Recent Progress in the Classification and Operation of Sacral Fractures

Most sacral fractures are caused by high-energy, violent injuries, often accompanied by lumbosacral plexus injuries, which can cause instability of the posterior pelvic ring or lumbosacral junction in severe cases. Currently, the most commonly used clinical classification methods are Denis classification, Tile classification, Isler classification, and Denis II classification. In recent years, lumbosacral vertebral injury classification and injury degree scoring systems have often been applied clinically as the choice of treatment methods. At present, the internal fixation and implantation methods of sacral fracture are developing in the direction of positive, efficient, safe, and minimally invasive. But different fixation methods have their own indications, which should be strictly followed. This article reviews the classification of sacral fractures and the latest progress in surgical treatment.


Introduction
Te sacrum is the center of the pelvic load, carrying the upper body load from the lumbar spine through the L5S1 intervertebral disc and the articular process above and transmitting the load through the sacroiliac joint to the lower limb or rami of the ischial bone. Te sacrum needs to accommodate the changing load conduction from sitting to standing position. On the other hand, the sacrum is the most enlarged part of the spine. Together with the iliac crest, the sacrum forms the cornerstone of the whole spine, especially the lumbar movement, and is the axis of the upper body rotation during running and jumping. Terefore, the joint of the lumbar spine, sacrum, and iliac crest with the sacrum as the center requires both strong stability and elastic micromovement. It is the strongest and most complex ligamentous bone structure complex in the human body. Its irregular bone structure and interlacing dense ligament structure confuse people, and its biomechanical mechanism and characteristics are still not completely clear. Traumatic sacral fractures are mostly high-energy injuries, accounting for 1% of fractures and 20%∼30% of pelvic fractures. With the development of imaging technology, especially MRI, the reports of an exercise-induced stress fracture, senile osteoporotic fracture, and fracture after radiotherapy are increasing gradually. Treatment of sacral fractures includes both nonoperative and operative treatments. Nonsurgical fractures were based on rest, analgesic fractures, and early activity. Surgical treatment can be divided into three categories: percutaneous sacroiliac screw fxation, posterior pelvic fxation, and anterior pelvic internal fxation. As for the treatment of unstable sacral fractures, there are still diferences on whether to choose surgical treatment as well as the method and timing of surgical treatment [1]. 95% of traumatic sacral fractures are associated with injuries, and common ones include sacral plexus injuries, fractures elsewhere in the pelvic ring, hip or lumbar fractures, pelvic organ injuries, sacrocaudal skin soft tissue injuries, and even open fractures (it is important to note that sacral fractures may be communicated through the damaged intestine). Tis article reviews and discusses the current status and latest progress of surgical treatment of sacral fractures.  [2] divided the sacrum into three regions according to sacral anatomy. Region I: sacral wing region; Region II: sacral foramen region; Region III: sacral canal region. According to the anatomical location of the fracture, the fracture was classifed into three types: ① Type I: Te fracture line is located in the lateral region of the sacrum and is the most common type, accounting for about 50% of patients with this type of fracture. L5 nerve roots pass anteriorly and can be damaged by fractures. ② Type II: fracture line transsacral foramen; Zone II fractures are characterized by injuries to the L5 nerves through the lower sacral nerve heel. Patients with neurological symptoms account for 21% to 28%, and zone II fractures account for 34% to 47.5% of sacral fractures [3]. ③ Type III: fracture line through the sacral canal. Te transverse fracture is type III because the fracture end passes through the sacral canal. Cauda equina syndrome involving sacral canal fracture displacement can damage the cauda equina, usually damaging bilateral nerve roots above S4, which is manifested as cauda equina syndrome, mainly involving intestinal dysfunction or bladder dysfunction [4]. Te advantage of this classifcation is that it points out the risk of nerve damage in diferent fracture types but does not provide any guidance for surgical management.

