Patent Application
20090018589
The present invention relates generally to devices and methods for the stabilization of bone structures, and more particularly to bone screws and medical devices used to stabilize bone structures.
Bone screws are used for a multitude of purposes, including attaching various medical devices to bone. One such application involves the use of bone screws to attach a plate to adjacent bone structures or fragments to promote fusion of the structures or fragments into a unitary structure.
As an example, bone screws can be used to attach an anterior cervical plate to the relevant portion of a patient's anterior cervical spine to achieve fusion. Anterior cervical spinal fusion is a common approach for the surgical management of cervical disk disease, in which two or more vertebrae are brought together under conditions whereby the vertebrae fuse together to form a unitary member of the spinal column. Frequently, if there is significant spinal cord compression or if there is more than one disk level involved, a small plate is affixed on the anterior surface of the cervical vertebrae to provide greater permanent stability. In carrying out the procedure, the members must be brought together under conditions that are critically controlled to prevent infection, maintain alignment of opposing members, and allow for the stress in the bone that is generated as the healing process matures. According to the present art, the plates commonly used for cervical spinal fusion are fabricated from titanium with holes through which screws are inserted to secure the plate to neighboring vertebrae.
An additional application of bone screws in the spinal field is in the stabilization of the lumber spine. In one typical procedure, a bendable rod is attached by bone screws to various vertebrae along a portion of the patient's spine. Such a rod may have apparatus, including holes, configured to receive bone screws to secure the rod to the patient's vertebrae.
As another example, bone screws can be used to attach sternal closure plates to a patient's sternum following a median sternotomy. Many surgical procedures require a median sternotomy, a procedure in which an incision is made along the sternum such that it can be separated laterally into two sides, to allow access to the tissues and organs located in a patient's thoracic cavity. Following the surgical procedure, the two sides of the sternum must be secured together, which may be accomplished by attaching a plating system to them. The plating systems used for sternal closure may have holes through which bone screws are inserted to secure the plating system to the two sides of the sternum.
It has been recognized that osseous trans-differentiation during the course of bone healing may have a significant role in improving the structural integrity of healed bone following surgery or trauma. This process of bone remodeling may, however, result in partial extrusion of the bone screws placed during surgery to fix the position of bones or fragments if the fusion does not take place. Post-operative screw extrusion can be dangerous and may require further surgery. Locking the screws into place to disallow any degree of post-operative movement has been found to be undesirable because it precludes trans-differentiation, but it can be important that the movement of the screws be somewhat restricted to prevent more than slight screw extrusion.
The design of the bone screw is important, as it affects the speed and ease of implantation during surgery, the stability of fastening following surgery, and the potential for screw extrusion, among other characteristics. For example, some prior art screw designs require a drilling operation and/or a tapping operation to prepare the bone for the screw prior to its insertion. Such operations have numerous disadvantages, including the danger associated with introducing an additional instrument into the spinal canal during surgery, the potential for fracture formation, and the removal of bone material. Further, shearing of the thread crests during insertion has been experienced with some prior art screw designs, leading to the release of tiny metal shards in the patient. Further, some prior art screw designs have caused problems with loosening, breaking, and extruding from the bone. Again, this can create a dangerous condition that requires further surgery. Therefore, it is desirable to have a new and improved bone screw.
In addition to the design of the bone screw alone, the design of the bone screw in relation to a corresponding medical device, such as an anterior cervical plate or sternal closure plate, can affect the potential for screw extrusion. It is desirable to have a corresponding bone screw and plate system that allows only a beneficial degree of post-operative screw extrusion.
An improved bone screw is provided for securing a medical device to bone. In one embodiment, the bone screw includes a screw head, a shaft with multiple regions therein, and a sharp tip. The shaft regions include a smooth cylindrical shaft region, a first cylindrical shaft region with external threading having a first pitch and first crest width, a second cylindrical shaft region with external threading having a transitional pitch and crest width, and a tapered shaft region with external threading having a second smaller pitch and second smaller crest width and further having at least one recessed flute. Each of the shaft regions is optional in a particular embodiment of the bone screw.
