EXTRAOCULAR MUSCLES
The vitrectomy surgeon needs to know the distance between the limbus and the ora serrata –considering that the insertion point of the rectus muscles corresponds the end of the retina– along with the width of the insertion point (Fig. 1.1). It should be noted that the anterior insertion point of the muscles coincides with the end of the retina and ora serrata. Identifying the insertion points of the recti is a key requirement for surgery.
FIGURE 1.1: Image of the extraocular muscles and their insertion points. The distance between the limbus and the equator is 9-12 mm and between the limbus and posterior pole is 29-32 mm. The circumference of the equator is 71-75 mm.
3The mean distance from the limbus to the ora serrata is 5 mm on the nasal side, and 7 mm on the temporal side.
It should be known that the insertion distance of the medial rectus with respect to the limbus is shorter on the nasal (5.5 mm) than the temporal side (6.9 mm) (Fig. 1.2). Moreover, the distance from the limbus to the equator is 9-12 mm and the distance between the limbus and posterior pole is 29-32 mm.1,2
FIGURE 1.2: Insertion points of the rectus muscles and their distance to the limbus in mm. The ora serrata of the retina occurs at this level.
The distance between the limbus and the vortex veins is 14-18 mm, and 10-14 mm posterior to the ora serrata. There are approximately seven vortex veins in each eye.4
The ciliary arteries and ciliary nerves enter the sclera at a distance of 1-2 mm from the optic nerve (Fig. 1.3).
GLOBE ANATOMY
The ocular globe is a slightly asymmetric sphere of mean axial length 24 mm. The mean thickness of the sclera is around 1 mm, yet ranges from 0.3 mm below the insertion point of the rectus muscles to 1.2 mm close to the optic nerve.
If we examine the eye in cross-section (Fig. 1.4), we can see the crystalline lens of 3.5 mm mean width and 9 mm horizontal diameter. The inner volume of the globe is 5.5 ml, of which in adults 4 ml are occupied by the vitreous humor.35
VITREOUS BODY
The normal vitreous is comprised of 4 ml of a clear gel occupying approximately 80% of the entire globe. Vitreous humor contains 99% water, type II collagen, hyaluronic acid, glycoproteins and proteoglycans, forming a delicate fibrillar network. The outer vitreous is known as the cortex and is 100 µm thick. The anterior cortex or anterior hyaloid membrane extends in front of the ora serrata and behind the crystalline lens as Wieger's ligament. The central, non-connected 6part of the ligament is known as Berger's space and is continuous with Erggelet's space, which gives way to the Cloquet's canal and ends in a funnel shape opening onto the optic nerve in the Martegiani's space.4,5
The posterior cortex, or posterior hyaloid, is joined to the margins of the papilla, foveola and retinal vessels.
CILIARY BODY
The ciliary body is comprised of two parts: the pars plicata and pars plana. The pars plicata is a circumferential zone extending 2.5 mm behind the limbus.
FIGURE 1.5: Image showing the pars plicata 2.5 mm behind the limbus followed by the pars plana, which is thicker at the temporal quadrants.
7It accommodates the circumferential and oblique muscles of the ciliary body and contains 70 to 80 ciliary processes. It is at this level that the zonule inserts.
The non-pigmented epithelium that coats the inner surface of the ciliary processes is morphologically equivalent to the retina. This epithelium secretes the aqueous humor.
The pars plana extends from the pars plicata above, to the ora serrata below. On the nasal side it is 3 mm thick and on the temporal side its thickness is 5 mm (the pars plana can be thicker in patients with myopia) (Fig. 1.5).
FIGURE 1.6: The vitreous base located 2-3 mm anterior and posterior to the ora serrata. The vitreous is firmly joined to the surrounding tissues.
BASE OF THE VITREOUS
The base of the vitreous body forms the tightest junction between the vitreous and the surrounding tissues. It is a circumferential area, 3-6 mm wide, that is joined to the pars plana and the peripheral retina and overlies the ora serrata, extending 2-3 mm behind it (Fig. 1.6).
SCLEROTOMY
Given the eye's anatomical features, a sclerotomy should not be performed less than 2.5 mm from the limbus (to avoid touching the pars plicata leading to intense bleeding) or more than 5 mm from the limbus (to avoid the ora serrata mainly in nasal quadrants with the risk of causing iatrogenic retinal detachment).
FIGURE 1.8: Appearance of the sclerotomy and its relationship with the ora serrata and the crystalline lens.
Thus, a sclerotomy6,7 for vitrectomy should be performed 4 mm from the limbus in phakic eyes and 3.5 mm from the limbus in aphakic or pseudophakic eyes to avoid the pars plicata anteriorly and the insertion of the vitreous base posteriorly (Figs 1.7 and 1.8).
In children under the age of 7, sclerotomies are undertaken at 2-3 mm because the pars plana is not fully developed and therefore narrower than in the adult.
REFERENCES
- Salzman M. Anatomy and histology of the human eyeball in its normal state. Chicago, University of Chicago Press, 1912;3–10.
- Greve MD. Vitreoretinal surgical anatomy. In: Peyman, Meffert, Conway, Chou, (Eds). vitreoretinal surgical techniques. Martin Dunitz, United Kingdom, 2001;2–6.
- Spencer WH, Vitreous. In: Spencer WH, (Ed). Ophthalmic pathology, and atlas and textbook, Vol 2, WB Saunders, Philadelphia, 1985;548.
- Michels RG, Retinal Detachment. The CV Mosby, St Louis, 1990;10–14.
- Charles S. Vitreous Microsurgery, 2nd edn, Williams and Wilkins Baltimore, 1987;68–69.
- Freeman HM, Tolentino FI. Atlas of vitreoretinal surgery. Thieme Medical Publishers New York, 1990;48–50.