fig3 is a cross sectional view of the primary elements of a stent and stent delivery device in accordance with one particular embodiment of the present invention . it will be understood by those of skill in the art that certain components that are not particularly relevant to the present invention , such as handles and an optional guide wire , are not shown for sake of clarity . the delivery device 200 has a proximal end 200 a and a distal end 200 b . the proximal end is the end that is in the physician &# 39 ; s hand during a medical procedure . the distal end is the end that is inserted into the lumen during a medical procedure . the device 200 includes an inner tube 201 and an outer tube 203 and a stent 205 captured in a radially constricted condition between the outer tube 203 and the inner tube 201 . for exemplary purposes , let us assume that the stent 205 is a bioabsorbable braided self expanding stent . a balloon 207 is located on the inner tube 201 at the distal end of the delivery device 200 and longitudinally aligned with the loaded stent 205 . while , in this particular example , the length of the balloon is shorter than the length of the stent , this is not necessary . the balloon may be longer than the stent or shorter than the stent . in fact , the balloon may comprise multiple , smaller balloons . the balloon may be inflated to a predetermined pressure or volume during manufacture of the delivery device and sealed with no further mechanism for inflating or deflating the balloon . alternately , the balloon may be inflatable and deflatable by the physician . in one such embodiment , an inflation tube runs from the proximal end of the delivery device 200 to the balloon . the inflation tube may or may not be adhered to the inner tube or integrally formed with the inner tube . in other embodiments , the inner tube 201 itself can serve as the inflation tube , without the need for a separate , third tube for inflation . such embodiments are particularly suitable in connection with stent delivery devices that are not used in connection with a guide wire and thus the inner tube may be closed at its distal end . at the proximal end of the delivery device , the inflation tube can be connected to a pump or syringe for pumping fluid into the balloon to inflate it . the inflation fluid may be a gas , such as nitrogen or air , or a liquid , such as saline , silicone solution , oil or contrast medium . alternately , the inflation medium could be a foam . the inflation medium should be biocompatible in case it escapes from the balloon while in the body lumen . the proximal end of the balloon 207 is sealed over the inner tube so as to be air tight , except through the bore in the inflation tube in embodiments in which the balloon is inflatable by the physician . the distal end of the balloon is also sealingly attached to the inner tube , such as by adhesive or a wire tightly wound around each end of the balloon . in one embodiment , the distal end of the balloon is rolled over and bonded to the inner tube . alternately , the distal end of the balloon may instead simply bonded to the inner tube without rolling over . the balloon 207 can be inflated anytime before the outer tube is moved relative to the stent and inner tube . this may be done prior to inserting the delivery apparatus into the lumen or after the delivery apparatus is inserted into the lumen and the stent is in the desired location for deployment , but before the outer tube 203 is withdrawn . the inflation medium may be applied at the proximal end of the inflation tube by syringe or by coupling the inflation tube to another tube coupled to a pump . the balloon 207 is inflated so that it presses the stent 205 against the inner surface of the outer tube 203 . the pressure or volume to which the balloon is inflated should be selected so as to exert sufficient frictional force against the stent as it is pressed against the inner wall of the outer tube , yet low enough that it is not impossible or difficult to slide the outer tube relative to the stationary stent and balloon . in certain embodiments of the invention , the balloon may be formed of a material , such as polyethylene terepthalate ( pet ) or nylon , or combinations thereof , that will expand to a certain nominal size and thereafter have minimal or no further expansion with increasing pressure up to burst pressure . in such embodiments , the balloon nominal radius should be no less than the inner radius of the outer tube minus the wall thickness of the stent . otherwise the inflated balloon would not contact the stent . while the balloon nominal radius may be larger than the inner radius of the outer tube , preferably , it is somewhere between the inner radius of the outer tube , on the one hand , and the inner radius of the outer tube , minus the wall thickness of the stent , on the other hand . in such embodiments , once any portion of the self expanding stent 205 is released from the outer tube 203 , it will expand beyond the diameter of the balloon and engage the wall of the lumen 233 , while the balloon diameter does not substantially further expand upon release from the outer tube . fig4 illustrates a partially deployed stent in accordance with such an embodiment . it can be seen that the released , distal portion of the stent 205 has expanded to contact the lumen 233 , but the released , distal portion of the balloon 207 remains at substantially the same diameter whether within the outer tube 203 or released from it . thus , the balloon 207 exerts no radial force on the stent 205 once the stent is released from the outer tube 203 . this type of embodiment is preferable because it often is undesirable for the balloon to exert expansive pressure on the body lumen . however , in some applications , it may be preferable that the balloon be expandable to a greater diameter upon release from the outer tube . in such cases , the balloon may be formed of a more stretchable or elastomeric