Patent Application
20100241120
1. Field of the Invention
The invention is directed to devices and methods to repair bone tissue, more particularly to an intramedullary nail and its surgical placement to repair a humerus.
2. Description of the Related Art
Intramedullary nails have been used to repair long bones since the 1930s, with the introduction of the Smith-Peterson nail to repair femur fractures. Since that time, improvements and variations of the intramedullary nail have been made. Screws have been introduced to fix the nails in place and to prevent rotation. Newer devices have been developed that comprise expansion devices such as wedges, deploying arms, anchor blades and tangs, which help fix the nail in place and to prevent rotation within the intramedullary canal. A discussion of the intramedullary nail art related to the present invention is discussed in U.S. Pat. No. 6,488,684, which is incorporated herein by reference. Regardless of the design of the nail or method of fixing the nail within the intramedullary canal, all intramedullary nails to date are placed into the intramedullary canal through the proximal or distal end of the long bone coaxial to the cross section of the canal, that is directly inline with the axis of the canal.
Intramedullary nails must be made of biocompatible materials. The most commonly used materials for intramedullary nails are stainless steel and titanium. Biocompatibility is the foremost concern when implanting any foreign object into the human body. The human body will actively attempt to destroy any unknown material that enters the body as a defense mechanism, known as the foreign body response. The biocompatibility of a material is closely based on the reactions between the surface of the material and inflammatory host response. Although all materials implanted into the body cause an inflammatory host response, certain materials, such as titanium, produce less of a response and are considered to be biologically inert. The more biologically inert a material is, the safer it is to implant in the body.
The body's response to a metal implant is different in bone than in soft tissue. Within two to three days after implantation, stem cells from the bone develop into osteoblasts, which along with fibroblasts form a layer near the implant surface. A collagen rich extra cellular matrix forms, which is followed by mineralization. In more vascularized tissue, the implant will be covered by a blood clot including leukocytes, thrombocytes, and various proteins. Inflammatory cells, such as monocytes and macrophages, arrive at the implantation site to remove debris and foreign materials. This foreign body response can begin to degrade the material causing even more inflammation and thrombosis. The implantation site will become painful and swollen as the body rejects the implant. Materials such as titanium and stainless steel are used in medical devices and implants because they are relatively biologically inert and do not induce a significant foreign body response.
A newer nickel titanium alloy called Nitinol is increasingly being used to repair damaged tissue. The unique properties of nickel titanium alloy were discovered in 1961 in a Navy lab. The common name, Nitinol, stands for Ni—Ti Naval Ordnance Laboratory. Nitinol is a shape memory metal, which undergoes martensite-austenite transition as a result of a change in temperature. This property of Nitinol has shown its usefulness in internal fixation of bone fractures, endoprostheses, spine surgery, cranial facial surgery and oral maxillofacial surgery (referenced in Youyi, Chu. Orthopedic Applications of NiTi Shape Memory Alloys in Chine. SMST-2000 Conference Proceedings. 2001, which is incorporated herein by reference), as well as in animal experiments on bone deformation (Kujala et al., Bone modeling controlled by a nickel-titanium shape memory alloy intramedullary nail, Biomaterials 23 (2002) 2535-2543; and Kujala et al., Comparison of the bone modeling effects caused by curved and straight nickel-titanium intramedullary nails, Journal of Materials Science: Materials in Medicine 13 (2002) 1157-1161, which are herein incorporated by reference).
Materials tested for use in the body are graded on the host's reaction induced by the material and the degradation of the material in the body environment. The largest concern in using Nitinol is the large concentration of nickel within the material. A small percentage of the population is allergic to nickel. In high doses, nickel is toxic to all individuals. However, the nickel molecules in the material are chemically bonded to the titanium molecules and do not leach out into the body in high doses. Most observations from extracted Nitinol implants show little or no corrosion with the highest corrosion rate being 0.46 mm/year (referenced in Youyi, Chu. Orthopedic Applications of NiTi Shape Memory Alloys in Chine. SMST-2000 Conference Proceedings. 2001, which is incorporated herein by reference). These tests conclude that the nickel in Nitinol does not pose a risk to the patient.