Roy-Camille
Parting. Roy-Camille et al. [5] studied 13 patients with high sacral fractures and referred to the fractures above the S2 vertebral body as high sacral transverse fractures. Since most of the injuries were caused by falling from a high altitude, it was also called a "jump (suicide) fracture." Te lower sacral transverse fracture is the one below S2. For better clinical application, Roy-Camille et al. subdivided high sacral transverse fractures into the following three types according to the location of the lumbar spine at the time of injury: ① type I: angular without displacement; type II: angular and displaced; and ③ type III: Te sacrum has complete displacement of the head and tail. In 1991, Strange-Vognsen and Lebech [6] reported a case of high sacral transverse fracture with obvious rectal and bladder symptoms. According to imaging analysis, it was a nondisplaced comminted fracture, and they added it to Roy-Camille type IV, that is, a neutral position fracture. Te improved Roy-Camille classifcation method can be used to guide the classifcation of transverse fractures above S4, predict the risk of neurological damage, and provide guidance for surgical treatment.

Gibbons Parting.
Based on the Denis classifcation, Gibbons et al. [7] studied and analyzed 44 patients with sacral fractures in 1990. According to the degree of involvement of the sacral foramina and the sacral canal, the fracture line morphology was associated with a nerve injury, which was divided into a longitudinal fracture and a transverse fracture. Tis type is further subdivided into 4 types, but this type is basically similar to the Denis type, so it is not repeated here. It is important to note that longitudinal fractures often cause longitudinal instability of the pelvis, and transverse fractures often exhibit symptoms of severe nerve damage. In order to evaluate the recovery of nerve function before and after treatment, Gibbons et al. summarized a large number of cases and divided cauda equina injuries in sacral fractures into four types: ① type 1: no nerve damage; ② type 2: only paresthesia; ③ type 3: lower extremity movement impaired; and ④ type 4: intestinal or bladder dysfunction. Te classifcation is of great signifcance for evaluating the degree of nerve injury and the recovery of the nerve before and after treatment. Anterior and posterior compression (APC) can also be divided into 3 types. Te general feature of APC I is the separation of the pubic symphysis. Te distinguishing features of APC I are: separation of the sacroiliac joint; the anterior and posterior ligaments of the sacroiliac joint are pulled but still intact; the common characteristic of APC (II-III) is the separation of the pubic symphysis or the longitudinal fracture of the anterior pelvic ring. Te distinguishing features of APC II were the widening of the sacroiliac joint, the fracture of the anterior ligament, and the integrity of the posterior ligament. APC III was characterized by complete hemipelvic separation without longitudinal displacement, sacroiliac joint separation, iliac wing, or sacral fracture. ③ Vertical shear type (VS): the general characteristics are pubic symphysis separation or a longitudinal fracture of the pelvic anterior ring. Te diferential characteristics are longitudinal displacement and anteroposterior displacement of the pelvis, usually through the sacroiliac joint or the iliac crest. ④ Compound (CM), the general characteristics of the pelvic anterior ring, posterior ring longitudinal or transverse fracture, and the diferential characteristics of longitudinal displacement and anteroposterior displacement, usually through the sacroiliac joint but also through the iliac crest mixed with other types. Burgess, think this kind of classifcation on the imaging model can also be passed on to the size and direction of iliac blood vessels and their branches around the force of the tag. Te force will be around the sacroiliac joint ligament injury, causing internal organs and blood vessels to be damaged. According to the size and direction of the force, we can determine the weight of the patients with varying degrees of injury and, based on these special cases, ofer treatment.

AO.
Te AO classifcation of pelvic fractures formulated by AO/ASIF in its Fixation Manual in 1991 is basically the same as the Tile classifcation. Te diference is that type B2 is divided into B2.1 lateral compression type (ipsilateral) and the B2.2 lateral compression type (barrel handle type). Type C1 is divided into C1.1 sacroiliac fracture, C1.2 sacroiliac dislocation or fracture dislocation, and C1.3 sacral fracture [10]. Te AO classifcation is used to determine the stability of the pelvic ring in the presence of sacral fractures. Type A fractures with stable pelvic rings included coccygeal fractures and sacrococcygeal dislocation (AO/OTA: 61-A3.1); no displaced sacral fracture below S 2 with no involvement of the pelvic ring (A0/OTA: 61-A3.

