Surgical Endoscopy

, Volume 26, Issue 6, pp 1751–1758

A comparative study of endoscopic full-thickness and partial-thickness myotomy using submucosal endoscopy with mucosal safety flap (SEMF) technique

Authors

  • Eduardo A. Bonin
    • Developmental Endoscopy UnitMayo Clinic
  • Erica Moran
    • Department of SurgeryMayo Clinic
  • Juliane Bingener
    • Department of SurgeryMayo Clinic
  • Mary Knipschield
    • Developmental Endoscopy UnitMayo Clinic
    • Developmental Endoscopy UnitMayo Clinic
Article

DOI: 10.1007/s00464-011-2105-3

Cite this article as:
Bonin, E.A., Moran, E., Bingener, J. et al. Surg Endosc (2012) 26: 1751. doi:10.1007/s00464-011-2105-3

Abstract

Background

Esophageal myotomy using submucosal endoscopy with mucosal safety flap (SEMF) has been proposed as a new treatment of achalasia. In this technique, a partial-thickness myotomy (PTM) preserving the longitudinal outer esophageal muscular layer is advocated, which is different from the usual full-thickness myotomy (FTM) performed surgically. The aim of this study was to compare endoscopic FTM and PTM and analyze the outcomes of each method after a 4 week survival period.

Methods

Twenty-four pigs were randomly assigned into group A (FTM, 12 animals) and group B (PTM) to undergo endoscopic myotomy. Lower esophageal sphincter (LES) pressure was assessed using pull-through manometry. For statistical analysis we compared the average esophageal sphincter pressure values at baseline, after 2 weeks, and after 4 weeks between groups A and B. The P value was set as <0.05 for significance.

Results

Eighteen animals were included for statistical analysis. Mean (SD) LES pressures were similar between groups A and B (nine animals each) at baseline [group A = 23 (10.4) mmHg; group B = 20.7 (8.7) mmHg; P = 0.79], after 2 weeks [group A = 19 (7.7) mmHg; group B = 21.8 (8.4) mmHg; P = 0.79], and after 4 weeks [group A = 22.6 (10.2) mmHg; group B = 20.7 (9) mmHg; P = 0.82]. LES pressures were significantly reduced in three animals after 4 weeks: one animal (1%) in group A and two animals (2.5%) in group B. An extended myotomy (3 cm below the cardia) was achieved in three animals and was responsible for the significant drop in LES pressure seen in the two animals from group B.

Conclusion

Esophageal myotomy using SEMF is a feasible yet challenging procedure in pigs. Full-thickness myotomy does not seem to be superior to partial-thickness myotomy as demonstrated by pull-through manometry. Endoscopic esophageal myotomy results are greatly influenced by obtaining adequate myotomy extension into the gastric cardia.

Keywords

AchalasiaGastrointestinalEndoscopyEsophagealDigestiveMyotomy

Surgical myotomy had been shown to provide superior long-term outcomes compared to endoscopic techniques for long-term treatment of achalasia [1, 2]. The laparoscopic modified Heller myotomy procedure has become the preferred surgical approach in the appropriately selected patient [3]. This procedure involves a surgical incision splitting the longitudinal and circular muscles of the esophagus from the distal esophagus onto the proximal stomach while preserving the mucosa. Extension of the myotomy 3 cm below the esophagogastric junction (EGJ) has been demonstrated to enhance long-term benefit [4]. In addition, partial fundoplication has been advocated since successful myotomy is associated with objective findings of GERD in 30–60% of patients [1, 5].

A new endoluminal approach using submucosal endoscopic dissection to perform a myotomy has been proposed [6]. A pioneering experimental study in pigs examined the feasibility of esophageal myotomy using the submucosal endoscopy with mucosal safety flap (SEMF) technique [7]. This technique has been recently translated into humans for the treatment of achalasia [8]. In this novel technique, originally a submucosal space was created with high-pressure CO2 injection (using CO2 bursts) and balloon dissection. The esophageal myotomy is performed within and at the end of this space. The myotomy, offset from a mucosal entry point, is securely sealed with the overlying mucosal flap, which closes the submucosal working space. The technical modification proposed by this previously published endoscopic method consists of partial-thickness myotomy involving the internal circular muscular fibers to decrease morbidity.

