Technical field The present invention pertains to the field of orthopaedic surgery, and more specifically relates to an implantable device for lengthening, and also for correction of rotational and angular malpositions. of skeletal bones. The invention advantageously can be applied in connection with stabilisation of fractures. post-traumatic bone defects. bone shortening, malpositions, congenital malformations and bone defects after tumour surgery.

Background of the invention Within the field of orthopaedic surgery. it is previously known with implantable devices e. g. for e. g. stabilisation of fractures, lengthening of skeletal bones, or for correction of different malpositions.

Many of the previously known devices require that suitable attachments means are driven into the skeletal bone on both sides of the bone portion which is to be influenced. and that subsequently a screw assembly is applied between the attachment means. wherein a screw or the like acting on the screw assembly, alternatively on the attachment means. is allowed to protrude through the skin of the patient. Thereafter. by means of turning the screw, the length of the screw assembly can be influenced in accordance with an adapted lengthening scheme. Accordingly. in case it is desired to correct a malposition of a skeletal bone which for some reason occurs. by means of a surs-tical operation. a screw assembly of the above- described type can be applied at one side of the skeletal bone with the aid of two attachment means, whereafter the skeletal bone between the two attachment means can be cut off. By means of lengthening the screw assembly in accordance with the chosen lengthening scheme, a gradual bending of the skeletal bone in question can be obtained. Finally. after an appropriate healing time. the screw assemble, the screw and the attachment means can be removed bv means of a surgical operation. In case oniv a linear lengthening

is desired, several screw assemblies in parallel can be applied around the skeletal bone in question.

Furthermore, for example in connection with stabilisation of fractures, it is previously known with so-called intramedullar, nails, which during use are intended to be inserted into the bone-marrow cavity of a tubular skeletal bone. It is also previously known with such devices for which the lengthening action can be activated and/or controlled from the outside after the implantation.

Accordingly. for example the patent publication US-A-5.626.5 81 discloses a nail assembly for bone lengthening comprising a first nail body portion and a second nail body portion.

The assembly further comprises a power package inside either the first or the second nail body portion, wherein the power package comprises a power-supplied hydraulic pump which forces hydraulic fluid from a hydraulic fluid container and into a sealed chamber between the first and second nail body portions, thereby forcing the first nail portion to be extended longitudinally in relation to the second nail portion and increasing the length of the nail. It is also reported that the disclosed assembly may comprise a spring of a heat- sensitive memory material such as nitinol, and a heating system with a battery-powered electrical heater acting on the spring. According to US-A-5.626.581. external activation and/or control of the assembly may be accomplished e. g. by means of providing the heating system with a timer or a suitable circuit for remote control which acts on the heating system.

Previously known arrangements for lengthening or correction of skeletal bones which require one or several permanent skin lead-throughs during the treatment time are associated with a large discomfort for the patient and a raised risk of infection.

The previously known implantable devices for lengthening and correction of skeletal bones which can be controlled from the outside, or which otherwise can be controlled without any permanent skin lead-throughs. often comprise a large number of mechanical and electronic components. and can be perceived as complicated or unnecessarilv expensive.

When the external dimensions. the durability and the different lengthening properties are concerned, it may become expensive to tailor-make devices for each individual case in accordance with the prior art. since this often requires a number of the many different components to be re-dimensioned. Furthermore, when devices having small external dimensions are concerne, problems with lack of internal space may arise.

Summary of the invention Accordingly, a first object of the present invention is to provide a simple and reliable, implantable device for lengthening and correcting malpositions of skeletal bones, wherein the dimensions and the lengthening process of the device can be tailor-made in advance for the configuration and type of tubular skeletal bone in question into which the device is to be implanted. and where the device also can be designed with very small external dimensions thanks to its simple design.

In accordance with claim 1, this first object of the invention is achieved by means of the device having an elongate shape comprising a first end, a second end, a length dimension and a cross-sectional dimension, and the device further comprising a lengthening menas for bringing the first and second ends into displacement from each other to a predetermined distance between each other after an activation. According to the invention. the lengthening means comprises an absorbent body. which is arranged for bringing the first and second ends to the predetermined distance bv means of a controlled swelling after the activation while the length dimension increases more than the cross-sectional dimension.

A second object of the present invention is to provide an implantable device which, in addition to the above-mentioned advantages, also enables correction of rotational and angular malpositions of skeletal bones.

