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Aim:
To compare the outcomes of IntraLase-enabled top hat penetrating keratoplasty (IEK) versus retrospective results of manual top hat penetrating keratoplasty (TH-PKP) and conventional PKP.
Patients/methods:
This non-randomised prospective study included 94 eyes: 23 eyes underwent IEK, 36 TH-PKP and 35 conventional PKP.
Preoperative and postoperative manifest refraction, uncorrected and best-spectacle corrected visual acuity (BSCVA), high-order ocular aberrations (HOA), endothelial cell counts and complications were analysed.
Results:
At 12 months of follow-up, the mean log MAR BSCVA was 0.32 (SD 0.31) in the IEK group, 0.53 (0.36) in the TH PKP group (pâ[euro]S=â[euro]S0.03) and 0.39 (0.30) in the conventional PKP group (pâ[euro]S=â[euro]S0.4). The mean spherical equivalent was similar between the groups and was less than -2.2 dioptres. The mean cylinder was similar in the IEK and conventional PKP group (3.6 (1.9) dioptres and 4.1 (1.8) dioptres, respectively), and was significantly lower than the TH-PKP group (5.1 (3.2) dioptres, pâ[euro]S=â[euro]S0.04). The complications rate and high-order ocular aberrations were similar between the three groups studied. The mean endothelial cell loss was significantly lower at 12 months of follow-up in the IEK and the TH-PKP groups versus conventional PKP (32.4% and 22.3% vs 40.8%, respectively) (pâ[euro]S=â[euro]S0.05). The mean time to suture removal was 4.1 (1.2) months in the IEK group and 3.9 (1.5) months in the TH-PKP group versus 9.7 (1.1) months in the conventional PKP group (p<0.0001).
Conclusions:
IEK is a safe and stable procedure. It results in higher endothelial counts and faster suture removal in comparison with the conventional PKP, and has less astigmatism and better BSCVA in comparison with the manual TH-PKP.
Full text
The concept of top hat wound configuration for penetrating keratoplasty (TH-PKP) was introduced to ophthalmology 50 years ago by Jose Barraquer, who named it "a keratoplasty in two planes." 1 Busin from Italy reported in 2003 his experience with this novel wound configuration. 2 This technique combined the visual outcomes of PKP with the wound-healing advantage of lamellar keratoplasty. This manual top hat dissection technique has not reached worldwide popularity due to the time required in preparing this top hat shape donor and recipient, with actually similar visual outcomes to conventional PKP. Recently, we analysed our outcomes with the manual TH-PKP in comparison with the conventional PKP, and found that TH-PKP speeds up suture removal and contributes to higher endothelial cell counts in the grafts 1 year after surgery. 3
An easier and more precise way to create the top hat wound configuration is to use of the femtosecond laser, which has been used in the last decade for flap creation in lamellar refractive surgeries. During the last 3 years, several groups 4 - 6 have determined the feasibility of using the femtosecond laser in a human cadaver eye model to create the "top hat" wound configuration for PKP. The mechanical stability and induced astigmatism of this novel procedure were compared with the traditional PKP procedure and were found to be more mechanically stable than those produced by the traditional method, 4 - 6 and even more stable than other wound configurations created by the femtosecond laser itself. 7 Several publications have appeared since 2007, reporting on the preliminary results of femtosecond laser top hat and zigzag wound configuration for PKP. 8 - 12 Those reports demonstrated biomechanically stable wound, excellent wound apposition and faster recovery of best-spectacle corrected visual acuity visual acuity (BSCVA) with moderate astigmatism.
The main aim of this study was to compare, for the first time, the visual outcomes and complication rate of femtosecond laser (IntraLase Corp. Irvine, California) enabled top hat PKP (IEK) versus retrospective data on non-laser, manual top hat PKP (TH-PKP) and conventional PKP cases.
