4.1 Domestic and foreign conditions of head preserving treatment for femoral head necrosis
ONFH is a multifactorial disease with multiple pathological mechanisms and ultimately the same outcome, resulting in bone marrow cell ischemia and bone cell necrosis. It is a common and difficult disease in orthopedics [6, 7]. For patients with early ONFH, femoral head preservation is the preferred target. The main methods include core decompression combined with or without bone graft, Vascularized Fibula Graft and allograft fibula transplantation or tantalum rod implantation. The clinical results of core decompression surgery are uncertain, lack of structural support, and lack of long-term mechanical support even in combination with bone grafting, resulting in bone collapse [8-10]. Vascularized fibula transplantation requires free peroneal artery and anastomosis with the external femoral artery of the affected hip, which has the disadvantages of the need for enlarged surgical procedure, high morbidity rate in the donor area, long recovery time and the risk of fractures in the proximal femur [1, 11]. However, allograft fibula transplantation is expensive and may be absorbed and rejected, which will increase the technical difficulty of THRA in the future. The above methods have some defects, such as large trauma and insufficient donor source. However, although the support effect of tantalum rods used recently at home and abroad is clear, the price is high and it is difficult for patients to accept. At present, there is no clear report that tantalum rods can help promote bone repair. Meanwhile, with the in-depth study of tantalum rods, the number of cases of their failure increases gradually and it is difficult to remove the titanium rod [12, 13]. Therefore, finding a minimally invasive method with the ability to repair the bone defect in the necrotic area of the femoral head and the ability to provide immediate and permanent structural support to prevent the collapse of the femoral head is the key to the early ONFH head preserving treatment .
4.2 Advantages of hydroxyapatite coated hollow titanium rod designed to support the femoral head and prevent collapse
The purpose and function of hydroxyapatite coated titanium rod is to provide strong and effective mechanical support in the early ONFH bone destruction area and prevent cartilage surface collapse of the femoral head, and to conduct bone grafting at the core decompression area, which is conducive to the biomechanical recovery and bone fusion of the head and neck, without hindering the long-term hip replacement. Hydroxyapatite coated hollow titanium rod has the following design advantages:①Hollow titanium rod adopts titanium alloy with elastic modulus close to human skeleton, which has good histocompatibility. Compared with tantalum rod, it has low cost and low price, which can reduce the hospitalization cost of patients. ② The hollow design of the titanium rod facilitates the entry and exit of the guide needle. During the placement, the guide needle can be used for accurate positioning to achieve precise titanium rod placement, which reduces the damage to bone tissue during positioning. The hollow design forms a channel in a short time and plays a role in decompression. The smooth inner surface of the hollow is not conducive to bone formation and can achieve long-term decompression effectiveness. ③The smooth rod design of the rod body and the 1/3 circular design of the rod head make the resistance of the rod less. The 1/3 circular design of the rod head corresponds to the circular structure of the femoral head, which can be more closely attached to the subchondral bone of the femoral head. ④The titanium rod can be embedded into the lateral cortical bone by using the thread design of the rod tail. The rod body diameter of titanium rod is 10 mm and the diameter outside the thick thread is 11 mm. This design with small head and large tail combined with the biting force of the thick thread effectively prevents the rod tail from protruding or the rod head penetrating into the cartilage surface. ⑤The overall length of titanium rod is 75~110 mm, increasing every 5 mm, which is in line with the average length of femoral neck in human body, and available for patients of different ages. The end of the rod tail is designed as an internal six-square notch with a depth of 3.5mm. The Embedded bite contact between the tool and the rod tail with a depth of 3.5mm is conducive to the entry or removal of the hollow titanium rod, making the surgical operation easier, less labor and not easy sliding buckle, and according to the custom of patients, most internal fixators are required to be taken out after bone healing, which is also in line with humanized application.
4.3 Necessity and effect of hollow titanium rod coated with hydroxyapatite on supporting and preventing collapse of femoral head
The ARCO stage belongs to the early ONFH, and its radiological and histopathological changes have obviously shown osteonecrosis and cystic changes of the femoral head. If the lesion continues to develop and cannot be effectively controlled, it is bound to cause fracture or absorption of the trabecular bone in the weight-bearing area, and the destruction of bone structure in the subchondral area, that is, the reduction of mechanical strength and supporting force in the local weight-bearing area of the femoral head, which will eventually lead to the collapse of the femoral head and ONFH. Clinical studies have found that after ONFH core decompression and lesion clearance, both the remaining femoral neck decompression tunnel and bone defect in the area of femoral head necrosis can cause the original weak femoral head to lack of bone structure support, resulting in stress concentration and accelerated collapse of the cartilage surface of the femoral head.
