US 5,443,452 AGrant
Seal Assembly for Access Device
Issue Date:1995-08-22
•19 Claims
•4 Drawing Sheets
Abstract
Access devices to facilitate the introduction of various surgical instruments into vessels, conduits, cavities, and other interior regions of the body are disclosed, which include a "duck bill" valve having at least three flexible fold sections or leaflets subtending 360.degree. degrees of the cylindrical wall portion. The valve is configured to provide reduced friction forces when an instrument is inserted, as well as zero closure characteristics when the instrument is removed.
Metadata
Assignee
- Applied Medical Resources
Inventors
- Charles C. Hart
- Donald L. Gadberry
Application Information
Application Number:US 0516090
Filing Date:1993-04-23
Priority Date:1992-07-02
Art Unit:338
Classifications
IPC:
A61M 5178
Field of Search:
604137167;246;247;256846;849
Patent Drawings (4 sheets)
Description
Background of the Invention
1. Field of the Invention
This invention relates generally to access devices and more specifically to seals and other closure mechanisms applicable to devices.
2. Discussion of the Prior Art
Access devices are commonly used in surgery to facilitate the introduction of various surgical instruments into vessels, conduits, cavities, and other interior regions of the body. These access devices include, for example, devices which facilitate introduction of a needle into a vessel, and trocars which facilitate the introduction of laparoscopic instruments into the abdomen of the body.
Some of these access devices are introduced into regions which include a fluid under pressure. In the case of a needle access device, this fluid may be a liquid such as blood. In the case of a trocar, this fluid may be a gas such as an insufflation gas. In either case it is desirable to provide for the introduction of the surgical instrument into the cavity without permitting the escape of the pressurized fluid.
Seal mechanisms are usually employed in an access device to prevent this escape of fluid. Such mechanisms have typically included an aperture or septum valve which has functioned satisfactorily to form a seal around the outer surface of an instrument positioned within the access device. However, when this instrument has been removed, such aperture or septum valves have been relatively ineffective in providing a complete closure or zero closure to prevent the escape of pressurized fluid. For this reason, a zero closure valve has typically been included in the seal mechanism. While zero closure valves are typically not relied on to seal with an installment in place, they nevertheless are expected to provide an effective closure when the instrument is removed.
In the past, zero closure valves have included flapper valves which permit the instrument to pass through the valve, but which are automatically biased to a closed position when the instrument is removed. One problem with the flapper valve is its complexity which generally equates to expensive and difficult manufacturing processes. An example of the flapper valve is disclosed by Moll et al. in U.S. Pat. No. 4,654,030.
Another type of zero closure valve is commonly referred to as a "duck bill" valve. This particular mechanism can form an effective zero closure, but it suffers from high friction forces which tend to inhibit insertion and removal of the instrument. The duck bill mechanism also tends to invert when the instrument is removed. An example of the "duck bill" valve is disclosed by Edwards, et al, in U.S. Pat. No. 4,566,493.
Summary of the Invention
The present invention relates to access devices having zero closure valves which are moldable and therefore inexpensive to manufacture, which offer minimal resistance to instrument insertion and removal, which offer significant zero closure forces, and which automatically return to an operable configuration if inverted. The valve includes multiple walls which form fold segments that are automatically biased to a closed state and are responsive to the fluid pressure to maintain that state. At least three fold segments are desired in order to provide inner surfaces which intersect along lines that contact the instrument upon insertion. The reduced area provided by these lines of contact significantly reduce instrument insertion forces and enable the valve to easily unfold to an accommodating open state.
In accordance with one aspect of the invention, a surgical instrument having a longitudinal configuration and extending between a proximal end and a distal end, includes a housing having an inner surface defining a chamber and a cannula extending distally of the housing and having at least one lumen communicating with the chamber. A valve disposed in the housing has a plurality of walls which form a first seal with the inner surface of the housing, and a second seal. The walls are foldable between a first position wherein the second seal closes the lumen of the cannula and a second position wherein the second seal opens the lumen of the cannula.
Another aspect the invention includes a method for actuating a valve having an axis extending between a distal end and a proximal end, the valve having a normally closed state and an open state. Steps of the method include providing the valve with an outer wall defining a channel and at least three generally planar inner walls extending distally inwardly from the outer wall. At least two of the three walls intersect to form a slit having a normally closed state. A further step includes orienting one of the two walls and the third wall to intersect along a line extending radially outwardly with progressive proximal positions. Inserting an elongate device distally into the valve contacts the line and opens the slit.
