Patent Publication Number: US-2023134455-A1

Title: Expanding, Conforming Interbody Spacer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 62/736,924, filed Sep. 26, 2018, and U.S. Provisional Application No. 62/751,432, filed Oct. 26, 2018. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to surgical implants, and more particularly to interbody spacers for vertebral implants. 
     2. Background and Related Art 
     In the area of spinal implants, there are certain difficulties that remain unaddressed. In particular, the problems of subsidence, endplate fractures, and stress shielding remain problems that can cause intervertebral implants to fail or to have reduced effectiveness at achieving the desired implant goals. These problems are heightened by the difficulties in properly sizing implants: to ensure that a correctly sized implant is used, the doctor must be careful in selecting among available implants, and there are costs associated with carrying implants of multiple sizes to be available at the time of implant surgery. Accordingly, either the doctor or hospital must incur the cost of purchasing and holding in inventory a large number of implants of varying sizes to ensure that a correctly sized implant is available, or they must have a reduced number of implant sizes with the risk that an implant of the correct size will not be available, such that an incorrectly sized implant must be used with reduced effectiveness. 
     Additionally, depending on surgeon experience, it may be difficult for the surgeon to select among available implant sizes an implant of ideal size, and some trial-and-error efforts may be used to select among available implant sizes. Where this is done, however, either incorrectly sized, but tried, implants are contaminated and wasted, or are required to pass through a sterilization process before being reused, if even possible. Accordingly, such trial-and-error efforts result in increased costs to the surgeon and/or hospital, which must then be passed on to patients. 
     Even when surgeons are able to use correctly sized implants, such implants still rarely have proper physical characteristics to promote bone ingrowth and to minimize problems with subsidence, endplate fracture, and/or stress shielding. Current implants are rarely shaped to conform to the endplates where they are placed. Additionally, current implants typically have stiffnesses that are significantly different from the stiffness of the vertebral endplates where they are placed, such that any nonconformities between the endplates and the implant lead to locations of increased stress and implant failure. 
     Accordingly, for reasons such as these, existing interbody implants fail to satisfactorily meet the requirements desired by surgeons and patients. 
     BRIEF SUMMARY OF THE INVENTION 
     Implementations of the invention provide expandable, conformable interbody spacers, methods for manufacturing interbody spacers, and methods for using interbody spacers. In accordance with certain implementations of the invention, an expandable, conformable interbody implant includes a frame, a first plurality of endplate-contacting segments adapted to extend in a superior direction from the frame, a second plurality of endplate-contacting segments adapted to extend in an inferior direction from the frame and a locking mechanism adapted to lock the first plurality of endplate-contacting segments and the second plurality of endplate-contacting segments in a variety of extended positions. 
     In some implementations, the first plurality of endplate-contacting segments is adapted to contact and collectively conform to an inferior endplate of a first vertebral body and wherein the second plurality of endplate-contacting segments is adapted to contact and collectively conform to a superior endplate of a second vertebral body. In some implementations, a load between the inferior endplate and the anterior endplate is substantially equally distributed among the first and second pluralities of endplate-contacting segments. 
     In some implementations, the locking mechanism exerts a lateral compression force among the first and second pluralities of endplate-contacting segments. In some implementations, the locking mechanism exerts a lateral compression force between the first plurality of endplate-contacting segments, the second plurality of endplate-contacting segments, and a plurality of cross webs. 
     In some implementations, the first and second pluralities of endplate-contacting segments have a limited amount of lateral motion within the frame before the locking mechanism is engaged to lock the first and second pluralities of endplate-contacting segments in their extended positions. In some implementations, when the first and second pluralities of endplate-contacting segments are in a retracted position, the implant has a smaller vertical profile for insertion. 
     In some implementations, the first and second pluralities of endplate-contacting segments are each interlocked with adjacent segments while permitting relative superior-inferior motion therebetween. In some implementations, the first and second pluralities of endplate-contacting segments each include a plurality of segments extending along a length of the implant. In some implementations, the first and second pluralities of endplate-contacting segments each include a plurality of segments extending across a width of the implant. 
     In some implementations, the first and second pluralities of endplate-contacting segments have a stiffness approximating the stiffness of vertebral bone. In some implementations, the first and second pluralities of endplate-contacting segments have a coil pack construction. 
     In some implementations, the implant is formed of biocompatible substances. 
     In some implementations, the implant includes an expansion mechanism adapted to apply a superior-directed force to each of the first plurality of endplate-contacting segments and an inferior-directed force to each of the second plurality of endplate-contacting segments before the locking mechanism is engaged. In some implementations, the expansion mechanism is adapted to continue providing the superior-directed force and the inferior-directed force while the locking mechanism is engaged. In some implementations, the expansion mechanism includes a bladder disposed in an internal cavity of the implant. In some implementations, the expansion mechanism is a mechanism such as a bladder, a plurality of corrugated layers adapted to be moved between nested and offset positions, a plurality of springs, a wire disposed on a plurality of pulleys, a plurality of threaded cylinders, or a plurality of dimpled layers adapted to be moved between nested and offset positions. 
