Laminotomies, laminectomies, and diskectomies are frequently performed through a small incision, using a series of serial dilators to access the appropriate spinal level with microscopic or endoscopic visualization. Once the largest dilator is in place, a tubular retractor is inserted and the inner dilators are removed. Specialized endoscopes or a microscope is used for visualization of the operative site. Instruments, similar to those used in open surgery, are utilized to perform the necessary decompression of the nerve tissue. A primary advantage of tubular retractor-based decompression procedures is the relatively minimal amount of soft-tissue damage caused by the surgical approach in comparison with a traditional open approach lumbar decompression. This facilitates same day surgery and rapid return to normal functioning.

A concern for those early in the learning curve of MIS decompression is the risk of inadequate decompression of the neural elements. This is largely due to the limited visualization of external spinal landmarks and the lack of experience in defining the location of the instruments relative to the spinal pathology. During an MIS decompression, the surgeon must rely on landmarks within the spinal canal to gain an understanding of the precise location of instruments within the surgical field. To the authors’ knowledge, there have been no studies published which have directly measured the incidence of inadequate decompression in MIS procedures. A 2010 review by Fourney et al.³ found reoperation rates in patients undergoing MIS decompression procedures to be higher than those who underwent traditional microdiskectomies, although the authors stated that the quality of the data they encountered was low and reported that most of the reoperations were the result of recurrent herniations rather than retained pathology. A recent meta-analysis by Shriver et al.⁴ found no significant differences in recurrent disk complications between lumbar open microdiskectomy, microendoscopic microdiskectomy, or percutaneous microdiskectomy.

The steps which can be taken in order to ensure an adequate decompression using an MIS approach are as follows. First, carefully study the advanced imaging studies and radiographs prior to surgery to accurately define the locations of stenosis or disk herniation that must be relieved during surgery. This will allow the development of an adequate preoperative plan including the correct location of the incision to reach the areas of pathology for the specific
case. Second, use fluoroscopic imaging to precisely localize the incision, tubular retractor position, and other instruments as required during surgery to be sure that all the necessary regions of the spinal canal that require decompression have been addressed. Third, use intraoperative visual and palpable landmarks to guide the decompression during surgery. The surgeon should expose the medial boarder of the pedicle which has been called “the signpost of the canal.” Referring back to the preoperative plan, the surgeon should have a good sense of the location of various areas of pathology relative to the pedicle. In cases of spinal stenosis, it is important to confirm by visualization and palpation the lack of residual compression within the lateral recess and foraminal zones. When performing a bilateral decompression through a single incision, the surgeon will be working across the spinal canal through the base of the spinous process. While drilling across to the contralateral side, the ligamentum flavum provides a useful guide. The ligamentum should be left intact during the drilling process to protect the underlying dura and allow the surgeon to follow the ligamentum across to the contralateral facet joint.

Unintentional durotomies or dural tears are a potential complication of any decompressive procedure. Polikandriotis et al.¹⁰ reported on 320 consecutive MIS laminotomies and foraminotomies for lumbar spinal stenosis and found a rate of dural tears of 2.2%. Wong et al. performed a retrospective study of 863 patients following lumbar decompression surgery. In this study, the authors divided the patients into those with MIS decompressions (n = 544) versus open decompression (n = 319) over a 5-year period. They found that spinal fluid leaks were significantly less frequent in MIS procedures (15/319 or 4.7%) compared to open procedures (49/544 or 9.0%). Additionally, the patients who sustained a dural tear during an MIS procedure were less likely to require reoperation for repair compared to those having open surgery.¹¹ A recent meta-analysis by Shriver et al.⁴ found no significant differences in dural tear between open and MIS lumbar microdiskectomies.

A number of strategies can be used to minimize the risk of a dural tear with MIS surgery. First, it is important to have a good three-dimensional understanding of the anatomy of the spinal canal with the viewing technology in use. Practice in cadaver models prior to tackling surgical cases can help to obtain this familiarization. Second, the authors believe that the use of a guide wire to localize the spinal lamina prior to serial dilation should be avoided. This reduces the risk that the guide wire will inadvertently penetrate the ligamentum flavum and cause a dural puncture. Instead, the small dilator can be utilized to directly palpate the laminar edge to allow docking on the laminar edge. Third, the surgeon should use a blunt instrument such as a ball-tipped dissector to palpate the edges of the dura and ensure the absence of adhesions that could lead to a dural injury. Fourth, the surgeon should leave the ligamentum flavum intact during the initial drilling to remove bone. This will protect the dura and reduce the chance of an inadvertent tear. Finally, the surgeon should try to keep the edge of the Kerrison rongeur firmly against bone to prevent the dura from entering the jaws of the instrument prior to biting bone. Having a variety of specialized instruments including 40 degree, 90 degree curved tipped Kerrison instruments is helpful in accessing various areas of the spinal canal in an optimal fashion.

