In an effort to provide immediate postoperative stability to the anterior cervical spine in the context of cervical trauma, Böhler and Gaudernak described instrumented fixation of the cervical spine using a heavy orthopedic plate and large screws. ⁴ The anterior plate system was further refined and commercially released by Caspar et al ⁵ during the early 1980s, with substantial improvement in design occurring since this first iteration. Current anterior plate systems have seen the replacement of bicortical screws by monocortical screws and locking mechanisms to prevent screw backout; the development of dynamic plates versus fixed (static) plates and, most recently, the introduction of low-profile interbody cages with incorporated fixation. Most plates used today are restricted backout plates that function in a dynamic fashion. As these technological advances have made plate insertion simpler, combined with increasing evidence that anterior cervical plate fixation improves graft fusion rates, particularly in multilevel ACDF, anterior cervical plate instrumentation has become widely adopted as a integral component of ACDF.

In parallel with advances in anterior cervical fixation devices, significant progress has been made with respect to interbody fusion. ⁶ Tricortical iliac crest autograft, as originally described by Smith and Robinson, ¹ is still considered the gold standard of anterior cervical fusion but its use is gradually being replaced by allograft or interbody cages composed of nonbiologic materials such as titanium, polyetheretherketone (PEEK) and carbon fiber reinforced polymers (CF-P).

The armamentarium of anterior cervical instrumentation methods has also expanded over the last decade to include motion-preserving procedures in the form of total cervical disk arthroplasty, as discussed in Chapter 12.

Classification of the devices available for anterior cervical procedures can be conveniently based on the goals of surgery and the function of the device. Anterior cervical instrumentation can be separated into two major groups. In the first group are the traditional fusion fixation devices, which include plates and interbody devices; the second group consists of the motion-preserving devices or arthroplasty devices ( ▶ Fig. 11.1).

11.1.1 Anterior Cervical Plates

The ability of anterior cervical plate fixation to improve fusion rates through enhanced stability at the fusion interface has been the main impetus behind their wide adoption. Anterior cervical plates also aid in the prevention of graft collapse, subsidence or extrusion, the maintenance or reconstruction of normal sagittal alignment and to avoid the need for postoperative external immobilization by using the plate as an internal fixation device.

The anterior plate and screws together form a cantilever beam, which has the ability to allow or prevent various amounts of axial loading to be transmitted through the plate–screw construct, thereby shielding or loading the intervertebral graft. The interaction or motion allowed between the screw heads and plate determines the amount of load sharing or stress shielding to which the interbody graft is exposed. Rigid internal fixation is considered desirable in promoting spinal arthrodesis, but anterior cervical spine fixation with a rigid plate has a significant biomechanical shortcoming. In accordance with Wolff’s law, bone heals best when there is a load applied to it. It is therefore important for the plate–screw construct to allow a certain amount of axial loading across the interbody graft to enhance fusion. Should the plate–screw construct provide complete stress shielding to the interbody device from all axial loading, as is the case with a rigidly fixed anterior plate, the construct has a higher chance of failing through graft nonunion. Conversely, if the plate–screw construct does not bear any axial load, as would be the case with a fully unconstrained or dynamic plate, the interbody device bears the entire axial load, which, depending on its elastic modulus, can result in subsidence or collapse of the graft with a kyphotic deformity, causing plate migration or screw failure or both. As one might expect, there are a variety of plate designs that provide for different degrees of load sharing ( ▶ Fig. 11.2).

The original Caspar plate design ⁵ was dynamic to some extent, as the screws did not lock to the plate, thus allowing motion at the plate-screw interface. As screw backout was a significant problem in this circumstance, bicortical screw placement became necessary. To eliminate the requirement for bicortical screw placement, subsequent plate designs rigidly locked screws to the plate. For instance, the Synthes cervical spine locking plate (CSLP) ( ▶ Fig. 11.3) used central locking screws to expand the head of the vertebral screws thus securing them to the anterior plate as a method to prevent screw backout.

The constrained plate is ideal in circumstances where rigid fixation is thought to be paramount; for example, in trauma with gross instability, in neoplastic lesions where fusion is unlikely to occur, and for reconstruction of deformity where instrumentation is required to maintain the reduction. Because of their fixed nature, however, in circumstances where a degree of graft resorption or subsidence into one end plate and away from the other occurs, the rigid nature of the constrained plate construct will keep the construct in a state of relative distraction and thus decrease the likelihood of successful bony union.

