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Abstract
Laser ablation of bone for the purposes of osteotomy is not as well understood as ablation of homogeneous, non-biological materials such as metals and plastics. Ignition times and etch rate can vary during ablation of cortical bone. In this study, we propose the use of two techniques to optimize bone ablation at 1064nm using a coaxial nitrogen jet as an assist gas and topical application of graphite as a highly absorbing chromophore. We show a two order of magnitude reduction in mean time to ignition and variance by using the graphite topical chromophore. We also show that an increase in volumetric flow rate of the assist gas jet does show an initial increase in etch rate, but increased pressure beyond a certain point shows decreased return. This study also demonstrates a 2nd order relationship between exposure time, volumetric flow rate of nitrogen, and etch rate of cortical bone. The results of this study can be used to optimize the performance of laser ablation systems for osteotomy. This is a companion study to an earlier one carried out by Wong et al. [ Biomedical Opt. Express 6, 1 (2015)].
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Osteotomy is a highly mechanical process that makes use of tools that apply friction and torque to remove and re-shape bone during surgery. Iatrogenic damage is an obvious concern, but in many situations it is considered ”part of the process” due to the forces transferred to surrounding areas during bone cutting. Despite care taken by the surgeon, peripheral damage is unavoidable. High risk osteotomies, such as craniotomies and laminectomies, would benefit from bone cutting modalities that reduce mechanical forces imparted onto the patient anatomy. Lasers, which have been extensively researched in bone cutting applications, are ideally suited for this task due to their precision and non-contact nature [1–7].
1.1. Topical chromophores
The use of topical chromophores have been studied in laser skin resurfacing applications to increase thermal ablative damage for improved resurfacing results. Sumain et al. proposed the viability of carbon-based topical chromophores on skin with 532nm laser irradiance for increased laser light absorption and subsequent heat transfer to dermal tissue [8]. The results showed that the coagulation depth was dependant on laser dose, and it was claimed that the use of the carbon-based chromophores were effective. However, no bare-skin irradiance baseline was established for comparison.
Indocyanine green (ICG) based topical dye has been used in select studies as a dermal topical absorber to increase thermal damage depth for skin resurfacing. Diven et al. showed that the use of ICG based dye on human cadaveric and guinea pig skin in conjunction with laser irradiation at 805nm produced significant penetration of thermal damage compared to irradiation on bare skin. It was reported that with application of an ICG-based dye, thermal-induced coagulation was seen to be as high as 416μm immediately after ablation on live guinea pig specimens. It is important to note that an inverse relation between irradiation power and coagulation depth was seen when samples were immediately compared via histology just after ablation; 3W of laser irradiation produced deeper damage penetration (416μm) than at 10W (390 μm). A baseline of 0μm damage at 10W was established when no dye was used. However, after 1 week and 1 month, depth of scarring was seen to positively correlate with irradiation power [9]. Mordon et al. experimented with the efficacy of ICG aqueous solution versus ICG emulsion (phosphatidylcholine and soybean oil) injections in laser photocoagulation of blood vessels in hamster skin flaps [10]. It was demonstrated that a 50 and 60 percent reduction in laser fluence at 805nm was achieved using the ICG emulsion compared to the aqueous solution to achieve vessel coagulation and skin ablation, respectively. Roh et al. experimented with various fluence and exposure parameters of a 1064nm ND:YAG laser for treatment of enlarged facial pores. One group of subjects in the study was treated with a topical lotion of suspended carbon particles prior to ablation. This study proved the efficacy of 1064nm ablation for pore size reduction, however no significant difference was seen with the application of the topical lotion. [11].
For osseous tissue, Kang et al. showed a three to six fold increase in ablation crater depth and volume by using a thin layer of distilled water on top of the target bone [12]. The thin layer of water was found to have decreased the damage threshold for the bone.