Isler Parting.
In 1990, Isler [11], through a retrospective analysis of 193 patients with pelvic fractures, found three diferent types of lumbosacral joint-related lesions, with the common feature that the fracture line traversed the S 1 articular process. Type 1: fractures outside the L 5 /S 1 facet: subluxation of the S 1 facet or L 5 subarticular process. Type 2: intraarticular L 5 /S 1 fractures, such as sacral fractures through the S 1 joint. Type 2 fractures are subdivided into type 3 fractures: ①2a: sacral fractures pass through the joints of S 1 , resulting in joint dislocation, but the degree of dislocation is small; ②2b: sacral fractures pass through the joints of S 1 , resulting in joint subluxation; ③2c: the sacral fracture passed within the S 1 joint, resulting in complete dislocation. Type 3: this is a complex injury characterized by hemipelvic dislocation resulting in fractures of the articular process, interarticular, lamina, and pedicle, accompanied by dislocation of the L 5 vertebra. Tis classifcation only analyzed the stability of the lumbosacral region but did not involve the stability of the pelvis or nerve damage. In order to be more applicable to clinical practice, Isler and Ganz [12] subdivided AO types A, B, and C into 3 categories and 3 subcategories based on the stability and integrity of the pelvic posterior ring in 1996 and added an anterior ring injury in addition to ABC type 3. In the classifcation of pelvic posterior ring injury and anterior ring injury, it is a complete classifcation of pelvic fracture showing the characteristics of pelvic ring injury. Tis classifcation is complicated and difcult to remember.

Spine-Pelvis Separation.
In 1996, Bents et al. [13] found an uncommon fracture type of sacrum in clinical work, which was diferent from stable lumbosacral junction fractures and dislocations in terms of severity and difculty of patient management. Bents et al. defned a lumbosacral injury as a traumatic spine-pelvis separation involving a lumbosacral fracture with dislocation, a bilateral fracture or dislocation between the sacroiliac joints, and a sacral transverse fracture with bilateral sacroiliac joint dislocation. Tis type of injury is the combination of a transverse fracture and a longitudinal fracture, as defned by Gibbons et al. H and U fractures are common. Such injuries are usually caused by high-energy injuries and often lead to instability of the lumbar vertebrae and pelvis.

Lumbosacral Injury Classifcation
System. It was proposed by Lehman et al. [14] in 2012 to guide the surgical treatment of complex sacral fractures in the clinic. Te lumbosacral injury classifcation system is a classifcation system designed after the spinal and lower cervical injury classifcation systems and the thoracolumbar injury classifcation system [15]. Tis classifcation is based on fracture morphology, the posterior sacroligament complex, and the presentation of the nerve injury. A summative score, called the cumulative injury severity score, is created from the three categories, with a total score between 1 and 10. If the total Emergency Medicine International score is greater than 4, surgery is recommended. Equal to 4 points, according to the specifc situation of the patient to decide; less than 4 points, conservative treatment is recommended. Te details are as follows: fracture morphology: the kyphosis ≤20°was 1 min, and the kyphosis >20°was 2 min. Axial compression fractures: 2 without sacral foramen and canal involvement, 3 with sacral foramen and canal involvement, 3 with transverse displacement or rotational displacement, and 4 with burst or vertical fracture. Te injuries to the posterior sacral composite ligament were 0 points complete, 1 point incomplete, and 2 points complete. Degree of nerve injury: no nerve injury (0 points); only paresthesia (1 point); lower limb motor dysfunction (2 points); intestinal or bladder dysfunction (3 points); progressive nerve injury (4 points); the total score 0-10 points. Tis classifcation system is novel but needs further validation before it can be widely used.
In addition, Schmidek et al. [16] and Sabiston and Wing [17] also classifed sacral fractures. Among the many methods of sacral fracture classifcation, Denis and Tile are the most commonly used. However, each of the above classifcations has its advantages in the diagnosis and treatment of fractures, but there is no lack of their onesidedness. Te importance of typing lies in guiding clinical treatment and providing a theoretical basis for the next treatment. As a clinical worker to fexibly use the knowledge, the patient's actual condition, and theoretical knowledge in a comprehensive analysis in order to achieve the best curative efect.

