Septal occluder and associated methods

RELATED APPLICATION

[0001] The present application is based on and claims priority from U.S. Provisional Patent Application Serial No. 60/340,858 filed on Dec. 19, 2001 and entitled PATENT FORAMEN OVALE (PFO) CLOSURE DEVICE WITH BIORESORBABLE COMPONENTS.

BACKGROUND OF THE INVENTION

[0002] A patent foramen ovale (PFO) as shown in FIG. 1 is a persistent, one-way, usually flap-like opening in the wall between the right atrium <b>10</b> and left atrium <b>12</b> of the heart. Since left atrial (LA) pressure is normally higher than right atrial (RA) pressure, the flap typically stays closed. Under certain conditions, however, RA pressure can exceed LA pressure creating the possibility for right to left shunting that can allow blood clots to enter the systemic circulation. In utero, the foramen ovale serves as a physiologic conduit for right-to-left shunting. After birth, with the establishment of pulmonary circulation, the increased left atrial blood flow and pressure results in functional closure of the foramen ovale. This functional closure is subsequently followed by anatomical closure of the two over-lapping layers of tissue: septum primum <b>14</b> and septum secundum <b>16</b>. However, a probe-patent foramen ovale has been shown to persist in up to 35% of adults in an autopsy series. Using contrast echocardiography (TEE), a PFO can be detected in approximately 25% of adults. These numbers are different because an autopsy allows direct visual inspection of the anatomy, whereas contrast echocardiography relies on the measurement of an indirect physiologic phenomenon.

[0003] The cause of ischemic stroke remains cryptogenic (of unknown origin) in approximately 40% of cases. Especially in young patients, paradoxical embolism via a PFO is considered in the diagnosis. While there is currently no proof for a cause-effect relationship, many studies have confirmed a strong association between the presence of a PFO and the risk for paradoxical embolism or stroke. In addition, there is good evidence that patients with PFO and paradoxical embolism are at increased risk for future, recurrent cerebrovascular events.

[0004] The presence of PFO has no therapeutic consequence in otherwise healthy adults. In contrast, patients suffering a stroke or TIA in the presence of a PFO and without another cause of ischemic stroke are considered for prophylactic medical therapy to reduce the risk of a recurrent embolic event. These patients are commonly treated with oral anticoagulants, which have the potential for adverse side effects such as hemorrhaging, hematoma, and interactions with a variety of other drugs. In certain cases, such as when anticoagulation is contraindicated, surgery may be used to close a PFO. To suture a PFO closed requires attachment of septum secundum to septum primum with a continuous stitch, which is the common way a surgeon shuts the PFO under direct visualization.

[0005] Non-surgical closure of PFOs has become possible with the advent of umbrella-like devices and a variety of other similar mechanical closure designs developed initially for percutaneous closure of atrial septal defects (ASD). These devices allow patients to avoid the potential side effects often associated with anticoagulation therapies.

BRIEF SUMMARY OF EMBODIMENTS OF THE INVENTION

[0006] Various embodiments of the present invention are directed to devices for closing septal defects such as PFOs. The closure devices generally include a proximal anchor member, a distal anchor member, and a flexible center joint connecting the two anchor members. The center joint can be a suture. Alternatively, the center joint can be a flexible elastomeric layer, which can, e.g., be used to promote tissue ingrowth or for drug delivery. The flexible material can also be covered with a biocompatible glue to promote adherence to tissue or growth factors to accelerate tissue ingrowth.

[0007] In accordance with some embodiments of the invention, the closure device is formed of bioresorbable components such that substantially no permanent foreign body remains in the defect.

[0008] In accordance with further embodiments of the invention, mechanisms are provided to collapse the closure device for facilitating device delivery, removal and/or repositioning.

