Abstract:
A spinal fixation apparatus comprising an upper and lower endplate. Depending distally in a longitudinal direction from the upper endplate is an inner sleeve that fits inside an outer sleeve that depends distally in a longitudinal direction from the lower endplate. The inner sleeve includes grooves located on its outer surface. A finger cage including resilient fingers fits around the outer sleeve. The fingers have ends that include male grooves. The ends of the fingers fit into apertures located in the outer sleeve. The apparatus further includes a locking mechanism that pushes the tips of the fingers through the apertures of the outer sleeve such that they mesh with the grooves of the inner sleeve. When the locking mechanism is unlocked the tips of the fingers disengage from the grooves of the inner sleeve, and the inner sleeve slide axially within the outer cage, adjusting the height of the device.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application Nos. 60/567,619 filed May 3, 2004; 60/567,618 filed May 3, 2004; 60,567,667 filed May 3, 2004; 60/567,554 filed May 3, 2004; 60/567,555 filed May 3, 2004; 60/567,556 filed May 3, 2004; 60/567,564 filed May 3, 2004; the entire disclosures of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to the field of orthopaedic implants. Specifically, the present invention provides a prosthetic implant to replace spinal vertebra and adjacent intervertebral discs. 
   2. Background 
   The vertebral column serves as the main structural support of the human skeleton. The vertebral column consists of a number of vertebrae separated by intervertebral discs. A vertebra approximates a cylindrical shape, with wing-like projections and a bony arch. The arches create a passageway through which the spinal cord runs. The vertebral column is held upright by fibrous bands of muscle and ligament. There are seven vertebrae in the cervical region, twelve in the thoracic region, five in the lumbar region, and five in the sacral region that are usually fused together. The integrity of the vertebral column is critical to protecting the fragile spinal cord, in addition to its duties in supporting the skeleton. 
   When a vertebra or intervertebral disc becomes damaged, either through trauma or disease, the spinal cord or nerve roots may be impinged, causing a great deal of pain. This pain can occur almost anywhere in a person&#39;s body, from the back or neck to the extremities. In such a case, patients often receive drug treatment in an initial attempt to relieve the pain, but if that is unsuccessful, it may be necessary to remove all or a portion of one or more vertebrae, including the anterior cylindrical body, which is the load-bearing portion of intervertebral discs. When this procedure is undertaken, it necessarily weakens the support structure of the spinal column, and the excised material must be replaced with some load-bearing material. Given the high propensity of spinal discs to degenerate in human beings, the need for an efficient and stable method of spinal fixation is tremendous. 
   It is possible to replace the excised vertebral bone with a bone graft. The bone graft may be an autograft, normally harvested from the patient&#39;s iliac crest, or, alternatively, may be an allograft—tissue obtained from a human donor. Once the bone graft is in place spanning the bony defect, it fuses over time with the remaining healthy vertebrae. Generally, a metal plate and screws are used to secure the bone graft to the natural vertebrae, providing support until the fusion process is complete. However, given that it generally takes between three and six months for bony fusion to occur, a more substantive prosthetic implant is often used to stabilize the spine, often in combination with bone graft that can grow out of the prosthesis and fuse with the adjacent bone over time. 
   A prosthetic spinal fixation device should accurately replace the height of the excised material, result in acceptable tension levels in the spine, maintain proper curvature of the spine, obtain balance through the spinal segments, and restore normal load-bearing characteristics throughout the spine. 
   Moreover, a prosthetic spinal fixation device should be easily adjustable to allow the surgeon to quickly select the height of the device during surgery to fit the needs of the patient. The desired height of the device will depend on the amount of bone that is removed from the patient, the size of the patient, as well as the location of the removed bone (i.e. cervical region or lumbar region). In addition, a one-size-fits-all device may reduce manufacturing costs because fewer different parts and/or models will be required to meet the needs of the marketplace. 
