CA1311689C

CA1311689C - Surgical materials and devices

Info

Publication number: CA1311689C
Application number: CA000556337A
Authority: CA
Inventors: Pertti Tormala; Timo Pohjonen; Pentti Rokkanen; Juha Laiho; Veli-Pekka Heponen; Seppo Vainionpaa
Original assignee: Materials Consultants Oy
Current assignee: Materials Consultants Oy
Priority date: 1987-01-13
Filing date: 1988-01-12
Publication date: 1992-12-22
Anticipated expiration: 2009-12-22
Also published as: ATE103189T1; ES2006795A6; MX167917B; PT86530A; AU1084288A; FI870111A0; KR950013463B1; EP0299004B1; AU602750B2; JPH11192298A; FI81498C; EP0299004A1; US4968317B1; CN88100127A; DE3789445D1; IN168204B; WO1988005312A1; JPH01501847A; JP3453314B2; KR890700366A

Abstract

ABSTRACT
At least partially fibrillated materials for fixation of bone fractures, osteotomies, arthrodesis or damaged joints and in augmentation or reconstruction of bone tissue and fixation devices manufactured of these materials forming rods, plates, screws, meshes intramedullary nails and clamps. The materials and/or devices comprise bio-resorbable polymers, copolymers or polymer mixtures.

Description

~311~9 NEW SURGICAL UAT~RIALS AND DEVICES
Surgical implants with good mechanical strength properties can be manufactured of bio-resorbable polymeric materials (resorbable composites) which contain resorbable reinforcing elements. "Bio-resorbable", "resorbable"
or "absorbable~ as used herein mean that the material is metabolized by living tissue. Such resorbable materials and implants manufactured thereof can be used, for instance, as rods, plates, screws, intramedullary nails etc. for fixation of bone fractures, osteotomies, arthrodesis or damaged joints. An advantage of such implants and materials is that they are resorbed (depolymerized to cell nutrients) after healing of the treated tissue. Thus the resorbable implants do not need to be removed as required in many cases for metallic implants.
V.S. Pat. 4,279,249 describes resorbable implant materials comprising ,, polyglycolide fibres as reinforcement and polylactide as a~ ing polymer (as . a resorbable matrix). In Finnish patent a~icatio~ FI ~
self-reinforced resorbable materials are described, where the resorbable polymer matrix has been reinforced with resorbable elements which have the same chemical element content as the matrix. Typical reinforcement elements are fibres or structures formed of such fibres.
Known materials with resorbable organic reinforcement elements have fairly high mechanical strength values. Such materials can thus be used in orthopaedics and traumatology in treatment of cancellous bone fractures, osteotomies, art ~ s or damaged joints. For example the self-reinforced materials of FI show bending strengths over 300 MPa (S. Vainionpaa, Thesis, Helsinki 1987), which values clearly are higher than even the average strength values of cortical bone. Further, the elastic moduli of Xnown self-reinforced resorbable composites are quite high, typically of the order of 10 GPa. The strsngths of these materials are thus clearly better than those of resorbable materials manufactured by fusion moulding techniques.
Implants manufactured of;resorbable polymers, copolymers or polymer mixtures, such as rods, profiles, plates etc., by fusion moulding techniques, such as by injection moulding or by extrusion, have mechnical properties of a level which is typical for thermoplastic polymers. The strength values (tensile, shear and bending strength) generally do not exceed 150 MPs, typically showing values between 40 and 80 MPa and moduli betwesn l and 6 :
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GPa. This is caused by the fact that the flow orientation, which exists in the flowing polymer melt relaxes as a consequence of the molecular thermal motion during the cooling of the moulded sample. When a crystallizable polymer is used, the sample sets to a partially crystalline, spherulitic structure. Thus the polymeric material manufactured by fusion moulding typically consists of folded crystalline lamellae (thickness 100-300 A, breadth about 1 ~m), which are surrounded by the amorphous polymer. The lamellae can be considered to consist of mosaic-like folded blocks (breadth of some hundreds of A). Such lamellae generally form ribbon-like structures, which grow from crystallization centres, (nuclei), to three-dimensional spherical spherulitic structures. Because the crystallized polymer material with the spherulitic mechanism does not normally show significant orientation of polymer molecules with strong covalence bonds, its mechanical strength values remain at the above mentioned levels. Only on the surface of the sample is molecular orientation present because of the rapid cooling in the mould (particularly in injection moulding).
ALthough the reinforced resorbable composites show considerably better strength properties than fusion moulded resorbable composites, it is often necessary to manufacture quite large implants, such as rods, intramedullary nails, screws or plates. This is necessary because one must sacure a load carrying capacity tbendine or shear load carrying capacity) of the implants a security margin high enough to ensure the stablity of the fixation when external or muscular stresses are directed to the fixated fracture, osteotomy, arthrodesis or damaged joint. These stresses can clearly exceed the weight of the patient. However, such large implants, upon which the security of the patient depends, can cause quite considerable operative traumas to the bone tissue and/or to the soft tissues when for example, the implant is located in a drill-hole in the bone or the implant is fastened on the bone surface. With increasing size of the implant, possibilities of foreign body reaction increase, which reaction may be exacerbated or its duratlon be prolonged because the implant and the resorption of the implant also cause physical and chemical stresses to the living tissue which correlate directly to the 9iZQ of the implant.
To the present? the elastic modulus values of resorbable implants manufactured of organic materials are at the best of the order of magnitude of . ..

