Design of novel plate for fixation of anterior pelvic ring injury: a digital anatomy study and finite element analysis

Abstract

Objective

To develop a novel internal fixation system for anterior pelvic ring injuries and to compare the biomechanical stability of this novel anatomical plate with conventional fixation devices in the management of Tile B1-type pelvic fractures using finite element analysis.

Methods

A total of 200 pelvic thin-slice CT datasets were collected from healthy adults, comprising 100 male and 100 female subjects. The three-dimensional (3D) pelvic models were reconstructed using Mimics software. The following measurements were obtained: inter-pubic tubercle distance (ITD), inter-obturator foramen inner border distance (IOFIBD), superior-inferior distance of pubic symphysis (SIDPS), pubis superior ramus length from mid-external pubic tubercle to lateral obturator margin (PSR-LOTM), pubic symphysis gap (PSG), pubic symphysis upper plane-superior ramus angle (SPS-SRA), pubic symphysis-coronal plane angle (PS-CPA), maximum superior-inferior diameter of pubic superior ramus (SID-PSR), and anterior-posterior diameter of pubic superior ramus (APD-PSR). Standard pelvis models were selected and imported into SolidWorks software to design two novel plates. Finally, a Tile B1 pelvic finite element model was established and fixed using two novel plates, a single superior pubic symphysis plate, and a combination of anterior and superior pubic symphysis plates. A load of 500 N was applied to the model in three directions (cranial–caudal, anterior–posterior, and lateral–medial), and the stiffness was determined based on the maximum displacements.

Results

The ITD was 48.93 ± 5.51 mm in males and 54.45 ± 5.15 mm in females; the IOFIBD was 46.67 ± 4.02 mm in males and 55.17 ± 4.46 mm in females; the SIDPS was 39.77 ± 4.12 mm in males and 36.76 ± 4.21 mm in females; the PSR-LOTM was 35.87 ± 4.04 mm in males and 37.62 ± 5.31 mm in females; the SPS-SRA was 135.67 ± 4.83° in males and 137.53 ± 4.46° in females; and the ITD + PSR-LOTM was 120.68 ± 10.66 mm in males and 124.70 ± 11.93 mm in females. Significant differences were observed between males and females for all these measurements (P < 0.05). These values were used as reference criteria for selecting standard pelvis models. The ITD + PSR-LOTM data were sorted in ascending order, and the medians of the 0–33%, 33–66%, and 66–100% intervals were used as references for selecting standard pelvis models. Based on the selected standard pelvis models, two designs and six length specifications (110, 115, 120, 125, 130, and 135 mm) of anatomical plates were developed. In the finite element analysis, the maximum displacements of the pelvis under three loading modes (cranial–caudal, anterior–posterior, and lateral–medial) were as follows: for Type A plate fixation, 0.357 mm, 0.192 mm, and 1.018 mm, respectively; for Type B plate fixation, 0.362 mm, 0.505 mm, and 1.133 mm, respectively; for single pubic symphysis superior plate fixation, 0.386 mm, 0.965 mm, and 1.232 mm, respectively; and for combined pubic symphysis anterior and superior plate fixation, 0.378 mm, 0.874 mm, and 1.151 mm, respectively.

Conclusions

We successfully developed two types of plates and designed different specifications to meet clinical needs. Preliminary biomechanical finite element analysis indicated promising fixation stability for Tile B1 type injuries, warranting further clinical investigation.

Introduction

Anterior pelvic ring injuries, encompassing pubic symphysis diastasis as well as fractures of the pubic body, and the superior and inferior pubic ramus, constitute approximately 24% of pelvic fractures [1–3]. These injuries are mainly triggered by high - energy trauma and are often accompanied by concurrent injuries to the abdominal and pelvic viscera, genitourinary structures, and femoral neurovascular bundles [4, 5]. Surgical intervention is typically recommended for pubic symphysis diastasis and bilateral pubic ramus fractures. Conversely, non - surgical treatment is preferred in the following circumstances: (1) stable Tile Type A fractures of the anterior pelvic ring; (2) pubic symphysis diastasis measuring less than 2.5 cm; and (3) minimally displaced fractures of the pubic body, as well as the superior and inferior pubic ramus. Surgical fixation methods for these injuries encompass: (1) Anterior ring external fixation, which is mainly employed for the temporary stabilization of unstable pelvic fractures. Nevertheless, it is restricted by its inability to achieve precise reduction and rigid fixation [6, 7]. (2) Percutaneous screw fixation presents advantages such as minimal invasiveness, shorter operative duration, and reduced blood loss when treating pubic symphysis diastasis and pubic ramus fractures [8]. However, considering that the pubic symphysis is a micromotion joint, the long - term outcomes of single or even multiple hollow screw fixations remain inconclusive. Therefore, this method should be used with caution in patients with pubic symphysis diastasis and osteoporosis. (3) Open reduction and plate fixation is regarded as the standard surgical approach for pubic symphysis diastasis, offering reliable stability. Biomechanical studies have shown that dual plating in multiple planes provides superior stability, and many scholars advocate for dual plating to promote early postoperative mobilization [9]. However, placing dual plates, usually above and anterior to the pubic symphysis, is technically demanding. This results in greater surgical trauma and frequent screw “interference,” which requires repositioning of the plates and prolongs the operative time [10].

