BONE CAP AND METHOD OF MAKING SAME

Abstract: A bone cap implant (10) of the type used to cover a remaining portion of a resected bone, such as metatarsal, metacarpal or phalangeal bones. The bone cap serves as an articulation surface for the proximal phalanx in the metatarsophalangeal, metacarpophalangeal or interphalangeal joints. A method of making the bone cap by forming an existent tubular sleeve (12) made of expanded PTFE integrally bonded to non-expanded PTFE (14) without the use of adhesives. A method of forming a structure onto an existent structure, such as one made of expanded PTFE, by sintering without the use of adhesives.

Description: BONE CAP AND METHOD OF MAKING SAME Background of the Invention The present invention relates generally to bone caps and a method of making orthopedic prosthetics. More specifically, the present invention relates to a metatarsal, metacarpal or phalangeal bone cap of the type used to cover a remaining portion of a bone after surgical removal of a degenerated or injured joint. The present invention also relates to a method of forming articles of polytetrafluoroethylene ("PTFE") in which expanded PTFE ("e-PTFE") is integrally bonded to non- expanded PTFE ("n-PTFE") . PTFE is a substantially chemically inert, biocompatible thermoplastic polymer composed of long linear carbon chains surrounded by fluorine atoms. PTFE is usually molded or extruded under extremely high pressure and temperatures. Expanded PTFE comprises a porous microstructure of "nodes" and "fibrils". The fibers originate from the nodes with the nodes being generally thicker than the fibrils. The fibril length is controlled during processing and determines the porosity of the material. PTFE is extremely hydrophobic as a result of the high electronegative charge of its polymer chains. This electrochemical property renders PTFE less thrombogenic than other biocompatible implant materials. Because of its highly non-reactive natural state, it has been regarded necessary to either derivatize the PTFE by stripping the fluorine atoms with an etchant to allow chemical bonding with the underlying carbon-chain backbone of the polymer or coat the PTFE with another more reactive polymer. Coating PTFE has typically been done by dipping the PTFE into liquid polymers which are, themselves, then capable of being bonded to other materials. In the field of internal biocompatible materials, however, the use of etchants, adhesives, silicones, latex or other potentially bioreactive materials is undesirable. PTFE, both in its expanded and non-expanded states, has been generally recognized as an ideal material for body implants, internal prosthetics, tissue grafts, etc. because of its high degree of biocompatibility and biochemical inertness. A wide variety of material compositions exist which employ either e-PTFE or n-PTFE, either alone, in combination with another material or in combination with each other. For example, U.S. Patent No. 4,147,824 discloses that it is known to make a plastic seal by compressing a layer of pure powdered n-PTFE in a mold, and then placing on top of the formed n-PTFE layer another layer of sinterable PTFE powder containing a filler material. The mixture is compressed in a second pressing and the two-layer material is sintered, after which the filler material is flushed out forming porosities in the second layer. U.S. Patent No. 4,283,448 discloses a process by which articles can be made by edge joining two or more pieces of e-PTFE by sintering the pieces under pressure to form the desired article without forming a non-porous seam at the joint. U.S. Patent No. 4,892,779 discloses a multilayered article made of one layer of a non-porous material fusion bonded, without adhesive, to a layer of microporous material having a large proportion of water insoluble filler at least 50 percent of which is siliceous. This reference contemplates that the microporous material may be PTFE, and that the PTFE may be stretched to increase void volume of the material. The stretched microporous material is then fusion bonded, without an extrinsic intervening adhesive, to a substantially nonporous material. Both the microporous and non-porous materials have generally opposed major surfaces such as are characteristic of sheets, films, foils and plates. Fusion bonding may be made by use of heated rollers, heated bars, heated lathes, heated bands, heated wires, flame bonding, RF sealing, and ultrasonic sealing. Finally, U.S. Patent No. 5,032,445 discloses a method for treatment of periodontal disease in which a periodontal implant is fashioned of a sheet of e-PTFE material having first and second surfaces. The first surface of the e-PTFE is permeable to tissue ingrowth, while the second surface is rendered impermeable to tissue ingrowth in desired areas by application of heat and pressure only to the areas of impermeability. This reference discloses that a single sheet of e-PTFE can be made to have two distinct opposing regions of tissue permeability and impermeability by application of heat and pressure to a