Prosthesis for the correction of flatfoot

Abstract: A prosthesis (1) for the correction of flatfoot, and designed for insertion inside a recess (2) or tarsal sinus defined between a first bone (3) or astragalus and a second bone (4) or calcaneus; characterized by the fact that it is made of bioreabsorbable material.

Description: The present invention relates to a prosthesis for the correction of flatfoot. The deformity commonly known as flatfoot is caused by excessive movement of the astragalus in relation to the calcaneus, and is corrected surgically by inserting, inside a recess defined between the above two bones, a prosthesis which provides for limiting movement of the astragalus in relation to the calcaneus. Currently used prostheses comprise a cylindrical body made of stainless steel or plastic, i.e. a material extraneous to the body and which in time may result in problems to the patient. During articulation of the foot bones, in fact, friction occurs between the bones and the prosthesis resulting in obvious discomfort to the patient as well as wear on the bones, so that a second surgical operation is required to remove the prosthesis. Moreover, the majority of patients having recourse to such a prosthesis are children and adolescents for whom a second operation is particularly distressing. From the document US-A-4 450 591, it is known a prosthasis formed of frusto-conical plug adapted to be inserted into the sinus tarsus of a person, for correcting pes plano valgus. The plug is made of a semi-rigid silicon material, and is embedded with a pair of tie lines protruding from its minor basis to be tied together around the deltoid ligament into a skin incision. Neither the tie lines nor the plug are bioreabsorbable. From the document FR-A-2 543 821, it is known another prosthesis for correcting a flatfoot, formed of a single piece of an elastic material and adapted to be inserted between the astragalus and the calcaneus. The prosthesis includes a frusto-conical portion provided with an axially elastic appendix, which terminates with a disk adapted to be located in contact with the inner side of the foot. It is an object of the present invention to provide a prosthesis for the correction of the flatfoot, designed to overcome the aforementioned drawback, and which more specifically, does not require a surgical operation for its removal. According to the present invention, there is provided a prosthesis for the correction of flatfoot, designed for insertion inside the tarsal sinus defined between the astragalus and the calcaneus, comprising a first body adapted to be axially inserted into said sinus; characterized in that said first body is substantially cylindrical and is formed of a bioreabsorbable polymeric material, said first body including an axial dead hole defined by a substantially cylindrical wall and by a substantially semispherical head; said dead hole including a threaded cylindrical portion and a slightly conical portion decreasing in diameter form said threaded portion to said head; an axial incision through said head and along said wall substantially as far as said threaded portion defining two deformable wings of said wall; a second substantially cylindrical body, also formed of bioreabsorbable material, comprising a shank having a head and a threaded portion close to said head; the threaded portion of said shank being screwed inside the threaded portion of said hole to cause said shank to engage said conical portion, thus deforming and parting outwards said wings in such a manner as to bring them into contact with said astragalus and said calcaneus, and to achieve a given position of the longitudinal axis of said astragalus in relation to the longitudinal axis of said calcaneus. A preferred, non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which: Fig.1 shows an exploded side view of a prosthesis in accordance with the teachings of the present invention; Fig.2 shows a side view of the Fig.1 prosthesis assembled for use; Fig.s 3 and 4 show respective side and underside views of the bone structure of a foot featuring the Fig.2 prosthesis. Number 1 in Fig.s 1 and 2 indicates a prosthesis for correcting the deformity known as flatfoot. As shown in Fig.s 3 and 4, this is achieved by inserting prosthesis 1 surgically inside a recess 2 or tarsal sinus defined between a first bone 3 or astragalus and a second bone 4 or calcaneus, the function of prosthesis 1 consisting in limiting movement of astragalus 3 in relation to calcaneus 4, the excessive movement of which is known to be the cause of the deformity. In the Fig.1 embodiment, prosthesis 1 comprises a first substantially cylindrical body 5 having an axial dead hole 6 as shown by the dotted line, a cylindrical wall 7, an outer annular flange 8 at a first axial end of wall 7, and a substantially semispherical head 9 closing a second axial end of wall 7. Between flange 8 and head 9, the outer surface of wall 7 presents a number of annular grooves 11, the functions of which are described later on. Hole 6 is defined by a threaded cylindrical portion 12 close to flange 8, and by a slightly conical portion 13 decreasing in diameter from portion 12 to head 9. Body 5 presents a longitudinal incision 10 formed through head 9 and along wall 7 substantially as far as threaded portion 12 of hole 6, and which defines two wings 7a and 7b. As shown in Fig.s 1 and 2, prosthesis 1 comprises a second cylindrical body 14 constituting a screw by virtue of presenting a head 15, and, in this case, a partially threaded shank 16. Head 15 is equal in diameter to shank 16, and presents a diametrical slot 17 for receiving the tip of a tool such as an ordinary screwdriver. Shank 16 is equal in diameter to portion 12 of hole 6, and presents a threaded portion 18 close to head 15, and a semispherical free end. In actual use, body 5 is first inserted inside recess 2; body 14 is then inserted inside body 5; and, using a tool, shank 16 is screwed inside hole 6. By virtue of incision 10 and the difference in diameter of shank 16 and portion 13 of hole 6, shank 16, when screwed inside hole 6, causes wings 7a and 7b to flex outwards, and is screwed inside hole 6 until wings 7a and 7b are so parted as to contact astragalus 3 and calcaneus 4 and achieve a predetermined position of axis A of astragalus 3 in relation to axis B of calcaneus 4. Annular grooves 11 define small spaces, for the growth of fibrous tissue, between prosthesis 1 and bones 3 and 4. Prosthesis 1 is made of bioreabsorbable material, such as poly-DL-lactic acid, poly-L-lactic acid, or other polymers, and, according to laboratory tests, undergoes initial structural changes, while at the same time maintaining its shape, roughly six months after insertion, presents initial fractures after about a year, and is totally reabsorbed by the organism by the third year. As the time lapse for correcting the deformity is less than that required for reabsorbing the prosthesis, the bioreabsorbable material employed provides for a prosthesis designed to effectively correct the deformity and which is then reabsorbed by the organism. The advantages of the present invention will be clear from the foregoing description. In particular, the prosthesis according to the present invention, by virtue of being made of bioreabsorbable material, provides for eliminating the problems posed by known prostheses, which, being made of plastic or stainless steel, i.e. of material extraneous to the organism, must necessarily be removed surgically. To those skilled in the art it will be clear that changes may be made to prosthesis 1 as described and illustrated herein without, however, departing from the scope of the present invention. In particular, the prosthesis may consist of a single, solid, slightly conical or club-shaped body, similar to prosthesis 1 in Fig.2.