     In some implementations, the frame includes openings on opposite ends thereof to permit access to an internal space of the implant. In some implementations, the implant is adapted to permit application of increased forces in any of an anterior area, a posterior area, a right lateral area, or a left lateral area. 
     According to further implementations of the invention, a method for using an expanding, conforming interbody implant, includes a step of affixing an expanding, conforming interbody implant to an inserter, the implant including a frame, a first plurality of endplate-contacting segments adapted to extend in a superior direction from the frame, a second plurality of endplate-contacting segments adapted to extend in an inferior direction from the frame, and a locking mechanism adapted to lock the first plurality of endplate-contacting segments and the second plurality of endplate-contacting segments in a variety of extended positions. The method also includes steps of placing the implant in a desired location using the inserter while the first and second pluralities of endplate-contacting segments are in a retracted position, supplying a force that causes the first and second pluralities of endplate-contacting segments to extend and generally conform to surfaces above and below the implant, and engaging the locking mechanism to secure the first and second pluralities of endplate-contacting segments in extended and conforming positions. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The objects and features of the present invention will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only typical embodiments of the invention and are, therefore, not to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIGS.  1 A and  1 B  show perspective views of an illustrative implant; 
         FIG.  2    shows a perspective transparent view of an illustrative implant; 
         FIG.  3    shows a perspective partially-transparent view of an illustrative implant; 
         FIG.  4    shows a perspective view of an illustrative implant; 
         FIG.  5    shows a perspective partially-transparent view of an illustrative implant; 
         FIG.  6 A- 6 E  show perspective views of various mechanisms to interlock segments of an implant; 
         FIGS.  7 A- 7 F  show perspective views of various methods for interlocking segments of an implant; 
         FIGS.  8 A and  8 B  show manners to expand an implant; 
         FIGS.  9 A- 9 C  show manners to expand an implant; 
         FIG.  10    shows an exploded view of a representative bladder; 
         FIG.  11    shows a perspective view of a portion of an implant; 
         FIG.  12    shows a perspective view of an implant; 
         FIG.  13    shows a perspective view of an implant; 
         FIGS.  14 A- 14 C  show perspective views of an implant; 
         FIGS.  15 A- 15 C  show perspective views of implants and components of implants; 
         FIGS.  16 A- 16 D  illustrate methods for expanding segments of an implant; 
         FIGS.  17 A- 17 C  illustrate aspects of an implant; 
         FIGS.  18 A- 18 C  illustrate features of certain implants; 
         FIG.  19    illustrates one construction of a block of an implant; 
         FIGS.  20 A- 20 D  illustrate implants and components thereof; 
         FIGS.  21 A- 21 B  illustrate aspects of certain embodiments of an implant; 
         FIGS.  22 A- 22 D  illustrate views of an alternate embodiment of an implant; 
         FIGS.  23 A- 23 C  illustrate mechanisms for locking segments of an implant; 
         FIGS.  24 A- 24 D  illustrate an alternate implant and enlarged views thereof; 
         FIGS.  25 A and  25 B  illustrate aspects of an implant; 
         FIGS.  26 A and  26 B  illustrate aspects of an implant; 
         FIGS.  27 A and  27 B  illustrate aspects of an implant and an implant inserter; 
         FIGS.  28 A and  28 B  illustrate aspects of an implant inserter; 
         FIGS.  29 A and  29 B  illustrate aspects of implant inserters; 
         FIGS.  30 A and  30 B  illustrate aspects of implant inserters; and 
         FIG.  31    illustrates aspects of an implant inserter. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A description of embodiments of the present invention will now be given with reference to the Figures. It is expected that the present invention may take many other forms and shapes, hence the following disclosure is intended to be illustrative and not limiting, and the scope of the invention should be determined by reference to the appended claims. 
     What is needed is an interbody implant with the ability to conform to the endplate shape, thereby minimizing problems of subsidence, endplate fracture, and stress shielding. Such an implant may utilize expanding segmented portions to permit the surfaces of the implant to generally conform to the vertebral endplates above and below the interbody space. Additionally, an interbody implant with an ability to expand reduces the carrying or inventory cost of the hospital and/or surgeon while also reducing the need for trialing by the surgeon. When such implants also include correct stiffness, they further reduce the possibility of subsidence, endplate fracture, or stress shielding. 
     Embodiments of the invention provide expandable, conformable interbody spacers, methods for manufacturing interbody spacers, and methods for using interbody spacers. In accordance with certain embodiments of the invention, an expandable, conformable interbody implant includes a frame, a first plurality of endplate-contacting segments adapted to extend in a superior direction from the frame, a second plurality of endplate-contacting segments adapted to extend in an inferior direction from the frame and a locking mechanism adapted to lock the first plurality of endplate-contacting segments and the second plurality of endplate-contacting segments in a variety of extended positions. 