These and other features and advantages of the invention will be more apparent with a discussion of preferred embodiments and reference to the associated drawings.
Description of the Drawings
FIG. 1 is a side elevation view of a trocar partially cut away to illustrate a zero closure valve in one embodiment of the present invention;
FIG. 2 is a perspective view of the zero closure valve illustrated in FIG. 1;
FIG. 3 is an enlarged side elevation view of the zero closure valve of FIG. 2;
FIG. 4 is a front elevation view of the valve taken along lines 4--4 of FIG. 3;
FIG. 5 is a rear elevation view taken along lines 5--5 of FIG. 3;
FIG. 6 is a side elevation view illustrating an instrument operatively positioned within the valve of FIG. 3;
FIG. 7 is a front elevation view taken along lines 7--7 of FIG. 6;
FIG. 8 is a rear elevation view taken along lines 8--8 of FIG. 6;
FIG. 9 is a diagram depicting closure forces typical for a valve of the present invention.
FIG. 10 is a perspective view similar to FIG. 2 of an embodiment including three fold sections;
FIG. 11 is a front elevation view taken along lines 11--11 of FIG. 10;
FIG. 12 is a rear elevation view taken along lines 12--12 of FIG. 10, and
FIG. 13 is a perspective view of a further embodiment of the invention wherein the a distal end of the valve has a convex configuration.
Description of Preferred Embodiments and Best Mode of the Invention
A trocar is illustrated in FIG. 1 and designated generally by the reference numeral 10. The trocar 10 is represented of many types of surgical access devices which include a housing 12 and a cannula 14 which is adapted to extend across a body wall into a body cavity. In the case of the trocar 10, the cannula 14 is configured to extend through an abdominal wall 16 into a cavity, such as the abdominal cavity 18. The housing 12 includes a chamber 21 which is defined by an inner surface 23. This chamber 21 of the housing 12 communicates with a lumen 25 in the cannula 14 which is defined by an inner surface 27.
With these feature, the trocar 10 is adapted to receive an instrument 28 having an elongate configuration and an outer surface 29. The instrument 28 is illustrated to be a pair of scissors having a length sufficient to pass through the trocar 10 and into the cavity 18 to perform a surgical operation. Although scissors are illustrated in FIG. 1, it will be understood that the instrument 28 may include any variety of devices such as needles, retractors, scalpels, clamps and various other surgical devices.
The housing 12 is configured to provide structural support for a seal mechanism, which in the preferred embodiment includes an aperture or septum seal 30 and a zero closure seal 32. It is the function of these seal. 30, 32 to prevent the escape of any pressurized fluid from the cavity 18 whether the instrument 28 is operatively disposed in the trocar 10 or whether the instrument 28 is removed from the trocar 10. In either case it is desirable that the valves 30, 32 be configured to produce minimal friction forces as the instrument 28 is inserted into and removed from the trocar 10.
With these conflicting requirements for an effective, low-friction seal, the valve 30 is typically provided with an aperture 34 which has a circular cross section. This shape for the aperture 34 enables it to closely engage the outer surface 29 of the instrument 28 and to form a seal round the instrument 28 when it is operatively disposed.
The valve 30 will typically be formed of an elastomeric material so that the aperture 34 is biased to seal against the outer surface 29. In order to avoid significant friction forces, this aperture 34 is preferably sized to a diameter slightly less than the diameter of the surface 29. However, since various instruments and various diameters for the outer surface 29 may be required in a particular surgery, the valve 30 will typically have a configuration wherein the size of the aperture 34 is expandable in order to accommodate a range of instrument sizes.
Particularly for instruments having larger diameters, the aperture 34 will have some minimum diameter greater than zero in a normal state. As a consequence, the valve 30 is open when the instrument 28 is not positioned in the trocar 10. Although it has been contemplated to provide septum valves 30 with apertures 34 which are closed in normal state, these valves generate significant friction forces for instruments 28 having typically larger diameters. As a consequence, septum valves, such as the valve 30, are usually no relied upon to provide full closure when the instrument 28 is removed.
This purpose is more typically served by a valve, such as the valve 32, which is commonly referred to as a zero closure valve. As noted, such valves of the past have included flapper valves, which are complex and expensive, and duck-bill valves which are characterized by high friction forces.
