The Effect of Autoclave Resterilisation on Polyester Vascular Grafts

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Eur J Vasc Endovasc Surg 18, 386–390 (1999) Article No. ejvs.1999.0891

G. Riepe∗1, M. S. Whiteley2, A. Wente1, A. Rogge1, A. Schro¨ der1, R. B. Galland2 and H. Imig1

1Department of General, Vascular and Thoracic Surgery, General Hospital of Hamburg–Harburg, Germany; 2Department of Surgery, Royal Berkshire Hospital, Reading, U.K.

Objectives: polyester grafts are expensive, single-use items. Some manufacturers of uncoated, woven grafts include instructions for autoclave resterilisation to be performed at the surgeon’s own request. Others warn against such manipulation. Theoretically, the glass transition point of polyester at 70–80°C and the possible acceleration of hydrolysis suggest that autoclave resterilisation at 135°C might be a problem. Materials and methods: a DeBakey Soft Woven Dacron Vascular Prosthesis (Bard) and a Woven Double Velour Dacron Graft (Meadox) were autoclave-resterilised 0 to 20 times, having been weighed before and after sterilisation. Tactile testing was performed. Mechanical properties were examined by probe puncture and single-ﬁlament testing, the surface was examined by scanning electron microscopy and the degree of hydrolysis by infra-red spectroscopy. Results: tactile testing revealed a change of feeling with increasing cycles of resterilisation. Investigation of weight, textile strength, single-ﬁlament strength, electron microscopy of the surface and infra-red spectroscopy showed no change of the material. Conclusions: changes felt are presumably a surface phenomenon, not measurably affecting strength or chemistry of material after autoclave resterilisation. We therefore feel that it is safe to use once-autoclave-resterilised surplus uncoated polyester grafts, provided that sterility is guaranteed.

Key Words: Polyester; Vascular grafts; Autoclave resterilisation.

Introduction The polyester, from which vascular grafts are made, has an amorphous (approx. 60%) and a crystalline Polyester (e.g. Dacron) vascular grafts are frequently section (approx. 40%). The theoretical objection to used in vascular surgery. They are expensive and a autoclave resterilisation is due to the material char- single-use item. Many of the modern knitted grafts acteristics of this polyester. The amorphous fraction are coated with albumin or collagen to become blood- has a glass transition point at 70–80 °C. Above this proof. This coating forbids autoclave resterilisation. temperature the molecular chains loosen and become On the other hand, woven polyester grafts do not vulnerable. Autoclave sterilisation at 134 °C could necessarily need coating to be blood-proof, due to therefore affect the physical properties of the polyester their highly dense textile structure. Some vascular and weaken the graft. Further, it is known that hy- surgeons resterilise excess material of uncoated poly- drolysis, the reverse reaction of the polyester synthesis, ester grafts to reduce costs. leads to degeneration of polyester grafts in the human Although this is not recommended, some manu- body after many years of implantation.1 It could also facturers of woven, uncoated polyester grafts include be shown that the hydrolysis of polyester vascular instructions for autoclave resterilisation to be per- grafts is extremely accelerated in boiling water.2 formed at the surgeon’s own request. Other graft We investigated whether repeated cycles of stand- manufacturers warn against this manipulation in oral ard, hospital-performed autoclave resterilisation af- communication. The increasing economic pressure on fected the physical properties of polyester vascular the one hand and theoretical danger of damaging grafts. polyester by autoclave resterilisation on the other hand induced our interest on examining the effects of auto- clavation. Methods

∗ Please address all correspondence to: G. Riepe, General Hospital of Harburg, Department of General, Vascular and Thoracic Surgery, Two different polyester grafts were tested. A 20-mm   Eissendorfer Pferdeweg 52, D-21075 Hamburg, Germany. DeBakey Soft Woven Dacron Vascular Prosthesis

1078–5884/99/110386+05 $12.00/0  1999 Harcourt Publishers Ltd. Autoclave Resterilisation 387

