Glaucoma is a progressive optic neuropathy which represents the final common pathway of a number of different disorders which affect the eye. Most, but not all, of these are associated with elevated intraocular pressure (IOP), which is the most important known risk factor for optic nerve damage, but is still only a risk factor and not the disease itself. The progressive loss of retinal ganglion cells and their axons leads to a characteristic clinical pattern of optic nerve head and corresponding visual field damage. Non-IOP independent mechanisms of glaucomatous damage are not confined to “normal tension” glaucoma, but can be operative in any patient, although they predominate when glaucomatous damage occurs at the lower end of the pressure spectrum. The goal of glaucoma filtering surgery is to reduce or eliminate the pressure dependent component of the disease process.

Filtration surgery to lower IOP has been in use for over a century. Limbal trephination, described by Elliot in 1909,1 became the most popular operation until the 1940s, when it fell out of favour because the very thin conjunctival bleb predisposed to late endophthalmitis. Thermal cautery combined with peripheral iridectomy,2 and posterior lip sclerectomy,3became the most widely used operations until guarded filtration procedures were developed. All of these full thickness procedures, however, were complicated by frequent flat anterior chambers, ciliochoroidal detachments, occasional choroidal haemorrhages, and …