Introduction to Dynamic Stabilization of the Spine

Chronic lower back pain is a very common, significant, and costly health problem in the United States and the entire world. It is estimated that more than ten million people in the U.S. suffer from chronic back pain at any a given time, that the annual prevalence of lower back pain is in the range of 15-45% of the population, and that thoracic and lumbar spinal disorders affect nearly three-quarters of the U.S. population some time during their lives. Chronic back pain can be debilitating, interfering with one's ability to work and enjoy recreational activities. It is the most common activity-limiting condition affecting people under the age of 45.

The leading cause of chronic lower back pain is the degeneration of the semi-flexible discs between spinal vertebrae.  There are non-invasive approaches to address chronic back pain, but sometimes they are inadequate and more invasive methods are required.  Historically, a common invasive method has been to fuse selected spinal vertebrae together in an effort to eliminate disc movement and stop the pain.  More than 150,000 lumbar fusions are done each year to immobilize selected vertebrae.  However, there are limitations associated with fusing vertebrae.  Fusion-related limitations include: undesirable restriction of natural spine movement (flexion, extension, lateral bending, and torsion) in fused segments; greater stress and degeneration affecting spinal segments adjacent to fused segments (a phenomenon called "transition syndrome"); bone loss in the immobilized segments; failure to stop the pain in approximately 20-25% of fusion cases; irreversibility of the procedure; and the invasiveness, health risks, and relatively long recovery period associated with the surgery.

Due to the limitations associated with the complete immobilization of selected vertebrae in fusion, there has been an increasing trend toward alternative methods of addressing back pain that preserve some spinal mobility. Dynamic stabilization is the term for methods that seek to maintain desirable spinal movement, but limit undesirable spinal movement. The ultimate form of dynamic stabilization would be to artificially recreate the natural biodynamics of a healthy spine.  Since the original spine is not entirely replaced, the challenge is to recreate natural biodynamics in an integrated manner with those portions of the original spine which are working properly and remain in place.  Due to the complexity of spinal biomechanics, this is not an easy goal.

With respect to limiting undesirable movement, dynamic stabilization seeks to: relieve the load and correct improper vertebral movement in areas where pain is caused by compression and improper vertebral movement; maintain proper rigidity, stabilization, and vertical support of the spinal column; avoid abnormal range of motion; and ensure the long-term durability of the spinal structure, including any implants.

With respect to allowing or enhancing desirable movement, dynamic stabilization seeks to allow normal biomechanical direction and range of motion including flexion (bending forward/anteriorly), extension (bending backward/posteriorly), lateral bending (right and left side bending), torsion (axial rotational movement), and limited longitudinal elongation or compression (so-called "shock absorber" functionality).  In addition to the mobility and comfort advantages for patients, allowing normal motion can also help avoid loss of bone density for diseased segments and more evenly distribute load across different portions of the spinal column to avoid creating stress-induced problems elsewhere.  Allowing desirable spinal movement is particularly important for young patients.

Dynamic stabilization can be implemented in the intervertebral space (such as with artificial discs), in the space posterior to the vertebrae (such as with flexible connecting rods), or in both places simultaneously.  With respect to the intervertebral space, malfunctioning disc tissue may be replaced with an artificial alternative. With respect to the space posterior to vertebrae, selected vertebrae may be connected by an elastic cord, a cord with spacer, a flexible rod, or by some other type of connecting member that allows some movement.  Both spaces may be addressed in combination to distribute loading in a manner that approximates the loading distribution in a healthy spine.  The patent-pending dynamic stabilization technology being developed by Orthonex currently focuses on dynamic stabilization outside the intervertebral space.  This technology may be used in combination with devices in the intervertebral space, such as artificial discs, but the opportunities and challenges for connectors outside this space are sufficiently different to merit focused review.

Current Approaches and the Unmet Clinical Need

Considerable progress has been made toward methods of dynamic stabilization to provide the correct balance of allowing desirable movement and limiting undesirable movement.  However, this remains a challenge.  Current treatment options have limitations.  There is still a need for a new approach to dynamic stabilization that addresses these limitations and provides better treatment options for the millions of people who suffer from chronic back pain.  This is one of the long-term goals of Orthonex.

