Abstract:
A self-expanding bone screw anchor assembly adapted to allow the screw body to engage the bone and the anchor to attach thereto. The anchor assembly is adapted to be universally deployed with a variety of installation tools.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a 35 USC §371 National Stage application of International Application No. PCT/US2010/055476 filed Nov. 4, 2010, now pending; which claims the benefit under 35 USC §119(e) to U.S. Application Ser. No. 61/258,362 filed Nov. 5, 2009. The disclosure of each of the prior applications is considered part of and is incorporated by reference in the disclosure of this application. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to devices for securing an implantable medical device into bone. More specifically, the invention relates to devices for use in spinal surgery to deploy spinal stabilization or fixation devices including bone screws. 
     BACKGROUND OF THE INVENTION 
     Bone in human and other mammal bodies is generally classified into two types, cortical bone, also known as compact bone and trabecular bone, also known as cancellous or spongy bone. Cortical bone is much denser with a porosity ranging between 5% and 10%. Cortical bone is found primary is found in the shaft of long bones and forms the outer shell around cancellous bone at the end of joints and the vertebrae. 
     In the vertebrae, each bone is generally heart shaped, with spinous, inferior and superior processes joined to the vertebral body via opposing pedicles. To stabilize or fix the spine, implantable medical devices can be employed between adjacent vertebrae, and attached via screws through the pedicles (i.e., via “pedicle screws”) and other osseous structures such as the lamina and facet joints. 
     The outer bony shell of the pedicles is formed of dense cortical bone, which surrounds spongier cancellous bone. In degenerative conditions such as osteoporosis or following injury, the cancellous bone can weaken and degrade. Cancellous bone can also be displaced through installation of a pedicle screw. 
     Screws of all sizes can loosen or shift position if placed in osteoporotic or otherwise weak cancellous bone. Pedicle screws are available which seek to avoid loosening or shifting in place through installation with an expandable anchor. In most such designs, the anchor includes deflectable wings at the distal tip which will push outwardly against the bone as the pedicle screw is inserted through the anchor in a fashion similar to screw anchors used in building walls. However, the cylindrical design of such anchors provides relatively minimal interaction between the body of the screw and bone. They also tend to rely primarily on friction to secure the anchor, which can close and lose essential contact as the cancellous bone further degrades. 
     Another drawback of many existing pedicle screw implants is the lack of adaptability to use with different surgical devices. In particular, pedicle screws of differing diameters, whether monoaxial or polyaxial in design, generally include a screw head for engaging an installation device, or a headless design. In both instances, the installation device must generally be matched in size and shape to the pedicle screw head or other proximal screw surface, requiring a selection of tools to be available for use with different patients. 
     SUMMARY OF THE INVENTION 
     In a first aspect, the invention provides a self-expanding bone screw anchor assembly adapted to allow the screw body to engage the bone and the anchor to attach thereto. The anchor may be fully or partially inserted, followed by insertion of the bone screw. 
     The anchor has a body with a partially open cylindrical design with a shaft extending between proximal and distal ends. At least two opposing vertical slots are disposed along the shaft body, through which helical turns of a screw body may protrude to engage surrounding bone. The material of the anchor body adjacent the slots form vertical walls which, at their distalmost end, are deflectable to form anchoring wings to engage the surrounding bone. 
     To strengthen the deflectable wings while limiting their ability to disengage from bone, a deflectable collar is provided within the interior diameter of the shaft proximal to the wings. Insertion of the screw through the collar causes it to expand and stiffen, reinforcing the shaft walls above the distal wings. 
     In another aspect, the distal wings are hinged to provide for controlled outward expansion on insertion of a bone screw. 
     In an optional aspect, one or more pairs of opposing outwardly deflectable flanges are provided along the shaft walls. To minimize the outer diameter of the anchor body during insertion into bone, the flanges are retracted within the diameter of the shaft until insertion of the screw through the lumen of the shaft. As it passes through the lumen, the screw body pushes each flange outwardly to engage the surrounding cancellous bone. Engagement of the flanges by the bone further secures the anchor and screw against dislodgment, even with some cancellous bone degradation. 
     In another aspect of the invention, the flanges are hinged to the shaft walls for controlled outward expansion on insertion of a bone screw. 
     In yet another aspect, the anchor includes a universally adaptable screw head or nut to facilitate its insertion into bone. The screw head is made of a deformable material which can stretch in diameter and shape to accommodate different sizes and shapes of installation tools and bone screws. 
     In another aspect, the screw head is a nut that can be removed following implantation of the anchor assembly, including a screw. 
     Additional systems, methods, features, and advantages of the invention will be apparent to one with skill in the art on consideration of this disclosure, including references cited therein, each of which is incorporated into the disclosure by this reference. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the invention, which is defined solely by the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following drawings may contain hidden features or elements shown in dotted lines and may include phantom views of various components or elements shown in dashed-dotted lines. In all drawings, representative dimensional characteristics are indicated. 
         FIGS. 1A and 1B  provide front (A) and side (B) views of the anchor assembly. In  FIG. 1A , line A-A represents the junction between the anchor body proper and the distal wings. 
