Treatment of Type 
Introduction Fragility fractures of pelvis (FFP) occurs primarily in the osteoporotic elderly and differs from their high‐energy trauma counterparts. In recent years its frequency is increasing secondary to an aging population and occurs 64% of all pelvic fractures in literatures . The choice of treatment of these fractures are based on FFP radiological classification which was developed by Rommens et al. . Type FFP IV fractures are the most instable lesions and accounted for 19.2% of all FFP . These injuries are spinopelvic dissociations associated with bilateral sacral fracture displacements which were often not so obvious, and the most common fracture pattern is the H‐type . There is consensus of surgeon that the surgery is necessary for this type fractures and the main goals of treatment are early, pain‐free mobilization, and restoration of previous patient self‐sufficiency. Therefore, the ideal internal fixation should have adequate mechanical strength, easy and safe to insert, and minimally invasive . Triangular fixation (TFX) is a classical type of internal fixation for the treatment of unstable sacral fractures in adults. TFX consists of unilateral or bilateral lumbopelvic fixations and iliosacral screws and has been widely used because it not only can reduce and fix sacral fractures simultaneously, but also formulate a tri‐dimensional interlocking structure to fix the fracture in order to obtain superior biomechanical fixed strength and maximum sacral stability . However, TFX also has a number of notable disadvantages: longer operating time; more intraoperative bleeding; a high rate of infection resulting from excessive exposure. These disadvantages are more lethal for the elderly whose physical conditions are even worse than adults and threaten their lives feasibly. In addition, the risk of vessel and nerve injury during implantation of the iliosacral screws and excessive intraoperative radiation exposure can cause extra iatrogenic damage. Recently many reports declared that the application of orthopedic robots can overcome these shortcomings above effectively . The robotic system can assist surgeons in planning the trajectory, position, and length of the inserted screws by importing intraoperative C‐arm images and inserting the implant minimally invasive and accurately, which leads to less bleeding and intraoperative fluoroscopy, less iatrogenic exposure damage and vessel or nerve iatrogenic injury, and better prognosis. However, the related reports of RoboTFX were mostly focused on the surgery of adults and the reports of osteoporotic elderly treatment were rarely. To address this shortcoming, we retrospectively analyzed data of FFP IV patients who, from March 2017 to December 2022, underwent RoboTFX surgery. We compared these clinical data with the most mature and advanced technology which has been reported until now‐robotic‐assisted minimally invasive iliosacral screws (RoboISS). Our objectives in undertaking this study were to: (i) evaluate the clinical outcomes of treating FFP IV with RoboTFX, (ii) discuss the advantages of this technique, and (iii) describe the key points of surgery. 

