Annals of Biomedical Engineering (Ó 2014) DOI: 10.1007/s10439-014-1029-1
Design, Development and Evaluation of an Inflatable Retractor for Atraumatic Retraction in Laparoscopic Colectomy CONOR O’SHEA,1 DENIS KELLIHER,1 EMMET ANDREWS,2,3 MI´CHEA´L O’RI´ORDA´IN,4 MICHAEL O’SHEA,1 TIMOTHY POWER,1 and PA´DRAIG CANTILLON-MURPHY1 2
1 School of Engineering, Electrical and Electronic Engineering, University College Cork, College Road, Cork, Ireland; Department of Colorectal Surgery, Cork University Hospital, Cork, Ireland; 3School of Medicine, University College Cork, Cork, Ireland; and 4Department of Colorectal Surgery, Mercy University Hospital, Cork, Ireland
(Received 18 February 2014; accepted 6 May 2014) Associate Editor Joel D. Stitzel oversaw the review of this article.
Abstract—Laparoscopic colectomy is the gold standard in the treatment of malignant tumours arising in the mucosa of the colon wall. The procedure is performed under general endotracheal anaesthesia and involves establishing a pneumoperitoneum with the patient in the Trendelenburg position. However this position can cause anaesthetic difficulties due to excess blood flow to the head and neck, increased pressure on the diaphragm and increased venous pressure. In the absence of steep head-down positioning, the bowels fall or ‘‘spill’’ into the operating field, obstructing the surgical space. The primary goal of this work is to design an atraumatic laparoscopic retractor to minimise the Trendelenburg position whilst effectively retracting the bowels from the operating field. This work details the design, evaluation and optimisation of a novel, hand held, inflatable, laparoscopic retractor, through physical experimentation, computer simulation, and pre-clinical animal investigation. The optimised design for the inflatable retractor performs in line with simulated expectations, and was successfully tested for safety and technical feasibility in vivo in a porcine model, where the bowels were effectively removed from the operating space whilst the model remained in the supine position. These initial results represent a promising approach for the mitigation of the Trendelenburg position, whilst effectively retracting the bowels during laparoscopic colectomy, using this atraumatic, inflatable retractor. Keywords—Laparoscopic colectomy, Laparoscopic instruments, Inflatable retractor, Bowel retraction, Atraumatic retractor.
Address correspondence to Conor O’Shea, School of Engineering, Electrical and Electronic Engineering, University College Cork, College Road, Cork, Ireland. Electronic mail: conor.oshea@umail. ucc.ie
INTRODUCTION Colorectal cancer is a malignant tumour arising in the wall of the large intestine or rectum, in most cases from dysplastic adenomatous polyps.2 Colon cancer is the third most common cause of cancer in men and women with over 103,000 new cases in the U.S. in 2012.19 However, when detected early, it can be a treatable malignancy.14 Laparoscopic surgery was first introduced in 1991 and is now the gold standard in clinical intervention.13 The short term benefits of laparoscopic surgery include reduced hospital stay, less postoperative pain, earlier return to normal activity, improved cosmesis, and overall reduction in healthcare costs.11,16,17 Laparoscopic colectomy is performed with the patient under general endotracheal anaesthesia. Once a pneumoperitoneum has been established to 12–15 mm Hg with CO2,6 a number of surgical cannulas are inserted through the abdominal wall and the patient is most commonly placed into a Trendelenburg position (TP). TP involves inclination of the patient’s body with his or her head down and legs elevated. To optimise surgical exposure, an angle of inclination of up to 40° may be required.10 This allows the small bowel to glide away from the pelvis, creating a working space within the abdominal cavity, which in turn allows the surgeon to mobilise the colon and retrieve the specimen extracorporeally through an enlarged port site. Post anastomosis, the bowel is returned into the peritoneal cavity, and the facial defect is closed.1 One of the most common challenges encountered during laparoscopy is that of the distended loops of bowel or overlaying organs spilling into the operating field and thus obstructing the surgeon’s view.4 Overcoming this requires changing the position of the
Ó 2014 Biomedical Engineering Society
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patient and the table as well as tedious pushing or pulling of the intrusive intestines using a bowel graspers or laparoscopic retractor. By placing the patient in the TP, the surgeon uses gravity to retract the distended loops of bowel. However, prolonged TP significantly increases intracranial pressure and intraocular pressure. In addition to the circulatory effects, steep head-down impacts on the cardiac system due to increased central venous pressure and on the respiratory system by decreasing total lung volume, pulmonary compliance, and functional residual capacity by 20%, which may lead to hypoxia, hypercapnoea and atelectasis.18,21 An increase in cerebral blood flow pressure could also impair cerebral circulation.21 More marked changes may be observed in obese, elderly, or debilitated patients. Noncrushing bowel graspers are the most common instrument used to manipulate and retain the internal organs (Fig. 1a). The surgeon relies on the haptic feedback from the graspers to delicately manipulate the internal organs. However, this feedback is severely limited due to the mechanical friction losses and variations in the transmission of forces over the working range.8,20 Furthermore, the relatively small tips on the laparoscopic graspers can generate high pressures locally on the soft tissue (up to 200 kPa),5,9 which may lead to injury or perforation. Conversely insufficient force will lead to tissue slipping out of the graspers. Any bowel injury should be treated at the time of recognition, however it is reported that up to 0.87% of cases remain undetected during the operation.3 Traditionally, barrier retractors are inserted into the body in a collapsed state, deployed within the body and either held by an assistant or may be fixed to the operating table. Numerous disposable and reusable retractors are available. The fan retractor, which was originally designed for use in upper gastrointestinal
surgery, works by rotating a dial in the proximal end of the device which in turn deploys a number of blades in a fan-like pattern once placed intra-peritoneally (Fig. 1b). The fan retractor must be observed closely when being closed as inadvertent organ injury may occur if trapped between the retracting blades. In addition, the fan retractor is made from many small components, and runs the risk of spoiling the operating field if breakage occurs.12 The Endo paddle retractor, which is a type of paddle retractor, operates in a similar manner. The Endo paddle retractor (Fig. 1c) comprises an inflatable paddle located on the distal end of a manually operated rod. After insertion, a balloon on the distal end of the rod is inflated providing a relatively wide palmated anterior surface which creates a soft interface between the balloon and internal organs. However, the small bowel may slip around the edge of the inflated surface and migrate into the operating field. The triangular retractor (Fig. 1d), which is a type of snake retractor, is primarily used in liver retraction. The snake retractor comprises hinged links along its shaft that can be manipulated into a curved profile by rotating a hand piece on the proximal end of the device. The snake retractor can fit through a 5 mm operating port. However, the possibility of pinching the soft tissue between its links when engaged makes the device unsuitable for bowel retraction. In addition, the contact area between the shaft and the organ is quite narrow. This work examines the design and both bench-top and pre-clinical investigations of a novel, hand held, inflatable, minimally invasive, disposable, laparoscopic retractor which improves on the manoeuvrability of current deployable retractors and minimises the TP. This inflatable retractor, called SecuRetract, which is described subsequently, may significantly alleviate the complications associated with the TP.