In the current study we performed the SEMF technique to assess subtotal versus full-thickness myotomy in a swine model in the setting of a 1-month survival study. The aim was to analyze the outcomes of each method, hypothesizing that full-thickness myotomy would be superior in providing LES pressures.

Materials and methods

This 4 week survival, Institutional Animal Care and Use Committee–approved study used 24 domestic cross-breed Yorkshire swine that weighed 50–55 kg. Animals were randomly assigned into study groups using a permuted block system: group A animals were to undergo full-thickness myotomy (12 animals) and group B partial-thickness myotomy (12 animals). Animals were fasted for solid food for 24 h and underwent standard general anesthesia (xylazine, 3 mg/kg intramuscular; Telazol, 5 mg/kg intramuscular). Animals were intubated and continuously monitored. Anesthesia was maintained using inhaled 1–2% isoflurane.

Lower esophageal sphincter (LES) pressure pre- and postmyotomy was assessed through manometric recordings. Pressures were recorded using a three-lumen water-perfused manometry catheter system (Narco Bio-systems, Austin, TX, USA) perfused with distilled water using a low-compliance pneumohydraulic system. Pressures within the catheter were transmitted to an external transducer and recorded using an electronic interface. The catheter assembly was pulled through the LES region three times. Each manometry recording was undertaken after a diagnostic endoscopic procedure with measurement of the distance from the snout and the esophageal Z-line. The manometry catheter was advanced using endoscopic assistance. Manometric recordings of LES pressure were taken on baseline, 2 weeks, and 4 weeks after procedure.

Endoscopic myotomy procedure

Two groups of animals were assigned to have either full-thickness myotomy, including the internal circular and longitudinal muscular layer (group A), or partial-thickness myotomy involving only the internal circular muscular layer located above the EGJ (group B). The partial-thickness myotomy technique was designed in order to avoid entering the pleural space. In all cases we attempted to perform the procedures by extending the myotomy caudally 3 cm below the EGJ (extended myotomy). Once baseline data collection was completed, the mucosa of the esophagus was lavaged with sterilized water and 20 ml of 10% povidone-iodine through the endoscope accessory channel. The endoscope was advanced 3–5 cm beyond the LES into the gastric cardia. The mucosa was marked posteriorly with injection of an inert sterile carbon particle solution (Spot, GI Supply Inc., Camp Hill, PA, USA). This tattoo served as a guide, in addition to distance estimates based on endoscopic measurements, for the length of the submucosal myotomy below the EGJ. The endoscope was then pulled back to 8 cm above the Z-line. A submucosal fluid cushion was established on the posterior esophageal wall with 0.83% hydroxypropyl methylcellulose (HPMC) followed by short bursts of CO2 for gas dissection within the distal esophagus 8 cm proximal to the EGJ. The needle puncture site was bluntly dilated by the tapered dilating part of the prototype distal needle catheter. A small electrosurgical mucosal incision was made into the bleb using a monopolar endoscopic dissecting knife (Hook-knife, Olympus, Tokyo, Japan) set at 40 W. A standard biliary retrieval balloon (Olympus America, Center Valley, PA, USA) was inserted into the insufflated submucosal layer from the puncture site and distended to tunnel the submucosal connective tissues along a posterior path. A 13-cm-long submucosal tunnel was created, crossing from the distal esophagus onto the gastric cardia. A standard diagnostic endoscope was inserted into the submucosal tunnel facilitating the balloon dissection and advanced fully into and through the submucosal tunnel to visually confirm whether the submucosal dissection had reached the site of tattooing. This distance was measured to compare the distance measured from within the gastric lumen (from the snout to the tattooed site). This was used to guide myotomy localization and incision length. Within 1 cm from the distal end of the created tunnel an endoscopically guided myotomy was made using the endoscopic hook knife. The myotomy was then extended in a distal-to-proximal fashion targeting a starting point up to 4 cm below the EGJ to 6 cm above (2 cm short of the entry point into the submucosal tunnel) (Fig. 1). The submucosal tunnel was closed by applying hemostatic clips (Resolution Clipping device, Boston Scientific, Natick, MA, USA; Quick clip, Olympus America, Center Valley, PA, USA) to the entry site.
https://static-content.springer.com/image/art%3A10.1007%2Fs00464-011-2105-3/MediaObjects/464_2011_2105_Fig1_HTML.jpg
Fig. 1