[0009] These and other features will become readily apparent from the following detailed description wherein embodiments of the invention are shown and described by way of illustration. As will be realized, the invention is capable of other and different embodiments and its several details may be capable of modifications in various respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not in a restrictive or limiting sense.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a cross-sectional view of a portion of the heart illustrating a PFO;

[0011]FIG. 2 illustrates a deployed PFO closure device with bioresorbable components in accordance with one or more embodiments of the invention;

[0012]FIG. 3 illustrates the PFO closure device of FIG. 2 in a collapsed state for passage through a delivery catheter or sheath;

[0013]FIG. 4 illustrates a closure device deployed to close a PFO in accordance with one or more further embodiments of the invention;

[0014]FIG. 5 illustrates a closure device deployed to close the PFO in accordance with one or more further embodiments of the invention;

[0015]FIGS. 6A and 6B are front and side views, respectively, of a PFO closure device in accordance with one or more further embodiments of the invention;

[0016]FIGS. 7A and 7B are front and side views, respectively, of a PFO closure device in accordance with one or more further embodiments of the invention;

[0017]FIGS. 8A and 8B are side and front views, respectively, of the PFO closure device of FIG. 6 deployed to close a PFO;

[0018]FIG. 9A illustrates a closure device having a retrieval mechanism in accordance with one or more further embodiments of the invention in a collapsed state for passage through a catheter or sheath;

[0019]FIG. 9B is a front view of the FIG. 9A device;

[0020] FIGS. <b>9</b>C-E illustrate deployment of the FIG. 9A device;

[0021] FIGS. <b>9</b>F-H illustrate removal of the FIG. 9A device;

[0022]FIG. 10A illustrates a closure device having a retrieval mechanism in accordance with one or more further embodiments of the invention in a collapsed state for passage through a catheter or sheath;

[0023]FIG. 10B is a front view of the FIG. 10A device;

[0024]FIGS. 11A and 11B illustrate an anchor member with an elastic hinge in accordance with one or more further embodiments of the invention;

[0025]FIG. 12 illustrates a PFO closure device made from a single material in accordance with one or more further embodiments of the invention;

[0026]FIG. 13 illustrates a PFO closure device having inflatable anchor members in accordance with one or more further embodiments of the invention;

[0027]FIG. 14 illustrates a PFO closure device with a wire connecting the proximal and distal anchor members in accordance with one or more further embodiments of the invention;

[0028]FIG. 15 illustrates a PFO closure device having a frame member in accordance with one or more further embodiments of the invention;

[0029]FIG. 16 illustrates a PFO closure device having frame anchor members in accordance with one or more further embodiments of the invention;

[0030]FIG. 17 illustrates a PFO closure device having frame anchor members in accordance with one or more further embodiments of the invention;

[0031]FIG. 18 illustrates the FIG. 17 device in a collapsed state for passage through a catheter or sheath;

[0032]FIG. 19 illustrates a frame anchor member having metal and polymer components in accordance with one or more further embodiments of the invention;

[0033]FIGS. 20A and 20B illustrate a PFO closure device having anchor members formed from a rolled material in accordance with one or more further embodiments of the invention in rolled and unrolled positions, respectively;

[0034]FIGS. 21A and 21B illustrate an alternate PFO closure device having anchor members formed from a rolled material in accordance with one or more further embodiments of the invention in rolled and unrolled positions, respectively;

[0035]FIG. 22A illustrates a closure device having frame anchor members and a generally “X” shaped joint member in accordance with one or more further embodiments of the invention;

[0036]FIG. 22B illustrates the proximal anchor member of the FIG. 22A device;

[0037]FIG. 22C illustrates the FIG. 22A device in a deployed state;

[0038]FIG. 23 illustrates a closure device having frame anchor members having a generally “+” shaped frame structure in accordance with one or more further embodiments of the invention; and

[0039]FIG. 24 illustrates a closure device having frame anchor members having a generally “G” shaped frame structure in accordance with one or more further embodiments of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0040] Various embodiments of the present invention are directed to methods and devices for closing septal defects such as PFOs, primarily by eliciting a healing response at the defect.

[0041] As shown in FIG. 2, a PFO closure device <b>18</b> in accordance with one or more embodiments of the present invention includes a distal anchor component or member <b>20</b> (which can be placed on the left atrial side of the PFO), a proximal anchor member <b>22</b> (to fix the device in place), a proximal attachment point <b>24</b> (for attachment and release from a catheter), and a central connecting member <b>26</b> (which can, e.g., be a simple suture in accordance with this embodiment).

[0042] In some embodiments, the distal anchor, the proximal anchor, and the connecting member are bioresorbable. These components can be fabricated from either a single bioresorbable polymer or by a laminated composite of two or more materials to provide a unique mix of properties such as, e.g., anchor members having stiff centers and flexible edges, and blood contacting surfaces having controlled porosity or surface texture to promote fast and thorough endothelialization, while minimizing thrombosis. In addition, the tissue contacting surface of the anchors can be designed to provide added stability by, e.g., being roughened.

