Patent Publication Number: US-2023149182-A1

Title: Expandable Intervertebral Spacers

Description:
FIELD 
     The disclosure relates to the field of implantable medical devices. More particularly, the disclosure relates to medical devices suitable for implantation in spaces between bones, such as spaces between the vertebral bodies in a spinal column of a vertebrate. Specific examples relate to expandable intervertebral spacers suitable for implantation between adjacent vertebral bodies of a spinal column. 
     BACKGROUND 
     Bone degeneration can be caused by trauma, disease, and natural processes, such as aging, which can have a negative impact on the lifestyle of an animal. For example, destabilization of a spine in a vertebrate, such as a human being, may result in alteration of the spacing between the adjacent vertebral bodies. This destabilization can place pressure on the surrounding nerves and tissues between the vertebral bodies causing pain, discomfort, and, eventually, nerve damage. 
     Implantation of a medical device into the space between adjacent vertebral bodies is a common and well-accepted clinical approach to alleviating the pain and discomfort caused by the destabilization of the spacing between discs. These medical devices, commonly referred to as intervertebral spacers, spacers, and cages, support the structure of the spine by maintaining a desired spacing and proper angular positioning of the spinal column. 
     Some intervertebral spacers are capable of expanding in situ during initial placement. These expandable intervertebral spacers were originally developed to eliminate the need for multiple trialing associated with placement of static spacers, which could lead to impaction, breakage, and pseudoarthrosis (Frisch R F, Luna I Y, Joshua G., Static versus Expandable Interbody Spacers: Preliminary 1-Year Clinical and Radiographic Results; J. Clin. Neurol. Neurosurg. Spine, 2017; 1(1):113). Expandable intervertebral spacers are inserted in an unexpanded configuration, which has a relatively minimal profile, and are subsequently expanded in situ. While expandable intervertebral spacers have provided benefits as compared to static spacers, known expandable intervertebral spacers have several drawbacks. For example, many known expandable intervertebral spacers have complicated structural configurations that are difficult to manufacture and assemble. Also, the structural elements that enable in situ expansion often occupy valuable space within the body of the spacer itself, which can reduce or even eliminate space needed for placement of bone cement or graft material. 
     A need exists, therefore, for improved expandable intervertebral spacers. 
     BRIEF SUMMARY OF SELECTED EXAMPLES 
     Various example expandable intervertebral spacers are described. 
     An example expandable intervertebral spacer comprises a main body defining a first opening, a first substantially planar lateral surface, a second substantially planar lateral surface, first and second curvilinear lateral surfaces adjacent and continuous with the first substantially planar lateral surface, and third and fourth curvilinear lateral surfaces adjacent and continuous with the second substantially planar lateral surface; a driving member defining a second opening; a first endplate defining a substantially planar bottom surface and first and second curvilinear bottom surfaces adjacent and continuous with the bottom surface; a second endplate defining a substantially planar top surface and first and second curvilinear top surfaces adjacent and continuous with the bottom surface; and an actuation member extending through the first opening and in the second opening; 
     Another example expandable intervertebral spacer comprises a main body defining a first opening, first and second substantially planar lateral surfaces disposed opposite each other with respect to said longitudinal axis, first and second curvilinear lateral surfaces adjacent and continuous with the first substantially planar lateral surface, third and fourth curvilinear lateral surfaces adjacent and continuous with the second substantially planar lateral surface, and an interior chamber bounded by the first and second substantially planar lateral surfaces and the first, second, third, and fourth curvilinear lateral surfaces; a driving member disposed in the interior chamber and defining an opening; a first endplate defining a substantially planar bottom surface and first and second curvilinear bottom surfaces adjacent and continuous with the bottom surface; a second endplate defining a substantially planar top surface and first and second curvilinear top surfaces adjacent and continuous with the bottom surface; and an actuation member extending through the first opening and disposed in the second opening. 
