WO2012036872A2

WO2012036872A2 - Methods and apparatuses for spinal fusion

Info

Publication number: WO2012036872A2
Application number: PCT/US2011/049289
Authority: WO
Inventors: Michael Macmillan
Original assignee: University Of Florida Research Foundation, Inc.
Priority date: 2010-09-16
Filing date: 2011-08-26
Publication date: 2012-03-22
Also published as: WO2012036872A3

Abstract

In one embodiment, a method for fusing the spine includes forming a passage through the ala of the sacrum to access the lumbosacral disc, and passing a spacer through the passage and into the lumbosacral disc space.

Description

METHODS AND APPARATUSES FOR SPINAL FUSION CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to co-pending U.S. Provisional Application serial number 61/383,416, filed September 16, 2010, which is hereby incorporated by reference herein in its entirety. BACKGROUND

Each vertebra of the spine comprises a generally cylindrical, columnar member known as the vertebral body. The tops and bottoms of the vertebral body are generally planar, bony plates known as endplates. Between the endplates of adjacent vertebra are fibrous intervertebral discs that connect to the top endplate of an inferior vertebral body and the bottom endplate of a superior vertebral body.

A variety of pathologic spinal problems require fusion of vertebrae. Fusion typically is achieved by removing the fibrous tissue of the intervertebral disc and facilitating growth of bone that bridges the distance from one vertebral body to another. Presently, the most common spinal fusion method involves first creating an opening in the outer covering of the intervertebral disc called the annulus. The annulus circumferentially encloses the intervertebral disc and can be surgically entered from many different approaches. Once the opening is created in the annulus, the fibrous tissue within the disc can be removed, the endplates can be scraped to stimulate bone growth, and a spacer can be placed between the endplates to stabilize and separate the vertebrae. Traditionally, the disc annulus is accessed from the patient's back. Unfortunately, such access can be undesirably traumatic due to the large amount of muscle and nerves that are disrupted during the procedure. In view of this trauma, surgeons have begun using a lateral approach in which the surgeon approaches the disc through the patient's flank. Although that approach is less traumatic to the patient, it is difficult if not impossible to access the lowest intervertebral disc, the lumbosacral disc (i.e., the L5-S1 disc), using a lateral approach because the illium of the pelvis obstructs such access. This is unfortunate because the lumbosacral disc is one of the most problematic discs for many patients and is often a good candidate for spinal fusion (i.e., fusion of the L5 vertebrae and the sacrum).

Because of the importance of being able to access the lumbosacral disc, surgeons have investigated alternative approaches. In one such approach, the lumbosacral disc is accessed through an abdominal incision. In another approach, the disc is accessed through an incision near the rectum. In both cases, the procedure is highly invasive and is unnecessarily disruptive to the tissues through which the surgeon must pass to reach the disc.

In view of the above discussion, in can be appreciated that it would be desirable to have an alternative method of accessing the lumbosacral disc for the purpose of fusing the L5 vertebrae and the sacrum.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed methods and apparatuses can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale. Fig. 1 is a partial lateral view of a patient's spine and pelvis that illustrates an example pathway to the lumbosacral disc.

Fig. 2 is a partial anterior view of the patient's spine and pelvis and the example pathway to the lumbosacral disc identified in Fig. 1.

Fig. 3 is a partial posterior view of the patient's spine and pelvis and the example pathway to the lumbosacral disc identified in Fig. 1.

Figs. 4A-4J are posterior views of the patient's sacrum, L5 vertebra, and lumbosacral disc that illustrate various steps in an embodiment of a method for accessing the lumbosacral disc and implanting a spacer within the disc space.

Fig. 5 is a perspective view of a first embodiment of a drilling guide tool that can be used to form guide holes through the sacrum.

Fig. 6 is a perspective view of a second embodiment of a drilling guide tool that can be used to form guide holes through the sacrum.

Fig. 7 is a lateral view of the patient's sacrum, L5 vertebra, and lumbosacral disc, illustrating guide holes drilled into the auricular surface of the sacrum.

Fig. 8 is a perspective view of an embodiment of a chisel that can be used to form a channel through the ala of the sacrum that leads to the lumbosacral disc.

Fig. 9 is a perspective view of an embodiment of a sleeve that can be inserted into the channel formed through the ala.

Fig. 10 is a perspective view of a first embodiment of a spacer that can be implanted into the lumbosacral disc space.

