Patent Publication Number: US-11389217-B2

Title: Vertebral fixation device

Description:
CROSS-REFERENCE TO A RELATED APPLICATION 
     This application is a Continuation Application of U.S. patent application Ser. No. 16/145,554, filed Sep. 28, 2018, now U.S. Pat. No. 10,660,678; which is a Continuation Application of U.S. patent application Ser. No. 15/089,560, filed Apr. 3, 2016, now U.S. Pat. No. 10,231,765; which is a Continuation Application of U.S. patent application Ser. No. 14/265,978, filed Apr. 30, 2014, now U.S. Pat. No. 9,402,666; all of which are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure relates to a vertebral fixation device. In more detail, the present disclosure relates to a vertebral fixation device having a body member and a strap that is configured for being mounted around a lamina of a vertebra, such as, e.g., a human vertebra. 
     BACKGROUND OF THE INVENTION 
     Vertebral fixation, also known as spinal fixation, is a neurosurgical procedure, in which two or more vertebrae are anchored to each other through a vertebral fixation device. It is an aim of such a neurosurgical procedure to reduce vertebral mobility and thus avoid possible damage to the spinal cord and/or the spinal roots. Indications for such a procedure are, amongst others, degenerative disk disease, spondylolisthesis, fracture, dislocation, spinal stenosis, fracture with instability, occipito-cervical dislocation, tumors and a revision of previous cervical spine surgery. 
     Common devices used to achieve vertebral fixation comprise permanent rigid or semi-rigid prostheses made of titanium. Known devices may include rods, plates, screws and various combinations thereof. 
     The medical community currently uses several different techniques for stabilizing the posterior region of the spine. The most radical of these techniques is spinal fusion. In recent years, spinal surgeons have begun to rely more heavily on mechanical implants which provide increased stability without so severely limiting the recipient&#39;s range of motion. A number of devices have been developed that allow the recipients an almost natural range of motion while still providing support. However, in many cases, the support offered by such devices is insufficient, leaving the physician with few other choices than spinal fusion. 
     A known device used to achieve vertebral fixation is shown exemplarily in  FIG. 1 . In  FIG. 1 , three vertebrae  20  are anchored to each other through a system comprising six spinal fixation devices  110 . Two devices  110  are attached to each vertebra to be anchored. Each device  110  is attached to a respective vertebra by means of a screw  140 . The devices  110  attached to different vertebrae  20  are coupled to each other via a connecting rod  130 . 
       FIG. 2  shows a top view of a vertebra to serve as a reference for the following description. In  FIG. 2 , the different portions of the vertebra are identified.  FIG. 3  shows a top view of a vertebra  20  similar to  FIG. 2 . In  FIG. 3 , however, two devices  110  have been attached to the vertebra  20 . The devices  110  are attached to the vertebra  20  by means of a respective screw  140 , which runs through the pedicle and the body of the vertebra  20 . In common techniques for spinal fixation, usually a tool, generally indicated by reference numeral  150  in  FIG. 3 , is used to fix the screw  140  to the vertebra  20 . 
       FIG. 4  shows a perspective view of a portion of a known spinal fixation system comprising two devices  110 . The devices  110  are attached to different vertebrae  20  by means of a respective screw  140 . Moreover, a rod  130  is connected between the devices  110  to anchor the vertebrae  20  to each other. 
     U.S. Pat. No. 8,162,946 B2 to Bacelli et al. discloses a surgical system for stabilizing a portion of a spine. The system comprises a spinal rod, an implant and an instrument. The instrument comprises a shaft, a moving part and a holder. The implant comprises a flexible tie and a bearing for bearing against the rod. 
     Moreover, U.S. Pat. No. 8,430,918 B2 to Bacelli et al. discloses a vertebral fixing system suitable for being mounted on a vertebra of the spine for connecting it to a rod. The system includes a connecting part suitable for being connected to the rod, a flexible ligature of elongated shape having two free ends suitable for connecting together the connecting part and at least a portion of a vertebra or a rib, and an adjustable locking member mounted on the connecting part. 
     In addition, US patent application No. 2009/0248077 A1 to Johns discloses a spine stabilization for the prophylactic treatment of adjacent level disease. A first vertebral level may be fused by advancing a pedicle screw in the first and second vertebra and coupling the bone screws to a rigid portion of a rod. A conformable ligature may be passed around a non-pedicle portion of a third vertebra and coupled to a dynamic portion of the rod using a blocking body. 
