Percutaneous Intradiskal Therapies

Abstract

Percutaneous intradiskal therapies are currently used in the treatment of two distinct medical problems: chronic low back pain and the herniated nucleus pulposus–related radicular syndrome (sciatica). Both conditions lead to substantial medical and economic losses, with low back pain being the major cause of disability in the United States for patients less than 45 years of age.

The prevalence of low back pain in the general population is 60% to 80%. Low back pain is the second most common reason for work absence.

In sciatica, the complaints are the result of a protrusion of nucleus pulposus through the annulus fibrosus, causing direct compression of the exiting nerve root and subsequent radicular complaints. Although the clinical picture of sciatica differs from that of low back pain, the socioeconomic impact of both entities is of equal magnitude. The annual incidence of sciatica is 5 per 1000, making it a common problem.

Disk Anatomy and Disease

The cause of both clinical entities is ascribed to disease of the intervertebral disk. This disk is a unique part of the musculoskeletal system capable of resisting axial load and rotational forces between two neighboring vertebral bodies. The disk is the largest avascular structure in the human body and consists of a cartilaginous outer margin, the annulus fibrosus, and a central gel-like structure, the nucleus pulposus. The annulus fibrosus is made up of fibrocartilaginous lamellae consisting of a.o. proteoglycans (20%) and collagen (60%, mainly type I). The outer layers of these concentric lamellae are incorporated in the posterior longitudinal ligament, resulting in a stable fixation of the disk. The junction of the disk with the vertebral bodies (synchondrosis) is made up of hyaline cartilage. The nucleus pulposus primarily consists of water (90%) and the remainder of proteoglycans (65%) and collagen (20%, mainly type II). The amount of collagen is lowest in the lumbar part of the spine. The amount of water in the nucleus pulposus varies according to a diurnal pattern and is also dependent on the mechanical forces exerted on the disk. Under normal circumstances, the disk can vary 1.2 mm in height. In degenerated disks, this might increase to 2.1 mm.

A reduction of intradiskal pressure can lead to shrinkage of the annulus fibrosus, which might result in tearing. The most important causes are a decrease in the intradiskal water content and also a decrease in the amount of proteoglycans. The proteoglycan level can decrease because of aging but is also influenced by a low intradiskal pH (in smoking) and the use of nonsteroidal anti-inflammatory drugs. Annular tears caused by aging are typically concentric in orientation as opposed to the radial tears caused by mechanical strain. Damage to the intervertebral disks can also be the result of degeneration of the vertebral bodies. Subcortical calcifications and ingrowth of bone marrow with enzyme-mediated lysis of the disk have been described as causative factors of disk damage. The innervation of the annulus fibrosus, which might be a key factor in the etiology of low back pain, was studied by Coppes and colleagues, who showed myelinated bundles of nerve fibers in the outer annulus. There are indications that in degenerative disk disease, centripetal growth of the fibers in the disk occurs, making them more vulnerable to mechanical and chemical stimulation. The chemical stimulation is attributed to cytokines (interleukin-1, -2, -8, -12, -15; interferon-α, tumor necrosis factor-α, transforming growth factor-β ₁ ) and synthesis of prostaglandins released in degenerative disk disease. Weinstein and colleagues reported that the presence of neuropeptides, substance P, and vasoactive intestinal peptides influenced pain at discography.

Degeneration of the annulus fibrosus might result in a full-thickness tear, in which case the nucleus pulposus might herniate through the defect. A herniation of the nucleus pulposus might lead to compression of the exiting nerve root and hence a radicular syndrome. Risk factors for intervertebral disk herniations are familial deposition, atherosclerosis of the abdominal aorta, smoking, and repeated/prolonged vibration. The radicular complaints can be intermittent and are accredited to the changes in intradiskal pressure. The intervertebral disk can be considered as a hydraulic system in which a volume increase of merely 1.0 mL will lead to an increase of intradiskal pressure of 2340 mm Hg (312 kPa). Model calculations show pressures up to 400 mm Hg on the nerve root. This can lead to changes in blood flow and edema and subsequent changes in nerve root signal conduction. Ingrowth of blood vessels into the annulus fibrosus is observed in cases of sciatica. This is visible in 12.5% of cases with complaints of 1 month's duration and 87% with complaints lasting longer than 6 months. The clinical importance of this phenomenon is unclear. The herniated nucleus pulposus–related radicular syndrome is often accompanied by low back pain. One of the possible causes is referred pain from the recurrent sinuvertebral nerve of Luschka, which traverses one or more disk levels.

Chemonucleolysis

Chemonucleolysis is the oldest percutaneous intervention for radicular syndrome. In chemonucleolysis, a proteolytic enzyme called chymopapain is injected into the center of the intervertebral disk under fluoroscopic guidance to induce hydrolysis of the proteoglycan molecules that form the nucleus pulposus. The fact that the resulting proteoglycan fragments have limited water-binding abilities leaves intradiskal water molecules free to diffuse into the surrounding tissues. The resulting loss of water causes a decrease of intradiskal pressure. The chymopapain molecules remain attached to the proteoglycan fragments, permanently limiting the ability of the nucleus pulposus to attract water. In addition to its proteolytic effects on nucleus pulposus tissue, chymopapain is also postulated to have anti-inflammatory properties, directly inhibiting the proinflammatory enzyme phospholipase A ₂ .

Automated Percutaneous Lumbar Diskectomy

Automated percutaneous lumbar diskectomy (APLD) is a minimally invasive intervention in which nucleus pulposus material is mechanically removed using an automated nucleotome, designed by Onik and associates in 1985. The initial results of the first series of patients were presented in 1987. The automated nucleotome is a 2-mm blunt-tipped device equipped with a reciprocating suction cutter. The nucleotome is introduced through a 2.5-mm diameter cannula into the center of the nucleus pulposus, where nucleus pulposus material is cut and aspirated at a rate of 180 times per minute. The resulting loss of nucleus pulposus mass causes a drop in intradiskal pressure.