low back pain ( lbp ) is thought to have originated from intervertebral lesion ( 739% ) . most patients with discogenic lbp respond well to conservative care , but 5% of patients suffer from constant pain313 ) . various treatment modalities , such as injection therapy , fusion surgery , total disc replacement , and thermal annular procedure , are used for discogenic lbp , and yet clinical efficacy is controversial125678111925 ) . until now , there were some thermal therapies for discogenic lbp , namely , intradiscal electrothermal therapy ( idet ) using thermocoil located at the nucleo - annular junction , disctrode using monopolar radiofrequency to apply ionic heat , and biacuplasty using cooled bipolar radiofrequency81624 ) . according to lee et al.17 ) , annuloplasty was performed using laser as a heat source and a 90% success rate was reported . first , using heat causes denervation of the nociceptor in the annulus , and ingrowth of the unmyelinated nerve ending that causes pain . second , enhancing stability is achieved through the stiffening of the annular fiber from denaturation and shrinkage of the collagen . however , their mechanism is still unclear12 ) . in order to satisfy the aforementioned rationale , the temperature must reach 45 in order to destroy the nociceptors and 60 to alter and shrink the collagen fiber9 ) . however , there are some problems and disadvantages that are associated with this modality . it can induce degenerated intervertebral disc and disc herniation from the puncture site after the annulus puncture . moreover , a probe tip that focuses heat was placed on the intra - annular area or nucleo - annular junction portion instead of the outer annulus with an ingrowth sensory - free nerve ending . in our experiment , we decided to apply heat to the posterior annulus of the intervertebral disc of a cadaveric lumbar spine model by using a 1414-nm nd : yag laser , which was developed recently . a 1414-nm nd : yag laser , compared with ho : yag laser , is easier laser control , a smaller manufacturing fee , and equivocal effectiveness . also compared with conventional ng : yag laser , it is more safe because of shallow tissue penetration . for this study , five discs from t12l1 to l45 were obtained from 4 fresh adult human cadavers . the donor criteria excluded subjects with severe degeneration , such as fissuring , bridging osteophytes , history of spine trauma , infection , tumor , and any other bony diseases . the specimen was initially prepared by isolating the vertebral column from t12 to l5 and by cutting at t1112 and l5s1 disc level . it was prepared as en bloc , and the adjacent muscles and soft tissues were removed . then , we removed the posterior elements of the spine by electric saw at the mid aspect of the pedicles . the cut vertebral column was placed in the water bath and maintained at a constant temperature of 371 by heat probe . in order to correct the temperature of the discs that are affected by the cerebrospinal fluid and blood circulation , we made some continuous artificial stir approximately 300 rpm via input and output circuit system of water bath ( fig . 1 ) . for the measurement of the disc temperature distribution around the laser probe , 5 thermocouple probes [ hyp1 ( hyp0 - 30 - 1/2-t - g60-smpw - m ) , omega , stamford , each probe was placed at 3 mm , 6 mm , and 9 mm laterally , and at 3 mm and 6 mm by depth as shown in fig . once all of the 5 probes constantly reached 37 , we decided to apply heat at 600 m fiber diameter of 1414 nm nd : yag laser system ( accuplasti , lutronic , goyang , korea ) placed at the midpoint of the posterior surface of the disc for 10 seconds . the pulse energy was kept at 200 mj , while the pulse rate was set at 20 hz . this recently developed 1414 nm nd : yag laser 's maximal pulse energy and rate was 600 mj and 40 hz . we used the selected parameter ( 200 mj and 20 hz ) for our experiment dose without causing carbonization and vaporization of the cadaveric disc , and also observed gross shrinkage and hyalinization in the water bath during the preliminary study . we recorded and analyzed the temperature by using the multichannel data acquisition system ( mv1000 , yokogawa electric corporation , tokyo , japan ) . the average temperature recordings of the 5 probes from 20 specimens , according to time , can be observed from fig . 3 . the temperature would rise upon irradiation and fall slowly with the cessation of irradiation . the most distinctive elevation of the temperature was observed from probes located 3 mm away from the laser fiber tip , both in depth and lateral positions . the distribution curves , as measured across time , were quite varied depending on the location of the probes . the temperatures that were collected from the probes , which were closer to the laser tip , were higher than the temperatures from the probes that were located further away , while the probes that were placed laterally showed higher temperatures than the probes that were placed deeper . the average maximum temperature recorded from the probes 3-mm and 6-mm deep from the laser tip were 47.04.3 and 38.70.9 , and each measured at 13.60.8 seconds and 24.83.0 seconds , respectively . the average maximum temperature recorded from the probes 3 mm , 6 mm , and 9 mm lateral to the laser tip were 69.513.3 , 37.80.7 , and 37.20.3 , and each recorded at 10.60.5 seconds , 26.63.4 seconds , and 38.84.4 seconds , respectively ( fig . the combination of the laser tip , 3-mm depth , and 16 levels ( 80% ) showed a temperature above 45 , which would result in a nociceptor ablation . meanwhile , the relationship between the laser tip and laterality showed all levels at temperatures above 45 , and 16 levels showed temperatures above 60 , where collagen denaturation can be achieved . the temperatures from the lateral probes showed higher temperatures than the depth probes , which could be related to the characteristic of 1414 nm nd : yag laser , where the water showed much greater heat absorption potential than the human tissue . the thermal procedures for discogenic lbp have been performed since the late 1990s , and the theoretical background was that the sources of discogenic pain were the nerve ingrowth and tissue regeneration into the annulus21 ) . therefore , achieving 45 or 60 through idet or biacuplasty , where nociceptive ablation and collagen denaturation were possible , was the key point to cadaveric and animal experiments for thermal procedures4141518202224 ) . bono et al.4 ) reported that upon using idet , the temperature above 60 was achieved within a 2-mm distance from the catheter , and the temperature