WO2021231867A1 - Retractor for gasless laparoscopy - Google Patents

Abstract

A retractor for laparoscopic surgery includes a rod including a distal portion for inserting in a subject and a proximal portion for lifting the skin of the subject. The distal portion includes at least one substantially semicircular segment, and the proximal portion includes a straight segment and extends substantially orthogonal to the distal portion.

Description

Retractor for Gasless Laparoscopy

Related Applications

[0001] This application claims priority from U.S. Provisional Patent Application No. 63/025,266, filed May 15, 2020, the disclosure of which is incorporated by reference in its entirety.

Background

[0002] Current practice of laparoscopic surgery creates an operating space by introducing carbon dioxide at a regulated pressure to push up the abdominal wall. If the pressure is not carefully regulated, this can lead to cardiovascular collapse. Particularly in low- and middle-income countries (LMICs), reliable sources of gas may not be available or affordable. Hence, there is an opportunity for alternative methods of creating a cavity for laparoscopic and other types of surgery.

Summary

[0003] The Summary is provided to introduce a selection of concepts that are further described below in the Detailed Description. This Summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.

[0004] One aspect of the present disclosure provides a lifting device for laparoscopic surgery, comprising consisting of, or consisting essentially of a rod, wherein the rod comprises a distal portion for inserting in a subject and a proximal portion for lifting the skin.

[0005] In some embodiments, the distal portion is disposed at least partially in a plane, and the proximal portion extends orthogonally from the plane.

[0006] Another aspect of the present disclosure provides a method of performing a gasless laparoscopic procedure using a lifting device as described herein.

[0007] Another aspect of the present disclosure provides a retractor for laparoscopic surgery, the retractor including a rod including a distal portion for inserting in a subject and a proximal portion for lifting the skin of the subject. The distal portion may include at least one substantially semicircular segment, and the proximal portion may include a straight segment and may extend substantially orthogonal to the distal portion.

[0008] In some embodiments, the distal portion includes a distal end tip at an endpoint of the-at least one substantially semicircular segment. The tip may be rounded and smooth. [0009] In some embodiments, the at least one substantially semicircular segment includes a first substantially semicircular segment including the tip at a first endpoint thereof and a second substantially semicircular segment having a first endpoint at a second endpoint of the first substantially semicircular segment. The proximal portion may extend from a second endpoint of the second substantially semicircular segment.

[0010] In some embodiments, the second endpoint of the second substantially semicircular segment is substantially centered between the first and second endpoints of the first substantially semicircular segment.

[0011] In some embodiments, the first substantially semicircular segment has a first diameter and the second substantially semicircular segment has a second diameter that is smaller than the first diameter. The second diameter may be about half the first diameter. [0012] In some embodiments, the distal portion resides in a plane and the proximal portion extends substantially orthogonal to the plane.

[0013] In some embodiments, a center of a circle defined by the first substantially semicircular segment is offset from a center of a circle defined by the second substantially semicircular segment.

[0014] In some embodiments, the first substantially semicircular segment has a first radius of curvature and the second substantially semicircular segment has a second radius of curvature that is smaller than the first radius of curvature.

[0015] In some embodiments, the proximal portion includes an engagement feature at a proximal end thereof, with the attachment feature configured for engagement to a stand to maintain the lifting the skin of the subject. The attachment feature may include a hook extending from the straight segment. The attachment feature may include threads at a proximal end of the straight segment.

[0016] In some embodiments, the rod is monolithic.

[0017] In some embodiments, the rod is rigid.

[0018] In some embodiments, the rod includes stainless steel. [0019] Another aspect of the present disclosure provides a kit including a plurality of the retractors as described herein. A first diameter of the first substantially semicircular segment and a second diameter of the second substantially semicircular segment are different for each of the plurality of retractors to accommodate different subjects having different heights and/or weights.

[0020] Another aspect of the present disclosure provides a system including a retractor as described herein and a stand. The stand includes: a planar base plate; a first elongated member extending orthogonally from the base plate; and a second elongated member extending orthogonally from the first elongated member. An engagement feature of the retractor is configured to engage the second elongated member such that an upward force on the retractor is maintained.

[0021] In some embodiments, the stand is configured to be placed on an operating surface beneath a subject’s back such that the subject’s weight stabilizes the stand.

[0022] In some embodiments, the second elongated member is adjustable vertically along to the first elongated member.

[0023] Another aspect of the present disclosure provides a method of performing a gasless laparoscopic procedure. The method includes: providing a retractor as described herein; inserting the distal end of the retractor into an incision in a subject; rotating the retractor toward the distal end to increasingly introduce the distal portion until all or substantially all of the distal portion is below an anterior abdominal wall of the subject; and applying vertical tension to the proximal portion of the retractor to lift the abdominal wall away from internal organs to provide adequate visualization inside the abdominal cavity of the subject. [0024] In some embodiments, the method further includes connecting an attachment feature at the proximal end of the retractor to a stand after applying vertical tension to the proximal portion of the retractor to maintain the vertical tension. The method may further include positioning a base plate of the stand under the subject’s back prior to inserting the distal end of the retractor into the incision to stabilize the stand with the patient’s weight. [0025] In some embodiments, the geometry of the retractor is such that applying vertical tension to the proximal portion of the retractor creates a cavity having a shape or volume of a truncated cone. [0026] In some embodiments, the retractor is one of a plurality of retractors each having a differently sized distal portion. The method may further include selecting one of the plurality of retractors based on the subject’s height and/or weight.

Brief Description of the Drawings

[0027] The accompanying Figures and Examples are provided by way of illustration and not by way of limitation. The foregoing aspects and other features of the disclosure are explained in the following description, taken in connection with the accompanying example figures (also “FIG.”).

[0028] Fig. l is a perspective view of a retractor according to some embodiments.

[0029] Fig. 2 is a bottom view of the retractor of Fig. 1.

[0030] Fig. 3 is a side view of the retractor of Fig. 1.

[0031] Fig. 4 is a perspective view of a retractor according to some other embodiments. [0032] Fig. 5 is a perspective view of a retractor system for laparoscopic surgery according to some embodiments.

[0033] Fig. 6 is a perspective view of a stand of the system of Fig. 5.

[0034] Fig. 7A is a perspective view of a retractor according to some other embodiments. [0035] Fig. 7B is a bottom view of the retractor of Fig. 7A.

