Patent Publication Number: US-2010114147-A1

Title: Directional soft tissue dilator and docking pin with integrated light source for optimization of retractor placement in minimally invasive spine surgery

Description:
STATEMENTS REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT AND CROSS-RELATED APPLICATIONS 
     This invention was not made with any government support. This application claims the benefit of U.S. Provisional Application No. 61/109,595 filed Oct. 30, 2008, the disclosure of which is expressly incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to the field of orthopedic surgery and more particularly to the area of spinal surgery. In particular, this invention relates to improved structures for both a soft tissue dilator and a docking pin for optimizing the placement of a retractor in a minimally invasive spine surgery. 
     In the past, surgery typically required large incisions to provide visual and instrument access to the surgical site. These large incisions resulted in significant blood loss, damage to muscle tissue, long healing times accompanied by prolonged pain, and significant scarring. Today, however, many surgeries are conducted using minimally invasive techniques. These techniques minimize patient trauma by creating a relatively small incision, followed by the introduction of dilators to increase the effective size of the incision. Following dilation, surgery is performed through a surgical port inserted into the dilated incision. Instead of cutting through the muscle surrounding the surgical site, dilation effectively splits the muscle. Splitting, rather than cutting, the muscle causes less damage to the muscle and leads to faster recovery times and reduced patient discomfort. 
     Dilators develop a channel from the subcutaneous layer of a patient to the site of operation. Initially, a small incision is made overlying the surgical area of interest. Then, a solid or cannulated pointed rod is inserted into the incision to penetrate the underlying structures and reach the surgical site. It is best if the rod can be positioned against a bony surface, inasmuch as the subsequent application of the dilators will attempt to push this rod forward. Conventional fluoroscopic techniques may be used before and/or after insertion of the initial rod to confirm placement at the desired surgical site. Thereafter, increasingly larger diameter dilators can then be sequentially inserted over each other to gradually enlarge the size of the channel. The increasingly larger diameters of the sequentially inserted dilators help to dilate the path of exposure, while lessening the magnitudes of the forces needed to create such path. The pointed tips of the dilators ease insertion and help to widen the base of the channel when the dilators are orbited around a central axis formed through the center of the dilator along its length at the level of the skin. 
     Most current dilators allow symmetric circumferential dilation with a constant center point, which is not always desirable. They also make precise placement of the working port relatively time consuming. If a starting guide wire or cannula is slightly off of the desired starting point, symmetric dilatation may cause stretching of the important structures, such as neural elements posteriorly or may bring final docking of the cannula too far anteriorly. Thus, it would be desirable to provide an improved structures for a soft tissue dilator having a variable center point that avoids these potential issues. 
     SUMMARY OF THE INVENTION 
     This invention is a new dilator system for insertion of cannula and working port for minimally invasive spinal surgery. Such an invention allows preferential anterior, posterior, superior or inferior dilatation for docking of the working port in an anatomically more desirable location. The new dilator system permits preferential dilation of one side of the tissue without excessively stretching the vital tissues in undesirable location. The improved dilator of this invention is interchangeable with symmetric dilators if necessary. It may also be made compatible with available neurophysiologic monitoring tools or illumination systems that are known in the art. The dilator tubes may be either incomplete or fully cylindrical in nature and may have asymmetric wall thickness such that they facilitate preferentially dilate tissues. Once the dilatation is completed, a fully circular or oval or any other desirable shaped working port can be placed and the dilating cannulas removed. The dilating cannulas may also be interchangeable with closed symmetric tubular dilators if desired. This invention also describes a combined light source and a docking pin for retention of the retractor blades in the desired location during the surgical procedure. Such a docking pin having an integrated lighting system eliminates the need for separate illumination source and a wire, pin, or shim to maintain the retractor in place. The combined device achieves both objectives and frees up some working space within the retractor. The docking pin with integrated locking system may be disposable and made of variable lengths to fit all sizes and body habitus. 
     The present invention describes incomplete or nearly complete cannulas of symmetric or variable thickness to allow preferential dilation of tissues towards more anatomically desirable location during minimally invasive exposures for direct lateral spinal fusion. This method avoids anterior migration of the tube or excessive posterior stretching of neural tissues. 
