referring now to the drawings and , more particularly , to fig1 - 8 , there is shown generally a specific , illustrative sternum retractor device , according to various aspects of the present invention . in one embodiment , illustrated in fig1 , the device comprises a plurality , e . g ., a pair , of relatively parallel , opposing jaws 1 a , 1 b , with a substantially u - shaped cross - section so as to house two sternal stumps and provide an abutment surface on which the force needed to keep them spread apart is applied . jaws 1 a , 1 b are attached to an actuator device 2 which comprises a stationary part in the form of a tubular sleeve 3 , closed at one end and having a very flat oval cross - section , in which a movable part in the form of a piston 4 is slidingly engaged in a watertight manner . an actuator fluid s is inserted in or extracted from the chamber inside sleeve 3 , delimited by piston 4 , through respective inlet and outlet ports 5 and 6 provided on the sleeve . the tubular sleeve 3 lies parallel to the two jaws 1 a , 1 b and the actuator device 2 is connected thereto by means of an articulated mechanism generically indicated by the numeral 7 . the articulated mechanism 7 comprises a bar 8 fixed crosswise to the tubular sleeve 3 and substantially at the end of said sleeve opposite from the end in which the piston 4 slides . the bar 8 extends laterally from both sides of the sleeve 3 and its ends are pivotally connected through pins 9 a , 9 b to the ends of respective first arms 10 a , 10 b . the first arms 10 a , 10 b are in turn revolvingly connected at their opposite ends with the respective jaws 1 a , 1 b through pins 11 a , 11 b integral to these jaws . therefore , the first arms 10 a , 10 b extend along the sides of the tubular sleeve when the retractor device according to the invention is in its closed condition , as shown in fig1 . the articulated mechanism 7 also comprises second arms 12 a , 12 b , with ends pivotally engaging with the pins 11 a , 11 b , while their opposite ends both engage with the same pin 13 , shown in fig4 , integrally attached to the piston 4 . more precisely , with reference to fig4 , the pin 14 projects perpendicularly from a bracket 15 extending along the piston 4 and at a distance therefrom that is greater than the thickness of the sleeve 3 , so as to enable the displacement of the sleeve 3 between the piston 4 and the bracket 15 . the end of the bracket 15 is attached to the free end of the piston 4 and , on the side facing towards the piston , it is formed with two shoulders indicated as 15 a and 15 b , with which it abuts against the free edge of the sleeve 3 and the crosswise bar 8 , respectively , when the retractor device is in its closed position . the axes of the pins 9 a , b , 11 a , b and 14 lie parallel to one another . when the actuator fluid is fed to the sleeve 3 , the piston 4 is displaced axially to emerge from the sleeve , causing the jaws 1 a , 1 b to move away in a direction orthogonal to the direction of said axial displacement of the piston 4 due to the effect of the articulated mechanism 7 . in fact , the displacement of the piston has the effect of making the pin 14 slide axially too , extending both the second pair of arms 12 a , b relative to the piston 4 , and the first pair of arms 10 a , b relative to the tubular sleeve 3 , up to a maximally divaricated position substantially corresponding to the one shown in fig2 , 3 and 4 . extracting the actuator fluid from the sleeve 3 causes the piston 4 to withdraw progressively in the sleeve 3 and , as a consequence , the two jaws 2 a , b to get close to one another until they return to the closed position shown in fig1 . in use , the sternum retractor device according to the present invention is attached to the two sternal stumps , which engage in the jaws 1 a , b , then actuator fluid is fed and the device is divaricated until it reaches the position best suited to the specific needs of the case . given the kinematic configuration adopted , the opening force increases rapidly with the opening of the mechanism : in fact , a progressively greater force needs to be exerted on the two sternal stumps to open the rib cage . as the patient &# 39 ; s conditions improve , the divarication of the device can be progressively reduced by withdrawing some of the actuator fluid from the sleeve 3 . the actuator fluid can advantageously consist of a biocompatible liquid , e . g . a physiological saline solution of known type compatible with this particular usage . the biocompatible liquid can be fed manually in the sleeve 3 , by means of a syringe for instance , generally shown in fig1 and indicated as 17 , and withdrawn by opening a shut - off valve 16 located at the outlet from the sleeve 3 and exploiting the force exerted by the patient &# 39 ; s rib cage which tend to close the opening in the sternum . in a second embodiment of the invention , illustrated in fig5 - 8 , the two jaws 20 a , 20 b are connected on opposite sides to a telescopic actuator device , generically indicated by the numeral 21 . the telescopic actuator device 21 is formed by a plurality ( four in the embodiment shown , identified as 21 a , b , c , d ) of progressively smaller - sized sleeves such that they can be contained one inside the other . as illustrated more clearly in the cross - sections in fig7 and 8 , where the retractor device according to the invention is shown both in its closed condition ( fig7 ) and in its opened condition ( fig8 ), the widest terminal sleeve 21 a is rigidly connected to one of the two jaws ( e . g . the jaw 20 a ), while