FIGURE 1. Current retractors used in laparoscopic surgery: (a) Laparoscopic bowel graspers traditionally used to manipulate the bowels during laparoscopic surgery (Medline Industries, Inc., Mundelein, Illinois). (b) Laparoscopic Fan retractor (LocaMed Ltd., Farnham, Surrey, U.K.). (c) Inflatable Endo PaddleTM retractor (Covidien Surgical, Mansfield, Massachusetts). (d) Snowden-Pencer triangular liver retractor (Cardinal Health, Dublin, Ohio).
Development of a Novel Laparoscopic Retractor
MATERIALS AND METHODS SecuRetract has been designed to comply with geometrical and mechanical requirements set forth in preliminary user surveys, and has been optimised for maximum strength and retracting force. The essential design requirements included:
The overall diameter of the retractor in its deflated state must be less than 5 mm. This is to accommodate insertion through a 5 mm trocar cannula which was chosen to minimise contact pressure and expedite postoperative healing. The device should be light weight, easily deployed and give an effective and reliable support to the hollow internal organs. The device must be atraumatic and resist gas leakage. It must be ergonomically designed to optimise its operation over a prolonged time. All these features have been taken into account in the design of the retractor as shown in Figs. 2 and 3. Several designs were developed and prototyped before the final design was recognised through continuous collaboration between the clinicians and the design engineers. The development cycle involved iterative handle and shaft design using SolidWorks (Dassault Syste`mes SolidWorks Corp., Waltham Massachusetts), which were subsequently developed in-house using a Dimension Elite 3D Printer (Stratasys Ltd., Minnesota, USA)
with a resolution of 0.178 mm, and a HURCO CNC machine (Hurco Companies, Inc., Indianapolis) which can achieve a resolution of 20 lm. The material used to create the printed handle prototypes was acrylonitrile butadiene styrene (ABS). Once the prototype handle and shaft were constructed, the balloons (provided by Creagh Medical Ltd., Co. Galway, Ireland) were attached to the distal tip. The assembled device was then evaluated and a new or alternative concept was developed until the final design was reached following end-user feedback. Proposed Clinical Use The final design, SecuRetract, can be inserted into the peritoneal cavity through a surgical cannula in its deflated state. Once the region of interest has been identified, the impeding organs are retracted by manipulating the device until it hooks around these organs. The device is then inflated, increasing the area of contact and finally withdrawn from the operating field, retracting the organs in the process (Figs. 2, 3). During the installation of SecuRetract, the patient may be placed in the TP to assist in placement. As the retractor is inserted, the distal tip, made from spring steel, may be curved creating a hook shape by actuating the control handle. The radius of curvature may be modified as required and the retractor is placed between the bowel and its attaching mesentery. The distal end of SecuRetract is then inflated increasing the contact area between the retractor and the bowels as
FIGURE 2. Proposed laparoscopic retractor engaging the small intestine in its deployed, inflated and curved position. The retractor may be equally used for left and right colectomy and is capable of being deflated and removed through a 5 mm instrument port as required.
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FIGURE 3. (a) SecuRetract in the initial deflated straight position and (b) SecuRetract in the inflated curved position. The retractor comprises a control handle, central shaft, and modular balloons. The control handle actuates the distal end of the device creating a curved profile.
well as creating a soft interface due to the cushioned effect of the modular balloons. As a result of the increased contact area and the curved profile of the retractor, a more effective withdrawal of the distended loops of the bowels can be obtained. Once the device has been positioned, the patient may return to the supine position where he or she can remain for the remainder of the procedure. Simulated and Mechanical Design The proposed solution comprises a control handle, a central shaft, and a number of inflatable modular balloons mounted to the distal end of the device (Fig. 4). The retractor is inserted in its deflated straight position with an absolute outside diameter of 4.8 mm. When inside the peritoneal cavity, the distal end, which comprises a leaf spring, curves by applying tension to an internal wire actuated by a thumb dial on the control handle. The curving portion of the device was simulated using the finite element analysis (FEA) tool, Strand7 (Strand7 Pty Ltd., Sydney, Australia) to optimise its performance. Stresses and strains, experienced in the spring steel as it deflects, were predicted. A pre-tensioned load was applied to the internal stainless steel wire in the model which represents the tension the wire experiences once the handle of the retraction device is engaged. The nonlinear static solver in Strand7, allowing for geometric stiffnesses, was used to analyse the large deformation of the beam during the tensioning of the wire, thus generating the final curvature of the beam in the retractor. The results predict the axial force in the wire as well as the bending stresses induced in the steel member, and can be used to determine if the selected material will succumb to
FIGURE 4. SecuRetract laparoscopic retractor prototype pictured in its inflated, curved position with non-return inflation valve.
permanent deformation as a result of exceeding its tensile yield stress. Physical modelling was subsequently carried out to validate the simulated results. The control handle comprises a thumb dial with an axis of rotation parallel to the shaft. As the dial is rotated it axially displaces a lead screw fixed in rotation which further displaced a wire that terminates at the distal end of the leaf spring. The degree of curvature of the leaf spring may be controlled by the thumb dial up to a fully curved position, creating a radius of approximately 80 mm. This orientation allows the lead screw to move longitudinally and prevents the dial from unravelling when released by the thumb. A perpendicular orientation was also considered but discounted for this reason. The distal end of the device comprises a number of elastomeric modular balloons mounted in series to co-axially extruded tubing which sheaths over the leaf spring and central shaft. The co-axial configuration facilitates inflation and the elastomeric material ensures elastic behaviour during the inflation and
Development of a Novel Laparoscopic Retractor
deflation cycles which allow the balloons to deflate completely. Experimental testing evaluated the maximum inflation pressure of the balloons before the material tensile strength is reached and also was used to debug any complications with the balloon assembly. The balloons may then be inflated with CO2 via an inflation lumen and a luer taper stop-cock. The extruded tubing and balloons are flexible enough to follow the curvature of the internal leaf spring which they cover and also provide an air tight envelope over the internal mechanism. During the procedure, it may be necessary to reposition the retractor. Therefore, SecuRetract can be deflated and removed back through the 5 mm surgical cannula without incurring damage. The device can subsequently be reinserted as required and is disposed of once the procedure is complete. Pre-clinical Investigation SecuRetract was laparoscopically deployed in vivo in a porcine model and its performance was recorded using laparoscopic video. The pre-clinical animal investigation endpoints were technical feasibility and safety. This investigation was approved by both the Irish Department of Health and UCC animal experimentations ethics committee. The porcine model is a very close anatomical model for the human digestive system, and is an ideal model for the technical evaluation of new medical devices in colorectal applications.
