: ACD, 2. Both the anterior and posterior radii of curvature measured by Mutti and colleagues were flatter than those obtained by Lotmar 33 and Borja et al. According to the Helmholtz theory, the ex vivo crystalline lens is considered to be in a state of maximum accommodation.
In contrast, the clinical data from a large-scale study was based on the crystalline lens in a state of cycloplegia. In our study, all the patients were under sedation and in the tonic accommodation state. Various anesthetic drugs induce myopia in monkeys, 35 which seems to be comparable to tonic accommodation night myopia in humans. In a past report, tonic accommodation among 6-year-olds was approximately 2. Therefore, it was believed that tonic accommodation in children 6 years of age and neonate was almost equivalent.
Our mean values for anterior and posterior radii of curvature in children 3 months and older fell in the middle of the range of the data reported by Mutti et al. However, our data for the crystalline lens of a neonate were similar to those for the isolated lens.
It was difficult to determine whether tonic accommodation in the neonate was amplified by triclofos sodium syrup or whether its distribution was individual dependent.
Changes in the anterior and posterior radii of curvature were significantly correlated with axial elongation in our study Fig. However, such correlations do not confirm that changes in crystalline lens shape and axial elongation are related. It is difficult to establish a reference point for two shapes that change simultaneously.
A valid comparison of the changes in the two shapes requires comparison of the entire contour of both shapes. Therefore, we used Fourier transformation of the shape contours. This value can extract a change in pattern with a large contribution from shape with diversity. Then, the PC can extract the eyeball shape of large contribution from various eyeball shapes, and can evaluate the correlation of two shapes such as the crystalline lens and the eyeball.
Moreover, this value can be visually reconstructed as shown in Figure 5 B. Our findings revealed that changes in the contours of the crystalline lens the PC1 of the lens and the eyeball PC1 of eyeball were significantly correlated Table 1. Multiple linear regression analysis showed that axial elongation was associated with the entire crystalline lens contour, and not a particular segment of the crystalline lens Table 2.
The equatorial lens diameter D was significantly correlated with ocular component dimensions, and with the radius of curvature of the crystalline lens. During development, the distance between the crystalline lens and the peripheral sclera i. For this reason, it was thought that the extension in the direction of the equator of the crystalline lens originated in peripheral scleral growth. A recent study noted that thinning and flattening of the crystalline lens were necessary for emmetropization.
Our results support this hypothesis. There were some limitations to this study. The possibility of the sample data being biased because the subjects were not prospectively randomly selected was a limitation. Another potential limitation is that because only horizontal images were examined, the overall height of the eyeball was not measured. It is necessary that 1 mm or smaller slice MR images are examined to reconstruct the sagittal image with the horizontal image. In a study using 1-mm-slice T2-weighted unprocessed MR images, Jones et al.
In our previous study, for a method using 3-mm-slice 3D-reconstructed MR images, the error margin for AL as determined by A-scan ultrasonography and 3D-reconstructed MR image was 0. In this study, we used 1. Therefore, moderate reliability of crystalline lens shape measurement was ensured. Thus, the correlation coefficient between SER and crystalline lens shape may have been reduced. In conclusion, we revealed that the crystalline lens shape changes dramatically during development.
In addition, axial elongation is related to the entire contour of the crystalline lens, and not to a particular segment. The mechanical stretching in the direction of the equator of the crystalline lens may be a key to resolving emmetropization. Chromatic aberration and accommodation: their role in emmetropization in the chick.
Vision Res. Bartmann M Schaeffel F. A simple mechanism for emmetropization without cues from accommodation or color. Contrast and spatial-frequency requirements for emmetropization in chicks. Schaeffel F Diether S. This process helps to explain the histologic patterns seen in cortical cataracts. Posterior subcapsular cataracts PSC develop due to the posterior migration of lens epithelial cells in response to external stimulus.
