Caution: Investigational device, limited by Federal (or United States) law to investigational use. The FSU device is currently not available in the United States. Information is presented on the website for general education.

Flexuspine's FSU (Functional Spinal Unit) Total Spinal Segment Replacement is designed to provide an alternative to fusion by reestablishing mobility to an affected segment of the lumbar spine. It is a comprehensive device comprised of an interbody disc component (Core) and posterior dynamic resistance component (Dampener) that was designed as a system to replace the natural kinematics of the motion segment.

To Learn more about:

FSU CORE – Size – Fit – Subsidence Resistance

FSU Wear Testing and Particulate Animal Study

FSU – Managing the Forces / Sharing the Load

FSU – Designing and Testing the Posterior Dampener

FSU CORE - Size – Fit – Subsidence Resistance

It is well known that artificial disc replacement devices should have as large a coverage area as possible to resist subsidence into the vertebral endplates.¹ Although the posterior implantation approach offers advantages over the anterior approach, such as direct access to the nerve roots for effective decompression, anatomic constraints limit the size of the device that can be implanted. The FSU Core component, however, has been designed to take advantage of the benefits of the posterior approach while still accommodating the implantation of a larger device. This is possible because the Core Component is part of a Total Spinal Arthroplasty device system that replaces not only the disc, but the complete three-joint complex (disc and facets). By removing the facets, a wider surgical window is available for the larger FSU Core disc component to be inserted.

 Benchmark Size Comparison
FSU Core versus
a typical Anterior TDR

 
A typical anterior total disc replacement was used as a benchmark for size comparison. As shown, the implant area of the FSU Core component is comparable to a typical anterior total disc replacement.²

Benchmark Fit Comparison
FSU Core versus
typical Anterior TDR
and Fusion Cages

The fit of the Flexuspine Core Component on the vertebral endplate is comparable in coverage to a typical anterior TDR and greater than common fusion cage footprints.

Weighted Area Comparison
FSU Core versus a typical Anterior TDR and Fusion Cages

Another advantage of the FSU Core Component is that the footprint is placed on the posterior lateral portion of the endplate, which is stronger than the central portion of the endplate, where the majority of interbody implants are placed¹. A detailed evaluation incorporating both the cross‐sectonal area of the FSU and typical interbody implants, along with the strength of the underlying bone¹ on which they are implanted, has been performed³. The area and strength of the underlying bone on which the implants are placed were used to calculate a weighted coverage area to provide an indication of subsidence resistance.

The study found that the weighted coverage area of the Flexuspine Core implant was greater than a similarly sized Anterior TDR due to
the FSU Core’s position on the
endplate bone.

*We are proud to mention that the information above was part of a more extensive poster exhibit that was voted “Highest Scored” poster at the 2008 SAS (Spine Arthroplasty Society) meeting.

FSU Wear Testing and Particulate Animal Study

The motion of any joint, human or man-made, will experience the effects of wear, which can produce microscopic particles. All arthroplasty devices exhibit wear, and, in the vast majority of cases, the body can manage the resultant particles. But depending on their size and amount, the body's cellular response to these foreign elements can sometimes lead to the destruction of surrounding bone leading to possible device loosening.⁴

Wear testing of the Flexuspine FSU was performed to characterize the volumetric wear potential of the FSU system. Testing in animals was also performed to evaluate the physiological response to wear debris comparable to that produced by the FSU system in vitro.

The FSU Core CoCr metal-on-metal articulation demonstrated wear comparable to metal-on-metal total disc replacements.⁵ The FSU silicone dampeners also showed minimal wear and sufficient durability under clincally relevant motions.

The particles produced
were well within the size
ranges of particles generated by other devices.¹ Particles equivalent to those produced in the wear testing were implanted
near the spine of rabbits without notable adverse effects.

Bushelow M, Walker J, Coppes J, Hinter M, Nechtow W, Kaddick C, "Comparison of Wear Rates: Metal/UHMWPE and Metal-on-Metal Total Disc Arthroplasty," Spine J, 2007, 7(5S): 97S-98S.

FSU – Managing the Forces / Sharing the Load

The spinal segment’s three joint complex (disc and facets) is designed to work as a unit to achieve stability and motion. The FSUCore and Dampeners) are designed to function seamlessly by sharing the anticipated load of the spine while continuing to encourage physiological motion. The interbody Core is considered to be the foundational support of the FSU. It features a shear face designed to provide stability to the spine segment, which is intended to help off-load the pedicle screws. This design feature is intended to deter pedicle screw loosening. In turn, the Dampeners (attached via pedicle screws) are designed to work with the Core providing resistance to flexion-extension, lateral bending, and axial rotation.

The Core . . . managing the forces of the three joint complex

The core is designed to allow motion while providing the stability necessary to protect the pedicle screws from excessive loading. This is particularly important for Total Spine Arthroplasty devices as the removal of the facets requires the device to play a greater role in shear. Due to the Core’s shear-resistant design, it has been shown in pre-clinical in vitro testing to be able to carry shear loads far surpassing anticipated in vivo loads.⁶

The Dampener . . . managing the forces of the three joint complex

Pedicle screw loosening can be a clinical concern when replacing the total three joint complex with a motion preservation device. A finite element model analysis was conducted to evaluate the stresses at the pedicle screw-bone interface.

