Insertion Slanting Strabismus Surgical Procedures FREE

The terms A-pattern and V-pattern strabismus are used to describe vertically incomitant horizontal strabismus in which there is a substantial difference in magnitude of the deviation between the midline upgaze and midline downgaze positions. A patient has an A pattern if the eyes are more converged (more esotropic or less exotropic) in upgaze than in downgaze and a V pattern if the eyes are more converged in downgaze than in upgaze.

In 1897, Duane¹ published what is probably the first report of a V pattern when he described a patient who had bilateral fourth cranial nerve palsies. It was not until Urrets-Zavalia^2- 3 described what is now known as the A and V patterns in 1948 that the general importance of measuring strabismus in upgaze and downgaze in the midline positions became recognized. In 1951, Urist⁴ brought attention to A- and V-pattern strabismus in the English language. Although numerous treatment approaches had initially been proposed for the A and V patterns,⁵ 2 approaches suggested by Knapp^6- 7 have been most predominantly used: (1) weakening of the oblique muscles if they are overacting (inferior oblique muscles for V patterns and superior oblique muscles for A patterns) and (2) vertically transposing the horizontal rectus muscles if there is no substantial oblique muscle dysfunction.

In 1970, Bietti⁸ described a procedure that consisted of performing a slanted recession of the horizontal rectus muscles to treat A and V patterns. It consisted of selectively recessing the superior and inferior poles of the horizontal rectus muscles asymmetrically to produce a slanted orientation of the new insertion. Subsequently, other researchers^9- 14 reported good results with this technique. However, in 2008, van der Meulen-Schot et al¹⁵ reported good results with an insertion slanting procedure in a series of patients with A or V patterns in whom they slanted the muscle in the opposite manner to that performed by Bietti and others. The fact that essentially opposite approaches could produce good results presents a conundrum that challenges our basic understanding of eye muscle physiologic features and strabismus surgery. In a thoughtful editorial on this subject, Bothun¹⁶ addressed some possible explanations for this apparent conundrum. However, a review of the previous publications and the underlying assumptions that the researchers made to justify their approach sheds further light on this subject. To understand this subject, a review of extraocular muscle mechanics is helpful.

The principles underlying rectus muscle transpositions for treating A and V patterns are shown in Figure 1 and Figure 2. When an eye moves into upgaze or downgaze, a mild amount of slack is induced in 1 edge of each horizontal rectus muscle because the end of the insertion to which that edge attaches moves posteriorly in the orbit—the superior edge slackens in upgaze and the inferior edge in downgaze. Scott¹⁷ calculated that when the lateral rectus muscle moves into downgaze, the length of the inferior edge will shorten from 40.0 to 37.1 mm; the superior edge will lengthen slightly, from 40 to 41.5 mm. If a muscle is infraplaced, there is even more slack in the muscle in downgaze than occurs for a nontransposed muscle as the insertion of the muscle moves farther posteriorly in the orbit. In upgaze, the muscle is more stretched. Because the slack caused by passive shortening of the muscle decreases the muscle's contractile force,¹⁸ an inferior transposition weakens the horizontal muscle's action mostly in downgaze. Thus, for medial rectus muscles, an inferior transposition would be an appropriate procedure for treating a V-pattern esotropia.^6- 7

In 1857, von Graefe wrote, “If the internal rectus is efficiently cut in the upper part, the muscle perceives less resistance when activated and will move back and down. The vertical rotary axis will take on a deviation to the inside and the muscle will take on a downward direction” (translated from the original German text).^19(p260) With this, von Graefe was describing the fact that tenotomy to slant the superior pole of a horizontal rectus muscle posteriorly infraplaces the muscle's insertion. Much later, several investigators^17,20- 21 carried this concept further by determining that for mathematical modeling one can consider the rectus muscles as inserting at a point in the middle of their insertion. Yet, if there is unequal tension along the edges of the muscle, one can still consider the insertion as being a point, but the position of the point shifts toward the edge under greater tension (Figure 3 and Figure 4). Thus, slanting the insertion of a horizontal rectus muscle so that the superior pole is recessed more than the inferior pole, as seen in Figure 4, is akin to inferiorly transposing the muscle. In theory, this type of muscle slanting could be used to treat a V-pattern esotropia if performed on the medial rectus muscles or an A-pattern exotropia if performed on the lateral rectus muscles because it is similar to an inferior transposition.