Fig. 11 is a perspective view of a second embodiment of a spacer that can be implanted into the lumbosacral disc space.

Fig. 12 is a side view of the spacer of Fig. 1.

Fig. 13 is a top view of the spacer of Fig. 11. Fig. 14 is a top view of an example arrangement of two spacers of the type illustrated in Fig. 1 1.

DETAILED DESCRIPTION

As described above, it would be desirable to have a method for accessing the lumbosacral disc that is less traumatic or invasive than known methods. Disclosed herein are methods and associated apparatuses with which the lumbosacral disc can be accessed through the sacrum. Specifically, the disc can be accessed by forming a channel through ala of the sacrum so as to use a transalar approach. In some embodiments, a passage is first formed through the illium to provide access to the auricular surface of the sacrum. A channel having dimensions similar to that of a spacer to be implanted in the lumbosacral disc space is then formed through the ala of the sacrum and the bottom of the lumbosacral disc. In some embodiments, the channel is rectangular and is formed by first drilling multiple guide holes into the sacrum using a guide tool specifically designed for that purpose, and then completing the channel using a rectangular chisel.

Once the channel has been formed, a sacrum sleeve can be placed inside the channel to maintain the shape of the channel and avoid unnecessary damage to the cancellous bone within the sacrum. After fibrous tissue within the disc space has been removed, one or more spacers are implanted within the disc space. In some embodiments, each spacer has a rounded bull-nose that facilitates its insertion into the disc space from below the disc. In some embodiments, two such spacers are implanted within the disc space and together generally form a V-shape with an apex that points toward the patient's abdomen. In the discussions that follow, various embodiments are described. It is to be understood that those embodiments are merely example embodiments of the disclosed inventions and that many other embodiments are possible. The present disclosure is intended to extend to all such embodiments.

Figs. 1-3 generally identify an example transalar path that can be traversed to access the lumbosacral disc (i.e., the L5-S1 disc). As is shown in those figures, the lumbosacral disc 0 can be said to be accessed along a path 12 that extends from a point A on the posterior surface of the illium 14, through the auricular surface 16 of the sacrum 18, through the ala 20 of the sacrum, to a point B within the lumbosacral disc. As is shown most clearly in Fig. 1 , the path 12 comprises a forward component (i.e., from the posterior to the anterior of the body). As is shown most clearly in Figs. 2 and 3, the path 12 also comprises a lateral component (i.e., from the side toward the center of the body). Accordingly, traversal of the path 12 from point A to point B may be described as diagonally forward and upward through the ala 20 of the sacrum 18.

Figs. 4A-4J illustrate various steps performed in an example method for forming a passage along the path 12 to the lumbosacral disc, and implanting spacers within the disc space using that passage. In those figures, portions of the vertebrae and sacrum (e.g., various bony processes) have been omitted from the drawings for purposes of clarity in explaining the method.

Beginning with Fig. 4A, a rigid guidewire 22 is passed through the ilium 14 of the pelvis, through ala 20 of the sacrum 18, through the bony endplate underneath the lumbosacral disc 0, and into disc space to define the center of the passage that is to be formed. In some embodiments, the guidewire 22 comprises a drill bit tip and is drilled along the desired path using an electric drill to form an elongated hole. In other embodiments, the hole can be drilled using a separate drill bit and the guidewire 22 can be passed through the hole. In either case, the passage of the guidewire 22 or drill bit through the pelvis and sacrum can be performed with the assistance of computer guidance to ensure the desired path is followed (e.g., the path 12 depicted in Figs. 1 - 3). By way of example, the guidewire 22 is made of steel and has an outer diameter of approximately 2 mm to 3.5 millimeters (mm).

Once the guidewire 22 has been positioned within the pelvis and sacrum 18 as shown in Fig. 4A, a passage 24 can be formed through the illium 14 to provide access to the auricular surface 6 of the sacrum 18. By way of example, the passage 24 can be a generally cylindrical passage having a diameter of approximately 12 mm to 16 mm and can be formed using a cannulated drill (not shown) that is passed over the guidewire 22. As is illustrated in Fig. 4B, a guide tube 26 can be positioned within the passage 24 to maintain clear access to the auricular surface 16. In some embodiments, the guide tube 26 comprises a hollow (see Figs. 5 and 6) cylindrical steel tube having an outer diameter that is similar to that of the diameter of the passage 24 (i.e., approximately 12 mm to 16 mm). Once positioned within the passage 24, the longitudinal central axis of the guide tube 26 is generally coincident with the guidewire 22. Once in position, guide tube 26 provides a window to the auricular surface 16 through which the channel to the disc 10 can be formed.

