Shelf-life of bioprosthetic heart valves: a structural and mechanical study.

This study was undertaken to evaluate the influence of storage conditions on the shelf-life of porcine bioprosthetic valves. Fifty-five unimplanted porcine bioprostheses have been evaluated. The valves were stored in 0.5% buffered glutaraldehyde solution for different periods of time (7, 23 and 32 months). Twenty-eight valves were refrigerated while the remaining valves were stored at room temperature. The pH of the glutaraldehyde solution at room temperature decreased with time of storage, while that kept in the refrigerator remained stable over the course of the study. Macroscopic observations showed that the valve tissues kept at room temperature, especially for the periods of 23 and 32 months, became darker and more yellow in colour, whereas the refrigerated specimens exhibited no such changes in appearance. Scanning electron microscopy analysis revealed no noticeable differences on the surfaces of the leaflets stored under different conditions. Mechanical tests, including stress-strain response, stress relaxation and fracture behaviour, were carried out. Analysis of variance showed that the storage temperature, but not the length of storage, had a significant effect on some mechanical properties. The stress relaxation at 1000 s (P = 0.05), the ultimate tensile strength (P = 0.01) and the strain at fracture (P = 0.04) were all higher after storage at room temperature compared to the results after refrigeration. No statistically significant changes in the denaturation temperature of the collagen were observed between the different storage conditions. In conclusion, the storage temperature appears to have some influence on the bioprosthetic tissue. The bioprostheses stored under ambient conditions experience changes which may influence their longterm in vivo performance.

Physical characterization of unimplanted gel filled breast implants. Should old standards be revisited?

Physical characterization of unimplanted gel filled mammary prostheses was undertaken using both destructive and non-destructive test methods. Physical properties measured included mass, volume, optical transmittance, coefficient of kinetic friction, mechanical stiffness, dynamic response to a stationary random vibration, bursting strength, elasticity of the envelopes, and nuclear magnetic resonance relaxation times. The data obtained from this study will provide reference points for quantifying the rate of degradation of prostheses in past and current clinical use. Some of the test methods are suitable for use as routine quality control procedures to improve the uniformity of the properties and performance of breast prostheses. This investigation also showed that some currently used prostheses do not comply with the relevant American Society for Testing and Materials standards, or even with the manufacturers' own stated claims or specifications.

Why make monofilament sutures out of polyvinylidene fluoride?

In recent years some clinical reports have associated suture failures with polypropylene monofilaments. Therefore there is interest in developing an alternative suture material that is less thrombogenic than polyester and similar in handling characteristics but less prone to mechanical failure than polypropylene. To this end, Peters Laboratoire Pharmaceutique has developed a new monofilament suture material from polyvinylidene fluoride (PVDF), which has been subjected to a special treatment to modify its crystalline form and level of crystallinity. The purpose of this study was to evaluate its mechanical, chemical, and biologic properties and to compare its performance, in a peripheral vascular application, to that of a polypropylene control. A series of in vitro tests were performed to study the morphology, tensile properties, creep, surface chemistry, thermal characteristics, and resistance to iatrogenic trauma. In addition, an in vivo trial was undertaken in which vascular prostheses anastomosed with either PVDF or polypropylene sutures were implanted as a thoracoabdominal bypass for 6 months in the dog. Histologic and degradation analyses were performed on the explants. The results from the mechanical tests on 4-0, 5-0, and 6-0 PVDF and polypropylene sutures demonstrated that although both materials have similar breaking strengths, the PVDF has a higher extension at break, has less delayed extension when under tensile creep testing, and suffers less trauma than the polypropylene when compressed by a standard needle holder. While chemical analyses found evidence of surface oxidation on both types of sutures, thermal analysis confirmed that the level of crystallinity of the PVDF polymer is higher than that of the polypropylene control. During the pilot study in animals, PVDF sutures were found to have good handling and frictional characteristics that facilitated the tying of knots. Histologic analysis of the explants found no inflammatory cells in the tissue surrounding either the PVDF or polypropylene sutures, and scanning electron microscopic examination of the cleaned suture surfaces found no evidence of degradation during 6 months in vivo. Though preliminary in nature, these findings indicate that monofilament sutures made from PVDF provide an attractive alternative to those made from polypropylene for use in cardiovascular surgery. In addition to providing acceptable in vivo behavior and being easy to manipulate and more resistant to iatrogenic injury, PVDF materials can be sterilized by beta or gamma radiation and so can reduce dependence upon ethylene oxide and chlorofluorohydrocarbons.