Endoscopic full-thickness myotomy using SEMF technique. AD Sequence of an extended myotomy being performed at the left lateroposterior wall at the level of the EGJ. Note the thick muscle bundle being sectioned (arrows) using an endoscopic hook-type cautery device. The dissected esophageal mucosa is opposing the muscle wall

Follow-up and data analysis

After the intervention, all animals were kept on a liquid diet (Ensure) for 48 h, progressing to softened diet on day 3 and then a normal diet as tolerated. All animals also received antibiotics for 5 days (enrofloxacin 10 mg/kg once a day) and antacid therapy for 30 days (omeprazole 40 mg bid). Follow-up endoscopy was performed initially 2 weeks later, again after 24 h fast, to record the postmyotomy LES pressures. At 4 weeks follow-up endoscopy was performed and a final set of LES pressures was recorded. The animals were euthanized after the procedure. Necropsy was performed to examine the thoracic, abdominal cavity, and operated site. Examined parameters at necropsy included presence of esophageal abnormalities and complications.

Other parameters such as intraoperative findings and clinical postoperative outcome were recorded. Endoscopic findings of the esophageal and gastric mucosal anatomy at follow-up were also recorded. Examined parameters at esophageal manometry included the pressure amplitudes obtained from each of the recording channels from three measurements at baseline, after 2 weeks, and after 4 weeks. For statistical analysis we compared the average pressure values at baseline, after 2 weeks, and after 4 weeks between groups A and B using a nonparametric test (Mann–Whitney test). Results were expressed as mean (SD). The P value was set as <0.05 for significance.

Results

Endoscopic myotomy using SEMF was performed in all 24 animals. Pneumothorax was a major complication for both full- and partial-thickness myotomy techniques and required a protocol change to include premyotomy tube thoracostomy.

The first five animals developed bilateral/tension pneumothorax which required urgent tube thoracostomy. Significant ventilation and oxygenation impairment persisted and the animals did not survive the postsurgical anesthetic recovery period. The procedures described below were all done using prophylactic right tube thoracostomy before starting the endoscopic procedures. Also, CO2 was added for endoscopic insufflation. The chest tube was removed after finishing the endoscopic procedure while the animal was under anesthesia.

Of the 19 subsequent animals, three developed significant intraoperative hypoxia despite adequate right tube thoracostomy drainage. One of them died in the postanesthesia recovery room despite recovering from hypoxia and achieving adequate anesthetic criteria for extubation. The other two animals underwent contralateral chest needle air decompression followed by chest tube insertion and survived for the required 4 week period without further complications.

Manometry mean pressures were similar between groups A and B at baseline, after 2 weeks, and after 4 weeks (none of them statistically significant) (Table 1). Manometry pressures were significantly reduced (below 5 mmHg) 4 weeks after myotomy in three animals: two animals (2.5%) in the partial-thickness group (20 and 28 mmHg at baseline) and one animal (1%) in the full-thickness myotomy group (30 mmHg at baseline).
Table 1

Manometry pressures at full-thickness and partial-thickness myotomy groups

Experimental groups (N)

Preoperative manometry pressures [mean (SD)]

Postoperative manometry pressures after 2 weeks [mean (SD)]

Postoperative manometry pressures after 4 weeks [mean (SD)]

Group A: full-thickness (12)

23 (10.4) mmHg

19 (7.7) mmHg

22.6 (10.2) mmHg

Group B: partial-thickness (12)

20.7 (8.7) mmHg

21.8 (8.4) mmHg

20.7 (9) mmHg

p value

0.79

0.79

0.82

There were three main technical components of the procedure: (1) opening and closure of the mucosal entry site, (2) submucosal tunneling, and (3) myotomy. The greatest technical issue encountered was tunneling across the EGJ into the cardia. This passage was challenged by the acute angulation into the cardia. Balloon dissection was not as readily performed as it was within the tubular esophagus. In order to avoid complications, the dissection had to be dramatically slowed and frequently supplemented by bursts of pressurized CO2 to define the submucosal plane followed by blunt dissection. Complications were classified as minor or major according to the need of an extra surgical intervention and life-threatening complications and could be aggregated within the three technical components of the procedure (Table 2).
Table 2

Major and minor complications related to the major components of the SEMF myotomy

 

Opening and closure of mucosal entry site

Submucosal tunneling

Myotomy

Minor complications

Bleeding (transient)

