Indications and Techniques in Annuloplasty

Michael Y. Wang

Treatment Indications and Strategy

Annular Anatomy and Function

Treatment following Microdiskectomy

The Rationale for Preventing Reherniations

Surgical Strategies

Treatment of Symptomatic Annular Tears

The Argument for Treating Annular Pathology

Interventional Strategies

Treatment after Placement of Intervertebral Prostheses

Conclusion

Treatment Indications and Strategy

Although annular repair techniques are in their infancy, this area has been the subject of intense recent investigation. Reconstruction or restoration of this component of the lumbar spine is appealing for several reasons. The annulus fibrosus gives the intervertebral disk its mechanical integrity. As such, disruption of its architecture, whether through degenerative processes or surgical intervention, has significant implications for the mechanical function of the lumbar spinal column. Currently, there are three major areas where annular repair may find clinical utility:

1. Prophylactic treatment following microdiskectomy to prevent subsequent degeneration or reherniations

2. Treatment of fissures and tears believed to be causing axial back pain symptoms

3. Replication of connective tissue barriers and mechanical structures following insertion of intervertebral prostheses

Annular Anatomy and Function

The annulus fibrosus is a fibrous ring composed primarily of type I collagen and elastin. Its fibers are oriented in a lamellar pattern around the circumference of this ring in roughly a dozen distinct layers. The fibers within each layer are organized in a parallel fashion oriented at 70 degree angles to the adjacent layers. These fibers have a high tensile strength and serve to contain the nucleus pulposus. The nucleus is composed primarily of type II collagen and proteoglycans, which retain a high water content to give it a deformable jellylike consistency. The annulus and nucleus thus work in concert like an inflated radial tire.

In the healthy intervertebral disk axial, compressive forces transmit pressure to the nucleus, resulting in annular tension. The two structures are thus well designed to handle these physical forces. However, progressive disk degeneration leads to mechanical incompetency of the disk. Early degeneration involves loss of nuclear water content. The decreased turgor within the disk causes slackening of the annulus. Axial compressive forces then result in compression and deformation of the annular tissue instead of tension, forces that the annulus withstands poorly. A cascade of pathology can then result, including fissuring of the annulus, nonuniform loading of the vertebral end plates, irritation of local nerve fibers, herniations of disk materials outside the annulus, abnormal facet joint loading, and loss of spinal alignment.

In addition, the annulus is intricately associated with the anterior longitudinal ligament (ALL) and posterior longitudinal ligament (PLL) to constrain motion in multiple planes. Both degenerative and iatrogenic events will frequently involve these ligamentous structures as well.

Attempts at annular repair thus focus on restoring one or more of the functional roles of the annulus. Specifically, annuloplasty can be directed at (1) repairing defects to appropriately retain nuclear material; (2) treating symptomatic, painful disruptions in the connective tissues; and (3) restoring the biomechanical functions of the annulus.

Treatment following Microdiskectomy

The Rationale for Preventing Reherniations

Approximately 500,000 lumbar diskectomies are performed each year in the United States. Although some of these procedures involve an intervertebral fusion or total disk replacement, most are for the removal of frank herniations causing nerve root compression.1 In this setting the herniations can be either contained within the annulus and result in a large, broad-based mass impinging on the nerve root, or the herniations may have ruptured through the annular ring and lie in proximity to the thecal sac and nerve root. One of the concerns regarding surgery for symptomatic disk herniations is the recurrence of symptoms from reherniation. This can result from defects in the annular wall from rupture and extrusion of free fragments or may be due to an annulotomy created at the time of surgery to remove a contained herniation. Several strategies have thus been developed to minimize the risk of reherniations. One approach is to reduce trauma to the annulus by limiting the surgical dissection to removal of only the free fragment, leaving the remainder of the disk undisturbed. This conservative approach, the Williams’ sequestrectomy, theoretically causes less annular disruption at the time of surgery, reducing reherniations.2 An alternative approach is to clean the disk space thoroughly at the time of surgery to ensure that there are no loose fragments that are prone to later herniation.

Despite these efforts, symptomatic disk reherniations are not infrequent, occurring in 5 to 15% of patients. In one study by Carragee et al, lumbar disk herniations were classified into four categories based on intraoperative findings: fragment-fissure herniations, fragment-defect herniations, fragment-contained herniations, and no fragment–contained herniations. As expected, annular competence was found to be associated with the rate of reherniation. Patients in the fragment-fissure group with small annular defects had the lowest rates of reherniation (1%), and patients in the fragment-defect group, with extruded fragments and large posterior annular defects, had a 27% rate of reherniation.³ These findings suggest that repair of the annulus may serve functionally to contain any reherniations.