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Question 1

How do electrosurgical devices work?

What is the difference between the “cut” and “coag” settings?

Answer 1

Electrosurgery uses radio frequency (RF) alternating current to heat the tissue by RF induced intracellular oscillation of ionized molecules that result in an elevation of intracellular temperature.

If tissue is heated to 60-99 degrees C, the simultaneous processes of tissue desiccation (dehydration) and protein coagulation occur. If the intracellular temperature rapidly reaches 100 degrees C, the intracellular contents undergo a liquid to gas conversion, massive volumetric expansion, and resulting explosive vaporization.

As waveforms of the electrosurgical devices change, so do the tissue effects; “pure-cut” is a uniform waveform resulting in rapid vaporization. Intermittent high voltage waveforms such as “coag” produce less heat and more coagulant.

Question 2

Define sensitivity, specificity, postive predictive value, and negative predictive value.

Answer 2

• Sensitivity is the proportion of true positives who are correctly identified as such
• Specificity is the proportion of true negatives who are correctly identified as such
• Positive predictive value is the probability that subjects with a positive test truly have the disease.
• Negative predictive value is the probability that subjects with a negative test truly don’t have the disease.

Question 3

How is disinfection classified?

What methods of sterilization can you describe?

Answer 3

Disinfection is classified as

1. Critical
   
   • A device that normally enters sterile tissue or the vascular system
   • Such devices must be sterilized, defined as the destruction of all microbial life.
   • e.g. biopsy forceps, sphincterotomes
2. Semi-critical
   
   • A device that comes into contact with intact mucous membranes and does not ordinarily penetrate sterile tissue.
   • These devices require high-level disinfection, defined as destruction of all vegetative microorganisms, mycobacteria, viruses, fungal and some bacterial spores, as represented by a 6-log-reduction in mycobacteria.
3. Non-critical
   
   • Devices in contact with intact skin
   • Require low-level disinfection.
4. Steam autoclave at 120° under 2atm up to 60 min.
5. Dry heat at 170° for up to 4 hours
6. Ethylene oxide gas (endoscopy)
7. Peroxyacetic acid (endoscopy)
8. Hydrogen peroxide gas

Question 4

Discuss surgical drains.

Answer 4

Classified by

Type:

• Passive
  
  • Penrose, Yates post lap chole
• Active
  
  • Blake’s drain post mastectomy
• Irrigating/sump
  
  • Malecot in rectal post Hartmann’s

Indication

• Therapeutic
  
  • Controlled drainage for fistula
• Indicative
  
  • Bile leak
• Prophylactic
  
  • Around revisited anastamosis

Question 5

Provide an overview of the pathophysiology of sepsis.

Answer 5

The normal response to infection serves to localise and control microbiological invasion. This occurs through the chemotaxis of neutrophils and macrophages, which in turn release inflammatory mediators.

When these inflammatory mediators (IL-1, IL-6, TNF-a) initiate a generalised repsonse, sepsis ensues. Sepsis is characterised by systemic vasodilation and resultant hypotension, increased vascular permeability and resultant oedema, and microcirculatory dysfunction and resultant tissue hypoxia. This downward spiral trend continues unless the anti-SIRS mediators (IL-10, IL-4) compensate to a sufficient degree.

Question 6

Describe assessment of perioperative risk.

Answer 6

Assessment of perioperative risk fasciliates decision making and informed consent. It also stratifies patients into risk categories, which is helpful for research purposes and audit of morbidity and mortality.

• Gross measurements
  
  • End-of-bed assessment by surgeon
  • 0-10m test or “get-up-and-go”
  • 6 minute walk test (cheap CPEX)
  • Metabolic equivalents (METs)
• Predictive scoring systems
  
  • POSSUM (12 physiological, 6 operative parameters)
    
    • P-POSSUM, V-POSSUM, CR-POSSUM, O-POSSUM - more studies needed.
  • ASA
• Measurement of physiological reserve
  
  • CPEX testing

Question 7

Describe the physiology of fever.

Answer 7

Afferent limb

• Exogenous and endogenous pyrogens trigger release of IL-1, TNF-a, IL-6 from monocytes.
• Thermoregulating neurons in hypothalamus stimulated.

Efferent limb

• Sympathetic vasoconstriction, heat preservation, core temp increased
• Peripheral efferent motor nerves contract causing shivering, increases core temp.