Although most cases are spontaneous, PSC also can develop secondarily to metabolic causes, such as diabetes, inflammation, uveitis, or from long-term topical or systemic corticosteroid use. PSC tend to occur in younger patients and progress more quickly than the other cataract subtypes.
The opacity is located at the posterior pole of the lens on the anterior surface of the posterior capsule Figure 16 and Slit lamp clinical photograph of a focal opacified area white granular appearance of posterior subcapsular cataract. Retroillumination clinical photograph of a focal granular area arrows of a posterior subcapsular cataract. An ultrastructural study involving 13 eyes with PSC showed the changes that took place as lens epithelial cells migrate from the equator of the lens to the PSC region.
The study concluded that there was a mitotic change as cells migrated towards the posterior pole. The equatorial region cells resembled normal lens epithelial cells but the cells near the PSC showed increased mitotic activity. The authors suggested that the cells were responding to some type of noxious stimuli at the posterior pole. The resultant activity caused the cells to mature into lens fibers or enlarge into bladder-like cells called Wedl cells and the formation of PSC Fig.
The migrating cells probably contribute to cataract formation by secreting extracellular materials, cytolysis, cell dissolution, and possibly release of lysosomal enzymes. Bladder cells bc or Wedl cells. Anterior subcapsular cataracts ASC develop because of the degeneration of anterior lens epithelial cells. They can develop secondary to trauma, medical treatment iatrogenic causes, or spontaneously.
The area of damage causes a migration of lens epithelial cells into the area and subsequent transformation of the cells into myofibroblasts in a process known as fibrous metaplasia. This results in an opacity on the anterior surface of the lens beneath the anterior capsule. Electron micrographs of the anterior lens epithelial cells in anterior subcapsular cataract. BM, basement membrane.
From Font, R. In one of the first studies on ASC, 5 lenses with ASC were examined by light and electron microscopy and confirmed the ability of the lens epithelium to undergo transformation to a fibrous plaque. The lens epithelial cells lost their normal cuboidal shape and elongated into a more spindle-shaped cell Fig. These cells were found frequently to be in contact with one another, resulting in the fibrous plaque known as ASC. This process can be broken down into two phases: a proliferative and a degenerative phase.
The proliferative phase was most evident near the periphery of the plaque, showing numerous spindle-shaped cells and mitotically active cells. It is followed by a degenerative phase, which results in an almost structure-less hyaline mass with fewer spindle-shaped cells.
Although the cause of ASC is varied, an association between ASC and the formation of synechiae after trauma or inflammation has been hypothesized. The synechiae would form between the posterior iris and anterior lens capsule, resulting in a stagnation of aqueous humor and accumulation of toxic metabolites that could produce a toxic effect on anterior lens epithelium.
Direct injury to the head or eye can cause significant mechanical disruption and lead to cataract formation. A Vossius ring can occur if the insult caused the posterior iris pigment epithelium to imprint on the lens capsule. The pigment deposition may abate and resolve completely with time. Severe blunt injury can cause stellate lenticular opacities in the cortex and capsule.
Such insult can lead to lens epithelium dysfunction, resulting in a significant edematous response to the superficial cortical lens. Vacuole pockets can then become trapped permanently within the lamellar zone, becoming integrated within the lenticular fibers while new layer are elaborated over the lesion.
Alternatively, blunt trauma can also cause cataract formation within all the lenticular layers, leading to a diffuse fibrous metaplasia Fig. Other forms of trauma that can lead to cataract formation include exposure to radiation, infrared, extreme heat and electrical injury. Traumatic cataract. Extensive anterior fibrous metaplasia arrows displaying prominent collagen staining blue in a traumatic cataract Trichrome stain, X Several pharmacological agents have been shown to cause cataract formation.
Long term corticosteroid therapy and anabolic steroid use are among the most common agents associated with cataract formation, particularly posterior subcapsular cataract. Psychotropic agents, particularly phenothiazine, induce deposition of pigmented material into the anterior lens epithelium in a very distinct axial configuration [14].