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10 GPa. This is a lower level of eLastic modulus than that of cortical bone, which is typically 20 GPa, but can even exceed 30 GPa. When the main goal of the surgeon is the best possible fixation lt is advantageous that the elastic modulus of the implant be as nearly as possible that of the bone. In an ideal case the elastic moduli of the bone to be spli~ed and that of the implant are equal. It is thus evident that efficient fixation of cortical or long bones requires resorbable organic composite materials which have higher elastic modulus values than those of currently known materials.
We have found, unexpectedly that by increasing the strength and elastic modulus values of resorbable polymeric materials by orientation of the molecular struct~re of the material in such a way that it is at least partially fibrillated, we achieve new macroscopic resorbable self-reinforced implant materials, which have considerably higher strengths and elastic moduli values than those Xnown heretofore. When the new materials are used as surgical fixation or splice materials or in the manufacture of devices of such materials, one can effectively decrease the operative trauma caused and at the same time obtain significantly better fixation than with the ~nown materials.
Here described are at least partially fibrillated (a~ fixation materials for treatment of bone fractures, osteotomies, arthrodesis or damaged joints (b) materials for reconstruction and augmentation of bone tissue and (c) fixation devices, reconstruction devices and augmentation devices such as rods, plates, screws, intramedullary nails, clamps and shaped surface units manufactured at least partially of the above materials using as raw materials resorbable polymers, copolymers or polymer mixtures. Further described is the application and use of at least partially fibrillated resorbable materlals and especially the use of rods, plates, screws, intramedullary nails, clamps or shaped surface sheets manufactured of the above mentioned materials in the fixation of bone fractures, osteotomies, arthrodesis or dama8ed joints or in augmentation or reconstruction of bone tissue.
The orlentation and fibrillation of spherulitic polymer systems is a process which has been studied extensively in the manufacturing of thermoplastic fibres. U.S. Pat 3 161 709 describes a three phase drawing process, where fusion moulded polypropylene filament is transformed to a fibre with high mechanical tensile strength.
The mechanism of fibrillation is described in, C. L. Choy et al. Polym.

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Eng. Sci., 23 1983, p. 910. When a semicrystalline polymer is drawn, the molecular chains in the crystalline lamellae become rapidly aligned in the drawing direction. Simultaneously the spherulites become elongated and broken up. Crystalline blocks are torn off from the lamellae and are connected by stressed tie-molecules originating from partial unfolding of the chains. The alternating amorphous and crystalline regions, together with the tie-molecules, therefore form long, thin (ca. 1~0 A width) microfibrils which are aligned in the drawing direction. Since the intrafibrillar tie-molecules are created at the interfaces between crystalline blocks, they lie mainly on the outer boundaries of the microfibrils. The tle-molecules which linked different lamellae in the starting isotropic material now connect different microfibrils, that is, they become intrafibrillar tie-molecules located at the boundary layers between the adjacent microfibrils.
In accordance with an aspect of the invention there is provided a method of manufacturing a surgical composite comprising a matrix selected from the group of resorbable polymer, resorbable copolymer, and/or mixtures thereof and further containing oriented, at least partially fibrillated structural units selected from the group of resorbable polymer, resorbable copolymer and/or mixtures thereof, characterized by the following method stages:
a) selecting the material forming the composite from the group of nonfibrillar resorbable polymers, resorbable copolymer and/or mixtures thereof;
b) exposing said nonfibrillar material forming the composite to a melt processing method while said nonfibrillar material forming the composite is in the molten state;
c) cooling said nonfibrillar material to a temperature below its melting temperature;
d) drawing said nonfibrillar material thereby causing the dimension in the drawing direction to increase and thereby inducing said oriented at least partially fibrillated structural units into the said material to provide said composite; and . ~

