Early Findings in a Pilot Study of Anterior Cervical Fusion
Object: The goal of this study was to assess the efficacy of bioabsorbable interbody spacers in cervical spine fusion.
Methods: The authors report on a prospective examination of 20 patients with degenerative cervical disc disease who underwent anterior cervical fusion at 28 total levels. The authors used Infuse bone graft (that is, recombinant human bone morphogenetic protein–2 applied to an absorbable collagen sponge and contained within a Cornerstone-HSR bioabsorbable spacer. Multiple-level fusions were performed in 30% of these patients. At 3 months postfusion, radiographs and computerized tomography scans demonstrated bridging bone in 100% of the patients. Improvement from baseline scores in physical functioning, mental health, and bodily pain was demonstrated at 3 months postoperatively according to results of the Short Form–36 Version 2 health survey. There were no device-related complications.
Conclusions: The results in this series indicate that the use of Cornerstone-HSR as a bioabsorbable interbody spacer in combination with Infuse bone graft may be an alternative treatment for cervical spine fusion.

Anterior cervical discectomy and interbody fusion with autograft has been a common treatment for management of degenerative or traumatic instability of the cervical spine. This procedure has reportedly yielded fusion success rates from as low as 56% to as high as 100%. The variability in reported results can be ascribed to multiple factors, including surgical technique, criteria used to define fusion, length of follow up, and number of levels fused. The success rates are lower when multiple levels are fused than for single-level fusions. Although various methods have been developed to decrease rates of nonunion after cervical fusion, grafting of autologous bone from the iliac crest remains the most common procedure. Nevertheless, use of autologous bone grafts from the iliac crest may be associated with postoperative complications such as hematoma, infection, numbness, donor site pain, lateral femoral cutaneous nerve injury, pelvic fracture, and muscle herniation. To avoid the morbidity associated with donor site complications, allograft may be used as an alternative source of bone; however, the use of allograft may result in slower recovery times. Delayed union or nonunion of the allograft can result in the need for revision surgery. The use of anterior plate fixation to provide immediate stability, and of intervertebral cages or spacers and rhBMP-2 has been introduced to minimize failure rates in cervical fusion.

Intervertebral cages were designed to prevent collapse and pseudarthrosis that was sometimes seen with fusions in which only bone graft was used. To be effective, intervertebral cages must be stable enough to be used as a stand-alone device. These cages were traditionally composed of autograft, cadaveric allograft, titanium, or stainless steel. The inability to assess fusion radiographically when metal cages are used has prompted the development of spacers composed of other materials. The optimal spacer would be rigid at implantation, become less so over time, and would degrade completely when the healing cycle has been completed.

Biomaterials such as carbon fiber, hydroxyapatite, and calcium phosphate ceramics were studied in animals and were found to be subject to fatigue fractures, collapse, and extrusion. Bioabsorbable materials have emerged as an alternative that could provide the attributes of immediate stability, relative radiolucency, gradual transfer of stress to the fusion site, and breakdown products that have no mutagenic or immunogenic properties. Bioabsorbable polymers have properties consistent with these criteria. Once the fusion has matured, the materials slowly resorb by bulk hydrolysis into carbon dioxide and water.

HYDROSORB devices, which are intended for support of weak bone tissue (distributed by Medtronic Sofamor Danek, Memphis, TN), have been used by some investigators as potential interbody devices. The HYDROSORB copolymer of polylactide, 70:30 PLa, has been used in various laboratory, preclinical, and clinical studies in a variety of clinically relevant situations. Laboratory tests have yielded evidence that the 70:30 PLa degrades over a period of 18 to 36 months.

Animal studies have been conducted to assess resorbable devices used for interbody fusion. The authors of one study investigated devices constructed of a 70:30 PLa polymer in 25 sheep over a period of 24 months; the animals were treated with and without added rhBMP-2. Comparable steadily increasing degrees of fusion were observed over the time period in mechanical and histological studies conducted on spines treated with the resorbable spacer filled with either rhBMP-2 or autograft bone. Additionally, as the implant resorbed at the 24-month period, the space was replaced with remodeled bone, there was normal bone mineralization adjacent to the device, and no osteolysis was found. These findings are comparable to those in another study, in which lumbar interbody devices composed of poly-L-lactide were investigated in 36 goats. The fusion was complete at approximately 2 years, and was faster and more complete than with identically shaped titanium cages. Tissue reaction in both studies was described as mild to moderate.

Limited clinical experience has been reported for the use of bioabsorbable interbody spacers in spinal fusion in humans. In these posterior lumbar interbody fusion procedures, local "morcellized" autologous bone was packed within and around the devices. At 1 year postfusion, results show clinical outcomes equivalent to those reported historically with allograft bone spacers. Preliminary results (1–9 months) in 60 patients who underwent transforaminal lumbar interbody fusion procedures in which two HYDROSORB bioabsorbable implants were used per level are encouraging. The majority of patients experienced symptomatic relief, and there were no implant failures.

The use of rhBMP-2 has been shown to initiate osteoinduction and to achieve spinal arthrodesis in animals and in humans. Clinical studies of lumbar and cervical fusion in humans have demonstrated consistent and clear osteoinduction when rhBMP-2 is impregnated onto an absorbable collagen sponge (Infuse) and placed in an intervertebral spacer. Here we present 3- to 6-month follow-up information on a series of 20 patients in whom a bioabsorbable interbody spacer was combined with Infuse interbody filler for anterior cervical fusions at one to three levels.