Other pharmaceutical agents known to cause lenticular cataracts include miotics, and amiodarone [15]. While age-related changes remains the leading factor for cataract formation, specifically senile cataract, other contributory factors include smoking, systemic disease, excessive exposure to sunlight and the aforementioned pharmacological agents []. In diabetic patients, cortical and PSCs appear to occur earlier, particularly among patients with poor glycemic control.
Hypocalcemia-induced cataracts usually initiate as small white dot opacities that can coalesce into larger flakes. Smoking, sun exposure and systemic disease management are modifiable risk factors, so taking measures to changes these factors can delay the onset and progression of cataract formation. Phytonutrients, such as xanthophyll carotenoids, lutein and zeaxanthin may play a potential role in limiting or neutralizing light induced oxidative changes within the lens [22].
Currently, there are several ongoing studies evaluating other possible protective agents. Although there is no definitive measure to prevent cataract formation, cataract surgery remains an extremely safe and highly successful intervention.
Two videos showing cataract surgical procedures from the anterior and posterior views of the anterior segment of the human eye. Surgery for cataracts has undergone extensive evolution. Ancient knowledge viewed the cataratous eye as an imbalance of humors that needed displacement to recover vision. Using a needle, the surgeon would proceed to displace the abnormal humor until the crystalline lens dislocated.
Modern cataract surgery has undergone significant changes and is now characterized by several steps: corneal incision, continuous curvilinear capsulorrhexis CCC , hydrodissection, phacoemulsification, cortical aspiration, and intraocular lens IOL implantation. Earlier surgical intervention to remove the entire cataractous lens required a 12 mm incision with subsequent suture closure. However, a small 2. The CCC technique was developed by Gimbel and Neuhann in the s and truly revolutionized the phacoemulsification technique [16].
CCC involves creating a tear in the anterior capsule then continuing the tear in a circular continuous fashion while minimizing shear forces exerted on the zonular fibers. After creation of the CCC, phacoemulsification is used to fragment and emulsify both the cortical and nuclear material.
Originally pioneered by Kelman in , phacoemulsification remains a vital part of cataract surgery [17,18].
The CCC opening is large enough to allow implantation of the entire optic and haptics of an intraocular lens IOL within the remnant lens capsular bag. The prior use of non-foldable polymethylmethacrylate lenses required a relatively large clear corneal incision for implantation.
However, the development of the foldable silicone and acrylic IOLs allowed insertion through a small incision mostly less than 4. Innovation is constantly improving these steps of cataract surgery, from novel IOLs with unique design to minimize the corneal incision, to use of the femtosecond laser to create an automated corneal incision, CCC and to fragment the nucleus prior to aspiration.
Details of the types of intraocular lenses, that are presently being used in cataract surgery, are presented in the following chapter in webvision by Jason Nguyen, and Liliana Werner. Epidemiology of cataract in childhood: A global perspective. Global prevalence of childhood cataract: a systematic review. The critical period for surgical treatment of dense congenital bilateral cataracts.
The critical period for surgical treatment of dense congenital unilateral cataract. American Journal of Ophthalmology. Lancet London, England. Early treatment of congenital unilateral cataract minimizes unequal competition. The Beaver Dam Eye Study. The MRI eye coil, with a viewing hole in the middle, was placed in front of and as close as possible to the measured eye without touching the skin or eyelashes and clamped in place.
The subject looked through the mirror at the center of a 31 mm diameter spoke-wheel target on a wall 6. The subject was instructed to look at the target during the measurements and to relax between measurements. The order of image acquisition was 1 a 16 s set of scout images, 2 an FSE image in the sagittal plane of the eye, 3 an FSE image in the transverse axial plane, 4 an MSE image in the sagittal plane, and 5 an MSE image in the transverse axial plane.
If the eye appeared tilted in the sagittal scout images, the vertical tilt of the mirror was adjusted appropriately and another set of scout images was obtained. The transverse axial scout images were used to manually select the slice plane for the first sagittal FSE image to correspond with the geometrical axis of the crystalline lens.