PATIENTS AND METHODS
In this prospective non-randomised study, we included 94 patients who underwent corneal transplantation at the cornea service of the Toronto Western Hospital, Toronto, Canada. Of these 94 patients, 23 consecutive patients underwent IntraLase-enabled keratoplasty (IEK), 36 underwent TH-PKP, and 35 underwent conventional PKP. Subjects who underwent additional procedure during corneal transplantation (ie, cataract surgery) were excluded from this study. The study was approved by the research ethics board of the University Health Network. The data presented here for the control groups (TH-PKP and conventional PKP) were retrospective in nature and were published previously. 3
The data collected included pre- and postoperative uncorrected (UCVA) and BSCVA, intra- and postoperative complications and postoperative clinical course. Endothelial cell density analysis was performed in all subjects at 6 and 12 months postoperative time gate using a non-contact specular microscope (ROBO, Konan storage system KSS 300; Konan Medical, Hyogo, Japan). Corneal topography and wavefront analysis of high-order aberrations were performed using a combined topographer and wavefront analyser (NIDEK, OPD Scan II ARK 10000). The second- to sixth-order aberrations were measured and are presented as root mean square (RMS (μm)) values and at a pupil diameter of 6 mm.
The patients were followed at 1 day, 1 week and 1, 3, 6 and 12 months postoperatively.
SURGICAL TECHNIQUE
All patients had a retrobulbar anaesthesia with complete akinesia of the lids.
INTRALASE-ENABLED KERATOPLASTY
The host cornea was measured for "white to white" diameter and corneal thickness at the planned incision, and the centre of the cornea was marked with a marking pen.
The donor cornea was removed from Optisol (Bausch & Lomb Surgical, Irvine, California) storage, mounted on an artificial anterior chamber and brought to the 60 kHz femtosecond laser, which was programmed to perform a top-hat-shaped cut, with certain depth and diameter (based on the recipient cornea measurements).
The cut pattern used on the host was identical in shape to that used on the donor, except that the outer diameter was oversized by 0.2 mm on the donor, and except for an uncut gap of 50 µm that was left between the posterior side cut and lamellar cut in the recipient's cornea only, in order to avoid corneal perforation during the laser cut. This gap was bluntly dissected during surgery using a Sinskey hook. The donor button was positioned by sliding the peripheral wing under the superficial stromal lip of the recipient bed. Four 10-0 nylon cardinal sutures were placed. Each suture exited the donor button wing and was then passed through the recipient stromal lip.
In the recipient cornea, the mean anterior side cut diameter was 7.51 (SD 0.45) mm, and the mean posterior side cut diameter was 8.78 (0.24) mm. The donor cornea was oversized by 0.2 mm in all cases. The mean posterior donor diameter was 8.97 (0.25) mm.
MANUAL TOP HAT PKP
The TH- PKP technique was performed as previously described by Busin, 2 with a modification in the suturing technique as described by us recently. 3 Briefly, the donor button was mounted on an artificial anterior chamber (Moria, Doylestown, Pennsylvania). The geometric centre of the cornea was marked, and a 7.0 or 7.5 mm Hanna trephine was used to make a circular, 0.4 mm deep incision from the epithelial side. A lamellar stromal dissection was carried out with a bevel-up crescent knife from the base of the incision all the way to the limbus. Following this, an 8.5 or 9 mm blade was placed in the Hanna trephine, and a concentric trephination process was carried out from the epithelial side, with the depth set for a full thickness cut. The donor button obtained this way consisted of a central, full-thickness part, 7.0 mm in diameter, surrounded by a peripheral lamellar wing of deep stroma and endothelium that was 0.75-1.0 mm in width. The recipient bed was prepared in a similar way: a 7.0 or 7.5 mm Hanna suction trephine was used to cut a circular incision 0.4 mm in depth. A lamellar stromal dissection was carried out with a bevel-up crescent knife from the base of the incision and extending about 1.5 mm peripherally. The anterior chamber was then entered, and curved Castroviejo corneal scissors were used to complete the excision of the corneal button at the periphery of the posterior lamellar stromal dissection. The donor button was positioned by sliding the peripheral wing under the superficial stromal lip of the recipient bed. Four 10-0 nylon cardinal sutures were placed. Each suture exited the donor button wing and was then passed through the recipient stromal lip.
The mean donor diameter was 8.8 (0.25) mm.