Limitations with current options can be grouped into three general categories: problems from reduced desired movement; problems from allowed undesirable movement; and other types of problems.  This webpage lists these problems grouped by category, reviews the main methods for dynamic stabilization outside the intervertebral space, and discusses the potential limitations of each method.  This will set the stage for discussion of Orthonex's technology for dynamic stabilization that addresses many of these problems.

The array of potential problems associated with too little movement after implantation includes the following.  The method may undesirably restrict natural spine movement in one or more directions (flexion, extension, lateral bending, and torsion).  The method may immobilize a section of the spine causing greater stress, unnatural movement, and degeneration of adjacent portions of the spine (a phenomenon called "transition syndrome").  The method may not be adjustable to meet the specific needs and features of different patients.  The method may not be adjustable after implantation to refine therapy or accommodate patient growth.

The array of potential problems associated with too much movement after implantation includes the following.  The method may allow multi-directional movement without the ability to selectively control the degree or range of movement in different directions (flexion, extension, lateral bending, and torsion).  The method may not provide sufficient vertical support of the spinal column.  The method may be prone to mechanical or material failure due to repeated flexing of materials or components.  The method may involve implants that are subject to slipping, shifting, or extrusion.  The method may cause scarring, pinching, or other damage to nearby soft tissue.

Other potential problems include the following.  The method may involve implantation of a relatively-large structure and a relatively-invasive surgical procedure.  The method may require a complex and time-consuming surgical procedure. The method may require an expensive array of parts in different sizes.  The method may be difficult or impossible to reverse due to invasion and destruction of body tissue.  The method may not stop the pain, as is the case in approximately 20-25% of fusion operations.

We now consider the twelve main methods (in the intellectual property literature) for dynamic stabilization (with intervertebral connectors outside the intervertebral space) and discuss the limitations of each method:

1.    Flexible Elastic Members Only

This method uses only flexible elastic members (such as elastic tethers, bands, cords, or cables) to connect vertebrae outside the intervertebral space.  An early example of this method is the Graf ligament.  The Graf ligament consists of securing elastic bands to spinous processes and/or pedicle screws.  It provides some stabilization while also allowing some flexion and torsion movement.  Devices and methods that appear to use this approach include: U.S. Patents 5,092,866 (Breard etal.), 7,125,410 (Freudiger), 7,229,441 (Trieu et al.) and U.S. Patent Application 20080269904 (Voorhies, R).  Potential limitations with this method include: inconsistency of motion control can occur when elasticity changes over time with repeated motion and material stress; it may limit the patient's ability for forward flexion; it may not sufficiently offload stress from a compressed disc or provide sufficient vertical support of the spinal column; it is difficult to adjust (non-invasively) after implantation in order to refine therapy or accommodate patient growth; it can allow multi-directional movement without the ability to selectively control the degree or range of movement in different directions; and it can involve a relatively time-consuming surgical procedure.

2.    Flexible Inelastic Members Only

This method uses flexible, but relatively inelastic, members (such as flexible rods, wires, inelastic fibers, or cables) to connect vertebrae outside the intervertebral space.  Devices and methods that appear to use this approach include: U.S. Patent 6,475,220 (Whiteside) and U.S. Patent Application 20060047282 (Gordon, Jeffrey).  Potential limitations with this method include: inconsistent motion control or breakage due to repeated flexing and material stress over time; the inelastic members may restrict flexion, extension, lateral bending, and torsion; it may immobilize a section of the spine which causes greater stress, unnatural movement, and degeneration of adjacent portions of the spine ("transition syndrome"); it may provide insufficient vertical support of the spinal column; it may not be adjustable after implantation to refine therapy or accommodate patient growth; it may allow multi-directional movement without the ability to selectively control the degree or range of movement in different directions (flexion, extension, lateral bending, and torsion); and it may cause scarring, pinching, or other damage to nearby soft tissue.