Methods 2.1 Inclusion and Exclusion Criteria Inclusion criteria: (i) bilateral sacral fracture; (ii) > 60 years; (iii) low energy trauma; (iv) bone mineral density (BMD) < −2.5 SD; (v) treated with RoboTFX or RoboISS; (vi) Time from injury to operation > 3 weeks; (vii) continuous follow‐up less than 15 months. Exclusion criteria: (i) open sacral fracture; (ii) associated severe internal medical disease. 2.2 Patients Data From March 2017 to December 2022, 22 consecutive cases of FFP IV fractures were included in the study. Subjects were divided into two groups according to operation employed (RoboTFX or RoboISS), whose medical records were retrospectively reviewed. The research was affirmed in accordance with the Declaration of Helsinki and approved by the ethic committee of Tianjin hospital (IEC no. 2023.JYL.062). All patients were approved to participate the research and sign the consent declaration. 2.3 RoboTFX Operation There were 2 males and 8 females with an average age of 72.1 ± 2.2 (60–95) years. BMD is −3.0 ± 1.7 (−2.5 to −4.0). Cause of injury were all of falling from a standing position. Fracture type (according to morphology): 8 cases of type H, 2 cases of type U; 3 factures had obvious displacement. All sacral fractures were associated with pubic rami fractures. Five patients had hypertension, 3 patients had arrhythmia, 3 patients had diabetes, and 2 patients had combined distal ulnar‐radial fractures. 2.4 RoboISS Operation There were 2 males and 10 females with an average age of 70.8 ± 1.4 (60–89) years. BMD is −2.7 ± 1.3 (−2.5 to −4.2) SD. Cause of injury: 11 cases of falling from a standing position, 1 case of falling from a bicycle; fracture type (according to morphology): 10 cases of type H, 2 cases of type U; 3 factures had obvious displacement. All sacral fractures were associated with pubic rami fractures. Five patients had hypertension, 2 patients had arrhythmia 2 had coronary heart disease, 3 patients had diabetes, 1 patient had proximal humeral fractures, and 1 patients had distal radial fractures. 2.4.1 Preoperative Treatment and Plan All patients accepted skeletal traction, and the definitive operations of sacral fracture were performed until the physiological condition of patients was confirmed stable. The standard preoperative imaging examination plan included dual‐energy X‐ray absorptiometry to diagnose and evaluate the level of osteoporosis, vessel ultrasonic examination to diagnose deep vein thrombosis (DVT), as well as anterior–posterior, inlet, and outlet x‐ray and 3D CT reconstruction of the pelvis to assess the displacement and degree of deformity of the sacral fracture. We created a radiological simulation of normal morphology for every injured sacrum in order to measure the anatomic data of the sacral vestibule and judge preoperatively whether the iliosacral screws were suitable for implantation. 2.4.2 Surgical Procedure All procedures were performed and the related clinical data were recorded by the same group of experienced orthopedic surgeons. Intravenous antibiotics were administered within 30 min before the skin incisions were made. 2.5 RoboTFX Operation Patients were administered general anesthesia and placed prone on a radiolucent table to ensure that pelvic anteroposterior (AP), inlet, outlet, and lateral views could be feasibly obtained. The navigation tracker of the orthopedic robot (TINAVI Medical Technologies, Beijing, China, Figure ) was percutaneously fixed on L3 spinous process. After L5intraoperative CT images were obtained using C‐arm (Siemens, Germany), the data were transmitted to the robotic planning system. Based on preoperative planning combined with L5 vertebral anatomic feature‐mapping, the length, angulations, and direction of unilateral pedicle screw insertion were designed, with screw placement simulated on the images of screen. After navigation planning was thus established, the robotic arm, covered in a sterile, plastic dressing, began to move to the proper position following guidance of the preplanned trajectory. A guide sleeve was placed onto the L5 bone surface via a 3 cm incision. After the trajectory was recalibrated, a guide pin was then inserted into the pedicle of one side through the sleeve. A cannulated universal pedicle screw, 6 mm in diameter (Kanghui Medical Instruments, China), was inserted along the pin, then the guide pin was removed. Following insertion of the screw, pelvic images of the lateral, iliac tangential (inlet‐obturator), and teardrop (outlet‐obturator) position were obtained through intraoperative fluoroscopy. These data were then transmitted to the robotic planning system. After the next round of navigation planning, the robotic arm moved to the next assigned position. A 5 cm incisions was made on the posterior superior iliac spine (PSIS) of the same side and the guide pin was inserted through the sleeve of robot. A 10mm diameter and 11 cm deep ipsilateral universal iliac screw (Kanghui Medical Instruments, China) was inserted along the guide pin. At this point, the direction of the screw from the PSIS to the anterior inferior iliac spine (AIIS) and between the medial and lateral lamina of the iliac wing was verified. Then a pre‐contoured rod, 6.5 mm in diameter (Kanghui Medical Instruments, China) was inserted through two incisions and connected to the pedicle and iliac screw. The same procedure repeated on the contralateral side of patient. The sequence of the screws and rods' insertion could change flexibly. If the intraoperative fluoroscopy showed the displacements of sacral fractures were unacceptable, the reduction of vertical displacement was achieved through distraction of the bilateral lumbopelvic devices then all connectors were screwed in place. One or two Schantz pins can be screwed into the lateral part of sacrum to reduce the horizontal or rotational residual displacement of the pelvic ring which is called the “joystick” technique , and the transversal connection instruments were fixed subfascially through two parallel incisions on L5 or PSIS. Once the quality of the reduction was confirmed by intraoperative fluoroscopy of AP, inlet, outlet, and lateral views, the related data of reduced sacrum were transmitted to the robotic planning system, and the navigation planning of the iliosacral screws was designed. The robotic arm then moved to the assigned position and made a 1 cm incision of one side. One 7.3 mm diameter fully threaded and cannulated screws (Kanghui Medical Instruments, China) was inserted along the robotic guide pin, serving as the iliosacral screw running from the ilium to the vertebral center of S1 and achieving direct fixation of the sacral fracture. Then the same procedure was manipulated on the contralateral side again (Figure ). 2.6 RoboISS Operation Patients were administered general anesthesia and placed prone on a radiolucent table with reduction frame. The intraoperative fluoroscopy of AP, inlet, outlet, and lateral views was obtained regularly to judge whether the displacements of sacral fractures were unacceptable or not. Once the reduction was necessary, several Schantz pins were inserted in the iliums of both sides and sacrum as the hand shank. The reduction of sacral fractures was performed by manipulating the pins under the fluoroscopy of C‐arm and the pins were connected with frames and tightened once the X‐ray showed the displacements of fracture were corrected. Then one S1 and one S2 iliosacral screws were inserted on each side with the help of robot. All patients had no blood transfusion and had regular blood test pre‐ and postoperatively in operation room to calculate the intraoperative blood loss. All of them accepted irrigation and vacuum drainage instruments before their wound was closed. All pubic rami fractures obtained operations simultaneously and fixed with locking reconstruction plate, biodegradable, or steel cannulated screw. All screws were coated with semi‐liquid state calcium phosphate bone substitution (Ruibang medical instrument, China) as augmentation before insertion (Figure ). 2.6.1 Postoperative Treatment All patients underwent the same postoperative management, with intravenously administered antibiotics continued for 72 h following surgery. The drainage instrument was removed 48–72 h postoperatively according to the amount of drainage fluid. Low molecular weight heparin was applied regularly as thromboprophylaxis during the immobilization period. Rehabilitation was planned and performed by a physical therapist as soon as the physical and wound condition of each patient permitted. Full weight bearing was permitted at least 12 weeks postoperatively, depending on the physical and bony quality conditions of patients. 2.6.2 Follow‐Up and Clinical Outcome Evaluation The follow‐up schedule was 4 weeks, 12 weeks, and every 3 months until 15 months postoperatively. Physical examinations and imaging exams to judge the degree of fracture healing, whether or not fixation had failed, and improvement in daily activity functionality. The clinical outcome for fractures was evaluated with Majeed pelvic outcome score. Clinical data pertaining to operation time, intraoperative bleeding (including visible and hidden blood volume, calculated with the formula noted by Nadler ), intraoperative fluoroscopy time, infection rate, favorable fracture healing rate (more than 1/2 of the vertical fracture line had bony fusion or covered with continuous callus by CT confirmation in last follow‐up was defined the favorable healing, or else was defined undesirable healing ), implant loosening rate (including the displacement of screws or the occurrence of radiolucent area around implant, which was confirmed by X‐ray or CT ), Majeed pelvic score were collected and compared statistically between two groups respectively. 2.6.3 Statistical Analysis All data were processed using SPSS20.0 statistical software (SPSS Inc. Chicago, IL, USA). Quantitative data were expressed as mean ± standard deviation (SD) and compared using Student's t ‐test. Categorical variables were compared using the Pearson's χ 2 test. The p value was set < 0.05 for significance. 

