Patent ID: 7632293

Claim:
A dynamic damping element for interconnecting at least two bone anchors, the damping element including a longitudinal axis and a progressive spring characteristic, the damping element comprising: a cylindrical rod having a flange formed on an end thereof, the flange including a plurality of first tabs; a hollow cylindrical bushing including a central bore, an external thread and a plurality of second tabs disposed circumferential on an outer surface of the bushing; a spring element coaxially aligned with the longitudinal axis of the damping element and located in-between the flange and the bushing, the spring element including a hollow space extending from a first opened end, the hollow space forming an end face, the first and second tabs engaging the ends of the spring element respectively, the spring element having an initial spring characteristic; a clamping sleeve received within the central bore of the bushing and the hollow space of the spring element, the clamping sleeve including a convex face directed towards the end face of the hollow space, the convex face being spaced at an axial distance from the end face of the hollow space when the clamping sleeve is inserted into the hollow space of the spring element; and a nut including an internal threading to threadably engage the external thread of the bushing so that rotation of the nut causes the convex face of the clamping sleeve to contact the end face of the hollow space, which in turn causes additional axial compression of the spring element resulting in a second spring characteristic, the second spring characteristic being higher than the initial spring characteristic, wherein the central bore of the bushing includes an internal taper and the clamping sleeve includes a corresponding external taper and at least one slot, rotation of the nut farther causing the external taper of the clamping sleeve to be pressed against the internal taper formed in the central bore of the bushing so that the clamping sleeve is radially compressed.