Design and Material Options of Nucleus Replacement

In contrast to the predominant ball-and-socket design with either metal-on-metal or metal-on-polyethylene for the current generation of TDR, the design and material options for nucleus replacement are quite variable. This variability is largely due to the fact that nucleus replacement preserves more of the anatomy of the motion segment, such as anulus and endplates, and more function. The first nucleus replacement with long-term clinical experience was the Fernstrom ball made of stainless steel (Fig. 52–1).^3,4 Although this design offered bipolar articulation with the endplates to provide segment motion, it caused high contact stress because of point loading and implant subsidence.

FIGURE 52–1 Fernstrom ball nucleus replacement.

Because the natural, healthy nucleus is a hydrogel with viscoelastic properties, hydrogel has become a popular material choice for nucleus replacement to mimic mechanical and physiologic properties of the natural nucleus. Preformed hydrogel and in situ curable (injectable) hydrogel have been used for nucleus replacement. Preformed hydrogel nucleus replacement has the benefit of a more consistent implant manufacturing process and more consistent and predictable implant properties, whereas the injectable hydrogel nucleus replacement has the advantage of easy implantation via a smaller annular portal.

Aquarelle developed by Howmedica (later Stryker) was the first preformed polyvinyl alcohol hydrogel used for nucleus replacement.⁵ Subsequently, several other preformed hydrogel materials have been used for nucleus replacement, including prosthetic disc nucleus (PDN) (later redesigned and named HydraFlex) (Fig. 52–2) by RayMedica⁶ and NeuDisc by Replication Medical (Fig. 52–3).⁷ PDN and NeuDisc used the same type of base hydrogel, an acrylic copolymer hydrogel (HYPAN), with PDN having the hydrogel core encased in a polyethylene jacket and NeuDisc having the hydrogel sandwiched in between the Dacron knitted meshes. The main advantage of using preformed hydrogel materials for nucleus replacement is that they not only mimic the viscoelastic and physiologic properties (imbibing and releasing water during the cyclic loading) of the nucleus, but also they have the ability to bear the mechanical load.

FIGURE 52–2 PDN nucleus replacement.

FIGURE 52–3 NeuDisc nucleus replacement.

Although there are many hydrogel materials, the mechanical loading requirement coupled with the biocompatibility and biodurability requirements make the choices of suitable hydrogel materials relatively limited. All three of the preformed hydrogel nucleus replacement devices have passed necessary static and fatigue mechanical tests for the intended application. To mimic the body fluid diffusion function, most preformed hydrogel nucleus replacement devices have tried to have the equilibrium water content in the range of 60% to 80%. The hydrogel nucleus replacement can be implanted either dehydrated or fully hydrated. The advantage of implanting at the dehydrated or semidehydrated stage is the reduced implant volume and dimension, which allows for easier implantation and reduced annular incision. If the implant is implanted at the dehydrated stage, the time required for rehydration must be taken into consideration. The main concerns and challenges for preformed hydrogel nucleus replacement devices have been potential implant extrusion because of the easy deformability and slipperiness of the hydrogel materials.

In addition to preformed hydrogel, some in situ curable hydrogel materials have also been used for nucleus replacement. Examples in this category are NuCore (Spine Wave)⁸ and BioDisc (Cryolife).⁹ NuCore is an injectable synthetic recombinant protein hydrogel, which is a sequential block copolymer of silk and elastin, with two silk blocks and eight elastin blocks per polymer sequence repeat (Fig. 52–4). The material has a water content and modulus similar to that of the natural nucleus. BioDisc is a biopolymer consisting of a protein-based hydrogel. During implantation, the dispensing device mixes the two components of predetermined ratio, and the glutaraldehyde cross-links the bovine serum albumin (BSA) molecules to each other and to the surrounding tissues.

FIGURE 52–4 NuCore nucleus replacement.

Although the injectable hydrogel nucleus replacement has the advantage of being able to implant the device through a small annular incision, the main challenge for injectable hydrogel nucleus replacement devices is whether the cavity can be adequately filled during surgery without risking the implant extrusion, especially when the disc is fully loaded during normal activities. Although both these devices are designed to bond to the surrounding tissues, their adhesion strength and durability in a clinical setting, especially when some of the bonding is to nucleus pulposus, have not yet been proven.

In addition to hydrogel elastomers, nonhydrogel elastomers have also been used for nucleus replacements. Nonhydrogel elastomer nucleus replacements can be preformed and cured in situ. Nonhydrogel elastomer nucleus replacements have the same benefits as hydrogel nucleus replacements with low modulus, but they do not have the ability to imbibe and release body fluid during the cyclic loading. Several nonhydrogel elastomers with implantable history for other applications, such as polyurethane and silicone, have been attempted for nucleus replacement. The only preformed nonhydrogel elastomer nucleus replacement with limited clinical experience is NewDisc by Sulzer (later Zimmer). NewDisc is made from polyurethane and shaped in the form of a spiral coil. An insertion instrument was designed to implant the device by stretching it into a strip and inserting it into the disc cavity, where it is coiled back to a circle shape inside the disc space.

DASCOR (Disc Dynamics) is the most clinically advanced in situ cured nonhydrogel elastomer nucleus replacement and is the only injectable nucleus device with a balloon for containment (Fig. 52–5).¹⁰ The injectable polymer is a two-part in situ curable polyurethane. The balloon is also made from polyurethane to facilitate adhesion between the curable polymer and the balloon. There are several clear advantages of using the balloon. First, it allows the polymer to be injected under certain pressure to fill the disc cavity completely without the risk of polymer leaking through the defected anulus. Second, the balloon prevents the direct contact between the uncured or semicured polymer and the surrounding wet tissue, so it avoids the leach of uncured monomers. The isolation of body fluid from the uncured polymer also can ensure better polymerization and a more consistent and stronger mechanical property for the final implant.

FIGURE 52–5 DASCOR nucleus replacement.

Another in situ curable nucleus replacement device is Percutaneous Nucleus Replacement (PNR; Trans1).¹¹ The PNR consists of two threaded titanium vertebral body anchors connected by a cylindric silicone rubber membrane. The two titanium anchors are fixed to the adjacent vertebrae via a trans-sacral approach, and the in situ curable silicone rubber is injected through the sacral anchor into the silicone membrane until the disc cavity is filled.

Some nonelastomeric materials have also been used for nucleus replacement based on the favorable clinical data of the Fernstrom ball. The main advantages of using these nonelastomers for nucleus replacement are their better mechanical strength and durability. The design of the Regain (Biomet) is similar to a modified Fernstrom ball and made of pyrolytic carbon (Fig. 52–6