persons skilled in the art will readily appreciate that various aspects of the present disclosure can be realized by any number of methods and apparatuses configured to perform the intended functions . stated differently , other methods and apparatuses can be incorporated herein to perform the intended functions . it should also be noted that the accompanying drawing figures referred to herein are not all drawn to scale , but can be exaggerated to illustrate various aspects of the present disclosure , and in that regard , the drawing figures should not be construed as limiting . finally , although the present disclosure can be described in connection with various principles and beliefs , the present disclosure should not be bound by theory . as used herein , the term “ elastomer ” generally defines a polymer that has the ability to be stretched to at least twice its original length and to retract rapidly to approximately its original length when released . the term “ elastomeric ” is intended to describe a condition whereby a polymer displays stretch and recovery properties similar to an elastomer , although not necessarily to the same degree of stretch and / or recovery . in accordance with various embodiments , a partial side view of a catheter system used to implant a medical device is shown and generally indicated at 100 in fig1 . the catheter system 100 includes a catheter shaft 102 and an expandable device 104 constrained to a delivery profile or constrained state suitable for endoluminal delivery of the device to a treatment site . the device 104 is held in the constrained state by a flexible , generally tubular constraining sleeve or restraining member 106 . the flexible restraining member 106 is held or maintained in a tubular shape by a removable stitch line 108 . when the stitch line 108 is actuated by pulling or tensioning in the direction indicated at 114 , the restraining member 106 will split open and allow the device 104 to expand . examples of restraining members and coupling members for releasably maintaining expandable devices in a constrained or collapsed state for endoluminal delivery to a treatment site can be found in u . s . pat . no . 6 , 352 , 561 to leopold et al , the content of which is incorporated herein by reference in its entirety . still referring to fig1 , as the catheter system 100 is advanced through a curved vessel , a gap 112 can form between the constrained device 104 and a compliant distal catheter tip 110 . described in greater detail below , the restraining member , in accordance with various embodiments , comprises a retractable section that extends over at least a portion of the compacted or constrained device and at least a portion of the catheter tip so as to cover or bridge a gap therebetween . the retractable section can retract away from the catheter tip sequentially or concurrently with at least a partial actuation or opening of the restraining member . referring to fig2 , a partial side view of a catheter system , in accordance with various embodiments , used to implant a medical device is shown and generally indicated at 100 . the catheter system 100 includes a catheter shaft 102 having opposite proximal and distal ends , and an expandable device 104 ( shown in dashed lines ) disposed near or at the distal end of the catheter shaft 102 . the device 104 is held in a constrained state suitable for endoluminal delivery of the device to a treatment site by a flexible , generally tubular constraining sleeve or restraining member 106 . the flexible restraining member 106 is held in the tubular shape by a removable stitch line 108 . when the stitch line 108 is actuated by pulling or tensioning in the direction indicated at 114 , the restraining member 106 will split open and allow the device 104 to expand . the restraining member 106 at its distal end incorporates a retractable section 200 that extends over at least a portion of both the device 104 and the catheter tip 110 . in various embodiments , the retractable section can be a generally tubular element . as the catheter system is advanced through a curved vessel , a gap 112 can form between the constrained device 104 and a compliant distal catheter tip 110 . as shown , the retractable section 200 extends over at least a portion of both the device 104 and the catheter tip 110 to bridge the gap 112 therebetween . the retractable section 200 can retract away from the catheter tip 110 sequentially or concurrently with actuation or opening of the restraining member . in various embodiments , a retracting element can be operatively coupled to the retractable section to facilitate retraction of the retractable section away from the catheter tip . the retracting element can be an elongated member , such as a tether , wire , string and the like coupled to the retracting section and extending through the catheter for access and selective actuation of the retracting element by the clinician at a proximal end of the catheter . in various embodiments , the retracting element , for example as illustrated at 201 in fig2 a , can be formed from an elastomeric material and operatively coupled to the retractable section 200 , such that the retracting element 201 is in a tensioned state while the retractable section 200 is releasably held or maintained over the device 104 and the catheter tip 110 to bridge the gap 112 therebetween . release or opening of the retractable section 200 allows the retracting element 201 to shorten as it moves toward a relaxed , untensioned state . the retractable section 200 is pulled or displaced away from the catheter tip 110 in response to the shortening of the retracting element 201 . in various embodiments , the retractable section can be formed from an