Question 8

What are the pro-inflammatory cytokines?

What are the anti-inflammatory cytokines?

Answer 8

Modulate SIRS:

IL-1, IL-2, IL-6, IL-8, TNF-a, PAF

Modulate CARS:

IL-4, IL-10, IL-13, TGF-b

Question 9

What are the adrenergic receptors and their activities?

Answer 9

a1

• smooth muscle constriction in blood vessels
• smooth muscle constriction in gut
• glycogenolysis and gluconeogenesis in liver

a₂

• smooth muscle contraction in blood vessels
• smooth muscle relaxation in gut
• increases Renin, increases plt agg., inhibits lipolysis

ß₁

• chronotropic and inotropic
• intestinal relaxation

ß₂

• relaxation of smooth muscle in gut and skeletal muscle
• glycogenolysis

Question 10

Categorise the surgically correctable causes of hypertension.

Answer 10

Cardiovascular

• Coarctation of the aorta
• Renal artery stenosis
• AAA
• A-V fistula

Adrenal

• Phaeochromocytoma
• CAH
• Cushing’s

Renal

• Renin-producing tumours
• Previous injury to kidney causing Paige kidney
• Polycystic disease

Endocrine

• Acromegaly
• Hyper/hypothyroidism

Neurologic

• Mass lesions causing hypertension

Pregnancy

• Pre-eclampsia

Question 11

What are the most common, and most deadly, types of cancer in New Zealand for men and women?

Answer 11

NZ Cancer Death

Male

• Lung
• Prostate
• CRC

Female

• Breast
• CRC
• Lung

Cancer Incidence

Male

• Prostate
• CRC
• Melanoma
• Lung

Women

• Breast
• CRC
• Melanoma
• Lung

Most deadly by mortality/diagnosis - Lung, Gastric, Liver

Question 12

Provide an overview of chemotherapeutic agents.

Answer 12

• Antimicrotubules
  
  • Cell arrest in metaphase
  • ^eg Paclitaxel
• Antimetabolites
  
  • Masquerade as purine analogues
  • ^eg 5-FU, Gemcitabine, MTX, Azathioprine
• Anthracyclines
  
  • Disrupt nucleic acid synthesis
  • ^eg Doxorubicin, Mitomycin
• Alkylating agents
  
  • Add Alkyl group to DNA strand to prevent replication
  • ^eg Platinum agents; Cisplatin etc
• Topoisomerase inhibitors
  
  • Inhibit topoisomerase
  • ^eg Irinotecan
• Antibodies
  
  • Variably affect cell surface receptors
  • ^eg Bevacizumab, Erlotinib, Sumatinib

Question 13

What are the phases of shock?

Answer 13

• Non-progressive phase

Reflex compensatory mechanisms are activated;

Baroreceptor reflexes, catecholamine release, RAAAS, ADH release, and sympathetic stimulation

Net effect of tachycardia, peripheral vasoconstriction, and renal conservation of fluid

• Progressive phase

With persistent oxygen deficit, intracellular aerobic respiration is replaced by anaerobic glycolysis and excessive production of lactic acid

Acidaemia results and the vasomotor response is blunted leading to vasodilation, peripheral pooling, and worsening shock

• Irreversible phase

In severe cases, widespread injury results in overwhelming lysozymal leakage and cell death

Bacterial superinfection catalyses this and multi-organ failure ensues

Question 14

What is a sentinel lymph node?

Answer 14

A sentinel lymph node is the first node encountered by a lymphatic channel that is draining a primary site.

There are often up to 3 “sentinel nodes”, as if there is more than one lymphatic channel draining a primary, there will be more than one sentinel lymph node.

It is not necessarily the hottest or closest.

Question 14

What is anaphylaxis?

Where is it commonly encountered in General Surgery?

Answer 14

• Anaphylaxis is an acute, potentially lethal, multisystem syndrome resulting from the sudden release of mast cell- and basophil-derived mediators into the circulation.
• It most often results from immunologic reactions to foods, medications, and insect stings, although it can also be induced through nonimmunologic mechanisms by any agent capable of producing a sudden, systemic degranulation of mast cells or basophils.
• The diagnosis is based on the involvement of multiple organ systems;
  
  • skin and mucosa
  • airway and lungs
  • gut (diarrhoea or vomiting)
  • cardiovascular (BP, HR, syncope)
• Most commonly encountered in General Surgery in response to perioperative medications
  
  • Antibiotics; 50%! beta-lactams most common
  • Blue dye; ~1/200
  • Chlorhexidine; up to 2%
  • Iodine; shellfish not transferrable
  • Latex; OT precautions.