CONVENTIONAL PKP
In the conventional PKP group, the donor button was cut with the 7.5-8 mm Hanna punch trephine. The donor buttons were punched from the endothelial side on curved blocks.
The recipient bed was prepared to closely conform to the shape of the donor button. A 7.5-8 mm Hanna suction trephine was used to cut a partial depth, circular incision in the cornea, centred at the geometric centre of the cornea. Excision of the recipient corneal button was completed with curved Castroviejo corneal scissors. The mean donor diameter was 7.75 (0.55) mm.
In all groups, either 16 interrupted sutures (eight sutures of 10-0 nylon, eight sutures of 10-0 vicryl) and a single continuous 16-bite 11-0 of nylon suture, or eight interrupted sutures (10-0 nylon) and a single continuous 16-bite 10-0 of nylon suture were placed.
Postoperatively all eyes were treated either with a topical combination of antibiotic and steroid (Tobradex, Alcon, Fort Worth, Texas) or a separate topical antibiotic and steroid eye-drops (0.1% dexamethasone sodium phosphate (Maxidex, Alcon, Fort Worth, Texas)) and moxifloxacin 0.5% (Vigamox; Alcon Laboratories, Fort Worth, Texas). The topical antibiotic was discontinued 1 week postoperatively, and the topical steroids were gradually tapered off based on our protocol (four times a day in the first month, three times a day in the second month, twice a day in the third month and once a day from then on). For subjects on topical combination, the drops were discontinued at 4 weeks and were switched to a topical steroid (Maxidex, Alcon, Canada).
In general, suture removal was done as a part of a protocol, at 3-6 months in the top hat PKP groups and at 12 months in the conventional PKP group. Suture removal was not selective. Loose sutures were removed upon diagnosis in all subjects.
STATISTICAL ANALYSIS
Baseline characteristics and outcome variables were compared between the groups with the Fisher exact test for categorical variables and Student t test for continuous variables. Proportions were compared using χ2 test for proportions. p Values less than 0.05 were considered statistically significant.
Rejection-free survival rates were plotted using the product-limit method of Kaplan and Meier, and compared using the logrank test. Survival of endothelial cells was plotted as a proportion of the preoperative cell count in the graft.
RESULTS
This study included a total of 94 eyes of 94 patients aged 23-83 years.
Table 1 summarises the baseline characteristics and the indications for transplants in the three study groups: IEK (n = 23), TH-PKP (n = 36) and conventional PKP (n = 35).
IEK (n = 23) | TH-PKP (n = 36) | Conventional PKP (n = 35) | p Value IEK versus TH-PKP | p Value IEK versus C-PKP |
||
Proportion of males | 53.2% | 68.6% | 54.2% | 0.35 | 0.84 |
|
Proportion of right eyes | 56.5% | 45.7% | 45.7% | 0.58 | 0.59 |
|
Age (SD) (years) | 42.2 (18.7) | 59.6 (20.4) | 63.6 (20.2) | 0.001 | 0.0002 |
|
Donor endothelial cell count (cells/mm2 ) | 2836 (292) | 2586 (201) | 2723 (416) | 0.0003 | 0.26 |
|
Follow-up (mean, months) | 9.9 (3.7) | 10.6 (2.7) | 39.8 (37.9) | 0.1 | 0.0001 |
|
Mean preop best-spectacle corrected visual acuity (logMAR) (median) | 1.45 (0.48) (20/400) | 1.66 (0.68) (counting fingers) | 1.36 (0.64) (20/400) | 0.20 | 0.56 |
|
Mean preop IOP | 13.7 (4.5) | 16.4 (3.9) | 15.5 (4.2) | 0.01 | 0.12 |
|
Reason for transplant | PBK | 4 (17.4%) | 14 (38.9%) | 13 (37.1%) | 0.14 | 0.18 |
Fuchs dystrophy | 0 (0%) | 1 (2.8%) | 7 (20%) | 0.82 | 0.06 |
|
Keratoconus | 12 (52.2%) | 9 (25%) | 8 (22.9%) | 0.06 | 0.04 |
|
Corneal scar | 6 (26.1%) | 6 (16.7%) | 3 (8.6%) | 0.58 | 0.15 |
|
Stromal dystrophy | 0 (0%) | 1 (2.8%) | 0 (0%) | 0.82 | - |
|
Previous graft failure | 1 (4.3%) | 4 (11.1%) | 3 (8.6%) | 0.66 | 0.91 |
Statistically significant values are shown in bold.