elastomeric material and tensioned or stretched such that the retractable section can be releasably maintained in a tensioned state while extending over the device and the catheter tip to bridge the gap therebetween , and released to allow movement of the retractable section toward a shortened , relaxed state sequentially or concurrently with opening of the restraining member . upon delivery , the restraining member is released allowing the restraining member to release or “ split - open ” and permit the compacted device to expand . the device can be expanded by a balloon or can expand due to an outward force applied by a compressed stent wire frame . the restraining member may remain with the device at the treatment site in the vasculature , captured between the device and vascular wall . as the restraining member is released , the retractable section of the restraining member retracts proximally away from the catheter tip . in some cases , the medical device has anchors or barbs that aid in securing the device to the vascular wall along with a blood sealing cuff . thus , retraction of the retractable section can further expose such anchors or barbs and / or sealing cuffs for engaging the vascular wall . referring to fig3 a , a catheter system 100 , in accordance with various embodiments , is shown having an expandable device 104 partially covered by a constraining sleeve or restraining member 106 . the restraining member 106 has a retractable section 200 a extending from a relatively non - elastic portion 300 . the retractable section 200 a is shown in a non - tensioned state having a relaxed , original longitudinal length . as shown in fig3 b , the retractable section 200 b of the restraining member 106 can be longitudinally tensioned ( stretched or elongated ) in the direction depicted by arrows 302 . the retractable section 200 b of the restraining member 106 can be stretched longitudinally to extend over the proximal end of the catheter olive or tip 110 to conceal or bridge a gap between the device 104 and the catheter tip 110 . once longitudinally tensioned to the desired stretched length , the retractable section of the restraining member can be longitudinally restrained in tension . the retractable section 200 b can , for example , be longitudinally tensioned or stretched to at least about 10 % longitudinal elongation or at least about 110 % of an initial or original ( relaxed ) length and held ( restrained ) in this stretched condition to bridge the gap between the device and the catheter tip . as illustrated in fig3 b , a releasable stitch line 108 maintains the retractable section 200 b in the elongated , tensioned state . as shown in fig3 c , the releasable stitch line 108 can be actuated or tensioned to allow the restraining member 106 to split open and release the expandable device 104 . as the restraining member 106 opens , the retractable section 200 c is free to retract in the direction depicted by arrows 304 toward a relaxed , non - tensioned state . the restraining member therefore shortens longitudinally in length and retracts proximally along the compacted device . in some cases , the medical device has anchors or barbs that aid in securing the device to the vascular wall along with a blood sealing cuff . by shortening in length , the restraining member can retract proximally to expose any optional anchors and / or sealing cuffs for engaging the vascular wall . as shown in fig3 d , the releasable stitch line can be actuated , allowing the device 104 to fully expand . the retractable section 200 a of the restraining member 106 is now longitudinally shortened as it moves toward the relaxed , non - tensioned state , as shown . since the retractable section 200 a is relaxed and non - tensioned , the retractable section retracts to a length shorter than a longitudinally tensioned or stretched length ( as illustrated in fig3 b , 200 b ). the restraining member 106 therefore does not cover or interfere with device sealing cuffs 306 or anchor barbs 308 , as shown in fig3 d . in various embodiment , a restraining member and retracting element or retractable section of the restraining member can be retained in an elongated and tensioned state by friction between the constrained device and the inner surface of the restraining member . opening of the restraining member by actuation of the stitch line as described above relieves the friction and allows the restraining member to longitudinally retract as the elastic element returns to a shorter , untensioned state . in various embodiments , a restraining member can include an elastic element that is held in an elongated tensioned state to conceal a gap along the catheter assembly , such as between the expandable device and an adjacent component of the catheter assembly , and that retracts toward a shortened relaxed state upon release or opening of the restraining member to reveal the gap and / or portions of the expandable device and / or adjacent component . in various embodiments , the restraining member can include proximal and distal elastic elements which can be held in elongated tensioned states to conceal proximal and distal gaps on opposite ends of the expandable device , and which retract toward shortened relaxed states upon release or opening of the restraining member to reveal the respective proximal and distal gaps and / or portions of the expandable device and / or adjacent components at opposite proximal and distal ends of the expandable device . elastic restraining members can comprise a variety of polymeric material , such as silicone . other exemplary biocompatible elastomers can include , but are not limited to , elastomeric copolymers of 6 - caprolactone and glycolide ( including polyglycolic