(1)
St. Johns, FL, USA
(2)
Helen Keller Foundation for Research and Education, International Society of Ocular Trauma, Birmingham, AL, USA
(3)
Consultant and Vitreoretinal Surgeon, Milos Eye Hospital, Belgrade, Serbia
(4)
Consultant and Vitreoretinal Surgeon, Zagórskiego Eye Hospital, Cracow, Poland
26.1 Internal Ocular Anatomy and Physiology1
The vitreous, bordered anteriorly by the lens, the zonular apparatus, and the ciliary body and posteriorly by the retina and optic disc, constitutes the largest volume2 (~4 ml) of the eye.
26.1.1 Vitreous Macroanatomy
The vitreous base 3 is a several mm thick, three-dimensional ring, extending up to 2 mm anteriorly and up to 3 mm posteriorly from the ora serrata.
The collagen fibers of the vitreous are interconnected with those of the retina here, making the separation of the two tissues impossible.
Weigert’s ligament is a disc, 8–9 mm in diameter, connecting the gel to the posterior lens capsule. Only in the epicenter is a small space left between the two tissues (Berger’s space).
Weiss ring is a condensation of the vitreous gel’s collagen fibers at the margin of the optic disc. If it detaches, it becomes visible in the vitreous cavity as mobile, truly ringlike structure (see Fig. 27.3).
The outermost part of the gel is called vitreous cortex, consisting of densely packed collagen fibers. The anterior part (anterior hyaloid membrane/face) is located anterior to the vitreous base; the part posterior to it is called the posterior cortical vitreous. The anterior cortex is 2 μ thick; the posterior is 100 μ. There is no cortex over the optic disc.
The anterior hyaloid face adheres to but is not interconnected with the posterior lens capsule.
The posterior hyaloid face is also adherent to (typically not interconnected with) the posterior retina, but is glued to it by an extracellular matrix.
The posterior adherence is stronger than elsewhere at the margin of the macula or the parafoveal area, along the major blood vessels, in areas of certain retinal degenerations,4 and especially at the margin of the optic disc (Weiss ring; see above).
Both the anterior (to the lens) and posterior (to the retina) adherences weaken with age, but pathologic connections may develop posteriorly at the sites of chorioretinal scars, which can be caused by various diseases, injuries, or even overly strong laser spots.
The premacular bursa 5 is an optically empty, fluid-only space measuring 7 mm in width and 0.6 mm axially, which also connects to the area of Martegiani6 in front of the optic disc. The superior extension of the premacular bursa fuses with Cloquet’s canal and courses through the vitreous, terminating behind the lens.7
26.1.2 Vitreous Biochemistry and Its Anatomical and Functional Implications
The vitreous is composed of ~98% water; the rest is made up of collagen fibers (mostly, but not exclusively, type II), hyaluronan, and many other molecules such as chondroitin sulfate, as well as a relatively small number cells (hyalocytes and fibroblasts, see Table 26.1).
