Modern artificial total knee surface arthroplasty (TKA) has been developed for more than 30 years. There is a large amount of long-term and systematic patient follow-up data both at home and abroad, and the results are encouraging. Nowadays, artificial total knee surface replacement has been recognized as one of the most successful surgical techniques in the field of orthopaedics, and occupies an irreplaceable position in the treatment of advanced knee lesions. The traditional artificial total knee joint surface replacement has a surgical incision that is usually about 20 CM, accompanied by extensive muscle soft tissue exposure. It is often necessary to cut the stopping point of the medial femoral muscle on the patella and turn the patella outward, which causes interference and damage to the knee extensor device and suprapatellar capsule, and the patient’s recovery time is often prolonged in the early postoperative period due to severe knee pain and weakness of the knee extensor device. Technical characteristics of minimally invasive total knee joint surface replacement: 1. The generation of minimally invasive total knee joint surface replacement is firstly due to the continuous improvement of surgical instruments. Traditional surgical instruments are large and bulky, requiring a large surgical incision and turning the patella in order to place the osteotomy template; while minimally invasive surgical instruments ensure surgical precision, they reduce the size to half of the traditional instruments, and the handles of some of the osteotomy templates are designed to be with offsets, which maximizes the need for minimally invasive surgery. In minimally invasive total knee arthroplasty, the skin incision is usually less than 15 cm, however, the length of the incision is closely related to the height and leg circumference of the patient, and the ability to achieve full visualization of the surgical field is the primary factor in determining the length of the incision, and the incision should be prolonged without hesitation if necessary, and the quality of the surgery should not be compromised in order to blindly pursue a small incision. 3. The shortening of the absolute length of the surgical incision will inevitably bring certain difficulties to the exposure of the surgical field, how to overcome this difficulty? This introduces another feature of minimally invasive surgery, the “moving window” technique. That is to say, the movement of the surgical incision is utilized to increase the scope of exposure of the surgical field. When the knee is in different angles of flexion and extension, the extent of the surgical incision is different, Bonutti et al. showed that the length of the original incision will increase by nearly 30% during 0°-90° of knee flexion. Intraoperatively, the corresponding surgical area is revealed sequentially through progressive extension and flexion of the knee. In addition, when surgical operations are performed on the medial side of the knee joint, the lateral pulling hook is relaxed, so that the surgical incision is moved inward, and vice versa. 4. Protecting the medial femoral muscle from invasion is one of the core features of minimally invasive total knee arthroplasty. That is, avoid cutting the quadriceps tendon as much as possible, reduce the scarring of the quadriceps muscle, and protect the muscle strength of the medial femoral muscle from being invaded, so that the knee-extension device will not be disturbed. Therefore, extensive and in-depth research has been carried out on the surgical approaches to the knee joint. At present, the following three approaches are mainly used: (1) Medial parapatellar approach This approach is similar to the traditional approach for total knee joint surface replacement, which is basically a shortened version of the traditional incision but does not completely cut off the medial femoral tendon, and the cut of the joint capsule starts at the proximal end of the upper edge of the patella (2-4 cm), and extends distally along the cut of the inner edge of the patella to the level of the tibial tuberosity. This incision is simple, easy to grasp, and relatively safe as it is far away from neurological and vascular structural areas; moreover, intraoperative flexibility is strong, and the quadriceps tendon incision can be extended proximally at any time according to surgical needs until it is transformed into a traditional surgical incision. However, due to the need to cut off part of the quadriceps tendon, this surgical incision will damage part of the medial femoral muscle and its blood flow, and the knee extension device will be interfered to a certain extent, and the postoperative pain is more obvious than that of the other minimally invasive approaches, and the recovery of the patient is slower. (2) Subfemoral approach The subfemoral approach was firstly proposed by Hofmann et al. The joint capsule was incised distally along the parapatellar side at the midpoint of the inner edge of the patella up to the level of the tibial tuberosity, and then the calf was rotated inward gently upward to lift up the belly of the vastus medialis muscle, and then the subfemoral edge was incised for 2-4 cm from the midpoint of the inner edge of the patella inwardly and superiorly, and then the synovium around the suprapatellar bursa was further loosened, so the patella could be pulled away laterally easily. The patella can be easily pulled away laterally. The advantages of the infrapatellar approach are that it is the most physiologic and anatomic approach, and it is the only surgical approach that can preserve the complete knee extension device. It minimizes the possibility of patellofemoral joint instability and patellofemoral mal-tracking, and protects the blood supply to the