Among strabismus surgeries, rectus muscle resection is one of the most commonly performed strengthening procedures. Accurate positioning of scleral sutures is important to achieve a good surgical outcome when performing rectus muscle resection.1 If scleral bites are too shallow, muscle slippage or loss might occur. Moreover, if the bites do not pass through the middle of the original insertion site, central muscle sagging may occur. In addition, deep scleral bites may cause scleral or globe perforation.1,2 In particular, surgeons or residents without substantial experience may be uncomfortable passing scleral sutures when the muscle stump is clearly removed from the original insertion. Thus, these surgeons and residents tend to perform scleral sutures with a small residual muscle stump. In this case, a needle may pass through the residual tendon fiber instead of the sclera. As a result, the connection between the resected muscle and original insertion can easily loosen. Eventually, the effect of the muscle resection procedure can be weakened, and the risk of having a stretched scar may be greater.2 Therefore, complete removal of the residual muscle stump and accurate scleral bites at the original insertion are important.
The current study was conducted to determine the degree of muscle migration that occurs during the process of wound healing or postoperative loosening in cases involving suturing to the residual muscle stump at the original insertion site during the superior rectus muscle (SRM) resection procedure. In addition, we sought to identify the histopathologic changes in these cases using a rabbit model.
Materials and Methods
All experiments were conducted in accordance with the tenets of the Association for Research in Vision and Ophthalmology Statement for the Use of Animals in Ophthalmic and Vision Research. The protocol was approved by the Institutional Animal Care and Use Committees of Dong-A University College of Medicine (DIACUC-16-019). In total, 24 eyes from 12 white rabbits (each weighing 2 to 3 kg) were used for this study. All rabbits had anatomically normal eyes and were healthy.
All surgical procedures were performed by one clinically practicing strabismus surgeon (WYR). Each rabbit was anesthetized with an intramuscular injection of ketamine hydrochloride (approximately 30 to 50 mg/kg) and xylazine hydrochloride (approximately 5 mg/kg). In addition, a topical anesthetic containing 0.5% proparacaine hydrochloride was administered. Preoperatively, an antiseptic treatment containing 10% povidone-iodine was applied to the eyelids and periorbital area, and each eyelid was opened with a speculum. The rabbits received topical administration of 2.5% phenylephrine eye drops to reduce bleeding. A peritomy of the upper limbal conjunctiva was conducted. Then the SRM was exposed using a Jameson hook, and the intermuscular membrane and check ligaments were dissected. The SRM was used in our study because it is the thickest extraocular muscle, making it easy to observe.3–5
The SRM was placed on a double-armed 6-0 polyglactin 910 suture (S-14, 8 mm, 1/4 circle) that was located at a distance of 3 mm from the original insertion site (Figure 1A). A security knot was placed at the center of the SRM by full-muscle thickness. Both needles connected to the security knot were passed at half-muscle thickness, and a full-thickness locking bite was placed on both edges of the muscle. The SRM was cauterized and cross-clamped with a Hartmann clamp for maximal hemostasis, followed by a transection of the SRM 1 mm anterior to the suture line, which was 2 mm posterior to the original insertion. After transection, the muscle stump immediately contracted because the extraocular muscles in rabbits are thin and have strong elastic properties, and the lengths of the muscles were all less than 0.5 mm (Figure 1B).
The superior rectus muscle (SRM) was placed on a double-armed 6-0 polyglactin 910 suture that was located at a distance of 3 mm from the original insertion site. (A) The SRM was cauterized and resected. In one randomly assigned eye in each rabbit, (B) the muscle stump was not removed, and (C) the transected muscle was sutured to the residual muscle stump (residual muscle stump group). For the other eye, (D) the muscle stump was completely removed and (E–F) the transected muscle was sutured to the original insertion in the sclera (control group).
