as used in this specification and the appended claims , the singular forms “ a ,” “ an ” and “ the ” include plural referents unless the context clearly dictates otherwise . thus , for example , the term “ a member ” is intended to mean a single member or a combination of members , and “ a material ” is intended to mean one or more materials , or a combination thereof . furthermore , the words “ proximal ” and “ distal ” refer to directions closer to and away from , respectively , an operator ( e . g ., surgeon , physician , nurse , technician , etc .) who would insert the medical device into the patient , with the tip - end ( i . e ., distal end ) of the device inserted inside a patient &# 39 ; s body first . thus , for example , the device end first inserted inside the patient &# 39 ; s body would be the distal end of the device , while the device end last to enter the patient &# 39 ; s body would be the proximal end of the device . as used in this specification and the appended claims , the terms “ upper ”, “ top ”, “ lower ”, “ bottom ”, “ front ”, “ back ”, “ rear ”, “ left ”, “ right ”, “ side ”, “ middle ” and “ center ” refer to portions of or positions on the implant when the implant is oriented in its implanted position . as used in this specification and the appended claims , the term “ axial plane ” when used in connection with particular relationships between various parts of the implant means a plane that divides the implant into upper and lower parts . as shown in the figs ., the axial plane is defined by the x axis and the z axis . as used in this specification and the appended claims , the term “ coronal plane ” when used in connection with particular relationships between various parts of the implant means a plane that divides the implant into front and back parts . as shown in the figs ., the coronal plane is defined by the x axis and the y axis . as used in this specification and the appended claims , the term “ sagittal plane ” when used in connection with particular relationships between various parts of the implant means a plane that divides the implant into left and right parts . as shown in the figs ., the sagittal plane is defined by the y axis and the z axis . as used in this specification and the appended claims , the term “ body ” when used in connection with the location where the device of this invention is to be placed to treat spinal disorders , or to teach or practice implantation methods for the device , means a mammalian body . for example , a body can be a patient &# 39 ; s body , or a cadaver , or a portion of a patient &# 39 ; s body or a portion of a cadaver . as used in this specification and the appended claims , the term “ parallel ” describes a relationship , given normal manufacturing or measurement or similar tolerances , between two geometric constructions ( e . g ., two lines , two planes , a line and a plane , two curved surfaces , a line and a curved surface or the like ) in which the two geometric constructions are substantially non - intersecting as they extend substantially to infinity . for example , as used herein , a line is said to be parallel to a curved surface when the line and the curved surface do not intersect as they extend to infinity . similarly , when a planar surface ( i . e ., a two - dimensional surface ) is said to be parallel to a line , every point along the line is spaced apart from the nearest portion of the surface by a substantially equal distance . two geometric constructions are described herein as being “ parallel ” or “ substantially parallel ” to each other when they are nominally parallel to each other , such as for example , when they are parallel to each other within a tolerance . such tolerances can include , for example , manufacturing tolerances , measurement tolerances or the like . as used in this specification and the appended claims , the terms “ normal ”, “ perpendicular ” and “ orthogonal ” describe a relationship between two geometric constructions ( e . g ., two lines , two planes , a line and a plane , two curved surfaces , a line and a curved surface or the like ) in which the two geometric constructions intersect at an angle of approximately 90 degrees within at least one plane . for example , as used herein , a line is said to be normal , perpendicular or orthogonal to a curved surface when the line and the curved surface intersect at an angle of approximately 90 degrees within a plane . two geometric constructions are described herein as being “ normal ”, “ perpendicular ”, “ orthogonal ” or “ substantially normal ”, “ substantially perpendicular ”, “ substantially orthogonal ” to each other when they are nominally 90 degrees to each other , such as for example , when they are 90 degrees to each other within a tolerance . such tolerances can include , for example , manufacturing tolerances , measurement tolerances