The present invention relates to interbody fusion devices disposed between two bony structures to maintain spacing therebetween and promote bony fusion. More specifically, the present invention relates to interbody fusion devices having threads or other structures defined on the outer surface to limit movement of the device between the bony structures. Most often, such devices find application in the spine for fusing adjacent vertebrae.
It is well known to utilize an artificial spinal fusion implant to be inserted in the space between two adjacent vertebra after removal of a damaged spinal disc or portion thereof. Common forms of such devices may be configured in a substantially cylindrical configuration. These cylindrical configurations can include truncated sidewalls or a tapering body portion. However, for bone fusion to occur, the invasion of new delicate blood vessels from the adjacent healthy bone is necessary for the creation of new living interconnecting bone. Motion around the implant can restrict or even prevent bone healing. Therefore, it is important to stabilize the implant upon insertion. In most applications, the outer body of the fusion device is provided with one or more structures to resist repulsion from the disc space when a load is applied to the spinal column. For example, U.S. Pat. No. 5,015,247 issued to Michelson discloses substantially cylindrical interbody fusion devices with an external thread disposed on the outer surface. The thread is interrupted at various locations. Further, the trailing portion of the thread may be twisted slightly. This twisted portion of the trailing edge acts as a locking thread to resist subsequent unscrewing of the fusion device. While such interrupted locking threads may be satisfactory in preventing unscrewing, the manufacturing process of twisting each of the individual teeth to create a locking thread may be costly and difficult to control from a quality aspect.
Thus, there remains a need for an improved interbody fusion device that incorporates features to resist undesired rotation after implantation while at the same time simplifying the insertion of the implant into a human body and minimizing manufacturing complexity.
The present invention provides an interbody fusion device having structures to limit rotation in at least one direction after the device is implanted between two bony structures. In one aspect of the present invention, an improved interbody fusion device is provided with an anti-rotation thread cut out. In this aspect, the fusion device includes a body portion with a thread pattern defined thereon. The thread pattern extends at least partially from a first end to an opposite second end. At least one thread is interrupted by a thread cut out on a trailing edge of the thread. The thread cut out includes an undercut portion extending beneath the outer surface of the interrupted thread to create a barb. When force is applied, attempting to unscrew the device, material from the thread path may be urged into the undercut area and retained there to resist accidental unscrewing. Preferably, the thread cut out also creates a relatively sharp pointed barb. The pointed barb may tend to impale bone tissue, further increasing resistance to unscrewing.
In a further aspect, the invention provides an interbody fusion device with an outer surface extending between a first end an opposite second end. A thread pattern is defined on the outer surface and extends at least partially between the first and second ends. A first thread adjacent the first end has a first height and a second thread adjacent the second end has a second larger height. The height of intervening threads between the first and second threads includes a substantially continuously tapering height increasing from said first thread to said second thread. In a preferred aspect, the outer surface of the fusion device has a substantially continuous outer diameter. When inserted into the disc space with the first end leading the insertion, the first thread travels through a thread path and each subsequent larger thread expands the thread path by engaging bone not contacted by the preceding thread. In this manner, resistance to further screwing of the device into the disc space is increased. Thus, the expanding thread pattern resists accidental movement of the device into the disc space as a result of further rotation.
In yet a further preferred aspect of the present invention, an interbody fusion device is provided incorporating both a thread pattern having a cut out on the trailing surface of the thread to resist accidental unscrewing and a tapering thread height to resist accidental advancing. This combination provides an improved interbody fusion device resistant to accidental rotation after implantation.
These and other objects of the present invention will be apparent from the following description of the preferred embodiments.