     Another example expandable intervertebral spacer comprises a main body defining a first opening and an interior chamber; a driving member disposed in the interior chamber and defining an opening; a first endplate defining a substantially planar bottom surface and first and second curvilinear bottom surfaces adjacent and continuous with the bottom surface; a second endplate defining a substantially planar top surface and first and second curvilinear top surfaces adjacent and continuous with the bottom surface; and an actuation member extending through the first opening and disposed in the second opening. 
     Additional understanding of the inventive expandable intervertebral spacers can be obtained by reviewing the detailed description of selected examples, below, with reference to the appended drawings. 
    
    
     
       DESCRIPTION OF FIGURES 
         FIG.  1    is a perspective view of a first example expandable intervertebral spacer. The expandable intervertebral spacer is shown in an unexpanded configuration. 
         FIG.  2    is a vertical sectional view of the first example expandable intervertebral spacer. The expandable intervertebral spacer is shown in an unexpanded configuration. 
         FIG.  3    is another perspective view of the first example expandable intervertebral spacer. The expandable intervertebral spacer is shown in an expanded configuration. 
         FIG.  4    is a vertical sectional view of the first example expandable intervertebral spacer. The expandable intervertebral spacer is shown in an expanded configuration. 
         FIG.  5    is a horizontal sectional view of the first example expandable intervertebral spacer. 
         FIG.  6    is a perspective view of the first endplate of the first example expandable intervertebral spacer. 
         FIG.  7    is a perspective view of the second endplate of the first example expandable intervertebral spacer. 
         FIG.  8    is a perspective view of the main body of the first example expandable intervertebral spacer. 
         FIG.  9    is a perspective view of the driving member of the first example expandable intervertebral spacer. 
         FIG.  10    is a partial magnified view of the second endplate of the first example expandable intervertebral spacer. 
         FIG.  11    is a partial magnified view of the driving member of the first example expandable intervertebral spacer. 
         FIG.  12    is a side view of the second endplate and the driving member of the first example expandable intervertebral spacer. The expandable intervertebral spacer is shown in an expanded configuration. 
         FIG.  13    is a magnified view of area A in  FIG.  12    when the expandable intervertebral spacer is in an unexpanded configuration. 
         FIG.  14    is a perspective view of a second example expandable intervertebral spacer. The expandable intervertebral spacer is shown in an unexpanded configuration. 
         FIG.  15    is another perspective view of the second example expandable intervertebral spacer. The expandable intervertebral spacer is shown in an expanded configuration. 
         FIG.  16    is an exploded vertical sectional view of the second example expandable intervertebral spacer. 
         FIG.  17    is a perspective view of a third example expandable intervertebral spacer. The expandable intervertebral spacer is shown in an unexpanded configuration. 
         FIG.  17 A  is a perimeter outline of a sectional view taken along line  17 A- 17 A in  FIG.  17   . 
         FIG.  18    is a vertical sectional view of the third example expandable intervertebral spacer. The expandable intervertebral spacer is shown in an unexpanded configuration. 
         FIG.  19    is another perspective view of the third example expandable intervertebral spacer. The expandable intervertebral spacer is shown in an expanded configuration. 
         FIG.  20    is a vertical sectional view of the third example expandable intervertebral spacer. The expandable intervertebral spacer is shown in an expanded configuration. 
         FIG.  21    is a perspective view of a fourth example expandable intervertebral spacer. The expandable intervertebral spacer is shown in an unexpanded configuration. 
         FIG.  22    is a vertical sectional view of the fourth example expandable intervertebral spacer. The expandable intervertebral spacer is shown in an unexpanded configuration. 
         FIG.  23    is another perspective view of the fourth example expandable intervertebral spacer. The expandable intervertebral spacer is shown in an expanded configuration. 
         FIG.  24    is a vertical sectional view of the fourth example expandable intervertebral spacer. The expandable intervertebral spacer is shown in an expanded configuration. 