IOP, intraocular pressure; logMAR, logarithm of minimal angle of resolution; PBK, pseudophakic bullous keratopathy.
Due to a significant difference in follow-up time between the three groups, data analysis of visual outcomes, endothelial cell counts and complication rates were performed at 12 months of follow-up.
In this series, other factors contributing to postoperative BSCVA worse than 20/40 included AMD (n = 2), end-stage glaucoma (n = 4), retinal detachment (n = 1), diabetic macular oedema (n = 2) and myopic macular scar (n = 1). These cases were eliminated from the visual and refractive outcome analysis.
Figure 1 shows a clinical picture of an IEK patient 1 week following surgery. Note the clear graft in its full-thickness part, surrounded by the hazier annular area of lamellar healing (top hat shape).
Figure 1Anterior segment picture of an IntraLase-enabled keratoplasty patient 1 week following surgery. Note the clear graft in its full-thickness part, surrounded by the hazier annular area of lamellar healing (top hat shape).
VISUAL OUTCOMES
Table 2 demonstrates the postoperative BSCVA in the three groups after 12 months of follow-up. The mean logMAR BSCVA was 0.32 (0.31) in the IEK group, 0.53 (0.36) in the TH PKP group and 0.39 (0.30) in the conventional PKP group. BSCVA was significantly better in the IEK group when compared with the TH PKP group (p = 0.03).
IEK (n = 16) | TH- PKP (n = 26) | Conventional PKP (n = 33) | p Value IEK versus TH-PKP | p Value IEK versus C-PKP |
||
Mean best-spectacle-corrected visual acuity (logMAR)(median) | 0.32 (0.31) (20/30) | 0.53 (0.36) (20/70) | 0.39 (0.30) (20/40) | 0.03 | 0.40 |
|
Mean IOP | 17.0 (4.4) | 17.6 (5.1) | 15.5 (5.0) | 0.64 | 0.24 |
|
Mean endothelial cell count (cells/mm2 ) | 1981 (474) | 1978 (277) | 1449 (516) | 0.97 | 0.0002 |
|
Mean endothelial cell loss (proportion of donor cell counts) | 32.4% (16.5) | 22.3% (9.1) | 40.8% (15.4) | 0.01 | 0.05 |
|
Time to sutures removal (months) | 4.1 (1.2) | 3.9 (1.5) | 10.7 (1.1) | 0.59 | <0.0001 |
|
Mean spherical equivalent (dioptres) | -2.2 (4.2) | -0.49 (3.96) | -0.89 (3.81) | 0.12 | 0.21 |
|
Mean cylinder (dioptres) | 3.6 (1.9) | 5.1 (3.2) | 4.1 (1.8) | 0.04 | 0.31 |
|
High-order aberrations (6 mm optical zone) | Total no of high-order aberrations | 3.9 (2.1) | 4.1 (3.2) | 3.1 (2.1) | 0.79 | 0.16 |
Total no of coma like aberrations | 1.9 (1.6) | 1.7 (1.6) | 1.5 (1.1) | 0.64 | 0.26 |
|
Total no of spherical aberrations | 1.1 (0.65) | 0.88 (0.74) | 0.49 (0.41) | 0.25 | 0.0001 |
|
Total no of trefoil like aberrations | 2.6 (1.4) | 2.5 (2.2) | 1.9 (1.6) | 0.84 | 0.09 |
Statistically significant values are shown in bold.
IOP, intraocular pressure; logMAR, logarithm of minimal angle of resolution.
The mean spherical equivalent and manifest astigmatism in the three groups studied after 12 months of follow-up are demonstrated in table 2 . The mean spherical equivalent was similar between the groups and was less than -2.2 dioptres.