[0036] Fig. 7C is a side view of the retractor of Fig. 7A.

[0037] Fig. 8A is a perspective view of a retractor according to some other embodiments. [0038] Fig. 8B is a bottom view of the retractor of Fig. 8 A.

[0039] Fig. 8C is a side view of the retractor of Fig. 8 A.

[0040] Fig. 9A is a perspective view of a retractor according to some other embodiments. [0041] Fig. 9B is a bottom view of the retractor of Fig. 9A.

[0042] Fig. 9C is a side view of the retractor of Fig. 9A.

[0043] Fig. 10A is a perspective view of a retractor according to some other embodiments. [0044] Fig. 10B is a bottom view of the retractor of Fig. 10A.

[0045] Fig. IOC is a side view of the retractor of Fig. 10A.

[0046] Fig. 11A is a perspective view of a Stainless Steel 316 ReadyLift prototype. The retractor is inserted through a small incision at the umbilicus and counter-rotated under the anterior abdominal wall. It has a rounded tip to prevent abdominal injury during insertion. When connected to the Bedside Stand through the curved hook, the user lifts and locks the system to retract the abdominal wall and provide intra-abdominal exposure.

[0047] Fig. 1 IB is a perspective view of a ReadyLift Bedside Stand placed on Medical Operating Table. The stand base is placed underneath the patient’s back so the patient’s weight will stabilize the entire lift system. The retractor is inserted into the abdomen and then secured onto the end of the horizontal rod of the stand. The entire system can be adjusted in the x, y, and z directions by loosening the clamps, repositioning, and then tightening the clamps. The system can also be rotated in the yaw and roll orientations following the same method.

[0048] Fig. 12 illustrates retractor radius derivation based on adjusted volumes achieved by pneumoperitoneum and ReadyLift. The goal is to find a relationship between the patient and the retractor where the chosen retractor provides an equivalent volume to that created by a pneumoperitoneum. At the vertical midplane of both volumes, the distance between the base of the abdomen and the retractor (h) can be found in relation to the side wall (s), body radius (r), and retractor radius (R) with Pythagorean’s Theorem (Eqn. 1). By equating the perimeter of the midplanes, the value of the side wall (s) can be found in relation to the body radius (r) and retractor radius (R) (Eqn. 2). These equations are then combined to find the value of (h) only in terms of the two radii (r, R) (Eqn. 3). The volume of the truncated cone (V) (Eqn. 4) is adjusted by substituting the previously determined equation for (h) to find the volume (V) in terms of the two radii (r, R) (Eqn. 5). To optimize the volume (V) as a function of the retractor radius (R), the derivative with respect to retractor radius (R) is taken and set to 0. Solving this equation results in a final relationship between retractor radius (R) and body radius (r).

[0049] Fig. 13 is a representative cross-sectional computed tomography image taken at the level of the umbilicus (upper arrow). The transverse abdominal radius is measured as the linear distance from the midpoint of the abdomen to the peritoneal border of the abdominal wall (lower arrow).

[0050] Figs. 14A-14F are charts illustrating a strong linear relationship was observed between Transverse Abdominal Radius and Age (Fig. 14A), Height (Fig. 14B), and Weight (Fig. 14C) (the coefficient of determination denoted R²=0.72, 0.73, 0.7, respectively). Comparatively, there was a poor relationship between Traverse Abdominal Radius and Body Mass Index (BMI) (Fig. 14D) and Abdominal Wall Thickness (Fig. 15E) (R²=0.26 and 0.47). In Fig. 14F, a new Normalized Summary Variable was defined to account for a patient’s height and weight together. The new variable had a stronger relationship than any individual factor (R²=0.75).

[0051] Fig. 15 is a chart providing a guide for selecting the correct retractor size in an operative setting based on the patient’s height and weight, which are readily accessible (polynomial regression, degree=2). Each box indicates the span of heights and weights covered by each retractor size. Each retractor size, and thus each box, is different from the others as shown in ANOVA with post-hoc Tukey HSD testing.

[0052] Fig. 16 illustrates finite element analysis simulation of ReadyLift prototype with 30 lb point mass acting on the mesh structure. Two different views of the same simulation are displayed. The displacement scale is shown with a maximum of 30 mm displacement. Circles outlined by box indicate the original position of retractor before the load was applied.

[0053] Fig. 17 is a Load-Deflection curve for ReadyLift prototype mounted on a custom hanging rack. Combinations of fractional and barbell weights were used ranging from 0.11 kg to 13.6 kg, which is the lifting force of a 20mmHg pneumoperitoneum. Images were taken with a static camera in two states: prior to and after weight placement on the base of the retractor. A ruler was affixed to the mounting rack to serve as a standard for measurement. Images were compared with ImageJ to determine the amount of deflection for each weight combination. Deflection remains linear until 13.6 kg, indicating that the material has not undergone permanent, plastic deformation.

Detailed Description

[0054] For the purposes of promoting an understanding of the principles of the present disclosure, reference will now be made to preferred embodiments and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the disclosure is thereby intended, such alteration and further modifications of the disclosure as illustrated herein, being contemplated as would normally occur to one skilled in the art to which the disclosure relates. [0055] Articles “a” and “an” are used herein to refer to one or to more than one (i.e. at least one) of the grammatical object of the article. By way of example, “an element” means at least one element and can include more than one element.

[0056] “About” is used to provide flexibility to a numerical range endpoint by providing that a given value may be “slightly above” or “slightly below” the endpoint without affecting the desired result.

[0057] The use herein of the terms "including," "comprising," or "having," and variations thereof, is meant to encompass the elements listed thereafter and equivalents thereof as well as additional elements. As used herein, “and/or” refers to and encompasses any and all possible combinations of one or more of the associated listed items, as well as the lack of combinations where interpreted in the alternative (“or”).

[0058] As used herein, the transitional phrase "consisting essentially of (and grammatical variants) is to be interpreted as encompassing the recited materials or steps "and those that do not materially affect the basic and novel characteristic(s)" of the claimed invention. Thus, the term "consisting essentially of as used herein should not be interpreted as equivalent to "comprising."

[0059] Moreover, the present disclosure also contemplates that in some embodiments, any feature or combination of features set forth herein can be excluded or omitted. To illustrate, if the specification states that a complex comprises components A, B and C, it is specifically intended that any of A, B or C, or a combination thereof, can be omitted and disclaimed singularly or in any combination.

