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Influence of suture regularity on corneal astigmatism after penetrating keratoplasty

2009; Wiley; Volume: 89; Issue: 5 Linguagem: Inglês

10.1111/j.1755-3768.2009.01729.x

1755-3768

Jesper Hjortdal, Anders Peter Søndergaard, W. Fledelius, Niels Ehlers,

Intraocular Surgery and Lenses

Purpose: To investigate whether suture regularity affects corneal astigmatism after keratoplasty. Methods: Twenty-one patients undergoing penetrating keratoplasty for various corneal diseases were included in the study. The grafts were sutured in place using a single-running Nylon 10-0 suture, taking 24 bites. Immediately after surgery, standard calibrated images of the grafted eye were captured and stored. Using a dedicated image analysis programme, stitches and needle points were identified, and a number of suture regularity variables were calculated. Corneal topographic images were obtained before suture removal (12 months after surgery) and 3 months after suture removal (18 months after surgery). Topographic measures of astigmatism [surface regularity (SRI), surface asymmetry index (SAI) and simulated keratometric astigmatism] were calculated and correlated with the computed suture regularity variables. Results: The average stitch length was 3.04 ± 0.28 mm and the distance between the outer needle points was 2.53 ± 0.09 mm. The SRI was 1.26 ± 0.36 and the SAI was 1.59 ± 0.67 after 12 months; these decreased to 1.03 ± 0.48 and 0.92 ± 0.46 after 18 months, respectively. Corneal astigmatism was 6.38 ± 2.99 and 5.87 ± 3.13 dioptres after 12 and 18 months, respectively. Suture regularity did not affect SAI, SRI or corneal astigmatism significantly 12 months after surgery. Eighteen months after surgery (3 months after suture removal), the standard deviation on the original stitch length was found to significantly increase corneal astigmatism. In addition, the size of the counter-clockwise angle between stitch and graft radian was correlated significantly with a lower SRI. Conclusion: The origin of corneal astigmatism after penetrating keratoplasty is multifaceted. Regular stitch length and stitch advancement on the surface appears to improve the optical quality of the graft after suture removal. Factors such as stitch depth, suture tension and variations in wound construction might also be important predictors of corneal astigmatism after penetrating keratoplasty. Corneal transplantation is one of the most successful tissue transplantation procedures, with more than 80% clear and functioning grafts more than 5 years after surgery in low-risk cases (Beckingsale et al. 2006). Unfortunately, normal visual acuity is rarely obtained and many patients need to use strong corrective spectacles or hard contact lenses to obtain functional vision (Claesson et al. 2002). The corneal graft can be sutured in place by various techniques: single sutures, running sutures, double-running sutures or a combination of single and running sutures. Today, most surgeons prefer a running-suture technique. The regularity of the suture placements could have an effect on post-keratoplasty astigmatism, but to our knowledge this issue has not been studied systematically. The purpose of the present study was to analyse whether suture regularity, as measured immediately after surgery, has any effect on post-keratoplasty astigmatism. Twenty-one patients (seven women, 14 men) undergoing penetrating keratoplasty for various corneal diseases were included in the study. Patient age ranged from 22 to 88 years and the mean age was 54 years. Preoperative diagnosis was keratoconus in eight patients, Fuchs' endothelial dystrophy in five, herpetic corneal scars in four, pseudophakic bullous keratopathy in one, Groenouw type I dystrophy in one and corneal scarring after bacterial keratitis in one. Surgery was performed under retrobulbar anaesthesia. Ink marks were made on the peripheral cornea of the recipient before trephination using a 12-blades radial marker. A motorized trephine (Draeger motor trephine; Draeger, Storz, Germany) with 8.00mm diameter and curved scissors were used to excise the diseased cornea. The 8.00-mm-diameter donor grafts were punched out from the endothelial side using a manual Moria trephine (Moria, Antony, France) in concave-shaped beeswax. The grafts were sutured in place with a single-running Nylon 10-0 suture (S&T AG, Neuhausen, Switzerland), taking 24 bites. Suturing was performed in the clockwise direction. At the end of surgery, suture tension was adjusted in order to produce a