Patent Application
20060276788
The spinal column is a highly complex system of bones and connective tissues that provides support for the body and protects the delicate spinal column and nerves. The spinal column includes a series of vertebrae stacked one atop the other, whereby each vertebral body includes a relatively strong bone portion forming the outside surface of the body (cortical) and a relatively weak bone portion forming the center of the body (cancellous). Situated between each vertebral body is an intervertebral disc that provides for cushioning and dampening of compressive forces applied to the spinal column. The vertebral canal containing the delicate spinal cords and nerves is located just posterior to the vertebral bodies.
Various types of spinal column disorders are known and include scoliosis (abnormal lateral curvature of the spine), kyphosis (abnormal forward curvature of the spine, usually in the thoracic spine), excess lordosis (abnormal backward curvature of the spine, usually in the lumbar spine), spondylolisthesis (forward displacement of one vertebra over another, usually in a lumbar or cervical spine) and other disorders caused by abnormalities, disease or trauma, such as ruptured or slipped discs, degenerative disc disease, fractured vertebra, and the like. Patients that suffer from such conditions usually experience extreme and debilitating pain as well as diminished nerve function.
The present invention involves a technique commonly referred to as spinal fixation whereby surgical implants are used for fusing together and/or mechanically immobilizing adjacent vertebrae of the spine. Spinal fixation may also be used to alter the alignment of the adjacent vertebrae relative to one another so as to alter the overall alignment of the spine. Such techniques have been used effectively to treat the above-described conditions and, in most cases, to relieve pain suffered by the patient. However, as will be set forth in more detail below, there are some disadvantages associated with current fixation devices.
One particular spinal fixation technique includes immobilizing the spine by using orthopedic rods, commonly referred to as spine rods, which run generally parallel to the spine. This is accomplished by exposing the spine posteriorly and fastening bone screws to the pedicles of the appropriate vertebrae. The pedicle screws are generally placed two per vertebra, one at each pedicle on either side of the spinous process, and serve as anchor points for the spine rods. Clamping elements adapted for receiving a spine rod therethrough are then used to join the spine rods to the screws. The aligning influence of the rods forces the spine to conform to a more desirable shape. In certain instances, the spine rods may be bent to achieve the desired curvature of the spinal column.
Another common spinal fixation technique is the use of a fixating plate with screws. A typical spinal fixation plate includes a relatively flat, rectangular plate having a plurality of apertures formed therein. A corresponding plurality of bone screws may be provided to secure the bone fixation plate to the vertebrae of the spine. These plates are generally attached to the anterior portion of the vertebral bodies. The screws may be rigidly constrained to the plate, or may be semi-constrained to allow for load sharing.
This invention relates generally to improvements in spinal fixation devices of the type designed for human implantation into adjacent spinal vertebrae, to maintain the vertebrae in substantially fixed spaced relation while promoting bone ingrowth and fusion therebetween. More particularly, this invention relates to screws and interconnecting rod or plate having an improved combination of enhanced mechanical strength together with osteoinductive and osteoconductive properties, in a device that additionally and beneficially provides visualization of bone growth for facilitated post-operative monitoring.
In typical posterior spinal fixation procedures, the space between the transverse processes of the two vertebral bodies are then usually filled with bone graft material, either autogenous bone material provided by the patient or allogenous bone material provided by a third party donor. In addition to this posterior lateral placement of fusion materials, such materials are often placed into the interbody space as well. The common method for a surgeon to analyze the growth of the bone in these areas is with the use of x-ray or magnetic resonance imaging (MRI).
In many anterior spinal fixation procedures, a graft is placed between the adjacent vertebrae in the interbody space. This graft is designed to enable or enhance bone growth between these vertebrae. The plate is then placed against the vertebral bodies, spanning the bone graft, and being directly adjacent to, if not touching, said bone graft. Again, the common method for a surgeon to analyze the growth of the bone in these areas is with the use of x-ray or magnetic resonance imaging.
