Developed by 4WEB Medical, the Truss Cage (TC) is a 3D-printed Ti-6Al-4V titanium alloy spinal implant designed to enhance bone growth and fusion after surgery. The need for spinal surgery typically arises from degeneration of intervertebral disks (see Fig. 1), often resulting from age-related spondylosis and spinal deformations like scoliosis. Because symptoms can include chronic pain sensitive to mechanical loading, different devices have been developed to enhance post-surgery stability of the spine in order to reestablish resilience.
Figure 1. Labeling of spinal column. See intervertebral disks and vertebrae. Retrieved from OrthoInfo.
The TC (see Fig. 2) is a novel type of interbody device due to its active promotion of osseointegration. Most interbody devices are static and do not promote bonding between bone and implant surfaces, but the TC is an example of active fusion devices that do promote bonding. Its effectiveness in enhancing spinal stability results from its (1) subsidence-resisting open truss structure and (2) bioactive and growth-stimulating “hierarchical” rough surface.
Figure 2. The 4WEB Truss Cage. Retrieved from Expert Review of Medical Devices.
Empirical testing of different truss designs concluded with an ideal truss structure consisting of ample open space for bone to grow into while maintaining strong load-bearing properties. By modifying sizes and arrangements of truss triangles, the equalizing of tension and compression across different struts determined a structure that could distribute physiological forces (see Fig. 3).
Figure 3. Magnitudes of tension (green) and compression (red) forces across truss structures. Retrieved from Expert Review of Medical Devices.
The truss structure’s importance is also based on the foundational mechanobiology theory of Wolff’s Law, stating that bone can adapt over time to repeated mechanical stresses. Due to its ability to redistribute forces, the TC follows this theory and guides the adaptation of surrounding native bone to form strong cancellous bone (see Fig. 4) throughout TC structural openings.
Figure 4. Compact bone versus spongy (cancellous) bone. Retrieved from My Family Physio.
During the manufacturing process, either acid etching or 3D-printed electron beam melting can be used to produce surface roughness (see Fig. 5), mimicking the lacunae that trigger local bone growth in our bodies, contributing to the fusion process.
The TC’s “hierarchical” property results from being able to manufacture roughness ranging from the nanoscale to the macroscale, consequently allowing for a range of pathways to guide bone growth at all levels. To compare the effectiveness of roughness and type of material, bone activity was measured for four different surfaces (see Fig. 6): rough titanium, smooth titanium, polyetheretherketone (PEEK), and tissue culture plastic (TCP) as the control surface. Because rough titanium produced the greatest amount of bone cell expression, its clear positive impact on bone growth led to the integration of surface roughness into the TC design. Many inert interbody devices use smooth metal surfaces, but development in rough surfaces is leading to more natural recovery and stronger fusion.
Figure 5. Increasing magnification of topography showing rough texture at micron scale. Retrieved from Expert Review of Medical Devices.
Figure 6. Comparison of osteogenic differentiation between TCP, rough titanium, smooth titanium, and PEEK. Retrieved from Expert Review of Medical Devices.
Because of its open truss structure, the TC minimizes bone-implant contact while also redistributing mechanical loading, resulting in minimal subsidence of native surrounding bone. With Quantitative Motion Analysis, thirty patients with the implanted TC were tracked for 12 months with a conclusion that there were no signs of significant subsidence. Alongside the significant design advantages, this mechanical success over time is another demonstration of the TC’s effectiveness, representing the most recent advances in bone-related biomechanics (see Fig. 7).
Figure 7. Timeline of cage designs. Retrieved from Expert Review of Medical Devices.
The TC is a major step away from previous designs, but clearly propels active integration of technology with the human body in unprecedented ways. These devices will be increasingly used in surgery, greatly enhancing not only the post-surgery recovery process and thus quality of life for many patients, but also the bridging of medicine with engineering.
Glossary
Cancellous bone: a spongy type of tissue located in the center of spinal vertebrae; see Fig. 4
Fusion: the permanent joining of two spinal vertebrae using metals plates, screws, and rods
Interbody: inserted between vertebrae
Lacunae: small divots produced by bone activity and which help stimulate bone growth
Osseointegration: the functional connecting of implant and bone
Spondylosis: change in the spine due to age
Subsidence: caving in
References
Hunt, J.P., Begley, M.R., Block, J.E. (2021). Truss implant technology™ for interbody fusion in spinal degenerative disorders: profile of advanced structural design, mechanobiologic and performance characteristics. Expert Review of Medical Devices, 18(8), pp. 707-715. Retrieved from https://doi.org/10.1080/17434440.2021.1947244.