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~ 3 ~ 9 e) exposing said composite to mechanical deformation to accomplish profiled structures onto the surface of the same.
In the description of embodiments of the invention which follows, reference will be made to the accompanying drawings wherein;
Figure la shows schematically, how a group of lamellae is transformed to a fibrillar structure (to a fibril which comprises a group of microfibrils as a consequence of drawing;
Figure lb shows schematically the interior molecular structure of microfibrils and the link between them;
Figure lc shows schematically the fibrillated polyn'er structure.
This Figure shows several fibrils (one of them in grey for clarity) which comprise several microfibrils having a length of several microns;
Figure 2 depicts a typical structure of fibres in a binding matrix;
Figure 3 further shows schematically the structural units in the fibrillated polymer;
Figure 4a to 4e shows plan views of some types of net or mesh structures in which rods of the fibrillated materials may be used;
Figure 5 shows a mesh structure in the form of an inverted trough;
Figure 6 shows sehematieally a plan view of a typieal operation on a mandible or ~aw bone;
Figures 7a and 7b show diagrammatieally details of a brea~ing test on a clamp formed of the fibrillated material;
Figure 8a is a perspective view of a porous hydroxy apatite rod with fibrillated reinforeing rods at its surface.

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li~ur~ ~h is a section on lin~ A--~ of Fi~ure 8a, and Fi~ure 8c is the rod of Fig~res 8a and 8b which has been filament wound.
With re~erence to Fi~ures la to lc the ~ibrillar structure starts to b~
fo~ed at relatively low dra~ ratios ~ (wh~re ~ = the len~th of the sample after drawing divided by the length of the sample before drawing).
IID-polyethylene is clearly fibrillated with a ~ value of 8 and polyacetal (POM) with a ~ value of 3.
When drawing of the fibrillated structure is further continued (this stage of the process is often called ultra-orientation), the fibril]ar structure is deformed by shear displacement of microfibrils, giving rise to an increase in the volume fraction of extended intrafibrillar tie-molecules. If the drawing is carried out at high temperature, the aligned tie-molecules w;ll crystallize to form axial crystalline bridges connecting the crystalline blocks.
The excellent strength and elastic modulus values of the fibrillated structure are based on the strong orientation of poLymer molecules and molecular segments in the direction of the drawing which is the direction of the longitudinal axes of the microfibrils.
Such high tensile strength fibrillated fibres cannot be used as fixation devices for bone fractures, osteotomies, arthrodesis or damaged joints, because the thin fibres are flexible and do not macroscopically show adequate bending strength or bending modulus and because of their small cross-sectional area do not have the necessary shear load carrying capacity.
Fibrillation of macroscopical polymeric samples, such as rods and tubes, is known for biostable polyacetal and polyethylene (see e.g. K.
Nakagawa and T. Konaka, Polymer 27, 1986, p. 1553 and references therein).
However, the fibrillation of macroscopial samples of resorbable polymers has not been known.
At least partial fibrillation of macroscopial polymer samples can be carried out by rapidly cooling a flowing fused polymer in a capillary tube to the solid state in such a way that the molecular orientation of the flowing molecules cannot relax to a total or partial state of random orientation Stronger fibrillation and therefore better mechanical properties can be achieved by mechanical deformation (orientation) of macroscopial polymer samples. Usually such deformation is effected by drawing or by hydrostatic ': _ 5 _ P~T 11823-1 ~ . ' :.. .. ... . . .

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extrusion of material in a physical condition (in solid state), where strong molecular structural changes of crystalline structure and amorphous structure to fibrillar state are possible. As a consequence of flbrillation the resorbable polymeric material which has been manufactured by in~ection moulding or extrusion and which material initially is mainly spherultic in its crystalline structure, changes first partially and later totally to a fibrillated structure which is strongly oriented in the direction of drawing or of hydrostatic extrusion. Such a resorbable material consists at least partially of oblong crystalline Microfibrils 7 of tie-molecules connecting the microfibrils and of oriented amorphous regions. In a partially fibrillated structure the amorphous regions between microfibrils form a more significant part of the material than in an ultraoriented material where in the extreme case the amorphous material exists only as crystal defects around the ends of the polymer molecule chains. When the degree of fibrillation increases in a material, its strength and elastic modulus values increase many times in comparison to the same values for non-fibrillated material.
Known resorbable composite materials comprise typically a randomly or unoriented binding material phase (matrix) which binds reinorcin~ elements having strongly oriented internal srructures. Such structure is shown 2Q schematically in Figure 2, where the oriented and unoriented molecular chains or their parts are shown by thin lines. The strength properties of the binding phase are significantly weaXer than those of the reinforcing elements. The strength properties of the composite in the direction o~
orientation of the reinforcement elements increase when the amount of the reinforcement elements in the material is increased. Because of practical difficulties the quantity of the reinforcement elements cannot axceed ca. 70 weight-% of the weight of the composite. Thus the stren~th properties of reinforcement elements cannot be utilized totally, because the composite also contains the weaXer binder matrix material, which also contributes on its part to the total strength of the composite.
By orientation and fibrillation it i5 possible to manufacture self-reinforced composites of resorbable polymers, copolymers and polymer blends, where nearly the whole mass of material has been oriented in a desired dlrection and where the amount of the amorphous phase is small. These materials show very hiBh mechanical strength properties in the direction of ~` ' . ' ~:

