Eur J VascSurg6, 21-25 (1992)
Reinforced Vascular Grafts: a Comparative Study Timothy R. Magee, Philip G. Niblett and W. Bruce Campbell Departments of Surgery and Clinical Measurements, Royal Devon and Exeter Hospital (Wonford), Exeter EX2 5DW, U.K. Reinforced vasculargrafts are designed to resist compression and flexion forces,for example in axillo-femoral or below-kneefemoropopliteal reconstruction. This study compared eight types of 6-mm reinforced grafts (five polytetrafluoroethylene (PTFE) and 3 Dacron). On a specially designed rig, graft diameter and flow were measured during compression by a standard series of weights up to 3.6 kg, and on another rig grafts wereflexed through 135°. Meadox Mierovel graft resisted compression best (p ( O.05) but there were no important differences between the other grafts (Vascutek; Bard; Goretex Standard, Thin Walled and Removable Ring; Impra Standard and Thin Wall). Flexion produced no significant changes in flow through the grafts. In addition, five general surgeons were asked to score the easeof removal of reinforcement, passage of sutures, and the tendency of sutures to cut out of each graft. Removal of reinforcement and passage of sutures were easierwith the PTFE grafts, but the Dacron was more resistant to sutures cutting out. This study shows that these grafts resist flexion and compression well. Their different handling characteristics and data on patency at different sites should remain the main determinants in choiceof graft. Key Words: Arterial occlusive disease;Arteries; Vascular grafts.
Introduction
Methods
Several vascular grafts are available in the U.K. with spiral or ring reinforcement to their walls. These grafts are used where compression or flexion might diminish flow and pose a risk of occlusion. For example, in axillo-femoral bypass the graft might be compressed when the patient lies on his/her side; while in below-knee femoro-popliteal bypass the knee must be allowed to flex fully without the graft kinking. Kinking has been cited as the cause of failure in 13% of unsupported below-knee femoro-popliteal polytetrafluoroethylene (PTFE) grafts. 1 The use of reinforced PTFE grafts has improved patency rates over unsupported grafts of the same material, and this has been attributed to their ability to withstand flexion without kinking. 2' 3 There have been no comparative studies on the physical properties of these grafts. This study aimed to provide objective comparison of reinforced grafts during compression and flexion, and also subjective data on their handling characteristics.
Eight 6-mm reinforced vascular grafts were examined; five PTFE and three Dacron. The Goretex grafts were reinforced by rings, the other grafts by spirals. The following grafts were examined. PTFE: Goretex Standard, Goretex Thin Walled, Goretex Thin Walled "Removable Ring", Impra Flex Standard and Impra Flex Thin Wall. Dacron: Meadox Microvel, Vascutek, Bard EXS.
Please address all correspondence to: W. B. Campbell, Departments of Surgery and Clinical Measurements, Royal Devon and Exeter Hospital (Wonford),ExeterEX2 5DW, U.K. 0950-821X/92/010021+ 05 $03.00/0© 1992Grune&StrattonLtd.
(1) Simple compression without flow (dry compression) All grafts were held in a specially designed rig and subjected to compression forces using known weights from 0 to 3.02kg in 13 increments. The compression was applied over a 2-cm length of graft. The diameter of each graft was measured with a micrometer screw-gauge (Moore and Wright, Sheffield, U.K.) as compression was increased. Three minutes was allowed to elapse after the application of each weight before three separate measurements were made. A single observer made all measurements; he was not blind to the graft type being used. This run
22
T.R. Magee et al.
(4) Handling characteristics
was repeated three times for each graft, the mean of these measurements was used in subsequent analysis.
Five consultant general surgeons were asked to assess the handling characteristics of all eight grafts, with respect to: (a) ease of removal of reinforcements; (b) ease of passage of a suture; (c) strength, as judged by attempting to pull the suture out, thus tearing the grafts. Unlabelled sections of each graft (5 cm) were mounted on a board and presented to the surgeons. The surgeons were asked to first remove the reinforcements from each graft. Then a 5/0 polypropylene suture (Prolene-Ethicon W 8890) was passed through each graft in turn using 6-inch Mayo-Hagar needleholders and 6-inch Debakey dissecting forceps. Graft strength was tested by trying to pull the suture out. Their impressions were recorded on a series of linear analogue scales and each graft was scored from 0 to 10 for each test. Summed scores for each graft were used to rate the ease of removal of reinforcements, ease of passage of a suture and strength. A sum of all scores for each graft (maximum of 50 for each test) was used to rate them for overall handling.
(2) Compression during flow The five PTFE grafts were placed in the same rig and water was passed through them driven by a constant pressure of 500 mmH20 from the header tank. Resistance to outflow was negligible, the water being carried away through a 6-mm tube. As water would leak through Dacron, only the five PTFE grafts were examined on the flow rig. Compression was applied to a 2-cm length of graft using known weights in 14 increments from 0 to 3.625 kg. Diameter was measured as detailed above. Flow through the grafts was measured by timing the passage of 500 ml into a measuring cylinder. Each graft was examined three times, and on each occasion three separate measurements of diameter and flow were made by a single observer.
