EXPANDED POLYTETRAFLUOROETHYLENE (e-PTFE)

Fluorocarbons are generally very stable compounds. PTFE is produced from tetrafluoroethyiene which in turn is produced from the fluorination of trichloromethane.

PTFE is an extremely long carbon chain protected by a dense sheath of fluorine, {CF2-C F2>n, making it inert to host responses. The polymer is mixed with lubricant, naphtha, to form a Teflon® paste that is shaped under high pressure. This material is made porous by a novel mechanical stretching process and the porosity controlled by sintering at high temperatures. Pore size and shape is modified by modulating the processing conditions and the post-sintering cooling rate. Forms of

Host responses cannot chemically react with the densely fluorinated carbon chain, hence the material is accepted and tissues continue to undergo a healthy turnover. The e-PTFE membrane has been shown to provoke an inflammatory response that is slightly more intense than a sham operation (Lam et al. 1995).

The aforementioned property of osteopromotive' is an index of the proliferative ability of bone, given the correct environment. e-PTFE is chemically able to provide this environment by being so inert as to not provoke a chemical response. However it does show a very high water contact angle, defining e-PTFE as a highly hydrophobic material (Lam

e ta l. 1995).

e-P TFE is a three dimensional matrix of nodes and interconnecting fibrils. The material elicits a similar response when implanted into different sites, with encapsulation and cell colonisation (Béllon et al.

1996). However the membrane can be produced with different porosities that in turn alter the host responses in certain environments (Hirabayashi et al. 1992). Figures 1.2, 1.3, and 1.4 are scanning electron micrographs of the membrane surface. Figure 1.2 was taken at a low magnification, showing the nodes and fibrils. Figure 1.3 demonstrates the nodes and inter-connecting fibrils, at a higher power. Figure 1.4 was taken through the section of the membrane and illustrates the ' " two porous layers, with the inner occlusive layer.

Figure 1.4 Photoelectronmicrograph of section through e-PTFE

The ability to vary the porosity allows a degree of control with regard to the amount of tissue ingrowth. e-PTFE is used in a very wide range of surgical applications including various forms for vascular surgery (Sise

et al. 1994, Chiesa et al. 1995), abdominal surgery (Bauer et al. 1987, Deysine 1992, Kennedy and Matyas 1994), thoracic surgery (Hyans et al. 1992, Arnold and Pairolero 1996), colo-rectal reconstruction (McMahen and Ripskein 1987), obstetrics and gynaecology (Pierluigi and Santilli 1988, Owen et al. 1989, Martin et al. 1990), experimental meniscal repair (Messner and Gillquest 1993), limb salvage (Kasabian

et al. 1995) and neurosurgery (Yamagata et al. 1993, Inoue et al. 1994, DiFazio etal. 1995).

e-P TFE has a long history in implant density. The use of this particular material is not requisite for guided bone regeneration, and there are other membranes that can function within this context (Kleinschmidt et al. 1993, Sandberg at al. 1993, Ashammakhi at al. 1995, Zellin at al.

1995, Piatelli a ta l. 1996, Pineda atal. 1996).

For guided bone regeneration the membrane should be biocompatible, occlusive to cells but not fluids, have the structural integrity to form a ‘space’, allow tissue ingrowth hence stability, and be clinically manageable (Hardwick at al. 1994). e-P TFE has the longest experimental record in guided bone regeneration. It has been shown that membranes constructed of different materials vary considerably in osteopromotive efficacy (Zellin at al. 1995).

In 1988 Dahlin at al. used expanded polytetrafluoroethylene (e-PTFE) as the barrier membrane’ to test the GTR principle with GBR. Having lifted mucoperiosteal flaps, 5mm holes were drilled bilaterally through the mandibles of 30 dogs. Controls had defects that where covered by just the mucoperiosteal flap, and the test sites had e-P T F E membranes (W.L. Gore and Associates, Flagstaff, Arizona) placed over the defects, prior to flap coverage and wound closure. Histomorphometric and gross analysis showed that the control defects were filled with fibrous connective tissue with a slight ingrowth of new bone at the defect margins whereas the test groups demonstrated new complete bone regeneration. Hence following further testing (Dahlin at al. 1990, Seibert

Dahlin (1993) used (^H) thymidine as a bone marker in rats. Ten days after surgery, endosteal cells within the bone adjacent to the membrane defects were undergoing mitosis. Incorporation of ^®Ca showed that test defects had a significantly higher quantity of mineralisation, even though the rate of uptake was slower. Maximal levels were reached after five weeks in the test specimens and two weeks in the controls. In rats, the potential for osteogenesis is high and so the rate of bone formation would be slower in man (Schmitz et al. 1986).

Linde (1993a) showed the osteopromotive effect of e-PTFE membranes can also cause bone to grow in anatomical sites where ordinarily bone is absent. He used a stiff dome-shaped material placed on top of the flat calvarium of rats, forming a sealed space into which osteogenic cells could migrate. The establishment of a blood clot beneath the membrane into an actual space is of paramount importance in allowing predictable osteogenesis. This study used a very porous form of e-PTFE, with 100pm pores, which also allowed angiogenesis. Moreover bone formation was also seen outside the membrane.

The variation in pore size and the ability of neovascularisation has been investigated (Brauker et al. 1995). Results suggest that larger pore diameters are able to support vascularisation by allowing cell entry.

With the rapid increase in awareness of the bone/e-PTFE interactions work into the applications of guided bone regeneration has spread into other fields, albeit slowly (Ashammakhi et al. 1995, Nyman et al. 1995, Piatelli e ta l. 1996, Bhumbra e ta l. 1998). Nyman eta l. (1995) concluded it was possible to use the principles of GBR to achieve bone union of segmental long bone defects.

Osteogenic cell origin is important, in aiding their migration and proliferation. In fractures, the cells derive from the periosteum and marrow. Under the defect, cell recruitment arises mostly from the endosteum, as well as from Haversian and Volkmann canals.

1.16 PRACTICAL ASPECTS OF GU ID ED BONE REGENERATION