Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B17/00—Surgical instruments, devices or methods, e.g. tourniquets
  • A61B17/56—Surgical instruments or methods for treatment of bones or joints; Devices specially adapted therefor
  • A61B17/58—Surgical instruments or methods for treatment of bones or joints; Devices specially adapted therefor for osteosynthesis, e.g. bone plates, screws, setting implements or the like
  • A61B17/68—Internal fixation devices, including fasteners and spinal fixators, even if a part thereof projects from the skin
  • A61B17/70—Spinal positioners or stabilisers ; Bone stabilisers comprising fluid filler in an implant
  • A61B17/7001—Screws or hooks combined with longitudinal elements which do not contact vertebrae
  • A61B17/7035—Screws or hooks, wherein a rod-clamping part and a bone-anchoring part can pivot relative to each other
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B17/00—Surgical instruments, devices or methods, e.g. tourniquets
  • A61B17/56—Surgical instruments or methods for treatment of bones or joints; Devices specially adapted therefor
  • A61B17/58—Surgical instruments or methods for treatment of bones or joints; Devices specially adapted therefor for osteosynthesis, e.g. bone plates, screws, setting implements or the like
  • A61B17/68—Internal fixation devices, including fasteners and spinal fixators, even if a part thereof projects from the skin
  • A61B17/70—Spinal positioners or stabilisers ; Bone stabilisers comprising fluid filler in an implant
  • A61B17/7001—Screws or hooks combined with longitudinal elements which do not contact vertebrae
  • A61B17/7035—Screws or hooks, wherein a rod-clamping part and a bone-anchoring part can pivot relative to each other
  • A61B17/7037—Screws or hooks, wherein a rod-clamping part and a bone-anchoring part can pivot relative to each other wherein pivoting is blocked when the rod is clamped
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B17/00—Surgical instruments, devices or methods, e.g. tourniquets
  • A61B17/56—Surgical instruments or methods for treatment of bones or joints; Devices specially adapted therefor
  • A61B17/58—Surgical instruments or methods for treatment of bones or joints; Devices specially adapted therefor for osteosynthesis, e.g. bone plates, screws, setting implements or the like
  • A61B17/68—Internal fixation devices, including fasteners and spinal fixators, even if a part thereof projects from the skin
  • A61B17/70—Spinal positioners or stabilisers ; Bone stabilisers comprising fluid filler in an implant
  • A61B17/7074—Tools specially adapted for spinal fixation operations other than for bone removal or filler handling
  • A61B17/7076—Tools specially adapted for spinal fixation operations other than for bone removal or filler handling for driving, positioning or assembling spinal clamps or bone anchors specially adapted for spinal fixation
  • A61B17/7077—Tools specially adapted for spinal fixation operations other than for bone removal or filler handling for driving, positioning or assembling spinal clamps or bone anchors specially adapted for spinal fixation for moving bone anchors attached to vertebrae, thereby displacing the vertebrae
  • A61B17/7079—Tools requiring anchors to be already mounted on an implanted longitudinal or transverse element, e.g. where said element guides the anchor motion
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B17/00—Surgical instruments, devices or methods, e.g. tourniquets
  • A61B17/56—Surgical instruments or methods for treatment of bones or joints; Devices specially adapted therefor
  • A61B17/58—Surgical instruments or methods for treatment of bones or joints; Devices specially adapted therefor for osteosynthesis, e.g. bone plates, screws, setting implements or the like
  • A61B17/68—Internal fixation devices, including fasteners and spinal fixators, even if a part thereof projects from the skin
  • A61B17/84—Fasteners therefor or fasteners being internal fixation devices
  • A61B17/86—Pins or screws or threaded wires; nuts therefor
  • A61B17/8605—Heads, i.e. proximal ends projecting from bone

Definitions

• the invention relates to an osteosynthesis fixation system of the type of those used to treat degenerative or traumatic vertebral deformities, comprising anchor screws for vertebrae, at least one anchor screw comprising a bone anchoring element provided at one of its ends with a screw head for coupling a connecting element to the bone anchoring element, the bone anchoring element comprising a threaded rod anchoring in the vertebra extended at the end by a coupling head with the screw head which comprises a body traversed longitudinally by a channel, said channel have, in the lower part, a receiving cavity of the coupling head of 1 anchoring element.