A preferred embodiment of the zero closure valve 32 is illustrated in FIGS. 2-5. In this embodiment, the valve 32 has an axis 41 which extends from a distal end 43 to a proximal end 45. A cylindrical outer wall 50 has an outer surface 52, and an inner surface 54 best shown in FIG. 5. The outer wall 50 typically extends from the distal end 43 toward the proximal end 45 where it engages an enlarged flange 56. This flange 56 extends radially outwardly of the wall 52 and is configured to form a seal with the housing 12 of the trocar 10.
Of particular interest to the present invention is the distal end of the valve 32 which is configured to provide reduced friction forces when the instrument 28 is inserted, as well as zero closure characteristics when the instrument 28 is removed. This distal end 43 includes three or more fold sections such as those designated by the reference numerals 61a, 6lb, 61c, and 61d in FIG. 2.
In the illustrated embodiment, each of these sections has components which are similar to those in other sections. For simplicity, the reference numerals associated with these components will be the same, followed by a letter which designates the associated folding section. When the similar components are referred to collectively, the letters will be omitted.
The folding section 61 includes a pair of walls 63 and 65 which have a respective outer surfaces 67, 70 and inner surfaces 72, 74 (best shown in FIG. 5). In a preferred embodiment the walls 63 and 65 are parallel and have a thickness such as 0.06 to 0.10 inches which is equivalent to the thickness of the outer wall 50.
The outer surfaces 67, 70, and inner surfaces 72, 74 are planar in this embodiment and extend at angles which are dependent in part on the number of folding sections 61. In the embodiment of FIG. 2, the walls 63, 65 associated with each folding section 61 intersect each other at a first angle, designated 71 in FIG. 2, and intersect the axis 41 at a second angle, designated 73 in FIG. 3.
The first angle 71 between the associated walls 63, 65 will typically be equal to 360.degree. divided by the number of folding sections 61. In the illustrated embodiment, this first angle 71 between the walls 63 and 65 is 90.degree. . In an embodiment including only three of the folding sections 61, the first angle 71 typically would be 120.degree.. In an embodiment including five of the folding sections 61 the first angle 71 typically would be 72.degree..
In a symmetrical embodiment, the second angle 73 between each of walls 63, 65 and the axis 41, would be equal. This angle, could be increased to accommodate a shorter axial length for the valve 32, or decreased to accommodate a longer axial length for the valve 32.
In the illustrated embodiment, the surfaces 67a, 70a provide the folding section 61a with a generally concave outer surface and intersect each other along a line 81a. This line of intersection 81a extends radially outwardly with progressive proximal positions until it contacts the surface 52 at a point 83a. As point 83 moves progressively toward the flange 56 the second angle 73 between each of the surfaces 67a, 70a and the axis 41 is reduced. The distance between the point 83 and the flange 56 should be sufficient to allow adequate flexure of the material without enabling a set or otherwise overstressing the material. In a preferred embodiment, the valve 30 has an overall length of 1.125 inches and the flange 56 has an axial thickness of 0.125 inches. The distance between the point 83 and the flange is 0.50 inches; it follows that the point 83 in this embodiment lies approximately half the distance between the distal end of the valve 30 and the flange 56.
The friction forces created when the instrument 28 is moved within the valve 32 are associated with the configuration of the inner surfaces 72 and 74. With reference to FIG. 5, it will be noted that these surfaces 72, 74 in the fold section 61 have a generally convex configuration and intersect each other along a line 85 which extends radially inwardly with progressive distal positions. The same relationship of the surfaces 72, 74 and lines 85 can exist for each of the fold sections 61. Thus for the fold section 6lb, the surfaces 72b and 74b have a convex configuration and meet to define an inner line 85b which extends radially inwardly with progressive distal positions along the valve 32.
Since the innermost extension of the surfaces 72 and 74 is the respective line 85, it follows that an instrument 28 introduced along the axis 41 contacts the valve 10 only along these lines 85a-d. The instrument 28 preferably makes no contact with the surfaces 72a-d and 74a-d. This result is highly advantageous since the area of contact between the valve 32 and the instrument 28 is greatly reduced. It follows that friction forces associated with the insertion and removal of the instrument 28 are also greatly reduced.