   While prosthetic corpectomy implants are known in the art, a need exists for improved implants that are more easily adjusted to achieve the necessary height to replace the excised bone during the implantation process, while also possessing the biomechanical properties necessary for long-term implantation in the human body and the immediate fixation ability to provide stability to the spinal column. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention is directed to an artificial corpectomy prosthesis for replacement of one or more spinal vertebrae. The apparatus is adjustable in height through use of a lockable telescoping finger mechanism that allows a surgeon to choose the appropriate height for the prosthesis when performing the corpectomy procedure. The prosthesis comprises a first endplate and a second endplate; a substantially hollow outer member; a substantially hollow inner member; a fixation device; and a locking mechanism. The sleeves are also referred to herein as cages or substantially open cylinders. Either endplate can accommodate the outer sleeve. All that is required is that one sleeve fits inside the other sleeve in a cylindrical enveloping relationship and that the sleeves are moveable with respect to each other. For the sake of brevity and ease of description herein, the outer sleeve will be described as attaching to the second, or lower endplate. Once implanted, bone graft can be packed within slots inside the components of the device, and inside the inner member such that the bone graft can grow into the body and fuse with healthy bone adjacent the prosthesis. 
   The first and second endplates serve as the respective ends of the prosthesis, and fit against the healthy vertebrae adjacent to the removed vertebrae and vertebral discs. Each of the first and second endplates includes a first and a second surface. 
   The first surfaces of each endplate interact with the healthy bone adjacent each end of the prosthesis. One or both endplates may include a feature located on its first surface that encourages and/or facilitates fusion of the prosthesis and the healthy bone. This feature can be in the form of physical protuberances (such as spikes, ridges, keels, knurling, and the like) or chemical or biological treatments such as hydroxyapatite and the like. This improves the fixation characteristics of the prosthesis and lengthens the overall life of the prosthesis. The footprint and angle of the first surface of both the first and second endplates can be varied depending on the location of placement in the spine. 
   Depending distally in a longitudinal direction from the first endplate is an inner member. The inner member has an outer surface that comprises a number of circumferential grooves. The inner member can also include a number of holes or slots spaced longitudinally or radially along its outer surface. 
   Depending distally in a longitudinal direction from the second endplate is a substantially hollow outer member. The inside perimeter of the outer member is large enough that the inner member can fit within the hollow defined by the perimeter of the outer member. The outer member includes one or more locking apertures and preferably also includes one or more ingrowth apertures. When the inner member is inserted into the hollow of the outer member, the locking apertures provide access to the grooves present on the outer surface of the inner member. 
   A finger cage is a hollow body that fits over the outer member. The finger cage includes one or more resilient fingers that are free to bend inwardly or outwardly. The free end of each finger includes one or more tabs. When the finger cage is fitted in place over the outer member, the fingers of the finger cage align with the locking apertures of the outer member such that the tabs of the fingers are inserted through the locking apertures of the outer member and into engagement in the circumferential grooves of the inner member. 
   A locking mechanism comprises a ring that fits around the fingers of the finger cage. The locking mechanism preferably has two positions—locked and unlocked. When in the locked position, the tabs of the fingers are forced through the locking apertures of the finger cage and mesh with the grooves of the inner member. Which grooves of the inner member the tabs of the fingers engage depends upon how far the inner sleeve is inserted into the outer sleeve. Therefore, when the locking mechanism is in the locked position, the inner member is prevented from sliding axially with respect to the outer member, and the height of the apparatus is thus fixed. 
   When the locking mechanism is in the unlocked position, the tabs of the fingers are not forced to mesh with the grooves of the inner member, and the inner member is free to slide axially with respect to the outer member. Therefore, when the locking mechanism is in the unlocked position, the height of the apparatus may be adjusted by the surgeon to fit the needs of the individual patient by sliding the inner sleeve axially with respect to outer sleeve. Preferably, the fingers of the finger cage are resiliently biased in an outward direction. In this manner, the apparatus is adjustable at all times until the locking mechanism is moved into a locked position. Therefore, the grooves, the finger cage, and the locking ring comprise a means for selectively altering and fixing the height of the prosthesis. 