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orientation: tensile strengths as high as 1000-1500 MPa and elastic moduli of 20-50 GPa. These strength values are clearly better than those of known resorbable composites and some ten times highPr than the strength valu~s of fusion moulded resorbable materials.
Figure 3 shows schematically the structural units which appear in the fibrillated structure of polymer fibres and also in macroscopial, fibrillated polymer samples such as rods and tubes. Crystalline blocks are separated from each other by amorphous material (e.g. free polymer chains, chain ends and molecular folds), tie-molecules connect crystalline bloc~s with one another (the amount and thickness of tie-molecules increases with increasing draw ratio ~) and possible crystalline bridges are present between crystalline blocks. During drawing tie-molecules can become oriented and group themselves into bridges tC. L. Choy et al. J. Polym. Sci., Polym. Phys. Ed., 19, 1981, p.
335-352).
The oriented fibrillated structure shown in Figures 1 and 3 develops at natural draw ratios of 3 to 8. When drawing is continued further as ultraorientation at a high temperature, the number of crystalline bridges can become large and in the extreme case bridges and crystalline blocks can form a continuous crystalline structure. The effects of tie-molecules and bridges are often similar and therefore their exact discrimination from one another is not always possible.
Orientation and fibrillation can be characterized experimentally by several methods. The orientation function f , which can be measured by x-ray diffraction, characterizes the orientation of molecular chains of the crystalline phase. f at natural drawing ratios (~ ~6) sttains the maximu~
value 1. Polymeric material with spherulitic structure shows f << 1.
Birefringence which can be measured with the polarization microscope is also a quantity, which describes molecular orientation of molecular chains.
As a rule it increases rapidly with increasing draw at natural draw ratios (~ <6), but thereafter during ultraorientation more slowly. This shows that the molecular chains of the crystalline phase are oriented into the drawing direction at naturaI draw ratios and the orientation of molecules in the amorphous phase continues further at the higher draw ratios tC.L. Choy et al.
Polym. Eng. Sci., 23, 1983, p. 910-922).
The formation of the fibrillated structure can be shown in many cases ~,:
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illustratively by studying the fibrillated material under the optical and/or electron microscope (see e.g. T. Konaka et al. Polymer, 26, 1985, p. 462).
Even single fibrils which c~nsist of microfibrils can be seen clearly in scanning electron microscopic views of the fibrillated structure.
Table 1 shows some known bio-resorbable polymers, which can be used in manufacture of resorbable materials and the novel devices herein. A
presupposition to efficient fibrillation is that the polymer exists in a partially crystalline form. Therefore such polymers, which because of their physical structure (e.~. configuration state) are not crystallizable, cannot be effectively fibrillated.

