Commentary on “Biomaterials in Spinal Implants: A Review”

In the current issue of Neurospine, Warburton et al. [1] reviewed the use of biomaterials in spinal implants focusing on their biocompatibility, mechanical characteristics, and real-world evidence. Clinical outcomes of spinal fusion surgery are correlated with the achievement of bony fusion. Other than the interbody fusion material, the stabilization and interbody screw fixation play an important mechanical role in restriction of motion and load-bearing. The most commonly used implants are metallic (titanium [Ti] or alloys) or polymers (polyetheretherketone, PEEK). To achieve best clinical outcomes, these must be combined with osteoconductive and osteoinductive materials. Ideally, a desired biomaterial implant will be which does not require additional allograft or autograft for fusion.

Most commonly used interbody cages are of Ti and its alloys and PEEK. When comparing Ti with PEEK, cage subsidence is higher in Ti cages, but rate of fusion is comparable between Ti and PEEK cages. PEEK in comparison to Ti has lower support for osteogenic tissues, hence lower level of bone integration [2].

Studies have also reported pseudoarthrosis after anterior cervical discectomy and fusion (ACDF) which is correlated with the graft choice and is statistically significant with PEEK, polymethymethacrylate, and trabecular metal compared to autografts. In their systematic analysis Iunes et al. [3] also reported that Ti, zero profile, and carbon cages have lower incidence of pseudoarthrosis.

Attempts have been made to combine characteristics of Ti and PEEK for implants to achieve improved outcomes. Assem et al. [4] reported safety, efficacy, similar fusion rates and clinical outcomes of Ti/PEEK spinal fusion cages compared to standard PEEK cages. Various modifications of Ti and PEEK interbody device such as hydroxyapatite-coating, composites with calcium silicate, bioglass, and β-tricalcium phosphate are available but lack enough preclinical and clinical studies.

Implant made of other materials such as silicon nitride, Tantalum, Nitinol, and bioabsorbable cages are available but are supported by results from few clinical studies. A multicenter retrospective study for ACDF showed comparable clinical efficacy for silicon nitride cages compared to existing evidence for other cage implants [5]. Single level locking standalone cages for ACDF have been investigated by several groups and show similar clinical and radiological outcomes as anterior plate constructs. Stand-alone cages have been also associated with lower incidence of dysphagia and adjacent segment degeneration [6].

There exist some contrary views for the tantalum implants for cervical fusions but for the lumbar fusions, trabecular metal has been demonstrated to obtain fusion and improved patient outcomes [7]. Ti and tantalum have also been shown to act as scaffold for mesenchymal stem cells in vitro and in animal studies which have clinical implications for spinal fusion and regenerative strategies [8,9].

Of further interest will be the cages of Nitinol with shape memory and super elastic properties. A single retrospective cohort study indicates similar fusion rates with TiNi implant for lumbar surgeries compared with alternative cages [10]. Bio-absorbable cages are another generation of cages which can avoid artefacts on scans, reduce long term toxicity. The clinical studies support the use of bioabsorbable interbody devices for structural support [11]. However, bioabsorbable posterior lumbar instrumented fusion cages, showed high osteolytic nature in 44% of the patients in a case series [12]. Bioabsorbable screws and plates are available as well but again, there is paucity of statistically significant clinical evidence in favor of biodegradable implants over traditional instrumentation.

When deciding on appropriate metal for rods, clinical evidence is not sufficient to favor single metal, and characteristics of each type of rod should be considered while choosing for spinal surgery. Rigid rods can cause adjacent segment disease and other adverse effects. Dynamic stabilization devices preserve motion but have high rate of implant failure and reoperation. A systematic review, reported lower incidence of adjacent segment disease with PEEK semirigid rods [13]. Nitinol has comparable wear resistance with CoCr and 100 times higher compared with Ti which makes Nitinol rods, stabilization system suitable for spinal instrumentation [14]. Memory rods have a special feature to return to their prebent shape on increase in temperature which would be particularly useful in scoliosis surgeries.

Artificial discs designed for disc replacement surgeries aim to mimic the characteristics of a natural disc. The ProDisc, activL Artificial disc, Charite, and Mobi-C implants have ultrahigh molecular weight polyethylene cores which provides them with low risk of free radical damage and lower friction. Success of another new generation M6 implant which has a polycarbonate urethane core providing a large degree of shock absorbance is yet to be clinically proven [15]. Tissue engineering approaches aiming at creating a scaffold which develop into similar structure as natural disc, are poised to decide the future of disc regeneration research.

Recently, 3-dimensional (3D) printing is in trend and being used for surgical planning, intraoperative surgical guides, patient specific and Off-The-Shelf surgical implants. The technology has a great potential to improve surgical implant characteristics, procedural accuracy, patient outcomes and reduce surgical time. The studies reporting 3D technology in spine are so far of low-quality with inherent bias and prevent from deriving a firm conclusion [16].

Low grade of clinical evidence is available for certain implant materials, but evidence is particularly lacking for surface modifications of implant materials. The biomaterials in spinal surgeries merit further investigation. Today, spine surgeons have at their disposal many spine implants such as pedicle screws, hooks, rods, plates and interbody fusion devices to choose from. New biomaterials are introduced without analysis of clinical outcomes. Their indications and applications are still being explored, so the use should be cautious. Complete understanding of the material and their behavior in physiological conditions is important to allow its use as an implant for spinal diseases. Other than efficacy and clinical outcomes, cost-effectiveness should also be an important factor in the decision-making process.