3.1 Characteristics of inferior pole fractures of patella and disadvantages of traditional surgical methods
Inferior pole fractures of patella are clinically common, accounting for approximately 9.3%-22.4% of all patella fractures. It has the following characteristics: Firstly, the fracture fragments are small and most of them are comminuted, Secondly, the inferior part of the patella is the insertion point of the patellar tendon, which is subjected to greater tensile stress. For that reason, the fracture is under a certain degree of particularity and difficulty in reduction and fixation  .
Up to this time, wire tension band fixation and resection of the inferior pole of patella, etc. are the preferred surgical treatments for inferior pole fractures of patella . Resection of the inferior pole of patella is rarely applied at present because it changes the biomechanical characteristics of the patella, causes weakened quadriceps muscle strength and patellofemoral arthritis and affects the function of the knee joint.
In contrast, Kirschner wire tension band internal fixation is widely applied. In this method, the steel wire is placed on the tension side of the patella, and the tension it is subjected to will change into the compressive stress at the fracture end during knee flexion therefore promote the fracture healing. In the traditional AO tension band technique, two Kirschner wires are fixed in the center of the bone while the “8”-shaped steel wire is fixed at both ends of the Kirschner wires. It is reported that although the tension band wire fixation method is reliable and achieves satisfactory clinical outcomes, it also has numerous drawbacks. First of all, this technique is not appropriate for small bones. Secondly, the fixation technique for comminuted fracture is not reliable. In addition, Kirschner wire tension band is prone to Kirschner wire withdrawal, which can compress the skin and cause pain, or on occasions pierce the skin. Finally, Kirschner wire tension band must be removed by a second operation, which can contribute to increased patient pain and medical costs.
3.2 Advantages and disadvantages of traditional anchor technique
Suture anchor was first applied in the treatment of rotator cuff tears and gradually applied in the treatment of inferior pole fractures of patella with the development of techniques in orthopedic. Fixation of comminuted fractures of the inferior pole of the patella with suture anchors can not only restore the integrity of the knee extension device, but also preserve the fracture of the inferior pole of the patella. Concurrently, bone-bone healing is also superior to tendon-bone healing. Ultimately, suture anchor does not need to be removed subsequently a secondary operation is avoided. In summary, this technique has obvious advantages over traditional wire tension band internal fixation and resection of the inferior pole of patella.
The traditional suture anchor method is a single row anchor fixation, which also has many drawbacks. First of all, single-row anchors rely only on sutures, which can not provide sufficient stability at the fracture end and prevent early ambulation postoperatively. Moreover, for comminuted fractures, fracture fragments can not be reduced well by wire anchors alone, which may easily cause displacement of fracture fragments.
3.3 Advantages of double-row anchor suture bridge technique
“Double-row anchor suture bridge technique” is a novel technique for arthroscopic repair of rotator cuff tears, which was first proposed by Park in 2006. Firstly, the sutures of the internal-row anchors passed through the tendon in a horizontal mattress and were knotted. After passing the suture tail through the external-row pressing screw, the external-row nails were inserted into the greater tubercle of the humerus at the lateral insertion point of the rotator cuff, so as to complete the uniform extrusion of the rotator cuff by the “suture bridge”. This technique is more stable than the traditional anchor fixation technique and has greater anti-pulling strength. Furthermore, due to the extrusion pressure of the “suture bridge” on the rotator cuff, the contact surface between tendon and bone is increased and the tendon and bone healing is promoted. In view of the advantages of suture bridge technique, this technique has gradually been widely applied in shoulder surgery, and its scope of application has also been expanded to be used in the treatment of some fractures. It has been reported in literaturethat some scholars have applied this technique to the treatment of avulsion fractures of the greater tuberosity of humerus and intercondylar spine avulsion fractures of the tibia, achieving satisfactory clinical results. However, there have been no reports about the application of this technique in the treatment of patellar fractures.
