Optimized Distance Holders Improve Precision in Fractional Laser Treatment

Abstract

Background: Fractional Photothermolysis (FP) relies on precise laser focus to create microscopic treatment zones (MTZs). Focal plane deviations, caused by handpiece pressure and movement, shift the treated tissue out of focus, increasing spot size and reducing lesion depth, compromising treatment efficacy. This study investigates an optimized spacer to mitigate load-induced deviations and improve clinical outcomes.

Methods: A difference frequency generation (DFG) laser system (IPG Medical) with 3050/3200 nm wavelength treated ex vivo human abdominoplasty skin, mounted on a load-adjustable platform (0-1000 g, ±0.1 g). Tissue was treated at 5 mJ/pulse with a nominal spot size of 40 µm using spacers ranging from 2 × 2 mm² to 8 × 8 mm² and an optimized 2 × 2 mm² grid spacer. Optical coherence tomography measured skin bulging and focal plane deviations, while histological analysis validated lesion depth and diameter. Knife-edge technique projections were used to assess theoretical beam behavior.

Results: Applied load caused skin bulging, resulting in a linear increase in lesion diameter and a linear decrease in depth. Smaller spacers (2 × 2 mm²) reduced focal deviations and maintained MTZ precision within the Rayleigh range. 2 × 2 mm² grid spacers further reduced bulging by redistributing the load and ensuring more uniform MTZ formation.

Conclusion: Load-induced focal shifts significantly impact FP efficiency, particularly on nonuniform skin surfaces. An optimized spacer enhanced precision by minimizing bulging, allowing treatments to remain within one Rayleigh length. These findings highlight the clinical importance of spacer selection in achieving consistent FP outcomes, particularly for small-spot laser applications. Clinicians should integrate smaller or grid-patterned spacers to enhance procedural accuracy.

Keywords: Rayleigh range; ablation zone; deviation; difference frequency generation laser system; focal distance spacers; focal plane; fractional photothermolysis; gaussian beam; knife‐edge technique; microscopic thermal zones; optical coherence tomography; thermal injury.

Figure 1

Depicting the OCT imaging setup shown with (A) low and (B) high load, respectively, showing increased skin bulging. (C) Depicts a laser treatment where the handpiece is pressed onto the skin with high load, resulting in skin bulging and focal displacement.

Figure 2

OCT images of skin bulging based on load using (A) 2 × 2 mm², (B) 4 × 4 mm², (C) 8 × 8 mm², and (D) 2 × 2 mm² grid spacers. Vertical comparisons depict increasing loads of 0, 400, and 1000 g, while horizontal comparisons show variations in spacer size. Skin bulging was shown to be greater with increasing load and also shown to be greater with increasing spacer size. The 2 × 2 mm² grid spacer exhibited the least bulging.

Figure 3

OCT image analysis of skin bulging in response to increasing spacer loads demonstrated a logarithmic relationship between load application and focal displacement across all spacer sizes. The most pronounced focal displacement occurred within the initial 200 g loading range. Additionally, a reduction in spacer size corresponded to a decrease in focal displacement, as evidenced by the visibly lower displacement of the red line (0.5 × 0.5 mm²) compared to the purple line (8 × 8 mm²). Notably, the grid spacer demonstrated superior accuracy in reducing skin bulging compared to the single‐opening spacer of equivalent size.

Figure 4

Histological analysis of focal displacement. The depth and diameter of MTZs are significantly influenced by focal displacement due to skin bulging. When the skin remains in focus (0 mm displacement), the MTZ depth reaches 760 µm with a diameter of 55 µm. However, at a focal displacement of 1.8 mm, the MTZ depth is markedly reduced to 45 µm, while the diameter increases to 153 µm. This shift results in shallower and broader MTZs, highlighting the critical impact of focal accuracy on treatment precision.

Figure 5

Correlation between histological analysis and laser beam characteristics. The relationship between ablation zone diameter and laser beam size as a function of focal plane deviation demonstrates a linear increase in histologically measured lesion diameter, whereas the measured beam diameter follows an exponential increase. Similarly, histological analysis of ablation zone depth reveals a relatively linear decrease, in contrast to the exponential decay observed in radiant exposure, as determined by the knife‐edge technique. These findings suggest that tissue response to laser energy deviates from theoretical beam propagation models, potentially due to nonlinear thermal diffusion and tissue‐specific optical properties.