Inclusion and Exclusion Criteria Inclusion criteria: (i) bilateral sacral fracture; (ii) > 60 years; (iii) low energy trauma; (iv) bone mineral density (BMD) < −2.5 SD; (v) treated with RoboTFX or RoboISS; (vi) Time from injury to operation > 3 weeks; (vii) continuous follow‐up less than 15 months. Exclusion criteria: (i) open sacral fracture; (ii) associated severe internal medical disease. 

Patients Data From March 2017 to December 2022, 22 consecutive cases of FFP IV fractures were included in the study. Subjects were divided into two groups according to operation employed (RoboTFX or RoboISS), whose medical records were retrospectively reviewed. The research was affirmed in accordance with the Declaration of Helsinki and approved by the ethic committee of Tianjin hospital (IEC no. 2023.JYL.062). All patients were approved to participate the research and sign the consent declaration. 

RoboTFX Operation There were 2 males and 8 females with an average age of 72.1 ± 2.2 (60–95) years. BMD is −3.0 ± 1.7 (−2.5 to −4.0). Cause of injury were all of falling from a standing position. Fracture type (according to morphology): 8 cases of type H, 2 cases of type U; 3 factures had obvious displacement. All sacral fractures were associated with pubic rami fractures. Five patients had hypertension, 3 patients had arrhythmia, 3 patients had diabetes, and 2 patients had combined distal ulnar‐radial fractures. 

RoboISS Operation There were 2 males and 10 females with an average age of 70.8 ± 1.4 (60–89) years. BMD is −2.7 ± 1.3 (−2.5 to −4.2) SD. Cause of injury: 11 cases of falling from a standing position, 1 case of falling from a bicycle; fracture type (according to morphology): 10 cases of type H, 2 cases of type U; 3 factures had obvious displacement. All sacral fractures were associated with pubic rami fractures. Five patients had hypertension, 2 patients had arrhythmia 2 had coronary heart disease, 3 patients had diabetes, 1 patient had proximal humeral fractures, and 1 patients had distal radial fractures. 2.4.1 Preoperative Treatment and Plan All patients accepted skeletal traction, and the definitive operations of sacral fracture were performed until the physiological condition of patients was confirmed stable. The standard preoperative imaging examination plan included dual‐energy X‐ray absorptiometry to diagnose and evaluate the level of osteoporosis, vessel ultrasonic examination to diagnose deep vein thrombosis (DVT), as well as anterior–posterior, inlet, and outlet x‐ray and 3D CT reconstruction of the pelvis to assess the displacement and degree of deformity of the sacral fracture. We created a radiological simulation of normal morphology for every injured sacrum in order to measure the anatomic data of the sacral vestibule and judge preoperatively whether the iliosacral screws were suitable for implantation. 2.4.2 Surgical Procedure All procedures were performed and the related clinical data were recorded by the same group of experienced orthopedic surgeons. Intravenous antibiotics were administered within 30 min before the skin incisions were made. 

Preoperative Treatment and Plan All patients accepted skeletal traction, and the definitive operations of sacral fracture were performed until the physiological condition of patients was confirmed stable. The standard preoperative imaging examination plan included dual‐energy X‐ray absorptiometry to diagnose and evaluate the level of osteoporosis, vessel ultrasonic examination to diagnose deep vein thrombosis (DVT), as well as anterior–posterior, inlet, and outlet x‐ray and 3D CT reconstruction of the pelvis to assess the displacement and degree of deformity of the sacral fracture. We created a radiological simulation of normal morphology for every injured sacrum in order to measure the anatomic data of the sacral vestibule and judge preoperatively whether the iliosacral screws were suitable for implantation. 