[0043] The distal anchor <b>20</b> is an elongated, preferably generally cylindrical, thin bar-like member with rounded, arcuately shaped ends. The tissue contacting surface of the anchor can be generally flattened to increase tissue surface contact. In size, the distal anchor component might, e.g., be 15-30 mm long and 2 mm in diameter with a circular cross-section. The proximal anchor <b>22</b> can be of similar dimensions and shape, although it can be shorter in overall length.

[0044] Other distal and proximal anchor structures are also possible. For example, the anchors can be formed of a generally flat material rolled to form a cylindrical shape as described below with respect to the embodiments of FIGS. 20 and 21.

[0045] For delivery and deployment, the distal anchor <b>20</b> and proximal anchor <b>22</b> are positioned to be generally aligned in a longitudinal, end-to-end manner within a delivery sheath or catheter <b>28</b> as shown in FIG. 3. These components, with the flexible connecting member <b>26</b> traverse the catheter or delivery sheath in this longitudinal orientation. The catheter or delivery sheath is inserted between septum primum and septum secundum into the left atrium <b>18</b>, and the distal anchor component <b>20</b> is ejected. Then, the catheter or delivery sheath <b>28</b> is withdrawn into the right atrium, and the proximal anchor <b>22</b> is ejected. The flexible central connecting member <b>26</b> extends between septum primum and septum secundum to join the distal anchor <b>20</b> and the proximal anchor <b>22</b>. Once ejected, the distal anchor and proximal anchor generally self-orientate to be essentially perpendicular to the axis of the central connecting member and in generally parallel planes to one another. The exact orientation will be governed by the individual patient anatomy.

[0046] An alternate delivery method for this device can be to deploy it directly through the septum primum as opposed to through the PFO.

[0047] The method of attaching the central connecting member <b>26</b> to the anchor and stop mechanism <b>22</b> to permit the distal anchor and the proximal anchor to be drawn together could be, e.g., via a friction fit or via a slip knot on the central connecting member. If a slip knot is used, the free end of the suture proximal to the knot can be held remotely and released after the knot has been placed in the appropriate location.

[0048] In one or more alternate embodiments of the invention shown in FIG. 4, the central connecting member <b>26</b> is mounted to permit free sliding movement of the proximal anchor <b>22</b> relative to the central connecting member <b>26</b>. A biasing spring <b>30</b>, which may be an expandable coil spring, can be formed at the outer end of the central connecting member <b>26</b> to bias the proximal anchor toward the distal anchor when both are deployed from the catheter or sheath.

[0049] In the embodiments illustrated in FIGS. 4 and 5, a metallic component may be used as the central connecting member <b>26</b> in order to provide an appropriate stop and apply compression force to the proximal anchor <b>22</b>. The metallic component could be a piece of shape memory wire that has one end molded or laminated into the distal anchor component <b>20</b>. In FIG. 4, the proximal anchor <b>22</b> slides on the central connecting member <b>26</b>, and once it is deployed, the biasing spring <b>30</b> formed on the end of the shape memory wire expands to bias the proximal anchor <b>22</b> toward the distal anchor <b>20</b>.

[0050] In the FIG. 5 embodiment, a shape memory wire forms a hook type anchor <b>32</b> made from two wires that exit through the center of the proximate anchor and curve in opposite directions when expanded to draw the proximate anchor toward the distal anchor.

[0051] While the embodiments of FIGS. 4 and 5 can leave a permanent foreign body when the bioresorbable components dissolve (if, e.g., a metallic component is used as the central connecting member <b>26</b>), one advantage of these devices is that no thrombogenic tissue scaffold (usually a vascular material) is placed on the left atrial side. Thrombus forming on the LA side of a PFO closure device can be released into the systemic circulation causing an embolic event within the coronary arteries, cerebral circulation, or distally in the vasculature, and most vascular graft materials utilized to close PFOs are highly thrombogenic.

[0052] The PFO closure devices may need to be capable of x-ray visualization and use with radiopaque fillers or marker bands fabricated from noble metals such as platinum or gold. These markers can be attached using a variety of common methods such as, e.g., adhesive bonding, lamination between two layers of polymer, or vapor deposition.