• the invention also relates to an anchor screw intended to be implemented with such a system.
• the osteosynthesis system and the anchor screw are intended in particular, but not exclusively, for the treatment of major trauma of the spine. STATE OF THE ART
• the spinal stabilization systems comprise bone anchoring elements intended to be fixed in the vertebrae concerned by the osteosynthesis (pedicle screws), connecting rods and means for coupling the connecting rods to these bone anchoring elements.
• the coupling of each bone anchoring element to the connecting rod is done either with the aid of a connector or directly.
• the bone anchoring element advantageously has a head arranged to receive the connecting rod.
• anchoring elements with a fixed head in order to control the reduction of the vertebrae.
• the reduction of the fracture or the straightening of the vertebrae is carried out firstly by fixing the bone anchoring elements in the vertebrae concerned by the osteosynthesis and then mounting the connecting rod on the head of the element. bone anchorage.
• the anchoring elements are fixed to extend substantially perpendicular to the vertebra on which they are attached while the connecting rod is to be placed substantially parallel to the spine.
• a tightening nut is then placed on each head, immobilizing the connecting rod while allowing a movement of rotation and translation of the latter on the head so as to allow the reduction manipulations before the final tightening of the nut and therefore complete immobilization of the connecting rod on the head.
• the recovery of the vertebrae is carried out gradually, by operating tractions, compressions and / or spacings on each of the bone anchoring elements implanted on the vertebrae to be straightened with the aid of adapted instruments.
• the mounting of the connecting rod on the heads of bone anchoring elements is frequently problematic.
• the major disadvantage encountered with fixed head anchoring elements is the establishment of connecting rods.
• the heads of each bone anchoring element may have different inclinations and directions so that it is difficult to place a connecting rod on all the heads of the bone anchoring elements. .
• the establishment of the connecting rods is all the more difficult as the deformation of the spine is important.
• the technique used does not always guarantee a correct recovery of the vertebrae.
• the anchoring elements with a multiaxial head have the disadvantage of not being able to serve as a lever for straightening the vertebrae, as is the case with anchoring elements with a fixed head, the screw head of these elements not being completely immobilized on the threaded rod when the clamping nut is mounted on the screw head in a clamping position permitting movement of the connecting rod. It is therefore necessary, once the bone anchoring elements, the connecting rod and the clamping nuts in place, to proceed with the reduction of the vertebrae either by the technique of positioning the patient or by bending techniques. of the connecting rod.
• these techniques are long and not easy and does not ensure a recovery of the threaded rod, and therefore the vertebra, in the axis of the screw head.
• the invention aims to remedy these problems by proposing an osteosynthesis system and an anchoring screw adapted to be implemented with the osteosynthesis system allowing easy placement of the connecting rods on the heads of the elements. bone anchorage while retaining the functionality of fixed head anchors (allow vertebral straightening).
• the invention also aims to provide an osteosynthesis system and an anchor screw to facilitate the recovery and ensure correct recovery.
• the invention also aims to provide a small footprint anchor screw.
• the invention relates to an anchoring screw intended to be implanted in a vertebra comprising a bone anchoring element.
