Abstract】 Objective To better carry out minimally invasive anterolateral muscle gap approach hip arthroplasty in China, to study the relevant anatomical structure of the national population, to observe the clinical results of the operation, and to discuss the operation-related techniques. Methods Minimally invasive total hip arthroplasty with anterolateral muscle gap approach was performed in 16 patients, and the clinical results and surgical operation experience were summarized. Results The surgical incision length ranged from 7 to 10 cm (average 8.8 cm). Intraoperative bleeding ranged from 250 to 450 ml (average 350 ml), and blood transfusion was performed in 4 cases (400 ml). Seven cases were found to have partial contusions of the muscle fibers of the anterior border of the gluteus medius intraoperatively and were trimmed. Follow-up ranged from 18 to 39 months (mean 27.7 months). Postoperative radiographs showed that most of the prostheses were well positioned, one case had a large anterior acetabular tilt angle and two cases had a large acetabular abduction angle, but none of them had complications or significant functional impairment. The hip Harris score was 39.1±6.7 preoperatively, 80.6±11.3 at 6 months postoperatively, 88.7±9.6 at 12 months postoperatively, and 91.4±13.5 at 24 months postoperatively (11 cases). No gluteus medius muscle weakness was observed in all patients. Conclusion Minimally invasive hip arthroplasty with anterolateral muscle gap approach has the advantages of simple access level, small surgical trauma, no stripping or cutting of muscles and tendons, and fast postoperative recovery, which has practical value and is suitable for promotion in small-sized national population. However, it should be avoided that the anterior tilt angle of the acetabular file is large due to the impact of the greater trochanter, and the soft tissue restriction at the distal end of the incision increases the abduction angle of the acetabulum. Avoid contusion of the skin proximal to the incision and the anterior border of the gluteus medius muscle by the medullary file. Pay attention to the accurate positioning of the skin incision and use surgical instruments dedicated to minimally invasive techniques. Keywords] Arthroplasty, replacement, hip; surgery, minimally invasive Artificial hip arthroplasty is becoming increasingly popular because of its good efficacy, but the traditional procedure is highly invasive, bleeding and has a long functional recovery time. For this reason, research on “minimally invasive” hip replacement techniques has received attention [1-3], and the surgical approach and methods can be broadly divided into two categories. “The other type is the use of a new surgical approach that does not strip the muscle tissue and enters from the muscle space, which largely reduces the interference with the local biological environment of the joint, and can be called “minimally invasive” hip arthroplasty [5]. The latter is considered to be more in line with the true meaning of minimally invasive surgery. The anterolateral hip gap approach is a representative of minimally invasive hip arthroplasty [6,7], but there are still problems such as damage to the superior gluteal nerve and difficulty in operating on the femoral side. The author carried out this technique clinically based on the results of anatomical studies and reported as follows. Clinical data and methods I. General data: In this group, there were 16 cases, 12 males and 4 females, aged 51-68 years (mean 61.3 years). Among them, there were 8 cases of femoral neck fracture (6 cases of subhead type and 2 cases of transcervical type), 5 cases of acetabular dysplasia combined with osteoarthritis of the hip joint, and 3 cases of ischemic necrosis of the femoral head. All of them were first-time joint replacement surgery, excluding the overly obese and muscular cases, and there was no obvious limitation of hip movement except for femoral neck fracture. All patients were operated by combined epidural lumbar anesthesia, and the surgery was performed by the same team of surgeons. Preoperative orthopantomogram of the pelvis and lateral radiograph of the affected hip were taken, and the expected size of the implanted prosthesis and the osteotomy position of the femoral moment were measured using a template. Postoperative time, intraoperative bleeding and postoperative drainage were recorded, and systemic and local complications were observed. The Harris score of hip function was recorded preoperatively, at 6, 12 and 24 months after surgery, and at a follow-up of 18 to 39 months (mean 27.7 months). The prostheses used were Biomet products, all of which were biologically based. Special minimally invasive hip replacement instruments (Figure 2) were used during surgery, including a small semicircular acetabular file, an eccentric distance acetabular file rod, a crank acetabular pounder, an eccentric distance femoral medullary enlarger, a notched mold femoral head, and a Hohmann pull hook. The affected limb was taken in the superior lateral position with the posterior half of the caudal end of the surgical bed disassembled to facilitate the postural manipulation during placement of the femoral prosthesis. The incision is located at the anterior projection of the gluteus medius muscle, that is, from the anterior border of the greater trochanter of the femur, pointing 6 to 8 cm posterior to the anterior superior iliac spine, with an incision of approximately 7 to 10 cm depending on the patient. 