Background

Lacrimal surgery existed in the Middle Ages for the treatment of dacryocystitis from a blocked nasolacrimal duct, albeit restricted to crude external drainage of abscess and extirpation of the lacrimal sac. It is not until the late 18th century that more modern techniques for draining the tear sac into the nose were introduced and it took some years for such dacryocystorhinostomy (DCR) surgery to be widely available (Fig. 52.1).

Fig. 52.1 Diagram of endonasal DCR by West.

West JM, A window resection in the nasal duct in cases of stenosis. Trans Am Ophthalmol Soc 1910;12:654–8. Kept at Royal Society of Medicine in London.

The spectrum of lacrimal disease was significantly more severe in the pre-antibiotic area. The French Impressionist artist Camille Pissarro (1830–1903) apparently suffered from recurrent chronic dacryocystitis, an illness which prevented him from working for some periods of time and which is said to have influenced his paintings.

The most common cause of a watering eye (epiphora) is a blocked nasolacrimal duct. Simple watering eye from a narrowed nasolacrimal duct (stenosis or partial nasolacrimal duct obstruction) is also an indication for DCR. The spectrum of disease has changed from dacryocystitis to simpler partial nasolacrimal duct obstructions and hence the threshold for surgery changed to include operating on lesser degrees of obstruction. The sophistication of lacrimal surgery has also advanced using more precise and delicate surgical techniques with miniature powered intranasal tools. This shift is very similar to the shift in cataract surgery also seen over the last 20 years, from intracapsular to extracapsular cataract extraction for dense white cataracts or dark nucleosclerosis, then to phaco-extraction for nucleosclerosis and subtle lenticular opacity. Patient expectation of surgery has increased and quality of life studies show that a real benefit can be obtained for patients with watering eyes by dacryocystorhinostomy surgery, whether it is by the external or the endonasal approach.

In many centers lacrimal surgery is now done almost exclusively endonasally and predominantly endoscopically (a microscope can be used instead), using powered tools and functional endoscopic sinus surgery instruments for adult and pediatric DCR. Endonasal endoscopic monitoring of syringe and probing and endoscopic placement of Jones glass bypass tubes are also standard.

However, the external approach DCR through the skin remains a useful operation where endonasal equipment and skills are not available, and can be particularly important for learning the anatomy of the medial canthus and sac area.

Definitions

There are various abbreviations and different operations.

Fig. 52.2 Dacryocystorhinostomy (DCR).

Courtesy of Santiago Ortiz-Perez.

Table 52.1 Etymology of the term Dacryocystorhinostomy

4 parts: dacro – cysto – rhino – stomy
Ancient Greek	Transliteration	Translation
	dakruon	tear
	kustis	bladder, sac
	rhis	nose
	stoma	mouth

History of modern DCR

Dacryocystorhinostomy was first described in 1893 by Caldwell using a nasal approach to lacrimal duct obstruction. West developed this further in 1913 with a window resection of the upper nasolacrimal duct. Toti advocated the external approach DCR, which was developed by Dupuy-Dutemps in the early 19th century and became the foundation for late 20th century external DCR.

The principle of DCR surgery, whether by the external skin incision approach or by the endonasal approach, is similar. It is to create a good opening for the entire vertical height of the lacrimal sac, into the lateral nasal wall through which the tears can drain unimpeded by mucosal or bony fragments or adjacent nasal structures.

Endonasal approach lacrimal surgery has developed further in the last 15 years using endoscopic techniques and visualization with a Hopkins rigid nasal endoscope (Fig. 52.3). Endoscopic endonasal dacryocystorhinostomy (EE-DCR) is a primary treatment for nasolacrimal duct obstruction with a number of advantages compared with the traditional external approach. These advantages include avoidance of a cutaneous scar, direct visualization of nasal anatomy, and preservation of medial canthal tendon and pump function. The success rate for primary endonasal DCR surgery is comparable to the external approach.

Fig. 52.3 Comparison of (A) external and (B) endonasal endoscopic approach DCR.

Courtesy of Santiago Ortiz-Perez.

Normal anatomy

Tear drainage

The tears normally drain through the puncta to the nose where they can be seen in the inferior meatus (Fig. 52.4).

Fig. 52.4 (A) Normal tear drainage. (B) Anatomy of lacrimal system.

Courtesy of Santiago Ortiz-Perez.

