Biomechanical comparison of three kinds of fixation in fracture-dislocation of the sacroiliac joint

Abstract

BACKGROUND:

Although there are several different kinds of fixation techniques for the sacroiliac fracture-dislocation, the treatment remains a challenge for orthopedic surgeons.

OBJECTIVE:

The purpose of this study was to compare the stability of sacroiliac fracture-dislocation fixed with two iliosacral (IS) screws, tension band plate (TBP), and minimally invasive adjustable plate (MIAP).

METHODS:

Five human cadavers (L4-pelvic-femora) were used to compare biomechanical stability. The pubic symphysis separation and left sacroiliac dislocation were created. The symphysis pubis was stabilized with a five-hole plate. The sacroiliac joint dislocation fracture was fixed with three kinds of internal fixation in a randomized design. The specimens were fastened in a biomechanical machine with a cycle vertical load. Displacements of the whole specimen and shifts in the fracture gap were recorded.

RESULTS:

Under different vertical loads, the shifts in the fracture gap and the displacements of the pelvis fixed with MIAP were similar to those in fractures fixed with two IS screws. However, the shifts in the fracture gap and the displacements of the pelvis fixed with MIAP were significantly smaller than those fixed with TBP.

CONCLUSIONS:

The stability of the sacroiliac joint dislocation fixed with MIAP was similar to that of dislocation fixed with two IS screws, and MIAP performed better than the TBP under vertical load.

Keywords

Biomechanics sacroiliac joint internal fixation screws plate

1. Introduction

Pelvic ring disruptions is a severe condition with a prevalence of about 20–37/100,000 in the general population [1]. Usually, pelvic ring disruptions are reported to result from high-energy trauma, including falls, automobile-pedestrian collisions, motorcycle crashes and motor vehicle crashes [2, 3]. Because there was a lack of proper surgical techniques and fixation devices in the past decades, the pelvic fractures were treated with skeletal traction and bed rest. However, the surgical outcomes were not satisfactory. As medical science developed, the attitude for treating pelvic fractures changed. Minimal invasive, rigid fixation and rapid recovery have become more popular options.

Posterior pelvic ring is a weight-bearing structure bearing 70% of the body weight [4], so the key of treating fracture-dislocation of sacroiliac joint is restoring the integrity of the posterior pelvic ring. Internal fixation is the definitive method used to treat pelvic fractures [5]. Currently, there are different methods for fixation of the sacroiliac joint fracture-dislocation, including the use of anterior or posterior sacroiliac plates, iliosacral (IS) screws, sacral bar, tension band plate (TBP), spinopelvic fixation system, pedicle screw-rod fixation system and so on [1, 6, 7, 8, 9]. Obviously, each kind of fixation has its own advantages and disadvantages. Selecting appropriate treatment for sacroiliac joint fracture-dislocation remains a challenging problem for orthopedic physicians.

To simply restore the integrity of the posterior pelvic ring, we introduced a novel minimally invasive adjustable plate (MIAP) that was based on the structural characteristics of the posterior pelvic ring (patent no. ZL201010130312.6). We investigated injuries with sacral fracture which were treated with MIAP and locking compression plate and were then followed up for at least 12 months [10]. We found that both methods can effectively stabilize the fractures. The MIAP method has the advantage that it is minimally invasive, less exposed to radiation, technically safe and saves time. Chen et al. [11] and Wu et al. [12] tested the stability of MIAP for treating sacral fractures with a biomechanical experiment. They constructed sacral fracture models, which were fastened to the Electroforce 3520-AT Bose biomechanical testing machine and fixed with three types of internal fixation under vertical load. They also demonstrated that the stability of sacral fractures fixed with MIAP was similar to the fractures fixed with iliosacral screws. The MIAP, due to its effective stability for treating sacral fracture, is also applied to the management of sacroiliac joint fracture-dislocation. Therefore, a biomechanical experiment was designed to compare the stability of sacroiliac joint dislocation fixed with three types of internal fixation, and provided a basis for the clinical application of MIAP.

2. Methods

This study was conducted in accordance with the Declaration of Helsinki and was approved by the Hospital Ethics Committee of the Third Hospital of Hebei Medical University in Shijiazhuang, Hebei, China.

2.1 Specimen preparation

Five embalmed adult male cadaver pelvises (average age 43.2 years), provided by Department of Anatomy of Hebei Medical University, were used for the biomechanical test. The inclusion criteria for pelvis specimens were as follows: (1) Specimens from patients with rheumatism, tuberculosis, anatomic variations, cancer and other diseases were excluded. (2) Specimens with osteoporosis (we used an osteocore 3 dual energy X-Ray osteodensitometer; Medilink Company, Parc de la Mediterranee, France) were excluded. (3) Hip joints and pubic symphysis must be intact. (4) The fourth and fifth lumbar vertebrae should be intact, and both femurs must be amputated at the transition of the upper to the middle third. The specimens were prepared by removing the soft tissues and the ligaments (anterior, posterior, interosseal sacroiliac ligament, sacrotuberous ligament, and sacrospinous ligament) were left intact. The specimens were stored at $-$ 20 ${}^{\circ}$ C and melted at room temperature 12 h before the test.