Ultrashort pulsed (USP) lasers have shown promise in laser osteotomy, since the peak-powers achieved using these types of lasers are easily able to induce non-thermal interaction (such as optical breakdown); because of this, little to no heat is generated at the ablation site. However, USP lasers do suffer some drawbacks. The very large field intensities achieved can potentially cause self-focusing due to spatial index variations caused by the Kerr effect [13]. This can be avoided by using high numerical aperture (NA) optics, but that would inherently lead to shorter stand-off (focusing) distances of the objective optics, which may potentially reduce or negate the general non-contact benefit of laser surgery. Given that the penetration depth of ultrashort pulses is extremely shallow (approximately 1μm due to highly efficient absorption by surface plasma), etch-rates of USP lasers tend to be comparatively slow. As well, USP lasers have much larger costs and system complexity compared to nanosecond pulsed fiber lasers.
For wavelength-multiplexed applications, high peak powers generated by USP lasers may provide challenges when optically coupling with coaxial monitoring beams, such as in applications using optical coherence tomography (OCT) based monitoring and control [14,15]. High peak powers could lead to potentially damaging leakage into common optical paths.
Thermal damage to an ablation target is a concern that is explored in multiple studies, as excessive thermally-induced necrosis will prevent or delay healing [16–19]. Ablation in the thermal regime is limited by excessive heat accumulation and diffusion into tissue. Therefore, reduced exposure time to the laser for a given target volume would be advantageous since it would limit the overall of heat deposition and diffusion into the surrounding tissue. An interesting study was done by Sotsuka et al., where bone ablation at 1070nm was compared with saw based osteotomy [20]. It was found that there no significant differences in healing times between the two. However, it was concluded that the laser ablation was a combination of thermal and plasma mediated ablation, therefore less charring was seen around the osteotomy site upon histological examination. Thermal ablation has the potential of being useful as well. Thermal ablation using radio frequency AC current is routinely used to treat bone lesions [21]. As well, laser ablation in the thermal regime has shown promise in applications of hemostasis [22], vascular stenosis [23] and cauterization [24], which could prove useful during osteotomy for reducing and/or stopping bone bleeds.
1.2. Graphite as a topical chromophore
In this study we explore the increase in efficacy of bone ablation at 1064nm in the thermal regime using graphite as a topical chromophore. Since 1064nm has a relatively low absorption cross-section for the various constituent materials of bone (ie. water, hydroxyapetite, collagen, etc.), most of the ablation response in the thermal regime happens after the surface incident to the beam has been heated enough for carbonization to begin. Once the black, carbonized tissue begins to form, the material removal process quickly accelerates due to the very large increase in light absorption by consequence. Before this, however, the time it takes for the carbonized tissue to first form (we refer to this as ignition time) is quite variable and unpredictable due to the inhomogeneity of bone tissue. Therefore, the use of a chromophore with a large absorption cross-section (similar to the carbonized tissue) as a topical absorber would significantly reduce the mean and variance of the ignition time. This is also important in isolating the effect of the gas-flow over the ablation site and reduce the effect of highly varying ignition times for the bone surface. Graphite was used as a topical chromophore for three main reasons:
- High optical absorbance
- Current use in many forms of bone surgery, where surgeons use sterile bone pencils to demarcate critical structures and cutting trajectories [25]
- Ease of application via a simple pencil applicator
1.3. Coaxial assist gas
Also in this study, we use a coaxial nitrogen gas jet to further increase the efficiency of ablation. Coaxial assist gas jets are commonly used in industrial laser machining applications. The gas flow helps to clear smoke, dross and plasma plume (assuming sufficient energy density) from the ablation site, allowing for incident photons to continue hitting the kerf bottom via an unobstructed path. Assist gas flow over the workpiece can also help in cooling the material and to mitigate the spread of heat damage and thermally induced cracking. Assist gas flow in the context of laser osteotomy is currently not well understood. Given the benefits seen in laser materials machining, the use of coaxial or blanket gas flow onto bone during laser ablation could have benefits such as increased etch rate and smaller heat affected zone (HAZ). Also, since pressurized gases such as nitrogen and oxygen are readily available in almost every operating room for multiple uses (anesthesia, insufflation, pneumatically actuated tools, etc.), the augmentation of laser osteotomy using assist gases for various surgical scenarios seems reasonable to investigate. This technique was first proposed by Wong et al. [26], where it was roughly determined which flow rates would be considered relatively safe in the context of neurosurgery. This was done by measuring the deflection of exposed sections of a porcine spinal cord (via laminectomy) using inline m-mode optical coherence tomography (OCT) whilst directly impinging the tissue with a nitrogen jet. The spinal cord was shown to compress almost 1mm for a flow rate of 70SCFH, using a nozzle with an exit diameter of 3mm, and a tissue-nozzle separation distance of roughly 2mm. It was thought that any deflection beyond this point would induce unwanted somatosensory evoked potentials (SSEPs) and motor evoked potentials (MEPs).
