Mechanical characterization of endoscopic surgical staples during an experimental Hernia repair

Clmicui Materiais 8

16 (1994) 81-89

1994 Elsevier Science Limited

Printed m Great Britain. ELSEVIER

All rights reserved 0267-6605:94/$7.00

nical Characterization of Endosco uring an Experimental Hernia Jamal Charara, Yves-Marie Dion & Robert Guidoin Quebec Biomaterials Canada (Received

GIL

Institute,

St-Franqois

3L5, and Department

4 October

d’Assise

of Surgery,

1993; sent for revision

Hospital,

10 1’Espinay

Lava1 University,

22 November

1993; accepted

Quebec

Street, City,

23 January

Quebec

City (Quebec),

Canada

1994)

Abstract: Recent developments in laparoscopic hernia repair techniques have led to the design of titanium staples. In a laparoscopic hernia repair, a polypropylene mesh is stapled over the direct and indirect hernia sites in the inguinal region. The effectiveness of these staples in holding the prosthetic mesh, and therefore providing adequate strength to the abdominal wall, has not been yet investigated. We have characterized the bursting strength (BS) of an experimental hernia mesh repair fixed with Prolene suture, which is used extensively for this procedure, and the BS of repairs fixed with two currently available staplers, the Endopath EMS endoscopic multifeed stapler and the Endo Hernia stapler. We first simulated abdominal wall hernias in 16 piglets by creating incisions on both sides of the abdomen of each animal. Each defect was then covered with a polypropylene mesh, which was fixed on one side with Prolene sutures and on the other side using either the Endopath EMS (Group 1) or the Endo Hernia stapler (Group 2). The abdominal tissue with the mesh covering the defect was then excised and the BS evaluated using an Instron machine. Since many mechanical characteristics contribute to the BS of a repair, we investigated these characteristics in vitro, including tensile properties of the staples and the prosthetic mesh as well as the suture-tearing resistance of the mesh. Polypropylene mesh exhibits the same elongation in the three directions, i.e. O”, 45” and 90”. This elongation was estimated at 136% (SD = 13 -0). Suture-tearing tests showed that polypropylene mesh was stronger than EMS and Endo Hernia staples, but was weaker than both Prolene and 3-O stainless steel sutures. During longitudinal tensile tests the EMS staple opened with a force of 45.35 N (SD = 3 -4) while Endo Hernia was broken with a force of 24 N (SD = 3 - 1). The mean BS of the meshes attached with Prolene was 1948.74 x lo2 Pa. Meshes fixed with the EMS staples had a mean BS of 1180.55 x lo2 Pa, and those fixed with Endo Hernia staples 886.84 x lo2 Pa. Our results suggest that a repair performed with Prolene sutures is stronger than one performed with staples (EMS or Endo Hernia) (P < 0,001). Furthermore, repair with EMS staples has a significantly higher BS than one performed using Endo Hernia staples (P = 0.006).

I INTRODUCTION

sutures or a relatively small mesh fixed in place with sutures.4-s As a result of recent developments in minimally invasive surgery, titanium staples have been designed for use in laparoscopic hernia repair. Among the various properti.es necessary for adequate abdominal wall reconstruction, the bursting strength of the repair is extremely important. The effectiveness of titanium staples in holding

Inguinal hernia repair is one of the most frequently performed surgical procedures. Repair of groin hernias is classically divided into two broad categories: the hernioplastic type, in which a large mesh is kept immobile in the preperitoneal space by the intraabdominal pressure,iP3 and the herniorrhaphic repair, probably the most frequent, which uses 81

82

J. Chavava, Y.-M. Dion, R. Guidoin

prosthetic meshes in place, and therefore providing adequate BS, has never been investigated. The purpose of this study was to characterize the BS resistance of an experimental hernia repair model using polypropylene mesh attached to tissue either with Prolene sutures, which are extensively used for hernia repair, or with titanium staples. We also investigated the mechanical characteristics that contribute to the strength of a repair, including the tensile properties of the staples and the prosthetic mesh as well as the suture-tearing resistance of the mesh.

