Radiographic Assessment of Interbody Fusion Using rhBMP-2

· (a - Medtronic Sofamor Danek Columbus, GA, USA

PURPOSE:
Radiographic imaging of a developing fusion mass is challenging with metallic interbody implants. Thin-cut CT imaging is the most efficacious method of identifying bone formation within second-generation cages. Recombinant human bone morphogenetic protein-2 (rhBMP-2) was shown to promote osteoinduction and fusion. To determine the osteoinductive capability of using rhBMP-2 with the LT-CAGE™ device, we prospectively evaluated the radiographic outcomes of patients who underwent a single-level ALIF with tapered fusion cages.

METHODS:
Forty-two patients underwent an ALIF and were randomized to receive autogenous iliac crest bone graft (20 patients; avg age, 44 years) or rhBMP-2 on a collagen sponge carrier (22 patients; avg age, 42 years) with the LT-CAGE device. Plain radiographs and thin cut CT scans interpreted by 2 independent, blinded radiologists were used to evaluate patterns of osteoinduction at 2 days after surgery and at 6, 12, and 24 months after surgery. Fusion was defined as an absence of radiolucent lines covering more than 50% of either implant, translation of 3 mm or less, and angulation of less than 5 degrees on flexion-extension radiographs, and continuous trabecular bone growth connecting the vertebral bodies. CT scans were used to assess new bone formation and bone remodeling within and around the cages. The changes in density within the cages were determined by measuring the Hounsfield units (HU) within each cage on the serial CT scans.

RESULTS:
In the rhBMP-2 group, immediate postoperative CT scans showed an average density of 156 HU within the central portion of the LT CAGE and the control group showed an average 533 HU. The investigational group showed an average increase to 325 HU at 6 months, an average increase to 447 HU at 12 months, and an increase to 522 HU at 24 months. In the autograft control group, density within the cage averaged 575 HU at 6 months, 643 HU at 12 months, and 750 HU at 24 months. Progression of densities within the cages correlated with evidence of fusion on standard plain radiographic measurements. One patient in the control group developed a pseudarthrosis at 12 months. In the rhBMP-2 group, new bone formation was identified outside of the cages in 82% of patients at 6 months, in 95% of patients at 12 months, and in 100% of patients at 24 months. In the autograft group, new bone formation was identified outside of the cages in 50% of patients at 6 months, in 80% of patients at 12 months; and in 95% of patients at 24 months.

CONCLUSIONS:
Progressive density on thin cut CT scans is evidence that rates of new bone formation and fusion in the rhBMP-2 group exceeded those of the autograft control group.