Annular Repair and Barrier Technologies

12 Annular Repair and Barrier Technologies

Pablo R. Pazmino and Carl Lauryssen

The intervertebral disc (IVD) serves as the core dynamic stabilizer to the complex spinal system. Injuries to the disc alter its intrinsic biomechanics and result in adverse load transfers toward the secondary surrounding structures, primarily the facet joints. This premise has garnered interest in annular repair and barrier technologies, which promise to provide a more biomechanically stable motion segment while diminishing the risks of recurrent disc herniations.

Since Fedor Krause’s first discectomy in 1909, management of disc herniations has been primarily directed toward excision.1 Annular repair, however, is not a new concept; the first known repair was performed in 1967 by Professor MG Yasargil using 7.0 suture in efforts to decrease adhesions.² Despite this report, this procedure attracted little or no attention, and initially clinical management was directed toward nucleotomy, in which a central portion of the disc was widely excised to prevent reherniation of disc material. Symptomatic reherniation rates after lumbar discectomies still range from 3 to 27%.^3,⁴ Outcome analysis after repeat surgical management to address these reherniations has shown diminished success rates compared with those of the index procedure.^3–5 Until recently, annular repair attracted little or no attention as the anulus was believed to be of little importance and largely incapable of healing. Advances in cell biology have demonstrated the crucial role of the nucleus pulposus (NP) in the development and establishment of the anulus fibrosus (AF).⁶ These studies have resulted in a renewed interest in the basic science of the NP and the AF in terms of their structure, physiology, and function.

New biologic techniques and advances in engineering may address annular deficiencies on a cellular, genetic, or mechanical level. These novel measures may lead to clinical applications using annular repair or the establishment of barrier technologies to preserve disc biology and function.

Anulus Structure and function

The AF makes up the peripheral border of each disc as a thick, convex attachment along the inner surface of the anterior and posterior longitudinal ligaments. The medial and lateral borders of the AF taper to a thin, free edge.⁷ The AF is made of both type I and type II collagen fiber bundles arranged circumferentially into distinct lamellar layers with varying thickness around the disc. Depending on the patient age, level, and location these laminates can range anywhere from 15 to 25 layers thick.⁸ There is a corresponding transition area along the posterolateral junction of the disc that represents an intersection of the maximum and minimum concentration of these laminate layers. This posterolateral quadrant also represents a change in the inclination of the fiber bundles themselves, which vary from 0 degrees and parallel to as much as 90 degrees of inclination along the interface.⁸ These inherent structural irregularities may help explain the frequency of herniations from the posterolateral quadrant, where circumferential tensile strains are highest in bending and extension modes.⁹

First described in the 1970s, the Kirkaldy-Willis “degenerative cascade” of disc disease has set the framework for the biochemical and structural changes associated with disc pathology.¹⁰ Essentially, degenerative changes begin as recurrent strains that lead to small circumferential tears in the AF. These tears then coalesce and form radial annular tears giving the NP a path toward the outer innervated annular lamellae.^10,¹¹ Throughout this process, there is a series of complex structural, physiologic, morphologic, and biochemical changes leading to mechanical dysfunction, and ultimately pain. Histologically dehydration, decreased cellularity, disorganization, and annular disruption have been described throughout the disc and its extracellular matrix.^7–9,¹¹ IVD cells exhibit both anabolic and catabolic responses to different types of mechanical stimuli, depending on the loading type, magnitude, duration, and anatomic zone of cell origin. In response to these stimuli, the chondrocytes regulate expression of cytokines, proteases, and matrix metalloproteinases, which subsequently remodel the extracellular matrix.^12,¹³ This modification of the matrix subsequently alters disc height, elasticity, and hydrostatic and interstitial pressures. With time, the disc loses its basic morphology through a loss of proteoglycans, fibrosis, calcification, and ankylosis. Progression through the cascade ultimately results in stress redistribution as demonstrated by ligamentum flavum hypertrophy, foraminal stenosis, vertebral endplate edema, and facet damage.

