Elsevier

The Spine Journal

Volume 3, Issue 2, March–April 2003, Pages 118-124
The Spine Journal

Clinical studies
Cervical plates: comparison of physical characteristics and in vitro pushout strength

https://doi.org/10.1016/S1529-9430(02)00464-3Get rights and content

Abstract

Background context: There are many cervical plates available to the spine surgeon today. A single plate design may not be appropriate for every clinical situation. It is important for the surgeon to understand the differences of these plating systems. Plate systems are known to fail by screw pullout from the bone, screw and plate breakage and a less frequent but clinically observed screw pushout from the plate. Pushout testing of the screws from the plate have not previously been subjected to study.

Purpose: This compares the features of cervical plating systems and the strength of the locking mechanisms to allow the surgeon to make a knowledgeable choice of plating system.

Study design: This is a review of descriptive geometric characteristics of cervical plate systems and a biomechanical evaluation of locking mechanism screw pushout strength.

Methods: Physical characteristics of each plate were determined. Features of plates and screws were cataloged. Each of the test plate systems had a different locking mechanism. Biomechanical testing of the locking mechanism–screw-plate constructs was performed to determine the pushout strength of the fixation screw from the plate-locking mechanism.

Results: Physical characteristics of the plating systems, including lengths, widths, shortest screw lengths and distance from edge of plate to nearest screw, were determined. Biomechanical testing showed significant differences in pushout strength, in part explained by the type of locking mechanism.

Conclusions: Biomechanical screw pushout data demonstrate that a significant range of pushout strengths exist across the available cervical plate systems today. Knowing the physical characteristics of the cervical plating systems available may allow the selection of a plate best suited for a given clinical situation.

Introduction

Anterior plating systems have become increasingly popular for fixation of the subaxial cervical spine. Internal fixation of the anterior cervical spine using instrumentation in patients with fracture instability is well recognized 1, 2. The role of instrumentation for cervical degenerative disorders has been more controversial but recent studies support the use of plating, especially for multiple-level fusions 3, 4, 5, 6, 7, 8. The plate systems have been recommended to decrease the pseudarthrosis rate in cervical surgery 9, 10, 11, 12, 13, 14, 15. Other advantages of the plate include prevention of graft extrusion and decreased need for external postoperative immobilization 7, 8, 14, 15, 16, 17, 18, 19. The disadvantages may include the cost, stress shielding of a bone graft, fixation failure from the bone with loosening of the screws and the risk of neurological or vascular injury 12, 15, 17, 18, 20, 21, 22, 23, 24. Currently, cervical plating systems incorporate a bone fixation screw secured by a locking mechanism. The locking mechanism makes the screw-plate construct more rigid without using bicortical fixation 25, 26 and prevents pushout of the bone screw. This screw-plate construct thus acts mechanically as a single-piece implant. Without the need for posterior cortical penetration in the vertebral body, there is a decreased risk of screw misplacement and resultant spinal cord injury [27]. These plating systems are subject to failure. These are not common complications but may include screw pullout from the bone or from the plate and hardware breakage.

There has been a proliferation of plating systems with variable locking mechanisms in recent years, all with their own purported advantages. No information exists in the spine surgery literature about the physical characteristics of these plating systems. The first purpose of this study was to consolidate physical information for 10 commonly used plates. Secondly, six of the systems were evaluated for screw pushout strength. The goal was to expand the spine surgeon's understanding of the versatility of each plating system.

Section snippets

Materials and methods

Test systems included cervical spine locking plate (CSLP) (Synthes, Paoli, PA); Orion, Premier, Zephr and Atlantis (Sofamor-Danek, Memphis, TN); Vuelock (EBI, Parsippany, NJ); PEAK and DOC (Depuy-AcroMed, Cleveland, OH); Aline (Surgical Dynamics, Memphis, TN) and the Blackstone (Blackstone Medical, Inc., Springfield, MA). These 10 plate systems are not inclusive of all plating systems available today. Additionally, only six of the systems were made available for the biomechanical pushout

Results

Table 1 summarizes height, width, screw length and distance from the end of the plate to the nearest screw hole. Heights ranged from 1.8 to 3.2 mm, widths form 14.9 to 20.7 mm, and end of the plate to near screw measurement ranged from 1.4 to 3.2 mm. Of note also, four systems had rescue screws for which the minimum length was longer than the minimum length standard screw. Table 2 summarizes the locking mechanism and angulation position of the screw to the plate. Some are fixed angle and some

Discussion

As the usage of cervical plates has increased, so too has the number of cervical plating systems. There is variability in these plates. Technical problems arise during surgery, and certain features within the plating systems more easily address these. For example, some patients have a smaller bone surface area of the anterior cervical spine and do not accommodate wide plates and longer screws. Lordosis is variable also. Therefore, optimal screw to plate angles vary as well. There is generally

Conclusions

All locking mechanisms significantly increased the pushout strength of the tested screw-plate systems. The expansion screw had the lowest pushout values. These data demonstrate a range of pushout strengths exists across the selections of available cervical plate systems today. Further study is needed to understand the optimal or minimal pushout strength to avoid this mode of failure. In addition, each plate has several unique physical characteristics, which permits the knowledgeable surgeon to

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  • Cited by (5)

    • Surgical treatment of dysphagia after anterior cervical interbody fusion

      2005, Spine Journal
      Citation Excerpt :

      Increased prominence of the instrumentation, graft or screws could cause esophageal irritation, even erosion and death [26]. All current anterior cervical plating systems are designed with minimal profile and the screw lock to the plate by various methods to prevent screw back out, which could cause esophageal irritation and dysphagia [27]. The PEAK Plate system of Depuy-Acromed used in this study is a contemporary system with a profile height of 3.2 mm and a width of 18.1 mm.

    FDA device/drug status: not applicable. Nothing of value received from a commercial entity related to this research.

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