(Bard, U.S.A.) and a 16-mm Woven Double Velour Probe puncture testing Dacron Graft (Meadox, U.S.A.). On both grafts we performed 0, 1, 5, 10 and 20 cycles of autoclave Probe puncture testing was performed on the complete sterilisation. The DeBakey Soft Woven Graft was graft wall. The probe puncture test is described in cut into 5 groups of 3×2 cm lengths for tactile ISO 7198-2 for cardiovascular implants and tubular testing and weighing before and after resterilisation. vascular prostheses.3 The test device was set up on a The Meadox Woven Double Velour Graft was cut ZWICK 1120 material-testing machine (Zwick GmbH into 5 sections of 5 cm. These samples underwent & Co, Ulm, Germany). The cylinder-shaped indenter probe puncture testing and longitudinal filament- measures 48 mm in diameter and has a polished, hemi- burst testing as well as examination by scanning spherical ending. The circular specimen holder is electron microscope (SEM) and Fourier-transformed equipped with a polyvinylchloride ring to prevent infra-red spectroscopy (FTIR). slipping of material. Specimens were slightly, man- ually stretched whilst clamping to remove their crimp- ing. The indenter penetrated the material with a velocity of 70 mm/min. Five tests were performed on Autoclave sterilisation each of the 5 samples having been resterilised 0×, 1×,5× and 20×. The sterilisation was performed at 134 °C, 2.4 bar steam pressure for 6 min. Together with the heating and cooling time, every sterilisation cycle took approx. 40 min. The material was packaged in special, sealed Longitudinal filament-burst testing paper bags, which were marked after each cycle. The sterilisation process was completely computer-con- For filament testing, single 12-lm-thick filaments were trolled. isolated from single yarns of the textile structure (Fig. 1). The titre [tex] of the fibre was measured by a VIBROSKOP (Lenzing AG, Lenzing, Austria). The longitudinal burst test of approx. 3-cm-long filaments was Weighing then performed on a VIBDODYN testing machine (Lenzing AG, Lenzing, Austria) at a velocity of 20 mm/ The grafts were weighed on highly accurate scales, min. Five tests were performed on each of the 5 ± precise down to 0.0001 g. It was necessary to use samples having been resterilised 0×,1×,5×,10× originally packaged graft material to prevent previous and 20×. surplus water-uptake from the surrounding air. The material was handled using fine forceps. After weighing, the samples were stored in vacuum-sealed bags before being led to the resterilisation process. The Scanning electron microscopy (SEM) same procedure was performed again after 1, 5, 10 or 20 cycles of resterilisation. SEM was performed on single filaments of the samples that underwent 0 and 5 resterilisation cycles. A Hitachi

Tactile testing

Tactile testing was performed by three surgeons, all having high experience with polyester vascular grafts. The 15 graft segments were divided into 5 groups of 0×,1×,5×,10× and 20× resterilisation. For the test, the surgeon was blindfolded and allowed to practice feeling the difference between an original and a20× resterilised graft between thumb and second finger first. He then had to feel the 15 graft segments of the five different resterilisation groups presented in a random order, deciding whether the segment felt ‘‘old’’ (rated as 1) or ‘‘new’’ (rated as 0). The results of the three surgeons were added for each group. Fig. 1. Single filament illustrated from one yarn in graft textile.

Eur J Vasc Endovasc Surg Vol 18, November 1999 388 G. Riepe et al.

Table 1. Results of tactile testing performed by three blindfolded surgeons (MW, GR, HI) rating the samples with 0 (feels like new), 1 (feels like old) or 0.5 (unsure). The tactile testing revealed a change of feeling with increasing cycles of autoclave resterilisation of the grafts.

Resterilisation Total No. of cycles MW GR HI (95% Conﬁdence Interval)

0× 000 0× 000 0× 0 0 0 0 (0.0–0.0) 1× 1 0 0.5 1× 001 1× 0 0.5 0 3 (0.025–0.641) × 5 101 × × 5× 000 Fig. 3. SEM of filaments after (a) 0 and (b) 5 resterilisation 5× 1 1 0 4 (0.120–0.769) showing no alteration on the surface of the filaments. 10× 101 10× 111 10× 1 1 0 7 (0.506–1.049) 20× 111 20× 111 Results 20× 1 1 1 9 (1.000–1.000) Tactile testing revealed a change of feeling with increasing cycles of resterilisation. All of the not-re- S-4500 cold-field emission electron microscope sterilised grafts were recognised as ‘‘new’’, all of the × (Hitachi, Kobe, Japan) was used. Random photographs 20 resterilised grafts were recognised as ‘‘old’’. The were made at several magnifications. results of the 5 groups are listed with the sum of the surgeons’ rating (‘‘new’’=0, ‘‘old’’=1, ‘‘unsure’’=0.5) and the 95% confidence intervals (Table 1). Weighing showed no weight gain down to 0.0001 g, indicating no water-binding of the polyester molecules Fourier-transformed infra-red spectroscopy (FTIR) during 1 to 20 cycles of autoclave resterilisation (Table 2). There was no change of strength of the polyester Filaments of samples that underwent 0 and 20 re- graft seen by probe puncture (Fig. 2a) nor in individual sterilisation cycles were examined by FTIR using a filaments by longitudinal burst testing (Fig. 2b). SEM BioRad 650 instrument (Bio-Rad Laboratories, U.S.A.). showed no change of surface of single filaments at up Hydrolysis of polyester chains increases the number to 10 000× magnification (Fig. 3). In infra-red spec- of carboxylic end-groups and lets the peak of infra- troscopy no significant increase of polyester molecule red absorbance of this group grow in the spectrogram. chain scissions by hydrolysis were seen.

