Anatomic Study and Assembled Navigation Template-Assisted Implantation for Anterograde Transpubic Screws

Objective: Research the anatomical parameters of the anterograde transpubic screw corridor and evaluate the safety of anterograde transpubic screw placement assisted by the assembled navigation template. Methods: A total of 50 normal subjects, including 25 males and 25 females, underwent pelvic CT scanning in our hospital from January 2020 to September 2020. A 3D model of the ilium was established. The ilium was divided into zone (cid:0), (cid:0) and (cid:0) according to Nakatani classication. The anterograde transpubic screw channel completely passes through zone (cid:0) and (cid:0) to form corridor A. The anterograde screw channel completely passes through zone (cid:0), (cid:0) and (cid:0) to form corridor B. The diameter and length of the inner circle, the distance from the center of the inner circle to the posterior superior and to the inferior iliac spine of corridor A and corridor B were measured, respectively. A total of 9 patients underwent anterograde transpubic screw and transverse sacroiliac screw placement assisted by the assembled navigation template in our hospital, including 5 males and 4 females, were retrospectively analyzed. Operative time, blood loss, incision length and uoroscopy times were recorded. Grading score and Matta score were evaluated after surgery. Results: In 50 normal subjects, the diameter of corridor A was 11.16±2.13 mm, and that of corridor B was 8.54±1.52mm, and the difference between the two corridors was statistically signicant (P=0.000). The length of corridor A was 86.39±9.35 mm, and that of corridor B was 117.05±5.91 mm, with signicant difference between the two corridors (P=0.000). The surface distance from the screw entry point to the posterior superior iliac spine in corridor A was 109.31± 11.06mm, and that in corridor B was 127.86± 8.23mm, the difference between the two corridors was statistically signicant (P=0.000). The surface distance from the screw entry point to the posterior inferior iliac spine in corridor A was 91.16±10.34 mm, and that in corridor B was 106.92±7.91 mm. The difference between the two corridors was statistically signicant (P=0.000). Nine patients successfully completed surgery, and a total of 18 sacroiliac transverse screws and 11 anterograde transpubic screws were inserted with the assistance of assembled navigation templates. The mean operation time of the 9 patients was 108.75±25.71 min, the blood loss was 141.11±50.21 ml, the incision length was 14±4.62 cm, and the intraoperative uoroscopy was 17.89±4.01 times. Matta scores were excellent in 5 patients and good in 4 patients. One of the anterograde transpubic screw was in Grade 1, and 10 were in Grade 0. One S1 screw was in Grade 1, and 8 S1 screws were in Grade 0. Nine S2 screws were in Grade 0. Conclusions: Majority of the patients can accommodate anterograde transpubic screw s with diameter of 6.5 mm. Anterograde transpubic screw placement assisted by an assembled navigation template is clinically feasible, and with low cortical breaches. transpubic corridor transpubic screws digital 3D modeling, conditions of transpubic screws in a population During prone while the sacroiliac and transpubic screws After of patients with severe open pelvic sacroiliac closed t-test was performed for the diameter of the corridors, the length of the corridor, and the surface distance from the screw entry point to the posterior superior iliac spine and inferior iliac spine. Descriptive statistics were performed on the general parameters and operative parameters of the patients. The and was regarded as signicant difference.


