J Korean Neurosurg Soc.  2017 Nov;60(6):611-619. 10.3340/jkns.2017.0178.

The Formation of Extragraft Bone Bridging after Anterior Cervical Discectomy and Fusion: A Finite Element Analysis

• Kwon SW
• Kim CH
• Chung CK
• Park TH
• Woo SH
• Lee SJ
• Yang SH

• KMID: 2395779
• DOI: http://doi.org/10.3340/jkns.2017.0178

Abstract

OBJECTIVE
In addition to bone bridging inside a cage or graft (intragraft bone bridging, InGBB), extragraft bone bridging (ExGBB) is commonly observed after anterior cervical discectomy and fusion (ACDF) with a stand-alone cage. However, solid bony fusion without the formation of ExGBB might be a desirable condition. We hypothesized that an insufficient contact area for InGBB might be a causative factor for ExGBB. The objective was to determine the minimal area of InGBB by finite element analysis.
METHODS
A validated 3-dimensional, nonlinear ligamentous cervical segment (C3-7) finite element model was used. This study simulated a single-level ACDF at C5-6 with a cylindroid interbody graft. The variables were the properties of the incorporated interbody graft (cancellous bone [Young's modulus of 100 or 300 MPa] to cortical bone [10000 MPa]) and the contact area between the vertebra and interbody graft (Graft-area, from 10 to 200 mm²). Interspinous motion between the flexion and extension models of less than 2 mm was considered solid fusion.
RESULTS
The minimal Graft-areas for solid fusion were 190 mm², 140 mm², and 100 mm² with graft properties of 100, 300, and 10000 MPa, respectively. The minimal Graft-areas were generally unobtainable with only the formation of InGBB after the use of a commercial stand-alone cage.
CONCLUSION
ExGBB may be formed to compensate for insufficient InGBB. Although various factors may be involved, solid fusion with less formation of ExGBB may be achieved with refinements in biomaterials, such as the use of osteoinductive cage materials; changes in cage design, such as increasing the area of polyetheretherketone or the inside cage area for bone grafts; or surgical techniques, such as the use of plate/screw systems.

Keyword

Spinal fusion; Cervical vertebra; Surgery; Grafts; Finite element analysis

MeSH Terms

Biocompatible Materials
Diskectomy*
Finite Element Analysis*
Ligaments
Spinal Fusion
Spine
Transplants
Biocompatible Materials

Figure

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  Fig. 1 Finite element model. The geometrical data for the current multi-segmental cervical model (C3–7) were reconstructed from computed tomography scans of a 26-year-old man with no pathologies.

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  Fig. 2 Interbody graft. The morphology of an interbody graft is simplified as a cylindroid shape. In the axial view, the depth/width ratio is 12/14, and the area of contact surface increases from 10 to 200 mm in increments of 10 mm2, starting from the center. The area of the endplate in the vertebra is 227.42 mm2. In the anterior-posterior and mid-sagittal views, the interbody graft is completely united with the cranial and caudal vertebrae. In the mid-sagittal view, the height of the interbody graft is identical to the original disc height (anterior, 7.36 mm; posterior, 4.46 mm).

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  Fig. 3 Measurement of interspinous motion between the flexion and extension finite element models. A compressive follower load of 73.6 N and a pure moment of 1.0 Nm were applied on both the flexion and extension motions, and the interspinous distance was measured between spinous processes C5–6 in flexion and extension models after implantation of an interbody graft. The inferior surface of C7 was rigidly fixed in all directions.

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  Fig. 4 Interspinous motion according to the interbody graft properties. The interspinous motion decreased as the contact area of the interbody graft (the Graft-area) increased. The interspinous motion was less than 2 mm when the area of the incorporated interbody graft was 190 mm2, 140 mm2, and 100 mm2 with properties of 100 MPa and 300 MPa cancellous bone and 10000 MPa cortical bone, respectively.

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  Fig. 5 Illustrative cases. A: Case 1. A 71-year-old man underwent C5–6 anterior cervical discectomy and fusion (ACDF) with a stand-alone MC+® cage (depth×width=14×17 mm) filled with demoralized bone matrix (DBM). Computed tomography (CT; middle inserted figure) scans and plain radiographs obtained 3 months postoperatively demonstrated that neither intragraft bony bridging (InGBB) nor extragraft bone bridging (ExGBB) formed. The interspinous motion was 2.27 mm. The interspinous distances are indicated in the figure in units of mm. All radiological measurements were performed in 150% magnified images. B: The same images were obtained 12 months postoperatively and showed both InGBB and ExGBB. ExGBB formed both anterior and posterior to the cage (middle inserted figure). There were lucent lines in InGBB and ExGBB, and the interspinous motion was 2.34 mm. C: At 24 months postoperatively, ExGBB was formed both anterior and posterior to the cage (middle inserted figure), and the posterior ExGBB encroached the spinal canal (arrow). The lucent line in InGBB was present, but the lucent line in the posterior ExGBB disappeared. The interspinous motion was 0.63 mm. D: Case 2. A 43-year-old woman underwent C4–5 ACDF using the same cage and DBM used in case 1. Twelve months postoperatively, CT scans and plain radiographs revealed the complete formation and remodeling of InGBB with the density of cortical bone without a lucent line. The interspinous motion was 0.38 mm without the formation of ExGBB.

Reference

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