The surface composition of a biomaterial can have an important influence on biologic responses. In this paper we report on a surface treatment using a gas discharge which deposits a thin fluorocarbon polymer coating onto tie surface of a synthetic vascular graft. The surface chemistry of the graft is significantly changed, while there is no measurable change in porosity, compliance or surface topography. Treatments with tetrafluoroethylene (TFE) gas yield dramatic improvements in both thrombo and emboli-resistance of the graft, based on in vitro measurements and ex vivo shunt tests in a baboon.

One of the most successfully applied classes of biomaterials is the porous textile vascular prosthesis. Experimental and clinical investigations have been continuously conducted during the past thirty years to study tissue healing, clinical longevity and improved design of these prostheses. The vascular device retrieval program has enabled classification of the 40 or more identified mechanisms of failure of arterial prostheses into the following major groupings: A. Failure of diagnosis B. Error in implantation C. Patient contribution D. Complications of healing E. Fabrication failure F. Infection

Studies of the pathology of explanted clinical and experimental valve specimens were done to determine the role of biomaterials in the failure of heart valve prostheses. From 7/80 through 12/84, 129 patients had valve prostheses removed at our hospital. Thrombosis/thromboembolism was the most frequent cause of the 39 mechanical valve failures (23%). Structural failure (poor durability) caused 12%. In contrast, with tissue-derived bioprostheses (90 patients), primary tissue degenerative failure was frequent (70%), and was usually associated with calcification (CALC). Infection caused 19% of all valve failures with a frequency not differing among valve types. Other causes of failure were not biomaterials-related. Thus, although frank materials degradation necessitates only a minority of mechanical valve removals, the suboptimal biologic properties of contemporary heart valve biomaterials contribute to some frequent complications (such as thrombosis/thromboembolism). Furthermore, degeneration due to mineralization is the overwhelming mode of failure of bioprosthetic valves.

To understand and potentially eliminate bioprosthetic valve CALC, the kinetics, morphology and host and implant determinants of this process were studied using subcutaneous implants in rats. In this model, in which the pathologic features were similar to those of actual experimental and clinical valve replacements, CALC of glutaraldehyde-treated porcine aortic valve and bovine pericardium began within 48 hours following implantation; mineral levels were comparable to those in failed clinical valves after 84–126 days. Calcific deposits were initially associated with connective tissue cells, but later involved collagen. Systemically-administered ethane-hydroxy-diphosphonate (EHDP), a drug used to treat some forms of metabolic bone disease, inhibited CALC by 97%, but caused significant bone growth retardation. However, EHDP given by site-specific controlled-release largely mitigated CALC at only 1% of the total effective systemic dose, but without adverse effects.

For the biomaterials scientist, the eye presents unique problems, principally because the maintenance of optical function is paramount. This paper concentrates on some current challenges in the development of devices to be placed in the optical path: from air to tear film, cornea, aqueous humour, pupil, lens, vitreous humour, and finally to the retina. Optical clarity during contact lens wear depends on control of deposits from tear film components (by patient performed cleaning procedures) and on continuous oxygen supply to the cornea whose clarity depends on intact epithelial and endothelial cells. Intracorneal lenses of contact lens material are possible if intracorneal fluid flow is not compromised. Corneal integrity can also be affected by adhesive endothelial cell damage during intraocular lens implantation, a factor leading to the development of so-called visco-elastic media for anterior chamber use. The associated questions of photodegradation of materials and protection of the optical tissues from photo-toxicity are critical in the eye. Laser surgery is also uniquely valuable in ophthalmology since many procedures can be performed without invasive access. Finally, intraocular lenses can also incur protein adsorption, cellular adhesion, and opacification of the lens capsule which compromise optical performance.

Implants are essential for the repair of retinal detachments. The implant buckles the wall of the eye and apposes the detached retina with the choroid, thus restoring light sensitivity to the retina. The scleral buckling also relieves traction on the retina from a shrinking vitreous body. The implant materials most commonly used are solid silicone rubber and silicone sponges, but both types have some disadvantages. A poly(hydroxyethyl acrylate-co-methyl acrylate) hydrogel implant with improved properties of softness and antibiotic absorption is also available for retinal detachment surgery. Proliferative vitreoretinopathy involves various conditions of retinal detachment complicated by vitreous fibrosis, which, after vitrectomy, may be treated with intraocular injection of fluids that support the retina against the choroid. For conditions requiring a long-term implant, silicone oil although controversial is the material of choice. Many other substances have been investigated but none better has yet been found.

