Original Article

The effect of scleral flap edge apposition on intraocularpressure control in experimental trabeculectomyWayne Birchall FRANZCO1 and Anthony P Wells FRANZCO1,2

1Wellington Hospital and 2Wellington School of Medicine, Wellington, New Zealand

ABSTRACT

Background: To assess the contribution of scleral flap edgeapposition to intraocular pressure (IOP) control in trab-eculectomy, using a previously described and validatedexperimental model of guarded filtration surgery.

Materials and methods: Twelve rectangular-flap trab-eculectomy operations each with two apical adjustablesutures were performed on six donor human eyes con-nected to a constant flow infusion with real-time IOPmonitoring. Three sizes of scleral flap were created:4 ¥ 4 mm, 16 mm2 (n = 4), 3 ¥ 3 mm, 9 mm2 (n = 4) or3 ¥ 2 mm, 6 mm2 (n = 4). Sutures were tied tightly toproduce high aqueous outflow resistance, and equilibriumIOP established. The lateral and posterior edges of thescleral flap were removed, the sutures tightened again, andthe new equilibrium IOP measured.

Results: Following flap closure and with intact flap edges, themean absolute IOP for all flaps (n = 12) was 19.5 � 3.9 mm Hg (mean � SD, range 12.4–27 mm Hg) and followingflap edge excision 18.7 � 4.4 mm Hg (range 5.6–27.9 mm Hg), demonstrating no significant difference betweenflaps with edge apposition compared with those without(P = 0.33). Mean relative IOP (% of baseline) was68.4 � 12.1% (range 40.9–94%) with intact flap edges and65.4 � 14.5% (range 18.5–97.2%) following flap edge exci-sion (P = 0.31). Flaps measuring 4 ¥ 4 mm and 3 ¥ 3 mmbehaved in a similar manner with minimal change in equi-librium IOP following excision of flap edges.

Conclusions: In this experimental model, scleral flap edgeapposition is not required for generating outflow resistance.Suture tension generated during tight flap closure producesapposition of the underside of the scleral trapdoor to the

underlying bed, and it is this apposition, which determinesIOP.

Key words: adjustable suture, experimental, glaucoma inci-sional surgery, intraocular pressure, trabeculectomy.

INTRODUCTION

Trabeculectomy, or guarded filtration surgery, is the mostcommon glaucoma procedure performed worldwide, andwas developed in an attempt to reduce overfiltration andhypotony-related complications commonly associated withfull thickness sclerostomy formation.1,2

Various modifications of surgical technique have beenproposed in an attempt to reduce complications and main-tain adequate long-term intraocular pressure (IOP) control.3–10 However, a limited number of clinical studies oftrabeculectomy suggest that variations in the shape5 andsize6,9 of the scleral flap and internal sclerostomy4,6 and tight-ness of scleral flap closure7,8 do not influence long-term IOPcontrol, which is largely determined by the healing responseof the subconjunctival tissues.11 Application of light cauteryto the lateral margins of the scleral flap to retract opposingtissue edges has also been suggested to facilitate filtration.12

None of these clinical studies involved the use of antime-tabolites such as mitomycin C (MMC), in which low IOP inthe first two postoperative weeks is a risk factor for moreprolonged hypotony.13 Tight scleral flap closure shouldreduce the risk of early hypotony,7 and in the context of theimpaired healing response in enhanced trabeculectomy, thisbecomes even more important. Scleral flap construction,suture placement and tension are the keys to avoiding com-plications associated with hypotony. Tight flap closure fol-lowed by postoperative manipulation of adjustable scleralflap sutures may allow more reliable and titratable reductionof IOP to desirable levels than suture removal, suture lysis orocular massage.14

� Correspondence: Mr Wayne Birchall, Northland Hospital, Maunu Road, Whangarei 0110, New Zealand. Email: w.birchall@xtra.co.nz

Received 31 March 2007; accepted 27 March 2008.