In accordance with claim 2. this second object of the invention is achieved bv means of the absorbent body of the implantable device further comprising one or several stiffening elements and/or one or several swelling zones having a raised intrinsic swelling capacity in comparison to the remaining absorbent body. which are arranged for controlling the

swelling so that the first and second ends strive towards a predetermined angle in relation to each other after the activation.

Further object of the invention will become evident from the following description, while the features enabling these further objects to be achieved are evident from the attached, dependent claims.

Brief description of the drawings In the following, the invention will be described in greater detail with reference to the attached drawings, in which Fig. I schematically shows a sectional view through an implantable device according to a preferred embodiment of the invention.

Fig. 2 schematically shows a sectional view through an implantable device according to an alternative embodiment of the invention Fig. 3A schematically shows a sectional view through an implantable device according to a particularly preferred embodiment of the invention. before activation.

Fig. 3B schematicallv shows a sectional view through the implantable device in Fig. 3A. after activation and subsequent swelling, and Fig. 4 schematically shows a sectional view through an implantable device according to another alternative embodiment of the invention.

Detailed description of preferred embodiments In the following, a particularly preferred embodiment. and a number of alternative and advantageous embodiments of an implantable device for lengthening and correcting malpositions of skeletal bones according to the invention will be described with reference to the attached Figs. I to 4.

The implantable device II : 11' : 21 ; III according to the invention has an elongate shape comprising a first end 12: 12' : 22 : 112. a second end 13 : 13': 23: 113. a length dimension 14; 24: 114 and a cross-sectional dimension 15: 25; 115. Thereby, "cross-sectional dimension"e. g. refers to the outer diameter in case of a circle-cylindrical device, but can also refer to another cross-sectional dimension depending on the geometrical shape of the cross-section.

Furthermore, the device 1 : 21 : 111 comprises a lengthening means 16:16'; 26; 116,216 for bringing the first 12;12'; 22; 112 and second 13; 13'; 23; 113 ends into displacement from each other to a predetermined distance 14'; 24'; 114' between each other after an activation. Thereb. predetermined distance"refers to the maximum distance of the implantable device which is desirable after it has been implanted into a tubular skeletal bone. The predetermined distance has to be adapted for each individual case.

According to the invention, and in the particularly preferred embodiment, the lengthening means comprises an absorbent body 16; 16'; 26; 116, which is arranged for bringing said first 12; 12' ; 22 : 112 and second 13; 13' ; 23 ; 113 ends to the predetermined distance 14' ; 24'; 114' by means of a controlled swelling after the activation, while the length dimension 14; 24 ; 114 increases more than the cross-sectional dimension 15; 25; 115.

In the particularly preferred embodiment of the implantable device 11'. 11" according to the invention. shown in figures 3A (before activation) and 3B (after activation and swelling), the absorbent body 16'. 16"comprises one or several stiffening elements 18'. 18", 19', 19", and/or one or several swelling zones 20'. 20"having a raised intrinsic swelling capacity in comparison to the remaining absorbent body 16'. 16". Thereby, the stiffening elements and/or the swelling zones are arranged for controlling the swelling so that said first 12'. 12" and second 13'. 13"ends strive towards a predetermined angle a in relation to each other after the activation. Thereby. the stiffening elements 18'. 18". 19', 19"advantageously are constituted of inserts of metal or plastic. while the swelling zones 20'. 20"advantageously comprise a high content of a superabsorbent. However. it is conceivable with embodiments with another choice of material. or where onlv stiffening elements or onlv swelling zones are present.

Accordingly, within the scope of the invention. it is conceivable with embodiments in which the first and second ends essentially are in the same plane (a ~O) before the swelling and are displaced in relation to the plane (a > 0) by the swelling, or inversely, embodiments in which the two ends initially are displaced in relation to a plane (a > 0) and are brought into the same plane (a =0) by the swelling.