The mean cylinder was similar in the IEK and conventional PKP group (3.6 (1.9) dioptres and 4.1 (1.8) dioptres, respectively), and was significantly lower than the TH-PKP group (5.1 (3.2) dioptres, p = 0.04).
High-order ocular aberrations were similar between the three groups studied, except for spherical aberrations, that were significantly higher in the IEK and TH-PKP groups, in comparison with the conventional PKP (1.1) (0.65 µm and 0.88 (0.74) µm versus 0.49 (0.41) µm, respectively; p = 0.0001).
COMPLICATIONS
Table 3 summarises the intraoperative and postoperative complications in the three groups.
Complications | IEK (n = 23) | TH-PKP (n = 36) | Conventional PKP (n = 35) | p Value IEK versus TH-PKP | p Value IEK versus C-PKP |
Rejection episodes | 1 (4.3%) | 4 (11.1%) | 7 (20%) | 0.66 | 0.19 |
Astigmatism treated with relaxing incisions/refractive procedure | 2 (8.7%) | 3 (8.3%) | 7 (20%) | 0.67 | 0.42 |
Graft failure | - | 1 (2.7%) | 2 (5.7%) | 0.79 | 0.66 |
Infectious keratitis | 2 (8.7%) | 1 (2.7%) | 1 (2.8%) | 0.67 | 0.69 |
Induced glaucoma | 2 (8.7%) | 3 (8.7%) | 6 (17.1%) | 0.63 | 0.60 |
Wound dehiscence | - | - | 2 (5.6%) | - | 0.68 |
Retinal detachment | - | 1 (2.7%) | - | 0.79 | - |
Complication rates were similar between the groups studied.
Figure 2 shows the Kaplan-Meier rejection-free survival curves of patients undergoing IEK versus TH-PKP versus conventional PKP. A lower rate of graft rejection was demonstrated in the IEK group in comparison with the TH-PKP group (hazard ratio = 0.39, p = 0.4, logrank test) and conventional PKP group (hazard ratio = 0.53, p = 0.5, logrank test), albeit not statistically significant.
![View Image - Figure 2<b>Kaplan-Meier rejection-free survival curves of patients undergoing IntraLase-enabled keratoplasty (IEK) versus top-hat penetrating keratoplasty (top-hat PKP) versus conventional PKP. A non-significant lower rate of graft rejection was demonstrated in the IEK group in comparison with the top-hat-PKP group (hazard ratioâ[euro]S=â[euro]S0.39, pâ[euro]S=â[euro]S0.4, logrank test) and conventional PKP group (hazard ratioâ[euro]S=â[euro]S0.53, pâ[euro]S=â[euro]S0.5, logrank test).</b>](https://media.proquest.com/media/hms/THUM/8/a8w3S?_a=ChgyMDI2MDEyMTA3MjAwNTY4OToyODYwMzESBzE0MTM1MzcaCk9ORV9TRUFSQ0giDjIxNi43My4yMTYuMTE0KgcyMDQxMDM5MgoxNzc4OTI4ODU3OgtJbmxpbmVJbWFnZUIBMFIGT25saW5lWgNJTEliBFRIVU1qCjIwMDkvMDEvMDFyCjIwMDkvMDEvMzF6AIIBPFAtMTAwNzA2Ny0yMDg2MTEtQ1VTVE9NRVItMTAwMDAzNDgvMTAwMDAyNTUvMTAwMDAxNTUtNjY5MzE2NJIBBk9ubGluZcoBDlNlYXJjaFJldHJpZXZl0gESU2Nob2xhcmx5IEpvdXJuYWxzmgIHUHJlUGFpZKoCLE9TOkVNUy1UZXh0UGx1c0dyYXBoaWNzQ29udGFpbmVyLXByZXBhcmVCb2R5ygIPQXJ0aWNsZXxGZWF0dXJl4gIA8gIA%2BgIBToIDA1dlYooDHENJRDoyMDI2MDEyMTA3MTkyMzE4NToyNTQyNjY%3D&_s=rU6qVrmvEP4OHlhdShXzy2efZKs%3D "View Image - Figure 2<b>Kaplan-Meier rejection-free survival curves of patients undergoing IntraLase-enabled keratoplasty (IEK) versus top-hat penetrating keratoplasty (top-hat PKP) versus conventional PKP. A non-significant lower rate of graft rejection was demonstrated in the IEK group in comparison with the top-hat-PKP group (hazard ratioâ[euro]S=â[euro]S0.39, pâ[euro]S=â[euro]S0.4, logrank test) and conventional PKP group (hazard ratioâ[euro]S=â[euro]S0.53, pâ[euro]S=â[euro]S0.5, logrank test).</b>")
Figure 2Kaplan-Meier rejection-free survival curves of patients undergoing IntraLase-enabled keratoplasty (IEK) versus top-hat penetrating keratoplasty (top-hat PKP) versus conventional PKP. A non-significant lower rate of graft rejection was demonstrated in the IEK group in comparison with the top-hat-PKP group (hazard ratioâ[euro]S=â[euro]S0.39, pâ[euro]S=â[euro]S0.4, logrank test) and conventional PKP group (hazard ratioâ[euro]S=â[euro]S0.53, pâ[euro]S=â[euro]S0.5, logrank test).
