Intervertebral spacer that dynamically promotes bone growth

Abstract

A dynamic intervertebral spacer includes a ring which is split on an anterior portion. A posterior portion of the ring acts as a torsion spring. After implantation, the ring is able to act as a spring between superior and inferior vertebral bodies, thus allowing dynamic bone growth in fusion procedures.

Claims

1. A dynamic intervertebral spacer comprising: a monolithic ring having an anterior portion, a posterior portion, a first lateral portion, a second lateral portion opposite the first lateral portion, and an open center portion; wherein the ring is split in the anterior portion and superior and inferior surfaces on a first side thereof are vertically offset from superior and inferior surfaces on a second side thereof; wherein the posterior portion of the ring is configured to act as a torsion spring to allow the vertical offset of the first and second lateral portions to decrease under load on the ring; and wherein the first and second sides of the ring each have at least one bone screw, wherein the bone screw(s) on one side are configured to attach to a superior vertebral body and the bone screw(s) on the another side are configured to attach to an inferior vertebral body.

2. The spacer as in claim 1, wherein the ring has first and second opposed faces at the split which move with respect to one another under load on the ring from adjacent vertebral bodies.

3. The spacer as in claim 2, wherein the opposed faces are planar.

4. The spacer as in claim 2, wherein the vertical offset resists the compression with a spring force in the range from 20 N/mm to 40000 N/mm.

5. The spacer as in claim 1, wherein the monolithic ring is configured to elastically resist flexion as a patient's spine goes through flexion and extension.

6. The spacer as in claim 5, wherein the monolithic body comprises a polymer.

7. The spacer as in claim 6, wherein the polymer is selected from the group consisting of polyether ether ketones (PEEK), polyaryl ether ketones (PAEK), and their composites, such as carbon fiber reinforced or with radiopaque compounds.

8. The spacer as in claim 5, wherein the monolithic body comprises a metal.

9. The spacer as in claim 8, wherein the metal is selected from the group consisting of titanium, and its alloys such as nitinol, cobalt chrome molybdenum and variants.

10. The spacer as in claim 1, wherein the vertical offset is in the range from 0.05 mm to 3.0 mm.

11. The spacer as in claim 1, wherein the superior surface has a convex geometry.

12. The spacer as in claim 1, wherein the ring includes an open center space extending from the superior to the inferior surface configured to receive a bone graft material.

13. The spacer as in claim 1, wherein a first portion of the anterior portion connected to the first lateral portion has the bone screw and a second portion of the anterior portion connected to the second lateral portion has another bone screw.

14. The spacer as in claim 1, wherein at least a part of the posterior portion has an attachment feature on at least one of the superior and anterior surfaces thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The novel features of the invention are set forth with particularity in the appended claims. A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description that sets forth illustrative embodiments, in which the principles of the invention are utilized, and the accompanying drawings of which:

(2) FIGS. 1A and 1B illustrate a first embodiment of a dynamic intervertebral spacer constructed in accordance with the principles of the present invention. FIG. 1A shows the dynamic intervertebral spacer oriented with the anterior portion downward to expose the superior surface of the spacer. FIG. 1B is a front or anterior elevation view showing the vertical offset of the right and left sides of the spacer.

(3) FIGS. 2A through 2C are alternative views of the dynamic intervertebral spacer of FIGS. 1A and 1B shown with the bone anchoring screws removed.

(4) FIGS. 3A and 3B illustrate a first alternative design of a dynamic intervertebral spacer of the present invention.

(5) FIGS. 4A and 4B illustrate a second alternative design for an intervertebral spacer of the present invention.

(6) FIGS. 5A-5C illustrate a second alternative design for an intervertebral spacer of the present invention. FIG. 5A is a view of the anterior side of the spacer while FIGS. 5B and 5C are views of the inferior and superior surfaces, respectively.

(7) FIG. 6 is a side or lateral view of the dynamic intervertebral spacer of FIGS. 1A and 1B shown implanted between a superior vertebral body and an inferior vertebral body.

(8) FIG. 7 is a view of the implanted dynamic intervertebral spacer FIG. 6 shown from a front or anterior perspective.

(9) FIG. 8 illustrates the implantation of a dynamic intervertebral spacer in combination with a dynamic bone plate.

DETAILED DESCRIPTION OF THE INVENTION

(10) Referring to FIGS. 1A and 1B, a dynamic intervertebral spacer 10 comprises a ring 12 (which forms a body thereof) having an anterior portion 14, a posterior portion 16, a right lateral portion 18, a left lateral portion 20, and an open center 22. The ring as illustrated has four sides with the posterior portion 16 being slightly wider than the anterior portion 14. The ring will also have a depth in a horizontal or anterior-posterior direction and a thickness in the vertical or superior-inferior direction. Representative dimensions for the exemplary ring for each of three intended implantation locations (lumbar, thoracic, and cervical) are set forth in Table I below.

