DEVICE FOR FIXING ARTICULATION FRAGMENTS HAVING CARTILAGE

FIELD OF THE INVENTION

The invention relates to a device for osteochondrosynthesis according to the preamble of claim 1 and to a kit comprising an embodiment of the device and a drill bit according to claim 35.

Such devices can be applied for various medical indications, particularly for the fixation of articulation fragments, i.e. fragments having bone - as well as cartilage portions.

Articulation fragments occur for example in the following cases:

1) in case of accidents, e.g. combined with injuries of ligaments and eventual dislocations;

2) in case of chronic articulation instabilities;

3) in case of disturbances of growth at adolescents (so called osteochondrosis or osteochondritis).

These cases mostly concern the knee (femur), the upper talocalcaneal joint (talus) and the hip joint (femur).

Articulation fragments typically measure between 2 and 30 mm and must be fixed anatomically precisely for a perfect function of the articulation. It is essential that the articulation is not immobilized because of the nutrition of the cartilage. A postoperative treatment with continuous passive kinetotherapy (CPM = continuous passive motion) is recommended. Further, the articulation fragment must be connected to its basis bone in a stable motionless manner.

Operations for the indications mentioned above are called "osteochondrosyntheses". Particularly, so called interfragmentary shearing motions are feared. To prevent such motions the fixations are performed using transfragmentary pins, which are inserted in the epiphysial bone from the side of the articulation.

The articulation fragments mostly are so small that often only a single pin may be positioned therein. A plurality of pins would also endanger the vitality of the bone portion. Furthermore, the articulation fragments are often positioned such that an

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approach from orthogonal to the upper surface of the fragment or from the articulation is difficult.

DESCRIPTION OF THE PRIOR ART

A tube shaped device for osteosynthesis, particularly to temporarily put a toe in splints is known from WO2004/089255, which is implanted by means of a guide rod. The various disadvantages of this known device are:

- the circular cross section of the tube allows the individual bones to rotate about the tube, i.e. there is no rotational securing of the implant;

- a costly surgical procedure (the inserted guide rod can bend and pass on a false path; the tube can get jammed on the guide rod; the tubular implant and the guide rod are weakened through a small dimension of the rod and the central cannulation in the tube; the application from distal, i.e. starting from the ball of the toe results in a loss of the distal interphalangeal articulation).

SUMMARY OF THE INVENTION

It is an object of the invention to provide a device for osteochondrosynthesis which allows for the indications mentioned above: a) a rotationally and slideably stable fixation of the articulation fragment by means of a single pin; b) the use of adequate drill means (alternately rotating, flexible drill bit) and the boring through a suitable drill sleeve.

The invention solves the posed objectives with a device for osteochondrosynthesis comprising the features of claim 1 , as well as with a kit including a device according to the invention and a drill bit comprising the features of claim 35.

The advantages achieved by the invention are essentially the following: a) the interfragmentary stability is significantly improved allowing a securing of the consolidation; b) the stability, particularly in rotation is held.

In a preferred embodiment the central axis of the rod is linear.

In a further embodiment the rod has an enveloping cylinder with a diameter of less than 7 mm, preferably less than 6 mm.

In another embodiment the rod has an enveloping cylinder with a diameter of more than 3 mm, preferably more than 4 mm.

The advantages of this relatively small diameter of typically 5 mm is that such a rod gives optimum results with fragments having bone and cartilage portions and is adapted to toe dimensions.

In yet a further embodiment the rod has an enveloping cone allowing an easier introduction.

In still another embodiment the enveloping cone has a cone angle of maximum 5,5°.

In a further embodiment the cross section remains constant over the entire length L of the rod.

In another embodiment the twisted rod has a peripheral surface which comprises at least two helical lines, preferably three helical lines.

In still a further embodiment the cross-section orthogonal to the central axis has a maximum dimension in the range of 1 to 3 mm, preferably in the range of 1 ,5 to 2,7 mm.

In yet another embodiment the pitch p of the helical line is less than 50 mm, preferably less than 40 mm.

In a further embodiment the pitch p of the helical line is more than 20 mm, preferably more than 25 mm.

Typically the pitch p is about 30 mm, i.e. one turn of the helically twisted rod corresponds to a length of about 30 mm.

In another embodiment the pitch p of the helical line is constant over its whole length.

The advantage of such configuration is that there can be made an introduction sleeve, which is not possible if the pitch is variable.

In yet a further embodiment the pitch p of the helical line is variable allowing the advantage of a slight axial compression if the pitch is high at the entering tip and low at the end of the implant (effect like in the Herbert screw (scaphoid bone).

In still another embodiment the helical line is running over more than 360°, preferably over more than 540°.

In again another embodiment the helical line is running over less than 900°, preferably less than 720°. Generally, a small pitch of the helical line renders the insertion of the rod in the bone more difficult.