In addition, several coatings have been created to further stabilize the material surface to ensure there is no corrosion or nickel leaching. One coating, calcium phosphate, has been used to create a more physiologically stable surface, which could help prevent nickel release. The layer (5-20 μm) is applied by dipping the implant in the calcium phosphate solution. The calcium phosphate creates a physiological surface to which leukocytes and platelets adhere more readily. This layer has been proven to decrease the foreign body response and prevent nickel release that may occur slightly within the first few days of implantation and during high load bearing situations (referenced in Choi et al., “Calcium phosphate coatings of nickel-titanium shape-memory alloys. Coating procedure and adherence of leukocytes and platelets”, Science Direct, 2003, which is incorporated herein by reference). However, in vivo tests of Nitinol, described in Choi et al., 2003, demonstrated that Nitinol performs similarly to stainless steel, even without any surface treatment (see also Shabalovskaya, S. A., “Surface, corrosion and biocompatibility aspects of Nitinol as an implant material,” Nitinol Sciences, Consulting, 2001, which is incorporated herein by reference.)
Due to the complex musculoskeletal anatomy of the shoulder and elbow, the repair of a fractured humerus bone is a complicated medical procedure. The current procedure is time consuming, invasive, and often accompanied by post surgical complications. First, an incision is made through the skin at the shoulder, exposing the rotator cuff. The rotator cuff is severed to expose the proximal portion of the humerus. A medical drill is used to cut through the ossified outer layer of the bone. A guide wire is dropped into the humerus bone canal to provide a path for the metal rod to follow as it crosses the fracture. A reamer is used to extract the marrow and enlarge the canal. An intramedullary nail, which is usually made of a titanium alloy, is hammered into the reamed out section of the bone following the path of the guide wire. Once in place, surgical screws are inserted through the bone into predrilled holes in the nail to hold the proximal and distal ends of the nail in place. The rotator cuff and initial incision through the skin are then sutured.
Although the procedure repairs the fractured bone, it often causes damage to the rotator cuff, possibly requiring additional surgery. Also, if the rod needs to be removed because of infection or other complications, the rotator cuff must be severed once again, causing even more damage. Thus, there is a great need for new and improved devices and surgical methods to repair a fractured humerus, which reduces surgical complications.
An object of the invention is directed to an intramedullary nail that can transition from a flexible state to a non-flexible state. In one embodiment, the nail comprises a shape memory metal, which is preferably a nickel-titanium alloy, having a martensite (bendable) phase at a physiologically safe temperature and an austenite (stiff) phase at a physiological temperature. In the martensite state, the nail can be bent into any shape to accommodate insertion of the nail into an intramedullary canal from any angle. In the austenite state, the nail assumes its therapeutically effective shape and becomes stiff. The nail may comprise a single cylinder of shape memory metal, or a combination of multiple cylinders of shape memory metal. In another embodiment, the nail comprises two or more interlocking links that fit into one another loosely to form a flexible nail and which can be tightened together to form a stiff nail. The nail may be made of any biocompatible material, such as stainless steel, titanium or the like, or other material coated with a biocompatible coating. In a preferred aspect of this embodiment, the nail comprises a wire threaded through the center of each interlocking link and fixed at the distal end on the nail. The wire can be drawn to tightened together the links and stiffen the nail.
In another object, the invention is directed to a method of delivering an intramedullary nail to the intramedullary canal of a long bone, comprising making an incision into a long bone at or near a distal or proximal end of the long bone, inserting a flexible nail through the incision into the intramedullary canal of the long bone, and stiffening the nail. Preferably, the long bone is a humerus and the incision is made off center (not in line) of the intramedullary canal and off set from the distal-most or proximal-most end of the bone.
In one embodiment of this invention, the flexible nail comprises two or more interlocking links that fit loosely together to allow limited rotation of each link relative to another link along the long axis of the nail, and a wire, cable or other flexible stiffening means which is attached to nail. In this embodiment, the flexible nail is inserted into an incision off-set from the center line of an intramedullary canal, and into the intramedullary canal of a bone to be repaired. When the flexible nail is in the appropriate position in the intramedullary canal, the wire, cable or other flexible stiffening means is drawn, pulled or tightened to force each link more closely together, such that a link is not able to rotate along the long axis of the nail relative to an adjacent link. The nail may be secured using any means, such as barbs, tabs, wedges, screws and the like, as is known in the art.