         FIG.  25    is a perspective view of a fifth example expandable intervertebral spacer. The expandable intervertebral spacer is shown in an unexpanded configuration. 
         FIG.  26    is another perspective view of the fifth example expandable intervertebral spacer. The expandable intervertebral spacer is shown in an expanded configuration. 
         FIG.  27    is an exploded view of the fifth example expandable intervertebral spacer. 
         FIG.  28    is a perspective view of a sixth example expandable intervertebral spacer. The expandable intervertebral spacer is shown in an unexpanded configuration. 
         FIG.  29    is another perspective view of the sixth example expandable intervertebral spacer. The expandable intervertebral spacer is shown in an expanded configuration. 
         FIG.  30    is an exploded view of the sixth example expandable intervertebral spacer. 
     
    
    
     DESCRIPTION OF SELECTED EXAMPLES 
     The following detailed description and the appended drawings describe and illustrate various example expandable intervertebral spacers. The description and illustration of these examples enable one skilled in the art to make and use examples of the inventive expandable intervertebral spacers. They do not limit the scope of the claims in any manner. 
     Each of  FIGS.  1 ,  2 ,  3 ,  4 ,  5 ,  6 ,  7 ,  8 ,  9 ,  10 ,  11 ,  12 , and  13    illustrates an example expandable intervertebral spacer  1000  or one or more components thereof. The expandable intervertebral spacer  1000  comprises a main body  1100 , a first endplate  1200 , a second endplate  1400 , a driving member  1600 , and an actuation member  1800 . The expandable intervertebral spacer  1000  is movable between a first, unexpanded configuration and a second, expanded configuration. In the first configuration, as illustrated in  FIGS.  1  and  2   , the driving member  1600  is disposed in a first, distal position and each of the first endplate  1200  and second endplate  1400  is disposed in a first position. In the second configuration, as illustrated in  FIGS.  3  and  4   , the driving member  1600  is disposed in a second, proximal position and each of the first endplate  1200  and second endplate  1400  is disposed in a second position. 
     In the second configuration, each of the first and second endplates  1200 ,  1400  is spaced such that the distance between the first and second endplates  1200 ,  1400  has increased as compared to the distance between the first and second endplates  1200 ,  1400  when the expandable intervertebral spacer  1000  is in the first configuration. The expandable intervertebral spacer  1000  moves between the first configuration and the second configuration through rotational movement of the actuation member  1800 , which forces the driving member  1600  to move linearly along a longitudinal axis of the expandable intervertebral spacer  1000 . In the illustrated example, clockwise rotational movement of the actuation member  1800  results in linear movement of the driving member  1600  toward the actuation member  1800 , which forces the first  1200  and second  1400  endplates to move away from the main body  1100 , moving the expandable intervertebral spacer from the first, unexpanded configuration to the second, expanded configuration. As described in detail below, this linear movement of the driving member  1600  forces the first and second endplates  1200 ,  1400  away from each other in opposing directions along an axis transverse to the longitudinal axis of the expandable intervertebral spacer  1000 . Counterclockwise rotational movement of the actuation member  1800  results in linear movement of the driving member  1600  away from the actuation member  1800 , which forces the first  1200  and second  1400  endplates to move toward the main body  1100 , moving the expandable intervertebral spacer from the second, expanded configuration to the first, unexpanded configuration. Although the reverse arrangement is possible, this structural arrangement is considered advantageous. 
     As best illustrated in  FIG.  8   , the main body  1100  defines a wall  1110  that defines an interior chamber  1112 . The wall  1110  defines slots  1114 ,  1116  for slidably receiving posts  1650 ,  1652  defined by the driving member  1600 . 