[0060] Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. For example, if a concentration range is stated as 1% to 50%, it is intended that values such as 2% to 40%, 10% to 30%, or 1% to 3%, etc., are expressly enumerated in this specification. These are only examples of what is specifically intended, and all possible combinations of numerical values between and including the lowest value and the highest value enumerated are to be considered to be expressly stated in this disclosure. [0061] As used herein, the term "subject" and "patient" are used interchangeably herein and refer to both human and nonhuman animals. The term "nonhuman animals" of the disclosure includes all vertebrates, e.g ., mammals and non-mammals, such as nonhuman primates, sheep, dog, cat, horse, cow, chickens, amphibians, reptiles, and the like. In some embodiments, the subject comprises a human or animal who is undergoing a laparoscopic procedure with a device as prescribed herein.

[0062] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. [0063] In some aspects, the present disclosure provides a laparoscopic retractor, also known as a "Ready Lift" retractor, which mechanically lifts the skin of a subject to create an open internal space. The ReadyLift retractor can improve surgical safety and eliminate the need for carbon dioxide and continuous pressure regulation. The device can be substituted for any procedure that includes gaseous inflation of a body cavity. The retractor is low-cost technique to allow for gas-independent laparoscopic procedures in LMICs. [0064] A retractor 10 for laparoscopic surgery according to some embodiments is illustrated in Figs. 1-3. The retractor comprises, consists of, or consists essentially of a body 12 such as a rod including a distal portion 14 and a proximal portion 16. As described in more detail herein, the distal portion 14 is for inserting the retractor 10 in a subject and the proximal portion 16 is for lifting the retractor 10 to thereby lift the skin of the subject. [0065] The rod 12 may be monolithic. The rod 12 may be rigid. The rod 12 may have a smooth outer surface (e.g., along its length). The rod 12 may be formed of any material suitable for surgical use, such as but not limited to stainless steel, Nitinol, plastic, etc. In some embodiments, the rod 12 has a diameter or thickness of 5 mm or less. In this way, the retractor 10 can be inserted through a 5-millimeter incision at the umbilicus and then counter-rotated beneath the anterior abdominal wall.

[0066] The distal portion 14 includes a distal end tip 18. In some embodiments, the tip 18 is rounded and/or smooth to prevent abdominal injury during insertion.

[0067] The distal portion 14 may include a first semicircular or substantially semicircular segment 20 including the tip 18 at a first endpoint 22 of the first semicircular segment 20. The distal portion 14 may include a second semicircular or substantially semicircular segment 24 having a first endpoint 26 at a second endpoint 28 (opposite the first endpoint 22) of the first semicircular segment 20. The first endpoint 26 of the second semicircular segment 24 and the second endpoint 28 of the first semicircular segment 20 may collectively be referred to as a first transition point 30.

[0068] The proximal portion 16 may extend orthogonally or substantially orthogonally away from the distal portion 14. As used herein, “substantially orthogonal” or “substantially orthogonally” may mean 90° ± 3° in some embodiments, or 90° ± 5° in some other embodiments. The proximal portion 16 includes a straight segment 32. In some embodiments, the distal portion 14 resides in a plane P and the straight segment 32 extends orthogonally or substantially orthogonally away from the plane P. The proximal portion 16 (or the straight segment thereof) may extend away from a second endpoint 34 (opposite the first endpoint 26) of the second semicircular segment 24. The second endpoint 34 of the second semicircular segment 24 from which the straight segment 32 extends may be referred to as a second transition point 36. As can be seen in Fig. 3, there may be a gradual bend at the second transition point 36.

[0069] The first semicircular segment 20 may have a first diameter D1 and the second semicircular segment 24 may have a second diameter D2. In some embodiments, the first diameter D1 is about two times as large as the second diameter D2. In some embodiments, the first diameter D1 is about 100 mm and the second diameter D2 is about 50 mm. However, as described in more detail herein, the diameters Dl, D2 may vary such that the retractors accommodate subjects of different sizes.

[0070] As used herein, the term “substantially semicircular segment” may mean that the arc length of the segment may deviate slightly from the arc length of an exact semicircular segment. The arc length of a semicircle is pϋ/2 and, in some embodiments, the arc length of a “substantially semicircular segment” is pϋ/2 ± 5%. For example, if the diameter is 100 mm, the arc length of the substantially semicircular segment may be 157 ± 7.85 mm. In some other embodiments, the arc length of a “substantially semicircular segment” is pϋ/2 ± 10%. For example, if the diameter is 100 mm, the arc length of the substantially semicircular segment may be 157 ± 15.7 mm.

[0071] In some embodiments, the distal portion 14 comprises, consists of, or consists essentially of the first (substantially) semicircular segment 20 and the second (substantially) semicircular segment 24. [0072] The second endpoint 34 of the second semicircular segment 24 and/or the straight segment 32 may be centered or substantially centered between the first endpoint 22 and the second endpoint 28 of the first semicircular segment 20. In some embodiments, the second endpoint of the 34 of the second semicircular segment 24 and/or the straight segment 32 may be centered or substantially centered in a circle defined by the first semicircular segment 20.

[0073] The retractor 10 may include an attachment or engagement feature at a proximal end 40 of the proximal portion 16. The engagement feature may be configured to engage or connect the retractor 10 to a structure such as a stand as described in more detail herein. [0074] As shown in Figs. 1 and 3, the engagement feature may include a hook 42 extending from the straight segment 32. In some other embodiments, and as shown in Fig. 4, the engagement feature may comprise threading or threads 44 at the proximal end 40 of the proximal portion 16.

[0075] Referring to Fig. 3, the proximal portion 16 (or the straight segment 32 thereof) may have an average height or length HI . In some embodiments, the height HI is between 50 and 150 mm.

[0076] Fig. 5 illustrates a retractor system 100 for laparoscopic surgery. The system 100 may include the retractor 10 as described herein.

[0077] The system 100 may include a stand 50. Referring to Fig. 6, the stand includes a planar base plate 52, a first elongated member 54 such as a first rod extending orthogonally or vertically from the base plate 52, and a second elongated member 56 extending orthogonally or horizontally from the first elongated member 54.

