The use of ultrasonography in determining the amount of extraocular muscle surgery in strabismus.

In twenty strabismus patients using ultrasonography a measurement was taken from the apex of the orbit through the centre of rotation of the eye to the front of the eye along the oculomotor axis which is at an angle of approximately 10 degrees to the anteroposterior axis of the eye. From this and from the diameter of the eye average values were derived for the distance from the equator of the medial rectus insertion of 6.27 mm (range 3.96-8. 92 mm) the lateral rectus insertion 8.77 mm, the length of the medial rectus 35.32 mm (range 32. 40-40. 25 mm) and the lateral rectus 37.82 mm (range 34. 79-42. 75 mm). There are sources of error in these measurements but they are probably not great, though corneal diameter and muscle position should be checked at operation. In addition a recession/muscle length ratio may be calculated and it is probably wise not to exceed a ratio of of 0.20 for the medial rectus and 0.25 for the lateral rectus with similar resection muscle length ratios suggested. The variability of the distance of the muscle insertions from the equator is greater than the muscle lengths and it seems important not to recess beyond the equator while alteration of muscle length may be less important. This suggests maximal recession of the medial rectus may vary from 4 up to almost 9 mm and from 6.5 to almost 11.5 mm for the lateral rectus. The value of making these measurements pre-operatively seems obvious and the considerable variation is the amount of squint surgery is stressed, particularly for muscle recessions.