"Net cage" technique in the treatment of inferior pole patella fracture: a novel patella internal xation technique

Fractures of the inferior pole of the patella can cause the knee joint extensor mechanism disorder. The fracture fragments are usually small and comminuted. Therefore, there are certain diculties in xation. The purpose of this study is to observe the effect of the "Net cage" technique in the treatment of the inferior pole of the patella. Methods This is a retrospective study that included 16 cases of inferior patella fractures (AO/OTA 34-A1) who underwent the "Net cage" technique from March 2017 to June 2020. Collecting their medical records and follow-up results, measuring indicators include the surgical complications of the xation method, knee joint function, the number of uoroscopies, fracture healing, and the incidence of soft tissue stimulation.


Introduction
Fractures of the inferior pole of the patella account for about 20% of all patella fractures undergoing surgical treatment [1] , and are usually caused by direct trauma to the bent knee joint [2] . Like other types of patella fractures, the purpose of surgery for inferior pole fractures is to rebuild the extensor mechanism while restoring the stability of the articular surface [3] ; the difference is that the fragments of inferior pole fractures are small and comminuted, making it di cult to maintain reduction and xation [4][5][6] . In order to better x the inferior pole fracture, many new xation methods have been developed, including suture xation, steel plate xation, and plate combined with tension band xation [7][8][9] . In addition, A partial patellectomy with repairment of the patella ligament and anchor suturing has been used as a method for comminuted distal pole fractures [9,10] , but its effectiveness has not been widely recognized.
Tension band wiring technology can convert the anterior tension forces produced by the extensor mechanism and knee exion into compression forces at the articular surface. It is widely used in the xation of patella fractures. However, for comminuted inferior pole fractures, traditional tension band wiring technology may not be able to effectively x it [6] . In order to solve these problems, we proposed a novel patella internal xation technique: "Net cage" technique. The following is a report of the surgical results.

Patients And Methods
This is a retrospective study. From March 2017 to June 2020, a total of 236 patients with patella fractures underwent surgical treatment were admitted to the Hong Hui Hospital of Xi'an Jiaotong University, 28 cases of (AO/OTA 34-A1) type, of which 5 patients underwent partial patella resection or other surgical treatments were excluded; 4 patients with femoral or tibial fractures that may affect the function of the knee joint were excluded; 3 patients did not complete su cient follow-up and were excluded except. In the end, 16 patients underwent "Net cage" technique surgery and completed at least 12 months of follow-up. Their demographic characteristics, injury mechanism, and fracture characteristics are as follows (Table 1): Among the 16 patients, there were 11 male, 5 female; their age ranged from 32 to 57 years, with an average of 41.507.39 years; 12 patients were injured by falls and 4 patients were injured by tra c accidents. This study was approved by the ethics committee of the Hong Hui Hospital a liated to Xi'an Jiaotong University, and all patients signed an informed consent form.
All patients underwent thorough preoperative assessments including standard knee X-rays, computed tomography (CT) scanning, and three-dimensional reconstruction to assess the degree and classi cation of fracture. In the 16 patients included in the study, the average length of the inferior pole fracture was 10.8mm (7.5~13.7). All patients were performed by doctor Ming Li.

