The natural intervertebral disc contains a jelly-like nucleus pulposus surrounded by a fibrous annulus fibrosus. Under an axial load, the nucleus pulposus compresses and radially transfers that load to the annulus fibrosus. The laminated nature of the annulus fibrosus provides it with a high tensile strength and so allows it to expand radially in response to this transferred load.
In a healthy intervertebral disc, cells within the nucleus pulposus produce an extracellular matrix (ECM) containing a high percentage of proteoglycans. These proteoglycans contain sulfated functional groups that retain water, thereby providing the nucleus pulposus with its cushioning qualities. These nucleus pulposus cells may also secrete small amounts of cytokines as well as matrix metalloproteinases (MMPs). These cytokines and MMPs help regulate the metabolism of the nucleus pulposus cells.
In some instances of degenerative disc disease (DDD), gradual degeneration of the intervertebral disc is caused by mechanical instabilities in other portions of the spine. In these instances, increased loads and pressures on the nucleus pulposus cause the cells within the disc (or invading macrophages) to emit larger than normal amounts of the above-mentioned cytokines. In other instances of DDD, genetic factors or apoptosis can also cause the cells within the nucleus pulposus to emit toxic amounts of these cytokines and MMPs. In some instances, the pumping action of the disc may malfunction (due to, for example, a decrease in the proteoglycan concentration within the nucleus pulposus), thereby retarding the flow of nutrients into the disc as well as the flow of waste products out of the disc. This reduced capacity to eliminate waste may result in the accumulation of high levels of proinflammatory cytokines and/or MMPs that may cause nerve irritation and pain.
As DDD progresses, toxic levels of the cytokines and MMPs present in the nucleus pulposus begin to degrade the extracellular matrix. In particular, the MMPs (as mediated by the cytokines) begin cleaving the water-retaining portions of the proteoglycans, thereby reducing their water-retaining capabilities. This degradation leads to a less flexible nucleus pulposus, and so changes the loading pattern within the disc, thereby possibly causing delamination of the annulus fibrosus. These changes cause more mechanical instability, thereby causing the cells to emit even more cytokines, typically thereby upregulating MMPs. As this destructive cascade continues and DDD further progresses, the disc begins to bulge (“a herniated disc”), and then ultimately ruptures, causing the nucleus pulposus to contact the spinal cord and produce pain.
One proposed method of managing these problems is to remove the problematic disc and replace it with a porous device that restores disc height and allows for bone growth therethrough for the fusion of the adjacent vertebrae. These devices are commonly called “fusion devices”.
Designs of intervertebral fusion devices are generally either box-like (i.e., Smith-Robinson style) or threaded cylinders (i.e., Cloward style). Smith-Robinson style implants have the advantage of possessing better contact area to the vertebral endplates, but rely on a coarse surface texture (such as teeth) to prevent their migration once implanted. Insertion then requires over distraction of the disc space to slide the implant in or to provide a smoother implant, which can migrate post-op.
One such box-like design is the Brantigan cage, which is disclosed in U.S. Pat. No. 4,743,256 (“Brantigan”). Brantigan discloses an improved surgical method for eliminating spinal back pain caused by ruptured or degenerated vertebral discs by spanning the disc space between adjacent vertebrae with rigid fusion devices, or “cages”, having surfaces facilitating bone ingrowth and bottomed on prepared sites of the vertebrae to integrate the implant with the vertebrae and to provide a permanent weight supporting strut maintaining the disc space.
One commercial box-like design is the injection-molded carbon fiber reinforced PEEK (CFRP) cage made by DePuy Spine. However, these cages are difficult to insert because of the interference fit that is required for intervertebral space distraction. In addition, the reinforced PEEK material that makes up the teeth is brittle and so is susceptible to breakage when applying impact or torque loads to the implant.