Before any lacrimal surgery is undertaken, a thorough knowledge of the normal nasal anatomy and the relationship of the soft tissues and bones of the lateral nasal wall is required. With this knowledge abnormal variations will then be easily recognized.

Nasal anatomy

Soft tissue and bones of the lateral nasal wall

The main soft tissue structures of the lateral nasal wall are:

• Turbinate: (superior rare), middle, and inferior (Fig. 52.5)

• Meatus: middle and inferior

• Other swellings produced by ager nasi (ethmoid), uncinate process, and bulla ethmoidalis

• Openings in middle meatus: frontal sinus, hiatus semilunaris and infundibulum, maxillary sinus additional opening (accessory opening).

Fig. 52.5 (A–D) Normal anatomy turbinates. (A) Left nasal space normal middle turbinate. (B) Right nasal space concha bullosa of the middle turbinate. This is an aerated middle turbinate. (C) Left nasal floor with Freer’s elevator underneath the inferior turbinate. The middle turbinate is seen in the distance. (D) Infract of the left inferior turbinate to show the inferior meatus and the opening of the nasolacrimal duct.

The main bones of the lateral nasal wall are (Fig. 52.6):

Fig. 52.6 (A–D) Anatomy of the lateral nasal wall. (A) Lateral nasal wall showing middle and inferior turbinates and position of underlying lacrimal sac. (B) The middle turbinate and part of the inferior turbinate have been removed to reveal the length of the nasolacrimal duct. (C) Diagram showing bones. (D) Blue probe in nasolacrimal duct showing its relation to the bones and its posterior directed angle.

• Maxilla

• Lacrimal bone

• Uncinate process

• Perpendicula plate of the ethmoid (middle turbinate)

• Inferior turbinate.

Etiology and assessment of watering eye

Blocked nasolacrimal duct in adults

The causes of a watering eye include hypersecretion and epiphora. Causes of hypersecretion (conjunctivitis, keratitis, corneal foreign body, etc.) should be excluded (Fig. 52.7A–D). Surgery of the lacrimal system is for epiphora, which is an outflow problem cause by a stenosis or obstruction of the lacrimal excretory system or very poor functional drainage, as for instance may occur in facial nerve paralysis.

Fig. 52.7 (A–D) Causes of hypersecretion. (A) Conjunctivitis. (B) Corneal epithelial erosions. (C) Corneal foreign body. (D) Herpes simplex keratitis.

The periorbital area should be examined and a mucocoele, dacyocystitis (Fig. 52.8), and fistula from the lacrimal sac should be excluded. Eyelid position, particularly punctual apposition to the globe, should also be recorded.

Fig. 52.8 (A&B) Dacryocystitis. (A) Right dacryocystitis. (B) Left previous acryocystitis with lacrimal sac fistula.

Canaliculitis is an often missed yet relatively common condition (Fig. 52.9). It is a simple diagnosis because the small swelling is medial to the punctum, which commonly has with some yellow discharge. It can be chronic and partially responsive to topical medication, with surgery the definitive treatment.

Fig. 52.9 (A&B) Canaliculitis. (A) Right inflamed swelling medial to the lower punctum. (B) Actinomycoses discharge from wide dilated ampulla in lower canaliculitis.

Serious causes of lacrimal outflow obstruction may rarely occur, such as a lacrimal sac or nasolacrimal duct tumor, and should be excluded by careful history and endonasal examination. Bloody tears may indicate a lacrimal sac tumor.

The most common cause of epiphora is chronic nasolacrimal duct mucosal inflammation and subsequent stenosis leading to complete obstruction, associated with nasal and sinus mucosal disease, chronic allergy, or upper respiratory tract infections. Rarer causes of epiphora include Wegener’s granulomatosis, sarcoidosis, previous maxillary sinus surgery, and intrinsic or extrinsic tumor. Rarely a lacrimal stone may cause obstruction with epiphora which then resolves after the stone is passed.