In one advantageous embodiment, the absorbent body 16: 16'. 16"primarily is constituted of a matrix of thermosetting melamine formaldehyde plastic on a needled reinforcement material of HT-rayon. Thereby, HT-rayon refers to rayon fibres or filaments having a high tenacity. Furthermore. the absorbent body 16 advantageously comprises micro-crystalline cellulose as a filler. The absorbent body preferably has been compression-moulded to its intended shape during heating. Within the scope of the invention. it is of course also conceivable with absorbent bodies with another matrix material, reinforcement material, or filler. Accordingly, the matrix material can consist of or comprise another polymeric resin suitable for the purpose, the reinforcement material can consist of or comprise another fibre material suitable for the purpose, and the filler can consist of or comprise a suitable superabsorbent. such as for example cellulose acetate.

In an alternative embodiment, in addition to an absorbent body of e. g. the type shown in Fig. 1. the device 21 further comprises first 28 and second 29 cylinder members having closed ends".'3 and opposite open ends 30,31. Thereby, the open end 30 of the first cylinder member 28 is at least partiallv slidably inserted into the open end 31 of the second cylinder member 29, so that the cylinder members 28.29 together form at least one internal cavity 32 within which the absorbent body 26 is arranged. Furthermore. a back stop device 33 is arranged between the open ends 30.31 for enabling the first 22 and second 23 ends of the implantable device 21 to be irreversibly displaced to the predetermined distance 24' after the activation. Thereby,"irreversibly''refers to the fact that the implantable device cannot be compressed in its longitudinal direction. but can only be extended until the first and second ends of the device have arrived at the above-mentioned predetermined distance between each other.

In another alternative embodiment, the device comprises valves 27', 27"for opening or closing a connection between the absorbent body 26 and the environment surrounding the device 21. wherein the valves 27'. 27"are arranged for being controlled by means of radio waves, ultrasonic. induction or magnetic control when accomplishing the activation and/or a deactivation. This embodiment is particularly advantageous when the device according to the invention comprises cylinder members of the above-mentioned type which enclose the absorbent body, but it is also conceivable with embodiments where the absorbent body only is enclosed by a relatively impervious membrane through which the valve or valves are arranged.

In still another alternative embodiment. the back stop device 33 comprises at least one spring-loaded member 34 of one of the cylinder members which is arranged for mechanical engagement with a member provided with steps 3 5 of the other cylinder member. This type of back stop device is schematically depicted in the attached Fig. 2.

In still another alternative embodiment, the cylinder members are slidably arranged with seals (not shown) against each other, wherein the internal cavity is divided into at least two portions by a stationary partition wall in which a connection having a check valve has been arranged.

In another advantageous, alternative embodiment of the device according to the invention, illustrated in Fig. 4. the absorbent body 116 is accommodated within a cylindrical member 129 comprising water inlet means for allowing water transport into the absorbent body during the controlled swelling. In this embodiment. the second end 1 13 is connected to a piston means 1'8 arranged for sliding displacement within the cylindrical member 129, wherein at least one spring member 216 is embedded in the absorbent body 116 for exercising a lengthening force in cooperation with the absorbent body on the first 112 and second 113 ends during the controlled swelling. The spring member 216 is preferably a helical steel spring. However. its also conceivable with embodiments with other geometrical spring shapes and/or materials.

In the alternative embodiment. the water inlet means can be constituted by one or several water-permeable portions 117 in the cylindrical member 129, by the piston means 128 being water-permeable. or by the piston means 128 consisting of a disc member with a pluraliy ofaxially extending through-apertures, which can be accomplished e. g. by means of punching or laser cutting. If desired, the water inlet means can be constituted by the piston means comprising one or several nonreturn valves. for allowing water transport only in a direction towards the absorbent body.

In the alternative embodiment, as illustrated in Fig. 4. the cylindrical member 129 preferably is sealed by a wall member 131 between the piston means 128 and the second end 113. thereby forming an internal cavity within the cylindrical member 129. The internal cavity is intended to be filled with an aqueous liquid H, O for providing at least a part of the water during the controlled swelling. It is also conceivable with embodiments in which opposing spring members (not shown) are provided on both sides of the piston means 128 in order to facilitate the control of the lengthening course of the implantable device.

In the described alternative embodiment, the absorbent body 116 and the spring member (s) 216 are adapted for bringing the first 112 and second 113 ends to the predetermined distance by means of exercising a pressure together on the first and second ends after the activation which is larger than about 500 N/cm2. Furthermore. the water inlet means 117, 128 advantageously are adapted to the remaining components 112. 116 of the implantable device 111, so that the water transport (into the absorbent body 116) during the controlled swelling generates a lengthening rate of about 1 mm/day. However. it is also conceivable with alternative embodiments with another pressure and/or lengthening rate.

