The current invention is directed to a reversibly expandable fixation device for application in osteoporotic vertebral bodies.
Internal fixation systems have been a major step forward for patients with healthy underlying bone structure; however, such systems have proven generally unreliable for patients with osteoporosis. Specifically, the poor quality of the bone into which the anchor screws and plates are driven in these osteoporotic patients often leads to screw loosening and mechanical failure of the implant. (See, e.g., Goldhahn, et al., J. Ortho. Res. 917-925, (2006), the disclosure of which is incorporated herein by reference.) This is particularly tragic as such patients are most often in need of the type of spinal fusion such systems are meant to address.
There have been a few attempts to address the need for improved fixation systems that can compensate for the inferior holding strength in osteoporotic bone and vertebrae. One solution has been to enlarge the implant/bone interface by increasing the surface area of the fixation screw. For example, Schroeder and colleagues attempted to provide a fixation screw with improved surface area by using a hollow perforated cylinder. (See, e.g., Schroeder, et al., SSO Schweiz Monatsschr Zahnheilkd 86:713-727, (1976), the disclosure of which is incorporated by reference.) Goldhahn addressed the question of improved fixation stability by creating a three-lobed fixation screw with an elliptical cross-section to increase the periphery volume of the implant. (See, e.g., Goldhahn, et al., WIPO Pub. WO 01/80754 A1, the disclosure of which is incorporated herein by reference.) Another proposed solution has been to insert a porous cannulated screw that would allow the injection of cement around the screw thereby enhancing the stability of the bone/implant interface. (See, e.g., Fransen, J. Neurosurg. Spine 7:366-369, (2007), the disclosure of which is incorporated herein by reference.)
However, all of these methods continue to rely on the structural stability of a threaded body. Inherently such screws have a large polar moment of inertia and are therefore prone to cutting through the cancellous bone under the types of repetitive loads typically experienced in spinal fusions. Accordingly, despite the longstanding interest in finding appropriate biomechanical solutions to the problem of cervical stability in osteoporotic patients, and the extensive research conducted in the area, researchers and practioners continue to search for an internal fixation system that provides maximal interface stability between the bone and implant.
The current invention is directed to a vertebral fixation screw for use in bone requiring additional stabilization, and more particularly to an expandable vertebral fixation screw.
In one embodiment, the expandable vertebral fixation screw comprises an expandable body capable of locking against anatomical structures within the pedicle of the vertebrae. In such an embodiment, the tip of the screw may be expandable.
In another embodiment, the screw is expanded using a central expansion rod.
In still another embodiment, the screw is expanded using a counter-rotation mechanism.
In yet another embodiment, the screw is cannulated to allow minimally invasive surgical techniques.
In still yet another embodiment, the screw is cannulated and/or porous to allow for the injection of cement or osseous integration materials into the pedicle.
The invention is also directed to a method of stabilizing a patient's spine using the expandable screws of the current invention.
These and other features and advantages of the present invention will be better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein:
a shows a schematic diagram of an exemplary expandable screw inserted into the vertebral body of a patient in an unexpanded state;
b shows a schematic diagram of an exemplary expandable screw inserted into the vertebral body of a patient in an expanded state;
a shows a schematic diagram of an exemplary expandable screw in an unexpanded state;
b shows a schematic diagram of an exemplary expandable screw in an expanded state;
a shows a schematic diagram of another exemplary expandable screw in an unexpanded state; and
b shows a schematic diagram of another exemplary expandable screw in an expanded state; and
The current invention is directed to a vertebral fixation screw for use in bone requiring additional stabilization, and more particularly to an expandable vertebral fixation screw.
Prior to describing the expandable screw of the current invention it is important to understand the environment the screw is to be engaged in. Vertebral fixation screws are generally driven into the cancellous bone found in the center of the vertebral body. For the purposes of this application this cancellous bone can be imagined as a scaffolding formed of horizontal, vertical and cross support members. Between these supports are large linking molecules, which are typically calcium salts. In healthy bone there is a large concentration of these molecules and supports such that a dense network is formed that can provide the support necessary for locking a threaded screw into place once inserted into the bone. The screw is locked into place because the interlocking molecules and scaffolding prevents the network of material from racking and twisting when placed under a load.
However, in unhealthy bone, such as that encountered with osteoporotic patients, there is a 50 to 60% reduction in the concentration of both the scaffold and linking molecules in this cancellous bone. Unless some alternative support can be provided there is simply not enough density in the bone to ensure that the screw doesn't twist and cut through the weak bone once place under load.
