There is tremendous interest in the use of lasers in spine surgery by the public and the lay press. The term “laser” often tops the list of spine-related searches on the Internet search engines. Since October 2011, “laser spine surgery” has become more popular among Google search terms than “minimally invasive spine surgery” or “artificial disk replacement” (Figure 1). A few studies exist on patient expectations of laser spine surgery, but, according to some publications and Internet marketing materials, a host of purported benefits to laser surgery exists, including less blood loss, less invasiveness, and more than 97% likelihood to recommend. The importance of this should not be underestimated, because patients increasingly use the Internet, especially Google, to obtain information about spine surgery.
Despite the dramatic increase in Internet queries for laser spine surgery, there is a paucity of peer-reviewed literature describing the advantages of laser spine surgery. Indeed, a PubMed search (in November 2017) revealed “no results” for the term laser spine surgery. A search for “Laser disc*” or “Laser disk*” produced 160 articles, including review articles, preclinical studies, meta-analyses, and systematic reviews (Figure 2). In contrast to the observed trend of increasing public interest, more studies were published in the interval between 1998 and 2007 (n = 58) and between 1988 and 1997 (n = 52) than in the last 10 years (n = 46). However, only 12 clinical trials and studies were available (Figure 2). This review summarizes the peer-reviewed, published literature on lasers in spine surgery so that practicing musculoskeletal providers (eg, physiatrists, general orthopaedic surgeons, and spine surgeons) can have a robust evidentiary background to use when counseling their patients.
Physics and Function of Lasers
Laser is an acronym for “Light Amplification by Stimulated Emission of Radiation.”1 Lasers produce light by excitation (“stimulated emission”) of a medium. As the material returns from an excited state to a relaxed state, photons are emitted.2 Lasers are distinguished from other light sources because lasers emit light coherently. Two types of coherence that characterize lasers exist: spatial coherence and temporal coherence.3 Spatial coherence is a measure of the correlation between the phases of a light wave at different points transverse to the direction of propagation and corresponds to uniformity. The spatial coherence property of lasers explains that laser light waves do not diverge or converge. Because of spatial coherence (and thus the lack of divergence or convergence), a laser can be focused onto a small point over a long distance without diffraction. Temporal coherence is a measure of the correlation between the phases of a light wave at different points along the direction of propagation and corresponds to monochromaticity. Because of the monochromaticity, lasers emit a very narrow spectrum (a single “color” of light) that can be precisely selected for a specific function in tissue. Lasers are characterized according to their wavelength in a vacuum, which is often a property of the lasing medium selected. Common lasers used in medical clinical practice use diode, argon, Co2, and Holmium:YAG lasing media. Laser light exhibits different properties depending on the lasing media. For example, a Co2 laser wavelength is well absorbed by water,4 whereas an Nd:YAG laser wavelength is poorly absorbed by water.
Lasers can have several different tissue effects in surgery, including cutting, coagulation, and ablation. The tissue effect of a laser is generally related to energy and time of exposure. The power of a laser (like any other energy source) is measured in watts. The watt (W) is a derived unit of power; in the International System of Units (SI), it is defined as 1 J/s of energy transfer. Laser power outputs vary between nanowatts (nW) (10–9 W) and terawatts (TW) (1,012 W).5 Spine procedures commonly use a power setting of 5 to 20 W/s and average 400 to 1,000 J of laser energy delivered.6 Thus, laser energy (E) is the product of a laser's peak power multiplied by the time of exposure (t).
In practice, lasers in spine surgery are most commonly used as an attachment to a surgical microscope (Figure 3 and Figure 4). The laser energy is directed by a joystick (Figure 5). The laser power settings are controlled from a video display panel (Figure 6). Lasers (in concert with surgical microscopes) ablate tissue in cranial neurosurgery and otorhinolaryngology applications (Figure 7). Lasers can also be applied through a fiberoptic handpiece wand (in percutaneous applications).
