Assembly and System for Orthopaedic Surgery
Publication Date: 2008-May-14
The IP.com Prior Art Database
An assembly for attaching a support element to a bone. The assembly comprises a fixation device, and a joint element. The fixation device comprises a fixation element for fixing the fixation device to a bone; and a clamp extending from the fixation element. The joint element has a convex surface and a bore extending through it for receiving a support element. The clamp comprises a concave surface at least partially corresponding to the convex surface of the joint element for receiving the joint element
ASSEMBLY AND SYSTEM FOR ORTHOPAEDIC SURGERY
The present invention relates to fixation devices and systems for attaching a support element to a bone. In particular, it relates to fixation devices and systems in which the relative position and orientation of the fixation device to the support element can be
Polyaxial screw systems are known in orthopaedic surgery. For example, in spinal surgery, a polyaxial screw may be used to attach a support element to immobilise segments of the spine and produce fusion between the segments of the spine.
One example of a polyaxial screw system is described in US-5782833. A polyaxial pedicle screw assembly is attached to a rod. The rod is secured between two or more embedded screw assemblies to immobilise segments of the spine. A screw, having a head and a threaded cylindrical shaft, is tightened into the bone to secure the system in place.
The head of the screw is spherically convex in shape and fits within a polyaxial housing having a stepped bore. A section of the housing has an inner spherically concave surface for cradling the bottom head of the screw and an aperture below it. In use, the axis of the screw relative to the polyaxial housing can be adjusted, the spherically convex and concave surfaces interact to hold the screw securely within the housing at that angle. The assembly
is tightened by inserting a support rod into a channel above the head of the screw and tightening the support rod, using a set screw onto the head of the screw. This forces the head of the screw against the polyaxial housing, locking the system.
Such systems allow the relative orientation of the support element and the screw to be
adjusted. However, the polyaxial housing must have a minimum size in order to accommodate both the spherical head of the screw and the support element. This acts as a limit beyond which the profile of the screw cannot be reduced. In some circumstances, it is advantageous to present a system having as low a profile as possible, i.e. one which extends a smaller distance from the surface of the bone.
According to the present invention, a fixation device is provided in which the polyaxial connection - enabling the relative position and orientation of a support element and a
fixation device to be adjusted - is provided between the support element and a clamp, rather than between a screw head and a polyaxial housing. This allows the profile of the device to be reduced because the polyaxial feature and the required clamping of the support element to the fixation device can be provided by a single clamp. In prior art devices, the profile was increased because the polyaxial connection was between a screw
head and a polyaxial housing and the support element needed to be clamped into the polyaxial housing. With the arrangement of the...