Twenty-four eyes from 12 rabbits were randomly divided into two groups. In one randomly assigned eye in each rabbit, the muscle stump was not removed (Figure 1B) and the transected muscle was sutured to the middle of the residual muscle stump, the length of which remained less than 0.5 mm (Figure 1C) (residual muscle stump group). The remaining contracted muscle stump was intentionally left attached to the remaining tendon for easy suturing to the sclera. For the other eye, the muscle stump was completely removed (Figure 1D) and the transected muscle was sutured to the original insertion (Figures 1E–1F) (control group). The transected muscle was tightly attached by a double-arm mattress suture, and each needle of the suture was passed at both ends of the residual muscle stump (residual muscle stump group) or the original insertion (control group). For a central security suture, a needle was passed through the central aspect of the muscle to prevent central sagging in both groups. Conjunctival suturing was performed with 8-0 polyglactin 910 sutures. Levofloxacin eye drops were applied once to each eye after surgery to prevent infection.
Postoperative Location Measurement
The distance from the corneal limbus in both groups was measured at 1, 2, and 4 weeks after surgery. The measurement was performed by a blinded observer who was unaware of the surgical information or the purpose of the current experiment. The distances were measured using a Castroviejo caliper at 0.5-mm increments at locations between the corneal limbus and the most anterior aspect of the reattached muscle at the central portion and the nasal and temporal edges throughout the conjunctiva. The measurements were repeated three times for accuracy. The mean distance was defined as the average of the measured values at each site.
All rabbits were killed using carbon dioxide gas at postoperative week 4. All eye enucleation and tissue handling procedures were performed by a masked observer. One pathologist (MSR) who did not receive any information about the study purpose or either group examined all specimens. Tissues were fixed in 4% paraformaldehyde solution and embedded in paraffin. Longitudinal serial sections through the muscle attachment sites were stained with hematoxylin–eosin to determine histopathologic differences. We performed immunohistochemical staining in both groups using antibodies to alpha-smooth muscle actin and CD34 to differentiate myofibroblasts from fibroblasts. In addition, isolated tissues were examined using Masson trichrome staining to evaluate collagen deposition and the degree of fibrosis, whereas elastic staining was used for evaluating elastic tissue.
All statistical analyses were conducted using the SPSS software program (SPSS for Windows, version 18.0K; SPSS, Inc., Chicago, IL). Wilcoxon signed-rank tests were conducted to compare results at postoperative weeks 1, 2, and 4 in both groups. Fisher's exact tests were used to verify the number of eyes that had movement from the original insertion site at postoperative week 4. A P value of less than .05 indicated statistical significance.
Preoperatively, all distances between the corneal limbus and the original insertion of the SRMs in both eyes of all 12 rabbits (a total of 24 SRMs) were 2 mm. In both groups, the center portion of the new attachment and its nasal and temporal edges were all precisely 2 mm from the corneal limbus immediately after SRM resection. The distances between the corneal limbus and the most anterior aspect of the SRM in both groups at postoperative weeks 1, 2, and 4 are listed in Table 1. The distances between the corneal limbus and the most anterior aspect of the SRM were not significantly different in both groups at postoperative week 1. However, the mean distance exhibited a significant difference in both groups at postoperative week 2 and, on average, the difference was greater at postoperative week 2 than at postoperative week 4 (Wilcoxon signed-rank test, P < .05).
Distance Between the Corneal Limbus and the Most Anterior Aspect of the Muscle (mm)
In the control group, the mean distances were as follows: 2.10 ± 0.35 mm at postoperative week 1; 1.99 ± 0.40 mm at postoperative week 2; and 1.81 ± 0.44 mm at postoperative week 4. There was no difference in the mean distance between postoperative weeks 1 and 2 in the control group (Wilcoxon signed-rank test, P = .203). However, the location at postoperative week 4 showed a slight anterior repositioning pattern (Wilcoxon signed-rank test, P = .039). By contrast, in the residual muscle stump group, the mean distances were as follows: 2.42 ± 0.52 mm at postoperative week 1; 2.97 ± 1.00 mm at postoperative week 2; and 3.14 ± 0.78 mm at postoperative week 4. Muscle slippage showed a sharp shifting tendency between postoperative weeks 1 and 2 (Wilcoxon signed-rank test, P < .05), and the difference was not significant at postoperative week 4 (Wilcoxon signed-rank test, P = .443).