Next, a passage, having dimensions similar to that of a spacer to be implanted in the lumbosacral disc space can be formed through the ala 20 of the sacrum 18. In some embodiments, the passage is a rectangular channel and is formed by first drilling a plurality of guide holes through the ala 20 that form a pattern or outline of what will be the rectangular cross-section of the channel. Fig. 5 depicts an embodiment of a drilling guide tool 28 that serves as a drilling template for forming the guide holes. As is shown in Fig. 5, the guide tool 28 is dimensioned so that it can be passed through the guide tube 26, which is positioned within the ilium 14 of the pelvis (see also Fig. 4C).

In the embodiment of Fig. 5, the drilling guide tool 28 comprises a frame member 30 in the form of an elongated hollow rectangular tube having a rectangular cross-section defined by short sides 32 and long sides 34. In some embodiments, short sides 32 are approximately 8 mm to 12 mm long and long sides 34 are approximately 10 mm to 16 mm long. Irrespective of their lengths, the sides 32, 34 together define as interior space having four 90° inner corners. As is shown in Fig. 5, the frame member 30 supports a plurality of drill tubes 36 that are positioned at various points along the inner periphery (inner walls) of the frame member so that the tubes generally form the outline of a rectangle having dimensions similar to that of the rectangular cross-section of the frame member (and of the rectangular channel to be formed). In some embodiments, the frame member 30 and the drill tubes 36 are made of steel and the tubes are secured to the inner walls of the frame member by welding, by adhesive, or by appropriate mechanical means.

In the example embodiment of Fig. 5, the guide tool 28 comprises six drill tubes 36 with one tube provided in each inner corner of the frame member 30 and one tube positioned at a halfway point along both of the long sides 34 of the frame member. It is noted, however, that such a configuration is merely exemplary and that the tool 28 could include a fewer or a greater number of drill tubes 36. For example, four tubes 36 could be provided, one tube being positioned at each inner corner of the frame member 30. As another example, eight drill tubes 36 can be provided, one tube being positioned at each inner corner of the frame member 30 and one tube being positioned halfway along the length of each side 32, 34 of the frame member. Such an embodiment is illustrated in Fig. 6. Generally speaking, the greater the number of drill tubes 36, the greater the number of guide holes that can be formed and the more easily the rectangular channel can be created. In some embodiments, each drill tube 36 has an inner diameter of approximately 2 mm to 3.5 mm so as to be able to receive a drill bit having similar dimensions.

As is further shown in Figs. 5 and 6, the drilling guide tool 28 comprises a central guidewire tube 38 that is aligned with a central longitudinal axis of the frame member and adapted to receive the guidewire 22 to help align the guide tool with the desired path and ensure that the guide holes that are formed using the tool are likewise aligned with the desired path. When the guide tool 28 is passed over the guidewire 22 (with the guidewire passing through the guidewire tube 38), the distal end 40 of the frame member 30 can be positioned against the auricular surface 16 of the sacrum 18. Drill bits 42 can then be separately or simultaneously passed through each of the drill tubes 36 and guide holes can be drilled through the ala 20 in the pattern defined by the positions and number of the drill tubes. The drilling of one such hole is depicted in Fig. 4C in which a drill bit 42 is shown drilled approximately halfway through the ala 20. Fig. 7 illustrates a pattern of guide holes 44 that can be formed through the auricular surface 16 using the drilling guide tool 28 shown in Fig. 5. As is shown in Fig. 7, the guide holes 44 form a pattern of perforations in the outline of the rectangular channel that is to be formed. That pattern extends from the auricular surface 16, through the ala 20, through the top surface of the endplate 46 below the lumbosacral disc 10, and into the disc space, as is depicted in Fig. 4D.

Once the desired guide holes 44 have been drilled, the drilling guide tool 28 can be removed from the guide tube 26 and a rectangular channel can be formed through the ala 20 of the sacrum 18. In some embodiments, the channel is formed using a rectangular impact chisel. Fig. 4E and Fig. 8 illustrate an example embodiment of such a chisel 48. As is shown in Fig. 8, the chisel 48 is formed as an elongated hollow rectangular tube having a rectangular cross-section defined by short sides 50 and long sides 52. Within the interior space 54 of the chisel 48 is a removable central tube 56 that is aligned with a central longitudinal axis of the space and adapted to receive and pass over the guidewire 22. The distal end of the chisel has a sharp beveled tip 58 designed to cut through and break apart bone. In some embodiments, the beveled tip 58 can be slightly concave so as to bow slightly inward along the two long sides 52 of the chisel 48.