Preclinical Studies of Lasers in Spine Surgery
Extensive preclinical data are available which describe the efficacy and risks of lasers when used to ablate disk. A host of preclinical studies describes the efficacy of lasers for nucleus pulposus ablation.7 One study reported that the ideal laser for disk ablation is a wavelength of 1908 nm and 5 W of power.7 Theoretically, disk ablation with a laser reduces intradiscal pressures, similar to the thermal ablation techniques such as intradiscal electrothermal therapy and biacuplasty. Indeed, in animal and cadaveric studies, laser disk ablation has been demonstrated to reduce intradiscal pressures.6 Furthermore, extensive laboratory data are available from studies on other organs, which report that in some contexts lasers may have beneficial direct tissue effects, including bactericidal properties, hemostasis, and sterilization with cutting, induction of neovascularization, reduced scar tissue, and even modulation of cell biology (photobiomodulation). However, limited preclinical data are available that report lasers to have a beneficial effect on intervertebral disk biology. Furthermore, numerous preclinical studies have demonstrated that laser disk ablation may cause thermal injury and premature degeneration to the remaining nucleus pulposus or annulus fibers.8,9 In one study using a guinea pig model, the authors reported that in the acute period after laser disk ablation, proteoglycan and collagen content decreased because of the ablative effect of ND:YAG laser on disk tissue.10,11 Additionally, an increase in cartilaginous fibrous tissue was noticed at 60 days after laser irradiation because of the proliferation of cartilaginous cells and fibrous tissues. These results led the authors to conclude that laser ablation does not induce a therapeutic, regenerative effect on disk cells.10,12
In vitro studies have also suggested that laser diskectomy may result in thermal injury to nerves.13,14 Perhaps one of the more important studies on the safety of laser usage around the nerves was performed by Robinson et al.15 In their study, they compared bipolar electrosurgical cautery with Co2 laser and reported that the use of a bipolar at a setting of up to 11 W did not lead to damage to the surrounding nerves, unless the nerves were directly between the forceps. Comparatively, the use of a laser with more than 2 W of energy led to acute damage to the nerve.15 These findings are of critical importance, because studies have reported that the ideal laser for disk ablation has 5 W of power.7 However, the danger of using a laser near neural tissue is still unclear, because one porcine study of percutaneous laser diskectomy (PLDD) found no neurologic deficits in the animals, and the authors measured the temperature of the ipsilateral spinal nerve and found it to not exceed 40°C.6
Finally, in addition to concerns about thermal injury to the disk fibers and neural tissue, thermal end plate damage has been described in in vitro studies as a complication of laser disk ablation.12,16 Turgut et al16 performed a surgical disk injury to 12 guinea pigs, followed by an irradiation of the disk with an Nd:YAG laser. The authors determined that in the early period after laser irradiation, the vessel counts decreased because of the extensive damage of Nd:YAG laser on disk tissue (P = 0.00). The authors concluded that the laser application had a notable negative influence on the end plate vascularity.16
Limited preclinical data are available supporting the use of lasers in aspects of spine surgery other than diskectomy. Spinal stenosis is commonly caused by bony hypertrophy (of the facet joints) and hypertrophic ligamentum flavum. However, lasers are not easily used for cutting of bone or ablation of other tissue (eg, ossified ligamentum flavum) because of the excessive heat transfer. In an ex vivo porcine model, lasers have been demonstrated to be effective in the ablation of ligamentum flavum. However, the authors reported tearing of the tissue and a large zone of necrosis surrounding the laser hole (which would include the dura of course). The authors identified that charring caused by the Ho:YAG laser was somewhat mitigated by continuously flushing the affected ligamentum flavum with saline during irradiation. The Nd:YAG laser was found to ablate ligamentum flavus with no gross visible indication of thermal damage to the surrounding ligamentum flavus.17 Last, lasers have also been used in vitro to ablate facet capsule, but again a concern exists about thermal injury to adjacent structures including facet joint cartilage and dorsal root ganglia.18
Although notable risks are associated with the use of a laser around the spine, undoubtedly theoretical benefits too exist; laser-assisted diskectomy may reduce the thermal injury to tissue compared with traditional cautery techniques. A recent porcine study compared laser craniotomy with traditional craniotomy and bipolar cautery (Malis) and found that with increasing power, the Co2 laser produced increased depth of ablation but not increased width when using it to cut through central nervous system tissue.19 Furthermore, when examined as a function of depth of coagulation, the total area of tissue affected by thermal energy was much higher when using the bipolar cautery than when using the Co2 laser, confirming the minimal adjacent injury expected with laser incisions. Therefore, when cutting through the neural elements, the Co2 laser produced less adjacent thermal damage than a conventional bipolar electrocautery instrument. However, to date, no preclinical study is available that supports the belief that a laser disk ablation reduces the microscopic tissue trauma associated with manual disk removal (with biting instruments such as pituitaries).
