Surgery – Instruments – Orthopedic instrumentation
Patent
1996-10-09
1998-09-22
Buiz, Michael
Surgery
Instruments
Orthopedic instrumentation
606 60, 606 72, 606 73, 606 75, A61B 1758
Patent
active
058108165
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD OF THE INVENTION
The present invention concerns anchors such as pedicle screws and hooks used for instrumentation of the spine, which are placed on the vertebrae to constitute anchor members linked together by one or more attachment rods.
The orthopedic hooks usually employed for instrumentation of the spine are shaped to hook onto a part of a vertebra. A hook of this kind comprises a hook body having an interior bearing face and an exterior fixing face provided with means for fixing it to an anchor point attachment rod, and a curved lug forming with the interior face of the body a U-shape profile to surround said part of a vertebra. The curved lug comprises a traction branch generally opposite the interior face of the body to which it is joined by a linking branch.
These orthopedic hooks used for instrumentation of the spine are placed on the vertebrae in three different ways.
In a first mode of use they are oriented upwards: they are slipped between two processes and the notched end of their lug bears on the pedicle of the vertebra with the hollow of their U-shape profile under the shoulder of the lower process and the lug on the inside.
In a second mode of use they are oriented upwards: they are slipped under the lamina of the vertebra and the hollow of their U-shape profile bears on the lower edge of the lamina of the vertebra.
In a third mode of use they are oriented downwards and the hollow of their U-shape profile bears on the upper edge of the lamina of the vertebra.
The problem is that, regardless of their position, the hooks have no or very little primary stability, i.e. they constitute a stable anchor member only on condition that they are combined with a second anchor member such as a another hook facing the opposite way or a fixing screw. Even in the presence of another anchor member facing the opposite way, a hook of this kind can slide medially on the lamina and escape from bearing engagement with the pedicle. It can turn inwards and enter the medullary canal of the vertebra, putting the spinal cord at risk.
In some cases anchorage is provided by a bone screw that is screwed into the pedicle of the vertebra. A pedicle screw of this kind has a screwthreaded external part with a close-pitched screwthread adapted to receive a nut, an anchor body or intermediate part of larger diameter and a coaxial screwthreaded internal part with a close-pitched screwthread adapted to screw into the pedicle and into the body of the vertebra. The screw is linked to a connecting rod by a clamp tightened by the nut screwed onto the screwthreaded external part. Alternatively, the screw is linked to a connecting plate or strip a portion of which has the screwthreaded external part of the screw passed through it and is clamped by the nut screwed onto said screwthreaded external part.
During the implementation step, the screwthreaded internal part is screwed into the bone, after which the connecting rod or plate is fitted to the screwthreaded external part and the nut is screwed on to reduce the deformation of the vertebrae and to fasten together the connecting rod or plate and the screw. Tightening the nut applies a torque to the pedicle screw which sometimes causes excessive tightening of the screw in the bone.
During reduction and during subsequent use the repeated movements of and stresses on the spine sometimes cause a small degree of unscrewing of the screwthreaded internal part, or axial withdrawal of the screw, or unseating due to repeated lateral stresses and oscillation that cause the screw to oscillate when only the intermediate part of its screwthreaded internal part is engaged in the cortical bone of the pedicle; this makes the anchorage fragile.
Documents GB-A-2 173 104 and FR-A-2 275 679 describe orthopedic anchor members in which a washer with short teeth is inserted between the anchor body and the bone of the vertebra, the teeth engaging in the bone. A washer of this kind does not prevent relative rotation and therefore does not achieve primary stabilization of the anchor.
Document U
REFERENCES:
patent: 4289123 (1981-09-01), Dunn
patent: 4793335 (1988-12-01), Frey et al.
patent: 4960420 (1990-10-01), Goble et al.
patent: 4988351 (1991-01-01), Paulos et al.
patent: 5269784 (1993-12-01), Mast
patent: 5314427 (1994-05-01), Goble et al.
patent: 5415659 (1995-05-01), Lee et al.
Chopin Daniel
Grosse Arsene
Roussouly Pierre
Taglang Gilbert
Buiz Michael
Eilberg William H.
Shai Daphna
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