Biomaterials can be classified in terms of their interfacial response with tissues: 1) nearly inert, 2) nearly inert microporous, 3) resorbable, and 4) surface active. Surface active biomaterials include bioactive glasses such as Bioglass®, bioactive glass-ceramics such as Ceravital® or A/W Glass-Ceramic, hydroxylapatite such as Calcitite® or bioactive composites such as Palavital® or stainless steel fiber reinforced Bioglass®. All of the above surface active biomaterials form an interfacial bond with adjacent tissue. However, the time dependence of bonding, strength of bond, mechanisms of bonding, and thickness of bonding zone differ. Depending upon composition, type of tissue, and type of load applied, and explanation for the compositional dependence is presented together with various medical and dental implant applications, including middle ear prostheses, maxillofacial reconstruction, endosseous ridge maintenance and augmentation, periodontal packing, and fixation of orthopedic joint prostheses.

Sintered ceramic, nonresorbable hydroxylapatite (HA) is a calcium phosphate biomaterial widely employed alone for augmentation or maintenance of the mandibular ridge and for filling alveolar bony defects. The small particles used for ridge augmentation or periodontal defects tend to scatter and the larger blocks used for extraction sockets don't fit well. Our recent studies suggest that these problems can be circumvented by composite plaster of Paris/HA implants. In addition to binding the HA particles, the plaster makes the implants easier to shape. Plaster is not only very biocompatible but may actually help prevent infection. It appears to act as a scaffold for incorporation of the HA particles into osseous tissue. In several days the plaster is resorbed but it is replaced at the same rate by infiltrating fibrovascular tissue; plaster definitely promotes the filling of osseous defects initially by connective tissue. This tissue not only maintains the form and integrity of the implant, but its vascular and collagen components promote healing and contribute to subsequent filling of the defects and approximation of the HA particles by new cancellous bone. These composite implants can be preformed on casts prior to surgery or tailored during the surgical procedure. Their use precludes the difficulties encountered when plaster alone is implanted (resorption) or when HA alone is used (particle scatter or nonpenetration of the material by host vascular and connective tissues). By having a resorbable component which permits penetration by fibrovascular tissue and a nonresorbable component which can act as a nidus for new cancellous bone formation, these composite alloplasts act like artificial bone. Their use to support a denture, or for immobilizing teeth and reducing pocket depths in advanced periodontal disease, gives better results than HA alone.

The fracture properties of selected commercial composite dental restorative materials and a model composite system were studied to determine the influences of the reinforcing phase, exposure to water, and particle/polymer adhesion on crack propagation. The content of inorganic fillers ranged from 36 to 62 volume percent. In the model system the polymer phase approximated that of the commercial products, a constant size distribution of quartz fillers was used, and polymer/particle adhesion was varied. The double torsion test method was employed to measure relationships between applied stress intensity factor and velocity of crack propagation during stable crack growth. In all systems, cracks propagated through regions of high stress concentration at the low end of the velocity range studied (10−7 m/sec to 10−3 m/sec). Wet materials fractured at lower stress intensities than dry materials at all velocities. At high velocities unstable (stick-slip) growth occurred in dry materials with strong filler/matrix interfaces and in wet specimens with initially strong interfaces and less than 41 volume percent filler. In wet conditions, materials with poorly bonded fillers fractured by slow crack growth at stress intensities 10% to 30% below the levels of composites with strong interfaces.

The best possible implantable biomaterial for bony contour abnormalities is autogenous bone grafts. However, some conditions may be present that require use of alloplastic materials. Such alloplastic material must be inert and there should be no reaction in the body to it. The composite graft described in this chapter is a combination of the two different materials described. To have a better take, the bone graft used is ground with a craniotome, and then mixed with blood. This produces an inductive matrix that promotes osteoneogenesis. The alloplastic material is a tray or a template where the bone is placed “in”, “on”, or “under” the implant to allow for complete solidification and better healing. The material used is polyetherurethane-impregnated terephthalate (Dacon) also referred to as osteomesh (R) (Xomed Corporation, Jacksonville, Florida). In a few instances the osteomesh was removed and we noted complete solidification of the bone (resembling skull in the case of the skull defect). For over eleven years, since the success in the experimental animal [1], this composite graft was used in clinical situations with good success [2] in over 100 patients. These patients treated were selected and a strict set of indications and contraindications were instituted and implemented, prior to the applications of this treatment in the individual patient.

This review is intended to place in perspective the requirements for biocompatibility of the merging high-technology materials and to describe the mechanisms by which these materials might interact with the tissues of the body.

Clinical treatment modalities have undergone very significant changes over the past 30 years, in part because of the development of new or improved synthetic substances (biomaterials) for the replacement of tissues. Most all aspects of medical care have been directly influenced. It is now possible to obtain biomaterials made from basic metallic, ceramic, carbonitic, or polymeric materials where the substance has been manufactured for a specific biomedical application. In past years, this was not possible and available industrial grade materials were often adapted or modified to accommodate applications in biological environments. These developments, along with the many other advances in both biomedical sciences and clinical applications, have provided an exciting situation with regard to the improvement of existing prostheses and materials or the development of new biomaterials for surgical implant devices. This presentation will summarize eight areas where defined needs for active biomaterials research continues, following an outline theme of: (1) the current clinical situation; (2) specific area of need; and (3) possible improvements through basic science and clinical research.

