Advances in Spinal Interbody Cages

Abstract

Since the late 1980s, spinal interbody cages (ICs) have been used to aid bone fusion in a variety of spinal disorders. Utilized to restore intervertebral height, enable bone graft containment for arthrodesis, and restore anterior column biomechanical stability, ICs have since evolved to become a highly successful means of achieving fusion, being associated with less postoperative pain, shorter hospital stay, fewer complications and higher rates of fusion when than bone graft only spinal fusion. IC design and materials have changed considerably over the past two decades. The threaded titanium-alloy cylindrical screw cages, typically filled with autologous bone graft, of the mid-1990s achieved greater fusion rates than bone grafts and non-threaded cages. Threaded screw cages, however, were soon found to be less stable in extension and flexion; additionally, they had a high incidence of cage subsidence. As of the early 2000s, non-threaded box-shaped titanium or polyether ether ketone IC designs have become increasingly more common. This modern design continues to achieve greater cage stability in flexion, axial rotation and bending. However, cage stability and subsidence, bone fusion rates and surgical complications still require optimization. Thus, this review provides an update of recent research findings relevant to ICs over the past 3 years, highlighting trends in optimization of cage design, materials, alternatives to bone grafts, and coatings that may enhance fusion.

Keywords: Cage subsidence; Interbody cage; Spine surgery.

Figure 1

Flow chart of search of published reports.

Figure 2

Scanning electron microscopy photographs of a porous titanium sheet at lower magnification (A) and higher magnification (B). The high porosity is created by a network of interconnecting micro and macropores. The sheet also has a triple pore structure (asterisks) that imitates trabecular bone.

Figure 3

A representation of a two‐screw anchored cage8. Investigators have found that two‐screw anchored cages result in a significantly greater range of motion than four‐screw cages, whereas four‐screw cages provide greater stability.

Figure 4

Representation of the finite element model used to determine which factors should be considered when creating the optimal cage design for lumbar fusion9. Investigators found that a compliant cage that allows for strong immediate bone growth hinders future bone formation.

Figure 5

(A) PEEK cage; (B) Titanium cage13.

Figure 6

Radiographic grading of subsidence: Grade III is the most severe15.

Figure 7

A composite silicon nitride cervical implant (photograph courtesy of Amedica Corporation, Salt Lake City, UT, USA)17.

Figure 8

A skeleton of a magnesium‐polymer cage19.

Figure 9

A beta tricalcium phosphate particle (A: ×30 magnification; B: ×2000 magnification)20.

Figure 10

INFUSE bone graft, which accompanies the LT‐CAGE Lumbar Fusion Device and contains bone morphogenic protein 2 (Medtronic Sofamor Danek, Memphis, TE, USA)23.

Figure 11

Scanning electron microscope images of (A) PEEK with HA coating at 80,000 magnification, (B) PEEK with hydroxyapatite coating at 10,000 magnification, (C) uncoated PEEK at 80,000 magnification, (D) uncoated PEEK at 10,000 magnification24.