US 20030004573 A1
A bone joining implant includes a tubular body. The tubular body has an axially extending outer surface defining an outer dimension of substantially uniform cross-section and including a smooth leading insertion portion and a bone engaging trailing portion.
 This Preliminary Amendment accompanies a request for a continuation application Examination of the claims in this Preliminary Amendment is requested. This application is a continuation application of U.S. patent application Ser. No. 09/609,129. Original claims 1-24 filed in the original patent application have been cancelled. New claims 25-33 have been added. Thus. claims 25-33 remain in this application for consideration.
 Attached hereto for the convenience of the Examiner is a courtesy copy of the Letter Submitting Formal Drawings and drawings as filed concurrently herewith. These drawings were submitted and approved in the parent application.
 This disclosure relates to surgical joining of bone bodies, and more particularly to instruments, implants and methods for self-alignment, instant fixation and staged bone fusion or arthrodesis of bone bodies, such as spinal vertebrae.
 This invention was specifically developed for the surgical joining of bone bodies, such as the fusing of contiguous spinal vertebrae so as to stabilize and prevent relative motion often resulting from a degenerative disc condition. Although the immediate effort leading to this disclosure is directed toward the lumbar, thoracic and cervical spine (anterior or posterior in approach), the described vertebral implants for immediate fixation and staged stabilization leading to arthrodesis (bone fusion) of bone bodies may be used in a bone fracture or osteotomy to fuse together resulting bone bodies, and across one or more joints or articulations. Furthermore, the implants may be used in the lumbar, thoracic and cervical spine.
 The use of fixation plates and screws to hold together disunited bone bodies has long been known to facilitate arthrodesis or bone-to-bone union, such as bone fusion, and healing of fractured bones. Typically, the separate bone bodies are formed when a single bone fractures, requiring bone reunion. Plates are secured across a fracture region with screws, joining together the bone bodies. The plates hold the bone bodies together in proximate relation, facilitating bone growth and fusion therebetween. In this manner, the bone bodies are supported in close proximity, or in direct contact which facilitates fusion therebetween. However, these techniques are not practical for certain joints such as joints formed between spinal vertebrae. Therefore, a significant number of stabilizing implants have been designed for joining together contiguous vertebrae.
 One early technique for achieving arthrodesis between adjacent vertebrae across a joint or articulation is the well-known Cloward Technique for use in the human cervical spine. A solitary dowel of bone is tapped into place in a prepared circular bed that is smaller than the dowel of bone. The dowel acts as a wedge, distracting the surrounding soft tissues of the joint, and separating the bone bodies or vertebrae joined there along. The intervertebral disc substantially comprises the soft tissues of the joint. The dowel of bone is inserted, or wedged into place, providing its own stability by putting an annulus of the disc on stretch. Additionally, simple friction of the inserted dowel between adjacent vertebral bodies stabilizes axial dislocation. However, a second surgical procedure must be performed to extract or harvest the dowel of bone, substantially adding trauma to the procedure, increasing costs, as well as increasing the threat of infection to the patient. Alternatively, bank bone from human donors can be used, but bank bone is less osteogenic and may introduce infection, or even transmission of Acquired Immune Deficiency Syndrome (AIDS) or hepatitis. Furthermore, bone morphogenic protein, hydroxy apatite, or other bone stimulating material may be utilized. Additionally, there has been a need to ensure the implant remains axially secured which has lead to further developments.