Finite Element Model of a ligamentous L4/L5 motion segment.⁹

Designing and Testing the Posterior Dampener

Our challenge was to create a posterior component that, when combined with the Interbody Core component, aims to provide the appropriate quantity and quality of motion to replicate the natural functionality of the spine.

The unique design of the FSU Posterior Dampener component is intended to provide clinically relevant displacements and physiological, direction-specific resistance to motion. 

Finding the right silicone for the Posterior Dampener was of paramount importance.

Extensive design work and mechanical testing was conducted to identify the ideal silicone dampener for use in a Total Spine Arthroplasty device. A large number of samples made of various sizes, shapes, and types of silicone were tested in both static compression and fatigue to determine the ideal material stiffness and type. This helped confirm that the material had the proper function and durability in vivo.

Step 1: Finding the right shape

As part of the design work for the silicone dampener, FEA modeling was performed (as shown to the right) to evaluate various shapes and sizes. A simple tube shape with sufficient diameter was found to work well. Since the length of the tube would also affect the in vivo strains on the dampener, motion simulation tests were performed to determine the appropriate length. Since a relatively long dampener was found to be needed, a dual action rod design was developed for the Posterior Dampener to allow for the length required without excessive bulk.

Step 2: Finding the right stiffness
Mechanical studies of the spine have shown that  the stiffness of the spine increases the farther it bends¹². Compression testing of the dampeners was conducted to determine if silicone also exhibited this behavior. The representative graphs below show that the stiffness of the dampeners increases
in a similar manner to the normal spine.

Testing the limits of the Posterior Dampener Design

Step 3: Finding the most durable silicone
Since the spine must move regularly to accomplish everyday tasks, the Posterior Dampener must be durable enough to withstand repeated loading. Multiple dampener sizes and several different silicone formulations were tested in compression for millions of cycles to identify the most durable dampener. As expected, some types and sizes of silicone did better than others.  Once the best type and size was identified, the final dampener design was tested in compression under greater than expected in vivo strains to simulate approximately 160 years of clinical use¹³.

The final dampener design was tested in cyclic compression to simulateup to 160 years of clinical use.

Step 4: Putting it all together
After the silicone dampener was tested extensively, the Posterior Dampener construct was tested to evaluate its strength and durability.

Dynamic Flexion Testing
Posterior rods were tested to determine the ability of the rod to withstand repeated loading. The testing showed that the posterior rod could withstand greater than expected in vivo loading as shown in the graph¹⁴.

Static Compression and Tension Testing
The Posterior Dampener was also tested in static compression, as would occur during extension, and static tension, as would occur during flexion, to evaluate the displacement capacity of the dampeners and the strength of the rod. The maximum displacements and loads allowed by the Posterior Dampener were well above the expected in vivo amounts as shown in the graphs¹⁴.

¹Grant JP, Oxland TR, Dvorak MF, “Mapping the structural properties of the lumbosacral vertebral endplates,” Spine, 2001, 26(8):889-96.
²Typical anterior TDR dimensions based off of Charite Artificial Disc Product Insert
³Phillips F, Gordon C, Sengupta D, and Gimbel J, “Subsidence resistance evaluation of posterior implanted total disc replacements,” SAS Global Symposium on Motion Preservation Technology, 8th Annual Meeting.
⁴Jacobs, "Wear Particles" JBJS (2007) 88:99-102
⁵Maverick and ProDisc L Total Disc Replacement Devices
⁶Pre-clinical in vitro test data available on file at Flexuspine.
⁷Lu WW, Luk KD, Holmes AD, Cheung KM, Leong JC, "Pure shear properties of lumbar spinal joints and the effects of tissue sectioning on load sharing," Spine, 2005, 30(8):E204-9.
⁸Serhan HA, Varnavas G, Dorris AP, Patwadhan A, Txemiadianos M, "Biomechanics of the posterior lumbar articulating elements,' Neurosurg Focus, 2007, 22(1):E1.
⁹Rundell S, Auerbach J, Balderston R, Durtz S, “Total Disc Replacement Positioning Affects Facet Contact Forces and Vertebral Body Strains,” Spine, 2008, 33(23):2510-7.
¹⁰Rundell S, Gimbel J, "Evaluation of Pedicle Screw Loosening in a Combined Facet and Total Disc Replacement System," SAS Global Symposium on Motion Preservation Technology, 2008, P236.
¹¹Panjabi MM, Oxland TR, Yamamoto I, Crisco JJ. “Mechanical behavior of the human lumbar and lumbosacral spine as shown by three-dimensional load-displacement curves,” J Bone and Joint Surg Am, 1994; 76(3):413-24.
¹²White A.A. and Panjabi M.M., Clinical Biomechanics of the Spine.  Lippincott, Philadelphia, c1978.
¹³Hedman TP, Kostuik JP, Fernie GR, Hellier WG, “Design of an intervertebral disc prosthesis,” Spine, 1991; 16(6 Suppl):S256-60.
¹⁴Phillips FM, Gimble JA, Lawthorne T, Havey RM, Voronov L, Patwardhan AG, “Effects of Total Spinal Arthroplasty on the Kinematics of the Lumbar Spine,” SAS Global Symposium on Motion Preservation Technology, 10th Annual Meeting