Trans-Spinous Microlaminectomy - a minimally invasive technique for decompression of the lumbar spine: A Human Cadaver Study

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Jon P. Kimball M.D.
Department of Orthopaedics and Rehabilitation
College of Medicine
University of Florida

Address:
Gainesville
Fl
USA

Michael Mac Millan M.D.
Department of Orthopaedics and Rehabilitation
College of Medicine
University of Florida

Address:
Gainesville
Fl
USA

Citation: J. P. Kimball & M. Mac Millan : Trans-Spinous Microlaminectomy - a minimally invasive technique for decompression of the lumbar spine: A Human Cadaver Study . The Internet Journal of Minimally Invasive Spinal Technology. 2008 Supplement I - to IJMIST Vol 1 No 2

Abstract

Objective: To determine the feasibility of a minimally invasive laminectomy technique for lumbar spinal stenosis and quantify increases in canal area after decompression.
Methods: Using computerized tomography (CT), stenotic lumbar levels were identified in 4 cadavers (L4-5 in 3, L2-3 in 1). A trans-spinous microlaminectomy was performed at each stenotic level and CT was repeated afterwards. The procedure entails splitting the spinal process in the sagittal plane to a point just about the laminar proper, then retracting the spinous process to expose the underlying lamina and remaining spinous process attachment. Through this approach, a standard laminectomy is performed. Metallic marker were used to ensure the pre- and post-procedure CT were at the same level. Area measurement software was used to measure canal areas at the level of interest before and after decompression.
Results: Canal space at the stenotic levels increased 2-6.8 fold (mean, 4.2 fold) after the procedure.
Conclusion: A trans-spinous microlaminectomy can be used to decompress the spinal canal effectively. This procedure may prove beneficial in decreasing complications and morbidity of surgical treatment of lumbar spinal stenosis, especially in older patients.

Introduction

Lumbar spinal stenosis is the most common indication for surgery of the spine in patients over age 65. ¹,²,³,⁴ This complex degenerative condition first described by Bailey and Casmajor in 1911 ⁵ has become widely recognized as a common cause of lower extremity pain and neurologic deficit.

The aim of surgical treatment of lumbar stenosis is decompression of the neural elements. This was first accomplished by aggressive resection of the posterior bony elements through a posterior midline approach. Although wide laminectomies were successful in decompressing the neural elements, the resection of bony elements such as the pars interarticularis, facet joints, and spinous processes were found to often result in significant morbidity and iatrogenic instability. ⁶

Lumbar stenosis pathophysiology is a complex combination of facet arthropathy, thickening of the ligamentum flavum, redundancy of the annulus of the intervertebral disc, and congenital narrowing of the spinal canal. Studies using noninvasive imaging have shown that the majority of these processes occur at the level of the interlaminar window. ⁷,⁸ Because of these observations, surgical efforts have focused on this area to allow for decompression while also preserving posterior stabilizing structures. Surgical techniques such as wide decompressive laminectomy, medial facetectomy, foraminotomy, and laminoplasty have been used for decades with varying degrees of success. ³,⁴,⁹,¹⁰,¹¹,¹²

These traditional open, midline approaches have been shown to create large soft tissue voids that can lead to local wound complications and also can cause direct tissue trauma resulting in paraspinal musculature denervation ¹³ and subsequent muscular atrophy. ⁶ The extensive exposures required for adequate visualization and decompression are associated with significant morbidities and complications. ⁶,¹³,¹⁴ Hurri et al ² demonstrated that the incidence of postoperative complications correlated best with the extent of tissue trauma.

This cadaveric study examines the results of midline and bilateral decompression of the lumbar spinal canal through muscle sparing, spinous process splitting approach. This approach had been used clinically in a private practice setting by the senior author with anecdotal good clinical results. In order to establish the objective results of this procedure, this cadaver study was undertaken to evaluate the extent of decompression with pre- and post-procedure computed tomography.