Percutaneous Sacroiliac Screw Fixation.
Te best indications for sacroiliac screw fxation are sacroiliac dislocation and Denis zone 1 fracture. However, the use of comminuted Denis zones 2 and 3 sacral fractures may be limited, and proximal anatomical reduction of the fracture is required before screw placement. Many scholars have studied the anatomical application of SIJS and how to place them safely. Coskun [18] et al. divided the medial and lateral sacral safety zones, and the average medial safety zone measured on MRI of 400 patients was 32.8 mm. Te average lateral safety zone was 17.7 mm. Although sacroiliac screw fxation is a well-established technique, there are anatomical variations in the sacrum, and vascular and nerve injuries have been reported frequently during screw placement. In addition, due to the small space available for S 2 vertebral placement, the risk of nerve injury is much higher than that of S 1 vertebral placement [19]. To solve this problem, Bagheri H. et al. [20] measured the S 1 -S 2 vertebrae, pedicle, sacral foramen, and sacral canal of 87 sacrum bones. Te pedicle depth of S 1 was 25.8 ± 2.3 mm, and the sacral alar depth was 50.1 ± 1.7 mm. Te measured anteromedial angle of the pedicle was 29.6°± 0.9°, and the sacral alar angle was 29.7°± 2.1°. Tis is helpful for the clinical placement of sacroiliac screws.
In addition, the comparison of the efectiveness and safety of SIJS and other internal fxation methods for sacral fractures is a research hotspot. Wenning et al. [21] reported the diferences in intraoperative operation, complications, and postoperative range of motion between sacroiliac screws and spinopelvic fxation. Twenty-nine patients and 48 patients received sacroiliac screws and spinopelvic fxation, respectively. Te spinopelvic fxation group had earlier weight-bearing, but the sacroiliac screw group had shorter operation times, shorter hospital stays, and lower postoperative wound infection rates. Computer 3D navigation and robotics make the application of SIJS more convenient. It has been reported [22] that the use of 3D fuoroscopic navigation (see the procedure chart) has a lower iliosacral screw misalignment rate than conventional techniques or 2D fuoroscopic navigation. In addition, it reduces radiation exposure and the correction rate compared to conventional techniques. However, the misalignment rate associated with 3D perspective navigation is between 0% and 31%, indicating that there is still room for improvement in navigation performance. With the application of CT-aided design and computer three-dimensional navigation technology, nail placement is more accurate, intraoperative fuoroscopy times are less, the operation time is shorter, and the incidence of postoperative complications is lower. Weil et al. [23] used A Renaissance robot mounted on A multidirectional bridge connected to the patient's spine and then planned the implant trajectory during preoperative or intraoperative 3D scans. Placing screw after inserting guide wire percutaneously. Te accuracy of sacroiliac screw placement, operation time, and fuoroscopy time were evaluated, and it was pointed out that the robotic system can implant internal fxators accurately and accurately for sacral fractures, which is a safe and reproducible method. Wu et al. [24] designed a new 3D template. Te new guidance template was attached to the iliac crest on the pelvic model before operation and then installed on the internal and external cannulas on the base to insert the Kirschner wire into the model. Sacroiliac screw placement was assisted by a 3D-printed template in 19 patients, and IS screws were directly inserted under fuoroscopy in 18 patients. Te investigators compared the quality of reduction, screw grade, operation time, and number of radiation exposures between the two groups and found that the combined templateassisted sacroiliac screw placement had higher accuracy, less fuoroscopy, shorter operation time, and avoided neurovascular injury caused by screw dislocation. Balling [25], under the guidance of 3D images, applied 120 navigation screws to 124 sacral fracture sites (the frst and second vertebrae) of 52 patients. After 2 years of follow-up, the average visual analogue scale (VAS) decreased from 8.9 ± 1.1 preoperatively (3.6 ± 1.7 postoperatively) to 1.8 ± 1.9. Te OSwestry Disability index (ODI) increased from 86.2 ± 4.9% preoperatively (28.5 ± 9.5% postoperatively) to 23.3 ± 13.7%. Krappinger et al. [26] proposed the use of preoperative CT scanning to develop a starting point and safe trajectory plan for safe and accurate fuoroscopic control of percutaneous iliosacral screw placement (SI screw) and used this technique to place 59 screws in 34 patients. Satisfactory outcomes were achieved in 97% of the patients. Richter et al. [27] used a fxed robotic 3D plate detector and a navigation system connected to the operating table for intraoperative navigation to improve the precision of SI screws. Fernandez-Fernandez et al. [28] 11 patients with complex posterior ring fracture combined with vertical instability were treated with iliac screw under CT localization. Te mean follow-up time was 33 months. Among them, 9 cases had an excellent Matta score, and 2 cases had a good Matta score. Te asymmetry index increased from 13.18 to 2.72. Te deformity index increased from 0.049 to 0.010. Seven patients were able to resume their previous activities. Only 2 patients with Denis 2 fractures had secondary displacement during follow-up. Four patients developed neurological complications as a result of the initial injury. Percutaneous transsacral screw fxation has become a popular fxation method due to its low complication rate and good clinical results. It has been suggested that partially threaded nails may cause iatrogenic nerve damage. Herman et al. [29] performed a retrospective study on 90 patients who received partially threaded screws under the assumption that they would not cause iatrogenic nerve damage, of which only 4 patients still had neurological dysfunction at the last follow-up. With the in-depth study of sacral anatomy, the progress of SJUS placement technology, and the development of artifcial intelligence, percutaneous sacroiliac screw fxation (SIJS), which uses a minimally invasive method to fx screws to the S 1 or S 2 sacral vertebrae via the iliac bone, has been widely accepted due to the advantages of less bleeding and low complications.
Currently, the common clinical treatment methods for Denis II sacral fracture and pelvic anterior ring fracture include open reduction plate fxation of the pelvic anterior ring fracture, ilio-lumbar pedicle screw internal fxation combined with external fxation, ilio-lumbar pedicle screw combined with pelvic anterior ring plate internal fxation, etc. Anterior pelvic ring plate internal fxation is only the reduction and fxation of anterior pelvic ring fracture. Trough reliable reduction and fxation of the anterior ring fracture, indirect reduction of the posterior pelvic ring can achieve the purpose of sacral nerve decompression to a certain extent. Although open reduction of the anterior pelvic ring has relatively small trauma, its stability is poor, and there is no reliable three-dimensional fxation of the lumbosacral part and the entire pelvic ring, nor can it completely decompress the injured sacral nerve. Although pedicle screw internal fxation combined with external fxation can efectively fx the anterior and posterior pelvic rings, the external fxation is difcult to care for, and the risk of infection is high, so it is difcult to obtain a satisfactory efect for the reduction of the anterior ring comminted fracture. It has been reported that external fxators can temporarily fx pelvic fractures, but using external fxators as the fnal fxation method can lead to severe malunion, with a malunion rate of up to 50%. Ilio-lumbar pedicle screw combined with anterior pelvic ring plate internal fxation is satisfactory for fracture reduction, can better solve the disadvantages of simple use of anterior ring plate fxation or ilio-lumbar pedicle screw internal fxation, and has the following obvious advantages: (1) Te posterior approach to surgery has clear visual feld, large operating space, solid internal fxation, large orthopedic force, good reduction efect on the sacrum, can restore the normal anatomical structure of the sacroiliac, and good indirect reduction efect on the anterior pelvic ring. (2) Te posterior internal fxation reduction can clearly explore the sacral nerve injury, fully relieve the sacral nerve compression, and provide a safe space for the sacral nerve. (3) It can not only restore the closed ring structure of the pelvis but also restore normal spine-pelvis physiological force line conduction. (4) It is a three-dimensional fxation of the spine and pelvis, which can limit fracture displacement and avoid further injury of sacral nerve. (5) In line with the anatomical and physiological structure of the spine and pelvis, fxation is efective and reliable, so patients can get out of bed early and reduce bed complications. Te results of the study by Zhang et al. [30] showed that all the patients in the study group had solid fracture fxation, no displacement, and no loosening or fracture of the nail rod and plate during the follow-up.