2.2 Structure of MIAP

MIAP (Tianjin Zhengtian Medical Instrument Company Ltd., Tianjin, China) is made up of an adjustable connection bar and two Z-shaped brackets. Each Z-shaped bracket is composed of an upper wing, a lower wing and a web plate. The connection bar consists of two custom-made eye bolts and a hexagonal tube that could be shortened or stretched by rotating the tube. The upper wing of the Z-shaped bracket is positioned on the dorsal surface of the posterior superior iliac spines and lower wing of which is put on the dorsal surface of the sacrum. Then some long cancellous screws were used to secure the two Z-shaped brackets on the sacrum and ilium, respectively (Fig. 1).

Figure 1.

The structure of MIAP.

Figure 2.

Image of the specimen after creating sacroiliac fracture-dislocation with incision on the symphysis pubis.

2.3 Modeling sacroiliac joint dislocation and fixation of specimens

For better testing the stability of internal fixation, the anterior and posterior pelvic rings were disrupted (Fig. 2). Anteriorly, a symphysis pubis rupture was made by cutting through symphysis pubis that was stabilized using a five-hole plate. Posteriorly, fracture-dislocation of the sacroiliac joint was manipulated by cutting the connection between the left sacroiliac joint. Three types of internal fixation were implanted randomly after reduction of the sacroiliac joint dislocation.

Three kinds of internal fixation were used as follows:

(1)
MIAP group: To choose an appropriate MIAP and expose the posterior side of ilium and sacrum. To insert some long cancellous screws into the sacrum and ilium after reduction (Fig. 3).

Figure 3.
Sacroiliac dislocation fixed with MIAP. A. Posterior view of the specimen. B. Anteroposterior radiograph of the pelvis.

(2)
IS screws group: To insert two 2.0 mm Kirschner wires into the first sacral vertebral body through ipsilateral external surface of the ilium. Fluoroscopy (inlet, outlet, and anteroposterior views) was used to confirm screw placement. To insert two 7.3 mm cannulated, partially threaded, and cancellous IS screws (Tianjin Zhengtian Medical Instrument Company Ltd., Tianjin, China) into the first sacral vertebral body along the kirschner wires (Fig. 4).

Figure 4.
Sacroiliac dislocation fixed with two IS screws. A. Posterior view of the specimen. B. Anteroposterior radiograph of the pelvis.

(3)
TBP group: To bend the TBP (Tianjin Zhengtian Medical Instrument Company Ltd., Tianjin, China) according to the structural characters of posterior pelvic ring. To choose and insert some suitable screws into external surface of the ilium (Fig. 5).

Figure 5.
Sacroiliac dislocation fixed with TBP. A. Posterior view of the specimen. B. Anteroposterior radiograph of the pelvis.

2.4 Measurements

The bilateral distal femurs and L4 vertebral body of specimens were fastened in the homemade clamp by TypeII denture-based self-curing denture acrylic (Shanghai Medical Instrument Company Ltd., Shanghai, China). The pelvic specimens were fastened in the Electroforce 3520-AT Bose biomechanical testing machine (BOSE Corporation, Eden Prairie, USA) at a neutral position. The ventral surface of the pubic symphysis and both anterior superior iliac spines were placed in the same plane. Two points in the lowest of the sacroiliac joint were chosen and marked as the fracture gap. A vertical load of 200 N was applied to eliminate creep after implanting each internal fixation. The vertical cyclic load was between 0 and 500 N, and increased at a rate of 10 N/S. The cyclic load was applied in 20 cycles. In the last three cycles, the displacement of specimen was recorded at vertical loads of 100 N, 200 N, 300 N, 400 N and 500 N by the machine. Shifts between the fracture gap were measured by grating displacement sensor at a vertical load of 500 N.

All these procedures were done by the same surgeon, including constructing fracture and fixing the model. In order to reduce the influence of prefixation on the subsequent operations, the direction and location of the internal fixation were designed prior to implantation and X-ray fluoroscopy was used after implantation.

2.5 Statistical analysis

The statistical analyses were performed using SPSS version 16.0 software (SPSS Inc., Chicago, IL, USA). The data were presented as mean $\pm$ SD. Model displacement and shifts between the joint were compared by ANOVA. The Student-Newman-Keuls test was used to compare significant differences between groups. Significance was set at $p<$ 0.05.