There was no significant difference in the number of eyes in which the new attachment site was more than 1 mm away from the original insertion. At postoperative week 4, there were 5 eyes in the residual muscle stump group and 1 eye in the control group (Fisher's exact test, P = .155). However, the number of eyes in the residual muscle stump group (11 eyes) in which there was a shift of more than 0.5 mm was significantly larger than that in the control group (4 eyes) (Fisher's exact test, P = .009).
All histopathologic examinations were conducted at postoperative week 4. Hematoxylin–eosin staining revealed a space that was filled with connective tissue in front of the SRM in the residual muscle stump group (Figure 2A). This space was not detected in the control group (Figure 2B). In Masson trichrome staining, this tissue was identified based on the deposition of fibrous tissue (Figure 3A), and the space in front of the SRM in alpha-smooth muscle actin immunostaining exhibited fibrous tissue deposition with scattered proliferating myofibroblasts (Figure 3B). Elastic staining and immunohistochemical staining using an anti-CD34 antibody were not significantly different between the two groups.
Representative histologic findings with hematoxylin–eosin staining (original magnification ×20) at postoperative week 4 in the (A) residual muscle stump group and (B) control group. (C–D) Schematized diagrams show the orientation. The residual muscle stump group showed a space that was filled with connective tissue in front of the superior rectus muscle (arrowhead) in comparison to the control group. C = cornea; S = sclera; M = extraocular muscle; Ct = connective tissue; Su = suture material; Ic = inflammatory cells; Ut = uveal tract
Representative findings with (A) Masson trichrome staining (original magnification ×27) and (B) alpha-smooth muscle actin immunostaining (original magnification ×40) at postoperative week 4 in the residual muscle stump group. The space in front of the superior rectus muscle revealed (A) fibrous tissue deposition and (B, arrowhead) scattered proliferating myofibroblasts.
A stretched scar is defined as an operated muscle tendon that is attached to the sclera by an amorphous scar segment due to an abnormal wound healing process.6,7 A stretched scar may lead to various surgical outcomes after strabismus surgery. In the current study, histopathologic changes in suturing to the residual tendon fiber were analyzed, and fibrous connective tissue and proliferating myofibroblasts were observed in front of the migrated SRM. The histopathologic findings of our study are similar to those of stretched scars described by Ludwig and Chow.6,7 Thus, we suggest that suturing to the residual tendon fiber contributes to fibrous connective tissue formation; this histopathologic change may lead to unexpected surgical results after rectus muscle resection. After strabismus surgery, histopathologic changes at the reattachment site between the rectus muscle and sclera, as well as scar formation around the site, are important factors that can influence surgical outcomes.4,6–8
Scleral bites may be sutured to residual tendon fibers, not to the sclera, when a muscle stump at the original insertion site is not completely removed during rectus muscle resection. This situation increases the likelihood that scleral bites can loosen because the presence of a residual muscle stump provides false security.2 In the current study, the number of eyes that had moved by more than 0.5 mm was significantly higher in the residual muscle stump group than in the control group. In particular, the distance between the corneal limbus and the most anterior aspect of the SRM was not different between the two groups at postoperative week 1, but a significant difference was observed at postoperative week 2. These results seem likely to be caused by gradual changes in the scar, such as stretching of the scar. To reduce this error during rectus muscle resection, we suggest complete removal of the entire remaining muscle stump at the original insertion.