This present study, which is a companion study to [26], investigates the effects that a nitrogen assist gas jet has on the etching efficacy of thermal bone ablation at 1064nm. The nozzle design and flow rate parameters were determined from insights gleaned from the results of [26]. We use a novel method of reducing variability in ignition behaviour of bone at this wavelength (1064nm) by using a graphite topical absorber; this is the first time, to the knowledge of the authors, such a technique for bone ablation has been reported in literature.
2. Methods
2.1. Ablation laser and in-line optical coherence tomography system
An in-house built fiber laser system with integrated in-line OCT-based depth measurement capability was used in this study to ablate the samples and monitor progression of ablation craters in real-time. The system used is similar to the one described in [14,27].
The 1064nm ablation laser light was produced by a high power fiber laser based on a design from [28]. It consisted of an all-fiber active cavity approximately 10m in length, constructed of Yb-doped fiber (Nufern LMA-YDF-10/130-VIII). Two fiber Bragg gratings (99.5% and 10% reflective at 1064nm for high and low reflectivity gratings, respectively) etched onto matching passive fiber (Nufern SM-GDF-10/125) were spliced onto the ends of the cavity with an APC connector terminating the output end. The cavity was pumped using a 976nm fiber coupled laser diode (RealLight M976±3-110-F105/22-D1) via a (2+1)×1 high power fiber combiner (ITF MM021112CC1A). All the fiber used in the construction of the fiber laser had a core diameter of 10μm. The output from the laser was coupled from a pigtail fiber into a collimator (Thorlabs F280APC-C), then into a Thorlabs AC254-150-C achromatic doublet lens for focusing. The lens spacing between the collimator and focusing lens was approximately 2.5cm. The focal length was approximately 14.5cm.
The OCT system consisted of a 1310nm centered swept wavelength MEMS laser (Santec HSL-20-50-S) with a bandwidth of 110nm, sweep rate of 50kHz (single-sided sweep, 62 percent duty cycle) and average power output of 18.5mW. An integrated Michelson interferometer (50/50 coupling ratio) with balanced detector system was used (Thorlabs INT-MSI-1300). The entire OCT system was constructed using SMF-28 fiber. The reference arm was constructed using a Thorlabs F280APC-C fiber-coupled free space collimator. The MEMS laser provides a k-clock output for sampling trigger.
We refer the reader to [14, 27] for details on the amalgamation of the OCT and fiber laser systems for inline ablation monitoring.
Table 1. Summary of fiber laser parameters.
Table 2. Summary of OCT system parameters.
3. Experimental results
3.1. Reduction of ignition-time variance using a topical chromophore
Three fresh porcine scapula from two pigs were used in this experiment. The scapula was chosen in this experiment due to the similarity in structure to that of human cranial bone. Surrounding muscle and periosteum were initially left on the bone but were removed minutes before ablation experiments were conducted. Approximately twenty holes were percussion drilled into the surface of each scapula; 10 holes were ablated onto the bare bone surface and the remaining 10 after the application of bone pencil graphite onto the bone surface. Figure 2 shows the ablation results with and without topical application of graphite on bone surface prior to ablation. The time was measured between initiation of laser exposure and when the ablation crater reached a depth of 0.5mm below bone surface. We see from the figure that there is a significant reduction in the mean and variance with the use of the topical chromophore. The mean time to 0.5mm depth for no graphite used was 6.525 seconds with a standard deviation of 7.167 seconds, where as with graphite the mean value was found to be 0.068 seconds with a standard deviation of 0.029 seconds; both values showed and improvement of 2 orders of magnitude.
