Abstract:
A sacral prosthesis for a partial or complete sacrectomy has a central body with a platform on a superior face thereof; first and second arms extending superiorly and laterally from the central body, with the arms including respective terminal portions for anchoring to a respective ilium. The terminal portions are held spaced from one another and the central body is suspended by the arms such that the platform is disposed inferiorly to the terminal portions at a fixed position relative thereto. The sacral prosthesis supports a spinal column against inferior displacement by supporting a vertebral body with the platform, while advantageously simultaneously preventing relative lateral displacement of the ilia. The sacral prosthesis may advantageously be formed of a radiolucent material.

Description:
BACKGROUND 
     The invention relates to a sacral prosthesis and a method for a sacral prosthesis. 
     Removal of the sacrum, while undesirable, is sometimes necessary. For example, a malignant or benign tumor may be found in the sacrum, and removal of the sacrum may be the best treatment option. However, the sacrum performs two structural functions: vertically supporting the spine and securing the ilia of the pelvis against relative lateral displacement. Thus, during a sacrectomy surgical procedure, a surgeon may find it desirable to install an artificial construct for performing these structural functions. Typically, such a construct is custom made and involves the use of rods anchored to the ilia via the Galveston technique and joined to other rods anchored to the vertebrae of the spine. Installing such a custom made construct increases the complexity and time required for the surgical procedure. 
     While the above approach may be appropriate for some situations, there remains a need for alternative surgical methods and devices for replacing the sacrum, advantageously ones that artificially replace one or both of the functions of the sacrum. 
     SUMMARY 
     In one illustrative embodiment, a sacral prosthesis comprises a central body comprising a platform on a superior face thereof to support an inferior face of a lumbar vertebral body; first and second arms spaced from one another and extending superiorly and laterally from the central body in respective diverging directions; the first and second arms including respective terminal portions for anchoring to a respective ilium; the terminal portions held spaced from one another a first distance by the first and second arms in combination with the central body; and the central body is suspended by the arms such that the platform is disposed inferiorly to the terminal portions at a fixed position relative thereto. The central body may be generally wedge-shaped and generally taper, both anteriorly-posteriorly and laterally, toward an inferior portion thereof. The terminal portions may be disposed superiorly to the platform and may comprise at least one aperture to accept a bone screw extending therethrough to mount the sacral prosthesis to the ilia. At least one fastener may engage the central body to secure the central body to the lumbar vertebral body. The sacral prosthesis may advantageously be formed of a radiolucent material. Various other aspects and embodiments are disclosed, which may be used alone or in any combination. 
     In another embodiment, a surgical method comprises surgically resecting at least a portion, and optionally all, of a sacrum at a surgical site; disposing a sacral prosthesis in the surgical site such that the central body is disposed inferiorly to a lumbar vertebral body and between the ilia; securing the terminal portions to respective ilium; and supporting a spinal column against inferior displacement by supporting the vertebral body with the platform, while the sacral prosthesis simultaneously prevents relative lateral displacement of the ilia. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a human pelvic girdle and partial spinal column. 
         FIG. 2  shows a perspective view of a sacral prosthesis according to one embodiment of the present invention. 
         FIG. 3  shows a bottom view of the sacral prosthesis of  FIG. 2 . 
         FIG. 4  shows a back view of the sacral prosthesis of  FIG. 2 . 
         FIG. 5  shows a side view of the sacral prosthesis of  FIG. 2 . 
         FIG. 6  shows an anterior perspective view the sacral prosthesis of  FIG. 2  joined to a pelvic girdle and a spinal column. 
         FIG. 7  shows a posterior view of the arrangement of  FIG. 6 . 
         FIG. 8  shows a perspective view of a sacral prosthesis according to another embodiment of the present invention with lockably adjustable length arms. 
