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 through open reduction and internal fixation. Because the fractures of the inferior pole fractures are usually very small and comminuted, it is difficult to carry out effective, stable fixation, [6, 11]and long-term external fixation of the brace can cause knee dysfunction. In order to better fix the fracture of the inferior pole of the patella, many fixation methods have been applied. Lazaro et al. reported: circular ligation and suture fixe 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. 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 fixation of fractures provides a new idea for our treatment[9, 13, 14], because the patella is a sesamoid, its surface is not flat, 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. reported that anchor suture fixation is to suture the inferior pole fracture and the patellar ligament together to reconstruct the knee extension function. However, this fixation may have insufficient strength, and the postoperative brace fixation is still insufficient for early functional exercise. Basket plate fixation and x-change acetabular revision mesh 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[17–19]. However, a large number of studies have shown that partial patella resection will cause many unsatisfactory results. Kaufer 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 fixation is considered to be the gold standard for optimal knee function recovery in patients with patella fractures. The quadriceps contraction of the knee joint can tear apart the fragments at the fracture site of the patella. The tension band wiring fixation converts this tension into compression so that the bone can withstand high tension loads and promote bone healing. Studies have described the internal fixation of displaced inferior patella pole fractures with anterior tension band wiring through cannulated screws and achieved good clinical results. However, this fixation method still requires a large fracture fragment to support the wire with screws, and it cannot be effectively fixed for small, comminuted inferior pole fixation. The traditional tension band wiring fixation was abnormal, and some complications were reported, such as bone needle withdrawal, steel wire fracture, internal fixation cutting. In order to solve these problems, we proposed an innovative fixation 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 fixation 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 fixation. The "net cage" structure composed of bone pins and steel cables can effectively fix the fractured mass and patellar ligament of the inferior pole to the broken end of the patella, providing strong fixation 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 fluoroscopy 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 firmness of the fixation). Meanwhile, the three bone spicules are held parallel under the distal fracture fragments, which is similar to the raft nail effect (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 firmly 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 fixation 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 fixation is prone to cutting between internal fixation and fracture fragments. When the two sets of tension bands are fixed, the contact area between the lower pole fracture block and the internal fixation 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 fixation. This has been confirmed in 34-C patella fractures.
In our study, the fractures of all patients were healed without internal fixation loosening, rupture, and other complications, and during the operation of the knee joint, we found that the “net cage” technology to fix the inferior pole of the patella fracture was very firm, 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 confirmed that “net cage” technology is beneficial 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 confirm the results of the study, and we are also conducting further studies on the biomechanics of the “net cage” technology.