spring would deflect by 80° from the normal position with a radius of curvature of approximately 80 mm (Fig. 6). The retraction force was also determined to be in excess of 5 N which was deemed to be sufficient to displace the bowels. De Visser et al.7 reported that the maximum pull force required to stretch the mesocolon for dissection was 4.7 N. The fully actuated position of the handle design is predetermined by an inbuilt stop which prevents excessive curvature at the distal end. However, the user may overexert the force on the dial forcing the internal displacement bolt against the internal stop. This may cause handle deformation and or force the two sides of the handle apart. The handle was therefore analysed to determine the magnitude of deformation. SolidWorks simulation tool was employed which assumes static loading and linear elastic behaviour. The material assigned to the handle was ABS (E of 2.3 GPa, ryield of 68 GPa) to correspond to the 3D printed prototypes. The results highlighted peak stress areas within the model which subsequently led to modifying the geometry and increasing wall thickness at specific locations until an acceptable deformation was obtained. An external force of 20 N (10 N per side) was applied. This figure represents the maximum translational force that the user may distribute from the dial to the axially acting displacement bolt determined from physical testing. The simulated results
RESULTS Simulated and Physical Results The bending stress and strain in the leaf spring was simulated using Strand7 as described in ‘‘Materials and Methods’’ section B, to optimise the blade parameters ensuring that the yield stress was not reached (Fig. 5). The simulated model was accurate in both geometrical scale and material properties and a Young’s modulus of 196 GPa was assumed as derived from experimental testing. Through simulation, it was possible to design a leaf spring which would experience a maximum of 450 MPa during the curving process. This provides a factor of safety (FOS) of 1.3 when compared to the yield strength of the spring steel (586 MPa). A prototype of the stainless steel shaft and leaf spring assembly, with a polyurethane tube sheathed over the shaft, was used to analyse the relationships between the applied axial force in the internal wire, with the resulting deflected angle and radius of curvature of the shaft. It was determined that by withdrawing the internal wire a set distance, it would experience an axial tensile force of approximately 20 N and the leaf
FIGURE 5. Simulated FEA model of the axial tension experienced by the internal wire used to displace the leaf spring. As the device is actuated, the internal wire experiences tension which in turn curves the leaf spring. The tensile force required to fully curve the blade is critical to the design of the control handle.
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identified a peak stress value of 3.98 MPa which is significantly less than the yield stress (FOS of 17.3), and a maximum resultant displacement of less than 0.04 mm (see Fig. 7).
The modular balloons were also evaluated. The optimum configuration comprises three elastomeric balloons closely positioned in a longitudinal array at the distal end of the device. The properties of the
FIGURE 6. (a) Deflected angle reference for leaf spring displacement. (b) Simulated and experimental axial tensile force in internal wire vs. corresponding deflected angle of SecuRetract’s distal tip.
FIGURE 7. FEA analysis of the handle illustrating (a) the maximum resultant displacement and (b) the areas of maximum stress for an external applied load of 10 N per handle part.
Development of a Novel Laparoscopic Retractor
elastomeric material were characterised though experimentation. Figure 8 illustrates the relationship of inflation pressure to inflated diameter. The ideal inflation diameter of 30 mm occurs at an inflation pressure of approximately 260 mmHg with a combined surface area of approximately 250 cm2. Taking into account that the abdomen is already inflated to approximately 15 mmHg, the total pressure applied to the inflatable is the sum of these two pressures. The burst pressure was rated at approximately 428 mmHg for an elongation at break of 800%. This results in a tensile strength of approximately 15.6 MPa. However, the level of inflation may be varied as appropriate to the procedure to reduce or increase the overall balloon diameter within a maximum working limit of 300 mmHg providing a factor of safety of over 1.4 Pre-clinical Investigation Results The model used was a female landrace pig weighing 26.7 kg. The animal was sedated for the duration of the procedure and was euthanized immediately following the procedure without any recovery. Three laparoscopic ports were inserted through the abdominal wall (2 9 5 mm and 1 9 12 mm trocars), the largest of which accommodated a laparoscopic camera (Olympus Evis Exera BF type 160 series) (Fig. 9a). The peritoneum was inflated to 15 mmHg CO2 with an Olympus UHI-3 insufflator (Olympus, Pennsylvania). SecuRetract was inserted into the upper right quadrant through a 5 mm trocar in its deflated form. Initially the retractor remained in its deflated state but was curved to form a hook shape (tip deflection of 80°). The device was then inflated and tested to assess the capability of retracting the intestines without the need for auxiliary
laparoscopic tools as shown in Fig. 9b. The animal remained in the supine position. SecuRetract was then deflated and removed back through the surgical port only to be reinserted in the left upper quadrant. With the aid of laparoscopic graspers, the intestines were manipulated to allow SecuRetract to hook around the loose organs. SecuRetract was subsequently disengaged and deflated and the animal was temporarily placed in a steep TP. The retractor was repositioned and inflated around the bowels which were retracted due to the effect of gravity. Once SecuRetract was in position, the animal was returned to the supine position. SecuRetract moved to mesentery manipulation for inferior mesenteric arch and lymph node access. The retractor was placed between the bowels and the mesentery holding the bowels to the posterior wall, thus providing a foothold to retract the bowels. Post laparoscopic procedure, the investigation concluded with an open surgical investigation to allow clear visualisation of the retraction process (Fig. 9c). The pre-clinical animal investigation proved successful in assessing technical feasibility of the retractor. The light weight retractor was successfully and repeatedly inserted thorough a 5 mm cannula. During the study, SecuRetract was repositioned several times, inflated and deflated, repositioned to different quadrants and did so without damage to the tissue or device. However, SecuRetract was incapable of independently retracting the bowels during installation. The bowels moved out of position as the retractor tried to hook around the organ during deployment. SecuRetract therefore requires a secondary laparoscopic instrument to assist in its initial placement. However, once achieved, SecuRetract can effectively prevent bowel migration into the operating field.