     
    
    
     DETAILED DESCRIPTION 
     The sacrum  16  is an inferior portion of the spinal column  10  that helps join the spinal column  10  to the pelvic girdle  20 . The sacrum  16 , in a healthy spine, provides a base that vertically supports the remainder of the spinal column  10  and typically directly supports the vertebra  12  in the lumbar region known as the L5 vertebra. In addition, the sacrum  16  helps maintain the pelvic girdle  20  against collapse. In particular, the sacrum  16  is disposed in the gap  26  between the right and left ilia  22 ,  24 , and helps keep the posterior portions of the ilia  22 , 24  spaced from one another. As indicated above, it is sometimes necessary or desirable to remove the sacrum  16  for a variety of reasons. Illustrative embodiments of the present invention provide a sacral prosthesis  30  and/or a related surgical method. 
     One embodiment of the sacral prosthesis is shown in  FIG. 1 , and generally indicated at  30 . The sacral prosthesis  30  comprises a central body  40  and a pair of arms  70   a , 70   b  extending from the body. The central body  40  is a somewhat elongated body that advantageously tapers from its superior portion  42  to its inferior tip portion  44 , typically in both the anterior-posterior and lateral directions. Thus, the central body  40  is advantageously somewhat triangularly shaped, so as to mimic the sacrum  16  it replaces, but is typically somewhat shorter in length. The superior face  62  of the central body  40  forms a platform area  60 . This platform area  60  is generally flat and may advantageously have a shape generally corresponding to the shape of the L5 vertebra  12 . The face  62  of the platform area  60  may, if desired, include suitable texturing, such as small ridges, teeth, knurling, or the like, to aid in bonding the relevant vertebra  12  to the central body  40 . In addition, one or more through holes  64  may extend through the central body  40  at the platform  60 . These through holes  64  accept fasteners  54 , as discussed further below, for securing the central body  40  to the spinal column  10 . The central body&#39;s anterior face  48  may be generally flat, but may include a shallow, generally vertically oriented, depression  49  if desired. The posterior face  50  may be gently convexly curved and may include one or more recesses  52  that connect to the through holes  64 . The lateral edges  46  of the central body  40 , and indeed the entire exterior of the central body  40 , are advantageously suitably contoured so as to remove sharp edges. The central body  40  may be solid or hollow, as is desired. 
     The arms  70   a , 70   b  extend away from central body  40 , proximate the platform area  60 . For ease of reference, the arm that is intended to mate with the patient&#39;s right ilium  22  will be referred to in this description as the right arm  70   b , and the arm intended to mate with the patient&#39;s left ilium  24  will be referred to in this description as the left arm  70   a . Thus, in  FIG. 2 , the right arm  70   b  is on the left of the figure, while the left arm  70   a  is on the right of the figure. The left arm  70   a  includes a medial first section  72   a  disposed proximate the central body  40  and a lateral second section  74   a  disposed distal from the central body  40 . The second section  72   a  includes a terminal portion  76   a  that is advantageously hooked-shaped with a hook gap  78   a  of sufficient width to fit over the corresponding ilium  24 . The terminal portion  76   a  may advantageously include one or more holes  79   a  for corresponding anchor screws  80 , as discussed further below. As can be seen in  FIGS. 2-4 , the left arm  70   a  is advantageously somewhat S-shaped overall, rather than straight. Nevertheless, a theoretical line  82   a  may be drawn from where the left arm  70   a  joins to the central body  40  to the point where the left arm  70   a  is to be anchored to the corresponding ilium  24 . For the illustrated embodiment, this anchoring point may be the anchoring hole  79   a  closest to the central body  40 . 
     The right arm  70   b  may be a substantial mirror image of the left arm  70   a  about central body midline  41 . As such, the right arm  70   b  includes a medial first section  72   b  disposed proximate the central body  40  and a lateral second section  74   b  disposed distal from the central body  40 . The second section  74   b  includes a terminal portion  76   b  that advantageously takes the form of a hooked section with a hook gap  78   b  of sufficient width to fit over the corresponding portion of the ilium  22 . Terminal portion  76   b  may also include one or more holes  79   b  for corresponding anchor screws  80 , as discussed further below. Again, a theoretical line  82   b  may be drawn from where the right arm  70   b  joins to the central body  40  to the point where the right arm  70   b  is to be anchored to the corresponding ilium  22 . For the illustrated embodiment, this anchoring point may be the anchoring hole  79   b  closest to the central body  40 . 