2 MATERIALS

PERPENDICULAR

t 45O

I/’

PARALLEL

AND METHODS Fig. 1. Schematic

2.1 Selection of materials

For this study we chose two commercially available endoscopic staplers, the Endopath EMS endoscopic multifeed stapler developed by Ethicon Endo-Surgery, and Auto Suture Company’s Endo Hernia stapler. Staple properties were compared with those of Prolene sutures. Polypropylene surgical mesh was used for the abdominal wall reconstruction. 2.2 Morphology of staples The morphology of fired staples was evaluated by scanning electron microscopy. Staples were sputter-coated with gold-palladium to improve their conductivity and to facilitate observation. A Jeol JSM 35CF scanning electron microscope operating at 15-25 kV accelerating voltage was employed. 2.3 Mechanical characterization

of the

polypropylene mesh and staples 2.3.1 Tensile strength tests

Longitudinal tensile stress/strain measurements of the polypropylene mesh were performed using an Instron Model 1130 tensile tester (500 N load cell, crosshead speed 200mm/min). The test specimen was 70mm x 20mm. These properties were evaluated at 45”, parallel and perpendicular to the longitudinal axis of the sample as illustrated in Fig. 1. The forces were measured in newtons. Longitudinal tensile strain was calculated as:

illustration

of the directions the mesh.

of pull for testing

knotted Prolene sutures was also tested, by passing two stainless steel wires through the suture (Fig. 2), on the same Instron tensile tester with a crosshead speed of lOOmm/min. The force required to tear the suture was measured in newtons. The tensile stress was then calculated in Pa by dividing the force by the area of the specimen upon which the load is applied. The thickness of the polypropylene mesh as well as that of sutures and staples was measured with a low-pressure thickness gauge, CSI Model CS-49, equipped with a micrometer and an electrical contact detector. 2.3.2 Suture/staple tearing strength

Measurement of the suture/staple tearing strength when attached to the polypropylene mesh was performed using an Instron Model 1130 tensile tester (500 N load cell, crosshead speed lOOmm/ min). Test specimens were cut in 15 mm x 15 mm squares with one corner bevelled at 45”. Tests += STAINLESS-STEEL SUTURE

PROLENE

Strain(%) = 100 x (I - &)/lo where I is the length at rupture and lo is the initial length. Longitudinal tensile strength of staples and of

Fig. 2. Schematic illustration of Prolene and staple tensile strength test. 3-O stainless steel wires were passed through the suture or staple and tested together on an Instron machine.

Mechanical

characterization

+’

of endoscopic

83

surgical staples

PROLENE MESH

HERNIA DEFEC STAINLESS

PROLENE

STEEL

_PROLENE OR STAINLESS-STEEL

MESH,

Fig. 4. Schematic illustration Fig. 3. Schematic illustration of suture tearing-strength test using either sutures or staples. The suture or staple was passed through the specimen, 5 mm from the edge. In the case of staples, a 3-O stainless steel wire was passed through the suture.

were conducted at 45”, parallel and perpendicular to the longitudinal axis of the sample. Specially designed clamps were used to hold the test specimens and to provide a groove to allow the suture or the staple to pass through the specimen wall, 5mm from the cut edge (Fig. 3). The test was conducted using 3-O stainless suture, Prolene suture, EMS and Endo Hernia staples. In the case of staples, a 3-O stainless steel wire was passed through the staple. We selected 3-O stainless steel sutures to assess the tearing strength of the mesh in case the Prolene sutures or staples broke. The force required to tear the suture or staple from the sample was measured in new-tons. 2.3.3 Bursting strength S was evaluated with an Instron tester, Model 1122, fitted with a compression cell (5000 N load cell, crosshead speed 200mm/min), a specially designed cylindrical indenter with spherical tip (7.62cm diameter) and a circular specimen holder (10cm diameter). The force was recorded in newtons and converted to Pa by dividing by the area of the specimen supporting the applied load. Five measurement were performed for each test type, and results were statistically analyzed using the independent group t-test. 2.4 IIEvivo experimental hernia model in pigs We performed a xyphopubic incision on 16 piglets, weighing 25-30 kg each, under general anesthesia. The peritoneum, skin and subcutaneous tissue were removed so that only the musculo-aponeurotic layers of the abdominal wall remained. Two 3.5 by 6cm elliptical incisions

of the hernia repair model.