Surgical Procedure All procedures were performed and the related clinical data were recorded by the same group of experienced orthopedic surgeons. Intravenous antibiotics were administered within 30 min before the skin incisions were made. 

RoboTFX Operation Patients were administered general anesthesia and placed prone on a radiolucent table to ensure that pelvic anteroposterior (AP), inlet, outlet, and lateral views could be feasibly obtained. The navigation tracker of the orthopedic robot (TINAVI Medical Technologies, Beijing, China, Figure ) was percutaneously fixed on L3 spinous process. After L5intraoperative CT images were obtained using C‐arm (Siemens, Germany), the data were transmitted to the robotic planning system. Based on preoperative planning combined with L5 vertebral anatomic feature‐mapping, the length, angulations, and direction of unilateral pedicle screw insertion were designed, with screw placement simulated on the images of screen. After navigation planning was thus established, the robotic arm, covered in a sterile, plastic dressing, began to move to the proper position following guidance of the preplanned trajectory. A guide sleeve was placed onto the L5 bone surface via a 3 cm incision. After the trajectory was recalibrated, a guide pin was then inserted into the pedicle of one side through the sleeve. A cannulated universal pedicle screw, 6 mm in diameter (Kanghui Medical Instruments, China), was inserted along the pin, then the guide pin was removed. Following insertion of the screw, pelvic images of the lateral, iliac tangential (inlet‐obturator), and teardrop (outlet‐obturator) position were obtained through intraoperative fluoroscopy. These data were then transmitted to the robotic planning system. After the next round of navigation planning, the robotic arm moved to the next assigned position. A 5 cm incisions was made on the posterior superior iliac spine (PSIS) of the same side and the guide pin was inserted through the sleeve of robot. A 10mm diameter and 11 cm deep ipsilateral universal iliac screw (Kanghui Medical Instruments, China) was inserted along the guide pin. At this point, the direction of the screw from the PSIS to the anterior inferior iliac spine (AIIS) and between the medial and lateral lamina of the iliac wing was verified. Then a pre‐contoured rod, 6.5 mm in diameter (Kanghui Medical Instruments, China) was inserted through two incisions and connected to the pedicle and iliac screw. The same procedure repeated on the contralateral side of patient. The sequence of the screws and rods' insertion could change flexibly. If the intraoperative fluoroscopy showed the displacements of sacral fractures were unacceptable, the reduction of vertical displacement was achieved through distraction of the bilateral lumbopelvic devices then all connectors were screwed in place. One or two Schantz pins can be screwed into the lateral part of sacrum to reduce the horizontal or rotational residual displacement of the pelvic ring which is called the “joystick” technique , and the transversal connection instruments were fixed subfascially through two parallel incisions on L5 or PSIS. Once the quality of the reduction was confirmed by intraoperative fluoroscopy of AP, inlet, outlet, and lateral views, the related data of reduced sacrum were transmitted to the robotic planning system, and the navigation planning of the iliosacral screws was designed. The robotic arm then moved to the assigned position and made a 1 cm incision of one side. One 7.3 mm diameter fully threaded and cannulated screws (Kanghui Medical Instruments, China) was inserted along the robotic guide pin, serving as the iliosacral screw running from the ilium to the vertebral center of S1 and achieving direct fixation of the sacral fracture. Then the same procedure was manipulated on the contralateral side again (Figure ). 

