INTERVERTEBRAL CAGE WHICH IS EXPANDABLE IN STEPS AND IMPLANTATION INSTRUMENT THEREFOR

Abstract

The invention relates to an intervertebral fusion implant for fusing two adjacent vertebrae (9, 9), comprising a base piece (2) and a top piece (3) which in each case are designed to come to rest against a facing end plate (92) of one of the adjacent vertebrae (9, 9), wherein the top piece (3) is vertically adjustable relative to the base piece (2), wherein a ratchet device (4), which effects a vertical adjustment in steps and secures a set height against reversion, is provided between the base piece (2) and the top piece (3). Thus a defined expansion can be achieved which is dependent upon the number of actuations. Precise control over the state of expansion is enabled in a purely haptic manner. Furthermore, implantation instruments are provided which are configured as parallel spreaders and have a stop as spreading path limiter or co-operating force and path measuring device. Thus it can be implanted more reliably, wherein the risk of overloading or injury to the vertebrae and the nerves and ligaments of the vertebrae is decreased.

Claims

1. An intervertebral fusion implant for fusion of two adjacent vertebrae (9, 9), comprising a base piece (2) and a cover piece (3) which are each designed to bear on a facing end plate (92) of one of the adjacent vertebrae (9, 9), the cover piece (3) being adjustable in height relative to the base piece (2), characterized in that a ratchet mechanism (4), provided between base piece (2) and cover piece (3), effects a stepped adjustment of height and secures an adopted height against reversal.

2. The intervertebral fusion implant as claimed in claim 1, characterized in that the base piece (2) has a bottom wall (22), for bearing on one of the adjacent vertebrae (9), and a vertical wall (21) rising from this bottom wall (22), and the cover piece (3) has a cover wall (33), for bearing on the other of the adjacent vertebrae (9), and a side wall (31) projecting downward from this cover wall (33), said vertical wall (21) and side wall (31) being parts of the ratchet mechanism (4), said ratchet mechanism (4) preferably being a multi-part mechanism and having a plurality of locking lugs (43) on one of its parts (31), and having a locking spring (42) on another of its parts (21), wherein two ratchet mechanisms (4) are preferably provided, the locking lugs (43) on one of the two ratchet mechanisms (4) having a spacing that is an integral multiple of the spacing, preferably double the spacing, between the locking lugs (43) on the other of the two ratchet mechanisms (4).

3. The intervertebral fusion implant as claimed in either of claims 1 and 2, characterized in that the angle of a supporting flank (45) of the ratchet mechanism (4) is in the range of 5 to 35, preferably approximately 10, wherein the locking lugs (43) moreover preferably overlap one another in an imbricated manner, wherein a counter-flank (47) is preferably oriented in the same direction as, but flatter than, the supporting flank (43).

4. The intervertebral fusion implant as claimed in one of the preceding claims, characterized in that a catch mechanism (5) is provided, which limits adjustment movement of the cover piece (3), wherein preferably the catch mechanism (5) is formed by a pawl (52), which interacts with a shoulder projection (53), and/or by a transverse pin on the cover piece (3), which transverse pin engages in slot-like apertures on the base piece (2).

5. An implantation instrument for an expandable intervertebral fusion implant as claimed in one of the preceding claims, characterized in that the implantation instrument (7, 8) has a cross section which at most is as great as that of the intervertebral fusion implant (1).

6. The implantation instrument as claimed in claim 5, characterized in that it is designed as a draw-key expander (8) with two spreading jaws (81, 81), with a draw key (82) preferably being arranged in the front area on the spreading jaws (81, 81), wherein a tensioning member (80), in particular an actuation spindle (89), is preferably provided for the draw key (82), which tensioning member (80) is routed rearward between the spreading jaws (81, 81).

7. The implantation instrument as claimed in either of claims 5 and 6, characterized in that the implantation instrument (8) has a display device (85), which indicates the attained height adjustment of the intervertebral fusion implant (1), preferably proportionally.

8. The implantation instrument as claimed in one of claims 5 through 7, characterized in that it is designed as a parallel expander, preferably with a cross lever (89) as reverse kinematics system.

9. The implantation instrument as claimed in claim 8, characterized in that the tensioning member (80) is actuated by a trigger lever (80), for which a limit stop mechanism (87) adjustable to predefined positions is preferably provided.