In one embodiment, the head includes a recessed portion for receiving a tool. In one embodiment, the tip has no threading or fluting.
The external threading comprises a forward face and a backward face. In one embodiment, the angle between the forward face and the central axis of the screw is substantially constant throughout the threading. In one embodiment, the angle between the backward face and the central axis of the screw is substantially constant throughout the threading. In a further embodiment, both angles are substantially constant throughout the threading.
In one embodiment, the shaft diameter is substantially equal in the smooth cylindrical shaft region and the first and second threaded cylindrical shaft regions. In one embodiment, the diameter of the threading is substantially constant along a majority of its length. In one embodiment, there is a ramping region between the smooth cylindrical shaft region and the first cylindrical shaft region. The ramping region has external threading that progressively increases in depth.
In one embodiment, the tapered shaft region includes three recessed flutes. In a particular embodiment, each of the three flutes spans an angle of about 90 degrees.
In one aspect, a bone screw is provided for engaging with a bone screw hole in an anterior cervical plate. In one embodiment, the bone screw includes a sharp tip, a shaft with multiple regions therein, and a head. The shaft regions include a tapered shaft region with external threading, having a threading diameter smaller than the diameter of the screw hole along at least a portion of this region; a first cylindrical shaft region with external threading, at least a portion of which is sized and shaped to engage with the screw hole; and a second cylindrical shaft region without external threading. Each of the shaft regions is optional in a particular embodiment. The diameter of the shaft in all shaft regions is smaller than the diameter of the screw hole.
In one embodiment, the head includes a recessed portion for receiving a tool. In one embodiment, the tip has no threading or fluting.
The external threading comprises a forward face and a backward face. In one embodiment, the angle between the forward face and the central axis of the screw is substantially constant throughout the threading. In one embodiment, the angle between the backward face and the central axis of the screw is substantially constant throughout the threading. In a further embodiment, both angles are substantially constant throughout the threading. In one embodiment, the diameter of the threading is substantially constant along said first cylindrical shaft region. In one embodiment, the diameter of the shaft is substantially equal in the first cylindrical shaft region and second cylindrical shaft region.
In one embodiment, there is a ramping region between said first cylindrical shaft region and said second cylindrical shaft region. The ramping region has external threading that progressively increases in depth.
In one embodiment, the tapered shaft region includes at least one recessed flute. In a further embodiment, the tapered shaft region includes three recessed flutes. In a particular embodiment, each of the three flutes spans an angle of about 90 degrees.
In another aspect, an anterior cervical plating system is provided. The plating system includes an anterior cervical plate with at least one bone screw hole, and a bone screw. The bone screw includes a head, a body, and a tip. The body has a region adjacent to the head that is unthreaded, and the rest of the body has external threading. The diameter of the head is larger than the diameter of the bone screw hole. The diameter of the body is smaller than the bone screw hole. At least a portion of the threading has a diameter that is larger than the diameter of the bone screw hole, and that portion engages with the bone screw hole when the screw is rotated through the hole. When the screw is fully inserted in the hole, the unthreaded portion of the screw is positioned within the hole.
The objects and advantages of the present invention will be better understood and more readily apparent when considered in conjunction with the following detailed description and accompanying drawings which illustrate, by way of example, preferred embodiments of the bone screw and in which:
As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure.
The bone screw can be further understood with reference to the exemplary, non-limiting embodiments illustrated in
The “front” of the screw will be used herein to refer to the end with the tip, and the “back” of the screw will be used herein to refer to the end with the head.
One embodiment of the bone screw is shown in perspective view in
The head 110 may be any shape, such as round, oval, pentagonal, and hexagonal. In one embodiment, as shown in
For other applications, such as with a lumbar spinal rod, the screw may have an approximately spherical head, or polyaxial head, as known in the pedicle screw field.