In summary, although several fixation constructs for anterior pelvic ring injuries are available, a single device that simultaneously provides dual-plane stability without excessive soft-tissue dissection has not yet been described. Therefore, the present study aimed to develop an anatomically contoured, integrated dual-plane plate for anterior pelvic ring injuries, and test the hypothesis that this novel plate would confer superior biomechanical stability compared with conventional single and dual reconstruction plates.

Materials and methods

Materials

The pelvic 64-slice spiral CT datasets (Siemens Healthineers, Erlangen, Germany; image resolution:290 × 290 px; slice thickness: 0.625 mm) were retrospectively collected from 200 adults (100 males, including 66 cases under 60 years old and 34 cases aged 60 or above; 100 females, including 65 cases under 60 years old and 35cases aged 60 or above; age range: 18–90 years, mean 49.68 ± 16.99) who underwent pelvic CT for non-orthopedic indications (e.g., renal calculus screening, soft tissue evaluation) at the imaging center of the first affiliated hospital of Chongqing Medical University during October 2017 and October 2018. Participants with pelvic deformity, fractures, tumors, or infection were excluded. All CT datasets were reconstructed into 3D pelvic models by Materialise’s interactive medical image control system (Mimics, version 17.0, Materialise Inc, Belgium) .

Anatomical measurement of the anterior pelvic ring

The pubic symphysis was anatomically aligned using the “Rotating” function in Mimics 17.0 to ensure standardized orientation prior to measurement (Fig. 1).

Fig. 1.

Anatomical measurement of the anterior pelvic ring

1. The linear distance between the most lateral points of bilateral pubic tubercles was measured, defined as the inter-pubic tubercle distance (ITD) which was marked as L1 (Fig. 1a).

2. The minimum horizontal distance between the medial borders of bilateral obturator foramina was measured, defined as the interobturator foramen inner border distance (IOFIBD) which was marked as L2 (Fig. 1a).

3. The vertical distance from the superior to inferior margin of the pubic symphysis was measured, defined as the superior-inferior distance of pubic symphysis (SIDPS) which was marked as L3(Fig. 1a).

4. A plane (p) was constructed perpendicular to the lateral edge of the obturator plane. The intersection of plane p with the superior pubic ramus was identified, and the midpoint between the anterior and posterior edges of the superior pubic ramus at this intersection was marked as point B. Using the “freehand curve drawing” function with a step length of 5 mm, the length of the superior pubic ramus from the midpoint of the pubic tubercle to point B was measured, defined as the pubis superior ramus length from mid-external pubic tubercle to lateral obturator margin (PSR-LOTM) which was marked as AB (Fig. 1b).

5. The distance between the midpoints of the bilateral pubic symphysis surfaces was measured, defined as thepubic symphysis gap (PSG) which was marked as H(Fig. 1c).

6. A re-cut layer was performed vertically along the anterior surface of the pubic symphysis from superior to inferior, with a slice interval of 1 mm. On the re-cut section where both the pubic symphysis and the superior pubic ramus were visible, the angle between the axis of the upper surface of the pubic symphysis and the axis of the superior pubic ramus was measured. The average value of this angle was recorded as pubic symphysis upper plane-superior ramus angle(SPS-SRA) which was marked as α(Fig. 1d).

7. On the standard anatomical sagittal plane, the angles between the anterior and posterior surfaces of the pubic symphysis and the human coronal plane were measured. The average value of these angles was recorded as pubic symphysis-coronal plane angle (PS-CPA) which was marked as β(Fig. 1e-f).

8. A re-cut layer was performed perpendicular to the anterior surface of the pubic body, with a slice thickness of 5 mm. The thickness of the pubic body was measured on each re-cut section (from superior to inferior, with a 5-mm interval), defined as the pubic body thickness (PBT) which was marked as D(Fig. 1g-h).