were then created on each side of the abdominai wall in order to remove a surface comparable to a large inguinal hernia. A polypropylene mesh measuring 5 cm by 8 cm was placed on the inner surface of each defect and was held in place on one side with interrupted Prolene sutures and on the other side with either Endopath EMS staples (Group 1) or Endo Hernia staples (Group 2) (see Fig. 4). Sutures and staples were evenly distributed around the hernia site. An average of 17 Prolene sutures was sufficient to achieve an adequate repair. For this study, 20 staples were applied through the mesh into healthy muscle parallel to the hernia defect. The stapling technique was similar to that used clinically. A large portion of the abdominal wall was then removed from each side and placed in a normal saline solution at 4°C for the BS tests, which were performed within 4 h. Photographs were taken during expansion and after bursting. The samples were also examined with a Tessovar macrophotography optical system (Zeiss, berkochen, Germany).

3 RESULTS 3.1 Morphology

of staples

Figure 5 shows photomicrographs of a fired Endopath EMS staple and a fired Endo Hernia staple. It is evident that the cross-section of the EMS staple is circular, while that of the Endo Hernia is almost square. The legs of the EMS staple are straight, those of the Endo Hernia have a corrugated shape. Measuring the heights of both legs on each staple, we found these to be 1.5 mm and 1.7mm on the EMS staple (see Fig. 5) and 0.95 mm and 1.2 mm on the Endo Hernia staple. The distance between legs, at mid-height, was 4.8 mm for the EMS staple and 47mm for the Endo Hernia.

84

J.

Charara, Y.-M. Dion. R. Cuidoin Table 1. Thickness materials used

and diameter

values

Parameter

Thickness Diameter Diameter Diameter Thickness

Fig. 5. Photomicrographs

showing the EMS staple (A) and the Endo Hernia staple (B); h is the height of the leg, d is the distance between legs at mid-height.

3.2 Mechanical characterization polypropylene mesh and staples

of the

3.2.1 Tensile properties

Thicknesses of the polypropylene mesh, sutures and staples are presented in Table 1. Figure 6 summarizes the strain results of the polypropylene mesh. The difference between these results was not significant, and the mesh presents comparable elongations in the three directions (mean = 136.6%, standard deviation (SD) = 13.0). Nevertheless, significant differences (P < 0.001) between tensile properties at the 0” and 45” directions were observed (Fig. 7). The average tensile strength was 180.2Pa (SD = 36.6). Figure 8 illustrates the longitudinal tensile strength test results for the different suture types.

of the different

Value (mm)

of the polypropylene of the 3-O stainless steel suture of the Prolene suture of the Endopath EMS staple of the Endo Hernia staple

0.59 0.22 0.36 0.52 0.40

The mean rupture force of Prolene (45.45N, SD = 4.4) was comparable to that of the Endopath EMS staple (45.35 N, SD = 3.4). These forces were significantly higher (P < 0.001) than those needed for the Endo Hernia staple (24.0 N, SD = 3.1). During this test the EMS staples opened, while the Endo Hernia staples broke. Since the cross-section of Endo Hernia staple is square, while that of EMS is circular, the contact area between the 3-O stainless steel wire was approximated as half the perimeter of the 3-O stainless steel suture x half the perimeter (Endopath EMS) or thickness (Endo Hernia) of the staple. Although the force needed to break the Endo Hernia staple was significantly lower (P < 0.001) than that needed to open the EMS staple, the difference between the stresses was not significant (see Fig. 9), i.e. 173.6 x 106Pa (SD = 22.1 x 106) for Endo Hernia and 160.6 x 106Pa (SD = 12.0 x 106) for EMS. However, the failure stress of Prolene (238.6 x lO”Pa, SD = 22.4 x 106) was significantly higher than that on the EMS (P = 0.001) and the Endo Hernia (P = 0.003) staples. Typical curves obtained during the tensile test of the three suture types are presented in Fig. 10. The 200 1

150

0 0

45 ANGLE

90

(degrees)

Fig. 6. Strain test results of the polypropylene

and 90”.

mesh at O”, 45”

85

Mechanical characterization of endoscopic surgical staples

x m

0 c

200

iii E bi 100 !I F6 f n

”

EMS

PROLENE 0

Fig. 7. Tensile

Fig. 9. Forces

90

45 ANGLE

(degrees)

stress test results of the polypropylene O”, 45” and 90”.