(3) Flow during flexion The five PTFE grafts were flexed through 135° to simulate full flexion of the knee. Flow measurements were made in the same way as during compression using the same head of pressure (500mmH20). Differences in diameters and flow for each compression were compared using Student's t-tests.
Results
(1) Simple compression without flow (dry compression) The changes in diameter of all eight grafts are shown in Figure 1.
• Bard EXS 0 Impra Flex Standard 6X' Q ~ ~ L
~
+ 0 • []
' E
4
,,
\
~"~.~O,,,,.,,,..~ X""
Irnpra Flex Thin Wall MeadoxMicrovel Vascutek Goretex Standord • GoretexThin Walled A Gor rex Thin Walled "Removable Ring"
£3
0
--2
-•
I
0
L
IOOO
L
L
2000 Weight ( g )
I
Fig. 1. Dry compression internal diameter. Eur J Vasc Surg Vol 6, January 1992
L
5000
I
4000
Reinforced Vascular Grafts
Meadox Microvel [mean (+S.D.) diameter 4.91 + 0.03 mm at I kg; 2.45 + 0.02 mm at 2 kg] resisted compression by weights of l k g and 2kg significantly better than Impra Flex Standard (mean diameter 3.69 + 0.02mm at lkg; 0.91 + 0.04mm at 2kg) and all other grafts (p < 0.05). There were no statistical differences in diameter reduction between the other grafts for any compression force.
0
23
(2) Compressionduringflow: changes in diameter All five PTFE grafts behaved similarly with no statistical differences between them (Fig. 2).
Changes in flow during compression There was no significant change in flow in any graft when compressed with weights up to I kg (Fig. 3).
Impra Standard
4- Impra Thin Wall [] Goretex Standard ~'~A
•
Goretex Thir~ Wall
A
Goretex ThmWall Removable Ring
/
E
o
k3
I000
3000
2000
4000
Weight (g)
Fig. 2. Compression during flow: internal diameter. 2000 O Impra Standard +
Impra Thin Wall
13 Goretex Standard •
Goretex Thin Wall
T
o tL
\. i
0
I
1000
i
_ _
2000
4000
3000
Weight ( g ) Fig. 3. Flow during compression: constant head of pressure 500 mmH20. Eur J Vasc Surg Vol 6, January 1992
24
T.R. Magee et at.
H o w e v e r , Impra Flex Standard (mean + S.D. 1092 + 12.1 ml min -1 at 2 kg) maintained flow significantly better than Impra Flex Thin Wall (829 + 3 . 5 m l m i n -1 at 2kg) and all other grafts w h e n compressed with a weight of 2 k g (p
Reinforced Vascular Grafts: a Comparative Study Timothy R. Magee, Philip G. Niblett and W. Bruce Campbell Departments of Surgery and Clinical Measurements, Royal Devon and Exeter Hospital (Wonford), Exeter EX2 5DW, U.K. Reinforced vasculargrafts are designed to resist compression and flexion forces,for example in axillo-femoral or below-kneefemoropopliteal reconstruction. This study compared eight types of 6-mm reinforced grafts (five polytetrafluoroethylene (PTFE) and 3 Dacron). On a specially designed rig, graft diameter and flow were measured during compression by a standard series of weights up to 3.6 kg, and on another rig grafts wereflexed through 135°. Meadox Mierovel graft resisted compression best (p ( O.05) but there were no important differences between the other grafts (Vascutek; Bard; Goretex Standard, Thin Walled and Removable Ring; Impra Standard and Thin Wall). Flexion produced no significant changes in flow through the grafts. In addition, five general surgeons were asked to score the easeof removal of reinforcement, passage of sutures, and the tendency of sutures to cut out of each graft. Removal of reinforcement and passage of sutures were easierwith the PTFE grafts, but the Dacron was more resistant to sutures cutting out. This study shows that these grafts resist flexion and compression well. Their different handling characteristics and data on patency at different sites should remain the main determinants in choiceof graft. Key Words: Arterial occlusive disease;Arteries; Vascular grafts.
Introduction
Methods
Several vascular grafts are available in the U.K. with spiral or ring reinforcement to their walls. These grafts are used where compression or flexion might diminish flow and pose a risk of occlusion. For example, in axillo-femoral bypass the graft might be compressed when the patient lies on his/her side; while in below-knee femoro-popliteal bypass the knee must be allowed to flex fully without the graft kinking. Kinking has been cited as the cause of failure in 13% of unsupported below-knee femoro-popliteal polytetrafluoroethylene (PTFE) grafts. 1 The use of reinforced PTFE grafts has improved patency rates over unsupported grafts of the same material, and this has been attributed to their ability to withstand flexion without kinking. 2' 3 There have been no comparative studies on the physical properties of these grafts. This study aimed to provide objective comparison of reinforced grafts during compression and flexion, and also subjective data on their handling characteristics.
Eight 6-mm reinforced vascular grafts were examined; five PTFE and three Dacron. The Goretex grafts were reinforced by rings, the other grafts by spirals. The following grafts were examined. PTFE: Goretex Standard, Goretex Thin Walled, Goretex Thin Walled "Removable Ring", Impra Flex Standard and Impra Flex Thin Wall. Dacron: Meadox Microvel, Vascutek, Bard EXS.