FIGURE 8. Inflation pressure vs. inflated diameter for a number of modular elastomeric balloons mounted towards the distal end of SecuRetract at room temperature (21 °C).
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FIGURE 9. SecuRetract used in a pre-clinical animal investigation. (a) External view of SecuRetract passing thorough laparoscopic port. (b) View from laparoscopic camera of the distal end of SecuRetract engaging the bowels. (c) View of SecuRetract’s placement around the bowels during a conversion to open surgery to better visualise the operation of the retractor. The curved, inflated end of SecuRetract is placed between the small bowels and the mesentery connecting the bowels to the posterior wall. This provides a foothold for the device to retract the organ.
When the animal was temporarily placed in the TP, the bowels were pulled back from the operating field using a laparoscopic instrument. SecuRetract was then positioned to hook around the retracted bowels, inflated and held in position. Once SecuRetract was in place, the animal was then returned to its supine position. SecuRetract maintained the bowels in their retracted position effectively away from operating field whilst providing little or no interference with other instruments.
DISCUSSION An essential part of any laparoscopic surgery is to ensure that the surgeon has sufficient exposure to visualise the operating field and surrounding structures to perform an effective and safe procedure. Laparoscopic retraction has greatly advanced since first introduced in 1991, and yet a truly safe and effective laparoscopic retractor has yet to be universally acknowledged.12,15 Through an iterative design process and continual consultation with end users, SecuRetract was developed. Initial end-user surveys were compiled from the clinical co-inventors to determine the initial design criteria. Once the current solution was reached,
SecuRetract was presented to a group of key opinion leaders in U.S. based hospitals to gain some feedback on the overall device design and functionality. It was noted that despite the handle being quite different in design to conventional laparoscopic instruments, the intuitive nature of its actuation may remove possible learning and adoption obstacles. In addition the presented solution may improve on current laparoscopic retraction devices due to its inflatable, manoeuvrable curved design. Simulated and physical modelling played a vital role in optimising the final design. The leaf spring on the distal end of the device was studied using finite element analysis which resulted in reduced material volume and increased retraction force. As predicted, the physical model closely resembled the simulated calculations (approx. 93%) reflecting the elastic nature of the spring steel. The handle was also scrutinised using FEA rendering a sufficiently rigid product whilst minimising the material used. As the handle is actuated, a tensile force of approximately 20 N is applied to the internal wire, which is sufficient to curve the distal portion of the retractor without exceeding the yield strength of the leaf spring. In addition, the engaged device commands sufficient rigidity to retract the soft bowels from the operating space (>5 N). A pre-clinical trial demonstrated that SecuRetract met the technical and clinical requirements set forth in the preliminary user survey. However, the trial did have its limitations. Due to the subjective nature of this trial, it was difficult to obtain quantitative data. The porcine gastrointestinal tract, albeit similar in anatomical make-up, was different in size and weight compared to a human’s. Therefore, the pre-clinical study was primarily a technical feasibility study of the device. It was shown that SecuRetract can effectively retain the bowels from the operating field during minimal invasive surgery without the foreseeable complications of competing devices or interfering with other laparoscopic instruments. However, SecuRetract may need the assistance of an auxiliary laparoscopic instrument in its initial placement. The inflated curved portion of the retractor has proven capable of safely retracting the internal organs in the supine position, generating a retraction force of over 5 N, thus providing evidence that the device may minimise the need for prolonged use of the TP. The pre-clinical trial also identified some possible design improvements which include device orientation, labelling and ergonomic modifications. The main proposed advantage of SecuRetract over alternative deployable retractors is that it can be adjusted and manoeuvred to suit any abdominal surgery. SecuRetract may also be inserted through a 5 mm port which in turn reduces recovery time when the
Development of a Novel Laparoscopic Retractor
facial defect is closed post-surgery. The inflatable nature of the device means that it will not perforate or damage soft internal organs or tissue. The long spherical balloon shape increases the contact surface with the bowel wall reducing the pressure on it. It also enhances the mechanical adherence with the bowel’s surface minimising space occupation. Due to its ability to hook around the bowels, SecuRetract can retract a far larger section of the bowels compared to the traditional laparoscopic retractors discussed in the introduction. Therefore, the procedure may be undertaken whilst the patient remains in the supine position. This will result in less operative complications, and consequently may decrease hospital costs. In addition, SecuRetract offers improved ease of laparoscopic access during surgery by means of its rapid deployment and retraction through a single 5 mm port. This feature is particularly amenable for extending the device’s use to clinical indications outside of colorectal surgery (e.g., upper gastrointestinal interventions). The curvature and length of the shaft may also be altered to provide a selection of SecuRetract devices as appropriate to a specific procedure. Further development is currently being undertaken to replace the carbon steel leaf spring with a stainless steel equivalent (grade 301) which obviates the risk of corrosion as well as increasing the functional factor of safety. The handle design may also be re-investigated pending a more substantial end-user survey. Finally the technical feasibility of applying a dark matte colour to the balloons will be assessed. This may minimise optical aberration reducing reflections from the balloon surface giving the surgeon a more comfortable visual environment within the abdominal cavity.
CONCLUSION While the results of this preliminary analysis and experimentation are encouraging, a clinical investigation will be required to investigate whether SecuRetract can effectively retract the small intestines in the supine position whilst providing a clear operating space within the human peritoneal cavity. A clinical investigation will also demonstrate whether or not SecuRetract outperforms existing laparoscopic retractors in reducing the time of intervention as well as minimising intestinal wall trauma. Subsequent to the pre-clinical investigation, a number of design improvements have been identified which will further enhance the operational performance and ease of use. Despite these important investigations and modifications, the current results point to a promising alternative laparoscopic retractor with the potential to help alleviate complications in laparoscopic colectomy.
ELECTRONIC SUPPLEMENTARY MATERIAL The online version of this article (doi:10.1007/ s10439-014-1029-1) contains supplementary material, which is available to authorized users.
ACKNOWLEDGMENTS The authors thank the Biological Services Unit and UCC Department of Physiology for facilitating the pre-clinical animal investigation, in particular Mr. Kieran McManamon, and Dr. Therese RuaneO’Hora.
