Cataract Surgery and Retinal Disease

In the past two decades, cataract surgery has become much safer and faster as more advanced technology has become available. There has been a dramatic linear increase in the rate of cataract surgery in the past 25 years due to better visual outcomes and shorter recovery times.

A patient complaining of visual distortion… is likely to be impacted more significantly by ERM, but may still benefit from cataract surgery

The decision to pursue cataract surgery and the appropriate timing of surgery can be quite challenging. The rationale for the surgery is the obvious fact that the presence of cataract hinders retinal examination for the surgeon and the patient’s vision. Optical coherence tomography (OCT) is an essential tool for retinal assessment of patients undergoing cataract surgery that allows the surgeon to ascertain visual potential post-surgery. However, the view could be hindered in advanced dense cataract including posterior subcapsular cataract. Cataract surgery can exacerbate retinal disease and ironically, some treatments such as intravitreal injections (anti-VEGF and/or steroids) and vitrectomy, can themselves cause faster progression of cataract. This article will review evidence on the association between various common co-existing retinal diseases and cataract surgery.

EPIRETINAL MEMBRANE

Both cataract and epiretinal membrane (ERM), also called membrane puckering, can cause blurring and distortion of vision. It can therefore be difficult to distinguish the cause of visual complaints and determine an appropriate timing/surgical approach i.e. cataract surgery, membrane peeling or both.

A large UK-based database study reported that eyes with primary ERM had an average visual improvement of 0.27 LogMAR following cataract surgery.¹ A separate study that compared outcomes of patients who underwent concurrent cataract surgery with a retinal peel alone, reported there were no significant differences in macular thickness or visual acuity between the groups following surgery.² This highlights the care that needs to be taken in determining the appropriate treatment for each patient, as cataract surgery will not always benefit the patient.

When determining the appropriate treatment, preoperative visual acuity and reported visual symptoms need to be considered. It has been reported that patients with co-existing cataract and ERM benefited most from cataract surgery when the preoperative visual acuity was worse than 20/40.¹ Each condition also produces different visual issues; visual distortion is more often associated with ERM while blurred vision is more often associated with cataract. A patient complaining of visual distortion, for example, is likely to be impacted more significantly by ERM, but may still benefit from cataract surgery alone if the visual acuity is 20/40 or worse. Once the cataract is removed, the patient may elect to proceed with an epi-retinal membrane peel or not, depending on their postoperative visual results and general health. Cataract surgery also helps to improve the retinal view necessary for a successful retinal peel.

Consideration should also be given to the possible impact of the surgery on the progression of underlying retinal pathology

In patients with pre-existing ERM who also require cataract surgery, combined phacoemulsification and vitrectomy has been reported to provide similar refractive outcomes as phacoemulsification alone.³ Both conditions can be safely treated simultaneously without exposing the patient to the extra risk of separate surgical procedures, as long as the patient consents to the combined procedure.

Although phacoemulsification does not appear to promote progression of preexisting idiopathic ERM,⁴ the rate of postoperative cystoid macular oedema (CMO) is higher in patients with ERM following cataract surgery than in those with no pre-existing ERM (i.e. 8.6% versus 1.38% respectively).¹ It should also be noted that there may be an increased risk of vitreo-macular traction and/or vitreous liquefaction following lens replacement due to forward movement of the vitreous fluid. It has been reported that 69% of patients develop posterior vitreous detachment (PVD) within a year following cataract surgery, with most developing PVD within one month.⁵ Additionally, there was no correlation found between incidence of PVD and any risk factors such as age or axial length, making the risk of PVD difficult to predict.⁵

POST-VITRECTOMY

Eyes that have undergone previous vitrectomy require extra caution and gentle intraoperative manoeuvring as the globe is softer and has unstable zonules with a deeper anterior chamber. These factors increase the risk of intraoperative miosis and fluctuations in anterior chamber depth. The posterior capsule can also be more prone to tearing. For these patients, extra caution is suggested to avoid touching the posterior capsule during cataract surgery. Implantation of a capsular tension ring and/or use of iris retractors is also recommended. Extra care should be taken if cataract develops rapidly after vitrectomy as there may have been injury to the posterior capsule and/or zonules during vitrectomy. These cases could be treated similarly to posterior polar cataracts, and hydro-dissection should be avoided.

