An anatomical structure resembling a pully is known as a trochlea; "trochlea" is a Greek word, the English meaning of the trochlea is pully.
In the human body, many pulley-like structures are known as a trochlea; for example, the medial part of an articular area at the lower end of the humerus is called the trochlea. A ligamentous structure attached to the hyoid bone acts as pully for the digastric muscle. The superior surface of the talus is also known as the trochlear surface.
The trochlea is a spine-like structure present at the medial angle of the orbit; the trochlea is a part of the orbital surface of the frontal bone. The orbital part of the frontal bone forms roof of the orbit and ethmoidal sinus.
The supratrochlear nerve and tendon of the superior oblique muscle pass through the trochlea, while the supratrochlear artery passes posterior to the trochlea.
The development of trochlea is not clear; recent studies describe the trochlea developing with superior oblique muscle tendon. It moves with the tendon and then develops as the fibrocartilaginous pully at 12 weeks of gestational age. One hypothesis describes the trochlea potentially originating from the common anlage with the sclera.
Supratrochlear artery (Image 2): The supratrochlear artery is a terminal branch of the ophthalmic artery (intracranial branch of the internal carotid artery).
Course: After emerging from the ophthalmic artery, the supratrochlear artery crosses trochlea along with the supratrochlear nerve; then, it crosses the supraorbital margin, pierces the orbital septum and enters the forehead. In the forehead, it runs between corrugator supercilii, orbicularis oculi, and the frontalis muscle. Above the level of the mid-forehead, it pierces frontalis muscle and becomes subcutaneous.
Branches and area of distribution: the bony branches of the supratrochlear artery supply the periosteum of the supraorbital rim and glabella. Muscular branches supply the muscle of the upper eyelid, corrugator supercilii, frontalis, and procerus muscle. Cutaneous branches supply the glabellar skin and skin of the medial forehead.
A study by Kleintjes describes nine branches of the supratrochlear artery:(i) the medial communicating branch that communicates with the contralateral artery, (ii) the lateral communicating branch that communicates with the supraorbital artery, (iii) Superior palpebral artery supplying the upper eyelid, (iv) branch to the brow, (v) periosteal branch, (vi) cutaneous branch supplying the skin of the glabella and median forehead, (vii) oblique branch, (viii) a vertical branch, (ix) medial and lateral vertical branches.
Supratrochlear vein: The supratrochlear vein drain the medial forehead, glabellar complex, and periosteum of the medial part of the supraorbital margin. It divides into two branches; one branch accompanies the supratrochlear artery, another branch unites with the supraorbital vein, and forms the angular vein.
Supratrochlear nerve: The supratrochlear nerve is a branch of the frontal nerve (branch of the ophthalmic division of the trigeminal nerve).
Course: The supratrochlear nerve passes forward relative to levator palpebrae superioris muscle; turns medially above the trochlea. Here it gives a descending branch to the infratrochlear nerve then enters into the forehead through the frontal notch. Once it exits the foramen, the supratrochlear nerve ascends to the forehead and runs close to the frontalis and corrugator supercilia muscles. At this junction, it divides and supplies the lower skin of the forehead next to the midline, conjunctiva, and the skin of the upper eyelid.
Branches and area of distribution: The descending branch of the supratrochlear nerve combine with the infratrochlear branch of the nasociliary nerve. The supratrochlear nerve gives sensory branches to the cornea, conjunctiva, the skin of the upper eyelid, the bridge of the nose, and the skin of the forehead. The supratrochlear nerve carries touch, pain, and temperature sensation from the area of the distribution.
The superior oblique muscle attaches to the trochlea, where it changes direction to insert on the eyeball. The origin of the superior oblique muscle from the body of the sphenoid near the margin of the optic canal. It changes direction at the trochlea. Then it runs backward and laterally on the superior surface of the eyeball behind the equator.
The action of the superior oblique muscle is intorsion, depression, and abduction of the eyeball.
Robert et al. reported that in 36% of cases, the supratrochlear nerve originated from the frontal nerve at the distal half of the orbit, while in 64% of cases, the supratrochlear nerve origin was from the frontal nerve at the proximal half the orbit. The supratrochlear nerve may exit the orbit through the supraorbital notch/foramen along with the supraorbital nerve. The supratrochlear nerve gives branches at 4 to 10 mm distance from the supraorbital rim; sometimes, it does not divide. In some cases, the supratrochlear nerve was absent, while in some instances an accessory supratrochlear nerve is also present.
The supratrochlear artery branches from the ophthalmic artery in 85 to 90% cases, while in 10 to 15% of cases, the supratrochlear artery and the supraorbital artery emerge as a single branch from the ophthalmic artery and then divide into supratrochlear and supraorbital arteries. In some cases, the supratrochlear artery is absent, where the paracentral artery may replace it. The branching pattern of the supratrochlear artery also has variation; in most cases, it divides into superficial and deep branches, whereas in some cases, the deep branch was not present. The supratrochlear artery also shows a variation in the number of branches.
For the midline forehead flap, the supratrochlear artery and its branches play a critical role along with the supraorbital artery. The supratrochlear artery forms anastomoses with the contralateral supratrochlear artery, the supraorbital artery, and the superficial temporal artery. Because of the rich vascularization, the skin supplied by the supratrochlear artery is often used as a skin graft/pedicle to repair the defect at the bridge of the nose or upper facial defect also.
Fracture of the roof of the orbit may lead to injury of the trochlea, which complicates as a compromised action of the superior oblique muscle; in severe injury, the position of the eyeball is also affected, leading to exophthalmos or enophthalmos.
The supratrochlear nerve block is performed in combination with a supraorbital nerve block as regional anesthesia, local infiltration, or field anesthesia for procedures such as repairing of scalp lacerations, craniotomies, and scalp lesion excision. Between 2 and 3 ml of 1% lidocaine is injected 1 cm medial to the supraorbital foramen to block the supratrochlear nerve.
The supratrochlear nerve may become entrapped at the supraorbital margin, supraorbital foramen, or within the corrugator supercilii muscle; the condition may present as chronic frontal migraine. The nerve is relieved by endoscopically or by injecting the botulinum toxin to relieve the migraine.
Embolia Cutis Medicamentosa (ECS) is a phenomenon following an intra-arterial filler injection. The drug or fluid may accidentally enter the supratrochlear or supraorbital artery while injecting the glabellar skin. Injecting fluid in the supratrochlear artery has been found as one of the causes of embolism of the ophthalmic artery and central artery of the retina; embolism of the central artery of the retina may lead to blindness.
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