Because of the stress shielding inherent in a fully constrained plate system such as the CSLP, subsequent iterations of locking plates used various mechanisms to prevent screw backout without rigid fixation of the screw to the plate. Most commonly, secondary screws on the plate with the ability to cover the head of the screw or bushings within the plate holes have been used to prevent disengagement from the plate. Dynamic or semiconstrained plates evolved to address the issue of stress shielding created by fixed plates. Semiconstrained plates are available from most manufacturers and are usually one of two types, rotational or translational. Rotational plates are dynamic plates designed to allow limited rotation of a screw head around the plane of the plate. With rotational plates, the angle that the screw makes with the plate changes as subsidence occurs. Translational dynamic plates are designed to allow for limited translation of the screw head along the y-axis within the plate as subsidence occurs. In this design the screw–plate angle does not change as subsidence takes place because the screw has the ability to slide down in the screw holes, thereby maintaining the original screw–plate angle. Translational plates are usually designed to allow for more translation to take place in the cephalad portions of the plate because this is where most subsidence typically occurs, especially with long constructs ( ▶ Fig. 11.4).

Combination translational and rotational plates have both features; they are designed to allow for y-axis translation, but as the screw reaches the maximum amount of translation allowed, x-axis rotation takes place. Surgical implantation of translational plates takes some planning because ideally screws should be positioned to allow for optimal translation; however, excessive subsidence in long constructs can result in “plate migration” affecting the disk space above and may result in late plate or screw failure or both. Some plate designs are extremely versatile in that they can be used as a fully constrained construct, a semiconstrained construct, or a hybrid construct. Materials used in plates have also evolved from titanium alloy to more recently developed thermoplastics such as PEEK. Absorbable plates made out of polylactide are also commercially available.

It is important to emphasize that despite the numerous theoretical advantages of anterior cervical instrumentation, the published regarding the clinically relevant advantages of anterior cervical plating following ACDF as compared with stand-alone ACDF. As delineated in the systematic review of Matz et al ⁷ that examined 17 class II and III studies comparing ACDF with and without anterior cervical plating, clinical outcomes are not appreciably different between these cohorts. A single study ⁸ suggested that quantitative arm pain was improved with anterior cervical plate fixation in the setting of two-level ACDF. Although clinical parameters appeared equivocal, Matz and colleagues, ⁷ in their systematic review, concluded that the use of anterior cervical plates provided improved radiographic outcomes such as better maintenance of cervical lordosis, a reduced risk of pseudarthrosis, and a decreased incidence of graft-related complications. These conclusions, as related to clinical outcome equivalence but superior rates of fusion, are supported by the small randomized study by Xie and Hurlbert ⁹ that compared anterior cervical diskectomy (ACD) with ACDF and ACDF with plating. Specifically, they noted that clinical outcome was not related to technique but that a higher rate of fusion was observed at 2 years in the instrumented group compared with ACDF and ACD (100%, 93%, 67%, respectively).

Similarly, despite the theoretical advantages of load sharing in the promotion of successful graft fusion as afforded by dynamic plate systems, a systematic review ¹⁰ that examined the outcomes of dynamic versus rigid plate fixation systems noted no significant differences in clinical outcome. Further, no difference in overall fusion rate was noted, although one prospective randomized study ¹¹ suggested that dynamic fixation systems provide faster arthrodesis and a higher fusion rate in multilevel cases.

11.1.2 Interbody Grafts and Cages

Autogenous ICBG, as described by Smith and Robinson, ¹ remains the gold standard anterior cervical interbody graft substrate; however, ICBG harvesting has limitations, largely related to high levels of both short- and long-term donor site morbidity, including pain, wound infection, hematoma, nerve injury, and iliac crest fracture. ¹² These complications provided the impetus to examine alternative graft/cage materials to ICBG. The ideal cage restores healthy alignment and disk height and provides early postoperative stability, high-fusion rates and low complication rates. ⁶

Standalone allograft, from either the iliac crest or the fibula, has been commonly used as an alternative to ICBG, with several commercially available, precut lordotic or parallel allograft spacers available for ease of use. The use of allograft, although it avoids graft site complications, is associated with slower rates of incorporation and marginally higher rates of pseudarthrosis and graft subsidence as compared with ICBG autograft. ^{13,​ 14} Interbody cages that can be filled with autograft, allograft, or osteoconductive materials have also been developed for use in the cervical spine.

