contralateral restriction of depression in abduction cannot fully account for the persistence of overdepression in adduction after partial posterior SO tenectomy, because we have seen this occur in the operated eye

14 after unilateral surgery. Also, we have observed that this finding is often present immediately after surgery. This would tend to rule out postoperative iatrogenic mechanical restriction in the contralateral eye as the cause. We do recognize, however, that since most SO weakening procedures are bilateral, both residual overdepression in adduction of the ipsilateral eye and limitation to depression in abduction of the contralateral eye could occur. Furthermore, these two conditions would be additive with respect to their e ect on versions in adduction.

We considered the anatomical e ects of the SO frenulum on the vertical and torsional force vectors of the SO tendon using basic two-dimensional trigonometry. We recognize that there are some obvious oversimplifications in our theoretical analysis. The geometric angles drawn on the scaled model are somewhat arbitrary. For example, our modeling of the anterior fibers of the unoperated SO tendon when the eye is adducted (see again Fig. 14.6b) assumes that the frenulum completely constrains the tendon. In reality, there is probably some elasticity of the frenulum that allows at least some forward slippage [6]. We assume this to be the case as common clinical observations confirm that the SO has a greater vertical and lesser torsional action in adduction than in the primary position. Nevertheless, prior investigation on the constraining e ect of the SO tendon frenulum suggests that our model is at least qualitatively sound, even if it is not exactly quantitatively accurate [6, 7]. In addition, we reduced a complex three-dimensional situation into a two-dimensional construct, and the abducting contribution of the SO tendon was ignored. We feel, however, that this would have minimal impact on our conclusions, as the abducting force of the SO muscle is relatively small. Thus, although the actual numbers we calculated are approximate, our qualitative analysis confirms what seems logical. Specifically, if we assume that the SO tendon is constrained by the frenulum in the primary and adducted fields of gaze, cutting the frenulum after a procedure such as a partial posterior tenectomy would collapse the angle the anterior fibers make with the anterior–posterior axis. This reduction in the angle makes the SO tendon a more e ective depressor in the adducted position. This may be an explanation for the residual overdepression in adduction in the ipsilateral eye after posterior partial tenectomy of the SO tendon.

Summary for Clinicians

■The SO frenulum is an important structure. How it is handled with superior rectus and SO surgery may a ect the surgical outcome.

■The frenulum should be severed for superior rectus recessions that exceed 10 mm, to allow for the desired recession e ect.

■The frenulum should be severed for all superior rectus resections to prevent the SO tendon incarceration syndrome.

■The frenulum should be left intact for superior rectus recessions that are less than 10 mm to prevent the SO tendon incarceration syndrome.

■With SO recessions using a suspension technique the handling of the frenulum is a matter of trade- o s. Severing the frenulum will involve a greater amount of recession, but may predispose to the SO tendon incarceration syndrome. Leaving the frenulum intact will prevent that restrictive strabismic syndrome but will limit the amount of recession obtained. Asymmetric handling of the frenulum with bilateral SO recession may predispose to an asymmetric response.

■The posterior tenectomy operation of the SO is e ective in collapsing up to 20 PD of A pattern but is less e ective in eliminating the overdepression in adduction.

References

1.Jampolsky A (1981) Superior rectus revisited. Tr Am Ophth Soc 79:233

2.Kushner BJ (2007) Superior oblique tendon incarceration syndrome. Arch Ophthalmol 125:1070–1076

3.Prieto-Diaz J (1988) Management of superior oblique overaction in A-pattern deviations. Graefes Arch Clin Exp Ophthalmol 226:126–131

4.Prieto-Diaz J (1989) Superior oblique overaction. Int Ophthalmol Clin 29:43–50

5.Castanera de Molina A, Fabiani R, Giner MG (1998) Downshoot in infra-adduction following selected superior oblique surgical weakening procedures for A-pattern strabismus. Binocul Vis Strabismus Q 13:17–28

6.Iizuka M, Kushner B (2008) Surgical implications of the superior oblique frenulum. J AAPOS 12:27–32

7.Jampolsky A (1986) Management of vertical strabismus. Symposium on pediatric ophthalmology: transactions of the new Orleans acad ophthalmol. Raven, New York, pp 141–171

8.Prieto-Diaz J (1996) Selective and moderated weakening of the superior oblique muscle. Memorias del IV Congresso del Consejo Latinoamericano de Estrabismus. Mayo, Buenos Aires, pp. 535–541

9.Harada M, Ito Y (1964) Surgical correction of cyclotropia. Jap J Ophthalmol 8:88–96

10.Prieto-Diaz J (1976) Tenectomia parcial posterior del oblicuo superior. Arch Oftalmol B Aires 51:267–271

11.Prieto-Diaz J (1979) Poseterior partial tenectomy of the SO. J Pediatr Ophthalmol Strabismus 16:321–323

References 193

12.Shin GS, Elliott RL, Rosenbaum AL (1996) Posterior superior oblique tenectomy at the scleral insertion for collapse of A-pattern strabismus. J Pediatr Ophthalmol Strabismus 33:211–218

13.Castanera de Molina A, ML GM (1997) Persistent SO “overaction” after surgical treatment of A-pattern anisotropies. In: M. Spiritus (ed) Transactions 24th meeting European strabismological association; Vilamoura, Portugal. Aeolus, Buren, The Netherlands