In summary, the preclinical data demonstrate unequivocally that lasers can be used to ablate disk tissue and other perispinal tissues (such as flavum). However, unique complications are inherent to lasers involving thermal injury to adjacent structures including residual disk, cartilage end plates, and, most importantly, nerves. Furthermore, none of the comparative preclinical studies demonstrate an advantage of laser disk removal over conventional disk removal.
Clinical Studies of Lasers in Spine Surgery
Percutaneous Laser Diskectomy
Lasers are most commonly used percutaneously or directly in surgery for disk ablation. Several commercial laser systems have been approved by the FDA for spinal diskectomy. PLDD has been extensively described in the literature for decades.20,21 It involves the placement of a needle into a specific location in the disk (under either fluoroscopic guidance or CT guidance similar to provocative diskography), the introduction of a fiberoptic laser wire through the needle, and activation of laser energy to ablate disk material without direct visualization. The procedure is intended to reduce intradiscal pressure to reduce end plate stresses, decrease back pain, and reduce disk protrusions.
A number of publications report the use of PLDD in managing symptomatic degenerative disk disease/internal disk derangement and protrusion-type disk herniations (but not sequestrations or extrusions), and these mostly describe acceptable outcomes with little to no complications.22-24 Most of these studies are uncontrolled, small, and retrospective case series with subjective outcome metrics (Table 1). Overall, patients undergoing PLDD were found to have smaller, more contained herniations; in almost all studies, patients with extrusions were excluded. Similarly, because PLDD does not include an open laminoforaminotomy, surgeons may be less likely to offer the technique to patients with large herniations. When considering all these factors, it becomes clear that selection bias may limit the generalizability of the results.
In addition to the retrospective studies, a single, nonblinded, prospective randomized trial exists, which compared open microdiskectomy (n = 58) with percutaneous laser disk ablation (n = 58) in patients with a contained disk herniation confirmed on MRI without an extruded fragment or spinal stenosis.25,26 In the PLDD group, a glass fiber of 600 μm was advanced into the disk, enabling the application of laser energy through a needle (Diode laser, 980 nm, 7 W, 0.6 s pulses, interval of 1 s; Biolitec) to a total energy delivered of 1,500 J (2,000 J for level L4–5).25 The authors found no difference in primary outcome measure (RMDQ at 1 year) between the groups. However, an overall reduction in visual analog scale (VAS) scores of leg pain was observed in favor of the surgery group with a mean between-group difference of −6.9 (95% confidence interval [CI], −12.6 to −1.3) (on a 100 mm scale).26 The authors also reported that the patients who underwent conventional surgery experienced earlier perceived recovery, with a median time to recovery of 8 weeks (95% CI, 3.2 to 12.8) for PLDD versus 6 weeks (95% CI, 5.2 to 6.9) for microdiskectomy. Also, a notable difference in the rate of secondary surgery was observed. The microdiskectomy group had a 21% revision surgery rate compared with 52% in the laser group. Therefore, the percutaneous laser microdiskectomy procedure had a much higher overall failure rate than the conventional microdiskectomy. In fact, the rate of failure of laser percutaneous diskectomy was substantially higher than the rate of 1-year pain recurrence (20% to 23%) or even 3-year pain recurrence (45% to 51%) after surgical or nonsurgical treatment in the Spine Patient Outcome Research Trial.27 Other methodologic issues with the study were noted, which may limit the generalizability of results.28 In the only prospective randomized PLDD study in the literature, the reported complication rate was 5% and consisted of transient nerve root injury (n = 3/53).26
Open Spinal Surgery