When the chisel 48 is used, portions of sacral bone, including soft cancellous bone, are separated from the remainder of the pelvis and are retained within the interior space 54 of the chisel. When the chisel 48 is removed from the sacrum, those portions of bone are likewise removed from the sacrum and can be stored for later implantation within the disc space. Once the channel has been formed, the guidewire 22 can also be removed from the sacrum.

Fig. 4F shows the rectangular channel 60 after it has been formed. As is shown in that figure, the channel 60 extends from the auricular surface 16, through the ala 20, through the endplate 46, and to the lumbosacral disc 10. Once the channel 60 has been formed, a rectangular sacrum sleeve 62 can be inserted into the channel to maintain its shape and dimensions, and to prevent unnecessary damage to the cancellous bone within the sacrum 18 during the remainder of the procedure. Fig. 4F shows the sleeve 62 in place within the channel 60.

Fig. 9 illustrates an example embodiment for the sacrum sleeve 62. As is shown in that figure, the sleeve 62 is formed as an elongated hollow rectangular tube having a rectangular cross-section defined by, short sides 64 and long sides 66. The sleeve 62 includes a beveled distal end 68. As is indicated in Fig. 4F, the beveled distal end 68 is angled so as to be generally parallel to the top surface of the endplate 46 of the sacrum 18 just below the lumbosacral disc 10 after the sleeve 62 has been inserted through the channel 60. In some embodiments, the beveled distal end 68 forms an angle Θ with a bottom edge 70 of the sleeve 62 of approximately 25° to 35°. If necessary, multiple rectangular cross-sectioned members (e.g., other sleeves) of increasing size can be sequentially inserted into the channel 60 to dilate the cancellous bone to the point at which the sleeve 62 can fit within the channel. By way of example, the sleeve 62 is made of steel.

Once the sacrum sleeve 62 has been put into position as shown in Fig. 4F, the fibrous contents of the lumbosacral disc 10 can be removed through the sleeve using appropriate tools, such as scraping, grasping, and suction tools. At this point, an at least partially at least partially open disc space is formed that is adapted to receive one or more spacers.

Fig. 10 illustrates an example embodiment of one such spacer 72. The spacer 72 can be made from any one of several different materials. By way of example, the spacer 72 is uniformily made from a single piece of a polymeric material, metal, or allograft bone. As is shown in Fig. 10, the spacer 72 comprises a body 74 having a generally rectangular cross-section defined by orthogonal sides including a top side 76, a bottom side 78, and opposed lateral sides 80. The length of the spacer 72 is defined by a front end 82 and a rear end 84. The front end 82 is angled inward from the bottom side 78 to the top side 76 of the body 74 and is rounded so as to define a bull-nose tip that assists in implantation of the spacer 72, as described below. In some embodiments, the bull-nose tip can form an angle φ with the bottom side 78 of the body 74 of approximately 45° to 60°. In some embodiments, the tip is slightly convex. In contrast to the front end 82, the rear end 84 of the body 74 can be generally perpendicular to the top, bottom, and lateral sides 76, 78, and 80. In some embodiments, the bull-nose tip comprises approximately 20% to 35% of the length of the spacer body 74, and the spacer 72 is approximately 25 mm to 45 mm long, approximately 8 mm to 16 mm tall, and approximately 12 mm to 18 mm wide (in the orientation of Fig. 10). In some cases, the width of the spacer 72 is at least half of the height of the spacer, and the length of the spacer is at least twice the height of the spacer.

As is further depicted in Fig. 10, the spacer 72 can comprise a vertical passage 86 that extends from the top side 76 to the bottom side 78 of the body 74. In some embodiments, the passage is rectangular, as depicted in Fig. 10. Irrespective of its shape, the passage 86 facilitates ingrowth of tissue (e.g., bone) into the spacer 72 to ensure fixation of the spacer within the disc space. In addition, lateral openings 88 can be provided in the lateral sides 80 of the body 74 that lead to the passage 86 and further facilitate ingrowth of tissue.