material , such as polyurethane or silicone . preferably , however , the balloon pressure still is low enough such that , when the balloon exits the outer tube it expands partially along with the stent , but not sufficiently to contact the body lumen . the proximal end of the balloon should taper down to a diameter less than the inner diameter of the outer tube as that it can be pulled back in the delivery device after the stent has been released . if the balloon is deflatable , than the taper is immaterial . the balloon should be formed of a material that is more compliant than the outer tube so that the balloon will take a greater set against the stent than the outer tube and hold it in place longitudinally with respect to the inner tube when the outer tube is moved longitudinally relative to the balloon and stent . if the outer tube was more compliant than the balloon , the stent would take more of a set on the outer tube and thus move along with the outer tube and slide along the surface of the balloon , rather than stay stationary with the balloon and allow the outer tube to slide relative thereto . the balloon should remain inflated until the stent is fully released from the outer tube and fully engages the wall of the lumen . at any point while the stent and balloon are still partially within the outer tube , the balloon is still holding the stent stationary relative to the inner tube so that the stent can be withdrawn back into the outer tube in case repositioning is necessary after partial release of the stent . fig5 illustrates the stent 205 and stent delivery device 200 in accordance with the first embodiment of the invention after the stent 205 has been fully released from the device 200 , but before the balloon 207 has been deflated . once the stent is fully released , the balloon can be deflated ( for embodiments for which the balloon is not fully sealed in the inflated state ) and the inner tube and balloon withdrawn proximally back into the tube as illustrated in fig6 . the stent delivery device can then be removed from the lumen and the procedure concluded . with respect to embodiments of the invention in which the balloon is fully sealed , the proximal end of the balloon should taper down to a diameter smaller than the inner diameter of the outer tube , as shown , so that the proximal end of the balloon will easily fit back into the outer tube . even if the remainder of the inflated balloon has a larger diameter than the outer tube , once the smaller end of the balloon is in the outer tube , the diameter of the remainder of the balloon will shrink when it engages the outer tube as it is pulled proximally relative to the outer tube . the single balloon of about equal length to the stent illustrated in the preceding figures is merely exemplary . fig7 a and 7b illustrate various potential embodiments of the invention with different balloon configurations . for instance , fig7 a shows an embodiment in which two balloons 801 , 803 are used to hold the stent 805 in place . the first balloon 801 is positioned adjacent the distal end of the stent 805 and the second balloon 803 is positioned adjacent the proximal end of the stent 805 . fig7 b shows another embodiment in which there is only one small balloon 812 positioned adjacent the proximal end of the loaded stent 805 . it should be understood that once the balloon 812 exits the outer tube 814 , it no longer serves its primary function of forcing the stent 805 against the outer tube 814 . accordingly , the balloon or at least part of a balloon should be as close to the proximal end of the stent as possible since that is the last part of the stent that can be released from the outer tube . once the proximal end of the stent is released , the balloon &# 39 ; s function is completed since the stent generally cannot be recaptured into the outer tube after that point . while the invention has hereinabove been described in connection with a standard type of self - expanding stent , it is equally applicable to other forms of stents and , in fact , any tubular self - expanding prosthesis that is delivered in the same general manner . for instance , the invention is equally applicable to stent - grafts and covered stents , both of which are stent - based medical prostheses that are well known to those of skill in the related arts . in fact , it is not even necessary that the prosthesis be self expanding . the invention can be useful in connection with any prosthesis that must be inserted into a small opening . having thus described a few particular embodiments of the invention , various alterations , modifications , and improvements will readily occur to those skilled in the art . such alterations , modifications and improvements as are made obvious by this disclosure are intended to be part of this description though not expressly stated herein , and are intended to be within the spirit and scope of the invention . accordingly , the foregoing description is by way of example only , and not limiting . the invention is limited only as defined in the following claims and equivalents thereto .
the invention is a method and apparatus for delivering a stent , including , but not limited to , a self expanding stent , into a body lumen . the apparatus comprises an outer tube having a proximal end and a distal end and sized to hold a stent therein in a radially constricted condition , an inner tube within the outer tube and having a proximal end and a distal end , and a holding element comprising one or more inflatable balloons carried on the inner tube at the location of the loaded stent . the balloon can be inflated prior to the procedure so that the balloon presses against the inner surface of the outer tube trapping the stent in its longitudinal position between the inner and outer tubes . accordingly , the outer tube can be slid proximally to release the stent or distally to retract the stent back into the delivery device without the stent inadvertently sliding .