acid ) with a mole ratio of 6 - caprolactone to glycolide of from about 35 : 65 to about 65 : 35 , more preferably from 35 : 65 to 45 : 55 ; elastomeric copolymers of 6 - caprolactone and lactide ( including l - lactide , d - lactide , blends thereof , and lactic acid polymers and copolymers ) where the mole ratio of 6 - caprolactone to lactide is from about 35 : 65 to about 65 : 35 and more preferably from about 30 : 70 to 45 : 55 ; other preferable blends include a mole ratio of 6 - caprolactone to lactide from about 85 : 15 to 95 : 5 ; elastomeric copolymers of p - dioxanone ( 1 , 4 - dioxan - 2 - one ) and lactide ( including l - lactide , d - lactide , blends thereof , and lactic acid polymers and copolymers ) where the mole ratio of p - dioxanone to lactide is from about 40 : 60 to about 60 : 40 ; elastomeric copolymers of 6 - caprolactone and p - dioxanone where the mole ratio of 6 - caprolactone to p - dioxanone is from about from 30 : 70 to about 70 : 30 ; elastomeric copolymers of p - dioxanone and trimethylene carbonate where the mole ratio of p - dioxanone to trimethylene carbonate is from about 30 : 70 to about 70 : 30 ; elastomeric copolymers of trimethylene carbonate and glycolide ( including polyglycolic acid ) where the mole ratio of trimethylene carbonate to glycolide is from about 30 : 70 to about 70 ; 30 ; elastomeric copolymers of trimethylene carbonate and lactide ( including l - lactide , d - lactide , blends thereof , and lactic acid polymers and copolymers ) where the mole ratio of trimethylene carbonate to lactide is from about 30 : 70 to about 70 ; 30 ; and blends thereof . examples of suitable biocompatible elastomers are described in u . s . pat . nos . 4 , 045 , 418 ; 4 , 057 , 537 and 5 , 468 , 253 . an optional external sleeve , or external sock may be incorporated to cover the retractable section of the restraining member . typical catheters used to deliver medical devices can comprise commonly known materials such as amorphous commodity thermoplastics that include polymethyl methacrylate ( pmma or acrylic ), polystyrene ( ps ), acrylonitrile butadiene styrene ( abs ), polyvinyl chloride ( pvc ), modified polyethylene terephthalate glycol ( petg ), cellulose acetate butyrate ( cab ); semi - crystalline commodity plastics that include polyethylene ( pe ), high density polyethylene ( hdpe ), low density polyethylene ( ldpe or lldpe ), polypropylene ( pp ), polymethylpentene ( pmp ); amorphous engineering thermoplastics that include polycarbonate ( pc ), polyphenylene oxide ( ppo ), modified polyphenylene oxide ( mod ppo ), polyphenelyne ether ( ppe ), modified polyphenelyne ether ( mod ppe ), thermoplastic polyurethane ( tpu ); semi - crystalline engineering thermoplastics that include polyamide ( pa or nylon ), polyoxymethylene ( pom or acetal ), polyethylene terephthalate ( pet , thermoplastic polyester ), polybutylene terephthalate ( pbt , thermoplastic polyester ), ultra high molecular weight polyethylene ( uhmw - pe ); high performance thermoplastics that include polyimide ( pi , imidized plastic ), polyamide imide ( pai , imidized plastic ), polybenzimidazole ( pbi , imidized plastic ); amorphous high performance thermoplastics that include polysulfone ( psu ), polyetherimide ( pei ), polyether sulfone ( pes ), polyaryl sulfone ( pas ); semi - crystalline high performance thermoplastics that include polyphenylene sulfide ( pps ), polyetheretherketone ( peek ); and semi - crystalline high performance thermoplastics , fluoropolymers that include fluorinated ethylene propylene ( fep ), ethylene chlorotrifluroethylene ( ectfe ), ethylene , ethylene tetrafluoroethylene ( etfe ), polychlortrifluoroethylene ( pctfe ), polytetrafluoroethylene ( ptfe ), polyvinylidene fluoride ( pvdf ), perfluoroalkoxy ( pfa ). other commonly known medical grade materials include elastomeric organosilicon polymers , polyether block amide or thermoplastic copolyether ( pebax ) and metals such as stainless steel and nickel / titanium alloys . semi - rigid restraining members can comprise appropriate materials listed above . medical devices incorporating stents can have various configurations as known in the art and can be fabricated , for example , from cut tubes , wound wires ( or ribbons ) or flat patterned sheets rolled into a tubular form . stents can be formed from metallic , polymeric or natural materials and can comprise conventional medical grade materials such as nylon , polyacrylamide , polycarbonate , polyethylene , polyformaldehyde , polymethylmethacrylate , polypropylene , polytetrafluoroethylene , polytrifluorochlorethylene , polyvinylchloride , polyurethane , elastomeric organosilicon polymers ; metals such as stainless steels , cobalt - chromium alloys and nitinol and biologically derived materials such as bovine arteries / veins , pericardium and collagen . stents can also comprise bioresorbable materials such as poly ( amino acids ), poly ( anhydrides ), poly ( caprolactones ), poly ( lactic / glycolic acid ) polymers , poly ( hydroxybutyrates ) and poly ( orthoesters ). it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention . thus , it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents .
a medical device constraint includes an elastic element having proximal and distal ends , a continuous lumen extending between the proximal and distal ends of the tubular elastic element ; and a medical device disposed at least partially within the continuous lumen , wherein the generally tubular element has a first state in which the tubular element is longitudinally held in tension to conceal a gap between the medical device and a distal tip and a second state in which the tubular element is longitudinally relaxed and spaced apart from the gap .