     However, known devices are usually difficult to attach to a vertebra and have the risk of causing damage to the spine. 
     It was therefore an objective of the present disclosure to provide a device that offers additional support to limit the compression on affected vertebrae while still offering a wide range of motion. It was a further objective to minimize the risk of damaging the spinal cord and the spinal roots when mounting the device to a vertebra during a surgical procedure. It was a further objective to provide a device that may easily be attached to a vertebra, and that may preferably be applied by the surgeon without requiring assistance to handle the device during the attachment process. It is a further objective of the present disclosure to allow a correction of the position of the implant after having attached the device to the vertebra. 
     SUMMARY OF THE INVENTION 
     The present disclosure solves the above-identified problems by providing a spinal fixation device according to appended claim  1 . Particular embodiments are addressed in the depending claims. 
     The present disclosure provides a spinal fixation device comprising a body member having a first side and an opposite second side, and an elongate flexible strap having a first section and a second section. The first section of the strap is attached or attachable to attachment means of the body member arranged at a first location thereof. The second section of the strap is further sized and shaped to pass through a passageway arranged at a second location of the body member. The strap has a series of teeth and the passageway of the body member has a latch configured for engaging one of the teeth. The teeth and the latch are configured to allow a movement of the second section of the strap through the passageway in a tightening direction from the first side to the second side of the body member, and to prevent a movement of the second section in the opposite direction. 
     To apply the spinal fixation device to a vertebra of a patient, such as a human being or an animal, the body member of the device may be positioned against the vertebra, and the strap may be arranged to extend from the attachment means of the body member, around a portion of the vertebra, and through the passageway of the body member. As the second section of the strap may only be pulled through the passageway in the tightening direction, but not in the opposite direction, the spinal fixation device may easily be attached to the vertebra by pulling the second section further through the passageway to reduce the length of the strap between the attachment means and the passageway. To tighten the strap around the vertebra, no further instruments are needed. Rather, the surgeon may easily and conveniently pull the second section further through the passageway for fixation. To this effect, he or she may grip a portion of the second section of the strap that extends on the second side of the body member. Moreover, the risk of causing damage to the spinal cord or the spinal roots is reduced. 
     In some embodiments, the first and the second location may be separate. For example, the first location and the second location may be spaced apart by at least 5 mm, preferably at least 10 mm. 
     The passageway may be in the form of a through hole extending through the body member. The passageway may be, e.g., in the form of a slit. In particular, the passageway may extend from the first side of the body member to the second side of the body member. The latch may, e.g., be arranged in the passageway, at an end of the passageway or on the first or second side of the body member. 
     In some embodiments, the body member further comprises connecting means arranged on the second side thereof. The connecting means may be configured for connecting the spinal fixation device to another device. In particular, the connecting means may comprise a threaded bore. A threaded bore provides a convenient way of connecting the spinal fixation device to another device having a correspondingly threaded rod, such as, e.g., a screw. In these embodiments, the threaded bore may have inner threads matching the outer threads provided on the threaded rod. The threaded bore and the threaded rod may, e.g., be tapered. 
     In some embodiments, the connecting means are arranged between the first location and the second location of the body member. These embodiments allow the forces acting on the connecting means to be more evenly distributed between the first and the second location of the body member. The risk of dislocating the spinal fixation device either during the surgical procedure or thereafter is therefore reduced. 
     In some embodiments, the spinal fixation device further comprises a connection element having connecting means configured for attaching to the connecting means of the body member, and further having coupling means for attaching a connecting rod. In particular, the connecting means of the connection element may be arranged at an end section thereof, and the connection element may define a longitudinal axis. The coupling means of the connection element may be configured for attaching the connecting rod in an orientation transversely, in particular perpendicularly or substantially perpendicularly to the longitudinal axis of the connection element. 
     In some embodiments, the teeth are provided on a first side of the second section of the strap facing the attachment means when the second section extends through the passageway of the body member. The teeth may thus be provided on a first side of the strap that faces towards the body member when the strap is attached to the body member. In this embodiment, the teeth are in an orientation towards the vertebra, providing additional stabilization and avoiding damage to soft tissue surrounding the vertebra. Accordingly, a second side of the strap that is opposite the first side may be flat, e.g., free of any protrusions. 
