Total hip arthroplasty (THA) is undoubtedly a thorough and durable treatment for hip arthritis, with excellent results especially in older patients. However, for young people with hip osteoarthritis, total hip arthroplasty has a significant disadvantage – a high rate of early revision, especially in males, those with more active jobs, and those who play sports after surgery. Hip surface replacements offer unique advantages for this problem. Hip surface replacement is a hip reconstruction technique with the advantages of less trauma, good postoperative mobility, and preservation of more lateral femoral bone volume, etc. The following is a brief description of its progress and trends in recent years. 1, clinical stability: hip joint table and replacement is not a new concept. As early as the 1950s, it was used in clinical practice, but for a long time, hip surface replacement was regarded as a type of treatment with low value due to immature surgical techniques, easy to cause femoral neck fracture, easy wear of polyethylene socket cup and low survival rate of prosthesis. In recent years, with advances in material science and improved technical details, clinical applications have gradually increased and encouraging clinical results have been achieved, and hip surface replacements have regained academic attention. de Smet et al. 2002 reported the short-term follow-up results of 310 hip surface replacements, with a mean follow-up time of 1.01 years and a prosthesis survival rate of 99.7%. 2004 Daniel et al. reported 446 hip surface replacements with a mean follow-up of 3.3 years and a prosthetic survival rate of 99.8%. Amstutz et al. reported 400 hip surface replacements with a mean follow-up of 3.5 years and a prosthetic survival rate of 94.4%. In 2005, Treaty et al. reported the follow-up of 14 first-stage hip surface replacements completed in 1997 and 1998, with a prosthetic survival rate of 98%. Of course, the follow-up period reported for the new generation of hip surface replacements is still relatively short, and its stability needs to be supported by medium- and long-term follow-up results. The RSA (roentgen stereophotogrammetric analysis) system can be used to indirectly determine implant stability by measuring implant deflection in all directions in early postoperative radiographs. glyn-Jones et al. used RSA to analyze 22 hips with BHR hip surface replacements and showed that at 2 years Itayem et al. also used RSA to analyze 20 BHR surface replacement hips, and the 2-year offset in all directions performed better than conventional THA. Of course, the accuracy of the RSA system for determining surface replacement stability needs to be tested, and there are many other factors that affect the long-term stability of the joint prosthesis. long-term stability of the joint prosthesis, but at least the hip surface replacement prosthesis performs well in terms of early excursions. Kishida et al? studied the postoperative stability of hip surface replacements using measurements of bone mineral density (BMD). He conducted a group-controlled trial in which 13 patients underwent BHR hip surface replacement and 12 underwent THA, and their BMD was measured 2 years after surgery using the DEXA system. with essentially the same pain, joint mobility and walking ability scores, the authors found that the BMD of patients in the surface replacement group was significantly better than that of the THA group, suggesting that patients with surface replacement had better bone quality conditions. 2. Biomechanical studies: Regardless of any hip surgery, restoring the length and biomechanical reconstruction of the lower limb is quite important. Relevant studies include lower limb length, displacement of the center of rotation, femoral eccentric distance (ofset), and determination of the neck stem angle. Unlike THA, hip surface replacement is characterized by the preservation of the femoral neck, and its adverse effects on the biomechanics of the lower extremity are quite small and close to anatomical reconstruction. Of course, from another point of view, since only the acetabular and femoral surfaces can be treated, the surgeon has little room for intraoperative adjustment. a specific study by Silva et al. on the biomechanics of hip surface replacements showed that they are smaller than THA in terms of the horizontal eccentric distance (ofset) of the femur. The results of Loughead et al. were similar in that the eccentric distance of the hip with a surface replacement was significantly smaller than that measured after THA. The remaining aspects such as limb length and molar cup eccentricity were excellent. On closer analysis, it is easy to see that the horizontal eccentric distance of the femur is more difficult to control for surface hip replacement, mainly because the range of surgical manipulation is only related to the joint surface. THA, on the other hand, allows direct adjustment of the eccentric distance by the femoral neck length. There are two main reasons for the small eccentric distance. Firstly, the femoral head prosthesis is placed on an eccentric position in order to reduce the lateral shear stress on the femoral neck during surface replacement, which increases the neck stem angle and shortens the eccentric distance. Second, Loughead et al. noted that the use of bone cement often results in uncontrollable changes – uneven cement thickness may shorten the eccentric distance or limb length by 1 to 2 lnln. However, the authors concluded that these differences, which exist in theory, do not necessarily limit activity. The results of the patients’ postoperative mobility and all scores are still better than those of total hip replacement across the board. The femoral neck stem angle for hip surface replacement has its own specificities. While the neck stem angle in conventional THA is essentially determined by the design of the prosthesis itself, the hip surface replacement depends on the inward and outward placement of the lateral femoral prosthesis. The long-standing surgical experience of many authors suggests that the femoral-side prosthesis should be placed with mild external rotation, the implication of which is to reduce the lateral shear stress on the femoral neck and femoral head. Reducing the incidence of postoperative femoral neck fractures. amstutz et al . In the surgical technique, it is specifically mentioned that the mild external rotation of the prosthesis, which constitutes a neck stem angle close to 140.(135. to 140.), is meant to reduce the lateral stress on the femoral neck and femoral head.Dc Smet et al? also concluded that the femoral head prosthesis should remain mildly valgus, and that correction to 137 is generally required regardless of whether the patient’s preoperative neck stem angle is valgus inward or valgus outward. The measurements in their experiments were an average increase of 2.9. of valgus angle after surgery. 