In young subjects, MR images during near viewing were obtained while fixating on a spoke-wheel target placed in a mount in front of and as close as possible to the eye, so that it could still be seen clearly and comfortably.
The near target was first removed from the mount to reveal a round hole in the mount. The subject was instructed to move the mount vertically and horizontally until the distant target appeared centered in the hole.
The mount was locked in place, and the near target was replaced. In this manner, the near target was subjectively aligned with the distant target, to maintain similar gaze direction for far and near scans. The subject was instructed to look at the near target and keep it in focus. The range of near target distances for different subjects was MR images during far and near viewing for a young subject and far viewing only for an older subject are shown in Figure 1.
External and internal boundaries in the eye were identified using a Canny edge filter available in Matlab Image Processing Toolbox. The eye image was rotated to orient vertically with cornea above and posterior sclera below Figure 1B. The angle of rotation was noted to check for any gaze deviations between far and near viewing in young subjects.
Adequate performance of the eye rotation algorithm has been reported previously Kasthurirangan et al. Figure 1. View Original Download Slide. A Transverse axial MR image of a 23 year old subject during far viewing.
B The same image has been rotated using custom-developed Matlab software to orient the eye with cornea above, sclera below, and a horizontal crystalline lens with no tilt. Dashed white lines indicate various automatically measured intraocular dimensions including anterior chamber depth, lens thickness, lens diameter, ciliary ring diameter, and axial length.
Conic curve fits to lens surfaces are indicated by thick white lines. C MR image of the same 23 year old subject during near viewing. D MR image of a 65 year old subject during far viewing. Figure 1 A Transverse axial MR image of a 23 year old subject during far viewing. A difficulty in the identification of crystalline lens pixels is that the iris obscures part of the anterior edge of the lens.
Therefore, the user manually defined two regions on either side of the pupil around the region of contact between the iris and the lens. These regions were removed from further analysis. Various biometric parameters were measured automatically from the MR images see Figure 1B.
Axial length was measured along a geometric axis of the eye Figure 1B. All measurements, except for ciliary ring diameter, were automatically performed. Statistical comparisons for accommodative trends were performed through paired t -tests and for age-related trends through unpaired t -tests. In general, the sagittal images were noisier than the transverse axial images.
In order to quantitatively evaluate this difference, signal-to-noise ratio and the intensity gradient across the anterior edge of the crystalline lens were compared between sagittal and transverse axial images in the same eyes.
Signal-to-noise ratio was calculated as the ratio of average pixel intensity within the crystalline lens signal and average pixel intensity anterior to the cornea i. The average signal-to-noise ratio was significantly larger in the transverse axial images than in the sagittal images 5. Peak intensity gradient across the crystalline lens was calculated in the following manner: 1 the intensity gradient along five lines of pixels across the anterior lens surface from the anterior chamber into the lens was calculated, 2 an average of the peak intensity gradients from the five lines was calculated, and 3 average peak intensity gradient was considered as the intensity difference across the anterior lens surface.
The increased noise was mainly due to motion artifacts, most likely due to blinks, affecting the sagittal images more than the transverse axial images. Paired comparisons revealed statistically significant differences between sagittal and transverse axial images for some ocular biometric parameters. Since ocular measurements with transverse axial images show some differences from sagittal images and the transverse axial images were sharper with well defined edges compared to sagittal images, further results are presented for transverse axial data only.
The MR images had an in-plane resolution of 0. Profile plots of intensity change along the anterior lens surface showed that the edge consisted of two pixels of rising intensity or gray values.
As an upper estimate, the uncertainty in defining a surface edge was of one pixel length, i. The error in measuring intraocular lengths i.
This suggests that the practical resolution of the MR images was 0. While the MRI technique was capable of imaging behind the iris, around the region of contact between the iris and anterior lens surface, the two structures could not be distinguished. This required removal of these data when fitting the anterior lens surface with conic curves. Examples of conic curve fits to the anterior and posterior lens surfaces are shown in Figures 2A and 2B for one eye of a young subject in the relaxed and accommodated states, respectively, and in Figure 2C for one eye of an older subject.