Assessment should include a thorough examination of the puncta, the fluorescein dye retention test, syringing and probing, and nasal endoscopy. Nasal endoscopy is done routinely preoperatively to check that there is no obvious nasal cause of the epiphora, determine the normal nasal anatomy, and see if there are any variations that may influence the success rate of surgery. It also allows an assessment of the nasal space to be carried out to see if there is enough space to introduce the instruments endonasally or whether there is a need for septoplasty or partial middle turbinectomy, in order to make more space. Additional imaging with radionucleotide (nuclear lacrimal scintigraphy) is often performed in partial obstruction in order to determine the level of the functional block, whereas dacryocystogram or CT dacryocystogram are reserved for assessing anatomic causes of a watering eye and the sinuses. All findings should be carefully recorded in the clinical record diagrammatically as well as with text, for clear communication.

Surgery for epiphora in adults

External approach DCR surgery

This technique is regarded as the gold standard against to compare all other tear drainage operations. The results vary between 70 and 98% success. Anatomic and functional results should be measured by recording of subjective resolution of symptoms of watering ± mucous reflux resolved. Success should be assessed 6 months after surgery and after the tubes have been removed.

Endoscopic endonasal DCR surgery [see Clip 6.13]

Ensocopic endonasal DCR (EE-DCR) can be performed in almost all patients for nasolacrimal duct obstruction. Septoplasty may be required, or other techniques such as partial middle turbinectomy, to create an amplified internal nasal space for instrument passage and tissue manipulation. It is important to maintain a good space in front of the ostium to prevent soft tissue adhesions, which could lead to surgical failure. Silicone tubes are placed. Local anti-metabolites such as mitomycin C reserved in high risk cases. The success rates are similar to external DCR.

Rongeurs can be used to remove the lower part of the thick maxillary bone to create a large ostium between the lacrimal sac and the nose. The higher thicker maxillary bone is removed with a diamond burr. Soft tissue mucosa can be fashioned with Blakesley forceps, ‘Thru’cut MacKay–Gruenwald nasal forceps, and a serrated oscillating blade or microdebrider.

When mechanical tools with a high speed diamond burr are used, surgery is under a general anesthetic due to continual saline irrigation of the burr head and bone and simultaneous constant aspiration of saline (Fig. 52.10). The diamond DCR burr is 2.5 mm wide and at an angle of 20° to its handle. It is used at 12 000 revs. The serrated oscillating blade (microdebrider) is 3.5 or 4 mm diameter wide and used at oscillating speeds of 1500 up to 5000, taking care not to debride soft tissue towards the orbit, to avoid inadvertent orbital damage.

Fig. 52.10 (A–D) Powered tools for endonasal endoscopic DCR. (A) Medtronic Xomed XPS console for high speed burr and oscillating blade. (B) High speed DCR diamond burr handle (also used for microdebrider). (C) DCR Diamond burr head. (D) Serrated oscillating blade (microdebrider).

Endonasal endoscopic DCR steps (Figs 52.11–52.13)

• Incise the nasal mucosal flap using a MVR blade or size 15 Bard Parker scalpel blade on a long handle.

• Elevate the nasal mucoperiosteum with a Freers elevator from the maxilla line (lacrimal line) anterior towards the middle meatus posterior. This reveals the bone of the lacrimal fossa.

• Perform partial anterior middle turbinectomy if indicated using turbinate scissors, curved Spring scissors ± a microdebrider.

• Fold the nasal flap over the anterior surface of the middle turbinectomy.

• Pass a small light pipe canula down the canaliculus into the lacrimal sac.

• See the light pipe on endonasal side through thick maxilla bone and thinner lacrimal bone and assess the vertical extent of lacrimal sac.

• Use bone rongeurs for the lower part (thinner) maxilla or proceed to entirely bone diamond burr use.

• Use a diamond burr to the maxilla to create ostium and expose the whole lacrimal sac mucosa.

• Carefully remove all small fragments of lacrimal bone by flipping them out first with a Freers elevator, then remove them with Blakesley forceps.

• Remove the uncinate process with pediatric Blakesley forceps.

• If ager nasi cell is encountered superiorly this is often removed using forceps or an oscillating blade.

• Use a serrated oscillating blade to remove small mucosal fragments and tidy up mucosal edges.

Fig. 52.11 (A–F) Diagrams showing endoscopic endonasal DCR steps (A) Location and size of nasal mucosal flap. (B) Nasal mucosal flap elevated. (C) The lower bone (LB) can be removed with ringeurs (a), and the upper maxillary bone (MB) (b) burred using a high speed diamond burr. (D) Once the bony osteotomy or rhinostomy has been made the lacrimal sac mucosa is tented up using a light source (LS) or Bowman probe (b). (E) The lacrimal sac mucosa is opened vertically with a keratome. Horizontal relieving incisions are made to reflect the flaps. (F) The O’Donoghue silicone tubes are passed and knotted. MT, middle turbinctomy; IT, inferior turbinctomy.