Question 14

What is ARDS?

Answer 14

• Acute respiratory distress syndrome is an acute, diffuse, inflammatory form of lung injury that is characterised by:
  
  • Respiratory failure with pulmonary oedema
  • An antecedent trigger
  • The abscence of congestive cardiac failure
• It is associated with a high mortality, usually from the underlying cause.
• Treatment of the underlying cause or inciting event is key, supportive care with escalating assisted ventilation characterises ARDS management.

Question 14

Describe the disease states associated with Hepatits B

Answer 14

• Hepatitis B is a dsDNS virus that belongs to the hepadnavirus family.
• It is a global health problems with >250million people affected, with over 600,000 people dying each year due to Hep-B related disease.
• Infection is either acute or chronic:
  
  • Acute infection is through transmission of bodily fluid.
  • The treatment for acute Hepatitis-B infection is supportive; only 1% will suffer liver failure and only 5% become chronic carriers.
  • In complicated cases patients can be treated with entecavir or tenofovir.
  • Chronic infection occurs when HepBSAg remains positive for more than 6 weeks
  • Most people with childhood-acquired chronic HBV infection will enter a long-term inactive phase associated with persistently normal serum ALT and low serum HBV DNA levels, which may last up to 30 years.
  • However, some people will enter an active phase of infection, which is associated with persistently elevated ALT levels, high levels of viral activity, liver inflammation and progressive liver fibrosis.
• Patients with chronic Hep are at increased risk of HCC and cirrhoisis (leading cause of cirrhosis in Maori and PI) so patient require surveillance with AFP and USS.
• Recently, oral antivirals have become available at GPs for (lifelong) treatment of Hep and subsequent risk reduction.

Question 14

Describe the pathogenesis of intra-abdominal adhesions

Answer 14

• Traumatic injury to the peritoneum (surgery) induces a similar pattern of response exhibited by most tissue
  
  • A reactive response, characterized by coagulation, haemostasis, release of pro-inflammatory cytokines, and chemo-attractants. PAF, IL-1, IL-2, and TNF are key molecules in this phase.
  • A regenerative response follows; infiltration with macrophages and fibroblasts generates a proliferative phase of recovery.
  • Finally, a remodelling phase ensues where matrix metaloproteinases degrade and remodel tissue substrate.
  • The early balance between fibrin deposition and degradation (ie, fibrinolysis) appears to be a critical factor in the pathogenesis of adhesions.
• The relative formation and breakdown of fibrin manifests as different phenotypes within a population, where some individuals form dense asdhesions and other, with a similar stimulus, form few if any.

Question 14

What are the pathological features of Cowden syndrome?

What tumours commonly occur?

Answer 14

P.A.T.H. B.T.Co.R.E

P= Papilloma

A = Acral keratoses

T = Tricholemomma

H = Hamartomatous polyps

Breast (80%)

Thyroid (20%)

Colorectal cancer (~15%)

Renal cell cancer (~15%)

Endometrial cancer (~15%)

Question 14

What is the in vivo half life of:

Monocryl?

PDS?

Maxon?

Prolene?

Answer 14

Monocryl = 2 weeks

PDS = 3 weeks

Maxon = 6 weeks

Prolene = 300 days

Question 14

What is the latest evidence with regard to bowel-prep for colorectal resection?

Answer 14

• Paradigm shift towards giving it again…
  
  • Polyethylene Glycol (“Moviprep”)
  • Neomycin 1g (3 doses)
  • Metronidazole 1g (3 doses)
• ACS-NSQIP study of ~45,000 patients
  
  • Reduced SSI
  • Reduced anastamotic leak
  • Reduced overall complications
  • Reduced mortality at 30 days

Question 15

How does a vacuum assisted dressing work?

Answer 15

• Commonly used adjunct for wound management
• Promote wound healing via direct and indirect effects:
  
  • Direct
    
    • Closes wound
    • Absorbs exudate
  • Indirect
    
    • Increases local blood flow
    • Decreasing inflammatory cytokines
    • Mechanical signalling for fibroblasts
    • Increased VEGF
    • Alters bacterial burden

Question 16

What is the contemporary definition of sepsis?

