STIFFNESS OF THE INFERIOR OBLIQUE NEUROFIBROVASCULAR BUNDLE

Purpose. To assess the mechanical ability of the inferior oblique neur ofibrovascular bundle (NFVB) to act as an ancillary origin for the inf erior oblique muscle after anterior transposition. Methods. Stress-str ain relations and Young's modulus of elasticity, a measure of tissue s tiffness, were determined for the NFVB in vitro, in situ, and in vivo in dynamic and static conditions. For comparison, similar studies were performed in vitro on the superior oblique tendon (SOT). Results. You ng's moduli for NFVB in situ (6.3 MPa [megapascals]) and in vivo (11.8 MPa) were approximately 2 and 4 times greater (P < 0.05), respectivel y, than those of isolated NFVB in vitro at 5% to 10%, dynamic strain ( 3 MPa). In dynamic conditions, Young's moduli in vitro for the NFVB an d the SOT were similar. Conclusions. The NFVB is a biomaterial that ha s stiffness properties similar to the SOT. Within the range of forces typical of normal eye movements (79 to 393 mN), the NFVB alone can tol erate forces of 98 mN at 0% to 10% strain and 393 mN at 15% to 20% str ain based on dynamic in vitro analysis. The greater measured stiffness in situ and in vivo suggest that the NFVB in the intact orbit potenti ally has a resting strain of 15% to 20%, and additional tissues in par allel with the NFVB also contribute to total stiffness. These data sup port the hypothesis that the NFVB, acting alone or in concert with adj acent orbital tissues, may form an ancillary origin for the inferior o blique muscle after anterior transposition.