10. The implantation instrument as claimed in claim 8 or 9, characterized in mechanisms for form-fit fastening of the base piece and/or cover piece (2, 3) are provided only on one of the spreading jaws, preferably as projections (84) engaging in apertures in the base piece (2).

11. The implantation instrument as claimed in one of claims 5 through 10, characterized in that it is provided with a force-measuring device, which interacts with a displacement-measuring device in such a way that it indicates the expansion path attained with a particular actuating force.

12. The implantation instrument as claimed in claim 11, characterized in that it is designed as a parallel expander, preferably with a cross lever as reversal kinematics system.

13. The implantation instrument as claimed in claim 11 or 12, characterized in that it has an actuation lever which acts on a transmission element supported on a rear end of the tensioning member, wherein the transmission element preferably comprises the force-measuring device.

14. The implantation instrument as claimed in claim 13, characterized in that the force-measuring device is designed as a spring element, and a force scale is preferably arranged on the transmission element, and more preferably a displacement scale is arranged on the tensioning member.

15. An arrangement composed of an intervertebral fusion implant as claimed in one of claims 1 through 4 and of an implantation instrument as claimed in one of claims 5 through 1

Description

[0041] The invention is explained in more detail below with reference to the attached drawing and on the basis of advantageous illustrative embodiments. In the drawing:

[0042] FIG. 1 shows a view of an illustrative embodiment of an intervertebral fusion implant in the implanted state on vertebral bodies;

[0043] FIG. 2 shows perspective views of base piece and cover piece as per the embodiment shown in FIG. 1;

[0044] FIG. 3 shows schematic views of the action of a ratchet mechanism;

[0045] FIG. 4 shows an enlarged view of teeth of the ratchet mechanism as per FIG. 3;

[0046] FIG. 5 shows an example of a catch mechanism in a variant of the illustrative embodiment;

[0047] FIG. 6 shows views of a release mechanism in the locked position and the unlocked position;

[0048] FIG. 7 shows a perspective view and a cross-sectional view of a securing element in the dismantled state and the assembled state;

[0049] FIG. 8 shows a perspective view of a grip head for an implantation instrument;

[0050] FIG. 9 shows cross-sectional views of the implantation instrument as per FIG. 8 in the relaxed state and partially expanded state;

[0051] FIG. 10 shows a plan view and cross-sectional details of the implantation instrument;

[0052] FIG. 11 shows a perspective view of a second embodiment;

[0053] FIG. 12 shows a two-part implantation instrument for an intervertebral fusion prosthesis as per a third embodiment;

[0054] FIG. 13 shows views of the second implantation instrument in a partially assembled state and a fully assembled state;

[0055] FIG. 14 shows a perspective view of a further illustrative embodiment of the intervertebral fusion implant;

[0056] FIG. 15 shows a cross-sectional view of FIG. 14;

[0057] FIG. 16 shows an enlarged detail of FIG. 14;

[0058] FIG. 17 shows a side view of a variant of FIG. 14;

[0059] FIG. 18 shows a plan view of a fourth embodiment of the implantation instrument;

[0060] FIG. 19 shows an enlarged detail of FIG. 18;

[0061] FIG. 20 shows another enlarged detail of FIG. 18;

[0062] FIG. 21 shows a plan view of a fifth embodiment of the implantation instrument;

[0063] FIG. 22 shows a detail of FIG. 21 in partial section; and

[0064] FIG. 23 shows a further detail of FIG. 21.

[0065] An intervertebral fusion implant, designated in its entirety by reference number 1, is provided for implantation in an intervertebral space 91 between two immediately adjacent vertebral bodies 9, 9. In a physiologically intact vertebral column, an intervertebral disk 90 is located in the intervertebral space between the vertebrae. This intervertebral disk 90 may undergo degeneration as a result of disease or wear, with the result that it has to be at least partially resected. In order to achieve sufficient support of the intervertebral space 91, despite the loss of intervertebral disk material, and to thereby prevent collapse of the vertebral column, the intervertebral fusion implant 1 is inserted into the intervertebral space 91. It provides a supporting action and thus facilitates fusion of the two adjacent vertebrae 9, 9 in a natural way through bone growth.