In a further embodiment the noncircular cross section is formed polygonally, preferably triangularly. Herewith the advantage of an optimization of the rotational securing of the rod is achieved. Instead of a triangular cross section a quadrilateral or pentagonal cross section could also be used.

In yet a further embodiment the corners of the polygonal cross section are rounded.

In another embodiment the polygonal cross section has concave sides.

In still a further embodiment the rod is provided with at least three longitudinal edges or longitudinal fins on its peripheral surface which run helical with regard to the central axis of the rod. The advantage achieved is that the longitudinal edges or longitudinal fins of the rod cut into the cartilage or bone tissue.

In a further embodiment the longitudinal edges or longitudinal fins are separated from each other through concave recesses running helical with regard to the central axis of the rod. This allows to achieve a better stability and avoids any other angles of the whole implant (solidity, resistance of the implant without angles). The only sharp angles would be the edges of the fins.

In yet another embodiment the concave recesses have a depth between 5% and 30% of the diameter of the enveloping cylinder.

In still a further embodiment the rod is provided with a front end determined for insertion in the bone, said front end being preferably configured bluntly. A blunt, e.g. planar embodiment of the front end is particularly suitable in an application in case of osteochondritis.

In another embodiment the rod comprises a coaxial head portion which has a cross section enlarging towards the rear end of the rod. In an application of the device in case of osteochondritis the following advantageous nailing effects can be achieved:

A) through the friction of the rod on a relevant length a sufficient stability is achieved, while

B) in the range of the head portion where the friction is weak due to its short length an axial stability is achieved by means of the positive fit realised through the edge effect of the head portion.

In a further embodiment the cross sectional area of the head portion orthogonal to the central axis progressively enlarges toward the rear end.

In again a further embodiment the head portion has a cylindrical enveloping surface coaxial to the central axis with a diameter equivalent to the maximum outer diameter of the rod.

In another embodiment the ratio between the length I of the head portion and the length L of the rod is between 1/20 and 1/3.

In yet another embodiment the bioresorbable material is essentially brittle and crumbling.

In still a further embodiment the bioresorbable material has a breaking elongation ε = (δl x 100/L) < 10% thus allowing that a better resorbability of the rod is achieved.

In a further embodiment of the invention the rod consists of a reinforced, preferably self- reinforced bioresorbable material. The bioresorbable material can be a Poly-L-Lactide (PLLA) or a Caprolactone. These materials show the advantage that they resorb faster via the synovial fluid. Preferably, the rod consists of a Copolymer of lactic acid and glycolic acid, preferably in a ratio of 3 : 1 to 5 : 1. Else, the rod can consist of a Copolymer from Poly-L-Lactide (PLLA) and Poly (DL-lactide-Co-Glycolid Acid) PLGA, preferably in a ratio 3 : 1 to 5 : 1 and typically in the ratio 4 : 1. Also, the rod can consist of a Copolymer of PoIy-L, D-Lactide.

In another embodiment the rod is tubularly configured. This configuration allows the advantage that a guide wire can be used during insertion of the rod in the bone. In one embodiment the inner diameter of the tubular rod is smaller than 1 ,6 mm, preferably smaller than 1 ,3 mm. The wall thickness of the tubular rod can be greater than 0,1 mm, preferably greater than 0,15 mm but smaller than 0,5 mm, preferably smaller than 0,4 mm. The length of the rod can be between 3 - 6 cm, preferably between 4 - 5 cm.

In yet a further embodiment the rod tapers at least at one end, preferably converging to a pointed tip such that the rod can easier be implanted.

In still another embodiment the rod has a smooth peripheral surface.

In a further embodiment the rod has a peripheral surface comprising an axial saw-tooth or fish-scales like three-dimensional structure, thus allowing the advantages of an unidirectional insertion in the bone only and that the surface structure facilitates insertion of the rod and prevents pull-out.

In another embodiment the noncircular cross section of the rod is centrally symmetric.

In a preferred embodiment a kit with a device according to invention and with a drill bit is provided whereby the drill bit has a diameter D being smaller than the maximum outer diameter of the rod and being preferably more than 30% smaller.

In a preferred embodiment the device according to the invention is used for fixation of articulation fragments, particularly such having bone and cartilage portions.

A BRIEF DESCRIPTION OF THE DRAWINGS

Several embodiments of the invention will be described in the following by way of example and with reference to the accompanying drawings in which:

Fig. 1 illustrates a perspective view of an embodiment of the device according to the invention;

Fig. 2 illustrates a sectional view of the embodiment of fig. 1 along line Il - II;

Fig. 3 illustrates a sectional view of another embodiment of the device according to the invention;

Fig. 4 illustrates a partial perspective view of a further embodiment of the device according to the invention;

Fig. 5 illustrates a lateral view orthogonal to one lateral surface of the embodiment of fig. 4; and

Fig. 6 illustrates a sectional view of the embodiment of fig. 4 along line III - III.

