The present invention relates to bone screws used in orthopedic spinal surgical procedures. Specifically, the invention is directed to a left-handed bone screw.
Several techniques and systems have been developed for correcting and stabilizing the spine and facilitating spinal fusion. Over the years, spinal and orthopedic implants have evolved toward progressively stronger and stiffer devices, since it is presumed that increased construct rigidity optimizes the bone fusion and provides more rapid and robust healing. The most widely used systems use a bendable rod that is placed longitudinally along the length of the spine. In such a procedure, a rod is attached to various vertebrae along the length of the spinal column by a number of bone anchor assemblies. A bone anchor element may be a hook that engages the vertebra laminae or a bone screw threaded into the vertebral bone.
In present day bone screw assemblies, rods are typically situated on opposite sides of the spine or spinous processes. Numerous bone screws are screwed into the pedicles of the vertebral bodies. Rods are then affixed to these bone screws through various connectors so corrective and stabilizing forces are applied to the spine. When stabilized, the vertebra is decortified where the outer cortical bone is removed to provide a foundation for bone grafts. Over time, these bone grafts fuse the damaged vertebrae together.
Bone screws are well known in the art. The threads, for example, of a bone screw anchor it to the bone and keep the screw from being axially pulled out of the bone. Such threads also cut a helical path into the bone as the screw rotates into the bone.
As shown in
There are many aspects of thread shape in the art. The thread can have a sharp or blunt apex. The pitch of the thread can be varied. The height of the thread can be deep or shallow. The thickness of the thread can change. In all screws, however, threads have a superior surface facing the screw head and an inferior surface facing the screw tip. The engagement of the superior surface with the bone provides resistance to screw pull-out. The screw thread can be further modified by varying the cross-sectional shape of the thread from the tip to the head of a screw. In most screws, the thickness of the thread near the tip has a narrow cross-section to cut into the bone as the screw rotates into the bone. The thread and core become thicker toward the head of the screw to increase thread strength and to displace bone matter downward against the superior thread surface. This is particularly advantageous in bones because of the hard cortical bone shell. The cortical bone is harder and more compact than the spongy cancellous bone in the bone center. The cortical bone provides the bulk of the bone's resistance to screw pull-out forces (axial load forces). The thread and core near the screw head engages the cortical bone and, thus, carry much of axial load on the screw.
A common problem among all bone screw assemblies is bone screw pull-out. When bone screws are weakened or begin to pullout, bone anchor or screw assemblies may slip. When the bone is strong and healthy, the initial fixation of the spinal and orthopedic screw is usually excellent, with more than adequate pull-out strength. With dense, sclerotic or osteoporotic bone, micro-motions resulting from the normal range of motion within the skeletal system may lead to a progressive degradation from the initially implanted state. In cases where the bone fails to heal, these micro-motions persist and cause the metallic screw to oscillate within the softer cancellous bone. When subjected to persistent toggling with the modulus mismatch of the metal to cortico-cancellous bone, the bone screw becomes loose.
During bone screw pullout, dislodgement and breakage occur because of extreme load, shear, stress and/or torsion. When bone screw failure occurs, it not only weakens the mechanical strength of the bone screw assembly but also lowers the biological potential for bone healing. When bone screws loosen, removal may be necessary because the earlier threads made in the bone by the bone screw are no longer usable. Given the importance of screw threads, it is surprising that there is no or little reference known on the use of right-handedness or left-handedness bone screws.
In summary, there is a need in the industry for improvements in bone screw and bone screw thread design. The present invention describes such an improvement.
The present invention provides an improvement to bone screws, such as vertebral pedicle bone screws, used in orthopedic spinal surgical procedures. Specifically, the present invention reverses the pitch angle to left-handedness from right-handedness.
When bone screw failure necessitates bone screw removal, the reverse thread of the present invention permits the screw to return through the previously tapped cortical and cancellous bone substrate and, more importantly, cuts and taps through the new bone providing a new bone screw with added strength and integrity.
The present invention is shown in
When the male threads 4 are reversed, the pitch angle 20 is shifted either one thread up or down. As the male threads 4 are reversed, the male threads 4 are shifted into new bone substrate 22. This is illustrated in
As shown in
As the left-handed bone screw 40 is rotated downward into the bone, the core thickness of the left-handed bone screw 40 increases. This core continually pushed cancellous bone 36 radiallly outward and, thus, slightly compresses the cancellous bone 36 in between the reverse threads 30. The cancellous bone 36 immediately below the inferior surface is displaced downward against the superior reverse thread 30 compacting the cancellous bone 36 between the reverse threads 30 and against the left-handed bone screw 40. As it does so, it fills in the void 24 left previously by the right-handed bone screw 2. The new cancellous bone 36 in the void 24 now has an opportunity to heal and strengthen the void 24. New cancellous bone 36 may even replace and fill in the void 24 left in the cortical bone 34, thereby, also strengthening it. This compression of the cancellous bone 36 between the reverse threads 30 also orients the bone to oppose screw pull-out. The compaction and orientation of the new bone improves the support provided by the cancellous bone 36, thereby increasing the pull-out resistance of the reverse thread 30 and the left-handed bone screw 40.
The left-handed bone screw 40 and its reverse thread 30 described herein can be customized for particular bone applications. For example, the taper, diameter and cross-sectional shape of the bone screw can be modified on the left-handed bone screw 40 just as the right-handed bone screw 2. The reverse thread 30 may also possess different pitch, height and thickness as other right-handed bone screws 2.
With the present invention, a surgeon can now use the previous right-handed bone screw hole with a left-handed screw having similar or the same size bone screw and similar pitch. The present invention avoids the use of a larger size right-handed bone screw that, in most cases, will further crack and damage the bone. The present left-handed invention also provides the new bone screw with increased strength and integrity. Furthermore, it increases the mechanical strength of the bone screw assembly and may also increase the biological potential for bone healing.
In the foregoing specification, the invention has been described with reference to specific preferred embodiments and methods. It will, however, be evident to those of skill in the art that various modifications and changes may be made without departing from the broader spirit and scope of the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than restrictive, sense; the invention being limited only by the appended claims.
This application claims priority to U.S. Provisional Application No. 61/536,501, filed on Sep. 19, 2011, the disclosure of which is incorporated herein by reference in its entirety for all purposes.
Number | Date | Country | |
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61536501 | Sep 2011 | US |