RETINAL VEIN OCCLUSION

Retinal vascular occlusion (RVO) remains the second most common retinal vascular disease after diabetic retinopathy.

In Australia, RVO affects 0.96% of the population, with the prevalence increasing with age and diabetes status.⁶ Most of these patients are elderly and have coexisting cataracts. The common treatment of RVO – intravitreal injections of anti-VEGF and/or steroids – exacerbates the risk of development and progression of cataract.

Patients with RVO have a high risk of postoperative CMO – in fact, a history of previous RVO leads to a more than 30-fold increase in the incidence of postoperative CMO.⁷ In a large study that excluded patients with other retinal or eye pathology, 27.4% of patients with preoperative RVO developed macular oedema within three months of surgery.⁸ The patients who developed CMO following surgery had worse visual acuity at three months post surgery compared to those patients who did not develop CMO, however there was no significant difference by six months post surgery.⁸

In a small study that followed three patients who developed postoperative RVO following cataract surgery, two patients were reported to have a history of previous RVO and all three patients had vascular risk factors (i.e. hypertension). The authors postulated that fluctuations in intraocular pressure (IOP) during the surgery contributed to RVO development.⁹

Due to the high risk of patients with a history of RVO developing CMO following cataract surgery, it is recommended that patients follow a prophylactic regimen. The use of nonsteroidal anti-inflammatory drugs (NSAIDs) following surgery has been shown to reduce the risk of CMO in patients with a history of RVO down to rates similar to patients with no history of RVO.⁷ It may be beneficial to commence ocular NSAID drops one week prior to surgery and to continue them, in addition to steroid eye drops, following cataract surgery. It is also highly recommended to wait until any associated macular oedema is resolved and or stable for at least three months prior to organising surgery. Anti- VEGF treatments can be administered either before or during the surgery to further minimise the risk.

AGE-RELATED MACULAR DEGENERATION

Although multiple studies have been conducted to ascertain whether a relationship exists between cataract or cataract surgery and either dry or wet age-related macular degeneration (AMD), the results are conflicting. It is difficult to differentiate between association and causation of AMD and cataract, and there is also inconsistency in controlling the risk factors statistically.

The Australian based Blue Mountains Eye Study reported no correlation between the presence of cataract and either early or late AMD once age and other risk factors were excluded.¹⁰ However, the Beaver Dam Eye Study, which monitored AMD incidence over a 20 year period, reported that both the presence of cataracts and cataract surgery increased the incidence of late AMD, but not early AMD, with the risk increasing with time following surgery.¹¹ 

The literature is equally conflicting in regards to the relationship between AMD and previous cataract surgery. A large study published by Ho et al. in 2018 reported that patients with a previous history of cataract surgery have a 2.5-fold increased risk of developing neo-vascular AMD over a five year period.¹² The Blue Mountains Eye study reported that patients who had undergone previous cataract surgery were three times more likely to develop late AMD within a 10 year period compared to phakic eyes.¹³ 

However, Dong et al.¹⁴ reported similar rates of progression (3.3–4.3%) to neo-vascular AMD in phakic and pseudophakic eyes over a 12 month period, suggesting that previously reported apparent increases in progression may be due to neo-vascular changes that were undetected at the time of surgery due to lens opacity.¹⁴ The Australian Cataract Surgery and Age-related Macular Degeneration study, also reported no difference in incidence of early or late AMD in phakic and pseudophakic eyes at three and five years following surgery.¹⁵ A meta-study of two randomised clinical trials and two case controlled studies reported that cataract surgery can improve visual acuity in patients with AMD, with no increase in the incidence of late AMD in the five years following the surgery.¹⁶ 

It has previously been reported that incidence of late AMD is higher following cataract surgery performed five or more years prior compared to surgery performed less than five years prior.¹¹ Although this was only observed for patients who were already at increased risk of incident late AMD,¹¹ studies that use a follow up period of less than five years to assess long term incidence of AMD following cataract surgery should be approached with caution. The Rotterdam Study found an association between cataract surgery and early AMD but not wet AMD.

The Age-Related Eye Disease Study (AREDS) report No.27 reported that patients with AMD of varying severity had statistically significant improvement in visual acuity following cataract surgery, which was maintained for an average of 1.4 years.¹⁷ Nutritional supplements did not affect visual acuity outcomes,¹⁷ and it is recommended that those who fit the criteria as per AREDS study for risk of AMD progression should take these supplements and continue to do so after surgery.