For the anterior lens surface, considerable data were unavailable along the region of contact between the iris and the anterior lens surface. Therefore, the conic curve fits were unreliable and the results on the anterior lens surface vertex radius of curvature and asphericity were excluded from the study. The posterior surface conic curve fits were good with r 2 values greater than 0. Figure 2. Examples of conic curve fits to anterior and posterior lens surfaces in one eye of a young subject during A relaxed and B accommodated states and C in one eye of an older subject during relaxed state.
These examples represent posterior lens radius of curvature values close to the average value within each condition. The anterior lens surface fits suffered from missing data at the region of contact between the iris and the crystalline lens. Results on anterior lens surface fits were excluded from the study.
The posterior lens surface fits were excellent with r 2 values greater than 0. Figure 2 Examples of conic curve fits to anterior and posterior lens surfaces in one eye of a young subject during A relaxed and B accommodated states and C in one eye of an older subject during relaxed state.
MRI and applanation A-scan measurements of axial ocular dimensions were compared in all eyes of young and old subjects for the relaxed accommodative state.
Figure 3. A significant linear relationship excluding the outlier was found with slope not different from 1 and intercept not different from zero. B Bland—Altman analysis of lens thickness data showed a mean difference close to zero and no obvious trends.
C Axial lengths measured with A-scan X -axis and MRI Y -axis show good correlation with slope not different from one and intercept not different from zero. In the MR images, the internal boundary of the retina was not always clearly visible, so axial length measurements were performed from the anterior border of the cornea to the posterior border of the eye, which would have resulted in an offset between A-scan and MRI axial length measurements. Paired differences showed that MRI axial length measurements were larger than A-scan length measurements by 0.
The slope in Figure 3B is close to 1, indicating good correspondence between A-scan and MRI axial length measurements. Ocular alignment during far viewing and near viewing in young subjects was checked by comparing eye rotation angle and axial lengths.
The average absolute difference in ocular alignment was 1. While there were some differences in eye rotation angle and axial lengths between unaccommodated and accommodated images in individual eyes, these differences were not systematic. Anterior chamber depth decreased significantly with accommodation and age Figure 4 and Table 1.
On average, anterior chamber depth decreased 0. Lens axial thickness increased 0. Figure 4. Box plot showing median and range of axial distances of anterior chamber depth, lens thickness, and anterior segment length for young subjects during far viewing YF and near viewing YN and for older subjects during far viewing OF.
Lens thickness increased with accommodation and age. Anterior segment length did not change with accommodation but increased with age. Figure 4 Box plot showing median and range of axial distances of anterior chamber depth, lens thickness, and anterior segment length for young subjects during far viewing YF and near viewing YN and for older subjects during far viewing OF.
Table 1 View Table. Lens equatorial diameter decreased 0. Therefore, although the relative changes in lens thickness and equatorial diameter were different between accommodation and age, the lens assumed a more rounded shape in either case. The ciliary ring diameter decreased with both accommodation mean change: 0. The circumlental space, the distance from the equatorial edge of the lens to the ciliary body tip, did not change with accommodation 1. Figure 5. Box plot showing median and range of equatorial values of crystalline lens diameter and ciliary ring diameter for young subjects during far viewing YF and near viewing YN and for older subjects during far viewing OF.
Ciliary ring diameter decreased with accommodation and age. Figure 5 Box plot showing median and range of equatorial values of crystalline lens diameter and ciliary ring diameter for young subjects during far viewing YF and near viewing YN and for older subjects during far viewing OF. Posterior lens surface curvature and asphericity. The vertex radius of curvature of the posterior lens surface decreased with accommodation mean change: 0.