(11) TABLE-US-00001 TABLE I Anterior Posterior Vertical Width Width Depth Thickness Offset LUM- 30-50 mm 25-45 mm 25-40 mm 8-15 mm .sup.0.1-3 mm BAR THO- 23-36 mm 15-29 mm 17-30 mm 4-9 mm 0.1-2.5 mm RACIC CER- 14-20 mm 14-20 mm 3-8 mm 3-8 mm 0.1-1.75 mm VICAL

(12) The specific geometry and dimensions set forth above are not critical and are meant to be exemplary only. Other geometries, such as circular, oval, triangular, rectangular, polygonal, and the like, may also find use. In all cases, however, there will be at least one break in the ring to form a gap 24 between opposed free ends of the ring. The free ends of the ring will be vertically offset by a small distance, typically in the ranges set forth above in the Summary, in order to allow the spacer to act as a spring when implanted between a lower surface of a superior vertebral body and an upper surface of an inferior vertebral body, will be described in more detail below. Representative vertical offsets are provided in Table I for each of the different implantation regions.

(13) The vertical offset and the dimensions of the spacer will correspond to a particular designed range of motion in the anterior/posterior direction for the spacer. A range of motion of 0.1-2 degrees is selected to achieve fusion quickly while a range of motion 3-6 degrees can be used to gradually transition to fusion over a period of about 2-3 years. A range of motion of 6 degrees or greater can be used to maintain motion without fusion.

(14) Referring now to FIGS. 2A through 2C, the ring 12 of the dynamic intervertebral spacer 10 may optionally be modified to promote bone ingrowth over certain selected regions thereof. As shown in FIGS. 2A and 2C, a superior bone attachment region 36 may be formed over the right lateral portion 18 and over a right side of the posterior portion 16 of the device. It will be appreciated that these attachment regions on the superior surface are elevated relative to the superior surface left lateral portion 20 and will be in contact with the lower surface of the superior vertebral body when the spacer is implanted. The surface modifications may be features, such as ridges, grooves, and the like, or may alternatively comprise coatings selected to promote bone ingrowth, such as titanium plasma spray or hydroxyapetite. The surface modifications will be in addition to, or in some cases in place of, use of a superior bone attachment screw 26 which is received in an inferior screw hole 52 in the anterior portion of the right lateral portion 18 of the ring.

(15) Surface modifications to promote bone ingrowth may also be provided on the walls of the open center 22 to promote bone attachment through the center of the ring 12 between the vertebrae. Coatings on the wall of the open center 22, such as titanium coatings on a polymer or PEEK ring, will encourage bone to grow through the ring to form a dynamic fusion.

(16) An inferior bone attachment region 38 will typically be formed over the inferior surface of the left lateral portion 20 of the ring 12, as shown in FIG. 2C. The inferior bone attachment region will typically have the same characteristics as the superior bone attachment region 36, and may be used together with or in place of an inferior bone attachment screw 28 which is received through the superior screw hole 50 on an anterior region of the left lateral portion 20.

(17) Referring specifically to FIG. 2B, the right lateral portion 18 and the left lateral portion 20 of the ring 12 are vertically offset to create an offset 46, as best seen in FIG. 2B. Exemplary vertical offsets are set forth in Table I above. It is this differential or offset which allows the ring 12 to act as a spring when implanted between superior and inferior vertebral bodies. In the particular ring design 12, the bending or spring constant of the ring will be defined by the torsional stiffness of the posterior portion 16. That is, the right lateral portion 18 and left lateral portion 20 will act as bending arms connected to the posterior portion 16, were the posterior portion acts as a torsional spring. Particular spring constants have been set forth above. The intervertebral spacer 10 provides compliant support between vertebrae during growth of bone through the spacer and provides a gradual transition from motion to fusion.

(18) Although the intervertebral spacer 10 has been designed with an anterior gap 24 and a posterior portion 16 acting as a torsion spring, the spacer may be configured with one or more gap and one or more torsion spring portions moved to anterior, posterior or lateral locations depending on the desired motion of the spacer.

(19) Referring now to FIGS. 3A and 3B, the gap between the right lateral portion and left lateral portion of the intervertebral spacer of the present invention may take a variety of forms and geometries. In an alternative ring construction 60 of FIGS. 3A and 3B, a right lateral portion 66 and left lateral portion 68 of the ring have a gap 74 which is linear in the anterior-posterior direction but non-linear in the superior-inferior direction. In particular, the gap 74 is has a step defined by an inferior tab 74a and a superior tab 74b which together form an opening which has two vertical portions joined by a horizontal portion. The right lateral portion 66 and left lateral portion 68 are vertically offset relative to each other, where the degree of vertical offset is limited by the tabs which will in turn also limit the degree of extension to prevent excessive extension. The ring 60 will also have an anterior portion 62, a posterior portion 64, and a superior bone attachment region 70 having any of the characteristics previously described as well as a superior region 72 which is free from any bone attachment features. Although not illustrated, the ring 60 will typically also be configured to receive bone attachment screws, and the inferior surface of the ring will also have a bone attachment region on the right lateral portion and region free from bone attachment features on the left lateral portion.