Current interbody devices are made from single materials (e.g., machined titanium, or molded and/or machined PEEK). Titanium has the disadvantage of being radiopaque (which can interfere with fusion assessment on x-ray) while also having a high modulus of elasticity (which can stress shield the bone graft). Injection molded CFRP is very brittle and susceptible to fracture during insertion. Unreinforced PEEK is much less brittle but also weaker than carbon-filled PEEK, requiring thicker-walled designs (diminishing space for bone graft). In addition, the teeth of an unreinforced PEEK cage are softer and so may allow more migration. Both PEEK and carbon-filled PEEK are radiolucent.
U.S. Pat. No. 6,824,565 (“Muhana”) discloses implant and instrument designs wherein some of the implant embodiments have planked designs and a mating inserter instrument. However, the disclosed inserter wraps around the exterior of the implant and partially into grooves on the implant. Moreover, the disclosed implant is derived from bone and is not hollow. The insertion technique disclosed by Muhana requires a cutting tool to prepare a channel for the implant.
US Patent Publication 2008-0154377 (Voellmicke) discloses a cage adapted to contain an inserter within its inner volume during insertion.
US Patent Publication 2009-0198339 (Kleiner) discloses an implantable intervertebral fusion cage including a removable means for retaining material inside of the cage during implantation. Embodiments are directed toward an implantable intervertebral fusion cage that includes at least one removable shield or veneer that is capable of retaining a surgically useful material, such as a spinal fusion-inducing material, inside of the fusion cage during implantation and/or until the shield or veneer is removed. None of the Kleiner shields cover the teeth of the cages.
U.S. Pat. No. 7,569,054 (Michelson) discloses disc space docking and distraction means. In particular, Michelson discloses an apparatus for use in human surgery has a tubular member with a passage for providing protected access to a surgical site. The passage has a minimum width transverse to the mid-longitudinal axis of the tubular member. Two opposed extensions extend from the distal end of the tubular member. The extensions each have a length and a maximum height perpendicular to the length. The maximum height of the extensions are less than the length of each extension and greater than one-half the minimum width of the passage. Each extension has an interior surface at least in part facing the mid-longitudinal axis of the tubular member. The interior surfaces of the extensions are spaced apart from one another along the length of each extension a distance no less than the minimum width of the passage. Each extension has opposed bone contacting surfaces configured to contact portions of bone.
Other relevant instruments include those disclosed in U.S. Pat. No. 7,008,431 (“Simonson”); U.S. Pat. No. 5,797,909 (“Michelson II”); U.S. Pat. No. 6,080,155 (“Michelson III”); U.S. Pat. No. 6,096,038 (“Michelson IV”); U.S. Pat. No. 7,300,440 (“Zdeblick”); and US Patent Publication 2009-0198339 (“Kleiner”).
In summary, the insertion of both smooth and toothed intervertebral cages has proven to be problematic due to high resistance forces (friction) and interference fit of the cage and intervertebral space. Whereas toothed cages are difficult to insert, cages with smooth upper and lower surfaces have demonstrated undesirable migration.
Current injection-molded PEEK or carbon fiber reinforced PEEK (CFRP) cages are difficult to insert because of the interference fit between the textured/spiked surfaces of the implant and the bony endplates.
The difficulty of direct, unshielded cage insertion and final positioning in the disc space also increases the likelihood of bony endplate damage, as the disc space preparation, FSU distraction forces and insertion trajectory are variable.
Consistent and accurate placement of the posteriorly inserted spinal fusion cages is difficult because light tamping and impaction are employed for final positioning. Cages have been over inserted via pushing or impaction through the annulus and into the adjacent body cavities and/or structures.
Most cages are filled with graft and/or bone inducing substances including BMP and collagen sponge. It has been found that the graft and/or BMP frequently drips or falls out of the graft retaining pockets. The uncontrolled delivery of the BMP/graft can irritate adjacent tissues and prompt bone formation in undesired locations including heterotopic bone.
Many spinal fusion procedures require either pre and or post packing of the disc space, thereby increasing patient risk and operative time.