9. A re-cut layer was performed perpendicular to the lateral edge of the obturator plane along the superior pubic ramus, with the re-cut layers range extending from the pubic tubercle to the lateral edge of the obturator foramen. The distance from the highest point of the posterior edge to the lower edge of the re-cut section of the superior pubic ramus was measured, defined as the superior-inferior diameter of pubic superior ramus (SID-PSR) which was marked as L4. Using the “freehand curve drawing” function with a step length of 5 mm, the surface distance from the highest point of the posterior edge to the anterior edge of the re-cut section of the superior pubic ramus was measured, defined as the anterior-posterior diameter of pubic superior ramus (APD-PSR) which was marked as L5(Fig. 1i-j).

Design of the plate

Based on anatomical measurement results, standard male and female pelvic models were selected. “Point cloud” files were imported into Geomagic Studio 2015 software (Geomagic Inc, North Carolina, USA). Standard pelvic models were reconstructed via triangular facet modeling and mesh optimization, followed by denoising, defect repair, and surface smoothing. The optimized models were then imported into Solidworks 2017 software (Dassault Systemes Inc, Massachusetts State, USA) for basal surface design of the plate. Considering the micromotion characteristics of the pubic symphysis, multi-planar fixation theoretically provides rigid stabilization for the pubic symphysis. To enable fixation of the superior pubic ramus, the plate was designed into two configurations with three components(Fig. 2a/d): (1) Interpubic tubercle portion: Covers the superior aspect of the pubic symphysis between bilateral pubic tubercles; (2) Superior pubic ramus portion: Covers the superior surface of the superior pubic ramus from the pubic tubercle to the lateral margin of the obturator foramen; (3) Pubic body portion: Type A (“Anterior-Superior Type”): Covers part of the anterior pubic body, with two lateral arms extended from the main plate body to the superior and middle anterior surfaces of the pubic body; Type B (“Posterior-Superior Type”): Differs from Type A in that the two lateral arms extended from the main plate body are positioned on the superior posterior surface of the pubic body.

Fig. 2.

(a) The bottom surface of anterior-superior type of the plate; (b) Add screw holes to the plate of anterior-superior type(Upper view); (c) Add screw holes to the plate of anterior-superior type(Anterior view). d. The bottom surface of posterior-superior type of the plate. e. Add screw holes to the plate of posterior-superior type(Upper view); f. Add screw holes to the plate of posterior-superior type(Posterior view)

Biomechanical analysis

The biomechanical stability of the new plates was compared with that of the single pubic symphysis superior plate and the dual anterior-superior pubic symphysis plates in a Tile B1 pelvic fracture model using finite element analysis. First, the pelvic model(a 25-year-old normal female) was imported into Geomagic Studio 15 software to construct a NURBS closed surface model. Subsequently, the Tile B1 pelvic fracture model was created using SolidWorks 2017 software. Finally, the two new types of plates, the single pubic symphysis superior plate, and the dual anterior-superior pubic symphysis plates were fixed to the Tile B1 pelvic fracture model (Fig. 3). The finite element models were imported into Abaqus 6.14 software (Dassault SIMULIA, USA), with material properties assigned based on available literature [8, 11]. The static analysis methods are as follows: (a) Superior-Inferior Loading (SIL): The bilateral acetabular cavities were constrained, and a vertical load of 500 N was applied to the surface of the S1 vertebral body; (b) Anterior-Posterior Loading (APL): The posterior parts of the bilateral iliac crests were constrained, and an anterior-posterior load of 500 N was applied to the anterior surface of the pubic symphysis; (c) Lateral-Medial Loading (LML): The bilateral acetabular cavities were constrained, and a lateral-medial load of 500 N was applied to the left iliac tubercle. The von Mises stress and displacement distributions were recorded.

Fig. 3.

a. The 3D finite element model of pelvis; b. The model of Tile B1 pelvic fracture; c. The model of Tile B1 pelvic fracture after reduction(Front view); d. The model of Tile B1 pelvic fracture after reduction(back view); e. The fixed model of the anterior-superior type of the plate; f. The fixed model of the posterior-superior type of the plate; g. The fixed model of the single plate; h. The fixed model of the double plate

Statistical analysis

Statistical analysis of the measurement data was performed using SPSS 19.0 software. All measurement data followed a normal distribution and exhibited homogeneity of variance. Data are presented as Mean ± SD. Independent samples t-test was conducted to compare the data between males and females, as well as between individuals of the same gender aged 60 and above versus those below 60, with a significance level of α = 0.05.