     Initially, a small diameter complete cannulated or as solid starter tube is placed with radiographic assistance over the desired location. Preferably the starting tube is cannulated, through which a guide wire may be placed to minimize unwarranted migration during the dilatation process. Once the starter tube is confirmed to be in a satisfactory position, several small complete circular dilators or incomplete dilators of symmetric or variable thickness can be placed sequentially for preferential directional dilatation of the tissues. The disclosed invention may be particularly useful during direct lateral exposure of spine or during posterior lumbar minimally invasive surgical techniques. 
     During direct lateral approaches, typically a transpsaos approach is utilized with symmetric dilatation of the surgical site access pathway. An asymmetric dilator may facilitate placement of initial dilator anterior to the psoas. The thicker wall of the dilating tube may be placed anteriorly if desired and once the tip is at the level of the disc, it may be rotated back and forth to provide posterior mobilization of the psoas muscle. After the last dilator tube is in place, a working port is placed over it and secured in place with means utilized in prior art. If necessary either prior to docking of the working port or during the dilation process, reverse directional tubes may be placed over the previously inserted dilator cannulas to make the dilated hole more symmetric. The dilators may be made compatible with neurophysiologic monitoring. The dilator may be made of any radiolucent or radio-opaque biomaterial. 
     Alternatively, the dilators may have symmetric wall thicknesses and projections provided on the inner walls thereof which are of variable height such that they facilitate preferential dilatation of the soft tissues. The projections may be linear in nature across entire length of the dilator tube, or they may be located co-linearly only across certain parts or randomly located through out the inner circumference of the tube. The projections may be in cluster of more than one if desired or may be different geometry. 
     Asymmetric dilatation may also be useful during lumbar minimally invasive surgery. Several surgeons place a pedicle screw prior to performing interbody work during a transforaminal interbody fusion surgical procedure. This sequence of pedicle screw fixation followed by interbody work is particularly desirable in cases of spondylolisthesis. During such procedures, it will be advantageous to use the percutaneous screw insertion tube as a guide for further asymmetric dilation. In this case, asymmetric dilators are placed sequentially over the distal pedicle screw tube (L5 pedicle screw tube for L4-5 transforaminal interbody fusion, for example), such that the center point of the dilator is placed directly over the disc space, providing straight access to the surgical site. Appropriate port may then be placed and secured in location for further surgical work. 
     If desired, the asymmetric dilators may be placed directly over a tap or a pedicle finder used to prepare the hole for the pedicle screw. Sequential asymmetric dilator placement may also be used for transforaminal interbody fusion, as described above. It also provides access to the corresponding facet joint (L4-5 facet joint with placement of dilator over tap/pedicle finder in L5 pedicle, for example). Such a technique facilitates placement of transfacet screw or performance of facet fusion. 
     It should be understood that the disclosed invention may be utilized for other orthopedic and non-orthopedic applications. 
     Various objects and advantages will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a first embodiment of a soft tissue dilator in accordance with this invention. 
         FIG. 2  is a side elevational view of the first embodiment of the soft tissue dilator illustrated in  FIG. 1 . 
         FIG. 3  is an end elevational view of the first embodiment of the soft tissue dilator illustrated in  FIGS. 1 and 2 . 
         FIG. 4  is a perspective view of a second embodiment of a soft tissue dilator in accordance with this invention. 
         FIG. 5  is a side elevational view of the second embodiment of the soft tissue dilator illustrated in  FIG. 4 . 
         FIG. 6  is an end elevational view of the second embodiment of the soft tissue dilator illustrated in  FIGS. 4 and 5 . 
         FIG. 7  is an end elevational view of a dilator member of a third embodiment of a soft tissue dilator in accordance with this invention. 
         FIG. 8  is an end elevational view of a portion of a dilator member of a fourth embodiment of a soft tissue dilator in accordance with this invention. 
         FIG. 9  is an end elevational view of a portion of a dilator member of a fifth embodiment of a soft tissue dilator in accordance with this invention. 
         FIG. 10  is an end elevational view of a portion of a dilator member of a sixth embodiment of a soft tissue dilator in accordance with this invention. 
         FIG. 11  is an end elevational view of a portion of a dilator member of a seventh embodiment of a soft tissue dilator in accordance with this invention. 
         FIG. 12  is an end elevational view of a portion of a dilator member of an eighth embodiment of a soft tissue dilator in accordance with this invention. 