3.    Springs or Spring-like Cut-Metal Members Only

This method uses springs or spring-like metal members with (helical) cuts to connect vertebrae outside the intervertebral space.  Devices and methods that appear to use this approach include: U.S. Patents 6,986,771 (Paul et al.), 6,989,011 (Paul et al.), 7,326,210 (Jahng etal.), 7,329,258 (Studer) and U.S. Patent Applications 20050065516 (Jahng, Tae-Ahn), 20050085814 (Sherman, Michael), 20050124991 (Jahng, Tae-ahn), 20050149020 (Jahng, Tae-Ahn), 20050154390 (Biedermann et al.), 20050203514 (Jahng et al.), 20050288672 (Ferree, Bret A.), 20060129147 (Biedermann et al.), 20060212033 (Rothman et al.), 20060247637 (Colleran et al.), 20070016193 (Ritl, Stephen), 20070123871 (Jahng, Tae-Ahn), 20070198088 (Biedermann et al.), 20070270860 (Jackson, Roger), 20070282443 (Globerman et al.), 20080021466 (Shadduck et al.), 20080045951 (Fanger et al.), 20080154307 (Colleran et al.), 20080177317 (Jackson, Roger), 20080221620 (Krause, William), 20080269904 (Voorhies, R), and 20080312693 (Trautwein et al.).  Potential limitations with this method include: inconsistency of motion control or mechanical failure due to repeated flexing of metal components; it may cause multi-directional movement without the ability to selectively control the degree or range of movement in different directions; it may not provide sufficient vertical support of the spinal column; it may not be adjustable before implantation to meet the specific needs and features of different patients; it may not be adjustable after implantation to refine therapy or accommodate patient growth; and it may cause scarring, pinching, or other damage to nearby soft tissue.

4.    Flexible Member with Flexible Inelastic Members Inside

This method uses a flexible member (such as a flexible cylinder or spring) that contains flexible, but inelastic members inside (such as inelastic wires, fibers, or cables) to connect vertebrae outside the intervertebral space.  Devices and methods that appear to use this approach include U.S. Patent Application 20070270821 (Trieu et al.).  Potential limitations with this method include: it may be prone to material failure due to repeated flexing of materials or components; the inelastic members may undesirably restrict natural spine movement in one or more directions (flexion, extension, lateral bending, and torsion); it may not provide sufficient vertical support of the spinal column; it may not be adjustable after implantation to refine therapy or accommodate patient growth; and it may cause multi-directional movement without the ability to selectively control the degree or range of movement in different directions (flexion, extension, lateral bending, and torsion).

5.    Flexible Member with Rigid Rod(s) Inside

This method uses a flexible member (such as a flexible cylinder or spring) that contains one or more rigid rod-like members to connect vertebrae outside the intervertebral space. For example, the device may be a flexible tube with multiple channels into which may be inserted rods with different shapes or degrees of flexibility, thereby customizing the shape and flexibility of the device.  Devices and methods that appear to use this approach include: U.S. Patent Applications 20040049190 (Biedermann et al.), 20040143264 (McAfee, Paul), 20040215191 (Kitchen, Michael), 20080125777 (Veldman et al.), 20080319486 (Hestad et al.), and 20090012562 (Hestad et al.).  Potential limitations with this method include: it may be prone to mechanical or material failure due to repeated flexing of materials or components (especially if the inner rigid members are thin); it may require an expensive array of parts (such as multiple size and shape rigid inserts); it may undesirably restrict natural spine movement in one or more directions (flexion, extension, lateral bending, and torsion); it may cause immobility of a section of the spine which causes greater stress, unnatural movement, and degeneration of adjacent portions of the spine ("transition syndrome"); and it may not be adjustable after implantation to refine therapy or accommodate patient growth.