• a bone anchoring element provided, at one of its ends, with a screw head for coupling a connecting element to the bone anchoring element, the bone anchoring element comprising a threaded anchor rod in the vertebra extended at the end by a coupling head with the screw head, the screw head comprising a body traversed longitudinally by a channel, said channel have, in the lower part, a cavity for receiving the coupling head of the anchoring element, characterized in that the receiving cavity comprises a first zone of increasing section towards the bone anchoring element and opening at the lower end of the screw head, a second zone of decreasing section in the direction of the anchoring element, the second zone, disposed above the first zone, having a communication opening with the first zone of section greater than the nominal external section of the threaded rod of the el bone anchoring element and of lower section than an outer section of the coupling head, and a third zone communicating with
• the coupling head of the bone anchoring element and the cavity receiving the coupling head of the screw head By the shape of the coupling head of the bone anchoring element and the cavity receiving the coupling head of the screw head, the latter is able to move from a state in which it is movable. on the bone anchoring element (clearance allowed between the screw head and the bone anchoring element) to a state in which it is fixed, the transition from the mobile state to the fixed state being effected by a rectification of the bone anchoring element until it is positioned in alignment with the screw head, the straightening of the bone anchoring element causing the recovery of the vertebra in which he will be implanted.
• straightening the bone anchoring element is meant placing the bone anchoring element so that its axis is substantially disposed in the axis of the screw head. In other words, the bone anchoring element is aligned with the screw head.
• the second zone is of frustoconical shape.
• the first zone is of frustoconical shape.
• the third zone comprises a wall portion of circular section greater than or equal to the end section. superior of the second zone.
• the third zone has an enlarged inner section relative to the wall portion of circular section. This thus offers possibilities of movement of the coupling head within the body of larger screw heads.
• the coupling head comprises a centering portion of section substantially equal to the wall portion of the third zone.
• the third zone comprises a frustoconical wall portion communicating with the second zone, the lower end of the wall portion having a section greater than or equal to the section of the upper end of the second zone.
• the coupling head comprises a frustoconical end portion s' widening in the direction of the threaded rod.
• the coupling head has an upper face provided with an annular retaining lug of the connecting element.
• the presence of a lug makes it possible to hold the connecting element in position when it is in place in the screw head and thus to prevent any displacement of the element in said body.
• the coupling head has a flat upper face extending substantially perpendicularly to the threaded rod.
• the second and third zones form a deflection chamber of the coupling head.
• the invention also relates to an osteosynthesis system comprising an anchoring screw as defined above the bone anchoring element and the body of the screw head respectively defining a longitudinal axis AA and BB, as well as means for straightening the bone anchoring element relative to the screw head.
• Said rectifying means are such as to straighten the threaded rod of the bone anchoring element and to place it in the same direction as that of the axis of the screw body.
• the rectifying means comprise a clamping nut.
• the rectifying means comprise a cradle for receiving the connecting rod, the cradle having a flat bottom end face.
• the invention also relates to an osteosynthesis system comprising anchoring screws intended to be respectively implanted in a vertebra, at least one of the anchoring screws comprising a bone anchoring element with longitudinal axis AA. , comprising a threaded anchor rod extended at the end by a coupling head, and a screw head for coupling a connecting element to the bone anchoring element, said screw head, mounted movably on the bone anchoring element, comprising a body of longitudinal axis BB traversed longitudinally by a channel having in the lower part a receiving cavity of the coupling head of the anchoring element, at least one connecting element intended to connect the anchoring screw therebetween, and means for clamping the connecting element on each anchoring screw, the system being remarkable in that the osteosynthesis system comprises means for straightening the anchor element. bone rage relative to the screw head (when clamping the connecting element on the anchoring screw, said rectifying means being arranged to straighten the threaded rod of the bone anchoring element and place it in the longitudinal axis BB of the body
• place in the longitudinal axis BB of the body of the screw head is meant to place the threaded rod so that the longitudinal axis AA of the anchor element is confused with the axis of the screw head.
• the rectifying means comprise a cradle for receiving the connecting element, said cradle having a lower end of conjugate shape with the upper end of the coupling head.
• the lower end of the cradle is of frustoconical shape.
• the cradle and the coupling head respectively have a lower face and a flat upper end face.
