Cervical Spine Construct Design

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Chapter 140 Cervical Spine Construct Design

Fundamental Concepts

Successful cervical spine instrumentation depends on several factors, including the nature and extent of the disease process, bone quality, and technical expertise of the surgeon. One of the most important, but often overlooked, elements in this process is determined well before the surgical procedure is undertaken. This is construct design.

The term construct is a neologism that has become entrenched in the spine literature. For the purpose of this discussion, a construct denotes the aggregate of biologic and/or nonbiologic materials that are implanted for the purpose of providing stability to a mobile or an unstable region of the spine. Construct design, then, is the process of contriving such an implant. For the most part, this chapter addresses the design of constructs, composed of bone and instrumentation for application in the subaxial cervical spine.

Without a sound construct design strategy, cervical fixation systems are doomed to failure. The meticulous technical application of a poorly conceived construct is a futile exercise, as prone to failure as the correct system improperly applied. Despite its importance, relatively little emphasis has been placed on this element of the procedure. This chapter presents a strategy to aid in the selection of certain instrumentation systems designed for specific clinical problems of cervical spine instability. The advantages and shortcomings of each type of construct are also discussed.

Benzel1 described an excellent method for preoperative mapping of thoracic and lumbar instrumentation procedures, using a “construct blueprint.” This approach is practical in this region of the spine, because the choice of implant components that may be applied here is vast. The design of thoracolumbar constructs entails the selection of the longitudinal member, cross-fixation mechanism, and implant-bone junction fixators. Each element may be different at various levels of a long construct, adding to the complexity of the system.

Additionally, the modes of possible construct application for the thoracolumbar spine are extensive. This refers to the desired forces that are applied by the surgeon at the implant-bone junction. Constructs may be placed in compression, distraction, neutral, translation, flexion, extension, and lateral-bending modes.2 In a single thoracolumbar construct, several modes of application may be required, depending on the structural demands at any given level. A systematic approach to the formulation of an operative plan is essential when designing constructs with this degree of complexity. The construct blueprint is a concise format capable of communicating complicated surgical strategies to all members of the surgical team.

The options concerning surgical approaches and types of fixation devices are more limited in the cervical region. The mode of application here is also less variable because most cervical constructs are applied in the neutral mode. Although this simplifies the cervical construct design scheme, the need for cogent preoperative planning is just as important. The format used to communicate the operative strategy is less important than the intellectual process of visualizing the biomechanical requirements of a given lesion and formulating an appropriate construct that satisfies these requirements.

The fundamental steps for appropriate construct design are (1) determine the need for instrumentation, (2) select the construct best suited to solve the instability problem, and (3) ascertain the need for postoperative orthotic stabilization to supplement the implant.

Indications for Cervical Construct Application

White and Hirsch3 outline four general indications for spinal stabilization: (1) to restore clinical stability to a spine in which the structural integrity has been compromised, (2) to maintain alignment after correction of a deformity, (3) to prevent progression of a deformity, and (4) to alleviate pain. Cervical spinal instrumentation may be applied in conjunction with a bone fusion in all of these scenarios. In rare instances, instrumentation may replace bone fusion as the principal means of cervical stabilization.

Optimally, internal fixation provides immediate postoperative stability to the region before the development of osseous fusion. This is beneficial in two respects. Instrumentation protects the neural elements from trauma and the spine from deformity until the bony fusion matures and can assume this role. Internal fixation also obviates, or at least significantly reduces, the requirement for postoperative external immobilization while the fusion mass heals. This technique improves patient comfort, which encourages accelerated mobilization after surgery. Additionally, this may enhance the probability of attaining successful bone fusion by ensuring compliance with postoperative immobilization.

Internal fixation may allow a reduction in the number of levels that require fusion by adding intrinsic strength and load-sharing properties to the construct. A shorter fusion facilitates the preservation of cervical motion and limits the resultant moment arm created by the fusion mass.