3.2. Gas flow experiments
As mentioned earlier, the experimental setup was similar to that found in [26]. However, the key difference is that high-powered fiber laser mentioned in the previous section was placed inline with the OCT to monitor the ablation depth; this is summarized in Fig. 1. Graphite was used as a topical chromophore during these experiments as well, since from the results of the previous section it is reasonable to conclude that the use of graphite during gas assisted ablation is able to isolate the ablation results almost entirely as a function of the assist gas as large variances in ignition time and initial heating would be eliminated. For all the experiments conducted in this section, the following laser parameters were used: Pavg = 8.18W, Ppeak = 523W, pulse repetition rate = 100kHz. To ensure consistent flow rate of the assist gas, a mass flow controller specifically calibrated for nitrogen gas (Alicat Scientific MCR-250SLPM) was connected directly upstream of the nozzle to control the gas flow in real-time. The system was tested using MDF (medium density fiberboard) to confirm the behaviour. MDF was selected because it ablates easily and consistently at 1064nm. We see from the results shown in Fig. 3, the etch rate shows linear increase with respect to flow rate, but shows a plateauing and even decrease in etch rate for higher flow rate values. This result agrees well with the findings of Gabzdyl [29], showing that the system does behave as expected.
Fig. 1 System diagram. The fiber laser system was built directly into the sample arm of the OCT based depth ranging system, allowing for real-time monitoring of the kerf bottom using m-mode imaging whilst ablating.
Fig. 2 Results of ablation times for 0.5mm depth with no graphite chromophore applied and with graphite chromophore applied to bone surface. We see a dramatic improvement in mean and variance of the time it takes to reach the target depth from when the laser was activated. Y-axis is in log scale.
Fig. 3 Effects of nitrogen flow during percussion ablation of medium density fiber board (MDF) for different exposure times.
3.3. Cortical bone experiments
Four fresh porcine femurs were stripped of their periosteum tissue layers and cleaned with water. Porcine femurs were chosen for this study due to the very thick layer of cortical bone (typically >3mm), thus ensuring that accidental breach into the cancellous layer did not occur. Marks were made with the graphite pencil on the surface of each bone. The flow rates used with were 0, 10, 30, 50, 70 and 90 SCFH (standard cubic feet per hour); the exposure times were 250, 500, 750 and 1000 ms. For each flow rate/exposure time combination, 10 holes were percussion drilled into the femurs, giving a total of 240 holes. The maximum achieved depths were analyzed immediately via b-mode imaging using a separate, calibrated swept-source OCT system. The results of this experiment are shown in Fig. 4. It can be seen that there is a significant drop in etch rate after the 250ms window; however there does appear to be an increase in etch rate with an increasing nitrogen flow rate. However, even though Fig. 4 does show that the etch rate increase does seem to be linear for the 250ms exposure time, the behaviour seems non-linear with respect to flow rate for the other exposure times. The data was fitted to a least squares 2nd order polynomial model with a coefficient of determination of R2 = 0.98, suggesting a good fit. Below is the fitted polynomial describing the behaviour:
where νetch is the etch rate in μm/s, νflow is the nitrogen flow rate in SCFH, and τex is the exposure time in milliseconds. To further investigate the effect of gas flow on the etch rate of cortical bone, the same procedure was used to drill holes into three more porcine femurs. This time, the m-mode data from the OCT system inline with the fiber laser (see figure 1) was recorded to monitor the kerf bottom as a progression of time during ablation. For these experiments, only the nitrogen flow rate was varied; 10 holes were percussion drilled into the femurs for each flow rate. Figure 5 shows the means of each set of flow rate trials. It was found that the assisted gas flow for up to 30SCFH showed a clear improvement in the depth achieved, but beyond that the improvement was reduced to near no-flow condition. It was also found that higher flow rates did result in a slight increase in slope of the m-mode progression (corresponding to the etch rate) during the initial, nearly linear portion.Fig. 4 Mean etch rate as a function of exposure time and flow rate. Etch rate is quite high during the smallest exposure window and highest nitrogen gas flow. Etch rate is seen to increase as a function of flow rate specifically, but an increase flow does not linearly translate to increase in etch rate.