[0078] In some embodiments, the horizontal rod 56 is connected to the vertical rod 54 by a clamp 58. The clamp 58 may be configured to allow the horizontal rod 56 to be adjusted in a vertical direction along the vertical rod 54 and/or lock the horizontal rod 56 in place. In some embodiments, a support 60 is on the horizontal rod 56. The support 60 may be configured to receive and hold the engagement feature (e.g., hook) of the retractor 10. For example, the support 60 may include a hook 62 that is configured to receive and hold the hook 42 (Figs. 1 and 3) of the retractor 10. The support 60 may be movable horizontally along the horizontal rod 56 and/or be lockable in place. [0079] In use, the retractor 10 can be inserted through an incision (e.g., a 5-millimeter incision at the umbilicus) and then counter-rotated beneath the anterior abdominal wall. Vertical tension on the retractor 10 (e.g., opposite gravity) lifts the abdominal wall to provide adequate visualization inside the abdominal cavity. The retractor 10 can be used in conjunction with the portable lifting stand 50, which can be referred to as a "ReadyLift Bedside Stand". The stand 50 can be placed adjacent to the subject or on a medical operating table T. When connected to the Bedside Stand through the curved hook 42 (Figs. 1 and 3), the user lifts and locks the system to retract the wall and provide intra-abdominal exposure. In some embodiments, the stand base 52 is placed underneath the patient’s back so the patient’s weight will stabilize the entire lift system. The retractor 10 is inserted into the abdomen and then secured onto the end of the horizontal rod 56 of the stand. The entire system can be adjusted in the x, y, and z directions by loosening the clamp 58 and/or the support 60, repositioning, and then tightening the clamp 58 and/or the support 60. For example, the horizontal rod 56 may be moveable vertically along the vertical rod 54. The system can also be rotated in the yaw and roll orientations using the clamp 58 and/or the support 60.

[0080] In some embodiments, the stand 50 may include an attachment or engagement feature such as a threaded passageway in the horizontal elongated member 56 and/or the support 60. The threads 44 of the retractor 10 (Fig. 4) can be threadingly received in the threaded passageway to secure the retractor 10 to the stand 50.

[0081] The retractor 10 and the stand 50 may be autoclaved or immersed in a disinfectant solution for sterilization.

[0082] As described herein, varying sized retractors can be provided to accommodate subjects of different sizes. Figs. 7-10 illustrate differently sized retractors compared to the retractor 10 shown in Figs. 1-3. The primary difference between the retractor 10 (Figs. 1- 3), the retractor 10a (Figs. 7A-7C), the retractor 10b (Figs. 8A-8C), the retractor 10c (Figs. 9A-9C), and the retractor lOd (Figs. 10A-10C) is the diameter of the semicircular segments 20 and 24. Other optional differences will be noted below. In the drawings, like reference numbers refer to like elements throughout.

[0083] Referring to Figs. 7A-7C, the retractor 10a includes the first substantially semicircular segment 20 having a first diameter D3 and the second substantially semicircular segment 24 having a second diameter D4. In some embodiments, the first diameter D3 is about 60 mm and the second diameter D4 is about 30 mm.

[0084] In some embodiments, the rod 12 of the retractor 10a has a diameter or thickness of 5 mm or less. In some other embodiments, the rod 12 of the retractor 10a has a diameter or thickness of 4 mm or less.

[0085] Referring to Fig. 7C, the proximal portion 16 (or the straight segment 32 thereof) may have an average height or length H2. In some embodiments, the height H2 is between 50 and 150 mm.

[0086] Referring to Figs. 8A-8C, the retractor 10b includes the first substantially semicircular segment 20 having a first diameter D5 and the second substantially semicircular segment 24 having a second diameter D6. In some embodiments, the first diameter D5 is about 140 mm and the second diameter D6 is about 70 mm.

[0087] In some embodiments, the rod 12 of the retractor 10b has a diameter or thickness of 5 mm or less.

[0088] Referring to Fig. 8C, the proximal portion 16 (or the straight segment 32 thereof) may have an average height or length H3. In some embodiments, the height H3 is between 50 and 200 mm.

[0089] Referring to Figs. 9A-9C, the retractor 10c includes the first substantially semicircular segment 20 having a first diameter D7 and the second substantially semicircular segment 24 having a second diameter D8. In some embodiments, the first diameter D7 is about 160 mm and the second diameter D8 is about 80 mm.

[0090] In some embodiments, the rod 12 of the retractor 10c has a diameter or thickness of 5 mm or less.

[0091] Referring to Fig. 9C, the proximal portion 16 (or the straight segment 32 thereof) may have an average height or length H4. In some embodiments, the height H4 is between 50 and 200 mm.

[0092] Referring to Figs. 10A-10C, the retractor lOd includes the first substantially semicircular segment 20 having a first diameter D9 and the second substantially semicircular segment 24 having a second diameter D10. In some embodiments, the first diameter D9 is about 180 mm and the second diameter D10 is about 90 mm. [0093] In some embodiments, the rod 12 of the retractor lOd has a diameter or thickness of 5 mm or less.

[0094] Referring to Fig. IOC, the proximal portion 16 (or the straight segment 32 thereof) may have an average height or length H5. In some embodiments, the height H5 is between 50 and 200 mm.

[0095] One of the retractors 10, 10a, 10b, 10c, and lOd may be selected based on a patient’s height and weight as described in more detail in the Example below. Each of the retractors 10, 10a, 10b, 10c, and lOd may be included in the system 100 illustrated in Fig. 5.

[0096] In some embodiments, a kit includes a plurality of retractors (e.g., one each of the retractors 10, 10a, 10b, 10c, and lOd) to accommodate subjects of different sizes.

[0097] Another embodiment of the present disclosure provides a method of performing a laparoscopic procedure, using the disclosed device to create a cavity in a subject. In an example usage, an incision is made in a subject, close to the region where the procedure is to be performed. The distal end of the retractor is inserted into the incision and the retractor is gently rotated toward the leading distal end of the retractor. This increasingly introduces the retractor inside the subject, until substantially most of the planar or distal portion of the retractor is inside the subject. A tension force is then applied to the proximal end of the retractor, causing the distal end to press against the inner surface of the skin and to lift the skin away from the internal organs. The proximal end of the retractor can be connected to a stand as described herein. The prescribed procedure can then continue in a conventional manner. After the procedure is completed, the retractor can be withdrawn and the cavity 'deflated' (i.e., allowed to return to a natural state) by rotating the device in the opposite direction.