Clinical Instability

The most frequent indication for cervical instrumentation is instability. To paraphrase an oft-quoted general definition, instability requires the loss of spinal biomechanical integrity such that the spine is unable to prevent initial or additional neurologic deficit, major deformity, or incapacitating pain under physiologic loads.3 The precise definition of spinal instability is difficult to establish and may vary according to the specific clinical setting.

In practice, it is essential to precisely determine 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 anulus fibrosus 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 and affects the decision to instrument the spine and also dictates the selection of an appropriate construct.

The etiology of spinal instability is important. Symptomatic cervical instability may result from trauma, degenerative disease, neoplasia, or infection. Iatrogenic instability may also occur, particularly after cervical laminectomy for spondylotic disease. Construct design is influenced by the nature of the disease process that produced the instability. 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 load-bearing 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 instrumentation to maintain structural integrity. Bone fusion may not be attainable, because of the rapid progression of disease. In these situations, the instrumented construct must be designed to bear physiologic loads for the remainder of the patient’s life.

Maintenance of Alignment

Cervical constructs are often required to maintain spinal alignment. Internal fixation may be indicated to prevent deformity or to preserve normal alignment after reduction. Unlike thoracolumbar instrumentation, cervical constructs are generally applied in the neutral mode. Thus, deformity reduction is essential before stabilization. This is often accomplished by applying axial skeletal traction. Many constructs designed for use in the thoracolumbar spine can apply significant compressive, distractive, translatory, 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. Internal cervical fixation may be used to maintain reduction, but application of reductive forces with these constructs should be avoided in most instances.

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.4 This complication 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 physiologic loads. Progressive postoperative deformity may be prevented by spinal reconstruction, using bone graft and instrumentation to reconstitute the axial spine.

Construct Selection

Cervical constructs should be designed to solve case-specific problems of spinal instability. This requires an understanding of the nature, extent, and causes of instability; load-sharing and load-bearing demands; bone integrity; and biomechanical attributes of various internal fixation systems. Implant cost and facility of application are also important concerns. Constructs may fail as a result of poor design, usually because biomechanical expectations of the implant were unreasonable. Two general rules help guide the selection of a cervical construct and limit unrealistic expectations: (1) the graft and implant must correct the specific preoperative instability, and (2) the long-term success of a cervical construct ultimately relies on the quality of the osseous fusion.

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 (ALL), 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 do not provide long-term stability due to the “plastic” properties of bone at the implant-bone interface. As with most biologic materials, bone deforms and reforms in response to stress.5 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 thus depends on osseous fusion. No internal fixation system currently available can compensate for a poorly designed bone graft.6

Cervical spine implants may be considered as rigid, semirigid, or dynamic.5 Rigid implants attempt to achieve complete immobilization of the instrumented motion segments. Ventral plate systems, with locking screws and dorsal screw-rod and hook-rod systems, provide rigid fixation. Luque rods and rectangles (Zimmer, Warsaw, IN), secured with segmental sublaminar or facet wires, and most lateral mass plate devices are examples of semirigid cervical implants. Rigid immobilization may be potentially detrimental to bone fusion because of stress shielding and stress-reduction osteopenia.5,7 This concern has led to the development of dynamic instrumentation, such as nonfixed moment arm cantilever beam screw-plate implants and axially dynamic ventral fixators.8

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-hook devices can be placed in the compression or distraction modes as well. However, the vast 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.5

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 load-bearing types: (1) distraction fixation, (2) tension band fixation, (3) three-point bending, (4) fixed moment arm cantilever beam, and (5) nonfixed moment arm cantilever beam fixation.2 Applied moment arm cantilever beam fixation, a technique occasionally applied in the thoracolumbar spine, is not used in the cervical spine. Assigning an implant to one of these fundamental load-bearing 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.

Simple Distraction

Simple distraction fixation occurs when a distraction force is applied by a cervical construct, usually from a ventral, interbody location.2 Interbody strut grafts, with or without ventral plate instrumentation, are examples of this type of fixation. These devices principally resist axial loads. Dorsally applied distraction fixation is rarely used because it is prone to create a kyphotic deformity.