The article by van der Meulen-Schot et al¹⁵ in 2008 describes good results with recession and insertion slanting surgery in a series of 52 patients with A- or V-pattern esotropia or exotropia in whom they slanted the recessions in a manner consistent with the aforementioned principles described by von Graefe and others (Simonsz/von Graefe method). For a V-pattern esotropia, they recessed and slanted the superior pole of the medial rectus muscles back farther than the inferior pole and applied similar reasoning for A-pattern esotropia and for A- or V-pattern exotropia. However, the recession and slanting procedure previously described by Bietti⁸ in 1970 involved slanting the rectus muscle insertions in an opposite manner to that used by Simonsz and coauthors. Bietti also described good results in his series of 68 patients with A and V patterns. For example, for V-pattern esotropia, Bietti recessed the inferior pole of the medial rectus more than the superior pole and vice versa for A patterns (Bietti method). Subsequent to Bietti's publication, numerous investigators^9- 14 reported using the principles of the Bietti method to successfully treat A and V patterns. Other researchers extrapolated from the concept of the Bietti method to treat convergence excess esotropia^22- 25 or convergence insufficiency exotropia,^26- 28 in some cases expanding the principles to include biased resections (resecting more from the superior or inferior aspect of the muscle, respectively). Of the numerous investigators reporting results using the Bietti method, only Rüssmann⁹ and Choi and Hwang²⁹ found it to be ineffective.

Bietti justified his approach by stating, “I thought it to be appropriate, in moderate cases (between 10 and 20 PD [prism diopters]) of A-V pattern to use a way, based on an entirely empirical concept, of weakening more than one part of the horizontal action, relative to the other one, in the same direction in which the insertion is shifted vertically”^8(p582) (translated from the original Italian). He did not offer a mechanistic explanation as to why recessing the inferior pole of a medial rectus muscle would give more effect in downgaze than in upgaze. He did, however, cite Boeder²⁰ as hypothesizing that there might be selective segmental innervation to the horizontal rectus muscles in upgaze or downgaze. According to this hypothesis, there might be increased active contraction of the inferior portion of the medial rectus in downgaze, and, hence, selectively recessing the inferior pole of that muscle's insertion would produce more weakening in downgaze. Boeder²⁰ speculated about this, but he did not attempt to prove this concept. Although Bietti primarily justified his method on empirical grounds,⁸ subsequent investigators have tried to explain the approach on specific mechanical factors. Reviewing these articles reveals that they are based on either misquoting or misinterpreting the 1975 study by Scott.¹⁷ Ohba and Nakagawa quoted Scott as saying “ . . . horizontal muscle tensions are different between upper and lower margins of the muscle with different directions of gaze; i.e. horizontal tension at the upper margin is stronger than at the lower margin in the case of upward gaze.”^12(p433) In fact, Scott said essentially the opposite. He wrote, “In 30 degrees upgaze the lower fibers stretch from 40 mm to 41.5 mm and the upper fibers relax from 40 mm to 37.1 mm which puts the entire strain on the lower fibers (more so if they are stiff).”^{17(pp186-187)} This misquote was repeated by subsequent investigators.¹³ A 1990 study²⁷ correctly cited Scott's findings but misinterpreted their implications. It properly reported the fact that the muscle fibers will shorten on the edge of a horizontal rectus muscle toward the direction the eye rotates, for example, superior fibers shorten on upgaze; however, the authors incorrectly interpreted that to mean that the shorter fibers were under greater tension. Subsequently, numerous other researchers^{10,14,22,25,28} repeated this same misinterpretation. In fact, muscle fibers that are passively shortened are under decreased tension. This is the basis for the weakening effect of an extraocular muscle recession or the force vector changes after transposition seen in Figure 2.

Numerous investigators^{10- 11,14,16,26} have claimed that insertion slanting procedures have a less adverse effect on torsion than do standard transposition procedures. The concept that insertion slanting procedures have a less adverse effect on torsion, however, is unfounded. In 1 study,³⁰ the effect of muscle transposition to treat A or V patterns on objective fundus torsion was studied prospectively and found to be small. There was only 2.5° to 3.5° of torsional change per eye after surgery, which was never clinically noticeable. On the other hand, I study recently examined a patient who had previously undergone a biased resection of the right medial rectus muscle (resecting the inferior fibers only) combined with a slanting posteriorly of the nasal fibers of the right inferior rectus muscle. This would inferiorly transpose the medial rectus muscle and temporally transpose the inferior rectus muscle according to the Simonsz/von Graefe method of reasoning. Immediately after surgery, the patient reported diplopia secondary to a subjective excyclotropia. It is unclear whether the torsional diplopia would have resolved as a result of sarcomere remodeling because the next day the surgeon performed a Harada-Ito procedure on the ipsilateral superior oblique muscle to decrease the extorsion. Similarly, Spielmann³¹ intentionally used insertion slanting surgery to induce a torsional change.

In summary, for the identical A and V patterns, different researchers would do opposite forms of insertion slanting, yet both groups report good results (Figure 4). Although Simonsz and coworkers cited numerous other researchers who reported success with the Bietti method of insertion slanting, they did not comment on the fact that their own approach was opposite to the method that the previous researchers used.¹⁵ How can we reconcile the conundrum that contradictory treatment approaches have been reported to be equally successful?

Probably, insertion slanting does not have a substantial effect on the outcome of strabismus surgery. The logic behind the Simonsz/von Graefe method makes sense, but the effect is miniscule and is negated by sarcomere remodeling. The main effect comes from the recession or resection. The published reasoning to support the Bietti method is based on a misinterpretation of the existing literature and does not make sense given what is known about oculomotor mechanics. The recession or resection procedure probably works despite the slanting.