[0053]FIGS. 6A and 6B illustrate a closure device <b>50</b> in accordance with one or more further embodiments of the invention. The device <b>50</b> includes proximal and distal anchor members <b>52</b>, <b>54</b> connected with a flexible (and preferably stretchable elastomeric) center joint or connecting element <b>56</b>. The anchor members <b>52</b>, <b>54</b> are preferably cylindrical in shape with rounded ends. In size, the distal anchor member <b>54</b> might, e.g., be about 15-30 mm long and about 2 mm in diameter with a circular cross-section. The proximal anchor <b>52</b> can be of similar dimensions and shape, although it can be shorter in overall length. The anchor members <b>52</b>, <b>54</b> are preferably made from a rigid (preferably bioresorbable) polymer (regular or shape memory), or biological tissue. Biocompatible metal can also be used.

[0054] Other distal and proximal anchor structures are also possible. For example, the anchors can be formed of a generally flat material rolled to form a cylindrical shape as described below with respect to the embodiments of FIGS. 20 and 21.

[0055] The center joint <b>56</b> of the FIG. 6 device (as well as the center joints of the devices shown in FIGS. <b>7</b>-<b>10</b>, <b>12</b>-<b>18</b>, and <b>21</b>-<b>24</b>) are preferably elastomeric and resilient and are made from thrombogenic or inflammatory materials including, e.g., polyester, biological tissue, bioresorbable polymer, small diameter springs (e.g., Nitinol), or spongy polymeric material. Alternatively, the center joint can be made of multiple strands of material <b>58</b> such as, e.g., polymer fibers as shown in the closure device <b>60</b> of FIGS. 7A and 7B. The center joint can be textured, porous or in a form of a single or double-sided hook material such as Velcro. These kinds of surfaces produce inflammatory responses and therefore, promote faster tissue ingrowth and faster defect closure. The entire device or parts of it can be made from bioresorbable polymers.

[0056]FIGS. 8A and 8B are front and side views, respectively, of the device <b>50</b> in a PFO defect. The proximal and distal anchor members <b>54</b>, <b>52</b> are longer than the defect width, thereby inhibiting the device from being embolized.

[0057] In accordance with further embodiments of the invention, a closure device can include a delivery/removal mechanism to facilitate device delivery, removal or repositioning. A device <b>70</b> shown in FIGS. 9A and 9B includes a removal string <b>72</b> and a delivery string <b>74</b>. The removal string is movably secured and slides freely inside of the proximal anchor member <b>76</b>. The string extends from one end of the proximal member <b>76</b> and is fixed to an opposite end of the distal anchor member <b>78</b>. By pulling on the free end of the removal string <b>72</b>, the whole device <b>70</b> can be collapsed and pulled into the delivery sheath <b>79</b> as shown in FIG. 9A. The strings can, e.g., be sutures or wires such as Nitinol wire.

[0058] The delivery and removal strings are manipulated separately in order to deploy or remove the device. FIGS. <b>9</b>C-E illustrate device deployment using the delivery string <b>74</b>, which is preferably attached generally to the center of the proximal anchor member <b>76</b>. The delivery sheath <b>79</b> containing the device <b>70</b> is first inserted between the septum primum and septum secundum into the left atrium as shown in FIG. 9C. As shown in FIG. 9D, the distal anchor <b>78</b> is then ejected from the delivery catheter <b>79</b>. Tension is then applied to the delivery string <b>74</b>, and the delivery sheath is withdrawn into the right atrium and the proximal anchor <b>76</b> is ejected. Applying tension to the delivery string enables the proximal anchor <b>76</b> to be properly deployed in the right atrium, and keeps the anchor <b>76</b> from being ejected into the left atrium. Upon successful deployment of the device <b>70</b>, both strings are released and the delivery system is withdrawn. No tension is applied to the removal string during delivery.

[0059] FIGS. <b>9</b>F-H illustrate removal of the device <b>70</b>. As shown in FIG. 9F, tension is applied to the removal string, while the delivery sheath <b>79</b> is moved toward the device <b>70</b>. The applied tension causes the proximal anchor <b>76</b> to be withdrawn into the delivery sheath as shown in FIG. 9G. The distal anchor <b>78</b> is also withdrawn into the delivery sheath as further tension is applied to the removal string. The device can then be redeployed if desired or removed.