Timing of cataract surgery in patients with wet AMD is crucial, and should only proceed once the retina is responding to treatment with anti-VEGF injections and is stable. It is imperative to discuss in detail with patients the guarded visual prognosis and the need for ongoing injections post surgery in order to manage patient expectations. These patients also require regular follow ups with proper retinal assessment, including OCT, to monitor macular degeneration and manage accordingly.

MACULAR HOLE

Macular holes are often caused by vitreomacular traction and require treatment with vitrectomy and membrane peel prior to cataract surgery. Cataract often develops and/ or progresses following these treatments. One study reported progression rates for nuclear sclerosis in post-vitrectomy eyes of 81% over six months, 98% over one year and 100% over two years (i.e. 4.5-fold, 4.9-fold and 12.5-fold rates over the same periods in untreated eyes respectively).¹⁸ Although it is recommended that macular hole is repaired prior to cataract surgery, post-vitrectomy eyes are more susceptible to complications, the most common of which is a capsular tear. This can be avoided by taking care to not touch the capsule during phacoemulsification, and by inserting a capsular tension ring.

Combined cataract surgery and macular hole repair has been successfully perfomed.^19-22 In a study that compared the outcomes of combined cataract surgery and macular hole repair (pars plana vitrectomy with internal limiting membrane peeling and gas tamponade) with that of consecutive treatments, similar rates of hole closure (100% and 96% respectively) and good visual outcomes for both groups were reported.²⁰ However, the visual recovery following the separate procedures was shown to be longer than for the combined treatments.²⁰

Macular holes can occur rarely as a complication of cataract surgery, resulting from tractional force across the fovea.

DIABETIC MACULAR OEDEMA

Although most patients with mild to moderate non-proliferative diabetic retinopathy (NPDR) experience an improvement in visual acuity following cataract surgery, the visual outcomes are generally not as good as patients with no diabetes.²³ The risk is greater for patients with severe NPDR and proliferative diabetic retinopathy (PDR) who are at risk of progression. In a large Australian study, 28.2% of eyes developed diabetic retinopathy (DR) within the 12 months following phacoemulsification, while only 13.8% of phakic eyes developed DR in the same period.²⁴ Within the same study, a pair-wise comparison showed progression of DR in 35.6% of pseudophakic eyes following surgery while only 20% of the phakic eyes showed DR progression.²⁴

To avoid DR progression, it has been recommended to perform cataract surgery when DR is either in its early stages or to achieve stability for three months prior to surgery if possible.²⁵ If the cataract is severe enough to preclude treatment, pan retinal photocoagulation (PRP) or anti-VEGF treatment can be performed either intraoperatively or in the early postoperative period within one to two weeks. It is also important to use NSAIDS along with steroid eye drops postoperatively in diabetic patients to prevent inflammation and CMO.²⁵

RETINAL DYSTROPHY

It has been reported that 23.4% of patients with retinitis pigmentosa have coexisting cataract.²⁶ One study, which evaluated outcomes of cataract surgery in eyes with a range of macular dystrophies, reported an improvement in average visual acuity from 1.09 LogMAR preoperatively to 0.61 LogMAR postoperatively, with a satisfaction rate of 93.3%. However, a higher incidence of zonular weakness, posterior capsular opacification (66.7%) and CMO (13.3%), compared to the general population, was also reported.²⁷

PATIENT MANAGEMENT

Retinal Surgery 

Determining whether retinal surgery is required before cataract surgery is vital. For patients with ERM, the impact due to cataract and ERM on vision can be differentiated by asking what visual symptoms are present. If the patient complains of blurred vision and if no foveal distortion can be observed using optical coherence tomography (OCT), this suggests that the cataract is responsible for the bulk of deterioration in vision.

If the patient complains of distorted vision with or without loss of foveal architecture, this suggests that the ERM is causing more problems and needs to be addressed before proceeding to cataract surgery. However, if the patient is elderly with poor general health and has significant cataract, visual improvement will be noticed with cataract surgery alone. It is important to discuss the visual prognosis as part of informed consent. For patients with significant ERM, a retinal peel or vitrectomy can be performed concurrently with the cataract surgery, and the use of NSAIDS with corticosteroids is highly recommended to prevent CMO.