Clinical and Experimental Ophthalmology 2008; 36: 353–357doi: 10.1111/j.1442-9071.2008.001738.x

© 2008 The AuthorsJournal compilation © 2008 Royal Australian and New Zealand College of Ophthalmologists

To evaluate the importance of scleral flap tissue edgeapposition relative to flap-bed apposition, we assessed theeffects on IOP of removing small segments of the edges ofscleral flaps of various sizes secured with adjustable sutureswhile making real-time measurements of IOP in a previouslydescribed experimental steady state infusion model,14–16

which has been shown to have clinical relevance.17

METHODS

Six grossly normal phakic human donor eyes with no historyof previous intraocular surgery or glaucoma were availablefor study. The experimental design, surgical technique andmethod of adjustable suture placement and tying have beendescribed in detail elsewhere.14,18,19 In brief, the anteriorchamber of each eye was intubated with a 24G intravenouscannula connected via non-expansile tubing to a continuousflow syringe pump (Harvard Apparatus, Holliston, MA,USA, http://www.harvardapparatus.com) and a Powerlabreal-time IOP monitoring system (AD Instruments Pty Ltd,Castle Hill NSW 2154, Australia, http://wwwadinstruments.com) (Fig. 1).

The IOP sensor was calibrated and zeroed with respect toatmospheric pressure prior to initiation of each trabeculec-tomy site. An infusion via the system was then commencedand flow rate required to keep the IOP of the eye underinvestigation between 23 and 30 mm Hg determined. Theinfusion rate varied from eye to eye and ranged from 10 mL/min to 18 mL/min.

After the IOP was stable, conjunctiva and loose episcleraltissue was removed from one quadrant to prepare a site forthe procedure. Three sizes of scleral flap were created inotherwise identical procedures: 4 ¥ 4 mm (area 16 mm2),3 ¥ 3 mm (area 9 mm2) or 3 ¥ 2 mm scleral flap (area 6 mm2).Initially, a half thickness incision was made behind and tan-gential to the limbus using a 0.3 mm guarded blade (Beaver)and this incision converted to a scleral tunnel with a crescentblade (Alcon, Fort Worth, TX, USA) with the dissection

extending to clear cornea. The sides of the scleral flap werecut with a 45 degree blade (Alcon) up to the limbus.

A 1 mm diameter hemispherical block of corneo-trabecular tissue (area 0.76 mm2) was excised using a trab-eculectomy punch (Moria Instruments, Antony, France,http://www.moria-surgical.com) and a peripheral iridectomyperformed with scissors. Two 4-throw adjustable sutureswere placed at the corners of the flap as previouslydescribed.

The IOP was allowed to reach equilibrium while main-taining the initial infusion rate, and the equilibrium IOPrecorded. The time to reach equilibrium varied from 4.1 to7.9 min (mean 6.4 min) for all flaps. The two sutures wereuntied and the equilibrium IOP for the un-sutured flaprecorded. A small broad based triangular segment wasexcised from the centre of each of the three cut edges of thescleral flap (Fig. 2a) with the length of the base of the tri-angle approximately one-third the length the flap edge. Theflap sutures were retied firmly and the equilibrium IOPre-measured.

If there was sufficient space for another trabeculectomyand the condition of the eye under investigation was suitablewithout visible scleral dehydration, the scleral flap wasclosed tightly with additional sutures, the surface of thescleral flap dried and cyanoacrylate adhesive applied to theflap edges to seal the wound. If the IOP could be maintainedabove 23 mm Hg at the initial infusion rate, the above stepswere repeated at a new site. Owing to procurement difficul-ties, multiple trabeculectomies (between one and three) wereperformed on each eye.

Data were collated in Microsoft Excel Spreadsheets andstatistical analysis performed using Excel Data Analysissoftware. The two-tailed t-test for paired samples was used tocompare IOP outcomes before and after flap side-excision.

RESULTS

Twelve procedures were performed in six eyes. For all flapsizes, absolute IOP outcomes and IOP relative to baseline are

Figure 1. Experimental set-up.The syringe pump (right) provides aconstant flow of balanced salt solu-tion into the donor eye. Intraocularpressure is measured via a transducerattached to a personal computer fordata acquisition and analysis.