Surgical Annular Repair

Prior studies have shown that a less-disruptive annulotomy technique for discectomy demonstrated an improved capacity of the disk to withstand multidirectional flexibility and internal hydrostatic pressurization.14 The annulotomy method has also been shown to correspond to the timing and strength of subsequent annular healing.¹⁴ Primary direct suture approximation of annular incisions using Vicryl (Ethicon, Somerville, NJ) suture and fascial grafts has not been shown to alter the rate or strength of annular healing.¹⁵ Despite these initial findings, investigation into methods for direct annular repair are under way. The Xclose tissue anchor device (Anulex Technologies Inc., Minneapolis, MN) utilizes T-anchors on either side of the incised AF to reapproximate and allow for subsequent repair of the annular surgical incision (Fig. 12.1). Preclinical studies demonstrate no adverse effects on spine flexibility, and maintenance of initial closure throughout cyclic loading in axial rotation, lateral bending, flexion, and extension.¹⁶

Barrier Technologies

Favorable long-term outcomes after discectomy correlate strongly with the preservation of the remaining nucleus and disc height. This can be accomplished through minimal tissue resection during the index discectomy; however, this is associated with an increased risk of recurrent herniations.¹⁷ Recently, technologies have emerged that aim to supplement the native AF or serve as barrier technologies in scenarios of annular resection after discectomy.

Fig. 12.1 The Xclose tissue anchor device. (Courtesy of Anulex Technologies Inc.)

The Barricaid device (Intrinsic Therapeutics Inc., Woburn, MA) is implanted through an annular defect, as small as 5 square millimeters, after a standard discectomy procedure (Fig. 12.2). The device consists of an expanded polytetrafluoroethylene (ePTFE) mesh supported by a nitinol frame. The nitinol frame consists of a nickel-titanium alloy base, which serves to reinforce the surrounding AF while closing the defect. The frame functions as a shape memory alloy exhibiting pseudoelasticity through a solid-state phase change. Initial use with the Barricaid device has demonstrated a decrease in the rate of recurrent herniations, sustained intradiscal pressures, and maintenance of disc height in comparison with nonimplanted patients.^18–20

The Inclose Surgical Mesh System (Anulex Technologies Inc.) is designed as an annular barrier and scaffold for repair (Fig. 12.3A). The mesh consists of a low-profile polyethylene terephthalate (PET) monofilament braid that expands beneath the annular defect upon insertion (Fig. 12.3B,C). Afterward, the Inclose is secured in place with nonabsorbable surgical suture anchors. Cadaveric and animal models demonstrated lack of wear particulates and negligible positional movement and effects on spinal flexibility.²¹ Early findings have shown improved outcomes (Oswestry Disability Index, Visual Analog Scale, SF-12 Health Survey), and no recurrent herniations.²²

Other devices concentrate on annular reconstruction to decrease the rate of reherniations, such as the Barricaid annular reconstruction device (ARD; Intrinsic Therapeutics Inc.; Fig. 12.4A). The ARD is implanted through an annular defect up to 10 mm wide, after a limited discectomy procedure (Fig. 12.4B). The ARD consists of a woven polymeric mesh supported by a titanium bone anchor. The mesh serves to reconstruct the annular defect and to lower the incidence of reherniation while helping to maintain disc height. The bone anchor resists the expulsive intradiscal pressures, which are transferred from the mesh and anchor into one of the two adjacent vertebral bodies (Fig. 12.4C). The single attachment point of the anchor differentiates the procedure from fusion devices and encourages natural disc motions (Fig. 12.4D). Initial short-term clinical use of the Barricaid ARD has demonstrated a decrease in the rate of recurrent herniations, improvement in sciatic and low-back pain, and the maintenance of disc height in comparison with nonimplanted patients.^18–20

Recent injectable technologies have been investigated in animal models as an avenue for percutaneous treatment modalities, which stimulate the natural annular repair process. Artefill (Artes Medical, Inc., San Diego, CA) is currently a Food and Drug Administration approved aesthetic injectable that is being investigated as a treatment for mid-annular concentric defects.²³ Artefill contains 20% ArteFill precision-filtered polymethylmethacrylate (PMMA) microspheres suspended in an 80% ArteFill purified bovine collagen gel. Histologic findings and magnetic resonance imaging (MRI) have demonstrated outer annular healing occurred along involved treatment levels in a sheep lumbar spine model.²³