Figs. 1 1 -13 illustrate an alternative embodiment of a spacer 90 that can be implanted within the disc space. The spacer 90 is similar in many ways to the spacer 72. Therefore, the spacer 90 can be uniformily made from a single piece of a polymeric material, metal, or allograft bone and comprises a generally rectangular body 92 defined by orthogonal sides including a top side 94, a bottom side 96, and opposed lateral sides 98. The length of the spacer 90 is defined by a front end 100 and a rear end 102, and the front end is rounded and angled inward from the bottom side 96 to the top side 94 of the body 92 so as to define a bull-nose tip that assists in implantation of the spacer. With reference to the side view of Fig. 12, the bull-nose tip can form an angle a with the bottom side 96 of the body 92 of approximately 45° to 60°. In some embodiments, the tip is slightly convex. Unlike the bull-nose tip of the spacer 72, the bull-nose tip of the spacer 90 is also angled or tapered in the lateral direction, as is most clear in the top view of Fig. 13. As is shown in that figure, the bull-nose tip is laterally tapered so as to form an angle β with a lateral side 98 of the spacer body 92. In some embodiments, the angle β is approximately 30° to 60°. As is described below, the taper, when provided, facilitates the fitting of two spacers 90 within the disc space.

Like the rear end 84 of the spacer 72, the rear end 102 of the body 92 can be generally perpendicular to the top, bottom, and lateral sides 94, 96, and 98. In some embodiments, the bull-nose tip comprises approximately 20% to 35% of the length of the spacer body 93, and the spacer 90 is approximately 25 mm to 45 mm long, approximately 8 to 16mm tall, and approximately 10 mm to 16 mm wide (in the orientation of Fig. 1 1 ). In some cases, the width of the spacer 90 is at least half of the height of the spacer, and the length of the spacer is at least twice the height of the pacer.

As is further depicted in Figs. 11 and 13, the spacer 90 can comprise a vertical passage 04 that extends from the top side 94 to the bottom side 96 of the body 92. In some embodiments, the passage 104 is rectangular. Irrespective of its shape, the passage 104 facilitates ingrowth of tissue (e.g., bone) into the spacer 92 to ensure fixation of the spacer within the disc space. In addition, lateral openings 106 can be provided in the lateral sides 98 of the body 92 that lead to the passage 104 to further facilitate ingrowth of tissue.

Figs. 4G and 4H illustrate, as an example, passage of the spacer 72 through the sacrum sleeve 62, and entry of the spacer into the now open disc space 1 10, respectively. As indicated in those figures, the spacer 72, whose dimensions can be similar to the inner dimensions of the sleeve 62, can be pushed through the sleeve using an appropriate push rod 1 12 or other narrow elongated member. Once the spacer 72 reaches the end of the sleeve 62, it can be passed through the endplate 46 of the sacrum 18 and into the disc space 1 10. As is depicted in Fig. 4H, the bull-nose tip of the spacer 72 facilitates such insertion. Specifically, the rounded and angled surface of the tip prevents the tip from snagging on the bottom endplate of the superior vertebra (i.e., the L5 vertebra) and further ensures that there is room for passage of the spacer into the disc space. Therefore, the tip of the spacer 72 can slide across that endplate and adopt a generally horizontal orientation illustrated in Fig. 4I.

With reference to Fig. 4J, the same procedure described above in relation to Figs. 4A-4I can be performed on the opposite side of the sacrum 18 to implant a further spacer 72 within the disc space 1 10. Once the two spacers 72 have been implanted within the disc space 1 10, biological material, such as the excavated cancellous bone removed from the sacrum 18, can be pushed into the disc space to encourage ingrowth of bone. In addition, the L5 vertebra can be fixed to the sacrum, for example using pedicle screws. Over time, bone will grow within the disc space 1 10 and the L5 vertebra will eventually fuse to the sacrum 18.

In embodiments in which tapered spacers, such as spacers 90, are used, the two spacers can together form a V-shape whose apex is pointed toward the abdomen. An example of such a V-shape is illustrated in Fig. 14. As is apparent from that figure, the V-shape is enabled by the tapers of the bull-nose tips of the two spacers 90. In particular, the two spacers 90 have opposite tapers that enable the spacers to be positioned close together to form a relatively sharp apex 1 14 between them. The taper also facilitates the fitting of both spacers 90 within the disc space 1 10.