Most commercially available spinal fixation systems are made from titanium alloys and have enjoyed clinical success as well as rapid and widespread use due to improved patient outcomes. However, traditional titanium-based implant devices exhibit radio-opaque characteristics, presenting difficulties in post-operative monitoring and evaluation of the fusion process using x-ray or fluoroscopic imaging. Radio-opacity presents a problem in that it does not allow structures located between the device and the imaging machine to be seen. Additionally, metallic implants cause scattering, or shadowing, and distortion of MRI's and CT's. These poor radiolucent properties can make it difficult, if not impossible to assess the bone growth using traditional means. In some cases, surgeons must use costly thin slice CT reconstruction to analyze the new bone growth. This is especially a problem for characterizing the bone growth between the transverse processes and in the interbody space, due to the titanium rod or plate being directly adjacent to the fusion material. Moreover, traditional titanium-based implant devices are primarily load bearing but are not osteoconductive, i.e., not conducive to direct and strong mechanical attachment to patient bone tissue, leading to potential micro-motion between the implant and the host bone, causing possible poor fusion, instability and bone resorption.
Another group of commercially available spinal fixation devices are made from various polymeric materials such as PEEK or polyurethane. However these devices have issues which make them difficult to use. One such problem is a lack of load bearing strength, which might lead to failure of the implant after surgery. Another issue is with intraoperative placement of the device, and postoperative radiographic analysis. Since these polymers are radiotransparent, they offer a solution to assessing bone growth via traditional radiographic imaging. However, this radiotransparency makes it extremely difficult for the surgeon to know where the device is located, both during and after implantation. Some devices utilize a radiographic marker to aid in this assessment, but exact location and orientation of the markers within the device still make it difficult for accurate assessment.
Autologous (patient) bone fusion has been used in the past and has a theoretically ideal mix of osteoconductive and osteoinductive properties. However, supply of autologous bone material is limited and significant complications are known to occur from bone harvesting. Moreover, the costs associated with harvesting autograft bone material are high, requiring two separate incisions, with the patient having to undergo more pain and recuperation due to the harvesting and implantation processes. Additionally, blood supply to the posterior lateral portion of the spine is generally low, meaning there is a lack of natural osteoinductive cells and growth factors, making it difficult to sustain bone growth in the area. This can cause pseudoarthrosis, which may lead to loosening or breakage of the implant and result in patient pain. It is also difficult to keep the autologous cancellous bone material in the proper placement between the transverse processes.
Ceramic materials provide potential alternative structures for use in spinal fusion implant devices. In this regard, monolithic ceramic constructs have been proposed, formed from conventional materials such as hydroxyapatitie (HAP) and/or tricalcium phosphate (TCP). See, for example, U.S. Pat. No. 6,037,519. However, while these ceramic materials may provide satisfactory osteoconductive and bio-active properties, they have not provided the mechanical strength necessary for the implant.
Thus, a significant need exists for further improvements in and to the design of spinal fixation devices, particularly to provide a high strength implant having high bone ingrowth and fusion characteristics, together with substantial radiolucency for effective and facilitated post-operative monitoring.
Hence, it is an object of the present invention to provide an improved spinal fixation device made from a bio-compatible, load bearing and imaging compatible material, with or without an open pore structure, which has radiolucency similar to that of the surrounding bone. Specifically, to provide a spinal fixation device with radiolucency that enables the surgeon to see the exact location and orientation of the implant utilizing traditional radiographic imaging, while still allowing for assessment of the bone growth in and around the device. It is also an object of the present invention to provide a substrate of adequate bio-mechanical strength for carrying biological agents which promote bone ingrowth, healing and fusion.
In accordance with the invention, an improved spinal fixation system is provided for human implantation into a pair of adjacent vertebrae, to restore and maintain the spinal anatomy in a predetermined and substantially fixed spaced relation while promoting bone ingrowth and fusion. In this regard, the improved fixation device of the present invention is designed for use in addressing clinical problems indicated by surgical treatment of bone fractures, skeletal non-unions, weak bony tissue, degenerative disc disease, discogenic back pain, scoliosis (abnormal lateral curvature of the spine), kyphosis (abnormal forward curvature of the spine, usually in the thoracic spine), excess lordosis, and spondylolisthesis.
