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The use of interbody fusion cages as a treatment for degenerative lumbar diseases has become widespread. Different interbody fusion cages have been developed to restore disc height and facilitate successful interbody fusion. Early subsidence after interbody fusion cage implantation may represent settling of the components into a stable position and does not portend failure of the implants. However, any later subsidence is more concerning and should be prevented.
Interbody fusion cages are routinely implanted during spinal fusion procedures to sustain or restore height of the intervertebral space, and consecutively prevent or relieve stress and compression on nerve roots. Therefore, the employment of the implant helps to prevent or relieve pain and / or sensory as well as sensitive deficiencies. Another reason for implanting an interbody fusion cage is to facilitate and support arthrodesis of a degenerated or unstable vertebral segment [1] and finally to stabilize these segments to prevent further aggravation of degeneration of the index level, as well as adjacent levels.
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Advanced three-dimensional (3D) printing technology
Due to an aging population, incident of lumbar degenerative diseases, such as spinal stenosis and degenerative spondylolisthesis, is increasing. When there is no response to conservative treatments, lumbar spinal fusion is an established surgical technique. Mainly used techniques include posterior lumbar fusion (PLF), posterior lumbar interbody fusion (PLIF), transforaminal lumbar interbody fusion (TLIF), anterior lumbar interbody fusion (ALIF), oblique lumbar interbody fusion (OLIF) and extreme lateral interbody fusion (XLIF). [2]
The purpose of the spinal cage during spinal fusion is to fuse two vertebral bodies and to keep the height between them. Commonly used interbody fusion cages are produced via milling and turning, via injection molding, or via an additive production process, as 3D printed cages, and are suitable for several surgical approaches. As an additive manufacturing technique, 3D printing can further enhance the biomechanical properties of a structural cage.
Titanium alloy cages improve the opportunity for surface bone formation
Much effort has been put on investigating materials which are able to fulfill the requirements for implantation into the body. Key materials which have been used in the creation of interbody cages include titanium (Ti) and its alloys, polyetheretherketone (PEEK) and carbon fiber-PEEK. However, today titanium alloy and PEEK are preferred in current designs. [3] Titanium alloy has the characteristic of providing a good scaffold or basis for the settling of osteocytes, the deposition of osteoid on its surface and new bone formation. Therefore, it can support better bone fusion. Studies have shown that the presence of titanium at the interface between the host and the device may be beneficial because of the hydrophilic and osteoconductive nature of titanium compared the hydrophobic nature of PEEK alone. [4] Also, titanium alloy implants have a surface structure that is comparably resistant to microbial adhesion. [5] Cages must be able to withstand biomechanical loads and must also leave enough space to place a bone graft for fusion. Titanium alloy fulfills these necessary requirements since it withstands high axial loads.
Secondary stability to prevent subsidence
In contrast to the good abilities of titanium alloy to work as scaffold, PEEK does not show this ability. Usually PEEK cages are surrounded by a connective tissue layer, and do not act as direct scaffold or substrate for bone formation. The probability of a non-fusion after implantation of a PEEK cage is higher than that of the titanium alloy spinal fusion cage. [6] However, due to the difference in elastic modulus of the cortical bone, subsidence of titanium alloy cages may occur as a complication. Subsidence means the spinal cage settles or migrates into the vertebral bodies on both sides, cranial and caudal, and reduces the height (which was intended with the reconstruction / implantation) between vertebral bodies significantly and with clinical impact. If the height is reduced ≥ 3 mm, the process is recognized as a serious complication. [7] Implant settling is a normal occurrence during the interbody fusion process owing to the early, normal, osteolytic phase of osteogenesis. [8] However, any further migration of cages into the adjacent end-plates after the process of implant settling is undesirable. This further migration and subsidence can be only avoided by secondary stability, which means osseointegration of the cage and solid fusion of the vertebral segment.
[1] Oh, K.W.; Lee, J.H.; Lee, J.H.; Lee, D.Y.; Shim, H.J. The correlation between cage subsidence, bone mineral density, and clinical results in posterior lumbar interbody fusion. Clin. Spine Surg. 2017, 30, E683-E689.
[2] Schnake, K.J., Rappert, D., Storzer, B. et al. Lumbale Spondylodese – Indikationen und Techniken. Orthopäde 2019, 48, 50-58.
[3] Phan K et al., Integral Fixation Titanium/Polyetheretherketone Cages for Cervical Arthrodesis: Evolution of Cage Design and Early Radiological Outcomes and Fusion Rates. Orthop Surg. 2019 Feb;11(1):
[4] Walsh WR, Bertollo N, Christou C, Schaffner D, Mobbs RJ. Plasma-sprayed titanium coating to polyetheretherketone improves the bone-implant interface. Spine J, 2015, 15: 1041-1049.
[5] Cabraja M, Oezdemir S, Koeppen D, Kroppenstedt S. Anterior cervical discectomy and fusion: comparison of titanium and polyetheretherketone cages. BMC Musculoskelet Disord, 2012, 13: 172.
[6] Lim et al., Design and Biomechanical Verification of Additive Manufactured Composite Spinal Cage Composed of Porous Titanium Cover and PEEK BodyAppl. Sci. 2019, 9, 4258.
[7] Anderson, P.A., Rouleau, J.P., 2004. Intervertebral disc arthroplasty. Spine 29 (23), 2779-2786.
[8] Sunil V Manjila SV, Mroz M, Michael P Steinmetz MP, Lumbar Interbody Fusions (ebook). Elsevier Health Sciences (2018).