This study focused on the difference in biomechanical stability between the number of LC-II screws placed and the number of reconstructive plates used to fix a posterior pelvic ring fracture type C1.1. In this study, one and two LC-II screws with two reconstruction plates were used to fix a posterior pelvic ring fracture type C1.1. The differences in pelvic mechanical stability provided by the three different fixation methods for fixing a posterior pelvic ring fracture type C1.1 were evaluated in terms of load displacement relationship, compression stiffness and ultimate load.
The load-displacement relationship reflects the value of longitudinal displacement of the fracture end under different pressure gradients, reflecting the ability of the three groups to resist displacement under different pressures; in this study we found that when the axial load was less than 1000 N, there was no difference in the ability of one LC-II screw and two LC-II screws fixation to resist longitudinal slippage of the pelvic ring; 1000 N axial load could meet the postoperative activity of some of the lighter weight patients There was no difference in the resistance to slip between one screw and two screws for the lighter weight patients. When the axial load was increased to 1000 N, the two LC-II screws and plate had greater slip resistance than the one LC-II screw, which is the mechanical characteristic required to fix a posterior pelvic ring fracture of type C1.1.[15] showed that in unstable pelvic fractures, the placement of two sacroiliac screws increased stability against rotation and vertical displacement when the pelvic ring was loaded to 1000 N, which is similar to the results of the present study.
The compression stiffness represents the ability of the overall structure to resist elastic deformation under axial loading in different fixation methods, and is the ratio of load to displacement; the ultimate load is the value of the load when the posterior ring of type C1.1 pelvis is transformed from the elastic to the plastic state under the action of external forces in different fixation methods, and represents the critical value at which the overall structure becomes unstable;
It is evident from the results of this study that two LC-II screws and plates fixing a posterior pelvic ring fracture type C1.1 have greater compressive stiffness and ultimate load, meaning that the fracture end can withstand greater external forces before elastic deformation occurs.
The LC-II screw is placed in a direction that is nearly perpendicular to the posterior iliac fracture line, with maximum stability and minimum shear forces achieved when the screw length is perpendicular to the fracture line. Hollow screw fixation is an intramedullary fixation[16] It is a central fixation and provides relative stability. The semi-threaded hollow compression screw has a high resistance to vertical shear and torsion and is placed intraoperatively to assist in repositioning and to create compression at the fracture end.[17] The two hollow screws have an anti-rotation effect. The two hollow screws are more resistant to rotation, which is related to the fact that they form a plane of fixation and the fracture does not rotate around one of the screws as the force increases, making it more biomechanically stable.
Type C1.1 pelvic fractures are unstable fractures and the aim of surgical treatment of type C1.1 pelvic fractures is to stabilise the fracture, restore stability to the pelvic rings and promote early functional exercise. Previous studies have shown that the anterior and posterior rings account for 40% and 60% of pelvic stability respectively.[18] Simonian et al.[19, 20] showed that simultaneous fixation of the anterior and posterior pelvic rings is often necessary to treat C1.1 pelvic fractures. In the present cadaveric study, anterior pelvic ring pubic branch fractures were fixed with retrograde pubic branch screws and biomechanical tests showed that[21–23] Starr et al.[24] suggested retrograde fixation of medial suprapubic branch fractures close to the pubic symphysis, with the ideal entry point being below the pubic tuberosity and lateral to the pubic symphysis.
To our knowledge, no author has described a posterior pelvic ring fracture of type C1.1 fixed with LC-II screws. Previously, posterior pelvic ring fractures of type C1.1 were often described as posterior iliac wing fractures or crescentic fractures of the ilium, but it is worth noting that this is not an accurate description. Crescentic fracture[25] is a dislocation of the sacroiliac joint via the ilium, the mechanism of injury is mostly lateral compression and is a B2.2 or B2.3 (AO/OTA) type pelvic fracture, whereas a C1.1 posterior pelvic ring fracture is a complete fracture located anterior to the sacroiliac joint and is extra-articular and does not require strict anatomical repositioning and strong fixation.
There is no standard fixation technique for posterior pelvic ring injuries of type C1.1, and Starr et al.[11] introduced a closed reduction percutaneous screw fixation technique for posterior iliac fractures, with screws placed from the anterior inferior iliac spine to the posterior inferior iliac spine, which is a slightly different direction to our LC2 screw placement, which is safer in comparison[26, 27] This is a slightly different direction to our placement of the LC2 screw, which is safer than the LC2 screw placement and less likely to penetrate and cause complications. Posterior transiliac internal fixator (TIFI) iliac screw technique[28, 29] The TIFI iliac screw technique provides sufficient mechanical strength to provide good fixation for a C1.1 posterior pelvic ring fracture, however, it requires fixation in the prone position under direct vision and loss of joint mobility. In contrast, anterior-posterior placement of the LC-II screw fixes the posterior pelvic ring, allowing simultaneous manipulation of anterior pelvic ring injuries without intraoperative position changes[11] The LC-II screw is placed anteriorly to posteriorly fix the posterior pelvic ring. To ensure accurate screw placement, the ideal entry point for the LC2 screw should be along the intersection of the lateral wall of the AIIS (anterior inferior iliac spine) and the body of the ilium, inserted below the line between the midpoint of the AIIS and the upper flat area of the PSIS[27] .
Satisfactory closed repositioning is a prerequisite for percutaneous internal fixation, if not, it is difficult to complete closed internal fixation, and even if it is completed, the efficacy of internal fixation is poor. Traction, rotation of the affected hemipelvis, placement of Schanz pins in the iliac wing, etc. can be used for closed repositioning.[11] The pelvic repositioning frame can be used for closed repositioning[30]. The advent of the pelvic repositioning robot has also greatly facilitated the development of minimally invasive techniques, however, for difficult to reposition or comminuted fractures, internal fixation with an incisional plate under direct vision is still required.
Conclusion: The results of this study show that fixation with two LC-II screws can achieve the fixation strength of a reconstructed plate and that fixation with one LC-II screw and two LC-II screws is similar at lower energy loads. This study establishes the effectiveness of fixation with two LC2 screws and allows the use of one LC2 screw for fixation depending on the patient's situation. The results of this study will facilitate clinical work in deciding on the fixation of posterior pelvic ring fractures of type C1.1 according to the patient's specific situation.