Osteonecrosis of the femoral head (ONFH) is a kind of pathological state with multiple causes, which leads to the decrease of blood supply of the subchondral bone of the femoral head, the death of bone cells and the collapse of the articular surface 1, 2. For the early and middle stage of non-invasive avascular necrosis of the femoral head, the first choice is to retain the femoral head. For the treatment of early avascular necrosis of the femoral head, the main principle is to effectively improve the blood supply of the femoral head and the function of the hip joint, and delay the development of the pathological process of necrosis of the femoral head. At present, several important factors for the treatment of avascular necrosis of the femoral head are as follows: ① increase of new bone regeneration and neovascularization. ② Remove the dead bone thoroughly. ③ Reduce the internal pressure of femoral head and promote the internal venous return of hip joint. ④ Maintain and increase the surface supporting force of the femoral head, prevent or treat the collapse of the femoral head3, 4.
There are many methods for the early necrosis of the femoral head, including hyperbaric oxygen, shock wave and other conservative treatment5-7. Surgical operations include osteotomy, vascularized bone flap implantation, and core decompression7-10. At present, it is generally recognized that core decompression can reduce the pressure in the bone, remove the necrotic bone tissue, increase the growth of blood vessels in the femoral head, and promote the crawling replacement of new tissue, which can be rapidly pain relieved, but the clinical satisfaction rate of using core decompression alone is not excited. The main reason is that although simple decompression can relieve the pressure in the femoral head, it can reduce the internal supporting force of the femoral head. Therefore, bone grafting or artificial materials are needed to provide enough biological support for the joint surface of the femoral head. According to Hungerford et al. 11, core decompression combined bone grafting are suitable for the treatment of early avascular necrosis of the femoral head, which can effectively prevent the collapse of the femoral head and delay the time of hip replacement in the later stage. However, core decompression combined bone grafting often fail to provide effective bone support for the femoral head of patients. If the necrosis area is too large, the autogenous cancellous bone particles implanted after curettage of necrotic bone cannot provide effective subchondral support for the patients with core decompression combined bone grafting, and the autogenous iliac bone absorption implanted after operation is also the main reason for the collapse and deformation of the femoral head12.
Tantalum is a blunt metal with good biocompatibility. Bone and vascular tissue can be seen to grow rapidly in the hip joint with tantalum coating 13, 14. Core decompression combined with tantalum rod placement can release the internal pressure of the femoral head through core decompression, so as to alleviate the pain. Tantalum metal has the same elastic modulus as bone, and has the structure of bone trabecula, with high porosity. The tantalum rod designed by Zimmer company in the United States has a cylindrical structure with an aperture of 430 μ m and a diameter of 10 mm, with a porosity of 75% - 80%, and a length of 70-130 mm (5 mm increase). The elastic modulus is equivalent to that of human fibula (3GPa), which has a good support for the femoral head 15. Tantalum rod can bear physiological load of human body, has good biocompatibility and good friction stability. The placement of tantalum rod can play a role in filling the bone at the core and supporting the femoral head, reducing the stress distribution of the surrounding bone tissue, and effectively preventing the surface of the femoral head from collapsing. At the same time, the porous structure of tantalum rod can induce osteoblasts to grow in, accelerate the regeneration of blood vessels and promote the process of vascularization, which is conducive to the regeneration of femoral head and the repair of femoral head.
Many studies have reported that tantalum rod implantation has achieved good early clinical effect in the treatment of avascular necrosis of the femoral head4, 16, 17. However, the long-term clinical effect, weight-bearing time and effect of porous tantalum implant are still controversial18-20. In the past few years, the failure rate of tantalum rods in ONFH has been reported to vary from 2% to 56%16, 21. Once the subchondral bone collapses, the progression of the disease is difficult to reverse. The collapse and deformation of the femoral head, the narrowing of the joint space, and the deterioration of the joint function occur in turn. Hip replacement has become the only treatment option for these patients. Moreover, at present, only animal studies have confirmed the bone ingrowth of porous tantalum rods 13, 22, proving that porous tantalum rods implantation is related to rapid and extensive bone growth, but the bone ingrowth in human body is still unclear.
In this study, the survival data of patients with avascular necrosis of the femoral head treated by tantalum rod implantation were reviewed and analyzed, and the clinical and imaging results were evaluated. The effect of tantalum rod implantation on avascular necrosis of the femoral head and the related factors leading to its conversion to THA were analyzed. In addition, pathological sections of the femoral head with tantalum rod were evaluated to confirm the degree of bone growth of tantalum rod in human body, and to evaluate the mid-and-long term efficacy of porous tantalum rod in the treatment of early avascular necrosis of the femoral head.
2.1 Patient Selection
From January 2008 to December 2013, patients with avascular necrosis of the femoral head underwent tantalum rod implantation in two hospitals were analyzed retrospectively. Inclusion criteria: non traumatic avascular necrosis of the femoral head, International Association of bone circulation stage (ARCO) I ~ II patients. Exclusion criteria: skin lesions in the surgical area, active infections, coagulation disorders, and cases previously treated with any other type of treatment. This retrospective study was approved by the ethics committee of our institute, and we confirm that all methods are carried out in accordance with relevant guidelines and regulations. All patients signed the informed consent of operation.