The preoperative donor endothelial cell density was obtained from the records of the Eye Bank of Canada, Ontario Division. Endothelial cell counts from the donor cornea averaged 2836 (292) cells/mm2 in the IEK group versus 2586 (201) cells/mm2 and 2723 (416) cells/mm2 in the TH-PKP and conventional PKP groups, respectively. Figure 3 and table 2 present the endothelial cell counts at various time intervals after surgery as a fraction of the preoperative cell counts in the graft (endothelial cell loss). The mean endothelial cell counts were significantly higher at 12 months of follow-up in the IEK and TH-PKP groups versus the conventional PKP group (p = 0.0002) (1981 (474)/mm 2 and 1978 (277)/mm2 vs 1449 (516)/mm2 , respectively). This represents 32.4% and 22.3% vs 40.8% endothelial cell loss, respectively (p = 0.05).
Figure 3Endothelial cell survival curves of patients undergoing IntraLase-enabled keratoplasty versus top-hat penetrating keratoplasty (top-hat PKP) versus conventional PKP. Values are represented as a proportion of the preoperative graft cell counts.
The mean time to suture removal ( table 2 ) was 4.1 (1.2) months in the IEK group and 3.9 (1.5) months in the TH-PKP group versus 9.7 (1.1) months in the conventional PKP group (p<0.0001).
DISCUSSION
This series presents the largest series to date reporting on the IEK procedure, and the only one that compares its outcomes to the manual TH and the conventional PKP techniques. Our results demonstrated that the IEK group of patients benefited from the advantages of both the TH-PKP and the conventional PKP procedures. The IEK procedure enabled good visual and refractive outcomes (similar to the PKP group), faster suture removal and higher endothelial cell counts (similar to the TH-PKP group).
Several studies have appeared lately on the outcomes of femtosecond laser penetrating keratoplasty. 8 - 12 Table 4 summarises the visual outcomes of these recent studies, in comparison with our results.
Article | Method/wound shape | No of eyes/follow-up | Best-corrected visual acuity | Cylinder | Suture removal | Endothelial cell counts cells/mm2 |
Buratto and Bohm 8 | 15 kHz femtosecond laser (IntraLase)/top hat | 7/3 | 20/60 to 20/20 | 2 to 5 D | Partial at 3 months | 2314 |
Price and Price 9 | 15 kHz femtosecond laser (IntraLase)/top hat | 6/12 | 20/400 to 20/25 | 2.25 to 8 D | Complete 7.0 (1.9) | 2030 (600) |
Farid et al 10 | Femtosecond laser (IntraLase)/Zigzag | 13/3 to 9 | 8 eyes>20/30 | <4.25 D | Sutures in situ, adjustment of sutures in five patients | NA |
Por et al 11 | 10 kHz Femtec/straight Conventional cut | 8/9.5 | 20/80 to 20/20 | 2.56 (0.5 to 4) | Not mentioned | NA |
Holzer et al 12 | 10 kHz Femtec/straight conventional cut | 5/up to 10 months | NA | NA | NA | NA |
Present study, Bahar et al | 60 kHz femtosecond laser (IntraLase)/top hat | 23/6.9 (3.7) | 20/300 to 20/20 Median 20/30 | 3.6 (1.9) | 4.1 (1.2) Interrupted | 1981 (474) |
As can be seen in the table, our visual and refractive outcomes are comparable with the IEK series reported. Furthermore, the visual (BSCVA) and the astigmatism results reported here for the IEK group are similar to those reported for the traditional PKP. 13 - 18 One would expect a lower astigmatism with the more exact match of donor to recipient cut, especially with the use of the radial alignment incisions. However, other factors such as the difference in steepness between recipient and donor corneas, and the wound tension created by the stitches we use, have not been addressed here.
