1. Technical Field
The present invention relates to devices and methods for use in orthopedic spine surgery. In particular, the present invention relates to a bone plate system that provides a low profile anterior vertebral body plate and at least one spike fixation element, the system being used for fixation and stabilization of the spine, the spike fixation elements being located proximate to at least one end of the plate to provide a secure attachment to the underlying bone.
2. Background Art
Disease, the effects of aging, or physical trauma resulting in damage to the spine has been treated in many instances by fixation or stabilization of the effected vertebra. The use of plates and screws for fixation and stabilization of the cervical vertebra has been widely accepted as a reliable practice and has proven to be highly successful clinically.
The various plates, which are attached to the anterior vertebral bodies of the spinal column by bone screws have some common features such as relatively planar body profiles that define multiple holes or slots through which the screws fit and are threaded into the bone. A common problem with plates affixed to bone using conventional bone screw attachments is the unintentional backing out of the screws from the underlying bone with a resulting loosening of the plate from the bone. Various means have been used to prevent the screws from becoming loose or detached from their necessary secured or locked attachment to the vertebral plate. Among the differences between the conventionally used plates and screws is the manner in which the screws are locked into place in the hole or slot of the plate after the screws have been secured to the bone.
These conventional devices attached to the underlying bone using only bones screws can be generally grouped into three basic categories with regard to how the screws are captured or secured in the plates.
Early plate designs were standard bone plates having holes through which screws were passed and screwed into the bone. These plates had no special provision for attaching the screws to the plate and as such were susceptible to having the screws back out of the plate over time. Later innovations included providing a screw head containing a threaded hole configured to receive a set screw, which was inserted into the screw head to expand the diameter of the screw head and frictionally engage the wall of the plate hole, thereby resisting forces which tend to cause the screw to back out. While such mechanisms have worked to some degree, the addition of a small additional part, the set screw, at the time of surgery presents the potential hazard of dropping the set screw into the surgical field or otherwise misapplying the set screw to the screw head, for example, cross threading.
Later alternative approaches included providing plates and/or screws with special features, which were specifically to hold the screw in position once the screw is inserted through the plate and screwed into the bone. One direction taken in this effort has been to design plates that incorporate or attach individual retaining rings, cams or snap features associated with each plate hole configured to hold the inserted screw in place relative to the plate.
Another approach is to add a cover to the plate after the screws have been placed. Such a design undesirably adds steps to the surgical procedure, thickness or height to the overall construct, and is susceptible to misapplication. Yet another direction taken in this effort to provide plates with locking elements is to provide dedicated overlying features, which are attached to the top side of the vertebral plate for the purpose of covering at least a portion of the screw head and thereby holding the screw in a seated and locked position. Generally these plates are designed to provide a variety of screw covering features that are pre-attached to the plate, and which can be selectively slid or rotated into position once it has been inserted. In some devices, such covering plates cover multiple screw heads. These plates typically require an increase in the overall composite thickness of the plate in order to accommodate the additional locking feature attached to the top side of the plate. This is a particularly unacceptable condition due to the use of such plates in an area of the spine where a thin, smooth surfaced profile for the plate assembly is preferred. Another major problem with such plates is that the overlying locking element cannot always be properly positioned over the screw head if the screw shaft was, due to anatomical necessity, positioned through the plate and into the bone at an angle such that the screw head does not fully seat in the plate recess provided on the top side of the plate. Further, when one of the overlying locking elements of such a plate loosens or becomes disengaged it is then free to float away from the top side of the plate and migrate into the soft tissue adjacent to the top side of the vertebral plate.
Yet another approach is to provide machine threads in the plate hole with corresponding threads on the screw head. Thus the screw has a first, bone engaging thread on its shaft and a second machine thread on the screw head. As the thread shaft is screwed into bone the screw head approaches the plate hole and the machine thread engages the thread in the hole. In practice there is nothing to prevent the same forces that urge the screw to back out of bone, allow the plate to loosen from the bone, and perhaps allow the loosened bone screw to interfere with and damage adjacent soft tissue.
These plates have been widely used; however, an inherent problem associated with such plates is the use of the additional very small retaining elements that can become disengaged from the plate and migrate into the surrounding soft tissues just as can the conventional unsecured bone screw. Further, difficulty experienced in accessing and disengaging the small locking elements and removing the screws from this type of plate has caused some concern for the continued use of such plates.
There is therefore, an unfulfilled need for an anterior cervical plate system that can maintain a relatively low profile while providing security from the bone screw backing out problems associated with conventional plate and bone screw systems. Further there is a need for a vertebral plate that does not have additional separate small locking elements as found in conventional devices with the predictable problems of those locking elements becoming disengaged from the plate and migrating away from the top side of the plate into the surrounding soft tissue.