Although the aforementioned studies reported using a laser as an alternative to traditional open surgery, the term laser spine surgery is often thought to indicate when lasers are used as an aid in conventional spine surgery. Despite the widespread interest in this topic by the public, to date, no high-level, prospective clinical trial is available which compares laser-assisted spine surgery with conventional surgery. A number of retrospective studies demonstrated equivalence or non-inferiority to standard techniques (Table 2). Lee and Lee29 reported on the outcome of Co2 laser disk ablation in a consecutive series of microdiskectomies for far lateral L5/S1 disks, in that study, the laser was only used after Wiltse approach was performed and a traditional foraminotomy was completed. The authors reported no device-specific complications and a recurrence rate of 3.6% and that overall 75% of patients were satisfied at 1 year.29 In another study of 21 patients undergoing laser-assisted conventional microdiskectomy for recurrent disk herniations, Kim et al30 reported no device-specific complications and overall notable improvement in back and leg pain. The authors reported possible improved dissection and reduced durotomies with laser dissection, but the laser was used only to make an annulotomy. A traditional foraminotomy was performed, and once annulotomy had been performed with the laser, traditional instruments were used to remove any free fragments. In one of the few comparative studies, Ahn et al31 reported on a retrospective series of patients who underwent posterior cervical laminoforaminotomy and diskectomy. In that study, 24 patients had laser-assisted diskectomy and 23 underwent conventional posterior cervical laminoforaminotomy. No difference was observed in Neck Disability Index or VAS scores between the groups, but blood loss was markedly less in the laser group. A recent study described the use of a flexible, handpiece-delivered Co2 laser to perform ablation of contralateral ligamentum flavum in 39 patients undergoing unilateral hemilaminotomy. The authors used a 9- to 11-W continuous power setting on a Co2 laser with 10,600 nm wavelength. Specifically, the laser was used to “desiccate the ligamentum flavum and cauterize bleeding bone.” According to the authors, the desiccated ligamentum flavum was more easily removed. No complications were reported in the study patients, although the authors do ominously report that “injury to nerve roots, dural tear, devitalization of the muscle, and directly burning nervous tissue are all risks with the laser. One must also remember that reflection of the laser onto other instruments and, for example, heating up the sucker, can cause complications.”32 One other retrospective study describes the use of lasers in decompression of patients with spinal stenosis. At 2 years, only 66% of patients with anterior pathology and 33% of patients with posterior pathology were found to have an improvement in Japanese Orthopaedic Association scores.33 Therefore, limited literature is available on laser-assisted spine surgery, which mostly consists of retrospective case series that do not report standardized outcome measures.
Another possible use of the laser in open surgery is as a tool in performing a diskectomy for an interbody fusion. A recent retrospective study described a similar use of a Co2 laser to open the disk and perform a total diskectomy in preparation of a fusion.4 Two nonrandomized groups of patients undergoing transforaminal lumbar interbody fusion were compared in a nonblinded fashion. In that study, 24 patients underwent Co2 laser–assisted, one-level transforaminal lumbar interbody fusion (TLIF) surgeries and 30 patients underwent standard, one-level TLIF surgeries without the laser. No neural thermal injuries or other intraoperative laser-related complications were encountered in this cohort of patients. At a mean follow-up of 17.4 months, statistically significant reduced lower back pain scores (P = 0.013) were reported in the laser-assisted patient group (mean, 1.7) than in the standard fusion patient group (mean, 2.1). However, this difference may not be clinically significant. Lower extremity radicular pain intensity scores were similar in both groups. No notable difference was observed in surgical times between the groups. Thus, the laser did not speed the end plate prep for diskectomy or reduce the overall surgery time (which would include time for retraction).