6.    Non-Contiguous Rigid Segments Connected by Flexible Member(s)

This method uses non-contiguous rigid segments (such as plastic/metal cylinders/ovals) whose only connection is one or more flexible members (such as wires, cords, or cables) in order to connect vertebrae outside the intervertebral space. Devices and methods that appear to use this approach include: U.S. Patents 6,296,643 (Hopf et al.), 6,299,613 (Ogilvie etal.), 6,616,669 (Ogilvie etal.), 7,083,621 (Shaolian etal.), and 7,326,210 (Jahng et al.)and U.S. Patent Applications 20040167520 (Zucherman et al.), 20050065516 (Jahng, Tae-Ahn), 20050124991 (Jahng, Tae-ahn), 20050149020 (Jahng, Tae-Ahn), 20050203514 (Jahng et al.), 20050245929 (Winslow et al.), 20060265077 (Zwirkoski, Paul), 20070123871 (Jahng, Tae-Ahn), 20070233075 (Dawson, John), and 20080114357 (Allard et al.).  Potential limitations with this method include: lack of solid or multiple-tensile connections between the rigid segments makes it very difficult to control the amount of flexion, lateral bending, or longitudinal support; it may be prone to mechanical or material failure due to repeated flexing, especially at the junctions between rigid and flexible members; it may not be adjustable to meet the specific needs and features of different patients; it may not be adjustable after implantation to refine therapy or accommodate patient growth; it may involve implants that are subject to slipping, shifting, or extrusion; and it may cause scarring, pinching, or other damage to nearby soft tissue.

7.    Contiguous Rigid Segments Connected by a Central Flexible Member

This method uses contiguous rigid segments (such as plastic or metal cylinders) that are connected through their centers by a flexible member (such as a wire, cord, or cable) to connect vertebrae outside the intervertebral space.  Devices and methods appear to use this approach include: U.S. Patents 6,290,700 (Schmotzer), 7,083,621 (Shaolian etal.), and 7,326,210 (Jahng et al.)and U.S. Patent Applications 20050065516 (Jahng, Tae-Ahn), 20050124991 (Jahng, Tae-ahn), 20050149020 (Jahng, Tae-Ahn), 20050203514 (Jahng et al.), 20060265077 (Zwirkoski, Paul.), 20070123871 (Jahng, Tae-Ahn), 20070288011 (Logan, Joseph), and 20080269904 (Voorhies, R).  Potential limitations with this method include: the central location of the connecting flexible member provides poor leverage and control over flexion and lateral bending; also, due to the central location of the connecting flexible member, changes in tension of the connecting member do not control the direction of curvature of the multi-segment structure; also due to the central location of the connecting flexible member, it is difficult to have rounded contiguous connections such as a ball-and-socket joint that would otherwise help to avoid mechanical failure; and it may cause scarring, pinching, or other damage to nearby soft tissue.

8.    Contiguous Rigid Segments Connected by One Type of Non-Central Member

This method uses contiguous rigid segments (such as plastic or metal cylinders) that are connected outside their centers by one type of flexible member (such as wires, cords, or cables) to connect vertebrae outside the intervertebral space. This method is new and there appear to be few, if any, devices or methods that currently use this approach.

9.    Telescoping Rigid Members with Springs

This method features telescoping rigid components (such as telescoping concentric hollow cylinders) with springs or other flexible members, inside or outside those telescoping components, to connect vertebrae outside the intervertebral space. Devices and methods that appear to use this approach include: U.S. Patents 5,480,401 (Navas), 5,540,688 (Navas), 7,361,196 (Fallin etal.) and U.S. Patent Applications 20050171543 (Timm et al.), 20050177156 (Timm et al.), 20050288672 (Ferree, Bret), 20060036256 (Carl et al.), 20060036259 (Carl et al.), 20060036324 (Sachs et al.), 20060084983 (Kim, Daniel), 20060084985 (Kim, Daniel), 20060084988 (Kim, Daniel), 20060085069 (Kim, Daniel), 20060085070 (Kim, Daniel), 20060247637 (Colleran et al.), 20070161991 (Altarac et al.), 20070173832 (Tebbe et al.), 20080097441 (Hayes et al.), 20080154307 (Colleran et al.), and 20080177317 (Jackson, Roger).  Potential limitations with this method include: when the telescoping members are directly connected to vertebrae and are largely-parallel to the longitudinal axis of the spine, then these members can restrict natural flexion and lateral bending movement of the spine; it may be prone to mechanical or material failure due to repeated flexing of materials or components; it may be difficult to adjust to customize the device for different patients; it may be difficult to adjust over time to refine treatment or respond to patient growth; when there are multiple telescoping members (such as several for each intervertebral span) that are each individually connected to the vertebrae using screws, then these multiple intrusions can further stress the structural integrity of vertebrae that are already weakened by injury or disease; and it may cause scarring, pinching, or other damage to nearby soft tissue.

