The present invention relates to an anchorage arrangement for a connecting rod for stabilizing the spine according to the preamble of claim 1.
For stabilizing the spine, pedicle screws are inserted in the spine and their head are interconnected by a rod. In order to maintain a dynamic behaviour, elastic rods are known as an alternative to rigid metallic rods.
Since elastic rods are often manufactured from plastic materials and plastic materials are generally susceptible to creep, plastic rods cannot simply be clamped by a press-fit as this is easily possible with metal rods, for example.
Therefore, for a reliable long-term clamping of plastic rods (i.e. made of polymeric material), particular devices are required.
Only few methods to this end are known in the art so far. These will be explained below and their disadvantages as compared to the present invention will be pointed out.
The inventions according to patent specifications EP 1 364 622 B1 (Freudiger) and EP 1 527 742 B1 (Freudiger) show positive or form-fit anchorages. The rod and its seat in the screw head are provided with geometrically mutually fitting grooves. However, positioning the grooved surfaces requires a precise introduction to avoid canting. Furthermore, grooved surfaces do not allow a continuous positioning.
The invention according to patent specification EP 1 759 646 B1 (Freudiger) shows a frictional anchorage with an indirect form-fit. The connecting rod is smooth and is clamped between two members, the screw head and a laterally guided spacer, whose contour nominally deviates from the rod contour in the area of contact. Therefore, this nominal contour deviation locally penetrates into the rod such that high stress intensities result in the clamped areas and relative movements due to shear deformation in the non-clamped areas. Furthermore, rod material may flow off between the clamping areas if they do not form a closed cage.
Accordingly, it is an object of the present invention to propose an anchorage arrangement for a connecting rod exerting less stress on the rod while maintaining positioning accuracy.
Such an anchorage arrangement is defined in claim 1. The further claims define preferred embodiments thereof.
The invention will be further explained by means of preferred exemplary embodiments with reference to the Figures. They show schematically:
a An exemplary cross-section having 4 sides.
b An exemplary cross-section having 5 sides.
c An exemplary cross-section having 6 sides.
a A typical cross-section having 5 sides and a taper.
b The opening between the screw head and the spacer whose contour is congruent with the rod.
c The rod in the mounted condition in the variant having an external nut and a separate spacer.
a The rod opening with a circumferential clamping ridge.
b A spacer with a clamping ridge.
a The rod opening with two circumferential clamping ridges.
b A spacer with two clamping ridges.
A plastic rod 1 is anchored in a pedicle screw 2 such that it can be introduced in a continuous manner without creating stress peaks in the anchoring area due to local clamping as a result of deviating contours and without any alteration of the rod cross-section by flowing of the material.
Furthermore, the anchorage is able to transmit both tensile and compressive as well as shearing and torsional forces from the rod to the screw.
This is accomplished in that the clamping action is uniform peripherally (cf.
Consequently, the invention relates to the rod cross-section defined in claim 1 with at least two plane-parallel sides, thereby allowing a uniform clamping action on the entire circumference, peripherally. Due to the uniform clamping action, maximal care is kept on the rod in the anchorage area, thereby allowing it to transmit the forces arising in the patient to the upper and lower pedicle screws safely and durably. Furthermore, the clamping action may be assisted by circumferential ridges in one or multiple planes. The clamping action is achieved either by an external or internal nut with a separate spacer or by an external or internal nut with an integrated spacer.
Another advantage of the plane-parallel cross-section is the variability of the stiffness in different directions. Thus, for example, the stiffness in the anterior/posterior direction can be increased in a dedicated manner with a minimum volume increase, which may be an essential advantage for patients suffering from vertebral slippage (spondylolisthesis).
Furthermore, the plane-parallel sides may most preferably be provided with a taper whose flanks each form an angle 37 of at least 10° with respect to the frontal plane of the rod in order to facilitate its introduction into pedicle screws that do not extend parallelly.
By way of example,
b illustrates a second exemplary cross-section having 5 sides with the same properties as that of
c illustrates a third exemplary cross-section having 6 sides with the same properties as that of
a illustrates an exemplary rod 1 having 5 sides of which 2 sides 35, 36 are plane-parallel and 2 tapering flanks 42, 43 are bevelled. The bevel angle 37 of the tapered flanks 42, 43 is at least 10° with respect to the frontal plane 45 of the rod on either side. Hence, the frontal plane 45 is a virtual plane orthogonal to the parallel sides 35, 36.
b illustrates a corresponding pedicle screw 2 with the opening contour 3 that is congruent with the rod 1 of
c illustrates the rod 1, the screw 2, the spacer 5, and an external nut 7 as the clamping element 6. The nut 7 is screwed on a thread 59 with the nut 7 exteriorily surrounding the head 51 in the assembled situation in a sectional view.
a illustrates an opening 3 in the screw head 51 with a central ridge 8.
b illustrates the corresponding spacer 5 with side notches complementary to the ridge 8. The spacer 5 is provided with a ridge 9 which completes the ridge 8 to a circumferential ridge once the spacer 5 is inserted in the opening 3.
a illustrates an opening 3 in the screw head 51 with an upper ridge 10a and a lower ridge 10b.
b illustrates the corresponding spacer 5 with upper side notches and lower side notches. The notches are complementarily shaped to the ridges 10. As above, this spacer 5 is provided with an upper ridge 11a and a lower ridge 11b complementing the ridges 10a/b to a complete circumferential ridge.
In these variants, the circumferential ridge or ridges constitute the area of contact, the contour of which is substantially identical to the contour of the rod.
On the basis of the foregoing description, the one skilled in the art may conceive numerous variations of the invention without leaving the scope of protection which is defined by the claims. Such modifications may be:
Number | Date | Country | Kind |
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1113/09 | Jul 2009 | CH | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/CH10/00168 | 7/1/2010 | WO | 00 | 1/9/2012 |