region on one side of the permeable e-PTFE sheet, thereby creating a region of non-porous impermeability on the otherwise permeable e-PTFE sheet. The method of forming a bond between an article made of e-PTFE and an article to be formed of n-PTFE, without the use of an intervening adhesive, appears to be unknown in the art. Turning now to the bone cap device, the present invention provides a bone cap having a microporous sheath or tubular portion and a non-porous end portion. Both the microporous sheath and the non-porous end portion are made entirely of PTFE without filler materials, such as carbon fiber or silicious materials. The sheath is made of e-PTFE to facilitate tissue ingrowth which anchors the cap to the bone. The end portion is made of n-PTFE to inhibit bone ingrowth and function as an articulation surface between the bone cap and an adjacent bone. The n-PTFE end portion and the e-PTFE sheath are fused together without the use of intervening adhesives by the method of the present invention. A bone cap of the above-described construction appears unknown in the art. A wide variety of bone caps are known in the art. For example, U.S. Patent No. 4,007,494 discloses a bone cap having an inner porous cap and an outer non-porous cap cover but without a sheath. The porous cap is formed from a porous polymeric material to enable tissue ingrowth to anchor the bone cap. The porous cap surrounds all aspects of an excised bone, including the intramedullary surface. The non-porous cap cover resides above the outer surface of the bone cap and restricts tissue ingrowth beyond the porous cap. The head may also include a stem for insertion into the medullary canal of the bone. U.S. Patent Nos. 4,362,681 and 4,756,862 disclose a bone cap comprised totally of bioengineering thermoplastics with select porous areas. Patent No. 4,756,862 discloses a process for preparing a sintered bioengineering thermoplastic material. Patent No. 4,362,681 discloses a process for preparing a prosthetic device, such as a hip joint, which involves sintering a particulate thermoplastic material onto a load bearing functional component to provide a porous coating. U.S. Patent No. 4,129,470 discloses a method for making implants using PTFE sintered with carbon or graphite fibers. The implant preferably has a porous structure of carbon or graphite fibers bonded together by sintered PTFE in a manner that exposes a maximum amount of fiber surface. U.S. Patent No. 4,351,069 discloses the formation of an implant using a sintering technique to form a porous coating on the outer surface of the implant. U.S. Patent No. 5,098,779 discloses an implant made of porous PTFE with a stiffening agent. The stiffening agent alters the porosity of the PTFE in order to allow the PTFE to be shaped by carving. Various methods have been employed to bind e-PTFE sheets or films to other substrates or to impregnate e-PTFE sheets or films with adhesives. For example, U.S. Patent No. 4,531,916 discloses a dental implant having an e-PTFE gingival interface between a root structure and a cervical segment and the bone, and U.S. Patent No. 4,304,010 discloses a tubular PTFE prosthesis having a porous elastomer coating. Patent No. 4,531,916 discloses that e-PTFE interface is either attached to the cervical and root segments by adhesion, compression between the cervical and root segments or molding the cervical and root segments of a biocompatible polymer with the e-PTFE interface in place. Patent No. 4,304,010 discloses a method for coating a e-PTFE tube with an elastomeric coating. The elastomeric coating is applied in solution or as a liquid compound to the outside surface of the PTFE tube. The elastomer coating is then dried and cross-linked. The porous e-PTFE tubing and the porous elastomer coating are bonded to each other as a result of a part of the elastomer entering the pore spaces of the PTFE tubing. Each of the known methods for joining a structure made of e-PTFE to another structure involve creating a junction between a formed e-PTFE structure with another formed structure. The junction may be formed by impregnating an adhesive into the microporous structure of an e-PTFE sheet and subsequently adhering another material onto the adhesive- impregnated e-PTFE, such as a metal film to form a circuit board, as disclosed in U.S. Patent No. 4,916,017. A junction may also be formed by wrapping a bundle of e-PTFE fibers axially along a length of wire, then wrapping the bundle of fibers with an e-PTFE tape and sintering the entire resulting structure to meld the fibers and tape into a unitary structure as disclosed by U.S. Patents No. 5,059,263. Similarly, a junction can be created between sheets or films of e-PTFE surrounding a plurality of parallel wires to form a ribbon cable by sintering compressing the sheets and sintering the e- PTFE sheets to each other, as disclosed by U.S. Patent No. 4,988,835 and U.S. Patent No. 4,978,813. Edge joining by sintering adjacent articles made of e-PTFE to create a porous seam while compressing the e-PTFE articles, is taught