     The teeth may further, e.g., have a triangular cross section. In some embodiments, each of the teeth may have two sides, each side defining an angle of inclination with respect to the longitudinal direction of the strap that is smaller than 90°. Alternatively or additionally, the cross section of the teeth may define an angle bisector that defines an angle with respect to the longitudinal direction of the strap that is smaller than 90°. 
     In some embodiments, the attachment means comprise a further passageway through the body member. In these embodiments, the first section of the strap may have a widening part that is sized and shaped to prevent a passage of the widening part through the further passageway of the body member. Alternatively, the first section of the strap may be attached to a widening element that is sized and shaped to prevent a passage of the widening element through the further passageway of the body member. The widening part of the first section of the strap or the widening element, respectively, may, in particular, be arranged over the second side of the body member, between the further passageway and the second side of the body member, and/or within the further passageway. 
     In some embodiments, the attachment means further comprise a first threaded bore extending from the second side of the body member to a position in the further passageway. The first threaded bore may, in particular, extend transversely to the further passageway. The spinal fixation device may further comprise a first screw configured for extending through the first threaded bore and into the further passageway, such that the first section of the strap is clamped between an end section of the first screw and an inner surface of the further passageway. The first screw may have outer threads matching the inner threads of the first threaded bore. In these embodiments, the first section of the strap may easily be fixedly clamped inside the further passageway to prevent a passage of the first section through the further passageway. In some embodiments, the widening part or the widening element comprises a cylinder. 
     In some embodiments, the attachment means comprise a further latch configured for engaging one of a further series of teeth provided on the strap, e.g., on the first side of the strap. In particular, the further latch and the further series of teeth may be configured for allowing a movement of the first section of the strap through the further passageway in a direction from the second side of the body member to the first side of the body member, and to prevent a movement of the first section of the strap in the opposite direction. The further series of teeth may, e.g. have a triangular cross section. The further series of teeth may be inclined with respect to the longitudinal direction in an opposite direction as compared with the series of teeth. In some embodiments, each of the further series of teeth may have two sides, each side defining an angle of inclination with respect to the longitudinal direction of the strap that is greater than 90°. Alternatively or additionally, the cross section of the further series of teeth may define an angle bisector that defines an angle with respect to the longitudinal direction of the strap that is greater than 90°. In these embodiments, the further series of teeth may be provided on the first section and series of teeth may be provided on the second section of the strap. In some embodiments, teeth may be provided along an entire length of the strap. 
     In some embodiments, the spinal fixation device further comprises locking means for locking the second section of the strap in the passageway of the body member. In particular, the locking means may be adapted for clamping the second section of the strap in the passageway of the body member. 
     In some embodiments, the locking means may comprise a second threaded bore extending from the second side of the body member to a position in the passageway, and the second threaded bore may extend transversely to the passageway. In particular, the spinal fixation device may further comprise a second screw configured for extending through the second threaded bore and into the passageway, such that the second section of the strap is clamped between an end section of the screw and an inner surface of the passageway. The second screw may have outer threads matching the inner threads of the second threaded bore. 
     In some embodiments, the spinal fixation device further comprises a cover configured to be attached to the second side of the body member. The cover may, for example, be configured to cover grooves, bores and holes provided on the second side of the body member. 
     In some embodiments, the cover is configured for clamping the second section of the strap between the second side of the body member and the cover to prevent a movement of the second section of the strap. Additionally or alternatively, the cover may be configured for clamping the first section of the strap between the second side of the body member and the cover to prevent a movement of the first section of the strap. 
     In some embodiments, the strap is made of a biocompatible plastic material or a metal sheet, such as titanium or another biocompatible metal material. 