mesh at

EMS staple test curve shows a slight dip prior to the force at rupture, while that of Endo Hernia shows two dips. The EMS staple test presents a slight slipping of the stainless steel wire during flattening under tensile force. Also; the corrugated shape of the Endo Hernia staple (see Fig. 5B) causes stress concentrations during extension. The two decreases in this case are due to slipping of the stainless steel wire at both ends of the clip, which did not necessarily occur at the same time. 3.2.2 Suture/staple tearing strength Figure 11 shows the results of the suture tearingstrength test using different sutures and staple types. The polypropylene mesh was always disrupted when tested with both 3-O stainless steel or Prolene sutures, and the results were similar in the three directions for each type. The mean suturetearing resistance of the polypropylene mesh and 3-O stainless steel was 42.85 N (SD = 7.1) and 55~73 N (SD = 7.5) respectively. The difference was significant (P < 0.001).

engaged

ENDO HERNIA

during the tensile strength

test.

Staples failed during testing on all but three occasions, when EMS staples were used at 90”. EMS staples opened at the mesh side, while Endo Hernia staples all broke. The E support a significantly higher ( P < 0.001) tearing force (33.13 N, the Endo Hernia staple (19. I 1 Nj; S Comparison between these results and those of the longitudinal tensile test shows that Prolene broke at 45.45 N, while it can d upt the polypropylene mesh at 55.73 N. The ff’erence between these values is significant (P = 0X)01).Although staples failed in both tests, differences between their forces at failure were also significant (P = 0.01 for both EMS and Endo Hernia). These differences may be due to high stress concentration factors. Figure 12 shows typical tearing curves, which demonstrate main and secondary peaks. The main peak corresponds to the breaking of the deeper knot, which resists tearing better because of its 50

PROLENE ---- ENDOPATH EMS --- END0 HERNIA

40

s

30

s ;

20

10

0 PRQLENE

Fig. 8. Tensile

strength

EMS

END0 HERNIA

results of the different tested.

suture

types Fig. 10. Typical

tensile strength

curves of the suture types.

J. Charara, Y.-M. Dion, R. Guidoin

86

60

ST.

EMS

PROLENE

STEEL

END0

HERNIA

Fig. 11. Suture tearing-strength results, in the three directions, of the polypropylene mesh using 3-O stainless steel sutures, Prolene sutures, EMS staples and Endo Hernia staples. Mesh was always disrupted during tests with Prolene and 3-O stainless steel sutures. When tested with staples, staple flattening or breaking was observed.

strong intermeshing with the other knots. The secondary peaks correspond to the breaking of the edge knots and the side yarns to which they are attached. 3.2.3 Bursting strength The average force needed to burst the entire polypropylene mesh was 2657.5 N (SD = 190.3). Bursting strength was evaluated at 5825.55 x lo2 Pa (SD = 417.16 x 102). 3.3 Characterization model

of the experimental hernia

Figure 13 shows a specimen during bursting, which

A

. . . . . STAINLESS-STEEL PROLENE ---- ENDOPATH EMS ----ENDOHERNIA

TIME (S) Fig. 12. Typical suture tearing-strength curves of the polypropylene mesh using 3-O stainless steel sutures, Prolene sutures, EMS staples and Endo Hernia staples.

was mostly due to tissue failure, while Fig. 14 shows a specimen after bursting. Figure 15 summarizes and compares the bursting results. The mean BS of the mesh attached with Prolene sutures was 1948.62 x lO”Pa (SD = 169.85 x 102) for Group 1 and 1949.41 x lO”Pa (SD = 289.97 x 102) for Group 2 (P = 0.988) with a total mean of 1948.74 x lo2 Pa (SD = 233.31 x 102). The mean BS of the mesh fixed with Endopath EMS staples was 1180.55 x 1O’Pa (SD = 161.85 x 102), that of the mesh fixed with Endo Hernia staples was 886.84 x lO”Pa (SD = 163.85 x 102). The difference between the BS of the mesh fixed with Prolene sutures and that of the mesh fixed with staples was statistically significant (P < 0.001). Differences in bursting strength of a mesh fixed with the Endopath EMS staples and a mesh fixed with Endo Hernia staples were significant (P = 0.006). If we consider that polypropylene mesh was fixed with 17 Prolene knots or with 20 staples, the contribution of each Prolene knot to BS was 114.63 x 102Pa, while those of EMS and Endo Hernia staples were 59.02 x lo2 Pa and 44.34 x lo2 Pa respectively.

