A precise navigation device for fixation of patella fractures with modified K-wire tension band: design and application


 Background Traditionally, the technique of modified tension band wires (MTBW) has been the most commonly used surgical procedure. The purpose of this study is to design a precise navigation device that can obtain a standard position of K-wires for (MTBW), and to compare the precise MTBW (P-MTBW) by a navigation device with the conventional MTBW (C-MTBW) by hands in a retrospective study.Methods The device is designed by solidworks2012 software (USA), which can provide a precise guidance for obtaining a parallel K-wires. In addition, it can set the distance between two k-wires and the level of k-wires below patellar anterior surface. From June 2014 to August 2018, a total of 112 patients are employed in this retrospective study. The patients are divided into P-MTBW group and C-MTBW group according to the surgical technique with or without the precise navigation device. We need to record and analyze the operation time and the number of fluoroscopy, postoperative internal fixation imaging, knee function and complications.Results There were 54 patients in P-MTBW group and 58 patients in C-MTBW group. There were statistically significant differences(P＜0.001) in the operation time between P-MTBW group (39.5±4.7; range, 32–49 minutes) and C-MTBW group (53.7±6.8; range, 42–71 minutes). The number of intraoperative fluoroscopy was significantly less (P＜0.001) in P-MTBW group(4.2±1.4) versus that of C-MTBW group (8.3±2.7). According to Iowa knee score, there was no significant difference (P=0.268) in function between the two groups. According to our own evaluation criteria for MTBW, anyone in the P-MTBW group was excellent and 26 patients were excellent, 20 patients were good and 2 patients were fair in the C-MTBW group.Conclusion The navigation device can reduce operation time and intraoperative fluoroscopy frequency. P-MTBW fixation is an accurate and effective surgical procedure for patella fractures.


Introduction
Patella fractures account for approximately 1% of fractures on adults [1,2]. The transverse fracture is the most common type of patella fracture, which often causes functional disability of the knee extensor apparatus with displacement of the fractured fragments [3].
Traditionally, the technique of modi ed tension band wires (MTBW) has been the most commonly used surgical procedure and is considered the gold standard for transverse patella fracture [4][5][6][7]. This technique consists of two parallel K-wires perpendicular to the fracture line and a 8-shaped wire passing anteriorly over the patella and behind the K-wires. According to AO principle [8,9], only when the two Kwires remain absolutely parallel can convert the tension force of the anterior patella into a compressive force across the articular surface, thus promoting fracture healing. Although the classic MTBW technique is reserved for transverse fractures, certain comminuted fractures can also be treated with a tension band construct if the articular surface is intact enough to allow for compression [10].
Parallel k-wires are the key to the tension band technique, usually depending on the experience and hand feeling of the surgeon. There are no reports about a device that remains two K-wires absolutely parallel, hence we designed a precise device to simplify surgery. This device can provide a precise guidance to obtain a parallel K-wires placement in any con guration, which can set the distance between two k-wires and the level of k-wires below patellar anterior surface, thus reducing the operation time and intraoperative uoroscopy. The goal of this study is to compare the precisely modi ed tension band wiring (P-MTBW) by a navigation device with the conventionally modi ed tension band wiring (C-MTBW) by hands in a retrospective study. We hypothesize that the P-MTBW would exhibit a better performance regarding surgical time, intraoperative uoroscopy, functional score and fewer complications than that of C-MTBW.

Materials And Methods
Design of the guide device

Clinical Study
This study protocol was approved by our hospital Ethics Committee. This retrospective study reviews patients with patellar fractures who underwent MTBW surgical treatment from June 2014 to August 2018. The inclusion criteria are shown as follows: (1) transverse fractures with or without a single additional fragment; (2) 18 to 65 years without previous knee surgery; (3) the articular displacement is greater than 2 mm or fragment separation is greater than 3 mm on radiography; (4) C1 and C2 type with consideration to AO classi cation; (5) MTBW with or without a navigation device; (6) follow-up at least a 12-month. According to these criteria, 112 patients are included in the study. The patients are divided into C-MTBW and P-MTBW according to the surgical technique with or without the precise navigation device.
There are 58 patients treated with the C-MTBW method and 54 patients treated with P-MTBW by the precise navigation device. We need to record and analyze the operation time and the number of uoroscopy, postoperative internal xation imaging, function and complications.
All patients underwent the standard MTBW technique. Approach and reduction techniques depend on the standard technique according to the AO principle, while the difference is that the P-MTBW group uses a self-designed precise navigation device to guide the K-wires. Adjusting the device parameters according to the width and thickness of patella measured before surgery. The distance of K-wires is set at one third of the widest diameter of the patella, and the level of the K-wires is set according to the thickness of the outer third of the patella. After setting the parameters of device, K-wire is implanted through the sleeve.