This feature relating to reduced friction is dramatically illustrated in FIGS. 6-8. As the instrument 28 is inserted into the valve 32 in its normally closed state, the fold sections 61 unfold to permit the instrument 28 to pass through the valve 32 in its open state, As the instrument 28 is inserted, its outer surface 29 contacts the valve 32 along the inner lines 85a-d forcing the fold section 61a-d radially outwardly to create and expand an opening 92 into the cannula 14. This opening 92 is best illustrated in the end views of FIG. 7 and FIG. 8. As the instrument 28 is introduced along the axis 41 it contacts the inner lines 85a-d along relatively short section designated 96a-d. Depending on the diameter of the instrument 28, this section 96a-d may be very short relative to the overall length of the line 85a-d, and in some cases, the section 96 of the line 85 may be substantially a point of contact. The narrower the line 85 and the shorter the section of contact 96, the lower the friction forces associated with insertion and removal of the instrument 28.
Although the seal 32 of the present invention is of particular interest for the low friction characteristics which it presents to the instrument 28, a primary purpose of the valve 32 is to provide a closure mechanism before the instrument 28 is inserted and after it is removed. In this case, the valve 32 provides a zero closure mechanism which is not possible with normal septum seals.
The zero closure state is best illustrated in FIG. 2 where it can be seen that the walls 63 and 65 which define the fold sections 61 extend distally to a surface 101 which is defined in a radial plane in this particular embodiment. As the instrument 28 is inserted into the valve 32, the valve moves from its normally closed state an open state as illustrated in to FIGS. 6-8. The inner surfaces 72, 74 in adjacent fold sections 61 intersect along a slit 103 which extends from the inside of the seal 32 and through the distal surface 101. In a preferred embodiment each slit 103 extends from the inner surface 54 of the wall 50 to the axis 41. There is one slit for each of the fold sections 61. Thus in the illustrated embodiment including four of the fold section 61, there are four slit 103. If these four slits are equally spaced around the axis 1, alternate slits are coplanar and the slits 103 collectively define a cross.
Since a primary function of the valve 32 is to provide the zero closure mechanism i is important to consider the nature of the closure forces which exist in the absence of the instrument 28. The seal 32 is of particular advantage when it is disposed so that the surface 101 faces any fluid pressures such as those which may be associated with an insufflation gas, commonly used in laparoscopic surgery. Oriented in this direction, the outer surfaces 67, 70 associated with the walls 63 and 65, respectively, are oriented with at least a component facing distally of the valve 32. This presents the surfaces 67, 70 such that a fluid pressure on these surfaces will act to close the slits 103. In a particular embodiment wherein the surfaces 67, 70 are parallel to the slits 103, this component of force is directed perpendicular to the respective slit 103.
This can be better understood with reference to FIG. 9 wherein the wall 65b is shown in cross-section. In this diagram, an arrow 110 is representative of the axial direction from which fluid forces would emanate. As these forces impinge upon the outer surface 67b of the wall 65b, they create a force along arrow 112 which is perpendicular to the surface 67b. This arrow 112 can be separated into an axial component, represented by an arrow 114 and a radial component 116. It is the radial force represented by the arrow 116 which is perpendicular to the slit 103. Thus, response to the fluid pressure along the arrow 110 results in a component force along the arrow 16 which tends to move the slit 103 to a closed position.
It can now be seen that as the angle 73 between the surface 65b and the axis 41 increases, the length of the arrow 116 decreases providing a reduced closing force. This reduction in closing force must be compromised with the length of the valve 32 which can be reduced as the angle 73 increases. In the same manner, closing forces associated with the arrow 116 can be increased with a reduction in the angle 73 which will typically require a longer valve
As previously mentioned, the concept of this invention is not to be limited by the number of folding sections 61. An embodiment of particular interest might include only three folding sections 61a-c as illustrated in FIGS. 10-12. In such an embodiment, the surfaces 67, 70 would be larger and for that reason could provide increased closing forced on the slits 103. Fewer folding sections 61 would also increase the circumferential distance separating the slits 103. This would equate to longer lever arms resisting the outward movement of the lines 81 as the valve 32 unfolds. Ultimately this could produce reduced friction forces opposing the entry of the instrument, 28. With increased closing forces and decreased friction, this embodiment including three fold sections 61, could be of particular interest.