Fig. 52.12 (A–F) Right endoscopic endonasal DCR. (A) Local anesthetic injection nasal mucosa lateral nasal wall. (B) Nasal mucosal flap reflected over middle turbinate. (C) Maxilla bone diamond burr. (D) Sac fully exposed with light pipe seen. (E) Sac fully opened and common canaliculus seen. Useful tips for endoscopic endonasal DCR surgery include using the serrated blade for mucosal debriding and use of viscoelastic to dilate the lacrimal sac and help subsequent cut down onto it with a keratome.

Fig. 52.13 (A–H) Left endoscopic endonasal DCR and tubes. (A) Flat made with MVR blade or 15 blade on long handle. (B) Serrated oscillating blade 3000 revs (range 1500 to 5000) uncinate process. (C) Light pipe in lacrimal sac ready to burr maxilla bone anterior. (D) Maxilla bone burr with diamond burr. (E) Further maxilla bone burring and sac filled with HPMC. (F) Sac fully exposed, filled with viscoelastic and ready to open then keratome used to open sac, viscoelastic seen. (G) The silicone passed via the common canaliculus and the nasal mucosal flap fashioned.

Inside the lacrimal sac

The addition of viscoelastic mixed with fluorescein 2% can be particularly helpful for defining the inside of the lacrimal sac as it provides color definition (Fig. 52.14).

Fig. 52.14 (A) Dilated expanded left sac after filled with HPMC and fluorescein 2%. (B) Sac opened and fluorescein draining. (C) Probe seen in common canaliculus opening with fluorescein staining inside mucosal lining of lacrimal sac.

When the lacrimal sac is opened great care must be taken to ensure that the entire sac vertically is opened and that the common canaliculus can be seen. If necessary a 30° endoscope can be used to look ‘inwards’ into the sac, which is in fact ‘outwards’ towards the lateral nasal wall. The sac may contain a mucocele and this must be fully drained with all mucous sucked out using a fine sucker.

Sometimes a lacrimal sac stone is found within the lacrimal sac lumen and the pieces, if part solid, must be fully picked out using a combination of the sucker and Blakesley forceps, checking that all material is removed as even the nasolacrimal duct may contain stone.

Acute dacryocystitis

Endoscopic endonasal DCR is used as a primary procedure for draining an acute or chronic dacryocystitis. The patient with acute dacryocystitis has usually received antibiotics systemically for a few days and, if the dacryocystitis is still present, the option is to drain externally and pack with ribbon gauze for 2 days and do a subsequent DCR (endoscopically or external) about 6 weeks later, or to proceed directly to endoscopic endonasal DCR. The latter is very effective.

Postoperative management after DCR surgery, external or endoscopic endonasal

• During the first week after surgery there may be a little bleeding from the nose. In order to minimize this, the patient is advised to sit up rather than lie down, and to sleep with extra pillows at night. Patients must not blow their nose for 1 week immediately after the DCR operation. After that they are encouraged to blow their nose to clear clots.

• Topical eye drops (antibiotic) are prescribed for 3–4 weeks and possibly antibiotic tablets for 5–7 days is there is evidence of a lacrimal sac infection.

• Anti-inflammatory nasal spray twice daily for 2–3 months.

• Avoid aspirin for at least 10 days after surgery – advised paracetamol or codeine based tablets for mild discomfort or ache.

• Avoid exercise and especially swimming for 2 weeks after surgery.

• They should not apply makeup for the first 2 weeks after surgery.

• Can fly from 7 days after surgery.

• Homeopathic medicines which promote fast healing are avoided.

• The tubes are kept in for 6–8 weeks (occasionally a longer period is advised). They are removed as a simple outpatient local anesthetic procedure, cutting the tubes at the corner of the eye and retrieving them atraumatically from the nose, often with the aid of a nasal endoscope to see them.

• Follow-up visits:

(i) Removal of sutures (if external DCR) at 6–10 days

(ii) Removal of tubes at 6–8 weeks

(iii) Last follow-up visit with syringing at 6 months.