by U.S. Patent No. 4,283,448. Accordingly, methods of binding e-PTFE sheets or films to other formed structures made of e-PTFE by compression and sintering is well known. Additionally, the use of adhesives to laminate e-PTFE sheets or films to another material, such as a metal film in a circuit board, is well known. The creation of a bond between existent e-PTFE structures, such as sheets or films, is common to each of these known methods. It is also known that regions on a sheet of e-PTFE may be made non-porous, or converted to n-PTFE, by application of heat and pressure to the region to be altered. U.S. Patent No. 5,032,445 discloses a method to alter a region to make it impermeable to tissue ingrowth. The method of this patent entails providing a sheet of e-PTFE having first and second surfaces. A region of the second surface is made impermeable to tissue ingrowth by applying plates heated between 300-400° C under pressure to the e-PTFE sheet. Application of heat causes the e-PTFE region on the second surface to coalesce into a non-porous condition in intimate contact with the opposing e-PTFE first surface without the use of adhesives. A similar method is disclosed by PCT International Application WO 90/06150. This reference discloses that a catheter may be made having a porous tip by two different methods. A first method entails forming a tube of n-PTFE and expanding an intermediate portion of the tube to create an e- PTFE region. A second method entails forming a tube of e- PTFE, covering an intermediate portion of the tube with a material of low thermal conductivity and sintering the unprotected regions of the e-PTFE tube. Another related method is taught by U.S. Patent No. 4,701,362 for forming reinforced through-holes in an e-PTFE sheet. This patent discloses exposing a sheet of e-PTFE to a point heat source, such as a laser or heating rods, which creates holes in the e-PTFE sheet by sintering an annular collar surrounding the through-hole. The sintered annular collar is formed of non-porous PTFE. Each of these methods entail sintering regions on an existent e-PTFE structure to convert the sintered regions to non-porous n-PTFE. There appear to be no known methods for forming a structure onto an existent e-PTFE structure. The bone cap of the present invention is an example of one type of device capable of manufacture by the method of the present invention. Those skilled in the art will recognize that the method of the present invention can be used to form a wide variety of devices made of either e-PTFE, n-PTFE or a combination thereof. Summary of the Invention The preferred embodiments of the present invention relate to a bone cap having a tubular section made of porous e-PTFE and an enclosed end section made of non-porous n-PTFE. The tubular section is porous to enable tissue ingrowth and revascularization, while the non-porous end section prevents bony ingrowth and protects the excised end of the bone. The method of the present invention provides a method for making an article of e-PTFE, n-PTFE or a combination thereof, in which a starting structure, such as a tubular body, is provided and a second structure, such as an end cap, is integrally formed onto the starting structure. The method entails mounting the starting structure onto a mandrel, filling a mold with powdered, unsintered PTFE, inserting the mandrel into the mold such that a region of the starting structure is immersed in the mold, applying positive pressure to the mandrel, securing the starting structure onto the mandrel and securing the mold/mandrel assembly together. The mold/mandrel assembly is mounted in an oven well to expose only the powdered, unsintered PTFE and the bonding region of the starting structure to sintering temperatures. The powdered, unsintered PTFE coalesces with the region of the starting structure exposed to the unsintered PTFE, thereby forming a unitary formed structure onto the starting structure. Brief Description of the Drawings FIG. 1 is a perspective view of a bone cap in accordance with the present invention. FIG. 2 is a side-elevational cross-sectional view taken along line 2-2 of FIG. 1. FIG. 3 is a side-elevational view illustrating the inventive bone cap surgically attached to an excised bone. FIG. 4 is a flow diagram representing the method of the present invention. FIG. 5 is a side elevational cross-sectional view of a mold and mandrel assembly employed in making the inventive bone cap using the inventive method. FIG. 6 is a side elevational partial cross-sectional view diagrammatic view illustrating a sintering well in accordance with the present invention. FIG. 7 is an enlarged view of region 7 of FIG. 6. FIG. 8 is a perspective view of a sintering oven for use in the method of the present invention. FIG. 9 is a diagrammatic cross-sectional view of a junction region between n-PTFE and e-PTFE regions of an article made in accordance with the inventive method. Detailed Description of the Preferred Embodiments Bone caps are implant devices designed to be placed over distal shafts of the