Please address all correspondence to: W. B. Campbell, Departments of Surgery and Clinical Measurements, Royal Devon and Exeter Hospital (Wonford),ExeterEX2 5DW, U.K. 0950-821X/92/010021+ 05 $03.00/0© 1992Grune&StrattonLtd.
(1) Simple compression without flow (dry compression) All grafts were held in a specially designed rig and subjected to compression forces using known weights from 0 to 3.02kg in 13 increments. The compression was applied over a 2-cm length of graft. The diameter of each graft was measured with a micrometer screw-gauge (Moore and Wright, Sheffield, U.K.) as compression was increased. Three minutes was allowed to elapse after the application of each weight before three separate measurements were made. A single observer made all measurements; he was not blind to the graft type being used. This run
22
T.R. Magee et al.
(4) Handling characteristics
was repeated three times for each graft, the mean of these measurements was used in subsequent analysis.
Five consultant general surgeons were asked to assess the handling characteristics of all eight grafts, with respect to: (a) ease of removal of reinforcements; (b) ease of passage of a suture; (c) strength, as judged by attempting to pull the suture out, thus tearing the grafts. Unlabelled sections of each graft (5 cm) were mounted on a board and presented to the surgeons. The surgeons were asked to first remove the reinforcements from each graft. Then a 5/0 polypropylene suture (Prolene-Ethicon W 8890) was passed through each graft in turn using 6-inch Mayo-Hagar needleholders and 6-inch Debakey dissecting forceps. Graft strength was tested by trying to pull the suture out. Their impressions were recorded on a series of linear analogue scales and each graft was scored from 0 to 10 for each test. Summed scores for each graft were used to rate the ease of removal of reinforcements, ease of passage of a suture and strength. A sum of all scores for each graft (maximum of 50 for each test) was used to rate them for overall handling.
(2) Compression during flow The five PTFE grafts were placed in the same rig and water was passed through them driven by a constant pressure of 500 mmH20 from the header tank. Resistance to outflow was negligible, the water being carried away through a 6-mm tube. As water would leak through Dacron, only the five PTFE grafts were examined on the flow rig. Compression was applied to a 2-cm length of graft using known weights in 14 increments from 0 to 3.625 kg. Diameter was measured as detailed above. Flow through the grafts was measured by timing the passage of 500 ml into a measuring cylinder. Each graft was examined three times, and on each occasion three separate measurements of diameter and flow were made by a single observer.
(3) Flow during flexion The five PTFE grafts were flexed through 135° to simulate full flexion of the knee. Flow measurements were made in the same way as during compression using the same head of pressure (500mmH20). Differences in diameters and flow for each compression were compared using Student's t-tests.
Results
(1) Simple compression without flow (dry compression) The changes in diameter of all eight grafts are shown in Figure 1.
• Bard EXS 0 Impra Flex Standard 6X' Q ~ ~ L
~
+ 0 • []
' E
4
,,
\
~"~.~O,,,,.,,,..~ X""
Irnpra Flex Thin Wall MeadoxMicrovel Vascutek Goretex Standord • GoretexThin Walled A Gor rex Thin Walled "Removable Ring"
£3
0
--2
-•
I
0
L
IOOO
L
L
2000 Weight ( g )
I
Fig. 1. Dry compression internal diameter. Eur J Vasc Surg Vol 6, January 1992
L
5000
I
4000
Reinforced Vascular Grafts
Meadox Microvel [mean (+S.D.) diameter 4.91 + 0.03 mm at I kg; 2.45 + 0.02 mm at 2 kg] resisted compression by weights of l k g and 2kg significantly better than Impra Flex Standard (mean diameter 3.69 + 0.02mm at lkg; 0.91 + 0.04mm at 2kg) and all other grafts (p < 0.05). There were no statistical differences in diameter reduction between the other grafts for any compression force.
0
23
(2) Compressionduringflow: changes in diameter All five PTFE grafts behaved similarly with no statistical differences between them (Fig. 2).
Changes in flow during compression There was no significant change in flow in any graft when compressed with weights up to I kg (Fig. 3).
Impra Standard
4- Impra Thin Wall [] Goretex Standard ~'~A
•
Goretex Thir~ Wall
A
Goretex ThmWall Removable Ring
/
E
o
k3
I000
3000
2000
4000
Weight (g)
Fig. 2. Compression during flow: internal diameter. 2000 O Impra Standard +
Impra Thin Wall
13 Goretex Standard •
Goretex Thin Wall
T
o tL
\. i
0
I
1000
i
_ _
2000
4000
3000
Weight ( g ) Fig. 3. Flow during compression: constant head of pressure 500 mmH20. Eur J Vasc Surg Vol 6, January 1992
24
T.R. Magee et at.
H o w e v e r , Impra Flex Standard (mean + S.D. 1092 + 12.1 ml min -1 at 2 kg) maintained flow significantly better than Impra Flex Thin Wall (829 + 3 . 5 m l m i n -1 at 2kg) and all other grafts w h e n compressed with a weight of 2 k g (p