         FIG. 13  is a perspective view of a ninth embodiment of a soft tissue dilator in accordance with this invention. 
         FIG. 14  is a side elevational view of the ninth embodiment of the soft tissue dilator illustrated in  FIG. 13 . 
         FIG. 15  is an end elevational view of the ninth embodiment of the soft tissue dilator illustrated in  FIGS. 13 and 14 . 
         FIG. 16  is a side elevational view of an illuminating docking pin in accordance with this invention. 
         FIG. 17  is a side elevational view of a portion of an alternative embodiment of the illuminating docking pin illustrated in  FIG. 16 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, there is illustrated in  FIGS. 1 ,  2 , and  3  a first embodiment of a soft tissue dilator, indicated generally at  10 , in accordance with this invention. The first embodiment of the dilator  10  includes a first dilator member  11  that, in the illustrated embodiment, is solid and generally cylindrical in shape and defines an axis. The illustrated first dilator member  11  includes an outer surface  11   a  that defines a generally circular cross-sectional shape, as best shown in  FIG. 3 . However, the first dilator member  11  need not be solid (it may be cannulated, for example) and may be formed having any desired cross-sectional shape. The illustrated first dilator member  11  has a leading end surface  11   b  that is flat and circular in shape. However, the leading end surface  11   b  may be formed having any desired shape (such as a tapered point) or combination of shapes (such as a combination of flat and tapered surfaces). 
     The first embodiment of the dilator  10  also includes a second dilator member  12  that is disposed about and supported on the first dilator member  11  for both axial and rotational sliding movement relative thereto. The second dilator member  12  is hollow and includes an inner surface  12   a  and an outer surface  12   b.  In the illustrated embodiment, both the inner surface  12   a  and the outer surface  12   b  of the second dilator member  12  are each generally cylindrical in shape and, therefore, define respective cross-sectional shapes that are generally circular and define respective axes, as best shown in  FIG. 3 . However, as also shown in  FIG. 3 , the inner surface  12   a  and the outer surface  12   b  are not oriented concentrically relative to one another. Rather, the wall thickness of the second dilator member  12  varies circumferentially such that the inner surface  12   a  and the outer surface  12   b  are oriented eccentrically relative to one another. The purpose for this eccentric orientation will be explained below. In the illustrated embodiment, the inner surface  12   a  of the second dilator member  12  is disposed about and supported on the outer surface  11   a  of the first dilator member  11  for both axial and rotational sliding movement relative thereto. The second dilator member  12  is provided with a leading end surface  12   c  that extends from the outer surface  12   b  thereof to the inner surface  12   a.  In the illustrated embodiment, the leading end surface  12   c  includes an outer tapered portion and an inner flat portion. However, the leading end surface  12   b  may be formed having any desired shape or combination of shapes. 
     The first embodiment of the dilator  10  further includes a third dilator member  13  that is disposed about and supported on the second dilator member  12  for both axial and rotational sliding movement relative thereto. The third dilator member  13  is hollow and includes an inner surface  13   a  and an outer surface  13   b.  In the illustrated embodiment, both the inner surface  13   a  and the outer surface  13   b  of the third dilator member  13  are generally cylindrical in shape and, therefore, define respective cross-sectional shapes that are circular and define respective axes, as best shown in  FIG. 3 . However, as also shown in  FIG. 3 , the inner surface  13   a  and the outer surface  13   b  are not oriented concentrically relative to one another. Rather, the wall thickness of the third dilator member  13  varies circumferentially such that the inner surface  13   a  and the outer surface  13   b  are oriented eccentrically relative to one another. The purpose for this eccentric orientation will be explained below. In the illustrated embodiment, the inner surface  13   a  of the third dilator member  13  is disposed about and supported on the outer surface  12   a  of the second dilator member  12  for both axial and rotational sliding movement relative thereto. The third dilator member  13  is provided with a leading end surface  13   c  that extends from the outer surface  13   b  thereof to the inner surface  13   a.  In the illustrated embodiment, the leading end surface  13   c  includes an outer tapered portion and an inner flat portion. However, the leading end surface  13   b  may be formed having any desired shape or combination of shapes. 