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summarized in Figure 3. Minimal outflow resistance prior toflap closure was confirmed by a mean IOP after fashioning ofthe initial scleral flap and sclerostomy of 0.72 mm Hg (range0–1.9 mm Hg).

For all flaps (n = 12) following flap closure mean absoluteIOP was 19.5 � 3.9 mm Hg (range 12.4–27 mm Hg) withintact flap edges and 18.7 � 4.4 mm Hg (range 5.6–27.9 mm Hg) following flap edge excision (P = 0.33). Meanrelative IOP (% of baseline) was 68.4 � 12.1% (range 40.9–94%) with intact flap edges and 65.4 � 14.5% (range 18.5–97.2%) following flap edge excision (P = 0.31).

Flaps measuring 4 ¥ 4 mm and 3 ¥ 3 mm behaved in asimilar manner with minimal change in equilibrium IOP fol-lowing excision of flap edges. Mean IOP values (range)before and after edge excision were 18.5 mm Hg, 69.6%(15.6–21.2 mm Hg, 65–78.5%) and 19.2 mm Hg, 72.3%(15–25 mm Hg, 62.5–92%) for 4 ¥ 4 mm flaps and23.2 mm Hg, 77.6% (16.9–27 mm Hg, 57–94%) and 16–27.9 mm Hg, 74.9% (16–27.9 mm Hg, 53–97%) for 3 ¥ 3 mmflaps.

The equivalent values were lower for 3 ¥ 2 mm flapsat 17 mm Hg, 58% (12.4–21.7 mm Hg, 40.9–77.5%) and14.5 mm Hg, 49% (5.6–20 mm Hg, 18.5–63.1%). Thisgroup contained one procedure with the lowest initial IOP of40.9% (the only one with initial IOP <50% of baseline) andthe largest drop and lowest IOP following edge excision of

18.5%. In contrast to the others this flap thickness was notapparently even, being thinner to one side and less than halfscleral thickness.

With this flap excluded, values for this flap size were18.5 mm Hg, 63.8% (15.9–21.7 mm Hg, 57.2–77.5%)before and 17.5 mm Hg, 59.3% (15.6–20 mm Hg, 54–63.1%) after edge excision.

DISCUSSION

The results of this study suggest that, for scleral flaps at leasthalf scleral thickness with relatively tight suture closure,apposition of the flap tissue edges is not required to maintaina reasonable IOP following flap closure. In the first weeksafter trabeculectomy, the scleral flap regulates the outflow ofaqueous from the eye, and the principal resistance to outflowis therefore the apposition of the flap to the underlyingscleral bed around the sclerostomy.

As with most studies of this nature, this study is limited bya small number of eyes and procedures, resulting fromlimited by availability of donor eyes. Several procedures hadto be completed on each individual eye, so the trabeculec-tomy sites could not be considered truly independent. It ispossible that a subtle change in IOP resulting from the exci-sion of flap edges could be detected in a much larger studyusing similar methods, but we feel that the data presented

Figure 2. Diagram of a scleral flap(3 ¥ 3 mm flap illustrated) showing(a) position of flap sutures and cutaway flap edges (black triangles) (b)area of maximal scleral flap apposi-tion (grey triangle) providingresistance to aqueous flow (dottedarrows). Bold arrows representhypothetical diagonal ‘tension lines’within the flap (see Discussion).

Figure 3. Box plots showing abso-lute intraocular pressure (left) andintraocular pressure relative to base-line (right) for all flaps (n = 12). Boxrepresents upper and lower quartiles,horizontal line median and whiskersupper and lower decile values. Smallsolid box represents mean intraocu-lar pressure.

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here demonstrate that apposition of flap edges does not havea major effect on control of IOP in the early stages followingtrabeculectomy.

There are some other limitations to this work. The donoreyes did not have impaired aqueous outflow, so the infusionflow rates needed to be higher than physiological aqueousflow rates to give an IOP commensurate with a typical glau-coma eye prior to trabeculectomy. Outflow facility was notmeasured; this was because (i) the time-consuming natureof the testing may have resulted in tissue deterioration/dehydration affecting results in later procedures on a giveneye, and (ii) because there was significant non-sclerostomyaqueous outflow.

