Purpose
To analyze the medial canthal tendon and to clarify the true anatomic nature of the posterior limb of this tendon.
Design
Observational anatomic study.
Methods
Eleven postmortem eyelids of 9 Asian cadavers (6 right and 5 left eyes; age average, 77.2 years) were analyzed. Axial sections in parallel to the eyelid margin starting at 1 mm above the upper eyelid margin were made. The sliced specimens were dehydrated and embedded in paraffin, cut into 7-μm thickness sections, and stained with Masson trichrome. To demonstrate the hardness felt when the Horner muscle is pulled, 3 additional postmortem eyelids of 2 Asians (2 right and 1 left eyes; age, 70 and 75 years at death) were analyzed. The pulling process was documented with a video camera.
Results
The posterior limb of the medial canthal tendon was not detected in any of the specimens. The Horner muscle originated via its tendon from the posterior lacrimal crest and the anterior area of the medial orbital wall. The lacrimal diaphragm around the posterior lacrimal crest ran almost parallel to the Horner muscle and usually was difficult to distinguish from the tendon of the Horner muscle. The medial check ligament supported the posterior aspect of the Horner muscle and was inserted into the medial orbital wall. The hard sensation that was felt when the Horner muscle was pulled was demonstrated in the video.
Conclusions
The posterior limb of the medial canthal tendon was not detected in any of the specimens. This anatomic structure seems to be a part of the Horner muscle.
The medial canthal tendon is regarded as a 2-limb (anterior and posterior) structure. Although the anterior limb is considered the main structure of the medial canthal tendon, it is actually the posterior component that is thought to be the pivotal structure that supports the medial canthus and allows apposition of the eyelid to the eyeball. However, the anatomy of the medial canthal area remains controversial, and there are a number of different anatomic models of this area.
It was previously stated that the posterior limb of the medial canthal tendon is located in front of the Horner muscle and is connected the posterior lacrimal crest and the tarsal plate. The posterior limb is less conspicuous than the anterior limb, and the Horner muscle was sometimes regarded as the posterior limb of the medial canthal tendon. Although the anterior limb of the medial canthal tendon is well documented, the actual existence of this posterior limb is uncertain.
The medial aspect of the tarsal plate is not supported by the anterior or the posterior limbs of the medial canthal tendon, but rather by the Horner muscle and the medial rectus capsulopalpebral fascia, which links the medial rectus muscle fascia and the medial aspect of the tarsal plate. The anterior limb of the medial canthal tendon influences medial canthal fixation through the pretarsal orbicularis oculi muscle from the anterior surface of the tarsal plate. The contribution of the posterior limb was not referred to, however, in relation to the medial canthal support mechanism. The purpose of the present study was to analyze the anatomic features of the medial canthal tendon and to clarify the true nature of the posterior limb of this tendon.
Methods
We analyzed full-thickness sections of eyelids fixed in 10% buffered formalin. The study included 11 postmortem eyelids of 9 Asians (6 right and 5 left eyes; age range, 68 to 92 years at death; average age, 77.2 years). None of the cadavers had any history or clinical evidence of a previous eyelid or orbital trauma, surgery, tumor, Graves’ orbitopathy, cerebral nerve palsy, strabismus, or any other periocular pathologic features.
The cadavers were used after obtaining the appropriate consents and approvals, and all cadavers were registered with the cadaveric service of Aichi Medical University. All methods for securing human tissue were humane and complied with the tenets of the Declaration of Helsinki.
We removed the orbital contents aided by binocular loupes (high-resolution prismatic HRP ×2.5, 340 mm/13 inches; Heine, Herrsching, Germany). After performing a full-thickness 360-degree incision of the periosteum around the circumference of the orbit, the periosteum was elevated and finally detached near the orbital apex. Nerves, blood vessels, and the nasolacrimal duct arising from the orbital wall were cut. The lateral orbital wall then was removed at approximately 3 cm posterior to the orbital rim and the retrobulbar content was incised with a sharp scalpel in a coronal plane. The removed orbital content was incised perpendicularly at the center of the eyelids and the medial half was used for analysis.
We then made axial sections in parallel to the eyelid margin. The sections started at 1 mm above the upper eyelid margin. The sliced specimens were dehydrated, embedded in paraffin, and then cut into 7-μm thickness sections. The sliced sections then were stained with Masson trichrome. Microscopic photographs were obtained with a digital camera system attached to the microscope (Moticam 2000; Shimadzu Rika Kikai, Tokyo, Japan).
To demonstrate the hardness felt when the Horner muscle is pulled, we analyzed an additional 3 postmortem eyelids of 2 Asians (2 right and 1 left eyes; age, 70 and 75 years at death) fixed in 10% buffered formalin. The Horner muscle and its attaching tarsal plate were exposed, while all the other orbital soft tissues, including the fibrous structure on the anterior and the posterior surface of the Horner muscle, were removed completely. The pulling process was documented with a video camera (Handycam DCR-PC300K; Sony, Tokyo, Japan).
Results
The posterior limb of the medial canthal tendon was not detected in any of the specimens ( Figures 1 and 2 ). In all specimens, the Horner muscle originated via its tendon from the periosteum around the posterior lacrimal crest and the anterior area of the medial orbital wall ( Figures 1 and 2 ). The lacrimal diaphragm around the posterior lacrimal crest ran almost parallel to the Horner muscle and usually was difficult to distinguish from the tendon of the Horner muscle ( Figures 1 and 2 ). The most anterior part of the Horner muscle attached to the lacrimal diaphragm ( Figures 1 and 2 ). The medial check ligament supported the posterior aspect of the Horner muscle and inserted into the medial orbital wall through the periosteum ( Figures 1 and 2 ). A hard sensation was felt when the Horner muscle was pulled (see the video clip in Supplemental Material at AJO.com ).