In another embodiment, the flexible nail comprises a shape memory metal having a martensite state at a physiologically safe temperature (e.g., ≧0° C.) and an austenite state at physiological temperature (e.g., ˜37° C.). In a preferred aspect of this particular embodiment, the shape memory metal is a nickel-titanium alloy. In this embodiment, the nail is cooled to the martensite state (i.e., a martensite temperature) and bent to a shape to accommodate the insertion of the nail into an incision that is not aligned in-line to the intramedullary canal (off-center incision). The nail is then inserted through the off-center incision and into the intramedullary canal using a jig cooled to a martensite temperature. After the nail has been placed into the intramedullary canal, the nail warms to an austenite temperature, taking on its therapeutically effective shape and becoming stiff. The nail may be secured using any means, such as barbs, tabs, wedges, screws and the like, as is known in the art.
Applicants have invented an intramedullary nail device that will provide support and alignment for any fractured long bone (especially a humerus) during the healing process. Along with providing this alignment and structure, the subject intramedullary nail device can transition between a flexible state and a rigid state, which allows for greater flexibility in selecting an incision site for insertion of the nail into an intramedullary canal. This especially opens the possibility of repairing the humerus without damaging the rotator cuff and surrounding tissue. The intramedullary nail device also enables a surgical procedure that would be less invasive, allowing for the healing time of the patient to be reduced. The current intramedullary nail insertion procedure followed in the art takes approximately three hours to perform. The subject intramedullary nail device, which can transition from flexible to stiff, could potentially drastically reduce this time to under an hour.
While the subject intramedullary nail device can be used in the repair of any bone and is not intended to be limited to any particular bone, it is especially useful in the repair of fractures in the humerus, given the complex anatomy of the elbow and shoulder. Due to the fact that the new surgical procedure and intramedullary nail device of the instant invention prevents the damage of the rotator cuff, the chance of additional surgery is greatly reduced. This alone can help the patient save money and pain from an additional procedure. Furthermore, the time needed for rehabilitation is expected to be drastically decreased, resulting in additional cost savings to the patient, the community and third party payers. The new surgical procedure and intramedullary nail device of the instant invention can help the surgeon and hospital by reducing the manpower and time required per surgery.
One embodiment of the instant intramedullary nail device, which is graphically described in
This particular device described in this example will allow the surgeon to decide when he or she would like to stiffen the rod after it has been inserted into the intramedullary cavity, therefore removing any time constraint imposed by a martensite-austenite transition event. Any removal of a time constraint makes for a surgeon friendly device.
In some cases, removal of an intramedullary nail is necessary. In this particular example, the linkage system design allows for easy removal of the nail. By simply loosening the screw and wire, and thus relieving any tension in the system, the links unlock, allowing the links to rotate relative to each other and making the entire device flexible once again.
In some situations, some pins may shear at removal, thus causing some links to separate and remain inside the bone. Thus, in one aspect of this embodiment, a plastic polymer sleeve can surround the device, trapping and links that may separate during removal. The sleeve also makes the device smoother, allowing for easier insertion and removal.
It is envisioned that gaps in the links may potentially allow bone to grow into the system of links, thus potentially making removal of the rod difficult. One of many ways to solve that problem is by using a plastic polymer sleeve as described above (supra). The sleeve would be tough enough to prevent bone growth between gaps, but soft enough to allow movement when inserting the device into the body. The sleeve may be fixed to the bottom end of the nail, allowing the links to move freely within the sleeve.
Given that patients vary in long bone length and canal diameter, this particular device can be fitted for various sized patients. Individual links could be added or removed to accommodate the size of the patient.
The shape transformation of a material, such as for example a nickel-titanium alloy (e.g., Nitinol), occurs between the austenite and martensite phases. By varying the amount of nickel and titanium along with small amount of other metals, the transformation temperature range of Nitinol can be altered. An exemplary and non-limiting Nitinol alloy that is useable at physiologically safe and useful temperature range, i.e., “body temperature Nitinol”, has an austenite start temperature of 15° C. and an austenite finish temperature of 35° C. To obtain its memory capability, a set process should be followed. The material is set in a mold and baked at around 500° C. for about an hour, then quenched. Following the quenching process, the metal can be deformed by tension/compression, bending or torsion, and will return to its original, set shape when heated.