The head may have a recessed portion for receiving a rotatable tool, such as a drill or screwdriver. The shape, depth, and other dimensions of the recessed portion may vary to correspond with a given rotatable tool. In one embodiment, as shown in
Shown adjacent to the head 110 in
In one embodiment, the pitch and crest width vary in multiple regions within the threaded portion of the body 124, as shown in
One sub-region of the body 124 is a first threaded region 114 with external threading having a first pitch and a first crest width. The first threaded region 114 may comprise any number of revolutions. The number of revolutions in the first threaded region 114 does not have to be an integer; that is, the region could end with an incomplete revolution. In one embodiment, the first threaded region 114 comprises between about 1 and about 6 revolutions of thread. In a particular embodiment, the first threaded region 114 comprises about 3 revolutions. The pitch of the thread remains substantially constant throughout the first threaded region 114. The pitch in the first threaded region 114 is larger than the pitch in the third threaded region 118. In one embodiment, the pitch in the first threaded region 114 is between 3 mm and 9 mm, and in one particular embodiment it is about 6 mm.
The crest width of the threading also remains substantially constant throughout the first threaded region 114, and is larger than the crest width in the third threaded region 118. In one embodiment, the crest width in the first threaded region 114 is between 0.3 mm and 0.5 mm. Advantageously, a crest width of this relatively large size resists shearing upon contact with a screw hole, such as in a spinal stabilization plate.
The first threaded region 114 has a shaft diameter, shown in
The angle between the forward face 450 and the central axis, shown close-up in
The first threaded region 114 may comprise a portion in which the depth of the threading, measured from the outer surface of the shaft to the crest, progressively increases. After the unthreaded region 112, the threading may ramp out of the screw shaft and increase in depth. After the initial ramping, the threading diameter of the first threaded region 114 may be substantially constant.
Alternatively, there may be a separate ramping region 126 between the unthreaded region 112 and the first threaded region 114, in which the depth of the threading progressively increases and the pitch and crest width may differ from the pitch and crest width in the first threaded region 114. A ramping region 126 is shown in part in
The region shown in
The transitional region 116 may comprise any number of thread revolutions. The number of thread revolutions in the transitional region 116 does not have to be an integer; that is, the region could end with an incomplete revolution. In one embodiment, the transitional region 116 comprises between less than 1 revolution and about 3 revolutions of the thread. In a particular embodiment, the transitional region 116 comprises about 1 revolution. The transitional region 116 has a shaft diameter, shown in
The angle between the forward face 450 and the central axis, shown close-up in
The region shown in
In a particular embodiment, there are 3 flutes 122, each spanning about a 90 degree angle, as shown in
Despite that the threading may not be continuous, the pitch of the threading that is present in the third threaded region 118 remains substantially constant. The pitch in the third threaded region 118 is smaller than the pitch in the first threaded region 114. In one embodiment, the pitch in the third threaded region 118 is between 1 mm and 5 mm, and in one particular embodiment is about 3 mm. The crest width of the threading also remains substantially constant throughout the third threaded region 118, and is smaller than the crest width in the first threaded region 114. In one embodiment, the crest width in the third threaded region 118 is between 0.05 mm and 0.2 mm. The small crest width may be advantageous because it allows the threading to have sharp crests that bite into the bone during insertion of the screw 100. In one embodiment, the crest width is about 0.13 mm. For certain applications, such as those requiring a larger screw, a larger crest width may be desired. The diameter of the shaft, measured from its outer surface to the central axis, may taper over the entire third threaded region 118 or a portion of the third threaded region 118. The tapering occurs in the direction toward the tip 120.
The angle between the forward face 450 and the central axis, shown close-up in
In one embodiment, both the forward angle and the backward angle remain substantially constant throughout all threading along the body 124 of the screw. These angles should be chosen to contribute to the screw having maximal bite in the bone and gaining sufficient purchase for secure placement in the bone. In a particular embodiment, the forward angle remains about 60 degrees and the backward angle remains about 87 degrees throughout the threading.