Answer 16

• Sepsis is a clinical syndrome that has physiologic, biologic, and biochemical abnormalities caused by a dysregulated inflammatory response to infection.
• Sepsis and the inflammatory response that ensues can lead to multiple organ dysfunction syndrome and death.
• Sepsis is considered present when 2 of the q-SOFA varibles are present;
  
  • RR >22/minute
  • Altered mentation
  • SBP <100mmHg

Question 17

Describe some of the virulence factors assoicated with common surgical pathogens

Answer 17

Virulence factors may be classified into those that promote adhesion, those that promote immune evasion, those that promote spread, and those that have toxic effects.

• Adhesion factors:
  
  • Vibrio cholera, H pylori, and Streptococcus pyogenes all have adhesion molecules
• Immune evasion:
  
  • Encapsulated bacteria; Neisseria, Haemophilus, Streptococci, E. coli, Klebsiella
• Facilitate spread:
  
  • Hyaluronidase elaborated by Staphylococcus aureus, Streptococcus pyogenes, and Clostridium perfringins
  • `DNAase by staph aureus
  • Protease and collagenase by clostridium perfringins
• Toxins:
  
  • Alpha toxin by staph aureus and clostridium
  • Super-toxins by Staph and Strep that cross-link and hyper-activate lymphocytes causing massive cytokine release
  • Endotoxins on the surface of gram negative e. coli

Question 18

What are the over-arching consequences of severe sepsis/SIRS?

Answer 18

* Myocardial depression * Vasoplegia * Endothelial dysfunction

Question 19

What is PEEP?

Answer 19

* Positive End Expiratory Pressure * PEEP mitigates end-expiratory alveolar collapse and may reduce the incidence of ventilator-associated pneumonia and lung injury * By preventing alveolar collapse, PEEP reduces the proportion of the lung that is shunted and improves the efficiency of gas exchange.

Question 20

What is the Bohr effect?

Answer 20

* Haemoglobin's oxygen binding affinity is inversely related to acidity and the concentration of CO₂ * This has the effect of oxygen release in the tissue, where there are locally high levels of acid and CO₂ * The corollary to this is increased uptake in the lung, which is low in CO₂

Question 21

What is the Haldane Effect?

Answer 21

* The Haldane effect describes one of the properties of Haemoglobin; that increased oxygenation displaces CO₂ from the haemoglobin molecule i.e. oxygenated haeoglobin has a reduced affinity for CO₂ * The corollary to this is that de-oxygenated haemoglobin has an increased affinty for CO₂

Question 22

What are the anti-inflammatory cytokines?

Answer 22

IL-10 Il-4, IL-13, TNF-B

Question 23

Explain the principle of fractionated, ionizing radiation.

Answer 23

Ionizing radiation is defined as radiation energy with sufficient strength to eject an orbital electron from an atom when the radiation is absorbed. Ejected electrons interact directly with target molecules in the cell and indirectly with water to produce free radicals. These interactions lead to double strand DNA breaks that overwhelm repair pathways. Small (fractionated), frequent doses allow normal cells to repair the damage (sub-lethal damage) while cancer cells (predominantly in the G2 or mitotic phase) are most affected.

Question 24

How are tumour markers categorised?

Answer 24

Protein tumour markers * CEA, AFP, Ca19-9, PSA, Ca125, HCG DNA based markers * RET, APC, p53, BRAF V600E Epigenetic changes * Cancer-specific methylation patterns.

Question 25

Discuss physiological scoring in the ICU.

Answer 25

APACHE-IV and SAPS II APACHE-IV (Acute Physiology And Chronic Health Evaluation) is one of the most commonly used ICU scoring systems that is based on ~20 physiological parameters in the ICU setting. APACHE-II through IV predict hospital mortality and length of stay. SAPS-III (Simplified Acute Physiology Score) is calculated from ~20 routinely collected variables. Similarly, it predicts LOS and ICU mortality.

Question 26

Name some bacterial virulence factors

Answer 26

Hyaluronidase Lipase Collagenase (many sp.) Streptokinase M-proteins (GAS)Q Alpha-toxin (clostridial sp.)

