Abstract:
A honeycomb sandwich cast for supporting human or animal body portions. The cast has inner and outer cast layers which, in the finished cast, are rigid, and a central honeycomb core having honeycomb cells which are perpendicular to the skin of the underlying body portion. The honeycomb is securely attached, e.g., mechanically locked or bonded to the cast layers to form an integral, high strength, low weight honeycomb structure. An indentable, resiliently compressible fabric such as reticulated plastic foam may optionally be positioned between the faces of the honeycomb core and the cast layers. With use of such a foam, when the outer layer is applied normally by wrapping a bandage-like fabric about the body portion and the surrounding honeycomb core, a compressive force is generated which presses portions of the foam structure into the honeycomb cell openings to form a more secure interlock and prevent relative movement between the components of the cast. The foam structure can subsequently be rigidified to form a mechanical interlock between it and the honeycomb.

Description:
BACKGROUND OF THE INVENTION 
     Orthopedic splints or casts are used to support body portions, normally limbs, but frequently also portions of the torso such as the rib cage, for immobilizing the underlying body portions and facilitate healing of various parts of the anatomy. Such casts have both medical and veterinary applications. 
     In the past, a common splint was a plaster cast. They were formed by supplying bandages covered with plaster of paris, immersing the bandages in water immediately prior to their application, and thereafter wrapping the bandages about the body portions in question, say a leg. After the plaster of paris hardens, a rigid cast is formed. Casts often carry weight and are subject to a certain amount of abuse during daily use. They must therefore be relatively thick, in the order of one inch or more. This represents great bulk and weight which has to be carried around by the patient and greatly restricts his mobility. Moreover, plaster of paris is brittle, breaks upon impact, and deteriorates when subjected to moisture. Even though its characteristics are substantially less than one might desire, plaster casts are widely used because of the simplicity with which they can be applied and because they are relatively inexpensive. 
     Some of the shortcomings experienced with plaster casts were overcome by orthopedic splints made from a plastic webbing or fabric which is wrapped about the leg. The fabric is solidified by first applying a solvent so that surface areas of the fabric are liquefied. After the solvent evaporates, the dissolved fabric material solidifies again and forms a rigid matrix since portions of the fabric loops in contact with each other fuse together. This cast or splint and certain improvements and variations thereof are illustrated and described in U.S. Pat. Nos. 2,308,483; 2,373,802; 2,489,252; and 2,632,442. (hereinafter sometimes referred to as &#34;fabric patents&#34;). 
     As is more fully set forth in U.S. Pat. No. 2,489,252, suitable plastic materials include cellulose plastic such as cellulose esters, cellulose ethers; polystyrene plastics; and vinyl ester plastics such as polyvinyl acetate, polyvinyl chloride, polyvinyl chloride acetate and copolymers of vinyl acetate, and vinyl chloride. Solvents for such materials include ethyl acetate, acetone, methyl acetate, methyl formate, and others. 
     Another approach utilizing plastic materials as a replacement for plaster casts is the impregnation into a backing web such as a gauze or fabric of a fusible plastic material. The plastic impregnated web is wrapped around the broken limb after which it is heated, molded and cooled to form a relatively rigid cast. As more fully described in the U.S. Pat. No. 3,692,023, suitable fusible plastic materials may include blends of cyclic ester polymers such as poly-epsilon-caprolactone and poly-(vinyl alkyl ethers) such as poly-(vinyl ethyl ether). Use of heat fusible materials eliminates the need for solvent activation. 
     Splints made in this manner are substantial improvements over plaster casts insofar as their overall weight, strength, and moisture resistance is concerned. However, they require relatively expensive raw materials so that such splints are not always as economical as one might desire. 