Introduction
Pelvic fractures often result in instability of the anterior or posterior ring [1][2][3][4]. To maintain the stability of the pelvis, the current treatment methods mostly involve simultaneous internal xation [5]. For posterior pelvic ring fractures, sacroiliac screws have become a popular technique for many orthopedic surgeons because of their good biomechanical performance [6]. However, there are few anatomical and biomechanical studies on anterior ring instability, especially for pubic ramus fractures. Common anterior ring xation techniques include plate xation, anterior ring percutaneous xation (In x) and transpubic screw xation [7]. Through mechanical experiments, researchers have con rmed that transpubic screws can maintain the stability of the anterior ring of the pelvis [8]. Based on its advantages of good mechanical stability and low trauma, transpubic screws are considered to be the best surgical method for pubic fracture [9]. An anatomic study of 260 transpubic screw corridors found that all males were able to accommodate the insertion of screws with a diameter of 6.5 mm, while 15% of females were unable to accommodate the insertion of screws with a diameter of 6.5 mm [10][11]. Most men could even accommodate the insertion of a 7.3 mm diameter screw [12].
However, most of these studies were based on two-dimensional images of patients, which are not intuitive and accurate, and lack analysis of Asian populations and clinical application. At present, the clinical application is mostly seen in retrograde transpubic screws, which require changing the patient's position during surgery, increasing the risk of infection [13][14].
In this study, a method of rapid transpubic screw corridor extraction was developed; the cancellous corridor of transpubic screws was analyzed and measured through digital 3D modeling, and the anatomical conditions of transpubic screws in a population in Southwest China were determined. During surgery, the patient was placed in the standard prone position, assisted by an assembled navigation template, while the sacroiliac and transpubic screws were inserted. After surgery, the safety of the screws was evaluated based on cortex penetration.
This study aimed to (i) explore a rapid method to obtain a transpubic screw corridor, (ii) evaluate the feasibility of transpubic screw placement, and (iii) assess the safety of the assembled navigation template for screw placement.

Measures of anatomical conditions
For the study, pelvic computed tomography (CT) datasets with a slice thickness of 0.625 mm were collected from our hospital between December 2019 and October 2020, and residual trauma and tumorous, in ammatory or high-grade degenerative alterations were excluded. Ultimately, 50 adults (including 25 males and 25 females) were included in the study. Informed patient consent for further research applications of image data was obtained.
The CT data were imported into Mimics 22.0 (Materialise, Leuven, Belgium). Based on the CT values of the bones, a pelvic mask was obtained (covering cancellous and cortical bones). The bilateral iliac crests were separated from the bone mask, and 3D models were constructed. The pubis was divided into zones , and according to the Nakatani classi cation [15].
In transparent mode, by rotating the ilium to the appropriate position, corridor A (approximately an oval) was formed by pubis zones and ( Figure 1A, B), and corridor B (a quadrilateral) was formed by pubis zones , and ( Figure 1D, E).
The diameter of the tangent circle and the length of the two corridors were measured. The distance along the iliac crest surface from the center of the tangent circle of the two channels to the posterior superior iliac spine and to the posterior inferior iliac spine was also measured ( Figure 1C, F).

Patients
The inclusion criteria were as follows: de nite reducible public fracture with zone and are associated with sacral fractures (Denis type / ) or reducible sacroiliac dislocation, the superior ramus pubis screw channel can accommodate screws of at least 6.5 mm diameter, and tile classi cation B/C. The exclusion criteria were as follows: patients with severe systemic diseases and open pelvic fractures, fracture and sacroiliac dislocation cannot be closed reduction.
A total of 9 patients with fractures in the pubis in our hospital from January 2020 to September 2020 were enrolled in this research, including 5 males and 4 females. Their ages ranged from 21 to 65 years, and their average age was 47 years. Six of the patients were injured from tra c accidents, 3 of them were injured from falling, and the others were injured for other reasons. CT scans were performed on the pelvises of all patients before surgery ( Table 1).

Design of an assembled navigation template
The 3D model of the patient's pelvis was constructed in mimics 22.0. Corridor A or B was determined according to the location of the fracture in the superior ramus of the pubis, and then a virtual screw with a diameter of 6.5 mm was inserted along corridor A or B (Figure 2A). At the same time, sacroiliac screw diameters of 7.3 mm via S1 and S2 were inserted simultaneously according to Wu The base is designed with internal threaded holes in the direction of the virtual screw for assembly with the navigation pipe. The navigation pipe with thread at the bottom is designed along the direction of the virtual screw ( Figure 2B, C). The navigation pipe is designed with a hole along the virtual screw that can accommodate the diameter of 2.6-mm K-wires. Finally, the navigation pipe was assembled with the base through a threaded structure ( Figure 2D). Then, an assembled navigation template to assist simultaneous transpubic screw S1 and S2 screw placement was formed.