metatarsal or metacarpal bones following metatarsal or metacarpal head resection. The bone cap implant provides a smooth surface for articulation against the proximal phalanx. Bone cap implants are indicated in degenerative or inflammatory joint disease, dislocation or subluxation of the lesser metatarso- or metacarpophalangeal joints, painful joints with limitation of motion, or revision of previous procedures in the presence of sufficient bone stock. Bone cap implants are also designed to be placed over the proximal phalangeal shaft following phalangeal head resection. The phalangeal bone cap implant provides a smooth surface for articulation against the middle phalanx. Phalangeal bone caps are generally employed in patients with angular deformity, impaired joint function and stability, pain and destroyed articular surfaces; all indications commonly found in the proximal interphalangeal joints in flexion contractures of the toes. FIGS. 1-3 in the accompanying drawings illustrate bone cap 10 of the present invention. Bone cap 10 may be used to truncate the metatarsus, metacarpus, or the phalangeal bones of both the hand or foot, with the principal difference simply being the size of the bone cap 10. Bone cap 10 consists of a tubular sleeve 12 and an end cap 14 which encloses one end of the tubular sleeve 12. A second end of the tubular sleeve 12 is open to allow access to a lumen 16. As illustrated in FIG. 3, bone cap 10 is surgically fitted onto either the metatarsal bone 18 or the phalangeal bone 20. Bone cap 10 is fitted onto the resected metatarsal head 19 or the phalangeal head 21 to provide an articulation surface for the metatarso-phalangeal joint 22 or the interphalangeal joint 23, respectively. For purposes of illustration reference will hereinafter be made only to the metatarsal bone 18. Bone cap 10 is affixed onto the distal shaft of the metatarsal bone 18. The resected metatarsal head 19 is placed into the lumen 16 and the bone cap 10 is axially positioned onto the distal shaft of the metatarsal bone 18. The resected head 19 should rest adjacent or in close proximity to the inside surface of end cap 1 . The surgical implant procedure can be performed on an outpatient basis using intravenous sedation and local anesthesia. A dorsilinear incision is made directly over the metatarsophalangeal joint, extending from approximately the distal third of the shaft of the metatarsal to a point midway along the shaft of the proximal phalanx. The incision is deepened by both sharp and blunt dissection and bleeders are coagulated as encountered. The subcutaneous tendons and neurovascular bundles are retracted out of the operative site. The dorsal capsule of the metatarsophalangeal joint is entered by a linear incision made at a point just proximal to the anatomical neck and distally to the metatarsophalangeal joint. The metatarsal head is dissected free of its attachments and a transverse osteotomy is made in the area of the anatomical neck. The remaining portion of the bone is smoothed with a fine diamond rasp. Care should be exercised to minimize the disruption to the periosteum surrounding the distal metatarsal. The area is preferably irrigated with an antibiotic irrigating solution to remove osseous debris. The soft tissue is then gently moved proximally from the distal portion of the metatarsal shaft. A sizer is placed over the distal aspect of the prepared metatarsal shaft to check for fit. The appropriate size bone cap implant 10 is then manipulated into position such that the tubular sleeve 12 resides over the bone shaft and the interior surface of the end cap 14 abuts adjacent to, or in close proximity to, the excised metatarsal shaft. If desired, the implant may be secured by sutures passing through the proximal end of the tubular sleeve 12 and tacked to adjacent soft tissue. The surrounding capsular structure and wound is closed using appropriate sutures. Bone cap 10 is preferably made of a porous biocompatible material forming the tubular sleeve 12 and a non-porous biocompatible material forming end cap 14. The bone cap 10 is preferably made by bonding the tubular sleeve 12 to the end cap 14 without additional copolymers, additives, or adhesives, thereby eliminating the possibility of leaching potentially bioreactive substances. E-PTFE is preferably used for the tubular sleeve 12, while n-PTFE is preferably used for the end cap 14. The use of PTFE for the joint implant of the present invention is advantageous in that it provides a chemically inert and biocompatible implant which has a high tensile strength and a low coefficient of friction. The porosities formed by the node and fibril structure of e-PTFE which comprises the tubular sleeve 12 promote tissue incorporation and revascularization, and implant anchoring. The non-porous n-PTFE acts to inhibit tissue ingrowth into the joint region. After implantation of the bone cap implant 10, tissue ingrowth will occur into the node and fibril structure of the e-PTFE of the tubular sleeve 12. The end cap 14, being non- porous, will prevent tissue