     The prior art has also made attempts to improve orthopedic splints by using more lightweight materials such as corrugated cardboard, air inflatable splints or splint sections, and reusable splints. As far as the present invention is concerned, splints employing corrugated materials are the most pertinent. U.S. Pat. Nos. 3,750,660 and 3,656,475 are exemplary. In essence, they show sandwich constructions in which an inner and an outer sheet are separated by a corrugated material having flutes or cells which run essentially parallel to the skin over which the splint is applied. These splints, although of light weight and relatively inexpensive, are primarily temporary splints for field applications. Their strength is very limited, they are normally incapable of resisting moisture, and, on the whole, they are not for permanent use, that is, use for several days up to several months. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved orthopedic splint or cast and a method for applying it. Generally speaking, the cast of the present invention is a honeycomb sandwich cast having a first, inner cast layer placed over the skin of the body portion to be supported, a central honeycomb core, and an outer cast layer. The cast layers are securely interlocked with and/or bonded to the honeycomb core via a suitable adhesive to prevent relative movements therebetween. To further this purpose, the cells of the honeycomb extend perpendicular to the underlying skin and to the cast layers. Optionally a separate locking member such as an indentable or resiliently compressible material may be interposed between faces of the honeycomb and the respective cast layers. The compressible material may be a reticulated plastic foam or other suitable compressible material. When the outer cast layer is applied, a compressive force is generated between the resiliently compressible material and the honeycomb core, thereby forcing portions of the foam into the cells and forming a combined frictional-mechanical interlock. To provide a positive mechanical interlock, the foam can be rigidified after the application of the outer cast layer. Alternatively, the compressible material can be bonded to the core and the cast layers or the cast layers can be directly bonded to the honeycomb core to define the desired honeycomb sandwich structure. 
     This sandwich construction of the cast permits the use of relatively thin inner and outer cast layers, which may be constructed of a variety of materials such as conventionally applied plaster of paris or, preferably, of a water impervious, moisture resistant material such as the bonded plastic fabric disclosed in the above-referenced fabric patents. A strength increase can be obtained both by increasing the thickness of the honeycomb core, which adds very little to the weight of the cast, and by increasing the thickness of the respective cast layers. Moreover, a strength increase is observed by rigidifying the interlocking compressible layers. This can be done by coating the compressible layer initially with plaster of paris and then hardening the plaster of paris as by spraying it with water just prior to or after the outer cast layer is applied. 
     By orienting the honeycomb cells so that they are perpendicular to the cast layers and the underlying skin, the honeycomb direction exhibiting strength is utilized for rendering the cast rigid, supporting weight as when the cast is used as a walking cast on the lower leg of a patient, or resisting impact against the outer cast layer. Moreover, the undulated ribbon defining the honeycomb has sharp edges at the honeycomb faces which project with a relatively large unit pressure into the interlocking foam which such a layer is utilized. They are therefore relatively deeply imbedded therein and form a secure interlock without any further action. And by rigidifying the deformed foam, or other compressible material, a positive mechanical interlock is obtained. 
     Adequate bonds can also be obtained directly between the honeycomb core and the rigid inner and outer layers without resort to a separate intermediate layer. In the case of solvent activated plastic webbing or fabric such as those taught in the previously referenced U.S. Pat. No. 2,489,252, a sheet of solvent activated adhesive between the core and fabric may be used to enhance the bond. In the case of heat softened plastic webbing such as that taught in previously referenced U.S. Pat. No. 3,692,023, a hot melt adhesive may be used to obtain good bonds. Finally, where conventional plaster layers are used, additional plaster may be applied to insure good bonds to the honeycomb. 
     In short, adequate bonds can be obtained by directly contacting the honeycomb core to the heat, solvent or water activated rigid layers or a separate bonding layer such as a thin adhesive layer or a resilient thicker layer may be employed. 