 A step forward from the Cloward Technique was provided by Bagby (U.S. Pat. No. 4,501,269) wherein a metal dowel uses the same principle. A perforated cylindrical hollow implant is inserted between prepared surfaces across a vertebral joint. The inserted implant immediately stabilizes the joint by spreading the bony surfaces apart in wedged opposition to surrounding tissue. This initial stabilization is more substantial because a metal dowel, unlike a bone dowel, will not be absorbed or fatigue in use. Over time, fusion occurs through and around the implant which is filled with bone fragments. Use of the metal dowel eliminates the need for a second operation to harvest a dowel of bone. Bone fragments to be inserted in the implant are retrieved during preparation of the circular beds in each vertebra. Furthermore, such a metal implant avoids the disadvantage of having to use bone bank to obtain donor bone. The Bagby implant described in U.S. Pat. No. 4,501,269 has a smooth outer surface, interrupted only by numerous openings or fenestrations through which bone ingrowth and through growth can occur. Ends of the implant are substantially closed, with one end receiving an end cap such that bone fragments are contained therein. Bone morsels or bone grafts are typically harvested when preparing the circular bed in each vertebra, after which they are placed into the fenestrated metal cylindrical implant. The Bagby implant is then driven or tapped into place in a manner similar to the placement of Cloward's Bone Dowel, which was solely directed for use in the cervical spine. However, the original Bagby implant relies completely upon stretch of the annulus to stabilize the vertebrae during bone remodeling and fusion.
 Improvements have also been made to “Cloward's Technique” wherein two dowel bone grafts are posteriorly inserted (Wiltberger's Technique) between adjacent lumbar vertebral bodies. Furthermore, threaded surfaces have been added to such bone grafts in order to keep the grafts in place (Otero-Vich German Application No. 3,505,567, published Jun. 5, 1986). More recently, a number of U.S. Patents have proposed combining the threaded features from threaded bone grafts with a metal implant, resulting in rigid threaded implant structures for placement between adjacent spinal vertebrae.
 One threaded metal fusion implant disclosed in Michelson (U.S. Pat. No. 5,015,247) provides a cylindrical fusion implant having an outer diameter sized larger than the space between adjacent vertebrae to be fused. Threads provided on the exterior of the member engage the vertebrae to axially secure the implant therebetween. The implant has a plurality of openings configured along the cylindrical surface to promote bone ingrowth. However, the threads per se of the implant do not function as a fastener to fix together the adjacent vertebral bodies. Instead, the implant functions as a wedge, imparting a distraction force across the disc which stabilizes the articulation formed therebetween by stretching the annulus of the disc. In fact, the threaded implant relies solely on the annulus to provide stabilization between the vertebrae, in direct response to wedge-induced distraction created therebetween. Distraction of the annulus stabilizes the two vertebrae, enabling ingrowth to later occur within the implant. Therefore, through-growth and fusion (arthrodesis) occur between the adjacent vertebrae subsequent thereto depending on the immobilizing potential of an intact healthy annulus which may or may not be present.
 Several additional problems result from the provision of threads on a cylindrical fusion implant. One significant problem with threaded metal fusion implants is that it is very difficult to thread the implant into alignment with prepared bone beds in adjacent vertebral bodies. In practice, such alignment can prove difficult, and the consequences of misalignment can detrimentally affect the ability to achieve fusion between the vertebral bodies and the ability to subsequently achieve arthrodesis. Aligned placement of such an implant is likely to lead to a higher incidence of arthrodesis. Additionally, for cases where a fusion implant does not have a physical retention mechanism for retaining the implant between bone beds, such implant may not be sufficiently mobilized to prevent movement. Such movement will also detrimentally affect the successful incidence of arthrodesis. Yet another problem results in that threads take up additional space which can make implantation in areas having limited anatomical space very difficult, such as in the posterior approach in the lumbar spine. Additionally, the threads effectively make the wall thickness greater which further separates bone provided inside the implant with bone provided outside the implant, which can delay initial bone union.
 For bone fusion to occur with any of the above devices, the invasion of new delicate blood vessels from the adjacent healthy bone is necessary for the creation of new living interconnecting bone. Where complete stabilization does not occur instantaneously upon implantation, motion can disrupt the in growth of delicate blood vessels. Disruption of the vessels then restricts or even prevents bone healing therebetween. The same problem occurs with any of the above mentioned implant techniques, including the threaded techniques of Otero-Vich and Michelson. Even when the annulus is completely on stretch, the threads per se of these constructions do not function in the manner of conventional screws, extending through one object and into another. Namely, they do not function to fasten together adjacent bodies by coaction of the implant with each body. For example, the threads merely act as a series of ridges that engage with each adjacent bone body, while the implant body functions as a wedge. The implant distracts apart the vertebral bodies which stretches the annulus, and stabilizes the articulation as a consequence thereof, while the thread functions solely to prevent axial dislodgement. Furthermore, the presence of threads requires the implant to be screwed in place via a torquing process, instead of tapping the implant directly into position.