     In some embodiments, the first plurality of endplate-contacting segments is adapted to contact and collectively conform to an inferior endplate of a first vertebral body and wherein the second plurality of endplate-contacting segments is adapted to contact and collectively conform to a superior endplate of a second vertebral body. In some embodiments, a load between the inferior endplate and the anterior endplate is substantially equally distributed among the first and second pluralities of endplate-contacting segments. 
     In some embodiments, the locking mechanism exerts a lateral compression force among the first and second pluralities of endplate-contacting segments. In some embodiments, the locking mechanism exerts a lateral compression force between the first plurality of endplate-contacting segments, the second plurality of endplate-contacting segments, and a plurality of cross webs. 
     In some embodiments, the first and second pluralities of endplate-contacting segments have a limited amount of lateral motion within the frame before the locking mechanism is engaged to lock the first and second pluralities of endplate-contacting segments in their extended positions. In some embodiments, when the first and second pluralities of endplate-contacting segments are in a retracted position, the implant has a smaller vertical profile for insertion. 
     In some embodiments, the first and second pluralities of endplate-contacting segments are each interlocked with adjacent segments while permitting relative superior-inferior motion therebetween. In some embodiments, the first and second pluralities of endplate-contacting segments each include a plurality of segments extending along a length of the implant. In some embodiments, the first and second pluralities of endplate-contacting segments each include a plurality of segments extending across a width of the implant. 
     In some embodiments, the first and second pluralities of endplate-contacting segments have a stiffness approximating the stiffness of vertebral bone. In some embodiments, the first and second pluralities of endplate-contacting segments have a coil pack construction. 
     In some embodiments, the implant is formed of biocompatible substances. 
     In some embodiments, the implant includes an expansion mechanism adapted to apply a superior-directed force to each of the first plurality of endplate-contacting segments and an inferior-directed force to each of the second plurality of endplate-contacting segments before the locking mechanism is engaged. In some embodiments, the expansion mechanism is adapted to continue providing the superior-directed force and the inferior-directed force while the locking mechanism is engaged. In some embodiments, the expansion mechanism includes a bladder disposed in an internal cavity of the implant. In some embodiments, the expansion mechanism is a mechanism such as a bladder, a plurality of corrugated layers adapted to be moved between nested and offset positions, a plurality of springs, a wire disposed on a plurality of pulleys, a plurality of threaded cylinders, or a plurality of dimpled layers adapted to be moved between nested and offset positions. 
     In some embodiments, the frame includes openings on opposite ends thereof to permit access to an internal space of the implant. In some embodiments, the implant is adapted to permit application of increased forces in any of an anterior area, a posterior area, a right lateral area, or a left lateral area. 
     According to further embodiments of the invention, a method for using an expanding, conforming interbody implant, includes a step of affixing an expanding, conforming interbody implant to an inserter, the implant including a frame, a first plurality of endplate-contacting segments adapted to extend in a superior direction from the frame, a second plurality of endplate-contacting segments adapted to extend in an inferior direction from the frame, and a locking mechanism adapted to lock the first plurality of endplate-contacting segments and the second plurality of endplate-contacting segments in a variety of extended positions. The method also includes steps of placing the implant in a desired location using the inserter while the first and second pluralities of endplate-contacting segments are in a retracted position, supplying a force that causes the first and second pluralities of endplate-contacting segments to extend and generally conform to surfaces above and below the implant, and engaging the locking mechanism to secure the first and second pluralities of endplate-contacting segments in extended and conforming positions. 
     Existing interbody implant designs have at most one or two moving elements, allowing at best for two points of adjustment (e.g., height and lordosis). The innovative designs of embodiments of the implant  10  discussed herein use multiple height-independent segments  12  to conform to individuals&#39; endplate shape, as is illustrated in  FIGS.  1 A and  1 B .  FIG.  1 A  illustrates one embodiment of the implant  10  having a plurality of height-independent segments  12  divided along the length of the implant. The illustrated embodiment shows how segments  12  are placed on both upper and lower surfaces of the implant  10  to allow conformity to both superior and inferior endplates of the intervertebral space. 
       FIG.  1 B  illustrates a portion of another embodiment of the implant  10  having a plurality of height-independent segments  12  divided along both the length and width of the implant. While not shown in this embodiment in  FIG.  1 B , the full implant  10  would have such segments  12  on both the upper and lower surfaces of the implant  10  to permit implant conformity to both the superior and inferior endplates of the intervertebral space. 
     Even where prior adjustable-height implants provided some adjustability for height or lordosis, the mechanisms for such adjustability had significant downsides. In particular, it was typical for such implants to use the same mechanical feature for lifting the implant or adjusting the height as for holding and carrying the patient load. This mechanical feature might be a ramp, wedge, or the like, but tended to collect the load to a very small portion of the implant, requiring it to be extremely strong and stiff, leading to subsidence and stress shielding. 