Spinal Pelvic Internal Fixation.
Troughout the history of posterior spinal pelopexy, postoperative wound exudation, incision infection, and foreign body sensation caused by internal fxators have always been difcult problems for surgeons. Traditional open spinopelvic fxation has drawbacks and complications, such as relatively high infection rates (10 to 15%), wound dehiscence, and instrumentation problems. Complications associated with wounds are relatively common due to overexposure. Two reports have shown the advantages of minimally invasive spinopelvic fxation over traditional incisions. With the development of artifcial technology, minimally invasive internal fxation combined with computer and robot navigation has been increasingly applied in the treatment of orthopedic patients. Liu et al. [31] treated 12 patients with traumatic spinopelvic separation in a single center since March 2016, all of whom were treated with minimally invasive lumbar and pelvic fxation assisted by the third-generation Tianji orthopedic robot (TINAVI Medical Technologies, Beijing, China). Compared with the control group, surgery time is shorter, intraoperative bleeding is less, average length of hospital stay is shorter, and complication rates are lower. Tsai et al. [32] reported the imaging and clinical results of a single-center retrospective study with more than 1 year of follow-up, in which 21 patients and 17 patients were treated with traditional open fxation and minimally invasive fxation, respectively. Tere were no signifcant differences in postoperative imaging assessment and functional scores between the two groups. Te minimally invasive fxation group had shorter operation time, less intraoperative bleeding, and a lower incidence of complications. Compared with the traditional open surgery group, the postoperative appearance is more beautiful. Decker et al. [33] also proposed a minimally invasive U-shaped spinopelvic stabilization technique: LPS from L3/4 to the iliac crest with a robust crossrod/cross-connector between the iliac crest screws. Two iliac crest screws were connected with a 5.5 mm transverse rod. A 5.5 mm transverse connector was used to attach to the pedicle screw. Tere were no soft tissue-related complications in the 10 patients who underwent the operation, and there was no Emergency Medicine International 5 abnormal injury in postoperative function and mobility. Koshimune et al. [34] reported the results of minimally invasive spinopelvic fxation for the treatment of unstable bilateral sacral fractures. In this study, three of the eight patients who received conventional fxation had MRSA infections, while those who received minimally invasive fxation did not. Of the 16 patients, 15 had bone union. In conclusion, minimally invasive spinopelopexy is associated with less operative time, less bleeding, and lower infection rates than traditional open spinopelopexy. Many scholars have compared spinopelvic fxation with other fxation techniques. Shetty et al. [35] reported the imaging and clinical results of a single-center retrospective study followed up for more than 2 years, in which 40 cases of noncomminuted longitudinal fractures with normal neurological function and satisfactory closed reduction were treated with sacroiliac screw internal fxation. Twenty-seven cases of comminuted or high transverse fractures with malformed anatomy or nerve loss were treated with spinopelvic fxation. It is pointed out that spinopelvic fxation techniques can be used for comminuted fractures, including vertical instability, unacceptable closed reduction, neurological dysfunction, lumbar deformity, and high transverse fractures. Spinopelvic fxation also has its disadvantages. Petryla et al. [36] followed up 16 patients with pelvic and spinal separation in a single center for 1 year and pointed out that only one-third of patients with pelvic and spinal separation reached the preinjury level one year after injury. Spinopelopexy has been continuously improved since it was shown to provide good reduction and sufcient strength for bilateral sacral fractures with vertical instability. Prost et al. [37] proposed minimally invasive lumbar iliac and iliosacral fxation techniques to treat posterior pelvic ring injuries. Te operation was performed with the patient lying prone. Tis included the insertion of pedicle screws into L4 or L5 and fuoroscopy-guided screw fxation of the iliac crest. Intraoperative distraction can be performed according to the amount of displacement. An iliosacral screw was then inserted percutaneously to reset it in the transverse plane, creating a triangular structure. Okuda et al. [38] proposed crab-shaped internal fxation: percutaneous pedicle screws were bilaterally inserted into the L5 or L4 pedicle; 4 iliac screws were bilaterally inserted into the iliac crest; and titanium rods were used to connect the screw directions from the left to the right and from the head to the tail. Tis provides a feasible method for pelvic and spinal internal fxation, which can reduce the vertical displacement and bone healing of unstable pelvic ring fractures. Peng et al. [39] combined the Starr Frame navigation robot and the Da Vinci robot for minimally invasive surgical treatment of pelvic fractures. Tis treatment can be used to treat pelvic fractures and sacral nerve injuries, as well as early and late nerve repair for sacral fractures.