or the like . a spinal implant 10 is described herein that is particularly adapted for placement between the spinous processes of the l5 vertebra and the s1 vertebra . however , it is to be understood that even though the following description of implant 10 is provided with reference to the l5 spinous process and the s1 spinous process , implant 10 may be used between other adjacent spinous processes and the discussion of the l5 spinous process may be interpreted to include any superior spinous process and the s1 spinous process may be interpreted to include the adjacent inferior spinous process . implant 10 includes an upper saddle 20 defined by a pair of sidewalls 21 a and 21 b joined by a bottom wall 22 . upper saddle sidewalls 21 a and 21 b may flare slightly outwardly away from the sagittal plane toward the top of implant 10 while upper saddle bottom wall 22 may be concavely curved . implant 10 may have a variable radius , which may be from about 3 . 0 mm on the ventral face 12 to about 2 . 0 mm on the dorsal face 45 . this allows implant 10 to engage the l5 spinous process , which is usually thicker at the base . as shown in fig5 , upper saddle bottom wall 22 may be oriented at about a 10 degree angle in the sagittal plane . the angle could be as large as about 20 degrees . the surfaces forming upper saddle sidewalls 21 a and 21 b and upper saddle bottom wall 22 may be generally parallel to the sagittal plane . this configuration for upper saddle 20 allows upper saddle 20 to receive and support the spinous process of an l5 vertebra therein . the height of upper saddle sidewalls 21 a and 21 b should be chosen so that upper saddle sidewalls 21 a and 21 b prevent the upper portion of implant 10 from moving laterally out of engagement with the spinous process of the l5 vertebra . upper saddle sidewalls 21 a and 21 b may extend between ⅓ and ½ of the base of the spinous process so they engage the lamina by about 2 to 3 mm . upper saddle sidewalls 21 a and 21 b may not have a constant cross - section . this allows upper saddle 20 to accommodate the variable thickness of the spinous process . implant 10 also includes a lower saddle 30 defined by a pair of sidewalls 31 a and 31 b joined by a top wall 32 . as described in more detail below , lower saddle 30 has a configuration to provide clearance of implant 10 over the s1 spinous process . as such , lower saddle 30 would not engage the spinous process of the s1 vertebra . lower saddle sidewalls 31 a and 31 b flare outwardly away from the sagittal plane toward the bottom of implant 10 . upper saddle sidewalls 21 a and 21 b flare out and may have a variable angle . the angle starts at about 40 degrees at the upper portion of upper saddle 20 and varies so that the angle is about 25 degrees at about the lowermost portion of upper saddle 20 . lower saddle sidewalls 31 a and 31 b flare out and have a constant angle between about 25 degrees and about 35 degrees . lower saddle top wall 32 may be concavely curved or may have another configuration that allows the lower portion of implant 10 to be fixed to the s1 pedicles and minimize any interference between the s1 spinous process and the rear of implant 10 . lower saddle top wall 32 is inclined between about 30 degrees and about 35 degrees in the sagittal plane . implant 10 has outer sidewalls 11 a and 11 b that extend on either side of implant 10 from the upper portion of implant 10 to the lower portion of implant 10 . outer sidewalls 11 a and 11 b flare outwardly away from the sagittal plane from the upper portion of implant 10 to give implant 10 a generally triangular - like shape . in addition , the overall shape of implant 10 transfers load from the l5 spinous process to the s1 pedicles instead of to the s1 spinous process or the s1 laminae . this is especially helpful where implant 10 is used in the l5 - s1 level since the small size and shape of the s1 spinous process may not provide adequate support for an implant . the front face 12 of implant 10 may have a curved profile that tapers from about 0 degrees along the middle of front face 12 to about 35 degrees adjacent to sidewalls 11 a , 11 b . implant 10 may have a curvature radius of between about 20 mm and about 30 mm . the generally triangular shape , where the base is larger than the top results in a constant pressure applied along the cross - sectional area of implant 10 . the shape of implant 10 also provides a better fit in the l5 / s1 space and therefore offers stability for implant 10 . the rear of implant 10 has a stepped configuration and includes a shelf 40 separating the rear of implant 10 into an upper portion and a lower portion . shelf 40 may be curved and is located so it is generally aligned with or above channels 34 a and 34 b . shelf 40 acts as a transition between the upper and lower portions of the rear of implant 10 and ensures that implant 