     Embodiments of the invention separate the conforming mechanism (the lift mechanism) from the shape-locking or height-locking mechanism. This separation allows the implant to have a reduced stiffness in the biological load path, thereby allowing the implant  10  to more-closely approximate the stiffness of bone. As illustrated in the embodiment shown in  FIG.  2   , in some embodiments, the segments  12  are lifted or separated into conformance with the vertebrae by a lift mechanism such as an inflatable bladder (not shown). Once the segments  12  are at the desired location (e.g., achieving a desired height and/or lordosis), a frame  14  of the implant  10  surrounding the segments is translated relative to the segments  12 , such that teeth  16  of the lateral edges of the segments  12  engage corresponding teeth on portions of the inside surface of the frame  14 , thereby providing multiple less-rigid load paths between the opposing segments  12  (e.g., in the superior-inferior axis). If desired in some embodiments, the lift mechanism may then be removed from the implant  10 . 
     As another example, as illustrated in  FIG.  3   , in alternate embodiments, each segment  12  has a strut (indicated by the arrows) along each side thereof. The struts of adjacent segments  12  interdigitate with struts of segments  12  on the other side of the implant  10  (superior struts adjacent to inferior struts). A light clamping force through the stack of struts (e.g., in the lateral axis), the conformed shape is locked and superior-inferior loads are transmitted locally from endplate to endplate, rather than being collected into a single overly rigid structure. 
     By providing a design where forces are transmitted through the implant  10  in as many paths as possible, rather than by collecting forces into a single rigid frame structure, stress shielding is reduced. This is illustrated in  FIGS.  5  and  6 A- 6 E , which illustrate how embodiments of the implant  10  permit the distribution of load along a variety of load paths  18  from the conforming superior surface of the implant  10  to the conforming inferior surface of the implant  10 . 
     Embodiments of the invention embrace the use of additive manufacturing techniques (e.g., 3D printing) that allow achievement of various design objectives, including the manufacture of interlocking segments  12  that remain interlocked but permit some measure of sliding relative to each other such that the individual segments  12  can conform to the vertebral endplates. In some embodiments, other than additive manufacturing techniques are used to manufacture some or all of the implant  10 , including the segments  12  and/or the frame  14 . In other embodiments, additive manufacturing techniques are used to manufacture both the segments  12  and the frame  14 . Accordingly, embodiments of the invention are not limited to a single manufacturing technique. 
       FIGS.  6 A- 6 E  illustrate various illustrative embodiments of manners in which adjacent segments  12  may be made interlocking while allowing a certain amount of sliding motion relative to each other in a generally superior-inferior direction. Other manners of providing slidable interlocking have been illustrated in  FIGS.  1 - 5   , and the manners illustrated in the Figures are not intended to be exhaustive, but illustrative of manners in which slidable interlocking of adjacent segments may be achieved. 
     As illustrated in  FIG.  6 A , adjacent segments  12  of one type of embodiment are joined to different elements of nested coils, allowing relative translation, but preventing lateral separation of segments  12 . As illustrated in  FIG.  6 B , adjacent segments  12  of another type of embodiment are joined by pin-slot features  22  that again allow relative translation but prevent lateral separation of segments  12 . As illustrated in  FIG.  6 C , adjacent segments  12  of another type of embodiment are joined by a grid of torsion bars  24  that also help balance the load across the implant surface. As illustrated in  FIG.  6 D , adjacent segments  12  of another type of embodiment include small-diameter coils  26  linked by larger-diameter coils  28  to form an implant surface, as illustrated in more detail in  FIGS.  17 A- 17 C . As illustrated in  FIG.  6 E , adjacent segments  12  of another type of embodiment are joined by compliant flexures  30 . 
       FIGS.  7 A- 7 F  illustrate various alternative illustrative embodiments of manners in which adjacent segments  12  may be made interlocking while providing a certain amount of sliding motion relative to each other in a generally superior-inferior direction.  FIG.  7 A  illustrates one way in which a surface of the implant  10  may be formed of interlocking chain mail links  32  in some embodiments.  FIG.  7 B  illustrates one way in which segments  12  may be formed to have interlocked bars  34  in certain embodiments.  FIG.  7 C  illustrates one way in which adjacent segments  12  may have articulating joints  36  in some embodiments.  FIG.  7 D  illustrates one manner in which adjacent segments  12  may be formed with dovetail joints  38  in some embodiments.  FIG.  7 E  illustrates one way in which adjacent segments  12  may be formed with interlocking shapes  40  in certain embodiments.  FIG.  7 F  illustrates on manner in which adjacent segments  12  may be formed with corresponding T-slots  42 . 
     As discussed previously, prior expandable interbody implants cannot conform effectively because they typically have a single lift mechanism (e.g., a ramp, a wedge, or the like) that performs all the lifting at a single point. Embodiments of the present invention, however, provide lifting at multiple points to achieve conformance with the shape of the vertebral endplates. In certain embodiments of the invention, the lift mechanism is configured to apply equal lift force or bone contact pressure at all segments  12 . 
     In certain embodiments, the lift mechanism includes an inflatable balloon or bladder (similar to a kyphoplasty balloon) temporarily or permanently disposed within a central cavity of the implant. After the implant  10  is placed in the vertebral space, the inflatable balloon or bladder is inflated until the segments  12  contact the endplates of the vertebrae, and additional inflation may be provided to achieve additional height and/or lordosis. Then, the adjustment of the segments  12  is locked, such as using one of the methods discussed herein, and the balloon or bladder may be deflated and potentially removed from the implant. 