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Glycolide/L-lactide copolymers (PGA/PLLA) Glycolide/trimethylene carbonate copolymers (PGA/TMC) Polylactides (PLA) Stereocopolymers of PLA:
Poly-L-lactide (PLLA) Poly-DL-lactide (PDLLA) L-lactide/DL-lactide copolymers Copolymers of PLA:
Lactide/tetramethylglycolide copolymers Lactide/trimethylene carbonate copolymers Lactide/~-valerolactone copolymers Lactide/~-caprolactone copolymers Polydepsipeptides PLA/polyethylene oxide copolymers Unsymmetrically 3,6-substituted poly-1,4-dioxane, 2-5-diones Poly-~-hydroxybutyrate (PHBA) PHBA/B-hydroxyvalerate copolymers (PHBA/HVA) Poly-B hydroxypropionate (PHPA) Poly-p-dioxanone (PDS) Poly-~-valerolactone Poly ~aprolactone Hethylmethacrylate-~-vinyl pyrrolidone copolymers Polyesteramides Polyesters of oxalic acid Polydihydropyrans Polyalkyl-2-cyanoacrylates Polyurethanes (PU) Polyvinylalcohol (PVA) Polypeptides Poly-~-malic acid (PMLA) Poly-g-alkanoic acids , ' ~ 3 ~
Reference: P. Tormala; S. Vainionpaa, and P. Rokkanen in IVA's Beijer Symposium "Biomaterials and Biocompatibility", Stockholm, Sweden, August 25-26, 1987.
Ultraoriented, resorbable polymer materials are an advantageous sp~cial case of oriented, self-reinforced resorbabl~ composite materials in which the oriented reinforcement eléments (crystalline blocks, tie-molecules and crystalline bridges) form and/or ~roup themselves during the mechanical deformation and where the phase which binds the above mentioned structural units includes at least the following structural elements: an amorphous phase; the interfaces between crystalline blocks; and the interfaces between crystalline bridges and microfibrils, which structural elements are also typically oriented strongly in the direction of deformation.
The resorbable, at least partially fibrillated, implant materials and osteosynthesis devices described, differ in several unexpected ways from known resorbable implant materi.als and devices. The new materials and devices, as a consequence of strong orientation and of at least partiaily fibrillated structures, have excellent tensile, bending and shear strength and elastic modulus properties. This makes possible the application in orthopaedics and traumatology of thinner and smaller rods, plates, screws, nails and clamps.
ete. than earlier was known. Operative trauma and foreign body load caused by the implant to living tissues is advantageously decreased. Further, the excellent mechanical strength and elastic modulus properties make possible the use of novel materials, implants and devices in those demanding fixation operations for long bone fractures, osteotomies and arthrodesis. It has also been found, unexpectedly, that the lnventive implants retain their mechanical properties in hydrolytic conditions longer than implants of equal size manufactured from known materials. The inventive materials and devices can also be used in treatment of such slowly recovering bone fractures, osteotomies and arthroaesis, where known materials and implants cannot be ; 30applied.
The, at least partially fibrillated, rods, tubes, plates and profiles, ~ ' ~ ,' ' ' ' -- .
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etc. described can be used as such as fixation devices, for example as described in the Finnish patents FI 69402 and 69403, or the materials can be formed into different kinds of fixation devices, such as screws, rods with scaly covering, profiled structures and clamps or other formed structures, because it has also been found unexpectedly that the oriented resorbable materials can be hot-worked mechanically at high temperature without losing the fibrillated structure. This allows the manufacturing of especially strong and tough screws embodying the invention.
The fibrillated resorbable matPrials can contain different kinds of additives or auxiliary materials to make the processing of the material easier (stabilizors, antioxidants or plasticizers) or to change its properties (plasticizers or powder-like ceramic materials) or to improve ease of handling and identification (colours).
The stiff and strong resorbable fixation materials described can be used as rods, plates or other profiles in manufacturing larger fixation devices as reinforcement elements for example by packing fibrillated rods into a cylindrical, oblong in~ection mould and by filling the mould then by injecting into it a suitable molten resorbable matrix polymer. When the injection is carried out from one end of the oblong mould, the injected melt flows ln the direction of resorbable reinforcement elements. ~hen the matrix material (polymer melt) flows and solidifles rapidly, it has advantageous molecular orientation in the direction of that of the reinforcement elements.
The stiff and strong fi~ation rods or plates described can also be used to construct stiff contoured surface net or mesh-like structures and plate-like structures, which more resemble metallic mesh in their mechanical properties than organic textile fibres. Figure 4 shows schematically some types of net structures which are constructed of stiff, stron~ resorbable rods. Some of the rods are depicted in white and some in black for clarity.
Such meshes or nets can be applied to treatment of comminuted fractures by bringing the comminuted parts of broken bone together and by bending the net around the bone parts to support them and by fixing the net with resorbable sutures or clamps. Such nets can be manufactured by hot-pressing to curved surface plates or tapered or trough or box-like sheets etc. or corresponding structures, which can be applied to the reconstruction of bone etc. in such a way that a defect in bone tissue (a hole, a cavity, a cyst, etc.) is filled .