     The first embodiment of the dilator  10  additionally includes both a fourth dilator member  14  and a fifth dilator member  15 . The fourth dilator member  14  is similar in structure and operation to the second and third dilator members  12  and  13 , respectively, and includes an inner surface  14   a,  an outer surface  14   b,  and a leading end surface  14   c.  In the illustrated embodiment, the inner surface  14   a  of the fourth dilator member  14  is disposed about and supported on the outer surface  13   a  of the third dilator member  13  for both axial and rotational sliding movement relative thereto. Similarly, the fifth dilator member  15  is similar in structure and operation to the second, third, and fourth dilator members  12 ,  13 , and  14 , respectively, and includes an inner surface  15   a,  an outer surface  15   b,  and a leading end surface  15   c.  In the illustrated embodiment, the inner surface  15   a  of the fifth dilator member  15  is disposed about and supported on the outer surface  14   a  of the fourth dilator member  14  for both axial and rotational sliding movement relative thereto. 
     As discussed above, the first embodiment of the dilator  10  can be used to facilitate the performance of a surgery using minimally invasive techniques, wherein a relatively small incision is made in a patient. Specifically, the first embodiment of the dilator  10  can be used to increase the effective size of the relatively small incision to facilitate the desired surgical procedures. To accomplish this, the first dilator member  11  is initially inserted through the relatively small incision in a conventional manner until the leading end surface  11   a  thereof is positioned at a desired surgical site within the patient. X-rays or other conventional fluoroscopic techniques may be used before and/or after insertion of the first dilator member  11  to confirm placement of the leading end surface  11   b  at the desired surgical site. Thereafter, the second dilator member  12  is inserted axially over the first dilator member  11  to gradually enlarge the size of the channel extending from the incision to the surgical site. The larger diameter of the second dilator  12  helps to dilate the size of the channel, while lessening the magnitude of the force needed to accomplish this. The tapered leading end surface  12   c  of the second dilator member  12  eases insertion and helps to widen the base of the channel. Similarly, the third, fourth, and fifth dilator members  13 ,  14 , and  15  can be sequentially inserted axially over the second dilator member  12  to further gradually enlarge the size of the channel. It will be appreciated that any number of such dilator members  12  through  15  may be used as deemed necessary for the particular surgical procedure to be performed. 
     Unlike known dilator members, however, the eccentric shapes of the dilator members  12  through  15  permit asymmetric dilatation of the channel in a quick and easy manner. Such asymmetric dilatation of the channel can be accomplished simply by rotating one or more of the dilator members  12  through  15  relative to one another. Such rotation can be performed either before or after the dilator members  12  through  15  have been inserted axially thereon. The eccentric structures of the dilator members  12  through  15  permit asymmetric dilatation of the channel in a quick and easy manner. 
     Although the first embodiment of the dilator  10  is disclosed as having five different dilator members  11  through  15 , it will be appreciated that this invention may be practiced with a greater or lesser number of such dilator members. Furthermore, although each of the second through fifth dilator members  12  through  15  of the first embodiment of the dilator  10  is disclosed as having eccentric inner and outer surfaces, it will be appreciated that this invention may be practiced with only some (or only one) of such second through fifth dilator members  12  through  15  having eccentric inner and outer surfaces. 
       FIGS. 4 ,  5 , and  6  illustrate a second embodiment of a soft tissue dilator, indicated generally at  20 , in accordance with this invention. The second embodiment of the dilator  20  is similar to the first embodiment of the dilator  10  and includes a first dilator member  21  that, in the illustrated embodiment, is solid and cylindrical in shape. The illustrated first dilator member  21  includes an outer surface  21  a that defines a circular cross-sectional shape, as best shown in  FIG. 6 . However, the first dilator member  21  need not be solid and may be formed having any desired cross-sectional shape. The illustrated first dilator member  21  has a leading end surface  21   b  that is flat and circular in shape. However, the leading end surface  21   b  may be formed having any desired shape, such as a tapered point. 