patella, resulting in less intraoperative bleeding. Patients have less postoperative pain. Since the suprapatellar bursa is not touched, the postoperative adhesion is less, and since the medial femoral muscle is preserved, the patient’s knee extension strength recovers very quickly after the operation, which can significantly reduce the patient’s bed rest time, and thus reduce the generation of complications. However, the infrapatellar approach also has some limitations. The exposure of the surgical area is often unpredictable and is affected by many factors, such as the patient’s patellar position, length of the femur, history of previous knee surgeries, location of the quadriceps stop and quadriceps strength, etc. Also, there are important vascular and neurological structures such as the genioglossal artery and its branches, the interosseous vessels and the saphenous nerve, etc, which have a certain limitation on the lengthening of the incision. When intraoperative visualization is insufficient, it is difficult to extend the surgical incision. Therefore, when choosing this incision, the indications should be strictly controlled. Patients with excessive obesity, short femur, overdeveloped thigh muscles, hypertrophic changes of bone joints and osteoporosis are often not suitable for this surgical route. (3) Medial femoral intermediate approach As mentioned above, the medial parapatellar approach can obtain good exposure of the three compartments of the knee joint, but it is more traumatic to the quadriceps muscle; and the medial femoral infraspinatus approach can preserve the knee extension device, but it is not possible to predict the degree of exposure of the surgical area, and intraoperative dexterity is poor, and it is difficult to improve the exposure of the surgical area by lengthening the incision. The medial femoral intermediate approach is a compromise between the advantages of the previous two approaches. This approach was firstly proposed by Engh et al. After incision of the skin and subcutaneous tissue, the parapatellar support band and the joint capsule were incised downward from the inner upper edge of the patella to the level of the tibial tuberosity in a state of knee flexion of 90°, and the medial muscle belly was incised obliquely and superiorly inwardly for 2-4 cm from the inner upper pole of the patella in the direction of the fibers of the medial femoral muscles, and the synovium around the suprapatellar bursa was further loosened, so that the patella could be fully moved outward to reveal the surgical area. The patella can be moved outward sufficiently to reveal the surgical area. The advantages of the medial femoris intermedius approach are similar to those of the inferior femoris intermedius approach: it can reduce the trauma to the quadriceps muscle and alleviate postoperative pain; it can reduce bleeding and patellar ischemia by avoiding the descending artery of the knee and preserving the patellar blood flow; it can improve patellar trajectory and stability by preserving more quadriceps tendon attachments, and it is less intrusive to the knee-extension device, and the recovery of quadriceps muscle strength and control is faster in the postoperative period, so it can shorten bed-resting and hospitalization time. Hospitalization time can be shortened. Compared with the infrapatellar approach, the medial femoral approach is more flexible, and when there is difficulty in intraoperative exposure, the incision can be extended obliquely along the direction of the medial femoral muscle fibers to the upper inside to increase the scope of exposure; however, it is not possible to extend the incision fearlessly, and care should be taken to avoid damaging the saphenous nerve and the descending artery of the knee. Therefore, this surgical approach is not suitable for patients with overweight or overdeveloped thigh muscles. In addition, Parentis et al. found that the postoperative electromyography of patients who underwent the medial femoral muscle intermediate approach showed the phenomenon of blockage of the medial femoral muscle activation current, but the long-term clinical significance of this phenomenon needs to be further investigated. In contrast, both clinical observations and Cybex testing showed faster postoperative recovery of quadriceps muscle strength in patients with the medial femoral intermediate approach compared with the medial parapatellar approach. In addition to the improvement of the surgical approach, another feature of minimally invasive total knee arthroplasty is that the patella is not turned during most of the surgical procedure, and is only turned in knee extension during patellar surface treatment, when the knee extensor device is in a state of laxity, which avoids excessive pulling on the quadriceps tendon and protects the knee extensor device. It has been found that the cause of poor postoperative quadriceps function is the excessive intraoperative pulling and turning of the patella. Minimally invasive total knee arthroplasty is not just about shortening the incision, but also about minimizing soft tissue damage, protecting the knee extensor device and suprapatellar bursa from interference, and avoiding excessive pulling and turning of the patella during surgery. Thus, the purpose of minimizing trauma and facilitating recovery is truly achieved. At the same time, in order to pursue a small incision, the soft tissues should not be overly pulled, resulting in greater trauma than traditional surgical methods. It is also important not to blindly pursue a small incision, resulting in inadequate surgical exposure, poor prosthesis position and abnormal force lines in the lower extremities. Only in this way can the real purpose of minimally invasive total knee arthroplasty be achieved.