The neurofibrovascular bundle of the inferior oblique muscle as its ancillary origin

Abstract

Purpose: To establish that the neurofibrovascular bundle (NFVB) of the inferior oblique muscle (IO) has ligamentous qualities that enable it to function as an ancillary origin to the muscle. Also, to show that the NFVB does function as the ancillary origin for the IO muscle, particularly when recessing and anteriorly transposing its insertion.

Methods: Fresh (no formaldehyde preservative) cadaver and patient eyes were studied anatomically, histologically, and physiologically. Eighteen orbits were dissected to isolate the IO, the inferior rectus (IR), and the NFVB to demonstrate the linear course of the NFVB and its adjacent fibrous bands. The shape of the muscle was documented. Coronal sections of the two whole, intact orbits were analyzed histologically. Light and electron microscopic sections of an autopsy specimen and a surgical specimen were used to evaluate the capsule of the NFVB and the adjacent fibrous bands near the anterior portion of the NFVB and their attachment to the IR and IO muscle capsules. The elastic modulus was measured in six in situ and six in vitro cadaver NFVB specimens and in six in vivo surgical cases at the time of denervation of the NFVB. For additional comparison, four in vitro cadaver superior oblique tendons were similarly tested. Six eyes that developed recurrent IO overaction following an anterior transposition procedure were surgically explored to determine what structure was serving as its ancillary origin.

Results: Gross anatomic and microscopic studies showed a linear orientation of the NFVB with adjacent fibrous bands anteriorly joining the IO and IR muscle capsules. The surgical specimens of the anterior portion of the NFVB show about 50% nerve and 50% fibrocollagenous capsule with the collagen fibers aligned parallel to the NFVB. The elastic modulus was highest (stiffest) in surgical specimens of the NFVB and in situ cadaver NFVB, followed by in vitro cadaver NFVB and, finally, in vitro cadaver superior oblique tendon. In patients who have undergone anterior transposition surgery, the NFVB served as the ancillary origin of the IO.

Conclusions: The name of the neurovascular bundle should be changed to the NFVB, since it has a prominent fibrocollagenous capsule and it is encased in fibrous tissue bands anteriorly. The NFVB has a linear course in the orbit from the apex to the IO muscle and is relatively stiff. The associated fibrous band extends posteriorly from the IO muscle capsule, encasing the nerve anteriorly and attaching 3 to 7 mm posteriorly into the capsule of the IR. The NFVB binds the mid posterior portion of the IO posteriorly. Its ligamentous qualities enable the NFVB to function as an ancillary origin for the IO.