In embodiments of the implantable device according to the invention where the absorbent body is enclosed by and stabilise by cylinder members of the above-discussed type, the absorbent body 26 : 116 particularly advantageously comprises cellulose acetate or another superabsorbent. When the cylinder members 28,29: 129 are concerned. they particularly advantageously are constituted of titanium. However. it is also conceivable with embodiments where the absorbent body comprises another suitable absorbent material, or

where the cylinder members are made of another material suitable for insertion into a shaft of a bone.

In all embodiments of the implantable device, it is particularly advantageous that the absorption properties of the absorbent body 16: 16', 16"; 26 : l l 6 are adapted so that the activation can be accomplished by means of wetting with water originating from a body fluid (BF).

In a particularly advantageous embodiment of the invention, the absorbent body 16; 26 is enclosed by a liquid-permeable membrane 17. 27 having permeation properties which are adapted for providing a controlled permeation rate of the water into the absorbent body during wetting. The liquid-permeable membrane 17 ; 27 is advantageously constituted of cellophane, a stretched polymer film or the like and, in addition to a cohesive effect, provides a possibility to control the rate at which the absorbent body is wetted and, as a consequence, also the swelling rate of the absorbent body and the lengthening rate of the device. In this embodiment, the liquid-permeable membrane preferably is selected in such a way that only water molecules and no other components of the body fluid can penetrate the membrane. However, it is also conceivable with embodiments where the implantable device according to the invention lacks a liquid-permeable membrane.

In another advantageous embodiment, the absorbent body 16 : 26. or the absorbent body and the liquid-permeable membrane 17; 27 is/are enclosed by a biodegradable. initially water- impervious, barrier layer (not shown), which advantageously comprises a hydrophobic material which is sensitive to body enzymes, or a macromolecular material which can be hydrolysed. Thereby, the barrier layer can be constituted of or comprise for example polylactate or polyurethane-urea (PUU). In this embodiment, the possibilities to control the permeation process and. consequently, also the swelling process of the absorbent body after activation are increased.

The present invention should bv no means be regarded as being limited to what has been disclosed in the foregoing in connection with the different embodiments, or to what is

shown in the attached drawings, but the scope of the invention is defined by the appended claims.

Accordingly, within the limits of what is allowable for the case in question, it is also conceivable with embodiments where the above-discussed cylinder members and/or the absorbent body does/do not have a circular cross-section.

When embodiments of the device according to the invention where activation and deactivation can be influenced from the outside after implantation are concerned, a method comprising the following steps can be utilised for the lengthening and/or correction: a device according to the invention having appropriate dimensions and lengthening properties for the tubular skeletal bone in question is selected, the device according to the invention is implanted into the skeletal bone using techniques which are well-known to the skilled person. the device is left in its place for 14 days, a lengthening of about 0.5 mm length increase is activated twice a day until the desired lengthening/correction L (mm) is achieved, and the device is left in its place within the skeletal bone for an approximate time of 5 x L weeks before the device is removed by means of techniques which are previously known per se.

In case the intention is to utilise a device according to the invention the activation and deactivation of which cannot be influenced from the outside, a method comprising the following steps can be utilised: analvsis of the lengthening/correction which is required for the skeletal bone in question, a device according to the invention having appropriate dimensions and lengthening properties is selected, the device is implanted using techniques which are previously known per se, the device is left in its place within the bone for a certain stabilisation time counted from the point in time when the desired lengtheningi'correction has been achieved. and

the device is removed using techniques which are previously known per se.

In both cases. the status of the skeletal bone should be regularly supervised by means of radiographs.

Accordingly, when the swelling process of the absorbent body 16 : 16' ; 26; 116 is concerned. after the activation having been accomplished by means of an external signal or the device according to the invention coming into contact with body fluid after the implantation, the swelling process is controlled from the outside and/or is predetermined by the choice and adaptation of the components included in the device according to the invention. so that the swelling process at least will comprise: (i) an initial phase during which no swelling takes place and recovery from trauma can take place : (ii) a lengthening phase during which the device according to the invention at a controlled rate gradually adopts the desired length/shape; and (iii) a healing phase during which no further swelling of the absorbent body takes place and healing/stabilisation of the skeletal bone in question can take place.