Various surgical instruments and implants have been described in the above disclosure. It is noted that, in some embodiments, two or more of those instruments and/or implants can be provided together in a fusing system. For example, a prepackaged spinal fusion kit can be provided that includes two or more of the following components: a guidewire, a guide tube, a drilling guide tool, a drill bit, a chisel, a sacrum sleeve, a spacer, a scraping tool, and a push rod. In such a case, each of the instruments and/or spacers would be sized and configured for use with each other, and possibly for a particular type and/or size of patient.

Claims

Claimed are:

1 . A method for fusing the spine, the method comprising:

forming a passage through the ala of the sacrum to access the lumbosacral disc;

passing a spacer through the passage; and

implanting the spacer within disc.

2. The method of claim 1 , wherein forming a passage comprises forming a rectangular channel through the ala.

3. The method of claim 2, wherein forming a rectangular channel comprises first forming guide holes in a rectangular pattern through the ala. 4. The method of claim 3, wherein forming a rectangular passage further comprises chiseling out the bone within the rectangular pattern of guide holes.

5. The method of claim 1 , further comprising forming a window through the illium of the pelvis to access the auricular surface of the ala before forming the passage.

6. A drilling guide tool comprising:

an elongated rectangular tube defining an interior space;

a plurality of drill tubes provided within the interior space, each drill tube being adapted to receive a drill bit; and a central guidewire tube positioned along a central longitudinal axis of the rectangular tube, the guidewire tube being adapted to pass over a rigid guidewire.

7. The guide tool of claim 6, wherein the rectangular tube comprises short sides and long sides that together define inner corners of the tube and wherein a drill tube is provided in each inner corner.

8. The guide tool of claim 6, wherein the drill tubes are arranged in a rectangular pattern within the interior space.

9. The guide tool of claim 6, wherein a drill tube is provided along at least one of the sides between two comer drill tubes.

10. A spinal fusion kit for use in fusing the L5 vertebra to the sacrum, the kit comprising:

a drilling guide tool adapted to guide a drill bit in forming a rectangular pattern of guide holes extending from the auricular surface of the sacrum to the lumbosacral disc;

a chisel adapted to, guided by the guide holes formed through the sacrum, form a rectangular channel that extends from the auricular surface to the lumbosacral disc; and

a sacrum sleeve adapted to fit within the rectangular channel and extend from the auricular surface to the lumbosacral disc.

1 1. The kit of claim 10, wherein the drilling guide tool comprises an elongated rectangular tube defining an interior space and a plurality of drill tubes provided within the interior space, each drill tube being adapted to receive a drill bit, and a central guidewire tube positioned along a central longitudinal axis of the rectangular tube, the guidewire tube being adapted to pass over a rigid guidewire.

12. The kit of claim , wherein the drill tubes of the drilling guide tool are arranged in a rectangular pattern within the interior space of the rectangular tube. 13. The kit of claim 10, wherein the chisel comprises a tube having a rectangular cross-section.

14. The kit of claim 10, wherein the sacrum sleeve comprises a tube having a rectangular cross-section. 5. The kit of claim 10, further comprising a rigid guidewire adapted to extend from the auricular surface to the lumbosacral disc and to guide both the drilling guide tool and the chisel. 16. The kit of claim 10, further comprising a guide tube adapted to be positioned within a passage formed through the ilium of the pelvis, the guide tube being sized such that the drilling guide tube can pass through the guide tube.

17. The kit of claim 10, further comprising a spacer that can be passed through the sacrum sleeve and implanted within the lumbosacral disc.

18. The kit of claim 17, further comprising a push rod adapted to push the spacer through the sacrum sleeve. 19. A spacer adapted for implantation within an intervertebral disc, the spacer comprising:

a generally rectangular body having a plurality of orthogonal sides; and an angled bull-nose tip. 20. The spacer of claim 19, wherein a width of the body is at least twice a height of the body.

21. The spacer of claim 19, wherein a length of the body is at least twice a height of the body.

22. The spacer of claim 19, wherein the angled bull-nose tip is approximately 20 to 35 percent of a length of the body.

23. The spacer of claim 19, wherein the bull-nose tip forms an angle of approximately 45 to 60 degrees with a bottom side of the body.

24. The spacer of claim 19, wherein the bull-nose tip is convex.

25. The spacer of claim 19, wherein the bull-nose tip is laterally tapered so as to form an angle of approximately 30 to 60 degrees with a lateral side of the body.

26. The spacer of claim 19, wherein the body comprises at least one passage or opening that facilitates ingrowth of biological tissue.