Table 1 provides the mean values of the various biometric parameters measured in the study. Figures 6A and 6B show the changes in lens dimensions with accommodation and age, respectively, using actual measured average dimensions. The curve representing the posterior crystalline lens surface was based on the average vertex curvature and average conic constant derived from Equation 1. The specific changes in lens axial and equatorial dimensions, ciliary body location, and posterior lens surface curvature can be seen in Figure 6.
Figure 6. Changes in crystalline lens and ciliary body apex with A accommodation and B age. In B , data for older subjects open square and dashed lines in red are plotted using the same scheme as A.
The overall changes in lens size and shape can be observed in the figures. The anterior lens surface could not be well fitted with conic curves due to missing data at the region of iris overlap. The anterior pole of the crystalline lens moved forward with accommodation and aging.
The ciliary body apex moved inward with accommodation and age, but no forward movement in either case was observed. Figure 6 Changes in crystalline lens and ciliary body apex with A accommodation and B age. The MRI technique was successfully employed to study changes in crystalline lens shape with age and accommodation. The crystalline lens became thicker and more spherical in shape with both accommodation and age.
However, equatorial diameter of the crystalline lens decreased with accommodation and increased with age. A significant change in the posterior surface radius of curvature was seen with accommodation.
Age- and accommodation-related changes in crystalline lens and ciliary muscle position are discussed in detail below. One of the aims of the study was to describe the three-dimensional shape of the lens by obtaining images along sagittal and transverse axial sections. Unfortunately, the sagittal images were noisier than the transverse axial images, possibly due to eye movements and blinks. In the interest of accuracy, only results from the transverse axial images have been provided.
At the region of contact between the iris and the anterior crystalline lens surface, the two surfaces were indistinguishable leading to exclusion of the anterior lens surface data from further analysis Figure 2.
The radius of curvature and asphericity values for the posterior lens surface alone are provided. As reported previously, anterior chamber depth decreased with accommodation and age and the lens thickness increased with accommodation and age see Figure 4 and Table 1 ; Atchison et al. The anterior segment depth did not change with accommodation but increased with age Figure 4.
With accommodation, the decrease in anterior chamber depth 0. A drawback of the current study was the inability to measure accommodative response in diopters during MR imaging. However, an increase in lens thickness of 0. The mean accommodative changes reported in the current study correspond to about 5 D of response accommodation. With aging, the increase in lens thickness 0.
Some previous studies have reported that anterior segment depth increases with accommodation Bolz et al. Koeppl et al. A possible reason for the lack of significant change in anterior segment depth in the present study could be due to the resolution limits of the MRI technique 0. Another plausible reason, given the similar magnitude of change in anterior chamber depth and lens thickness, could be the supine posture of the subjects compared to the erect posture of subjects in past studies personal communication with Dr.
Adrian Glasser. In a supine posture, the crystalline lens may sag to its deepest position due to the effect of gravity, even in the unaccommodated state. Therefore, with accommodation, no further backward movement could have been possible. The difference between MRI and A-scan measured anterior segment depths is similar to the expected accommodative change up to 0.
This lends support to the idea that erect vs. It is of interest to evaluate this effect of lens position during erect and supine postures and with accommodation in a future study. A significant change in posterior lens surface radius of curvature was seen with accommodation.
Therefore, although there is no posterior movement of the posterior pole of the lens in this study i. In the current study, the decrease in equatorial diameter 0.
In addition, the ratio of crystalline lens thickness to diameter increased, approaching 0. It is interesting to note the similarity in magnitude of the change in lens axial thickness and equatorial diameter for a certain magnitude of accommodative response, suggesting that the changes in the two parameters per diopter of accommodation may also be equivalent.
A significant increase in crystalline lens diameter with age was seen. The change in equatorial diameter 0. Previous reports have shown no change in crystalline lens equatorial diameter with age in humans Strenk et al. In the present study, mean crystalline lens diameters in two groups of subjects separated in age by about 40 years were compared. In past reports, linear regression analysis was undertaken to study age-related changes in lens diameter Strenk et al.