     The cast or splint of the present invention is a substantial improvement as compared to the prior art and it applies the well known advantages of a sandwich structure to orthopedic splints. Its weight is normally no more than about one-third to one-fourth of the weight of a plaster cast having a comparable strength. Further, the great reduction in weight renders the splint economically competitive with the much heavier prior art casts even though it employs base materials which, on a weight basis, are more expensive than plaster of paris, for example. Furthermore, the splint can be constructed of moisture resistant and water-impervious materials so that the heretofore common deterioration of the casts during daily use, particularly when exposed to water, is eliminated. The splint materials can be impact resistant materials such as the earlier mentioned plastic fabrics, so that the splint does not deteriorate even during extended use. Thus, a splint constructed in accordance with the present invention can be worn for many months without requiring replacement. Such replacement was frequently necessary with prior art casts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal, sectional view of the splint of the present invention applied to immobilize a human leg; 
     FIG. 2 is an enlarged, cross-sectional view taken on line 2--2 of FIG. 1 and illustrates the relative positioning and placement of the components making up the honeycomb sandwich splint of the present invention; 
     FIG. 3 is an enlarged, fragmentary, cross-sectional view illustrating in greater detail the construction of the splint of the present invention; 
     FIG. 4 is a further enlarged, cross-sectional view illustrating the interlock between the honeycomb core, the rigid inner cast layers, and the indentable, resiliently compressible foam interposed between the two; 
     FIG. 5 is a schematic, fragmentary view illustrating the manner in which a splint constructed in accordance with the present invention may be applied to a human arm; 
     FIG. 6 is a schematic, fragmentary view illustrating the manner in which the splint of the present invention can be applied to a human torso; and 
     FIG. 7 is a fragmentary, schematic view illustrating the manner in which the splint of the present invention can be applied to an animal foot. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to FIG. 1, a cast or splint 2 constructed in accordance with the present invention is shown applied to a human leg 4, having a broken tibia 6 and requiring the immobilization of knee 8 and ankle 10. The cast is intended as a walking cast and is provided with a suitable walking heel or shoe 12 which supports the cast and the leg on the ground when the patient takes a step. The cast is a sandwich cast defined by inner and outer cast layers 14, 16 and an intermediate honeycomb core 18 having core cells 20 which are oriented perpendicular to skin 22 of the patient&#39;s leg 4 and, therewith, also to the cast layers. 
     The honeycomb core may be of a conventional construction and have repeating hexagonal cells defined by undulated ribbons which are bonded to each other at their common nodal points. Such honeycomb is relatively stiff, and for application of the cast to relatively sharply curved body portions, such as the human leg, it is preferred to employ three-dimensionally deformable honeycomb. The construction of such honeycomb is fully set forth in U.S. Pat. No. 3,227,600 and it is commercially available from the Hexcel Corporation of Dublin, California. The honeycomb may be constructed of a variety of materials but for purposes of light weight and moisture resistance it is preferred to use honeycomb constructed of aluminum foil. If desired, however, impregnated paper honeycomb, plastic honeycomb, and the like may be substituted. 
     Referring now to FIGS. 1 through 4, the construction and application of cast 2 is described in detail. After the body portion to be supported by the cast, e.g., leg 4, has been set in the position in which it is to be immobilized, a protective sheath such as a stockinette 24 is applied to separate the skin (and any hair growing thereof) from the actual cast, as is conventional in the art. Thereafter, padding material such as crepe paper, webbing, foam, or the like, may be applied to cushion and protect the leg and skin from the cast and to prevent possible rubbing between them. 
     The first cast layer 14 is now applied, preferably by using a bandage comprising a fabric or webbing of a plastic material which can thereafter be solvent or heat softened as, for example, described in the above-referenced issued fabric patents. Normally, the bandage is supplied in rolls of up to six inches in width and the roll, if solvent activated, is briefly immersed in suitable solvent. Thereafter the bandage is quickly wrapped about the leg and stockinette. In normal practice it is most desirable to form the inner cast layer with two passes of the roll over the leg. The layer then will have a thickness of between 1/32 and 1/16 inch. 
     Upon completion of the formation of the inner cast layer, the core is bonded to the cost layer. In one form of the invention, an indentable, resiliently compressible material of the required width and length is placed over the inner layer. The material may comprise reticulated plastic foam such as reticulated urethane foam. Applying the core or foam to the inner cast layer before the latter fully dries, or cools, that is before the solvent thereon fully evaporates, or the fusible plastic fully solidifies assures the formation of a bond between the two. In the case of foam, surface bonding together with frictional forces generated between the foam and the inner cast layer prevents relative movement. 
