Patent Publication Number: US-2006015183-A1

Title: Skeletal reconstruction device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefit of U.S. Provisional Application No. 60/587,072 filed Jul. 9, 2004, and entitled “SKELETAL RECONSTRUCTION DEVICE,” the specification of which is incorporated herein by reference in its entirety. 
    
    
     DESCRIPTION  
      Several embodiments of a novel skeletal reconstruction implant are discussed and illustrated herein. The implant is generally comprised of a mobile portion, plate portions, and a motion limiter portion.  
      The primary embodiments of the implant are useful for spinal column reconstruction; however, the device can also be used for non-spinal orthopedic applications wherever it is desired to join two bones or bone parts. In spinal applications, the mobile portion of the device may be adjusted to assume different angles in the sagittal plane to restore the normal lordosis or kyphosis, and/or in the coronal plane to correct any scoliosis deformations. The device may also be adjusted along the longitudinal axis or transverse plane. Preferably, the mobile portion allows bone surface engagement portions on the plate portions to be oriented at any angle in all planes within the physiological spine curvature. Alternatively, the mobile portion may be restricted by the motion limiter portion to permit only selected motion, for example around a single plane or around a single axis with incremental change. In a preferred embodiment, the motion limiter portion may be formed to prevent or lock out generally all the motion otherwise allowed by the mobile portion. This complete locking mode is particularly useful when a fusion between bone portions is desired.  
      Relative motion occurring at the mobile portion provides for a multitude of possible angular and translatory orientations between a first bone surface engagement portion and a second bone surface engagement portion of the implant. This motion serves to feature the device as a variable angle or multi-positional reconstruction device. In preferred embodiments, this variation in angle or position is adjustable in-situ to match the unique physiological orientation of the bone portions within which the implant is located.  
      For example, if the device is used in the intervertebral space between two vertebrae, the upper plate portion and lower plate portion can be inserted first into the intervertebral space with the mobile portion orientating the bone surface engagement portions to match the unique curvatures of the vertebral bone portions of the patient&#39;s spinal column. The surgeon may then choose a motion limiter to lock the bone surface engagement portions in this orientation. For example, if the device is used in the patient&#39;s spinal column between the 4 th  and 5 th  lumbar vertebrae which has a 5 degree lordotic angle and 0 degree scoliotic angle between vertebral bone portions, the surgeon may choose a motion limiter wedge with these similar angular features to place and lock between the bone plate portions of the device. This motion limiting wedge will then serve to lock the implant in an orientation which matches the physiological spine curvature. Towards this end, the motion limiter may be formed such that it is interchangeable or removable.  
      There are instances when the surgeon may not wish to use a motion limiter to drive the implant to match the physiological space between the bone portions. Alternatively, the motion limiter may be used to drive a predetermined therapeutic angulation or orientation between the bone portions. For example, to reduce a problematic scoliosis, the surgeon may choose a motion limiting wedge which forces a reduced scoliosis. In any event, the bone surface engagement portions of the implant can be adjusted to a variety of orientations by the motion limiter portion.  
      The plate portion of the implant typically serves as a platform to seat features of the implant that interface with the bone portions and with the mobile and/or motion-limiting portions of the implant. The plate portion has a bone surface engagement portion for interfacing with a bone portion. The plate portion may also have a porous surface suitable for tissue in-growth such as bone tissue. Each bone surface engagement portion may include spikes, pegs, keels, or other bosses that protrude into the surface of the bone portions to be fused. The porous surface and protrusions assist in seating the plate portions into the exposed bone portion to prevent motion therebetween and for fixing the plate portion to the bone. For best fit, the bone surface engagement portions of the plate portion may be contoured or shaped to complement the shape of the bone portion with which it will mate. Therefore, the bone surface engagement portion may be flat, may be curved concavely or convexly, or may assume any other shape that complements with the bone portion.  
      A joint plate surface is located on a part of the plate portion that is not directly interfacing with the bone portion. This joint plate surface is shaped to complement or to be a component of the mobile portion of the implant. For example, the joint plate surface may have a concave profile to create a ball joint with a convex joint spacer or with an opposing convex surface formed on a separate plate portion.  