Removal of tubes [see Clip 6.14]

The tubes are removed simply under topical eye and nasal anesthesia (Fig. 52.15).

Fig. 52.15 (A–C) Right post DCR. Removal of tubes. (A) Tubes seen emerging from the ostium on the lateral nasal wall. (B) The tubes are identified at the medial canthus then divided. (C) After the tubes have been retrieved from the nose the FEDT is performed and the lacrimal system irrigated with fluorescein and saline.

• First look inside the nose to visualize the ostium with the tubes emerging.

• Check that there are no peri- or intraostium granulomas that require treatment.

• Check that the tubes move well at the ostium on blinking.

• Divide the tubes at the medial canthus with scissors, with the patient looking laterally.

• Retrieve the tubes endonasally with forceps, using endoscopic visualization.

• Syringe any retained mucous through and check for patency.

• Perform the functional endoscopic dye test (FEDT) with a fluorescein 2% drop placed in the conjunctival fornix seen emerging rapidly from the ostium for a positive test.

Canaliculitis

Actinomycosis canaliculitis responds well to canalicular marsupialization (Fig. 52.16). If possible, try to open the canaliculus whilst preserving the punctual annular ring. The slit canaliculus can be subsequently re-sutured after intubation with a Mini-Monaka.

Fig. 52.16 (A&B) Canaliculotomy and removal of sulphur granules for actinomycosis canaliculitis. (A) The Bowman probe is in the canaliculus and an MVR blade used to cut down onto the probe, keeping the punctum intact. (B) The sulphur granules are removed. Subsequently the canaliculus is re-sutured using 8.0 Vicryl and there is intubation of the canaliculus through to the lacrimal sac with a Mini-Monaka stent.

Canalicular bypass surgery: conjunctival DCR with Jones tube

Conjunctival DCR (CDCR) with a glass tube is the treatment of choice for a watering eye from severe extensive canalicular obstruction e.g. after trauma, antimetabolite chemotherapy, and rarely for functional lacrimal pump failure.

CDCR with the insertion of a Lester Jones Pyrex glass bypass tube is a procedure which creates a lacrimal drainage route from the conjunctiva into the nasal space, bypassing the canaliculi and sac. The proximal end of the Jones tube lies at the medial canthus, and the distal end at the anterior end of the middle meatus (inside the nose).

This operation is usually done as a primary or secondary endonasal endoscopic assisted procedure after failed primary DCR with canalicular obstruction. This enables accurate positioning of the tube inside the nose and alterations to its position if needed. The neck of the glass tube is fixed to the lower medial eyelid with a fine 6.0 polypropylene suture. Initial management postoperatively is in order to check the position of the tube both proximally and endonasally. The patient is encouraged to do the sniff test each day, which is done by holding the nose and sniffing in and out to see if air passes up the tube to the conjunctival fornix.

Re-do DCR [see Clips 6.15 and 6.16]

Failed DCR is usually due to a combination of factors including, most commonly, mucosal scarring across the ostium (membranous scarring) and/or a sump syndrome and/or inadequate bony removal (Figs 52.17 and 52.18). Endoscopic endonasal revision is a relatively simple procedure. Since the principle cause of failure is a scarred ostium, this must first be confirmed by probing and syringing of both the upper and lower canaliculus. Simultaneously, the endoscopic appearance of the ostium and surrounding structures is noted when a probe is in situ and it is possible to see if the membrane is densely scarred or thin. Some DCR ostiums fail because of chronic inflammation with granuloma formation within the ostium and sac remnant, which can be treated in a small number of cases with silver nitrate cautery. Usually the inflammation leads to scarring and need for subsequent re-do DCR.

Fig. 52.17 Left failed DCR with very small ostium and scarred membrane. Membranectomy using Thru’cut forceps. Also see Video 3. (A) Light pipe seen in sac behind thick scarred membrane obstructing its passage. (B) MVR blade used to cut down a small window of nasal mucosa. (C) Mucosal flap retracted showing emerging light pipe from sac. (D) McKay–Gruenwald nasal forceps (through cut with teeth) used to bite and remove the small nasal flap of scarred tissue.

Fig. 52.18 (A–D) Left failed DCR with small membranous flap over lower half of lacrimal sac opening. (A) Light pipe emerging from common canalicula opening but scarred naso-mucosal flap obstructing function. (B) MVR blade used to divide scar. (C) Microdebrider used to remove scarred membrane. (D) Ostium unimpeded now and tubes emerging from common canalicula opening.

