Free electron laser ablation of articular and fibro-cartilage at 2.79, 2.9, 6.1, and 6.45 ?m: Mass removal studies

Enhanced Tissue Ablation Efficiency with a Mid-Infrared Nonlinear Frequency Conversion Laser System and Tissue Interaction Monitoring Using Optical Coherence Tomography

We report development of optical parametric oscillator (OPO)-based mid-infrared laser system that utilizes a periodically poled nonlinear crystal pumped by a near-infrared (NIR) laser. We obtained a mid-infrared average output of 8 W at an injection current of 20 A from a quasi-phase-matched OPO using an external cavity configuration. Laser tissue ablation efficiency is substantially affected by several parameters, including an optical fluence rate, wavelength of the laser source, and the optical properties of target tissue. Dimensions of wavelength and radiant exposure dependent tissue ablation are quantified using Fourier domain optical coherence tomography and the ablation efficiency was compared to a non-converted NIR laser system.

Ablation of articular cartilage with an erbium:YAG laser: an ex vivo study using porcine models under real conditions-ablation measurement and histological examination

BACKGROUND AND OBJECTIVES

The use of an erbium:YAG laser in arthroscopic surgery has the advantage of a precise treatment of soft tissue. Due to the high absorption in water, the laser energy is perfectly matched to smoothing the hydrous, fibrillated articular cartilage surface. In minimal invasive surgery, the workspace is filled with aqueous liquids for enlargement. This appears contrary to the absorption characteristics of erbium:YAG laser radiation in water. The purpose of this study was to evaluate the ablated volume per pulse of cartilage lesions and the potential side effects including thermal damage and tissue necrosis.

STUDY DESIGN/MATERIALS AND METHODS

Twenty-four osteochondral specimens of porcine knee joints were irradiated with an Er:YAG laser completely submerged in water, with distances to the cartilage surface of 1, 3 and 5 mm and pulse durations of 75 and 100 microseconds. To keep a constant peak power of approximately 6 kW, pulse energies of 450 and 580 mJ were used at a pulse repetition rate of 15 Hz. After a histological preparation, ablated volumes, depths, and widths of the cuts were investigated. Additionally, laser protocols were correlated with different markers of cartilage tissue damage and apoptosis.

RESULTS

Ablation could be observed for every measurement. The influence of the distance showed a statistical significance (P < 0.001) for the volume, depth, and width of the cuts. For the pulse duration, statistical significance (P < 0.001) was found only for the volume and the depth. We observed no loss of proteoglycan or collagen type II. The total cell number, cell morphology, and number of apoptotic cells in an area close to the cutting edge and in a corresponding unaffected area of the same specimens revealed no differences regardless of the applied protocol.

CONCLUSION

The use of an Er:YAG laser demonstrates the successful application in liquid environments for cartilage removal without any damage of the surrounding tissue.

A comparison of mass removal, thermal injury, and crater morphology of cortical bone ablation using wavelengths 2.79, 2.9, 6.1, and 6.45 µm

BACKGROUND AND OBJECTIVE

Previous investigations have reported evidence of wavelength dependence on cortical bone ablation. This study used mid-infrared laser wavelengths generated by a free electron laser (FEL) and mass removal measurements to further examine the ablation efficiency of a wavelength (2.79 microm) not previously reported and three wavelengths (2.9, 6.1, and 6.45 microm) previously demonstrated by crater morphology alone to be efficient for cortical bone removal.

STUDY DESIGN/MATERIALS AND METHODS

The wavelengths examined were provided by an FEL emitting 4 microseconds macropulses consisting of 1-2 picoseconds duration micropulses delivered at 350 picoseconds intervals. The mass removal measurements were conducted by a microbalance, and the collateral thermal injury and crater morphology of cortical bone were examined by light microscopy following standard histologic processing.

RESULTS

The study demonstrated that the highest mass removal was achieved at lambda = 6.1 microm followed by, in order, lambda = 2.9, 6.45, and 2.79 microm. The zones of thermal injury and crater morphology created in cortical bone at the selected wavelengths were examined at the radiant exposure of 28.3 J/cm2. Ablation using lambda = 6.1 microm provided the largest crater size and the least collateral thermal injury. The greatest amount of collateral thermal injury was produced by lambda = 2.79 microm at both the sides and base of the ablation crater.

CONCLUSIONS

The mass removal of cortical bone produced by FEL ablation at selected mid-IR wavelengths was measured as a function of incident radiant exposure. The ablation efficiency was found to be dependent upon wavelength. The lambda = 2.79 microm did not offer any improvement over the other wavelengths evaluated, suggesting that a potential shift in the dynamic optical properties of water during tissue irradiance with the FEL does not present an advantage to the cutting of cortical bone. The lambda = 6.1 microm provided the highest ablation efficiency with deepest crater and the least amount of collateral thermal injury.

Mid-IR laser ablation of articular and fibro-cartilage: a wavelength dependence study of thermal injury and crater morphology

BACKGROUND AND OBJECTIVE

The aim of this study was to evaluate areas of collateral thermal injury and crater morphology for evidence of wavelength-dependent effects on the ablation of articular cartilage and fibro-cartilage (meniscus) using selected mid-IR wavelengths produced by a free electron laser.

STUDY DESIGN/MATERIALS AND METHODS

Two types of cartilage, articular cartilage and fibro-cartilage were used in the study. The wavelengths (lambda) evaluated were 2.79, 2.9, 6.1, and 6.45 microm generated by a free electron laser (FEL) using a 4 microseconds macropulse configuration. The zone of thermal injury and crater morphology produced by laser ablation were examined by light microscopy following standard histologic processing.

RESULTS

The zone of thermal injury and crater morphology created in cartilage by the FEL at selected mid-IR wavelengths were examined as a function of incident radiant exposure. Ablation using lambda = 6.1 microm provided the largest crater size for both articular and fibro-cartilage at all radiant exposures. For the zones of collateral thermal injury in articular cartilage, lambda = 6.1 microm produced the least thermal injury at the radiant exposure of 7.6 J/cm2. When the radiant exposure is increased to 20.4 J/cm2, both lambda = 6.1 and 6.45 microm produced less thermal injury than the ablation using lambda = 2.79 and 2.9 microm. The greatest amount of collateral thermal injury was produced by lambda = 2.79 microm for both tissue types.

CONCLUSIONS

The results demonstrate that crater depth and collateral thermal injury produced in articular cartilage and fibro-cartilage are wavelength-dependent with 6.1 microm providing the largest craters at all radiant exposures. The least amount of thermal injury was created in articular cartilage using lambda = 6.1 microm at the radiant exposure of 7.6 J/cm2. Both 6.1 and 6.45 microm wavelengths demonstrated similar amount of thermal injury at 20 J/cm2 that was less than lambda = 2.79 and 2.9 microm at similar fluences. These observations are explained based on the absorption by water and protein in the tissue types studied. It is further observed that the use of crater dimensions may not provide a reliable estimate for the amount of tissue removal provided by an ablation procedure.