Tarsus And Septum Orbitale

    The tarsal plates are crescentic-shaped, dense condensations of connective tissue that maintain the structural integrity of the eyelids while accommodating the contour of the globe. The upper tarsus is approximately 29 mm long and extends from the lateral commissure to the punctum medially. It is 10 mm wide in the central eyelid, narrowing medially and laterally. The lower tarsus is a similar length to that of the upper but is only 4—5 mm wide at its center. The septum orbitale, or orbital septum, consists of a thin fibroelas-tic membrane of varying consistency that is notably non-distensible adjacent to the inferolateral orbital rim. It extends from the bony margin towards the tarsus and represents the continuation of the orbital periosteum. At the junction where the periosteum and septum fuse along the orbital rim is a dense white fibrous band termed the arcus marginalis. The arcus marginalis is clearly evident inferomedially where the rim has a sharp edge, whereas it is less defined on the rounded inferolateral rim.

    In the upper eyelid, the septum attaches to the levator aponeurosis, generally 2—5 mm above the superior edge of the tarsal plate.lj However, this septal-levator attachment can vary from the level of the superior tarsal border to almost 10 mm above. Below this level there is a blending of the connective tissue lamellae from both the septum and the levator aponeurosis called the prctarsal extension of the levator or the conjoined fascia. This fascia continues inferiorly to attach to the lower third of the anterior surface of the tarsal plate and sends fibrous slips forward to invest fascicles of the pretarsal orbicularis. In the lower lid, the septum similarly attaches to the capsulopalpebral fascia (which is the equivalent of the levator aponeurosis of the upper lid) below the inferior edge of the tarsus.
This line of fusion between the two structures occurs in an oblique direction approximately 5 mm from the tarsal plate medially and 10 mm from the plate laterally. It is usually visible on operative dissection as a thickened white line in the septum that expands out laterally as it attaches to the inferolateral orbital rim by a triangular fascial band known as the arcuate expansion. The lower lid septum can therefore be considered as being divided into an upper part, reinforced by the capsulopalpebral fascia on its deep surface, and a lower unsupported part. Orbital fat distension and bulging of this relatively weak lower septum orbitale results in the characteristic lower lid fat bags of aging.

    The septum attaches medially to the spine at the lower end of the anterior lacrimal crest, called the lacrimal tubercle. It then extends from the lower eyelid to the upper eyelid medially by passing behind the attachments of the medial orbicularis at the posterior lacrimal crest. Superomedially the arcus marginalis forms the inferior portion of the supraorbital groove and laterally it blends with the lateral canthal raphe.