Early Results Using the Atlantis Anterior Cervical Plate System

Bryan Barnes M.D., Regis W. Haid, M.D., Gerald Rodts, M.D., Brian Subach, M.D., Michael Kaiser, M.D., Department of Neurosurgery, Emory School of Medicine, Atlanta, Georgia; and Department of Neurosurgery, College of Physicians and Surgeons, Columbia University, New York, New York

Neurosurg Focus. 2002;12(1) 

In This Article

Discussion

In most reviews, fusion rates range from 87.5 to 97% for anterior cervical procedures (both corpectomy and discectomy), in which plate and bone screw constructs are used to treat degenerative spondylosis.[3,4,5,8,11,18,19,23,25,32,33,34] The initial results obtained using the Atlantis cervical plating system seem to be comparable with those reported in most series, but the system provides the additional benefits of fixed, variable, and hybrid constructs, which we believe has allowed for a greater degree of tailoring of the Atlantis plate to individual patient needs.

The proliferation of ACP systems and subsequent results have elucidated both biomechanical advantages and disadvantages associated with each type of plating system. In general, we prefer to categorize ACPs as restricted backout and unrestricted backout systems, with the unrestricted backout systems representing those systems that require bicortical screw purchase because of the lack of screw-locking mechanisms. Examples of unrestricted backout systems include the Caspar and Morscher plate systems.[6,7,8,9,14,15,17,18,23] Hardware failure in ACP systems, defined as any broken or loosened screw or plate, ranges from 2 to 44% in unrestricted backout systems.[6,19,22]

Generally, lower hardware failure rates, ranging from 0 to 18%, have been reported for restricted backout systems with unicortical locking screws, which provide greater screw pullout protection.[19,30] Examples of restricted back-out systems include the Synthes, Orion, and Codman plate systems. Although restricted backout systems were developed to improve the high rates of hardware failure demonstrated in cases in which unrestricted backout systems were used, the authors of many in vitro models and several clinical reports have advanced the idea that restricted backout systems allow the graft to be "shielded" from load bearing because the plate absorbs more of the load of the construct when screws are constrained from backing out.[7,11,12,15,31] Thus, less compressive force may be sustained by the graft in restricted backout constructs; application of the law of Wolff to this situation would theoretically reduce the rate of fusion because there would be less compressive force on the graft.

As a result of concern about graft loading biomechanics, plate systems such as the Codman and Aline constructs have been developed. These plate systems allow for screws to rotate at the screw-plate interface, creating a semiconstrained system that allows for variable screw angulation, as well as less shielding of the graft by the plate.

The Atlantis plate is a restricted backout system with a floating washer design. The system has the option of fixed, variable, and hybrid constructs (Fig. 1). A fixed construct uses two or more sets of fixed-angle screws at a 12° cephalad-caudad angle and a 6° convergent (medial) angle (Fig. 2). We prefer the fixed construct for cases such as trauma that require rigid fixation. Biomechanically, the fixed construct is "constrained:" the fixed-angle locked screws permit no rotation or translation, creating a rigid fixation device. Screws may be bicortical if desired.

Photographs showing the Atlantis ACP system constructs. Upper: Fixed construct, providing constrained system. Center: Hybrid construct in which inferior (fixed-angle) screws act as buttress while variable-angle (superior) screws rotate at the plate-screw interface. Lower: Variable construct, with rotation at both superior and inferior aspects of plate-screw interface.

Drawing depicting the screw angles of the fixed-screw construct.

In the Atlantis hybrid construct two fixed-angle screws are used at the inferior portion of the construct at the aforementioned predetermined angles, whereas two variable-angle screws are used for the superior portion of the construct (Fig. 3). Screws placed between the fixed-and variable-angle screws are generally those of the variable-angle type. Variable-angle screws can be directed from 22° distal to 2° proximal in a caudad-cephalad direction; variable screw angles range from a 17° medial convergent to a 4° lateral divergent angle. The resulting hybrid construct is semiconstrained, allowing for rotational pivoting at the screw-plate interface; the lower (fixed) screw acts as a "buttress." Because of the rotational pivoting, the overall construct undergoes "controlled subsidence," with increased load placed on the graft compared with that occurring in a constrained construct (Fig. 4). We use the hybrid construct in a majority of cases performed for cervical myelopathy and/or radiculopathy, because we believe that graft loading for this particular construct is optimum for most cases of degenerative cervical spondylosis.

