Cervical Spine Construct Design

Clinical Instability

The most frequent indication for cervical instrumentation is instability. In practice it is essential to determine precisely the nature and extent of spinal instability. The nature of instability refers to the status of specific structures that normally confer stability on each motion segment in the cervical region. This concern addresses the competency of the ligamentous structures, bony elements, and annulus fibrosis of the intervertebral disc. Identification of the incompetent elements allows the severity of segmental spinal instability to be estimated. The extent of instability denotes the number of unstable motion segments, as well as whether the instability is predominantly ventral, dorsal, or both. Defining these concepts precisely is of fundamental importance, because they have a significant impact on all factors associated with the selection of an appropriate construct.

The etiology of spinal instability is important. Symptomatic cervical instability may result from congenital abnormalities, trauma, degenerative disease, neoplasia, or infection. Iatrogenic instability also occurs, particularly after cervical laminectomy for spondylotic disease or to access intraspinal tumors. Construct design is influenced by the nature of the disease process that produced the instability, as the long-term structural demands placed on a construct are often determined by the progression or remittance of the underlying disease. Posttraumatic instability may demand the least of a construct: short-term immobilization is often all that is required to promote adequate healing. After the injury heals, the loadbearing and load-sharing properties of the construct are no longer required to maintain stability. Spondylotic and iatrogenic instability may require more from a construct, owing to the slowly progressive nature of the process. Instability arising from spinal neoplasia often mandates long-term participation by the instrumentation to maintain structural integrity. Osseous fusion may not be attainable because of the rapid progression of disease or adjuvant use of radiation therapy: the instrumented construct must be designed to bear physiological loads for the remainder of the patient’s life.

Maintenance of Alignment

Internal fixation may be indicated to prevent deformity from occurring or to preserve normal alignment after reduction. Unlike thoracolumbar instrumentation, cervical constructs are generally applied in the neutral mode, thus deformity reduction should occur before stabilization. Many constructs designed for use in the thoracolumbar spine can apply significant compressive, distractive, translational, and rotatory forces to a region of spinal deformity, thus affecting reduction. As a rule, most cervical instrumentation systems cannot apply the magnitude of force required to reduce a deformity and are used predominantly to maintain reduction.

Prevention of spinal deformity may also be accomplished by the timely use of internal fixation. Progressive kyphosis or spondylolisthesis may result from spinal decompression procedures. If individuals at risk for this complication are identified preoperatively, cervical deformity may be preventable. Patients exhibiting a loss of the normal cervical lordotic configuration are prone to develop postlaminectomy kyphosis, which may be avoided by proper internal stabilization at the time of decompression. Similarly, operative resections that compromise principal load-bearing elements may render the spine incompetent to withstand physiological loads, and deformity may be prevented by spinal reconstruction, using bone graft and instrumentation to reconstitute the axial spine.

Pain Management

Spinal stabilization may be indicated to relieve incapacitating pain by reducing motion of dysfunctional segments. Fusion of the cervical spine purely for amelioration of axial pain may benefit certain patients greatly, but selecting them is a significant clinical challenge. Such a procedure should be carefully considered and only performed after extensive nonoperative treatment measures have failed.

General Considerations

In most cases, cervical constructs are used to maintain clinical stability. This may be accomplished most efficiently by matching the implant with the major site of instability—that is, if the instability is primarily dorsal in location, a dorsal construct should be considered for stabilization. Similarly, ventral instability, created by incompetence of the anterior longitudinal ligament, vertebral body, or intervertebral disc complex, is most effectively treated by the application of a ventral construct. It is unreasonable to expect that a construct will function with optimal stability when implanted in a biomechanically disadvantageous position.

Internal fixation systems provide immediate postoperative stability to the instrumented region, but they do not provide long-term stability, owing to the “plastic” properties of bone at the implant–bone interface. As with most biological materials, bone deforms and reforms when stress is applied. Eventually, even the most rigid construct allows a small degree of motion. Repetitive loading gradually increases the amount of movement and can ultimately lead to implant failure unless bony fusion occurs. The long-term stability of all constructs is thus dependent on osseous fusion: no internal fixation system currently available can compensate for a poorly designed bone graft.

Cervical spinal implants may be considered as rigid, semirigid, or dynamic. Rigid implants attempt to achieve complete immobilization of the instrumented motion segments. Ventral plate systems, with locking screws and dorsal rod and hook/rod systems, provide rigid fixation. Rigid immobilization may be potentially detrimental to bone fusion because of stress shielding and stress-reduction osteopenia. This concern has led to the development of dynamic instrumentation, such as non–fixed-moment arm cantilever beam screw-plate implants and axially dynamic ventral fixators.

Modes of Application

The modes of application available for cervical constructs are more limited than those available for use in other spinal regions. Thoracolumbar implants may be placed in distraction, compression, neutral, translation, flexion, extension, and lateral-bending modes. In contrast, cervical spine constructs are generally applied in the neutral mode. This is not universally true, because certain cervical plate systems and wire constructs may provide a modest degree of compression. Theoretically, cervical rod/screw (or hook) devices can be placed in the compression or distraction modes as well. However, the majority of cervical constructs in clinical use are applied in the neutral mode at the time of surgery. Biomechanical conditions change as the spine is loaded after surgery. Most “neutral” implants must resist axial compression when the upright posture is assumed. These constructs then function in a distraction mode.

Cervical construct designs are also more limited in their mechanism of load bearing than their thoracolumbar counterparts. Generally, cervical constructs conform to one of five fundamental loadbearing types: (1) distraction fixation, (2) tension-band fixation, (3) three-point bending, (4) fixed-moment arm cantilever beam, and (5) non–fixed-moment arm cantilever beam fixation. Applied moment arm cantilever beam fixation, a technique occasionally applied in the thoracolumbar spine, is not usually used in the cervical spine. Assigning an implant to one of these fundamental loadbearing types is somewhat artificial, because a given construct may exhibit features of several mechanical types. However, it permits classification of implants by their principal biomechanical attributes.