Question 27

Why does smoking affect wound healing?

Answer 27

There are more than 4000 chemical in cigarette smoke, including nicotine and CO. Nicotine causes vasoconstriction of the dermal-subcutaneous vascular plexus. Smoking also causes platelet aggregation which results in thrombosis of capillaries, this is detrimental to wound healing which relies heavily on neovascularisation. Blood viscosity is also increased in smokers, impairing oxygen delivery to newly forming tissue.

Question 28

What is Positron Emission Tomography?

Answer 28

* Positron-emission tomography is a nuclear medicine functional imaging technique that is used to observe metabolic processes in the body as an aid to the diagnosis of disease. * Various radio-ligands may be used to detect altered metabolic activity in the body * FDG-PET detects areas of altered glucose metabolism common in malignant cells * Ga-DOTATE-PET detects Octroetide-expressing cells such as Neuroendocrine tumours * Besides being used to detect primary rumours, it is also used to detect distant metastases, to differentiate benign from malignant disease or recurrent cancer from scar tissue, and to evaluate response to therapy.

Question 29

What are the Cluster of Differentiation (CD) antigens associated with GIST?

Answer 29

* c-kit or CD-117 (\>95% of GIST) * CD-34 (70%) * Negative for desmin (smooth muscle tumours) * Negative for S100 (paraganglionomas)

Question 30

What is the Osmolarity of Gastrograffin?

Answer 30

~1900mOsm i.e. around 6 times higher than serum Osm.

Question 31

What is the effective radiation dose and comparable background radiation dose for: Chest X-ray Mammogram Barium swallow CT abdo/pelvis CTA PET?

Answer 31

Chest X-ray: 0.1 mSv = 10 days Mammogram: 0.4mSv = 7 weeks Barium swallow: 6mSv = 2 years CT abdo/pelvis: 10 mSv = 3 years CTA: 20 mSv = 7 years PET: 25mSv = 8 years

Question 32

How does Argon Plasma Coagulation work?

Answer 32

APC uses a coaxial flow of argon gas to conduct a monopolar current to target tissue; this is highly effective for superficial coagulation. A high frequency current is applied to the gas which ionizes the argon and applies the current to the target tissue.

Question 33

How does the Harmonic Scalpel technology work?

Answer 33

Ultrasonic mechnical vibration causes breakage of intracellular hydrogen bonds, resulting in denaturation and coagulation. This occurs at a low temperature, minimising smoke.

Question 34

How is oxygen delivery calculated? How is content of oxygen per blood volume calculated?

Answer 34

DO₂ = CO x CaO₂ CaO₂ = (1.34 x Hgb x SaO₂) + (0.003 x PaO₂)

Question 35

Discuss the pathophysiology of Human Papilloma Virus in the context of oropharyngeal and anogenital cancer.

Answer 35

**High risk HPV strains (16 &18)** suppress apoptosis and activate cell growth when the HPV E6 and E7 proteins disrupt regulatory cell cycle and DNA repair pathways; **E6 inactivates the tumour suppressor gene p53** **E7 inactivates the retinoblastoma tumour suppressor gene.**

Question 36

Which oral anti-diabetic medications need to be discontinued prior to surgical procedures?

Answer 36

* Sodium-glucose co-transporter-2 (SGLT2) inhibitors (“gliflozins”) are oral medications that act by promoting glucose excretion in urine and are used in the treatment of Type 2 Diabetes Mellitus. * They have been associated with euglycaemic DKA and also with Fournier's gangrene. * Metformin should be stopped immediately pre-op due to the increased risk of lactic acidosis and renal impairment.

Question 37

What is the flow rate of crystalloid in a 14G and 18G cannula?

Answer 37

* 125ml/min for a 14G * 60ml/min for an 18G

Question 38

Scalpel versus diathermy for incisions?

Answer 38

* A meta-analysis in Annals of Surgery in 2011 over over 3000 patients showed: * No difference in wound infection rates * Reduced use of analgesia in the first 24 hours post surgery in the diathermy group * (Clinically insignificant) differences in blood loss and time of incision for diathermy compared with scalpel.

Question 39

What factors influence choice of incision of the abdominal wall?

