EXTERNAL FIXATION DEVICE

Abstract: This external fixation device for osteosynthesis comprises pins (3) adapted for being passed through the bone fragments (1) being joined, a plurality of at least two clamps (5) for fastening said pins (3), and at least one longitudinal rod (4) interconnecting said individual clamps (5) to form a single frame with the bone fragments (1) disposed in required positions relative to each other. The clamps (5) are designed in such a way that they allow movement of the axially clamped extremity (30) of said pin (3) on a trajectory (24) approximating the arc of a circle and maintaining approximately orthogonality between said clamped pin extremity (30) and said trajectory (24), whereby the center (25) of said circle is located approximately within the near bone cortex (7) where said pin (3) enters said bone fragment (1) and the radius (26) of said circle corresponds to the free length (12) of said pin (3).

Description: EXTERNAL FIXATION DEVICE __ Background of the Invention This invention relates to an external fixation device for osteosynthesis, comprising pins adapted for being passed through the bone fragments being joined, a plurality of at least two clamps for fastening said pins and at least one longitudinal rod interconnecting said individual clamps to form a single frame with the bone fragments disposed in required positions relative to each other . Various proposals have been made for orthopaedic external fracture fixing apparatus, so-called "fixateur externes" comprising a range of components which are variably connectable with each other and with the fragments of a fractured bone by way of pins to form a support network holding the bone fragments in a desired positional relationship for the purposes of re-union. The principle of external fixation entails load transfer from bone to an external frame by means of pins (or wires in the case of Ilizarov-type devices) . Length of the pins must allow for clearing soft tissues covering the fractured bone, as well as for post-surgical treatment. The resulting distance between the treated bone and the external frame produces undesirable mechanical conditions at pin-bone interface. Parametric analyses of stress concentrations for commenly used frame configurations showed extremly high stress levels ar pin-bone interface ( "Parametric Analyses of Pin-Bone Stresses in External Fracture Fixation Devices" R.Huskes, E.Y.S.Chao, and T.Ξ.Crippen, Journal of Orthopaedic Research, Vol 3, 341-349, 1985). None of the known external fixation devices has been able to reduce the stress concentration factors to an acceptable level . High stress levels at pin-bone interface are likely to contribute to, if not even to directly induce, pin loosening and the known clinical complications that follow. Most of the stress concentration at pin-bone interface is produced by pin angulation within cortices under functional loading as well as preloading. Summary of the Invention The invention as claimed is intended to remedy these drawbacks. It solves the problem of how to design an external fixation device which couples the reaction force with the reaction moment between the pin and the rod in such a way as to keep the pin angulation at zero within bone cortices. This completely eliminates stress concentration at pin-bone interface due to bending moments on the pin. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming part of this disclosure. For the better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which are illustrated and described preferred embodiments of the invention. Brief Description of the Drawings In the drawings: Fig. 1 is a schematic view of an external fixation device according to the state of the art applied to a fractured bone; Fig. 2 is a diagrammatic view of loads and deformations in a device according to the state of the art; Fig. 3 is a sectional view of the pin-bone interface showing stress distribution in the device according to Fig. 1 ; Fig. 4 is a diagrammatic view showing conditions necessary for eliminating bending moment on the pin between bone cortices in a device according to the invention Fig. 5 is a sectional view showing reduced stresses at pin-bone interface in a device according to the invention; Fig. 6 is a sectional view of a device according to the invention showing loads and deformations of the unilateral frame ; Fig. 7 is a sectional view and a cross-sec ion of a device according to the invention showing ..a clamp with a gliding kinematic pair; Fig. 8 is a sectional view of a device according to the invention showing a clamp designed as a four-bar linkage; Fig. 9 is a sectional view of a device according tc the invention showing a. clamp designed as a flexible four-bar linkage; Fig. 10 is a sectional view of a device according to the invention showing a clamp made as a composite structure; Fig. 11 is a sectional view of a device according to the invention showing a coupled pair of clamps. Description of the Preferred Embodiments Fig. 1 represents a fractured bone 1 fixed by means of an external fixation device, whereby pins 2 are inserted through bone fragments 1 and fixed to longitudinal rods 4 by means of clamps 5. Transmission of force 6 is achieved partly or completely in case of comminuted fracture 2 through external frame 3,4,5- Fig. 2 shows, a deformation of the pin 3 under load 6 to position 3a. Reaction forces 8 in case of symmetrical bilateral frame shown will equal one half of the axial load 6. The corresponding diagramm shows bending moment of the pin 3 , Reaction moments S depend on the pin length and bone diameter. Between bone cortices 7 bending moment is constant. Fig. 3 shows the resultant stress distribution 10 and 11 at pm-pone interfaces due to mainly high bending moment between cortices 7. Fig. 4 shows deformations of the pm 3 into position 3a due tc load 6 provided that reaction moments 21 are high enough to eliminate bending moment 22 between cortices 7. The magnitude of compensating bending moment 21 is equal to the product of the reaction force 8 with the free pm length 12. Fig. 5 shows the resultant stress distribution 23 in the device according to Fig. 4. Fig. 6 shows the kinematic constraint necessary to produce conditions described by Fig. 4. If the clamped extremity' 30 of the pm 3 is forced to move on a circular trajectory 24 with the center 25 within * the near cortex 7 the reaction moment 21 will be equal to the product of the reaction force 8 multiplied by the radius 26 of the circle 24. Radius 26 is equal to the free pm length 12. Trajectory 24 should remain fixed with reference to the longitudinal rod 4. Fig. 7 shows a preferred embodiment of the clamp 5. It consists of a glider 27 which clamps pm extremity 30. Glider 27 is free to move within the circular groove 29 machined in the clamp body 28. To reduce the friction rollers 13 may be interposed between gilder 27 and clamp Cody 23. Fig. 8 shows a further preferred embodiment whereby kinematics of the p extremity 30 is achieved by the four-bar linkage 32. The latter consists of a pm bar 3 clamping the p 2 a rod bar 34 clamped to the longitudinal rod 4 and two side bars 35 and 36. The four bars are connected by means of joints 3-7. The center of rotation of the bar 33 with respect to the bar 34 lies at the intersection 25 of the axis 38 and 39 of the side bars 35 and 36. For small deformations pin extremity 30 will move on approximately circular trajectory 24. Fig. 9 shows a further preferred embodiment whereby the four-bar linkage 32 is realized by connecting the bars 33,34,35,36 by flexible sections 40. The whole four-bar linkage 32 may be manufactured from one single piece of metal . Fig. 10 shows a further preferred embodiment whereby circular kinematic trajectory 24 is realized by connecting sections 41 and 42 of clamp 5 with a composite structure 43 consisting of fibers 44 within an elastomer matrix 45. If only two pins 3 are used in each bone segment 1 an additional kinematic constraint is needed to prevent rotation of the bone segment 1 in the frame plane. This may be achieved by an additional bar linkage 31 connecting pin extremities 30 or the respective elements 33 of the clamp mechanism as shown in Fig. 11.