1/4 of the incision is on the greater trochanter and 3/4 is proximal to the greater trochanter (Figure 3). The subcutaneous tissue is separated from the deep fascia along the incision line and the gap between the anterior border of the gluteus medius and the broad fascial tensor muscle is palpated with the fingers (Figure 4a). The lower outer portion of this muscle gap (near the anterior border of the lateral femoral muscle) is well defined, so this gap is separated inwardly from the anterior border of the lateral femoral muscle (Figure 1b). To protect the inferior branch of the superior gluteal nerve, this muscle gap is separated during surgery until the muscle fibers of the gluteus medius join with those of the vastus medialis tensor fasciae. The hip capsule can be accessed by inserting the fingers deeper into this muscle gap. The affected limb is fully abducted and externally rotated to relax the gluteus medius (minor) muscle, and the Hohmann pull hook is placed below the gluteus medius (minor) muscle (surface of the joint capsule) and gently retracted, while the other pull hook is placed immediately below the femoral neck against the joint capsule. The anterior joint capsule is incised in a “U” shape with the medial base, and the pulling hook is repositioned inside the joint capsule, and the hip is flexed and externally rotated to reveal the femoral neck. A two-step osteotomy is performed: the first osteotomy is performed with a narrow saw blade under the femoral head; the affected limb is further externally rotated and a second osteotomy is performed at the low level of the femoral neck according to the preoperative measurements, using a femoral end osteotomy template or determining the direction of the osteotomy line according to the anatomical line of the femoral neck into the greater trochanter. The femoral neck bone block and femoral head are removed (Figure 4b). Hohmann pulling hooks are placed at the anterior and posterior edges of the acetabulum, with the posterior hooks pressing the femur posteriorly, the anterior hooks distracting the broad fascial tensor and the joint capsule flap, and a third hook is placed proximal to the acetabulum, traction and external rotation of the lower leg distally to facilitate exposure of the acetabulum (Figure 4c). The round ligament is cleared and the synovial layer of the joint capsule and acetabular glenoid labrum are excised. The acetabulum is repaired with a special eccentric-spaced acetabular file deep to the lunate fossa. The position of the acetabular prosthesis is determined with an acetabular positioning guide rod or by reference to the anatomic landmarks of the autologous acetabulum, with 45 degrees of abduction and 20 degrees of anterior tilt, and the acetabular prosthesis is fitted with a cranked acetabular pounder, which can be screwed to reinforce the medullary socket fixation and implant a suitable acetabular liner. The femur is prepared by placing the foot and lower leg in a sterile pocket in front of the assistant’s body, who places the hip in the posterior extension, adduction and external rotation position and pushes the femur proximally in the longitudinal direction. The Hohmann hook is placed in the posterior corner of the greater trochanter and the gluteus medius is retracted posteriorly and superiorly. The proximal femur is cleared and the femoral stem is palpated to determine the direction of marrow expansion. The femoral medullary cavity is prepared to the appropriate size with an eccentric moment medullary file after slotting with a box bone cutter. Use the prosthesis trial mold to determine the appropriate neck length and joint tightness. The femoral prosthesis stem and head of the appropriate size are implanted. The limb is returned to its original position to reset the hip joint. Suture the remaining fibrous layer of the joint capsule, place the drainage tube, and close it layer by layer. Postoperative treatment: After surgery, the affected limb should be placed in an abducted neutral position with a T-shaped shoe. After the patient woke up from anesthesia, the patient could perform hip joint activities and quadriceps active diastolic exercises. After the operation, the patient was allowed to go to the ground for 3 d, and the affected limb was allowed to bear weight gradually, and the patient abandoned the crutches and walked 4-6 weeks after the operation. Clinical results All cases in this group were operated under the preoperative design procedure, and the incision length was 7-10 cm (average 8.8 cm). The operative time ranged from 60 to 135 min (average 95 min). Intraoperative bleeding ranged from 250 to 450 ml (mean 350 ml), and postoperative incisional drainage ranged from 150 to 300 ml (mean 250 ml). Blood was transfused in 4 cases (all 400 ml). The postoperative hospital stay ranged from 8 to 25 days (mean 16 days). 