Claims: A prosthesis for the correction of flatfoot, designed for insertion inside the tarsal sinus (2) defined between the astragalus (3) and the calcaneus (4), comprising a first body (5) adapted to be axially inserted into said sinus (2); characterized in that said first body (5) is substantially cylindrical and is formed of a bioreabsorbable polymeric material, said first body (5) including an axial dead hole (6) defined by a substantially cylindrical wall (7) and by a substantially semispherical head (9); said dead hole (6) including a threaded cylindrical portion (12) and a slightly conical portion (13) decreasing in diameter form said threaded portion (12) to said head (9); an axial incision (10) through said head (9) and along said wall (7) substantially as far as said threaded portion (12) defining two deformable wings (7a, 7b) of said wall (7); a second substantially cylindrical body (14), also formed of bioreabsorbable material, comprising a shank (16) having a head (15) and a threaded portion (18) close to said head (15); the threaded portion (18) of said shank (16) being screwed inside the threaded portion (12) of said hole (6) to cause said shank (16) to engage said conical portion (13), thus deforming and parting outwards said wings (7a, 7b) in such a manner as to bring them into contact with said astragalus (3) and said calcaneus (4), and to achieve a given position of the longitudinal axis (A) of said astragalus (3) in relation to the longitudinal axis (B) of said calcaneus (4). A prosthesis as claimed in claim 1, characterized in that said material is capable of undergoing initial structural changes while maintaining its shape roughly six months after insertion; said material being also capable of presenting initial fractures after about one year after said insertion, and of being totally reabsorbed by the organism by the third year after said insertion. A prosthesis as claimed in Claim 2, characterized in that said material is a poly-L-, or poly-DL-lactic acid. A prosthesis as claimed in any previous Claim, characterized in that said wall (7) is slightly conical-or club-shaped. A prosthesis as claimed in any previous Claim, characterized in that said substantially cylindrical wall (7) is provided with an outer annular flange (8) opposite to said semispherical head (9), and is provided with a number of annular grooves (11) which, in use, define spaces for the growth of fibrous tissue. A prosthesis as claimed in any previous Claim, characterized in that the head (15) of said second body (14) has the same diameter of said shank (16) and is provided with a diametrical slot (17) far receiving the tip of a tool, said shank (16) having the same diameter as the threaded portion (12) of said hole (6) and terminating with a substantially semispherical free end.

Inventors: GIANNINI SANDRO, GIANNINI, SANDRO
Applicants: HOWMEDICA INTERNATIONAL DIV NE, HOWMEDICA INTERNATIONAL DIV.NE PFIZER ITALIANA S.P.A.
Application year: 1993
Main classification: A61F2
IPC codes: A61L27, A61F2, A61L31, A61B17