     In some embodiments, the series of teeth comprises at least five, in particular at least ten teeth arranged along a longitudinal direction of the strap, such that the second section of the strap is attachable at the second location of the body member in at least five, in particular at least ten different positions. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
       Further features and benefits of the present disclosure will be apparent to the skilled person when reading the following detailed description in conjunction with the appended figures, in which like reference numerals refer to like elements, and in which 
         FIG. 1  shows a spinal fixation system known in the art, 
         FIG. 2  shows a top view of a human vertebra, 
         FIG. 3  shows known fixation devices attached to a vertebra, 
         FIG. 4  shows a perspective view of a fixation system attached to vertebrae, 
         FIG. 5  shows a schematic drawing of a spinal fixation device according to an embodiment attached to a vertebra, 
         FIG. 6  shows a perspective view of a spinal fixation device according to an embodiment, 
         FIG. 7  shows a perspective view of a spinal fixation device according to an embodiment, 
         FIG. 8  shows an exploded view of the spinal fixation device of  FIG. 7  having a cover, 
         FIG. 9  shows a perspective view of a strap of a spinal fixation device according to an embodiment, 
         FIG. 10  shows a perspective partial view of a spinal fixation device according to an embodiment, 
         FIG. 11  shows a schematic cross-section of the spinal fixation device of  FIG. 7 , and 
         FIG. 12  shows a schematic cross-section of a spinal fixation device according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 5  shows a schematic view of a spinal fixation device according to an embodiment. The device comprises a body member  10  and a strap  30 . The body member  10  may, e.g., be made of a solid material, such as a metal or a biocompatible plastic material. The body member  10  may be positioned against the vertebra  20 , in particular, against a lamina  21  of the vertebra  20 . The strap  30  extends from a first location of the body member  10  around the lamina  21  and back to a second location of the body member  10 . As will be described in more detail below with reference to the following figures, a first and a second passageway are provided in the body member  10  at the first and second locations, respectively, to receive a first and a second section of the strap  30 , respectively. A widening element  34  in the form of a cylinder is attached to a first end of the strap  30 , e.g., by welding, glue or cold pressing. The cylinder  34  rests in a groove  12  formed at the first location on the second side of the body member  10 , as shown in more detail in  FIG. 6 . The widening element  34  prevents passage of the first end of the strap  30  through the first passageway in the body member  10 . The strap  30  can therefore not be pulled entirely through the first passageway, such that there is no need for an external device to hold the end of the strap  30  in place during the surgical procedure. The strap  30  passes through the first passageway, passes under the lamina  21 , reenters the body member  10  through the second passageway and gets out through a groove  14  formed at the opening of the second passageway on the second side of the body member  10 , as also shown in  FIG. 6 . 
       FIG. 6  shows a perspective view of the spinal fixation device of  FIG. 5 . As previously described, the body member  10  may be mounted on the lamina  21  of a vertebra  20  by means of the flexible strap  30 . The body member  10  has a first side  101  and an opposite second side  102 . Two passageways  13 ,  15  in the form of slit-shaped through-holes extend through the body member  10  from the first side  101  to the second side  102 . A first passageway  13  ends in a first groove  12  arranged on the second side  102  of the body member  10 . Similarly, a second passageway  15  ends in a second groove  14  arranged on the second side  102  of the body member  10 . The strap  30  has a first section  301  located at a first end thereof. The first section  301  of the strap  30  is attached on one side to a cylinder  34 , e.g., by welding, glue or cold pressing. The cylinder  34  rests in the groove  12 . The strap  30  passes through the first passageway  13 . The cylinder  34  attached to the first section  301  of the strap  30  is sized and shaped to prevent a passage of the first section  301  through the first passageway  13 . 
     Starting from the cylinder  34  attached to the first section  301 , the strap  30  extends through the first passageway  13  and forms a loop section  35  that is located on the first side  101  of the body member  10 . A second section  302  of the strap  30  extends through the second passageway  15  in the body member  10 . The first section  301  of the strap  30  may be at least partially wound around the widening element  34 . A first side of the strap  30  is provided with a series of teeth  37  along a longitudinal direction of the strap  30 . Each tooth  37  extends transversely, in particular perpendicularly to the longitudinal direction of the strap  30 . A latch  18  is provided on the second side  102  of the body member  10  above the second groove  14  and is fixed by means of screws  19 . The latch  18  is configured to engage the teeth  37  provided on the first side of the strap  30 . In more detail, the latch  18  is provided on a side of the second groove  14  that faces towards the first groove  12 . To tighten the strap  30  around the lamina  21  of a vertebra  20 , a portion of the second section  302  of the strap that is located above the second side  102  of the body member  10  may be gripped by the surgeon and may easily be pulled away from the body member  10 , causing a movement of a portion of the strap  30  through the passageway  15  from the first side  101  to the second side  102  of the body member  10 . The engagement between the latch  18  and the teeth  37  further prevents a passage of the strap  30  through the passageway  15  in a reverse direction from the second side  102  to the first side  101  of the body member  10 . 
