Many types of spinal irregularities can cause pain, limit range of motion, or injure the nervous system within the spinal column. These irregularities can result from, without limitation, trauma, tumor, disc degeneration, and disease. Often, these irregularities are treated by immobilizing a portion of the spine. This treatment typically involves affixing a plurality of screws and/or hooks to one or more vertebrae and connecting the screws or hooks to an elongate rod that generally extends in the direction of the axis of the spine.
Treatment for these spinal irregularities often involves using a system of pedicle screws and rods to attain stability between spinal segments. Instability in the spine can create stress and strain on neurological elements, such as the spinal cord and nerve roots. In order to correct this, implants of certain stiffness can be implanted to restore the correct alignment and portion of the vertebral bodies. In many cases, an anchoring member such as a pedicle screw along with a vertical solid member can help restore spinal elements to a pain free situation, or at least may help reduce pain or prevent further injury to the spine.
Some systems are directed toward immobilization of the vertebral bodies by implantation of bone screws, couplings, and elongate rods. Examples of such systems include U.S. Pat. Nos. 5,690,630, 5,669,911, and 5,672,176. It is well known that difficulties can arise during a surgical procedure when attempting to connect a vertical solid member, such as a rod, to a bone screw. As a result, it may be desirable to use a variable angled coupling element to connect a rod to a bone screw. This lessens the need to modify the system, such as by bending the rod, by repositioning the screw, or the like. The coupling element acts as an elbow which can “reach out” to the rod and allow for easier adjustment and installation of the rod in the patient.
Typically, a conventional polyaxial screw described by the prior art often has a cap of some kind is used to compress the rod onto the coupling element. This compression of the rod enables the locking mechanism within the coupling element to fix the angle between the bone screw and said element. Therefore, the vertical compression of the rod is paramount in the design of the bone screw system.
Conventional polyaxial screws also typically require that compression of the coupling element and the bone screw be achieved through the clamping of some form of taper within the coupling element. This is achieved usually by means of slits which are placed within the members of the coupling element. When these elements are placed in compression the tines created by the slits contract on the head of the screw by means of a cylindrical taper. In addition to the references mentioned above, additional examples of such systems requiring pressure from the rod to lock the position of the polyaxial screw head also can be found in U.S. Pat. Nos. Re 37,665, 5,733,286, and 5,476,464. Some systems, such as described in U.S. Pat. No. 6,248,105, describe the possibility of separately fastening the connecting body to the spherical head and elongated rod.
While these designs may provide an advantage of reducing assembly time over earlier screw designs by requiring clamping of only one fastener to hold the rod and coupling element by applying pressure against the head of the bone screw, such systems lack the ability to separately unlock or release one element, such as the rod or the coupling element.