Preoperative preparation
The navigation template base, navigation pipes and pelvis were saved in STL format and imported into 3D printers (3DS, projet 3600) and (3D talk, FDM 460) ( Figure 3A, B). The operative procedure was simulated based on the 3D-printed object, K-wires were inserted under the assistance of a navigation template, and the consistency with the preoperative design was evaluated ( Figure 3C, D). The navigation template and pelvic model were hypothermically disinfected before surgery.

Surgical technique
The operations were performed by the same surgeon with more than 10 years of experience. All patients were informed of the experimental design before the surgery and signed an informed consent form.
Under general anesthesia, the patient was placed in the standard prone position on a radiolucent operation table. First, traction reduction was performed for some patients, Reduction of fracture or dislocation was con rmed by intraoperative C-arm uoroscopy. Second, posterior superior iliac spine and screw insertion points were located on the body surface. Third, the soft tissue of the posterior superior iliac spine was stripped, and the template base was inserted through the above incision and completely attached to the corresponding target spot ( Figure 4A). Fourth, threaded navigation pipes were inserted through a small incision and assembled and xed with the navigation template base in the wound ( Figure 4B). Fifth, several K-wires 2.5 cm in diameter were drilled through navigation pipes ( Figure 4C). Pelvic inlet, outlet and lateral sacral uoroscopy images were obtained to con rm the positions of the Kwires ( Figure 5A, B, C). Sixth, the navigation pipes were removed after the K-wire positions were con rmed ( Figure 4D), and cannulated screws with diameters of 7.3 mm and 6.5 mm were screwed into the sacrum and pubis along the K-wires, respectively ( Figure 4E). Finally, the K-wires and template base were removed ( Figure 4F).

Evaluation criteria
Anatomical parameters of anterograde transpubic screws The tangent circle diameter for corridor A or B was measured to evaluate whether a 6.5-mm-diameter screw could be inserted into the pubic ramus. The length of corridor A or B was measured to evaluate the length of the transpubic screw. The surface distance from the screw entry point to the posterior superior iliac spine and to the posterior inferior iliac spine was measured as a reference for the covering area of the navigation template base.

Operated information for patients
Screw safety was assessed using the grading score [17]. The quality of the reduction was assessed by the Matta score [18].

Statistical analysis
All statistical analyses were performed in SPSS 19.0 (SPSS Inc., Chicago, IL, USA). Student's t-test was performed for the diameter of the corridors, the length of the corridor, and the surface distance from the screw entry point to the posterior superior iliac spine and inferior iliac spine. Descriptive statistics were performed on the general parameters and operative parameters of the patients. The con dence interval was set at 95%, and P<0.05 was regarded as a signi cant difference.

Anatomical measurement
A total of 50 normal adults were enrolled in this study.

Clinical indicators
A total of 9 patients completed the surgery successfully, with 18 sacroiliac screws and 11 anterograde transpubic screws implanted under the assistance of assembled navigation templates ( Figure 5D-I). There were no complications, such as vascular and nerve injury, and 1 patient developed super cial wound infection. After wound treatment and dressing changes, the wound healed. The 9 patients had an average surgical time of 108.75±25.71 min, a blood loss of 141.11±50.21 ml, an incision length of 14±4.62 cm, and a radiation of 17.89±4.01 times. One anterograde transpubic screw was Grade 1, and 10 screws were Grade 0. One S1 screw was Grade 1, and 8 S1 screws and 9 S2 screws were Grade 0 (

Discussion
Anatomical conditions of anterograde transpubic screws Retrograde transpubic screw xation can provide stability of the anterior pelvis with fewer surgical exposures than plate xation [19][20]. There have been many studies on the anatomic measurement of retrograde transpubic screws, and taking reproductive structures as the marking point, the entry point of retrograde transpubic screws was located, which greatly reduced the di culty of screw placement [21][22][23]. Zhang LH developed a navigation apparatus to assist with the placement of percutaneous retrograde transpubic screws to minimize trauma [24]. However, the reduction failure rate is up to 15% after percutaneous screw xation for pubic ramus fractures, and failure is particularly common in elderly patients and females . Fourth, anterograde transpubic screw xation can avoid many problems, such as large trauma due to anterior ring xation and injury of the lateral femoral cutaneous nerve. Fifth, the assembled navigation template can signi cantly improve the safety of screw placement, and no screw penetrated the cortex more than 2 mm in our study.