The body temperature Nitinol of this particular non-limiting embodiment contains about 55.5% nickel, 0.05% carbon, 0.005% hydrogen, 0.05% oxygen, 0.05% copper and the remaining, titanium. It also posses two different Young's Moduli: 12×106 psi in its austenite phase and 6×106 psi in the martensite phase (see Buehler, W. J. and R. C. Wiley, “Nickel-Base Alloys”, U.S. Pat. No. 3,174,851, Mar. 23, 1965, which is herein incorporated by reference).
The image depicted in
The device could be cooled below 15° C. to its malleable martensite form in order to set it to its desired shape for insertion. After the hole is drilled into the bone and the cavity is reamed, the rod would be slowly inserted into the humerus. As the device warms to 37° C., the Nitinol nail would transition into the originally manufactured shape, which would conform to the straight intramedullary canal cavity. Once the rod is in the bone the proximal and distal screws would be inserted to fix the nail to the long bone shaft.
This particular exemplary nail is made with the same or very similar dimensions to a standard titanium nail for a particular bone (e.g., humerus), but is made instead with body temperature Nitinol, allowing for insertion points away from the rotator cuff for humerus application. The device has certain mechanical properties to properly fix the humeral fracture and allow for proper healing. During the healing process, the rod must be able to withstand the stresses caused by forces on bone. Nitinol has very similar mechanical properties to medical titanium, which has been proven to have adequate stiffness to satisfy the conditions needed for proper healing of a bone. Using a 0.5-inch deflection test, the particular Nitinol rod compares very closely with a titanium rod. When the Nitinol was warmed up to body temperature (austenite phase) it required 83 pounds of force to deflect the rod 0.5 inches, while the titanium rod with identical dimensions required 87 pounds. From this test one skilled in the art would reasonably expect that the Nitinol rod, while in the austenite phase, would satisfy the structural demands of fixing and supporting a fractured humeral bone.
Equations were developed to calculate the deflection from AB, BC and CD. The total deflection was then found based on these three calculations. Only the fracture, which is a very small portion, was subject to bending. The length of the fracture was modeled to be very small as the rod compresses the bone together to close the fracture tightly. At every other section away from the fracture, the bone was taking most of the load. Calculations were made according to Gere, James, Mechanics of Materials, Brooks/Cole, 2001, pg. 646, examples 9-10, which is herein incorporated by reference. The following values were used in those calculations: D1 (bone diameter)=0.866 inch; D2 (outer diameter of nail)=0.354 inch; D2i (inner diameter of nail)=0.157 in; L1=4.99 inch; L2=0.00 inch (
Another calculation was performed to determine the force needed to overcome the yield strength of Nitinol in martensite form. Accordingly, the yield strength of the Nitinol in martensite form was calculated to be 20 ksi. The device is modeled as a cantilever beam supported at the top. It was determined that nine (9) pounds of force would need to be applied, which would yield a stress of about 21 ksi, to overcome the yield strength of the martensite Nitinol to bend it. Nine (9) pounds of force can easily be applied by the surgeon to shape the rod before surgery.
A calculation was performed to determine the force needed to break the bone during nail insertion. It was determined that a force of 122 lb is needed to break the bone during insertion. This is far more than what would be applied to insert a martensite nail, making the procedure safe.
The force needed to buckle the Nitinol nail of this example was calculated to bee around 400 pounds, which is far more than would be applied during the insertion process, making this device safe from buckling.
A heat transfer calculation was performed on the Nitinol nail of this example.