DISCLOSURES The authors of this paper have no conflict of interest or financial ties to disclose.
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Design, Development and Evaluation of an Inflatable Retractor for Atraumatic Retraction in Laparoscopic Colectomy CONOR O’SHEA,1 DENIS KELLIHER,1 EMMET ANDREWS,2,3 MI´CHEA´L O’RI´ORDA´IN,4 MICHAEL O’SHEA,1 TIMOTHY POWER,1 and PA´DRAIG CANTILLON-MURPHY1 2
1 School of Engineering, Electrical and Electronic Engineering, University College Cork, College Road, Cork, Ireland; Department of Colorectal Surgery, Cork University Hospital, Cork, Ireland; 3School of Medicine, University College Cork, Cork, Ireland; and 4Department of Colorectal Surgery, Mercy University Hospital, Cork, Ireland
(Received 18 February 2014; accepted 6 May 2014) Associate Editor Joel D. Stitzel oversaw the review of this article.
Abstract—Laparoscopic colectomy is the gold standard in the treatment of malignant tumours arising in the mucosa of the colon wall. The procedure is performed under general endotracheal anaesthesia and involves establishing a pneumoperitoneum with the patient in the Trendelenburg position. However this position can cause anaesthetic difficulties due to excess blood flow to the head and neck, increased pressure on the diaphragm and increased venous pressure. In the absence of steep head-down positioning, the bowels fall or ‘‘spill’’ into the operating field, obstructing the surgical space. The primary goal of this work is to design an atraumatic laparoscopic retractor to minimise the Trendelenburg position whilst effectively retracting the bowels from the operating field. This work details the design, evaluation and optimisation of a novel, hand held, inflatable, laparoscopic retractor, through physical experimentation, computer simulation, and pre-clinical animal investigation. The optimised design for the inflatable retractor performs in line with simulated expectations, and was successfully tested for safety and technical feasibility in vivo in a porcine model, where the bowels were effectively removed from the operating space whilst the model remained in the supine position. These initial results represent a promising approach for the mitigation of the Trendelenburg position, whilst effectively retracting the bowels during laparoscopic colectomy, using this atraumatic, inflatable retractor. Keywords—Laparoscopic colectomy, Laparoscopic instruments, Inflatable retractor, Bowel retraction, Atraumatic retractor.
Address correspondence to Conor O’Shea, School of Engineering, Electrical and Electronic Engineering, University College Cork, College Road, Cork, Ireland. Electronic mail: conor.oshea@umail. ucc.ie
INTRODUCTION Colorectal cancer is a malignant tumour arising in the wall of the large intestine or rectum, in most cases from dysplastic adenomatous polyps.2 Colon cancer is the third most common cause of cancer in men and women with over 103,000 new cases in the U.S. in 2012.19 However, when detected early, it can be a treatable malignancy.14 Laparoscopic surgery was first introduced in 1991 and is now the gold standard in clinical intervention.13 The short term benefits of laparoscopic surgery include reduced hospital stay, less postoperative pain, earlier return to normal activity, improved cosmesis, and overall reduction in healthcare costs.11,16,17 Laparoscopic colectomy is performed with the patient under general endotracheal anaesthesia. Once a pneumoperitoneum has been established to 12–15 mm Hg with CO2,6 a number of surgical cannulas are inserted through the abdominal wall and the patient is most commonly placed into a Trendelenburg position (TP). TP involves inclination of the patient’s body with his or her head down and legs elevated. To optimise surgical exposure, an angle of inclination of up to 40° may be required.10 This allows the small bowel to glide away from the pelvis, creating a working space within the abdominal cavity, which in turn allows the surgeon to mobilise the colon and retrieve the specimen extracorporeally through an enlarged port site. Post anastomosis, the bowel is returned into the peritoneal cavity, and the facial defect is closed.1 One of the most common challenges encountered during laparoscopy is that of the distended loops of bowel or overlaying organs spilling into the operating field and thus obstructing the surgeon’s view.4 Overcoming this requires changing the position of the
Ó 2014 Biomedical Engineering Society
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patient and the table as well as tedious pushing or pulling of the intrusive intestines using a bowel graspers or laparoscopic retractor. By placing the patient in the TP, the surgeon uses gravity to retract the distended loops of bowel. However, prolonged TP significantly increases intracranial pressure and intraocular pressure. In addition to the circulatory effects, steep head-down impacts on the cardiac system due to increased central venous pressure and on the respiratory system by decreasing total lung volume, pulmonary compliance, and functional residual capacity by 20%, which may lead to hypoxia, hypercapnoea and atelectasis.18,21 An increase in cerebral blood flow pressure could also impair cerebral circulation.21 More marked changes may be observed in obese, elderly, or debilitated patients. Noncrushing bowel graspers are the most common instrument used to manipulate and retain the internal organs (Fig. 1a). The surgeon relies on the haptic feedback from the graspers to delicately manipulate the internal organs. However, this feedback is severely limited due to the mechanical friction losses and variations in the transmission of forces over the working range.8,20 Furthermore, the relatively small tips on the laparoscopic graspers can generate high pressures locally on the soft tissue (up to 200 kPa),5,9 which may lead to injury or perforation. Conversely insufficient force will lead to tissue slipping out of the graspers. Any bowel injury should be treated at the time of recognition, however it is reported that up to 0.87% of cases remain undetected during the operation.3 Traditionally, barrier retractors are inserted into the body in a collapsed state, deployed within the body and either held by an assistant or may be fixed to the operating table. Numerous disposable and reusable retractors are available. The fan retractor, which was originally designed for use in upper gastrointestinal
surgery, works by rotating a dial in the proximal end of the device which in turn deploys a number of blades in a fan-like pattern once placed intra-peritoneally (Fig. 1b). The fan retractor must be observed closely when being closed as inadvertent organ injury may occur if trapped between the retracting blades. In addition, the fan retractor is made from many small components, and runs the risk of spoiling the operating field if breakage occurs.12 The Endo paddle retractor, which is a type of paddle retractor, operates in a similar manner. The Endo paddle retractor (Fig. 1c) comprises an inflatable paddle located on the distal end of a manually operated rod. After insertion, a balloon on the distal end of the rod is inflated providing a relatively wide palmated anterior surface which creates a soft interface between the balloon and internal organs. However, the small bowel may slip around the edge of the inflated surface and migrate into the operating field. The triangular retractor (Fig. 1d), which is a type of snake retractor, is primarily used in liver retraction. The snake retractor comprises hinged links along its shaft that can be manipulated into a curved profile by rotating a hand piece on the proximal end of the device. The snake retractor can fit through a 5 mm operating port. However, the possibility of pinching the soft tissue between its links when engaged makes the device unsuitable for bowel retraction. In addition, the contact area between the shaft and the organ is quite narrow. This work examines the design and both bench-top and pre-clinical investigations of a novel, hand held, inflatable, minimally invasive, disposable, laparoscopic retractor which improves on the manoeuvrability of current deployable retractors and minimises the TP. This inflatable retractor, called SecuRetract, which is described subsequently, may significantly alleviate the complications associated with the TP.