Drawing depicting the screw angles of hybrid construct.

Lateral radiograph obtained 14 months postoperatively, demonstrating construct settling after placement of the hybrid ACP for one-level corpectomy. The plate chosen for the construct is of a longer than optimal length, resulting in a reactive osteophyte in the anterior superior VB. In general, we use the shortest possible plate to avoid this phenomenon.

The Atlantis variable construct illustrated in Fig. 5 uses entirely variable-angle screws at the range of angles previously mentioned. This construct is a semiconstrained system, which allows for rotational motion at the plate-screw interface on both ends of the construct.

Drawing showing the screw angles for the variable-screw construct.

Fusion rates in our patients ranged from 90 to 100% for all subgroups; the mean rate was 93.5%, which we consider to be comparable with most series of plate-related anterior cervical fusion procedures for myelopathy and/or radiculopathy.[4,5,8,16,20,26,29] The mean fusion rate for single-level corpectomies in our series (91.6%) compares favorably with a recent report of 87% in single-level corpectomy in which the Atlantis system was used (Fig. 6).[11]

Postoperative lateral radiograph obtained at 17 months, revealing excellent incorporation and fusion of fibular allograft in a case in which the hybrid ACP was implanted.

Clinically, satisfactory outcomes were significantly worse in patients who had undergone multilevel ACDF procedures and combined anterior-posterior procedures, compared with other groups. We have not identified any factors that account for this difference (Fig. 7).

Postoperative radiographs. Left: The hybrid ACP system, at 13 months after four-level ACDF. Center: The hybrid system, 15 months after two-level ACDF. Right: Twelve-month postoperative lateral radiograph, after multilevel corpectomy with posterior lateral mass plates.

Our rate of hardware failure was 2.6% (two cases), and there were no episodes of plate breakage or graft extrusion. Both patients in whom screw failures occurred were asymptomatic, and no morbidity was associated with the two cases. The rate of hardware failure in this series is on the lower end of the 0 to18% reported for all types of plate systems; in particular, an overall screw loosening rate of 2.4% has been reported in the literature.[6,12,15,19,20,22]

The significance of reduced lordosis in all subgroups compared with age-matched asymptomatic controls can be attributed to the underlying degenerative cervical spondylotic disease. Although we believe that the Atlantis construct helps to restore cervical lordosis, preoperative radiographs were not available in a vast majority of cases, thereby obviating a statistically meaningful comparison of pre-and postoperative lordosis. The unavailability of preoperative radiographs is an unfortunate consequence of practicing at a tertiary referral center: many patients undergo their initial radiological studies at outside institutions. We can only conclude that after over 1 year mean follow up, significantly less lordsis was present in our patients than in age-matched controls.

Segmental lordosis was significantly reduced, compared with overall cervical lordosis, at follow up in three subgroups: those who had undergone single-level ACDF, multilevel ACDF, and single-level corpectomy. It seems logical that single-level constructs (that is, in corpectomy or discectomy) would not be lordotic compared with over-all cervical lordosis, because the length of the construct covers only one level in the cervical spine. The finding that multilevel ACDF constructs are segmentally less lordotic than the overall cervical spine is more difficult to explain, although 14 of the 20 constructs were placed in two-level ACDF procedures. In our opinion, a two-level ACDF construct is essentially the same length as that used in a single-level corpectomy, and this fact can partially explain the lack of segmental lordosis demonstrated in the multilevel ACDF subgroup.

The significance of our findings of reduced overall lordosis and, in some cases, segmental lordosis is limited because preoperative radiographs were unavailable for comparison, although the authors of another study have found that ACP system can maintain lordosis.[17]

The results presented in this paper demonstrate the radiographic and clinical efficacy of the Atlantis ACP system at minimum 1-year follow up in 77 patients. We have found the flexibility of the Atlantis ACP system advantageous in tailoring anterior cervical constructs to individual patient biomechanical needs. There is a paucity of peer-reviewed literature providing minimum 1-year follow-up data in cases in which a single ACP system is used; we hope to continue to follow our patients to extend our database and report long-term outcome data regarding the Atlantis system.

Abbreviations used in this paper: ACDF = anterior cervical discectomy and fusion; ACP = anterior cervical plate; ANOVA = analysis of variance; CT = computerized tomography; VB = vertebral body.

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