Optimizing bone ablation using a Laser and Automated Microjet Irrigation System

This short review is based on the research article titled “Optimizing deep bone ablation by means of a microsecond Er:YAG laser and a novel water microjet irrigation system”. It was authored by a group of researchers – Lina M. Beltran Bernal, Ferda Canbaz, Antoine Droneau, Niklaus F. Freiderich, Philippe C. Cattin, and Azhar Zam, and published in the Biomedical Optics Express journal on 1st December, 2020. The study explores the process of calibrating the irrigation system using a pneumatic pressure controller, and the Er:YAG laser setting for deep bone ablation. At the end of the study, they proposed a system that is able to meet all factors involved, and thereby enable an optimal deep bone ablation.

Abstract

The microsecond Er:YAG pulsed laser with a wavelength of λ = 2.94 μm has been used extensively in the field of medicine, specifically for ablation of dental tissues. Bone & dental tissues have similar compositions, consisting of mineralized and rigid structures, the Er:YAG laser proves to be a promising tool for laserosteotomy applications. This study examines the use of Er:YAG laser in deep bone ablation.

This is performed with the aim to optimize its performance and point out any shortcomings. The calibration settings of the laser and tissue irrigation were optimized in an independent manner. This research suggests an automated irrigation feedback system that is able to deliver water depending on the detected tissue temperature. A thin water jet with a diameter of 50 μm was used in the irrigation system. During the ablation procedure, this water jet was able to penetrate the crater, with a laminar flow length of 15 cm. This ensured the irrigation of deeper layers, not possible by other spray systems. On optimization of the irrigation system, the ablation was observed independent of the irrigation water. Line cuts – 21mm deep were observed, without any visible damage to the surrounding tissue.

This automated procedure proposed by the group of researchers, has the ability to enable deeper & quicker bone ablation in laserosteotomy applications. 

Introduction

Since the very early ages, surgery of the bone is still heavily dependent on the use of various mechanical tools – for example types of oscillating saws and burs [1,2]. The primary disadvantage of these tools is the massive mechanical stress they cause – leading to cases of irreversible damage in patients. This leads to extended healing times, and changes in the life of these patients.[3] Similarly, the bio-compatibility and extent of contamination of these mechanical tools pose a high risk of infection. Conventional tools, in most cases are made of metal, because of which corrosion and wear resistance are key factors that need to be evaluated. Previous studies have already established the many advantages of using lasers for bone ablation, over mechanical tools [4-10]. Lasers are able to make contactless cuts, avoiding all mechanical stress related issues and it also provides a higher degree of safety during surgical procedures. Also, if a robot assisted technology is used to operate the laser, it results in highly precise cuts. [11]. The first known studies involving laser ablation of hard tissue started in the field of dentistry. Photothermal ablation is the ablation mechanism for microsecond lasers operating in the mid-infrared region[27]. In this type of ablation, the water molecules present in the tissue vaporize, resulting in high level ablation efficiency. The absorption peaks of both Water and hydroxyapatite, two of the main components of bone, are near 3 μm [18,28], thus ablation becomes more efficient when Er:YAG lasers are used.

Extremely high temperatures generated during photothermal ablation of bone cause the remaining tissue to dry out. Many effects of heat are observed in the tissue with rising temperature. The bone eventually starts drying out, followed by carbonization, and finally completely damaging the tissue. Due to these reasons, an irrigation system to re-hydrate and cool down the tissue is necessary.

Aim & Objectives

Given below are the key objectives of this study concerning the deep bone ablation : 

• Optimize the irrigation system and Er:YAG laser parameters for deep bone ablation, using a penumatic flow controller.
• Observe the effect of consistently cleaning the laser path on the ablation process as a whole.
• The limitations of the laser are explored, in terms of beam quality and parameters (beam spot size, energy, repetition rate) and their influence on ablation performance
• The ablation process was also examined for both hole and line ablation.

Materials & methods

An Er:YAG laser with a wavelength of 2.94 μm was used for this experiment. It possesses a maximum average power of 9W, and the pulse energy and peak power of the lase are 10-900 mJ and 0.1-2.25 kW respectively.

Figure 1 demonstrates the schematic diagram and Fig 2 shows the photograph of the experimental setup used in this study. All stages were in a static position for hole ablation experiments, the motorized stage was used only for line ablation experiments. 

To cool the tissue, an irrigation system was utilized, and pressured air was used for clearing of debris from the bone surface. An irrigation feedback mechanism was performed with the help of an infrared camera, this is explained in detail in the original research article.

For the irrigation system, two different systems were used to execute the ablation procedure. The first one was an ESI Elveflow microfluidic system, consisting of a pneumatic pressure controller, the Elveflow OB1 MK3+. A tygon tubing coil is used to deliver the water, with an internal diameter of 500 μm, a maximum pressure of 2 bar, and a laminar flow regime of 1 cm long. The second irrigation system uses a new, and specifically designed nozzle, which is able to deliver a water jet 50 μm in diameter, a pressure of 10-800 bar, and characterized by a laminar flow regime of approximately 15 cm.

Key Findings

One pulse ablation

The data obtained from one pulse ablation experiments can be used to determine how the ablation will evolve as the tissue is exposed to additional pulses. Fig 3 shows the depth of ablation observed with a single laser pulse as a function of incident peak fluence. The pressurized air was able to remove a portion of the debris on higher ablation rates.

Testing an automated irrigation feedback system by an IR camera

For optimization of ablation, the irrigation system should be configured to each laser setting – for example the energy and repetition rate of the laser. A temperature threshold was identified with a value of 104°C and the irrigation system would be activated in order to avoid the risk of carbonization of the tissue. For the study the time duration of integration of the camera is 20ms.

Hole & Line Ablation

As demonstrated in Fig 4, the ablation was deeper for cases when pressurized air was used, compared to when it was not. Also, when ablation is done with multiple pulses, the debris and irrigation water prevent ablation due to the shielding effect, but also adhere to the lens that focuses the laser beam. 

Also, studying ablation of shapes is also extremely vital for understanding the applicability and performance of real bone surgery. In this study, ablated lines using the best four irrigation systems

Optimized laser settings to execute bone ablation

According to Fig. 5, the deepest ablation is achieved by raising the average intensity. Thus, the energy as well as the repetition rate should be increased, while the beam

size should be decreased (Eq. (6)). Although, by increasing the average intensity and decreasing the spot size will cause faster divergence (decrease in depth of focus), resulting in superficial ablation. Thus, a balance between spot size and depth of focus of the beam has to be maintained.

Conclusion

As a conclusion to this review article, this study involving bone ablation using a microsecond Er:YAG laser was executed by closely studying the different factors that contribute to the optimizing the procedure. Firstly, findings from the one-pulse ablation process were used to estimate the ablation process after introduction of several pulses. Within the estimation, various characteristics of the laser are also taking into account. The requirement of an ideal automated irrigation system was also observed. Additional information from experiments focused on thermal tissue damage, the ablation process could be modified by adjusting the threshold temperature within the software. 

A thin water jet of 50 μm ar 30 bar pressure is observed to be the best option for deep ablation. The water was delivered deep into the tissue, to cool the entire region. The use of a pressurized air/ suction system to blow off debris and excess water optimizes the ablation process. This is well documented for line ablation in the study.

These exciting results were achieved by the researchers with the help of a pneumatic pressure controller within the automated irrigation system in order to assist ablation. For an in-depth analysis of this study, please refer to the complete article available here.