ingrowth into the metatarsophalangeal joint. Clinical Examples A total of fifteen bone caps were implanted in five patients in accordance with the present invention over a three month period. Patients were selected based on adequate vascular status, presence of fibro-osseous unions (joint fusions) , progressive joint deformity and the presence of intolerable pain with little or no peripheral inter-phalangeal joint (PIPJ) range of motion available. The structural joint changes of those patients selected no longer responded to conservative treatment. In addition, those patients with hammertoes often exhibited PIPJ subluxations/dislocations, often with varus deformities. The metatarsal bone caps were indicated in forefoot reconstruction procedures, degenerative or inflammatory metatarsophalangeal joint (MPJ) diseases, lesser MPJ subluxations and dislocations, painful prominent metatarsal heads, or painful arthritic joint disease with limited range of motion. Following surgery, patients were evaluated at one, two, four and twelve week intervals for signs of infection, edema, pain, implant failure and bony resorption. All patients received preoperative prophylactic Ancef W. All surgeries were performed on an outpatient basis under IV sedation and local anesthesia. The joint implant of the present invention was implanted in patients according to the surgical procedure previously described. The clinical trials showed patients with a significant post-operative increase in functional metatarsal parabola, with a shorter post-operative recovery period and an earlier return to wearing normal footwear. The tests further demonstrated minimal to non-existent post-operative fibrositis, foreign body reaction, infection, and dislocation. Patients also exhibited limited to non-existent post-operative edema or pain. The method of the present invention is illustrated with reference to FIGS 4-8 of the accompanying drawings. For purposes of illustration, the inventive method will be described with reference to making the inventive bone cap 10, in which an end plug, i.e., the end cap 14, is fused to an end of an tube made of e-PTFE, i.e., tubular sleeve 12. Those skilled in the art should understand and appreciate, however, that the method may be used to make other structures. The method may also be used, for example, to form a hard sleeve made of n-PTFE onto the end of an tube made of e-PTFE to create a machinable surface or to form a molded structure of n-PTFE onto a sheet or film of e-PTFE. FIG. 4 sets forth process steps of the inventive method used to make bone cap " 10 and illustrate application of the method to form a PTFE structure onto an existent PTFE structure. As illustrated in FIGS. 5 and 6, there is provided a mold assembly 40 and a heater 50. Mold assembly 40 consists of a mold 42 having an interior mold cavity configured to form a desired object. In accordance with the method to make the inventive bone cap 10, mold cavity 43 is formed as a concave- ended cylindrical cavity. The mold 42 is configured to concentrate thermal energy in a lower aspect 44 of mold 42. Concentration of thermal energy in the lower aspect 44 is important because the unsintered PTFE will be placed in the lower aspect 44 of mold 42, while the sintered e-PTFE will substantially reside in other areas of cavity 43. In this manner, the sintered e-PTFE is distant from the concentration of thermal energy to minimize coalescing of e-PTFE to the n- PTFE state. A mandrel 46 and push rod 48 are provided. Mandrel 46 serves as a support for the existent structure onto which the structure to be formed is molded. The push rod 48 facilitates application of positive pressure to a powdered, unsintered body of n-PTFE deposited into the mold cavity 43. Applying compressive force is needed to create a formed body and ensure proper coalescence of both the formed body and the area between the formed body and the e-PTFE existent structure when sintered. A sintering oven 50 is illustrated in FIGS 6-8. For high volume production it is preferable to simultaneously fabricate multiple articles. Those skilled in the art will understand that multi-cavity molds 42, mold assemblies 40 and sintering ovens 50 facilitates process scale-up and increases device throughput. Sintering oven 50 preferably consists of a plurality of heating wells 54 formed as a planar body 42 made of a thermally conductive material, such aluminum. A thermally non-conductive material is provided as an insulating surface member 53. Insulating surface member 53 should be of sufficient thickness to allow the mold assembly 40 to seat with only the lower region 44 of the mold 42 being exposed to the heating coils 56. The insulating surface member thermally insulates substantially the entire length of the mold assembly 42, exposing only a bottom portion of the mold 44 and mold cavity 43 to heat. In this manner the thermal energy is concentrated in an area to be exposed to sintering heat. Each of the heating wells 54 is formed into both the insulating surface member 53 and the conductive planar body 