RoboISS Operation Patients were administered general anesthesia and placed prone on a radiolucent table with reduction frame. The intraoperative fluoroscopy of AP, inlet, outlet, and lateral views was obtained regularly to judge whether the displacements of sacral fractures were unacceptable or not. Once the reduction was necessary, several Schantz pins were inserted in the iliums of both sides and sacrum as the hand shank. The reduction of sacral fractures was performed by manipulating the pins under the fluoroscopy of C‐arm and the pins were connected with frames and tightened once the X‐ray showed the displacements of fracture were corrected. Then one S1 and one S2 iliosacral screws were inserted on each side with the help of robot. All patients had no blood transfusion and had regular blood test pre‐ and postoperatively in operation room to calculate the intraoperative blood loss. All of them accepted irrigation and vacuum drainage instruments before their wound was closed. All pubic rami fractures obtained operations simultaneously and fixed with locking reconstruction plate, biodegradable, or steel cannulated screw. All screws were coated with semi‐liquid state calcium phosphate bone substitution (Ruibang medical instrument, China) as augmentation before insertion (Figure ). 2.6.1 Postoperative Treatment All patients underwent the same postoperative management, with intravenously administered antibiotics continued for 72 h following surgery. The drainage instrument was removed 48–72 h postoperatively according to the amount of drainage fluid. Low molecular weight heparin was applied regularly as thromboprophylaxis during the immobilization period. Rehabilitation was planned and performed by a physical therapist as soon as the physical and wound condition of each patient permitted. Full weight bearing was permitted at least 12 weeks postoperatively, depending on the physical and bony quality conditions of patients. 2.6.2 Follow‐Up and Clinical Outcome Evaluation The follow‐up schedule was 4 weeks, 12 weeks, and every 3 months until 15 months postoperatively. Physical examinations and imaging exams to judge the degree of fracture healing, whether or not fixation had failed, and improvement in daily activity functionality. The clinical outcome for fractures was evaluated with Majeed pelvic outcome score. Clinical data pertaining to operation time, intraoperative bleeding (including visible and hidden blood volume, calculated with the formula noted by Nadler ), intraoperative fluoroscopy time, infection rate, favorable fracture healing rate (more than 1/2 of the vertical fracture line had bony fusion or covered with continuous callus by CT confirmation in last follow‐up was defined the favorable healing, or else was defined undesirable healing ), implant loosening rate (including the displacement of screws or the occurrence of radiolucent area around implant, which was confirmed by X‐ray or CT ), Majeed pelvic score were collected and compared statistically between two groups respectively. 2.6.3 Statistical Analysis All data were processed using SPSS20.0 statistical software (SPSS Inc. Chicago, IL, USA). Quantitative data were expressed as mean ± standard deviation (SD) and compared using Student's t ‐test. Categorical variables were compared using the Pearson's χ 2 test. The p value was set < 0.05 for significance. 

Postoperative Treatment All patients underwent the same postoperative management, with intravenously administered antibiotics continued for 72 h following surgery. The drainage instrument was removed 48–72 h postoperatively according to the amount of drainage fluid. Low molecular weight heparin was applied regularly as thromboprophylaxis during the immobilization period. Rehabilitation was planned and performed by a physical therapist as soon as the physical and wound condition of each patient permitted. Full weight bearing was permitted at least 12 weeks postoperatively, depending on the physical and bony quality conditions of patients. 

Follow‐Up and Clinical Outcome Evaluation The follow‐up schedule was 4 weeks, 12 weeks, and every 3 months until 15 months postoperatively. Physical examinations and imaging exams to judge the degree of fracture healing, whether or not fixation had failed, and improvement in daily activity functionality. The clinical outcome for fractures was evaluated with Majeed pelvic outcome score. Clinical data pertaining to operation time, intraoperative bleeding (including visible and hidden blood volume, calculated with the formula noted by Nadler ), intraoperative fluoroscopy time, infection rate, favorable fracture healing rate (more than 1/2 of the vertical fracture line had bony fusion or covered with continuous callus by CT confirmation in last follow‐up was defined the favorable healing, or else was defined undesirable healing ), implant loosening rate (including the displacement of screws or the occurrence of radiolucent area around implant, which was confirmed by X‐ray or CT ), Majeed pelvic score were collected and compared statistically between two groups respectively. 

Statistical Analysis All data were processed using SPSS20.0 statistical software (SPSS Inc. Chicago, IL, USA). Quantitative data were expressed as mean ± standard deviation (SD) and compared using Student's t ‐test. Categorical variables were compared using the Pearson's χ 2 test. The p value was set < 0.05 for significance. 

Results 3.1 Comparison of Patients' Information There were no significantly statistical differences in patients demographic data, such as gender, age, BMD, injury mechanism, and fracture type between the two groups (Table ). 3.2 Clinical Data of Patients All operations were undertaken from 3 to 15 days (average 5.7 ± 1.7 days) following primary injuries. All patients were followed up continuously 15 months postoperatively. The average surgical time was 125.3 ± 15.5 (55–190) min in Group RoboTFX, 137.1 ± 17.2 min in Group RoboISS. The average amount of intraoperative bleeding was 320.4 ± 25.2 (50–550) mL in Group RoboTFX, 302.4 ± 21.5 (50–500) mL in Group RoboISS. The average intraoperative fluoroscopy time of the two groups was as follows: 23.3 ± 4.5 (15–35) s in Group RoboTFX, 40.3 ± 3.8 (10–75) s in Group RoboISS. No patients experienced loss of reduction, 5 of 40 screws had implant loosening in Group RoboTFX, meanwhile 13 of 48 screws had implant loosening in Group RoboISS. Four of 20 vertical sacral fractures were healed undesirable including 2 nonunions and the favorable healing rate of 80% in Group RoboTFX, meanwhile 8 of 24 fractures were undesirable including 4 nonunions and the favorable healing rate was 66.7% in Group RoboISS. There were no occurrences of wound infection in both groups, and Majeed scores for the last follow‐up were 76.2 ± 3.4 in Group RoboTFX and 74.2 ± 2.7 in Group RoboISS. 3.3 Comparison of Clinical Data of Two Groups Comparing patients' clinical data of two groups, neither operation time, intraoperative bleeding, favorable fracture healing rate, and Majeed score of the two groups were statistically significantly different ( p > 0.05). However, intraoperative fluoroscopy time and implant loosening rate in the RoboTFX group were both significantly better than those of the RoboISS group ( p < 0.05) (Table ). 