[0098] The ReadyLift retractor is advantageously easy to manufacture with materials that can be locally sourced. Production of these devices can be accomplished in LMICs, making the product more sustainable and potentially creating jobs and business in these settings. Currently, if neither gas nor lift laparoscopy can be performed, some localities are attempting insufflation with room air or hand-assisted techniques, which can lead to additional issues such as increased discomfort and longer recovery time, as well as increased risk of spreading airborne contaminants. [0099] Additionally, the lack of sharp bends reduces the stress when bending the metal into shape and helps maintain structural integrity. The ReadyLift also contains a rounded tip to both facilitate entry through the abdominal wall and limit potential injury during the insertion procedure. The semicircular base provides sufficient contact area on the anterior abdominal wall and allows for the user to rotate the retractor itself. This technique can provide additional exposure in specific areas of the abdomen without having to remove and reinsert the retractor through a different entry point. Further, the retractor is not limited to just one port and can be moved to different entry points to create optimum exposure depending on user needs. Multiple retractors can also be used in conjunction if the sidewall needs to be further retracted. Moreover, the retractor is easily sterilized; due to its smooth and one-piece design, it can either be autoclaved or immersed in a disinfectant solution. [0100] The curved hook of the retractor is designed to be integrated with many commercially available table-mounted lifting systems. Existing solutions, such as the Bookwalter Retractor or the Kent Retractor stand can be easily modified to accept the ReadyLift retractor and be ready for use in minutes.

[0101] Moreover, it is noted that, although the retractor is described in reference to human subjects, the device is equally applicable to non-human subjects. This includes, for example, veterinary surgery. The size and diameter of the retractor can be adapted to the physical traits of non-human subject using the principles described herein.

[0102] The following Example is provided by way of illustration and not by way of limitation.

Example

ReadyLift Prototype

Introduction

[0103] Since the first laparoscopic appendectomy was performed in 1981, minimally invasive procedures have rapidly become the standard of care in high-income countries, and 3.5 million cases are now performed per year in the United States. Many abdominal, pelvic, and thoracic surgeries can be performed endoscopically, with superior outcomes to open surgery. However, patients in low- and middle-income countries (LMICs) rarely have access to laparoscopic surgery due to the high cost of equipment, lack of consumable supplies, and surgical workforce shortages.

[0104] Laparoscopic surgery is standardly performed by achieving pneumoperitoneum, where compressed carbon dioxide (CO2) is insufflated into the abdomen to create a working volume. A commonly reported barrier to the implementation of laparoscopic surgery in LMICs is the inconsistent availability of compressed CO2 and frequent power- outages that may inhibit the ability to maintain consistent intraabdominal pressure, and therefore surgical exposure. In many parts of the world, gasless or lift laparoscopy is being practiced as a viable alternative to traditional laparoscopy. For surgeons who have access to gas laparoscopy, it is the standard of care, provides the best exposure, and should continue to be used. For surgeons who do not have access to gas laparoscopy, lift laparoscopy remains a viable alternative.

[0105] There are isolated reports of basic surgical procedures that have been performed using lift laparoscopy, but the methods, safety profile, and surgical outcomes of using gasless devices are not well described. Previous work on lift laparoscopy has included retractors and lifting mechanisms of various designs. These designs are often cumbersome, requiring several anchoring devices that are inserted subcutaneously or under the peritoneum, and then must be placed on traction. When several anchoring points are used, this reduces the maneuvering space of the laparoscopist and increases the number of incision sites for the patient.

[0106] This Example describes the development and testing of the ReadyLift retractor, a semicircular, biocompatible, stainless steel retractor for gasless laparoscopic surgery. The ReadyLift can be inserted through a single 5-millimeter incision at the umbilicus and then counter-rotated beneath the abdominal wall. It can be paired with the ReadyLift Bedside Stand or a commercially available table-mounted retractor system. Vertical tension on the retractor lifts the abdominal wall to provide visualization inside the abdominal cavity. The ReadyLift device is electricity-independent and does not require a consistent supply of medical -grade CO2, making it ideal for use in LMICs.

Methods

Mathematical Optimization of the Semicircular Retractor Design [0107] Fig. 11A shows the ReadyLift retractor that has been designed for this study. Additionally, the ReadyLift Bedside Stand was developed as an alternative to table- mounted lifting systems and directly interfaces with the ReadyLift retractor (Fig. 1 IB). The stand is constructed of 0.75-inch diameter aluminum rods and an aluminum baseplate. The baseplate is designed to be placed under the patient’s back, such that the stand will be stabilized by the patient’s weight. It has degrees of freedom in all three translational directions as well as yaw and roll adjustment to allow for optimal retractor positioning. [0108] For the ReadyLift retractor, it was assumed that the volume achieved by pneumoperitoneum is hemispherical and the volume created by the ReadyLift retractor is a truncated cone (Fig. 12). This assumption was based on the observation that when prototypes were inserted into an abdominal laparotomy model (Simulab Corporation, Seattle, WA), the resulting shape of the volume was conical. Based on the dimensional relationship between the volumes achieved by the pneumoperitoneum and the retractor, a new equation was derived that would maximize the volume of the truncated cone (Fig. 12). The following equation identifies the relationship between the retractor radius (R) to provide optimal abdominal exposure as a function of the patient’s transverse abdominal radius (r).

R = .814 * r (1)

[0109] This is distinct from the work by Izumi et al. who described a semi-looped retractor, including a mathematic derivation that assumes the abdomen is semi-cylindrical and the volume created by the retractor is a truncated pyramid.

ReadyLift Design for Patient Diversity

[0110] Equation 1 demonstrates that the retractor radius (R) for best exposure will directly depend on the patient’s transverse abdominal radius (r). Previous research has suggested that for an “average” adult patient, the radius of the retractor should be 8.5cm. However, to achieve optimal exposure in patients of many ages and sizes, retractors of different sizes will be needed. Therefore, a retrospective study was developed to collect cross-sectional computed tomography (CT) data from subjects varying in age and weight. The study was deemed exempt from review by the Institutional Review Board at Duke University. Using cross-sectional imaging, data was taken at the level of the umbilicus. The transverse abdominal radius (r) was measured as the linear distance between the midpoint of the abdomen to the peritoneal edge of the lateral abdominal wall, as shown in Fig. 13. Additionally, each subject’s height, weight, age, and abdominal wall thickness were recorded. Study data were stored and managed using REDCap (Research Electronic Data Capture) hosted at Duke University.