the opposite , narrowest terminal sleeve ( 21 d ) is rigidly connected to the other jaw ( e . g . the jaw 20 b ). the sleeves 21 a , b , c , d are slidingly connected to one another with a watertight connection , since the wider sleeves contain an internal flange ( 22 a , b , c ) that slides to form a watertight seal against the outer surface of the narrower sleeves , which in turn have an external flange 23 a , b , c that slides to form a watertight seal against the inner surface of the wider sleeves . here again , the mutual sliding between the sleeves 21 a , d is achieved by feeding / withdrawing an actuator fluid , such as a biocompatible liquid , in / from the internal chamber delimited by the sleeves 21 a , d through inlet / outlet ports . the insertion of actuator fluid in the sleeves 21 a , d causes the divarication of the jaws 20 a , b and the consequent moving away of the sternal stumps from one another , while the withdrawal of actuator fluid from said chamber enables the two jaws 20 a , b to move closer together , due to the effect of the force exerted by the rib cage . here again , the actuator fluid can be inserted manually , e . g . by means of a syringe , up until a maximum divarication has been reached corresponding to the maximum extension of each sleeve from the wider sleeve containing it , and then releasing the volume of fluid needed to achieve the degree of divarication required case by case . it will be clear to a person skilled in the art that , in the two above - described embodiments of the invention , as an alternative to said manual methods , there may be automatic systems for feeding and withdrawing the actuator fluid , such as a metering pump on a feed line and a solenoid valve on a discharge line , the operation of which is governed by a microcontroller . it should be noted that , in both the embodiments , the actuator has a very flat oval cross - section in order to restrict its overall dimensions in the direction orthogonal to the sternal plane . since the retractor device according to the invention is designed to be placed up against the heart , it is made using entirely biocompatible materials and all its components have rounded and / or bevelled edges to avoid any injury to the cardiac muscle even in the event of accidental contact . the two jaws 1 a , b are shaped so as to guarantee an effective juxtaposition with the profile of the sternum and are wide enough to contain the surface pressure exerted thereon . small vertical holes are created in the supporting surfaces to make easier the fixing of the retractor device to the sternal stumps and thus avoid any displacement of the retractor that might potentially be caused by movements of the rib cage and / or of the patient . the device will be made in different sizes to guarantee that its overall longitudinal dimensions do not exceed the length of the sternum , and its dimensions in the direction orthogonal to the sternal plane do not extend too far beyond the edge of the surgical site in order to facilitate as far as possible the application of the sterile cover . retractor devices may consequently be designed for use in neonatal surgery , for children and for various adult body sizes . the actuator fluid is fed and withdrawn at the bottom of the sleeve 3 comprising the actuator , remaining fixed in position so that the fluid delivery tube emerges longitudinally with respect to the sterile cover on the patient &# 39 ; s abdomen . the retractor device according to the present invention enables numerous advantages to be achieved by comparison with the currently - adopted solutions . in the first place , it enables a safe and efficient sternum divarication to be achieved with a relatively standardisable procedure . moreover , it enables the divarication amplitude to be adjusted from the outside without any need to reopen the surgical site , thus avoiding the removal of the protective membrane at any possible need . depending on the patient &# 39 ; s specific clinical needs , the operator can therefore increase or reduce the distance between the sternal stumps simply by taking action on the external syringe ( or other actuator fluid insertion / withdrawal means ) without removing the membrane . this versatility consequently also enables action to be taken promptly to deal with any deterioration in the patient &# 39 ; s hemodynamic situation ; on the other hand , it also enables a patient to be gradually “ weaned ” off the expanded sternal closure . the retractor device according to the invention is made so as to guarantee absolutely sterile conditions and preferably of non - colonisable material . it may also be made in a resterilisable version or in a disposable version . as concerns the primary use of the device according to the present invention for temporarily divaricating the sternum , it will be clear to a person skilled in the art that the device has other potential uses in the field of plastic surgery , in undermined lesions where a loss of substance makes it necessary to maintain the surface margins of the lesion open to enable in - depth wound irrigation from the outside , while reducing the distance between the margins as the tissue progressively grows back . the system is likewise applicable in all those modem surgery settings in which soft tissues need to be divaricated to enable the in - depth irrigation of a wound . various modifications and alterations may be appreciated based on a review of this disclosure . these changes and additions are intended to be within the scope and spirit of the invention as defined by the following claims .