Modified pedicle screw-rod versus anterior subcutaneous internal pelvic fixation for unstable pelvic anterior ring fracture: A retrospective study and finite element analysis

Objectives: This study compared the stability and clinical outcomes of modified pedicle screw-rod fixation (MPSRF) and anterior subcutaneous internal pelvic fixation (INFIX) for the treatment of anterior pelvic ring fractures. Methods: In a retrospective review of a consecutive patient series conducted in a level 1 trauma university hospital, 63 patients with Orthopaedic Trauma Association (OTA)/Arbeitsgemeinschaft für Osteosynthesefragen (AO) type B or C pelvic ring fractures were treated by MPRSF (n=30) or INFIX (n=33). The mean follow-up was 20 months. The main outcome measure was the incidence of complications and adverse outcomes, and fixation stability was evaluated by finite element analysis (FEA). Results: The 2 groups did not differ in terms of injury severity score, OTA classification, cause of injury, and time to pelvic surgery (P>0.05). However, the MPSRF group had a higher satisfactory rate according to the Tornetta and Matta grading system than the INFIX group (73.33% vs 63.63%) as well as a higher Majeed score (81.5±10.4 vs 76.3±11.2), which was statistically significant at 6 months’ post-surgery (P<0.001). FEA showed that MPRSF was stiffer and more stable than INFIX and had a lower risk of implant failure. Conclusions: Both MPSRF and INFIX have acceptable biomechanical stability for the treatment of unstable pelvic anterior ring fractures. However, MPRSF has better fixation stability and lower risk of implant failure, which can lead to better clinical outcomes.


Introduction
Anterior pelvic structures are more fragile and prone to fracture than dorsal parts as they bear higher loads [1]. Clinically, high-energy pelvic ring fractures leading to decreased functionality account for approximately 1.5-3.9% of all fractures [2]. The high rates of morbidity and mortality are of concern to orthopedists and an economic burden to patients and society.
Anterior pelvic external fixation is a rapid and temporary corrective measure for unstable pelvic fractures in cases where there is hemodynamic instability [3]. However, it is associated with complications such as pin tract infection (in 2-50%), fixator loosening (in 0-20%), loss of reduction (0-30%), and restriction of daily activities, particularly in obese patients [4,5]. Moreover, open reduction has the potential disadvantage of extensive exposure including muscle stripping, as well as risk of damage to neurovascular structures. Therefore, minimally invasive fixation is increasingly being used as an alternative to external fixators [6,7]. Insertion of supra-acetabular pedicle screws connected via a subcutaneous contoured rod tunnelled just below the belly crease (socalled bikini area) [8], which is known as the subcutaneous anterior pelvic fixation (INFIX) technique, has the advantages of convenience, minimal invasiveness and blood loss, and less discomfort for patients with anterior pelvic ring injury [9,10]. However, there is limited evidence for the efficacy of INFIX and many aspects require improvement such as the persistence of pubic pain, soft tissue irritation, loss of reduction, and especially fixator loosening [5,11].
We previously showed that modified pedicle screw-rod fixation (MPSRF) can lead to more rapid recovery from anterior pelvic ring fractures [12]. However, it is unclear how it compares to INFIX in terms of strength and stability. To address this question, in the present study we compared the two methods by assessing radiologic reduction, functional outcomes, and associated complications. We also examined the postoperative biomechanical characteristics of the implants when patients assumed single-/dual-leg standing and sitting postures by finite element analysis (FEA), a computational method for assessing and predicting the outcome of surgery that eliminates variations in bone quality, fracture pattern, bone anatomy, and fixation location upon application of physiological loads [13] that has been used in various pelvic fracture models [14,15].

