The concept of intervertebral fusion for the cervical and lumbar spine following a discectomy was generally introduced in the 1960s. It involved coring out a bone graft from the hip and implanting the graft into the disc space. The disc space was prepared by coring out the space to match the implant. The advantages of this concept were that it provided a large surface area of bone to bone contact and placed the graft under loading forces that allowed osteoconduction and induction enhancing bone fusion. However, the technique is seldom practiced today due to numerous disadvantages including lengthy operation time, destruction of a large portion of the disc space, high risk of nerve injury, and hip pain after harvesting the bone graft.
Presently, at least two devices are commonly used to perform the intervertebral portion of an intervertebral body fusion: the first is the distraction device and the second is the intervertebral body fusion device, often referred to as a cage. Cages can be implanted as standalone devices or as part of a circumferential fusion approach with pedicle screws and rods. The concept is to introduce an implant that will distract a collapsed disc and decompress the nerve root, allow load sharing to enhance bone formation and to implant a device that is small enough to allow implantation with minimal retraction and pulling on nerves.
In a typical intervertebral body fusion procedure, a portion of the intervertebral disc is first removed from between the vertebral bodies. This can be done through either a direct open approach or a minimally invasive approach. Disc shavers, pituitary rongeours, curettes, and/or disc scrapers can be used to remove the nucleus and a portion of either the anterior or posterior annulus to allow implantation and access to the inner disc space. The distraction device is inserted into the cleared space to enlarge the disc space and the vertebral bodies are separated by actuating the distraction device. Enlarging the disc space is important because it also opens the foramen where the nerve root exists. It is important that during the distraction process one does not over-distract the facet joints. An intervertebral fusion device is next inserted into the distracted space and bone growth factor, such as autograft, a collagen sponge with bone morphogenetic protein, or other bone enhancing substance may be inserted, either before or after insertion of the device into the disc space, into the space within the intervertebral fusion device to promote the fusion of the vertebral bodies.
Intervertebral fusion and distraction can be performed through anterior, posterior, oblique, and lateral approaches. Each approach has its own anatomic challenges, but the general concept is to fuse adjacent vertebra in the cervical thoracic or lumbar spine. Devices have been made from various materials. Such materials include cadaveric cancellous bone, carbon fiber, titanium and polyetheretherketone (PEEK). Devices have also been made into different shapes such as a bean shape, football shape, banana shape, wedge shape and a threaded cylindrical cage.
It is important for a device that is utilized for both intervertebral body fusion and distraction to be both small enough to facilitate insertion into the intervertebral space and of sufficient height to maintain the normal height of the disc space. Use of an undersized device that cannot expand to a sufficient height can result in inadequate fusion between the adjacent vertebrae and lead to further complications for the patient, such as migration of the device within or extrusion out of the disc space. Addressing these issues can require the use of multiple devices of varying sizes to be used serially to expand the disc space the proper amount, which increases the time required to carry out the procedure, increasing the cost and risk associated with the procedure.
Accordingly, there is a need in the art for a device of sufficient strength that can distract from a beginning size small enough to initially fit into the disc space to a height sufficient to reestablish and maintain the normal height of the disc space.