Horizontal squint surgery. The mathematical approach.

STANDARDIZATION of the surgical treatment of strabismus has long been the ambition of surgeons. Even for the most experienced the number of millimetres of recession or resection to perform is always a difficult decision. Because of individual variation in response to similar surgical procedures, Cooper (1961) believed it difficult to predict the result. Knapp (1958) considered accurate estimation impracticable, and Scobee (1952) and Stallard (1965) regarded it as impossible. The general principles of muscle surgery are discussed often enough, but precise detail regarding the amount of muscle to move or remove is frequently omitted and the decision is left to the surgeon's skill and experience in each individual case. Lyle (1950), Scobee (1952), Castanera Pueyo (1958), and Cooper (1961), who have all described the conditions that may influence the result of the operation, vary considerably in their estimation of the relationship between the amount of recession or resection and the degree of correction of squint. Lyle (1950) stated that a 5 mm. recession of the medial rectus produced an average final reduction of between 70 and 120, and that the average effect of resection of the lateral rectus "is to reduce the angle of deviation by about 10°", adding that "it is usually combined with either recession or tenotomy of the medial rectus, and then the combined effect is about 25 per cent. greater than that of each operation separately". Castanera Pueyo (1958) stated that for each millimetre of recession of the medial rectus there should be a correction of 30, but for each millimetre of resection of the lateral rectus only 10 or even less. Stallard (1965) held that roughly 1 mm. of recession of the medial rectus corrected about 40 of strabismus and 1 mm. of adjustment to the lateral rectus about 20. Among the most interesting attempts to standardize the surgical treatment of horizontal strabismus, reference must be made to the work of Urist (1951). His surgery, as he states, "is theoretically based on the concept that the position of the eyes is determined by a state of balance between convergence and divergence innervations", and his operative procedure is predetermined by the amount of deviation present for distance vision and upward gaze compared with that of near vision and downward gaze.