• the receiving cavity formed in the screw head comprises a first zone of increasing section in the direction of the bone anchoring element and opening at the lower end of the screw head, a second zone of decreasing cross-section. direction of the anchoring element, the second zone, disposed above the first zone, having a communication opening with the first zone of section greater than the external nominal section of the threaded rod of the anchoring element bony and of lower section than an outer section of the coupling head, and a third zone communicating with the second zone and arranged with the latter to allow movement of the screw head on the coupling head, and in that the head coupling of the bone anchoring element comprises a contact portion having an outer surface of shape conjugate to the inner surface of the second zone of e the receiving cavity.
• the first, second and third zones form, with the contact portion, the rectifying means.
• the first and / or second zone is (are) of frustoconical shape.
• the invention also relates to an anchoring screw intended to be implemented in an osteosynthesis system as previously described.
• the anchor screw contains all the characteristics of the anchor screw described above.
• FIG. 1 shows a perspective view in longitudinal section of a screw of anchoring according to a first embodiment of the invention
• FIG. 2 shows a detail view of the anchor screw of Figure 1
• FIG. 3a and 3b show front views of the anchoring screw of Figure 1, respectively in the deflection position and in the clamping position;
• FIG. 4 represents a perspective view in longitudinal torque of an anchor screw according to a second embodiment of the invention.
• FIG. 5 shows a detail view of the anchor screw of Figure 4.
• FIGS. 6a and 6b show front views of the anchoring screw of Figure 4, respectively in the deflection position and in the clamping position;
• FIGS. 7a and 7b show front views of an alternative embodiment of the anchoring screw of Figure 4, the anchor screw being shown respectively in the deflection position and in the clamping position;
• FIGS. 8a and 8b show longitudinal sectional views of an anchor screw according to another embodiment, the anchor screw being shown respectively in the deflection position and in the clamping position;
• FIGS. 9a and 9b show longitudinal sectional views of an anchor screw according to another embodiment, the anchor screw being shown with a cradle, respectively in the deflection position and in a fixed position;
• FIGS. 10a and 10b show longitudinal sectional views of an anchor screw according to another embodiment, the anchor screw being shown with a cradle, respectively in the deflection position and in a fixed position;
• FIG. 11 shows a view of the coupling head shown in Figure 9A in contact with a connecting rod
• FIGS. 12a and 12b show longitudinal sectional views of an anchor screw according to another embodiment, the anchor screw being shown with a cradle, respectively in the deflection position and in a fixed position.
• identical or similar elements of the different embodiments are identified by identical reference signs throughout the figures.
• an anchor screw 1 to be implanted in a vertebra according to a first embodiment.
• the anchoring screw 1 comprises a bone anchoring element 2 provided at one of its ends with a screw head 3.
• the bone anchoring element 2 of longitudinal axis AA, comprises a threaded rod 4 intended to be engaged in the vertebra.
• Said threaded rod 4 is provided at one of its ends with a head 5 configured to allow the coupling of the threaded rod with the screw head 3. It will be discussed later threaded rod 4 and 5 coupling head relationship with the threaded shank and the end of the shank coupling with the screw head 3.
• the bone anchor 2 is cannulated.
• the screw head 3 has, in known manner, a screw head body 30 of longitudinal axis BB, having two lateral arms 31, 32 arranged to define a transverse channel 6 in the shape of a U intended to receive. a connecting element, of the connecting rod type.
• the screw head body 30 is traversed longitudinally by a channel 7.
• the axis of the longitudinal channel corresponds to the longitudinal axis BB of the screw head body 30.
• the screw head body 30 also has notches 33, 34 for the attachment of a specific instrument, such as for example a guide tube used in the context of minimally invasive procedures.
• each of the lateral arms 31, 32 comprises a longitudinal extension 310, 320.
• the extensions 310, 320 delimit a cavity longitudinal threaded through 8. They further comprise notches 311, 321 for the attachment of a specific instrument, such as for example a guide tube used in the context of minimally invasive procedures.
• Each arm 31, 32 is connected to an extension 310, 320 corresponding by a rupture zone 10 allowing the removal of the extension once the intervention is completed. This has the advantage of avoiding leaving unnecessary parts in the body after surgery.
