Reconstructive surgery
12.2.5 Reconstructive surgery
12.2.5.1 Plastic surgery
The many different biomaterials used in plastic surgery include collagen, silicone (polydimethyl- siloxane), Teflon, polyethylene, Dacron and polyglocolic acid. Polysiloxanes are widely used in reconstructive plastic surgery because of their lack of any tissue reactivity, mechanical properties and structure. Polysiloxane may be either heat vulcanized or vulcanized at room temperature for more delicate structures and is ideal for soft tissue replacement where repeated flexure occurs. In applications where strength is also required, steel-wire reinforcement is necessary. The material is used not only for implants but also for explants, where it is molded to fit a specific shape, colored to give a skin match and fixed with tissue adhesive. Common examples are ear prosthesis or facial reconstruction after loss due to injury or cancer surgery. Silicone mammary implants are now widely used for cosmetic reasons or after mastectomy for breast cancer. These have an outer shell with the appropriate shape and resilience covering an inner volume of silicone gel. In these reconstructions Dacron may be used as a lining for the ear and as a backing in a breast implant to provide a better fastening of the implant to the surrounding tissue.
12.2.5.2 Maxillofacial surgery
Disease or injury to the oral and facial area may be repaired by implants from a range of biomaterials. Often the effective prostheses can improve the confidence and well-being of the patient beyond the immediate functional repair. The jaw, and jaw bone area, can utilize many of the metal implants already discussed. Commercially pure titanium in perforated sheet form has been used because of its biocompatibility and ease of manipulation and fixing.
Polymers, particularly silicones and polyurethanes, may be used to replace flexible tissues of the nose, cheek and ear regions of the face. Polysiloxanes have been used for onlays in the area of the molar bone in the lateral side of the mandible or over the forehead to smooth it. Reinforced with stainless steel wires, they can also replace the mandible. A porous composite of polytetrafluoroethylene (PTFE) strengthened with carbon fibers may be used to replace damaged bone structures. Both PTFE and carbon are biocompatible, and fibrous tissue growth into the pores ensures bonding of the artificial and natural bone. Composites of hydroxyapatite in a polyethylene matrix HAPEX have been used for patients who had either fractured the orbital floor supporting the eye or had lost an eye. A great advantage of such material implants is that they can be shaped during the operation and inserted on the base of the eye socket and bonded firmly to the supporting bone.
594 Physical Metallurgy and Advanced Materials
12.2.5.3 Ear implants
HAPEX has been also used successfully for other clinical replacements, mainly middle-ear implants. These transmit sound from the outer to inner ear, where the vibrations are translated into electrical sig-
nals to be processed by the brain. Middle-ear malfunction can lead to deafness (conduction deafness) which may be cured by implant surgery. Otosclerosis (middle-ear deafness) may result from fibrosis of the middle ear caused by repeated infections or from an hereditary disease. A stapedectomy removes
a small amount of bone and immobilized tissue and replaces it with biomaterial. The implant has a hydroxyapatite head on a HAPEX shaft which can be trimmed and shaped in the operating theatre using an ordinary scalpel to fit the individual patient.
12.2.5.4 Other surgical applications
Polymers are also used in a wide variety of general surgical applications. Polyurethane has good tissue and blood contact properties and is used in both short-term applications, e.g. catheters, endotracheal tubes, vascular tubing, hemodialysis parts, and long-term applications, e.g. heart-assist devices. Dacron is used in composite form with a poly(2-hyroxyethyl methacrylate) matrix for orthopedic tendon reconstruction. Varying the composite mix can alter the properties to match the requirements. Reinforced Dacron fabric is used for reconstructing the trachea and, in woven Dacron form, for small bowel repair or replacement. It is also used in the genitourinary systems, in mesh form, in repairing hernias and abdominal wall defects. Polysiloxanes are used in neurological surgery, e.g. in valves to drain fluids produced intercranially and also as tubes to drain other canals such as the middle ear. Not having a porous structure, these tubes resist bacterial contamination.
Shape–memory–effect (SME) alloys (see Chapter 8, Section 8.6.4), particularly Ni–Ti, have been used in several biomedical applications because of their unique behavior and for their biocompatibility. In orthopedics, for example, pre-stressed fracture bone plates can be made to shrink on heating to provide a rigid, compressive load fixing. By contrast, Ni–Ti rods can be programmed to provide traction on local heating. In other applications Ni–Ti has been used in artificial heart muscles, teeth- straightening devices, intrauterine contraceptive devices and as a filter in the vena cava (inserted cold the filter opens its mesh at the temperature of the deoxygenated blood flowing back to the heart).
In spinal surgery, commercially pure titanium cables and screws have been used for the correction of scoliosis by gradual tightening of the cable to straighten the spine. A big advantage of Ti for these devices is its resistance to crevice corrosion.