In order to overcome the shortcomings of the traditional suture anchor method, we modified the technique of “double-row anchor suture bridge” in shoulder arthroscopy and applied it to the treatment of the fracture of the inferior pole of patella. Specifically, two internal row anchors were screwed into the proximal end of the patella fracture. Subsequently, the sutures were passed through the inferior pole fracture fragments via the bone tunnel and the patellar ligament was sutured with horizontal pad and knotted. The tail line of the suture was arranged to cover the bone fragments at the lower part of the patella. After passing the tail of the suture through the external extrusion anchor, the external extrusion nail was driven into the upper part of the patella (see Figures A-B). This technology not only strengthens the stability of the fracture, but also gathers the lower pole fragments and maintains the reduction of the fracture. Since the sutures of the anchors wrap around the bone fragments below the patella in the shape of a parachute, it is also known as “parachute technique”.
Although Kim KS reported that the suture bridge anchor technique was used to fix the fracture of the lower pole of the patella, the technique in this study was different from theirs. First of all, their technology need two extrusion anchors, while the technique implemented here only used one, which saved the cost of surgery. Simultaneously, the extrusion anchors were reported to be driven into the both sides of the patella according to Kim KS. However in this study, the extrusion anchors was driven into the bone at the upper end of the patella as this part of the bone is thicker and stronger, which can obtain better fixation strength.
The technique used in this research is to treat the fracture of the lower pole of the patella. Patients were able to exercise the flexion and extension function of the knee joint early after operation. No loosening of internal fixation and re-displacement of the fracture were found. All fractures healed and achieved satisfactory clinical results. It was discovered that comminuted fractures benefited from utilizing this technique, which could not only increase the stability of the fixation, but also close and reduce the fracture block. In this group, All comminuted fractures were healed well without fracture displacement and loss of reduction.
3.4 Points of attention and surgical experience
First of all, the two anchored implant points should be lower than the center of the patella section, close to the subchondral bone and need to be screwed to a sufficient depth in order to achieve the maximum holding force. For patients with relatively loose bones, the screw can be screwed deeper until it approaches or even breaks through the cortical bone at the upper end of the patella to increase the holding force of the anchor. If the anchor is found to be loose when screwed in, it is estimated that sufficient stability can not be obtained therefore it is necessary to replace or assist other internal fixation methods. At the initial stage of the operation, one case was found to have a loose screw anchor during the operation hence other internal fixation methods were implemented in time. In the coronal position, the two anchors should be positioned equally on either side of the midline of the patella to facilitate fracture reduction and stress distribution. All the patients in this group were operated on according to the above methods. On that account, no loosening of anchor or loss of fracture block reduction was found.
Secondly, after the suture passes through the fracture of the lower pole, one end of the tail line needs to be sewed into the patellar ligament for 2-3 stitches and then tied into the other end for fixation. Direct knotting may result in cutting of bone, especially in comminuted fractures. After all the four wires are knotted, the position of the tail wire should be adjusted and the broken bones of the bottom pole should be covered as much as possible, which is resembling a parachute bag, in order to achieve the optimal fixing outcome. All cases in this group were performed according to the above methods and no tendon cutting or loss of fracture block reduction was found.
Moreover, intraoperative fluoroscopy is required to ensure that the extrusion screw is in the center of the upper pole of the patella when the external extrusion screw is inserted and attention should be paid to the matching perforator to open the bone canal. In case of too small perforator and insufficient bone canal diameter, splitting of the bone canal wall may occur when the extrusion anchors are inserted. In severe cases, iatrogenic fractures may occur. Meanwhile, proper tail line tension needs to be adjusted for driving the extrusion anchors. Too little tension may make it impossible to achieve favorable fixation and too much tension may result in the cutting of the bone tunnel wall with sutures and displacement of the inferior pole fracture.
Finally, after the fracture is fixed, the two patellas are circulated with non-absorbable ETHIBOND suture to strengthen the fixation. Bend and extend the knee joint to confirm that the fractured end is firmly fixed and not separated. If instability is found, a brace is recommended for postoperative protection.