FIGURE 1. Current retractors used in laparoscopic surgery: (a) Laparoscopic bowel graspers traditionally used to manipulate the bowels during laparoscopic surgery (Medline Industries, Inc., Mundelein, Illinois). (b) Laparoscopic Fan retractor (LocaMed Ltd., Farnham, Surrey, U.K.). (c) Inflatable Endo PaddleTM retractor (Covidien Surgical, Mansfield, Massachusetts). (d) Snowden-Pencer triangular liver retractor (Cardinal Health, Dublin, Ohio).
Development of a Novel Laparoscopic Retractor
MATERIALS AND METHODS SecuRetract has been designed to comply with geometrical and mechanical requirements set forth in preliminary user surveys, and has been optimised for maximum strength and retracting force. The essential design requirements included:
The overall diameter of the retractor in its deflated state must be less than 5 mm. This is to accommodate insertion through a 5 mm trocar cannula which was chosen to minimise contact pressure and expedite postoperative healing. The device should be light weight, easily deployed and give an effective and reliable support to the hollow internal organs. The device must be atraumatic and resist gas leakage. It must be ergonomically designed to optimise its operation over a prolonged time. All these features have been taken into account in the design of the retractor as shown in Figs. 2 and 3. Several designs were developed and prototyped before the final design was recognised through continuous collaboration between the clinicians and the design engineers. The development cycle involved iterative handle and shaft design using SolidWorks (Dassault Syste`mes SolidWorks Corp., Waltham Massachusetts), which were subsequently developed in-house using a Dimension Elite 3D Printer (Stratasys Ltd., Minnesota, USA)
with a resolution of 0.178 mm, and a HURCO CNC machine (Hurco Companies, Inc., Indianapolis) which can achieve a resolution of 20 lm. The material used to create the printed handle prototypes was acrylonitrile butadiene styrene (ABS). Once the prototype handle and shaft were constructed, the balloons (provided by Creagh Medical Ltd., Co. Galway, Ireland) were attached to the distal tip. The assembled device was then evaluated and a new or alternative concept was developed until the final design was reached following end-user feedback. Proposed Clinical Use The final design, SecuRetract, can be inserted into the peritoneal cavity through a surgical cannula in its deflated state. Once the region of interest has been identified, the impeding organs are retracted by manipulating the device until it hooks around these organs. The device is then inflated, increasing the area of contact and finally withdrawn from the operating field, retracting the organs in the process (Figs. 2, 3). During the installation of SecuRetract, the patient may be placed in the TP to assist in placement. As the retractor is inserted, the distal tip, made from spring steel, may be curved creating a hook shape by actuating the control handle. The radius of curvature may be modified as required and the retractor is placed between the bowel and its attaching mesentery. The distal end of SecuRetract is then inflated increasing the contact area between the retractor and the bowels as
FIGURE 2. Proposed laparoscopic retractor engaging the small intestine in its deployed, inflated and curved position. The retractor may be equally used for left and right colectomy and is capable of being deflated and removed through a 5 mm instrument port as required.
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FIGURE 3. (a) SecuRetract in the initial deflated straight position and (b) SecuRetract in the inflated curved position. The retractor comprises a control handle, central shaft, and modular balloons. The control handle actuates the distal end of the device creating a curved profile.
well as creating a soft interface due to the cushioned effect of the modular balloons. As a result of the increased contact area and the curved profile of the retractor, a more effective withdrawal of the distended loops of the bowels can be obtained. Once the device has been positioned, the patient may return to the supine position where he or she can remain for the remainder of the procedure. Simulated and Mechanical Design The proposed solution comprises a control handle, a central shaft, and a number of inflatable modular balloons mounted to the distal end of the device (Fig. 4). The retractor is inserted in its deflated straight position with an absolute outside diameter of 4.8 mm. When inside the peritoneal cavity, the distal end, which comprises a leaf spring, curves by applying tension to an internal wire actuated by a thumb dial on the control handle. The curving portion of the device was simulated using the finite element analysis (FEA) tool, Strand7 (Strand7 Pty Ltd., Sydney, Australia) to optimise its performance. Stresses and strains, experienced in the spring steel as it deflects, were predicted. A pre-tensioned load was applied to the internal stainless steel wire in the model which represents the tension the wire experiences once the handle of the retraction device is engaged. The nonlinear static solver in Strand7, allowing for geometric stiffnesses, was used to analyse the large deformation of the beam during the tensioning of the wire, thus generating the final curvature of the beam in the retractor. The results predict the axial force in the wire as well as the bending stresses induced in the steel member, and can be used to determine if the selected material will succumb to
FIGURE 4. SecuRetract laparoscopic retractor prototype pictured in its inflated, curved position with non-return inflation valve.
permanent deformation as a result of exceeding its tensile yield stress. Physical modelling was subsequently carried out to validate the simulated results. The control handle comprises a thumb dial with an axis of rotation parallel to the shaft. As the dial is rotated it axially displaces a lead screw fixed in rotation which further displaced a wire that terminates at the distal end of the leaf spring. The degree of curvature of the leaf spring may be controlled by the thumb dial up to a fully curved position, creating a radius of approximately 80 mm. This orientation allows the lead screw to move longitudinally and prevents the dial from unravelling when released by the thumb. A perpendicular orientation was also considered but discounted for this reason. The distal end of the device comprises a number of elastomeric modular balloons mounted in series to co-axially extruded tubing which sheaths over the leaf spring and central shaft. The co-axial configuration facilitates inflation and the elastomeric material ensures elastic behaviour during the inflation and
Development of a Novel Laparoscopic Retractor
deflation cycles which allow the balloons to deflate completely. Experimental testing evaluated the maximum inflation pressure of the balloons before the material tensile strength is reached and also was used to debug any complications with the balloon assembly. The balloons may then be inflated with CO2 via an inflation lumen and a luer taper stop-cock. The extruded tubing and balloons are flexible enough to follow the curvature of the internal leaf spring which they cover and also provide an air tight envelope over the internal mechanism. During the procedure, it may be necessary to reposition the retractor. Therefore, SecuRetract can be deflated and removed back through the 5 mm surgical cannula without incurring damage. The device can subsequently be reinserted as required and is disposed of once the procedure is complete. Pre-clinical Investigation SecuRetract was laparoscopically deployed in vivo in a porcine model and its performance was recorded using laparoscopic video. The pre-clinical animal investigation endpoints were technical feasibility and safety. This investigation was approved by both the Irish Department of Health and UCC animal experimentations ethics committee. The porcine model is a very close anatomical model for the human digestive system, and is an ideal model for the technical evaluation of new medical devices in colorectal applications.