Although the above issues are in theory problematic, notonly this laboratory flow model of trabeculectomy is wellestablished,14,18,19 but findings from it have been validated forpostoperative titration of IOP following glaucoma surgery.17

Adjustable sutures were used in this study because wefavour their use in clinical practice and our group has previ-ously demonstrated the clinical application of these suturesand the laboratory model used to study them.17–19 They canbe tied firmly, but with an upper limit of maximum tension,and are likely to give better reproducibility of suture tensionthan fixed sutures, where the second throw may shorten theknot and suture loop by a variable and non-titratable amount.Adjustable sutures in the experimental setting could also beuntied to allow the interventions to the scleral flap.

One flap in the 3 ¥ 2 mm group appeared thinner at oneside with associated increased outflow and was the only flapwith initial IOP <50% of baseline and poor IOP maintenanceafter edge excision. Mean IOP before and after edge excisionwas slightly lower in this group even after exclusion of thissingle technically inferior flap. In a previous study using thesame techniques with larger numbers we found comparableIOP control between 4 ¥ 4 mm and 3 ¥ 2 mm flaps.18

Considering all flaps, edge excision resulted in smallreductions in IOP in six, a small increase in IOP in four andstatic IOP in two. Although we attempted to standardizesuture tension by tight closure with adjustable sutures, smallchanges in IOP in either direction after edge excision couldbe explained by small differences in suture tension as well assmall changes in the biomechanical properties of the flap.The ability of the scleral flap to lift from its bed and allowflow from the sclerostomy to the bleb will depend upon flapdeformability (which will be influenced by flap constructionand thickness18 and scleral elasticity), tension within the flapdetermined by suture position and tension,14 and the positionof the flap hinge and side-cuts relative to the sclerostomy.18,19

The data presented here support our hypothesis that it isthe flap-bed interface that controls IOP with the principalresistance to aqueous egress beneath the flap created byapposition of the scleral flap to its bed. Using the example ofa square flap divided into four equal triangles, apposition islikely to be greatest beneath the triangular area (around thesclerostomy) bounded by the flap hinge and the intersectingdiagonals between the flap corners and diametrically oppo-site ends of the flap hinge (Fig. 2b). The degree of ‘tension’

along these diagonal lines will depend on the ‘tightness’ ofthe sutures and stretching of the flap. Subtle ‘tension’ lines arevisible in this model with flattening of the flap along thediagonals and slight lifting and gaping of the flap edges, andare more marked when non-adjustable sutures are tied tightlyresulting in more stretching of the flap along these diagonallines.

Creation of significant tension along the diagonals maynot be essential per se in the presence of a thick (and rela-tively rigid) flap well anchored at the corners as it will berelatively resistant to stretching and lifting off the scleral bedmaintaining good ‘apposition’. Conversely, good aqueousoutflow resistance may be difficult to achieve with twoadjustable sutures if a scleral flap is cut too thin, In additionto placing additional sutures in this situation, converting tofixed and tighter corner sutures may increase outflow resis-tance by stretching the flap along the diagonals and we havedemonstrated this principle in our experimental model withsignificant elevation of IOP when the sutures are tightened(W Birchall, unpubl. obs., 2004).

Ideally, the scleral flap should be at least half scleral thick-ness to reduce flap deformability, risk of dehiscence andsuture cheese-wiring, which could contribute to excessiveaqueous outflow10. With the exception of a single 3 ¥ 2 mmflap, all flaps were of even thickness and at least half scleralthickness. Previous studies using this experimental modelsuggest that thin or asymmetric flaps and flaps which sitpoorly in the scleral bed at the corners may not exert suffi-cient flow resistance and result in low IOP maintainance18,19

and support the importance of good surgical technique.In conclusion, this study demonstrated scleral flap edge

apposition is not essential for good intraoperative control ofaqueous outflow resistance and maintenance of IOP in thepresence of a well-constructed flap with relatively tightsuture closure.

ACKNOWLEDGEMENT

This work was supported by funding from the Capital VisionResearch Trust.

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