4 DISCUSSION Inguinal hernia repair using a prosthetic patch is becoming a popular treatment modality.’ The ideal prosthesis should be incorporated with the tissues, does not stimulate visceral adhesion formation, and maintains adequate strength.“>” Although many prosthetic materials have been used in hernia repair, polypropylene is among the most widely used material for abdominal wall reinforcement. 12-14 In the present study, the intra-abdominal mechanical bursting forces were simulated in vitro. Although these tests do not reflect actual conditions, the goal of this study was to compare performances of suture and staples during bursting. However, these bursting forces comprise the tensile force on the mesh, the tensile force on the suture or the staple, and the suture-tearing force on the mesh. Evaluation of the physical properties of mesh was not the purpose of this study, but having these data is valuable for an adequate evaluation of the strength of the total repair. Polypropylene mesh presents the same elongation in the three directions, i.e. O”, 45” and 90”. Differences observed in the tensile stress test at 0”

87

Mechanical characterization of endoscopic surgical staples

Fig. 13. Photomacrograph

showing

an experimental

hernia repair during bursting

and 45” can be explained by the tensile characteristics of the mesh, which are not the same in the three directions. However, these kinds of forces could be applied on the mesh during the period following implantation when fibrous tissue has not infiltrated the mesh. This infiltration will change the properties and the behavior of the mesh. The tensile test performed on different types of fixation showed that the EMS staple is as strong as rolene. While the force needed to open the EMS staple is twice that needed to break the Endo Hernia staple, the stresses generated by these forces are equivalent. This is due to the staple shape and size. Since the cross-section of the Endo Hernia staple is square, the force generated by the stainless steel wire during loading is applied to the front area of the clip, which is 0.40mm thick. In the case of the EMS staple, which has a circular cross-section, the load is applied to the front half perimeter area, which is 0.82mm thick

Fig. 14. Photomacrograph

showing

test using an Sntron machine,

Model

i 122.

(7r x diameter/2). Therefore, theoretically the shape of the Ethicon clip has the advantage of reducing by half the longitudinal stress applied by the stainless steel wire. The suture tearing-strength test showed that polypropylene mesh was stronger than EMS and Endo Hernia staples but weaker than both Prolene and 3-O stainless steel. Because of the elasticity of the materials, diameters of the Prolene and mesh fibers decrease during extension” Consequently the contact areas between them change with time, leading to higher stress concentration. It is therefore difficult to estimate the real stress applied during tearing tests. As discussed earlier, however, stress concentrations due to the staple shape explain the lower resistance of the Endo ernia staple compared to the Endopath EMS. Comparing force values in longitudinal tensile tests and in tearing-strength tests in the sutures showed significant differences because of stress

a hernia model repaired with polypropylene mesh and Proiene was mostly due to tissue disruption.

sutures after bursting.

Bursting

J. Charava, Y.-M

END0

HERNIA

Fig. 15. Bursting strength test results. Significant differences are noted between each type of suture material, the strongest being the Prolene suture and the weakest the Endo Hernia staple.

concentration. Although, as discussed above, these stresses are difficult to estimate, they should nevertheless be the basis for comparison. Intra-abdominal pressure has been identified as one of the major factors of recurrence of the defect.5;‘2 A variety of studies have evaluated the acute rise in the intra-abdominal pressure following daily activities such as straining, coughing or walking. 15-” In humans, coughing or straining generates a greater rise in intra-abdominal pressure than lifting a 25 kg weight or jogging.]’ The highest intra-abdominal pressure recorded in clinical studies, 293.30 x 102Pa,15 occurred during coughing. Nordin et al.” reported intra-abdominal pressure of 269.31 x lo2 Pa during straining. Postoperatively, coughing can occur as soon as the patient awakes in the operating room, so the repair can be stressed well before fibrocytes infiltrate the mesh. Therefore it appears important that in herniorrhaphy-like procedure, the mesh should be adequately fixed to adjacent tissues. However, the slow speed of the indenter used during the BS tests of the repair (200 mm/min) simulates straining rather than coughing. Although different animal models have been proposed to test the behavior of hernia patches, studies addressing the behavior of sutures are rare. Jenkins et aZ.,12 using rats, replaced 4 x 4 cm defects with a variety of materials and abdominal wall BS was tested at 3 and 4 weeks. The BS characteristics were tested by infusing normal saline into the abdominal cavity. Flow was regulated at a constant rate (45 ml/min) by a blood pump. Intraabdominal pressure was monitored by placing an angiocatheter opposite the infusion needle and recorded by a pressure transducer. Bursting was due to the disruption of the prosthesis-tissue inter-