     The two arms  70   a , 70   b  extend away from the central body  40  at an angle so as to diverge therefrom. As can be seen in  FIG. 4 , the arms  70   a , 70   b  diverge such that lines  82   a , 82   b  are divergent and form an included angle Θ of approximately 75°-85°. In addition, as can be seen in  FIG. 5 , the lines  82   a , 82   b  may be disposed at a slight upward angle from the central body  40 . This orientation is believed to facilitate installation and to route the arms  70   a , 70   b  away from sensitive neurovascular structures such as the sacral nerve roots. Further, as with the central body  40 , the exterior surfaces of arms  70   a , 70   b  are advantageously suitably contoured so as to remove sharp edges. 
     The anchoring point  79   a  on one arm  70   a  is laterally spaced from the anchoring point  79   b  on the other arm  70   b  by distance D. The prosthesis  30  is advantageously designed to be rigid enough so that this distance D remains relatively fixed. Thus, it is contemplated that arms  70   a , 70   b  will be relatively substantial structures. To this end, it is contemplated that the sacral prosthesis  30  will be manufactured from a suitable biocompatible material, such as titanium, stainless steel, or carbon fiber reinforced polymer. It should be noted, however, that manufacturing the sacral prosthesis  30  from a radiolucent material may be advantageous because sacrectomy patients often require follow-up radiological studies such as CT scans and MRIs. As such, the sacral prosthesis  30  may be advantageously manufactured from a radiolucent material, such as carbon fiber reinforced polyetheretherketone known as PEEK Optima™, available from Invibio Limited of Lancashire, England. The sacral prosthesis  30  may be molded as a single piece, or may be made from separate pieces and welded or otherwise joined together, or may be formed using any other suitable known manufacturing technique. 
     The sacral prosthesis  30  is installed during a surgical procedure sometimes known as a sacrectomy. For this procedure, the surgeon prepares the surgical site and removes the sacrum  16  in a conventional fashion, typically using a combined anterior-posterior approach. See the article entitled “Surgical Treatment Of Primary Sacral Tumors: Complications Associated With Sacrectomy” by Dr. Mehmet Zileli et al., Neurosurgical Focus, volume 15, November 2003, which is incorporated herein by reference. The sacral prosthesis  30  is then positioned with the terminal portions  76   a , 76   b  hooking over the respective ilia  24 , 22  and the central body  40  disposed inferior to, and abutting, the L5 vertebra  12 . Suitable screws  80  are then inserted through holes  79   a , 79   b  and into the respective ilia  24 , 22  to anchor the sacral prosthesis  30  to the pelvic girdle  20 . Advantageously, the screws  80  pass through both exterior surfaces of the corresponding ilium, and are therefore installed using a bicortical technique. If desired, suitable holes in the ilia  22 , 24  may be drilled or otherwise created prior to installing the screws  80 . Fasteners  54  are then inserted through their respective recesses  52  in the posterior face  50  of central body  40  so as to extend through the respective through hole  64 . Again, if desired, suitable holes in the vertebra  12  may be drilled or otherwise created prior to installing the fasteners  54 . These fasteners  54  are then tightened into the L5 vertebra  12  to secure the sacral prosthesis  30  to the spinal column  10 . Of course, the sequence may be reversed, such that the sacral prosthesis  30  is first secured to the spinal column  10  and then secured to the ilia  22 , 24 . Either way, the sacral prosthesis  30  is advantageously secured to both the spinal column  10  and the ilia  22 , 24 . The surgeon then closes the surgical site in an appropriate manner. 