The improved fixation system comprises a bone screw and interconnecting rod or plate formed from a bio-compatible material composition having a relatively high bio-mechanical strength and load bearing capacity. These components may be porous, open-celled, or dense solid. A preferred material of the high strength substrate block comprises a ceramic material. The screws and rods may be porous, having a porosity of about 10% to about 80% by volume with uniformly distributed pores throughout and a pore size range of from about 5 to about 500 microns. When the component is porous, the porosity of the device is gradated from a first relatively low porosity region emulating or mimicking the porosity of cortical bone to a second relatively higher porosity region emulating or mimicking the porosity of cancellous bone. This structure mimicking of the porous properties of cancellous bone is called a bio-mimetic structure. In a second embodiment, the device is a dense solid comprised of a ceramic, metal or polymer material. This dense solid substrate would then be attached to a second highly porous, bio-mimetic region emulating or mimicking the porosity of cancellous bone. Preferably, the porous region would be integrally formed around or on the face of the substrate.
In the method where a dense, solid material is used as the substrate block, the block will be externally coated with a bio-active surface coating material selected for relatively high osteoconductive and bio-active properties, such as a hydroxyapatite or a calcium phosphate material. The porous portion is internally and externally coated with a bio-active surface coating material selected for relatively high osteoconductive and bio-active properties, such as a hydroxyapatite or a calcium phosphate material. The porous region, however, may be in and of itself a bio-active material selected for relatively high osteoconductive and bio-active properties, such as a hydroxyapatite or a calcium phosphate material.
The thus-formed fixation device can be made in a variety of shapes and sizes to suit different specific implantation requirements. Preferred shapes include a rod or plate with a lordotic curvature. This rod has a dense inner cylinder of high strength for supporting spinal loading. The dense inner cylinder is surrounded along its axis by a structure of open porosity. The plate component is made of a dense body of high strength for receiving the screws and supporting load. The face of the plate which lies adjacent to the vertebral body is covered with a structure of open porosity. In turn, the porous structure has osteoconductive materials coating throughout the pores. This preferred embodiment aids in the fusion along the rod or plate, which is placed between transverse processes or adjacent to the interbody space. Additional preferred shapes include that of a bone screw. The bone screw is comprised of a dense substrate of high strength for spinal loading. Portions of the threaded shank of the screw are surrounding by a structure of open porosity. In turn, the porous structure has osteoconductive materials coating throughout the pores. This enables bone growth into the screw itself, thereby aiding in the fixation of the device to the vertebral body.
The resultant spinal fixation device exhibits relatively high mechanical strength for load bearing support, while additionally and desirably providing high osteoconductive and osteoinductive properties to achieve enhanced bone ingrowth and fusion. Importantly, these desirable characteristics are achieved in a structure which is substantially radiolucent so that the implant does not interfere with post-operative radiographic monitoring of the fusion process.
In accordance with a further aspect of the invention, the spinal fixation device may additionally carry one or more therapeutic agents for achieving further enhanced bone fusion and ingrowth. Such therapeutic agents may include natural or synthetic therapeutic agents such as bone morphogenic proteins (BMPs), growth factors, bone marrow aspirate, stem cells, progenitor cells, antibiotics, or other osteoconductive, osteoinductive, osteogenic, bio-active, or any other fusion enhancing material or beneficial therapeutic agent.
Other features and advantages of the invention will become more apparent from the following detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.
The accompanying drawings illustrate the invention. In such drawings:
As shown in the exemplary drawings, an osteoconductive spinal fixation device referred to generally in
The bone screw 14 is comprised of a dense body or shank which has at least one threaded portion or segment 22 for engaging and anchoring to bone. The preferred embodiment is manufactured from a high strength ceramic material, allowing for load carrying abilities, as well as substantial radiolucency and non-magnetic characteristics. Proximal to the threaded portion 22 is a head section 26 that is design for receiving the rod 12. The portion of the rod being received into the head section 26 of the screw 14 is that of the dense, strong mechanical portion located at the end 20 of the rod. This dense end 20 of the rod is fixated to the screw head 26 by means of a locking screw 16.
Portions of the bone screw 14 disposed axially adjacent to and preferably axially between thread segments 22 are of a porous construction 24 to define another open lattice conducive to bone ingrowth and fusion. This open-celled construction 24 is coated internally and externally with a biologic coating selected for relatively high osteoconductive and bio-active properties, whereby the coated construction 24 provides a scaffold conducive to cell attachment and proliferation to promote bone ingrowth and fusion attachment. This aids in the fixation of the bone screw 14 to the host skeletal structure S1. The substrate may also carry one or more selected therapeutic agents suitable for bone repair, augmentation and other orthopedic uses.