Triangle Fixation Technique.
Te posterior pelvic ring fracture is treated with a spinal and pelvic longitudinal fxation system combined with a transverse sacroiliac screw or plate, which is called the "triangle fxation technique." Tis technique has been proven to be the most biomechanically stable, providing multiplanar mechanical stability and enabling patients to walk with early loading. Te disadvantage lies in the need for extensive dissection of paravertebral muscles, trauma, bleeding, postoperative wound exudate infection, and other problems. Jindal et al. [40] reported the early and midterm efcacy of spinal pelvic internal fxation and transverse iliac screw internal fxation in the treatment of unstable transsacral fractures. Tis investigator reported the radiological and clinical fndings of a single-center retrospective study with a mean follow-up of 3.1 years in which 22 patients underwent trigonometry and noted that trigonometry is also a reliable treatment for unstable transsacral fractures that allows for early weightbearing and promote faster functional recovery. Tian et al. [41] described a modifed TOS procedure for the reduction and fxation of unstable vertical sacral fractures based on the principle of TOS. It consists of vertical and transverse rods and a pedicle screw system for fxation. In this report, the clinical outcomes were investigated radiologically, and the reduction and fxation of vertically unstable sacral fractures were examined to further assess the biomechanical function and surgery-related complications after fnal reduction and stabilization. Kanezaki et al. [42] proposed a minimally invasive trigonometry and followed up on the clinical outcomes of 10 patients with unstable sacral fractures treated with minimally invasive trigonometry (MITO) for at least 1 year. According to the Majeed functional rating system, the clinical outcomes of 8 patients were "excellent." Tian et al. [43] 18 patients with traumatic spine-pelvis separation were treated with modifed bilateral triangular fxation. After an average follow-up of 11 months, the excellent and good rate of Matta function was 89%. Terefore, Tian et al. believed that modifed triangular internal fxation combined with internal fxation is an efective and advanced surgical option for patients with traumatic spine-pelvis separation.