10 will fit properly in the patient &# 39 ; s anatomy . the upper rear portion of implant 10 is defined by the rear wall 45 , which flares outwardly from the top of implant 10 . rear wall 45 is curved such that it does not compete for engagement with upper saddle 20 but rather allows implant 10 to rest freely on the l5 lamina . this allows for easy implantation on the l5 level . the thickness of implant 10 gradually increases from the top of implant 10 to shelf 40 . this taper may be between about 30 degrees and about 50 degrees . the bottom rear portion of implant 10 has a thinner profile and provides clearance so that lower saddle 30 does not engage the inferior spinous process . this results in practically no load being transferred from implant 10 to the inferior spinous process . indeed , lower saddle 30 may be configured such that it is spaced from and does not engage the inferior spinous process when implant 10 is implanted in the patient . the wider bottom portion of implant 10 allows two lower lobes 33 a and 33 b to be defined along the bottom portion of implant 10 adjacent to either side of lower saddle 30 and provides an area through which implant 10 may be fixed to the spine . each lower lobe 33 a and 33 b defines a channel 34 a and 34 b extending through implant 10 . channels 34 a and 34 b allow a fixation device 60 , such as a cortical screw or similar device , to extend therethrough to fix implant 10 in the desired location on the spine . as such , the internal diameter of channels 34 a and 34 b should be sufficient to allow passage of fixation device 60 therethrough , but should not be so large as to allow too much “ play ”, or too big of a gap , between fixation device 60 and channels 34 a and 34 b . for example , channels 34 a and 34 b could have an internal diameter that is about 0 . 5 mm to about 1 mm greater than the outer diameter of fixation device 60 . channels 34 a and 34 b flare outwardly from adjacent the mid - line of implant 10 and adjacent the top of the bottom portion of implant 10 so that fixation device 60 can be located therein and extend to the pedicles of the s1 vertebra . for example , channels 34 a and 34 b may extend at an angle α of about 60 degrees away from the sagittal plane toward the rear of implant 10 and at an angle β of about 5 degrees toward the top of implant 10 in a direction from the front of implant 10 toward the rear of implant 10 . alternatively , angle α could be between about 45 degrees and about 60 degrees , while angle β could be between about 5 degrees and about 10 degrees . this orientation for channels 34 a and 34 b allows fixation device 60 to extend there through and engage the pedicles of the s1 vertebra . the pedicles have good bone quality and provide superior support for spinal stabilization systems . the wider bottom portion of implant 10 , and indeed the overall configuration of implant 10 , also allows implant 10 to withstand higher forces being placed on it and helps to ensure compression forces placed on implant 10 are evenly distributed throughout the body of implant 10 . implant 10 may be formed from two portions . an inferior portion 300 and a superior portion 200 . inferior portion 300 may be made from a solid or relatively stiff material such as peek , a high durometer polycarbonate - urethane (“ pcu ”), stainless steel , titanium or other hard , durable biocompatible material . by forming inferior portion 300 from a relatively stiff material , fixation device 60 can firmly affix inferior portion 300 to the spine while ensuring that inferior portion 300 will not be pulled from fixation device 60 during flexion or other movement of the spine . such pulling through of a spinal implant from a fixation device is more likely if the implant were formed from a softer , more flexible material . conversely , superior portion 200 may be formed from a softer more flexible material , such as silicone , a low durometer pcu or some other flexible biocompatible material . superior portion 200 may have a durometer of between about 63a and about 85a . forming superior portion 200 from a flexible material prevents subsidence , which may occur when the superior spinous process engages a hard material such as metal . more importantly , forming superior portion 200 from a flexible material allows implant to act as a shock absorber in extension while providing adequate stabilization to the l5 / s1 level and allowing a more normal range of motion . as shown in fig1 , inferior portion 300 may be designed to extend only below , or inferior to , superior portion 200 . in an alternate embodiment shown in fig1 a , inferior portion 300 ′ includes superiorly extending lateral portions 320 a and 320 b . this configuration provides implant 10 with a varying durometer laterally across implant 10 where the sides are stiffer than the central portion of implant 10 . appropriate