Muscle slippage or loss is also one of the most important complications that can occur in the rectus muscle resection procedure.9–11 If muscle loss occurs, severe incomitant restrictive strabismus with diplopia will result and severely limit eye movement.12 Thus, accurate scleral sutures are important to prevent complications.1 The current study used a rabbit model to determine the degree of muscle migration and postoperative loosening in cases involving suturing to the residual muscle stump at the original insertion site during the SRM resection procedure. Based on our findings, compared with the control group, the residual muscle stump group demonstrated rapid migration between postoperative weeks 1 and 2. These results indicate that in rectus muscle resection, suturing to the remaining muscle stump without completely removing the muscle stump at the original insertion site can lead to muscle loosening between postoperative weeks 1 and 2, which might eventually cause inaccurate surgical results. According to several studies, abnormal attachment, such as a stretched scar in the healing process after strabismus surgery, may lead to late overcorrection or an increased risk for reoperation.6,7,13–15 We believe that abnormal attachment or healing, such as a stretched scar, in the new attachment sites at postoperative week 2 led to the results obtained in the current study, and we experimentally demonstrated these results in a rabbit model.
The rectus muscle should be precisely attached at the desired location to achieve a good surgical outcome after strabismus surgery.5,12,16 Repka et al.17 conducted an experimental animal study in which they determined the position at which the muscle reattaches to the sclera following a hang-back recession, and the results suggested that the site of reattachment varied as a result of postoperative pseudotendons. In our previous study,5 we determined the manner in which a sagging muscle reattaches to the sclera in a rabbit model, and extraocular muscles that were not securely attached to the sclera reattached at various locations. These results are similar to those of Repka et al. It is important to securely suture to the sclera and to reduce any inflammation or fibrosis at the space between the muscle and sclera through wide attachments (muscle and sclera) when performing any strabismus surgeries, regardless of whether the procedure involves recession or resection.
The current study has some limitations. First, there are anatomical differences between humans and rabbits. Second, the small number of samples (12 eyes) in each group limits our ability to generalize these results. Third, muscle fibers might be initially positioned marginally further back in the residual muscle stump group. Nevertheless, we consider this setting necessary to design an experiment. In addition, this setting could be possible if residents or surgeons without substantial experience perform scleral sutures with a small residual muscle stump. Fourth, the surgical outcomes may reflect measurement errors due to caliper use. However, to improve accuracy, the measurements were repeated three times by a blind observer at 0.5-mm increments.
In our animal study, we evaluated the surgical results that can occur if the scleral suture is not properly performed during rectus muscle resection, a commonly performed muscle tightening procedure among strabismus surgeries. Specifically, we were able to identify unexpected, diverse attachments of the stretched scar based on histopathologic examinations. We think that these results will be helpful for surgeons or residents without substantial experience.
Postoperative loosening associated with a stretched scar frequently occurred when the residual muscle stump was sutured at the original insertion during SRM resection in a rabbit model. These conclusions are based on the finding that proliferation was observed in the fibrous connective tissue at the muscle reattachment site observed in histopathologic examinations. We think that our results support the view that secure reattachment between the muscle and sclera is important for increasing the success rate of strabismus surgery. Therefore, we believe that the complete removal of the muscle stump and accurate scleral suturing are critical for decreasing errors during rectus muscle resection.
- Ferris JD, Davies PE. Strabismus Surgery. Philadelphia: Saunders Elsevier; 2007.
- Wright KW. Color Atlas of Strabismus Surgery. New York: Springer; 2007.
- Ryu WY, An SH. Reduction in rectus muscle recession depending on the distance between rectus muscle suture placement and scleral insertion in rabbit eyes. J Pediatr Ophthalmol Strabismus. 2015;52:294–298. doi:10.3928/01913913-20150609-03 [CrossRef]
- Ryu WY, Jung HM, Roh MS, et al. The effect of a temperature-sensitive poloxamer-alginate-CaCl2 mixture after strabismus surgery in a rabbit model. J AAPOS. 2013;17:484–489. doi:10.1016/j.jaapos.2013.07.003 [CrossRef]
- Ryu WY, Bae JB. Changes in sagging extraocular muscle following surgical recession of the superior rectus muscle in rabbit eyes. J Pediatr Ophthalmol Strabismus. 2013;50:162–168. doi:10.3928/01913913-20130129-02 [CrossRef]
- Ludwig IH, Chow AY. Scar remodeling after strabismus surgery. J AAPOS. 2000;4:326–333. doi:10.1067/mpa.2000.107899 [CrossRef]
- Ludwig IH. Scar remodeling after strabismus surgery. Trans Am Ophthalmol Soc. 1999;97:583–651.