     Since the foam layer should be permeable, reticulated or open core foam is preferred, although open cell and closed cell foam can be employed. Furthermore, since the foam layer is subsequently compressed by the intermediate honeycomb core 18 and should partially protrude into the open core cells, it should have a thickness of at least about 1/16 inch. A range of about 1/32 to 1/8 inch is fully acceptable. Thinner foams normally provide insufficient indentability, and padding, whereas thicker foam layers add undesirably to the bulk of the finish formed cast and permit excessive limb movement. 
     After the inner cast layer has been wrapped with inner foam layer 26, honeycomb core 18 is applied. When the cast exhibtis relatively sharp curvatures, a three-dimensionally deformable honeycomb core is employed. It is cut to a width so that when wrapped around the leg it substantially fully encircles it and to a length so that it extends from above the knee to at least the ankle. Excess length may be provided to extend the same honeycomb core over the heel to the patient&#39;s foot. Alternatively, a second, smaller core section may be applied over the foot. 
     The core is manually held in position or may be temporarily tightened as with string or tape. Thereafter an outer cast layer is applied over the core, or when an outer foam layer 28 is wrapped about outer face 30 of the core the outer cast layer 16 is formed over the outer foam layer. Again, the outer cast layer if it is solvent activated is applied by first briefly soaking a plastic fabric roll in a suitable solvent and thereafter wrapping it around the outer foam layer to form the outer cast layer 60. Normally, two or three passes are preferred to give the outer cast layer sufficient strength to resist normally expected impact from sharp corners, edges, and the like during the every day use of the cast. After the outer cast layer has been fully formed by making the desired number of passes with the roll, drying of the outer cast layer and set-up in the case of solvent activated rolls may be speeded up by directing a hot air stream against the cast, while in the case of heat softenable rolls, fusion and set-up is accomplished by heating the material to above its softening temperature and allowing it to cool. 
     While the outer cast layer is applied, or separately thereafter, walking heel 12 is conventionally applied by positioning it against the under side of the cast and securing it thereto as by wrapping it with a plastic fabric after it has been briefly placed in a suitable solvent. 
     This completes the formation of the cast and after all solvent has evaporated, the plaster has set or the softenable material has fully solidified, the cast has reached its full strength and rigidity and is ready for use, e.g., for walking thereon. The finish formed cast is a honeycomb sandwich which exhibits the beneficial strength-to-weight ratio attained with such structures. The rigidly spaced apart inner and outer layers form a structure capable of resisting large tension, compression and bending forces. If the inner and outer cast layers were not so spaced apart, the strength of the cast would be greatly reduced and with the indicated layer thicknesses such a cast would be incapable of providing the desired support and leg immobilization. 
     As is illustrated in FIG. 4, the purpose of the foam layers 26, 28 is to form a mechanical interlock between the core and the cast layers not always obtainable using only surface bonding techniques. When the outer cast layer is formed, the roll of plastic fabric is tensioned and wrapped around the leg while under such tension. This generates an inwardly directed radial compressive force against the outer foam layer 26, core 18, and inner foam layer 26 which is supported by the already rigidified inner cast layer 14. Sharp edges 32 of the honeycomb core 18 exert a large unit pressure against the foam layers and indent therein so that the resiliently compressible foam between the honeycomb edges springs back and protrudes into core cells 20. Since the foam layers are immovably retained to the cast layers, foam protrusions 34 in the honeycomb cells have the effect of forming an interlock between the foam and the core so that relative movement between them along their interfaces is prevented. For many applications no more is needed to form an effective, immovable connection between all layers of the cast. 
     In instances where the applied loads are relatively large, as on walking casts for example, it may become desirable to form a more positive interlock between the core and the foam. Such an interlock can be established by rigidifying the foam after it has been indented, that is, after the outer cast layer 16 has been applied. In one form of the invention, foam layers 26, 28 are therefore covered with plaster of paris. It is most desired that the plaster of paris adheres to all ligaments. This can be accomplished by dusting the foam immediately prior to use with plaster of paris or by adhering the plaster of paris with a suitable bonding agent which prevents a premature set-up of the plaster. 
     Plaster coated foam is applied to the cast in the same manner as described above. However, after its application, the foam is wetted as by spraying water over the cast after the inner and outer cast layers have set up and hardened. The plaster adhering to the foam will then harden the foam in its deformed position (as shown in FIG. 4) so that foam protrusions 34 become essentially non-compressible. A secure mechanical interlock between the plaster and the foam is thereby established. 