Table 26.1
Selected anatomical and functional features of the eyeball and their clinical implications*
Feature | Clinical implication |
|---|---|
AC depth | Primarily determined by the corneal contour but maintained by the aqueous, it quickly reforms if the corneal wound is not gaped. This is one of the reasons why a prolapsed iris should be pulled, not pushed, back into the AC |
Extraocular muscles insertion into the sclera | This is the line posterior to which the surgeon must be extremely careful not to penetrate the sclera with a needlea. The difficulty of the suture placement is due to thinness of the sclera and to the curvatures of the sclera and the needle mirroring, not mimicking, each other |
ILM | This is the only part of the retina that is inelastic, which explains the high success rate of ILM peeling in eyes with a posterior RD in a highly myopic eye. The ILM also provides a scaffold on which cells can proliferate – hence the sparing of the ILM-denuded area if reproliferation occurs in PVR and the lack of EMP recurrence after ILM peeling |
IPM | The glue between the neuroretina and the RPE does not reform intraoperatively. If, during PPV, the retina is reattached by F-A-X but then the BSS is reinjected, the retina will redetach again in the area of the previous detachment |
Long posterior ciliary nerve | To prevent damaging the nerve and thus cause iatrogenic mydriasis, fewer and lighter spots during laser cerclage should be delivered in the horizontal meridians |
Macula | Traction on the macula by an anomalous PVD gives host to numerous conditions ranging from VMTS to edema |
Optic disc | Over 100 million nerve fibers are packed into a very small areab; this is where the surgeon can do the most damage if he is not careful. An obvious example is diathermy for a bleeding vessel in PDR: sufficient distance must be kept from the disc and the power of the diathermy lowered to the minimum |
Pars plana | The external anatomical landmarks are important to remember since this is the only safe area through which the vitreous cavity can be accessed |
Pars plicata | It is crucial to be cleansed of vitreous, fibrin, membranes, capsular remnants etc. in eyes with severe trauma or proliferation (PVR, PDR) |
Posterior pole | The most valuable part of the retina. The surgeon may need to sacrifice the more peripheral retinac for it in diseases such as (recurring) PVR |
Premacular bursa | A structure that is not directly visible to the surgeon intraoperatively. Preoperatively, it may be demonstrated by OCT |
PVD | A very often misused term, referring to the separation of the posterior hyaloid face from the retina. In truth, the preoperative diagnosis is unreliable (see vitreoschisis below). Even intraoperatively, and even with the use of TA, what appears as a PVD may still be vitreoschisis if the inner surface of the posterior wall of the schisis cavity is too smooth for the crystals to stick to it. The preoperative diagnosis of “no PVD” is therefore always correct, while that of “PVD” may not be |
Retinal tear | An adherent vitreous is pulling on the retina with every move of the patient’s eyeball or head. A tug of war develops between this traction force vs the combined resistance of the RPE pump, the IPM, and the retina itself. It is the outcome of this struggle that determines whether a retinal tear results. Once a retinal break is formed, the risk of RD significantly increases unless the retinal area under traction is completely torn (operculum) |
Syneresis | The breaking down of the molecular structure of the vitreous gel, resulting in the presence of gel/fluid admixture in the vitreous cavity, is typically the first step in the development of an RD |
Vitreoschisis | Not removing the posterior wall of the schisis cavity can lead to several postoperative complications ranging from EMP to RD |
Vitreous base | Its significance lies in the fact that the vitreous here is inseparable from the peripheral retina. Even in a normal eye, the line of no-separation moves posteriorly as the person ages. Even in younger age, in certain pathologies such as RD, the surgeon often finds VR adhesion in a much wider area than the vitreous base itself That the vitreous cannot be separated from the retina at the vitreous base explains why truly 100% vitreous removal is impossible; at the vitreous base even when the VR surgeon refers to his action as “vitrectomy,” in reality he does “vitreotomy”: shaving the vitreous as much as possible, but still leaving a thin vitreous “skirt” behindd |
Vitreous cortex (posterior) | This structure is typically invisible to the surgeon intraoperatively, unless the vitreous is stained (ICG) or marked (TA). Preoperatively, it may or may not be demonstrated by ultrasonography or OCT |
Weigert’s ligament | The adhesion between the posterior capsule and anterior hyaloid face weakens with age. This explains why ICCE in a young person has disastrous consequences: the prolapsing anterior gel exerts traction on the vitreous base and thus on the peripheral retina |
Weiss ring | It is commonly assumed, even by experienced VR surgeons, that the presence of a Weiss ring corresponds to a PVD. In truth, the Weiss ring means only that the vitreous separated at the disc; the cortical vitreous may still be adherent to the retina elsewhere |
*Listed in alphabetical order. See the text and the appropriate chapters for more details.
aIf the eye undergoes scleral buckling, for instance.
b10 mm2.
cLike a pawn for the king in chess.
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