above 45 was achieved at 9-mm to 14-mm distance away from the device . shah et al.22 ) reported that by using idet on the cadaveric model , they have managed to reach 55 in the outer annulus , while denaturation , shrinkage , and coalescence of the annular collagen were demonstrated on the histologic finding . however , kleinstueck et al.15 ) , reported that during their cadaveric study of idet , temperatures above 60 were only achieved at approximately 1 mm to 2 mm around the catheter tip , while the temperatures adequate to damage nociceptor fibers were observed at 6 mm around the catheter tip without affecting the posterior annulus , where it is clinically relevant . these results have shown that the idet , where the catheter is inserted and positioned in the inner border of the posterior annulus and then heated , has its limits in giving a direct thermal control on the free nerve endings from gray rami communicans and sinuvertebral nerves that innervated to the disc in cases of disc degeneration . we have performed annuloplasty and used a laser for minimally invasive endoscopic procedures at the same time as one of thermal energy sources , and chose 1414 nm pulse wave nd : yag laser to evaluate the temperature distribution . for laser irradiation , an nd : yag laser with a wavelength of 1414 nm operating in the 200-mj to 600-mj range was used . we can observe the epidural space by epiduroscope and remove lesions , such as herniated disc , fibrosis , and adhesion , by laser . the laser power level was set to 200 mj to 300 mj ( 10 hz to 20 hz ) for denervation and coagulation , and 300 mj to 600 mj ( 10 hz to 20 hz ) for vaporization and ablation . the adequate temperature for treatment in thermal therapy is considered to be above 60 at minimum , where collagen fibril shrinkage can be managed23 ) . there have been a few studies describing the effects of thermal therapy on collagen . in their research on the effect of nonablative laser energy on in vitro rabbit femoropatellar joint capsule , hayashi et al.10 ) observed the fusion of collagen and pyknotic nuclear changes in a fibroblast , and reported that there was no evidence of charring or removal of tissue . high - power energy usage will result in laser that can induce tissue coagulation and charring , thereby leading to destruction . therefore , it is essential to select a proper energy setting to ensure that the temperature , albeit above 60 , is not high enough to damage the tissue . jayasree et al.12 ) reported that when using a 1064-nm nd : yag laser for a disc decompression , it is ideal to use an initial delivery of 40 w laser and a reduced power of 10 w to 15 w thereafter . however , this setting is a parameter used in disc ablation , and it is too high to apply in laser annuloplasty . there were no investigations on specific parameters for such purpose , hence the pre - research was done to find the parameter that selectively induces shrinkage without gross annular charring or coagulation , as well as a pulse energy of 200 mj , a pulse rate of 20 hz , and a duration of 10 seconds . the fact that the lateral surface of annulus have shown a higher temperature than the deeper portion of the annulus was quite a surprise , considering the laser directionality . although the thermal properties of the tissue is determined by its water content , the spread of energy via thermal conductivity is influenced by the structure and complicated geometry of the tissue16 ) . also , transforaminal epidural lqser annuloplasty ( tela ) actually is outside- in technique under epiduroscopic vision for the ablation of pain sources from the degenerative disc , while our experiment is close to idet , which is actually inside posterior annulus - out ablating procedure without visionary assistance . that might be the reason why proper high temperature was conducted within 3 mm of laser tip . the temperature increase and distribution within a laser - irradiated tissue depends on the volume and thermal properties of the tissue25 ) . in comparison to the former nd : yag laser , the 1414-nm wave nd : yag laser has a higher absorption rate of water . furthermore , there was a higher heat conduction within the water bath than the annulus tissue , thereby showing a higher conduction laterally than longitudinally . since it is a newly developed laser with a new wave length , further research about thermal distribution and histological change of tissue will be required . in this study , the thermal distribution after laser irradiation in the annulus has shown a temperature increase in the relatively smaller radius , as compared to the previous studies on idet and its temperatures4141520 ) . similar to the idet , it is hard to apply heat to the entire annulus . however , it seems suitable for the treatment of focal discogenic pain source , such as annular tear or high intensity zone , by endoscopy or fluoroscopy . unlike idet , it will provide an extradiscal approach , thereby preserving a wide extent of healthy annulus without secondary damage due to a disc puncture . furthermore , it can be applied to an endoscopic device that is compatible with percutaneous laser disc decompression or percutaneous endoscopic laser discectomy . but our study has a several limitation . even though depth of tissue penetration is 0.4 mm , by using a 1414-nm nd : yag laser , we did n't measured the temperature of surrounding root and dural sac which severe complication was arisen , if damaged . as one solution to this , there is a tela procedure . the tela has recently been introduced and considered as a new method for the treatment of discogenic lbp . the advantages of tela as compared with back surgery , include relatively non - invasiveness , short operating time , reduction of risks related to general anesthesia , possibility of communication with the patient during the procedure ( leading to reduced risks of accidental nerve damage ) , and easier access into ventral epidural space . and the results of this study are limited through the use of cadaveric spine . although our experiments were performed in a water bath to make closely similar situation of human spine as possible . however , this does not reflect the impact of soft tissue such as muscles and connective tissue around disc . when measuring the temperature distribution of the annulus during laser annuloplasty with 1414-nm nd : yag laser , it could reach a sufficient temperature to ablate the nociceptor and induce collagen denaturation within a relatively narrow radius of 3 mm from the laser tip . it is also thought to be one of the most useful thermal procedures in clinical applications .