Three materials have primarily been incorporated in the design of cage implants: titanium, PEEK, and carbon fiber–reinforced polymers (CF-P). Titanium is a robust biomaterial with high corrosion resistance and low density and can undergo surface modification (i.e., electron spray) to improve osseointegration. PEEK has the advantage of radiolucency and an elastic modulus close to bone, thus avoiding the stress shielding associated with titanium implants. Although initial experience with CF-P and titanium cages was positive, their use has largely been superseded by PEEK after publication of multiple studies reporting improved fusion rates and lower subsidence rate with PEEK cages. ⁶ The next iteration in interbody cage design is focused on using the improved bioactivity of titanium in combination with the superior elastic modulus and radiolucency of PEEK through the creation of composite titanium/PEEK spacers. At present, limited data are available on the efficacy of such a design compared with more established cage options.

11.1.3 Zero-Profile Cage-Plate Devices

Recently, advances in interbody cage design have seen the incorporation of anterior plating into a stand-alone cage. Such zero-profile cages use a low-profile plate design integrated with a PEEK interbody cage, with the overarching aim of reducing dysphagia rates and other plate associated complications while maintaining satisfactory clinical and fusion outcomes. ¹⁵ The Zero-P (Depuy-Synthes Spine, Raynham, Massachusetts) was the first zero-profile cage approved by the United Stated Food and Drug Administration (in 2008) and consists of a PEEK graft body attached to a titanium anterior plate containing four holes for screws 14 or 16 mm long. Similar devices have now been introduced by multiple vendors, including Biomet (ALTA ACDF), Stryker (AVS Anchor-C), Medtronic (PEEK Prevail), Precision Spine (Vault C), and LDR (ROI-C). Clinical experience continues to develop with these new devices, but recent reports of satisfactory medium-term clinical results related specifically to the Zero-P ¹⁶ suggest that such zero-profile devices are a reasonable alternative to traditional anterior plating techniques with the potential for a decreased complication profile (as discussed to follow).

11.2 Patient Selection

Anterior cervical plating with screw fixation is indicated in the treatment of degenerative, traumatic, infective, and neoplastic conditions involving the cervical spine. The vast majority of patients treated have various combinations of radiculopathy and myelopathy, with or without axial neck symptoms, as part of the degenerative disease process. Anterior cervical plating is generally considered the standard of care in patients undergoing multilevel diskectomies or single-level or multilevel corpectomies.

11.3 Preoperative Preparation

The preoperative preparation of patients undergoing anterior cervical instrumentation is reviewed in previous chapters related to anterior cervical diskectomy and corpectomy.

11.4 Operative Procedure

After the surgical exposure and decompression of the anterior cervical spine, as described in previous chapters related to anterior cervical diskectomy and corpectomy, stabilization of the spine is undertaken. Once the decompression has been completed, the length of the graft is estimated by measuring the rostrocaudal extent of decompression under gentle distraction. We prefer to use PEEK interbody cages or commercially machined allograft spacers filled with local autologous bone graft to reconstruct the anterior cervical spine, but autologous ICBG continues to be an excellent option.

Multiple diskectomies are the preferred method of decompression, as opposed to consecutive corpectomies, in cases of multilevel spondylosis. Multi-level ACDF allows for an increased number of points of screw fixation, thus significantly increasing the stability of the operative construct compared with cervical corpectomy. Multiple diskectomies also have an improved ability to maintain or restore cervical lordosis through the use of multiple lordotic interbody spacers. Cervical corpectomy is reserved for situations where there is significant ventral neural compression located behind the vertebral body. We always perform supplementary posterior fixation in cases of multilevel corpectomy because of the relative lack of anterior fixation points available with such a construct.

After the graft or grafts have been sized appropriately, they are tamped gently into place, again while gentle distraction is provided along the longitudinal axis of the neck ( ▶ Fig. 11.5). It is optimal to countersink the graft by 1 to 2 mm when no plate is used. When a plate is used, however, the graft should be flush with the adjacent bodies to optimize contact among the plate, vertebral body, and graft. Distraction is released, and the graft is probed to ensure firm seating and proper positioning. Gentle palpation with a blunt nerve hook in the space lateral to the graft is done to confirm the seating depth of the interbody graft.

Related posts:

Minimally Invasive Decompression of the Cervical Spine Posterior Cervical Wiring Techniques Cervical Disk Arthroplasty Costotransversectomy Carpal Tunnel Release Endoscopic Carpal Tunnel Release Via a Biportal Approach

Stay updated, free articles. Join our Telegram channel

Join