At the front end of the screw is a tip 120. The tip 120 has a sharp point and can be without threading or fluting. Advantageously, the sharp tip 122 acts similar to an awl and eliminates the need for the target area on the bone to be pre-drilled in preparation for the screw's insertion. The length of the tip 122 may vary. In one embodiment, the length of the tip 122 is less than 1 mm. For certain applications, such as those requiring a larger screw, a longer tip may be desired. The side angle of the tip 120, defined as the angle between the central axis and one side of the tip 120, may vary. In one embodiment, the side angle is about 35 degrees, as shown in
The bone screw may be made of any biocompatible material. In one embodiment, the bone screw is made of titanium alloy. The bone screw may be employed with any type of medical device.
The features of the bone screw may have several functional advantages, some of which are discussed above. Additional advantages may be gained by the combination of the bone screw with a bone screw hole in a medical device, for example an anterior cervical plate or sternal closure plate.
A portion of the external threading on the screw can engage with a screw hole on a plate or other medical device. One embodiment of a bone screw designed for combination with an anterior cervical plate is shown in
Once the diameter of the external threading exceeds the diameter of the screw hole, which can occur in the tapered shaft region 856 or in the first cylindrical shaft region 854 of the screw 800, the screw 800 cannot pass further through the hole without the threading engaging with the edges of the hole. In one embodiment, the threading diameter is substantially constant in the first cylindrical shaft region 854. As described above, and shown close-up in
With further rotation, the threading continues to engage with the hole throughout the first cylindrical shaft region 854, and the rest of the screw 800, except for the head 850, may pass through the hole. At the point the unthreaded second cylindrical shaft region 852 reaches the hole, there is no threading engaged with the hole, meaning the screw 800 is disengaged from the hole. Thus, when fully inserted such that only the head 850 remains above the plane of the hole, the screw 800 is disengaged from the hole. The dimensional relationship of the unthreaded region 852 to the hole may vary, but in one embodiment, the unthreaded region 852 has a diameter small enough to avoid a tight fit in the hole. The length of the unthreaded region 852 also may vary, but preferably the length of the unthreaded region 852 is greater than the depth of the screw hole edges. That is, the screw can be inserted such that only the unthreaded region 852 extends through the screw hole, with the screw head 850 and threading to the top and bottom, respectively, of the hole.
The transition from the threaded first cylindrical shaft region 854 to the unthreaded second cylindrical shaft region 852 may be advantageous in multiple respects. Given the disengagement of the screw 800 from the plate when the unthreaded region 852 passes through the hole, the screw 800 is not locked in place with respect to the plate. This may allow some movement of the screw 800 with respect to the plate, as can be caused by trans-differentiation during the bone healing process. Further, the unthreaded region 852 may be of a sufficient length to cause disengagement when the screw 800 is inserted with up to 15 degrees of variance from an angle perpendicular to the plane of the hole. This allows for beneficial engagement to occur given a range of normal variability in the insertion of a bone screw. Further, when the screw is fully inserted such that the unthreaded region 852 extends through the hole, as shown for one embodiment in
Between the threaded first cylindrical shaft region 854 and the unthreaded second cylindrical shaft region 852, there may be a ramping region with external threading that increases in depth, the depth being measured from the outer surface of the shaft to the thread crest. The ramping region may comprise any number of thread revolutions. The number of thread revolutions in the ramping region does not have to be an integer; that is, the region could end with an incomplete revolution. In one embodiment, the ramping region comprises less than a full revolution of threading.
The bone screw and corresponding medical device may be made of any biocompatible material. In one embodiment, the bone screw and medical device are made of titanium alloy.
The bone screw is not limited to use with the devices described herein, and may be employed with any type of medical device. The medical device may have any number of bone screw holes.
