The Scleral Lens Vault
The Scleral Lens Vault
Scleral Lenses and the Conjunctiva
BY GREGORY W. DENAEYER, OD, FAAO
The scleral tissue supports the haptic section of a scleral contact lens, which allows it to vault up over the cornea. However, the lens haptic rests on the bulbar conjunctiva, which has clinical implications concerning the fit of a scleral lens.
The bulbar conjunctiva is a transparent tissue that is loosely attached to Tenon’s capsule. It has two to five layers of stratified columnar epithelial cells and an underlying stroma made of loose connective tissue (Snell and Lemp, 1989). Mucin-producing goblet cells, nerves, and blood vessels are diffusely distributed throughout the conjunctiva.
The thickness of the bulbar conjunctiva has been recently studied by Zhang et al (2013) using optical coherence tomo-graphy (OCT). Measuring 711 eyes of Chinese subjects, they determined the average bulbar conjunctival thickness to be 240 microns. There were no significant differences between sexes, and the full thickness decreased with age.
Scleral Lens Fit
Ideally, the haptic section of a scleral contact lens should evenly rest on the bulbar conjunctiva/sclera without lift, impingement, or excessive compression. The sclera is nonrotationally symmetrical (van der Worp, 2010), and for most fits the bulbar conjunctiva acts as a cushion that is more forgiving than if the scleral lens were directly fit on the rigid scleral surface. A scleral lens haptic that is too tight will cause vascular blanching and limbal congestion of the bulbar conjunctiva.
Figure 1. Bulbar conjunctival impression ring after removal of a scleral lens.
A scleral lens will gradually depress into the bulbar conjunctival tissue after application secondary to its spongy nature. Settling of a scleral lens will result in loss of corneal vault. Thus, some loss of vault has to be taken into account during the fitting process; otherwise, this will result in a lens that has insufficient corneal clearance.
The question is: how much settling will occur during a specified time frame? Caroline and Andre (2012) reported that a scleral lens with a diameter of 16.5mm will settle approximately 96 microns (range: 70 to 180 microns) after eight hours of wear. A study by Kaufmann et al (2013) showed that scleral lenses with diameters of 15.8mm and 18.2mm settled 133 microns and 88 microns, respectively, after an eight-hour wear period. Nau and Schornack (2013) reported that 15mm scleral lenses settled 107 microns after two hours of lens wear.
Scleral lenses may further settle over weeks or months of initial wear secondary to adaptive changes of the bulbar conjunctiva. Additionally, the Zhang et al (2013) study suggests that less settling will occur in our older patient population.
For the most part, the bulbar conjunctiva is helpful in providing a cushion on which the scleral haptic can land. However, the spongy nature of the conjunctiva can significantly influence the short- and long-term vaulting relationship between a scleral lens and the cornea. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #220.
Contact Lens Spectrum, Volume: 29 , Issue: March 2014, page(s): 48