Postoperative Management And Evaluation
For all patients, an elastic bandage is used for 48 hours after surgery of reducing swelling. The knee joint is protected by the knee adjustable brace for four weeks, gradually increasing the exion and extension range of the motion. Postoperative follow-ups are arranged for one and two weeks, 1, 2, 3, 6 and 12 months, and the amount duration is longer than one year. Common complications include incision infection, failure of internal xation, fracture displacement and K-wire irritation. After one year of surgery, the knee function was evaluated according to the Iowa knee score criteria.
Postoperative imaging usually evaluates the reduction of the fracture and ignores the assessment of the internal xation position. We believe it is necessary to establish a standard evaluation strategy for K-wires of MTBW technique. All patients should have standard x-rays of the knee on two planes after surgery. An AP viewing and a lateral view are evaluated according to the angle between the K-wires. If the angle at both views is less than 5°, it is de ned as excellent; If the angle at one view is between 5° and 15 °, it is de ned as good; If the angle at both views is between 5° and 15 °, it is de ned as fair; If the angle at any view is more than 15 °, it's bad.

Result
There were 54 patients in P-MTBW group and 58 patients in C-MTBW group, 3 cases in P-MTBW group and 5 cases in C-MTBW group lost to follow-up. Demographics of both groups were no statistically signi cant differences (Table 1). There were statistically signi cant differences (P 0.001) in the operation time between P-MTBW group (39.5 ± 4.7; range, 32-49 minutes) and C-MTBW group (53.7 ± 6.8; range, 42-71 minutes). The number of intraoperative uoroscopy was signi cantly less (P 0.001) in the P-MTBW group (4.2 ± 1.4) versus that of C-MTBW group (8.3 ± 2.7). According to Iowa knee score, there was no signi cant difference (P = 0.268) in the function between the two groups (Table 1).
According to our own evaluation criteria, all patients in the P-MTBW group were excellent, and 26 patients were excellent, 20 patients were good, and 2 were fair in the C-MTBW group. The comparison between the two groups was statistically signi cant (P 0.001) ( Table 2).
There was no signi cant difference in the incidence of complications(P = 0.751)between the two groups, and none of all patients in this study were infected. One patient experienced failure of internal xation due to steel wire breakage, ve patients had K-wire irritation and three presented with fracture displacement in the P-MTBW group. In the C-MTBW group, three patients had internal xation failure due to wire breakage and k-wire withdrawal, six patients had K-wire irritation and two presented with fracture displacement. The removal rate of internal xation was not calculated in this study because of custom.

Discussion
Although there are new techniques such as cannulated screw tension band [11,12] system [13] and patella plate [14], the MTBW is still the most widely accepted surgical method for the treatment of patellar transverse fracture [7]. The MTBW technique is easy to operate on and economical to reduce the burden on patients, and the key of MTBW technique is to keep two k-wires parallel [8,15]. The manual operation is not enough to keep two k-wires parallel and needs to be repeatedly adjusted, hence we design and create this precise guiding device. In addition, it can set the distance between two k-wires and the level of k-wires below patellar anterior surface. According to the anatomic parameters of preoperative measurement, it can set distance accurately in the navigation device. Our study shows that the navigation device is accurate and effective, which can reduce the operation time and intraoperative uoroscopy frequency and is bene cial to the treatment of patellar transverse fracture.
So far, there is no research on evaluating the quality of MTBW technique, hence it is necessary to establish an evaluation strategy for K-wires of MTBW technique. The MTBW technique should be evaluated according to the angle between the two K-wires in an AP view and a lateral view after surgery. There is only 45% (26/58) excellence in C-MTBW group and all is excellent in P-MTBW group. The level of the K-wires and the distance between two k-wires are not included in the evaluation strategy, because a complex evaluation strategy is not easy to accept and understand.
According to AO principle [8], the ideal level for the K-wires lies in the center of the patella, approximately 5 mm below its anterior surface. In practice, the K-wires are closer to the articular than to the anterior surface. Hsu [16] reported the depth of K-wires involving 170 patients treated with MTBW,37 (22%) patients at super cial third and 133 (78%) patients at the middle third of the patella, and super cially placed Kirschner wires increased the rate of minor loss of reduction.
There is a recently published study by Yang [15] et al show that the wide distance between them and the K-wires at the deep level might be more helpful for MTBW. Ling [17] performed a nite element analysis, and posteriorly placed K-wires make optimal stability possible. We believe that it should be located 5 mm below the anterior surface of the lateral third, because the anterior surface of the patella is irregular and the lateral part of the patella is thinner.
There are some limitations in our study. First, it is not a prospective and randomized study. Second, all the operations are open, and the closed reduction and internal xation guided by the guidance should be further carried out in the later. Finally, we don't distinguish the level of K-wires and the distance between the K-wires.
In conclusion, the navigation device can reduce the operation time and intraoperative uoroscopy frequency. P-MTBW xation is an accurate and effective surgical procedure for treatment of transverse patellar fractures. The evaluation strategy for MTBW technique is highly necessary, therefore we can adjust the position of k-wires during the operation. The participant enrolled into the study agreed the use of data for research.

Availability of data and material
All of the data are available in contact with the correspondent author.

Competing interests
No Funding This study was sponsored by the Chongqing Yuzhong District Science and Technology Project (No.

20170126)
Authors' contributions FM and GW are responsible for the whole study, FM and HL for manuscript writing, and XT, YL, and XH are assisting in the surgery. All authors have read and approved the manuscript for submission. Figure 1 Design appearance of the navigation device The physical appearance of the navigation device during the operation