Surgical base unit and retractor support mechanism

[0001] The present application claims priority from pending U.S. Provisional Application Ser. No. 60/598,739, filed Aug. 4, 2004, which is incorporated herein by reference.

[0002] The present invention pertains to devices and methods for locally securing a support base at the site of an incision by anchoring to a pedicle screw inserted into one or more bones, which support base may be used for exerting relative forces on the bones as well as for supporting a further surgical retractor.

[0003] Fusion of lumbar vertebrae is performed under circumstances including those in which degenerative disk disease, scoliosis, or other deformity indicates that stopping motion between adjacent vertebral segments is desirable. The fusion procedure entails inserting a bone graft between vertebral elements thereby inducing bone growth that ultimately fuses the adjoining vertebral elements. In order to accomplish the fusion of lumbar vertebrae in a minimally invasive manner, procedures, such as those performed in accordance with the Pathfinder™ System of Abbott Spine of Austin, Tex., provide for insertion and retention of a stabilizing rod between pedicle screws, one of which is anchored into each of the adjoining vertebrae, so as to maintain them at a fixed relative displacement after the graft has been inserted and until the biological response completes the fusion process.

[0004] It should be noted that lumbar fusion surgery may require relative positioning of adjacent vertebrae in various planes, whether by relative distraction or compression of the inter-vertebral space, or by reduction, i.e., displacement of a vertebra in a plane transverse to the axis of the spine, i.e., in a direction that will be referred to herein as ‘vertical,’ as referred to a prone patient.

[0005] Using minimally invasive surgical procedures, lumbar fusion surgery is begun as now described with reference to FIG. 1. A first incision 10 is performed in the patient's back 12, parallel to the spine 14 and to one side. Though this incision, pedicle screws 16 and 18 are inserted, by known procedures, into the pedicles of adjoining vertebrae 20 and 22, such as lumbar vertebrae L4 and L5. Rod 24 is inserted through slotted guides in each of pedicle screws 16 and 18 and secured with respect to each of the pedicle screws by closure tops that are threaded for retention in a corresponding helical thread of the pedicle screws. The rest of the procedure proceeds via a second incision 26 made parallel to the first incision 10 and laterally opposite to the first incision on the other side of the spine. Edges of the second incision are held apart by a retractor 28 so that a graft 30 may be inserted between vertebrae 20 and 22. Retractor 28 is typically tethered, via flexible arm 32, to a mounting 34 fixed with respect to the operating table. This method of mounting retractor 28 is unwieldy and not as convenient for the surgeon's unfettered access to the surgical site as might be desired.

[0006] It is desirable for reasons both of convenience and sterility that a method and apparatus be provided whereby forces may be applied to a bone, such as a vertebra, with respect to a local fulcrum, and also that a method and apparatus be provided for supporting a retractor 28 locally to the region of the surgery.

[0007] In accordance with preferred embodiments of the present invention, a method is provided for exerting force on a first bone with respect to a fulcrum. The method has steps of inserting a pedicle screw with a central threaded socket into the first bone, and then driving a threaded closure top into the central threaded socket of the pedicle screw. A base structure which is mechanically coupled to the fulcrum is engaged with the closure top so that a force may be exerted by acting against the fulcrum in a direction that is either axial or transverse with respect to the pedicle screw. The direction that is axial with respect to the pedicle screw may also be referred to, herein, as a ‘vertical’ direction, taken with respect to a prone patient.

[0008] In accordance with other embodiments of the invention, the step of engaging the base structure to the closure top may include coupling a leg of the base structure to the closure top. The step of driving the threaded closure top may include driving the closure top by means of a leg extension of the base structure, and the leg extension and closure top may, together, form an integral unit. In an alternate embodiment, the step of driving the closure top may include driving an extended shank of the closure top. A second bone may serve as the fulcrum.

[0009] In accordance with other aspects of the invention, a method is provided for securing a retractor to a bone. This method has steps of:

• • [0010] a. inserting a pedicle screw into the bone, the pedicle screw having a central threaded socket;
  • [0011] b. driving a threaded closure top into the central threaded socket of the pedicle screw;
  • [0012] c. engaging a base structure the closure top; and
  • [0013] d. supporting a retractor from the base structure.