Textile arterial prostheses: is water permeability equivalent to porosity?

Porosity and water permeability are two distinct terms that describe different characteristics of vascular prostheses. The porosity is a measure of the void fraction within the prosthesis wall and is believed to give a rough prediction of the capacity of the graft to anchor newly formed surrounding tissue after implantation, whereas the water permeability indicates the rate at which water can flow through the prosthesis wall and, when measured under physiological pressure conditions, provides the surgeon with information about the need for preclotting prior to implantation. The literature has not always clearly distinguished between these two terms, and some authors in fact have suggested that they both refer to the same property of a prosthesis. In an attempt to clarify the issue, porosity and water permeability measurements were made on 34 commercial vascular prostheses having different textile constructions. Linear regression analysis demonstrated that these two characteristics are only weakly related (r = 0.59). It is therefore recommended that the current draft standards for such devices reference both properties: porosity and water permeability.

Knee menisci. Correlation between microstructure and biomechanics.

Light and scanning electron microscopy techniques were used to define the microstructure of human knee menisci. Two structurally different regions were shown: a mesial part that included the innermost two thirds and a peripheral part formed by the remaining outer one third. The organization of collagen bundles of the mesial part demonstrated a radial pattern. Those of the peripheral part were larger and circumferential. The articular surfaces of the mesial part were lined by thinner bundles parallel to the surface, while the outer portion was covered by synovium. This structural organization suggested specific biomechanical functions: mainly compression mesially with tension peripherally and a direct translation of forces from the inner wedge-shaped part to the outermost region. The covering layer is well suited for surface to surface motion. Outward displacement of the menisci by the femoral condyles is resisted by solid anchorage of the peripheral circumferential fibers to the intercondylar bone. The resistance to such displacement would force the femoral condyles inwards. Such an organization of menisci has implications in knee joint stability and in the pathology of meniscal injuries.

[Influence of bandages on the strength of impact of punches in boxing]. / Influence des bandages sur la force d'impact des coups de poing à la boxe.

In boxing, a punch can be directed to the head or the torso of the opponent; however, because of the gravity of the ensuing injuries, the limit of tolerance of the head and face to repeated impacts (Gurjian, 1955, 1962; Patrick, 1955; Hodgson and Nakamura, 1968) is particularly important to keep in mind. The objective of this research was to compare the impact forces which could be attained barehand versus those obtained with different types of bandages made up of gauze and diachylon. For the purpose of this study, 22 boxers were selected. The impact force was measured with a Kistler force plate, covered with a synthetic mattress. A system of photoelectric cells was used to measure the velocity of the punch. Each boxer had to impact the target with 10 punches while his hand was covered with one of four types of bandages as well as barehand. This study has shown that the bandaged hand increases significantly the impact force of the punch. The force increases in relation to the thickness of the bandage. The use of additional pieces of diachylon has a greater influence than gauzes. Therefore, it can be concluded that the bandaging increases the rigidity of the hand and facilitates the transfer of the force in comparison to a similar punch without a bandage or with a thinner one.

[A study of the menisci of the knee by scanning electron microscopy (author's transl)]. / Les ménisques du genou. Etude en microscopie électronique à balayage et corrélation biomécanique.

Sixty menisci have been studied by scanning electron microscopy. In the frontal plane of the middle third of the meniscus, two distinct parts can be discerned - a central part corresponding to the medial two thirds and a peripheral part corresponding to the lateral third. Collagen fibres can be arranged in four different ways; radial fibres in the central portion; a marginal area underlying each surface of the meniscus made up of mixed collagen fibres; circumferential fibres in the peripheral part and a fascicular layer of fibres made up of loose connective tissue arising from the articular capsule and penetrating into the circumferential fibrous layer. The authors consider that the radial fibres are best adapted to pressure, whilst circumferential fibres are submitted to traction forces; the marginal areas are best adapted to gliding forces. Both menisci comprise a single functional unit.

Cardiovascular sutures as assessed by scanning electron microscopy.

Nine types of commercial sutures were investigated with respect to their suitability in cardiovascular surgery involving anastomosis of arterial prostheses. Polyester, nylon, silk and polypropylene in the form of multifilaments and monofilaments with or without coatings were included. The gross structure of the suture and the needle, the uniformity of the fibres and the coatings, the mechanical properties, before and after surgical use and the quality of the resulting anastomoses in canine models were evaluated. Although all surveyed products were potentially usable for the purpose intended, some appeared more suitable. Intraoperative damage to the sutures was found to be extensive and may play an important part in the long-term security of the anastomoses.