Also included in the invention is a preferred method for inserting the instrument 28 through the valve 32. In this method, the valve has the axis 41 and three or more of the fold sections 61. Each of these sections defines an opening such as the slit 103 and is movable between a first position wherein the slit 103 is in a closed state as illustrated in FIG. 2 and a second position wherein the slit 103 is in an open state as illustrated in FIG. 6. As the instrument 28 is introduced into the valve 32, it contacts the walls 63, 65 of the valve along the lines 85. With further insertion of the instrument, the fold sections 61 move from the closed first position to the open second position. When the instrument 28 is ultimately removed from the valve 32, the fold sections 61 return from the open second state illustrated in FIG. 6 to the closed first state illustrated in FIG. 2.
Orienting the valve 32 so that the outer surfaces 70, 72 face in the distal direction makes the valve 32 particularly valuable for use in a pressurized environment. For example, when the cannula 14 is introduced into the abdominal cavity for laparoscopic surgery, a commonly used insufflation gas will exert a pressure on the distally facing outer surfaces 67, 70. As previously disclosed with reference to FIG. 9, the forces generated by the insufflation gas will tend to force the valve 32 to its zero closure state.
A further embodiment of the invention is illustrated in the perspective view of FIG. 13. This embodiment is similar to that illustrated in FIGS. 2 and 10 except that the distal end 43 of the valve 30 is generally curved. This curve is preferably convex so that the surface 101 at the distal end of the valve 30 extends decreasingly, radially, outwardly with progressive proximal positions along the axis 41. In a preferred embodiment, the surface 101, forms the quadrant of a circle. Thus the surface 101 is substantially perpendicular to the surface 52 at the axis 41 and substantially parallel to the axis 41 at the surface 52.
This particular embodiment having a curved distal end 43, has been found to offer improved characteristics for returning to its normal shape in the event of an inversion. While this feature is present to some extent in all of the foregoing embodiments, this particular embodiment appears to offer the greatest resistance to inversion and the greatest tendency for returning to its normal shape.
Although the invention has been disclosed with reference to specific embodiments and methods, it will be apparent to those skilled in the art that the concept can be structured in other embodiments, and the method can be carried out with other steps. For example, and without limitation, other embodiments might include more than four of the fold sections 61. Similarly, the outer surfaces 67, 70 defining the walls 64, 65 of the fold sections 61 could be angled in different directions. This angle of orientation may affect not only the closing forces but also the axial dimension of the valve 32 as previously discussed.
While the illustrated embodiments all terminate in a distal surface 101 which is disposed in a radial plane, it will be apparent that this distal surface can be disposed at other angles relative to the axis 41. Similarly, the symmetry illustrated and disclosed for the fold section 61, the surfaces 67 and 70, the length of the slits 103, and the length of the lines 81 and 85, could be varied to produce or asymmetrical embodiment.
The inner surfaces 54, 72 and 74 may also be varied in their respective shapes. Whenever possible, these various configurations should produce the point or line of contact, such as the line 85. As noted, this reduced area of contact will significantly reduce the friction forces associated with the insertion and removal of the instrument 28.
Given these wide variations, which are all within the scope of this concept, one is cautioned not to restrict the invention to the embodiments which have been specifically disclosed and illustrated, but rather encouraged to determine the scope of the invention only with reference to the following claims.
Claims
We claim:
1. A surgical device having a longitudinal configuration and an axis extending between a proximal end and an opposing distal end, the device comprising: a housing disposed at the proximal end of the device and having an inner surface defining a chamber; a cannula extending distally of the housing and having at least one lumen communicating with the chamber of the housing; a first valve disposed in the housing and having a plurality of walls forming a first seal with the inner surface of the housing and a second seal; the walls being foldable between a first position wherein the second seal closes the at least one lumen of the cannula and a second position wherein the second seal opens the lumen of the cannula; and a second valve disposed proximally of the first valve and having an aperture therein which is adapted to receive a surgical instrument therethrough and to form a seal about an outer surface of said instrument.
2. The surgical device recited in claim 1 wherein the walls of the first valve form at least one fold segment including a first wall and a second wall disposed at an acute angle to the first wall.
3. The surgical device recited in claim 2 wherein each of the first wall and the second wall include an outer surface which faces distally of the first valve.
4. The surgical device recited in claim 1 wherein the walls of the first valve are biased to the first position.
5. The surgical device recited in claim 1 wherein the walls of the first valve form a first slit and a second slit which intersects the first slit.
6. The surgical device recited in claim 5 wherein the first valve has an axis and the first slit intersects the second slit at a particular angle along said valve axis.