     The second embodiment of the dilator  20  also includes second through fifth dilator members  22  through  25 . Similar to the dilator members  12  through  15  of the first embodiment of the dilator  10  described above, the dilator members  22  through  25  of the second embodiment of the dilator  20  have respective inner surfaces  22   a  through  25   a,  outer surfaces  22   b  through  25   b,  and leading end surfaces  22   c  through  25   c  provided thereon. As also described above, some or all of the inner surfaces  22   a  through  25   a  of the dilator members  22  through  25  of the second embodiment of the dilator  20  are oriented eccentrically relative to the outer surfaces  22   b  through  25   b  thereof. However, unlike the dilator members  12  through  15  of the first embodiment of the dilator  10  described above, the dilator members  22  through  25  of the second embodiment of the dilator  20  are not formed in a circumferentially complete manner. Rather, as best shown in  FIG. 6 , each of the dilator members  22  through  25  of the second embodiment of the dilator  20  is circumferentially incomplete, having removed portions  22   d  through  25   d  provided thereon. The circumferentially incomplete portions  22   d  through  25   d  of the dilator members  22  through  25  of the second embodiment of the dilator  20  provide additional space at the surgical site if needed. 
       FIG. 7  illustrates a dilator member, indicated generally at  30 , of a third embodiment of a soft tissue dilator in accordance with this invention. The dilator member  30  is hollow and includes an inner surface  30   a  and an outer surface  30   b.  In the illustrated embodiment, both the inner surface  30   a  and the outer surface  30   b  of the dilator member  30  are generally cylindrical in shape and, therefore, define respective cross-sectional shapes that are circular. In this embodiment of the invention, however, the inner surface  12   a  and the outer surface  12   b  are oriented concentrically relative to one another. Thus, the wall thickness of the dilator member  30  does not vary circumferentially. Rather, a plurality of projections  30   c,    30   d,    30   e,  and  30   f  are provided on the inner surface  30   a  of the dilator member  30 . 
     In the illustrated embodiment, four of such projections  30   c,    30   d,    30   e,  and  30   f  are provided on the inner surface  30   a  of the dilator member  30 . However, it will be appreciated that this invention may be practiced with any number of such projections, including only a single projection. It will also be appreciated that any number of such projections may be provided on the outer surface  30   b  of the dilator member  30 , either alone or in combination with projections provided on the inner surface  30   a  of the dilator member  30 . Furthermore, the projections  30   c,    30   d,    30   e,  and  30   f  may extend axially throughout some or all of the length of the dilator member  30  as desired. 
     The projections  30   c,    30   d,    30   e,  and  30   f  are sized to provide an eccentric orientation relative to another dilator member (not shown) when the dilator member  30  is inserted axially over such other dilator member in the manner described above. In the illustrated embodiment, the projection  30   c  has a first size, the projections  30   d  and  30   e  each have a second size that is smaller than the first size, and the projection  30   f  has a third size that is smaller than the second size. Thus, when the dilator member  30  is inserted axially over another dilator member in the manner described above, the projections  30   c,    30   d,    30   e,  and  30   f  support the dilator member  30  eccentrically relative to such other dilator member. The projections  30   c,    30   d,    30   e,  and  30   f  may be sized as desired to accomplish any desired eccentric orientation. 
     Each of the illustrated projections  30   c,    30   d,    30   e,  and  30   f  has a cross-sectional shape that is generally arcuate. However, it will be appreciated that some or all of the projections  30   c,    30   d,    30   e,  and  30   f  may be different shapes.  FIGS. 8 through 12  illustrate some of such alternative shapes. In  FIG. 8 , a dilator member, indicated generally at  31 , includes a projection  31   a  having a cross-sectional shape that is generally rectangular with convex sides. In  FIG. 9 , a dilator member, indicated generally at  32 , includes a projection  32   a  having a cross-sectional shape that is generally rectangular with concave sides. In  FIG. 10 , a dilator member, indicated generally at  33 , includes a projection  33   a  having a cross-sectional shape that is generally rectangular with a central recess. In  FIG. 1 , a dilator member, indicated generally at  34 , includes a projection  34   a  having a cross-sectional shape that is generally rectangular with radial sides. In  FIG. 12 , a dilator member, indicated generally at  35 , includes a pair of projections  35   a,  each having a cross-sectional shape that is generally rectangular with radial sides. Any other desired shape or combination of shapes is contemplated to be within the scope of this invention. 