The aim is to control retinal disease, medically and/or surgically, as much as possible prior to proceeding with cataract surgery in order to optimise visual results.

Cataract Surgery 

Informed consent explaining the risks and reasonable visual outcomes is essential; patients need to understand that retinal disease means that postoperative visual outcomes may not be ideal. A detailed discussion needs to take place to emphasise this and the discussion must be documented.

In a study that monitored AMD progression following surgery, 8.4% of patients who were diagnosed with nonneovascular AMD prior to surgery were found to have neovascular changes that went undiagnosed until after cataract surgery due to lens opacity.¹⁴ This emphasises how the presence of cataract can hinder proper retinal examination, which can lead to the misdiagnosis of retinal disease that may be contributing to vision loss. In these cases, vision loss may be wrongfully attributed to cataract, and the patient referred prematurely for cataract surgery. OCT angiography (OCT-A) plays a role in pre-operative retinal assessment and post-operative management. In cases of AMD and other neo-vascular diseases, OCT-A provides detailed images regarding the retinal vasculature, including network structure and reduced choroidal flow.²⁸ Vasculature changes can also be monitored following surgery in order to detect the early stages of inflammation.²⁹

For patients with AMD, it is recommended that an anti-VEGF injection is administered approximately one to two weeks prior to surgery to allow the medication to reach its full potency, and readministered two to four weeks following surgery to allow for wound stabilisation. Following this, injections can be administered as per the preoperative schedule.

Risk Factors 

Ultraviolet light (UVL) and exposure to sun are usually risk factors for AMD. UVL blocking lenses could be considered in patients with AMD having cataract surgery. It is also important to advise these patients to wear UV protective sunglasses and or hats when outdoors.

It is imperative to control other risk factors including blood sugar levels prior to organising cataract surgery to avoid rapid progression of diabetic retinopathy and the risk of infections. Undertreated diabetic retinopathy and/or inadequate response to treatment has a negative effect on postoperative macular edema, progression of diabetic retinopathy and visual outcomes.

Postoperative Risk Factors 

Following cataract surgery, there is anterior movement of vitreous. This is due to postoperative loss of lens volume, possibly along with changes in the levels of the intraocular angiogenic factors and other cytokines which may play a role in causing progression of pre-existing retinal disease. One study in diabetics from Japan showed that increased levels of VEGF in aqueous humour at the time of cataract surgery correlated with an increased risk of postoperative macular edema.³⁰

Intraocular Lenses and Advanced Macular Disease 

Advanced macular disease due to macular degeneration and/or dystrophy can cause visually debilitating central scotomas, which usually do not affect the peripheral vision. These patients do not notice improvement in their reading vision despite flawless cataract surgery. Visual rehabilitation in these patients is usually done by cumbersome hand-held/stand magnifiers, hand held telescopes, closed circuit televisions, and high-plus glasses in conjunction with high minus contact lenses to create a telescopic effect. Over the years many implantable devices have been used in advanced stages of AMD with variable success.

The Scharioth macula lens (Medicontur), a one-piece foldable intraocular hydrophilic acrylic lens with a central magnifying portion, is now being successfully implanted in the ciliary sulcus of pseudophakic eyes. It improves near vision in patients with AMD. The overall diameter of the lens is 13.0mm with four symmetric haptics and it has a central portion of 1.5mm diameter with an add of +10.0 Dsph and a neutral remaining optical zone. The Scharioth macula lens does not affect peripheral vision and does not reduce binocularity at a normal reading distance.

The Eyemax (London Eye Hospital Pharma) enhanced breadth of focus IOL uses the interesting concept of enhancing both longitudinal and transverse asphericity to increase breadth of focus. This was recently discussed at the European Society of Cataract and Refractive Surgeons (ESCRS) meeting in Vienna, Austria. It is also an injectable, hydrophobic acrylic lens designed to improve the quality of the retinal image in all areas of the macula up to 10 degrees from fixation. This lens can be inserted into the capsular bag and has an overall diameter of 11 mm, and optic diameter of 6.0mm with modified C-loop haptics. Bilateral implantation achieves better visual outcome due to summation of images and neuroadaptation.