spring would deflect by 80° from the normal position with a radius of curvature of approximately 80 mm (Fig. 6). The retraction force was also determined to be in excess of 5 N which was deemed to be sufficient to displace the bowels. De Visser et al.7 reported that the maximum pull force required to stretch the mesocolon for dissection was 4.7 N. The fully actuated position of the handle design is predetermined by an inbuilt stop which prevents excessive curvature at the distal end. However, the user may overexert the force on the dial forcing the internal displacement bolt against the internal stop. This may cause handle deformation and or force the two sides of the handle apart. The handle was therefore analysed to determine the magnitude of deformation. SolidWorks simulation tool was employed which assumes static loading and linear elastic behaviour. The material assigned to the handle was ABS (E of 2.3 GPa, ryield of 68 GPa) to correspond to the 3D printed prototypes. The results highlighted peak stress areas within the model which subsequently led to modifying the geometry and increasing wall thickness at specific locations until an acceptable deformation was obtained. An external force of 20 N (10 N per side) was applied. This figure represents the maximum translational force that the user may distribute from the dial to the axially acting displacement bolt determined from physical testing. The simulated results
RESULTS Simulated and Physical Results The bending stress and strain in the leaf spring was simulated using Strand7 as described in ‘‘Materials and Methods’’ section B, to optimise the blade parameters ensuring that the yield stress was not reached (Fig. 5). The simulated model was accurate in both geometrical scale and material properties and a Young’s modulus of 196 GPa was assumed as derived from experimental testing. Through simulation, it was possible to design a leaf spring which would experience a maximum of 450 MPa during the curving process. This provides a factor of safety (FOS) of 1.3 when compared to the yield strength of the spring steel (586 MPa). A prototype of the stainless steel shaft and leaf spring assembly, with a polyurethane tube sheathed over the shaft, was used to analyse the relationships between the applied axial force in the internal wire, with the resulting deflected angle and radius of curvature of the shaft. It was determined that by withdrawing the internal wire a set distance, it would experience an axial tensile force of approximately 20 N and the leaf
FIGURE 5. Simulated FEA model of the axial tension experienced by the internal wire used to displace the leaf spring. As the device is actuated, the internal wire experiences tension which in turn curves the leaf spring. The tensile force required to fully curve the blade is critical to the design of the control handle.
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identified a peak stress value of 3.98 MPa which is significantly less than the yield stress (FOS of 17.3), and a maximum resultant displacement of less than 0.04 mm (see Fig. 7).
The modular balloons were also evaluated. The optimum configuration comprises three elastomeric balloons closely positioned in a longitudinal array at the distal end of the device. The properties of the
FIGURE 6. (a) Deflected angle reference for leaf spring displacement. (b) Simulated and experimental axial tensile force in internal wire vs. corresponding deflected angle of SecuRetract’s distal tip.
FIGURE 7. FEA analysis of the handle illustrating (a) the maximum resultant displacement and (b) the areas of maximum stress for an external applied load of 10 N per handle part.
Development of a Novel Laparoscopic Retractor
elastomeric material were characterised though experimentation. Figure 8 illustrates the relationship of inflation pressure to inflated diameter. The ideal inflation diameter of 30 mm occurs at an inflation pressure of approximately 260 mmHg with a combined surface area of approximately 250 cm2. Taking into account that the abdomen is already inflated to approximately 15 mmHg, the total pressure applied to the inflatable is the sum of these two pressures. The burst pressure was rated at approximately 428 mmHg for an elongation at break of 800%. This results in a tensile strength of approximately 15.6 MPa. However, the level of inflation may be varied as appropriate to the procedure to reduce or increase the overall balloon diameter within a maximum working limit of 300 mmHg providing a factor of safety of over 1.4 Pre-clinical Investigation Results The model used was a female landrace pig weighing 26.7 kg. The animal was sedated for the duration of the procedure and was euthanized immediately following the procedure without any recovery. Three laparoscopic ports were inserted through the abdominal wall (2 9 5 mm and 1 9 12 mm trocars), the largest of which accommodated a laparoscopic camera (Olympus Evis Exera BF type 160 series) (Fig. 9a). The peritoneum was inflated to 15 mmHg CO2 with an Olympus UHI-3 insufflator (Olympus, Pennsylvania). SecuRetract was inserted into the upper right quadrant through a 5 mm trocar in its deflated form. Initially the retractor remained in its deflated state but was curved to form a hook shape (tip deflection of 80°). The device was then inflated and tested to assess the capability of retracting the intestines without the need for auxiliary
laparoscopic tools as shown in Fig. 9b. The animal remained in the supine position. SecuRetract was then deflated and removed back through the surgical port only to be reinserted in the left upper quadrant. With the aid of laparoscopic graspers, the intestines were manipulated to allow SecuRetract to hook around the loose organs. SecuRetract was subsequently disengaged and deflated and the animal was temporarily placed in a steep TP. The retractor was repositioned and inflated around the bowels which were retracted due to the effect of gravity. Once SecuRetract was in position, the animal was returned to the supine position. SecuRetract moved to mesentery manipulation for inferior mesenteric arch and lymph node access. The retractor was placed between the bowels and the mesentery holding the bowels to the posterior wall, thus providing a foothold to retract the bowels. Post laparoscopic procedure, the investigation concluded with an open surgical investigation to allow clear visualisation of the retraction process (Fig. 9c). The pre-clinical animal investigation proved successful in assessing technical feasibility of the retractor. The light weight retractor was successfully and repeatedly inserted thorough a 5 mm cannula. During the study, SecuRetract was repositioned several times, inflated and deflated, repositioned to different quadrants and did so without damage to the tissue or device. However, SecuRetract was incapable of independently retracting the bowels during installation. The bowels moved out of position as the retractor tried to hook around the organ during deployment. SecuRetract therefore requires a secondary laparoscopic instrument to assist in its initial placement. However, once achieved, SecuRetract can effectively prevent bowel migration into the operating field.