   If it is desired to prevent rotation of the inner member relative to the outer member, it is possible to incorporate a security mechanism between them to be activated after the desired height of the apparatus has been selected. The security mechanism could, as but one example, consist of a key inserted through a slot on the outer mechanism that is aligned with one of the slots or openings of the inner sleeve such that when the key is inserted, the inner sleeve is not free to rotate within the outer sleeve. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description of various exemplary embodiments in conjunction with the accompanying drawings, in which; 
       FIG. 1  is a side view of one embodiment of the present invention; 
       FIG. 2  is a side view of a second embodiment of the present invention; 
       FIG. 3  is a partial perspective view of a finger cage of one embodiment of the present invention; 
       FIG. 4  is a partial perspective view depicting the inner member of one embodiment of the present invention; 
       FIG. 4(A)  is a side view depicting the inner member of one embodiment of the present invention; 
       FIG. 4(B)  is a top view depicting the first endplate of one embodiment of the present invention; 
       FIG. 5  is a partial perspective view depicting a locking ring of one embodiment of the present invention; 
       FIG. 6  is a top view depicting the locking ring shown in  FIG. 5 ; and 
       FIG. 7  is a perspective view depicting an outer cage of one embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   While the present invention will be described more fully hereinafter with reference to the accompanying drawings in which particular embodiments and methods are shown, it is to be understood from the outset that persons of ordinary skill in the art may modify the invention herein described while achieving the functions and results of this invention. Accordingly, the description that follows is to be understood as illustrative and exemplary of specific embodiments within the broad scope of the present invention and not as limiting the scope of the invention. In the following descriptions, like numbers refer to similar features or like elements throughout. 
   Ideally, an artificial corpectomy prosthetic provides the surgeon with an easy adjustment mechanism that allows the length of the prosthetic to be modified during surgery with a minimum of time and effort to fit the needs of the patient. The prosthetic must allow a significant range of adjustment because the size and number of vertebrae removed will vary from patient to patient. 
   Referring now to  FIG. 1 , one embodiment of a prosthetic is shown which includes an adjustable telescoping mechanism that allows the height  100  of the prosthetic to be expanded or contracted during surgery, and also prevents both accidental compression or expansion of the device once implanted in the patient. 
     FIG. 1  shows an embodiment with a short outer cage  30 .  FIGS. 2 and 7  show a second embodiment with a longer outer cage  30 . Referring now to  FIGS. 4 ,  4 A, and  4 B, the apparatus  10  comprises an inner member  20 ; an outer cage  30 ; a finger cage  40 ; and a lock ring  50 . In the embodiment shown, the inner member  20  includes a first endplate  21  that serves as the uppermost surface of the prosthesis and fits against the healthy vertebra adjacent to the uppermost removed bone. The first endplate  21  includes an upper surface  21   a  and a lower surface  21   b . The upper surface  21   a  of the first endplate  21  may include spikes  21   c , ridges, or some other protrusion or surface treatment in order to facilitate fusion between the prosthesis and the adjacent bone, thus providing better fixation of the prosthesis and increasing the life of the implant. In other embodiments, the first endplate  21  also can include an aperture  21   d , preferably located at or near the center of the first endplate  21 . In one embodiment, a cupped insert (not shown) is placed in the aperture  21   d . The purpose of the aperture  21   d , and the cupped insert, will be explained below. 
   Referring now to  FIG. 7 , the outer cage  30  includes a second endplate  31  that serves as the lowermost surface of the prosthesis and fits against the healthy vertebra adjacent to the lowermost removed bone. The second endplate  31  includes a lower surface  31   a  and an upper surface  31   b . The lower surface  31   a  of the second endplate  31  may include spikes  31   c , ridges, or some other protrusion or surface treatment in order to facilitate fusion between the prosthesis and the adjacent bone, thus providing better fixation of the prosthesis and increasing the life of the implant. The second endplate  31  also can include an aperture (not shown), preferably located at or near the center of the second endplate  31 . 
   As can be seen in  FIG. 4 , the inner member  20  further comprises a hollow inner sleeve  22  that depends distally in the longitudinal direction from the lower surface  21   b  of first endplate  21 . The inner sleeve  22  has an outer surface  22   a  that includes a number of circumferential grooves  23 . Preferably, the grooves are separated by thin ridges and are approximately one (1) millimeter wide, though many spacings and numbers are possible. The inner sleeve  22  further comprises a number of slots  24  that are spaced longitudinally along the outer surface  22   a  of the inner sleeve  22 . 