 Hence, some recent designs have resulted in an implant that produces immediate fixation per se between bone bodies following insertion and independent of the annulus. Such designs show promise particularly for cases where the annulus structure is substantially or completely weakened or damaged at surgery. Where the annulus is damaged so significantly as to lose structural integrity, the wedge-effect of prior art threaded implants will not produce any distraction forces across the annulus. Also, when the implant is used to arthrodese and change angulation, a healthy annulus cannot be totally corralled to be placed on stretch. As a result, there is no form of stabilization or fastening between bone bodies sufficient to enable the occurrence of arthrodesis therebetween when the annulus is weakened or inadequate. Additionally, there exist additional shortcomings with such recent designs as discussed below.
 One such design that produces immediate fixation is disclosed in Bagby (U.S. Pat. No. 5,709,683) as a bone joining implant having a spline or undercut portion that engages in assembly with each bone body to be joined. However, such design requires the preparation of bone beds that are engaged in interlocking relation with a bone bed engaging portion provided by such undercut portions.
 Many of the previously described implants can be inserted between vertebrae while such vertebrae are distracted with a distraction tool. One such tool uses a threaded pin which is inserted laterally into each bone body, with such pins being rigidly secured therein. Such tool distracts the vertebrae by separating the pins and vertebrae which stretches the annulus. A drill is then used to drill out bone beds within the vertebrae, after which the implant is inserted therein. However, such procedure does not always impart sufficient distraction and takes time and space to implement. Therefore, techniques that provide further distraction are desired.
 For the case of vertebral inner body implants which lack the presence of any external threads, the implant is typically tapped into place between bone beds prepared in adjacent vertebral bodies. However, complete tapping of such an implant extending in an anterior to a posterior direction can be somewhat risky as the leading end of the implant is the spinal cord. Accordingly, improvements are desired to minimize any risks resulting from completely tapping an implant into place between pairs of adjacent vertebral bodies.
 Therefore, there is a present need to provide an implant device that more accurately aligns itself with prepared bone beds between bone bodies upon implantation, enhances arthrodesis by encouraging bony fusion adjacent the implant, and ensures retention between adjacent bone bodies during insertion. There is also a need to provide such a device that facilitates accurate aligned placement and staged stabilization leading to bone fusion, in a manner that is relatively simple, more reliable, less complicated, has fewer parts, and leads to quicker and more thorough bone fusion and remodeling therebetween. The final stage of bone fusion through and around the implant substantially eliminates any need for the implant to maintain the fusion, thus allowing the bone union to provide primary support therebetween.
 A self-aligning, self-fixating, and self-distracting vertebral fusion device is disclosed according to four distinct embodiments. Although not necessary, an additional feature is provided by less than all of the embodiments which encompasses bone joining features that entrap bone projections to instantly fix adjacent bone bodies together, such as instantly fixing adjacent vertebral bodies via the implant.
 According to one aspect of the invention, a bone joining implant includes a tubular body. The tubular body has an axially extending outer surface defining an outer dimension of substantially uniform cross-section and including a smooth leading insertion portion and a bone engaging trailing portion.
 According to another aspect of the invention, a vertebral fusion device includes a perforated fusion body. The perforated fusion body has an insertion portion with an axially extending uniform cross-sectional dimension adjacent a leading end and a bone fixating trailing portion adjacent a trailing end.
 According to a third aspect of the invention, a vertebral fusion implant includes an elongate, axially extending fusion body. The fusion body includes an insertion portion having an axially extending uniform cross-section and a threaded trailing portion provided at a trailing end of the fusion body. The insertion portion self-aligns the fusion body with bone beds of adjacent vertebrae during implantation. The threaded trailing portion self-fixates the fusion body between the bone beds.