round, non-astigmatic reflex from a surgical keratoscope mounted on the operating microscope. Following suture adjustment, standard calibrated images of the grafted eye were captured and stored. The surgical keratoscope was used to ensure that the suture line was perpendicular to the operating microscope. An image-analysis program was developed. Using the program, stitches and needle points were identified and a number of suture regularity variables were calculated (stitch length, stitch direction, variations in needle entrance and exit points in relation to the graft–recipient interface). In Fig. 1, a detailed explanation of the various parameters is presented. (A) Image obtained immediately after penetrating keratoplasty. (B) After manual identification of stitch points, a circle identifying the best-fit inner (ISR) and outer circle (OSR) was calculated. A circle with a diameter of 8.00 mm identifies the graft size (red). (C) Digitalized suture line with circles. (D) Calculated stitch parameters: stitch length (SL) is distance from inner to outer stitch points, OSCD is distance from outer stitch point to graft–host interface, ISCD is distance from inner stitch point to graft–host interface, OISL is the outer stitch length, IISL is the inner stitch length, CW is the angle of clockwise stitches with respect to the graft radian and CCW is the angle of counter-clockwise stitches with respect to the graft radian. Corneal topographic images (TMS-3; Tomey, Nagoya, Japan) were obtained before suture removal (12 months after surgery) and 3 months after suture removal (18 months after surgery). Topographic measures of astigmatism, surface regularity index (SRI), surface asymmetry index (SAI) and keratometric astigmatism were calculated and correlated with the computed suture regularity variables [Pearson correlation analysis and multiple linear regression analysis using a stepwise method (spss v. 11.5; SPSS Inc., Chicago, Illinois, USA)]. SAI measures the difference in corneal powers at every ring (180° apart) over the corneal surface, and SRI is a measure of local fluctuations in central corneal power (Dingeldein et al. 1989). Surgery and follow-up were uneventful in all cases. Tables 1 and 2 present a summary of the suture-related variables calculated from postoperative images obtained at the end of surgery. The SRI was 1.26 ± 0.36 and the SAI was 1.59 ± 0.67 after 12 months, decreasing to 1.03 ± 0.48 and 0.92 ± 0.46 after 18 months, respectively. These changes were statistically significant (p < 0.05; paired t-tests). Corneal astigmatism was 6.38 ± 2.99 and 5.87 ± 3.13 dioptres after 12 and 18 months, respectively. This small decrease in astigmatism after suture removal was not significant. Correlation analysis revealed that suture regularity did not affect SAI, SRI or corneal astigmatism significantly 12 months after surgery (Tables 3 and 4). Eighteen months after surgery (3 months after suture removal), the standard deviation on the original stitch length was found to correlate significantly with increased corneal astigmatism (p < 0.01) (Fig. 2). The counter-clockwise angle between stitches and graft radian (CCWS) was found to be significantly correlated with increased SRI after 18 months (p < 0.05) (Fig. 3). None of the other stitch parameters correlated significantly with astigmatism, SRI or SAI 18 months after surgery. Relation between keratometric astigmatism computed from corneal topography 18 months after penetrating keratoplasty and the standard deviation on stitch lengths measured immediately after keratoplasty. Relation between surface regularity (SRI) computed from corneal topography 18 months after penetrating keratoplasty and the counter-clockwise angle between stitch and graft radian, as measured immediately after keratoplasty. Penetrating keratoplasty has for many years been the standard procedure in almost every indication for keratoplasty. In principle, the surgical procedure is fairly simple: the opaque cornea of the recipient is removed and a clear donor button is sutured in place. But because most complications occur in the time after surgery and are often related to improper suture technique (wound leakage, loosening sutures), a systematic and precise suture technique is critical. Over a longer postoperative perspective, the presence of corneal regular and irregular astigmatism often compromises the visual benefit for the patient. During recent decades, surgeons have therefore tried to develop the procedure in terms of suture technique and wound construction. In the present study, we studied whether suture