     As best illustrated in  FIG.  6   , the first endplate  1200  has a first endplate first end  1202 , a first endplate second end  1204 , a lengthwise axis  1201  extending between the first endplate first end  1202  to the first endplate second end  1204 , a first extension  1206 , a second extension  1208 , and a third extension  1210 . A first endplate exterior surface  1212  and a first endplate interior surface  1214  are defined by a first endplate body  1216 . The first endplate  1200  has an axial length  1203  that extends from the first endplate first end  1202  to the first endplate second end  1204 . The first endplate  1200  defines first endplate opening  1218  that extends from the first endplate exterior surface  1212  to the first endplate interior surface  1214 , extending through the entire thickness of the first endplate body  1216  and providing access to the interior of the expandable intervertebral spacer  1000 . First endplate opening  1218  provides a window through which graft material can be introduced before, during, or after placement of the intervertebral spacer  1000  between adjacent vertebral bodies. 
     The first endplate exterior surface  1212  and the first endplate interior surface  1214  lie within a shared plane and, as such, are substantially parallel to each other but directly oppose each other within the structure of the first endplate body  1216 . As illustrated in  FIG.  6   , the first endplate  1200  defines first endplate passageway  1220  that extends from the first endplate exterior surface  1212  to the first endplate interior surface  1214 , extending through the entire thickness of the first endplate body  1216 . When the expandable intervertebral spacer  1000  is in its first configuration, the second endplate third extension  1410  is disposed in the first endplate passageway  1220 . Similarly, when the expandable intervertebral spacer  1000  is in its first configuration, the first endplate third extension  1210  is disposed in the second endplate passageway  1420 . The first endplate exterior surface  1212  can be smooth. Alternatively, the first endplate exterior surface  1212  can define a set of protruding ridges that extend away from the first endplate exterior surface  1212 . In particular embodiments, the first endplate exterior surface  1212  defines a porous and/or abrasive surface, such as a surface that has been roughened after formation or a surface originally formed as a rough surface, such as a surface produced through  3 D-printing. 
     A first protrusion  1270  extends away from the first endplate interior surface  1214  and defines a first plurality of steps  1272 , a second plurality of steps  1274 , and a curvilinear depression  1276  between the first plurality of steps  1272  and the second plurality of steps  1274 . Each of the first  1272  and second  1274  plurality of steps includes a series of steps of increasing length measured on an axis transverse to lengthwise axis  1201 . 
     A second protrusion  1280  defines a third plurality of steps  1282  and a third protrusion  1290  defines a fourth plurality of steps  1292 . Each of the third  1282  and fourth  1292  plurality of steps includes a series of steps of substantially equal length. First endplate opening  1218  is disposed between the second  1280  and third  1290  protrusions and, as a result, between the third plurality of steps  1282  and the fourth plurality of steps  1292 . 
     The second endplate  1400  has a similar structure to the first endplate  1200 , with the second endplate third extension  1410  and the second endplate passageway  1420  positioned on opposite sides relative to the lengthwise axis to allow for structural interaction with the first endplate third extension  1210  and the first endplate passageway  1220 . Thus, the second endplate  1400  has similar structural features referenced with the same numbers as for the first endplate  1200 , increased by 200. As best illustrated in  FIG.  7   , the second endplate  1400  has a second endplate first end  1402 , a second endplate second end  1404 , a lengthwise axis  1401  extending between the second endplate first end  1402  to the second endplate second end  1404 , a first extension  1406 , a second extension  1408 , and a third extension  1410 . A second endplate exterior surface  1412  and a second endplate interior surface  1414  are defined by a second endplate body  1416 . The second endplate  1400  has an axial length  1403  that extends from the second endplate first end  1402  to the second endplate second end  1404 . The second endplate  1400  defines second endplate opening  1418  that extends from the second endplate exterior surface  1412  to the second endplate interior surface  1414 , extending through the entire thickness of the second endplate body  1416  and providing access to the interior of the expandable intervertebral spacer  1000 . Second endplate opening  1418  provides a window through which graft material can be introduced following placement of the intervertebral spacer  1000  between adjacent vertebral bodies. 