This thematic presentation will consider the following general areas: (1) the minimization of interfacial wear and implant biodegradation with examples of total joint replacements and occlusal surfaces of teeth; (2) adhesives for soft and hard tissues with examples of total joint replacements, cements, cardiovascular and plastic surgery reconstructions; (3) percutaneous devices for orthopaedic, dental, cardiovascular, and general surgery; (4) bone and skin replacements for the treatment of major bone lesions and burns; (5) ligaments and tendons for joint and hand reconstructions; (6) spinal instruments for orthopaedic procedures; (7) conduits of various diameters for general and cardiovascular applications; and (8) valves for cardiovascular and urological procedures.

The material factors which relate to the degradation and/or leaching of ions or molecules are described and the possible biological pathways which they may activate are described, i.e. cytotoxic, immune, tumor and nonspecific inflammatory response. Cytotoxicity is the only biological response which may be measured with high sensitivity by an in vitro assay prior to implantation. All other biological pathways require some degree of in vivo involvement. Three examples of biological response to material factors associated with devices which require evaluation by in vivo assays are discussed, namely: surface topology (texture), mechanically induced factors at the device/tissue interface caused by differences in compliance, and dystrophic calcification in connective tissue and vascular devices.

This article reviews the problem of ultrahigh molecular weight polyethylene component surface degradation in total hip and total knee prostheses, including its clinical implications. Several factors affecting surface damage have been identified from a combination of the observations of in-vivo degradation made on retrieved components, experimental measurement of contact stresses on polyethylene components as a result of contact with their metallic counterpart, and analytical studies of both contact stress and the stresses beneath the polyethylene surface. Both the observations of in-vivo surface degradation and the analytical studies demonstrate that surface degradation is a more severe problem in total knee replacements than in total hip replacements. The performance of polyethylene components, as affected by design factors such as material thickness and material modification, is also considered.

Owing to the interest in ever increasing lifetimes for total hip prostheses, possible failure due to aseptic loosening of the acetabular component is an issue that may require increased attention. Mechanical factors, especially the deformation behavior of the material and the stress distribution at the implant-cement and the bone-cement interfaces, presumably play an essential role in total hip failure. Therefore the experimental model of the present work makes use of rosette strain gauge techniques to evaluate the deformation behavior of cemented polyethylene cups under quasi-static loading. Standard tests in UHMWPE beam specimens confirmed that the present method is a reliable tool for strain measurements in the polymer that was tested. On the other hand, the apparent elastic modulus of UHMWPE can be shown to be markedly dependent on the testing method, the geometry, and, owing to the visco-elastic properties, dependent on the strain rate. Thus in the present work a correlation between the measured strain and the ultimate strength was not attempted. Instead a parametric analysis quantifying the influence of the cup thickness and the cement distribution on the strain pattern was performed.

Loosening is the dominant long-term problem facing joint replacement surgeons and patients. A probable cause of endoprosthesis loosening is the strain singularity at the material interfaces. The concentration of shear at the bone-cement interface leads to micromotion which precipitates a soft-tissue membrane and resorption of the cancellous bone.

A more compliant cement would substantially reduce the interfacial stresses and serve as a “pillow” between the prosthetic stem and the cancellous bone. We have developed a surgically-workable formulation of a reduced modulus acrylic bone cement — polybutylmethylmethacrylate (PBMMA) — to test this hypothesis. Materials property testing and in vivo implantation are discussed.

Fatigue phenomena of high-density alumina ceramic hip joint endoprosthesis with ceramic-to-ceramic articulating components were analyzed for retrieved ex-vivo objects and for in-vitro tests in hip-joint simulators. Scanning electron microscope investigations were employed. The cyclical compression between ball and cup causes severe shear stresses to form below the bearing surfaces. Locally changing subsurface maxima of contact stresses can generate fatigue cracks starting at subsurface microstructural defects and grain boundaries. The fatigue fractures are regarded as being identified with the tribological surfaces of the simulator specimens and of the ex-vivo prostheses retrieved because of fracture.