       FIGS. 13 ,  14 , and  15  illustrate a ninth embodiment of a soft tissue dilator, indicated generally at  20 ′, in accordance with this invention. The ninth embodiment of the dilator  20 ′ is similar to the second embodiment of the dilator  20  and includes a first dilator member  21 ′ that, in the illustrated embodiment, is solid and cylindrical in shape. The illustrated first dilator member  21 ′ includes an outer surface  21   a ′ that defines a circular cross-sectional shape, as best shown in  FIG. 15 . However, the first dilator member  21 ′ need not be solid and may be formed having any desired cross-sectional shape. The illustrated first dilator member  21 ′ has a leading end surface  21   b ′ that is flat and circular in shape. However, the leading end surface  21   b ′ may be formed having any desired shape, such as a tapered point. 
     The ninth embodiment of the dilator  20 ′ also includes second through fifth dilator members  22 ′ through  25 ′. Similar to the dilator members  22  through  25  of the second embodiment of the dilator  20  described above, the dilator members  22 ′ through  25 ′ of the ninth embodiment of the dilator  20 ′ have respective inner surfaces  22   a ′ through  25   a ′, outer surfaces  22   b ′ through  25   b ′, and leading end surfaces  22   c ′ through  25   c ′ provided thereon. However, all of the inner surfaces  22   a ′ through  25   a ′ of the dilator members  22 ′ through  25 ′ of the ninth embodiment of the dilator  20 ′ are oriented concentrically relative to the outer surfaces  22   b ′ through  25   b ′ thereof. Also, the dilator members  22 ′ through  25 ′ of the ninth embodiment of the dilator  20 ′ are not formed in a circumferentially complete manner. Rather, as best shown in  FIG. 15 , each of the dilator members  22 ′ through  25 ′ of the ninth embodiment of the dilator  20 ′ is circumferentially incomplete, having removed portions  22   d ′ through  25   d ′ provided thereon. The circumferentially incomplete portions  22   d ′ through  25   d ′ of the dilator members  22 ′ through  25 ′ of the ninth embodiment of the dilator  20 ′ provide additional space at the surgical site if needed. 
     Referring now to  FIG. 16 , there is illustrated an illuminating docking pin, indicated generally at  40 , in accordance with this invention. The illuminating docking pin  40  includes a tip portion  41 , an elongated body portion  42 , and an end portion  43 . The tip portion  41  of the illuminating docking pin  40  is conventional in the art and may be formed having any desired geometry, such as beveled, diamond tip, spear shaped, drill tip, or threaded. The length of the body portion  42  of the illuminating docking pin  40  maybe varied as desired to accommodate the thickness of the tissue between the skin and the lateral aspect of the spinal column. The body portion  42  of the illuminating docking pin  40  may also formed having any desired cross-sectional shape, such as round, oval, trapezoid, etc., and combinations thereof. The end portion  43  of the illuminating docking pin  40  may be utilized for driving the pin  40  in the desired location to secure a retractor (not shown) in place. If desired, the body portion  42  of the illuminated docking pin  40  may have one or more outwardly extending projections  42   a  provided thereon to prevent the pin  40  from being driven too deep and to help keep the retractor in place. 
     The illuminating docking pin  40  includes a light source for providing light to the surgical site, which is located near the tip portion  41 . The light source of the illuminating docking pin  40  may be embodied as a passageway  44  that extends from a first port  44   a  located near the end portion  43  to a second port  44   b  located near the tip portion  41 . The second port  44   b  may, for example, be located from about 1.5 cm to about 5.0 cm away from the tip portion  41  of the illuminating docking pin  40  or elsewhere as desired. The passageway  44  is effective to transmit ambient light from the first port  44   a  through the body portion  42  and the second port  44   b  to the surgical site located near the tip portion  41 . As shown in  FIG. 16 , the first port  44   a  may be located directly adjacent to the end portion  43  of the illuminating docking pin  40 . Alternatively, as shown in  FIG. 17 , the first port  44   a ′ may be spaced apart from the end portion  43 ′ within the body portion  42 ′ of the illuminating docking pin  40 ′. On the other hand, as also illustrated in  FIG. 16 , a separate source of light  45  (such as a battery powered lamp) may be disposed within the passageway  44  for this purpose. 
     The above detailed description of this invention is given for explanatory purposes. It will be apparent to those skilled in the art that numerous changes and modifications other than those cited can be made without departing from the scope of the invention. Accordingly, the whole of the foregoing description is to be construed in an illustrative and not a limitative sense, the scope of the invention being defined by the appended claims.