[0066] The intervertebral fusion implant 1 comprises a base piece 2, which bears on an upper end plate 92 of the lower adjacent vertebra 9. Moreover, it comprises a cover piece 3, which bears on a lower end plate 93 of the upper adjacent vertebra 9. In order to ensure a secure bearing of the base piece 2 on the upper end plate 92, on the one hand, and of the cover piece 3 on the lower end plate 93 of the upper vertebra 9, it is necessary to suitably adjust the height of the intervertebral fusion implant 1. It has to be expanded until the base piece 2 and the cover piece 3 bear firmly on the respective end plates 92, 93.

[0067] To permit a movement of base piece 2 and cover piece 3 away from each other in the sense of an expansion, a ratchet mechanism 4 is provided. The latter comprises a multiplicity of locking teeth 43 on the cover piece, which interact with a locking spring 42 on the base piece 2.

[0068] In order to keep the intervertebral fusion implant 1 according to the invention compact, and thus be able to implant it through an access prepared by minimally invasive surgery, while at the same time allowing it to exert a sufficient supporting action in the intervertebral space 91, the intervertebral fusion implant 1 according to the invention is adjustable in height. For this purpose, it has a base piece 2 and a cover piece 3, which is arranged to be movable in height relative to the base piece 2. The base piece 2 is substantially trough-shaped and comprises a bottom wall 22, which functions as a contact face on the end plate 92 of the lower of the adjacent vertebrae 9, and an upwardly projecting vertical wall 21 on each of the two lateral faces. The base piece 2 therefore has an overall U-shape in cross section. At the upper end of the vertical wall 21, the locking spring 42 is arranged as a part of the ratchet mechanism 4. Moreover, approximately at the middle of an upper end of the vertical wall 21 there is a projection 52, which forms a part of a catch mechanism 5. In the bottom wall 22, there are a plurality of openings 24 via which a hollow space 10, in the interior of the trough-shaped configuration between the side walls 21, is connected to the space underneath the bottom wall 22. This promotes the growth of bone into the intervertebral fusion implant 1.

[0069] Moreover, the depicted illustrative embodiment of the intervertebral fusion implant 1 according to the invention comprises a cover piece 3. The latter has a generally box-shaped configuration, with a substantially plane top face 33, which functions as a contact face for the lower end plate 93 of the upper adjacent vertebra 9. A side wall 31 is arranged projecting downward on each of the lateral faces. On their outward surface, these side walls 31 carry a plurality of locking teeth 43, which form a part of the ratchet mechanism 4 and interact with the locking spring 42 on the base piece 2. In the illustrative embodiment shown, an end wall 30 is provided on an end face. It is optional and can also be omitted. Moreover, a bearing shoulder 53 is also formed approximately at the middle on the side walls 31. Its dimensions are chosen such that it interacts with the projection 52 on the base piece 2. The arrangement of the bearing shoulder 53 is chosen here such that it comes to bear on the projection 52 in the position of maximum extension of the cover piece 3 relative to the base piece 2. A limit stop is thus obtained. The projection 52 and the bearing shoulder 53 thus interact as a catch mechanism 5. A plurality of apertures 34 are formed on the top face 33, in order to allow bone substance to grow into the interior of the cover piece.

[0070] For implantation, the cover piece 3 is fitted into the hollow space 10 of the trough-shaped base piece 2. The implant is thus in the state of minimal expansion, as is shown in the uppermost of the three views arranged above one another in FIG. 3a. In this state, the implant is introduced into the intervertebral space 91 until it more or less reaches the position shown in FIG. 1b. In a next step, it can then be expanded until it is set to the height of the intervertebral space 91. This is done by the cover piece 3 being moved upward by means of an implantation instrument 8, wherein the locking spring 42 successively engages with different locking teeth 43 on the lateral faces 31 of the cover piece 3 and thereby secures the attained position of expansion. This expansion procedure is depicted in the three views in FIG. 3a.

[0071] The locking teeth 43 are sawtooth-shaped. Each tooth 43 has two flanks 45, 47. Here, the shorter flank 45 is the load-bearing flank, which comes into engagement with the locking spring 42 in such a way as to transmit force. This load-bearing flank 45 is shaped facing downward and is thus oriented in the same direction as the other, non-load-bearing flank 47. This results in a certain degree of imbrication of the locking teeth 43 when viewed from the side. In the illustrative embodiment shown in FIG. 4, the angles and are chosen such that the angle is about 60 and the angle is 8. The top face 46 of the locking spring 42 is inclined by said angle . Thus, together with the downwardly facing orientation of the load-bearing flank 45, this means that a loading force F results in an inwardly directed lateral force L, which draws the locking spring 42 into the teeth 43 and thus secures it against undesired escape from the locking teeth 43. A self-securing effect is thus obtained in the sense that the securing force L increases the greater the loading force F.