       FIGS.  8 A and  8 B  illustrate alternate methods for providing distributed lifting to segments  12  of embodiments of the implant  10 . According to the method illustrated in  FIG.  8 A , a tension wire, such as a nitinol wire  44  acts on pulleys  46  on opposite-facing segments  12 , such that the segments  12  can lift with equal force per segment  12 . In another type of embodiment, illustrated in  FIG.  8 B , the implant  10  is built up from corrugated layers  48 , where each layer  48  is allowed to conform by being springy. In such embodiments, a collapsed height is achieved when alternating layers are shifted and allowed to nest, and a raised height is achieved by shifting the alternating layers to an offset position such as is shown in  FIG.  8 B . 
       FIGS.  9 A- 9 B  illustrate certain alternate methods for providing distributed lifting to the various segments  12  of the implant  10 . In embodiments such as illustrated in  FIG.  9 A , a central plate  50  has slits which pinch on rails on each segment  12 . As the central plates  50  are separated, the attached segments  12  move with the central plates  50 ; however, when the force on any segment  12  exceeds the friction generated by the pinch force (e.g., when that segment  12  contacts the vertebral plate with sufficient force), that segment  12  stops traveling. Other segments continue traveling until a conformed shape and distributed load have been achieved, wherein positions of the segments are locked as previously discussed. 
     In embodiments such as illustrated in  FIG.  9 B , stacks of diamond- or almond-shaped springs  52  may be provided in the implant  10 . When the springs  52  are forced into each other laterally, they will expand vertically. A single force applied to the end of the stack causes all the stacks to experience an expansion and lift. In embodiments such as illustrated in  FIG.  9 C , each segment  12  may be provided with its own spring  54  to lift it into contact with the bone of the vertebral endplate. In some such embodiments, the segments  12  are held in a retracted position, compressing the springs  54 , until the implant  10  is in a desired position, after which the segments  12  can be released to allow the springs  54  to cause the segments to expand and lift, thereby conforming to the vertebral endplates. 
       FIG.  10    illustrates in exploded view one embodiment of a shape-controlled bladder or balloon (hereafter bladder  56 ) adapted to provide lift or separation to the segments  12  of the implant  10 . In this and similar embodiments, the bladder  56  is formed of multiple layers that allow the bladder  56  to expand vertically (in the superior-inferior direction) while self-constraining against unwanted lateral expansion, which would put unnecessary loads on the frame  14 . Kyphoplasty balloons tend to expand equally in all directions, but for an expanding interbody implant, it would be more desirable for the bladder  56  to only (or largely only) expand vertically. In the illustrated embodiment, the bladder  56  is formed of various layers that are bonded on alternate edges (inner and outer edges) to form a bellows-like construction, and the layers have internal reinforcement that prevents or reduces lateral expansion. In alternate embodiments, other internal reinforcements prevent or minimize lateral expansion. 
     Initially, the bladder  56  is sized to fit in a flat, rectangular cavity. In some embodiments, the bladder  56  is designed to receive two cycles of inflated pressure of approximately 400 pounds per square inch (psi) (approximately 2,800 kilopascals (kPa)) for five minutes each, or approximately 200 psi (approximately 1,400 kPa) for one hour. The bladder  56  of some embodiments is flexible enough to be removed from an approximately 0.150 inch to approximately 0.170 inch (approximately 3.81 to approximately 4.32 mm) hole. The dimensions of the cavity will vary based on implant footprint and height, but in one illustrative embodiment, the cavity has dimensions approximately as follows (prior to inflation of the bladder  56 ): a length of approximately 0.743 inches (approximately 18.9 mm), a width of approximately 0.308 inches (approximately 7.82 mm), and a height of approximately 0.036 inches (approximately 0.914 mm). The access hole for the implant  10  in this illustrative embodiment may be approximately 0.170 inches in diameter (approximately 4.32 mm in diameter), and a feed tube for the bladder  56  may be approximately 0.105 inches in diameter (approximately 2.67 mm in diameter). 
     In some embodiments, the implant  10  is configured to provide an adjustable base height for the implant, with conformability at a selected height. An example of such an embodiment is illustrated at  FIGS.  11 - 13   .  FIG.  11    shows that the embodiment of the implant includes two frame halves  60 . Each frame half  60  carries a set of static blocks  62  which move with the frame half  60  in superior/inferior direction, but are able to slide laterally in the frame half  60 . The implant  10  also includes height bars  64  that include ramps that interact with ramps on the frame halves to increase and/or measure the base height of the implant  10 . 
       FIG.  12    shows additional components of the implant assembly, namely moving blocks  66 . The internal bladder  56  (not shown) causes the moving blocks  66  to conform to the vertebral endplates. The internal bladder  46  may also contribute to the force causing the increase in the base height of the implant. Once conformity to the endplate has been achieved, end screws  68  are tightened to compress the static and moving blocks together and lock the conformed shape. Thereafter, the bladder  56  may be removed and replaced with a compliant core, as illustrated in  FIG.  13   . In some embodiments, the height bars  64  may be released to allow the implant  10  to settle onto the compliant core (e.g., the static blocks  62  rest on the compliant core and transfer forces therethrough). 
