The use of metallic super-elastic alloys, such as Ni—Ti (nitinol) and other bi- or tri-metal alloys, has been documented in a variety of technical applications, including fasteners, connectors, gaskets, clamps and seals. Many such uses have required temperature in order to activate the material and change its physical state, while others have used mechanical forces that impart stress to cause a super-elastic physical deformation in the material. Of particular concern to the instant inventor is the application of the super-elastic material to connectors. The use of non-corrosive, metallic super-elastic material offers a decided advantage in high performance connecting assemblies, versus more conventional connectors requiring threaded fasteners, springs, clamps or other holding or securing mechanisms. Particularly it can withstand more wear than alloys used in conventional connectors due to its harder surface characteristics. It can also withstand extreme vibrations and not loosen due its elastic preloaded condition without using conventional adhesives to hold the assembled components and/or the connector itself together. Adhesives used with conventional connectors make them very difficult to disassemble, whereas it is generally possible to make a super-elastic connector completely reversible.
U.S. Pat. Nos. 5,395,193 and 5,584,631 to Krumme et al., discuss the use of nickel-titanium shape memory retainers in an optimized elastic condition that have super-elastic or pseudo-elastic properties. These fasteners are said to be useful for eyeglass assembly; they are placed onto a pin to retain components together.
It is commonly known that nitinol (formally known as Nickel Titanium Naval Ordinance Laboratory, but other super-elastic nickel-titanium alloys being included in this definition) tubing, wire or rod can be used as a mechanical drive shaft. The use of metallic super-elastic alloys, such as Ni—Ti (nitinol) and other bi- or tri-metal alloys, has been documented in a variety of technical applications, including fasteners, connectors, gaskets, clamps and seals. Many such uses have required temperature in order to activate the material and change its physical state, while others have used mechanical forces that impart stress to cause a super-elastic physical deformation in the material. Of particular concern to the instant inventor is the application of the super-elastic material to connectors. The use of non-corrosive, metallic super-elastic material offers a decided advantage in high performance connecting assemblies, versus more conventional connectors requiring threaded fasteners, springs, clamps or other holding or securing mechanisms. Particularly it can withstand more wear than alloys used in conventional connectors due to its harder surface characteristics. It can also withstand extreme vibrations and not loosen due its elastic pre-loaded condition without using conventional adhesives to hold the assembled components and/or the connector itself together. Adhesives used with conventional connectors make them very difficult to disassemble, whereas it is generally possible to make a super-elastic connector completely reversible.
U.S. Pat. No. 5,683,404 to Johnson, entitled “Clamp and Method for its Use”, further discusses shape memory materials that are “pseudo-elastic”, defining these materials to be super-elastic, because of their ability to exhibit super-elastic/pseudo-elastic recovery characteristics at room temperature. In other words, a material is super-elastic when, if sufficient stresses are applied, such materials exhibit martensitic activation (i.e., deform from an austenitic crystal structure to a stress-induced structure postulated to be martensitic in nature), returning thence to the austenitic state when the stress is removed. The alternate crystal structures described give the alloy super-elastic or pseudo-elastic properties. Poisson's Ratio for nitinol is about 0.3, but this ratio significantly increases up to approximately 0.5 or more when the shape memory alloy is stretched beyond its initial elastic limit it is at this point that stress-induced martensite is said to occur. i.e., the point beyond which the material is permanently deformed and thus incapable of returning to its initial austenitic shape. A special tool is employed by Johnson to impart an external stretching force that deforms the material which force is then released to cause the material to return to its original condition. While the device is stretched, a member is captured by it and securely clamped when the stretching force is released. This device is intended for use in clamping and does not contemplate traditional connecting operations of the kind addressed by the present invention. Another use envisioned by Johnson is in connecting the modular components of a medical device, as described in his U.S. Pat. No. 5,858,020, by subjecting a thimble component made of shape memory material to an external stretching stimulus to elongate and thereby reduce its transverse dimension. Upon release of the stretching force, this component returns towards its original rest dimension, contacting and imparting a force on another component. This is a sequential stretching and relaxation of the super-elastic material rather than a simultaneous activation and retention operation. Also, special structures are necessary on the thimble to allow the stretching force to be imparted.
In U.S. Pat. No. 5,197,720 to Renz, et al., a work piece is held within a clamping tool by an expansion element made of shape memory material that is activated by mechanical force. In this way, torque is transmitted through the shape memory member. This device is useful for bringing parts together for holding the work piece in order to perform an operation. It does not, however contemplate a use as a connector. U.S. Pat. No. 5,190,546 to Jervis discloses insertion into a broken bone cavity of a split member made of shape memory material using a SUPER-elastic alloy. The split member holds the walls of the bone cavity when radial compressive forces acting on it are released. In order for the radial compressive force to reduce the diameter, the component must be split, allowing the reduction in dimension for insertion. It does not act as an interposed member in a connecting assembly.
Others have sought to utilize the properties of shape memory materials as locking, connector and bearing elements, e.g. U.S. Pat. No. 5,507,826 to Besselink et al, U.S. Pat. No. 5,779,281 to Kapgan, et al., and U.S. Pat. No. 5,067,827 to Arnold, respectively; however, such approaches have required temperature to be applied during use. U.S. Pat. No. 5,277,435 to Kramer, et al. and U.S. Pat. No. 5,876,434 to Flomenblit, et al. similarly has relied upon temperature to activate the shape memory effect. Such dependence on extrinsic activation by temperature introduces an added process step and may further be disadvantageous in certain other applications.