   Referring again to  FIG. 7 , the outer cage  30  further comprises a hollow outer sleeve  32  depending distally in the longitudinal direction from the upper surface  31   b  of second endplate endplate  31 . The outer sleeve has an outer surface  32   a  and an inner surface  32   b . The perimeter of the inner surface  32   b  of outer sleeve  32  is designed such that the inner sleeve  22  can fit within the outer sleeve  32 . When the inner sleeve  22  is in place within the outer sleeve  32 , it is free to slide axially relative to the outer sleeve  32 . The outer sleeve  32  also includes locking apertures  33  that allow access to grooves of inner sleeve  22  when inner sleeve  22  is in place within the outer sleeve  32 . 
   As shown in  FIG. 3 , the finger cage  40  includes a ring  41  that fits over the outer sleeve  32  and rests on a ledge  31   f  that is formed at the intersection of the outer sleeve  32  and the second endplate  31 . Depending distally in a longitudinal direction from the ring  41  are one or more fingers  42 . Preferably, there are a plurality of fingers  42 . Each of the fingers  42  has a first end  42   a  and a second end  42   b . The first end  42   a  is the point of attachment with the ring  41 . The fingers  42  are preferably resilient, allowing the second end  42   b  to bend inward under force toward the center of the ring  41  while maintaining the capability to return to their original position after the force is removed from the finger  42 . Preferably, the fingers  42  are resiliently biased outwardly. 
   Each of the fingers  42  has a tab  43  at its second end  42   b . Each tab  43  has an interior side  43   a  and an exterior side  43   b . The interior side  43   a  of each tab  43  has incrementally spaced male nubs  44 . Each of the exterior sides  43   b  of tabs  43  have locking grooves  45 . 
   When the ring  41  of the finger cage  40  is in place on the ledge  31   f  of the outer cage, the tabs  43  of the fingers  42  align with locking apertures  33  of the outer sleeve  32 . In resting position, with no force applied to the fingers  42 , the tabs  43  are located within the apertures  33 , but are not forced to engage the circumferential grooves  23  of the inner sleeve  22 . However, when inwardly-directed force is applied to the tabs  43  of fingers  42 , the tabs  43  push through the locking apertures  33  of the outer sleeve  32  and the male nubs  44  of tabs  43  meshingly engage with the grooves  23  of the inner sleeve  22 . When the male nubs  44  of finger cage  40  engage the grooves  23  of the inner sleeve  22 , the inner sleeve  22  is locked in place within the outer sleeve  32 , and the height  100  of the apparatus  10  is fixed. 
   To effectuate the locking action, the apparatus  10  includes a locking ring  50  (see  FIGS. 5 and 6 ). The locking ring  50  fits over the second end  42   b  of fingers  42  such that it fits within the locking grooves  45  located on the exterior side  43   b  of tabs  43 . The locking ring  50  rotates about the grooves  45  about the longitudinal axis of the apparatus  10 . Preferably, the locking ring  50  is rotatable between two positions—locked and unlocked. 
   The locking ring  50  has an interior surface  51  and an exterior surface  52 . The interior surface  51  is in contact with the locking groove  45  of the tabs  43 . The interior surface  51  of locking ring  50  is cammed such that the thickness of the locking ring  50  varies over the course of its circumference in a manner allowing the locking ring to be rotated relative to the finger cage  40  between a first locked position and a second unlocked position. Referring to  FIG. 6 , when the locking ring is in a first position within the locking groove  45  of the finger cage  40 , a thinner portion  51   a  of the locking ring  50  is in contact with each of the locking grooves  45  of the exterior side  43   b  of tabs  43 . In this position, the locking ring does not apply inwardly-directed force to the fingers  42 . Therefore, the male nubs  44  of tabs  43  do not mesh with the grooves  23  of the inner sleeve  22 . Conversely, when the locking ring is rotated to a second position, a thicker portion  51   b  of the locking ring  50  is in contact with each of the locking grooves  45  of the exterior side  43   b  of tabs  43 . In the second position, the locking ring  50  squeezes the resilient fingers  42  inward, bringing the male nubs  44  of tabs  43  into contact with the grooves  23  of the inner sleeve  22 . When the locking ring  50  is in the second, or locked, position, the inner sleeve  22  cannot slide axially relative to the outer sleeve  32 . Therefore, the height  100  of the apparatus  10  is fixed. 