Although lasers have been approved by the FDA mostly for spinal diskectomy, lasers are also used by some doctors to perform facet joint medial branch rhizotomies either as a stand-alone procedure or as an adjunct to open microdiskectomy. The purpose of rhizotomy in addition to microdiskectomy is to treat patients who have symptomatic lumbar radiculopathy along with facetogenic axial back. Iwatsuki et al34 reported a small retrospective study on a series of 17 patients who underwent percutaneous posterior facet capsule and medial branch ablation (as a stand-alone procedure without open microdiskectomy). At 1 year, a reduction in VAS score of pain was noted from an average of 7.4 preoperatively to 3 postoperatively. To the authors' knowledge, only a single small retrospective published study has described the outcome of microdiskectomy with conventional radiofrequency facet ablation.35 In that study, no difference was noted between the microdiskectomy and rhizotomy groups versus microdiskectomy alone. In addition, a recent prospective randomized controlled trial has called into question the general efficacy of rhizotomy to manage back pain.36
Similar to the preclinical data, laser-specific complications exist that are mostly related to thermal injury. Several cases describe thermal injury to the adjacent vertebral end plates in patients who underwent laser diskectomy.37-40 In severe cases, the end plate thermal injury has been reported to cause permanent lesions on MRI and even subchondral osteonecrosis.37 In addition to the aforementioned imaging studies, histologic evidence of thermal injury has also been noted in revision surgical cases after laser spine surgery. A case series was reported on 13 patients who underwent revision surgery after percutaneous laser diskectomies with an Ho:YAG laser.38 All patients were noted to have either laser-induced clefts in the bony end plates or signal change consisting of a separation between the end plate and vertebral body. The disk tissue resected during salvage operations contained carbonized lesions, suggestive of thermal injury. The authors concluded that the Holmium laser used in the study injured the vascular supply of the subchondral bone and cartilage.
Also, several authors have reported thermal nerve root injury after laser decompression.41 One recent case report described thermal injury to an L5 nerve after PLDD.42 The intraoperative findings in the revision surgery included carbon spots on the dura mater of the nerve root and a disk herniation strongly adherent to the nerve roots. The authors concluded that the area was damaged by excessive heat during laser irradiation.
Other laser-specific complications include the risk of visceral organ injury because of overpenetration of the disk space by the laser radiation. In one case report, a patient was noted to develop an iatrogenic iliac artery injury when a laser was used in an open microdiskectomy.43
In summary, most of the clinical studies of lasers in spine surgery are uncontrolled, retrospective studies characterized by small numbers of patients and variable outcome reporting. Even in those studies, no specific advantages were reported on the use of a laser in open or percutaneous cases. The reported success rate does not approximate the greater than 90% success rate often quoted on the Internet. As suggested by the laboratory studies, a host of laser-specific complications has been reported in patients undergoing laser diskectomy mostly related to thermal injury of neurologic structures or remaining disk. The only high-level prospective randomized controlled trial in the literature showed no advantage for laser surgery, improved leg pain in the conventional surgery group, and a lower revision surgery rate in the conventional surgery group. One limitation of this review is that our approach excluded descriptions of lasers used topically for therapeutic purposes (outside surgery) or lasers used in endoscopic surgical applications such as epiduroscopy. This review was focused on whether lasers would benefit conventional lumbar surgical techniques, such as microdiskectomy or lumbar laminectomy. Because of the paucity of evidence, the North American Spine Society Coverage Recommendations Task Force and the Canadian Agency for Drugs and Technology44 have both issued recommendations against laser spine surgery.45
Lasers have been used in medicine for decades. The special features of lasers (spatial and temporal coherence) have been exploited extensively in other fields of medicine, such as ophthalmology and dermatology. A host of preclinical studies has demonstrated that lasers can be used for disk ablation, and indeed some lasers are approved by the FDA for diskectomy. Patients are intensely interested in lasers in spine surgery likely because of the aura of new technology and the information on some Internet websites of high success rates (more than 90%), the absence of scar, and minimally invasive surgery. However, the purported advantages of lasers are not supported by robust preclinical or clinical research. The available clinical studies do not show a notable advantage for laser surgery, and the available small patient series of nonblinded, retrospective studies are heavily subject to bias. The advantages of laser spine surgery described in the research studies are not consistent with patients understanding of its purported benefits. Numerous laser-specific complications are present about which patients should be informed before undergoing a laser procedure. In the absence of definitive evidence showing a benefit to laser spine surgery and with substantial evidence suggesting potential harm of laser diskectomy, the current evidence suggests strongly against the use of lasers in spine surgery.
References printed in bold type are those published within the past 5 years.
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