connection means may be used to connect inferior portion 300 to superior portion 200 . for example , a tab 310 may extend from the upper wall 320 of inferior portion 300 which engages a slot 210 that may be formed in the bottom portion of superior portion 200 , or vice versa . tab 310 may have a generally elongated cross section when view from the top of inferior portion 300 . as shown in fig1 , tab 310 may extend only along a portion of upper wall 320 . alternatively , as shown in fig1 a , tab 310 ′ may extend across substantially the entire width of upper wall 320 ′. the specific dimensions of the tab may be varied as necessary . in addition , the cross - section of the lower portion of tab 310 may be smaller than the cross - section of the upper portion of tab 310 . see fig5 and 6 . slot 210 may be formed with a configuration and dimensions that will allow tab 310 to be received in slot 210 with an interference fit . the configuration for tab 310 and slot 210 ensures that inferior portion 300 is locked to superior portion 200 with no relative movement between them . in addition to the use of a single slot 210 and tab 310 , other connection means may be used to connect inferior portion 300 to superior portion 200 . for example , a tab in the form of a helical screw could engage a tapped hole , the tab could take the form of a barb , multiple slots and tabs could be used , appropriate adhesives could be used , a tongue and groove configuration could be used , or any other connection system known to those of skill in the art could be used . another mechanism to connect inferior portion 300 to superior portion 200 is to overmold superior portion 200 over inferior portion 300 . an advantage of a two - piece implant as described herein , is that the inferior portion may be implanted and fixed in placed first and then the superior portion may be located between the inferior portion and the superior spinous process . once the inferior portion is properly located in the interspinous space adjacent to the s1 vertebra , fixation devices , such as cortical screws , may be placed through channels 34 a and 34 b and driven into the s1 pedicles to fix the inferior portion in place . thereafter , the superior portion may be fitted between the l5 vertebra and the inferior portion of the implant . this may make implantation of the implant easier than if the implant were a single piece . if desired , a tether 90 , or other fixation device , may be used to connect the superior portion of the implant to the superior spinous process . implant 10 may also define a curved passage 80 that extends between outer sidewalls 11 a and 11 b of implant 10 . the curve of passage 80 may be defined by a radius of curvature of about 20 millimeters where the openings 85 a and 85 b to passage 80 are closer to the top of implant 10 than the nadir of passage 80 . openings 85 a and 85 b are generally perpendicular to outer sidewalls 11 a and 11 b . other radii of curvature may also be used to define passage 80 . the nadir of passage 80 may be substantially aligned in the sagittal plane with the bottom most portion of upper saddle bottom wall 22 and the uppermost portion of lower saddle top wall 32 . a tether 90 may extend through passage 80 . the curve of passage 80 facilitates tether 90 being threaded through passage 80 with a standard curved surgical needle . as shown in fig9 and 10 , tether 90 may extend across the superior portion of the superior spinous process when implant 10 is located in the interspinous space . tether 90 thus helps to maintain implant 10 in the proper position in the patient &# 39 ; s anatomy during extension and flexion . it is to be understood that other fixation devices may be used instead of a tether 90 . for example , a pin , rod , screw or other similar mechanical device may be used and would extend through upper saddle 20 and into the upper spinous process . while various embodiments of the flexible interspinous process device have been described above , it should be understood that they have been presented by way of example only , and not limitation . many modifications and variations will be apparent to the practitioner skilled in the art . the foregoing description of the flexible interspinous process device is not intended to be exhaustive or to limit the scope of the invention . it is intended that the scope of the invention be defined by the following claims and their equivalents .
medical devices for the treatment of spinal conditions are described herein . the medical device includes a main body that is adapted to be placed between the l5 vertebra and the sacrum so that the main body acts as a spacer with respect to the l5 vertebra and the sacrum to maintain distraction therebetween when the spine moves in extension . the main body is formed from two pieces , an upper body portion and a lower body portion .