The magnitude of the changes reported in the present study could have been missed in past studies using regression analysis, due to the wide individual variation and lack of sufficient subjects in clearly delineated age groups. A potential confounding factor leading to the observed age-related changes in equatorial diameter in the current study could have been the level of tonic accommodation in the younger subjects even when a far target was used to relax accommodation.
This and previous studies measured unaccommodated lens diameters during far viewing and without any cycloplegic agent Strenk et al. A greater baseline tonic accommodation in younger subjects leading to decrease in lens diameter compared to older subjects could have resulted in an increase in lens diameter with age as seen in the current study. Nevertheless, the present study is comparable to the past studies because none of the studies used a cycloplegic agent.
A future study should consider using cycloplegic agents to truly measure changes in unaccommodated lens equatorial diameter with age. The increase in the lens thickness to equatorial diameter ratio shows that the crystalline lens becomes more rounded with age. In the current study, no forward movement of the ciliary body was observed with accommodation or aging Table 1.
An MRI study in humans showed a more forward positioning of the ciliary muscle with age 0. It is likely that the lack of any measurable forward ciliary movement during accommodation or with age in the current study was due to the limited resolution of our MRI technique 0. Centripetal movement i. The circumlental space, the space between lens equatorial edge and ciliary body tip, did not change with accommodation and decreased with age as shown previously in humans Strenk et al.
In young Rhesus monkeys, the circumlental space has been shown to remain stable or decrease only slightly during Edinger-Westphal EW nucleus stimulated accommodation, while significant changes were observed during supramaximal or pharmacological e. The lack of changes in circumlental space in young humans in the current study may be because the ciliary muscle effort was within the maximum accommodative amplitude of the subjects. This decrease was due to a combination of increase in lens equatorial diameter 0.
Following cataract surgery, the ciliary body has been shown to move outward i. This observation, along with the findings of the current study, suggests that the axial growth of the lens with age may increase the natural tension in the anterior zonular fibers during relaxed accommodation, in turn exerting an inward pull on the ciliary body leading to a reduction in circumlental space.
This force may be partly or fully released following cataract surgery due to the removal of lens material and collapse of the capsular bag. The magnitude of the repositioning of the ciliary muscle following cataract surgery will be quite informative in determining the success of accommodating IOLs.
The objective of the current study was to describe the overall biometric shape of the lens surface and not a central optically relevant region. All previous studies on the lens surface shape had considered only a central zone and are not directly comparable to the current study.
With accommodation, the radius of curvature of the posterior lens surface decreased as reported previously with a variety of optical methods in humans and Rhesus monkeys Brown, ; Dubbelman et al. The conic constant of the posterior surface did not change with accommodation Table 1. Previous studies were based on optical techniques and it was not clear if the observed changes in the posterior lens surface were due to any optical artifacts when measuring through an accommodating lens.
The current study has used MRI to demonstrate clear changes in the posterior lens surface during accommodation using a non-optical imaging technique. The anterior lens surface curvature could not be reliably measured in the current study primarily due to the loss of data at the region of contact between the iris and anterior lens surface.
With age, the radius of curvature of the posterior lens surface did not change and the conic constant increased. The trends in radius of curvature are largely supported by literature Atchison et al. The mean posterior lens radius of curvature of 6. Dubbelman et al. It is difficult to compare the findings of the current study directly with Dubbelman et al.
The current study shows that the posterior lens surface becomes more spherical with age. The various changes in the crystalline lens identified during accommodation are shown in Figure 6A based on the mean data from Table 1. During accommodation, the anterior chamber depth decreases, and the crystalline lens increases in thickness and decreases in diameter with a reduction in the radius of curvature of the posterior lens surface.
The increase in lens thickness is equal to the decrease in anterior chamber depth with no change in anterior segment depth. The decrease in equatorial diameter is equal in magnitude to the increase in lens thickness. The ciliary body moves inward without any forward movement, while the circumlental space remains unchanged.
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