CONCLUSION

Cataract surgery can improve vision in patients with retinal diseases, but it is essential to monitor and treat the underlying retinal pathology. Consideration should also be given to the possible impact of the surgery on the progression of underlying retinal pathology. Detailed informed consent and additional planning in the peri-operative period may optimise surgical outcomes and help manage patient expectations.

Clinical Associate Professor Smita Agarwal is a comprehensive ophthalmologist and the Head of the Ophthalmology Department at Shellharbour and Wollongong Public Hospitals in NSW. She is a Clinical Associate Professor at the Graduate School of Medicine, and University of Wollongong, and she lectures at the University of Sydney. She teaches registrars and interns at public hospitals, regularly conducts seminars for doctors and optometrists, and presents lectures at Australian and International ophthalmology meetings and conferences. 

A/Prof Agarwal has clinics in Wollongong, Nowra and Warilla and recently set up a world class facility for refractive corneal surgery (SMILE, LASIK, PRK), a first for the NSW south coast. She also works at several public and private hospitals at Shellharbour, Wollongong and Nowra. She specialises in cataract and refractive surgery and has interest in corneal cross-linking for progressive keratoconus, glaucoma, and age related macular degeneration. 

Originally from India, A/Prof Agarwal completed her ophthalmology training at Monash Medical Centre gaining FRANZCO fellowship in 2006. She completed an advanced fellowship in Melbourne and a Graduate Diploma in Refractive Surgery from the University of Sydney. A/Prof Agarwal has published several research papers in peer-reviewed journals. To earn your CPD points from this article answer the assessment available at mivision.com.au/ cataract-surgery-&-retinal-disease 

