The problem of tissue substitutes from a mechanical point of view

A biomaterial is a synthetic material used to replace a part of a living system or function in intimate contact with living tissue. Biocompatibility is important for the successful acceptance of the artificial implant by the surrounding tissues and the body as a whole. The properties of biomaterials change with the level of loading. The mechanical properties of connective biomaterials and bone tissues differ. Bone tissues that are mineralized have the greatest mechanical strength and stiffness.

History and the present
The use of biomaterials was impossible until the development of aseptic surgical technique in the 1860s, as earlier surgical procedures had failed due to infection. Infection problems can be exacerbated because the implant sometimes makes the area inaccessible to the body's immunologically competent cells.

Another reason was the ill-conceived mechanical constructions (they were too thin and had tension concentrators in the corners). Materials like vanadium steel that had good mechanical properties but corroded quickly in the body. In the 1930s, the situation improved with the introduction of stainless steel and Co-Cr alloys.

Corrosion minimization

 * use suitable metals resistant to corrosion,
 * prevent implantation of different types of metal in the same area,
 * minimize voids and crevices during construction,
 * during production, use the same construction of the same variant of the given alloy on the part,
 * prevent the transfer of metal from the instrument to the tissue or implant during surgery,
 * admit that a metal that resists corrosion in one body environment may corrode in another part of the body.

Modern implants are often made from several types of biocompatible materials. In addition to metallic materials such as titanium alloys, cobalt - chromium alloys or stainless steel, polymer materials are widely used in implantology. They are used for structural elements replacing the original bone tissue.

Biocompatible materials
At the beginning of every technical application aimed at improving the quality of life of patients, there is a long and demanding development. The whole multidisciplinary process consists of finding suitable materials, structural arrangement and medical procedures.

The input material affects the properties of the implant. The characteristics of the material are influenced by its production process and heat treatment, when the same type has different mechanical properties. Another area of ​​the process that affects function and properties can be machining operations, where powerful computer-controlled machines are used to guarantee precision and repeatability of implant manufacturing. However, the most important area is the final surface treatment of the individual components. These processes qualitatively change the basic characteristics of the used basic material.

The spectrum of materials used to replace part of a living system is very wide, from fully synthetic to materials based on completely natural resources, in various forms, from nanofibers to porous stem cell carriers to composites replacing parts of teeth or entire sections of the dentition. New materials can be fully or partially absorbable, non-toxic for the organism or non-absorbable, so-called passively biocompatible.

For example, valve replacements, or prostheses, are differentiated according to their composition into mechanical ones, which are made of alloys of noble metals and plastics, and biological valves, or bioprostheses, the basis of which is biological tissue modified by a complex procedure, originating from an animal. Both types of valve prostheses have a number of advantages, but also disadvantages. Mechanical prostheses are not subject to wear, but require permanent anticoagulant treatment, which, on the other hand, is not permanently needed for bioprostheses; but they are subject to wear and tear and therefore have a limited lifespan (roughly 10-15 years). The operator individually selects the optimal type of prosthesis for the patient.

Requirements for the implats

 * tight approximation of physical properties - flexibility and texture,
 * undistortable and unalterable properties after implantation,
 * no counter-reaction of the tissues,
 * non-carcinogenic, non-toxic, non-allergenic,
 * sterilised,
 * static and dynamic strength,
 * simple design in terms of implantation and reimplantation,
 * ability to absorb energy and absorb shock,
 * contact surfaces are not subject to excessive wear and are characterised by low friction.

Principles of inoperation
The most important thing is to indicate the implant correctly so that it brings relief to the patient. Biological acceptance of the implant involves the reactions occurring between the implant and the surrounding tissues. These are mainly determined by the specificity of the materials, biomechanical and biological properties. The type of biomaterial used and its biomechanical and biological properties influence the survival time of the implant in the body.

Oscillating saws are still used for orthopaedic surgery. However, bone is very sensitive to the effect of high temperatures on the body. The consequence of exceeding critical values' is irreversible damage to bone tissue. Conventional tools, such as saws and other instruments that use a rigid member in machining bone, generate temperatures in some cases in excess of 100°C. These thermal traumas are responsible for the lack of bone tissue ingrowth into the prosthesis, which prolongs the healing and recovery process.

Wear on the implant
The primary limiting factor in implant life is abrasion, caused by movement between opposing components in workload. The mechanical consequence of abrasion is progressive wear and thinning of the articulating surfaces. The wear products of the implant cause adverse tissue reactions that can lead to significant bone loss in the vicinity of the implant and subsequent loosening of the fixed endoprosthesis.

Výše uvedený důvod je indikací vyžadující tzv. reimplantační revizní operaci, během níž je uvolněná endoprotéza operačně vyměněna za revizní endoprotézu. Reoperace jsou poměrně náročné a finančně nákladné, často s neuspokojivými výsledky. Počet revizních operačních zákroků cementovaných a necementovaných endoprotéz neustále narůstá.

Bacterial infection
The aforementioned reason is an indication requiring a so-called reimplantation revision surgery, during which the loose endoprosthesis is surgically replaced with a revision endoprosthesis. Reoperations are quite demanding and costly, often with unsatisfactory results. The number of revision surgeries for cemented and uncemented endoprostheses is steadily increasing.

In the post-traumatic and post-operative period, the risk of infection is high. Inflammations, abscess and sepsis may occur. These health complications are caused by the production of enzymes and enterotoxins, cytotoxins. The infections are treatable with Antibiotics, but 80% of the strains are resistant to penicillin. Deep inflammation is treated with surgery and antibiotics.