Anterior cervical discectomy and fusion (ACDF) is a common type of neck surgery that involves removing a damaged disc to relieve pressure on the spinal cord or nerve root and thereby alleviate associated pain, numbness, weakness or tingling. The damaged disc is removed from between two vertebral bones along with simultaneous fusion surgery. The fusion involves placing a bone graft or “cage” and/or implants where the disc was originally located to stabilize and strengthen the area. The use of cages for ACDF is important postoperatively to the alignment of the cervical spine and to maintain the intervertebral disc height. Few studies, however, have examined the impact of the underlying cancellous or “spongy” bone contact with regards to handling large loads from the cage. Moreover, it is still not clear whether a cage with or without screws will be the best choice for long-term fusion as the micromotion or sliding distance and subsidence or penetration of the cage still take place repeatedly. Now, for the first time, researchers have evaluated the effect of the range of motion, cage migration and subsidence using variable angle screws.

For the study, the researchers at Florida Atlantic University (Boca Raton, FL, USA) developed five finite element models from a cervical spine model. The first model was an intact spine model, and the second model was an altered model with cage insertion and a 2-level static plate. The other three models were altered models with the same cage insertion and a 2-level dynamic plate. They compared ACDF cages with and without screws on the biomechanical characteristics of the human spine, implanted cage, and associated hardware by comparing the micro motion and subsidence. Results of the study showed that the cage-screw and anterior plating combination model has promising potential to reduce the risk of micro motion and subsidence of implanted cages in two or more level ACDFs. This method could increase the stiffness of the construct and reduce the incidence of clinical and fusion failure following ACDF, which in turn, could decrease the need for revision surgeries or supplemental posterior realignment.

According to the researchers, because of the biomechanical stability of the current construct, there has been no need for a rigid cervical collar, which is typically used by other surgeons. A lower screw rotational angle resulted in superior biomechanical performance and lower incidence of migration and subsidence compared with a higher rotational angle in multilevel applications, regardless of loading. The researchers believe the underlying mechanism may be due to the cage-screw bonded to the bone and the constrained bottom of C5 vertebra making it more rigid. Researchers demonstrated that the cage-screw was able to prevent subsidence in all loading scenarios better than the cage without screws.

“Anterior cervical discectomy and fusion is widely used to treat patients with spinal disorders, where the cage is a critical component to achieve satisfactory fusion results. The risk factors for cage migration are multifactorial and include patient factors, radiological characteristics, surgical techniques and postoperative factors,” said Frank Vrionis, M.D., senior author of the study and director of the Marcus Neuroscience Institute. “Our results showed that the plate used in our study provided directional stability and obtained excellent fusion, indicating promising clinical outcomes for patients with degenerative cervical spine disease.”

“Our research aims to develop a platform for next generation patient-specific spine surgery by combining intelligent image process, AI technology/machine learning, finite element simulation and 3D printing to help surgeons design a surgery plan for each patient,” added Chi-Tay Tsai, Ph.D., study co-author and a professor, FAU Department of Ocean and Mechanical Engineering , and director, FAU Spine Biomechanical Laboratory. “Our clinical and biomechanical data have shown that the outcomes with the rigid static plate are very good. The main reason may be due to greater contact area between the cortical and cancellous bone in the cage-screw than that in unanchored cage constructs.”

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