References 

1. Hardin JS, Gauldin DW, Soliman MK, Chu C, Yang YC, Sallam AB. Cataract surgery outcomes in eyes with primary epiretinal membrane. JAMA Ophthalmol 2018; 136: 148-154 
2. Yiu G, Marra KV, Wagley S, Krishnan S, Sandhu H, Kovacs K, Kuperwaser M, Arroyo JG. Surgical oucomes after epiretinal membrane peeling combined with cataract surgery. J Ophthalmol 2013; 97: 1197-1201 
3. Shi L, Chang JS, Suh LH, Chang S. Differences in refractive outcomes between phacoemulsification for cataract alone and combined phacoemulsification and vitrectomy for epiretinal membrane. Retina 2018 
4. Hayashi K, Hayashi H. Influence of phacoemulsification surgery on progression of idiopathic epiretinal membrane. Eye 2009; 23: 774-779 
5. Mirshahi A, Hoehn F, Lorenz K, Hattenbach L. Incidence of posterior vitreous detachment after cataract surgery. J Cataract Refract Surg 2009; 35: 987-991 
6. Keel S, Xie J, Foreman J, van Wijngaarden P, Taylor HR, Dirani M. Prevalence of retinal vein occlusion in the Australian National Eye Health Survey. Clin Exp Ophthalmol 2018; 46: 260-265 
7. Henderson BA, Kim JY, Ament CS, Ferrufino-Ponce ZK, Grabowska A, Cremers SL. Clinical pseudophakic cystoid macular edema. Risk factors for development and duration after treatment. J Cataract Refract Surg 2007; 33: 1550-1558 
8. Cho HJ, Hwang HJ, Kim HS, Lee DW, Kim CG, Kim BY, Kim JW. Macular edema after cataract surgery in eyes with preoperative retinal vein occlusion. Retina 2018; 38: 1180-1186 
9. Ting DSJ, Hegde V, “Cataract surgery and retinal vein occlusion: is there an association?,” presented at the European Association for Vision and Eye Research Conference, Nice, France, August 2013 
10. Wang JJ, Mitchell P, Cumming RG, Lim R. Cataract and age-related maculopathy: the Blue Mountains Eye Study. Ophthalmic Epidemiol 1999; 6: 317-326 
11. Klein BEK, Howard KP, Lee KE, Iyengar SK, Sivakumaran TA, Klein R. The relationship of cataract and cataract extraction to age-related macular degeneration: the Beaver Dam Eye Study. Ophthalmology 2012; 119: 1628-1633 
12. Ho J, Xirasagar S, Kao L, Lin H. Neovascular age-related macular degeneration is associated with cataract surgery. Acta Ophthalmol 2018; 96: e213-e217 
13. Cugati S, Mitchell P, Tan AG, Smith W, Wang JJ. Cataract Surgery and the 10-year incidence of agerelated maculopathy: the Blue Mountains Eye study. Ophthalmology 2006; 113: 2020-2025 
14. Dong LM, Stark WJ, Jeffreys JL, Al-Hazzaa S, Bressler SB, Soloman SD, Bressler NM. Progression of age-related macular degeneration afetr catraact surgery. Arch Ophthalmol 2009; 127: 1412-1419 
15. Wang JJ, Fong CS, Burlutsky G, Cugati S, Tan AV, Rochtchina E, Arnold J, Smith W, Mitchell P. Risk of agerelated macular degeneration 4 to 5 years after cataract surgery. Ophthalmology 2016; 123 
16. Kessel L, Erngaard D, Flesner P, Andresen J, Tendal B, Hjortdal J. Cataract surgery and age-related macular degeneration. An evidence-based update. Acta Ophthalmol 2015; 93: 593-600 
17. Forooghian F, Agron E, Clemons TE, Ferris FL, Chew EY, Age-Related Eye Disease Study Research Group. Visual acuity outcomes after cataract surgery in patients with agerelated macular degeneration: age-related eye disease study report no. 27. Ophthalmology 2009; 116: 2093-2100 
18. Cheng L, Azen SP, El-Bradey MH, Scholz BM, Chaidhawangul S, Toyoguchi M, Freeman WR. Duration of vitrectomy and postoperative cataract in the vitrectomy for macular hole study. Am J Ophthalmol 2001; 132: 881-887 
19. Miller JH, Googe JM, Hoskins JC. Combined macular hole repair and cataract surgery. Am J Ophthalmol 1997; 123: 705-707 
20. Muselier A, Dugas B, Burelle X, Passemard M, Hubert I, Mathieu B, Berrod JP, Bron AM, Creuzot-Garcher C. Macular hole surgery and cataract extraction: combined vs consecutive surgery. Am J Ophthalmol 2010; 150: 387-391 
21. Lahey JM, Francis RR, Fong DS, Kearney JJ, Tanaka S. Combining phacoemulsification with vitrectomy for treatment of macular holes. Br J Ophthalmol 2002; 86: 876-878 
22. Yagi F, Takagi S, Tomita G. Combined idiopathic macular hole vitrectomy with phacoemulsification without face-down positioning. J Ophthalmol 2012; 2012 
23. Somaiya M, Burns JD, Mintz R, Warren RE, Uchida T, Godley BF. Factors affecting visual outcomes after smallincision phacoemulsification in diabetic patients. J Cataract Refract Surg 2002; 28: 1364-1371
24. Hong T, Mitchell P, de Loryn T, Rochtchina E, Cugati S, Wang JJ. Development and progression of diabetic retinopathy 12 months after phacoemulsification cataract surgery. Ophthalmology 2009; 118: 1510-1514 
25. Rice J. Cataract and diabetic retinopathy. Comm Eye Health 2011; 24: 9 
26. Liew G, Strong S, Bradley P, Severn P, Moore AT, Webster AR, Mitchell P, Kifley A, Michaelides M. Prevalence of cystoid macular oedema, epiretinal membrane and cataract in retinitis pigmentosa. Br J Ophthalmol 2018 
27. Davies EC, Pineda R. Cataract surgery outcomes and complications in retinal dystrophy patients. Can J Ophthalmol 2017; 52: 543-547 
28. Jia Y, Bailey ST, Wilson DJ, Tan Q, Klein ML, Flaxel CJ, Potsaid B, Liu JJ, Lu CD, Kraus MF, Fujimoto JG, Huang D. Quantitative optical coherence tomography angiography of choroidal neovascularization in age-related macular degeneration. Ophthalmology 2014; 121: 1435-1444 
29. Pilotto E, Leonardi F, Stefanon G, Longhin E, Torresin T, Deganello D, Cavarzeran F, Miglionico G, Parrozzani R, Midena E. Early retinal and choroidal OCT and OCT angiography signs of inflammation after uncomplicated cataract surgery. Br J Ophthalmol 2018 
30. Funatsu H, Yamashita H, Noma H, Shimizu E, Mimura T, Hori S. Prediction of macular edema exacerbation after phacoemulsification in patients with nonproliferative diabetic retinopathy. J Cataract Refrac Surg 2002; 28: 1355