[0072] In order to prevent a situation in which the cover piece is moved too far up during the expansion and the locking spring 42 thus leaves the area of the locking teeth 43, a catch mechanism 5 as shown in FIG. 5 is alternatively provided. It comprises a projection 52 on each locking spring 42, specifically at the middle thereof. A downwardly facing stirrup 53 is arranged on the cover piece 3, underneath the locking teeth 43, its dimensions being chosen such as to enclose the projection 52. When the maximum expansion is reached, the stirrup 53 engages under the projection 52 and prevents a further expansion movement.

[0073] For the purpose of height adjustment, the expansion movement generally takes place uniformly on both sides of the ratchet mechanism 4, as is also depicted in FIG. 3a. However, it may also be desirable for the adjustment to be non-uniform in order to adopt a defined tilt angle. A desired tilt angle can be adopted in a defined manner depending on whether the adjustment on one side takes place across one or two extra locking teeth 43. For this purpose, the surgeon simply has to count off the number of clicks, i.e. the number of locking procedures in the ratchet mechanism 4 made by the engagement of the locking spring 42 in the different locking teeth 43. For a finer adjustment of the tilt, more finely stepped teeth 43 can also be provided, twice as many teeth 43 of half the size of the teeth 43. Preferably, only one of the two ratchet mechanisms is provided with the finer teeth, as is shown in FIG. 5. Optionally, the start points of the teeth 43, 43 are mutually offset between the two ratchet mechanisms.

[0074] It may be necessary to return the cover piece 3 from its expanded position to a less expanded position or to a position of no expansion at all, as is used for the implantation. For this purpose, a resetting mechanism 6 is expediently provided. This comprises an instrument with forceps-like grippers 61 which, with corresponding gripper tips 62 at their front end, engage in receiving openings 60 formed for this purpose on the top face of the vertical walls 21. By spreading the grippers 61 in a forceps-like movement, the two vertical walls 21 lying opposite each other are moved away from each other, as a result of which the locking spring 42 on the respective vertical wall 21 is no longer in engagement with the locking teeth 43 on the cover piece 3. The ratchet mechanism 4 is thereby released. The cover piece 3 can be freely moved downward and in this way reset. For this purpose, the material of the base piece 2 is preferably chosen such that it is able to deform elastically, in particular the bottom wall 22. This is shown in FIG. 6.

[0075] In order to achieve additional securing of the attained height adjustment after successful expansion, a separate blocking element 70 can be provided. It is preferably shaped such that it can be pushed with a good fit into the hollow space 10 between cover piece 3 and base piece 2, which hollow space 10 has been enlarged by the expansion. It will be appreciated that different blocking elements 70 have to be used depending on the expansion status. The blocking element 70 preferably has a central aperture 74, which is flush with the apertures 34 on the cover piece 3 and the apertures 24 on the base piece 2. In this way, the hollow space 10 can be more easily reached and accessible for the inward growth of bone substance. In the inserted state, the blocking element 70 secures the cover piece 3 against a reverse movement back into the base piece 2, as is shown in FIG. 7b.

[0076] For the height adjustment by means of expansion, an implantation instrument is provided. It is designed as a draw-key expander 8 with two spreading jaws 81. They are provided at their front end with seats 84, which engage with a form fit in the apertures 34. At the front end, the spreading jaws 81 are indented in a V-shape, and a wedge-shaped draw key 82 is fitted into this indent. The draw key 82 is connected to a tensioning member 80, which extends along the longitudinal axis of the implantation instrument 8, over the entire length of the latter (see FIG. 10). It begins with its front end on the draw key 82 and terminates, at the rear end of the implantation instrument 8, in a handwheel 88, in which it is mounted via a spindle thread 89. When the handwheel 88 is turned, the tensioning member 80 is moved rearward by means of the spindle thread 89, as a result of which the draw key 82 retreats into the V-shaped depression between the two spreading jaws 81. The spreading jaws 81 are thereby driven apart from each other (see FIG. 9, where the top view shows the non-expanded initial state and the lower view shows an expanded state). Depending on the pitch of the spindle thread 89, the surgeon can obtain a desired spreading action by means of a defined number of rotations of the handwheel 88, as a result of which the cover piece 3 is adjusted in height in relation to the base piece 2. For additional control of the attained state of expansion, a display mechanism 85 is provided. It comprises a scale 86 arranged on the spreading jaws 81, and an indicator pin 87 arranged on the tensioning member 80. The indicator pin 87 is drawn rearward together with the tensioning member during the spreading procedure, as a result of which its position relative to the scale 86 changes, and the state of expansion attained can thus be read off.