     The second endplate exterior surface  1412  and the second endplate interior surface  1414  lie within a shared plane and, as such, are substantially parallel to each other but directly oppose each other within the structure of the second endplate body  1416 . As illustrated in  FIG.  7   , the second endplate  1400  defines second endplate passageway  1420  that extends from the second endplate exterior surface  1412  to the second endplate interior surface  1414 , extending through the entire thickness of the second endplate body  1416 . When the expandable intervertebral spacer  1000  is in its first configuration, the first endplate third extension  1210  is disposed in the second endplate passageway  1420 . Similarly, when the expandable intervertebral spacer  1000  is in its first configuration, the second endplate third extension  1410  is disposed in the first endplate passageway  1220 . The second endplate exterior surface  1412  can be smooth. Alternatively, the second endplate exterior surface  1412  can define a set of protruding ridges that extend away from the second endplate exterior surface  1412 . In particular embodiments, the second endplate exterior surface  1412  defines a porous and/or abrasive surface, such as a surface that has been roughened after formation or a surface originally formed as a rough surface, such as a surface produced through  3 D-printing. 
     A first protrusion  1470  extends away from the second endplate interior surface  1414  and defines a first plurality of steps  1472 , a second plurality of steps  1474 , and a curvilinear depression  1476  between the first plurality of steps  1472  and the second plurality of steps  1474 . Each of the first  1472  and second  1474  plurality of steps includes a series of steps of increasing length measured on an axis transverse to lengthwise axis  1401 . 
     A second protrusion  1480  defines a third plurality of steps  1482  and a third protrusion  1490  defines a fourth plurality of steps  1492 . Each of the third  1482  and fourth  1492  plurality of steps includes a series of steps of substantially equal length. Second endplate opening  1418  is disposed between the second  1480  and third  1490  protrusions and, as a result, between the third plurality of steps  1482  and the fourth plurality of steps  1492 . 
     As best illustrated in  FIG.  9   , the driving member  1600  has a driving member first end  1602 , a driving member second end  1604 , a lengthwise axis  1601  extending between the driving member first end  1602  to the driving member second end  1604 , a driving member interior surface  1615 , a driving member interior chamber  1616 , and a driving member outer surface  1618 . Additionally, the driving member  1600  has an axial length  1620  that extends from the driving member first end  1602  to the driving member second end  1604 . 
     The driving member second end  1604  defines a passageway  1630  that threadably receives actuation member  1800 . 
     The driving member  1600  defines a first plurality of steps  1672 , a second plurality of steps  1674 , and a third plurality of steps  1676  that are continuous with each other. Similarly, the driving member  1600  defines a fourth plurality of steps  1678 , a fifth plurality of steps  1680 , and a sixth plurality of steps  1682  that are continuous with each other. Each of the first  1672 , second  1674 , fourth  1678 , and fifth  1680  plurality of steps includes a series of steps of substantially equal length. 
     The driving member  1600  also defines a seventh plurality of steps  1684 , an eighth plurality of steps  1686 , a ninth plurality of steps  1688 , and a tenth plurality of steps  1690 . Each of the seventh  1684 , eighth  1686 , ninth  1688 , and tenth  1690  plurality of steps includes a series of steps of substantially equal length. Driving member interior chamber  1616  is disposed between the seventh  1684  and ninth  1688  plurality of steps on one side and the eighth  1686  and tenth  1690  plurality of steps on the opposite side. 