52. Each heating well 54 accepts a mold 42 therein and exposes the lower region 44 of the mold 42 to a concentrated heat source. In accordance with the best mode of the present invention there is disclosed a resistive heating coil 56 surrounding each of the plurality of heating wells 54. Heating coil 56 is connected, via wires 58, to a voltage source 60 which provides electricity to the resistive heating coils 56. It is preferable to provide a digital temperature gauge 64 and thermocouple 62 in thermal or electrical connection with the heating coils 54 or heating wells 56. The provision of the temperature gauge 64 permits temperature readouts to ensure that adequate sintering temperatures are reached in the heating well 54 to sinter the n-PTFE to the e-PTFE sleeve. The presence of a thermocouple 62 is desirable. Thermocouple 62 can act as a controller for the voltage source 60 to increase or decrease voltage output to the resistive coils 56 in order to reach a pre-set temperature point. Additionally, thermocouple 62 can be used to output a control signal to automated process controls (not shown) to actuate mechanical loading or unloading of the mold assemblies 40 into the heating wells 54. Finally, it is contemplated that cooling means 66, such as cooling coils, can be provided in the insulating member 53 adjacent to the mold assembly 40. Cooling means 66 may be linked to a heat exchanger (not shown) to provide a recycling source of a cooling fluid medium. Providing cooling means 66 facilitates formation of a distinct thermal boundary in the mold between the e-PTFE sleeve region and the n-PTFE end cap region to be sintered. Protecting the e-PTFE region from heat generated by the heating coils 56 further guards against contraction of the porous e-PTFE to non-porous n-PTFE. The cooling means may, for example, consist of tubular members which act as a fluid conduit to conduct a recycling cooling fluid to and from the heat exchanger. The inventive bone cap 10 is preferably made in accordance with the process 30 illustrated in FIG. 4. A tubular e-PTFE sleeve is selected 31 and then fitted onto cylindrical mandrel 32. The tubular sleeve is adjusted 33 such that a first end extends beyond a first end of the mandrel about 0.125 inches, this extension is then inverted into the lu inal opening in the mandrel. The second end of the tubular sleeve is secured onto a second end of the mandrel by use of a wire tie, clip or other fastening means. A first body of powdered, unsintered PTFE, of predetermined volume or weight, is poured 34 into the mold cavity. The first inverted end of the tubular member on the mandrel is then inserted into the mold cavity 35. The tubular sleeve and mandrel assembly is pressed into the mold and into the first body of powdered, unsintered PTFE residing in the mold cavity. A second body of powdered unsintered PTFE, of predetermined volume or weight, is poured into the mold cavity through the luminal opening of the mandrel 36. A push rod is inserted through the lumen of the mandrel and positive pressure 37 is applied to the push rod and mandrel to compress the PTFE into a solid plug with the e-PTFE tubular sleeve in intimate contact with the solid plug of unsintered PTFE. The mold assembly is transferred to the sintering oven and placed into a heating well. The mold is exposed to sintering heat 38 which sinters the n-PTFE plug to the e-PTFE sleeve, causing the n-PTFE plug and the contacted region of the e-PTFE sleeve to coalesce into a unitary sintered structure. After the mold assembly reaches sintering temperature, the mold assembly is removed from the sintering oven and cooled 39. After cooling the push rod and mandrel are removed, and the resulting bone cap implant removed from the mandrel, inspected and trimmed a desired length. EXAMPLE 1 A mold was formed by drilling a 1.0 inch aluminum rod concentrically along its longitudinal axis with a 0.375 inch drill to a depth of 0.50 inch. A mandrel was formed by machining a 2.25 inch long aluminum tube to a 0.28 inch outside diameter and a 0.215 inside diameter. A solid stainless steel rod was machined to 0.187 inches to form a push rod. A 0.50 inch hole was drilled into a 4" x 6" x 0.250" aluminum sheet to provide a heat sink away from the mold. A standard propane torch was used to heat the protruding mold. Approximately 0.5 teaspoon of unsintered, powdered PTFE resin was pored into the mold cavity. A piece of e-PTFE tube was fitted over the outer diameter of the aluminum mandrel. A first end of the e-PTFE tube was adjusted to extend beyond a first end of the mandrel by 0.125", then inverted into the lumen of the mandrel. A second end of the e-PTFE tube was secured to the outer diameter of the aluminum mandrel by tying with brass wire about the circumference of the e-PTFE tube. The first end of the e-PTFE tube and the first end of the mandrel were pressed into the mold and into the powdered, unsintered PTFE in the mold cavity. A second small amount, approximately 0.125 teaspoon, of unsintered, powdered PTFE was introduced into the