FIGURE 8. Inflation pressure vs. inflated diameter for a number of modular elastomeric balloons mounted towards the distal end of SecuRetract at room temperature (21 °C).
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FIGURE 9. SecuRetract used in a pre-clinical animal investigation. (a) External view of SecuRetract passing thorough laparoscopic port. (b) View from laparoscopic camera of the distal end of SecuRetract engaging the bowels. (c) View of SecuRetract’s placement around the bowels during a conversion to open surgery to better visualise the operation of the retractor. The curved, inflated end of SecuRetract is placed between the small bowels and the mesentery connecting the bowels to the posterior wall. This provides a foothold for the device to retract the organ.
When the animal was temporarily placed in the TP, the bowels were pulled back from the operating field using a laparoscopic instrument. SecuRetract was then positioned to hook around the retracted bowels, inflated and held in position. Once SecuRetract was in place, the animal was then returned to its supine position. SecuRetract maintained the bowels in their retracted position effectively away from operating field whilst providing little or no interference with other instruments.
DISCUSSION An essential part of any laparoscopic surgery is to ensure that the surgeon has sufficient exposure to visualise the operating field and surrounding structures to perform an effective and safe procedure. Laparoscopic retraction has greatly advanced since first introduced in 1991, and yet a truly safe and effective laparoscopic retractor has yet to be universally acknowledged.12,15 Through an iterative design process and continual consultation with end users, SecuRetract was developed. Initial end-user surveys were compiled from the clinical co-inventors to determine the initial design criteria. Once the current solution was reached,
SecuRetract was presented to a group of key opinion leaders in U.S. based hospitals to gain some feedback on the overall device design and functionality. It was noted that despite the handle being quite different in design to conventional laparoscopic instruments, the intuitive nature of its actuation may remove possible learning and adoption obstacles. In addition the presented solution may improve on current laparoscopic retraction devices due to its inflatable, manoeuvrable curved design. Simulated and physical modelling played a vital role in optimising the final design. The leaf spring on the distal end of the device was studied using finite element analysis which resulted in reduced material volume and increased retraction force. As predicted, the physical model closely resembled the simulated calculations (approx. 93%) reflecting the elastic nature of the spring steel. The handle was also scrutinised using FEA rendering a sufficiently rigid product whilst minimising the material used. As the handle is actuated, a tensile force of approximately 20 N is applied to the internal wire, which is sufficient to curve the distal portion of the retractor without exceeding the yield strength of the leaf spring. In addition, the engaged device commands sufficient rigidity to retract the soft bowels from the operating space (>5 N). A pre-clinical trial demonstrated that SecuRetract met the technical and clinical requirements set forth in the preliminary user survey. However, the trial did have its limitations. Due to the subjective nature of this trial, it was difficult to obtain quantitative data. The porcine gastrointestinal tract, albeit similar in anatomical make-up, was different in size and weight compared to a human’s. Therefore, the pre-clinical study was primarily a technical feasibility study of the device. It was shown that SecuRetract can effectively retain the bowels from the operating field during minimal invasive surgery without the foreseeable complications of competing devices or interfering with other laparoscopic instruments. However, SecuRetract may need the assistance of an auxiliary laparoscopic instrument in its initial placement. The inflated curved portion of the retractor has proven capable of safely retracting the internal organs in the supine position, generating a retraction force of over 5 N, thus providing evidence that the device may minimise the need for prolonged use of the TP. The pre-clinical trial also identified some possible design improvements which include device orientation, labelling and ergonomic modifications. The main proposed advantage of SecuRetract over alternative deployable retractors is that it can be adjusted and manoeuvred to suit any abdominal surgery. SecuRetract may also be inserted through a 5 mm port which in turn reduces recovery time when the
Development of a Novel Laparoscopic Retractor
facial defect is closed post-surgery. The inflatable nature of the device means that it will not perforate or damage soft internal organs or tissue. The long spherical balloon shape increases the contact surface with the bowel wall reducing the pressure on it. It also enhances the mechanical adherence with the bowel’s surface minimising space occupation. Due to its ability to hook around the bowels, SecuRetract can retract a far larger section of the bowels compared to the traditional laparoscopic retractors discussed in the introduction. Therefore, the procedure may be undertaken whilst the patient remains in the supine position. This will result in less operative complications, and consequently may decrease hospital costs. In addition, SecuRetract offers improved ease of laparoscopic access during surgery by means of its rapid deployment and retraction through a single 5 mm port. This feature is particularly amenable for extending the device’s use to clinical indications outside of colorectal surgery (e.g., upper gastrointestinal interventions). The curvature and length of the shaft may also be altered to provide a selection of SecuRetract devices as appropriate to a specific procedure. Further development is currently being undertaken to replace the carbon steel leaf spring with a stainless steel equivalent (grade 301) which obviates the risk of corrosion as well as increasing the functional factor of safety. The handle design may also be re-investigated pending a more substantial end-user survey. Finally the technical feasibility of applying a dark matte colour to the balloons will be assessed. This may minimise optical aberration reducing reflections from the balloon surface giving the surgeon a more comfortable visual environment within the abdominal cavity.
CONCLUSION While the results of this preliminary analysis and experimentation are encouraging, a clinical investigation will be required to investigate whether SecuRetract can effectively retract the small intestines in the supine position whilst providing a clear operating space within the human peritoneal cavity. A clinical investigation will also demonstrate whether or not SecuRetract outperforms existing laparoscopic retractors in reducing the time of intervention as well as minimising intestinal wall trauma. Subsequent to the pre-clinical investigation, a number of design improvements have been identified which will further enhance the operational performance and ease of use. Despite these important investigations and modifications, the current results point to a promising alternative laparoscopic retractor with the potential to help alleviate complications in laparoscopic colectomy.
ELECTRONIC SUPPLEMENTARY MATERIAL The online version of this article (doi:10.1007/ s10439-014-1029-1) contains supplementary material, which is available to authorized users.
ACKNOWLEDGMENTS The authors thank the Biological Services Unit and UCC Department of Physiology for facilitating the pre-clinical animal investigation, in particular Mr. Kieran McManamon, and Dr. Therese RuaneO’Hora.
DISCLOSURES The authors of this paper have no conflict of interest or financial ties to disclose.
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