Answer 39

* Need for rapid entry * Certainty of the diagnosis * Body habitus * Location of previous scars * Potential for significant bleeding * Minimizing postoperative pain * Cosmetic outcome

Question 40

How is pain classified? Which pathophysiological pathways are targeted with analgesia?

Answer 40

Nociceptive - pain evoked from tissue stimulus Neurogenic - abnormality in nerve tissue Psychogenic Transduction - Noxious stimuli in target tissue * NSAIDS, Corticosteroids Transmission - Generation of action potential * LA, regional blocks, neurotomy, spinal opioids Modulation - communication between cord and descending, ascending pathways * Paracetamol, parenteral opiods, spinal opioids, tramadol, TCAs, acupuncture.

Question 41

Discuss the investigation and diagnosis of VTE in surgical patients. What is the clinical utility of the Well's score?

Answer 41

Clinical history and examination are insufficient to exlude PE D-Dmier testing allows for exclusion of DVT in "low-risk" patients only. Well's score is _NOT VALIDATED in hospitalised patients_. i.e. Get the CTPA/USS. Do not delay treatment.

Question 42

Fogarty catheter sizes

Answer 42

The size of a fogarty catheter is measured by the inflated balloon in the french system in the range of these catheters the inflated balloon size in mm is approx twice the French size

Question 43

Basics of radiation safety

Answer 43

* The portable xray machine is composed of 2 components * the columnator * generates the xrays * the image intensifier * receives the image * Patient should be positioned as close to the intensifier as possible * The dose of radiation received is subject to the inverse square law * intensity = 1/distance² * this applies to patient and staff * As an operator * always ensure PPE * ensure all staff do the same * wear a dosimeter * wear xray eye protection

Question 44

Suture choice for elective laparotomy fascial closure

Answer 44

2-0 PDS on a CT-2 needle

Question 45

Clindamycin

Answer 45

Lincosamide antibiotic Reversibly binds to 50S ribosomal subunits preventing peptide bond formation thus inhibiting bacterial protein synthesis Important actions: * bacteriostatic or bacteriocidal depending on organism and concentration * suppresses toxin production * decreases penicillin binding protein production * thereby increasing the ratio of beta lactam antibiotic to receptor Important negatives: relatively high rates of c diff

Question 46

Local anaesthetic

Answer 46

* agents that bind irreversibly to sodium channels to block depolarisation and propagation of action potentials * structure = 3 parts - hydrophobic part, hydrophilic part, and a connecting chain between the two (an ester like cocaine or an amide like lignocaine, bupivacaine, ropivacaine) * amides are more resistant to hydrolysis so last longer * don't work well in acidic environment because they are bases; activation requires molecule to pass through cell membrane and in acid environment they are charged and can't do this * smaller diameter fibres (sensory) are more sensitive than thicker (motor) fibres * if given with adrenaline, adrenaline vasoconstricts so delays absorption systemically from the site - so greater dose can be given for longer effect * toxicity: agitation, restlessness, tremor --\> seiszure, CNS depression, respiratory failure, cardiac toxicity * doses in mg/kg, max in 8hrs: * lignocaine 3mg plain, 7mg w/o * bupivacaine 2mg plain, 2.5mg w/o * ropivacaine 3mg plain, 4mg w/o * how to work out volume: * 1% = 1g in 1000mL so 10mg/mL * 0.5% = 0.5g in 1000mL so 5mg/mL * 0.25% = 0.25g in 1000mL so 2.5mg/mL * dose adjustment for: * \>70 * renal impairment for Bupivacaine * hepatic impairment - 60% less dose

Question 47

Provide an overview of tissue reconstruction. Give examples of reconstructive techniques in common use, their advantages, and disadvantages.

Answer 47

**Skin grafts** *- Simple, acceptable cosmesis, totally reliant on the bed they are grafted onto.* * Split thickness * Full thickness **Regional flaps** *- Offer the ability to reconstruct myocutenous defects and cover vital structures. Require long, anatomically reliable pedicles.* * Myocutenous flaps * Pectoral * Latissimus * Platysmal **Free flaps** *- Offer the ability to reconstruct myocutaneous and bony defects as well as cover vital structures* * Myocutaneous * Radial forearm * TRAM * DIEP * Osseous * Fibular free flap * Iliac crest **Enteric flaps** *- Pliable, mucus-secreting, but high oxygen requirement.* * Jejunal interposition flaps * Colonic interposition flaps