When and how to strengthen the superior oblique muscle

To review the history of procedures used to strengthen the action of the superior oblique (SO) muscle and methods of quantifying surgical dosage and to determine the relationship between congenital onset and tendon laxity measured at the time of surgery.

We reviewed medical records over a 10-year period of 30 patients who had undergone SO tendon tuck for SO muscle palsy using intraoperative assessment of forced ductions to determine surgical dosage. We also designed and tested a modified Bishop tucker, which can simultaneously measure tendon shortening and the force required. This allowed development of length tension curves between 0 and 200 g for individual SO tendons in patients with and without evidence of muscle palsy.

In distinction to most other procedures on the extraocular muscles, intraoperative forced ductions are used to determine appropriate surgical dosage. Patients undergoing SO tendon tuck using a uniform and repeatable forced duction method receive a greater amount of tuck (mean, 3 mm) when there is known congenital onset. Patients with congenital SO muscle palsy received a mean tuck of 10.8 mm (range, 8-16 mm), whereas patients with equivocal or known adult-onset SO palsy received a mean of 7.8 mm (range, 4-12 mm; p = 0.002).

Patients with congenital SO muscle palsy have increased tendon laxity when measured directly during SO tendon tuck. The excursion of presumed normal SO tendons through the trochlea is variable and may be less than previously thought. Modifying the Bishop tucker to provide length and tension data provides information that may be useful in determining surgical dosage.