The present invention relates to spinal stabilization systems and particularly to semi-rigid devices for fixation to the vertebrae.
Degenerative spinal column diseases, such as disc degenerative diseases (DDD), spinal stenosis, spondylolisthesis, and so on, often need surgical operation if conservative pain management approaches prove inadequate. Typically, spinal decompression is the first surgical procedure that is performed. The primary purpose of decompression is to reduce pressure in the spinal canal and on nerve roots located therein by removing certain tissue of the spinal column to reduce or eliminate the pressure and pain caused by the pressure. If the tissue of the spinal column is removed the pain is reduced but the spinal column is weakened. Therefore, fusion surgery (e.g., ALIF, PLIF or posterolateral fusion) is often necessary for spinal stability following the decompression procedure. However, following the surgical procedure, fusion takes additional time to achieve maximum stability and a spinal fixation device is typically used to support the spinal column until a desired level of fusion is achieved. Depending on a patient's particular circumstances and condition, a spinal fixation surgery can sometimes be performed immediately following decompression, without performing the fusion procedure. The stabilization surgery is performed in most cases because it provides immediate postoperative stability and, if fusion surgery has also been performed, it provides support of the spine until sufficient fusion and stability has been achieved.
Conventional methods of spinal fixation utilize a rigid spinal fixation device to support an injured spinal segment and prevent movement of the injured part. These conventional spinal fixation devices include: fixing screws configured to be inserted into the pedicle or sacrum of the spinal vertebrae to a predetermined depth and angle, rods or plates configured to be positioned adjacent to the injured spinal segment, and coupling elements for connecting and coupling the rods or plates to the fixing screws such that the injured spinal part is supported and held in a relatively fixed position by the rods or plates.
U.S. Pat. No. 6,193,720 discloses a conventional spinal fixation device, in which connection members of a rod or plate type are mounted on the upper ends of at least one or more screws inserted into the spinal pedicle or sacrum of the backbone. The connection units, such as the rods and plates, are used to stabilize the injured part of the spinal column which has been weakened by decompression. The connection units also prevent further pain and injury to the patient by substantially restraining the movement of the spinal column. However, because the connection units prevent normal movement of the spinal column, after prolonged use the spinal fixation device itself can cause ill effects, such as “junctional syndrome” (transitional syndrome) or “fusion disease” resulting in further complications and abnormalities of the spinal column. In particular, due to the high rigidity of the rods or plates used in conventional fixation devices, the patient's treated segments are not allowed to move after the surgical operation, and the movement of the spinal motion segments located superior or inferior to the instrumented vertebral level is increased. Consequently, such spinal fixation devices may eventually lead to decreased mobility of the patient and increased stress and instability to the spinal motion segments adjacent to the instrumented level.
It has been reported that excessive rigid spinal fixation is not helpful to the fusion process due to decreased or abnormal load sharing caused by rigid fixation. Thus, load sharing semi-rigid spinal fixation devices have been developed to eliminate this problem and assist the bone fusion process. For example, U.S. Pat. No. 5,672,175, U.S. Pat. No. 5,540,688 and U.S. Pub No 2001/0037111 disclose dynamic spine stabilization devices having flexible designs that permit axial load translation (i.e., along the vertical axis of the spine) for bone fusion promotion. However, because these devices are intended for use following a bone fusion procedure, they are not well-suited for spinal fixation without fusion. Thus, in the end result, the problems resulting from fusion still persist with these devices.
To solve the above-described problems associated with rigid fixation, non-fusion technologies have been developed. The Graf band is one example of a non-fusion fixation device that is applied after decompression without bone fusion. The Graf band is composed of a polyethylene band and pedicle screws to couple the polyethylene band to the spinal vertebrae requiring stabilization. The primary purpose of the Graf band is to prevent sagittal rotation (flexion instability) of the injured spinal motion segments. Another non-fusion fixation device called “Dynesys” is similar to the Graf band except it uses a polycarbonate urethane (PCU) spacer between the screws to maintain the distance between the heads of two corresponding pedicle screws and, hence, adjacent vertebrae in which the screws are fixed. Early reports by the inventors of the Dynesys device indicate it has been successful in many cases. However, due to the mechanical configuration of the device, the surgical technique required to attach the device to the spinal column is complex and complicated.