- Kennedy JB, Larochelle MB, Pedler MG, Petrash JM, Enzenauer RW. The effect of amniotic membrane grafting on healing and wound strength after strabismus surgery in a rabbit model. J AAPOS. 2018;22:22–26. doi:10.1016/j.jaapos.2017.08.007 [CrossRef]
- Yang SM, Kim SH, Cho YA. The difference in muscle slippage according to scleral suture techniques in rectus muscle resection of rabbit eyes. Eye (Lond). 2008;22:564–568. doi:10.1038/sj.eye.6702803 [CrossRef]
- Simon JW. Complications of strabismus surgery. Curr Opin Ophthalmol. 2010;21:361–366. doi:10.1097/ICU.0b013e32833b7a3f [CrossRef]
- Wan MJ, Hunter DG. Complications of strabismus surgery: incidence and risk factors. Semin Ophthalmol. 2014;29:421–428. doi:10.3109/08820538.2014.959190 [CrossRef]
- Lenart TD, Lambert SR. Slipped and lost extraocular muscles. Ophthalmol Clin North Am. 2001;14:433–442. doi:10.1016/S0896-1549(05)70241-8 [CrossRef]
- Cho YA, Ryu WY. The advancement of the medial rectus muscle for consecutive exotropia. Can J Ophthalmol. 2013;48:300–306. doi:10.1016/j.jcjo.2013.03.003 [CrossRef]
- Maxfield SD, Hatt SR, Leske DA, Jung JH, Holmes JM. Factors associated with atypical postoperative drift following surgery for consecutive exotropia. J AAPOS. 2017;21:360–364. doi:10.1016/j.jaapos.2017.07.201 [CrossRef]
- Jung JH, Leske DA, Holmes JM. Classifying medial rectus muscle attachement in consecutive exotropia. J AAPOS. 2016;20:197–200. doi:10.1016/j.jaapos.2016.02.008 [CrossRef]
- Plager DA, Parks MM. Recognition and repair of the slipped rectus muscle. J Pediatr Ophthalmol Strabismus. 1988;25:270–274.
- Repka MX, Fishman PJ, Guyton DL. The site of reattachment of the extraocular muscle following hang-back recession. J Pediatr Ophthalmol Strabismus. 1990;27:286–290.
Distance Between the Corneal Limbus and the Most Anterior Aspect of the Muscle (mm)a
|Time||Residual Muscle Stump Group||Control Group||Pb|
|1 week postoperatively|
| Center||2.42 ± 0.70||2.08 ± 0.42||.160|
| Nasal edge||2.17 ± 0.69||2.04 ± 0.33||.672|
| Temporal edge||2.67 ± 0.58||2.17 ± 0.39||.063|
| Average||2.42 ± 0.52||2.10 ± 0.35||.097|
|2 weeks postoperatively|
| Center||2.96 ± 1.20||1.85 ± 0.53||.025|
| Nasal edge||2.79 ± 1.08||1.83 ± 0.49||.032|
| Temporal edge||3.17 ± 0.83||2.29 ± 0.45||.027|
| Average||2.97 ± 1.00||1.99 ± 0.40||.026|
|4 weeks postoperatively|
| Center||3.08 ± 0.93||1.58 ± 0.36||.002|
| Nasal edge||2.96 ± 0.94||1.83 ± 0.58||.007|
| Temporal edge||3.38 ± 0.74||2.00 ± 0.60||.002|
| Average||3.14 ± 0.78||1.81 ± 0.44||.002|