Pellucid marginal degeneration is a slowly progressive corneal ectatic condition often leading to severe visual impairment of working age people.
The peripheral corneal thinning location requires a large graft size, eventually a peripheral lamellar crescentic keratoplasty followed by a central penetrating keratoplasty.
Besides surgically more challenging, corneal transplantation is associated to increased risk of vascularization and rejection.
At a certain stage, ICRS implantation can be a good option to restore good corrected visual acuity, deter progression and postpone or avoid corneal graft challenges and complications.

http://eyewiki.aao.org/Intrastromal_Corneal_Ring_Segments_%28ICRS%29
http://eyewiki.aao.org/Pellucid_marginal_corneal_degeneration
Akaishi L, Tzelikis PF, Raber IM. Ferrara intracorneal ring implantation and cataract surgery for the correction of pellucid marginal corneal degeneration. J Cataract Refract Surg. 2004 Nov;30(11):2427-30.