Lumbar fusion cage is a commonly used surgical instrument to solve patients' lumbar instability and other problems, from the first generation of metal interbody fusion cage was invented, a variety of different materials of instruments are still in the process of innovation, but various materials of fusion cages have their shortcomings. Metal is the traditional material of interbody fusion cage, mainly titanium alloy, but also cobalt or magnesium alloy and other materials. The former is widely used, with sufficient strength, toughness, strong fracture resistance and good histocompatibility. However, the large elastic modulus of titanium causes the problem of stress shielding. At the same time, titanium cage has a high incidence of cage subsidence, and the cost is high. The magnesium metal fusion cage could also belongs to the absorbable material and has a suitable elastic modulus. The degraded magnesium can be excreted from the body through urine, but the biodegradation rate of magnesium alloy is too fast. It may not be able to provide sufficient support before the bone fusion fully reaches the load-bearing standard. Metal interbody fusion cage will also affect the imaging judgment, so it is difficult to evaluate the specific conditions after interbody fusion in detail.
The PEEK interbody fusion cage which was born at the turn of the century is also widely used at present. Compared with metal fusion cage, its biggest advantage is that it has good ability to pass through X-ray, good compatibility to MRI, biological inertia and elastic modulus are also more suitable. The clinical study of titanium alloy fusion cage shows that PEEK material has higher fusion rate and lower sedimentation rate. However, due to the halo effect caused by poor osseointegration, the practical application of PEEK is still limited in some cases(2). Recently, in order to integrate the advantages of these two kinds of fusion cages, a titanium-coated PEEK interbody fusion cage has emerged, which not only does not curb the early osseointegration, but also facilitates imaging judgment. It also responds to the problem of surface inertia of metals or polymers. Recent clinical studies have found that this kind of fusion cage can significantly improve the rate of vertebral fusion in the short term (3) (4) (2).
In addition to the two mainstream fusion cages, absorbable, degradable and biological interbody fusion cages are also developing rapidly, and some of them are used in practical clinical work. In addition to the magnesium mentioned above, the absorbable fusion cage is mainly made by polylactic acid and polyglycolic acid, which can be degraded with metabolism in the body. The ideal absorbable fusion cage has good mechanical strength and suitable elastic modulus, and the degradation rate should match the bone fusion speed. However, after degradation, polylactic acid fusion cage is easy to appear local acidic environment in the surgical area, leading to inflammation or other problems. In addition, there are carbon fiber and other polymer materials, high hardness, light weight, not easy to cause vertebral body subsidence, but poor toughness, easy to fracture inside of the body. These material innovations of physical and chemical modification to improve bone bonding ability are still under continuous testing and development(4) (5).
The calf bone interbody fusion cage is a new type of fusion cage developed in the past few years, which is made of 10–20% of the total protein removed from calf bone. it has sufficient strength to support spinal pressure and has the structural characteristics of cancellous bone, which provides a good environment for bone fusion. The elastic modulus of deproteinized calf bone is close to that of human bone tissue, so it is not easy to have problems such as lumbar subsidence and stress shielding in theory. And the surface of calf bone material is not inert, so it has strong adhesion ability with surrounding bone tissue in vivo. The results of this study show that, from the point of view of postoperative patient orientation, calf bone interbody fusion cage has no worse therapeutic effect than metal fusion cage, which can effectively relieve pain and improve the quality of life of patients.
Inevitably, this study also has some limitations. This new type of calf bone interbody fusion cage has not been widely used, so the long-term effect of this device has not been verified. The longest follow-up time in this study is 2 years. Secondly, this study can only prove on the level of patients' self-perception that the new fusion cage can achieve the same therapeutic effect as the metal interbody fusion cage after lumbar degenerative diseases. As for the other theoretical advantages of this fusion cage, such as the incidence and degree of vertebral subsidence, stress shielding to new bone and other (long-term) problems, still need further follow-up, observation and control study to draw a conclusion. Considering that there are some differences in the structure and fineness of the operation area in different parts of spine, the shape, size and material of the most suitable interbody fusion cage in different parts are not the same. In addition, this study did not refer to the fusion rate, which is the key parameter to evaluate the nature of the interbody fusion cage, and did not collect the imaging data of the patients with calf bone fusion cage. it is difficult to objectively compare the advantages and disadvantages of the two kinds of fusion cages from the actual vertebral body stability, interbody fusion rate, sedimentation rate and other parameters.