One patient (2 hips) died of diseases unrelated to the operation, and three patients (3 hips) lost their visit. A total of 42 cases (52 hips; 5 females and 37 males) were analyzed. The average age of these patients was 40.3 years (range: 20-63 years). There were 30 males and 12 females with an average body mass index of (25 ± 3) kg / m2. The etiology of osteonecrosis was idiopathic or unknown in 10 hips, the use of corticosteroids in 19 hips and alcohol abuse in 23 hips. The demographic data and preoperative baseline characteristics of patients are shown in Table 1.
2.2 Surgical methods
After anesthesia, the patient was in supine position, the hip was sterilized and covered with towel, and the skin protection film was applied. A 4cm long incision was made 2cm below the trochanter to separate the fascial muscle layer by layer from the proximal and lateral cortex of the femur. The intersections of the horizontal line slightly above the trochanter and the lateral femoral cortex were used as the insertion points, and the center of the osteonecrosis area was used as the orientation of the insertion points. In order to eliminate the influence of the anteversion angle of the femoral neck on the position, the hip should be rotated 10 ° ~ 15 °, and the guide pin should be inserted in the central coronal plane of the femoral neck; the 3.2mm guide pin should be inserted 5 mm below the articular cartilage of the femoral head. Ream the hole along the direction of the guide pin, and use the hollow drill to gradually ream the marrow from 8mm diameter to 9mm; in the process of reaming, clean the hollow drill to prevent bone fragments from gathering in it. When the hollow drill enters the necrotic bone and does not penetrate the necrotic area, the necrotic tissue is removed through the bone tunnel with a multi-directional curette. Reaming with 10 mm hollow drill. Insert a depth gauge to determine the length of the required tantalum rod, and the measured length shall avoid the tail of the tantalum rod finally exposed outside the lateral cortex, so as to avoid the pain caused by its abrasion of surrounding soft tissue. Install the corresponding length of the tapping extension at the front end of the tapping, and rotate clockwise to make all the threads enter the femur. Screw in the tantalum rod (product of Zimmer company of the United States) so that its tail is slightly niche into the bone cortex; if there is resistance when transplanting, determine whether the tapping is in place or whether there is broken bone in the medullary canal. Fluoroscopy showed that the tantalum bar was in good position, and the incision was sutured layer by layer. The second generation cephalosporin antibiotics were given to prevent incision infection 30 minutes before operation and 24 hours after operation. In 12 weeks after operation, the patients' hip joint was fully limited in weight-bearing, and functional exercise was performed without weight-bearing. After 12 weeks, they walked with full weight-bearing.
The indications of THA were: continuous hip pain interfered with daily activities and deterioration of hip score, or radiation collapse of femoral head and intraarticular penetration of tantalum rod. The technology of tantalum rod transfer to THA after failure includes femoral neck osteotomy and implant cutting, both of which use a power saw to remove the tantalum rod from the rotor with a special ring drill. This procedure is performed as a routine hip replacement.
2.3 Clinical Assessment
The follow-up examination was arranged at 1 month, 3 months, 6 months and 12 months, and then every 6 months. The evaluation parameters included Harris hip score and imaging and MR examination of the diseased hip. The X-ray films of hip joint were used to evaluate the size of lesions, the consistency of femoral head, whether there was crescent sign and the degeneration of hip joint. MR images were used to assess changes in bone marrow edema and lesion size. According to the ARCO23 grading system, the initial stage and the degree of involvement of the femoral head were evaluated by radiology. More than 30% of necrosis is defined as large osteonecrosis. Myeloedema was defined as a low signal area on T1 weighted images, The high signal region is defined on T2-weighted image or inversion recovery image，in the femoral head, neck and intertrochanteric region24, 25. The clinical evaluation was conducted by a non surgeon observer throughout the study. Two radiologists who were not involved in the operation and did not know all the clinical information were measured radiographically. Each parameter is measured twice at appropriate intervals to prevent bias from affecting the results.
2.4 Analysis of bone implant specimens
The pathological section of the femoral head implanted with tantalum rod was completed in Shandong Key Laboratory of oral tissue regeneration by using the EXAKT hard tissue section system. The bone graft into the femoral head was fixed in 10% buffered formalin and prepared for histological analysis of thin sections without decalcification26. The section was mounted on a glass slide and gradually thinned by petrological grinding technology, stained by HE and Masson respectively, and examined by transmission optical microscope. This allows qualitative assessment of tissue response to tantalum implants, including calcification and fibrous tissue recognition, new bone formation, and vascular distribution and cellular structure of the femoral head.
2.5 Statistical Analysis
The data and charts were analyzed and processed by IBMS PSS Statistical 19.0 statistical software. Continuous variables were analyzed using independent sample T test. Categorical variables were analyzed using the Pearson chi-square or Fisher exact tests. Kaplan-Meier survivorship analyses were used with the endpoint defined as reoperation with THA. Test level was set at both sides α =0. 05, P < 0.05 was considered statistically significant.