Results

Sexspecific differences in measurement of the anterior pelvic ring

The ITD was 48.93 ± 5.51 mm in males and 54.45 ± 5.15 mm in females. The IOFIBD was 46.67 ± 4.02 mm in males and 55.17 ± 4.46 mm in females. The SIDPS was 39.77 ± 4.12 mm in males and 36.76 ± 4.21 mm in females. The PSR-LOTM was 35.87 ± 4.04 mm in males and 37.62 ± 5.31 mm in females(Table 1). The PSG was 2.85 ± 1.20 mm in males and 3.02 ± 1.14 mm in females, with no significant difference between the genders (P > 0.05) (Table 1). The angle α(SPS-SRA) was 135.67 ± 4.83° in males and 137.53 ± 4.46° in females, and the angle β (PS-CPA) was 50.45 ± 4.89° in males and 52.56 ± 4.59° in females (Table 1). In subgroup analyses across different age groups, there were no significant differences in ITD, SIDPS, AB, PSR-LOTM, α, or β between individuals of the same gender (Tables 2 and 3), The stability of pelvic anatomical parameters within the same sex may not be affected by age (with 60 years as the threshold). In the re-cut sections of the pubic body, 4–6 sections were obtained on one side, with an approximate elliptical shape. The sections near the obturator foramen were narrow and irregular. The thickness of the pubic body was greater in males than in females, with significant differences (P < 0.05), and the sections near the pubic symphysis decreased from superior to inferior, following a “thin-thick-thin” pattern, while the sections near the obturator foramen exhibited a trend of decreasing thickness from superior to inferior (Fig. 4). For the superior pubic ramus, 6–8 sections were obtained, with irregular shapes. The anterior-posterior and superior-inferior diameters of the superior pubic ramus were greater in males than in females, with significant differences (P < 0.05) (Table 1). The ITD + PSR-LOTM data were sorted in ascending order, and the medians of the 0–33%, 33–66%, and 66–100% intervals were used as references for selecting standard pelvis models. Based on the selected standard pelvis models, two designs and six length specifications (110, 115, 120, 125, 130, and 135 mm) were proposed.

Table 1.

The difference in the anatomical parameters of anterior pelvic ring between two genders (Mean ± SD, n = 200)

	Male	Female	t	P
L1(ITD)	48.93 ± 5.51 mm	54.45 ± 5.15 mm	-7.197	< 0.001
L2(IOFIBD)	46.67 ± 4.02 mm	55.17 ± 4.46 mm	-13.936	< 0.001
L3(SIDPS)	39.77 ± 4.12 mm	36.76 ± 4.21 mm	5.019	< 0.001
AB(PSR-LOTM)	35.87 ± 4.04 mm	37.62 ± 5.31 mm	-2.579	0.011
H(PSG)	2.85 ± 1.20 mm	3.02 ± 1.14 mm	2.854	0.303
α(SPS-SRA)	135.67 ± 4.83°	137.53 ± 4.46°	-2.772	0.006
β (PS-CPA)	50.45 ± 4.89°	52.56 ± 4.59°	-2.317	0.022
L4(SID-PSR)
Layer 1	14.71 ± 1.75 mm	11.46 ± 1.76 mm	12.881	< 0.001
Layer 2	15.36 ± 1.92 mm	12.12 ± 1.81 mm	12.073	< 0.001
Layer 3	16.67 ± 2.13 mm	12.98 ± 2.01 mm	12.362	< 0.001
Layer 4	18.52 ± 2.46 mm	14.47 ± 2.14 mm	12.193	< 0.001
Layer 5	21.10 ± 2.59 mm	16.32 ± 2.21 mm	13.786	< 0.001
Layer 6	23.66 ± 2.65 mm	18.64 ± 2.37 mm	13.832	< 0.001
L5(APD-PSR)
Layer 1	22.93 ± 2.64 mm	20.89 ± 3.14 mm	4.887	< 0.001
Layer 2	18.21 ± 2.43 mm	17.12 ± 3.09 mm	2.742	0.007
Layer 3	14.75 ± 2.09 mm	13.20 ± 3.01 mm	3.925	< 0.001
Layer 4	12.54 ± 2.04 mm	11.65 ± 2.39 mm	2.816	0.005
Layer 5	12.62 ± 2.20 mm	11.60 ± 2.28 mm	3.178	0.002
Layer 6	14.36 ± 2.59 mm	13.44 ± 2.37 mm	2.573	0.011

Table 2.

Comparison of key anatomical parameters between different age groups of males. (Mean ± SD, n = 100)

	< 60 years (n = 66)	≥ 60 years (n = 34)	t	P
L1(ITD)	49.52 ± 0.73 mm	47.77 ± 0.74 mm	1.520	0.132
L3(SIDPS)	39.97 ± 4.34 mm	40.32 ± 3.18 mm	-0.250	0.810
AB(PSR-LOTM)	35.96 ± 4.29 mm	35.70 ± 3.56 mm	0.308	0.759
α(SPS-SRA)	135.53 ± 4.81°	135.95 ± 4.92°	-0.408	0.684
β (PS-CPA)	50.48 ± 3.68°	50.37 ± 2.95°	0.152	0.880

Table 3.