Comparison of Patients' Information There were no significantly statistical differences in patients demographic data, such as gender, age, BMD, injury mechanism, and fracture type between the two groups (Table ). 

Clinical Data of Patients All operations were undertaken from 3 to 15 days (average 5.7 ± 1.7 days) following primary injuries. All patients were followed up continuously 15 months postoperatively. The average surgical time was 125.3 ± 15.5 (55–190) min in Group RoboTFX, 137.1 ± 17.2 min in Group RoboISS. The average amount of intraoperative bleeding was 320.4 ± 25.2 (50–550) mL in Group RoboTFX, 302.4 ± 21.5 (50–500) mL in Group RoboISS. The average intraoperative fluoroscopy time of the two groups was as follows: 23.3 ± 4.5 (15–35) s in Group RoboTFX, 40.3 ± 3.8 (10–75) s in Group RoboISS. No patients experienced loss of reduction, 5 of 40 screws had implant loosening in Group RoboTFX, meanwhile 13 of 48 screws had implant loosening in Group RoboISS. Four of 20 vertical sacral fractures were healed undesirable including 2 nonunions and the favorable healing rate of 80% in Group RoboTFX, meanwhile 8 of 24 fractures were undesirable including 4 nonunions and the favorable healing rate was 66.7% in Group RoboISS. There were no occurrences of wound infection in both groups, and Majeed scores for the last follow‐up were 76.2 ± 3.4 in Group RoboTFX and 74.2 ± 2.7 in Group RoboISS. 

Comparison of Clinical Data of Two Groups Comparing patients' clinical data of two groups, neither operation time, intraoperative bleeding, favorable fracture healing rate, and Majeed score of the two groups were statistically significantly different ( p > 0.05). However, intraoperative fluoroscopy time and implant loosening rate in the RoboTFX group were both significantly better than those of the RoboISS group ( p < 0.05) (Table ). 

Discussion In our study, compared to RoboISS, RoboTFX not only has the same operation time, intraoperative bleeding, favorable fracture healing rate, and Majeed score, but also can lessen intraoperative fluoroscopy time and implant loosening rate, increasing favorable fracture healing rate. 4.1 Advantages of RoboTFX in the Treatment of FFP IV Among the regular surgeries of treating sacral fractures, percutaneous iliosacal screwing is the standard procedure because of its minimally invasive, adequate stability, and high rate of healing . Currently, the combination of ISS and robot assistance has become popular because it has the least operating time, intraoperative bleeding, fluoroscopy frequency and infection rate, most accurate insertion, and highest rate of fracture healing among the different fixations of adults unstable sacral fractures except RoboTFX . However, this method has obvious limitation in the treatment of osteoporotic elderly. Elderly patients with osteoporotic bones have significantly low BMD, which results in a high loosening and failure rate of screw fixation . Griffin et al.  reported 13% failure rate with the use of ISS in treating unstable pelvic fractures. Furthermore, ISS cannot simultaneously ensure the reduction and fixation of sacral fracture. Therefore, the closed reduction with the help of reduction frame is necessary and the extra operative time of reduced adjustment and real‐time fluoroscopy is unavoidable. TFX combines indirect lumbopelvic fixation and direct screw fixation of sacral fractures, which can restore the normal load of the weight‐bearing axis, and form a tri‐dimensional interlocking structure that provides the maximum stability . The biomechanical study of Schildhauer et al.  concluded that triangular osteosynthesis was superior to iliosacral screw fixation. This technique avoids placing shear or rotational force on the injured area of the sacrum, thus not only preventing re‐displacement of the fracture, but also creating a stable base to improve the rate of fracture union and re‐enable ambulation earlier. Furthermore, TFX can make the reduction and fixation of sacral fractures simultaneously, which shorten the operation and fluoroscopy time. Compared with the other methods, TFX can obtain better quality of reduction, better chance of fracture healing, and improved prognosis . However, the traditional TFX also comes with several significant disadvantages. The implantation of fixation needs open procedure, the excessive exposure results in increasing of operation time, intraoperative bleeding, and high rates of wound‐related complications such as infection and wound dehiscence , which were not acceptable in the treatment of elderly . Furthermore, the healing of fracture may be affected because of the iatrogenic excessive soft tissue damage. RoboTFX can overcome those above‐mentioned advantages perfectly. Under the premise of fixing the fractures most stably, the little skin incisions and soft tissue dissection not only can shorten operation time and lessen intraoperative bleeding effectively, which were most important of elderly patients safe but also promote better healing of fractures than open procedure. 4.2 Indications and Surgical Experiences of RoboTFX Although RoboTFX has so many advantages, not all FFP IV are suitable for this procedure. Detailed planning and preoperative evaluation must be undertaken, and the indications and contraindications must be strictly controlled. We concluded the indications were as follows: Upper sacral dysplasia can occur in 30%–50% of all sacrum , which leads to a more narrow and oblique screw pathway in S1 than normal, making screw placement difficult and significantly increasing the risk of iatrogenic vessel and nerve injury. For this reason, the evaluation of sacral morphonology by CT is important, as only patients who have a safe pathway for iliosacral screws are suitable for TFX. If patients present with lumbosacral conjunction injury, only those with the relatively stable (Isler types I, IIA, and IIB ) and unnecessary for open operation of fusion are suitable for RoboTFX. If the patients are associated severe symptoms of nerve injury (Gibbons III, IV ), it indicates the lumbar or sacral nerves should be entrapped by the fragments of the fracture, and the preoperative MRN exam is necessary. If the entrapment is confirmed to be present, sacral laminectomy and neural decompression should be performed with open incision before attempting the vertical distraction by lumbopelvic fixation in order to avoid exacerbating nerve injury caused by more serious entrapment . Therefore, RobotTFX only can be performed for the patients who has no nerve entrapment. 4.3 Skill and Experience Melly et al.  reported that the ISS screw pathway of S1 will decrease by 40% if the residual sacral fracture displacement is 10 mm. This finding indicates that even with minimal displacement of sacral fracture, which is hard to avoid during surgery, the safe path of ISS can be narrowed greatly and the difficulty and danger of screw implantation increase sharply. Although the anatomic reduction of FFP is unnecessary, the position of sacral fracture must ensure the safe insertion of screws. If this is not the case, the reduction must be undertaken instead. Therefore the preoperative evaluation and intraoperative judgment of imaging must be meticulous and accurate. The direction of the iliac screw (direction onto the AIIS or supraacetabular) is not decisive for the fixation strength of the iliac screw, but the size of the implant. Screws with a diameter > 9.5 mm and a length > 80 mm showed a significantly more stable fixation . So we choose the thickest and longest iliac screws which we can obtain. Besides, the length of iliosacral screws should be as long as possible. No matter the vertical and rotational displacement of sacral fractures, the manipulation of reduction should be gentle and gradual because of the poor quality of FFP patients. Fully threaded cannulated screws should be used as ISS in order to avoid extra compression of fracture lines, which may lead to the extrusion of the sacral nerve. We coated the screws with semi‐liquid state calcium phosphate bone substitution before insertion as implant pathway augmentation to increase the stability of fixation . Bone substitution does not generate heat during solidification, and with the gradual degradation and absorption of the bone substitution, bony growth can achieve optimistically in the target area . 