ReadyLift Prototype Development and Testing

[0111] Digital models of the retractor were generated in Fusion 360 (Autodesk Inc., CA) to allow for future design optimization and simulative testing. To verify the design and material selection, finite element analysis simulations of the model were generated in Fusion 360. A static stress study was selected using Stainless Steel 316 as the material. A fixed constraint was placed on the upper portion of the retractor to indicate where the retractor interacts with the table-mounted retractor system. Standard intraabdominal pressure settings in gas laparoscopy are 15-20 mmHg (0.02-0.027 kg/cm²), which provides adequate surgical exposure even in morbidly obese adults. The largest abdominal radius measured on CT was less than 120 mm, giving a maximum abdominal surface area of 452 cm². An intraabdominal pressure of 15mmHg would then correspond to a 9kg load and 20mmHg would correspond to a 12kg load. A 13.6 kg maximum load has previously been used, and therefore this was also used as the maximum load.

[0112] The initial prototypes were 3D-printed in biocompatible ABS plastic on an Ultimaker 2+ 3D printer (Ultimaker, Netherlands) and tested in the Simulab Laparotomy Model to determine feasibility. To manufacture the retractor, 4.76 mm diameter 316 Surgical Stainless-Steel biocompatible rods and a Grizzly Industrial Compact Bender System (Grizzly Industrial, Inc.) were used. To achieve the exact dimensions, custom aluminum bending dies were created using the retractor radii from the derived equation and the CT study data. The outer diameters of the dies were reduced to the correct sizes using a Babin CNC Lathe ( BabinMachine Tool, Brockton, MA). The additional bends were completed by 3D-printing a ‘bending jig’ that was in the shape of the retractor. The stainless steel was gently heated using a laboratory heat gun to facilitate bending and then the metal was shaped around the jig to produce the desired shape.

[0113] The simulative data from finite element analysis was verified through physical load testing of the stainless-steel prototypes. A custom mounting rack was built to hang the retractor and model the lifting system. Individual weights ranging from 0.11 kg to 9.07 kg were hung in different combinations from 0.11 kg to 13.6 kg and the deflection at the end of the retractor was measured using a static camera. A ruler was affixed to the mounting rack to serve as a standard for measurement. Images were analyzed with ImageJ software to determine the amount of deflection for each weight combination.

[0114] To quantify the amount of intra-abdominal exposure that can be achieved with the ReadyLift, the prototype was placed into the Simulab Laparotomy Model. This model is rigid on 5 sides and flexible on the top, and therefore exposure can only be measured in one dimension. A laparoscopic camera was affixed to the skin layer of the Laparotomy Model next to the retractor entry point and a ruler was placed centrally inside the model. The ruler was located on the base of the model and parallel to the edges of the model box. The camera was rotated and positioned so that the ruler appeared horizontally on the viewing screen. Upward traction was applied to the ReadyLift retractor and exposure was assessed by taking images with the camera and analyzing the amount of ruler visible. The exposure, or horizontal field of view, was then compared between retracted and unretracted states.

Statistical Analysis

[0115] Linear regressions were computed among collected CT study variables to determine the strength of correlation. A similar analysis was performed for the load testing data. A Wilcoxon rank-sum test was performed to compare the exposure between retracted and unretracted states in the laparotomy model. A one-way ANOVA with assumption checking and post hoc testing was conducted to determine statistical difference between the various retractor sizes. The statistical analysis was performed in R (version 3.5.3). P values are reported with p < 0.05 considered statistically significant.

Results

ReadyLift Retractor Size Determination

[0116] When considering the clinical application of the ReadyLift retractor, multiple retractor sizes will be needed for optimal exposure in patients of various sizes. Therefore, 75 CT scans were reviewed, and transverse abdominal radius (r) was measured as previously described for a range of heights (58.5 cm - 190.5 cm) and weights (5.47 kg - 98.5 kg). The transverse abdominal radius is plotted as a function of age, weight, height, body mass index (BMI), and abdominal wall thickness (Fig. 14). Linear regression was applied, and the regression coefficient, R², is reported. Height (R² = 0.73), age (R² = 0.72), and weight (R² = 0.7) had the strongest relationship to transverse abdominal radius, respectively. Comparatively, BMI (R² = 0.26) and abdominal wall thickness (R² = 0.47) had the weakest correlation. Height and weight were normalized against their respective maximum values and then summed to create a new summary variable. The correlation between this new summary variable and the transverse abdominal radius was the strongest (R² = 0.75) (Fig. 14F).

[0117] Most patients in LMICs do not receive a CT scan prior to surgery; thus, surgeons will not have a measure of the patient’s specific transverse abdominal radius. To assist surgeons in selecting an appropriate retractor size, a user guide was created according to the patient’s weight and height (Fig. 15). Four distinct retractor sizes were established after dividing the patient population into 4 groups where the retractors have the following sizes: A = 49 mm radius, B = 62 mm, C = 76 mm, and D = 86 mm. Patient height and weight are plotted, and the span of each retractor size is overlaid in colored boxes.

[0118] A one-way ANOVA was conducted to ensure that the 4 selected retractor sizes were statistically different from one another. Shapiro-Wilk normality check and Levene’s test were carried out and the assumptions were met. There was a significant difference in retractor radius [F (3,79) = 232.4, p < 2e-16] between the groups. Post hoc comparisons using the Tukey HSD test were carried out. There was a significant difference between every combination of groups with all p < 0.001.

Finite Element Analysis and Load Testing

[0119] The results of the finite element analysis using a 13.6 kg point mass can be seen in Fig. 16 in two different viewing angles. Fusion 360 divided the retractor body into a mesh of 11,002 elements and 19,000 nodes for computation. The scale bar indicates a range of displacement from 0 mm to 30 mm. There was no deflection in the vertical region of the retractor while the semicircular base recorded some deflection.

[0120] Confirmation of the finite element analysis results was performed via load testing on a custom mounting rack for the retractor. The relationship between weight applied to the retractor and the resulting deflection can be seen in Fig. 17 (n=24). There is a strong linear relationship between these variables with a recorded R² = 0.96. At a load of 13.6 kg or 133.5 N, there is roughly 6 cm of deflection at the end of the retractor. After each application of weight, the retractor was visually assessed and no permanent, plastic deformation was observed. When analyzing the load-displacement curve in Fig. 17, the data still follow a linear nature. This indicates that the yield point where the metal changes from elastic to plastic deformation has not yet been achieved, confirming the visual assessments.