• These rupture zones 10 are constituted by notches whose depth and geometry are determined to allow a clean break when exerting, on one of the extensions, a lateral force exceeding a threshold value with a specific instrument.
• Such a screw head 3 is intended in particular for the treatment of major trauma. It is, of course, obvious that the anchoring screw 1 according to the invention is not limited to such an application, and that the screw head 3 can have a conventional shape (ie, not having an extension) like the illustrated heads. in Figures 4 to 6a, 6b without departing from the scope of the invention.
• the longitudinal channel 7 has, in the lower part, a receiving cavity 11 intended to receive the coupling head 5 of the anchoring element 2.
• the receiving cavity 11 has a first zone 12 of increasing section towards the bone anchoring element 2 and a second zone 13 of decreasing section towards the anchoring element, the second zone 13 being arranged above the first zone 12 and communicating with the latter.
• the first zone 12 opens directly into the second zone 13.
• the communication opening between said zones advantageously has a section greater than the external nominal section of the threaded rod of the bone anchoring element 2 and a section smaller than a section. external to the coupling head 5.
• the passage opening is dimensioned to allow the coupling of the screw head 3 with the bone anchoring element 2 (insertion of the bone anchoring element 2 from the top of the screw head 3).
• the receiving cavity 11 further comprises a third zone 14 communicating with the second zone 13 and situated above the latter.
• the third zone 14, communicating directly with the second zone 13, extends in the extension thereof to form a frustoconical shaped cavity.
• the receiving cavity 11 thus has substantially an hourglass shape.
• the first zone 12 and the third zone 14 have the function of allowing the displacement of the screw head 3 on the bone anchoring element 2, and thus facilitate the positioning of the rod.
• the second zone 13 has the function of allowing the straightening of the threaded rod 4 in the longitudinal axis of the longitudinal channel 7 of the screw head 3 and thus the straightening of the vertebra when a clamping means, of the nut type, is engaged in the corresponding channel 7.
• the first zone 12 and the third zone 14 will be designated as control cone 12 (or clearance cone 12) and clearance chamber 14, and the second zone 13 as cone 13 (or rectification cone). 13).
• the upper portion of the hourglass shape formed by the clamping cone 13 and the deflection chamber 14 has a height at least twice as high as the height of the lower part of the shape. hourglass formed by the travel cone 12.
• the upper portion has an angle at the top relative to the longitudinal axis BB less than the angle at the top of the lower portion.
• the coupling head 5 of the bone anchoring element 2 has an outer surface 15 of shape conjugate to the inner surface of the clamping cone 13. More particularly, the coupling head 5 has a substantially frustoconical shape s' flaring in a direction opposite to the threaded rod 4, of adjacent conicity, and preferably identical to the conicity of the clamping cone 13. When the anchoring screw is in fixed position (ie after straightening of the anchoring element) the wall of the clamping cone is thus in contact with a wall portion of the coupling head. This wall portion will be referred to hereinafter as the "contact portion" under the reference 20.
• the anchoring screw 1 Because of the hourglass shape of the receiving cavity 11 and the inverted frustoconical shape of the coupling head 5, the anchoring screw 1 jointly ensures the function of multiaxial screw and fixed screw. It allows a displacement of the screw head 3 on the bone anchoring element 2 and the straightening of the bone anchoring element 2 in the axis of the screw head 3 (axis corresponding to the axis of the hourglass shape) when the connecting rod and the clamping nut are placed on the screw head 3.
• the anchor screw 1 comprises a retaining means 16 of the connecting rod in position when the latter is mounted in the corresponding channel 6 of the screw head 3.
• the retaining means 16 is in the form of an annular lug 17 formed on the upper face of the coupling head. It is of course obvious that the invention is not limited to this type of retaining means and that it can be provided in particular a plurality of discontinuous lugs.