[0014] Other embodiments of the invention relate to an apparatus for exerting force on a bone with respect to a fulcrum. The apparatus, in accordance with those embodiments, has a base structure, mechanically coupled to the fulcrum and a leg extension coupled to the base structure, adapted for coupling to a pedicle screw inserted into the bone. Moreover, the apparatus has a simple machine for urging the leg extension in at least one of axial and transverse directions with respect to the pedicle screw by means of the base structure acting against the fulcrum. The simple machine may be a toothed ratchet drive, or a worm drive, and the leg extension may include a threaded end for insertion into a central threaded socket of the pedicle screw. The leg extension may also be rotatably coupled to the base structure, and frangibly coupled to the threaded end.

[0015] In accordance with yet further embodiments of the invention, a retractor is provided as a support for non-invasive spinal surgery. The retractor has an adjustable trestle having a cross-bar characterized by a length and two legs coupled to the cross-bar, each leg having an inner cavity for coupling to screws fixed in relation to bone tissue. The adjustable trestle may have one or more telescoping cross-bar sections and a mechanism, that may be a screw mechanism, for adjusting the length of the cross-bar.

[0016] Other embodiments of the invention provide a leg assembly for supporting a surgical retractor, wherein the closure top has both a threaded base for retention by a threaded socket in a pedicle screw and an extension coaxial with, and coupled to, the threaded base, adapted to be rotatably coupled to a retractor support. The leg assembly may further include a frangible coupling between the threaded base and the extension.

[0030] In accordance with preferred embodiments of the present invention, forces may be applied to a bone with respect to an effectively fixed fulcrum, or between adjacent bones, by a trestle-structure base unit designated generally by numeral 200, as now described with reference to FIG. 2A. Base unit 200 may function alone, or in conjunction with a retractor 500 (shown in FIG. 5) supported by the base unit.

[0031] Base unit 200 has two legs (or “posts”) 202, each of which is coupled, as described in further detail below, to a pedicle screw 16, 18, anchored to the skeletal system of the patient. Trestle base unit 200 may also serve to support one or more flexible arms 204 to which, in turn, the retractor 500 of FIG. 5 is coupled.

[0032] Support of legs 202 of base unit 200 with respect to the pedicle screws is described, now, with reference to FIG. 3A, in which a pedicle screw is designated generally by numeral 300. The application of forces between the components of the base unit will be described below. Pedicle screw 300 is retained within pedicle tissue by means of threads 302 at the distal end of the screw. A pedicle screw 300 may be retained, for example, in the sacrum, in cases where the L5 vertebral body has “slipped” with respect to the sacrum. Pedicle screw body 304 is notched to receive a rod 24, seen here from the end, with the axis of rod 24 directed into the plane of the page. Rod 24, once secured with respect to two pedicle screws, maintains a fixed separation between adjoining vertebrae so that a graft may be inserted between them, and, further, stabilizes the vertebrae until the graft has resulted in fusion of the two vertebrae. Rod 24 is secured with respect to pedicle screw 300 by advancing threaded end 306 of closure top 308 into the correspondingly threaded central socket of pedicle screw body 304.

[0033] In the embodiment of the invention depicted in FIG. 3A, closure top assembly 308 is an integral assembly with retractor leg extension 219, the combination comprising a disposable element of the apparatus, and forces may be applied by base unit 200 to a bone, via pedicle screw 304, as described below. After the surgical procedure has been completed, retractor leg extension 219 may be sheared off from closure top neck 306 at breakpoint 250. Alternately, closure top assembly 308 may be withdrawn after the procedure has been completed, and may be replaced with a standard closure top in order to retain rod 24.

[0034] Closure top 308 is advanced into pedicle screw body 304 by applying a torque to closure top 308 about central axis 310. In one embodiment of the invention, torque may be applied to closure top 308 by means of a torque pin (or “torque wrench”) 241, inserted into through-hole 242, as shown, in its inserted position, in FIG. 3C, or by means of knurled grip 226. In the former embodiment, torque pin 241 facilitates the rotation of retractor leg extension 219 about its axis at radial joint 240, as further discussed below. A front view of the pedicle screw/retractor leg extension assembly is depicted in FIG. 3D.