       FIGS.  14 A- 14 C  illustrate another type of embodiment of the implant  10 , this embodiment being a multi-piston embodiment that has a compliant/porous frame  70 . The frame  70  is fitted with multiple cross-members  72 , as illustrated in  FIG.  14 A . The cross members  72  are able to translate a short distance laterally within the frame  70 . The implant  10  also includes displacement-limited pistons  74  between the cross members  72 . The bladder  56  (not shown) is inserted between the upper and lower layers of pistons  74 . Under pressure from the bladder  56 , the pistons  74  move outward to conform to the vertebral endplate and provide a distraction force, as illustrated in  FIG.  14 C . ( FIG.  14 C  does not illustrate movement of the lower pistons  74 , but such pistons  74  would be present and move as well.) Once movement of the pistons  74  is complete and to be locked, screws  76  (or some other mechanism) may be actuated to compress the cross members  72  and pistons  74  to frictionally lock the conformed shape and height. The inserter tool and bladder  56  are removed and the implant  10  is post-packed with bone graft material, etc., if desired. 
     In additional embodiments, the implant  10  includes multiple compliant segments  12  supported by fingers  78 , as illustrated in  FIG.  15 A . The fingers  78  of upper segments  12  slide on fingers  78  of lower segments  12  on adjacent surfaces. Multiple segments  12  lock together in a full implant, as shown in  FIGS.  15 B and  15 C  (illustrating varying embodiments of the implant  10 ). During implantation, the bladder  56  causes the segments  12  of the implant  10  to assume the conformed shape and provides a distraction force. Then, screws  80  or some other locking mechanism is actuated to compress all fingers  78  together (e.g., through the lateral load path illustrated in  FIG.  15 C ) to frictionally lock the fingers  78  and thus the segments  12  together to maintain the conformed shape and height. The bladder  56  is removed and post packing occurs, if desired. 
     In alternate embodiments of the implant, something other than the bladder  56  is used as a lift mechanism. In some embodiments, a central area of the implant  10  is filled with a biocompatible but extremely hydrophilic material. After implantation, a saline solution is applied to the hydrophilic material such that the material swells at a certain pressure to cause the segments  12  of the implant to conform and lift in a manner similar to the manners illustrated and described herein. 
       FIGS.  16 A- 16 C  illustrate other mechanisms that may be used with embodiments of the invention.  FIG.  16 A  illustrates a threaded cylinder segment  82  with portions containing a right-hand thread and other portions containing a left-hand thread. A simple rotation causes the cylinder segment  82  to simultaneously apply upward and downward forces, and can also cause locking. Multiple instances of the cylinder segment  82  would be used in each implant  10 . 
       FIGS.  16 B- 16 D  illustrate how dimpled moveable layers  84  can be interleaved and nested then translated relative to each other to provide height control. By selecting and zoning the location of dimples to different areas of the moveable layers  84 , the implant  10  may be provided with height control of left/right as well as anterior/posterior areas of the implant  10  as the surgeon may desire during implantation. Static layers  86  of the implant  10  may be toothed together, as illustrated in  FIG.  16 D  to preserve the implant footprint across variations in implant height. 
       FIGS.  17 A- 17 C  illustrate an example of embodiments in which interlocking coils form the conforming surfaces of the implant  10 . In this type of embodiment, interlocking small-diameter coils  26  and large-diameter coils  28  are created in upper sets  88  and lower sets  90 . The upper sets  88  and the lower sets  90  are located in a dual-slot frame  92 , with the small-diameter coils  26  of the upper sets  88  interdigitated with the small-diameter coils  26  of the lower sets  90 . The implant  10  is inserted while in the position shown in  FIG.  17 A , then the upper sets  88  and the lower sets  90  are expanded in a fashion similar to that disclosed herein, and the height is locked by compressing (laterally) the interdigitated smaller-diameter coils  26 , as shown in  FIG.  17 B .  FIG.  17 C  shows the interlocking coils in more detail. The interlocking coils can be extended in their conformability by not typing the multiple leads of the same coils together, which leads to multiple nested structures which each have their own compliance and can translate on each other. 
       FIG.  18 A  illustrates how in some embodiments, a coil structure  94  could be disposed laterally instead of vertically (as illustrated in  FIGS.  17 A- 17 C ) to create a conformable surface.  FIG.  18 B  illustrates that in some embodiments, an implant  10  includes a plurality of rings  96  tuned to have a correct stiffness.  FIG.  18 C  illustrates that where a ramp mechanism  98  is used to cause height increases of the implant  10  (thereby reducing the inventory/carrying cost for carrying implants of varying height), and where space does not permit the ramp mechanism  98  to give the implant full height in a single stroke, shims  100  can be inserted between strokes to increase the height variability of the implant  10 . 