3. Results

3.1 General situation

All specimens were fastened at a neutral position without obvious fracture or obliquity. Evulsion, loosening and breakage of internal fixation weren’t observed. Under vertical load, the displacements of specimens were recorded by BOSE biomechanical workstation, and the shifts between the fracture gap were recorded simultaneously. According to the load-displacement scattergraph, the smooth straight line indicated that the specimens had elastic deformation (Fig. 6).

Figure 6.

Load-displacement scattergraph of the specimens indicates that the deformation of specimens was elastic.

3.2 Displacement of the specimens

Displacement of the pelvis fixed with either two IS screws or MIAP was significantly lower than the displacement of specimens fixed with TBP under vertical load ( $p<$ 0.05) (Table 1). However, the difference between the displacement of the specimens fixed with MIAP and two IS screws was not significant ( $p>$ 0.05) (Fig. 7).

Table 1
Displacement of the pelvis fixed with three kinds of internal fixation under different vertital load ( $\bar{x}\pm s$ , $n=$ 5)

Load (N)	IS screws (mm)	MIAP (mm)	TBP (mm)	F value/ $p$ value
100	0.673 $\pm$ 0.136	0.854 $\pm$ 0.206	1.863 $\pm$ 0.431	24.977/ $<$ 0.001
200	1.336 $\pm$ 0.270	1.789 $\pm$ 0.390	2.882 $\pm$ 0.776	11.442/0.002
300	1.894 $\pm$ 0.355	2.382 $\pm$ 0.373	3.832 $\pm$ 0.966	12.715/0.001
400	2.358 $\pm$ 0.327	2.865 $\pm$ 0.435	4.700 $\pm$ 1.033	16.698/ $<$ 0.001
500	3.094 $\pm$ 0.492	3.697 $\pm$ 0.375	5.533 $\pm$ 0.791	24.005/ $ï ¼ œ$ 0.001

IS screws: iliosacral screws; MIAP: minimally invasive adjustable plate; TBP: tension band plate.

Figure 7.

The displacement of the pelvis fixed with three types of internal fixation under different vertical load.

3.3 Displacement of the fracture gap

The shifts in the fracture gap of the pelvis fixed with the two IS screws and the MIAP were 0.893 $\pm$ 0.235 mm and 1.185 $\pm$ 0.327 mm respectively under vertical load of 500 N, which were significantly smaller than that of the pelvis fixed with TBP [2.267 $\pm$ 0.503 mm ( $p<$ 0.01)]. However, the difference was not significant between the MIAP group and the IS screw group ( $p>$ 0.05) (Fig. 8).

Figure 8.

The shifts in the fracture gap of the pelvis fixed with three types of internal fixation under a load of 500 N.

4. Discussion

Fracture-dislocation of the sacroiliac joint is rare and severe, and is associated with high-energy injuries. It remains a challenge for orthopedic surgeons due to the anatomical variability and complexity of the pelvis and intrapelvic structures [13]. There are several serious sequelae if the reduction and internal fixation are not ideal, which include posterior pelvic pain, back pain, leg length discrepancy and sitting imbalance [14]. Uchida K found that the relationship between poor radiographic findings and poor clinical outcomes was significant. The sacroiliac dislocation was very difficult to treat conservatively. It also considered that correct reduction and rigid internal fixation of the posterior pelvic ring can result in a satisfactory outcome [15]. There are several different techniques for fixation of the posterior pelvic ring, including SI screws, TBP, and sacral bars, which have their advantages and disadvantages.

Clinically, SI screws were commonly used for fixation of sacroiliac joint fracture-dislocations [16, 17]. It has the merits of slight trauma, little scar and early recovery. However, it also has some disadvantages. The procedure is performed conventionally under C-arm fluoroscopy, which exposes both patients and doctors to prolonged radiation and has a high rate of vessel and nerve injuries and screw malposition even for experienced surgeons [16, 17, 18]. Although computer-navigated equipment was used to reduce operation time, radiation exposure and malposition rate, the expensive equipment and huge facilities limited its widespread application in primary hospitals [19]. Traditional posterior TBP was first introduced by Albert et al. and widely used in clinical practice [20]. It also has advantage of easily operation and mastery. But the plate should be pre-bent to adapt to the irregular structure of the posterior pelvic ring, which reduces the strength [21]. Moreover, skin infections were common complications using TBP fixation. Ayoub found the skin infection rate of 12.5% when they treated the patients with unstable pelvic ring injuries using TBP fixation [22]. Suzuki reported the rate of skin infections was 2/19 [23]. To address limitations, we introduced a new fixator MIAP. The aim of this study was to compare the stability of the sacroiliac joint dislocation fixed with three kinds of internal fixation.