 According to a fourth aspect of the invention, a bone fusion device includes an axial extending body. The axial extending body has a cylindrical leading end portion communicating with a threaded trailing end portion. The threaded trailing end portion includes at least one thread segment extending radially outwardly of an outermost surface of the cylindrical leading end portion.
 Preferred embodiments of the invention are described below with reference to the following accompanying drawings.
FIG. 1 is a perspective view of a vertebral structure showing a vertebral interbody implant embodying this invention;
FIG. 2 is a perspective view of a pair of adjacent vertebrae and illustrating a drill guide and drill bit used to prepare a bore that initiates preparation of bone beds within the vertebrae;
FIG. 3 is a perspective view of the pair of vertebrae of FIG. 2, and illustrating a first hole saw used with the drill guide to cut a cylindrical kerf about the bore in order to further prepare the bone beds within the vertebrae;
FIG. 4 is a simplified side view illustrating a second hole saw of FIG. 3 used to further cut a further widened cylindrical kerf within the pair of vertebrae;
FIG. 5 is a perspective view of a kerf cleaning/deburring tool for further widening the kerf produced in FIG. 4 and cleaning debris from the cylindrical kerf formed within the vertebral bodies;
FIG. 6 is a perspective view taken from the leading, insertion end of the vertebral interbody implant of FIG. 1 for insertion within the prepared bone beds of FIG. 5;
FIG. 7 is a perspective view taken from the driven end of the vertebral interbody implant of FIG. 6;
FIG. 9 is a side view of the vertebral interbody implant of FIGS. 6 and 7;
FIG. 8 is a leading end view of the vertebral interbody implant of FIGS. 6-8;
FIG. 10 is a driven end view of the vertebral interbody implant of FIGS. 6-9;
FIG. 11 is an unrolled plan view of the outer peripheral surface of the vertebral interbody implant of FIGS. 6-10;
FIG. 12 is sectional view taken along line 12-12 of FIG. 11 further illustrating the smooth leading end and the threaded, retaining trailing 11 end of the vertebral interbody implant of FIGS. 6-11.
FIG. 13 a perspective view illustrating an implant insertion tool usable for inserting and threading the implant of FIGS. 12-16 within the prepared bone beds of FIG. 5;
FIG. 14 is a simplified frontal view illustrating a pair of vertebrae that have bone beds prepared according to the steps depicted in FIGS. 2-5 comprising a cylindrical kerf;
FIG. 15 is a simplified frontal view illustrating the vertebrae of FIG. 14 in an instantly fixed and slightly distracted position caused by inserting the implant of FIGS. 6-10 within the bone beds of FIG. 5;
FIG. 16 is a simplified, sagittal and centerline view of the implant of FIGS. 6-13 prior to insertion.
FIG. 17 is a simplified, sagittal and centerline view of the implant of FIGS. 6-13 during insertion.
FIG. 18 is a simplified, sagittal and centerline view of the implant of FIGS. 6-13 after insertion;
FIG. 19 is a surgical time simplified sagittal view of the implant of FIG. 18 received within the prepared bone beds of adjacent vertebrae and containing bone fragments immediately following implantation;
FIG. 20 is a healed time simplified sagittal view of the implant of FIG. 19 received within the prepared bone beds of adjacent vertebrae and illustrating the vertebra following bone remodeling and reorganization and 11 showing arthrodesis;
FIG. 21 is a coronal view of the implant and healed bone comprising vertebrae and taken along line 21-21 of FIG. 20 and showing arthrodesis;
FIG. 22 is a perspective view of an alternatively constructed vertebral interbody implant similar to the embodiment depicted in FIGS. 1-21 for insertion within prepared bone beds formed solely by generating a bore as shown in FIG. 2; and
FIG. 23 is a perspective view of an alternatively constructed vertebral interbody implant similar to the embodiment depicted in FIGS. 1-21 for insertion within prepared bone beds formed solely by generating a bore as shown in FIG. 2.
