Normal disc structure biomechanically allows both stability and flexibility within each individual spinal motion segment. As the disc degenerates the normal load carried by the nucleus diminishes with increasing load on the annulus, accelerating annular incompetence with diminished disc height. Discogenic low back pain might become a clinical manifestation of this process. Increased stimulation of nociceptors within the outer aspect of the annulus and vertebral endplates secondary to the increased loads likely contributes to the underlying pain syndrome of discogenic back pain (10,11). A microdiscectomy performed for herniated nucleus pulposus can further disrupt the load sharing function of the nucleus resulting in increased compressive loads on the annulus and facet joints accelerating the degenerative cascade (12).

Whereas a biologic replacement of the nucleus is appealing, a more practical biomechanical solution could achieve these goals. Replacement of the disc nucleus would be done to re-establish the normal load on the nucleus and annulus and restore disc height. This would help to restore the overall normal disc function of stability as well as allowing motion (13,14).

The ideal artificial nucleus has a number of basic requirements (13). Multiple surgical approaches could be used for any given nucleus replacement procedure based on specific patient indications. Ideally, the procedure would involve a small annular incision, minimizing the biomechanical impact and decreasing the risk of migration of the implant. The implant would have good conformity to the superior and inferior endplates, improving the loading characteristics. The procedure would allow for a minimally invasive or percutaneous type technique and be technically practical. If a postoperative clinical failure occurs, the revision strategy should be safe allowing for different surgical solutions, for example, explanation, arthrodesis in situ, or total disc arthroplasty.

The technique of prosthetic implantation should be adaptable to address the patient’s pathology and symptoms. If the patient has a typical posterolateral extruded herniation requiring excision, the prosthetic nucleus would ideally be implanted by a posterior laminotomy approach, yet this requires limited further disruption of the annulus. This limitation makes the nuclectomy as well as implantation more difficult. Implantation of a preformed nucleus replacement device with a fixed geometry is unlikely to be successful as a stand-alone device as placement requires an equivalent annular opening. If this were inserted without bony fixation, it is quite likely that the preformed device would extrude out through the annular defect. The annular defect created for placement of such a device or an existing large annular defect may be associated with extrusion. This could lead to the additional requirement of repair of the annular defect for them to be safely applied.

Revision surgery of a nucleus replacement prosthesis will be driven by the surgical approach of the index procedure. Repeated anterior surgery may require mobilization of the great vessels. However, if the initial retroperitoneal approach did not require mobilization of the great vessels, such as would occur with a small annulotomy, the revision could be accomplished with less risk than after a total disc arthroplasty. The potential for catastrophic bleeding in this approach needs to be considered (15,16). As clinical failure might occur, it also remains to be seen whether a disc prosthesis could be left in situ as an anterior spacer and pain relief would be achieved by adding a posterior fusion or other posterior stabilization.

Restoration of the nuclear load is a basic principle of nucleoplasty (8,13). This load is created by the interface of the nucleus implant with the vertebral endplates and annulus. A device with greater conformity to the endplates with large surface area coverage and appropriate modulus of elasticity should result in more balanced, even loading. This likely will result in less bony reaction and postoperative Modic changes. This is best accomplished by forming the implant in situ or allowing a preformed device to deform in situ under the influence of load, as with certain hydrogel prostheses. The ideal nucleus replacement should have a similar modulus of elasticity of the intact nucleus. A prosthesis with a modulus that is too low will have a high incidence of extrusion and will not re-establish normal biomechanical loading. Nucleus replacement with a relatively high modulus of elasticity would be associated with increased load on the endplates and possibly lead to subsidence or other untoward effects as reported by Fernstrom (18) with the use of the stainless steel ball endoprosthesis. The goal of minimal endplate wear would occur if the ideal prosthesis distributes the forces evenly over the endplates with a large surface area. Likewise, a lower coefficient of friction
between the implant and the endplates would be expected to lead to less endplate or prosthetic wear. An additional benefit of improved implant endplate conformity is likely a diminished risk of implant extrusion.

Disc Dynamics Inc. (Eden Prairie, MN) has developed an injectable in situ curable polyurethane nucleus replacement device called the DASCOR Disc Arthroplasty Device (32) (Fig. 14.1). The DASCOR device is made by mixing two parts of liquid polymer while delivering it through a catheter to an expandable polyurethane balloon that is placed in the disc space (Fig. 14.2).

The polymer cures in a few minutes, changing state from a liquid to a firm but pliable solid device. After 15 minutes, the delivery catheter is detached, leaving the final implant device.

There are several features to this system. The balloon catheter has a low profile and requires only a 5.5-mm annulotomy for introduction. The mixed liquid polymer is delivered to the balloon under controlled pressure, allowing the balloon to expand contouring and filling the entire disc space left by the nuclectomy procedure. This creates a large prosthetic footprint and a large volume device through a small annulotomy, thus making migration unlikely. In addition, the system creates an implant of variable size that conforms to the nuclectomy. The deployment of a large, pliable, endplate-contouring prosthesis maximizes the load transfer between the annulus and the artificial nucleus while minimizing endplate disruption. The DASCOR device also has the ability to distract the disc space. Therefore, the implantation of the device offers not only the ability to fill any given space established by nuclectomy but also the potential to re-establish normal disc height.