As is known to all, postoperative thigh pain is a common problem after the primary total hip arthroplasty (THA) with cementless prosthesis, which would be one of the main reasons for decreased surgical satisfaction. [10, 17] Although the conical femoral stem prosthesis could reduce the pain occurrence, it still has a high incidence due to unknown mechanisms. [18, 19] At present, researches on impact after THA between prosthesis, strikes and iliopsoas muscle (between the bones and abundant) [20–23], and those on the implant tip and femoral cortical impact, mainly focus on the intramedullary nail surgery. However, few studies have focused on the femoral prosthesis stem tip and femoral cortical impact problems after THA. [24, 25] In this study, the impact between the femoral stem tip and the femoral cortex and postoperative thigh pain were studied.
Our retrospective analysis of 172 patients undergoing primary THA showed that the cortical impaction of the distal femur was not uncommon after primary total hip arthroplasty, with an incidence of 14.5%, especially in women of small stature. Vision (as women get older), osteoporosis, femoral cortical thinning, the femur bow angle would be increased, when the biological type femoral stem (tapered straight shank) was used. In order to ensure better proximal pressure distribution, the larger femoral prosthesis stem would be chosen, and the femoral stem length would be increased, thereby increasing the point between the cortex and the impact of risk within the plants.[26–28] In addition, the existing biological femoral stem prosthesis is mainly designed based on patients in European or American, and therefore the LK. LCU femoral stem prosthesis of the same length used in this study was bound to increase the risk of collision when implanted into the body of Asians. For Asian patients with short stature, and femoral bow angle in the proximal with suitable cases, shorter creature type femoral line handle THA may be beneficial. However, Gielis et al.[29–31] have found that using short shank primary total hip replacement, thigh pain would follow the surgery, which would not be significantly reduced. [29–31] In addition, Boese et al.  have also reported that the mismatch between femoral morphology and femoral prosthesis geometry in total hip arthroplasty not only increases the surgical difficulty, but also bring serious implant-related complications. The selection of too large prosthesis during femoral stem implantation would lead to the difficulty of femoral stem implantation. Some physicians may reinsert the bone after further appropriate distention. However, excessive reaming could result in postoperative pain and other problems. The choice of a smaller femoral stem prosthesis may affect the stability of total hip arthroplasty. Moreover, it also increases the risks of complications such as postoperative pain and fractures around the prosthesis. [6, 33, 34] Therefore, it is important to find out the risk factors associated with femoral stem and femoral cortex impact after primary THA. Preoperative planning can be made in advance, and attention should be paid to the choice of femoral stem prosthesis. Therefore, the personalized femoral stem placement schemes should be developed, to facilitate the placement, decrease the operation difficulty and reduce the occurrence of postoperative complications.
The femoral stem implant angle is another risk factor with great influence, which is easily overlooked. As Teloken et al.  have believed that the placement angle of the femoral stem prosthesis should be less than or equal to 3° on anteroposterior hip X-ray. Otherwise, the prosthesis was considered to be in varus or valgus position. In this study, our results showed that the risk of collision between the femoral stalk tip and the cortex was significantly higher when the prosthesis was in varus, compared with when the prosthesis was not varus. Moreover, postoperative thigh pain was found to be mainly concentrated in the patients with inversion at 3-month follow-up. These patients had obvious postoperative thigh pain and lower postoperative Harris scores. Our results also showed that the contact area of such patients in Gruen zones 2 was relatively small. Similar findings have been obtained by Junya et al., who have shown that the contact area of Gruen 2 is negatively correlated with post-operative pain.  This may be largely related to the experience and technical level of the surgeon. Therefore, this phenomenon can be a good reminder to the surgeon. During THA, especially when the femoral stem is inserted, more attention should be paid to the implant angle. In order to avoid too large angle of implantation, which might result in the impact of the postoperative prosthesis and the distal femur cortex, and thus affect the surgical quality.
This study has some limitations. First, the relatively small sample size and the nature of retrospective study limited the level of evidence. Second, it was limited to LK-LCU type femoral stem prosthesis, which was difficult to explain the problems of other femoral stem prosthesis.