   It is also possible to include a secondary locking mechanism, such as a locking screw (not shown), that prevents rotation of the inner sleeve  22  within the outer sleeve  32  after the desired height  100  of the apparatus  10  is determined and the locking ring  50  is rotated to the second, or locked, position. In one embodiment, the outer sleeve  32  may include one or more locking apertures  34 . Once the locking ring  50  is rotated to the locked position, the inner sleeve  22  is rotated within the outer sleeve  32  such that the locking aperture  34  of the outer sleeve  30  is aligned with one of the slots  24  and can receive a screw in one of the holes therein. In another embodiment, the slots  24  of the inner sleeve  22  can align with one or more appurtenances in the inner surface  32   b  so that the appurtenances ride in the slots  24  and prevent rotation. 
   In another embodiment, shown in  FIG. 2 , the ring  41  of the finger cage  40  does not rest on a ledge  31   f  formed at the intersection of the second endplate  31  and the outer sleeve  32 , but instead on a plateau  32   c  that is located on the outer surface  32   a  of outer sleeve  32  (see  FIG. 7 ). Below the plateau  32   c , the outer sleeve  32  extends distally in the longitudinal direction to second endplate  31 . The same finger cage  40  and inner member  20  are used in this embodiment. This embodiment is preferred when it is necessary for the apparatus  10  to replace a large amount of bone because the elongated outer sleeve  32  allows for a significantly taller prosthesis with which the components of the shorter apparatus  10  other than the outer sleeve  32 , such as the finger cage  40  and the inner member  20 , are still compatible. 
   Preferably, the first and second endplates  20 ,  30  include bone openings  5 , allowing bone graft to be packed inside. This allows the bone graft to fuse to the prosthesis, then grow outward into the body after implantation and connect with the healthy bone adjacent to the prosthesis, improving fixation of the device within the body. The outer sleeve  40  and the inner sleeve  50  also preferably include openings  39  to allow more bone graft to be packed inside the hollow of the inner sleeve  50  such that it can grow out of the slots and fuse with adjacent bone. Further, as mentioned above, the first endplate  21  includes an aperture  21   d , while the second endplate  31  can include an aperture. The aperture  21   d  of the first endplate  21  and the aperture of the second endplate  31 , respectively, align with the hollow sections of the inner and outer sleeves  22 ,  32 , respectively. This allows bone graft to be packed inside the hollow sections of the inner and outer sleeves  22 ,  32  and grow out of the apertures  21   d , to fuse with the healthy bone adjacent the apparatus  10 . Further, cupped inserts (not shown) can be placed within the aperture  21   d  of the first endplate  21  and the aperture of the second endplate  31 , respectively. Bone graft can be placed in the cupped inserts, ensuring that bone graft will be in contact with the adjacent healthy bone even if the device is expanded after the bone graft is packed into the hollow sections of the inner and outer sleeves  22 ,  32 . The cupped inserts are preferably slotted, thus still allowing bone graft to be packed in the hollow sections of the inner and outer sleeves  22 ,  32  and grow through the cupped inserts to fuse with the healthy bone. Also, although it is not required, it is preferred that the prosthesis be made of titanium alloy, such as Ti-4Al-6Va. 
   The apparatus  10  is manufactured of materials that are suitable for implantation in the human body. Many such materials are known presently. In the preferred embodiment, the apparatus  10  is made of titanium alloy, such as Ti-4Al-6Va. 
   While there has been described and illustrated particular embodiments of an adjustable corpectomy apparatus, it will be apparent to those skilled in the art that variations and modifications may be possible without deviating from the broad spirit and principle of the present invention, which shall be limited solely by the scope of the claims appended hereto which shall be limited solely by the scope of the claims appended hereto.