Screw and rod fixation system

[0001] The present invention relates to spinal fixation devices and more particularly to a pedicle screw and rod fixation assembly useful in stabilizing a spine of a patient.

[0002] Over the years, several techniques and systems have been developed for correcting spinal injuries and/or degenerative spinal processes. Spinal correction frequently requires stabilizing a portion of the spine to facilitate fusing portions of the spine or other correction methodologies. Medical correction of this type is frequently employed for many spinal conditions, such as, for example, degenerative disc disease, scoliosis, spinal stenosis, or the like. Frequently, these corrections also require the use of implants, such as, bone grafts. Stabilizing the spine allows bone growth between vertebral bodies such that a portion of the spine is fused into a solitary unit.

[0003] Several techniques and systems have been developed for correcting and stabilizing the spine and facilitating fusion at various levels of the spine. In one type of system, a rod is disposed longitudinally along the length of the spine in the region of concern. The rod is arranged according to the anatomy and the correction desired. In this system, the rod is aligned along the spine and engages various vertebrae along its length. The rod engages, or more typically the parallel rods engage, the spine using fixation elements, such as, anchors attached to vertebral bodies by a bone screw.

[0004] Anatomy and correction frequently require aligning the rod and screw at various angles along the length of the portion of correction. In order to provide this alignment, polyaxial screws/anchors have been developed. Many variations of polyaxial screw and rod fixation systems exist on the market today.

[0005] Despite the fact that numerous polyaxial rod fixation systems are on the market, improvement of the devices is desirable. Accordingly, there is a need for a screw and rod fixation system that provides a strong, effective, and secure lock of the screw and rod in the desired position and angle. Additionally, there is a need for a screw head securing mechanism or device that is minimal in size and has a reduced amount of components to provide for a simpler, more effective, and less cumbersome device for fixing screws.

[0006] To attain the advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, a screw and rod fixation system is provided. The screw and rod fixation system comprises a bone screw having a threaded portion, a transition, and a head comprising a housing engaging surface and an insert engaging surface. The bone screw is provided in a housing with a bone facing surface and a top edge and a sidewall extending therebetween. A through hole extends from the top edge to the bone facing surface and a pair of opposing slots reside proximate the top edge. The housing has a screw head seating surface that is shaped to cooperatively engage the housing engaging surface to allow polyaxial orientation of the bone screw relative to a vertical axis of the housing. A housing couples to the bone screw and housing. The insert comprises a screw head engaging surface to cooperatively engage the insert engaging surface to allow polyaxial orientation of the bone screw relative to a vertical axis of the housing and an elongate rod rest shaped to seat a portion of the elongated rod, the elongated rod rest traversing the housing between the pair of opposing slots. An elongated rod traverses the housing between the pair of opposed slots and seated in the elongated rod rest.

[0007] The present invention further provides an insert useful in a device that couples spinal rods to vertebral bodies. The insert comprises a bone screw engaging surface and an elongated rest coupled to the bone screw engaging surface having an outer edge and a bottom surface opposite the elongated rest. A transition portion connects the bone screw engaging surface and the elongated rest. The insert cooperatively engage a bone screw to allow polyaxial orientation of the bone screw relative to the insert.

[0008] The foregoing and other features, utilities and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention as illustrated in the accompanying drawings.

[0020] Referring now to FIG. 1 and FIG. 2, and in accordance with certain embodiments of the present invention, a screw and rod fixation system 100 is shown. FIG. 1 shows a perspective view of system 100 while FIG. 2 shows a cross-sectional view of system 100. System 100 includes a bone screw 102, a housing 104, an insert 106, a rod 108, and a compressive member, such as, a setscrew 110. Housing 104 may contain one or more first mating surfaces 112. First mating surfaces 112 are designed to mate with a tool (not specifically shown). Also, setscrew 110 typically has one or more second mating surface 114 to mate with a tool (also not shown). As shown in FIG. 1, first mating surfaces 112 are actually slots on an outer surface 116 of housing 104. While shown as slots, first mating surfaces 112 may be any number of designs including one or more dimples, hex detents, or other equivalent mechanisms as are known in the art. Second mating surface 114 is shown with a hex shape to accept a hex driver useful in threading the setscrew. Of course, one of ordinary skill in the art would recognize other and equivalent first and second mating surfaces 112, 114 are possible.

