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General remarks on percutaneous implant design

As the failure of percutaneous devices is related to the skin reaction to the implant, the formation of a stable skin-implant junction may be the major factor in determining the success of these implants. Two factors are considered to be responsible for the formation and maintenance of an effective skin/implant seal:

bulletchemical and physical properties of the implant materials
bulletmechanical stresses at the skin/implant interface, generated by relative movement between implant and skin

Chemical and physical properties of the implant materials

Considering the influence of the physico/chemical properties, first some general remarks on the application of so-called biomaterials for implant manufacturing have to be made. The general accepted definition of biomaterials is: “Biomaterials are nonviable materials used in a medical device, intended to interact with a biological system”. This interaction concerns both the host response to the material and the material response to the livingtissue (called biocompatibility).

Materials used for the construction of percutaneous implants can be classified as:

bulletmetals Mainly stainless steel, titanium and titanium alloys are used. Metals are stiff and strong and are covered with an oxide layer. The stability of the oxide layer determines the corrosion resistance of the metal. Compared to stainless steel it is known that the surface of implants consisting of titanium and its alloys is surrounded by a highly adherent and protective oxide film. In contrast to stainless steel, released ions of these metals will form electroneutral hydroxides and oxides leading to a good tolerance.

bulletceramics Similar to metals ceramics are strong, but on the other hand brittle. Consequently, implants manufactured from these materials cannot be heavily loaded. For percutaneous devices hydroxyapatite, alumina ceramic and carbons were applied.

bulletpolymers Polymers are available with a wide variety of mechanical and physical properties. However for clinical applications limiting factors for the use of polymers are lack of tissue compatibility and lack of resistance to the biological environment. Frequently used polymers in percutaneous devices are polysiloxanes (Silicone), polyethylene terephtalate (Dacron®), polyurethane and polytetrafluoroethylene (Tefon®).

In addition to their bulk characteristics, physical and chemical surface properties of an implant also influence the final tissue reaction.

Mechanical stresses

Mechanical stresses are another cause for percuaneous implant failure. To reduce these interfacial stresses, several solutions have been investigated, like skeletal attachment of the implant. However, in most cases were percutaneous conduits are indicated, solely soft-tissue anchored systems can be used. For instance in peritoneal dialysis, the percutaneous implant has to be inserted in the highly mobile tissue of the abdominal wall. To find a solution for this application, most designs of percutaneous devices were provided with a micro- or macroporous cuff or flange to anchor the implant.

 

 

 

 

Picture showing Dacron® cuffs for anchoring of a percutaneous device

 

 

 

 

 

 

 

 

 

 

 

Permigration: the epidermis follows the immature connective tissue into the pores,

hereby gradually extruding the percutaneous implant

 

The purpose of the porosity is to allow ingrowth of connective tissue, resulting in a strong mechanical link with the device. Unfortunately, almost all designs eventually failed, due to the inability to obtain matured, well vascularized connective tissue inside the pores of the anchor. Instead, the porosity was filled with inflammatory cells. The epidermis follows the immature connective tissue into the pores, a process called permigration.

 

 

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Last updated (dd/mm/yy) 09.12.2004.