Patent Application
20090182384
The present application is directed to a pedicle screw assembly and, more specifically to a pedicle screw assembly with elements constructed of different materials.
Various conditions may lead to damage of vertebral members and/or intervertebral discs. The damage may result from a variety of causes including a specific event such as trauma, a degenerative condition, a tumor, or infection. Damage to the intervertebral discs and vertebral members can lead to pain, neurological deficit, and/or loss of motion. Elongated members, such as but not limited to rods, bars, and plates, may extend along the spine to redistribute stresses and/or restore proper alignment of the vertebral members. The elongated members may be substantially straight, or include a curved configuration to conform to the curvature of the spine.
One or more pedicle screw assemblies attach the elongated members to the vertebral members. The assemblies are usually connected to the vertebral members at points along the spine where the elongated members are to be located. The assemblies should securely connect with the elongated members and provide a strong anchor for maintaining the position of the elongated member. The connection with the elongated member often proves difficult because of the stresses imposed to restore proper alignment of the vertebral members.
The assemblies should be constructed of materials with sufficient strength to withstand the stress induced by the spinal realignment. However, the assemblies are often bulky, and the materials used may interfere with magnetic resonance imaging, as well as impose dangers on the patient.
The present application is directed to embodiments of a pedicle screw assembly to position an elongated member within a patient. The assembly may include a receiver with a channel sized to receive the elongated member and a chamber. The assembly may also include an anchor with a head sized to fit within the chamber. The assembly may include a compression member sized to fit within the chamber and include a first side that contacts against the head and a second side that contacts against the receiver. At least one of the receiver, anchor, and compression member may be constructed of a first material, and at least one of the receiver, anchor, and compression member may be constructed of a second material. The first and second materials may include different moduli of elasticity to prevent deformation of the assembly.
The present application is directed to embodiments of a pedicle screw assembly 10.
The number of elements of the assembly 10 constructed of the first and second materials may vary. In one embodiment, only one element is constructed of the first material, with the other elements being constructed of the second material. In another embodiment, multiple elements are constructed of the first material and multiple elements are constructed of the second material. In one embodiment, one or more of the elements is constructed of a third material. Likewise, the number of elements constructed of at least two different materials may vary.
The sidewalls 22 extend from the base 20 and are spaced apart to form a channel 24 sized to receive the elongated member 60. A seating surface 25 may form a lower portion of the channel 24. In one embodiment, the seating surface 25 is curved to substantially match the radius of the elongated member 60 positioned within the channel 24. In one embodiment, the receiver 20 may then be free to rotate and pivot about the head 51 when the elongated member 60 is secured within the channel 24. In another embodiment, the seating surface 25 is positioned such that the elongated member 60 contacts the head 51. For such an embodiment, when the elongated member 60 is secured in the channel 24 it engages the head 51 and locks the position of the receiver 20.
The sidewalls 22 may include threads 26 to receive the set screw 30. Threads 26 may be positioned on the interior of the channel 24 as illustrated in
The chamber 23 is positioned in a lower section of the base 21 and is sized to receive the head 51. The chamber 23 includes a central section with a width to accommodate the head 51. Upper and lower constrictions 27, 28 are positioned on each side of the central section to capture the head 51. Each constriction 27, 28 includes a width smaller than the head 51 to maintain the head 50 within the chamber 23. The constrictions 27, 28 may be formed by the receiver 20 itself, or may be formed by additional elements operatively connected to the receiver 20, such as the compression member 40, or a locking ring 75 (
An exterior surface 29 of the receiver 10 may be generally rounded. Other shapes may also be considered when advantageous for a particular application. For example, the exterior surface 29 may include a flat surface (not shown) to allow a reduced clearance between the receiver 10 and an adjacent receiver 10. A bore 81 may extend through the sidewall 22 and receive a second set screw (not shown) to secure the elongated member 60 within the channel 24.
Set screw 30 attaches to the receiver 20 to capture the elongated member 60 within the channel 24. In one embodiment, the set screw 30 is substantially disc-shaped and is sized to fit within the interior of the channel 24 between the sidewalls 22. Set screw 30 includes exterior threads 31 that engage with the sidewall threads 26. When fully mounted within the channel 24, set screw 30 may apply a compressive force through the elongated member 60 to the head 51 to lock the angular position of the anchor 50 relative to the receiver 20. In another embodiment (not illustrated), set screw 30 is attached to an exterior of the sidewalls 22 and includes a central opening that extends around the receiver 20.