Advantages of RoboTFX in the Treatment of FFP IV Among the regular surgeries of treating sacral fractures, percutaneous iliosacal screwing is the standard procedure because of its minimally invasive, adequate stability, and high rate of healing . Currently, the combination of ISS and robot assistance has become popular because it has the least operating time, intraoperative bleeding, fluoroscopy frequency and infection rate, most accurate insertion, and highest rate of fracture healing among the different fixations of adults unstable sacral fractures except RoboTFX . However, this method has obvious limitation in the treatment of osteoporotic elderly. Elderly patients with osteoporotic bones have significantly low BMD, which results in a high loosening and failure rate of screw fixation . Griffin et al.  reported 13% failure rate with the use of ISS in treating unstable pelvic fractures. Furthermore, ISS cannot simultaneously ensure the reduction and fixation of sacral fracture. Therefore, the closed reduction with the help of reduction frame is necessary and the extra operative time of reduced adjustment and real‐time fluoroscopy is unavoidable. TFX combines indirect lumbopelvic fixation and direct screw fixation of sacral fractures, which can restore the normal load of the weight‐bearing axis, and form a tri‐dimensional interlocking structure that provides the maximum stability . The biomechanical study of Schildhauer et al.  concluded that triangular osteosynthesis was superior to iliosacral screw fixation. This technique avoids placing shear or rotational force on the injured area of the sacrum, thus not only preventing re‐displacement of the fracture, but also creating a stable base to improve the rate of fracture union and re‐enable ambulation earlier. Furthermore, TFX can make the reduction and fixation of sacral fractures simultaneously, which shorten the operation and fluoroscopy time. Compared with the other methods, TFX can obtain better quality of reduction, better chance of fracture healing, and improved prognosis . However, the traditional TFX also comes with several significant disadvantages. The implantation of fixation needs open procedure, the excessive exposure results in increasing of operation time, intraoperative bleeding, and high rates of wound‐related complications such as infection and wound dehiscence , which were not acceptable in the treatment of elderly . Furthermore, the healing of fracture may be affected because of the iatrogenic excessive soft tissue damage. RoboTFX can overcome those above‐mentioned advantages perfectly. Under the premise of fixing the fractures most stably, the little skin incisions and soft tissue dissection not only can shorten operation time and lessen intraoperative bleeding effectively, which were most important of elderly patients safe but also promote better healing of fractures than open procedure. 