     Alternative methods of rigidifying the resilience of the compressible foam can of course be employed. Thus, the foam may be treated with suitable chemical agents to cause its hardening after it has been compressed. Such procedures are well within the purview of those skilled in the art and, therefore, not further described herein. 
     Such an interlock can also be established by bonding the foam to the honeycomb core and the adjacent cast layer as by applying a bonding agent or subjecting the foam to a solvent which renders it temporarily tacky. Upon the evaporation of the solvent, which is preferably the same as the solvent used in the plastic bandages used for forming the cast layers, the foam adheres to both the core and the cast layers. Alternatively, the cast layers can be directly bonded to the honeycomb core, thereby eliminating the intermediate foam layers. 
     For orthopedic applications, a honeycomb thickness of 3/16 to 1/4 inch for the above-described cast is normally sufficient for honeycomb having a cell size of between 1/8  inch to 3/8 inch and made of aluminum foil. The finished cast then has an overall thickness of about 1/2 inch to 3/4 inch. If desired, the honeycomb may be supplied in thinner form so that it can be applied in multiple layers as by spirally winding them about the leg to attain the desired thickness. In such instances, the honeycomb core may be a full length sheet or it may be furnished in narrower bands which are then wrapped about the leg in a fashion similar to the wrapping of a bandage. 
     Referring now briefly to FIGS. 5 through 7, various applications of the cast of the present invention are illustrated. In FIG. 5, a honeycomb sandwich cast 37 is formed on a human arm 36 by first forming an inner cast layer 38 as above described, thereafter positioning the honeycomb core 40 over the layer and then wrapping a roll 42 of the above-referenced plastic fabric, for example, as above described to complete the cast. Foam layers (not separately shown in FIG. 5) are positioned between the inner and outer cast layers and the intermediate honeycomb core. 
     In FIG. 6, a honeycomb sandwich cast 44 is formed to support and protect a human torso. The cast is applied by forming inner and outer cast layers and positioning therebetween a honeycomb core, in the illustrated instance a plurality of sectional cores 46 which are shaped and positioned to follow the counter and provide the desired cast rigidity. Intermediate foam layers as above described are included. 
     Casts 37 and 44 illustrated in FIGS. 5 and 6 both depart from the cast shown in FIGS. 1-4 in that the intermediate honeycomb core does not fully surround the body member to which the cast is applied. Instead, the honeycomb core is strategically positioned, as over the elbow in FIG. 5 and at certain locations distributed about the circumference of the torso in FIG. 6, to obtain the desired rigidity and cast strength. In both instances those portions of the arm and torso, respectively, over which it is difficult and/or bulky to apply a honeycomb core are covered with the combined inner and outer cast layer only. This is particularly advantageous in applications where relatively little load and force is applied to the cast. 
     Referring now briefly to FIG. 7, a honeycomb sandwich cast 48 constructed in accordance with the invention is applied to a body portion of an animal, say a leg 50 of a horse. The cast is applied as described above in connection with the description of FIGS. 1-4. In particular, prior to the application of an inner cast layer a protective wrap, such as a paper layer and s stockinette 52 are applied to the leg. Thereafter a plastic bandage is immersed in a solvent and rolled about the leg to form an inner cast layer 54. 
     Next a honeycomb core 56 is placed over the inner cast layer (after the application of an intermediate foam layer, not separately shown in FIG. 7) an outer foam layer is placed over the exterior surface of the honeycomb core and finally a plastic bandage 58 is wrapped about the outer foam layer to form an outer cast layer 60. After the outer cast layer has dried (as above described), the cast is completed and forms a high strength cast ready to support heavy loads. 
     In view of the relatively greater forces applied to a cast on the leg of a heavy animal such as a horse, the cast will normally have larger dimensions than a corresponding human cast. Thus, both the inner and outer cast layers may be substantially thicker than above described and honeycomb 56 may have a thickness of as much as 1/2 inch or more, giving the finished cast an overall thickness of up to one inch or more.