Selection of experimental materials
The purpose of this study was to explore a method for making cast specimens that could facilitate direct observation of the intraosseous artery of the tibia. Human tibia samples are relatively difficult to obtain, so it is more feasible to use animal tibia specimens instead. The selection factors of animal tibia specimens include tibia length, shape, vascular distribution, and caliber size. Throop et al. [11] reported that deer tibias are closer to human tibias in terms of length and morphology than tibias of other animals such as pigs and sheep, and mechanical test results obtained using them are also similar to those obtained using human tibias. They suggested that deer tibia should be used as a standard model for orthopedic implants. However, the report does not mention whether there is any difference in the distribution of intraosseous and extraosseous blood vessels between deer tibia and human tibia, and there is no similar literature for reference. Pigs are often replaced by goats and other animals in in vivo experiments because of their rapid growth and aggressiveness. However, there is high similarity between pig bone and human bone in terms of bone anatomy, shape, and composition [12]. Kotsougiani et al. [13] dissected the hindlimbs of 8 pigs and collected data on the source, length, and caliber of nutrient arteries in their tibias. The results they reported were highly similar to those derived from human tibias dissected by Anetai et al. [9]. Therefore, using porcine tibias as an experimental material to investigate the distribution of intraosseous arteries can help researchers to deepen their understanding of human intraosseous arteries.
Blood supply to the tibia and treatment of fracture
After studying the anatomy of the lower limbs of fresh cadavers, Nelson and Kelly [14, 15] proposed that the blood supply to the tibia can be divided into three parts; the epiphyseal artery, nutrient artery, and periosteal artery. After entering the medullary cavity, the nutrient artery is divided into ascending and descending medullary artery trunks. The branches of the medullary artery near the inner surface of the bone cortex enter into the bone cortex [16]. Part of the capillary network radiating from the ascending and descending branches can pass through the bone cortex, and anastomose with the capillaries of the periosteal artery [17]. In the current study the structure of ascending and descending medullary arteries in pig tibia was similar to that in humans. The peripheral branches entered the bone cortex, and some of them penetrated the bone cortex and anastomosed with the periosteal artery to form a circular structure. The nutrient artery plays an important role in the blood supply of the tibial shaft. Levack et al. [18] performed quantitative magnetic resonance imaging (MRI) of 8 fresh frozen lower limbs and confirmed that the bone cortical area of the tibial shaft was dominated by intraosseous blood supply. If the intraosseous blood vessels are injured, the probability of nonunion after fracture is increased. At present, the commonly used clinical tibial fracture fixation instruments can easily cause damage to intraosseous blood vessels. According to a retrospective study of 105 patients with tibial fractures treated with external fixators by Almansour et al. [19], at least 38% of the patients had damaged tibial nutrient arteries, which may be conservative given that damage to the nutrient arteries itself cannot be detected by computed tomography. This view is also supported by Brinker et al. [20], who reported a similar situation in patients treated with tibial intramedullary nailing. After recognition of the importance of blood supply for fracture healing, existing instruments have been redesigned to avoid damaging intraosseous blood vessels during implantation; but in practice, considering factors such as operation time and firm fixation of the fracture, surgeons still tend to use methods they are familiar with in an effort to avoid intra-operative complications. There is now a consensus among most surgeons that retaining the residual periosteum as much as possible during the operation is desirable [21], but it is difficult to popularize and implement the concept of protecting intraosseous vessels.
Significance of this study
The study of intraosseous vessels of the tibia has been challenging. Because of the hardness of the bone tissue, it is impossible to fully expose the blood vessels in the bone, as can be done via dissection in other organs. Although angiography, MRI, and other detection methods have verified the existence of intraosseous vessels, this is not sufficient for researchers who want to establish new surgical methods and instruments. They need a visual model that clearly shows the intraosseous vessels.
A method of vascular casting and exposure of the intraosseous artery of the tibia was established in the current study. The popliteal artery was separated and exposed, and after intubation, epoxy resin was injected into the popliteal artery, and the intraosseous vessels of the tibia were exposed by acid-base etching. The origin and 3D distribution of blood vessels in the tibia can be clearly seen, which is useful with respect to understanding diseases caused by impaired blood supply. It also helps doctors to protect the blood supply of the tibia during surgery, and reduce the occurrence of iatrogenic nonunion. There are benefits to understanding the potential mechanisms involved in tibial fracture healing. The transparent tibia model made by 3D printing and epoxy resin perfusion can establish a new standard model of orthopedic implants, facilitating the development of new surgical methods and instruments to avoid damage to intraosseous vessels. If the hardness and density of the model are correctly adjusted to make it closer to a real tibia, this type of model can be tested to simulate fracture and intraosseous vascular injury under various conditions. The tibia model which clearly shows the intraosseous vessels will also give surgeons and medical students a better understanding of the anatomy of the tibia, and play a positive role in promoting medical education.
The future of intraosseous vessels of the tibia
The treatment of tibial nonunion has always been a difficult problem in the field of orthopedics. According to relevant reports, the incidence of tibial nonunion after treatment is 7%–12% [2]. In a retrospective study by Rupp et al. [22], treatment times and costs incurred by patients with tibial nonunion were much greater than those of patients with normal healing. Compared with the huge pressure on patients and medical resources that may be caused by the treatment of tibial nonunion, the prevention of it has more practical significance and value. At present the standard treatment of tibial fracture is to provide a blood supply to the fracture end indirectly, such as via autogenous cancellous bone transplantation, vascular pedicled bone flap transplantation, or periosteal transplantation [23]. There is no surgical method to directly restore blood supply to the fracture end after intraosseous vascular injury. This has a lot to do with our lack of a clear understanding of the distribution of blood vessels in the tibia. A skin flap is a tissue mass with its own blood supply that can survive independently. The earliest application of it in human history can be traced back to the 6th century BC. With the rapid development of super microsurgery in recent years, the clinical application of skin flap transplantation has expanded from plastic surgery to other types of surgery. The caliber of blood vessels that can be anastomosed is constantly being revised downwards. Xu et al. [24] successfully anastomosed blood vessels with a caliber of only 0.2 mm, proving that it is possible to restore blood supply via microsurgery. The current study successfully demonstrated the intraosseous vessels of pig tibia. This method can be used to make human transparent tibia specimens, which will have more research value. With the combination of a microsurgical technique and a skin flap transplantation technique it is possible to establish a new surgical method for one-stage treatment of tibial fracture, and restore a blood supply to the fracture end to prevent nonunion.
Limitations
The present study had some limitations. There is currently no evidence that the distribution of intraosseous arteries in pig tibia is similar to that in humans, so the results of the study can not be directly applied in the clinic. If human tibia specimens can be obtained for study, the results will be more meaningful. Another limitation relates to the fact that the complete intraosseous blood circulation system of the tibia includes arteries and veins. There was no vein perfusion or casting in the present study, though we plan to do this in future experiments. Lastly, the sample size was small, and the anatomical characteristics of the intraosseous artery of pig tibia were not analyzed and summarized in detail. In follow-up experiments we will increase the sample size.