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with tissue compatible ceramic powder such as hydroxyapatite or tricalciumphosphate and the curved net is fixed on the defert as a cover, which immobili~es the ceramic particles and prevents their movement from the defect. Because such nets or meshes are stiff they function as significantly more effective immobilizers than the Xnown flexible fibres manufactured of resorbable fibres.
Figure 5 shows schematically a mesh or net structure which is manufactured of resorbable rods and which has been shaped to the ~orm of a hemi-cylinder or inverted trough e.g. by hot pressing. Such a structure can be used advantageously with ceramic materials to augment the bone tissue of alveolar ridges in the following way. First a subperiosteal tunnel is made surgically below the gingival tissue on the surface of the alveolar ridge.
The resorbable unit is pushed inside the tunnel so that the convex surface of the trough is directed towards the gingival tissue and the end surfaces of the sides of the trough are placed on the alveolar ridge. This situation has been described schematically in Figure 6 for an operation which is done to the right side of the mandible. After installation of the tube it can be filled with ceramic bone graft powder and after that the operation incision can be closed. If necessary, it is possible to place several such mesh units on the same alveolar ridge. The resorbable mesh prevents movement of the ceramic powder packed within it. At the same time bone and connective tissue cells grow from the bone tissue of alveolar ridge and from the surrounding soft tissues into the ceramic powder and immobolize lt with r~spect at least to the bone tissue of alveolar ridge. The mesh is resorbed at the same time or later.
Ceramic powders and pieces can be used in many other ways to augment or reconstruct the bone tissue ~as bone graft materials).
Ceramic materials (bioceramics), which are tissue compatible and/or ; which form chemical bonds with bone tissue and/or which promote the growth of bone tissue, are e.g. calciumphosphate: apatites like hydroxyapatite, HA, CalO(P04)6(0H)2 (R.E. Luedemann et al., Second World Congress on Biomaterials (SWCB), Washington, D.C., lg84, p. 224), substances sold under trade marks Durapatite, Calcitite, Alveograf and Permafraft; fluoroapatities;
tricalciumphosphates (TCP) (e.g. trade narm Synthograft) and ; dicalciumphosphates (DCP); magnesiumcalciomphosphates, ~ -TCMP (A. Ruggeri _t al., Europ. Congr. on Biomaterials (ECB), Bologna, Italy, 1986, Abstracts, , .. . . . . .
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' p. 86); mixtures of ~ and TCP (E. Gruendel et al.), ECB, Bologna, Italy, 1986, Abstracts, p. 5, p.32); aluminiumoxide ceramics; bioglasses like SiO2-CaO-Na2O-P205, e.g. Bioglass 45S (structure: 8iO2 45 wt-70, CaO
24, S %, Na20 24,5 7O and P2O5 6 %) (C.S. Kucheria et al., SW~C, Washington, D.C., 1984, p. 214) and glass ceramics with apatites, e.g.
MgO 4,6 wt-~o, CaO 44,9 %, SiO2 34,2 7O, P2O5 16,3 % and CaF 0,5 %
(T. Kokubo et al.) SW~C, Washington, D.C., 1984, p. 351) and calciumcarbonate (F. Souyris et al., EBC, Bologna, Italy, 1986, Abstracts, p. 41).
The applications of the above ceramic materials are synthetic bone grafts have been studied in different ways by using them for example both as porous and dense powder materials and as porous and dense macroscopial samples. Also ceramic powder - polymer composites have been studied in this means (e.g. W. Bonfield, et al. SWBC, Washington, D.C., 1984, p. 77).
The resorbable strong and stiff materiaLs described can be used in many different ways combined with porous bioceramics to biocomposites. The mechanical properties, especially the impact strength, bending strength and sheer strength of such composites are significantly better than the ~ o~ e~ponding properties of porous bioceramics. Finnish patent ~L~
-; ) 0G~j.73 describes several possibilities of combining resorbable polymeric materials and bioceramics. Thoss principlss can be applied too when the materials here described are used in combination with bioceramics.
Embodiments of the invention are illustrated by means of the following examples.

Poly-L-lactide (PLLA) (M = 600.000) was injection moulded to cylindrical rods with a diameter (0)4 mm. The rods were drawn to the drawing ratio 1 = 7 at temperatures from room temperature to Tm ~ 40C (where Tm =
the melting point of the polymer~. The fibrillated structure of the drawn rods was examined microscopically. Part of the rods were drawn further to a drawing ratio ~= 12 (ultraorientation). As reference samples were sinteled self-reinforced rods (0 = 1.5 mm) of PLLA fibres (tensile strength 800 MPa, 0 = 15 ,um) which rods were manufactured by a method described in Finnish patent application 851,828.
The following strength values were measured for the injection moulded, fibrillated and sintered self-reinforced rods: tensile strength, elastic , .~ ~