With the reconstruction of blood supply, in the process of ONFH repair broken bone and bone resorption is often greater than the speed of new bone formation, although after the lesion clearance on the cavity of the femoral head bone graft, but the implanted bone lacks immediate and permanent mechanical support, continued for a long time in the human body weight under the action of the bone graft only Microsoft support will gradually disappear, further cause and aggravate the collapse of the femoral head . Therefore, after ONFH lesion clearance, bone graft not only fills in the bone defect to restore anatomical tissue structure, accelerates its vascularization to promote bone repair, but also increase the intracavitary mechanical strength of femoral head lesion removal.
Hydroxyapatite coated hollow titanium rod was inserted into the femoral neck of the femoral head and the weight-bearing necrosis area of the femoral head. Hydroxyapatite coating could form a firm bone bond at the interface between the titanium rod and bone, and the irregular fracture zone in the femoral head was stabilized. To improve the biomechanical characteristics of the femoral head neck, provide structural and mechanical permanent support for the subchondral site where the articular surface of the femoral head may collapse. The latter support is conducive to recovery and plays an important role in maintaining the concentric circle structure corresponding to the acetabulum and the femoral head  (See figure 2) (FIG.7)
4.4 Feasibility analysis and efficacy evaluation of hollow titanium rod coated with hydroxyapatite for ONFH head preserving treatment
The changes in ONFH staging were observed by X-ray at 24 months after surgery. Table 1, table 2 and follow-up data showed that whether 6 patients with stage IIB or 10 patient with stage IIC were aggravated to stage and deteriorated, Finally, THRA was performed surgically and Its pathogenic factors were hormone. Postoperative X - ray observation of the other stages of the case stage stability, lesion repair, good support of titanium rod, femoral head articular surface without collapse, no hip degeneration. This indicates that this treatment method has a good effect on patients with stage II ONFH caused by causes other than hormonal ONFH (See table 2) (FIG.8).
According to postoperative pain and hip joint function can be known, both VAS and Harris scores improved significantly at each time point after surgery compared with before surgery, indicating that this treatment can alleviate pain, improve hip joint function and optimize score index. In particular, the early postoperative period (6~12 months) and the late postoperative period (24 months~the last follow-up) showed significant improvement.
VAS scores of individual patients increased and Harris scores decreased at 24 months after surgery, but there was no statistically significant difference between them and those at 12 months. Combined with interval personalized data analysis, it was found that there were cases of pain aggravation and hip function was affected during this period (See table 3).
The clinical effect of the last follow-up showed that the postoperative improvement rate of this group was 76.13%, among which the improvement rate of stage IIA was the best 100%, the improvement rate of stage IIB was 79.48%, and the improvement rate of stage IIC was the lowest 58.06%. Whether the patients with stage IIB or IIC aggravation and no change were patients with glucocorticoid-induced ONFH. The results showed that this method had the best effect in improving the curative effect and preventing the collapse of femoral head for ONFH stage II A and IIB lesions. For IIC period curative effect sure, but two years later risk of head collapse in glucocorticoid-induced ONFH exists. It is related to the hormone leading to a significant decrease of VEGF and BMP in the trabecular bone and bone marrow tissue of the femoral head and inhibiting the synthesis of BMP by osteoblasts and the differentiation of BMSC into osteoblasts decreased, and adipocyte increased [16, 17]. Postoperative histopathological observation also confirmed that hormonal ONFH was mainly characterized by bone destruction and absorption and granulation tissue formation, with weak bone repair and osteogenesis ability, and the inhibitory effect of postoperative hormone on osteogenesis activity persisted. That is, progressive bone loss and osteonecrosis occurred, resulting in the loosening of the internal fixation [18-20], and the failure of the titanium rod support caused the collapse of the femoral head. Therefore, the therapeutic effect of ONFH in stage IIC of hormone is relatively poor, so it should be carefully selected, while better clinical effect can be obtained for ONFH lesions caused by trauma, alcohol and idiopathic factors (See table 4).