THE BEAUTY OF TEACHING at a school of optometry is to be confronted daily with the curiosity and fresh perspective that students bring to concepts that established practitioners take for granted.
One fourth-year student is particularly interested in scleral lenses. Needless to say, he has millions of questions about all aspects of specialty lenses. Recently, he listened to a webinar about the complications of scleral lenses. The speaker was arguing that practitioners must avoid touching the limbus so as not to alter the corneal stem cells.
Although it is true that scleral lenses should not make contact with the limbus, calling this a problem due to “alteration of stem cells” does not accurately depict why this is a concern for fitters. Examination of ocular anatomy makes it clear that this is not that simple.
Corneal stem cells are crucial for maintaining the integrity and transparency of the cornea. They contribute to the constant renewal of the epithelial cell layers. They maintain the barrier between the conjunctiva and cornea, preventing blood vessels and conjunctival cells from encroaching onto the normally clear corneal surface (Hertsenberg and Funderburgh, 2015).
Scleral lens wear may not have a significant impact on these stem cells because they are located deep within the limbal area, in the Palisades of Vogt, below the surface where the lens is landing (Chee et al, 2006). The niches in which they reside offer a protective, nutrient-rich environment and shield stem cells from external insults, supported by structures like the eyelids and melanocytes (Park, 2023). Another argument is that stem cells are physiologically located more along the superior and the inferior limbus (Ruan et al, 2021), whereas most scleral compression is exerted nasally and temporally (Formentin et al, 2025).
It is not likely that the pressure of a scleral lens will alter these cells, because they are not directly insulted by contact with the lens. However, these vital cells can be harmed by other factors. Chemical and thermal burns, chronic inflammation (such as in Stevens-Johnson syndrome or ocular cicatricial pemphigoid), hereditary genetic disorders (like congenital aniridia), long-term contact lens use (hypoxic stress), severe ocular trauma, iatrogenic injury from surgery, radiation, or exposure to toxic agents can all damage the niches or directly harm the stem cells (Ahmad, 2012).
When a scleral lens touches the limbus or adjacent area, a mechanical insult is generated on the epithelial (not stem) cells. This causes their tight junctions to break and fluid to accumulate between the cells. This phenomenon has been described as bullae (Nixon et al, 2017). The breakdown of junctions and the accumulation of fluid allow neovascularization of the region. This is the real reason why touching the limbus with a scleral lens should never be tolerated (Figure 1).
My student was right to doubt the simplified mechanical explanation that he had heard in the webinar. As we enter the new year, it is essential that we all question outdated or oversimplified concepts and embrace those explanations that are evidence-based and scientifically proven.
References
1. Hertsenberg AJ, Funderburgh JL. Stem cells in the cornea. Prog Mol Biol Transl Sci. 2015;134:25-41. doi: 10.1016/bs.pmbts.2015.04.002
2. Chee KY, Kicic A, Wiffen SJ. Limbal stem cells: the search for a marker. Clin Exp Ophthalmol. 2006;34(1):64-73. doi: 10.1111/j.1442-9071.2006.01147.x
3. Park CY. An updated narrative review of treatment for limbal epithelial stem cell deficiency. Ann Eye Sci. 2023;8:15. https://aes.amegroups.org/article/view/7404/html
4. Ruan Y, Jiang S, Musayeva A, Pfeiffer N, Gericke A. Corneal epithelial stem cells-physiology, pathophysiology and therapeutic options. Cells. 2021;10(9):2302. doi: 10.3390/cells10092302
5. Formentin L, Choquechambi YC, Araujo NPF, Miyazawa SAM, Oliveira HMC, Santos RT. Analysis and optimization of the landing zone parameters of a sclera lens model. Arq Bras Oftalmol. 2025;88(3):S0004-27492025000300309. doi: 10.5935/0004-2749.2023-0332
6. Ahmad S. Concise review: limbal stem cell deficiency, dysfunction, and distress. Stem Cells Transl Med. 2012;1(2):110-5. doi: 10.5966/sctm.2011-0037
7. Nixon AD, Barr JT, VanNasdale DA. Corneal epithelial bullae after short-term wear of small diameter scleral lenses. Cont Lens Anterior Eye. 2017;40(2):116-126. doi: 10.1016/j.clae.2016.11.007