      The plate portions of each device may include apertures or other structure suitable for fasteners such as bone screws, or may include integrated fasteners, to secure the device to the surrounding bone mass of the bone portion.  
      The mobile portion of the implant may form several different kinds of joints. For example, the joint may be an articulating joint, such as a ball and socket joint, a hinge, or other variation of a mating concave-convex joint an elastomeric joint such as an elastomer situated between two plate portions, a pivot joint, a planar joint, or a joint incorporating a liquid or gas filled balloon, or any combination thereof. The motion occurring at the mobile portion provides for a multitude of possible angular and translatory orientations between the bone surface engagement portions of the implant.  
      Forms of the mobile joint may have a profile or configuration including concave, convex, or a combination of concave and convex, joint surface portions formed on each of the plate portions. A spacer portion of the implant, having convex or concave surface portions generally matching the curvature of the plate joint surface portions is formed on surfaces of the spacer portion. With the spacer portion situated between the two plates, two articulating joints are formed. The radii on the mating concave and convex joint surface portions may match or may be mismatched. If the radii are mismatched, the radius of the concave surface is generally larger than the radius of the convex surface, although either radius may be larger than the other. Another joint interface profile comprises a concave inner joint plate surface portion on one of the plate portions and a convex inner joint plate surface portion on the other plate portion. Joining these joint interface profiles forms an articulating joint between the two plates.  
      The motion limiter portion may be used to lock the bone surface engagement portions of the implant in a predetermined orientation or at least restrict their relative movement to a pre-determined range of motion. Depending on the type of motion limiter, it may be inserted or otherwise deployed before, during, or after implantation of the other portions of the device. The motion limiter may be integrated into a portion of the implant such as within the plate portion or spacer portion. It is preferred, however, that the motion limiter be a separate part so it can be applied at the surgeon&#39;s convenience after other portions of the implant are in place and the orientation between bone portions can be reevaluated.  
      The motion limiter may be in the form of a positional stop or lock. The stop portion is formed to block motion by the mobile portion of the implant. The final positional orientation of the bone surface engagement portions of the implant is then determined by the shape and/or position of the stop. The profile or configuration of a stop may vary, with some examples including a shaped insert such as a wedge, a sloped ring, a balloon filled with an incompressible material such as a curable polymer, a spring pin or otherwise deployable pin that springs into a predetermined recess to prevent movement between surfaces, a post or a cam which blocks the space within the mobile portion needed for motion to occur, or a plate which may support the plate portions in an immovable or limited-motion position. Such stops can be fixed or attached or mated to the implant in several different ways. Some examples include wedging, teeth engaging, screwing, snapping, camming, or locking into the implant. The stop may also be housed within the implant so as to not require separate attachment.  
      Another motion limiter or restrictor embodiment uses at least one strut placed in between the plate portions. The struts may be fixed or adjustable in length, and may be locked after a desired length is selected. The strut(s) may be connected to the plate portions to control the angulation of the plate portions and may be connected or otherwise fixed to the plate portions by a variety of methods. For example, screw threads or a ball and socket linkage may connect the strut(s) to the plate portion. If using screw thread, a variable angle screw head is preferred.  
      The motion limiter may also comprise an adhesive or other bonding agent such as calcium phosphate bone cement to lock the mobile portion in a pre-determined position. Such an agent may be used, for example, between the joint surface portions to bond them in fixed relation to each other. Similarly, along with several other biocompatible materials, the motion limiter portion may be made from bone or bone substitutes or other substances that enhance the growth of bone or provide a path for bone growth. One example is recombinant bone morphogenetic protein (BMP). By forming the motion limiter out of a suitable BMP, the material can enhance fusion and act as a motion limiting device. Alternatively, the motion limiter may be made from a collagen-based matrix.  
      The motion limiting portion of the implant may be made from a bioresorbable material such as a resorbable bone substitute or polymer. An example illustrating the benefit of this material occurs when the reconstructive device is used for delayed motion preservation. For example, it is often preferable after spinal reconstructive surgery that there is a period of immobilization at the surgical site during the early stages of healing. Therefore, it is beneficial to have an intervertebral motion preservation device, which is initially locked or limited in motion but will allow increased motion over time as healing progresses. By utilizing the bio-resorbable motion limiter, the reconstructive device can be implanted in a predetermined fixed or locked orientation. However, as the motion limiter is resorbed, the reconstructive device will regain a predetermined amount of motion and serve as a motion preservation device between the vertebral bone portions. Such a device can be used similarly at other joints of the body.  
