Introduction to Orthonex's Tensile Control Rod for Dynamic Stabilization

Orthonex's patent pending "Tensile Control Rod"  is an innovative device for dynamic stabilization of the spine.  The Tensile Control Rod is a longitudinal implantable device for dynamic stabilization of the spine that is attached posteriorly to the spine in a manner similar to attachment of a conventional spinal rod.  However, unlike a conventional spinal rod, the Tensile Control Rod allows desirable spinal movement and prevents undesirable spinal movement.  Further, the Tensile Control Rod enables adjustment of the range and dampening of spinal motion in different directions, including the possibility of adjustments during surgery and also non-invasively (via wireless control) after surgery. 

How Does Orthonex's Tensile Control Rod Work?

The Tensile Control Rod includes the following components: (1) a longitudinal sequence of incompressible segments that connect spinal vertebrae, contain non-central longitudinal channels, and provide spinal support; (2) substantially-inelastic members (e.g. tensile wires) that run through these non-central channels, connect the incompressible segments, and restrict spinal movement so that it remains within a desirable range of motion; and (3) motion-dampening members (e.g. springs) that also run through the non-central channels and advantageously-dampen spinal movement within the desirable range of motion. 

Figure 1 shows one example of how Orthonex's Tensile Control Rod may be designed to provide dynamic stabilization of the spine.  The far-left portion of Figure 1 shows the exterior of a rod-like version of this device that has been posteriorly attached, with pedicle screws, to a sequence of spinal vertebrae.  The mid-left portion of Figure 1 shows a close-up, primarily-opaque, side view of two of the cylindrical segments that are part of a longitudinal sequence of multiple cylindrical segments forming the interior of the Tensile Control Rod. 

In the mid-left portion of Figure 1, the upper cylindrical segment is shown with six channels that run longitudinally through the cylinder and intersect its cross-section in a roughly circular manner.   Since the mid-left portion is a primarily-opaque view of the segments, the channels are only visible where they open onto the bottom end portion of the upper cylindrical segment.  Two of the six channels do not appear in this view because they are obscured by a partial ball-like protrusion that protrudes from the bottom end portion of the upper cylindrical segment.  This ball-like protrusion fits into a socket in the top end portion of the lower cylindrical segment.  This ball-like protrusion and socket comprise a partial ball-and-socket joint between the upper segment and the lower segment.  In this example, within each of the longitudinal channels, there is a coaxial set of longitudinal members including a longitudinal spring and a longitudinal wire inside the spring.  These springs and wires run longitudinally through the upper and lower cylindrical segments and connect these segments to each other. 

The mid-right portion of Figure 1 shows a partially-transparent side view of the same two cylindrical segments.  This portion shows interior transparent views of four of the six longitudinal channels, each including a longitudinal spring with a wire running longitudinally through its center.  If this figure were fully transparent, then the rear two channels would also be shown.  However, such full transparency would clutter the figure too much, so the rear two channels are not shown.  This is why this figure is called a partially transparent, rather than a fully transparent, side view.  In this example, there is some slack in all of the connecting wires when the spine is in a baseline configuration with minimal longitudinal extension, tilting, and rotation.  This baseline configuration is represented by the longitudinal alignment of the two cylindrical segments.  The far-right portion of Figure 1 provides a cross-sectional view of the bottom of the top cylinder, showing the six channels, cross-sections of the springs with wires inside, and the ball portion of the ball-and-socket joint connecting the two cylinders.

In this example of the Tensile Control Rod, the rigidity of the two cylindrical segments, connected by a partial ball-and-socket joint, resists longitudinal compression of the spine, but allows some degree of tilting, rotation, and longitudinal extension of the spine.  The inelasticity of the connecting wires limits the degree of tilting, rotation, and longitudinal extension to a desirable range of motion.  The force resistance of the connecting springs dampens movement of the spine within this allowable range of motion.  Acting together, these components provide dynamic stabilization of the spine.  They advantageously allow dampened movement within a desired range of motion and prevent movement outside this desired range of motion.

Figure 2 shows how the Tensile Control Rod moves when the spine flexes and the cylindrical segments are tilted relative to each other.  The slack in different wires increases or decreases when the spine tilts.  When slack in a wire decreases to zero from this tilting motion, then the wire becomes taut and prevents further movement in that direction.  This is how the substantially-inelastic members, embodied by wires in this example, prevent undesirable movement of spinal vertebrae relative to each other.