FIG. 24 is a perspective view of an alternatively constructed vertebral interbody implant similar to the embodiment depicted in FIGS. 1-21 for insertion within prepared bone beds formed solely by generating a bore as shown in FIG. 2.
 This disclosure of the invention is submitted in furtherance of the constitutional purposes of the U.S. Patent Laws “to promote the progress of science and useful arts” (Article 1, Section 8).
 Reference will now be made to a preferred embodiment of Applicant's invention. Four exemplary implementations are described below and depicted with reference to the drawings comprising various self-aligning and self-fixating bone joining implants. A first embodiment is shown and described below in a first mounting configuration with reference generally to FIGS. 1-21. Second through fourth embodiments are shown and described below with reference to FIGS. 22-24, respectively. While the invention is described by way of several preferred embodiments, it is understood that the description is not intended to limit the invention to these embodiments, but is intended to cover alternatives, equivalents, and modifications which may be broader than these embodiments such as are defined within the scope of the appended claims.
 In an effort to prevent obscuring the invention at hand, only details germane to implementing the invention will be described in great detail, with presently understood peripheral details being incorporated by reference, as needed, as being presently understood in the art.
 A preferred embodiment bone joining implant in accordance with the invention is first described with reference to FIGS. 1, 6-13 and 15-21. Such an implant is further described below with respect to a cylindrical, open-ended vertebral inter-body implant having self-aligning and self-fixating features. Additionally, an optional feature includes a leading open end in the form of a cylindrical inner surface that entraps bone projections, and a tapered leading end that provides limited self-distraction. The self-aligning and self-fixating implant is designated in FIGS. 1, 6-13 and 15-21 generally with reference numeral 10.
 A first alternative implementation comprising a cylindrical vertebral inter-body implant similar to implant 10 above, but with a pair of end flanges and a central bulkhead flange, is depicted in FIG. 22. A second alternative implementation comprising a tubular, rectangular cross-section implant with a plurality of retaining tabs provided about a trailing surface portion is depicted in FIG. 23. A third alternative implementation comprising a cylindrical vertebral inter-body implant with a smooth leading outer surface portion and a plurality of retaining tabs provided about a trailing surface portion is depicted in FIG. 24.
 As shown in FIGS. 1, 6-13 and 15-21, implant 10 comprises a rigid, unitary body having a cylindrical leading edge 60 and a cylindrical outer surface 66, with an open leading end 80 (see FIGS. 6-10). As shown in FIG. 1, implant 10 is inserted within an aperture 18 that has been prepared between a pair of adjacent vertebral bodies 12 and 14 within a vertebral column 16.
 As shown in FIG. 1, aperture 18 is prepared within vertebral bodies 12 and 14, and along disc 16, according to the procedure and tools depicted in FIGS. 5-11 and described below in further detail. Aperture 18 forms a pair of vertebral bone bodies 22 and 24 that are formed to have a cylindrical configuration comprising a cylindrical kerf 158 (see FIG. 9). A leading cylindrical end of implant 10 is inserted into aperture 18, causing vertebral bodies 12 and 14 to be instantly fixed together (see FIGS. 14-18 below). An open leading end 80 (see FIG. 6) of implant 10 entraps an intact living bone projection 54 and 56 on each respective vertebral body (see FIGS. 14-18) which imparts immediate fixation between adjacent vertebral bodies 12 and 14 upon implantation.