Bleeding (transient)

Mediastinal emphysema

Bleeding (transient)

Small esophageal [8 (33%) animals] and gastric [4 (16%) animals] mucosal perforations at EGJ

Opening of the gastric serosa with pneumoperitoneum [1 (4%) animal]

Tearing of the entry site [4 (16%) animals]

Major complications

  

Inadvertent opening of the pleuraa (pneumothorax) 8 (66%) of the 12 animals in partial-thickness group

aIntentional opening of the pleura occurred as a technical component of the full-thickness myotomy procedure

Opening and closure of the mucosal entry site

We could perform a standard opening technique in all animals using submucosal injection with HPMC and mucosal electrocautery-assisted incision. Technical issues were seen in six animals and were related to the insertion of the endoscope into the submucosal space which resulted in linear mucosal tears in four cases, the largest being 4 cm in one animal. Despite this, closure of the mucosal entry site was achieved in all animals in which it was attempted (23 of 24). Closure was not attempted in the second animal of the experiment due to pneumothorax and significant hypoxia. The median number of clips used for closure was four (range = 2–9). Additional technical issues seen in six animals were related to attempts to close gaping mucosal openings. In one animal a two-channel endoscope and tandem use of a long-bladed tissue forceps with a large clip (Resolution clip, Boston Scientific, Natick, MA) were used to facilitate the closure Table 3.
Table 3

Overall postoperative findings and complications related to endoscopic myotomy using SEMF technique

Minor findings and complications

Permanence of clips at the mucosal entry site after 4 weeks [4 (16%) animals]

Gastric mucosal ulceration at the cardia [one (4%) animal]

Erosive esophagitis [6 (25%) animals]

Presence of pneumoperitoneum and intra-abdominal fluid [1 (4%) animal]

Major findings and complications

Severe hypoxia due to bilateral/tension pneumothorax leading to death [5(20%) animals]

Abscess at myotomy site [1 (4%) animal]

Submucosal tunneling

In all animals submucosal balloon tunneling was readily and expeditiously performed within the posterior esophageal wall down to the EGJ. Blunt dissection was then performed using a biopsy forceps with periodic CO2 burst insufflations which resulted in predictable success. Small mucosal perforations occurred at the esophageal and gastric mucosa at the level of the EGJ in four animals due to the short catheter point on the dissecting balloon and the acute angulation of the anatomy at this level. In one animal the balloon dissected into the chest leading to laceration of the pleura and pneumothorax. The median tunnel length was 9 cm (range = 4–14). Limited tunnel length (≤6 cm) was observed in four animals.

Myotomy

The myotomy procedure was partial-thickness (circular muscle layer) in 12 animals and full-thickness in 12 animals. Pneumothorax was observed in all full-thickness myotomy animals. The partial-thickness technique was associated with muscular microperforations leading to pleural opening in eight animals. In three animals the extension of the esophageal myotomy was less than 5 cm (limited full-thickness myotomy) (Fig. 2). A deep (extended) myotomy (3 cm below the cardia) was achieved in three animals, in which the tattoo site could unequivocally be seen in two. A significant drop in manometry reading (below 5 mmHg) was observed in those two animals, both among the partial-thickness group. For the other animal (full-thickness group), the tattoo could not be found and the extended myotomy was evaluated at necropsy (Fig. 3). In two animals who underwent full-thickness myotomy, the mucosal entry site was less than 2 cm from the myotomy.
https://static-content.springer.com/image/art%3A10.1007%2Fs00464-011-2105-3/MediaObjects/464_2011_2105_Fig2_HTML.jpg
Fig. 2

Limited full-thickness myotomy after endoscopic full-thickness myotomy using SEMF technique. Necropsy depicting full-thickness myotomy (large arrows) at the posterior esophageal wall after 4 weeks. Note the myotomy extension limited to the upper portion of the EGJ. The small arrows depicts the tattooed region

https://static-content.springer.com/image/art%3A10.1007%2Fs00464-011-2105-3/MediaObjects/464_2011_2105_Fig3_HTML.jpg
Fig. 3

Extended myotomy, necropsy after 4 weeks. The esophagus has been sectioned longitudinally at the myotomy site (A) and at a preserved area (B). The mucosa has been sectioned and retracted (white arrows) to expose the muscular layer. Note the healing area at the EGJ corresponding to disrupted muscle fibers (A)