[0021] Referring now to FIGS. 3 and 4, housing 104 is described in more detail. Housing 104 may be referred to as a coupling device, seat, or anchor. Housing 104 has a bone facing surface 302, at least one sidewall 304 having an outer surface 116 and an inner surface 306 (best seen in FIG. 4), first mating surfaces 112, a pair of opposed slots 308 in sidewall 304, a top edge 310, and a through hole 312 extending from top edge 310, to bone facing surface 302. Housing 104 can be considered to have a rod receiving section 402 and a screw receiving section 404.

[0022] Notice, housing 104 is shown with one cylindrically shaped sidewall 304. It is believed providing housing 104 as a cylindrical shape reduces the profile of the device, but other shapes are possible, such as cubic or the like. If housing 104 had multiple sidewalls 304, the edges between the multiple sides should be beveled or rounded to inhibit tissue trauma.

[0023] Referring first to rod receiving section 402, inner surface 306 has first threads 406 extending partially along inner surface 306. As will be explained below, first threads 406 allow setscrew 110 to be threaded on housing 104. Slots 308 generally have a width W1 sufficiently wide to accept rod 108. Slots 308 have an insert seating surface 408, which will be explained further below. An insert groove 410 is provided about insert seating surface 408. Insert groove 410 allows provides a mechanism to lock and stabilize insert 106, as will be explained below.

[0024] As shown in FIG. 4, rod receiving section 402 provides a channel for a single rod 108 to traverse housing 104 from slot 308 to slot 308. In some instances, it may be difficult to provide a single rod 108 that can traverse the entire length of the spine necessitating multiple rods. Housing 104 could be easily converted to provide multiple rod guides. For example, housing 104 could have a second set of opposed slots substantially parallel to slots 308. Alternatively (but not shown), a rod cap/seat could be installed below threads 406 and above insert seating surface 408. The rod cap/seat would provide a cap side operatively sized and shaped to fit about rod 108 opposite the insert seating surface 408. The rod cap/seat provide a second seating surface operatively sized and shaped to fit about a second rod aligned substantially above rod 108. Setscrew 110 would provide compressive force to the substantially aligned rod, which would correspondingly provide compressive force to rod 108 and insert 106, etc.

[0025] Referring to screw receiving section 404, inner surface 306 has a screw head seating surface 412. Surface 412 has a shape corresponding to the screw head, described below. Surface 412 has an upper lip 414 and a lower lip 416. Upper lip 414 and lower lip 416 could comprise a beveled edge and/or protrusions. Upper lip 414, lower lip 416, and surface 412 cooperative engage the screw head to allow a polyaxial engagement of the screw 102, which will be described below. As shown, upper lip 414 has a first diameter d1 and lower lip 416 has a second diameter d2. First diameter d1 is larger than second diameter d2. Inner surface 306 terminates at bone facing surface 302. As shown, inner surface 306 flares out from second diameter d2 such that the bone side of through hole 312 has a third diameter d3 larger than second diameter d2, but generally smaller than first diameter d1. Outer surface 116 is generally perpendicular to top edge 310 and bone facing surface 302 to conserve space. As shown, bone facing surface 302 may have a rounded transition 418 between bone facing surface 302 and outer surface 116 to inhibit trauma to tissue. Also, top edge 310 has beveled or rounded edges. Lastly, through hole 312 at top edge 310 has a fourth diameter d4. Diameter d4 is sufficiently large to accept the head of screw 102 and setscrew 110.

[0026] Bone screw 102 will now be described with reference to FIGS. 5 and 6. Bone screw 102 has a threaded portion 502, a transition portion 504, and a head portion 506. Threaded portion 502 can use any conventional thread, but as shown, threaded portion 502 has a shaft 508 and threads 510 machined such that shaft 508 has an increasing diameter from the tip 512 to transition portion 504. Further, threads 510 become relatively thicker towards transition portion 504. Designing threaded portion 502 in this fashion increases the frictional engagement of bone screw 102 in bone and generally increases the screw strength.

[0027] Transition portion 504 comprises the portion of bone screw 102 between threaded portion 502 and head portion 506. Transition portion 504 could be integrated into threaded portion 502. Transition portion 504 may be straight, curved, bowed, flared, or the like to transition threaded portion 502 to head portion 506.