     Moreover, the body member  10  comprises connecting means  11  in the form of a threaded hole or bore. In the embodiment shown in  FIG. 6 , the device further comprises a connecting element  40  having a threaded end section  401  with outer threads that match the inner threads of the threaded hole  11 . The connecting element  40  further has a second end section  402  defining a passageway adapted to receive a connecting rod  50 . The passageway in the end section  402  is arranged, such that the connecting rod  50 , when received in the passageway, extends transversely, in particular perpendicularly to a longitudinal axis of the connecting element  40 . The connecting element  40  and the rod  50  may be used to connect the spinal fixation device with other spinal fixation devices that are, e.g., attached to adjacent vertebrae. 
       FIG. 7  shows a perspective view of a similar spinal fixation device as the one shown in  FIG. 6 . To avoid repetitions, in the following, only the differences between the embodiments of  FIGS. 6 and 7  will be discussed. In addition to the features shown in  FIG. 6 , the device of  FIG. 7  further has threaded bores  16 ′,  17 ′ that extend from the second side  102 ′ of the body member  10 ′. Threaded bore  17 ′ extends from the second side  102 ′ of the body member  10 ′ to a position in the first passageway  13 ′, as is shown in more detail in  FIG. 11 . By fastening a screw  35 ′ in the threaded bore  17 ′, the strap  30  may be fixedly clamped between an end section of the screw  35 ′ and an inner surface of the first passageway  13 ′. 
     Similarly, threaded bore  16 ′ extends from the second side  102 ′ of the body member  10 ′ to a position in the second passageway  15 ′ (see also  FIG. 11 ). By fastening a screw  36 ′ into the threaded bore  16 ′, the strap  30  may be clamped by an end section of the screw  36 ′ and an inner surface of the second passageway  15 ′. By clamping the strap  30  inside the passageways  13 ′,  15 ′, the strap  30  may be fixated in its final position, i.e., after pulling the second section  302  of the strap  30  through the second passageway  15 ′ until the strap  30  runs tightly around the lamina  21  of the vertebra  20 . 
       FIG. 8  shows an explosion view of the spinal fixation device of  FIG. 7 , which further includes a cover  60 ′. The cover  60 ′ is attachable to the second side  102 ′ of the spinal fixation device to cover the grooves  12 ′,  14 ′ and the end sections  301 ,  302  of the strap  30  extending over the second side  102 ′ of the body member  10 ′. The cover  60 ′ has a central opening  61 ′ which can be aligned with the connecting means, such as, e.g., the threaded hole  11 ′ defined in the body member  10 ′, when attached to the body member  10 ′. The cover  60 ′ may be adapted to additionally clamp the first and second sections  301 ,  302  of the strap  30  between the cover  60 ′ and the second side  102 ′ of the body member  10 ′. 
       FIG. 9  is a perspective view of the strap  30 . A widening element  34  in the form of a cylinder is attached to the first section  301  of the strap  30 . The cylinder  34  extends perpendicularly to the longitudinal direction of the strap  30 . A length of the cylinder  34  is greater than a width of the strap  30 . Therefore, passage of the cylinder  34  through the passageway  13 ,  13 ′ in the body member  10 ,  10 ′ may be prevented, while still allowing a passage of the strap  30 . A first side of the strap  30  is provided with a series of teeth  37 . The teeth  37  extend perpendicularly to the longitudinal direction of the strap  30 . The teeth  37  are angled away from a second end section  302  of the strap towards the first end section  301 . The teeth  37  are thus configured for allowing a one-way movement of the strap  30  through the passageway  15 ,  15 ′ of the body member  10  by engaging the latch  18 ,  18 ′ of the body member  10 ,  10 ′. 
       FIG. 10  shows a photograph of a portion of a spinal fixation device according to an embodiment.  FIG. 10  shows a second section  302  of the strap  30  that extends through a passageway  15 ″ in the body member  10 ″. The body member  10 ″ is provided with a latch  18 ″ that is configured to engage one of a series of teeth  37  provided on the first side of the strap  30 . 