Materials And Methods

Study Design. After approval by the State Anatomical Board, the study was performed using four embalmed cadavers. The dorsal skin and subcutaneous tissues dorsal to the lumbar fascia were removed to minimize the degree of artifact on imaging studies and metallic markers were placed into the lumbar spinous processes for level identification. Axial computed tomographic (CT) images were obtained of the lumbar spines of all four specimens. Area measurement software (Cedara I-Contour, Mississauga, Ontario) and the CT imaging workstation were used to quantify the spinal canal area of each of the scanned levels. Of twenty possible lumbar levels in the four specimens, four levels were identified that had less than 200 square millimeters of spinal canal area at the level of the disc, as measured in the bone window setting. In three of the cadavers, the stenotic level identified was L4-5, and the other cadaver had an area of stenosis at L2-3. The identified spinous processes then underwent surgical decompression with the technique described below. After surgical decompression, the spines were re-scanned and the spinal canal area remeasured.

Surgical Technique. With the cadavers in the prone position, the metallic markers were used to identify the spinous processes of the stenotic levels. A single, midline, longitudinal incision was made along the top of the desired spinous process. Through this incision a high speed drill was used to divide the spinous process through its midline down to its base (Fig. 1). The drill bit was then directed out through the cortex on each side, thus detaching each half of the spinous process. The interspinous ligament above and below tends to remain attached to the cortical surface of each split spinous process. Each individual cortical surface of the spinous process was then retracted laterally, thereby revealing the amputated base of the spinous process along with the right and left spinal laminae (Fig. 2). The laminectomy was performed in a standard fashion using the high speed drill to remove the laminar bone (Fig. 3) and curved Kerrisons to decompress the associated right and left foramens (Fig. 4). CT was then repeated to establish and measure the extent of decompression of the spinal canal.

Figure 1: Schematic drawing shows how a high speed drill is used to divide the spinous process through its midline down to its base.

Figure 2 :Each half of the spinous process is detached with a high speed drill and then retracted laterally.

Figure 3: The laminectomy is performed using a high speed drill.

Figure 4: Bilateral neural foramen are decompressed with curved Kerrison rongeurs.

Results

CT imaging confirmed that the decompression was performed at each desired level. The pre- and post-procedure scans were evaluated carefully to ensure that the measured scan slices were at the same anatomic location and the same spinal location was being measured (Figs. 5 and 6). The metallic markers assisted in this identification (Figs. 5A,D; 6A, D).

Figure 5abcd: Computer tomography images of a stenotic level in one cadaver before the procedure. Arrows in A and D indicate the metallic markers used to mark the level of interest.

Figure 6abcd: Computer tomography slices of the same cadaver after the trans-spinous microlaminectomy. Arrows in A and D indicate the metallic markers which pinpoint the level previously imaged (Fig 5).

All the slices were assessed using both bone and soft tissue windowing algorithms. The absence of cerebrospinal fluid on the cadaveric specimens simplified the identification of the actual spinal canal space (Figs. 7 & 8). On the post-microlaminectomy images, the posterior extent of the spinal canal space had to be extrapolated from the contour of lateral and anterior margins of the spinal canal space (Figs 7B, 8B).

Figure 7ab: These computer tomography scan images show the same anatomic level of a cadaver before and after the trans-spinous microlaminectomy. The red dashes outline the spinal canal area which was measured (Table 1).

Figure 8ab: Computer tomography images from the same anatomic level of a second cadaver, before(A) and after (B) trans-spinous microlaminectomy. The spinal canal areas within the red dashes were measured (Table 1).

Using this method, the average cross-sectional area was increased from 128 mm2 (±37 mm2) on the initial CTscans, to 448 mm2 (±21 mm2) after decompression. Canal decompression averaged 250% (±39%) (Table 1).

Table 1: Canal cross-sectional area before and after midline laminectomy decompression (MLD).

