Corneal Densitometry After Small Incision Lenticule Extraction (SMILE) and Femtosecond Laser-Assisted LASIK (FS-LASIK): 5-Year Prospective Comparative Study

Assessing Changes in Corneal Densitometry in Patients After Small Incision Lenticule Extraction (SMILE)

PURPOSE

The aim is to assess for any lasting changes in corneal densitometry (CD) in patients who underwent small incision lenticule extraction (SMILE) and developed early transient postoperative corneal haze.

METHODS

This retrospective chart review analyzed 98 eyes from 49 patients who underwent SMILE at the Hoopes Vision Clinic and had one-year postoperative Pentacam CD (Oculus Optikgeräte GmbH, Wetzlar, Germany). These were compared to 78 eyes from 42 unoperated myopic control patients with documented CD measurements. The preoperative CD, measured in grayscale units (GSU), was compared between SMILE patients who developed early transient haze ("transient haze group"), SMILE patients who did not build haze ("non-haze group"), and patients who did not undergo any refractive surgery ("controls"). The postoperative CD was compared between the transient haze and non-haze groups. Then, the postoperative CD was compared to the preoperative CD for the non-haze group and transient haze group, respectively.

RESULTS

The only significant difference in preoperative CD was in the central layer of the cornea at the 6-10 mm annulus between the non-haze group and controls (17.31 ±3.77 vs. 16.18 ±3.92 GSU; p=0.04). Postoperatively, there were no differences in CD between the non-haze group and the transient haze group (p<0.05). Comparing postoperative CD to preoperative CD, the non-haze group had increased CD in the 2-6 mm annulus of the anterior (1.54 ±0.45 GSU; p=0.001), posterior (0.65 ±2.28 GSU; p=0.032), and full thickness (0.72 ±0.29 GSU; p=0.006) layers of the cornea in addition to an increased CD in the 6-10 mm annulus of the posterior corneal layer (1.19 ±0.55 GSU; p=0.049). The transient haze group had an increased CD in the 2-6 mm annulus of the anterior (2.61 ±1.09 GSU; p=0.021) and full thickness (1.44 ±0.68 GSU; p=0.039) layers of the cornea.

CONCLUSION

There is no lasting difference in CD between patients who develop postoperative transient haze and those who do not after undergoing SMILE.

Corneal densitometry: A new evaluation indicator for corneal diseases

Corneal densitometry (CD) uses the biological properties of the cornea to visualize the morphology of the cornea and determine the degree of corneal transparency. At present, it is an emerging metric that has shown promise in various clinical diagnosis and evaluation of eye diseases and surgeries. We introduce the different methodologies used to measure CD. Furthermore, we systematically categorize the diagnostic value of CD into high, medium, and low levels based on its clinical significance. By analyzing a wide range of conditions, including keratoconus, postrefractive surgery changes, and other corneal pathologies, we assess the utility of CD in each context. We also discuss the potential implications of these classifications for disease monitoring and prognosis evaluation. Our review underscores the importance of integrating CD assessments into routine clinical practice to enhance the accuracy and effectiveness of diagnostic processes for corneal disorders.

Refractive Surgery in Myopic Children

In this paper, we summarize the current knowledge on refractive surgery performed in the myopic pediatric population. We describe the main concerns about refractive surgery in myopic children and the indications for refractive surgery in this age group. We present a range of surgical procedures that are being used for the management of unilateral/bilateral myopia in children: corneal refractive surgery (PRK, LASEK, LASIK, FS-LASIK and SMILE) and intraocular refractive surgery (phakic intraocular lens implantation, refractive lens exchange or clear lens extraction), with both their advantages and drawbacks. We also describe the various complications and measures to prevent them.

Comparative Study of Primary SMILE, SMILE Enhancement, and Femtosecond Laser-Assisted LASIK on Higher Order Aberrations and Corneal Densitometry

PURPOSE

To compare differences in corneal densitometry (CD) and higher order aberrations (HOAs) in eyes that underwent small incision lenticule extraction (SMILE) and femtosecond laser-assisted laser in situ keratomileusis (FS-LASIK) for the treatment of myopia and myopic astigmatism at postoperative months 3, 6, and 12, and to evaluate their changes in a separate cohort of eyes after SMILE enhancement.