Fig. 5 Time progression of depth measured via inline m-mode OCT, as a function of flow rate. 30SCFH is shown to most dramatically improve ablation. 70SCFH is shown to improve slope initial ablation progression, just before 200ms.
4. Discussion and conclusion
It was found that the mean time of exposure to reach a crater depth of 0.5mm was significant between holes that did and did not have topical application of graphite. For the samples where no graphite was used, typically a very long period of heating of the cortical bone surface was seen before rapid material drilling could take place. This is thought to be due to the relatively poor absorption of 1064nm light in osseous tissue. Due to the non-homogeneity of bone, these periods were shown to have an extremely high variance; this is shown in Fig. 2. Using a poorly absorbing wavelength in conjunction with a topical chromophore to cut bone can have the advantage of isolating ablation activity solely to regions where and in close proximity of chromophore application. This way, a margin of safety can be realized so that inadvertent ablation due to unexpected/sudden beam shift (ie. getting bumped or knocked) would be less likely to occur. As well, the use of a chromophore can enhance the use of an overall lower-power laser, thereby adding another aspect of safety in case of failure or damage to delivery fiber.
It was observed that etching efficiency does reduce with respect to increase in exposure time. This could be because of the kerf bottom being out of the focal region of the laser for extended periods of ablation. However, it was shown in this study that for 30SCFH, the efficacy of ablation did increase for the same approximate exposure time. Generally, it is desirable to keep exposure time limited for a given volume to reduce unwanted heat effects; therefore, it is crucial to optimize etching efficiency to achieve maximal penetration in the minimal amount of time.
The results reported in this paper shows findings consistent with those reported by Aljekhadab et al. [30] where an increased etch rate was reported with the use of airflow over the ablation sample. As well, [30] also demonstrated cleaner and more well defined edges of craters compared to water-layer assist and no assist fluid. The use of nitrogen has some key advantages in its potential use as an assist agent in ablation over other gases and liquids. Since most of the atmospheric air is composed of nitrogen, there is no practical need to optically compensate for change in refractive index, as would potentially be the case if an optically dissimilar gas or liquid was to be used. This also means that unwanted absorption or lensing through the assist medium is eliminated. The use of nitrogen also suffocates the ablation region of oxygen, reducing the chance of flare up. As well, Nitrogen is readily available as a standard medical gas in operating rooms, therefore this does not have to be piped from outside the operating or brought in using a pressurized cylinder. We see the relationship in Fig. 4 between flow rate and exposure time vs etch rate to be strongly second order. We hypothesize that even though the nitrogen jet used in this study is a of relatively low Reynold’s number, turbulence at the mouth of the kerf increases with an increase in flow rate, thereby decreasing the laminar flow entering the kerf. This is thought to lead to a decrease in clearing efficiency of smoke and ejected debris during ablation. The use of a coaxial nitrogen jet can have some drawbacks. It has the potential to cause surface drying of the bone, which could lead to undesirable effects such as increased cell death and localized heating. As well, pressurized fluid flow may cause unwanted spattering of fluids; whether this would be worse than the use of a high-speed spinning drill bit is a matter of future study.
It has been noted by several studies that ablation of bone, specifically in the thermal regime, causes a delay in healing post laser osteotomy compared to traditional mechanical methods [31–33]. Although this study did not quantify the thermal damage conducted into the tissue as a result of laser energy deposition, the proposed techniques have the potential to mitigate some of this damaged-induced healing delay primarily in two ways. First, the use of a topical absorber such as graphite can dramatically reduce the exposure time of the bone to the ablation light, thereby reducing the overall time that excess heat can conduct into surrounding tissue. Second, inert assist gas flow may act to locally cool the area, potentially reducing the heat-affected zone around the ablation site. The hope of this study and such explorations is to help make low-cost, effective laser surgery more prevalent in clinical practice.
Funding
Natural Sciences and Engineering Research Council of Canada; Canada Foundation for Innovation.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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