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modulus and shear strenKth. The results of the measurements are given in Table 2.
Table 2. Strength properties of PLLA rods Sample Manufacturing Rod Tensile Elastic S~.ear N:0 method thickness strength strength strength _ (mm) (MPa) (GPa) ~Pa) 1 In;ection 4 80 5.5 70 moulding 2 Injection 1.4 560 14 360 moulding +
fibrillation ( ~ = 7) 3 Injection 1.2 800 17 470 moulding +
fibrillation ( ~ = 12) b Self-rein- 1.5 400 10 260 forcing (sintering) . . _ . . . _ . . . _ . _ _ . . _ Table 2 shows that the strength properties of the new fibrillated, resorbable rods are clearly better than the strength properties of the known resorbable materials.
EXAMPLE 2.
Resorbable rods of Example 1 (the len~th 25 mm) were applied to fixation of the arthrodesis of the proximal phalanx of thum~ by removing both joint surfaces, by joining the uncovered bone surfaces temporarily to each other by clamps to sn arthrodesis surface, by drilling two channels through the arthrodesis surface and by tapping the resorbable fixation rods into the drilling channels. 20 patients were operated upon, the average area of the arthrodesis surfaces was ca. 170 mmZ. The calculated shear load carrying capacity of the fixation was 1100 N, when two fibrillated rods N:o 2 were applied. The proportion of drilled channels (which describes the operative trauma) of the arthrodesis surface was 1.8 70. The corresponding values were for fibrillated rids N:o 3 1060 N and 1.3 70 and for sintered rods ~:o 4 920 N
and 2.1 %. Accordingly the fibrillated rods give a stronger fixation than the " : .- : :
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sintered rods. Also the operative trauma was smaller for the fibrillated rods. Injection moulded rods were not used in fixation, because they would have caused clearl~ ~reater operative trauma (ca. 15 %) than the other materials.
EXAMPLE 3.
Injection moulding was used to manufacture rods ~0 = 3.2 mm) of the following resorbable polymers: polyglycolide tPGA) (M = 100.000), glycolide/lactide copolymer (PGA/PLA, the molar ratio 87/13, M = 120.000), poly-B-hydroxybutyrate (PHBA) (M = 500.000) and poly-p-dioxanone (PDS) (Mw = 300.000).
Polarization microscopy and scanning electron microscope showed that, exclusive of a thin surface layer, the rods had a spherulitic crystalline structure. The melting points (Tm) of the materials of the rods were measured by differential scanning calorimetry (DSC) and the following va]ues were obtained for T : PGA (225C), PGA/PLA (180C), PHBA (175C) and PDS
(110C). The tensile strength~ of the rods were: PGA (60 MPa), PGA/PLA (50 MPa), PHBA (30 ~Pa) and PDS (40 UPa). The rods were fibrillated by drawing them at temperatures from room temperature to T - 10C to drawing ratios ~= 8-16. The diameters of the fibrillated rods were between 0.8 mm and 1.1 mm. The tensile strengths of the fibrillated rods were: PGA (600 MPa), PGA/PLA (500 MPa), PHBA (400 MPa) and PDS (300 MPa).
EXAUPLE 4.
Fibrillated PGA rods of Example 3 and self-reinforced, sintered rods ~0 = 1.1 mm; which were manufactured of PGA sutures Dexon*, size 3-0) which were 50 mm long, were hydrolyzed at 37C in distilled water 5 and 7 weeks. The shear load carrying capacities of fibrillated (f) and sintered (s) rods were after manufacturing f: 570 N and s: 300 N. After hydrolysis of 5 weeks the corresponding values were f: 160 N and s: 30 N. After 7 weeks hydrolysis the sintered rods had already lost their shear load carrying capacity, but the fibrillated rods still showed 75 N shear load carrying capacity.
EXAMPLE 5.
Fibrillated PGA rods of Example 3 (the length 50 mm, 0 1.1. mm) were bent in a mould to form clamps shown schematically in Figure 7a at a bending temperature of 180C. Corresponding self-reinforced clamps were manufactured ;~ of PGA sutures (Dexon*, size 3-0) by sintering the according to the method of Trade Mark :
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Finnish patent application 851828 at elevated temperature and pressure in a clamp mould. The tensile load carrying capacity of fibrillated and sintered clamps was measured by fixing the 10 mm long arms of clamps into holes which were in drawing jaws of a tensile testing machine and by drawing the clamps according to Figure 7b. The clamps were broken typically according to Figure 7b f rom the base of the arm. The novel f ibrillated clamps showed a mean tensile load carrying capacity of 300 N and the sintered clamps a corresponding value of 120 N.
EXAMPLE 6.
Fibrillated PLLA rods n:o 4 of Example 1 were compression moulded in a mould with a screw-like mould cavity at about 160C temperature to resorbable 30 mm long screws, with the core thickness of 1.1 mm and the height of threads 0.5 mm and the distance betwean the threads 0.8 mm. The tensile load carrying capacity of the screws was 300 N. The corresponding screws which were manufactured by injection moulding of PLl.A showed a tensile load carrying capacity of 80 N and the corresponding self-reinforced, sintered rods which were manufactured of PLLA fibres of Example 1 showed a tensile load carrying capacity of 150 N.
EXAMPLE 7.
Fibrillated PLLA rods N:o 3 (the length 60 mm, 0 1.2 mm) of Example 1 were coated with PDLLA (M = 100.000) by immersing the rods in a 5-~ acetone solution of PDLLA and by evaporating the solvent. T?le operation was repeated until the rods had at least 40 W-70 of PDLLA. The coated rods were compressed in a cylindrical mould (the length 60 mm and 0 4.5 mm) at 160C to cylindrical resorbable rods which showed a bending strength of 450 MPa and a bending modulus of 14 MPa.
EXAMPLE 8.
Porous hydroxyapatite (HA-) rods (open porosity about 50 %, 0 = 4 mm and the length 60 mm), which contained on their outer surfsce 6 longitudinal ~rooves shown schematically in Figure 8a and in a crosssectional Figure 8b ~the cross-section plane A-A of Pigure 8a), and tha novel resorbable reinforcing materials herein were applied to manufacture biocomposite rods (intramedullary nails). The reinforcing element materials were fibrillated PLLA rods (the length 60 mm, 0 1.0 mm) of Example 1. Also PLLA fibre bundles coated with PDLLA (ca. 0.1 mm thick, slightly twisted bundle of fibres; 0 of . .. . .