METHODS

In this prospective, randomized, paired-eye clinical trial, consecutive eligible participants were randomized to undergo SMILE or FS-LASIK in either eye. Main outcome measures were CD and HOAs preoperatively and at 3, 6, and 12 months postoperatively. A separate cohort of consecutive patients who had SMILE and underwent enhancement were also included for comparison.

RESULTS

For CD, no significant differences were found between SMILE and FS-LASIK up to month 12. For HOA measured by wavefront aberrometry, both SMILE and FS-LASIK had an increase in total root mean square (RMS) HOAs, spherical aberration (SA), and vertical coma up to month 12. SMILE had an additional increase in vertical quatrefoil, and FS-LASIK had an increase in horizontal coma at month 12. FS-LASIK had higher SA than SMILE, whereas SMILE had higher vertical quatrefoil than FS-LASIK at month 12. Central and posterior zone CD had significantly decreased after SMILE enhancement compared to after primary SMILE up to 2 years after enhancement. RMS HOAs, lower order aberrations, and SA were all increased after SMILE enhancement compared to after primary SMILE.

CONCLUSIONS

SMILE induced lower SA but higher vertical quatrefoil than FS-LASIK at 1 year. Both SMILE and FS-LASIK had similar increases in RMS HOAs and vertical coma up to 1 year. There were no differences in CD between both groups. SMILE enhancement additionally had decreased central and posterior CD but greater RMS HOAs and SA compared to primary SMILE. [J Refract Surg. 2024;40(5):e291-e303.].

Characterization the corneal transparency profile in SMILE-treated myopic patients with densitometry: A 7-year cohort study

PURPOSE

To investigate 7-year changes in corneal densitometry (CD) and its influencing factors following small incision lenticule extraction (SMILE) for moderate to high myopia.

METHODS

A total of 32 patients (52 eyes) with a mean spherical equivalent (SE) refraction of -6.30 ± 1.30 D who underwent SMILE were recruited for this prospective study. Pre- and postoperative CD and corneal aberrations were measured by an oculus Pentacam system, and patients were followed up to 7 years. The CD evolution over time and its influencing factors were explored.

RESULTS

No complications were observed after SMILE. There was a significant time effect on postoperative CD ( P < 0.05). The CD values at the 0-to-2 and 2-to-6 mm of the anterior layer increased at the first day ( P < 0.05) and declined to baselines at 1 year postoperatively; after that, CD values at any zone and any corneal layer presented a decreasing trend over time (all P < 0.05). Multivariate analysis showed that postoperative CD was positively correlated with age and optical zone (OZ), whereas negatively correlated with SE, corrected distance visual acuity (CDVA), and corneal aberrations (higher-order aberrations, Coma0 and Coma90) (all P < 0.05). Generalized estimating equation revealed that age, SE, CDVA, OZ, and Coma0 were the main factors influencing postoperative CD values (all P < 0.05).

CONCLUSION

CD manifested a long-term decreasing trend in moderate to high myopia treated with SMILE. Its main influencing factors are age, SE, CDVA, OZ, and Coma0.

Effect of sodium fluorescein strip application on cornea parameters commonly used in laser-assisted in-situ keratomileusis

