Resorbables: Do They Have a Role in the Cervical Spine

Hyun Bae

Nomaan Ashraf

Cervical disk degeneration causing axial neck pain and radiculopathy has historically been treated well with anterior cervical discectomy and fusion (ACDF) (1). The last several years have seen an increase in the usage of instrumentation as newer plate designs are able to more reliably enhance fusion (2). Additionally, reconstruction of the cervical spine is frequently being performed using interbody devices made of metal or other synthetic materials.

The move to utilize bioresorbable plates and interbody devices in the cervical spine stems from implant-related complications including implant failure, plate migration, stress shielding, adjacent segment degeneration, and compression of anterior soft tissue structures. Additionally, artifact from metal implants render postoperative imaging difficult while trying to assess bony union on radiographs or CT and make it nearly impossible to analyze the spinal canal and neural foramina using MRI. This frequently results in the utilization of myelography, which is an invasive test with procedure-related comorbidities.

Bioresorbable implants have been utilized for several decades in fracture fixation and craniofacial procedures (3). Bioabsorbable implants are commonplace in sports medicine surgeries, especially in shoulder and knee ligamentous reconstruction. Their use is now expanding to the realm of spinal reconstructive surgery. The development of bioabsorbable technology requires the dedicated effort of a multidisciplinary team of surgeons, materials scientists, and engineers.

The potential advantages of resorbable implants in spinal surgery include a more physiologic modulus of elasticity in relation to native vertebral bone, as compared with rigid metal devices. They also maintain predictable resorption through bulk hydrolysis as carbon dioxide and water and the elimination of long-term residual metal hardware with the potential attendant complications of migration or dislodgement. In the cervical spine, there are several critical biomechanical characteristics these implants would ideally demonstrate. They must mimic the stability of titanium implants in the early postoperative period while bony union is being attained. The device must then absorb at a rate that parallels the rate of fusion and gradually transmit forces to the bone in the area that the plate was placed. Finally, the implants must resorb without causing an inflammatory response that would antagonize adjacent neural structures or anterior soft tissue structures.

BIOMECHANICS OF THE POLYMERS

The cervical spine research on bioabsorbable implants has focused on alpha-polyesters or poly (alpha-hydroxyacids). These include polylactic acid (PLA), polyglycolic acid (PGA), and polydioxanone (PDS). Studies have revealed that PGA has a propensity to produce more of an inflammatory tissue reaction than PLA (4). Isomers of these compounds, such as poly-L-lactide, poly(D,L-lactide) (PDLLA), and poly(L-lactide-co-D,L-lactide), have also been used to achieve an optimal strength-to-degradation ratio. The rate of degradation has been associated with the degree of inflammatory response of the surrounding tissues, with faster resorption rates producing greater inflammatory reactions. PGA materials lose their mechanical strength by 6 to 8 weeks. The loss of the mechanical properties of this material occurs at varying rates, with the loss of shear strength being slower than loss of bending strength. The breakdown of these polymers has been linked to many types of inflammatory reactions in vivo. Synovitis, hypertrophic fibrous encapsulation, and osteolysis have all been seen as complications of polymer breakdown (5). One study found a 4.3% occurrence of aseptic tissue reactions in various procedures that used bioabsorbable implants (6).

During polymer breakdown through hydrolysis, the release of lactic acid monomers has raised questions about local pH changes and potential adverse physiologic effects on surrounding tissues. PLA resorption has been shown to be biocompatible with dura, spinal Schwann cells, neurons, and glial cells, producing no unfavorable effects in these highly pH sensitive tissues (7). The blood-brain barrier in coupled with the slow degradation rate of PLA and the small amounts of polymer in implants make cerebrospinal fluid acidosis unlikely. In compartments outside the cerebrospinal fluid, studies have documented normal
osteogenesis in both spinal and nonspinal applications of PLA devices (8).

Bioabsorbable polymers differ from common stainless steel implants in that they are more viscoelastic in nature. Therefore, they exhibit enhanced properties of creep and stress relaxation. Claes exhibited the importance of this property by demonstrating that when used in the form of an interfragmentary screw, bioabsorbable polymers lost 20% of their force 20 minutes after application due to stress relaxation. Reinforcing techniques have been developed to improve the mechanical characteristics of bioabsorbable implants. Self-reinforced absorbable components are polymeric materials in which the reinforcing elements and matrix material have the same chemical composition. The most effective way to manufacture the self-reinforced structure into the polymer is by the mechanical deformation of the nonreinforced material (9,10).

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