[0035] In an entirely different embodiment of the invention, torque is applied to closure top 308 by means of an extended shank 312 that is shown in FIG. 3B. In this alternate embodiment, a tool (such as a socket wrench, or retractor leg extension 219 that has an internal cavity of cross section matching the cross section of the extended shank) captures shank 312, which is the extension of closure top 308. Shank 312, in this case, is characterized by a cross section preferably shaped to facilitate application of torque to the closure top, thus the cross section may be that of a regular polygon, such as a pentagon or hexagon. It is to be understood that the invention is not limited with respect to the means provided for applying torque to drive closure top 308 into pedicle screw 300. For example, shank 312 may also have an interior slot by means of which it is driven with a tool into pedicle screw 300.

[0036] In accordance with alternate embodiments of the present invention such as the embodiment depicted in FIG. 3B, extended shank 312 of closure top 308 serves not only for advancing the closure top so as to secure rod 24 but also as a base on which to support leg 202 (or, in yet other embodiments of the invention, an extension 219 thereof) of retractor support structure 200 (shown in FIG. 2). Leg extension 219 is tubular, containing an interior cavity inserted over, and thereby engaged with, shank 312. The cross section of the interior cavity of leg extension 219 may be shaped to mate with the exterior cross section of shank 312. Alternatively, while not a favored embodiment of the invention, it is to be understood that the cross section of interior cavity may also circumscribe the exterior cross section of shank 312 so that the orientation of retractor support structure 200 is not constrained with respect to rotation about axis 310. Leg extension 219, in that embodiment, is inserted over shank 312 and within an extender sleeve 314 of pedicle screw 300, however a separate socket wrench would then typically be used to urge closure top 308 into pedicle screw 300.

[0037] Returning now to description of the embodiment depicted in FIG. 3A, leg extension 219 is coupled to support leg 202 by rotational coupling 240 that provides for rotation of leg extension 219 about its longitudinal axis, so that it may be rotated about axis 310, thereby providing torque to closure top 308 for insertion into pedicle screw 300.

[0038] Support of legs 202 of retractor support structure 200 by means of closure tops 308 having been described, retractor support 200 is now further described with reference to FIG. 2A. Legs 202 of retractor support 200 are traversed by a cross-bar, designated generally by numeral 206. In accordance with preferred embodiments of the invention, cross-bar 206 has a left sleeve section 210 and a right sleeve section 211, capable of telescoping adjustment with respect to one another so that the length 200 of cross-bar 206 may be adjusted along direction 212 so as to obtain a desired spacing 252 between legs 202 to match the spacing between centers of pedicle screws 300 after attachment to adjacent vertebrae. Adjustment of the length of cross-bar 206 may use any convenient mechanism known in the art. Ratchet handles 220 are used to actuate ratchet mechanisms 227, acting to provide sequential separation of cross-bar sleeve sections 210 and 211 by urging each with respect to inner sections 208, and thereby providing distraction or compression of the bones retaining the pedicle screws to which support legs 202 are attached. Ratchet mechanisms 227 may include a toothed ratchet drive that engages teeth in the inner sections 208. Ratchet levers 220 are locked to pedestals 218 for securing sleeve sections 210 and 211 at a determined relative disposition, once set by adjustment of ratchet mechanisms 227. Other means known in the mechanical arts for adjusting the separation of sleeve sections 210 and 211 (such as a worm drive, for example), and of securing them at a determined relative disposition, are similarly within the scope of the present invention.

[0039] In the embodiment of the invention depicted in FIG. 2A, inner transverse sections 208 are the horizontal legs of L-shaped members of which uprights 222 and 223 form the corresponding vertical legs. Uprights 222 and 223 preferably slide with respect to one another while retained in relative alignment by means of the tongue-in-groove configuration shown in the top view of FIG. 2G. Thus, the uprights 222 and 223 may be separated for sterilization purposes. Ratchet mechanism 227, activated by lever 224, acts to urge uprights 222 and 223 in relative vertical displacement, thereby exerting vertical forces, through legs 202, leg extensions 219, and pedicle screws 300, to reduce misalignment in the vertical plane of the bones to which the pedicle screws are attached. For example, misalignment of the spine (listhesis) may be reduced by elevation of a vertebra (typically L5) with respect to the sacrum as a fulcrum.

