The present invention relates to apparatuses and methods for use in performing spinal surgery and, in particular, to polyaxial bone screws for use in spinal surgery. Such screws have a head that can swivel about a shank of the bone screw, allowing the head to be positioned in any of a number of angular configurations relative to the shank.
Bone screws are utilized in many types of spinal surgery in order to secure various implants to vertebrae along the spinal column. Spinal implant screws typically have a shank that is threaded and configured for implantation into a pedicle or vertebral body of a vertebra. Such a screw also includes a head designed to extend beyond the vertebra and also defines a channel to receive a rod or other implant. In bone screws of this type, the head may be open, in which case a closure member must be used to close between opposite sides of the head once a rod or other implant is placed therein. Alternatively, the head may be closed, wherein a rod-like implant is threaded through the head of the bone screw.
When the head and shank of the bone screw are fixed in position relative to each other, it is not always possible to insert a bone screw in such a manner that the head will be in the best position for receiving other implants. Consequently, swivel head bone screws have been designed that allow the head of the bone screw to rotate or swivel about an upper end of the shank of the bone screw while the surgeon is positioning other implants and finding the best position for the bone screw head. However, once the surgeon has determined that the head is in the best position, it is then necessary to lock or fix the head relative to the shank. Different types of structures have been previously developed for such purpose.
Because bone screws are for placement within the human body, it is desirable for the implant to have as little effect on the body as possible. Consequently, heavy, high profile, bulky implants are undesirable and lighter implants with a relatively small profile both in height and width are more desirable. However, a drawback to smaller, lighter implants is that they may be more difficult to rigidly fix in position relative to each other and in a desired position. Reduced bulk may also reduce strength, resulting in slippage under high loading. Also, more component parts may be required to rigidly fix the implant in a desired position. A further drawback of smaller components is that they may be difficult to handle during surgery because of their small size, failing to provide adequate driving or gripping surfaces for tools used to drive the shank into bone.
One undesirable attribute of some of the swivel-head implants is the need for a multitude of components that may loosen or even disassemble within the body. It is most often undesirable for components to become moveable in the body after the completion of surgery. Loosening of components relative to each other may result in related undesirable movement of the bone or vertebra that the implant was intended to stabilize.