[0121] The ReadyLift retractor was applied to the Simulab Laparotomy Model to evaluate the level of intra-abdominal exposure that can be obtained. Data were collected during 2 states: unretracted and retracted. There were 8 images taken for each state and the internally placed ruler was used to determine the field of view. The unretracted state had a mean horizontal field of view of 2.51 inches while applying tension to the retractor resulted in a 57% larger mean horizontal field of view of 3.94 inches (p = 0.0008).

Discussion

ReadyLift Design Considerations

[0122] The ReadyLift retractor was designed to be easily manufactured, allow for gasless laparoscopic surgery, and be globally implemented. Each decision was carefully made with these goals in mind. In settings where insufflation is prohibitive due to inadequate supply chains or the cost of consumable medical-grade CO2 and insufflators, gasless laparoscopy has been said to be revolutionary. Both the ReadyLift retractor and ReadyLift Bedside Stand are easy to manufacture with materials that can be locally sourced in LMICs, making the product more sustainable and potentially creating jobs and business in these settings. [0123] The lack of sharp bends in the ReadyLift is intentional; it reduces the stress when bending the metal into shape and helps maintain structural integrity. The ReadyLift also contains a rounded tip to facilitate entry through the abdominal wall and limit potential injury during the insertion procedure. The semicircular base provides sufficient contact area on the anterior abdominal wall and allows the user to rotate the retractor itself. This technique can provide additional exposure in specific areas of the abdomen without having to remove and reinsert the retractor through a different entry point. The retractor is thin enough to use entry points created for 5-millimeter trocar ports at the umbilicus. Additionally, the retractor can be moved to different entry points to create optimum exposure depending on user needs. Multiple retractors can also be used in conjunction if the abdominal sidewall needs to be further retracted. The ReadyLift retractor is easily sterilized; due to its smooth and one-piece design, it can either be autoclaved or immersed in a disinfectant solution such as Cidex®. The Bedside Stand is similarly sterilizable, allowing for intra-operative repositioning as is standard with existing table-mounted retractor systems. Cidex® is commonly used in LMICs making the ReadyLift ideal for those settings.

[0124] When designing the ReadyLift retractor, it was assumed that the volume created by the ReadyLift retractor was a truncated cone and the resulting volume achieved in the pneumoperitoneum is hemispherical, which was based on our observations in abdominal models and in the operating room. With this assumption, an equation was derived to maximize the volume of the truncated cone. Specifically, the equation relates the retractor radius (R) to the patient’s transverse abdominal radius (r) with a relationship coefficient of 0.81. Conversely, previous work from Izumi et al. assumed the volume created by their retractor was a truncated pyramid and the volume achieved in the pneumoperitoneum was semi-cylindrical, which yielded a slightly larger relationship coefficient of 0.85. A lower coefficient allows for a smaller radius retractor to achieve the same exposure. Thus, less of the abdominal wall is impacted by lifting forces and more space exists for instrument and camera maneuverability.

[0125] The curved hook of the retractor is designed to be integrated with commercially available table-mounted lifting systems. Existing solutions such as the Bookwalter Retractor (, Symmetry Surgical Inc., Nashville, TN) or the Kent Retractor ( Takasago Medical Industry Co., Ltd., Tokyo, Japan ) can be used to stabilize the ReadyLift retractor. However, these systems are at times unavailable or cost-prohibitive in LMICs, and many operating tables in LMICs do not have side railings, which are required for commercially available table-mounted retractor systems. By using the weight of the patient to stabilize the system, the Bedside Stand does not require the operating table to have railings.

Impact of Gasless Laparoscopy

[0126] Currently, if neither CO2 insufflation nor lift laparoscopy can be performed, some surgeons in LMICs are attempting insufflation with room air or hand-assisted techniques which can lead to additional issues such as increased discomfort, longer recovery time and the potential for air emboli. Gasless laparoscopy can address many challenges presented by CO2 pneumoperitoneum. The Coronavirus Disease 2019 (COVID-19) pandemic intensifies the risk of using traditional CO2 methods as some viruses may be spread via droplet or airborne measures. Multiple societies such as the Society of American Gastrointestinal and Endoscopic Surgeons have provided recommendations regarding laparoscopic procedures where insufflation pressure should be kept at a minimum and devices to filter CO2 should be used peri-operatively to reduce particle aerosolization. These filters are another consumable supply that are difficult to obtain in LMICs. Furthermore, for high-risk patients with cardiorespiratory disease, insufflation is often difficult to tolerate and abdominal wall-lifting methods have been shown to alleviate intraoperative ventilatory function deterioration.

[0127] Though the ReadyLift demonstrates significant promise in improving the landscape of global laparoscopic surgery, the intent is not to replace traditional gas-based methods. When gas laparoscopy is available, it should be used as it provides better exposure in most situations. However, in cases where reliable supplies of CO2 and consistent power sources are unavailable, lift laparoscopy is a practical alternative. These cases are often seen in LMICs and simply having the ability to perform laparoscopy, even with abdominal wall lifting methods, increases access to laparoscopic surgery. Lift laparoscopy may be used as a bridge to increase access, whereby surgeons can learn and teach lift laparoscopy, and then convert to gas laparoscopy when it becomes available in their setting.

Future Work

[0128] The next steps in the development of the ReadyLift retractor include testing in a porcine model and clinical validation. Measurements of applied force, resulting pressure to the abdominal wall and exposure obtained will be evaluated. The retractor will be tested against various abdominal wall thicknesses and weights. Safety and efficacy will be assessed by gauging abdominal wall injury and by comparing the exposure achieved between the ReadyLift and traditional CO2 methods, respectively. Additionally, surgeon feedback will be obtained as well as safety data in performing standard operative tasks. It is possible that multiple retractors could be used to improve abdominal exposure. Since the umbilicus is a cosmetically pleasing location to place a 10-12 mm incision, it will be investigated if both a retractor and laparoscope can be placed at the umbilicus simultaneously. [0129] Data from the animal studies will be used to fine-tune the ReadyLift design in preparation for clinical trials. These trials would determine which cases are feasible to perform with the ReadyLift. Another consideration is the change in abdominal wall weight or elasticity over time, particularly with the rise of obesity; the effect of these changes on the ReadyLift would be beneficial to note. Surgeon training and experience with laparoscopy is variable in LMICs, and many surgeons may require training and mentoring in performing laparoscopic procedures. This training would be an important step in the adoption of the ReadyLift retractor into clinical practice.