In the event of a dural tear, it is the authors’ preference to consistently attempt to obtain a watertight closure of the tear. This can usually be achieved using a double-armed 6-0 suture which is sewn using a micropituitary instrument as the needle driver. The needle is most commonly passed in an inside to outside fashion and the edges of the tear are anatomically approximated. The authors also frequently use a small tissue patch to bolster the repair. Following completion of the repair, a Valsalva maneuver is performed to ensure the absence of any spinal fluid leakage. Tight closure of the fascia and skin is performed. In the event of an anatomic repair, routine postoperative mobilization is generally performed.¹,¹¹

Another risk of decompression surgery is the potential for iatrogenic instability. In a review by Guha et al.,¹² the authors found the incidence of postoperative instability to be lower in MIS decompression compared to open decompression. The reduced incidence of iatrogenic instability is believed to be due to the maintenance of stabilizing structures including the spinous processes, supra- and intraspinous ligament and the facet capsules.¹³ On the other side however, minimally invasive decompressive procedures performed at the upper segment of the lumbar spine, L1-L2, L2-L3 and some L3-L4, the lamina and spinous processes are tall but very narrow to such degree that the working tube is forced to drift laterally over the facet. Such position can predispose to a more aggressive facetectomy than desired and cause instability.

Minimally invasive transforaminal lumbar interbody fusion (MI TLIF) is a procedure which has been performed with increasing frequency and has been the subject of a number of good quality studies. In a review by Wong et al.¹⁴ involving more than 500 consecutive MI TLIF procedures, the overall rate of complications including dural tears, surgical site infections, instrumentation failure, and iatrogenic neurological deficits was comparable or better than the rate of complication reported for open TLIF procedures. Parker et al.¹⁵ reported a 3.4% lower incidence of surgical site infection in MI TLIF compared to open TLIF. Wu et al.¹⁶ reported a 5% lower rate of overall complications with MI TLIF compared to open TLIF, but also noted methodologic flaws that called for further research on the subject. According to a review by Joseph and Smith, the most common complications with MI TLIF are dural tears and malpositioned hardware.¹⁷

Wong et al. in a series of 513 patients found 26 patients (5.1%) who had an intraoperative dural tear during MI TLIF surgery. There were 19 cases (4.4%) of dural tear with single-level fusions and 7 cases (8.6%) with multilevel fusions. Of the 26 patients with MI TLIF complicated by dural tear, each was treated with repair and flat bed rest overnight, and none of the patients required additional procedural interventions to resolve the cerebrospinal fluid leak.¹⁴

Hardware malpositioning has been another concern during MI TLIF procedures. There has been speculation that superior facet violation (SFV) rates could be higher in MI TLIF compared to open procedures due to the lack of direct visualization; however, Lau et al.¹⁸ found no significant difference in the rates of SFV, except in patients with BMI>30. Dhall et al. retrospectively reviewed a series of 42 patients with single-level TLIF including 21 patients who underwent TLIF via the mini-open technique and the other 21 underwent TLIF via the traditional open approach. The authors reported one additional case of hardware malfunction in the MIS group compared to the open group and noted that it occurred early in their learning curve.¹⁹ Villavicencio et al. reported the results of 139 consecutive TLIF cases involving either open (n = 63) or a MIS (n = 76) approach. The interbody graft was found to be malpositioned in two open cases (3.2%) and three MIS cases (3.9%). The pedicle screws were malpositioned in two open cases (3.2%) and four MIS cases (5.3%).²⁰ Chrastil and Patel reported a significantly increased rate of postoperative interbody cage migration with the use of recombinant bone morphologic protein-2 (rhBMP-2) within the interbody space.²¹ Wong et al.¹⁴ recently reported a retrospective analysis of 513 patients following MI TLIF and found 11 patients (2.1%) with instrumentation failure.