[0077] A second alternative illustrative embodiment of the intervertebral fusion implant 1 is shown in FIG. 11. Although it is substantially the same as the one in FIG. 1, it differs in that it is not designed rectilinearly in a box shape but is instead substantially curved in the shape of an arc of a circle. As regards structure and function, the above description applies by analogy.

[0078] In a further embodiment, which is suitable in particular for a transverse implantation (as is indicated in FIG. 1a by a broken line in the area of the intervertebral disk 90), a further embodiment of the intervertebral implant is provided. In the area of its end face 30, it has an opening in which a part of the implantation instrument can be screwed (see opening indicated by broken lines in FIG. 2a). The implantation instrument 7 provided therefor is shown in FIGS. 12 and 13. The implantation instrument comprises a lever part 71 and a guide part 73. Moreover, a threaded rod 72 is provided, which is to be fitted into an axial bore 75 arranged along the longitudinal axis of the lever part 71. At its rear end, the lever part 71 has a toggle 75 protruding radially to the side. At its front end, the lever part 71 has a gripping jaw 71, which is designed to grip the bottom wall 22 of the base piece 2. At its front end, the guide piece 73 has a second gripping jaw 73, which is correspondingly designed to grip the cover wall 33 of the cover piece 3. At the rear end, the guide piece 73 has a guide bell 77, which is open on one side at the front and, at its rear, closed end, has a central handle 78. On its front edge surrounding the open end of the bell, it has a recess 79. The recess 79 is dimensioned such that, seen in the radial direction, it has a greater width than the toggle 75 of the lever piece 71.

[0079] For assembly, the threaded rod 72 is pushed through the axial bore 75 of the lever piece 71 and, with a thread 72 at its front end, is fastened in a screw thread of the opening 38 on the end face 30 of the cover piece 3. It is screwed tightly using a knurled head 76 at the rear end of the threaded rod 72. The lever part 71, with the cover piece 3 fastened thereon, is then mounted on the guide piece 73, specifically in such a way that its toggle 75 engages in the recess 79. The base piece 2 is fitted onto the front end of the actuation piece 73. In this way, the implant is fitted onto the instrument 7 in its assembly position, i.e. non-expanded.

[0080] The expansion takes place by means of the cover piece 3 being successively tilted relative to the base piece 2, such that the locking spring 42 always jumps one locking tooth 43 further on one side. This alternate tilting takes place by means of the toggle 75 being pivoted to and fro in the recess 79 from one radial end position to the other radial end position, as shown in FIG. 13c by the double arrow. In this way, the cover piece 3 is successively tilted and thus at the same time expanded. The above description applies accordingly for the further description.

[0081] A further alternative embodiment of the intervertebral fusion implant is shown in FIGS. 14 to 17. It is based on the embodiment shown in FIG. 2, and similar components are provided with the same reference numbers and to this extent require no explanation. This embodiment has an alternative catch mechanism 5. It comprises a transverse pin 52 which is arranged at the bottom of and protrudes from the side walls 31 (see also the detail in FIG. 16). The transverse pin 52 engages in a slot-like aperture 53 formed in the vertical walls 21. By virtue of their extended form in the vertical direction, they permit a height adjustment of the cover piece 2, specifically until the transverse pin 52 finally abuts the upper end of the slot-like aperture 53. A further movement is thereby prevented. The arrangement of the transverse pin 52 at the bottom on the side walls 31 has the advantage that the toothing 43 on the side walls can run through without interruption. It is therefore stiffer and stronger than the interrupted embodiment shown in FIG. 5.