Conclusions

[0130] The ReadyLift retractor was carefully designed to optimize exposure, retain structural integrity, and allow for laparoscopic procedures to be performed in LMICs. The ReadyLift can easily be combined with commercially available table-mounted lifting systems or the ReadyLift Bedside Stand. The patient’s height and weight can be used to determine the correct retractor size as patient diversity dictates a need for multiple retractors varying in size. Gasless laparoscopy using the ReadyLift retractor has great potential for eliminating the need for pressurized CO2, complex sterilization procedures, and a constant supply of power, thereby increasing access to surgery in LMICs.

[0131] One skilled in the art will readily appreciate that the present disclosure is well adapted to carry out the objects and obtain the ends and advantages mentioned, as well as those inherent therein. The present disclosure described herein are presently representative of preferred embodiments, are exemplary, and are not intended as limitations on the scope of the present disclosure. Changes therein and other uses will occur to those skilled in the art which are encompassed within the spirit of the present disclosure as defined by the scope of the claims.

[0132] No admission is made that any reference, including any non-patent or patent document cited in this specification, constitutes prior art. In particular, it will be understood that, unless otherwise stated, reference to any document herein does not constitute an admission that any of these documents forms part of the common general knowledge in the art in the United States or in any other country. Any discussion of the references states what their authors assert, and the applicant reserves the right to challenge the accuracy and pertinence of any of the documents cited herein. All references cited herein are fully incorporated by reference, unless explicitly indicated otherwise. The present disclosure shall control in the event there are any disparities between any definitions and/or description found in the cited references.

[0133] The foregoing is illustrative of the present invention and is not to be construed as limiting thereof. Although a few example embodiments of this invention have been described, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from the teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the claims. The invention is defined by the following claims, with equivalents of the claims to be included therein.

Claims

We Claim:

1. A retractor for laparoscopic surgery, the retractor comprising: a rod comprising a distal portion for inserting in a subject and a proximal portion for lifting the skin of the subject, wherein the distal portion comprises at least one substantially semicircular segment, and wherein the proximal portion comprises a straight segment and extends substantially orthogonal to the distal portion.

2. The retractor of claim 1 wherein the distal portion comprises a distal end tip at an endpoint of the at least one substantially semicircular segment.

3. The retractor of claim 2 wherein the tip is rounded and smooth.

4. The retractor of claim 2 wherein the at least one substantially semicircular segment comprises a first substantially semicircular segment comprising the tip at a first endpoint thereof and a second substantially semicircular segment having a first endpoint at a second endpoint of the first substantially semicircular segment, and wherein the proximal portion extends from a second endpoint of the second substantially semicircular segment.

5. The retractor of claim 4 wherein the second endpoint of the second substantially semicircular segment is substantially centered between the first and second endpoints of the first substantially semicircular segment.

6. The retractor of claim 4 wherein the first substantially semicircular segment has a first diameter and the second substantially semicircular segment has a second diameter that is smaller than the first diameter.

7. The retractor of claim 4 wherein the second diameter is about half the first diameter.

8. The retractor of claim 4 wherein the distal portion resides in a plane and the proximal portion extends substantially orthogonal to the plane.

9. The retractor of claim 4 wherein a center of a circle defined by the first substantially semicircular segment is offset from a center of a circle defined by the second substantially semicircular segment.

10. The retractor of claim 4 wherein the first substantially semicircular segment has a first radius of curvature and the second substantially semicircular segment has a second radius of curvature that is smaller than the first radius of curvature.

11. The retractor of claim 4 wherein the proximal portion comprises an attachment or engagement feature at a proximal end thereof, the attachment feature configured for engagement to a stand to maintain the lifting the skin of the subject.

12. The retractor of claim 11 wherein the attachment feature comprises a hook extending from the straight segment.

13. The retractor of claim 11 wherein the attachment feature comprises threads at a proximal end of the straight segment.

14. The retractor of claim 4 wherein the rod is monolithic.

15. The retractor of claim 4 wherein the rod is rigid.

16. The retractor of claim 4 wherein the rod comprises stainless steel.

17. A kit comprising: a plurality of the retractors of any one of claims 4 to 16, wherein a first diameter of the first substantially semicircular segment and a second diameter of the second substantially semicircular segment are different for each of the plurality of retractors to accommodate different subjects having different heights and/or weights.

18. A system comprising: the retractor of claim 11; and a stand comprising: a planar base plate; a first elongated member extending orthogonally from the base plate; and a second elongated member extending orthogonally from the first elongated member, wherein the engagement feature of the retractor is configured to engage the second elongated member such that an upward force on the retractor is maintained.

19. The system of claim 18 wherein the stand is configured to be placed on an operating surface beneath a subject’s back such that the subject’s weight stabilizes the stand.

20. The system of claim 18 wherein the second elongated member is adjustable vertically along to the first elongated member.

21. A method of performing a gasless laparoscopic procedure, the method comprising: providing the retractor of any one of claims 4 to 16; inserting the distal end of the retractor into an incision in a subject; rotating the retractor toward the distal end to increasingly introduce the distal portion until all or substantially all of the distal portion is below an anterior abdominal wall of the subject; and applying vertical tension to the proximal portion of the retractor to lift the abdominal wall away from internal organs to provide adequate visualization inside the abdominal cavity of the subject.

22. The method of claim 21 further comprising connecting an attachment feature at the proximal end of the retractor to a stand after applying vertical tension to the proximal portion of the retractor to maintain the vertical tension.

23. The method of claim 22 further comprising positioning a base plate of the stand under the subject’s back prior to inserting the distal end of the retractor into the incision to stabilize the stand with the patient’s weight.

24. The method of claim 21 wherein the geometry of the retractor is such that applying vertical tension to the proximal portion of the retractor creates a cavity having a shape or volume of a truncated cone.

25. The method of claim 21 wherein the retractor is one of a plurality of retractors each having a differently sized distal portion, the method further comprising selecting one of the plurality of retractors based on the subject’s height and/or weight.