     The presence of the sacral prosthesis  30  provides vertical support to the spinal column  10 . In particular, the spinal column  10  rests on the platform area  60  of the central body  40 . The central body  40  is in turn supported by the arms  70   a , 70   b  which are anchored to the ilia  24 , 22 . Thus, the spinal column  10  is vertically supported by the pelvic girdle  20  via the sacral prosthesis  30 . In addition, the presence of the sacral prosthesis  30  helps prevent the pelvic girdle  20  from collapsing. The sacral prosthesis  30  provides resistance against the two anchoring points (e.g., at  79   a , 79   b ) moving toward or away from each other. Thus, the sacral prosthesis  30  performs the dual functions of supporting the spinal column  10  and maintaining the pelvic girdle  20 . In addition, these two functions are provided by an easy-to-use device that requires neither extensive customization nor extensive assembly during surgery. As such, the surgical process is simplified. And, radiolucent versions of the sacral prosthesis  30  may aid in post-operative radiological studies by minimizing the scatter artifacts typically seen with metallic implanted materials. Further, some embodiments of the sacral prosthesis  30  allow the rectum to be protected more easily than in prior art sacrectomies. 
     The discussion above has assumed that the arms  70   a , 70   b  were of a fixed, non-varying length. However, in some embodiments, the arms  70   a , 70   b  may have a variable configuration. For example, as shown in  FIG. 8 , the each arm  70   a , 70   b  may include a lockable slide adjuster  90  that joins overlapping portions of the respective first sections  72   a , 72   b  and second sections  74   a , 74   b . Looking at the right arm  70   b , this slide adjuster  90  may include a slot  92  in the second section  74   b  with a screw  94  and slide nut (not shown) that is in a fixed position along the first section  72   b . The left arm  70   a  likewise comprises a locking slide adjuster  90 . With this arrangement, the length of the arms  70   a , 70   b  may be adjusted during surgery by sliding the second section  74   a , 74   b  relative to the first section  72   a , 72   b  and then locking the adjuster  90  by tightening the screw  94  to the slide nut. Of course, other lockable means for locking adjustment known in the art may alternatively be used, such as interlocking teeth and clamps, etc. The ability to vary the length of the arms  70   a , 70   b  allows the surgeon to easily adjust the sacral prosthesis  30  to the morphology of the patient. Thus, a single sacral prosthesis  30  design may be used for patients with differing gaps  26  between their ilia  22 ,  24 . 
     The distal second sections  74   a , 74   b  of the arms  70   a , 70   b  may, if desired, include malleable sub-sections (not shown) that allow the hook shape of the terminal sections  76   a , 76   b  to be custom formed during surgery. For example, the surfaces of the arms  70   a , 70   b  in these malleable sub-sections may include a plurality of transverse grooves, or be made of a malleable material such as Nitinol, that allow the arms  70   a , 70   b  to preferentially bend in these sub-sections. With such embodiments, the hook shapes in the terminal sections  76   a , 76   b  may advantageously be formed to some nominal configuration at manufacture, and then adapted to a patient&#39;s particular needs during surgery using conventional in situ benders or other tools known in the orthopedic arts. 
     Further, the central body  40  may optionally include one or more passages therethrough, advantageously in the inferior portion  44  thereof, for the passage of a cable or other means for aiding in positioning the coccyx if the coccyx is to be left in place at the conclusion of the surgical procedure. 
     The discussion above has assumed that the sacral prosthesis  30  is secured to the spinal column  10  via fasteners  54 , and that the L5 vertebra  12  directly abuts the platform area  60 . However, it should be understood that additional means may be employed to secure the sacral prosthesis  30  to the spinal column  10 , such as an application of appropriate bone cement between the platform area  60  and the inferior face  14  of the adjacent vertebra  12 . Alternatively, the platform area  60  may be coated with an osteoconductive coating. Indeed, it is believed advantageous for the sacral prosthesis  30  to become fused to the adjacent vertebra  12  in most situations, not just fixedly mounted thereto. 
     The discussion above has also assumed that the sacrum  16  is being totally removed; however, this is not required in all embodiments. In some embodiments, the sacrum  16  may be only partially resected. In such situations, the amount resected should advantageously be sufficient to allow the central body  40  of the prosthesis  30  to be positioned in the space formerly occupied by the resected material. 
     The present invention may be carried out in other specific ways than those herein set forth without departing from the scope and essential characteristics of the invention. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.