US20070043364A1 - Spinal correction system with multi-stage locking mechanism - Google Patents

Abstract

In a rod fixation system of the type used to treat various spinal conditions, the same fastener is used to lock both a polyaxial screw and a rod in position. A spinal correction device according to the invention, adapted for use with a rod and a polyaxial screw having a ball-shaped head and a threaded end, comprises a head-body and a cap. The head-body includes a lower internal cavity to receive the ball-shaped head of the screw, an upper bore to receive a fixation rod therethrough, and an upper end configured to receive the cap. The head-body and the cap are configured such that a first rotation of the cap locks the polyaxial screw in position, and a second rotation of the cap locks the rod in position. In the preferred embodiment, the head-body includes one or more gaps to facilitate flexion of the head-body as a rotational torque is placed on the cap. The gap or gaps may be horizontal, vertical or other angles relative to the axis of the entrapped rod.

Description

REFERENCE TO RELATED APPLICATION
• This application claims priority from U.S. Provisional Patent Application Ser. No. 60/691,708, filed Jun. 17, 2005, the entire content of which is incorporated herein by reference.
FIELD OF THE INVENTION
• This invention relates generally to spinal correction and, in particular, to a system incorporating a multi-stage locking mechanism.
BACKGROUND OF THE INVENTION
• There are many spine-related conditions, such as scoliosis, which require multi-level correction/fixation. Numerous plate and rod systems are available for this purpose, many of which have received patent protection. One of the deficiencies with existing systems, however, is that the mechanism used to lock the polyaxial screw is different from the mechanism used to lock the rod, thereby requiring additional parts and less manageable installation steps.
SUMMARY OF THE INVENTION
• Broadly, this invention resides in a rod fixation system to treat various spinal conditions, wherein the same fastener is used to lock both a polyaxial screw and a rod in position. A spinal correction device according to the invention, adapted for use with a rod and a polyaxial screw having a ball-shaped head and a threaded end, comprises a head-body and a cap. The head-body includes a lower internal cavity to receive the ball-shaped head of the screw, an upper bore to receive a fixation rod therethrough, and an upper end configured to receive the cap. The head-body and the cap are configured such that a first rotation of the cap locks the polyaxial screw in position, and a second rotation of the cap locks the rod in position.
• In the preferred embodiment, the head-body includes one or more gaps to facilitate flexion of the head-body as a rotational torque is placed on the cap. The gap or gaps may be horizontal, vertical or other angles relative to the axis of the entrapped rod. With appropriate selection of materials and cross-section, sufficient “flexion” may be established without the need for gaps.
• Tapered threads may be used on the head-body and/or cap to achieve subsequent tightening of the polyaxial screw and rod to a continuous rotational movement. An optional compression-enhancing bushing may be disposed between the rod and the ball-shaped head of the screw. Although the invention is described in conjunction with rod fixation, extensions to rod and plate or plate-only configurations are possible.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a drawing that shows a cross-section of a preferred embodiment of the invention;
• FIG. 2 is an oblique view of the embodiment ofFIG. 1;
• FIG. 3 is a drawing that shows a cross-section of an alternative embodiment of the invention;
• FIG. 4 is an oblique view of the embodiment ofFIG. 3;
• FIG. 5A is a bottom-up drawing ofcaps112,312;
• FIG. 5B is an oblique, partial cross-sectional view ofcaps112,312; and
• FIGURE SC is a side cross sectional view ofcaps112,312.
DETAILED DESCRIPTION OF THE INVENTION
• Broadly, this invention resides in a rod fixation system to treat various spinal conditions, wherein the same fastener is used to lock both a polyaxial screw and a rod in position. Although the invention is described in conjunction with rod fixation, extensions to rod and plate or plate-only configurations are possible.
• Reference is now made toFIGS. 1 and 2, which show from a cross-section and oblique view, respectively, a first preferred embodiment of the invention utilizing three primary components, namely, a head-body102, apolyaxial screw104, and a cap112 (not shown inFIG. 1). Thepolyaxial screw104 includes a proximal hemispherical head106 and a distal threadedportion108. The ball portion of the screw includes some type of turning aperture, such ashex depression107. Thescrew104 is held in position within the head-body102 through bushing120 and fastener122, facilitating assembly from the bottom. Thefastener122 may be threaded, press fit or otherwise secured in position.
• Above the hemispherical portion106 of thepolyaxial screw104, is a partially cylindrical passage to receive arod114. Note thegap116 formed within the head-body102 internally to the head-body, and around the upper portion of the spherical head106 ofscrew104.
• Broadly according to the invention, whencap112 is placed onto the head-body102, a first rotation of thecap112 locks the polyaxial screw in position, while allowing the rod a slight degree of movement. However, with further rotational force applied to thecap112, the rod is then locked in position, forming a rigid, unified structure. The polyaxial screw is screwed into position using ahex driver107 with thescrew104 aligned the head-body102, after which time the hex driver is removed, allowing the head-body to swivel and therod114 to be placed through the head-body.
• FIGS. 3 and 4 illustrate an alternative embodiment of the invention, utilizing four basic components, namely, those shown inFIGS. 1 and 2 with slight variation, and further including a bushing330 having aslot332 which aligns with aslot336 formed through head-body302. Polyaxial screw is shown at304, having a proximal at leastsemi-spherical head306. Therod314 is shown in phantom.
• The head-body302 includes anupper portion310 adapted to receivecap312 as discussed previously. The operation of the alternative embodiment ofFIGS. 3 and 4 is essentially the same as that ofFIGS. 1 and 2, namely, that through a first rotational torque oncap312, thepolyaxial screw304 is held in relative angular position with respect to head-body302, such that with a subsequent torque applied tocap312,rod314 is locked in position. The purpose of bushing330 is to provide additional compressive strength without the need for as much rotational torque as might be required in the preferred embodiment ofFIGS. 1 and 2.
• FIG. 5A is a bottom-up drawing ofcaps112,312.FIG. 5B is an oblique, partial cross-sectional view ofcaps112,312, and Figure SC is a side cross section. Note that due to the geometries involved, a first rotational torque through a predetermined angle, such as 90 degrees, causes the polyaxial screw to be locked in position, and that a subsequent rotational torque of an additional one-quarter turn of the rod to be locked in position.
• Those of skill in the art will appreciate that as an alternative to a geometric relationship of the type just described, tapered threads may be used on the head-body and/or cap to achieve subsequent tightening of the polyaxial screw and rod to a continuous rotational movement. Additionally, although gaps are shown in the head-body as being horizontal and vertical, mixtures of these and angles other than horizontal and vertical may be used and, indeed, with appropriate selection of materials and cross-section, sufficient “flexion” may be established without the need for gaps. In terms of materials, the assemblies described may be formed from any bio-compatible sufficiently strong materials, such as chrome, cobalt or other alloys, ceramics or combinations thereof.

