Tenon's capsule is a structure with definite body and substance in childhood which gradually atrophies in old age but not to the same degree as conjunctiva. Tenon's capsule has an anterior and posterior part. Anterior Tenon's capsule is the vestigial capsulopalpebral head of the rectus muscles. This covers the anterior half to two-thirds of the rectus muscles in their sheaths as well as the intermuscular membrane. Anterior Tenon's capsule is fused with the undersurface of conjunctiva and attaches to sclera at the limbus. The fused conjunctiva-anterior Tenon's capsule is movable over underlying posterior Tenon's capsule and episclera, the latter being the anterior extension of posterior Tenon's capsule. Episclera starts at the level of the insertion of the rectus muscles in a line around the globe, which is called the spiral of Tillaux. Episclera joins conjunctiva and anterior Tenon's capsule, fusing at the limbus.
Posterior Tenon's capsule is made up of the fibrous sheath of the rectus muscles together with the intermuscular membrane. According to Lester Jones, the tissues that make up posterior Tenon's capsule form at a later evolutionary stage than those forming anterior Tenon's capsule. Fibrous attachments between the inner surface of anterior Tenon's capsule and the outer muscle sheath (part of posterior Tenon's capsule) fuse at a point 15 to 20 mm behind the insertion of the medial and lateral rectus muscles to form a barrier to extraconal fat. A condensation of fibrous tissue and smooth muscle between the outer surface of anterior Tenon's capsule and the orbital wall medially and laterally is the location of the aforementioned pulleys of the horizontal rectus muscles. If the horizontal rectus muscle is separated completely from anterior Tenon's capsule, exposing extraconal fat, there will be no or reduced pulley effect on the eye muscle. This will result in up and down ‘slip’ of the muscle relative to the globe. It is not practical or even logical in the usual strabismus surgery to free pulleys outside anterior Tenon's capsule, but this could be done for special need. Eye muscle surgery is routinely performed entirely inside anterior Tenon's capsule with no fat exposure (Figure 25 A-C).
Figure 25 The Conjunctiva/Tenon’s Capsule Relationships
A Axial view of the orbit
1 Wall of the orbit
3 Anterior Tenon’s capsule
4 Posterior Tenon’s capsule
5 The muscle
6 Intermuscular membrane (posterior Tenon’s capsule)
7 Intraconal orbital fat
8 Extraconal orbital fat
9 Horizontal pulley
1 The limbal fusion of the conjunctiva and anterior Tenon’s capsule
2 Potential space between anterior
Tenon’s capsule and episclera
3 The muscle in its sheath (posterior Tenon’s capsule) inserting into the sclera
4 Postinsertional muscle footplates
7 Anterior Tenon’s capsule
|C Coronal section of B at X|
2 Anterior Tenon’s capsule
3 Muscle sheath
4 Extraocular muscle
5 Intermuscular membrane
6 Sclera substance
Posterior Tenon's capsule, composed of the muscle's capsule and the intermuscular membrane, unites the rectus muscles in a ring around the globe. The extent to which the intermuscular membrane is cut during surgery influences how far the rectus muscles, particularly the medial and to some extent the lateral, will retract during surgery. Dissection of posterior Tenon's capsule far posteriorly leads to exposure of intraconal fat, so called because it resides inside the muscle cone. Excessive dissection of anterior Tenon's capsule exposes extraconal fat and risks disruption of the pulleys of the medial and lateral rectus muscles.
|A When the layer of fused conjunctiva-anterior Tenon's capsule is retracted, the muscle insertion in its sheath is exposed. Fibrous attachments are seen between the undersurface of anterior Tenon’s capsule and the outer surface of the muscle. The fusion of the intermuscular membrane (posterior Tenon's capsule), as well as of the muscle to the sclera, is apparent. This fusion of the intermuscular membrane to the sclera must be incised before the bare sclera and subposterior Tenon's capsule space can be encountered. Only after entering subposterior Tenon's capsule space can the insertion of the rectus muscle be engaged cleanly on a muscle hook. This is the ‘free space’ used by the retina surgeon. The tip of the scissors in the photo points to this ‘free space.’
||B Posterior Tenon’s capsule attaches to sclera at the muscle’s insertion and in the intermuscular space forming the spiral of Tillaux.|
C The muscle hook is placed in a ‘hole’ created in intermuscular membrane adjacent to the muscle insertion and glides along bare sclera behind the rectus muscle insertion and is exposed at the opposite muscle border with a snip incision.
D The muscle hook is placed in a ‘hole’ created in intermuscular membrane adjacent to the muscle insertion and glides along bare sclera behind the rectus muscle insertion and is exposed at the opposite muscle border with a snip incision.With a limbal incision, the multiple layers and surfaces associated with the rectus muscles can be readily seen. Conjunctiva and anterior Tenon’s capsule shown here separated are actually fused and separated only with difficulty.
While extraocular muscle surgery is performed beneath anterior Tenon's capsule, it is done within the plane of posterior Tenon's capsule. The intermuscular membrane part of posterior Tenon's capsule must be fenestrated in order to place a muscle hook behind the insertion of a rectus muscle (Figure 26 A-D). How much more dissection is done in the intermuscular membrane beyond the minimum required to gain access to the muscle is the decision of the surgeon. It is probably wise to do as little cutting of posterior Tenon's capsule as is compatible with the conduct of the surgical procedure intended. Retinal detachment surgery, in contrast to extraocular muscle surgery, is carried out beneath posterior Tenon's capsule. This enables a view of the scleral surface far posteriorly to a point near the posterior ciliary vessels and the optic nerve.