U.S. Pat. No. 5.842.312 to Krumme, et al., entitled, “Hysteretic Damping Apparati and Methods”, employs shape memory tension elements to provide energy dissipation. Such elements can be placed between building structures, etc. which are subject to vibration, serving to absorb the energy created by their relative movement. However, this patent does not contemplate the vibration dampening effect of a SUPER-elastic material in the formation of a connector.
Nitinol is especially useful for transmitting torque while in a bowed or bent shape. These types of drive shafts have proven useful in orthopedic surgical applications where drilling or reaming of curved bones is necessary. One application is to use a drill or reamer with a nitinol drive shaft to clean out the center of a femoral bone before implanting a prosthesis or femoral nail. These bones typically have a bow with a 90-inch radius and require a flexible reamer for the procedure. Nitinol tubing can be used for this application since it is cannulated and can be passed over a guide wire that is placed down the femur before the reaming process begins. Since the tubing is solid it is very easy to clean after the surgical operation since there are no crevices for blood to get trapped in. Earlier designs utilized spring drive shafts and cleaning was extremely difficult since blood could get trapped between the windings of the spring. The earlier spring designs also had difficulties when run in the reverse direction since springs tend to be strong while being used in one direction, however when run in the opposite direction they tend to unwind. To prevent this unwinding problem several manufacturers have added an additional spring inside of the primary spring, which is wound in the opposite direction. Since one spring is inside of the other this contributes to the difficulties with cleaning and further obviates the need for an alternative shaft design.
With the market demand increasing for these novel nitinol drive shafts there have been many attempts to develop safe coupling methods for attachment to the shaft. One difficulty that engineers have been faced with is presented when nitinol tubing exceeds its torsional or fatigue stress limits; it has been known to fail catastrophically and fragment into several sharp pieces. This is dangerous when inside of a patient and poses severe concerns if these types of products are to be used reliably. Historically there have been no solutions offered to limit the stress in the drive shaft, which would eliminate the presently lingering concerns over breakage during use.
Another difficulty is presented with the attachment of the fittings to the nitinol drive shaft. The connection must be reliable and not create any unnecessary stress on the tubing. This will lead to early failure of the shaft. Typically in the orthopedic reamer example mentioned above one end of the nitinol tube has a stainless steel fitting which attaches to a power instrument and on the opposite end either a stainless steel reamer head or an intermediate modular fitting that connects to a reamer head. Several attempts to create reliable attachments have been made.
One approach to the above-mentioned attachment problem has been to use an epoxy to “glue” the fittings onto the nitinol shaft. However, temperatures in the sterilization process and the criticality of surface preparation have rendered this approach unreliable.
Another approach has been to attach the fitting to the shaft with a laser weld; however, the welding process embrittled the tubing and it was known to fail torsional demands in testing. A cross hole and pin were placed through the fittings, however this added approach further proved useless since the matching hole in the tubing created a tremendous stress riser in the tubing causing failure at very low torsional values. In the example mentioned it was known to fail anywhere between 2 to 4 N–M.
Yet another approach has been to press fit the nitinol shaft into a fitting with approximately 0.002-inch interference. Initial trials worked, however when put through rigorous fatigue tests the tubing placed too much hoop stress on the fittings causing them to fail rather than the shaft. The solution proposed to fix that problem was to add a long section on the fitting that was loosely fit around the tube. This would allow the stress to transition slowly into the area where the press fit was done. This worked successfully, however the solution created a need for the fitting to be extremely long in comparison to the reamer heads being used. This is undesirable since the reamer must follow the curvature of the bone and it did not solve the issue of limiting the torque in the shaft to ensure the safety of the drive shaft during use.
Thus, there remains a primary need to provide a coupling system that is safe and effective for use in surgical and industrial applications where flexible drive shafts are necessary.
Accordingly, there is a need to form a connecting assembly using a durable metallic, non-corrosive connector assembly, which are simple to install using relative motion to activate the assembly.
There is a further need to form a secure connection between components that minimizes the micro-motional wear characteristics of the assembly, enhancing its useful life.
There is another need to form a fastened assembly that does not require temperature for its activation.
There is still a need to form an assembly using a fastener that adjusts for differences in thermal coefficients of expansion or contraction of dissimilar materials comprising those components being fastened.
There is still a further need for a connector with elastic properties that allow more forgiving tolerances during manufacturing of the assembly components.
There is another need to use a nitinol drive shaft to replace the spring drive shafts in orthopedic instruments and many industrial tools to simplify the cleaning process and ensure consistent torque resistance in the forward and reverse directions.
There is a further need to provide a coupling system which will limit the torque in the nitinol drive shaft fitting to ensure that the coupling limits the torque before the tubing breaks.
There is yet a need for a coupling that will not place any unnecessary stress on the nitinol tubing causing it to prematurely break.
There is also a need to shorten the length of the fitting so that the reamer can follow the natural contour of the inside of the bone while transferring the stress smoothly so as to ensure the strength of the fitting.