lumen of the mandrel and the steel push rod was inserted into the lumen of the mandrel. Positive pressure was applied to the push rod and the mandrel to form a solid plug of the powdered, unsintered PTFE about the first end of the e-PTFE sleeve and mandrel. The mold assembly was placed in the 0.50 inch ' hole in the aluminum plate with about 0.125 inches of the mold protruding through the bottom of the aluminum plate. A propane torch was used to heat the bottom surface of the mold, with the blue tip of the flame held at the bottom of the mold for sixty seconds. The mold assembly was removed from the aluminum plate and quenched in room temperature water. The mold assembly was disassembled and the tubular sleeve removed from the mandrel The PTFE plug was found to be hard and sintered. The e-PTFE connected to the end plug was heat damaged and contained severe radial fissures along the tube length. However, it appeared that the n-PTFE and the e-PTFE had coalesced together forming a unitary structure. The test was repeated with heating for 50 seconds, with the same heat damage evident in the finished product. The foregoing is a general description of the inventive method for forming a solid body of n-PTFE onto an existent structure made of e-PTFE and is representative of the best mode known to the inventor. Those skilled in the art should recognize that description of the inventive method with reference to a process for making the inventive bone cap is for illustrative purposes and is not intended to limit the scope or application of the inventive method. Rather, the method may be used to form any type or desired configuration of n-PTFE onto an existent structure of e-PTFE. EXAMPLE 2 The same procedures were employed as in Example 1, except that the mold was made of brass. After heating the bottom of the brass mold for 50 seconds, the resultant tube and end plug showed no sign of heat damage. The resulting structure evidenced coalescence of the n-PTFE into an end plug integrally joined to the e-PTFE tube. EXAMPLE 3 The same procedures were employed as in Example 2, except that a small resistance oven was built using an inverted cone design of coils capable of 1 amp per volt. The inverted coil structure was set in an insulator and held in position by a refractory. A variac controller was set to output 8 volts yielding an oven bottom temperature of 1051° C with the heat extremely localized at the oven entrance. The mold assembly was placed in the oven entrance and heated until the n-PTFE plug reached sintering temperature. Dwell time was a function of the size of the mold. Plug temperature was measured with a hand held FLUKE thermometer and a 12" probe thermocouple. It was found that the sintering oven used with the brass mold, consistently yielded a well coalesced end plug of sintered n- PTFE integrally formed onto the end of the e-PTFE sleeve. Table 1, below, sets forth the preferred dimensions of a lesser metatarsal bone cap implant in accordance with the present invention. TABLE-! Table 2 , below sets forth the preferred outside dimensions for the mandrel and the push rod and the inside diameter of the mold cavity, used to make bone cap implant sizes 10-60. TABLE 2 It is preferable to have a radius on the end of the push rod to facilitate compaction of the powdered, unsintered PTFE into the bottom of the mold cavity. For the push rods identified in Table 2 , the radius of the push rod end was 0.0625 chord. In accordance with the best mode known to the inventor for making the mold assembly, the mold cavity is made of brass or a brass/stainless steel alloy having an interior finish of 8 of better. Both the push rod and the mandrel are preferably made of 300 series stainless steel having a finish of 32 or better. It has been found that the finished bone cap implant has a tendency to adhere to the mold cavity if the mold cavity is made of stainless steel or chromium. The foregoing description is provided to delineate the best mode known to the inventors for forming the inventive bone cap with the inventive process. Those skilled in the art should understand that the foregoing is illustrative in nature and is not intended to limit the scope or application of the inventive method to the manufacture of bone cap implants. Rather, the reference to the bone cap structure is provided for purposes of example only. It is intended that various structure configuration may be formed onto an existent structure using the method of the present invention in which an existent structure serves as a matrix for forming another structure therewith. Additionally, variations in mold cavity configuration, mandrel or die configuration, materials, temperature, pressure and time conditions may be made and still fit within the intended scope of the inventive process. While the preferred embodiment of the invention has been shown and described, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the true spirit and scope of the present invention. For that reason, the scope of the present invention is set forth in the following claims.