 More particularly, vertebrae 12 and 14 comprise neighboring bone bodies of a vertebral column 16 (see FIG. 1). A resilient articulation or joint is formed between vertebra 12 and 14 by disc 16 extending between vertebrae 12 and 14. Anatomically, the disc is made up of a central nucleus pulposus and an outer encircling annulus. The annulus and nucleus pulposus are composed of laminae of fibrous tissue and fibro-cartilage. The nucleus pulposus, located at the center of the disc, comprises a soft, pulpy, and highly elastic substance. The annulus is formed from laminae of fibrous tissue extending in a criss-crossing fashion to encircle the nucleus pulposus. Additionally, the intervertebral disc is adherent, by its cephalad and caudad surfaces, to a thin layer of hyaline cartilage that covers the top and bottom surfaces of adjacent vertebrae. In a healthy patient, adjacent vertebra 12 and 14 are spaced apart by disc 16. However, degenerative disc disease and localized trauma can cause degradation or complete loss of the soft tissue components between neighboring vertebrae. For example, the annulus can partially or completely tear which can seriously degrade the structural condition of the articulation. Additionally, fluid can escape from the nucleus pulposus. When any of the above happens, vertebrae 12 and 14, loaded by the normal weight bearing of a patient, are is pressed into closer adjoining positions, which can result in pinching of nerves that extend from between vertebrae of the spinal column (not shown).
 Therefore, there is a need to recover the disc spacing provided by a normal healthy disc 20 by way of inserting implant 10. Furthermore, there is a need to provide implant 10 with a fixation that aligns implant 10 during insertion and instantly interlocks adjacent vertebra 12 and 14 together upon being implanted. Furthermore, there is a need for such an implant 10 that retains itself in place upon insertion, and that facilitates staged stabilization resulting in arthrodesis to occur between the vertebral bodies, following initial implantation. Even furthermore, there is a need to instantly fix adjacent vertebrae together since relative motion can otherwise cause pinching of nerve tissue.
 As a result, implant 10 can be inserted, preferably in a central location between adjacent vertebrae 12 and 14 of patients who have bad, ruptured or degenerative discs. Furthermore, implant 10 can be axially oriented anterior to posterior, or even laterally.
 In summary, implant 10 is adapted for implantation between prepared bony surfaces or beds 22 and 24 and across the articulation formed by disc 20. A typical implantation might involve placement of one or more implants 10 as required in order to stabilize and fix the joint during bone ingrowth and through-growth of the implant structure. Bone growth is also accomplished outside of and surrounding the implant.
 Alternatively, a pair of somewhat smaller sized and laterally adjacent implants can also be used. However, such dual implant implementation uses individual implants that are sized smaller than the single implant 10 of FIG. 1. As a result, such dual implant implementation uses smaller sized apertures which do not invade as much cancellous bone as the aperture 18 (see FIG. 1) prepared for receiving the larger sized single implant implementation depicted in FIG. 1.
 A solitary implant 10 as shown in FIG. 1 invades cancellous bone since aperture 18 has a larger diameter. In contrast, smaller sized dual implants tend to invade mostly cortical bone on the end plates. However, cancellous bone is more desirable for bone growth during staged bony fusion since cancellous bone is more osteogenic than cortical bone. New growth bone, or callus bone, comprises soft cancellous bone that only becomes hard (cortical) over time via action of Wolff's Law of maturity.
 Applicant's implant depicted in FIGS. 6-10 generates a limited amount of self-distraction during insertion between a pair of vertebral bodies due to tapered portion 64. Such feature provides an additional desirable benefit.
 FIGS. 2-5 illustrate the various steps used to prepare aperture 18 and bone beds 22 and 24 within vertebral bodies 12 and 14, respectively (of FIG. 1). Such figures illustrate one technique for preparing a suitable pair of bone beds within adjacent vertebrae 12 and 14 for receiving implant 10 (of FIG. 1) such that self-alignment, self-fixation, self-distraction and immediate fixation are imparted between vertebral bodies 12 and 14.
FIG. 2 depicts a tool guide 26 and a drill bit 34 that are used to drill a bore 38 (see FIGS. 3-5) into vertebral bodies 12 and 14 and disc 20. Bore 38 is drilled partially into bodies 12 and 14 so as to leave sufficient intact living bone to create bone projections 54 and 56
FIG. 17 illustrates implant 10 after smooth insertion portion 72 has been received within bore 38, but prior to engaging interlocking trailing portion 72 therein. Additionally, FIG. 17 illustrates a substantial portion of disc 20 removed, or resected, prior to insertion of implant 10 so as to facilitate arthrodesis as discussed below.