To assess the effect of sodium fluorescein (NaF) strip on corneal parameters commonly used in Laser-assisted in-situ keratomileusis (LASIK). Eighty-six subjects (172 eyes) scheduled for LASIK were recruited between January and March 2022. The study and statistical analysis test were conducted in April 2022. Topographic measurements of corneal parameters, including central corneal thickness (CCT), anterior keratometric (K) readings (K1, flat keratometry; K2, steep keratometry), horizontal corneal diameter (white to white, WTW), and corneal asphericity (Q value), were obtained using a Scheimpflug device (Pentacam) before and 10 min after NaF strip treatmentThe Pentacam recorded a small significant increase in CCT (mean 538.88 ± 28.78 μm to 547.90 ± 29.94 μm; p < .001), with no differences in K1 and K2 (mean 42.24 ± 1.35D to 42.24 ± 1.35D, and mean 43.34 ± 1.50D to 43.32 ± 1.51D; P > .05, for all) as well as WTW(mean 11.58 ± 0.32 mm to 11.58 ± 0.32 mm, P > .05) before and after NaF strip intervention. Furthermore, there was no significant difference was observed in Q value (mean - 0.30 ± 0.13 to - 0.30 ± 0.14, P > .05). These results indicate that clinicians should avoid NaF strip application before obtaining precise topographic measurements of cornea parameters using the Pentacam.

Seven-year corneal densitometry changes after small incision lenticule extraction and femtosecond laser-assisted in situ keratomileusis

This study aims to investigate the 7-year changes in corneal densitometry (CD) after small incision lenticule extraction (SMILE) and femtosecond laser-assisted in situ keratomileusis (FS-LASIK). Forty-two eyes undergoing SMILE and thirty-one undergoing FS-LASIK were included in this retrospective study. CD was measured preoperatively and at 3 and 7 years postoperatively. Three concentric radial zones (0-2, 2-6, and 6-10 mm) of three different corneal depths (anterior 120 μm, posterior 60 μm, and central layer) were analyzed. In all three zones of the three corneal layers, no significant difference in CD was found between the two groups preoperatively and at 3 and 7 years postoperatively (all P > 0.05). All CD values in the three zones of the anterior and central layers significantly declined at 3 years postoperatively (all P < 0.001), and then increased at 7 years postoperatively compared to corresponding 3-year values (all P < 0.01). In the posterior layer, CD values in the 0-2-mm and 2-6-mm zones decreased at 3 years postoperatively (all P < 0.01) and continued to decrease at 7 years postoperatively (all P < 0.01). Similar CD trends were observed after SMILE and FS-LASIK. Posterior layer CD showed a decrease throughout the study period. In the anterior and central layers, CD values decreased and then increased at 7 years postoperatively. The transparency changes might have been due to age-related three-dimensional growth of collagen fibers and wound-healing reactions.

The influence of pterygium on corneal densitometry evaluated using the Oculus Pentacam system

Purpose

To assess the effect of pterygium on corneal densitometry (CD) values.

Methods

One hundred and nine patients (155 eyes) with primary pterygium were divided into a severe pterygium group (79 eyes) and a mild-to-moderate pterygium group (76 eyes) according to pterygium severity. Among them, 63 patients had monocular pterygium; and 25 patients (38 eyes) underwent pterygium excision combined with conjunctival autograft follow-up. A Pentacam anterior segment analyzer was used to obtain the CD values and corneal morphological parameters, including central corneal thickness (CCT), flat-axis keratometry (K1), steep-axis keratometry (K2), corneal astigmatism, irregular astigmatism, and spherical aberration. CD was subdivided into four concentric radial regions based on corneal diameter and three layers according to depth.

Results

CD values at 0-12 mm of the anterior 120 μm layer, 0-10 mm of the center layer and full thickness, and 2-6 mm of the posterior 60 μm layer were significantly higher in eyes affected by pterygium than in the contralateral unaffected eyes (all P < 0.05). CD values were significantly higher in the severe pterygium group than in the mild to moderate pterygium group (all P < 0.05). Corneal astigmatism, irregular astigmatism, K1, K2, CCT, and spherical aberration correlated with CD values in eyes with pterygium (all P < 0.05). CD values at 6-10, 0-12 mm in the anterior 120 μm layer and full thickness, 10-12 and 0-12 mm in the center layer were significantly decreased 1 month after pterygium surgery compared with those before surgery (all P < 0.05).

Conclusion

Patients with pterygium had increased CD values, particularly in the anterior and central layers. CD values were correlated with pterygium severity grading and corneal parameters. Pterygium surgery partially reduced the CD values.