(20) Referring now to FIGS. 4A and 4B, a further alternative ring 80 is similar to the previously described embodiments, but includes a gap region 94 which is linear in the superior to inferior direction and non-linear in the anterior to posterior direction. Particular, the ring 80 has an anterior portion 82, a posterior portion 84, a right lateral portion 86, and a left lateral portion 88. A superior bone attachment region 90 is formed over the right lateral portion 86 which is raised relative to the left lateral portion 88. A superior surface 92 of the left lateral portion 88 is free from bone detachment features. The gap 94 is shown to include two axial lengths in the anterior-to-posterior direction joined by a lateral length in the lateral direction. The use of non-linear gap regions is advantageous as in can help retain the bone graft material in the open centers of the rings.

(21) Referring now to FIGS. 5A-5C, dynamic and intervertebral spaces according to present invention may be formed from one or more ring structures, typically joined in a monolithic or integrated geometry. In particular, ring 100 has an anterior portion 102, a posterior portion 104, a right lateral portion 106, and a left lateral portion 108. In addition, a center region 110 is formed between the right lateral portion and left lateral portion, defining a right open region 112 and a left open region 114. One of the gaps 122 opens into the right open region 112 and the other of the gaps 122 opens into the left open region 114. In this way, the center region 110 is cantilevered from the posterior region 104 and is free to move in the vertical direction relative to both lateral portions 106 and 108. In the illustrated embodiment, the center region 112 is raised relative to the right and left lateral portions 106 and 108 when unconstrained so that, once implanted, a superior surface 120 of the center region 112 will engage the lower surface of an adjacent, superior vertebral body. Conversely, the inferior surfaces of the both the right lateral portion 106 and left lateral portion 108 will contact the superior surface of the inferior vertebral body. A vertical offset remains between the inferior surface of the center portion 110 and the superior surface of the inferior vertebral body, thus allowing the desired dynamic vertical movement of the vertebral bodies to promote bone growth. The ring 103 will typically include bone attachment screws (not shown), including at least one for each lateral region and one for the center region. Additional, bone attachment regions 116 and 118 will typically be formed on the inferior surfaces of the right lateral portion 106 and left lateral portion 108, as shown in FIG. 5B which is a bottom plan view of the ring 100. In contrast, the inferior surface 120 of the center region 110 will be free from such features as bone attachment is not desired. The bone attachment regions on the superior surface of the ring 100 will be arranged opposite to the arrangement on the inferior surface, i.e. the superior surface 121 of the center portion 110 will have bone attachment features while the superior surfaces of the right lateral portion 106 and left lateral portion 108, as shown in FIG. 5C which is a top plan view of the ring 100, are free from such attachment features.

(22) Referring now to FIGS. 6 and 7, implantation of the dynamic spacer 10 of FIGS. 1A, 1B and 2A-2C is illustrated. The posterior portion 14 of the ring 12 is directed toward the patient's posterior while the anterior portion 14 is directed toward the patient's anterior. The gap 24 (best seen in FIG. 7) is thus aligned with the anterior surfaces of the vertebral bodies, allowing movement as the patient's spine experiences flexion and extension.

(23) Referring now to FIG. 8, the dynamic intervertebral spacers of the present invention may be used in combination with other dynamic vertebral stabilization devices, such as a dynamic bone plate 130 which may be implanted after implantation of the dynamic intervertebral spacer 10.

(24) In use, the dynamic intervertebral spacer provides a transition between a full range of motion and complete spinal fusion. For example, if a patient prior to surgery has a natural range of motion of the natural disc in the anterior/posterior direction of about 6 degrees, a dynamic spacer may be implanted having a range of motion of about 3 degrees and the patient's range of motion immediately post-surgery is expected to be about 3 degrees. As the bone of the patient grows into and through the spacer, the range of motion may be decreased further to about 1-2 degrees or less upon complete growth of bone bridging between the two vertebrae. The reduction in range of motion gradually over time can improve patient outcomes and allow the patient's natural anatomy to better accommodate the fusion. Allowing some motion between the vertebrae promotes bone growth and can accelerate the timeframe until complete fusion is achieved. The amount of motion allowed by the dynamic spacer can be selected depending on the anatomy and/or range of motion of the particular patient. A method of selecting a dynamic spacer may include steps of measuring the natural range of motion of the patient at the level of the desired surgery and selecting a dynamic spacer with a range of motion limited to an amount less than the natural range of motion. The selected spacer may have a range of motion of about 2 or more degrees less than the natural range of motion.

(25) A timeframe for transition to complete fusion can vary and depends on the amount of motion. For motion of about 3 degrees to about 6 degrees, the transition to fusion is gradual and is expected to take 1-3 years. For motion of less than 3 degrees, the fusion transition happens more quickly and is expected to take less than two years or less than one year.

(26) In another use, the dynamic intervertebral spacer provides a limited range of motion which continues to provide motion without complete spinal fusion. A method of selecting a dynamic spacer for a non-fusion may include steps of measuring the natural range of motion of the patient at the level of the desired surgery and selecting a dynamic spacer with a range of motion limited to approximately the natural range of motion. The selected spacer may have a range of motion of within about 2 degrees of the natural range of motion.

(27) Modification of the above-described assemblies and methods for carrying out the invention, combinations between different variations as practicable, and variations of aspects of the invention that are obvious to those of skill in the art are intended to be within the scope of the invention disclosure.