Influencing Factors Relating the Demarcation Line Depth and Efficacy of Corneal Crosslinking

The recent article of Spadea et al.^1,2 and Toker et al.³ evaluated the efficacy of accelerated corneal crosslinking (AXL) under different treatment protocols, and the relationship between CXL efficacy and the demarcation line (DL) depth based on available measured data.^1,2 The sudden-drop of DL depth at high intensity (>45 mW/cm²) has the similar feature as the AXL efficacy reported by Wernli et al.⁴ However, it is clinically unclear whether DL depth is proportional to the crosslink depth or the CXL efficacy.^1–3 Moreover, controversial results of AXL efficacy were reported due to inconsistent protocols which were not optimized. To improve the efficacy of AXL, Lin proposed a new protocol called riboflavin (Rf) concentration-controlled method (CCM),⁵ in which extra Rf drops are applied to the cornea, at every crosslink time (T*), during the UV exposure, having a frequency defined as Fdrop = N − 1, with N = 0.365 [I₀/C₀]^0.5. This formula provides the optimal protocol that higher intensity (I₀) and/or lower Rf concentration (C₀) requires larger Fdrop to compensate its faster Rf depletion, and lower steady-state efficacy, comparing to the low-intensity Dresden protocol. 

This correspondence intends to further analyze the clinically measured DL depth via the key influencing factors of CXL, including Rf concentration profile (pre- and intraoperatively),⁶ Rf initial diffusion depth, UV light effective dose, epithelial absorption (for epi-on case), and most importantly, the administration protocols of Rf during CXL, governed by Fdrop and the waiting period after each Rf drop.⁵ The measured DL-depth of various protocols will be analyzed and compared to a combined-efficacy formula.^5,7 

The measured DL depth may be compared to a combined type-I CXL efficacy given by^5–7 c-Ceff = 1-exp (−4R), with R = (62/t') [C₀F(z)/I₀³]^0.5, where F(z) = 1–0.5z/D, is the Rf initial concentration profile defined by a diffusion depth (D); and the resupply Rf drops every t' − minutes. R is the efficacy ratio between the noncontrolled Dresden protocol and the optimal protocol (via CCM) having R = 1.0, and c-Ceff = 0.98, for all range of UV intensity I₀ = 3 to 60 mW/cm². 

Figures 1 and 2 show that the DL depth (based on measured data^1,2) follows the similar decreasing-trend as the calculated CXL efficacy in AXL. Figure 2 shows the theoretical curves (1, 2, 3, 4) calculated from the combined efficacy (c-Ceff) formula,^5,6 with time of resupply Rf drops t' = (4, 2, 1, 0.5) minutes. The measured DL-depth (z*) and theorized efficacy are compared to conclude the following features: 

1.  
   In Figure 1, curve B for extended dose (6.2 J/cm²) has a larger z* than curve A (dose of 5.4 J/cm²). It is also predicted by Lin's formula for crosslink depth⁵ which is proportional to light dose, rather than light intensity. Modern protocol has also suggested to use a higher dose of 7.2 J/cm², replacing the Dresden 5.4 J/cm² (operation manual of KXL, Avedro Inc.).
2.  
   Epi-on is less efficient than epi-off due to limited diffusion depth (D) and the extra absorption of the epithelium; and epi-on CXL with iontophoresis has a larger z* due to improved diffusion depth (D), comparing data 1 and data 2 of Figure 1. This feature is also shown in Lin's efficacy-formula,⁷ where higher D has higher value of F(z) = F(z) = 1 − 0.5z/D and leads to higher efficacy.
3.  
   Curve 2 of Figure 2 represents the conventional protocol having resupply the Rf drops every 4 minutes. Therefore, it has the similar profile as curve A of Figure 1. This feature indicates that CXL efficacy is proportionally related to the DL depth. The recent article of Spadea et al.² also concluded that the depth of DL is an indirect measurement of CXL penetration within the stroma, although “the deeper, the better” requires further clinical long-term studies.
4.  
   Both CXL efficacy and DL depth have a cutoff maximum intensity, under the noncontrolled Dresden protocol, as reported by Wernli et al.⁴ It should be noted that the theoretical curve 4 (with controlled t' = 0.5 minute, or Fdrop = 3 to 4) gives the optimal efficacy comparable to CCM.⁵
5.  
   In contrast to the conventional belief (by Hafezi and Kling et al.) that oxygen-mediated type-II plays the critical role of CXL, Kamaev et al.⁸ kinetic model showed that CXL is predominated by type-I, while oxygen (or type-II) only plays a limited and transient role. Lin's 3-pathway model⁹ showed mathematical details of the role of oxygen, supporting the claim of Kamaev et al.⁸ In addition, a recent clinical study of Lombardo et al.¹⁰ showed a simple-exponential kinetic of RF concentration also implied that, in ambient environment (with approximately 21% partial pressure of oxygen), nonoxygen-mediated type-I mechanism is predominant.
6.  
   The conventional Dresden protocol, extra Rf drops (with a frequency Fdrop = 10–15) instilled during the UV exposure. This too-often Fdrop will reduce the effective dose and CXL efficacy. Some modern protocols propose not to apply any extra Rf drops (with Fdrop = 0), however, has less efficacy than that of optimal CCM⁵ which requires Fdrop = 3 to 5 for high intensity AXL (18–50 mW/cm²).
7.  
   Extension of exposure time (or dose) in AXL may increase the crosslink depth (z) and improve the efficacy governed a crosslinked stroma volume⁷ = depth × strength. Higher Rf concentration (C₀) achieves higher efficacy, predicted by c-Ceff formula,⁷ was also clinically reported by O'Brart et al.¹¹ Therefore, new clinical studies of DL-depth for a wider range of C₀ = 0.1% to 0.3%, and under the CCM protocol might lead to a breakthrough of the AXL efficacy and justify the accuracy of CCM for optimal efficacy.

Figure 1

 

Measured DL-depth in CXL under conventional dose 5.4 J/cm² (curve A) and extended dose 6.2 J/cm² (curve B). Also shown are the epi-on cases with (data 1) and without (data 2) iontophoresis.²

Figure 2

 

Theoretical CXL efficacy versus UV light intensity for resupply of Rf drops at various time, t' = (4, 2, 1, 0.5) minutes, for curves (1, 2, 3, 4).

Acknowledgments

Disclosure: J. Lin, New Vision, Inc. (F, E, S)