7 cases were found to have intraoperative contusions of some muscle fibers at the anterior border of the gluteus medius due to pulling, which were trimmed or sutured (Figure 4d). 5 cases had mild extrusion or contusion of the skin incision edge, and the wound was closed after skin edge trimming. There were no other systemic or local complications in this group. The patients walked normally during the follow-up period. Postoperative radiographs showed that most of the prostheses were well positioned (Figure 5), one case had a large anterior acetabular tilt angle and two cases had a large acetabular abduction angle, but none of them had complications or significant functional impairment. The hip Harris score was 39.1±6.7 preoperatively, 80.6±11.3 at 6 months postoperatively, 88.7±9.6 at 12 months postoperatively, and 91.4±13.5 at 24 months postoperatively (11 patients). No gluteus medius weakness was found in all patients, and Trendelenburg’s sign was negative. DISCUSSION Hip arthroplasty is one of the most successful treatment techniques in joint surgery. However, there are still problems such as long incision, large surgical trauma, bleeding, postoperative scarring that affects aesthetics, susceptibility to fat liquefaction and infection, difficulty in rehabilitation exercises, long recovery time, and heavy psychological burden on patients. Therefore, “minimally invasive” hip arthroplasty, which aims to reduce soft tissue damage and accelerate postoperative recovery, has received widespread attention [3-7]. At present, “minimally invasive” hip arthroplasty includes anterior approach, anterolateral approach, lateral approach, posterior lateral approach, and double-incision (triple-incision) technique [1,3-7]. Among them, the anterior, lateral and posterior lateral approaches have reduced the surgical incision due to improved instruments and techniques, but they cannot overcome the inherent defects of these approaches. The anterior approach damages the lateral femoral cutaneous nerve, is difficult to visualize and manipulate the lateral femur, and may require extensive tissue stripping and release, or even severance of the external rotator muscle [8]. The posterior approach requires severance of all external rotators and resection of the posterior joint capsule, which increases the likelihood of posterior hip dislocation. The effectiveness of postoperative repair of the external rotators and posterior joint capsule in preventing posterior dislocation is controversial [9]. The lateral approach requires partial severance of the gluteus medius (small) muscle attachment fibers on the greater trochanter, which destroys the stable structure of the hip joint and can cause persistent claudication and gluteus weakness if the gluteus medius (small) muscle heals poorly [10]. None of the above approaches that damage muscle tissue are in line with the theory of minimally invasive techniques [11]. Most authors point out that simply reducing the incision does not represent the essence of minimally invasive techniques, which should cause no significant damage to muscle tissue, preserve the hip capsule, substantially reduce surgical trauma, and accelerate postoperative functional recovery. Accordingly, at present, the only truly “minimally invasive” hip replacements are the anterolateral (muscle gap) approach and the double-incision minimally invasive hip arthroplasty. The double-incision minimally invasive hip replacement technique does not require dissection of the muscles and is easier to expose and manipulate the acetabulum, but the femoral side requires an additional incision and the position and size of the prosthesis must be confirmed under X-ray fluoroscopy, so it is not widely accepted [12]. All operations of the anterolateral (muscle gap) surgical approach are located in the triangular muscle gap between the vastus lateralis tensor, gluteus medius, and lateral femoral muscles. No disruption of the muscles around the hip joint, no destruction of the posterior joint capsule, and no additional incisions and fluoroscopic X-ray monitoring are required, which is more in line with the concept of minimally invasive techniques [7,10]. The above factors are the reasons why we conducted an applied anatomical study of the anterolateral (muscle gap) surgical approach in the national population and adopted this surgical approach in clinical practice. The results of our cases showed that the anterolateral muscle gap approach for minimally invasive total hip replacement surgery has the following advantages: ① Smaller incisions, better appearance, and easy acceptance by patients. The incision length in this group was 7-10 cm (average 8.8 cm), which was significantly smaller than the conventional incision of about 20 cm; ② less bleeding, 250-450 ml (average 350 ml) of intraoperative bleeding and 150-300 ml (average 250 ml) of postoperative incisional drainage in this group. The complications of bleeding