Other Fixation
Methods. In addition to spinopelvic fxation and triangular fxation, Joo and Grauer [44] pointed out that the use of posterior superior margin screws as an auxiliary fxation method is a feasible choice and can be used as a reference for fxation scafolds in complex pelvic and spinal reconstruction. A technical report describing posterior pedicle plates was provided by Boudissa et al. [45], which evaluated the clinical and radiological outcomes of 10 patients with vertically unstable pelvic fractures. Te excellent and good rate of fracture reduction was 70%, the excellent and good rate of the Hannover and Majeed scores was 80%, and the average Majeed score was 71.8 ± 17 points. Complications included early postoperative sepsis requiring surgical irrigation in 3 cases and stent removal in 4 cases due to discomfort. Trough biomechanical tests, Shinohara et al. [46] believed that the posterior internal fxation of the spine had more biomechanical strength than the traditional posterior plate fxation. For the treatment of longitudinal sacral fractures, Fathy Saoud et al. [47] proposed that percutaneous iliac screw fxation with the posterior superior iliac spine as the starting point is a new option for the treatment of sacral fractures. Fifty patients with longitudinal sacral fractures received "ileal" internal fxation. Te investigators believe that minimally invasive "ileal" fxation provides a safe, rapid, and easy fxation method for such fractures. It is suitable for critically ill patients because it is fast and causes less blood loss. Tile C pelvic ring injuries are challenging for surgeons to manage. Most of these injuries can be managed by percutaneous reduction techniques, and the posterior ring can be stabilized by percutaneous transpedicle screw fxation. However, for a signifcant Denis II comminuted area or signifcant lateral/vertical displacement of the semipelvis through a complete sacral fracture. In this case, percutaneous treatment can be dangerous. Martin et al. [48] proposed a minimally invasive technique for indirect reduction and temporary stabilization that is soft tissue-friendly and allows the state of reduction to be maintained during fnal fxation surgery.