7. The surgical device recited in claim 6 wherein the particular angle is in a range of about 60.degree. to about 120.degree..
8. A surgical device adapted to extend into a cavity pressurized with a fluid, the device comprising: a tube having an elongate configuration and an inner surface defining a lumen along the length of the tube; a first valve disposed in the tube and having an axis extending between a proximal end of the first valve and a distal end of the first valve, the first valve forming a seal with the inner surface of the tube; a first pair of walls included in the valve, each of the first pair of walls being disposed in an associated plane which is disposed at an acute angle to the axis of the first valve, the first pair of walls forming a first openable slit; a second pair of walls included in the first valve, each of the second pair of walls being disposed in an associated plane which is disposed at an acute angle to the axis of the first valve, the second pair of walls forming a second openable slit; the first slit being disposed at an angle not greater than 120.degree. to the second slit; and a second valve disposed proximally of the first valve and having an aperture therein which is adapted to receive a surgical instrument therethrough and to form a seal about an outer surface of said instrument.
9. The surgical device recited in claim 8 wherein at least one of the first pair of walls faces distally and is adapted to respond to the pressure of the fluid in the cavity to facilitate closure of the first slit.
10. The surgical device recited in claim 8 wherein at least one of the first pair of walls is disposed to face in a direction such that the force produced on the wall by the pressure of the fluid has a component directed to close the first slit.
11. The surgical device recited in claim 10 wherein the first slit and the second slit are disposed in a particular plane and the component of force is directed generally parallel to the particular plane.
12. The surgical device recited in claim 11 wherein the particular plane is a radial plane.
13. The surgical device of claim 8 wherein the tube forms a housing of a trocar.
14. A trocar assembly adapted to provide access for a surgical instrument across a body wall and into a body cavity, comprising: a cannula having a distal end and a proximal end, the cannula being adapted to be positioned across the body wall with the distal end extending into the body cavity; a housing disposed at the proximal end of the cannula and forming with the cannula a lumen sized and configured to receive the instrument; a first valve disposed in the housing and across the lumen, the first valve having characteristics for opening the lumen when the instrument is inserted into the trocar and for closing the lumen when the instrument is withdrawn from the trocar assembly; an outer wall included in the first valve and forming a seal with the housing; a plurality of fold sections, each including a pair of walls extending distally inwardly of the outer wall and forming a slit having an open state when the instrument is inserted into the lumen and a closed state when the instrument is withdrawn from the lumen; and a second valve disposed proximally of the first valve and having an aperture therein which is adapted to receive said instrument therethrough and to form a seal about an outer surface of said instrument.
15. The trocar assembly recited in claim 14 wherein the outer wall forms a cylinder having an axis.
16. The trocar assembly recited in claim 15 further comprising at least three of the fold sections each forming a slit wherein all of the slits intersect at point disposed along the axis of the cylinder.
17. The trocar assembly recited in claim 16 further comprising four of the fold sections each forming a slit wherein the slits of alternate sections are disposed along a line.
18. The trocar assembly recited in claim 17 wherein the four fold sections form two of the lines and the two lines intersect at the point along the axis of the cylinder.
19. The trocar assembly recited in claim 18 wherein the two intersecting lines are substantially perpendicular.
Patent Citations (17)
| Patent | Date | Inventor | Cited By |
|---|---|---|---|
| US3861416 | 1975-01-01 | Wichterle | |
| US4143853 | 1979-03-01 | Abramson | |
| US4177814 | 1979-12-01 | Knepshield et al. | |
| US4222126 | 1980-09-01 | Boretos et al. | |
| US4364127 | 1982-12-01 | Pierce et al. | |
| US4375864 | 1983-03-01 | Savage | |
| US4424833 | 1984-01-01 | Spector et al. | |
| US4475548 | 1984-10-01 | Muto | |
| US4492253 | 1985-01-01 | Raftis | |
| US4535819 | 1985-08-01 | Atkinson | |
| US4566493 | 1986-01-01 | Edwards | |
| US4673393 | 1987-06-01 | Suzuki et al. | |
| US4765588 | 1988-08-01 | Atkinson et al. | |
| US4798584 | 1989-01-01 | Hillsted | |
| US4809679 | 1989-03-01 | Shimonaka et al. | |
| US5010925 | 1991-04-01 | Atkinson | |
| US5141498 | 1992-08-01 | Christian |