Distribution of 50-layer corneal densitometry values and related factors

PURPOSE

To investigate the distribution of 50 layers of corneal densitometry and related factors.

METHODS

Clinical data, including age, sex, central corneal thickness, corneal keratometry, and diopters, were collected from 102 healthy participants (102 eyes) in this retrospective study. The cornea was divided into 50 layers, and densitometry of each layer at 19 points was measured by the Pentacam. The value versus the depth curve was plotted. Paired-sample t test and one-way analysis of variance were used to compare densitometry in different regions or depth. Statistical significance was defined as P < .05.

RESULTS

The densitometry values of the Bowman membrane (10-14% depth), anterior stroma (14-30% depth), epithelium (0-10% depth), and Descemet membrane (94-98% depth) decreased sequentially, and the densitometry values of the middle and posterior stroma (30-94% depth) and endothelium (98-100% depth) were the lowest. The higher the degree of astigmatism, the higher the second densitometry peak (R = 0.277, P < .001). The densitometry values of the vertex and superior parts of the cornea were higher than those in the periphery and inferior parts, respectively (all P < .001). In the Bowman membrane, the densitometry is lowest in the inferior nasal part, while in the Descemet membrane, it is lowest in the inferior temporal part.

CONCLUSION

Two densitometry peaks appeared near the Bowman membrane and Descemet membrane. For different depths, the distribution of densitometry within a layer is different. We provide a methodological reference and data basis for corneal research based on local changes in densitometry, and help understand the details of corneal structure from an optical perspective through detailed layering and zoning analysis of densitometry.

Corneal optical density: Structural basis, measurements, influencing factors, and roles in refractive surgery

The cornea is the main refractive medium of the human eye, and its clarity is critical to visual acuity. Corneal optical density (COD) is an important index to describe corneal transparency. Intact corneal epithelial and endothelial cells, regular arrangement of collagen fibers in the stroma, and normal substance metabolism are all integral for the cornea to maintain its transparency. In the last two decades, the Pentacam Scheimpflug imaging system has emerged as a breakthrough for the measurement of COD (also called corneal densitometry). It has been found that a wide variety of factors such as age, refractive status, and corneal diseases can affect COD. Different corneal refractive surgery methods also change COD in different corneal regions and layers and affect visual acuity following the surgery. Thus, COD has gradually become a significant indicator to evaluate corneal health, one on which the attention of clinicians has been increasingly focused.

Age-related changes in Scheimpflug corneal densitometry and their correlations with corneal topographic measurements in a healthy Chinese population

Background

Objective measurement of corneal densitometry (CD) values can be used to assess corneal transparency and health status, to investigate corneal diseases, and to review anterior segment surgeries. However, literature regarding the association between CD and corneal parameters in healthy adolescent and older individuals is limited. This study investigated age-related changes in Scheimpflug CD values and their correlations with age, sex, and corneal topographic parameters.

Methods

This retrospective cross-sectional observational study included 347 eyes from 181 consecutive healthy Chinese participants aged between 5 and 90 years. They were divided into 9 age groups: 5-9, 10-19, 20-29, 30-39, 40-49, 50-59, 60-69, 70-79, and 80-90 years. CD and corneal topographic measurements were measured using the Oculus Pentacam. To evaluate CD, the cornea was divided into 3 layers according to depth (anterior 120 µm, central, and posterior 60 µm), and into 4 annular regions according to diameter (0-2, 2-6, 6-10, and 10-12 mm).

Results

CD across different depths and regions was positively correlated with age (all P values <0.001). Of the 3 layers of corneal thickness, CD was highest in the anterior 120 µm and lowest in the posterior 60 µm (all P values <0.05). Among the 4 annular regions spanning the corneal diameter, the lowest CD values were 6-10, 2-6, and 0-2 mm at 5-29, 30-69, and 70-89 years, respectively. The highest CD values were 10-12 mm at 5-79 years, and 6-10 mm at 80-90 years (all P values <0.05). CD values of 10-12 mm in the anterior 120 µm corneal layer were significantly lower in men than in women (Z=-2.353; P=0.019). CD of 0-10 mm in each layer was not significantly different between sexes (all P values >0.05). Corneal topographic measurements, including flat-axis keratometry (K1), steep-axis keratometry (K2), and spherical aberration, were slightly positively correlated with age and CD (all P values <0.05). However, central corneal thickness (CCT) and thinnest corneal thickness (TCT), and age or CD showed no correlation (all P values >0.05).