      As a safety feature, the mobile portion of the implant may be designed to imitate a normal functioning intervertebral disc. Therefore, failure or absence of the motion limiter will not lead to complete implant failure. In this case, and in a backup mode of operation, the implant can adequately serve as a motion preservation device much like the normal human disc in the long or short term or until fusion across the implant occurs.  
      In the event the implant is used for multi-segment vertebral body replacement, each segment may include a mobile portion, so the motion or orientation of one vertebral body to the other at each segment can be adjusted.  
      Portions of the implant may be perforated or otherwise have passages to permit bone to grow into the implant as well as to grow through the implant from one bone portion to another. When present, these passages assist in obtaining optimal fusion between bone portions.  
      The implant can be manufactured from a variety of biocompatible materials. A non-exhaustive list of these materials includes PEEK and other biocompatible polymers, bone or bone substitutes, BMPs, stainless steel alloys, cobalt chrome, titanium and titanium alloys, or combination of these materials. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
      The embodiment in  FIG. 1  shows a mobile portion  150  of implant  100 . The mobile portion  150  is typically located between bone surface engagement portions  110 ,  120  and facilitates motion in at least one plane or around at least one axis between each bone surface engagement portion  110 ,  120 . The motion provided by the mobile portion  150  of the implant  100  enables the bone surface engagement portions  110 ,  120  to orientate congruently to bone portions  160  to maximize the surface contact between the bone surface engagement portions  110 ,  120  and bone portions  160 . Maximizing this surface to surface-contact favors greater boney in-growth into porous surfaces  140  and consequently stronger fixation between the bone surface engagement portions  110 ,  120  and the bone portions  160 . Both the porous surfaces  140  and the bone surface engagement portions  110 ,  120  may be formed on upper and lower plate portions  170  and  180 .  
      In the event the implant forms an articulating joint, the joint can have several different profiles or configurations. One embodiment of a spinal fusion implant  100  according to this invention (note  FIGS. 1 and 2 ) comprises a first bone surface engagement portion  110  and a second bone surface engagement portion  120  for fixation to distinct bone parts. These bone surface engagement portions  110 , 120  are formed on the upper  170  and lower  180  plate portions. Joint plate surfaces  190  are also formed on the upper  170  and lower  180  plate portions, and complementing joint spacer surfaces  200  are formed on a joint spacer  210 . In this embodiment, joint motion may occur bi-modally between each joint plate surface  190  and joint spacer surface  200 .  
      The motion limiter portion  220  in this embodiment comprises an upper limiter  240  and a lower limiter  250 . Although the limiters  240 ,  250  in this embodiment are ring shaped, it is noted that they can be of a multitude of profiles provided they ultimately support or fix or lock the plate portions  170 ,  180  at a desired orientation. As shown, these limiters  240 ,  250  may include a split wall  270 , in this case, to allow elastic compression of the limiter  240 ,  250  before mating a retaining ridge  280  in a receiving groove  260  of the joint spacer  210 . The split wall  270  is not necessary as the limiter  240 ,  250  and the retaining ridge  280  may be sized for a snap-fit into the receiving groove  260 . The limiter  240 ,  250  may utilize many other types of connections such as a threaded connection, or a push and turn bayonet-type of connection. Also in this embodiment, note that the limiters  240 ,  250  may include locking tabs  230  and locking recesses  290 , or other types of bosses or features, to prevent rotation or other movement of the limiters  240 ,  250  relative to other components of the device.  
      The motion limiters  220  are sloped at a preferred angle α ( FIG. 2 ). A kit of this fusion device may include a plurality of interchangeable motion limiters  240 ,  250  each being sloped at a different value for the angle α. Such sloping may occur in more than one plane. The proper combination of sloped motion limiters  220  at varying predetermined angles enables the implant to best match the angle β between the bone portions  160 . A kit may also include other components of the device in a variety of sizes and thicknesses as desired due to varying needs for different patients, and may include instruments for insertion of the implant. The spinal fusion implant  100  may be sized to adhere to all or part of the bone portions  160 . In the case where the device is used for a spinal operation, the device may be sized to occupy all or part of an intervertebral space.  