Surgical revision of failed DCR is aided by additional procedures to make more room in the nose in front of the ostium, such as partial middle turbinectomy and anterior ethmoidectomy. The serrated oscillating blade microdebrider is used to revise mucosal scarring, with re-intubation with silicone O’Donoghue tubes.

Functional DCR failure can occur despite anatomic patency of the ostium if there is marked canalicular cheese-wiring (whole canaliculus slit open) which affects the functional drainage. Similarly, canalicular obstruction which cannot be overcome surgically is also an indication for conjunctival DCR.

Surgery in children

Congenital blocked nasaolacrimal duct in children

Etiology

See Figure 52.19 for the causes of pediatric watering.

Fig. 52.19 Causes of pediatric watering. (A) Delayed opening of valve of Hasner. (B) Blocked nasolacrimal duct. (C) Delayed opening of valve of Hasner with widened nasolacrimal duct and lacrimal cyst in inferior meatus. It is often associated with congenital amniocele.

Management

Primary syringing and probing

The management of CNLDO is usually to wait until 1 year of age before doing a primary syringing and probing as 90% of children will resolve their watering eyes by that age. A primary syringing and probing is done under a short general anesthesia. If there is a significant nasolacrimal duct block a DCR is indicated, but the majority of cases can be cured by syringing and probing, or with Ritleng intubation.

Secondary endoscopic monitoring of syringe and probing ± nasolacrimal duct intubation

Children who have had failed syringings and probing with recurrent watering should have an examination under general anesthesia, with endoscopic endonasal monitoring of the syringing and probing to determine if there is any other pathology such as an intranasal inferior meatal cyst or a more significant block. Investigations are done with a 4 mm Hopkins endoscope or a finer pediatric rigid endoscope to look into the inferior meatus. The Freers elevator is used to infracture the inferior meatus and create space to see the Bowman probe.

First the lacrimal system is syringed with fluorescein and saline then probed with a size 1 Bowman probe. The probe end is visualized at the floor of the nose and then removed. The system is syringed again (Fig. 52.20).

Fig. 52.20 Standard instruments for endoscopic monitoring of syringe and probing.

By monitoring the probing false passages can be detected and the appearance of the opening of the inferior meatus examined. A mucocele is drained internally via the valve of Hasner.

The indication for Ritleng or bicanalicuar Crawford intubation is when there is a tight but patent lacrimal system which may close off without intubation. Tubes are left in for up to 3 months.

Intranasal cyst and amniocele in neonate

An intranasal cyst can cause difficulties in breathing and noisy breathing (Fig. 52.21). This can be picked up on taking a careful history. It can be associated with a congenital amniocele in an infant so there may be a small swelling over the lacrimal sac area.

Fig. 52.21 (A–C) Right pediatric intranasal inferior meatal cyst probing and cutdown. (A) Right gray round cyst full of mucous in neonate. (B) MVR blade used to cut down onto cyst. (C) Cyst opened and all mucous drained. No Ritleng tube was required.

Pediatric DCR

This can be done endonasally or externally from the age of 18–24 months for failed syringing and probing ± intubation where the obstruction is significant.

Summary

Surgery of the lacrimal system in both adults and children has undergone many changes since the late 18th century and advanced rapidly over the last 15 years. The use of the Hopkins rigid endoscope has radically altered our approach to examination and assessment, to the type of surgery done, and to postoperative management in clinic. Now both the inside of the nose and the internal surgical site can be seen as well as the external aspect. The degree of refinement to instrumentation now means that endoscopic techniques are minimally invasive yet give results similar to external approaches. External DCR is still used in certain patients when there is significant septal deviation or nasal disease, which cannot easily be resolved by septoplasty surgery or other nasal surgery, even with the help of an ENT surgeon. If there is a lacrimal sac tumor then the sac should be taken out whole using an external approach dacryocystectomy.

Future of lacrimal surgery

The advent of mini endoscopes will likely lead to improved transcanalicular DCR outcomes. A better understanding of the inferior meatus and nasolacrimal duct may lead to more innovative retrograde surgery.

Acknowledgment

The author would like to acknowledge the help of Gurjeet Jutley in the preparation of references for this chapter.