         FIGS. 2A and 2B  provide side view of the anchor assembly with, respectively, symmetrical and asymmetrical unexpanded wings. 
         FIG. 3  provides a side view of the unexpanded collar. 
         FIG. 4  provides a top view of the unexpanded collar. 
         FIGS. 5A and 5B  provide side and front views of a wing in a non-deflected state. 
         FIG. 6  provides a front view of the anchor body deflectable flanges. 
         FIG. 7  provides a side view of the anchor body deflectable flanges. 
         FIGS. 8A and 8B  provide top and side views of the anchor screw head. 
         FIGS. 9A through 9J  provide a side view of various embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 1A and 1B , the anchor assembly comprises elongate body  1 , defining lumen  2  therethrough. The lumen provides a bore through the device having an inner diameter that allows for the insertion of a screw body such as bone (pedicle) screws or setting rods and seating therein. 
     Elongate body  1  has a partially open cylindrical design with a shaft extending between proximal and distal ends  3  and  4 . At least two opposing vertical slots  5  and  6  are disposed along the shaft body, through which helical turns of a screw body may protrude to engage surrounding bone, thereby stabilizing the screw within the bone. Slots  5 ,  6  extend vertically to about ⅔ to ¾ the length of the elongate body and circumferentially to up to 75% of the body diameter. The remaining material forms opposing walls of the shaft body  7  and  8 , which walls are curved (e.g., in substantially a “c” shape). 
     At the distalmost end (below line A-A of  FIG. 1A ), walls  7 ,  8  are tapered inwardly as well as outwardly deflectable on insertion of a bone screw; i.e., the ends can extend away from body  1  to form wings  10  and  11 . As discussed further hereinbelow, wings  10 ,  11  may be symmetrical or asymmetrical, as shown in expanded state in  FIGS. 2A and 2B . In either configuration, the wings are outwardly deflectable from the anchor body to form an angle therefrom of 10 to 60°, most preferably between 25 and 45°. 
     As detailed in  FIGS. 3 and 4 , to strengthen the deflectable wings while holding preventing distortion of the shaft walls, to maximize the deflecting force exerted on the wings, a deflectable collar  9  is provided within the diameter of elongate body  1  proximal to wings  10 ,  11 . Prior to insertion of a screw through lumen  2 , collar  9  rests in an unexpanded inverted shape that is preferably concave in its center (forming a loose  FIG. 8 ) at the vertical openings of the shaft to minimize the hole diameter into which the anchor must be seated in the bone. Insertion of the screw through the collar exerts compressive force against the internal surface of the collar, causing it to expand and stiffen, forming a shape (preferably circular) which protrudes from vertical slots  5 ,  6 , thereby engaging surround bone and reinforcing the shaft walls above wings  10  and  11 . Most preferably, the sides of collar  9  are slightly offset from horizontal; i.e., by between 5° and 10° (for example, by 8° as shown in  FIG. 3 ). Alternatively, when unexpanded, the collar may assume any shape (e.g., straight parallel walls) whose walls do not protrude through vertical slots  5 ,  6  prior to expansion, and through which a bone screw may be inserted. 
     Wings  10 ,  11  may be deflectable in response to a compressive force exerted by insertion of the screw through lumen  2 , wherein the deflection is provided by virtue of flexibility in the material used in body  1  as described below. Preferably, however, wings  10 ,  11  are each provided with a hinge  12  for controlled outward expansion on insertion of a bone screw through the length of lumen  2 . 
     As shown in  FIG. 2A , wings  10 ,  11  are symmetrical in placement and length on body  1 . Alternatively, wings  10 ,  11  may be asymmetrically disposed on body  1  as illustrated in  FIG. 2B . The asymmetrical configuration allows wings  10 ,  11  to be displaced at specific and independent distances and angles in the superior, inferior, medial, and lateral directions. 
     To allow for deflectability of wings  10  and  11 , elongate body  1  is preferably a biocompatible polymer with a degree of flexibility. Those of ordinary skill in the art will be familiar with materials available for use in bone screw anchors which include, without limitation, a medical grade polymer or titanium. Numerous medical grade polymers exist such as nylon, polyetheretherketone (PEEK), high density polyethylene, ultra-high molecular weight or high modulus polyethylene, polyethylene glycol, polyvinyl chloride (PVC), polymethyl methacrylate (PMMA), polyvinyl alcohol (PVA), etc. Additional materials of use in the invention include titanium, nitinol and stainless steel. 
     If hinged, however, deflection of wings  10 ,  11  may occur by rotation of hinge  12  at each wing. Hinge  12  attach wings  10 ,  11  to walls  7 ,  8 . Hinge  12  may secured to walls  7 ,  8  by any means suitable for deployment in an implantable medical device, including a biocompatible metal or polymer pin, groove and socket hinges, and similar structures. 
     Referring to  FIGS. 3 and 4 , collar  9  may be formed of the same material or a more elastic one, having sufficient rigidity to retain the unexpanded concavity described herein while being expandable on insertion of a screw body therethrough. Such materials include, without limitation, those described above with respect to suitable materials for body  1 , including wings  10 ,  11 . 