Three-Point Bending

Three-point bending fixation occurs when forces are applied to the spine at three or more sites along the length of the construct.2 Dorsally directed forces are applied at the rostral and caudal ends of the construct. An equal but opposite ventrally directed force is applied at the fulcrum, usually in the center of the construct. Three-point bending instrumentation is most often utilized dorsally in the cervical spine and includes fixation of multiple motion segments. Three-point bending forces may be applied with Luque rods and rectangles secured with sublaminar wires or cables, hook-rod implants, and, to a lesser degree, with lateral mass screw-plate or screw-rod instrumentation.

Cantilever Beam Fixation

A cantilever is formed by a projecting beam supported at one end only.2 When the cantilever is rigidly attached to the supporting longitudinal member, a fixed moment arm cantilever beam is created. This variety of load bearing is accomplished by ventral cervical plate systems secured with locking screws and rigid lateral mass rod-screw instrumentation. A fixed moment arm cantilever beam device contributes some axial load-sharing properties to the construct. Nonfixed moment arm cantilever beam fixation employs a dynamic attachment of the cantilever to the longitudinal member. Lateral mass plates and nonfixed moment arm cantilever beam screw-plate implants and axially dynamic ventral fixators are representative of this type of load bearing.

The classification of spinal implants by a mechanism of load bearing is somewhat artificial. In practice a single implant may function by using several of the fundamental load-bearing mechanisms simultaneously. For example, the lateral mass plate is capable of stabilization by three such mechanisms. Dorsal tension band, three-point bending, and nonfixed moment arm cantilever beam fixation are all accomplished by this device.

Construct Materials

A variety of biologic and prosthetic materials have been used for cervical spine stabilization. Most constructs are composed of a bone graft, coupled with a metal prosthesis. Occasionally, bone and/or metal components may be supplemented or replaced by methyl methacrylate.

Bone Grafts

Autograft and allograft bone have both been used extensively in spinal stabilization. Some studies report that fusion rates with allograft bone are comparable to those obtained with autograft bone.911 Other studies have maintained that autograft bone is superior.3,12,13 This is particularly evident with dorsal cervical constructs, in which the bone graft is not placed under compression. Certainly, fusion rates with autograft bone meet or exceed those reported with allograft. The use of autograft bone eliminates the concern of infectious disease transmission (including HIV and hepatitis virus transmission) that may be associated with allograft bone.

The iliac crest provides a versatile and abundant source of bone graft material for incorporation into cervical spine constructs. Favorable attributes of this type of graft include ease of procurement in both the supine and prone positions, strength, and relative expendability of the donor site.3,14 The tricortical structure of the iliac crest is responsible for much of the strength inherent in this graft, thereby providing excellent axial load-bearing capability. The abundant cancellous bone provides ample substrate for osseous remodeling. Although all commonly used configurations of iliac crest grafts can sustain high compressive loads, the Smith-Robinson–type graft is probably superior to other styles of grafts in this respect.15 The principal disadvantage associated with iliac crest harvest is donor site morbidity, which may be substantial. Complications include pain, wound hematoma, infection, meralgia paresthetica, hip dislocation, and fracture of the anterior superior iliac spine.

Fibula is another commonly used site for graft material. It is particularly well suited for multilevel ventral reconstruction procedures, because the thick cortical bone in this graft resists high axial compressive loads. The relatively small amount of cancellous bone present in the fibula graft may delay bone remodeling, however. This may be partially overcome by packing additional cancellous bone in the center of the graft, as well as surrounding the outer cortical surface with the cancellous bone. Donor site morbidity arising from graft harvest may be significant, because one sixth of body weight is borne by the fibula.5 This may be principally a theoretical concern, however, as fibular bone has been used quite successfully in many cases of spinal reconstruction.

Rib grafts have also been used, particularly with dorsal cervical constructs. The native configuration of rib is advantageous because it conforms well to the cervical lordotic curve. There is minimal morbidity in harvesting rib compared with iliac crest. This is an excellent graft to use for dorsal fusions.16

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