     As best illustrated in  FIGS.  2  and  4   , in the assembled expandable intervertebral spacer  1000 , the first  1672 , second  1674 , and third  1676  pluralities of steps of the driving member  1600  interface with the first plurality of steps  1272  and second plurality of steps  1274  of the first endplate  1200 . The fourth  1678 , fifth  1680 , and sixth  1682  pluralities of steps of the driving member  1600  interface with the first plurality of steps  1472  and second plurality of steps  1474  of the second endplate  1400 . The seventh  1684  and eighth  1686  pluralities of steps of the driving member  1600  interface with the third plurality of steps  1282  and the fourth plurality of steps  1292  of the first endplate  1200 . Similarly, the ninth  1688  and tenth  1690  pluralities of steps of the driving member  1600  interface with the third plurality of steps  1482  and the fourth plurality of steps  1492  of the second endplate  1400 . 
     Each of  FIGS.  10 ,  11 ,  12 , and  13    illustrate the structural detail of some of the various pluralities of steps and the structural interaction between interfacing pluralities of steps. For example,  FIG.  10    illustrates the second protrusion  1480  and the third plurality of steps  1482 . Each of the steps in the third plurality of steps  1482  includes a surface that lies on a plane that is disposed at a non-parallel angle to the lengthwise axis  1401  of the second endplate  1400 . Similarly, as illustrated in  FIG.  11   , each step of the seventh  1684  and ninth  1688  pluralities of steps of the driving member  1600  includes a surface that lies on a plane that is disposed at a non-parallel angle to the lengthwise axis of the driving member  1600 . This structural arrangement is considered advantageous at least because, as best illustrated in  FIG.  12    and  FIG.  13   , it increases the resistance required to achieve relative movement between pluralities of steps as compared to structures in which the non-parallel surfaces are disposed on planes parallel to the respective lengthwise axes. This is considered advantageous at least because it increases tactile feedback during transitioning of the expandable intervertebral spacer between non-expanded and expanded configurations while also providing a desirable degree of resistance to transition between expanded and non-expanded configurations. It is noted that, while  FIGS.  10 ,  11 ,  12 , and  13    focus on the structural interaction between the third plurality of steps  1482  of the second endplate  1400  and the ninth plurality of steps  1688  of the driving member  1600 , each plurality of steps in the expandable intervertebral spacer  1000  can include steps with the structural arrangement. Indeed, it is considered advantageous that all steps in the expandable intervertebral spacer  1000  have this structural arrangement so that all interfaces between complimentary pluralities of steps behave in the manner described above. 
     Each of  FIGS.  14 ,  15 , and  16    illustrates another example expandable intervertebral spacer  2000 . The expandable intervertebral spacer  2000  is similar to the expandable intervertebral spacer  1000 , except as described below. Thus, the expandable intervertebral spacer  2000  has a main body  2100 , a first endplate  2200 , a second endplate  2400 , a driving member  2600 , and an actuation member  2800 . The expandable intervertebral spacer  2000  is movable between a first, unexpanded configuration and a second, expanded configuration. 
     In this example, the endplates  2200 ,  2400  lack the passageways and include complimentary extensions. Furthermore, the driving member  2600  includes a divided wall portion on each side of the driving member  2600 . 
     Each of  FIGS.  17 ,  18 ,  19 , and  20    illustrates another example expandable intervertebral spacer  3000 . The expandable intervertebral spacer  3000  is similar to the expandable intervertebral spacer  1000 , except as described below. Thus, the expandable intervertebral spacer  3000  has a main body  3100 , a first endplate  3200 , a second endplate  3400 , a driving member  3600 , and an actuation member  3800 . The expandable intervertebral spacer  3000  is movable between a first, unexpanded configuration and a second, expanded configuration. 
     In this embodiment, a first side of the driving member  3600  defines three step members  3650 ,  3652 ,  3654 . Similarly, the second, opposite side of the driving member  3600  defines three step members (not illustrated in the drawings). The first step member  3650  defines a first plurality of steps  3650   a  and a second plurality of steps  3650   b . Similarly, second step member  3652  defines a first plurality of steps  3652   a  and a second plurality of steps  3652   b  and third step member  3654  defines a first plurality of steps  3654   a  and a second plurality of steps  3654   b . The step members on the second, opposite side of the driving member has an identical structure. 