Discussion

There is a recognized need for a less invasive treatment of spinal stenosis. This is driven by two factors. First, the disease usually presents late in life, at a time when patients are less able to tolerate complex surgical procedures. Second, the aging spine is more prone to incipient instability, and extensive exposures and decompressions may lead to clinically significant instability. Three strategies for addressing the central and bilateral components of spinal stenosis that purport to be less invasive are in current use.

The earliest attempts were to address the right and left sides independently of each affected level with bilateral “keyhole” foramino-laminotomies. Excellent clinical results have been reported both by Grob ¹⁵ and McCulloch and colleagues. ¹⁶,¹⁷,¹⁸,¹⁹ Two problems have limited the widespread application of this method. The first is that two separate procedures at each level require more time in surgery. Also, the unilateral spinous process sparing approaches make visualization of the ipsilateral foramen difficult. A perpendicular visual angle prevents the surgeon from seeing far enough into the foramen to ensure decompression and protect the nerve root.

A second strategy evolved from the use of tubular retractors. Guiot, Khoo, and Fessler ²⁰ first described the use of tubular retractors to perform microendoscopic, bilateral foramino-laminotomies for lumbar stenosis via a unilateral transmuscular approach. For this technique, the cannula is used in the standard fashion to address perpendicularly the disease on one side. It is then tilted towards the spinal canal and the contralateral foramen is decompressed within the spinal canal. These procedures are demanding technically and care is required to prevent ipsilateral facet complex disruption, nerve root injury, and dural tears.

Finally, techniques such as spinous process osteotomies as described by Weiner et al ²¹ have been employed to reduce stripping of the paraspinal muscles. However, this approach still involves standard retractor systems which have been shown to cause paraspinal musculature trauma or denervation.

Two newer procedures are intended to be minimally invasive solutions for spinal stenosis. The first involves interspinous distraction of the affected level and is based on the clinical observation that patients lean forward to relieve the symptoms of pseudoclaudication. Interspinous spacers designed to create a local kyphosis are inserted at the level of stenosis. Patient results in a prospective randomized study were encouraging. ²² However, a careful comparison of long-term results with outcomes from other known decompressive procedures will be needed to determine their true clinical value.

An approach described recently by Lin et al ²³ that also uses the spinous process to access the lumbar neural elements. In their procedure the cortical confines of the spinous process are left intact and the lumbar lamina is decompressed through the excavated process. Similar to our technique, they report the ability to access both foramens through this approach. Among 18 patients, the increase in the spinal canal at the area of interest in their study was 2- to 6.8- fold (mean 4.2-fold).

Rosen and colleagues ²⁴ examined the efficacy and safety of minimally invasive lumbar spinal decompression in 50 patients aged 75 years and older. With follow-up averaging only 7 months, they reported significant decreases in disability (Owestry Disability Index) and in pain (VAS). This indicates that minimally invasive lumbar spine decompression, at least in the short term, was of value even in this older population.

One obvious concern is the fate of the residual cortical surfaces. There are two reasons why these remaining bony elements do not threaten the exposed dural surface. One reason is simply the large distance from the spinal canal to the remaining bone. The other is that the intraosseous excavation of the spinous process purposely leaves the attachments of the supraspinous ligaments, the interspinous ligaments, and the periosteum of the lumbar muscles. These multiple attachments prevent migration of the spinous process cortical surfaces.

Conclusion

The pathology of spinal stenosis arises from all of the structures in the intervertebral segment and affects all of the contained neural elements. Laminectomy is a well documented procedure that addresses both the central and foraminal components of this disease. Because of the typical age of patients afflicted with spinal stenosis, less invasive approaches to perform a laminectomy have obvious clinical advantages. Our approach to decompression via a trans-spinous microlaminectomy could be effective in decompressing the lumbar neural elements without disrupting the paraspinal muscles. Clinical studies are required to substantiate the clinical effectiveness of this technique.

Corresponding Author

Michael Mac Millan, MD
Orthopaedics Sports Medicine Institute
3450 Hull Road
Gainesville, FL 32607
Phone: 352-273-7435
Fax: 352-273-7388
Email: [macmim@ortho.ufl.edu]

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