It is known that bony ingrowth implants without intimate apposition results in diminished ingrowth. In this study the use of bone morphogenetic protein (BMP) and cancellous bone graft to augment ingrowth in interface voids is investigated in the canine. Eight CoCr surface replacement hip implants of chamfered cylinder design with beaded CoCr porous layer were implanted in four adult canines and followed from 23 to 56 days. Prior to implantation, cylindrical defects 4mm diameter by 3mm deep were created in the femoral head and acetabular surfaces and filled with 10mg of BMP in one hip and cancellous bone graft in the contralateral hip. Animals were followed with serial radiographs to sacrifice after which microradiographs of ground sections and histologic specimens were analyzed. All defects were 100% filled with viable new bone without differences in density, organization, or ingrowth in the BMP and cancellous specimens. Femoral specimens uniformly showed unexpected seating gaps which were bridged to varying degrees with viable new organized bone. In five femoral cases seating defects of 2mm or more were completely bridged with ingrowth at the porous layer. In one acetabular case a 6mm gap was 90% bridged. No correlation of results was noted with time to sacrifice. It was concluded that new bone present in the seating gaps was most likely due to spillage of material from the cylindrical defects. Although the amount of ingrowth required for durable fixation is unknown, incongruencies of implant fit are common. As BMP showed no discernible advantage over cancellous graft in results of this study we advocate liberal use of readily available cancellous graft (from reamings or femoral head) to fill interface voids.

A stemmed bone ingrowth hip resurfacing arthroplasty designed for use with bone deficient femoral heads raises concerns of stress shielding and is investigated in this study. Ten CoCr bone ingrowth hip resurfacing devices were implanted in five adult canines. Design consisted of a 6mm deep cylindrical cavity with porous beaded surface of 150um pore size and Imm nominal depth. A 3mm diameter smooth CoCr stem was eccentrically fixed to the component for additional fixation with threaded nut at the lateral cortex. Femoral head was reamed for interference fit with hole for the stem placed superiorly. Acrylic fixed polyethylene cups were used for acetabular components. Animals were followed with serial radiographs to sacrifice at 73 to 267 days. After sacrifice microradiographs of ground sections and histologic specimens were evaluated for bone ingrowth, remodeling, and femoral neck attenuation. All specimens showed well ingrown bone at the porous layer. Bone remodeling showed preservation of proximal femoral neck cancellous trabecular pattern and medial cortical support. Resorption of bone at the implant rim and femoral neck thinning were, however, present. Two specimens had neck thinning of 10% or less, four of 10 to 25%, three of 25 to 50%, and one greater than 50%. There was no correlation with time to sacrifice for parameters evaluated. Results indicate preservation of the compressive stresses through the femoral head. Resorption of bone at the rim and femoral neck attenuation, however, indicates load sharing between the implant and bone. Though a lower modulus material such as titanium would decrease load sharing, the geometrical design insures the presence of stress distribution anomalies.

Metal alloys, ceramics, and thermoplastics are commonly used as materials for endoprostheses, especially artificial hip joints. Prostheses made of these materials still retain disadvantages which have been responsible for failures in the past. A new material, without the abovementioned disadvantages, has been developed. It is a carbon fiber reinforced thermo set based on the triazin resin. Its principle mechanical properties can be matched with those of bone. The new material (referred to as TCF) has a higher bending and fatigue strength than cast metallic alloys if endless fibers are applied. Its creep rate at 37°C under high load (3000 N), as well as its wear rate against metallic or ceramic wear partners, is negligible. Experiments with animals based on ten years of experience show good osteocompatibility of the TCF. Toxic reactions of the tissue caused by the TCF implants themselves, or by fine wear debris found in membranes of connective tissue (or lymph nodes), were not observed at all. Tissue reacts on large particles of TCF wear debris in the same manner as it does on particulates of the same size of polyethylene. It can be expected that the favorable biomechanical and biological properties of the TCF material will make it a good alternative to other materials already in use for construction of endoprostheses, especially hip joints.

Commercially pure titanium type T5 and titanium type TAl alloy have been used at Stanmore in major bone and joint replacement prostheses since 1963 and 1971 respectively. Because laboratory studies indicated cast CoCrMo alloy to be a superior bearing material both titanium materials have been joined to cast CoCrMo alloy bearing elements to form composite prostheses with no apparent adverse clinical effects.

Laboratory studies and clinical experience have shown the inadequacy of the fatigue properties of titanium type T5 and cast CoCrMo alloy for highly stressed intramedullary stems. Nevertheless, although titanium type TAl alloy has superior fatigue properties highly stressed components of this material are not immune to fatigue failure.

Wear of U.H.M.W.P.E. appears to be greater when articulated with titanium type TAI alloy than it is when articulated with cast CoCrMo alloy and Al2O3 counterfaces. In addition, capsular tissue which rubs on titanium type TAl alloy can become heavily pigmented. However, to date no clinical complications appear to have resulted from either of these shortcomings. Whether a surface treatment for titanium type TAl alloy such as ion implantation will both improve its wear characteristics with U.H.M.W.P.E. and reduce the amount of metallic material liberated into the soft tissues has yet to be established.