Improvement of Visualization
The availability of small-caliber, rigid-rod endoscopes; three-dimensional cameras; and developing image modification technology has provided operating surgeons with a superior means of visualization and tissue differentiation than the naked eye and microscope. The "aquarium effect" of the technology provides clearer and larger images of anatomical structures.
Although the microscope permits visualization of the dorsolateral aspect of the contents of the spinal canal, spinal endoscopy via a posterolateral approach makes it possible to visualize the medial, lateral, and ventral surfaces of the nerve root and the dural sac without undue manipulation and retraction of neural tissue.
Recurrent herniation following open laminotomy and discectomy is not uncommon. Atken and Bradford (7) reported an incidence of reherniation up to 24%. Balderston et al. (8) reported recurrent herniation of 12% in two groups of patients who underwent open discectomy with simple fragment extraction or fragment removal and curettage of the disc space. They reported similar outcomes in both groups. Recurrent disc hernia-tions at the site of previous surgery may be diagnosed with enhanced magnetic resonance imaging (MRI) studies. However, many of these herniations are asymptomatic and do not require surgical management. During open translaminar discectomy, annular fenestration is performed at the apex of the herniation. Therefore, the containing ability of the annular ring is further weakened. This invites expulsion of nuclear tissue into the epidural space, particularly when the spine is exposed to flexion and rotational forces during the patient's work and activities. The posterolateral annulus appears to be a more desirable area for annulotomy. The natural axial width and intact fibers of the posterolateral annulus (Fig. 13A in Chapter 2) combined with its inherent contractibility may minimize the incidence of reherniation through the surgically induced annular fenestra-tion. The anatomical position of the facet joints also inhibits undue transmission of external forces to the posterolateral boundary of the annulus fibrosis, therefore limiting expulsion of nuclear tissue through the posterolateral annular defect.
In an animal model, Hampton et al. (9) reported on the healing potential of a surgically induced defect in the annular fibers of 10 dogs. The dogs were sacrificed within 3-12 wk postoperatively. Dissection of the surgical site demonstrated that the defect was filled with a solid plug of fibrous structures. Postoperative imaging studies by my colleagues and I on patients who had undergone percutaneous posterolateral discectomy confirmed these findings. Markolf and Morris (10) reported a decrease in compressive stiffness and an increase in creep and the relaxation rate of the intervertebral disc in cadaveric specimens that were exposed to annular fenestration and then followed by exposure of the spinal unit to compressive forces. In younger specimens, extrusion of nuclear tissue had a tendency to seal off the annular defect and to restore normal function of the spinal unit.
Reduction in Incidence of Nerve Root Tethering and Formation of Epidural Scar Tissue
Although perineural and epidural scar tissue may not be pain-producing structures, tethering of the nerve root and dural sac has a tendency to inhibit smooth mobility and gliding of these structures in flexion and extension (Fig. 5A,B; see also Figs 9 and 12 in Chapter 2). Nerve root tethering may be responsible for recurrence of sciatic pain when the patient has resumed normal physical activities following the surgical procedure (11). Epidural application of a fat graft, Gelfoam, and other artificial materials has proven to be unsatisfactory in the prevention of epidural and perineural fibrosis after open spine surgery. I have emphasized avoiding manipulation of the content of the spinal canal and suggested subligamentous access to contained and nonmigrated extraligamen-tous herniations (12).
Protection of Epidural Venous System and Minimization of Nerve Root Trauma
The delicate venous system of nerve roots may be further traumatized by undue intraoperative manipulation and retraction. Haaland et al. (13), Park (14,15), and other investigators (16) have emphasized the importance of patency of neural venous systems in the prevention of venous stasis, which is invariably followed by neural edema, ischemia, and the development of pain (Fig. 6A) in a clinical setting. The presence of neural edema has been objectively demonstrated with postoperative MRI studies following an exercise program (Fig. 6B).
During arthroscopic or endoscopic spinal surgery, the paraspinal muscles, namely the erector spinalis, sacrospinalis, quadratus lumbrorum, and psoas major, are not severed, stripped, or retracted. A small soft-tissue dilator with a 4.9-mm outer diameter (od) has a tendency to separate the muscle fibers and descend toward the annulus at the index level. This reduces the postoperative morbidity and eliminates potential denervation and muscle injury (17-20). The derangement of the muscle fibers and massive scar formation may be readily observed in postoperative MRI studies of patients who have been exposed to traditional open spinal surgery (Fig. 7).
When posterolateral arthroscopically or endoscopically assisted disc extraction is attempted, the facet joints and bony structures are not disturbed. Therefore, the incidence of postoperative instability, spondylolisthesis, and rapid collapse and narrowing of the disc spaces is reduced (21).
When a subligamentous approach for removal of a disc herniation is utilized, the contents of the spinal canal are not manipulated nor disturbed. The absence of epidural and perineural fibrosis (Fig. 8A-D) facilitates accurate postoperative imaging evaluation of the contents of the spinal canal if it becomes necessary (22). My clinical observation has been that even when transforaminal access for retrieval of sequestered fragments under fluid medium was attempted, the incidence of postoperative scar formation appeared to be less prevalent than with open laminotomy and discectomy.
Facilitation of Accurate Intraoperative Documentation
The entire operative procedure may be documented for future reference or teaching purposes via either intraoperative photography or videotape.
The use of regional anesthetics during arthroscopic and endoscopic spinal surgery combined with minimal postoperative morbidity has eliminated the need for hospital-ization and lengthy postoperative rehabilitation. Currently, most minimally invasive operative procedures are being performed in short-procedure units on an ambulatory basis. This has contributed to the cost-effectiveness of minimally invasive spinal surgery.
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