Fig. 2. Graphs showing the probe puncture strength and the ﬁlament burst strength vs. cycles of autoclave resterilisation. In both tests no change of the mechanical properties could be seen.

Eur J Vasc Endovasc Surg Vol 18, November 1999 Autoclave Resterilisation 389

Table 2. The weight of graft samples before and after autoclave to increasing economic pressure. They furthermore resterilisation showing no weight gain. The weight loss after resterilise surplus graft material to reduce costs. Al- vacuum drying after resterilisation of graft B is explained by the prolonged contact to air moisture in the opened package before though not officially recommended, the manufacturers resterilisation. of the tested graft material provide information for autoclave resterilisation in the adjoining instruction Autoclave Graft A Graft B resterilis- leaflet (Table 4). There is little independent data re- 4 ation Weight Weight Differ- Weight Weight Differ- garding the effect of resterilisation of these grafts. before after ence before after ence Explant retrieval studies have shown that de- No. of cycles [g] [g] [%] [g] [g] [%] generation of polyester takes place in vivo leading 5–10 1× 0.3608 0.3608 0.00 0.3271 0.3268 −0.09 to graft failure after many years of implantation. 5× 0.2807 0.2808 0.04 0.2854 0.2852 −0.07 Fatigue, cellular interaction and hydrolysis must be × − 10 0.4444 0.4444 0.00 0.3318 0.3313 0.15 discussed as possible mechanisms of degradation. Hy- 20× 0.3763 0.3763 0.00 0.3231 0.3227 −0.21 drolysis, the reverse reaction of the polyester synthesis, disrupts the polyester molecules. This could be sim- Table 3. An example of the prices of vascular grafts in Germany ulated in vitro by boiling polyester grafts in water for in Deutsche Mark [DM] (1997). two weeks.2 It has been shown that molecule-chain scissions take place in vivo as well, correlating with Approx. Diameter Length price an approx. 30% decrease of the probe puncture Graft type (mm) (cm) (DM) strength after 10 years of implantation in human be- ings, explaining observed, rare graft ruptures after Tube Woven 16 60 860,– 1 Bifurcation Woven 16/9 40 830,– 10–20 years. Above the glass transition point at Tube Woven Coated 16 30 1456,– 70–80 °C, the molecular chains in the amorphous sec- Bifurcation Woven Coated 16/9 40 1865,– tion of polyester loosen and become more vulnerable.11 Therefore, theoretically, the watery milieu and the high temperature at 134 °C could affect the physical Discussion properties of the polyester and weaken the graft during autoclave sterilisation. The polyester vascular grafts currently available are The palpable difference between new and re- expensive and intended for single use. A brief sterilised woven grafts encouraged us to examine the overview over the approx. prices of grafts is given in material closely. The tactile difference was only re- Table 3. producible between 0 and 20 cycles of resterilisation. In the past, surplus graft material was frequently Differences between 1, 5 and 10 cycles were not sig- resterilised by surgeons. The tubes cut from long tubes nificant. An explanation for felt changes could not be or from shortened bifurcation grafts were stored for found by the applied material tests. It is presumed that later use again as tube grafts or as patch material. Of the repeated autoclave resterilisation at the pressure of course, the cutting of material which was designed 2.4 bar might flatten the velour-like surface of the for tube-grafting into patches may damage the radial polyester graft. This would remove the smooth feeling structure of the textile, thus changing radial strength of new velour-like grafts and reveal the cobblestone- or suture-retention strength. This problem is not the like structure of the woven fabric to the fingers. subject of this paper. The introduction of the even Despite the fact that our results showed no meas- more expensive, coated, primarily waterproof grafts urable change of the material, we do not know whether made autoclave resterilisation impossible. However, frequent resterilisation may accelerate the in vivo de- many surgeons prefer not to use coated grafts, due generation process. Hydrolysis is a process affecting

Table 4. Graft manufacturers’ instructions for autoclave resterilisation of grafts.

Graft type Instructions for resterilisation

DeBakey Soft Woven ‘‘Resterilization of this prosthesis/fabric is not recommended. However, if resterilization is required, Dacron Vascular exposure to 30 minutes of accumulated standard autoclave cycles at 121 °C (250 °F), 103 kPa (15 psi) or Prostheses ‘ﬂash’ autoclave cycles at 132 °C (270 °F), 186 kPa (27 psi) will not affect the physical properties of this (Bard, U.S.A.) material. Subsequent exposure to steam sterilization should be avoided. Do not permit this material to come in contact with metal or glass surfaces during the sterilization process....’’ Woven Double Velour ‘‘By steam: remove graft from plastic tube, insert in hospital envelope and autoclave at 250 °F (120 °C) for Dacron Graft 20 minutes at 15 psi (1055 gm/cm2). In an emergency it may also be ﬂashed at 270 °F (133 °C) for 4 (Meadox, U.S.A.) minutes.’’