• the coupling head 5 comprises a cavity 18 opening on the upper end 19, and more particularly on the upper face forming the base of the frustoconical shape of the coupling head 5.
• the cavity 18 is arranged to receive the end of an instrument, such as the end of a screwdriver-type drive instrument.
• Figures 4, 5, 6a and 6b illustrate an anchor screw 100 according to another embodiment of the invention.
• the deflection chamber 14 has a circular section and extends in the extension of the second zone 13 in a direction substantially parallel to the longitudinal axis BB of the screw head 3.
• the clearance chamber 14 opens directly into the clamping cone 13.
• the clearance chamber 14 and the coupling head 5 are dimensioned so as to provide a slight clearance between the inner wall 36 of the clearance chamber 14 and the largest outer diameter of the coupling head 5.
• For light play means sufficient clearance to allow the passage of the element d bone anchoring 2 in the screw head 3 and the displacement of the screw head 3 on the bone anchoring element 2 while ensuring optimal external dimensions of the screw head 3.
• the clamping cone 13 has a generator having a length of between 1.5 millimeters and 3 millimeters, and preferably about 2 millimeters.
• the upper part of the receiving cavity 11 formed by the clamping cone 13 and the deflection chamber 14 have a height substantially equal to the height of the lower part of the receiving cavity 11. formed by the travel cone 12.
• the cone clamp 13 has a meanwhile height lower than that of the clearance chamber 14.
• the upper portion has an angle at the top relative to the longitudinal axis BB substantially similar to that of the lower part. It is of course obvious that this embodiment is not limited to this dimensioning, other dimensions can be provided according to the desired deflection, the size of the anchor screw, etc., without departing from the scope of the present invention. 'invention.
• FIGs 7a and 7b illustrate an alternative embodiment of the anchor screw 100 illustrated in Figures 4, 5, 6a and 6b (anchor screw referenced 110).
• the coupling head 5 comprises a portion having a frustoconical shape conjugate to the inner wall of the clamping cone 13 of the screw head 3. This portion of the coupling head 5 thus forms the portion contact portion 20 intended to come into contact with the wall of the clamping cone 13 during the straightening of the bone anchoring element 2.
• the contact portion 20 is advantageously extended by an end portion 21 of inverted frustoconical shape relative to that of the contact portion 20.
• the outer section of the base of the end portion 21 is equal to the outer section of the base of the contact portion 20.
• the advantage to provide a prolonged contact portion 20 by a frustoconical end portion 21 is to increase the volume of the coupling head 5 while allowing a movement of the screw head 3 on the coupling head 3. E n increasing the volume of the coupling head 5, any rupture zones present at the cavity 18 and the coupling head when it is devoid of end portion are eliminated.
• the anchor screw 110 comprises means for retaining the connecting rod on the screw head 3.
• the retaining means comprises a pin annular formed on the upper face (end face) of the end portion 21.
• the advantage of the inverted frustoconical shape of the end portion 21 is not to limit the movement of the screw head 3 on the coupling head 5 of the bone anchoring element 2.
• the clearance chamber 14, as in the embodiment described above, has a circular section and extends substantially in parallel. to the longitudinal axis of the screw head 3.
• the clamping cone 13 has a height substantially equal to the height of the deflection cone 12. It also has an angle at the vertex with respect to the longitudinal axis BB less than that of the cone. 12. Obviously, it is obvious that this embodiment is not limited to this dimensioning, other dimensions can be provided according to the desired travel, the size of the anchor screw, etc., without leaving of the scope of the invention.
• the illustrated screw head 3 is an arranged head for the treatment of major trauma (similar configuration to the screw head of the anchor screw of Figure 1). It is of course obvious that the screw head 3 may have a conventional shape as illustrated in FIG. 4.
• Figures 8a and 8b illustrate an anchor screw 120 according to another embodiment.
• the clearance chamber 14, as in the embodiment described above has a circular section 23 and extends substantially parallel to the longitudinal axis of the screw head 3. However, it communicates with the clamping cone 13 via a centering zone.