      In an alternative articulating joint profile, the mobile portion  410  of an implant  400  may not have a spacer. In one such embodiment ( FIG. 3 ), concave  430  and convex  420  joint plate surfaces are formed on an upper plate portion  450  and on a lower plate portion  440  of the implant. Although shown with the concave joint plate surface  430  on the upper plate portion  450 , the joint plate surfaces  420 ,  430  may be reversed so that the convex joint plate surface  420  is formed on the upper plate portion  450  with the concave joint plate surface  430  being on the lower plate portion  440 . Note that, as shown in  FIG. 3A , all plate portions  440 ,  450  may include apertures, slots, or other passageways  580  for bone screws (not shown) or other fasteners for securing the plate portions  440 ,  450  against the bone portions. Note also that apertures, tunnels, or other passageways  580  may be used to promote fusion by opening a path for bone growth through the implant and between the bone portions. Although not shown in  3 A, these passageways  580  may also extend through the joint plate surfaces of the implant. The passageways may be straight or curved, and may vary in diameter.  
      A similar embodiment to that of  FIG. 3  is illustrated in  FIG. 4  with the motion limiter portion  460 . In this embodiment, the motion limiter  460  is shown as two fixation struts  470 , although three fixation struts  470  located at generally three points of an isosceles triangle around the perimeter of the motion portion  410  are preferred. The fixation struts  470  may be either fixed or adjustable in length, as selected by the surgeon. If fixed, the user may select from a variety of lengths of fixed-length fixation struts. These struts may then be positioned within a strut access  480  of the plate portions  440 ,  450 . To ease insertion, the fixation struts  470  may have a bayonet connection between their ends such that each end is pushed together, and a twist of 90 degrees, for example, will lock the fixation strut  470  ends together. It is preferred that the fixation struts  470  have a poly-axial head to articulate within a complementary shaped seat within the plate portions  440 ,  450 . Examples of poly-axial heads are semi-spherical or chamfered head profiles. The fixation seat  490 , on the other hand, may be chamfered, radiused, or have a single line contact formed by having a smaller through diameter the strut access  480  than for a diameter of the fixation seat  490 . In any case, the fixation strut  470  and fixation seat  490  preferably cooperate to firmly support the fixation strut(s)  470  in the desired orientation. Fixation struts  470  that are variable in length can be used without the need for an assortment of fixed length fixation strut  470  sizes. As an example, the variable fixation struts  470  shown in  FIG. 4  may be threaded together and relative rotation between the struts  470  threadably advances one relative to the other to shorten or lengthen their combined length. Similarly, the struts  470  may be in the form of a releasable plate (not shown) spanning between the plate portions  440 ,  450  to hold the plate portions  440 ,  450  in a predetermined orientation.  
      Yet another form of the motion limiter portion  500  is illustrated in  FIGS. 5 &amp; 6  in a preferred embodiment of the reconstructive device. In this embodiment, the upper and lower plate portions  440 ,  450  of the device are stabilized in a pre-determined orientation through the insertion of a positional wedge  540  between the plate portions  440 ,  450 . This wedge  540  may come in a variety of pre-determined angulations in all planes and as with many of the motion limiter portions, it may be inserted after the plate portions  440 ,  450  or at the same time or along with the plate portions  440 ,  450 . Assuming insertion after the plate portions  440 ,  450 , the sloped shape of the wedge  540  eases insertion between the plates  440 ,  450  by driving the plate portions  440 ,  450  apart while concurrently sliding between them. Once in position, the positional wedge  540  is secured between the plate portions  440 ,  450  by common locking, fastening, or other attachment methods. For example, in the embodiment shown in  FIG. 6 , wedge-fastening holes  530  are provided in the positional wedge  540 . These holes line up with complementing plate-fastening holes  510  situated on the upper and lower plate portions  440 ,  450 . These holes  510  may be threaded and may house fasteners spanning between the positional wedge  540  and the plate portions  450 ,  440 .  