Claims (9)

1. A spinal correction device adapted for use with a rod and a polyaxial screw having a ball-shaped head and a threaded end, the device comprising:

a head-body and a cap, the head-body having a lower internal cavity to receive the ball-shaped head of the screw, an upper bore to receive a fixation rod therethrough, and an upper end configured to receive the cap; and

wherein the head-body and the cap are configured such that:

a first rotation of the cap locks the polyaxial screw in position, and

a second rotation of the cap locks the rod in position.

2. The device ofclaim 1, wherein the head-body includes one or more gaps to facilitate flexion of the head-body as a rotational torque is placed on the cap.

3. The device ofclaim 1, further including a compression-enhancing bushing disposed between the rod and the ball-shaped head of the screw.

4. A spinal correction device adapted for use with a fixation rod, comprising:

a polyaxial screw having a ball-shaped head and a threaded end;

a head-body and a cap, the head-body having a lower internal cavity to receive the ball-shaped head of the screw, an upper bore to receive the fixation rod therethrough, and an upper end configured to receive the cap; and

wherein the head-body and the cap are configured such that:

a first rotation of the cap locks the polyaxial screw in position, and

a second rotation of the cap locks the rod in position.

5. The device ofclaim 4, wherein the head-body includes one or more gaps to facilitate flexion of the head-body as a rotational torque is placed on the cap.

6. The device ofclaim 4, further including a compression-enhancing bushing disposed between the rod and the ball-shaped head of the screw.

7. A spinal correction device, comprising:

a fixation rod;

a polyaxial screw having a ball-shaped head and a threaded end;

a head-body and a cap, the head-body having a lower internal cavity to receive the ball-shaped head of the screw, an upper bore to receive the fixation rod therethrough, and an upper end configured to receive the cap; and

wherein the head-body and the cap are configured such that:

a first rotation of the cap locks the polyaxial screw in position, and

a second rotation of the cap locks the rod in position.

8. The device ofclaim 7, wherein the head-body includes one or more gaps to facilitate flexion of the head-body as a rotational torque is placed on the cap.

9. The device ofclaim 7, further including a compression-enhancing bushing disposed between the rod and the ball-shaped head of the screw.

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