• the centering zone 23, of circular section extends substantially in the longitudinal axis BB of the screw head 3.
• the centering zone 23 is intended to be placed opposite a circular portion of the coupling head 5, called the centering portion 22, during the straightening operation.
• retaining means are provided on the upper face of the centering portion 22.
• the centering portion of the coupling head 5 and the centering zone of the screw head 3 thus make it possible to align the threaded rod 4 with respect to the longitudinal channel 7 of the screw head 3.
• the clearance chamber 14 has an internal section greater than the internal section of the centering zone 23. This change of section is designated by the reference 28.
• the deflection position is then obtained by a slight sliding of the screw head 3 on the threaded rod 4 towards the anchoring end of the latter.
• the coupling head 5 also comprises an end portion of frustoconical shape inverted relative to that of the contact portion 20similar to the end portion 21 of the head of coupling 5 illustrated in Figures 7a and 7b.
• the straightening of the bone anchoring element 2 is performed by the connecting rod during assembly of the clamping nut in the screw head 3.
• the presence of a cradle between the coupling head 5 and the connecting rod is not necessary. It may of course be envisaged to provide the implementation of the anchoring screws described above with a cradle 24 having a flat contact surface 26 without departing from the scope of the invention as illustrated in Figures 9a and 9b. It is understood in this case that the coupling head 5 will have an end surface 27 preferably flat.
• the advantage of using a cradle having a planar lower contact face and a coupling head 5 having a flat end face is to offer an optimum lever arm regardless of the axis of implantation of the rod.
• anchorage 4 in the vertebra Indeed, and as illustrated in FIG. 9a, the point of contact 25 between the cradle 24 and the coupling head 5 when the screw head 3 is inclined with respect to the bone anchoring element 2 is located on the edge from the upper end of the coupling head 5.
• Such a point of contact can not be obtained with a connecting rod.
• the point of contact 29 obtained with a connecting rod 35 has a distance Y with the longitudinal axis BB less than the distance X obtained with the cradle 24.
• the coupling head 5 is formed of a plurality of frustoconical portions. These frustoconical portions are arranged relative to each other to give the coupling head 5 a shape close to that of a hemisphere.
• the screw head 3 is then rotatably mounted on the bone anchoring element 2.
• the presence of a cradle 24 as described above is necessary, in association with the connecting rod and the nut. in order to completely lock the rotational movement of the screw head 3 and to intervene in the straightening of the bone anchoring element 2 in the axis of the screw head 3 during the recovery of the vertebra.
• Figures 12a and 12b show another embodiment of an anchor screw 150 provided with a cradle 24, the screw being shown respectively in the deflection position and in a fixed position.
• the elements screw head 3, bone anchoring element 2 and cradle 24
• the cradle 24 and the coupling head 5 of the anchoring element 2 differ in the configuration of the contact zones implemented by the cradle and the coupling head.
• the cradle 24 has a chamfered end face 26 (chamfer 26 A).
• the cradle 24 thus has a lower end portion of frustoconical shape.
• the coupling head 5 has an upper end face 27 provided with a frustoconical bore 270 conjugated to that of the end portion of the cradle 24. As illustrated in Figure 12b, the bore 270 is dimensioned and arranged so that the coupling head 5 cooperates with the end portion of the cradle 26 when the bone anchoring element 2 is placed to extend in the axis of the screw head 3. The bore 270 opens into the receiving cavity 18 of the end of the instrument.
• the cradle and the bone anchoring element may also be provided to subject the cradle and the bone anchoring element to a chemical treatment, such as anodizing.
• a chemical treatment such as anodizing.
• it can be provided an inverted configuration for the contact faces of the cradle and the coupling head.
• the coupling head has an upper end portion of frustoconical shape, the taper decreasing in the opposite direction to the threaded anchoring rod while the cradle has an end face provided with a boring of conjugated shape with the frustoconical end portion of the coupling head when the bone anchoring element is placed in the axis of the screw head.

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