Indications and Surgical Experiences of RoboTFX Although RoboTFX has so many advantages, not all FFP IV are suitable for this procedure. Detailed planning and preoperative evaluation must be undertaken, and the indications and contraindications must be strictly controlled. We concluded the indications were as follows: Upper sacral dysplasia can occur in 30%–50% of all sacrum , which leads to a more narrow and oblique screw pathway in S1 than normal, making screw placement difficult and significantly increasing the risk of iatrogenic vessel and nerve injury. For this reason, the evaluation of sacral morphonology by CT is important, as only patients who have a safe pathway for iliosacral screws are suitable for TFX. If patients present with lumbosacral conjunction injury, only those with the relatively stable (Isler types I, IIA, and IIB ) and unnecessary for open operation of fusion are suitable for RoboTFX. If the patients are associated severe symptoms of nerve injury (Gibbons III, IV ), it indicates the lumbar or sacral nerves should be entrapped by the fragments of the fracture, and the preoperative MRN exam is necessary. If the entrapment is confirmed to be present, sacral laminectomy and neural decompression should be performed with open incision before attempting the vertical distraction by lumbopelvic fixation in order to avoid exacerbating nerve injury caused by more serious entrapment . Therefore, RobotTFX only can be performed for the patients who has no nerve entrapment. 

Skill and Experience Melly et al.  reported that the ISS screw pathway of S1 will decrease by 40% if the residual sacral fracture displacement is 10 mm. This finding indicates that even with minimal displacement of sacral fracture, which is hard to avoid during surgery, the safe path of ISS can be narrowed greatly and the difficulty and danger of screw implantation increase sharply. Although the anatomic reduction of FFP is unnecessary, the position of sacral fracture must ensure the safe insertion of screws. If this is not the case, the reduction must be undertaken instead. Therefore the preoperative evaluation and intraoperative judgment of imaging must be meticulous and accurate. The direction of the iliac screw (direction onto the AIIS or supraacetabular) is not decisive for the fixation strength of the iliac screw, but the size of the implant. Screws with a diameter > 9.5 mm and a length > 80 mm showed a significantly more stable fixation . So we choose the thickest and longest iliac screws which we can obtain. Besides, the length of iliosacral screws should be as long as possible. No matter the vertical and rotational displacement of sacral fractures, the manipulation of reduction should be gentle and gradual because of the poor quality of FFP patients. Fully threaded cannulated screws should be used as ISS in order to avoid extra compression of fracture lines, which may lead to the extrusion of the sacral nerve. We coated the screws with semi‐liquid state calcium phosphate bone substitution before insertion as implant pathway augmentation to increase the stability of fixation . Bone substitution does not generate heat during solidification, and with the gradual degradation and absorption of the bone substitution, bony growth can achieve optimistically in the target area . 

Contradictions of RoboTFX Although RoboTFX has many advantages, not all FFP IV patients can accept this treatment. The contradictions include (i) The abnormality of upper sacrum exists and affects the safe insertion of iliosacral screws; (ii) The patient has the symptom of neural injury and the open operation of neural decompression is necessary. 

Limitations and Strengths The sample size of this study is related small, the follow‐up time is not long, and some subtype injuries of FFP IV (IVa and IVc) were not included, which may lead to deviation in the results and affect the validity of the conclusions. The current reports of FFP IV all focus on RoboISS or manual therapy of TFX, and no study of robotically minimally‐invasive manipulation combined with TFX has been published. This study is the first report of the world as we know and has the highly creativity and clinically academic value. 6.1 Prospect of Clinical Application The clinical outcomes of RoboTFX treated on unstable sacral fractures were better than other techniques which has been proved in previous study. Furthermore, its advantages such as less operating time and bleeding, less iatrogenic injury, and infection rate were more suitable for osteoporotic elderly patients. Our study has proved RoboTFX can improve the clinical outcomes of FFP IV patient significantly and we believe it will has the optimistic prospect of clinical application. 

Prospect of Clinical Application The clinical outcomes of RoboTFX treated on unstable sacral fractures were better than other techniques which has been proved in previous study. Furthermore, its advantages such as less operating time and bleeding, less iatrogenic injury, and infection rate were more suitable for osteoporotic elderly patients. Our study has proved RoboTFX can improve the clinical outcomes of FFP IV patient significantly and we believe it will has the optimistic prospect of clinical application. 

Conclusion RoboTFX had the advantages of least operating time, intraoperative bleeding, intraoperative fluoroscopy time, lowest infection rate, most accurate insertion, and highest rate of fracture healing among all treatments of FFP IV. Our study confirms previous efforts and supports RoboTFX as an effective and advanced choice. However, the indications of this operation should be strictly controlled. 

Wei Tian: writing and revising the study. Feng‐Shuang Jia , Jia‐Ming Zheng and Zhao‐Jie Liu: collecting and analyzing the data. Jian Jia: designing the study and supervising the process. 

The research was affirmed in accordance with the Declaration of Helsinki and approved by the ethic committee of Tianjin Hospital (IEC no. 2023‐062). All patients were approved to participate the research and sign the consent declaration. 

The authors declare no conflicts of interest. 