In order to place less of a time constraint on the surgeon, the applicant invented an improved nail insertion device (a.k.a. improved jig), which allows the intramedullary nail to remain at a temperature below the austenite start temperature while in the jig (
According to the present non-limiting example, the Nitinol nail and improved jig both have a 4 mm I.D., and the guide wire is around 2 mm in O.D. Therefore, there is an approximately 1 mm clearance space between rod and guide wire. The clearance space provides a path for the chilled saline to flow along. A tube may be fixed to the top of the improved jig to bring in the chilled saline during the procedure. Calculations demonstrate that a slow, steady flow of saline into the rod, creating a convection boundary, will allow the Nitinol device to reach an equilibrium temperature of about 5.65° C., which is below the austenite finishing temperature of the metal, thus allowing the rod to remain flexible (
The saline will be able to flow out of the bottom end of the rod and into the body via the fracture. Any excess saline will flow out of the top of the device. The chilled saline itself should be at a physiologically safe temperature, e.g., approximately 1-5° C. This is just above freezing, preventing the destruction of skin and muscle tissue, but cool enough to prevent the rod from completely reaching its austenite phase. A hole in the locking screw centering device, which protrudes from the improved jig (
The mechanical properties of Nitinol may be significantly changed during the course of fabrication and machining. Due to the alloy's elasticity, high titanium content, and work-hardening rate, the alloy presents challenges in the production of a finished part. According to this example, which is not intended to be limiting, the Nitinol alloy comprises about 55.5% nickel, 0.05% carbon, 0.005% hydrogen, 0.05% oxygen, 0.05% copper and the remaining (44.345%) titanium. However, one skilled in the art may use another formulation in the practice of this invention, so long as the austenite start temperature is at a physiologically safe temperature, wherein physiologically safe temperature means a temperature that does not cause or facilitate permanent tissue damage over the time course required to deliver the Nitinol nail to the intramedullary canal.
During manufacturing, once the alloy is melted into its composition of Nickel and Titanium, it is usually forged and rolled into bar or slab form. Hot-working has been found to break down the cast structure and improve mechanical properties. Once hot-worked, the alloy is then cold-worked. The cold-working process can be challenging because of the alloy's work-hardening rate. Cold-working and heat-treating must be done to achieve final dimensions and desired physical and mechanical properties. The alloy is difficult to form at ambient temperatures due to Nitinol's super elastic properties and its tendency to return to its original shape once deformed. Also, when trying to heat treat a part made of Nitinol, the part should be fully constrained in the desired shape to prevent the part from trying to return to its original shape.
Once the metal has been formed and heat treated, the sample can be machined to its desired shape. Conventional techniques of milling, turning, and drilling can be used to machine Nitinol to its desired shape. Carbide tools with chlorinated lubricant are recommended for these operations.
In this particular example, gun drilling was chosen to drill the center of the Nitinol nail, mainly due to the fact that a normal drill bit may not be strong enough and may become brittle with the increasing heat from friction. Gun drilling includes three main components: a carbide tip, a heat treated alloy shank, and a steel driver. These components are hollow, allowing coolant to pass through the entire configuration. This coolant to keeps the drill bit from overheating during the cutting process. Once a hole has been started in the center of the tube, the drill is positioned and forced through the workpiece, creating thin curled chips of the Nitinol. The coolant not only cools the tool, but also carries the chips away from the drill area.
After gun drilling, the rod would be finished on a lathe and tapered at the end. Also, the top of the rod may possess threads to accept the insertion device (jig). An end cap will be placed in this threaded hole after insertion in order to reduce the risk of bone in-growth. After manufacturing, the rod may be bent, placed in a mold and baked to achieve a slight bend at the top to allow the device to conform to the bone cavity and any angled hole that may be drilled into the bone. In this particular curved embodiment, markings may be made on the Nitinol nail to indicate the correct orientation of the device, such that the pre-bent portion of the rod coincides with the angled hole drilled into the bone cavity.
In another embodiment, the Nitinol intramedullary nail comprises a braided rod made of many small diameter cylinders composed of body temperature Nitinol (
A particular advantage to a Nitinol nail comprising a multiplicity of small diameter cylinders is that the nail possibly may be easier to remove from the intramedullary canal. That is, chilled saline may be injected into the nail to allow the nail to enter the martensite state and become more malleable. Because the particular nail of this example is made up of many different cylinders, the chilled saline (chilled saline is at a temperature below the austenite start temperature) injected into the tubes making up the nail will come into contact with more surface area than a single hollow nail would. As the saline flows through the tubes, it cools the Nitinol. The mechanical properties will be altered to allow the surgeon to bend the device to the point where it can be removed from the bone.