Claims: r Claims 1. External fixation device for osteosynthesis, comprising pins (3) adapted for being passed through the bone fragments (1 ) being joined, a plurality of at least two clamps (5) for fastening said pins (3), and at least one longitudinal rod (4) interconnecting said individual clamps (5) to form a single frame with the bone fragments ( 1 ) disposed in required positions relative to each other, characterised in that said clamps (5) are designed in such a way that they allow movement of the axially clamped extremity (30) of said pin (3) on a trajectory (24) approximating the arc of a circle and maintaining approximately orthogonality between said clamped pin extremity (30) and said trajectory (24), whereby the center (25) of said circle is located approximately within the near bone cortex (7} where said pin (3) enters said bone fragment (1 ) and the radius (26) of said circle corresponds to the free length (12) of said pin ( 3 ) . 2. External fixation device according to claim 1 , wherein said clamp (5) comprises a circular groove (29) suitable to receiving glider (27) in which extremity (30) of said pin (3) is clamped in orthogonal relationship and allowing free movement of said glider (27) within and along said circular groove (29) . 3. External fixation device according tc claim 1 , wherein said clamp (5) comprises a four-bar linkage (32) consisting of a bar (33) in which the extremity (30) is clamped, of a bar (34) which is clamped to the longitudinal rod (4) and two side bars (35,361 interconnecting said two bars (33,34) by joints (37) in such a way that the axis (38,39) of said two side bars (35,36) are intersecting within the near cortex (7) . 4- External fixation device according to claim 3, wherein said joints (37) are replaced by flexible sections (40) forming a single framework together with said bars (33,34,35,36) . 5-. External fixation device according to claim 1 , wherein said clamp (5) consists of an element ("41 ) connected to said pi'n (3), an element (42) connected to said longitudinal rod (4) and an element (43) interconnecting said two elements (41) and (42* . whereby element (43) is made of a composite material with elastomer matrix (45) and fibres (44) arranged in such a that said element (41) is free to move on said trajectory (24) . 5. External fixation device according to one of the claims 1 to 5. wherein said plurality of clamps (5) is limited to the number of two per bone fragment and said clamped p extremities (30 of said pins (31 are connected with a bar (31 ) assuring synchronism to achieve rotational stability m the plane formed by said bone (1 ) and said longitudinal rod (4; .

Inventors: TEPIC SLOBODAN, TEPIC, SLOBODAN
Applicants: SYNTHES AG
Application year: 1987
Main classification: A61B17
IPC codes: A61B17