A learning curve was identified as we progressed in experience (Fig. 4). Initial technical issues were related to pneumothorax and the most difficult component was performing the necessary extended myotomy.
https://static-content.springer.com/image/art%3A10.1007%2Fs00464-011-2105-3/MediaObjects/464_2011_2105_Fig4_HTML.gif
Fig. 4

Identified learning curve of endoscopic myotomy using SEMF technique (24 cases)

Postoperative outcome

Eighteen animals survived the initial 24 h postanesthesia recovery period and thrived well during the following 4 weeks before being euthanized for necroscopic examination. Adequate healing of the mucosal closure site with slight mucosal scarring was observed in all animals at endoscopic examination prior to euthanasia. Retained clips were found at the mucosal closure site in four animals. One animal developed gastric mucosal ulceration at the cardia with significant improvement after 4 weeks. Erosive esophagitis was documented in six animals (three partial-thicknesses and three full-thicknesses). At necropsy the only finding of concern was a localized abscess at the esophageal wall with no extension to the pleural cavity in one animal that had undergone a full-thickness myotomy. This animal had no clinically detectable signs of infection at follow-up. Air and clear peritoneal fluid was seen in one and two animals, respectively. At necropsy the tattoo site was found in all animals and an extended cardia myotomy was documented in three.

Discussion

Submucosal endoscopy with mucosal safety flap (SEMF) myotomy has emerged as a scarless and less invasive surgical myotomy option for treatment of achalasia [6, 7]. Theoretical advantages are obviating the need for abdominal or thoracic access, faster recovery, less pain, and improved cosmesis. An important anatomic advantage is the elimination of dissection at the level of the diaphragmatic hiatus thereby preserving the phrenoesophageal ligaments, which may be beneficial to avoid postmyotomy reflux. Furthermore, a SEMF myotomy performed along the posterior esophageal wall, which is technically the easiest approach, allows the anterior wall to be surgically accessed for revision. The SEMF procedure has been recently translated into humans for the treatment of achalasia [810]. This feasibility study has confirmed the ability to clearly identify and successfully incise the internal circular muscular layers using currently available endoscopic devices, creating a safe partial myotomy. This procedure has been safely performed by a single operator in 56 patients with favorable functional results at short-term follow-up [10]. Initial published experience in humans is more than encouraging despite a relatively short follow-up. Others are rapidly adopting this method.

The SEMF technique requires creation of an esophageal mucosal opening for access (despite the fact that opening of the esophageal mucosa has been regarded as a complication of surgical esophageal myotomy); however, no difference in outcomes is seen when an inadvertent defect is detected and properly closed. In our series, the most common complication of creating the mucosal opening was tearing and enlargement of the entry incision. Attempting to pass the endoscope into a suboptimally sized opening was responsible for most of the entry point tears. Closure of the mucosal entry point was challenged more by a wider rather than a longer defect. Lengthening of the mucosal entry point by tearing could theoretically limit the cephalad extent of the myotomy, which may or may not be a disadvantage. The finding of a localized abscess in one animal could have been related to contamination of the submucosal tunnel.

In the present animal series, pneumothorax was a recurrent complication. This expectedly occurred in all full-thickness myotomies but also in animals undergoing partial-thickness myotomy. Despite limitations of the pig model because of the pig’s thinner esophageal wall and closer pleural contact to the esophageal adventitia as compared to humans, inadvertent opening of the pleura had a significant impact on morbidity and mortality for this procedure and should be a cause for cautionary planning in human cases.

Esophageal myotomy itself is a surgically challenging procedure. Standard esophageal myotomy for achalasia is a complete splitting of the esophageal muscle fibers 7 cm above and 1–2 cm below the EGJ. Incomplete myotomy is a term used to describe a myotomy with limited extent and is attributed to 30% of dysphagia recurrence in patients with achalasia [11]. The most critical measure for reducing dysphagia recurrence has been to deliberately extend distal myotomy 3 cm below the EGJ [4, 12]. The extended distal myotomy has been shown to reduce postoperative recurrent dysphagia compared to the 1–2 cm more common standard myotomy [4]. This procedure is characterized by performing the myotomy below Belsey’s fat pad, a superficial landmark located at the esophageal junction level that distinguishes the starting point for the myotomy at the cardia, resulting in disruption of the complex crisscrossing muscular fibers that contribute to the LES sphincter mechanism. Careful dissection of this area is recommended to avoid bleeding and excessive cauterization.