       FIG.  19    illustrates one manner of constructing the compliant blocks of the embodiments illustrated in  FIGS.  11 - 13   . The manner of constructing illustrated in  FIG.  19    may be used for both the static blocks  62  and the moving blocks  66 . The construction method uses multiple nested coils as described in U.S. Patent Application Publication No. 2017/0156880 to Halverson and Hawkes, published on Jun. 8, 2017, which is incorporated herein by reference for all it discloses. The diameter of the nested coil structure illustrated in  FIG.  19    may be smaller than that disclosed in the prior publication so as to improve spatial density and coil stability in a small structure. 
       FIG.  20 A  illustrates an alternate type of embodiment of the implant  10 . In this type of implant  10 , the stack of segments  12  is lightly loaded with a clamping force. The lifting mechanism incrementally lifts each segment  12  to a given height, then the lifting mechanism is withdrawn. Any segments  12  experiencing a load greater than the frictional force exerted by the initial clamping mechanism will then retreat until other segments  12  come into contact with the bone and the load is evenly distributed over the segments  12 . The clamping force is then increased to a final value to fix the shape and height of the implant  10  and its segments  12 . Post packing of bone graft material, etc., may then occur as desired. 
       FIGS.  20 B- 20 D  illustrate another type of embodiment, in which interdigitation is extended with one or more middle layers  102  such that the segments  12  on each side of the implant  10  can move out farther and still be shape locked. In such embodiments, there are two clamping paths, as illustrated by the arrows shown in  FIG.  20 C . In some embodiments, the clamping paths can be varied in orientation, as illustrated by the arrows of  FIG.  20 D . 
     In some embodiments, a sliding caliper could be used to measure the endplate shape and build a custom implant  10  out of compliant segments of a correct height, as illustrated in  FIG.  21 A . In some embodiments, a special inserter could be used to assemble the implant  10  in situ, thereby retaining the small-access benefits of the implant  10  being expandable. 
       FIG.  21 B  illustrates that in some embodiments, the screw-based mechanism for locking stacks of blocks or fingers can be replaced with cams, ramps, or wedges. 
       FIGS.  22 A- 22 D  illustrate a type of embodiment where the implant  10  is formed of multiple compliant layers  104  that clamp against each other and against a ground layer to maintain an arched/bridged/conformed shape.  FIGS.  22 A- 22 C  each illustrate a configuration of alternate layers  104 , and  FIG.  22 D  illustrates an embodiment of the assembled implant  10 . 
       FIGS.  23 A- 23 C  illustrate alternate types of clamping mechanisms (other than screws such as end screws  68  or screws  76  or  80 ) to permit locking of the height of the implant  10 . In the illustrative embodiment of  FIG.  23 A , the implant  10  is provided with a face thread  106  instead of a male-female thread to avoid the radial space loss of the thread overlap area. Such an embodiment provides for a larger driver and better access for the bladder  56  and post packing with graft material than a single male-female thread. The face thread  106  can be left handed on one face and right handed on the other face to reduce the ramp angle of the thread and thus reduce frictional losses. Multiple starts are possible to save space as well. 
     In some embodiments, as is illustrated in  FIGS.  23 B and  23 C , a conventional threaded locking mechanism can be replaced by a quarter-turn mechanism which always locks to the same kinematic position, thereby saving the surgeon the trouble of worrying that he or she didn&#39;t tighten the screws enough or that some factor caused a drive to torque out too early. 
     As discussed above, embodiments of the implant may be manufactured using additive manufacturing methods. In such embodiments, clearance between adjacent parts is tuned such that the implant  10  can be manufactured (e.g., printed) as an assembled unit without having adjacent surfaces fuse.  FIGS.  24 A- 24 C  illustrate considerations that may be used in determining clearances between adjacent parts during fabrication.  FIG.  24 A  shows an embodiment of the implant  10 .  FIG.  24 B  shows a close-up view of the implant  10  of  FIG.  24 A , showing that clearance will be considered between a segment  12  and its containing pocket, between a segment  12  and its adjacent segments  12 , between a segment  12  and any travel limiters, between a cross bar and the frame  14  of the implant  10 , and between the frame  14  and an end bar. The clearances required may be different at each location. 
     For improved avoidance of component fusion between adjacent parts during 3D printing, the implant  10  can be designed with a separate end portion  108 , as shown in  FIG.  24 C . The implant  10  is printed without the end plate  108  and with the segments  12  and cross webs spaced out. After support removal, the end plate  108  can be inserted and held in place by mechanical means or by welding or bonding, as also illustrated in  FIG.  24 C . In some embodiments, the end plate can be fabricated in an inverted or arc shape that comes into a desired planar shape when loaded by the locking mechanism force, thereby distributing load evenly across the back of the segment pack, as illustrated in an exaggerated manner in  FIG.  24 D . 
