Chapter 139 Cervical Spine Fusion Using Dynamic Ventral Cervical Plating
The benefits of rigid implants (i.e., internal fixation) in the axial skeleton include rigid stabilization, maintenance of alignment, minimal postoperative immobilization, earlier return to function, and potentially enhanced fusion rates.1 A potential shortcoming of rigid implants is that they may stress-shield the bone graft and result in nonunion or implant failure or both. Stress-shielding refers to an implant-induced reduction of bone healing–enhancing stresses and loads, resulting in stress reduction osteoporosis or nonunion (Fig. 139-1). This hypothesis is in keeping with Wolff’s law, which postulates that the form and function of bone is a result of changes in the internal architecture according to “self-ordered” mathematical rules.2 In contemporary terms, skeletal morphology is substantially controlled by mechanical function, and bone remodeling, both locally and throughout the skeleton, is influenced by the level and distribution of the functional strains within the bone.3,4 A corollary to Wolff’s law is that bone heals optimally under compressive, as opposed to tensile, forces. Experimental studies in the thoracolumbar spine show that a 70% or greater axial load should be transmitted through the spine, not the implant, optimally to enhance arthrodesis and provide acute stability.5
In an attempt to improve on the shortcomings of rigid implants, there has been a resurgence of interest in dynamic implants, in particular, for use in the cervical spine. The concept of dynamic implants is not new. Dynamic hip arthroplasties have been employed successfully for femoral neck fractures. These dynamic implants allow for the femoral neck to shorten or collapse along its axis so that the bone is subject to optimal bone-healing compressive forces.6 Advocates of dynamic implants hypothesize that implants that permit a limited and controlled type of deformation may be desirable. Some experts have termed this controlled dynamism. In the spine, allowing for some axial deformation but not angular deformation (kyphosis) may be optimal. Occasionally, the failure of a rigid implant may permit fusion because the bone graft and vertebral bodies are subsequently exposed to the appropriate bone healing–enhancing forces. In this case, the implant has “dynamized by failing” (Fig. 139-2).
The first ventral cervical plate and screw system was introduced by Bohler7 in 1964. This system ultimately culminated in the development of the Caspar (Aesculap, Center Valley, PA) and Orozco (Synthes, West Chester, PA) plate systems in the early 1980s. These early ventral cervical plates were dynamic implants and are classified as having unrestricted backout properties (i.e., nonlocking and nonrigid) because of a lack of fixation at the screw-plate interface. These implants permit a significant transfer of load through the bone graft, increasing the likelihood of fusion. The nonfixed moment arm nature of the screw causes degradation of the screw-bone interface with cyclic loading. This effect can be minimized with bicortical screw purchase, which requires C-arm fluoroscopy. The main disadvantage of these plates is that the nonlocking and nonrigid (i.e., variable angle) screws led to high rates of screw backout and screw breakage with graft subsidence (Fig. 139-3).