In our experience, dissection of EGJ was one of the most critical, time-consuming, and difficult steps of the procedure. There was a trend to perform limited dissection at this area due to concerns for inadvertent gastric mucosal perforation and the anatomic challenge of the sharp angulation crossing into the cardia. The submucosal connective tissue was less responsive to balloon dissection, requiring supplemental gas and blunt dissection which was challenged by a less readily identifiable plane of dissection. In the two animals with inadvertent perforation of the gastric serosa, the resultant pneumoperitoneum could be managed conservatively with needle decompression, which could certainly translate into an easily performed step in human procedures. Successful dissection into the cardia side of the LES was predictably heralded by the presence of several large vessels extending across the submucosal plane, which interestingly correlates to human anatomy. These serve as a critical landmark in the distal SEMF dissection. The deliberate effort at extended dissection into the cardia later in the study resulted in ulceration of the cardia in one case. We attributed this to mechanical trauma to the gastric mucosal flap. Despite tattooing to ensure targeting the distal submucosal dissection, the tattoo was unequivocally reached in only two instances. Limited myotomy, tattoo marking more distal than intended, and loss of orientation during dissection were regarded as reasons for not finding the tattoo markings. We observed that the extension of myotomy beyond the LES seems to increase with experience, in our case after performing 15 procedures.

Finally, gastroesophageal reflux disease is a concern for patients undergoing myotomy. Although we have not performed a specific test to detect gastroesophageal reflux and the animals had normal peristaltic esophagus prior to procedure, distal esophagitis was seen in three animals that had partial-thickness and in three animals that had full-thickness myotomy as early as 4 weeks after the procedure. A second procedure using transoral gastroplication could be a future option for patients presenting with such complication [13].

Intraoperative manometry has been used for assessing completeness of myotomy by measuring any residual high pressure across the gastroesophageal junction. In the present series, the manometry results between the two groups did not favor either partial- or full-thickness myotomy. In fact, we observed significant reduction in LES pressure (manometric pressure below 5 mmHg) in only three animals. Interestingly, in two of these animals an extended myotomy (the tattoo site was identified) was achieved. As extended myotomy was performed in only 3 of the 18 animals, this would explain why most animals in this series did not reach significant reduction in LES pressure after myotomy.

In conclusion, the present study demonstrates that intraluminal endoscopic esophageal myotomy is a feasible yet challenging procedure, chiefly for obtaining adequate extension into the gastric cardia. Although full-thickness and partial-thickness myotomy were associated with pneumothorax requiring prophylactic thoracostomy tube drainage, this has not been seen in the current human experience, likely because of the anatomic differences in the thoracic compartment between the porcine model and human. Advanced endoscopic expertise is needed to achieve the necessary extended cardia myotomy.

The take home message for the surgeon is that the intraluminal approach to myotomy simplifies the entire procedure. Creating the submucosal tunnel is efficient and promptly allows access to the EGJ, where the most critical phase of the myotomy—extension into the gastric cardia—can be focused upon with careful dissection. Finally, with care to avoid trauma to the overlying mucosal flap, a predictably safe and simple completion to the procedure is possible.

Conclusion

The present study demonstrates that esophageal myotomy using SEMF is a feasible yet challenging procedure in the swine model. Full-thickness myotomy does not seem to be superior to partial-thickness myotomy as demonstrated by pull-through manometry. Endoscopic esophageal myotomy results are greatly influenced by obtaining adequate myotomy extension into the gastric cardia. Proper dissection and extension of myotomy beyond the LES seems to increase with experience.

Acknowledgment

This study was supported by a NOSCAR grant.

Disclosures

Dr. Gostout and the Developmental Endoscopy Unit receive research grants from Olympus America and Olympus Tokyo. Dr. Gostout is an advisor to Apollo Endosurgery; both he and the Mayo Foundation maintain an equity position in Apollo Endosurgery, Inc. Dr. Gostout and the Mayo Foundation hold intellectual property (patent) on the SEMF procedure and devices. Drs. Bonin, Moran, and Bingener as well as Mary Knipschield have no conflicts of interest or financial ties to disclose.

Copyright information

© Springer Science+Business Media, LLC 2012