Eur J Vasc Endovasc Surg Vol 18, November 1999 390 G. Riepe et al. the material surface. Minor, invisible surface altera- References tions leave the core of the fibre undamaged and do not change mechanical properties. Future observations 1Riepe G, Loos J, Imig H et al. Long-term in-vivo alterations of polyester grafts in humans. Eur J Vasc Endovasc Surg 1997; 13: of all patients receiving polyester graft material and 540–548. an exact record of the material employed are necessary. 2Ludwig M. Untersuchung der Degeneration von Geefa¨ß- Furthermore, it must be taken into account that re- prothesen mittels Infrarotspektroskopie. Diplomarbeit Uni- versita¨t Dortmund, Germany, 1995. peated resterilisation increases the risk of con- 3International Organization for Standardization. Cardio- tamination.12 vascular implants – tubular vascular prostheses – Part 2: sterile For the meantime we feel safe to implant once- vascular prostheses of biological origin – specification and methods of  tests. Committee Draft ISO/CD1994; 7198-2: 30–31. AAMI, 3330 autoclave-resterilised surplus uncoated Dacron Washington Blvd., Suite 400, Arlington, VA 22201-4598, U.S.A. grafts, using the standard autoclave sterilisation pro- 4Helmus MN, Botan EA, Malone et al. Sterility and pyrogenicity cedure in our hospital, provided that sterility is guar- issues of synthetic vascular grafts. Vascular grafts update: safety and performance. In: Kambric HE, Kantrowitz A, Sung P, eds. anteed. American Society for Testing and Materials. Philadelphia: ASTM STP, 1986; 898: 236–249. 5Rudakova TE, Zaikov GE, Voronka OS, Daurova TT, Deg- tyareva S. The kinetic specificity of polyethylenterephthalate in Acknowledgements the living body. J Polymer Science Polymer Symposium 1979; 66: 277–281. 6Berger K, Sauvage LR. Late fiber deterioration in Dacron arterial The research for this paper was performed in Hamburg, enabled grafts. Ann Surg 1981; 193: 477–491. by the ESVS Educational Travel Grant of Mr. Whiteley. 7Maarek JM, Guidoin R, Aubin M, Prud’homme RE. Molecular We wish to thank: weight characterization of virgin and explanted polyester arterial Dipl. Ing. Roman Nassutt, Dr. rer. nat. Michael Morlock and Prof. prostheses. J Biomed Mater Res 1984; 18: 881–894. Dr. sc. tech. Erich Schneider (Section of Biomechanics, Technical 8King MW, Guidoin R, Blais P, G arton A, Gunasekera KR. University of Hamburg–Harburg, Germany) for the assistance on Degradation of polyester arterial prostheses: A physical or chem- the probe puncture test; ical mechanism? In: Fraker AC, Griffin CD, eds. Corrosion and Dipl. Phys. Joachim Loos and Prof. Dr. rer. nat. Ju¨ rgen Petermann degradation of implant materials: second symposium. Philadelphia: (Material Sciences, University of Dortmund, Germany) for the elec- ASTM STP, 1985; 859: 294–307. tron microscopic and infra-red spectroscopic examinations and their 9Pourdeyhimi B, Wagner D. On the correlation between the information on the material characteristics of polyethylene- failure of vascular grafts and their structural and material prop- terephthalate; erties: a critical analysis. J Biomed Mater Res 1986; 20: 375–409. Mrs. Dipl. Ing. Regina Grewe and her team (Textil Laboratory, 10 Vinard E, Eloy R, Descotes J et al. Stability of performances of NYLSTAR, Neumu¨ nster, Germany) for the assistance on the filament vascular prostheses. Retrospective study of 22 cases of human examinations. Mrs. Marie-Louise Krauß and the staff of the Central implanted prostheses. J Biomed Mater Res 1988; 22: 633–648. Section of Sterilisation in the General Hospital of Harburg for the 11 Mark HF, Bikales NM, Overberger CG, Menges G. Encyclopedia resterilisation of the graft material in this paper. of polymer science and engineering. Volume 12 – Polyesters to poly- peptide synthesis. New York, Chichester, Brisbane, Toronto, Sin- gapore: Wiley & Sons, 1988: 225–234. 12 Gerlach E. Kunststoffe in der Gefa¨ßchirurgie – Behandlung und Verhalte. Die Schwester/Der Pfleger 1986; 7/25: 574–576.

Accepted 7 April 1999

Eur J Vasc Endovasc Surg Vol 18, November 1999