In another aspect of this particular embodiment, the individual small diameter cylinders are fused together, preferably by soldering. Alternatively, a plastic sleeve may be molded around the braided nail to make an effectively single piece nail. An advantage to a plastic sleeve is that it can also serve as a barrier between the bone and rod to prevent in-growth of biological material into the nail. Plastics are available that have been used in prosthetics and are FDA approved (supra).
Prior to the surgery, the operating room should be supplied with all the necessary equipment for the surgery including the intramedullary nail, sterile chilled saline, and an appropriate sized guide wire. A section of a patient's limb is measured and a intramedullary nail of the proper size is selected. A nail insertion system, which comprises the Nitinol nail, a drill guide and a threaded jig (insertion device), is assembled. The nail insertion system is submerged in the sterile chilled saline at least about ten minutes prior to the surgery. The patient is positioned and immobilized as is appropriate for the limb or section of limb to which the surgery is to be performed.
In a situation is which the intramedullary nail is to be used in the repair of a humerus, the following procedures may be carried out. However, the use of an intramedullary nail of the instant invention shall not be limited in scope to only repairing a humerus. While the following procedure describes the use of a Nitinol nail, the nail as described in Example 1 (e.g. link and wire system, supra) may also be used in this procedure.
The patient is positioned with his or her legs parallel to the floor and the upper body semi-reclined with the affected shoulder slightly over the edge of the operating table. The patient's head can be immobilized with one or two strips of adhesive tape to prevent movement while traction is being applied to the arm. A deltopectoral incision is made near the area of the humerus that is to be drilled. A deltoid splitting incision may used as an alternative with care given to avoid the axillary nerve. The superficial tissues are divided and the deltopectoral groove is blunt dissected to deepen the incision (use of a retractor may be needed to view the site to be drilled). A hole is drilled in the proximal region of the humerus large enough to fit the width of the Nitinol nail to be used (9-12 mm). Preferably, the hole should be drilled at an angle of between 30-35° pointing towards the distal end of the humerus relative to the central axis of the humerus. A bulb ended guide wire is inserted into the humeral canal. A flex reamer device having a diameter of preferably 0.5 mm larger than the diameter of the Nitinol nail is placed over the bulb ended guide wire and is used to clear away the soft tissue within the humeral intramedullary canal. The reamer is removed from the canal. A slipcover is placed over the bulb ended guide wire and inserted into the humeral intramedullary canal. The bulb ended guide wire is then removed from canal. A standard guide wire is then inserted through the slip cover and into the canal. The slip cover is removed from the canal in advance of the insertion of the Nitinol nail into the intramedullary canal.
After the canal has been reamed and guide wire inserted, the nail insertion system is removed from the chilled saline bath. The nail is fed into the insertion point and is hammered down the canal over the guide wire. The chilled saline is slowly injected to increase flexibility of the Nitinol nail. Once the nail is past the fracture point and almost completely in the humeral canal, the guide wire is removed.
Any necessary adjustments to the position of the nail are made before inserting any proximal and distal screws. The proximal end of the humerus is secured by inserting a titanium screw through a drill guide that is attached to the nail insertion system and screwing the screw through the humerus and into the predrilled holes in the nail. The hole in the distal end of the nail may be located using ray techniques, a hole drilled through the distal part of the humerus aligned with the distal hole in the nail, and a screw is inserted through the distal humeral and nail holes to secure the distal end of the humerus. The jig, drill guide and threaded hammer cover are removed and disassembled after the rod is secured at the proximal and distal ends. The incision is then closed with an appropriate suture.
Table 1 illustrates how the Nitinol nail, compared to a standard titanium nail, can drastically reduce the time of surgery to fix a fractured humerus.
Preferred embodiments of the invention are described in the preceding description and examples. Other embodiments within the scope of the claims herein will be apparent to one skilled in the art from consideration of the specification or practice of the invention as disclosed herein. It is intended that the specification, together with the examples, be considered illustrative and exemplary only, with the scope and spirit of the invention being indicated by the claims which follow.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US2004/032546 | 10/4/2004 | WO | 00 | 5/18/2010 |