     In this embodiment, first endplate  3200  defines a series of facets  3250 ,  3252 ,  3254 . Similarly, second endplate  3400  defines a series of facets  3450 ,  3452 ,  3454 . The main body  3100  defines a series of facets  3150 ,  3152 ,  3154  on a first side and a structurally identical series of facets  3170 ,  3172 ,  3174  on the second, opposite side of the main body  3100 . Lateral facets  3150  and  3170 , top facet  3450  and bottom facet  3250  define flat, substantially planar surfaces while lateral facets  3252 ,  3254 ,  3452 ,  3454 ,  3152 ,  3154 ,  3172 ,  3174  define curvilinear surfaces having an outwardly-directed radius with respect to a longitudinal axis of the expandable intervertebral spacer  3000 . As best illustrated in  FIGS.  17 A , the faceted structure of the main body  3100 , first endplate  3200 , and second endplate  3400  cooperatively define a low profile shape for the expandable intervertebral spacer  3000  when it is in the unexpanded configuration. This structural arrangement, with its combination of substantially planar surfaces and curvilinear surfaces extending around the circumference of the expandable intervertebral spacer  3000 , is considered advantageous at least because it provides lateral, top and bottom planar surfaces while also having the ability to fit through an element with a circular cross-sectional profile, such as a distractor used in minimally invasive placement procedures. Also, this structural arrangement, in combination with the three step members on each side of the driving member  3600 , provide critical structure for achieving a desired degree of controlled expansion in a low profile design. 
     As best illustrated in  FIG.  20   , the first  3200  endplate defines a first protrusion  3270  having only a first single step  3272 , a second protrusion  3280  having only a second single step  3282 , and a third protrusion  3290  having only a third single step  3292 . Each of the single steps  3272 ,  3282 , and  3292  interfaces with a mating plurality of steps on the driving member  3600 . Similarly, the second  3400  endplate defines a first protrusion  3470  having only a first single  3472 , a second protrusion  3480  having only a second single step  3482 , and a third protrusion  3490  having only a third single step  3492 . Each of the single steps  3472 ,  3482 , and  3492  interfaces with a mating plurality of steps on the driving member  3600 . Also, each of the single steps  3472 ,  3482 ,  3492  and steps of the mating pluralities of steps on the driving member  3600  have a surface disposed at a non-parallel angle to the relevant longitudinal axis of the defining component, as described above in connection with the first example expandable intervertebral spacer  1000 . The inclusion of single steps only, in the absence of additional steps on the relevant protrusion, with this desired structure is considered advantageous at least because it provides the desirable tactile feedback and a desirable amount of resistance against movement between configurations. 
     Each of  FIGS.  21 ,  22 ,  23 , and  24    illustrates another example expandable intervertebral spacer  4000 . The expandable intervertebral spacer  4000  is similar to the expandable intervertebral spacer  1000 , except as described below. Thus, the expandable intervertebral spacer  4000  has a main body  4100 , a first endplate  4200 , a second endplate  4400 , a driving member  4600 , and an actuation member  4800 . The expandable intervertebral spacer  2000  is movable between a first, unexpanded configuration and a second, expanded configuration. 
     In this embodiment, a first side of the driving member  4600  defines three step members  4650 ,  4652 ,  4654 . Similarly, the second, opposite side of the driving member  4600  defines three step members (not illustrated in the drawings). The first step member  4650  defines a first plurality of steps  4650   a  and a second plurality of steps  4650   b . Similarly, second step member  4652  defines a first plurality of steps  4652   a  and a second plurality of steps  4652   b  and third step member  4654  defines a first plurality of steps  4654   a  and a second plurality of steps  4654   b . The step members on the second, opposite side of the driving member have identical structure. 