The resultant illustrative fixation device 10 exhibits relatively high bio-mechanical strength similar to the load bearing characteristics. In addition, the fixation device 10 exhibits relatively high osteoconductive and bio-active characteristics attributable primarily to the surface coating, again similar to natural bone. Importantly, the fixation device 10 is also substantially radiolucent and non-magnetic, so that the device does not interfere with post-operative radiological or other imaging methods of analysis of bone ingrowth and fusion.
The rod 12 is shown in
Still another preferred embodiment of the bone screw 610 is illustrated in
An osteoconductive spinal fixation device referred to generally in
Each bone screw 1014 is comprised of a dense body which has a threaded portion 1016 for engaging and anchoring to bone. The preferred embodiment is manufactured from a high strength ceramic material, allowing for load carrying abilities, as well as substantial radiolucency and non-magnetic characteristics. The plate component 1012 has apertures for receiving the head section 1018 of said screw 1014, allowing the threaded portion 1016 to pass through the aperture. The portion of the plate receiving the head of the screw 1018 is that of the dense, strong mechanical portion. To aid in direct visualization intraoperatively, the plate 1012 may have a window 1020 to view the bone graft.
Portions of the bone screw 1014 may also be of porous construction, as demonstrated previously in various embodiments depicted in
The resultant illustrative fixation device 1010 exhibits relatively high bio-mechanical strength similar to the load bearing characteristics. In addition, the fixation device 1010 exhibits relatively high osteoconductive and bio-active characteristics attributable primarily to the surface coating, again similar to natural bone. Importantly, the fixation device 1010 is also substantially radiolucent and non-magnetic, so that the device does not interfere with post-operative radiological or other imaging methods of analysis of bone ingrowth and fusion.
The spinal fixation devices depicted in
The improved fixation device of the present invention thus comprises an open-celled porous structure which is coated with a bio-active surface coating, and has the strength required for the weight bearing capacity required of a fusion device. The capability of being infused with the appropriate biologic coating agent imparts desirable osteoconductive and bio-active properties to the device for enhanced interbody bone ingrowth and fusion, without detracting from essential load bearing characteristics. The radiolucent or non-magnetic characteristics of the improved device beneficially accommodate post-operative radiological or other diagnostic imaging examination to monitor the bone ingrowth and fusion progress, substantially without undesirable radio-shadowing. In addition to these benefits, the present invention is easy to manufacture in a cost competitive manner. The invention thus provides a substantial improvement in addressing clinical problems indicated for surgical treatment of scoliosis (abnormal lateral curvature of the spine), kyphosis (abnormal forward curvature of the spine, usually in the thoracic spine), excess lordosis (abnormal backward curvature of the spine, usually in the lumbar spine), spondylolisthesis (forward displacement of one vertebra over another, usually in a lumbar or cervical spine) and other disorders caused by abnormalities, disease or trauma, such as ruptured or slipped discs, degenerative disc disease, fractured vertebra, and the like.
The fixation device of the present invention provides at least the following benefits over the prior art:
[a] a porous osteoconductive scaffold for enhanced fusion rates;
[b] a bio-mimetic load bearing superstructure providing appropriate stress transmission without fatigue failure;
[c] a pore structure and size suitable for ingrowth and vascularization, [d] the ability to absorb and retain an osteoinductive agent such as autologous bone marrow aspirate or BMPs;
[e] bio-inert and bio-compatible with adjacent tissue and selected for ease of resorption;
[f] radiolucent and MRI compatible;
[g] fabricatable and machinable into various shapes;
[h] sterilizable; and
[i] low manufacturing cost.
A variety of further modifications and improvements in and to the fixation device of the present invention will be apparent to those persons skilled in the art. In this regard, it will be recognized and understood that the fixation device can be formed in the size and shape of a plate with screws for implantation into a bone regeneration/ingrowth site. Accordingly, no limitation on the invention is intended by way of the foregoing description and accompanying drawings, except as set forth in the appended claims.