[0060] Alternatively, the delivery string <b>74</b> can be omitted, and the removal string <b>72</b> be used for both device deployment and removal. The delivery sheath <b>79</b> containing the closure device is first inserted between the septum primum and septum secundum into the left atrium in a similar manner to that shown in FIG. 9C. The distal anchor <b>78</b> is then ejected from the delivery catheter <b>79</b> in a similar manner to that shown in FIG. 9D. Tension is applied to the removal string <b>72</b>, and the delivery sheath is withdrawn into the right atrium, and the proximal anchor <b>76</b> is ejected. Applying tension to the removal string enables the proximal anchor <b>76</b> to be properly deployed in the right atrium, and keeps the proximal anchor <b>76</b> from being ejected into the left atrium. The elasticity of the center joint connecting the anchor members helps properly position the proximal anchor at the defect. Upon successful deployment of the closure device, the string <b>72</b> is released and the delivery system is withdrawn.

[0061] As shown in FIGS. 10A and 10B, in another embodiment, strings <b>80</b> (suture, Nitinol wire, etc.) are attached to both ends of the proximal anchor member <b>82</b> of a closure device <b>84</b>. Both anchor members are flexible and can fold as shown in FIG. 10A in order to be delivered to or removed from the defect.

[0062] In accordance with a further embodiment of the invention, as shown in FIGS. 11A and 11B, each of the proximal and distal anchor members can include two elements <b>90</b> separated by an elastic hinge <b>92</b>. The elastic hinge <b>92</b> can facilitate folding of the members as shown in FIG. 11B. The hinge <b>92</b> can be molded or made from a material such as, e.g., Nitinol or other shape memory materials, which can be a different material from the elements <b>90</b>.

[0063] In accordance with some embodiments of the invention, an entire closure device can be made from a single sheet of a material as shown, e.g., in the closure device <b>100</b> of FIG. 12. Two opposite ends of the sheet can be rolled to form the proximal and distal anchor members. Glue or heat bonding can be used to maintain the rolled-up configuration of the anchor members <b>102</b>, <b>104</b>.

[0064] As shown in FIG. 13, in accordance with some further embodiments of the invention, one or both anchor members <b>110</b>, <b>112</b> of a closure device <b>114</b> can be inflatable. The anchor members can be inflated with, e.g., saline or other physiological fluid during or before the delivery of the device. A tube <b>116</b> can communicate with cavities in the anchor members. An inlet <b>118</b> can be provided at one of the members for introducing fluid therein.

[0065] In accordance with some further embodiments of the invention, a wire <b>120</b> such as, e.g., an S-shaped wire, can be provided to connect the proximal and distal anchor members <b>122</b>, <b>124</b> of a device <b>126</b> as shown in FIG. 14. The wire can be used to provide additional clamping force while the device is in a PFO defect. Other wire shapes are also possible.

[0066] In accordance with further embodiments of the invention, one or more frame structures can be used as the anchor members of a closure device. For example, FIG. 15 shows a closure device <b>130</b> having a frame structure <b>132</b>. Also, FIG. 16 shows a closure device <b>136</b> having frames <b>138</b>, <b>139</b>. The frames can be, e.g., a metal (e.g., Nitinol wire) or polymer frame.

[0067] FIGS. <b>17</b>-<b>19</b> illustrate closure devices in accordance with some further embodiments of the invention. A closure device <b>140</b> shown in FIG. 17 includes anchor members <b>142</b>, <b>144</b> having a frame structure. The frame shape can be polygonal as shown in the figure or it can alternatively be a circular shape. Other frame shapes are also possible as, e.g., will be described below with respect to FIGS. <b>22</b>-<b>24</b>.

[0068] A recovery suture can be attached to opposite ends of the proximate anchor member <b>142</b> to collapse the anchors for delivery in a catheter <b>146</b> as shown in FIG. 18 or for retrieval or repositioning. The anchor members can be made from a metal, preferably Nitinol, or polymers. Alternatively, as shown in FIG. 19, an anchor member <b>148</b> can include both metal and polymer components.