Comparison of key anatomical parameters between different age groups of females. (Mean ± SD, n = 100)

	< 60 years (n = 65)	≥ 60 years (n = 35)	t	P
L1(ITD)	54.53 ± 5.30 mm	54.37 ± 4.67 mm	0.155	0.877
L3(SIDPS)	36.43 ± 4.50 mm	37.87 ± 3.39 mm	-1.660	0.100
AB(PSR-LOTM)	35.13 ± 5.39 mm	34.88 ± 4.81 mm	0.229	0.819
α(SPS-SRA)	137.48 ± 4.47°	137.72 ± 4.22°	-0.253	0.801
β (PS-CPA)	52.50 ± 3.81°	52.65 ± 2.94°	-0.198	0.844

Fig. 4.

The thickness of pubic body on different layers. On the same layer, the differences between male and female of the three parameters were statistically significant (p < 0.05)

Features of the novel plate

The novel plate was designed with a base thickness of 1.5 mm, incorporating rounded edges with a radius of 0.3 mm and rounded corners with a radius of 2 mm. It consists of three distinct portions (Fig. 2):

1. Interpubic tubercle portion: The main body of the plate has a width of 10 mm and a length of 50–55 mm, with a slight curvature. Six elliptical standard screw holes (long axis 6 mm, short axis 4 mm) were added (R4-6, L4-6), with an approximate spacing of 3 mm between holes. No pre-bending slots were included between R5 and L5 (Fig. 2b and e).

2. Superior pubic ramus portion: This section is 35–40 mm in length and 10 mm in width. Two locking screw holes (diameter 3.5 mm) were placed near the lateral edge of the obturator foramen (R1-2, L1-2), with the outer edge of the most lateral screw hole (R1, L1) not exceeding the lateral edge of the obturator foramen. Near the pubic tubercle, one elliptical standard screw hole (long axis 6 mm, short axis 4 mm) was added (R3, L3). The spacing between the three screw holes is approximately 5 mm, and pre-bending slots were included (Fig. 2b and e).

3. Pubic body portion: Type A Plate: The lateral arms extending to the anterior aspect of the pubic body are 20 mm in length and 10 mm in width. One elliptical standard screw hole (long axis 6 mm, short axis 4 mm) was added near the main body of the plate (R7, L7). Two locking screw holes (diameter 3.5 mm) were placed in the upper-middle part (R8-9, L8-9), with pre-bending slots and an approximate spacing of 3 mm between holes (Fig. 2c). Type B Plate: The lateral arms extending to the posterior-superior aspect of the pubic body are 12 mm in length and 10 mm in width. One locking screw hole (diameter 3.5 mm) was added near the main body of the plate (R10, L10), along with one elliptical standard screw hole (long axis 6 mm, short axis 4 mm) (R11, L11). The spacing between the screw holes on the lateral arm is approximately 2 mm, and no pre-bending slots were included (Fig. 2f).

Finite element analysis

The pelvic model consisted of 164,440 nodes and 123,924 elements. The Type A plate had 79,396 nodes and 44,809 elements, while the Type B plate had 117,315 nodes and 70,199 elements. The single plate had 13,199 nodes and 7,322 elements, and the dual plates had 19,822 nodes and 10,862 elements. Under the three loading modes, the maximum von Mises stress in the pelvic model was 60.51 MPa, and the maximum von Mises stress in the fixation devices was 279.8 MPa. These stress levels did not compromise the integrity of the finite element models (Table 4). Under the same loading conditions, the maximum displacement of the Type A plate fixation model (0.357 mm) was lower than that of the other fixation models (Figs. 6–7). The maximum displacement was almost located at the loading points (Fig. 6). Under different loading modes, stress concentrations were observed around the implants for all four models. The two new types of plates exhibited uniform stress distribution without stress concentration (Fig. 5).

Table 4.

The max von mises stresses on pelvis and fixator under three loading modes(MPa)

	Loading mode	Max von Mises stress (MPa)
		Type A plate	Type B plate	Single plate	Dual plates
Pelvis	Cranial–caudal	22.23	12.75	16.19	14.46
	Lateral–medial	33.20	37.14	60.51	34.73
	Anterior–posterior	50.46	52.57	54.19	57.48
Fixator	Cranial–caudal	33.97	27.83	20.06	31.82
	Lateral–medial	244.40	220.40	279.80	207.70
	Anterior–posterior	153.60	201.30	137.30	143.60

Fig. 6.