U.S. Pat. Nos. 5,282,863 and 4,748,260 disclose a flexible spinal stabilization system and method using a plastic, non-metallic rod. U.S. patent publication No. 2003/0083657 discloses another example of a flexible spinal stabilization device that uses a flexible elongate member. These devices are flexible but they are not well-suited for enduring long-term axial loading and stress. Additionally, the degree of desired flexibility versus rigidity may vary from patient to patient. The design of existing flexible fixation devices are not well suited to provide varying levels of flexibility to provide optimum results for each individual candidate. For example, U.S. Pat. No. 5,672,175 discloses a flexible spinal fixation device which utilizes a flexible rod made of metal alloy and/or a composite material. Additionally, compression or extension springs are coiled around the rod for the purpose of providing de-rotation forces on the vertebrae in a desired direction. Prior flexible rods such as that mentioned in U.S. Pat. No. 5,672,175 typically have solid construction with a relatively small diameter in order to provide a desired level of flexibility. Because they are typically very thin in an effort to provide suitable flexibility, such prior art rods may be prone to mechanical failure.
Additionally, in a conventional surgical method for fixing the spinal fixation device to the spinal column, a doctor incises the midline of the back to about 10-15 centimeters, and then, dissects and retracts the soft tissue to both sides. In this way, the doctor performs muscular dissection to expose the outer part of the facet joint. Next, after the dissection, the doctor finds an entrance point to the spinal pedicle using radiographic devices (e.g., C-arm fluoroscopy), and inserts securing members of the spinal fixation device (referred to as “spinal pedicle screws”) into the spinal pedicle. Thereafter, the connection units (e.g., rods or plates) are attached to the upper portions of the pedicle screws in order to provide support and stability to the injured portion of the spinal column. Thus, in conventional spinal fixation procedures, the patient's back is incised substantially and as a result the back muscles important for maintaining spinal column stability are incised or injured, leading to significant post-operative pain to the patient and a slow recovery period.
To reduce patient trauma, minimally invasive surgical procedures have been recently developed which are capable of conducting spinal fixation surgery through a relatively small hole or “window” that is created in the patient's back at the location of the surgical procedure. Through this smaller incision or window, two or more securing members (e.g., pedicle screws) of the spinal fixation device are screwed into respective spinal pedicle areas using a navigation system. Thereafter, special tools are used to connect the stabilizing members (e.g., rods or plates) of the fixation device to the securing members. Alternatively, or additionally, the surgical procedure may include inserting a step dilator into the incision and then gradually increasing the diameter of the dilator. Thereafter, a tubular retractor is inserted into the dilated area to retract the patient's muscle and provide a visual field for surgery. After establishing this visual field, decompression and, if desired fusion procedures, may be performed, followed by a fixation procedure, which includes the steps of finding the position of the spinal pedicle, inserting pedicle screws into the spinal pedicle, using an endoscope or a microscope, and securing the stabilization members (e.g., rods or plates) to the pedicle screws in order to stabilize and support the weakened spinal column.
While these minimally invasive surgical procedures have done much to reduce the trauma and ill effects associated with spinal surgery, the nature of the implant itself can aggravate even a minimally invasive procedure. The nature of these fixation devices often requires significant manipulation at the surgical site, thereby complicating the procedure.
Therefore, conventional spinal fixation devices have not provided a comprehensive and balanced solution to the problems associated with addressing the effects of spinal diseases. Many of the prior devices are characterized by excessive rigidity, which leads to the problems discussed above, while others, though providing some flexibility, are not well-adapted to provide long-term stability and/or varying degrees of flexibility. The need exists for an improved dynamic spinal fixation device that provides a desired level of flexibility to the injured parts of the spinal column, while also providing long-term durability and consistent stabilization of the spinal column.