Claims: What is claimed is: 1. A bone cap implant, comprising: a) a tubular sleeve member having a lumen of sufficient size to accommodate a resected bone shaft therein, said tubular sleeve member being made of a biocompatible microporous material having pore densities sufficient to serve as a tissue scaffold; and b) an end cap member integrally joined to and enclosing one end of said tubular sleeve member, said end cap member being made of a biocompatible, substantially non-porous material, whereby said end cap member inhibits tissue ingrowth into the biocompatible, substantially non-porous material. 2. The bone cap implant of Claim 1, wherein said tubular sleeve member consists essentially of expanded polytetrafluoroethylene. 3. The bone cap of Claim 1, wherein said end cap member consists essentially of non-expanded polytetrafluoroethylene. 4. The bone cap of Claim 1, wherein said end cap member is integrally joined to a terminal end of said tubular sleeve member without an intervening adhesive. 5. The bone cap of Claim 1, wherein said end cap member is integrally joined to said tubular sleeve member substantially without an intervening seam. 6. A bone cap implant of the type used to truncate a resected bone, comprising: a) a tubular sleeve member having a lumen of sufficient size to accommodate a resected bone shaft therein, said tubular sleeve member comprising sintered expanded polytetrafluoroethylene having pore densities sufficient to serve as a tissue scaffold; and b) an end cap member integrally joined to and enclosing one end of said tubular sleeve member, said end cap member comprising substantially non-porous sintered non- expanded polytetrafluoroethylene. 7. The bone cap of Claim 6, wherein said end cap member is integrally joined to said tubular sleeve member without intervening adhesive. 8. The bone cap of Claim 6, wherein said end cap member is integrally joined to said tubular sleeve member substantially without an intervening seam. 9. A bone cap implant made by a process comprising the steps of: a) mounting a tubular sleeve member made of a microporous biocompatible sintered plastic material onto a tubular mandrel; b) charging a mold cavity with a first volume of a powdered, unsintered plastic material; c) seating said tubular mandrel into said charged volume within said mold cavity; d) charging the mold cavity with a second volume of a powdered, unsintered plastic material by introducing the second volume into and through the tubular mandrel; e) compressing the tubular mandrel and the first and second volumes of a powdered, unsintered plastic material into the mold cavity, thereby forming a solid plug surrounding a terminal end of the tubular sleeve member; and f) heating the mold cavity to a sintering temperature of the plastic material while localizing the heating to the region of the mold cavity retaining the solid plug, thereby sintering the solid plug and joining the solid plug to the tubular sleeve. 10. The bone cap implant of Claim 9, wherein said tubular sleeve member consists essentially of expanded polytetrafluoroethylene. 11. The bone cap implant of Claim 9, wherein said end cap member consists essentially of non-expanded polytetrafluoroethylene. 12. The bone cap implant of Claim 9, wherein said end cap member is integrally joined to said tubular sleeve member without intervening adhesives. 13. The bone cap implant of Claim 9, wherein said end cap member is integrally joined to said tubular sleeve member substantially without an intervening seam. 14. A method of joining a sinterable microporous plastic material to a sinterable non-porous plastic material, without the use of intervening adhesives, comprising the steps of: a) mounting an existent structure made of a microporous plastic material onto retention means for retaining the existent structure's form; b) charging a mold cavity with a volume of the non-porous plastic material to be joined to the existent structure; c) placing the retention means and the existent structure into intimate contact with the charged volume of the non-porous material within the mold cavity; d) locally heating the area of intimate contact between the retention means, the existent structure and the charged volume of non-porous material within the mold cavity, thereby sintering the charged volume of non-porous material to the existent structure and forming an integral junction therebetween. 15. The method of Claim 14, wherein said step of mounting the existent structure onto the retention means further comprises the step of providing an existent structure consisting essentially of expanded polytetrafluoroethylene. 16. The method of Claim 15, wherein said step of mounting an existent structure consisting essentially of expanded polytetrafluoroethylene further comprises the step of securing the existent structure to the retention means to prevent shrinkage of the existent structure. 17. The method of Claim 14, wherein said step of charging a mold cavity with a volume of the non-porous plastic material to be joined to the existent structure further comprises the step of charging the mold cavity with powdered, unsintered polytetrafluoroethylene. 18. The method of Claim 14, wherein said step of placing the retention means, the existent structure and the volume of non-porous plastic material further comprises the step of creating intimate contact between a delimited region of the existent structure and the volume of non-porous plastic material. 19. The method of Claim 18, wherein said step of placing the retention means, the existent structure and the volume of non-porous plastic material further comprises the step of charging the mold cavity with a second volume of non-porous plastic material, thereby exposing all surfaces of the existent structure within the delimited region to the non- porous plastic material. 20. The method of Claim 19, wherein said step of locally heating the area of intimate contact further comprises the step of exposing only the delimited region of the existent structure, and the volume and second volume of non-porous plastic material to sintering heat. 21. The method of Claim 20, wherein said step of locally heating the area of intimate contact further comprises the step of cooling the mold cavity in non-delimited regions of the existent structure residing within the mold cavity. 22. The method of Claim 14, wherein said step of locally heating the area of intimate contact further comprises the step of providing a sintering oven comprising at least one heating well having means for generating heat associated with the at least one heating well and at least one means for regulating a temperature within the at least one heating well. 23. The method of Claim 22, wherein said step of providing a sintering oven further comprises the step of providing a resistive heating coil thermally associated with the at least one heating well and a voltage source to provide electrical energy to the resistive heating coil.

Inventors: SKIBA JEFFRY B, SKIBA, JEFFRY, B.
Applicants: INT POLYMER ENG INC, INTERNATIONAL POLYMER ENGINEERING, INC.
Application year: 1993
Main classification: A61B19
IPC codes: A61F2, A61L27, B29C35, B29C43, B29C67, B29C70