[0028] FIG. 6 shows head 506 of screw 102 in more detail. Head 506 has a housing engaging surface 602, an insert engaging surface 604, a top edge 606, and at least one third mating surface 608, which in this case comprises a plurality of grooves. Insert engaging surface 604 has a shape corresponding to the insert, as is explained further below. Third mating surface 608 is designed to allow a surgeon to thread screw 102 into bone using a driver.

[0029] Housing engaging surface 602 has a shape corresponding to screw head seating surface 412. As shown, screw head seating surface 412 is convex in shape to mate with the concave shaped housing engaging surface 602. Housing engaging surface 602 can rotate relative to screw head seating surface 412. If designed as protrusions, upper lip 414 and lower lip 416 may provide limits on the rotation or angle of screw 102 relative to housing 104.

[0030] Insert 106 will now be described with reference to FIGS. 7, 8, and 9. Insert 106 comprises a screw head engaging surface 702, a transition section 704, and an elongated rod rest 706. Screw head engaging surface 702 has a shape that corresponds to insert engaging surface 604. In this case, insert engaging surface 604 is concave shaped to mate with convexly shaped screw head engaging surface 702. The corresponding surfaces 604 and 702 allow polyaxial orientation of screw 102 relative to insert 106. Moreover, insert 106 and housing 104 remain aligned or substantially aligned. Transition section 704 is a provides a transition between screw head engaging surface 702 and elongated rod rest 706. Transition section 704 is somewhat narrower than the largest diameter of surface 702 to increase the rotational engagement between the screw and insert.

[0031] Elongated rod rest 706 traverses housing 104 between opposing slots 308. Elongated rod rest 706 has a bottom surface 708 shaped to fit in insert seating surface 408. Elongated rod rest 706 is shaped to receive rod 108. Optionally, rod rest 706 may be solid as shown or substantially void as a matter of design choice; however, it is believed providing a substantially solid rest provides a more stable construct.

[0032] Referring now to FIG. 9, a top elevation view of insert 106 is provided. As shown, elongated rod rest 706 has a plurality of tabs 902 formed by slots 904. Each tab 902 has a protrusion 906. Insert 106 is placed in housing 104 by compressing tabs 902 as allowed by slots 904 toward rest 706. When compressed, protrusions 906 are a distance d5 apart, where d5 is less than W1. When compressed, insert 106 can be fitted into through hole 312 of housing 106 until screw head engaging surface 702 rests on insert engaging surface 604. Once surface 702 and surface 604 are in place, the compression on tabs 902 is released and tabs 902 return to the uncompressed state such that protrusion 906 are a distance d6, where d6 is greater than d5 and W1. Protrusions 906 engage groove 410 (FIG. 4) to lock and stabilize insert 106 in housing 104.

[0033] With reference to FIG. 10, placement of system 100 will be explained. First, screw 102 is inserted through through hole 312. Screw 102 is partially driven into bone, not specifically shown in the FIG, until bone facing surface 302 seats on the bone and housing engaging surface 602 engages screw head seating surface 412. Screw 102 may be angled relative to a vertical axis V of housing 104. Also, housing is aligned such that opposing slots 308 will align with rod 108. Screw 102 also may have a polyaxial orientation relative to insert 106. Rod 108 is placed on elongated rod rest 706 and setscrew 110 is threaded into through hole 312 until setscrew 110 contacts rod 108 placing the construct under sufficient compression to stabilize and lock the device in place. As shown, threads 406 substantially terminate at a top surface 1002 of rod 102. This allows sufficient thread to provide the necessary force without providing unnecessary thread. Thread 406 could stop substantially at or above top surface 1002, but in that case, setscrew 110 should be designed with a boss. The thread design reduces the spreading of the housing 104 when the setscrew is tightened to rod 108.

[0034] The above construct is very stable in part because of the surface-to-surface contacts provided by various engaging surface. Further stability is provided in some instances by the insert coupling with a groove in the housing an providing a surface-to-surface contact between the rod and insert.

[0035] While the invention has been particularly shown and described with reference to an embodiment thereof, it will be understood by those skilled in the art that various other changes in the form and details may be made without departing from the spirit and scope of the invention.