The anchor 50 secures the receiver 20 to a vertebral member. Anchor 50 includes the head 51 and a shaft 52 with helical threads 53 on an outer surface. The head 51 is positioned at an end of the shaft 52 and may include a variety of shapes. Anchor 50 may also be constructed as rivets and pins each with a first end that attaches to the receiver 20, and a second end that attaches to the vertebral members.
The compression member 40 is positioned between the elongated member 60 and head 51. The compression member 40 includes a first side 41 that forms a bearing surface to contact the head 51 and a second side 42 that contacts the elongated member 60. In one embodiment, the second side 42 includes a curved surface that substantially matches the curved shape of the head 51.
The assembly 10 is formed with at least one of the elements constructed at least in part of a different material than the other elements.
A variety of different materials may be used for the assembly 10. In one embodiment, the different materials are selected to provide different physical properties to particular elements. In one embodiment, one or more of the elements is constructed of titanium and one or more elements are constructed of cobalt-chrome. In one embodiment, each of the different materials contains less than 1% of nickel.
In another embodiment, at least one of the elements is constructed of stainless steel. It may be desirable for the entire assembly 10 to be constructed of stainless steel, however, stainless steel may exhibit undesirable properties as an implant material. Because stainless steel is relatively heavy and an entire assembly 10 constructed of stainless steel may be burdensome to the patient. Stainless steel also presents problems with magnetic resonance imaging (MRI). Stainless steel is a ferromagnetic material, and elements constructed of stainless steel may be physically moved by the strong magnetic fields produced during an MRI. Stainless steel may also produce artifacts (areas of empty space in the MRI image) around the elements. Additionally, stainless steel elements may increase infection rates, and patients with an allergy to nickel may not tolerate stainless steel receivers. Therefore, a limited number of the elements are constructed of stainless steel to take advantage of the desirable properties, while the other elements are constructed of different materials to reduce the undesirable properties.
The assembly 10 may be constructed of a variety of different materials. Examples include but are not limited to titanium, cobalt chrome, and stainless steel.
The individual elements may also be constructed of two or more different materials. In one embodiment, the receiver 20 includes the base 21 constructed of a first material 91, such as titanium, and the sidewalls 22 constructed of a second material 92, such as cobalt-chrome. The different materials 91, 92 may be necessary because the sidewalls 22 are exposed to forces applied through the elongated member 60 and/or the set screw 30. The forces may cause the sidewalls 22 to splay outward from the channel 24 causing the set screw 30 and the elongated member 60 to loosen or even escape from the receiver 20. Therefore, sidewalls 22 are constructed of the second material 90 to provide greater resistance to these forces.
The different materials are discrete sections that are connected together to form a unitary element. Further, the sections are connected together to form a complete element prior to insertion into the patient. This prevents the sections of the elements from separating while being inserted into the patient.
A variety of different methods and structures may be included to connect the sections.
The sections may also include mating surfaces to facilitate the connection between the different materials.
The sections constructed of the different materials may be connected together in a variety of manners. Examples include but are not limited to diffusion bonding, electron beam welding, and biocompatible adhesive. Diffusion bonding is a solid-state joining process capable of joining a wide range of metal combinations. The process may be applied over a variety of durations, applied pressure, bonding temperature, and method of heat application. The bonding is typically formed in the solid phase and may be carried out in vacuum or a protective atmosphere, with heat being applied by radiant, induction, direct or indirect resistance heating. Electron beam welding is a fusion welding process in which a beam of high-velocity electrons is applied to the materials being joined. The sections melt as the kinetic energy of the electrons is transformed into heat upon impact. Pressure is not necessarily applied, though the welding is often done in a vacuum to prevent the dispersion of the electron beam. A biocompatible adhesive is applied to one or both sections and forms a permanent connection. In addition, multiple connection methods may be used on the same sections (e.g., diffusion bonding and biocompatible adhesive).
In one embodiment, the assembly 10 includes a compression member 40. In another embodiment as illustrated in
Spatially relative terms such as “under”, “below”, “lower”, “over”, “upper”, and the like, are used for ease of description to explain the positioning of one element relative to a second element. These terms are intended to encompass different orientations of the device in addition to different orientations than those depicted in the figures. Further, terms such as “first”, “second”, and the like, are also used to describe various elements, regions, sections, etc. and are also not intended to be limiting. Like terms refer to like elements throughout the description.
As used herein, the terms “having”, “containing”, “including”, “comprising”, and the like are open ended terms that indicate the presence of stated elements or features, but do not preclude additional elements or features. The articles “a”, “an” and “the” are intended to include the plural as well as the singular, unless the context clearly indicates otherwise.
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.