The displacement nephogram of different fixations under three loading modes

Fig. 7.

The max displacement on pelvis and fixator under three loading modes (mm)

Fig. 5.

The von Mises stress distribution of different fixator under three loading modes

Discussion

Currently, fixation methods for anterior ring injuries mainly include single plate, double plates, and percutaneous screw fixation, but these approaches remain controversial in terms of surgical trauma, technical difficulty, and biomechanical stability [12–15]. Chen et al. [16] noted that while double plating can enhance local stability, it is associated with a higher infection rate (possibly due to greater soft tissue dissection and prolonged surgical exposure), and the failure rate of internal fixation shows no significant difference compared to single plating. Yao et al. [12] concluded in their biomechanical study that double plating fixation is superior to single plating. Additionally, Yin et al. [17] found that minimally invasive fixation (e.g., screws, INFIX) offers comparable strength to single reconstruction plating but has a lower anatomical reduction rate than traditional plating, along with a risk of nerve injury. At present, there is no integrated design plate specifically for anterior pelvic ring injuries in clinical practice. Based on thin-slice CT data from 200 normal adult pelvises, this study designed and validated through finite element analysis a novel integrated dual-plane anatomical plate, aiming to balance stability with minimally invasive benefits. Compared to previous studies, this plate adopts a three-dimensional coverage concept of “pubic tubercle region-superior pubic ramus-anterior/posterior surface of pubic body,” theoretically achieving comparable or even superior anti-displacement performance to double plates while reducing the number of plates required.

The measurement results of this study indicate that the ITD in females is (54.45 ± 5.15) mm, which is significantly larrger than that in males (48.93 ± 5.51) mm. Similarly, the PSR-LOTM in females is (37.62 ± 5.31) mm, and the SPS-SRA-α in females is (137.53 ± 4.46)°, both of which are greater than those in males (135.67 ± 4.83)°. These differences may be attributed to the fact that the pelvic inlet plane in females is wider than that in males, this is consistent with previous research findings [18]. These measurement values provide an anatomic basis for the design of the length and curvature of the main body of the plate, and lay the foundation for the differentiated design of bone plates between different genders. The IOFIBD in males is (46.67 ± 4.02) mm, which is smaller than that in females (55.17 ± 4.46) mm. This may also be related to the wider pelvis in females. This measurement value suggests that the width of the anterior part of the pubic symphysis in the plate design should not exceed this distance. The SIDPS in males (39.77 ± 4.12 mm) was approximately 3.01 mm larger than that in females (36.76 ± 4.12 mm), indicating that the screw insertion depth above the pubic symphysis in male patients should not exceed this value and can be set at 35–38 mm (90%-95% of SIDPS), while in females it should be controlled within 32–35 mm to avoid screw penetration through the inferior edge of the pubic symphysis and bladder injury. Additionally, this measurement suggests that the length of the anterior arm of the plate should not exceed this value. Differences in the male pelvis create problems and issues for the patient, especially in robotic surgery. When the pelvis is deeper than normal, the use of robotic arms becomes impossible. This can lead to changes in the surgical plan and can have negative consequences for the surgeon and the patient. Therefore, precisely determining the pelvic dimensions before surgery will allow for the appropriate design of the plate/screw sizes to be used. The PSG was (2.85 ± 1.20) mm in males and (3.02 ± 1.14) mm in females. This value can guide the reduction of the pubic symphysis separation during surgery. There is a significant difference in the PS-CPA-β between males (50.45 ± 4.89)° and females (52.56 ± 4.59)°, which may be related to the anatomical differences corresponding to the barrel-shaped pelvis in females that is favorable for childbirth. This value can guide the angle of screw insertion above the pubic symphysis during surgery. The thickness of the pubic body and the anteroposterior and vertical diameters of the pubic ramus are larger in males than in females in different osteotomy planes, which may be related to the fact that male bones are coarser than female bones. The thickness of the pubic body can guide the length of the screw inserted in front of the pubic symphysis during surgery. Meanwhile, the thickness of the section near the pubic disc shows a “thin-thick-thin” pattern from top to bottom, while the thickness of the section near the obturator foramen shows a downward trend from top to bottom. The closer to the obturator foramen, the smaller the thickness in the middle and lower parts. This suggests that the design of the lateral arm parts of the anterior and posterior parts of the pubic body in the bone plate should be close to the inner side and upper middle part of the pubic body. The anteroposterior diameter of the pubic ramus can guide the width selection of the bone plate covering the pubic ramus, while its vertical diameter can guide the length of the screw inserted into the pubic ramus during surgery. We found that the length of the main body of the plate, namely ITD + PSR - LOTM, is (120.68 ± 10.66) mm in males, which is smaller than that in females (124.70 ± 11.93) mm. The difference between males and females can serve as a basis for selecting standard pelvises. To ensure that the designed bone plate can cover the majority of the population, we divided ITD + PSR - LOTM into three intervals: small, medium, and large. Selecting the median value of each interval can effectively screen out representative standard pelvises. For males, the median values of the 0–33%, 33–66%, and 66–100% intervals are 109.97 mm, 119.97 mm, and 131.43 mm, respectively. For females, they are 115.69 mm, 124.13 mm, and 135.27 mm. In summary, we selected six pelvises with lengths of 110 mm, 115 mm, 120 mm, 125 mm, 130 mm, and 135 mm as the standard models for bone plate design(Fig. 8).