[0040] Sleeve sections 210 and 211 are coupled to legs 202 as further described below with reference to FIG. 5. In preferred embodiments of the invention, leg 202 is coupled to leg extension 219 (which, in preferred embodiments, is part of an integral assembly 308 that includes the closure top) at a snap-lock joint 221. Above the snap-lock joint 221, leg 202 has a rotating coupling 240. One embodiment of rotating coupling 240 is depicted in FIG. 2B where the outer ring of bearing 260 is affixed to retractor leg 202, while the inner ring receives leg extension 219, such that leg extension 219 is free to rotate 360° about longitudinal axis 262.

[0041] An exemplary frangible mechanism for engaging an upper leg 270 to a lower leg 264 is shown in FIG. 2C, where lower end 264 of the retractor leg (the lower end may be leg extension 219) contains two locking dogs 266, disposed 180° apart about axis 262, and having angled ends 268. The upper end 270 of retractor leg 202 has two opposed retaining slots 272 for receiving locking dogs 266 of the lower end 264. Locking dogs 266 are biased, by springs, for example, to engage slots 272 thereby maintaining contact between the upper and lower leg segments until the dogs are manually pushed out of retaining slots 272. Another exemplary mechanism for engaging upper leg 270 to lower leg 264 is the representative quick disconnect depicted in FIG. 2D and designated generally by numeral 280. Here, lower leg 264 contains a radial cut 282 that receives dogs 284 coupled to upper leg 270 when sleeve 286 is forced over the dogs by insertion of lower leg 264 into upper leg 270 forcing outer sleeve 287 against spring 288. Lower leg 264 is released by pushing outer sleeve 287 back against spring 288, allowing dogs 284 to release radial cut 282 in the lower leg 264. Quick-disconnect 280 may advantageously provide for free rotation of lower leg 264 with respect to upper leg 270. Other coupling modalities, such as the snap lock 290 depicted in FIGS. 2E and 2F are also within the scope of the present invention.

[0042] Legs 202 may emanate as right cylinders directly from cross-bar sections 210 and 211, or, in accordance with other embodiments of the invention, may emanate in an arcuate manner, as depicted in FIG. 4 which shows retractor support structure 200 in cross-section as viewed from either the left or right side. While axis 254 (shown in FIG. 2A) of the cross-bar is oriented into the page in FIG. 4, wheel 216 is visible in FIG. 4, as is extender sleeve 314 of pedicle screw 300. The closure top threaded section 306 and extended shank 312 of the closure top are apparent as well. Wheels 214 and 216 are shown, whose purpose is for securing flexible arms 204 which hold each half of the retractor assembly 500, as shown in FIG. 5. Flexible arms 204 are locked to base unit 200 by screws 228 or other connectors.

[0043] In accordance with further embodiments of the invention, a flexible arm 204 extends from one or both sides of the retractor support structure and supports retractor 500, one embodiment of which is now described with reference to FIG. 5. Retractor 500 is shown in FIG. 5 as viewed from above. The retractor is supported, at the site of the surgery, by means of flexible arms 204 which couple it rigidly, but adjustably, to the retractor support structure described in the foregoing sections. Retractor 500 serves to retain spreaders 502 and 504 at a spacing prescribed by the surgeon to provide access to the region between vertebrae for excision of a disk or insertion of a graft. In an exemplary embodiment of the invention, the placement of pull pins 512 and 514 with respect to perforated spacing members 516 and 518 respectively, govern the coarse placement of the retractor in two orthogonal directions. Fine lateral adjustments of the position and opening of spreaders 502 and 504 is provided by ratchets 508 and 510.

[0044] The retractor support mechanism herein described may advantageously serve for procedures of posterior lumbar interbody fusion (PLIF) and translateral interbody fusion (TLIF). Its applicability, however, is not so limited and it may advantageously be applied in surgical circumstances where local support of a retractor relative to bone is desirable.

[0045] Moreover, the described embodiments of the invention are intended to be merely exemplary and numerous variations and modifications will be apparent to those skilled in the art. All such variations and modifications are intended to be within the scope of the present invention as defined in the appended claims.