In situations where poor bone quality is encountered a rescue screw would typically be used. For example, rescue screws have been used in situations where a screw has been inserted into the cortical tables of the skull, but does not have sufficient purchase and is therefore loose, or in the vertebrae if poor fixation has been encountered and a larger screw is not available. Currently, there are only two choices; one is to put in some bone cement, and the other would is to take out the poorly seated screw and put in a screw having a bigger diameter. However, both of these solutions still depend on anchoring the screw within the weak cancellous bone. Moreover, there is an limit to the size of screw that can be inserted into a vertebral body limiting the efficacy of such solutions.
The screw of the current invention allows for the expansion of the tip of the screw after insertion in those cases where it is not possible to anchor the screw into the cancellous bone of a vertebral body. By expanding the tip of the screw you allow for the anchorage of the screw against the far end of the pedicle shaft in the cortical bone, thus avoiding the inherent problems in anchoring into the cancellous bone of such patients. Moreover, by expanding the screw and catching the anterior portion of the pedicle, it is possible to obtain a seating strength that is greater than 50% of the maximum theoretical pullout strength. As a comparison, studies from the 1980s show that one could only obtain a 100% pullout strength if the screw were driven all the way though the front of the vertebrae. However, such a technique is never recommended as it raises the possibility that the screw could be driven into the back of the aorta, a possibly fatal complication.
In addition, by actively and mechanically anchoring the screw against the wall of the bone the current invention creates immediate stability, which allows quicker recovery for the patient and also allows the screw to become osseously integrated with the bone. The immediate stability of the screw is important because if the bone is allowed to shift within the pedicle, cleavage lines will form around the screw and its thread pattern preventing successful and complete integration with the surrounding bone.
Turning to the structure of the expandable screw of the current invention, as shown in
The one feature of the screw that is critical to the operation of the current invention, as shown in
Turning now to the operation of the expandable screw of the current invention. It should first be understood that although two possible expansion mechanisms are discussed below, any expansion mechanism, such as a molly bolt or expansion bolt, that allows for the reversible expansion of at least the tip of the threaded portion of the screw may be used with the expandable screw of the current invention.
For example, in a first exemplary embodiment as shown in
In a second exemplary embodiment, as shown in
Although the above discussion has focused only on the expansion of the threaded portion of the screw, as will be understood, to reverse the expansion a surgeon would simply need to reverse the rotation of the internal threaded portion, which would in turn contract the divided components of the threaded portion of the screw. Such reversibility is essential since in osteoporotic patients it is often necessary to perform revision surgeries if a patient's condition deteriorates. In addition, such reversibility allows for removal of the device should complications arise during the procedure.
Although the above discussion has focused on the mechanical components of the screw, it should be understood that the current invention also contemplates the incorporation of osseous integration materials to assist in the fixation of the screw into the vertebral body. Such fixation material can take two basic forms. First, as previously discussed the screw (10) of the current invention may be made hollow and/or porous (38), as shown in
All of the above components, including can be made of any suitable surgical material, such as, for example, stainless steel or titanium.
Finally, the inclusion of a hollow cannula (38) in the center of the screw allows for the use of the screw with percutaneous minimally invasive surgical methodologies. It should be understood that the current invention is also directed to the use of the expandable fixation screw of the current invention in such a minimally invasive surgical technique. In such an embodiment, a small incision is made and an anchor guidewire is attached to a point along the vertebral body where insertion of the screw is desired. The anchor guidewire may be positioned by any conventional means, such as, for example, by a suitable catheter. In such an embodiment, the hollow cannula is provided with an outlet (40) in the tip of the threaded shaft (14). The guidewire can be threaded through the hollow cannula (38) of the screw thereby allowing for the screw to be directed down the guidewire through the opening in the patient to the insertion point without the surgeon needing to confirm the placement of the screw visually. Once in position the screw can then be operated as set forth above.
Although specific embodiments and exemplary embodiments are disclosed herein, it is expected that persons skilled in the art can and will design alternative reversible expandable fixation devices and methods that are within the scope of the following claims either literally or under the Doctrine of Equivalents.
The current invention claims priority to U.S. Provisional Patent Application No. 60/864,478, filed Nov. 6, 2006, the disclosure of which is incorporated herein by reference.
Number | Date | Country | |
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60864478 | Nov 2006 | US |