6.4.8MISS Rectus Muscle Transposition Surgery

Rectus muscle transpositions are indicated for strabismus associated with compete paralysis. They generate new force vectors that can replace the paralysed muscle. So far, partial or full rectus muscle transpositions are usually performed through large limbal openings. As an alternative, surgery can be performed through two radial cuts placed between the paralysed muscle and the two adjacent muscles, which will be transposed. As with ParksÕ fornix incision, such openings must be displaced over the muscle insertion to allow performing all surgical steps. In older patients the conjunctiva will tear and so radial cuts cannot be used for older patients. Unfortunately, transposition surgery using limbal openings often induces considerable conjunctival and lid swelling during the direct postoperative period and may even lead to corneal surface problems. To avoid these complications, a MISS transposition technique, which can also be used in inelastic conjunctiva, has been developed [13]. Several small keyhole openings are to be placed exactly where the surgical steps are to be performed. For a partial muscle transposition of both adjacent muscles, four openings will sufÞce; for complete transposition six openings are necessary. A blunt 20G sub-TenonÕs anesthesia cannula is used to safely displace the needles and later the muscle to the paralysed muscle. The rest of the conjunctiva remains untouched. The cannula is introduced through the cut where the transposed muscle is anchored. Then, it is advanced until it comes out from the opening lying next to the muscle which is to be transposed. The needles are inserted in the cannula. Next, the cannula is retracted, allowing the needles to be safely displaced. It is important to take into account the anatomy of the oblique muscles to correctly undercross or overcross the transposed muscles. This access and tissue dissection technique minimizes anatomical disruption and, postoperatively, allows all openings to remain covered by the lids except during excessive gaze positions. The muscle transposition technique explained here in detail is KaufmannÕs modiÞed Hummelsheim procedure [7]. As in the Hummelsheim technique, half of both rectus muscles next to the weak rectus muscles are transposed. The modiÞcation consists in an undercrossing of the weak muscle by the transposed halfmuscles. For large squint angles in primary position,

surgery has to be combined with ipsilateral rectus muscle recession or botulinum toxin injection. For smaller squint angles, both a combined rectus muscle recession and plication, transposition without undercrossing, or transposition of less than 50% of the Þbers should be considered. All surgical steps can be performed without the help of an assistant. An operating microscope will enhance visibility of all surgical steps. A limbal traction suture will allow optimal exposure of the Þrst of the two rectus muscles, which will be split and transposed. Then, a small parainsertional radial cut is performed at the muscle part which is to transposed (Fig. 6.11a). The anterior margin of the cut is at the level of the tendon insertion. The size of this opening is approximately 4 mm. In patients with reduced elasticity of the conjunctival tissue, slightly larger openings will be necessary to split the muscle without tearing the conjunctiva. With blunt Wescott scissors the episcleral tissue is separated from the muscle sheath and the sclera. Then the muscle is hooked. Now, on the side of the opening, a meticulous dissection of the check ligaments and intermuscular membrane is performed (Fig. 6.11b). This dissection is performed 7 mm behind the insertion. The resulting tunnel will help to split the muscle easily. A curved ruler is used to measure the muscle insertion width and to determine its midpoint (Fig. 6.11c). Starting at the midpoint, the muscle is gently split using a small muscle hook with a mark 15 mm away from the hook (Fig. 6.11d). Splitting is stopped as soon as the mark reaches the insertion, which will ensure that exactly 15 mm of the muscle has been split. Two single sutures are applied to the muscle tendon which is to transposed. These sutures are placed as close as possible to the insertion (Fig. 6.11e). Then, the split half-muscle is detached from the insertion using a Wescott scissor (Fig. 6.11f). If necessary, hemostasis is performed. After applying a new limbal traction suture, the same technique is used to split the other rectus muscle (Fig. 6.11g). After desinsertion of the second split half-muscle, the minimally invasive transposition can be performed. Therefore, a new limbal traction suture is applied, which will help to expose the weak rectus muscle. Now, two small parainsertional keyhole cuts are performed along the muscle margin (Fig. 6.11h). The size of these openings is 3 mm. Afterwards, a blunt 20G sub-TenonÕs anesthesia cannula is inserted through one of these openings and advanced under the weak rectus muscle to come out through the