3, material science research: hip surface replacement prosthesis materials have undergone many changes before forming the current form. Compared to the 1980s, it uses metal-on-metal (MOM) as the contact surface and eliminates the PE liner. The significance of this is the reduction of debris generated by wear and tear. The advantage of using a metal material for hip surface replacement, in terms of resistance to wear, is undoubtedly its most important feature. This advantage is unmatched by any other material prosthesis. Simulation tests have confirmed that the wear of MOM is significantly less than that of metal-to-PE material ‘”J. Based on prostheses recovered from humans, Sieber et al. suggest that the wear rate is 1/6th of that of conventional metal-to-PE material. large-diameter prostheses, usually 36 mm or more, that approach the size of the original femoral head. A larger diameter prosthesis improves joint stability and reduces the incidence of dislocation – understood on the other hand, a stable joint ensures mobility in all directions. At the same time, another advantage of a large diameter prosthesis is that it increases the contact area between the metal shell and the femoral head, making the bond stronger and less likely to loosen. In THA and early hip surface replacements, the body’s own hypersensitivity to polyethylene wear particles is always a problem; PE wear particles promote the production of multinucleated giant cells, leading to secondary osteolysis and consequent loosening of the prosthesis. Metal particles, on the other hand, do not trigger hypersensitivity reactions. This is usually thought to be related to the size of the wear particles. Electron microscopic results show that metal wear particles are less than 50 nm in diameter, whereas metal-to-PE particles are 1O times larger 15,16]. It should be noted that metal particles still stimulate autoimmune reactions, for example, it can promote the production of II.6, ILl, TNF, and these autoimmune reactions can also cause aseptic loosening and bone resorption around the prosthesis, but the effect is much weaker compared to the hypersensitivity reaction caused by PE particles . 4. Problems: MOM surface replacement can lead to increased metal ion concentrations in blood or urine, which is currently a common concern in academia. clarke et al. measured 22 patients with MOM hip surface replacement. According to their measurements, patients with surface replacement prostheses using metal-on-metal as the articular surface had significantly higher concentrations of cobalt and chromium in their blood, and their postoperative blood concentrations of metal ions were, not only higher than normal, but also significantly higher compared to postoperative patients with THA with the same MOM surface. The authors suggest that this is related to the large diameter design of the prosthesis, which results in an increased contact surface between bone and metal. Although metal prostheses cause an increase in the concentration of metal ions in the blood or urine, it is still inconclusive how much this change affects the body. High concentrations of co, cr, and Ni ions have been shown to be associated with malignancy in animal models, but there have been no reports of surface replacement prostheses causing tumors in humans. Moreover, most animal models are not instructive for the study of human tumors. In addition, to date there is still no accepted and consistent protocol, method or technique for how to measure metal ions in blood. All existing methods also vary greatly in how they define high or low ion concentrations. Therefore, the issue of elevated metal ion concentrations in the blood after MOM surface replacement still requires long-term and systematic studies. Hip surface replacement requires careful selection of the right patient. In terms of the characteristics of hip surface replacement, preservation of the bone volume on the femoral side and a large diameter prosthesis provide good function. Younger patients as well as those with a requirement for joint mobility are preferred. Recent reports in the literature on hip surface replacements show that patients under 55 years of age are operated on. In addition, a comparison of a Swedish registry of total hip patients of the same age group with the same disease diagnosis by Daniel et al. also showed that the revision rate of hip surface replacement was significantly lower than that of total hip replacement in the young population. However, it is undeniable that increased activity is necessarily an influential factor in the longevity of the prosthesis. This is a contradiction to the urgent desire of young patients who need to improve their hip mobility. In the absence of long-term follow-up results, there is still a need to be cautious about how to increase the patient’s activity and by how much. Also considering the effects of metal ions, caution should be used in patients with renal dysfunction and in those allergic to metallic materials. In summary, the history of hip surface replacement has been a rather tortuous one, with the limitations coming more from the prosthesis itself than from the surgical technique. And recent improvements in prosthesis design have brought renewed attention to the value of the application of surface replacement technology. For young patients with osteoarthritis, surface replacement is a recommended procedure in order to achieve both pain relief and return to activity. The superiority of metal-to-metal hip surface replacement over conventional total hip arthroplasty is due to: a significantly lower incidence of dislocation and increased hip mobility; less surgical blood loss and a decreased incidence of vascular embolic complications; fewer discrepancies in limb length; and less production of wear particles. It is also technically more convenient and feasible for future THA revision than conventional THA because it preserves the structure of the femur itself to the greatest extent possible. The results of recent research on hip surface replacement show that its future development is worthy of anticipation, although this technology has not yet reached full maturity and there are still many issues that need to be addressed. First of all, the results that we have seen so far are only short-term experimental results, even as short as one year, and in the absence of long-term clinical validation, no definitive conclusions can be drawn. Meanwhile, other related studies need to be strengthened, and there is still a lack of reports on complications after hip surface replacement such as deep vein thrombosis and femoral neck fractures. However, with the advancement of technology, we will definitely see a wider application of hip surface replacement technology.