     The implant  10  of some embodiments is designed with angled surfaces to facilitate self-supporting 3D printing. The implant  10  of some embodiments is also designed with droop-reducing or droop-compensating features. Additionally, the implant  10  of some embodiments includes segments  12  with minimum-area internal horizontal surfaces to minimize the amount of support material required during 3D printing. These features are illustrated in the view of FIG.  25 A. The implant frame  14  of some embodiments, as illustrated in the view of  FIG.  25 B , has openings  110  at both ends. The openings  110  permit the inner cavity to be accessed from either side, increasing ease of support removal and also make it possible to install the bladder  56  into the inner cavity by a pull-through approach rather than trying to push it in from one end. 
       FIGS.  26 A and  26 B  illustrate one embodiment of cross webs  112  used in embodiments of the implant  10  to support the individual segments  12  and to stabilize the segments  12  when compressed to keep the segments  12  at their set heights. The cross webs  112  of this embodiment are planar in nature and have bosses to receive and stabilize the segments. This reduces the stroke required of the locking mechanism. The cross webs  112  are joined top and bottom for direct load transfer in the superior-inferior direction rather than sharing load through the frame  14 . In this way, there are no support points on the frame that can slip off. As more-clearly shown in  FIG.  26 B , coils  114  of some embodiments are grouped in pairs to provide rotational stability without causing excessive loss of shape-matching ability. 
     In some embodiments, the plate that compresses the segment stack and the frame  14  are each female threaded with a slotted thread such that they can both simultaneously engage a locking screw having both left and right hand threads. This is advantageous in that the required axial length is reduced and the screw (an embodiment of which is illustrated in  FIG.  27 A ) has positive control over the return of the compression plate instead of relying on the spring force of the segment stack. 
       FIG.  27 B  illustrates one embodiment of an inserter end  116  adapted for securing and inserting the implant  10  into the intervertebral space. The inserter end  116  includes sets of opposed claws  118  that engage pockets on the surface of the implant  10 . The claws  118  are able to flex inwardly as they enter the pockets due to slits  120  in the inserter end  116  that provide compliance and flexibility to the claws  118 . Once the claws  118  are fully engaged in the pockets, a center portion  122  of the inserter is advanced such that the claws  118  can no longer collapse inwardly to release the implant  10 . The implant  10  is thus retained until the center portion  122  is withdrawn. 
     Because multiple items (the feed tube of the bladder  56 , the locking driver, and the claw expander) have to fit through the inserter end  116 , radial space is at a premium. Accordingly, in some embodiments, driving interfaces that can transmit relatively large torques while occupying relatively little radial space are used.  FIG.  28 A  shows one embodiment that uses a triple-square drive  124 . Hex, Torx, TorxPlus, or some variation thereof are used in alternate embodiments, as is a driver having castellations on the face thereof. 
       FIG.  28 B  shows a perspective view of one embodiment of an inserter  126  (with the inserter end  116  omitted). To permit proper placement of the implant  10 , it is important to be able to hammer on the back of the inserter  126  without crushing the feed tube of the bladder  56  and to rotate the driver to activate the implant shape-locking mechanism without twisting up the feed tube of the bladder  56  or depressurizing the bladder  56 . The inserter  126  of  FIG.  28 B  achieves these objectives by having the top face of the inserter  126  open such that a feed tube  128  can exit from the drive and out to the side without passing through a hammering surface  130 . 
     A thumb wheel  132  engages the driver and allows for initial tightening of the locking mechanism by continuous rotation. For final tightening, a counter-torque is attached to flats  134  of a tail of the instrument and a slotted driver is introduced, still allowing the feed tube  128  to pass and remain under pressure. The slotted driver is limited to a small range of angular motion to prevent the feed tube  128  from being sheared off. Accordingly, final tightening is an incremental process. 
       FIG.  29 A  shows an alternate version of the inserter  126 . In this embodiment, a gearbox  136  is used to move the rotation connection off to the side of the feed tube  128 . A cap for hammering is provided to the back of the inserter  126 . Another solution, as shown in  FIG.  29 B  would utilize an inserter  126  similar to that of  FIG.  28 B , but fits the thumb wheel  132  with a torque-limiting clutch and then uses a wrench  138  of some sort to rotate the thumb wheel to achieve final torque. In the embodiment of  FIG.  29 B , it is important that the clutch not overtighten the locking mechanism, but always be able to unlock it. The clutch face shown in  FIG.  30 A  has different entry and exit angles to the depressions in the race, thus allowing for different release torques in the forwards and backwards directions. 
       FIG.  30 B  illustrates an alternate version of the inserter  126 . In this embodiment, the inserter  126  has a dual-state handle  140  to protect the feed tube  128  from hammering when the handle  140  is in a straight position, but providing improved access to a driver  142  when the handle  140  is rotated out of the way. 
     As illustrated in  FIG.  31   , the inserter  126  of some embodiments is designed such that it does not need to be detached from the implant  10  before post packing the implant  10  with bone graft or the like. Instead, after the shape of the implant  10  has been locked, the driver and the bladder  56  can be removed through the internal portion  122  that expands the claws  118  to engage the implant  10 . Some sort of fusion-promoting substance can then be packed into the implant through the lumen of the internal portion  122 . 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims, rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.