What are orthopedic internal fixation materials?

Orthopedic internal fixation materials are therapeutic devices used to treat bone diseases. Orthopedic internal fixation materials can effectively repair injured bone tissue and can also be used to fix bones. Generally, orthopedic internal fixation materials are made of imported metal fixation equipment and can be implanted in human bone joints to achieve the effect of treating orthopedic diseases without causing much harm to the body.

What are orthopedic internal fixation materials?

Currently, metal internal fixation materials still play an important role in assisting in repairing or replacing diseased or damaged bone tissue. The high specific strength and fracture toughness of metal internal fixation materials make them suitable as fixation devices. Commonly used orthopedic internal fixation materials include stainless steel, titanium, cobalt-chromium alloy, etc. However, these metal biomaterials may release toxic metal ions and/or particles during corrosion or wear, leading to an inflammatory cascade, a reduction in biocompatibility, bone dissolution, and even implant failure. In addition, their elastic modulus does not match well with natural bone tissue, resulting in a stress shielding effect, inhibiting new bone formation and reconstruction, and reducing the stability of the implant. If they can exist permanently in the body, they need to be removed through a secondary surgery, which increases the patient's pain and medical expenses. In addition, its degradation rate is difficult to control, and the degradation products can easily cause sterile inflammation.

Magnesium is a light metal with a density of about 1.74g/cm3 (only 2/3 of aluminum alloy and 1/4 of steel). Its fracture toughness is much greater than that of ceramics. Compared with other commonly used metal implants, its elastic modulus and specific strength are closer to those of natural bone. Magnesium ions can also stimulate the formation of hard callus at the fracture end, induce osteogenesis, promote fracture healing, and stimulate cartilage formation. In addition, magnesium is a cofactor for many enzymes and stabilizes the structure of DNA and RNA.

Zreiqat et al. found that the expression of type I collagen was also significantly increased, and the expression of integrin α5β1 and β1 ligand was also significantly increased, suggesting that magnesium ions can promote the proliferation and adhesion of osteoblasts. Mg2+ is key to human metabolism and exists in natural bone tissue. It ranks fourth among human cations. A normal 70kg adult contains about 1 mol, of which about half is present in bone tissue. The level of magnesium in extracellular fluid ranges from 0.7 to 1.05 mmol/L and is maintained in balance by the kidneys and intestines. When the serum magnesium content exceeds 1.05mmol/L, it can cause muscle paralysis, hypotension and respiratory distress. When it is as high as 6-7mmol/L, it can cause cardiac arrest. However, the incidence of hypermagnesemia is relatively rare because it is often excreted in the urine. Magnesium corrodes in the body's electrolyte environment to form a soluble, non-toxic oxide that is excreted in the urine.

Therefore, magnesium and its alloys, as degradable, load-bearing orthopedic implants, can maintain mechanical integrity after a 12-18 week bone healing period and will eventually be replaced by natural tissue. However, human body fluids are more complex than the natural environment. The chloride content in body fluids is about 150 mmol/L, and other anions also have a corrosive effect on magnesium. Pure magnesium will quickly degrade and corrode in a physiological pH (7.4-7.6) and high Cl- environment (the corrosion reaction is: Mg(s)+2H2O→Mg(OH)2(s)+H2(g)(1); Mg(s)+2Cl-(aq)→MgCl2(2); Mg(OH)2(s)+2Cl-→MgCl2(3)). Magnesium implants lose their mechanical integrity before the fracture is completely healed, and the rate at which hydrogen is produced during the corrosion process exceeds the processing speed of the host tissue.

Despite some early successful attempts, this metal implant has not yet been widely used in clinical practice. However, there are still some methods, such as applying alloying elements and protective coatings, to reduce its corrosion rate. Of course, these treatments must also use non-toxic and biocompatible materials.