Medial Canthus

    The medial canthus involves the integration of the pretarsal and preseptal orbicularis oculi, the septum orbitale, the medial end of Lockwood's ligament, the medial horn of the levator aponeurosis and the check ligament of the medial rectus muscle. The precise anatomical arrangement of the various components is debated, but in practice they are not individually identified at operation. These structures attach to the medial orbital wall via the medial canthal tendon, which is intimately associated with the lacrimal sac.
    The pretarsal orbicularis muscle inserts medially by a superficial head and a deep head. The superficial head from each lid blends with the fibrous continuation of the tarsal plates to form the anterior part of the medial canthal tendon. The deep head from each lid is also known as the pars lacrimalis, or Horner's muscle. Its fibers begin at the medial end of the tarsal plates and insert into the posterior lacrimal crest just behind the lacrimal sac. The preseptal muscle also inserts medially by a superficial and deep head. The superficial head from each lid inserts into the upper and lower borders of the medial canthal tendon. The deep heads insert into the fascia overlying the lacrimal sac and the medial orbital wall above and below Horner's muscle. Closing the eyelids leads to traction on the deep heads which pulls the lacrimal sac fascia laterally, producing a negative internal pressure.


    The medial canthal tendon inserts into the frontal process of the maxilla in a tripartite manner: anterior and posterior horizontal elements and a vertical clement. The anterior horizontal insertion point is onto the anterior lacrimal crest, level with the upper part of the lacrimal sac. The posterior part leaves the deep surface just before the anterior lacrimal crest and inserts into the posterior lacrimal crest behind the sac. The tendon has a definite inferior margin but the superior border blends with the periosteum having vertically orientated fibers that insert into an ill-defined portion of the medial orbital rim at or close to the nasofrontal suture. This vertical component of the medial canthal tendon is thought to be responsible for suspension and fixation of the medial canthus, while the horizontal components are relatively weak and contribute little to medial canthal stability.