The use of the IntraLase certainly facilitates the creation of a top hat wound configuration, and enables a more exact match of recipient to donor ( fig 4A ).
Figure 4Visante optical coherence tomography of IEK (A) and top-hat penetrating keratoplasty (TH-PKP) (B) 6 months postoperatively, demonstrating the perfect match of recipient to donor in the IEK cornea (A). In contrast, the manual TH-PKP incision may create tags of tissue protruding to the anterior chamber (B), due to the lack of precise match of cut between the two pieces of tissue. Why are some of the values shown in bold in the tables?
In contrast, the manual TH-PKP incision may create tags of tissue protruding to the anterior chamber ( fig 4B ), due to the lack of precise match of cut between the two pieces of tissue.
Top hat PKP offers the advantage of earlier visual rehabilitation: the wound seems to stabilise sooner, and sutures may be removed at around 3-5 months, rather than 10-12 months in the conventional PKP group. The shortest time to suture removal was in the TH-PKP group of this study. It is possible that the manual top hat creates a rougher surface and therefore may contribute to better and faster peripheral healing. We believe that this point should be studied further.
Another significant advantage that was demonstrated in this study and in our previous study 3 is the higher endothelial cell density of the top hat PKPs in comparison with the conventional PKP. This could be a manifestation of the larger diameter of posterior donor surface (8.5 or 8.87 mm diameter of corneal endothelium in the top hat groups vs 7.75 mm in the conventional PKP group) that is transplanted in this technique. A larger posterior transplant substitutes a much larger portion of the recipient endothelial layer. Spreading of endothelia cells across the surgical wound over a much smaller residual posterior surface of the recipient cornea is substantially reduced and results in a higher central/paracentral endothelial cell counts. An interesting observation is also the lowest endothelial cell loss in the TH-PKP group (22.3% in the TH-PKP group vs 32.4% in the IEK group). We believe that this finding should be further studied. The creation of a top hat shape donor cornea was significantly easier with the IEK than with the manual TH-PKP corneas. It is possible that the femtosecond laser energy induces some damage to the endothelium during the performance of the incision cut. This point should be addressed in a laboratory study, using vital stains of endothelial cells.
Although not statistically significant, rejection rates seem to be lower in the IEK group compared with the manual groups, 1 year following the operation ( fig 2 ). This could be a result of the perfect match between the donor and recipient which minimised the exposure of the donor tissue antigens to the anterior chamber fluid. On the other hand, IEK and TH-PKP patients seem to have a tendency to early rejections (not statistically significant). This could be the result of the larger endothelium layer in the top hat configuration. This observation should be further investigated in a larger sample series and longer follow-up time.
This study is limited by the relatively small sample size and short follow-up. The IEK eyes were significantly younger and had more keratoconus as the primary diagnosis. This may contribute to better visual results, faster healing, less comorbidity and fewer graft failures, compared with the other two series.
In summary, the IntraLase reliably cuts donor and recipient corneas for PKP, providing good visual outcomes. IEK resulted in higher endothelial counts and faster suture removal in comparison with the conventional PKP, and had a lower astigmatism and better BSCVA in comparison with the manual TH-PKP.
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Competing interests: None.
Ethics approval: Ethics approval was provided by the research ethics board of the University Health Network.
Patient consent: Obtained.
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