Front Fixation Technology.
Te anterior approach was considered a forbidden area for the surgical treatment of sacral fractures in the past because of its complex anatomy, difcult exposure, small operating space, and risk of injury to the lumbosacral trunk, sacral plexus, and blood vessels.
Previously, scholars at home and abroad also performed presacral anatomy: the area of 2 cm saved from the lateral part of the L4 nerve to the sacroiliac pass was a safe area for surgery, and plate and screw fxation could be performed. Terefore, anterior plate fxation was initially indicated only for sacroiliac dislocations and partial Denis I fractures. In recent years, Huang et al. reported [49] that the modifed para-rectus abdominis approach was used to fx the anterior sacral plate, and the common iliac arteries and veins and the lumbosacral plexus were safely raised and retracted with a rubber catheter to protect the common iliac arteries and veins and the lumbosacral plexus under direct sight during the operation, forming the "ffth window" of the modifed para-rectus abdominis approach. Te two ends of the pelvic reduction forceps can be placed on sacral cone 1 and the iliac crest to reduce the sacral fracture. Te two plates are fxed simultaneously on the frst cone, the sacral wing, and across the sacroiliac joint to the iliac crest to complete the sacral fracture fxation. Tis technique innovatively fxes the plate to the sacral 1 cone, broadens the scope and indications of anterior plate fxation for sacral fractures, and provides a new fxation idea for the anterior treatment of traumatic sacral fractures. Management of LC-1 pelvic injuries, especially in patients with a complete sacral fracture (LC-1PICSF, OTA 61-B2.1), remains controversial. Te specifc indications for fxation alone are unknown, and outcome data are scarce compared with combined fxation. Huang et al. [49] reported the imaging and clinical results of a single-center retrospective study with more than 2 years of follow-up. Tirty-six patients and 32 patients were treated with anterior internal fxation alone or combined anterior and posterior fxation, respectively. Tere were no signifcant diferences between the two groups. Compared with combined fxation treatment, anterior fxation alone has a shorter operation time, less intraoperative fuoroscopy, and less intraoperative bleeding. Compared with anterior fxation alone, patients treated with combined fxation achieved full weight-bearing earlier.

Summary and Prospect
At present, the surgical treatment of unstable sacral fractures is a major challenge. Te timing of surgery and fxation methods are still controversial, but the overall trend is towards minimally invasive treatment. Te authors concluded that as a new anterior sacral approach for the treatment of sacral fractures, the para-rectus abdominis approach has the advantages of a small skin incision, satisfactory exposure, no need to cut muscle, short wound closure time, no nerve traction injury, less bleeding, and a low infection rate. Exploration and decompression of the superior sacral plexus (L4 to S1) in the presacral region have a signifcant visual feld advantage, which can remove scar tissue under direct vision and reduce or remove free bone debris. It is a feasible treatment method without further destabilizing the posterior pelvic ring. However, for the inferior sacral plexus (S2 to S4), sacral nerve injury, and lumbosacral separation, a combined anterior and posterior approach is recommended for nerve exploration and decompression and fracture fxation due to the complex anatomical structure of the anterior sacral venous plexus, the difculty of anterior sacral nerve exploration and decompression, and the inability to efectively stabilize the lumbosacral joint.

Data Availability
Te data used to support the fndings of this study are included within the article.

Conflicts of Interest
Te authors declare that they have no conficts of interest.