Conclusions

With age, CD, keratometry, and spherical aberration gradually increased, while the corneal thickness did not change significantly.

Comparison of Corneal Densitometry and Visual Quality after Small Incision Lenticule Extraction (SMILE) and Laser Epithelial Keratomileusis (LASEK): One-Year Comparative Study

Purpose

To investigate changes in corneal densitometry (CD) and visual quality following small incision lenticule extraction (SMILE) and laser epithelial keratomileusis (LASEK) in patients with mild-to-moderate myopia.

Methods

A retrospective analysis was performed on 24 and 25 patients (46 eyes each) who underwent SMILE and LASEK, respectively, for mild-to-moderate myopia. The visual quality and CD values were recorded. Using the Pentacam Scheimpflug system, CD values were collected in three concentric optical zones at the depths of the anterior, central, and posterior layers. Efficacy, safety, predictability, corneal wavefront aberrations, and QoV scores were measured to evaluate visual quality. A correlation analysis was performed between changes in CD and clinical characteristics.

Results

There were no statistical differences in efficacy and safety indices between the two groups. At 3 months postoperatively, a pronounced reduction in several zones was observed in the LASEK group (p < 0.05), whereas no obvious change was observed in the SMILE group. There were obvious changes in the CD values in several zones in the SMILE and LASEK groups (p < 0.05) after 1 year. The magnitude of the CD changes in the anterior and central corneal layers was smaller in the SMILE group than in the LASEK group (all p < 0.05). Lower HOAs, spherical aberration, and horizontal comas of the anterior and whole corneal surfaces were observed in the SMILE group. QoV scores were similar between the two groups.

Conclusion

CD decreased in the SMILE and LASEK groups after 1 year; there was a smaller reduction in SMILE than in LASEK. SMILE and LASEK did not differ significantly in terms of safety and effectiveness in correcting mild-to-moderate myopia.

Long-term changes in corneal densitometry and associated factors following small incision lenticule extraction for moderate and high myopia

Purpose

To investigate long-term trends in corneal densitometry and associated influencing factors following small incision lenticule extraction (SMILE).

Methods

A comparative study was performed among 72 eyes of 38 patients undergoing SMILE. Eyes were divided into moderate myopia [mean spherical equivalent (SE),−4.22 ± 0.68D] and high myopia (mean SE,−7.63 ± 1.09 D) groups. Visual acuity, manifest refraction, corneal topography and corneal densitometry (CD) were evaluated preoperatively and 3 years postoperatively.

Results

The efficacy indices at last postoperative follow-up (42.47 ± 0.51 months) were 0.92 ± 0.21 and 0.97 ± 0.22, the safety indices were 1.12 ± 0.17 and 1.14 ± 0.21 for high and moderate myopia, respectively (all P > 0.05). CD values in the 0–6 mm zone of the posterior corneal layer was statistically significantly lower than the preoperative values in both groups. Postoperative CD values (0–2 mm zone) in the posterior layer of the high myopia group were statistically significantly lower than in the moderate myopia group (P = 0.025); CD values (2–6 mm zone) in the anterior layer were higher in the high myopia group (P = 0.026). Correlation analyses showed that CD values in the 0–2 mm middle layer were negatively correlated with lenticule thickness in high myopia (r = −0.411, P = 0.016); there was a negative correlation between the CD values (0–6 mm) and corneal oblique trefoil in this group (P < 0.05).

Conclusion

Corneal transparency in the 0–6 mm zone of the posterior cornea increased following SMILE in moderate and high myopia. Long-term CD values in high myopia may be correlated with oblique trefoil and lenticule thickness.