Surgical tips
First, the base of the navigation template should cover important targets, such as the posterior superior iliac spine and the posterior inferior iliac spine. Second, the navigation template base will be warped if it is too long, therefore, the navigation template base should be shortened as much as possible while still covering important targets. The directions of the navigation pipes should not intersect to prevent interference between screws, and K-wire holes are designed based on the navigation template to strengthen the xation between the navigation template and the iliac crest. Third, the entry point of the transpubic screw should be located on the body surface under uoroscopy before surgery to reduce the tension of skin and soft tissue. Fourth, the soft tissue around the target should be thoroughly removed during the operation to ensure the tight attachment of the navigation template base. Fifth, it is recommended to perform pelvic inlet, outlet and lateral sacral uoroscopy. Finally, navigation K-wires for sacroiliac screws are recommended to be inserted rst to enhance the stability of the navigation template base, followed by navigation K-wires for transpubic screw insertion.
Limitations A total of 50 normal persons were included in this study, and there was not enough diversity in the sample. The sample size will be expanded in the future to obtain more objective and accurate nail track parameters. In addition, once the intraoperative incision deviates from the screw entry point are located on the body surface, soft tissue tension will reduce the accuracy of the navigation wire, therefore, we are developing a metal mesh that can be attached to the skin for the positioning of the entry point.

Conclusion
This research demonstrates that the majority of patients can accommodate transpubic screws with a diameter of 6.5 mm.
Anterograde transpubic screw placement assisted by an assembled navigation template is clinically feasible, and with low cortical breaches.

Consent for publication
The participant has consented to the submission of the case report to the journal.
Patients signed informed consent regarding publishing their data and photographs.

Availability of data and materials
The patients' data and software were authorized to our research.

Competing interests
The authors have no con ict of interest. This study was approved by the ethics committee of our hospital.   Figure 1 <p>The pubic was divided into zone , zone and zone according to the Nakatani classi cation and the corridor for transpubic screw in formed. A, B. the corridor A (an oval) was formed by the pubis of the zone and . C. The diameter(D) of the tangent circle, length of the two corridors, distance along the Iliac crest surface from the center of the tangent circle of the two channels to the posterior superior iliac spine (SD1) and to the posterior inferior iliac spine (SD2) was measured. <strong>&nbsp;</strong>D, E. the corridor A (a quadrilateral) was formed by the pubis of the zone , and . f. The diameter(D) of the tangent circle, length of the two corridors, distance along the Iliac crest surface from the center of the tangent circle of the two channels to the posterior superior iliac spine (SD1) and to the posterior inferior iliac spine (SD2) was measured. <strong>&nbsp;</strong></p> Figure 2 <p>The design of assembled navigation template. A, a virtual screw with diameter of 6.5 mm was inserted along corridor. B. The navigation base is designed with internal threaded holes in the direction of the virtual screw for assembly with the navigation pipe. C. The navigation base is designed with 5 holes of 2.7 mm for 2.5 screws to reinforce the attachment of the base to the iliac crest. D.
Sketch of the assembled navigation template.</p> Figure 3 <p>The transpubic screw placement simulation based on 3D printing. A. the 3D printed assembled navigation template and pelvic. B.
The assembled navigation template was assembled and attached to the surface of the iliac crest. C, D. K wires were inserted under the assistant of a navigation template.</p> Figure 4 <p>The operation procedure A. The template base was inserted through the incision on posterior superior iliac spine. B. The template base was completely attached to the corresponding target spot. C. navigation pipes were inserted and assembled with the navigation template base and K wires were inserted along the navigation pipes. D. The K wires were inserted along the navigation pipes and navigation pipes were removed. E. The cannulated screws were inserted along the K wires. F. The K-wires and template base were removed</p>