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single fibres lS ,um and tensile strength 800 MPa) were applied as shown below. A S-qO (w/v) acetone solution of PDLLA (M = lO0.000) was spread into the grooves of HA-rods and the fibrillated resorbable rods which were immersed in the same solution were pushed into the grooves. The rods adhered into the grooves when the acetone was evaporated. HA-rods with the fibrillated PLLA
rods in their grooves were coated with PLLA fibre bundle Scoated with PDLLA) by filament winding method. ThP filament winding was carried out at 150C
temperature in such a way that the HA-rods were coated with several fibre bundle layers with different directions so that the fibre bundle layer was at the most 0.4 mm thick. The filament winding was carried out in such a way tha~ areas of rod surface without fibres remained between fibre bundles.
These uncovered areas of HA-rods could be seen on the surface of biocomposite rods as is shown schematically in Figure 8c. The resorbable reinforced coating of rods was pressed smooth in a cylindrical mould ~0 = 5.0 mm). These biocomposites showed a bending strength of 140 MPa, when the bending strength of mere ~A-rods was 12 MPa.
The above biocomposite rods were applied to fixation of osteotomies of rabbit femurs in the following way. The osteotomy was done with a diamond saw to the uncovered proximal part of the rabbit femurs about 1 cm from the neck of the femur. The osteotomy was fixed with clamps. A drill hole ~0 = 5 mm) was drilled through the greater trocanter vertically into the intramedullary channel of femur. The biocomposite rod was tapped into the drill hole so that th upper end of the rod was located on the level of the bone surface. The clamps were removed and soft tissues were closed with a resorbable suture.
The animals were returned to their cages and after anesthesia they could move immediately freely. 20 test animals were used. The follow-up time of 6 moll~hs showed that all the osteotomies had healed well. Histological examinations of bone-biocomposite test sample showed growth of bone tissue from femoral bone into the open porosity of HA-rods.

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Claims

1. Surgical composite comprising a material selected from the group of resorbable polymer, resorbable copolymer, and mixtures thereof and further containing oriented, at least partially fibrillated, structural units (fibrils) which have been induced into the material providing said units while said material is in its original nonfibrillar state by drawing said material in solid state.

2. The surgical composite of claim 1 being in the form of a device selected from the group consisting of rods, plates, screws, nails, tubes and clamps.

3. The surgical composite of claim 1 which contains at least partially ultraoriented structural units.

4. The surgical composite of claim 1 having shear strength value of at least 200 MPa and shear modulus value of at least 4 GPa.

5. The surgical composite of claim 1 having a bending strength value of at least 200 MPa and bending modulus value of at least 4 GPa.

6. The surgical composite of claim 3 having a bending strength value of at least 200 MPa and bending modulus value of at least 4 GPa.

7. The surgical composite of claim 3 having shear strength value of at least 200 MPa and shear modulus value of at least 4 GPa.

8. The surgical composite of claim 1 wherein the composite comprises a matrix and reinforcement fiber that are the same chemically.

9. Use of the surgical composite of any one of claims 1, 2, 3, 4, 5, 6, 7 or 8 in fixation of bone fractures, osteotomies, arthrodesis or damaged joints or in reconstruction or augmentation of bone tissue formed as rods, plates, screws, intramedullary nails or clamps and as reconstruction and augmentation materials or devices for bone tissue formed as plates or curved surface units.

10. A method of manufacturing a surgical composite comprising a matrix selected from the group of resorbable polymer, resorbable copolymer, and/or mixtures thereof and further containing oriented, at least partially fibrillated structural units selected from the group of resorbable polymer, resorbable copolymer and/or mixtures thereof, characterized by the following method stages:
a) selecting the material forming the composite from the group of nonfibrillar resorbable polymers, resorbable copolymer and/or mixtures thereof;
b) exposing said nonfibrillar material forming the composite to a melt processing method while said nonfibrillar material forming the composite is in the molten state;
c) cooling said nonfibrillar material to a temperature below its melting temperature;
d) drawing said nonfibrillar material thereby causing the dimension in the drawing direction to increase and thereby inducing said oriented at least partially fibrillated structural units into the said material to provide said composite; and e) exposing said composite to mechanical deformation to accomplish profiled structures onto the surface of the same.

11. The method of claim 10 characterized by the following method stages:
f) after step (b);
g) cooling the said non fibrillar material to a temperature that is below its melting temperature (Tm) and below its glass transition temperature (Tg);

h) reheating the said nonfibrillar material to a temperature above the glass transition temperature but below its melting temperature; and i) continuing with steps (d) and (e).

12. The method of claim 10 characterized in that said composite is exposed to a compression molding process to accomplish said profiled structures.

13. The method of claim 10 characterized in that the material is drawn by using a draw ratio exceeding the natural draw ratio of said nonfibrillar material to provide ultraoriented structural units into said material.

14. The method of claim 10 characterized in that said melt processing method comprises extrusion or injection molding.