10.    Telescoping Rigid Members with a Flowable Substance Inside

This method features hydraulic or pneumatic telescoping members (such as concentric hollow cylinders) with a flowable substance (such as a liquid or a gas) inside to connect vertebrae, including some applications for artificial discs inside the intervertebral space. Devices and methods that appear to use this approach include: U.S. Patents 4,932,975 (Main et al.), 5,375,823 (Navas), and 6,835,207 (Zacouto etal.) and U.S. Patent Applications 20040152972 (Hunter, Mark), 20060085073 (Raiszadeh, Kamshad), 20060085074 (Raiszadeh, Kamshad), 20070173855 (Winn et al.), and 20080288073 (Renganath et al.).  Most of the devices related to telescoping members involve artificial discs in the intervertebral space. Intervention in the intervertebral space is often insufficient; stabilization is often required outside (alone or in combination) the intervertebral space.  With respect to stabilization outside the intervertebral space using telescoping members, most current devices appear to involve direct attachment of telescoping members to the vertebrae. Further, most of the current or proposed devices appear to involve one or two relatively-large telescoping members per vertebral space spanned.

There are potential limitations with these applications of telescoping members in the related art. First, directly attaching one or two telescoping members to the vertebrae restricts natural flexion and bending, especially if the telescoping members are parallel to the longitudinal axis of the spine. The telescoping action of concentric cylinders only occurs in a straight line; it allows straight-line contraction or extension, but not flexion or bending.  Second, if you use a large number of telescoping members (ten, for example) to create an angled configuration that allows flexion and bending, but you connect each member directly to the bone (creating ten holes for ten screws, for example), then you can weaken vertebrae that are already weakened by injury or disease. Third, multiple telescoping members that are individually attached to the vertebrae form an irregularly-shaped moving structure that is difficult to isolate from body tissue and fluids with a protective barrier.

11.    Telescoping Rigid Members with Gears or Other Components

This method features telescoping rigid components (such as concentric hollow cylinders) moved by gears or other methods that are neither springs nor flowable substances to connect vertebrae outside the intervertebral space.  Devices and methods that appear to use this approach include: U.S. Patent Applications 20060004447 (Mastrorio et al.), 20060247637 (Colleran et al.), 20070233098 (Mastrorio et al.), 20070282443 (Globerman et al.), 20070288011 (Logan, Joseph), 20080045951 (Fanger et al.), and 20080269904 (Voorhies, R).  In addition to the limitations associated with telescoping rigid members filled with a flowable substance discussed above, rigid members with gears or other methods may be prone to mechanical failure due to repeated movement of materials or components.

12.    Integrated Configurations of Differentially-Flexible Materials

This method features (generally-solid) structures with integrated configurations of differentially-flexible materials to connect vertebrae outside the intervertebral space. For example, they may be a relatively-solid composite rod with a rigid core and flexible outer layer, or vice versa.  Devices and methods that appear to use this approach include: U.S. Patent 7,326,210 (Jahng et al.) and U.S. Patent Applications 20050065516 (Jahng, Tae-Ahn), 20050149020 (Jahng, Tae-Ahn), 20070293862 (Jackson, Roger), 20080177317 (Jackson, Roger), 20080177388 (Patterson et al.), 20080319486 (Hestad et al.), and 20090012562 (Hestad et al.).  This broadly-defined method can have a very wide range of possible designs, so it is difficult to pin down specific advantages and limitations. Due to this design flexibility, this method has considerable upside potential, but is also vulnerable to almost all of the potential limitations that listed above. It may be particularly vulnerable to mechanical or material failure due to shearing of the differentially-flexible materials within a solid member with repeated movement over time. It may also undesirably restrict natural spine movement if the rigid material is dominant or fail to offer sufficient motion control if the flexible material is dominant. It also may be difficult to adjust after implantation to refine therapy or accommodate patient growth.

Disclaimer: Classification of devices and methods into the general method categories listed above is subjective and is subject to change and/or correction.  The potential limitations discussed in the context of each general method category may not apply to particular devices and methods that have been classified into a given category.   Products based on Orthonex technology are not yet approved by the FDA and are not currently available for patient care.  The information provided by this website does not constitute medical advice and should not be used for medical decision making.