[0082] The embodiment shown in FIG. 14 is moreover provided with a structuring. On its top face 33, and also on its bottom face 22, it has furrows 29 extending in the transverse direction from one side to the other. These are approximately 2 mm wide and serve to provide better anchoring of the intervertebral fusion implant on the end plates 92, 93 of the vertebra 9. The edge area of the top face 33 and of the bottom face 22, later coming to lie in the anterior or posterior area, remains free of the furrows 29, so as not to impair the load-bearing capacity in these areas. A large number of fine grooves 28 are formed in the longitudinal direction and extend from one end face to the other. They have a spacing of approximately 1 mm and likewise serve to provide better anchoring. Moreover, the top face 33 and also the bottom face 22 are provided with a coating of titanium spray 27. It is finely particulate and therefore does not block the grooves 28. This coating promotes the attachment and incorporation of bone substance. It will be noted that the structuring or the coating is not limited just to this embodiment and can equally well be provided in the other embodiments too.

[0083] As can be seen from FIG. 17, the intervertebral fusion implant can have a wedge shape with a wedge angle . It is therefore better suited for implantation between vertebrae that are arranged in a more strongly curved portion of the spinal column. It will be noted that the wedge shape is not limited just to this embodiment and instead can equally well be provided in the other embodiments too.

[0084] A further embodiment of an implantation instrument 8 is shown in FIGS. 18 to 20. In terms of its front area designed to receive the intervertebral fusion implant, is substantially identical to the instrument shown in FIG. 10. Similar components have the same reference numbers and to this extent require no explanation. The instrument 8 is designed as a parallel expander. It is actuated like forceps, the spreading jaws 81, 81 being moved rectilinearly away from each other. A particular aspect is that the instrument 8 has a cross lever 89, with which a reverse kinematics system is formed. It functions in the sense that, when the instrument 9 is pressed together in the rear grip area, the spreading jaws 81, 81 in the front area move away from each other. This greatly simplifies handling. Otherwise, a mechanism with a draw key 82 is provided for the expansion, as also in the instrument shown in FIGS. 9 and 10.

[0085] Moreover, the implantation instrument 8 has a limit stop mechanism 87. The latter limits the adjustment path of the draw key 82 and, consequently, the expansion stroke. For this purpose, a rotary wheel 88 is provided at the rear end of the instrument 8 and can be adjusted to predefined positions. Depending on the position, it forms a limit stop 87 that protrudes to a greater or lesser extent and that is provided for the tensioning member 80, which is actuated via a trigger lever 80. To obtain expansion of the implant after it has been inserted, the surgeon only needs to actuate the trigger lever 80, specifically until the rear end of the tensioning member 80 bears with its rear end on the limit stop 87 that is adjustable via the rotary wheel 88. The implant is thus expanded in the desired manner. Markings 86 arranged on the rotary wheel indicate the respective adjustable expansion positions. By adjusting the handwheel 88 to the desired expansion position, it is automatically ensured that the implantation instrument 8 stops when this position is reached. The danger of overexpansion, with the resulting risks to the patient, is thus eliminated.

[0086] A variant of the above-described implantation instrument is shown in FIGS. 21 to 23. Components similar to those of the instrument according to FIG. 18 have the same reference signs and to this extent require no explanation. The instrument 8 has a housing 99 on which a grip and a bracket for mounting the trigger lever 80 are arranged. Moreover, a force-measuring device 94 and a displacement-measuring device are provided. The force-measuring device 94 comprises a sleeve 96 and a barrel spring 98. The barrel spring 98 lies with one end on the rear edge 96 of the sleeve 96 and is supported with its other end on a disk 80 which is arranged on the rearward end of the tensioning member 80. Arranged on the sleeve is a scale 94 which, with the front end of the housing 99 as reference, indicates the force applied via the trigger lever 80. Moreover, a depth scale 85 is provided on one of the spreading jaws 81 in order to define the depth of insertion of the spreading jaws.

[0087] For the displacement-measuring device 95, a displacement scale 97 is formed on a viewing hole 99 of the housing 99. It cooperates with a marking 95 on the circumference of the tensioning member 80 and thus allows the displacement path to be read off directly. The displacement scale 97 is calibrated such that it does not indicate the stroke of the tensioning member 80 but instead the expansion travel of the spreading jaws 81, 81.

[0088] This therefore not only effectively eliminates the danger of overexpansion but also the danger of introducing excessive force, with the resulting risks to the patient.