Relationship between limbal incisions and the structures of the anterior chamber angle

Posterior corneoscleral limbus: Architecture, stem cells, and clinical implications

The limbus is a transition from the cornea to conjunctiva and sclera. In human eyes, this thin strip has a rich variation of tissue structures and composition, typifying a change from scleral irregularity and opacity to corneal regularity and transparency; a variation from richly vascularized conjunctiva and sclera to avascular cornea; the neural passage and drainage of aqueous humor. The limbal stroma is enriched with circular fibres running parallel to the corneal circumference, giving its unique role in absorbing small pressure changes to maintain corneal curvature and refractivity. It contains specific niches housing different types of stem cells for the corneal epithelium, stromal keratocytes, corneal endothelium, and trabecular meshwork. This truly reflects the important roles of the limbus in ocular physiology, and the limbal functionality is crucial for corneal health and the entire visual system. Since the anterior limbus containing epithelial structures and limbal epithelial stem cells has been extensively reviewed, this article is focused on the posterior limbus. We have discussed the structural organization and cellular components of the region beneath the limbal epithelium, the characteristics of stem cell types: namely corneal stromal stem cells, endothelial progenitors and trabecular meshwork stem cells, and recent advances leading to the emergence of potential cell therapy options to replenish their respective mature cell types and to correct defects causing corneal abnormalities. We have reviewed different clinical disorders associated with defects of the posterior limbus and summarized the available preclinical and clinical evidence about the developing topic of cell-based therapy for corneal disorders.

Computational Model of Sclerotic Scatter

PURPOSE

A clinical examination technique to detect pathology within the anterior eye is known as "sclerotic scatter" (SS). Its propagation pathway has not been thoroughly investigated. Although conventionally theorized to occur by "total internal reflection" (TIR) within the cornea, existing data suggest that this may be an incomplete explanation.

METHODS

An anterior eye model representative of a human eye has been constructed using nonsequential ray tracing (OpticStudio 18.1). Three generations of the model were constructed to support the analyses of the pathway of light in SS. A design of experiment methodology involving the key parameters was used to determine the slit-lamp setup for optimum clinical visualization.

RESULTS

Most of the light directed into the temporal limbus in SS is lost (52%) into the sclera or reemitted back to the clinician. Only 0.006% of light that transits the central cornea undergoes TIR off the anterior cornea and more significantly 0.000125% off the posterior cornea. The optimal slit-lamp setup parameters to maximize the clinician's visualization are also summarized.

CONCLUSIONS

The propagation of light within SS primarily does not occur by TIR off the posterior cornea but rather the direct transcameral propagation of light. SS also represents an inefficient usage of light, with approximately half of the light creating a potential glare source for clinicians. We have formulated a recommended set of parameters for the slit-lamp setup to maximize clinical visualization. We also describe the transcameral pathways involved in SS that create a corneal "backlighting" effect. Almost a century after Graves original description, the optics of this phenomenon are described here.

Limbus-insertion distance of superior rectus in primary angle closure glaucoma: an anterior segment OCT study

OBJECTIVE

To compare the distance from the corneoscleral limbus to the insertion site of the superior rectus of primary angle-closure glaucoma (PACG) patients with normal controls and to identify potential characteristics associated with limbus-insertion distance.

DESIGN

Cross-sectional study.

PARTICIPANTS

A total of 92 subjects (eyes) were enrolled in the study: 35 were nonglaucoma controls and 57 were PACG patients.

METHODS

The limbus-insertion distance of the superior rectus was measured by anterior segment optical coherence tomography (AS-OCT) and intraoperative surgical caliper. The anterior chamber parameters were also measured using AS-OCT to identify potential characteristics associated with limbus-insertion distance.

RESULTS

The limbus-insertion distance of the superior rectus was 0.40 ± 0.08 mm shorter in the PACG group than in the normal control group (p < 0.001). Anterior segment parameter values, including anterior chamber depth (ACD), anterior chamber width (ACW), anterior chamber area (ACA), and anterior chamber volume (ACV), were smaller in the PACG group than in the normal control group, but the lens vault was larger (all p < 0.001). A shorter limbus-insertion distance of the superior rectus was significantly associated with shorter axial length and "crowded" angle (smaller ACD, ACW, ACA, and ACV) (all p < 0.05).

CONCLUSIONS

This is the first study to demonstrate that the limbus-insertion distance of the superior rectus was shorter in PACG eyes than in normal eyes. The shorter limbus-insertion distance may be another anatomic characteristic of PACG eyes.

The Sclerotic Scatter Limbal Arc Is More Easily Elicited under Mesopic Rather Than Photopic Conditions

INTRODUCTION

We aimed to determine the limbal lighting illuminance thresholds (LLITs) required to trigger perception of sclerotic scatter at the opposite non-illuminated limbus (i.e. perception of a light limbal scleral arc) under different levels of ambient lighting illuminance (ALI).

MATERIAL AND METHODS

Twenty healthy volunteers were enrolled. The iris shade (light or dark) was graded by retrieving the median value of the pixels of a pre-determined zone of a gray-level iris photograph. Mean keratometry and central corneal pachymetry were recorded. Each subject was asked to lie down, and the ALI at eye level was set to mesopic values (10, 20, 40 lux), then photopic values (60, 80, 100, 150, 200 lux). For each ALI level, a light beam of gradually increasing illuminance was applied to the right temporal limbus until the LLIT was reached, i.e. the level required to produce the faint light arc that is characteristic of sclerotic scatter at the nasal limbus.

RESULTS

After log-log transformation, a linear relationship between the logarithm of ALI and the logarithm of the LLIT was found (p<0.001), a 10% increase in ALI being associated with an average increase in the LLIT of 28.9%. Higher keratometry values were associated with higher LLIT values (p = 0.008) under low ALI levels, but the coefficient of the interaction was very small, representing a very limited effect. Iris shade and central corneal thickness values were not significantly associated with the LLIT. We also developed a censored linear model for ALI values ≤ 40 lux, showing a linear relationship between ALI and the LLIT, in which the LLIT value was 34.4 times greater than the ALI value.

CONCLUSION

Sclerotic scatter is more easily elicited under mesopic conditions than under photopic conditions and requires the LLIT value to be much higher than the ALI value, i.e. it requires extreme contrast.

Transconjunctival corneoscleral tunnel "blue line" cataract incision

PURPOSE

To report the technique and astigmatic results of the blue line cataract incision.

SETTING

The Buzzard Eye Institute, Las Vegas, Nevada, USA.

METHODS

This prospective study included 411 eyes of 271 patients who had cataract extraction by phacoemulsification with a self-sealing 3.0 mm blue line cataract incision. The blue line incision is performed with a diamond knife transconjunctively, 2.0 mm behind the surgical limbus.

RESULTS

Mean patient age was 68 years (range 40 to 94 years) and mean preoperative astigmatism, 0.96 diopter (D) +/- 0.78 (SD). Uncorrected visual acuity at 1 day was 20/40 or better in 47% of patients. Mean spherical equivalent was -0.57 +/- 0.78 D at 6 months. Mean postoperative astigmatism measured with a subtraction method was 1.00 +/- 0.84 D at 6 months. Vector analysis showed an induced astigmatism of -0.47 +/- 1.00 D at 1 month, -0.58 +/- 0.81 D at 3 months, and -0.57 +/- 0.99 D at 6 months. No complications such as wound leakage or hyphema occurred.

CONCLUSION

The blue line incision combines the efficiency of the clear corneal with the safety of the scleral tunnel cataract incision and appears to be relatively astigmatically neutral.