     As best illustrated in  FIG.  24   , the first  4200  endplate defines a first protrusion  4270  having a first single  4272 , a second protrusion  4280  having a second single  4282 , and a third protrusion  4290  having a third single  4292 . Each of the single steps  4272 ,  4282 , and  4292  interfaces with a mating plurality of steps on the driving member  4600 . Similarly, the second  4400  endplate defines a first protrusion  4470  having a first single step  4472 , a second protrusion  4480  having a second single step  4482 , and a third protrusion  4490  having a third single step  4492 . Each of the single steps  4472 ,  4482 , and  4492  interfaces with a mating plurality of steps on the driving member  4600 . 
     Each of  FIGS.  25 ,  26 , and  27    illustrates another example expandable intervertebral spacer  5000 . The expandable intervertebral spacer  5000  is similar to the expandable intervertebral spacer  1000 , except as described below. Thus, the expandable intervertebral spacer  5000  has a main body  5100 , a first endplate  5200 , a second endplate  5400 , a driving member  5600 , and an actuation member  5800 . The expandable intervertebral spacer  5000  is movable between a first, unexpanded configuration and a second, expanded configuration. 
     In this embodiment, each of the main body  5100 , the first endplate  5200 , the second endplate  5400 , and the driving member  5600  define a plurality of openings extending through a thickness of the respective member. Also, a retaining member  5900  comprising a c-shaped member is held captive in a circumferential channel  5190  defined by the main body  5100  and is disposed within a circumferential channel  5890  defined by the actuation member  5800 . The retaining member  5900  maintains the axial position of the actuation member  5800  relative to the main body as it is rotated, forcing the driving member  5600  to move laterally in response due to a threaded engagement between the actuation member  5800  and driving member  5600 . 
     Each of  FIGS.  28 ,  29 , and  30    illustrates another example expandable intervertebral spacer  6000 . The expandable intervertebral spacer  6000  is similar to the expandable intervertebral spacer  1000 , except as described below. Thus, the expandable intervertebral spacer  6000  has a main body  6100 , a first endplate  6200 , a second endplate  6400 , a driving member  6600 , and an actuation member  6800 . The expandable intervertebral spacer  6000  is movable between a first, unexpanded configuration and a second, expanded configuration. A retaining member  6900  is held captive in a circumferential channel  6190  defined by the main body  6100  and is disposed within a circumferential channel  6890  defined by the actuation member  6800 . The retaining member  6900  maintains the axial position of the actuation member  6800  relative to the main body  6100  as it is rotated, forcing the driving member  6600  to move laterally in response due to a threaded engagement between the actuation member  6800  and driving member  6600 . The upper side of the driving member  6600  defines a proximal full width plurality of steps, a distal full width plurality of steps, and a first, second, third, and fourth lateral plurality of steps, each of which interfaces with corresponding plurality of steps defined by the second endplate  6400 . Similarly, the lower side of the driving member  6600  defines a proximal full width plurality of steps, a distal full width plurality of steps, and a first, second, third, and fourth lateral plurality of steps, each of which interfaces with corresponding plurality of steps defined by the first endplate  6200 . Each step in all plurality of steps has the non-parallel structural arrangement described above. Also, as an alternative to pluralities of steps, the endplates can define one or more single step structures that interface with the corresponding plurality of steps defined by the driving member, as described above. 
     In this embodiment, main body  6100  defines passageway  6180  disposed adjacent the threaded opening  6170  that receives the actuation member  6800 . Passageway  6180  provides access to the interior chamber  6112  defined by the main body. 
     Those with ordinary skill in the art will appreciate that various modifications and alternatives for the described and illustrated examples can be developed in light of the overall teachings of the disclosure, and that the various elements and features of one example described and illustrated herein can be combined with various elements and features of another example without departing from the scope of the invention. Accordingly, the particular examples disclosed herein have been selected by the inventors simply to describe and illustrate examples of the invention and are not intended to limit the scope of the invention or its protection, which is to be given the full breadth of the appended claims and any and all equivalents thereof.