The bulbar conjunctiva, fused to the underlying anterior Tenon's capsule, loosely covers the anterior part of the globe from the fornices above and below and from the canthi medially and laterally. The bulbar conjunctiva and anterior Tenon's capsule have multiple, fine imbedded arterioles and veins. These are branches of the anterior ciliary circulation and of the marginal arcades of the vessels of the lids. Their number and configuration vary from person to person. This circulation furnishes a small but probably significant blood supply to the anterior segment. The fused conjunctiva and anterior Tenon's capsule attach firmly to the sclera at the limbus (Figure 22). The combined conjuctiva and underlying anterior Tenon's capsule is thick and has substance in infancy and childhood but becomes much thinner and more friable in adulthood and old age.
The plica semilunaris is a fold in the conjunctiva located far medially in the palpebral fissure and is mostly below the midline. The caruncle, located just medial to the plica, is about 3 mm in diameter, covered with squamous epithelium, and often contains small hairs (Figure 23). The relationship of the plica and caruncle to each other and to the palpebral fissure is an important cosmetic factor in strabismus surgery. When repairing the conjunctiva, care should be taken not to alter the position of these structures. It is particularly important that the plica not be displaced laterally, making it more obvious as a reddened, unsightly mass seen in the palpebral opening.
Figure 22
The topographic landmarks of the conjunctiva important to the strabismus surgeon are the following:
A The fusion of the conjunctiva and anterior Tenon’s capsule with the sclera at the limbus
Figure 23
A The limbus
B The plica semilunaris
C The caruncle
A fat pad is present in the inferior fornix extending to within 12 to 14 mm of the limbus (Figure 24). This fat pad is beneath conjunctiva and its posterior condensations behind the orbital septum, and is outside both layers of Tenon's capsule in the extraconal space. A transconjunctival incision made medially and laterally in the inferior cul-de-sac should be posterior to the attachment of posterior Tenon’s capsule or at least 8 mm from the limbus in order to expose bare sclera. However, it should also be anterior to the inferior fat pad, no more than approximately 12 mm from the limbus. There is no comparable fat pad superiorly.
Figure 24
A The shallow lower fornix with a visible fat pad beneath conjunctiva
B The deep upper fornix with no visible fat under conjunctiva
C The inferior fat pad shown frontally
D Saggital section of the orbit shows the relationships of the vertical rectus muscles, fat compartments, oblique muscles, orbital septa, and lids. It should be noted that on the superior aspect of the globe, the extraconal fat lies above the levator palpebrae and behind the orbital septum. Surgery on the superior oblique muscle is not associated with extraconal fat because of the barrier of the levator palpebrae muscle. Pulling the lower lid down allows the inferior fat pad to prolapse.
During extraocular muscle surgery, all incisions should be limited to the bulbar conjunctiva; they should not extend into the fornix or palpebral conjunctiva. An incision carried too deeply into the fornix causes unnecessary bleeding and serves no purpose. Transconjunctival incision in the palpebral opening over the insertion of the medial or lateral rectus should be avoided, if possible, because it can lead to unsightly scarring, which in extreme cases can even restrict motility.
When prior surgery has left the conjunctiva reddened and unsightly or scarred so that it limits motility, the conjunctiva can be recessed with or without removal of conjunctival tissue. In these cases, the sclera is left uncovered. Sclera is rapidly re-covered with epithelium when it is left exposed, remaining comfortable in the process. It is not necessary to use a mucous membrane graft to cover exposed sclera. As long as one of the opposing surfaces behind the lids is covered with epithelium, a symblepharon will not develop.
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