Fig. 8.

Flowchart of preoperative decision-making and intraoperative surgical procedure for the novel anatomical plate for the anterior pelvic ring

Double-plate fixation for symphyseal pubis separation exhibits excellent biomechanical properties and can attain the maximum fixation strength [8]. Therefore, when designing a new type of steel plate, the characteristics of multi - planar fixation should be considered to achieve three - dimensional coverage corresponding to the upper part between the pubic tubercles, the superior pubic ramus, and the anterior or posterior aspect of the pubic body. The main body and the two side arms of the steel plate have a width of 1 cm and a thickness of 1.5 mm. The edges of the steel plate are rounded, and the ends are chamfered. This not only ensures the strength of the steel plate but also facilitates its insertion during surgery and reduces irritation to the genital branch of the genitofemoral nerve, the spermatic cord, or the round ligament of the uterus. The side arm anterior to the pubic body of the Type A steel plate is integrally designed with the steel plate’s main body. Meanwhile, it can also fix the pubic body fracture, expanding the fixation range of the steel plate. Given that the middle part of the pubic body is relatively thin and the fixation screws are short, two 3.5 - mm locking holes are added here to enhance the fixation strength. A pre - bent groove is added to the side arm, enabling it to fit better on the anterior surface of the pubic body. Since the angle between the pubic body and the human coronal plane is approximately 50°, and the abdominal soft tissues obstruct the view, inserting screws posterior to the pubic body is relatively difficult. Therefore, the side arm designed posterior to the pubic body of the Type B steel plate is relatively short, only 12 mm. One locking hole is designed on the side arm near the main body, which can improve the fixation stability. The other hole is oval - shaped, which is beneficial for adjusting the screw’s direction to facilitate its smooth insertion. When using the Type B steel plate for fixation, the surgical exposure range required is small, which can avoid damaging the soft tissues anterior to the pubic body. During surgery, the reduction effect of the anterior pelvic ring can also be evaluated according to the fitting degree of the steel plate, or the steel plate can be used as a reduction template. The Type A steel plate is more suitable for patients with anterior pelvic ring injuries accompanied by pubic body fractures and osteoporosis.

The types of pelvic fractures are determined by the direction of the violent force and can be classified into three categories: anterior - posterior compression, lateral compression, and vertical shear [19]. Therefore, this study simulated the forces in these three directions to assess the stability of the pelvic ring. Nevertheless, in real - world scenarios, the occurrence of violent events is typically more intricate and challenging to simulate. Under physiological circumstances, the pelvis primarily serves to transfer the weight of the upper body to the lower limbs. In our experiment, a force of 500 N was chosen as it mimics the upper - body compression on the sacrum of a 70 - kg standard - weight male in an upright posture. For safety redundancy, we selected a normal adult female pelvis to create a finite element model. The finite element analysis results revealed that the maximum stress on the pelvis was 60.51 MPa, which is significantly lower than the compressive ultimate strength of cortical bone (170–200 MPa) [20, 21]. This finding suggests that none of the fixation methods led to overloading of the bone tissue. The maximum von Mises stress of the fixing device is 279.8 MPa, which is lower than the typical yield strength of orthopedic implants (e.g., titanium alloy > 600 MPa) [22]. This indicates that the fixing device meets the standards of commonly used clinical materials. This requirement complies with the standards of commonly used clinical materials. Under three - dimensional loads, the maximum displacements of the Type A steel plate (0.357 mm in the cranial–caudal direction, 0.192 mm in the anterior - posterior direction, and 1.018 mm in the medial - lateral direction) are all lower than those of other fixation methods (e.g., traditional double steel plates: 0.378 mm/0.874 mm/1.151 mm). Particularly under the cranial–caudal load, it demonstrates its advantage in resisting vertical displacement.