A Type C left sacroiliac dislocation combined with anterior symphysis pubis separation was created. For the anterior pelvic ring disrupture, some surgeons considered the conservative treatment could be taken if the separation of symphysis pubis was less than 2.5 cm [24]. But, some researchers advocate open reduction and internal fixation [25]. In this study, we fixed the symphysis pubis using a five-hole plate. The sacroiliac complex was stabilized by the rigid internal fixation in order to achieve better effects, which has an important load-bearing function [26]. In our study, the performances of used TBP and IS screws were compared with that of the MIAP. In order to avoid inaccuracies caused by bone density of specimens, age, and implanting sequence of internal fixation.

These specimens were fixed at a standing neutral position with a peak load of 500 N, which was equal to the weight of upper body of an adult [27, 28]. In this study, we used twenty cycles of loading to test the stability of posterior pelvic ring, which was fixed with three kinds of internal fixation. Schildhauer examined 10,000 cycles [29], where Sagi only performed 10 cycles in biomechanical tests [30].

In most biomechanical researches, the displacement of the fracture gap was recorded in single or multiple directions [12, 31, 32, 33, 34]. The shifts in fracture gap were not limited to the vertical shifts (z-axis), but also rotational shifts (x-axis) and translational shifts (y-axis) [35]. We measured the fracture gap on the level of posterior inferior iliac spine using grating displacement sensor, which was the calculated value of three direction shifts. The vertical displacement reflected the total displacement of the entire specimen was recorded by bio-mechanical testing machine, and three kinds of fixation were fixed in the same specimen in sequence. Therefore, the data were comparable.

The results of the study indicate that using IS screws, TBP and MIAP in the treatment of sacroiliac dislocation restored most of stability of the posterior pelvic ring. Two IS screws, placed in the first sacral vertebral body, were regarded as the strongest fixation in stabilizing the posterior pelvic ring [36]. Yinger et al. [34] and Padalkar et al. [37] considered that two IS screws were stiffer than TBP, which was consistent with our results. Compared with MIAP and IS screws simultaneously fixed in the sacrum and ilium, TBP was only fixed in the ilium for stabilizing the posterior pelvic ring. Therefore, the stability of sacroiliac joint dislocation fixed with MIAP and two IS screws was significantly better than that fixed with the TBP in this study. IS screw fixation considered as a “two point fixation” technique provides strongest fixation. Because IS screw penetrates through three layers of the cortical bone when it inserts into the S1 vertebral body [38, 39]. MIAP simulates the structure of the sacroiliac complex and is attached to the dorsal part of sacroiliac joint without pre-bending. It is also fixed the sacrum and ilium through the long cancellous screws and restores the function of sacroiliac complex. In our study, the stability of posterior pelvic ring fixed by two IS screws was better than that fixed by the MIAP, however, the difference was not statistically significant.

This study had several limitations. Although three kinds of internal fixation were inserted into the same specimen in sequence for reducing the influence caused by individual difference of specimens, the use of pre-fixation could affect the holding power of the subsequent fixation. Therefore, we accurately located the direction and position of the screws using X-ray, in order to minimize the influence between screw channels. There were only five specimens in our study. The power analysis of this study showed only 44.3% power to detect a significant difference at the $p<$ 0.05 level. It furthermore showed that a large sample size (more than 12 specimens) would be required to detect any difference with 80% power. In the future, artificial pelvises should be used in the biomechanical test. In this study, the fractures of the anterior pelvic ring were fixed by rigid fixation. Different anterior pelvic ring injury modes should be created in future studies.

5. Conclusion

The stability of sacroiliac joint dislocation fixed with MIAP was similar to that of dislocation fixed with two IS screws, and better than that of dislocation fixed with TBP under vertical load. MIAP was easy to operate and was used in clinical treatment of sacroiliac joint dislocation.

Footnotes

Acknowledgments

This study was funded by the Nature Science Foundation of Hebei Province (Grant no. H2016206291).

Conflict of interest

None to report.

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Biomechanical comparison of three kinds of fixation in fracture-dislocation of the sacroiliac joint

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSIONS:

Keywords

1. Introduction

2. Methods

2.1 Specimen preparation

2.2 Structure of MIAP

2.5 Statistical analysis

3. Results

3.1 General situation

Table 1 Displacement of the pelvis fixed with three kinds of internal fixation under different vertital load ( x ¯ ± s , n = 5)

5. Conclusion

Footnotes

Acknowledgments

Conflict of interest

References

Table 1
Displacement of the pelvis fixed with three kinds of internal fixation under different vertital load ( $\bar{x}\pm s$ , $n=$ 5)