[0069] In accordance with one or more further embodiments of the invention, the distal and proximal anchors can be formed of a flat sheet-like member rolled to form a cylindrical shape as shown, e.g., in the device <b>170</b> of FIG. 20A. The anchors <b>172</b>, <b>174</b> can unroll to form sheet-like members when deployed as shown generally in FIG. 20B. The sheet-like member can be made of a material having shape memory properties such as, e.g., shape memory polymeric materials. Alternately, the sheet-like member can include metal struts made of shape memory metals such as, e.g., Nitinol or Nitinol alloys. The shape memory materials allow the device to be delivered in a delivery sheath or catheter with the anchors in the rolled configuration of FIG. 20A. The anchors attain the sheet-like geometry of FIG. 20B once deployed due to their shape memory properties. The anchor members <b>172</b>, <b>174</b> can be connected to each other with a connecting member <b>176</b>, which can, e.g., be a suture similar to that used in the FIG. 2 device.

[0070]FIGS. 21A and 21B illustrate a closure device <b>180</b> having rolled anchor members <b>182</b>, <b>184</b>, which are similar to the anchor members <b>172</b>, <b>174</b> of the device of FIGS. 20A and 20B. The anchors <b>182</b>, <b>184</b> are connected to each other by a connecting member or joint <b>186</b>, which can be a sheet of flexible material similar to the connecting members previously described with respect to FIGS. 6 and 7.

[0071]FIG. 22A illustrates a closure device <b>200</b> in accordance with one or more further embodiments of the invention. The device <b>200</b> includes distal and proximal anchor members <b>202</b>, <b>204</b>, each of which has a polygonal or circular frame structure. The anchor members are connected by a connecting member <b>206</b>, which can be made from a flexible material similar to that previously described in connection with FIGS. 6 and 7. The connecting member <b>206</b> can be made of two sheets of flexible material connected at their centers, generally forming an “X” shape in the side view of the device. As shown in FIG. 22B, the proximal anchor member <b>204</b> can include one or more recovery wires or sutures attached to the frame structure for use in device deployment of recovery. FIG. 22C illustrates the device <b>200</b> as deployed.

[0072]FIGS. 23 and 24 illustrate closure devices <b>220</b>, <b>230</b>, respectively, in accordance with further embodiments of the invention. Each device <b>220</b>, <b>230</b> includes distal and proximal anchor members having a frame structure. The anchor members are connected by a flexible joint <b>222</b>, which can be made from a flexible material similar to that previously described in connection with FIGS. 6 and 7. The FIG. 23 device <b>220</b> includes distal and proximal anchor members <b>224</b>, <b>226</b> generally having a “+” shape. The FIG. 24 device <b>230</b> includes distal and proximal anchor members <b>232</b>, <b>234</b> generally having a “G” shape.

[0073] The closure devices described herein can optionally be used along with suturing or stapling techniques where the anchors or flexible joints of the devices can be sewn or stapled to septum primum or secundum for better dislodgment resistance. Also, the flexible joint can, if desired, be covered with biocompatible glue to adhere to the tissue or can be loaded with drugs or growth factors to promote healing. The glue and also certain drugs can also optionally be stored in any cavities in the anchor members (e.g., in the cylindrical members of FIGS. 6 and 7) and released after deployment. Noble metal markers can also be attached to the closure devices for a better x-ray visualization.

[0074] The various closure devices described herein can include a number of advantageous features. The closure devices preferably have an atraumatic shape to reduce trauma during deployment or removal. In addition, the devices can be self-orienting for ease of deployment. Furthermore, because of the flexible center joint, the devices generally conform to the anatomy instead of the anatomy conforming to the devices, which is especially useful in long tunnel defects. In addition, the devices can preferably be repositioned or/and removed during delivery. The devices also generally have a relatively small profile after deployment. The flexible center joint of the devices can encourage faster tissue ingrowth and therefore, faster defect closure. Furthermore, there are generally no exposed thrombogenic components on the left and right atrial sides. The devices can also advantageously include bioresorbable components, which can disappear over time.

[0075] Other benefits of the devices can include possible use of a relatively small diameter delivery sheath, use of reduced or no metal mass in the device, ease of manufacturing, cost effectiveness, and overall design simplicity.

[0076] Having described preferred embodiments of the present invention, it should be apparent that modifications can be made without departing from the spirit and scope of the invention.