Surgical Technique
The patient was placed in a supine position under general anesthesia, antibiotics were used prophylactically, and then a tourniquet was placed on the base of the patient's thigh. The pressure of the tourniquet was 300mmHg. Surgical incisions are selected from the anterior median longitudinal incision of the knee joint, and the skin, subcutaneous tissue, and super cial fascia are cut layer by layer to remove blood clots between the fractures, avoiding the peeling of the prepatellar fascia and periosteum to prevent the separation of comminuted fractures.
After the fractured end is revealed, three 1.8mm bone spicules were driven parallel from the subchondral bone area of the fracture section to the upper pole of the patella ( Fig. 1a-1c, during this process, you can directly touch the articular cartilage surface of the patella with your hands to assist in determining the position of the bone spicule). Use Bone tenaculum to maintain the reduction of the fracture of the inferior pole of the patella and pass the 3 bone spicule retrogradely out of the patella ligament, so that the bone spicule is supported under the bone block of the inferior pole in a row of needles ( Fig. 1d), bend the tail of the bone spicule at the upper pole of the patella for 180 °, pull down the bone spicule, and buckle the hook end of the bone spicule at the upper pole of the patella into the quadriceps femoris tendon, and then use two steel cables or steel wires to cross in pairs to x the fracture in a "net cage" technique( Fig. 1e). Recheck the reduction of the fracture through the C-arm uoroscopy (Fig. 1f) and ex the knee to check the stability of the inferior pole fracture. After con rming satisfaction, the drainage tube is left in the wound, and the wound is closed layer by layer.
There is no need to wear a brace or protective device after the operation. Patients are encouraged to perform early functional exercises and assist the patients to perform functional exercises through the CPM machine. X-rays are reviewed every month after the operation to understand the healing of the fracture.
After the fracture has healed, for patients who have a strong request to remove the internal xation, we only need to cut the upper part to expose the wire cable tightening device or wire knot and remove the bone spicule, so we need to pay attention to the placement of the wire cable tightening device or wire knot on the upper part of the patella.

Outcomes Measures
The primary results of this study were surgical complications of xation methods ( xation failure/displacement, implant fracture/dislocation) and knee function. Secondary outcomes included the number of uoroscopies and the incidence of soft tissue irritation.
All patients were followed up regularly at the 4th week and every month. During the postoperative followup, the anterior and posterior X-rays, lateral X-rays, and knee function were examined. Knee function was assessed by knee range of motion (the patient was instructed to lie prone on the checklist with a standard goniometer to measure ROM) and the Bostman score at the last follow-up. The criteria for fracture healing include clinical manifestations (no pain, no tenderness, and the ability to walk independently without the aid of tools) and radiology (trabecular connection of the two main fragments). Typical case: a 42 years old female with a fracture of the inferior pole of the patella caused by a fall was treated with "net cage" technology. She was followed up regularly after operation. One year after operation, the reexamination showed that the patient achieved good clinical effect and function. The results are as follows.