     In an alternative embodiment which provides additional stability to the anchor (especially for use with significantly degraded bone), body  1  is further stabilized within the bone on insertion by engagement of flanges along body  1  with surrounding bone. To that end, one or more pairs of opposing outwardly deflectable flanges  13  and  14  are provided along walls  7 ,  8  (see  FIGS. 6 and 7 ), situated on body  1  above line A-A (see  FIG. 1A ). To minimize the outer diameter of elongate body  1  during insertion into bone, one or more pairs of flanges  13 ,  14  are retracted within lumen  2  so as not to extend beyond the outer diameter of body  1  until insertion of a screw body through lumen  2 . As it passes through lumen  2 , the screw body pushes each flange pair  13 ,  14  deflect outwardly to engage the surrounding bone. 
     Each flange  13 ,  14  is attached to walls  7 ,  8  by a hinge  12  for controlled outward expansion on insertion of a screw body through lumen  2 . The wings are outwardly deflectable from the anchor body to form an angle therefrom of 10 to 60°, most preferably between 25 and 45°. 
     To allow for universal application of the anchor, a screw head or nut  15  is provided per  FIGS. 8A and 8B  to facilitate insertion of the anchor into bone using any suitable installation tool (e.g., a surgical screwdriver). Anchor head  15  is made of a deformable material, such as a polymer with elastic properties such as rubber, or any of the materials described with regard to the composition of body  1 . 
     Head  15  has a oval-shaped bore  16  therethrough that includes opposing slits  17  and  18  at the opposing ends of the oval, which allows bore  16  to stretch in diameter and shape to accommodate different sizes and shapes of installation devices and bone screws. 
     To minimize the volume occupied by the anchor assembly (including the screw) after installation, anchor head  15  may be a removable nut; i.e., one that is reversibly threaded onto the proximal end of the anchor and can be removed after the anchor and screw are fully introduced into the bone. 
     On insertion and rotation of a screw body through lumen  2 , the rotation can exert a rotation force on body  1 . Engagement of wings  10 ,  11  and flanges  13 ,  14  with surrounding bone will stabilize the anchor position. To further stabilize the anchor assembly during its implantation, anchor head  15  may be further provided with a pair of opposing recesses  21  and  22 , as shown in  FIG. 8B . Recesses  21 ,  22  provide a structure for engagement of a retaining tool, such as a hemostat, to hold the anchor assembly in place as it is being deployed. 
     Following insertion of a bone screw into the anchor body, the helical turns of the screw face will engage surrounding bone, stabilizing the anchor and screw in place within the bone. The inner diameter of lumen  2  is therefore adapted to allow for insertion of a screw through lumen  2  whereby the helical turns of the screw face will protrude from vertical slots  5 ,  6  to engage with surrounding bone. 
     It will be appreciated that the outer surfaces along body  1 , wings  10 ,  11  and flanges  13 ,  14  can be treated to be especially well suited to a particular application (e.g., fixation or stabilization of bone at various levels of the spine). For example, the surfaces may be smooth, sanded, or micro-abrasive blasted to create a rough surface finish. A rough surface finish has the advantage of significantly increasing the friction coefficient between the device-bone interface. Additionally, said porous surfaces can promote bone growth and further strength the device-bone interface. 
     The anchor assembly may also be coated with a material, drug, or biologic to increase bone growth. For example, the device may be coated with hydroxyapatite, small molecules, or stem cells. 
     The invention having been fully described, its construction and use is illustrated by the examples below. The scope of the invention, however, is not limited by the examples but is defined instead by the appended claims. 
     EXAMPLES 
     Example 1 
     Construction of an Anchor Assembly According to the Invention 
     The device must be constrained to the dimensions of the anatomy of human vertebrae in order to attain a beneficial interaction. Thus, dimensions of an anchor assembly in accordance with the invention adapted for use in an average human L3 (lumbar level 3) vertebrae are provided in Figures illustrating each element of the anchor assembly. It will be appreciated that the dimensions illustrated are representative, and may be altered to accommodate variance in patient size, variance in level of the vertebra, and variance in pedicle screw size. 
     Example 2 
     Method for Use In L3 Vertebra 
     1. Surgeon inserts a 4 mm gear shift (pedicle probe) into pedicle and vertebral body to create a pilot hole. During this step, the surgeon assesses the quality of the patient&#39;s bone. 
     2. Surgeon determines that the quality of the patient&#39;s bone is poor. 
     3. Surgeon decides to augment the pedicle screw fixation by application of the device to reconstruct the strength of the pedicle. 
     4. Surgeon expands the pilot hole with a 6 mm gear shift. 
     5. Surgeon inserts the device into the 6 mm pilot hole. 
     6. Surgeon locks the device in proper orientation by attaching tool or tapping with mallet. 
     7. Surgeon inserts a pedicle screw into the device, causing the wings and, if present, flanges of the device to displace outward and improve the fixation strength of the pedicle screw.