       FIG. 11  shows a cross sectional view through the spinal fixation device of  FIG. 7  that is attached to a lamina of a vertebra  20 . As can be seen from  FIG. 11 , the second side  102 ′ of the body member  10 ′ has a flat central portion  104 ′ that extends parallel to the first side  101 ′ of the body member  10 ′. The second side  102 ′ further has peripheral portions  103 ′ that are angled with respect to the central portion  104 ′. The threaded bores  16 ′,  17 ′ extend from a respective one of the peripheral portions  103 ′ of the second side  102 ′ to a respective position in the respective passageway  13 ′,  15 ′, as described in more detail above. The latch  18 ′ is provided on the second side  102 ′ of the body member  10 ′ above the passageway  15 ′ and is fixed by means of screws  19 ′. 
     As can also be seen from  FIG. 11 , the teeth  37  provided on the first side of the strap  30  are angled towards the first section  301  of the strap. In more detail, each of the teeth  37  provided on the second section  302  has two sides, each side defining an angle of inclination with respect to the longitudinal direction of the strap  30  that is smaller than 90°, and the cross section of the teeth  37  defines an angle bisector that defines an angle with respect to the longitudinal direction of the strap  30  that is smaller than 90°. This way, the second section  302  may easily be pulled through the passageway  15 ′ in the body member  10 ′ from the first side  101 ′ to the second side  102 ′, while a movement of the second section  302  through the passageway  15 ′ in the opposite direction is prevented. Hence, the strap  30  may easily be tightened around a lamina  21  of the vertebra  20  by pulling a portion of the second section  302  that is located above the second side  102 ′ of the body member  10 ′ away from the body member  10 ′. After the strap  30  has thus been tightened around the lamina  21  of the vertebra  20 , the latch  18 ′ and the teeth  37  prevent a loosening movement of the strap  30 , i.e. a passage of the second section  302  through the passageway  15 ′ from the second side  102 ′ towards the first side  101 ′ of the body member  10 ′. In this position, the strap  30  may further be fixated by screws  34 ′,  35 ′ that are inserted and fastened in the threaded bores  16 ′,  17 ′. 
     As shown in  FIG. 11 , the connecting element  40 ′ further comprises a stem portion  402 ′ and a removable cap  403 ′. The stem portion  402 ′ has an end section  401 ′ with outer threads matching the inner threads of the threaded hole of the body member  10 ′ that serves as connecting means  11 ′. The cap  403 ′ is further adapted to be screwed onto a further end section of the stem portion  402 ′, and to define a passageway for a connecting rod (not shown) between the stem portion  402 ′ and the cap  403 ′. 
     While  FIG. 11  shows a cylinder  34  attached to the first section  301  of the strap  30 , the skilled person will understand that the first section  301  of the strap  30  may alternatively or additionally include a widening part that is sized and shaped to prevent a passage of the widening part through the passageway  13 ,  13 ′. The skilled person will further understand that the first section of the strap may be fixed to the body member by other means. For example, as shown in the embodiment of  FIG. 12 , a second latch  180 ′″ may be arranged above the first passageway  13 ′″, e.g., by means of screws  19 . The latch  180 ′″ is configured to engage additional teeth  38 ′″ provided on the first section  301 ′″ of on the strap  30 ′″ to prevent a movement of the first end section  301 ′″ of the strap  30 ′″ through the passageway  13 ′″ from the second side  102 ′″ to the first side  101 ′″ of the body member  10 ′, and to allow a movement in the opposite direction. The additional teeth  38 ′″ are angled with respect to a longitudinal direction of the strap  30 ′″ in an opposite direction as compared to the teeth  37 ′″ provided on the second section  302 ′″ of the strap  30 ′″. The additional teeth  38 ′″ further have a triangular cross section. Each of the additional teeth  38 ′″ has two sides, each side defining an angle of inclination with respect to the longitudinal direction of the strap  30 ′″ that is greater than 90°, and the cross section of the additional teeth  38 ′″ defines an angle bisector that defines an angle with respect to the longitudinal direction of the strap  30 ′″ that is greater than 90°. Similarly, each of the teeth  37 ′″ provided on the second section  302 ′″ has two sides, each side defining an angle of inclination with respect to the longitudinal direction of the strap  30 ′″ that is smaller than 90°, and the cross section of the teeth  37 ′″ defines an angle bisector that defines an angle with respect to the longitudinal direction of the strap  30 ′″ that is smaller than 90°. 
     It will be evident to the skilled person that modifications, variations and substitutions of the features of the described embodiments are possible.