In Figure 2, the cylindrical segments have been tilted relative to each other by movement of the spine.  In this example, wires on the right side of the segments have been pulled taut by the tilting movement while wires on the left side of the segments have become more slack.  At some point, the tautness of the wires on the right side prevents further tilting of the segments in this direction.  This is how the substantially-inelastic longitudinal members connecting the segments, wires in this example, prevent undesirable motion of the vertebrae relative to each other. 

The tilting of the segments in Figure 2 does not affect only the degree of slack in different wires.  This tilting also affects the extension of different springs.  For example, left-ward tilting in this example causes springs on the right side of the segments to become more extended and the springs on the left side of the segments to become more compressed.  This is how the motion-dampening longitudinal members, springs in this example, provide advantageous dampening of vertebral motion within the range of motion allowed by the substantially-inelastic members.

The relative slack or tension of each of the tensile members (wires in this example) may be adjusted during surgery to achieve the desired range and dampening of spinal movement.  This may include spinal distraction for gradual correction of scoliosis or other spinal deformity.  The tensile members may also be wirelessly and non-invasively adjusted in the weeks, months, or even years after surgery by remote communication with an implantable actuator or based on automatic interaction between implanted sensors and the actuator. In this respect, Orthonex's Tensile Control Rod can be one of the first "smart devices" for dynamic stabilization of the spine, allowing non-invasive adjustment of the range and dampening of spinal movement after surgery.

Potential Advantages Over Current Devices and Methods for Dynamic Stabilization of the Spine

Orthonex's Tensile Control Rod addresses many of the limitations of current devices and methods for dynamic stabilization of the spine.

In contrast to methods using flexible elastic members only or flexible inelastic members only, the Tensile Control Rod provides good support for the spinal column, provides selective control of the degree and range of movement in different directions, and has potential for adjustment after implantation. 

In contrast to methods using springs or spring-like cut-metal members only, the Tensile Control Rod provides good support for the spinal column, provides selective control of the degree or range of movement in different directions, has potential for adjustment after implantation, and has no flexing metal springs or spring-like structures that may weaken and break with repeated movement. 

In contrast to methods using flexible members with flexible inelastic members inside or flexible members inside, the Tensile Control Rod provides good vertical support for the spinal column, provides selective control of the degree or range of movement in different directions, and has potential for adjustment after implantation.

In contrast to methods using flexible members with rigid rods inside, the Tensile Control Rod does not have flexing (thin) rods that may weaken or break with repeated movement, does not require an expensive array of parts such as multiple size and shape rigid inserts, allows natural spine movement in multiple directions (flexion, extension, lateral bending, and torsion), and has potential for non-invasive adjustment after implantation. 

In contrast to methods using non-contiguous rigid segments connected by flexible members, the Tensile Control Rod: provides good vertical support for the spinal column, provides selective control of the degree or range of movement in different directions; and has potential for non-invasive adjustment after implantation.

In contrast to methods using contiguous rigid segments connected by a central flexible member, the Tensile Control Rod provides good leverage for selective control of the degree or range of movement in different directions, allows the use rounded of rounded (e.g. ball-and-socket) joints between rigid segments because there is no connecting flexible member in the segment center; and has potential for non-invasive adjustment after implantation.

In contrast to methods using contiguous rigid segments connected by only one type of non-central flexible member, the use of both inelastic and motion-dampening longitudinal members (such as wires and springs) allows: firmer restriction of movement outside the desired range of motion; and more precise control of motion-dampening within the desired range of motion.

In contrast to methods using telescoping members with springs or gears, the Tensile Control Rod allows natural flexion and lateral bending movement of the spine, is less prone to mechanical or material failure, and has the potential for non-invasive adjustment after implantation.

In contrast to methods using telescoping members with a flowable substance inside that are directly attached to the vertebrae, the Tensile Control Rod provides selective control of the degree or range of movement in different directions without weakening the vertebrae with a large number of holes, and avoids having an irregularly-shaped moving structure which is difficult to isolate from surrounding body tissue and liquids.

In contrast to methods using integrated configurations of differentially-flexible materials, the Tensile Control Rod provides good vertical support for the spinal column, is less prone to mechanical or material failure because there are no shearing or large-scale flexing members, provides selective control of the degree or range of movement in different directions, and has the potential for non-invasive adjustment after implantation to refine therapy or accommodate patient growth. 

To summarize, Orthonex's Tensile Control Rod has many potential advantages over current methods of dynamic stabilization.  It has considerable potential to restore normal spinal biomechanics and offers new possibilities for adjustment before and after implantation.  It can be a useful addition to the treatment options available for the millions of people suffering from chronic lower back pain.