Discussion
The main treatment for inferior pole fractures of the patella is to restore the function of the quadriceps muscle and prevent the occurrence of osteoarthritis [11] through open reduction and internal xation. Because the fractures of the inferior pole fractures are usually very small and comminuted, it is di cult to carry out effective, stable xation, [6,11] and long-term external xation of the brace can cause knee dysfunction. In order to better x the fracture of the inferior pole of the patella, many xation methods have been applied. Lazaro et al. reported [12] : circular ligation and suture xe the fracture of the inferior pole of the patella, the wire band cannot effectively close the proximal articular surface of the patella, and the fracture fragments can easily enter the joint cavity. Zhu et al [9] . reported that the microplate and tension band wiring in the treatment of inferior pole fractures of the patella have achieved certain clinical effects.
Although the plate xation of fractures provides a new idea for our treatment [9,13,14] , because the patella is a sesamoid, its surface is not at, and there is no guarantee that the plate can be placed in a suitable position; in addition, the inferior pole of the patella and the gap between the patellar ligaments is very small, and placing a steel plate may increase the damage to the tissue structure. Assaf et al [15] . reported that anchor suture xation is to suture the inferior pole fracture and the patellar ligament together to reconstruct the knee extension function. However, this xation may have insu cient strength, and the postoperative brace xation is still insu cient for early functional exercise. Basket plate xation [13] and xchange acetabular revision mesh [16] may be promising alternatives, but there may be complex operations and greater trauma.
Partial resection of the patella is also used to treat fractures of the inferior pole of the patella, especially for comminuted fractures of the inferior pole of the patella [17][18][19] . However, a large number of studies have shown that partial patella resection will cause many unsatisfactory results. Kaufer [20] proved that the knee extension mechanism function of the knee joint after partial patella resection is reduced by 30%; resection of the inferior pole of the patella changes the length of the patellar tendon and makes the patella oriented moving downward, the end of the patellar ligament moves backward, changing the local mechanical distribution, which may interfere with the physiological function of the patellofemoral joint and cause a series of long-term complications [4,21] . Therefore, the patella should be preserved as much as possible.
Tension band wiring xation is considered to be the gold standard for optimal knee function recovery in patients with patella fractures [22] . The quadriceps contraction of the knee joint can tear apart the fragments at the fracture site of the patella. The tension band wiring xation converts this tension into compression so that the bone can withstand high tension loads and promote bone healing [2] . Studies have described the internal xation of displaced inferior patella pole fractures with anterior tension band wiring through cannulated screws and achieved good clinical results [6] . However, this xation method still requires a large fracture fragment to support the wire with screws, and it cannot be effectively xed for small, comminuted inferior pole xation. The traditional tension band wiring xation was abnormal, and some complications were reported, such as bone needle withdrawal, steel wire fracture, internal xation cutting. In order to solve these problems, we proposed an innovative xation method: "net cage" technique and achieved good clinical results in many years of clinical practice. We believe that this technique can provide a way for the clinical treatment of inferior patella fractures.
Our xation technology is not just two sets of tension bands. First of all, a fracture of the inferior pole of the patella can be regarded as a special avulsion fracture. It does not participate in the articular cartilage surface of the patella. We only need to reconstruct the continuity of the patella and the patellar ligament to restore the knee extension mechanism of the knee joint. The concept is similar to anchor suture xation [15] .
The "net cage" structure composed of bone pins and steel cables can effectively x the fractured mass and patellar ligament of the inferior pole to the broken end of the patella, providing strong xation for the fracture of the inferior pole (see Fig. 1e). Secondly, during the operation, we can directly touch the articular surface with our hands during the process of inserting the 3 bone spicules from bottom to top. This not only reduces the number of intraoperative uoroscopy but also ensures more precise insertion of the bone spicules. Drive 3 bone spicules parallel to the articular surface into the subchondral bone area. This area has a stronger bone condition and reduces the risk of disengagement and failure of the tension band due to the withdrawal of the spicules (due to the injury mechanism, fractures of the inferior pole of the patella are often compressed and comminuted in layers, and the bone mass is poor, which potentially reduces the rmness of the xation). Meanwhile, the three bone spicules are held parallel under the distal fracture fragments, which is similar to the raft nail effect [23] (similar to the tibial plateau fracture plate) to better support the inferior pole fracture fragments. It forms a cross mesh structure with the two tension band steel cables on the upper surface of the fracture fragments. The three-dimensional wrapping is similar to the "net cage" structure, which rmly hoops the inferior pole fracture fragments on the patella body, to restore the continuity of patellar ligament and reconstruction of knee extension device. Finally, we choose two sets of tension bands for xation for many reasons: 1. The "net cage" structure composed of the two sets of tension band pins and steel cables is smaller, which can prevent fracture fragments from leaking from the "net cage" structure; 2. One set of tension band xation is prone to cutting between internal xation and fracture fragments. When the two sets of tension bands are xed, the contact area between the lower pole fracture block and the internal xation increases, and the pressure on the contact surface will decrease accordingly, reducing the occurrence of cutting. 3. Even if one set of tension bands is withdrawn or loosened, the other set of tension bands will still play a role to avoid complete failure of internal xation. This has been con rmed in 34-C patella fractures.
In our study, the fractures of all patients were healed without internal xation loosening, rupture, and other complications, and during the operation of the knee joint, we found that the "net cage" technology to x the inferior pole of the patella fracture was very rm, and there was no need routine to wear a brace after the operation. The patient can actively or passively move the knee joint early so that the patient can obtain better knee joint function. In our study, 14 patients recovered the relative motion of the knee joint of the uninvolved limb. At the last follow-up, the knee function of 16 patients showed that the average ROM was 133.75°, which con rmed that "net cage" technology is bene cial to patients Recovery of knee joint function.
Our study also has limitations. First, this is a retrospective study. The relatively small sample size and the lack of a control group may cause bias in the results. Second, although good results have been achieved in clinical applications, there is a lack of biomechanical research on this technology. Third, this technique also needs to wear a brace after the operation in elderly patients with severe fractures and porosity, which may increase the occurrence of joint stiffness to a certain extent. In this regard, we look forward to conducting a large-sample, multi-center randomized controlled study to con rm the results of the study, and we are also conducting further studies on the biomechanics of the "net cage" technology.   Figure 1 Intraoperative process of "net cage" technique in the treatment of inferior patellar fracture