Previous studies have indicated that a displacement of ≤ 1 mm is considered a sign of reliable fixation [23]. In this study, for all models, except under the lateral - medial loading, the displacement remained below 1 mm. Furthermore, the minimal displacement of the Type A fixation model may decrease the risk of postoperative micromotion, which is vital for the stability of the pubic symphysis in Tile B1 - type injuries. In this study, the displacements of all fixation models under the lateral load exceeded 1 mm. This could be attributed to the pelvis being prone to “diagonal - type” deformation under the lateral load, resulting in stress concentration in the pubic symphysis area and causing cumulative displacement [24]. Meanwhile, the total number of elements in this model is approximately 120,000 (pelvis) + 70,000 (steel plate). However, some research proposes that a model requires more than 200,000 elements to converge, suggesting that the low mesh density may diminish the structural stiffness response, particularly for the lateral load sensitive to bending [25]. The displacement of the Type A steel plate in the cephalo - caudal direction is 0.357 mm, which is superior to that of the external fixator (0.772 mm) in previous studies [19]. Additionally, the stress distribution of the two new - type steel plates is more uniform. Bi - planar fixation can enhance the anti - torsional stiffness by optimizing the screw layout, implying that the mechanical advantages of Type A may originate from its innovative structural design. The displacement of the Type B steel plate fixation model is still superior to that of the traditional method. This suggests that the new - type steel plates can improve stress transfer through three - dimensional structural design and achieve stability equivalent to multiple steel plates with a small number of steel plates. The core advantage of multiplanar fixation lies in its three-dimensional biomechanical design, which effectively constrains multidirectional displacement of fracture fragments while conforming to skeletal anatomy and physiological load-bearing axes. This configuration disperses forces across multiple fixation units, better accommodating the complex biomechanical demands of human movement. Consequently, it delivers enhanced stability in clinical applications (particularly for complex fractures or weight-bearing regions), reduces fixation failure rates, and creates a more reliable mechanical environment for fracture healing [26–28].

In conclusion, We successfully developed two types of plates and designed different specifications to meet clinical needs. Preliminary biomechanical finite element analysis indicated promising fixation stability for Tile B1 type injuries, warranting further clinical investigation.

In this study, we did not account for other factors that might influence the experiment, including pelvic muscles, cartilage, fascia, as well as the patient’s age, gender, and other relevant factors. Similar to other finite - element analysis models, we only incorporated relevant and significant ligaments into the pelvic model for finite - element analysis. Meanwhile, the finite - element analysis of this experiment was conducted under static conditions, which fails to reflect the biomechanical state during human locomotion. Moreover, the screws were assumed to have a fixed relationship with the pelvis, and the friction between them was not taken into consideration. Additionally, we did not investigate the long - term dynamic stress during the patient’s postoperative rehabilitation. This study simulated vertical loads; however, Tile B1 injuries are characterized by rotational instability. Subsequent torsion tests (e.g., torque analysis) are necessary to comprehensively assess the anti - rotational performance. At the same time, the design of this plate is based on anatomical data measurements from the local region, which may limit its range of application. Therefore, the stability of the newly designed plate for anterior ring fixation still requires validation through multi - center clinical trials.

Abbreviations

3D

Three-dimensional

APD-PSR

And anterior-posterior diameter of pubic superior ramus

CT

Computed tomography

DICOM

Digital imaging communication in medicine

IOFIBD

Inter-obturator foramen inner border distance

ITD

Inter-pubic tubercle distance

PS-CPA

Pubic symphysis-coronal plane angle

PSG

Pubic symphysis gap

PSR-LOTM

Pubis superior ramus length from mid-external pubic tubercle to lateral obturator margin

SID-PSR

Maximum superior-inferior diameter of pubic superior ramus

SIDPS

Superior-inferior distance of pubic symphysis

SPS-SRA

Pubic symphysis upper plane-superior ramus angle

STL

Stereolithography. NURBS: Non-Uniform Rational B-Splines

Author contributions

All authors contributed to the article. Study design has been suggested by SG. Data were collected and analyzed by KY and MW. The manuscript was drafted and designed by KY. SG and MW revised the manuscript. All authors read and approved the final manuscript.

Funding

The authors declare that sponsors had no such involvement.

Data availability

No datasets were generated or analysed during the current study.

Declarations

Ethics approval and consent to participate

Ethical approval has been granted in this study.

Consent for publication

All patients involved in this study gave their consent for the anonymized data to be used for scientific purposes and published in a scientific journal.

Competing interests

The authors declare no competing interests.