The device for the fixation of a joint fragment illustrated in figs. 1 and 2 essentially consists of a helically twisted rod 1 with a linear central axis 2 and a noncircular (here triangular) cross section 3, which is made of a preferably self-strengthening PoIy-L ₁ D- Lactide (SR-PLA 96/4). Copolymers from Poly-L-Lactide (PLLA) and Poly-(DL-lactide- Co-Glycolide acid) (PGLA), preferably in a ratio 4:1 are suitable as well. Also a mixture of 96% Poly-L-Lactide (PLLA) and 4% Poly-D-Lactide has proved to be advantageous. The helically twisted rod 1 has an enveloping surface formed as an enveloping cylinder 30 coaxial to the central axis 2. The length of the rod 1 is 3,75 cm. The surface of the rod 1 is completely smooth. The front end 23 of the rod 1 which is determined to enter in the toe is tapered such that it terminates rounded. The noncircular cross section 3 is shaped triangularly so that the peripheral surface 10 of the rod 1 is provided with three longitudinal edges or longitudinal fins 6,7,8. The longitudinal edges or longitudinal fins 6,7.8 extend helically around the central axis 2 of the rod 1 such forming helical lines at

the peripheral surface of the rod 1. The sides of the triangle are concavely shaped, such that the longitudinal edges or longitudinal fins 6,7,8 are mutually separated through concave recesses 9. Similarly to the longitudinal edges or longitudinal fins 6,7,8 the recesses 9 extend helically around the linear central axis 2 of the rod 1. The cross section 3 orthogonal to the central axis 2 has a maximum dimension 12 of between 1 to 3 mm, preferably between 1 ,5 to 2,7mm and a minimum dimension of between 1 ,0 - 2,5 mm, typically 1 ,6 mm. The recesses 9 have a depth of between 0,1 to 0,5 mm.

The bore to be performed in the intramedullary channel of the respective bone has a bore diameter 18 which is advantageously smaller than the maximum dimension 12 of the cross section of the rod 1 such enabling the longitudinal fins 6,7,8 to cut into the wall of the bored intramedullary channel allowing a rotational securing of the rod 1.

An alternative embodiment of the rod 1 is illustrated in fig. 3 which differs from the embodiment illustrated in figs. 1 and 2 only therein that the noncircular cross section 3 is shaped elliptically instead of triangularly. The lines formed by the principal vertices of each subsequent cross section 3 form helical edges.

The embodiment illustrated in figs. 4 - 6 depicts a rod 1 including a helically twisted portion with a triangular constant cross section 3 and a head portion 25 coaxially arranged to the central axis 2 and terminally arranged at the rear end 24 of the rod 1. The central axis 2 which is here formed by the line joining the gravity centres of the axial subsequent cross sections extends linearly in the embodiment shown here. The cross section orthogonal to the central axis 2 of the head portion 25 enlarges towards the rear end 24. The peripheral surface 10 of the rod 1 is provided with three plane lateral surfaces 26,27,28 and three longitudinal edges or longitudinal fins 6,7,8. The triangular cross section 3 is equilateral and has a circumcircle with the maximum outer diameter 12 of the rod 1.

The head portion 25 is distinguished by an axial curvature of the plane lateral surfaces 26,27,28 adjacent the portion of the rod 1 with constant cross section such that the distance a between the central axis 2 and each one of the lateral surfaces 26,27,28 progressively increases toward the rear end 24 of the rod 1. Furthermore, the head portion 25 has a length I and an enveloping surface with the maximum outer diameter

12 and where the lateral surfaces 26,27,28 run into at the rear end 24 of the rod 1 so that the end face orthogonal to the central axis 2 is a circular area.

For better comprehension of the device according to the invention a description of the basic points of a possible surgical procedure follows:

Description of a fixation of a joint fragment

Example of an osteochondritis dissecans tali:

1. Osteotomy of the medial malleolus

2. Checking the instability of the osteochondral fragment or reposition

3. Boring of the fragment and the talus body and measurement of the depth of the bone hole.

4. Impacting the rod through a guiding device corresponding to the rod. The helicoidal shape of the cross-section of the rod allows for axial stability

5. Sawing or cutting off the rod on the level of the cartilage.

The non-circular profile of the fragment produces the necessary rotational stability. The insertion of a second rod is not necessary. Anyhow, mostly there is no space for this and such a second rod would additionally endanger the vitality (vascularity).

While various descriptions of the present invention are described above, it should be understood that the various features can be used singly or in any combination thereof. The scope of the present invention is accordingly defined as set forth in the appended claims.