Dion, R. Guidoin

face or to intra-abdominal saline extension through the inguinal canal. In our study, bursting was due to disruption at the mesh-tissue interface, which is related to tissue weakness. Differences in bursting values are attributed to the contribution of each staple to total strength. A hernia repaired with Prolene sutures is still stronger than one repaired with staples. Indeed, staples penetrate less tissue (0.95 mm in the case of the Endo Hernia staple and 1.5 mm in the case of the EMS staple) than sutures (7-8mm). The staple can therefore be dislodged with less force than the latter. This is a function of the contact area between the tissue and the staple in the tearing direction. In the case of the EMS staple this area is about 1.23 mm2 in any tearing direction (it has a circular cross-section); the contact area of the Endo Hernia staple depends on the angle between the applied force and the staple direction. The latter is about 0.38 mm* at a 0” or 90” angle and higher at different angles. This geometric difference may explain the higher values of BS of the EMS staple compared with that of the Endo Hernia repair. We must stress that the conditions in vivo are at 37 “C in the presence of biological fluids. Although the hernia repairs were stored and impregnated in saline solution prior to testing, bursting was Therefore, conducted at room temperature. changes in the repair behavior may occur as a result of temperature difference. CONCLUSIONS

An in vitro testing protocol was developed to quantify the strength of a hernia repair using prosthetic mesh. Hernia repaired using a mesh fixed with Prolene is more resistant than one fixed with staples. Endopath EMS staples produce a significantly higher bursting strength than Endo Hernia staples. Since failure of a repair is frequently due to tissue weakness, staples should be used with care. REFERENCES Stoppa, R., Petit, J., Abourachid, H., Henry, X., Duclaye, C., Monchaux, G. & Hillebrant, J.P., Procede original de plastie des hernies de l‘aine: l’interposition sans fixation d‘une prothese en tulle de Dacron par voie mediane sous piritoneale. Chirurgie, 99 (1973) 119923. Stoppa, R., Petit, J. & Henry, X., Unsutured Dacron prosthesis in groin hernias. Int. Surg., 60 (1975) 411-2.

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characterization

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13. Elliott, M.P. & Juler, G.L., Comparison of Marlex mesh and microporous Teflon sheets when used for hernia repair in the experimental animal. Am. J. Surg., 137 (1979) 342-4. 14. Brown, G.L., Richardson, J.D.; Malangoni, M.A., Tobin, G.R., Ackerman, D. & Polk, H.C., Comparison of prosthetic materials for abdominal wall reconstruction in the presence of contamination and infection. Ann. Surg., 201 (1985) 705-11. 15. Hemborg, B., Moritz, U., Hamberg, J., Holmstrom, E., Lowing H. & Akesson, I., Intra-abdominal pressure and trunk muscle activity during lifting. Band. 9. Rehab. Med., 17 (1985) 15-24. 16. Nachemson, A.L., Andersson, G.B.J. & Schultz, A.B., Valsalva manoever biomechanics: effects on lumbar trunk loads of elevated intra-abdominal pressures. Spine, 11 (1986) 476-9. pressure response to loads 17. Grew, N.D., Intra-abdominal applied to the torso in normal subjects. Spine, 9 (1980) 149-54. 18. Nordin, M., Elfstrom, G. & Dahlquist, P., Intra-abdominal pressure measurements using a wireless radio, pressure pill and two wire corrected pressure transducers: a comparison. &and. J. Rehab. Med., 16 (1984) 134-46. 19. Twardowski, Z.J., Khanna, R., Nolph, K.D., Scalamogna, A., Metzler, M.H., Schneider, T.W., Prowant, B.F. & Ryan, L.P., Intra-abdominal pressures during natural activities in patients treated with continuous ambulatory peritoneal dialysis. Nephron, 44 (1986) 129-35.