and blood transfusion were reduced; however, a few elderly patients still had anaemia for a long time after surgery, and although the degree of anaemia was not serious, a small amount of blood transfusion was still preformed for a few patients in order to improve the general condition of the patients; (3) the surgery was less traumatic and the anatomical level of the surgical approach was simple, which avoided the stripping and destruction of muscle tissue and the resulting low tension of the soft tissues around the hip joint, easy dislocation, long time in bed, and slow recovery, etc. The problems of disruption of muscle tissue and the resulting low tension of soft tissues around the hip, easy dislocation, long bed rest and slow recovery were avoided [13]. In this group of cases, functional exercises were performed on the ground in 3 d. The patients with artificial hip replacement are mostly elderly people, who may have their own organ diseases or degenerative factors, which makes the surgery more risky [14]. Minimally invasive total hip replacement with an anterolateral muscle gap approach reduces surgical trauma and the risk of disturbance to the internal environment and systemic complications; ⑤ It reduces the cost of patient care and economic burden because it shortens bed rest and hospitalization time. Of course, minimally invasive hip arthroplasty also has its limitations. Because of the small surgical field of view and unclear anatomical landmarks, inaccurate placement of the prosthesis can easily occur. For example, when grinding the acetabulum and placing the acetabular prosthesis, if the joint capsule is not loosened enough, the greater trochanter will rebound strongly and hit the acetabular file and mounting rod, resulting in a large anteversion angle when the acetabulum is filed and repaired; meanwhile, because of the small incision, the skin and soft tissue restrictions at the distal end of the incision tend to increase the acetabular abduction angle, so the skin incision must be accurately positioned and special surgical instruments such as the eccentric distance acetabular file and mounting rod must be used. The skin incision must be accurately positioned and special instruments such as eccentric spaced acetabular files and mounting rods must be used. This technique presents some difficulties in the operation of the femur, especially when the incision is not accurately positioned (often the surgical incision is anterior) and the release of the joint capsule is not adequate, the restricted posterior extension and internal retraction of the femur can cause the medullary file to damage the skin proximal to the incision and the anterior border of the gluteus medius. Therefore, it should be clear that this technique is not suitable for all patients. For example, overly obese, muscular, stiff hip joint, severe deformity of the acetabulum and artificial joint revision are not suitable for this procedure. It should also be carried out with caution in cases of congenital hip dislocation, acetabular dysplasia and osteoporosis. Minimally invasive total hip replacement should pay attention to the following issues: ① Pay attention to the selection of patients, especially in the early stage of this technology, the indications should be strictly selected, and patients who are not suitable for minimally invasive technology should not be forced to perform it, and the accuracy and therapeutic effect of prosthesis installation should not be neglected due to the pursuit of small incision surgery. ②The operator must be familiar with the local anatomy and have rich experience in routine hip replacement surgery. ③Appropriate special surgical instruments must be available to ensure adequate field exposure when small incisions are used. (4) Intraoperative attention should be paid to avoid damage to the inferior branch of the superior gluteal nerve. Protection of the muscle fiber connection at the upper inner corner of the triangular muscle gap can avoid damage to the inferior branch of the superior gluteal nerve. ⑤ Intraoperative fine manipulation and careful hemostasis are required to reduce bleeding in the operative field and to ensure a good field of view within the small incision, and special illumination is required if necessary. ⑥Patients can go down to the ground early after surgery, but they should not cross their legs, bend their hips more than 90°, and do not extremely pronate in the short term, and they should gradually carry out functional exercises. (7) Since minimally invasive surgery patients can move to the ground earlier, they should also pay attention to postoperative analgesic treatment. Although there are still some problems with minimally invasive hip arthroplasty, minimally invasive techniques are undoubtedly the future trend of orthopedic development [15]. Minimally invasive hip arthroplasty with anterolateral muscle gap approach does not damage muscles, has faster postoperative functional recovery and high patient satisfaction, and is a recommended technique for the relatively small size of the national population [7,10].