      Alternatively, and as another example, the positional wedge  540  may have locking tabs  550  that span across the front of the upper and/or lower plate portions  450 ,  440 . Fasteners or other connectors may be used to secure the locking tabs  550  to the plate portions  450 ,  440 . As yet another example, the wedge  540  and/or the plate portions  440 ,  450  may include locking ridges, teeth, steps, bosses  560 , locking recesses  570 , or other features that interlock once the wedge is inserted between the plate portions  440 ,  450 . The positional wedge  540  is particularly well-suited to be made from bone or a bone substitute due to its simple shape, and may be bioresorbable.  
      As discussed previously, a motion limiter may be a balloon filled with an incompressible or minimally compressible filler material such as a curable polymer. For example, the positional wedge  540  may be a wedge-shaped balloon. This balloon typically has an entry site that is punctured or has a valve to provide an entry for the inflating filler material. The preferred material of choice for balloon inflation is a curable polymer or bone cement, though it may be of any variety of fluids such as saline. It is preferred that a variety of sizes and angulations of balloons are provided. When filled, the balloon distends to a predetermined shape, thereby positioning the plate portions  440 ,  450  to a predetermined orientation. Alternatively, the plate portions  440 ,  450  may be first positioned in a desired orientation, followed by curing material in the balloon to retain this orientation. However, the orientation of the plate portions  440 ,  450  can be completed at any stage of implantation. The balloon is only one example of how the motion limiter may be inserted or otherwise deployed before, during, or after implantation of the plate portions and/or motion portions of the implant.  
      As another example of a device including an alternative articulating joint profile, one or more components of the joint surface portions may be formed on an insert that is slid into, attached, fixed or otherwise housed within a plate portion, such as device  600  shown in  FIGS. 7 and 8 . The device  600  has similar upper  630  and lower  640  plate portions. The mobile portion  610  of the implant includes an insert spacer  700 , which has a joint insert surface  690  articulating with a joint plate surface  710 . Insert spacer  700  may be inserted or otherwise held by one of the plate portions  630 ,  640 . The insert spacer  700  may include features to secure the insert spacer  700  to the plate portion  640  such as one or more insert locking tab  650  that fall into a recess (not shown) in the lower plate portion  640 . The insert spacer  700  may additionally include insert rails  720  while the lower plate portion  640  has complementing insert guides  730 , though this configuration may be reversed. The insert spacer  700  may be sized and formed such that, although secured within the lower plate portion  640 , the spacer  700  has some ability to slide within a plane along its generally flat bottom, thus adding additional degrees of freedom of motion between the plate portions  630 ,  640 .  
      The motion limiter portion  620  of the device  600  includes angulation posts  680 , which mate with the insert spacer  700 . The motion limiter portion  620  is secured within the insert spacer  700  once the insert spacer  700  is slid into the lower plate portion  640 . This is due to the angulation posts  680  protruding through the insert spacer  700  and the lower plate portion  640  blocking release of the insert spacer  700 .  
      The motion limiter portion  620  may provide an angulation. Towards this end, the angulation posts  680  may be provided with a variety of slopes or angles and with differing heights, thereby providing a pre-determined and desired amount of angulation. At least one angulation post  680  preferably includes an anti-rotation tab  670  received in a locking recess  660  on the mating upper plate portion  630 . Similar features common to preventing rotation between two bodies may be used.  
      For all embodiments, these bone surface engagement portions  110 ,  120  of the implant, regardless of whether they include protrusions, may have a porous surface  140  with porosity in the range of 100-1000 um for optimal bone in-growth into the implant. For example, the porous surface  140  may comprise a porous material such as porous nitinol or tantalum, a porous coating such as sintered metal particles, or other similar functioning material that the bone can grow into to assist in fixation of the implant  100  with the boney segment  160 .  
      While there have been illustrated and described particular embodiments of the present invention, it will be appreciated that numerous changes, modifications, and combination of features will occur to those skilled in the art, and it is intended in the appended claims to cover all those changes, modifications, and combinations which fall within the true spirit and scope of the present invention.