regularity in standard penetrating keratoplasty with a single-running Nylon 10-0 suture had any influence on postoperative corneal topography 12 and 18 months after keratoplasty (3 months after suture removal). Corneal topography was chosen as a non-biased and objective method for studying the optical quality of the graft surface. The surface regularity and asymmetry indices have been proven to correlate with visual acuity (Dingeldein et al. 1989). An image-analysis program was developed to compute a number of suture-related parameters for each case. Because many of the parameters are interrelated, multiple linear regression analysis was used for the final evaluation of statistical correlations between suture parameters and corneal topographic descriptors. Some improvement was observed in corneal topography by suture removal. Corneal astigmatism decreased slightly, but not significantly, whereas SRI and SAI decreased and thereby improved significantly between 12 and 18 months after surgery. There was no significant correlation between any of the suture-related parameters and corneal regularity 12 months after keratoplasty. When sutures had been removed, two suture-related parameters influenced the optical quality of the graft 18 months after surgery. The standard deviation of the length of the stitches correlated positively with the amount of simulated keratometric astigmatism. This finding suggests that the surgeon should attempt to make the stitches equal in length when the graft is sutured. The other significant suture parameter was the counter-clockwise angle between the stitch and the graft radian: a large angle was found to correlate with a significantly lower SRI. This suggests that the surgeon should try to advance each stitch on the surface, rather than in the graft and host tissues, during suturing. However, the influence of the suture parameters was limited. Approximately 20–25% of the variation in corneal astigmatism and SRI could be explained by the two suture-related parameters. At least in theory, a large variation in the distance between stitches should result in considerable astigmatic or irregular optical properties of the corneal surface. Ink marks on the peripheral corneal surface, as used in the study, are helpful in maintaining regularity between stitches. Therefore, the variation in suture-related parameters between patients may have been too small to influence post-penetrating keratoplasty corneal topography profoundly. In register-based studies, recipient age and diagnosis have been found to have limited influence on post-keratoplasty corneal astigmatism (Claesson & Armitage 2007). The present study included patients with a wide range of diagnoses and ages. In a larger, more homogenous sample of patients, some additional suture parameters might have been shown to be significant. Only suture regularity in the surface plane was studied. The stitch depth of each suture could be neither measured nor evaluated; nor could the tension in the suture be measured. Adjustment of suture tension was attempted at the end of surgery in order to reduce immediate postoperative astigmatism. It is known that suture adjustment in the first months after surgery can reduce astigmatisms, but the effect is lost when the sutures are removed (Serdarevic et al. 1995). In this study, the measured suture parameters failed to have an effect on the 12-month optical quality of the graft. Perhaps an uneven suture tension, induced at the end of surgery, can explain this finding. The influence of the original suture regularity may not have become evident until the sutures were removed. Improvement of the optics of the grafted cornea after penetrating keratoplasty is ongoing: femto-second lasers are currently used for making advanced wounds with 'top-hat', 'mushroom', 'zig-zag' and other configurations (Buratto & Böhm 2007). Follow-up studies are needed to assess the significance of these efforts. In this respect, the present study may provide background information. An alternative keratoplasty procedure such as Descemet's stripping automated endothelial keratoplasty (DSAEK), in which most of the recipient corneal integrity is preserved, is superior to penetrating keratoplasty when it comes to speed of visual recovery and corneal astigmatism (Hjortdal & Ehlers 2009). However, many patients have combined corneal disease affecting the endothelium as well as the stroma. Thus, penetrating keratoplasty procedures might still be the best surgical option for a considerable number of patients both currently and in the future.