Patients and methods
This retrospective study was reviewed and approved by the Ethics Committee of Zhongshan Hospital. All procedures were performed in accordance with the Declaration of Helsinki and strictly adhered to institutional guidelines. A total of 63 patients with anterior pelvic ring injury admitted to Zhongshan Hospital or its Qingpu branch were enrolled from January 2014 to January 2017, with a minimum follow-up of 13 months. Inclusion criteria were unstable anterior pelvic ring fracture with a stable posterior ring (either intact or recovered after fixation) diagnosed by the senior trauma surgeon, hemodynamic stability, and full consciousness. Patients with an open contaminated wound, who were lost to follow-up before 3 months, had a pathologic fracture, or were < 16 years of age were excluded (Fig. 1).
The pelvis of each patient was examined by preoperative radiography (including anteriorposterior [AP], inlet, and outlet views) and computed tomography (CT) in order to fully evaluate fracture-dislocation. Imaging data were analysed by 2 senior orthopedists according to the Arbeitsgemeinschaft für Osteosynthesefragen (AO) and Orthopaedic Trauma Association (OTA) modified tile type classification. Post-surgery clinical outcome was assessed based on Tornetta and Matta grade, Majeed score, and complications.

Surgical technique
If required, surgical reduction of the posterior pelvic ring injury was performed prior to fixation as previously described [12]. For MPSRF, a 3-to 4-cm oblique incision was made over each anterior inferior iliac spine (AIIS). A bony tunnel from the AIIS to the posterior superior iliac spine was created with a pedicle finder. A polyaxial pedicle screw with a diameter of 7 mm and length of 60 mm was inserted into the tunnel to a depth of about 2 cm from the bone surface to avoid compression of vascular tissue. A subcutaneous tunnel was created from the incisions at bilateral AIIS to the Pfannenstiel incision over the deep fascia. The curved titanium rod was inserted to connect the 3 bilateral pedicle screws via the subcutaneous tunnel. After confirming that there was sufficient space between the rod and bone by fluoroscopy, the screws were tightened with a torque screwdriver, with those at bilateral AIIS tightened before the one at the pubic tubercle. A representative case is shown in Supplementary Fig. 1 (S1).
The INFIX was inserted in the same manner. Briefly, 2 polyaxial pedicle screws were placed at bilateral AIIS. A subcutaneous tunnel was created from 1 side of the AIIS to the other, and the precontoured titanium rod was connected to the 2 screws via the tunnel.
Fracture reduction was performed by the same method as described above. A representative case is shown in Supplementary Fig. 2 (S2).

Postoperative management and follow-up
Functional exercises of the lower limbs and joints were initiated in bed as early as possible after the operation to prevent deep vein thrombosis; regular wound and dressing care was performed in the outpatient clinic until sutures were removed at 2 weeks postsurgery. Crutch-assisted partial weight bearing was permitted at 6 and 10 weeks for AO/OTA type B and C injuries, respectively, as long as the pain was tolerable. Full weight bearing was allowed if osseous union was confirmed by radiography. Physical therapy was prescribed for muscle strengthening and gait training. Hardware removal was performed between 12 and 14 months after the surgery.

FEA
In order to define the solid geometry of the pelvis, we constructed a model of the pelvis of a healthy 32-year-old male (height, 175 cm and weight, 73 kg) based on images from a CT scan. The material properties of the model were obtained from previous studies (Table 3) [1]. A 2-cm gap was created at the right superior and inferior rami to simulate injury using Geomagic Studio software (3D Systems Inc, Rock Hill, SC, USA). 3-Dimensional models of the rod and screws were constructed using Creo v3.0 software (Parametric Technology Corp, Needham, MA, USA). The materials for the different models and implants were assumed to be elastic, isotropic, and linear. A value of 0.33 was set as Poisson's ratio (y) for both cortical and cancellous bone. Results A consecutive series of 63 patients with type B (type B1, n = 11; type B2, n = 32; type B3, n = 9) and type C (type C1, n = 6; type C2, n = 5; and 0 type C3) fractures were enrolled.
Average age and sex ratio did not differ between the 2 groups (P > 0.05). The mean injury severity score of patients who underwent INFIX was 25 points (range, 19-29 points); that of patients who underwent MPSRF was 22 points (range, 19-29 points). The 2 groups were similar with respect to OTA classification and causes of injury as well as time to pelvic surgery (P > 0.05). However, the INFIX group had longer operation time and greater blood loss than the MPSRF group. There was also a statistically significant difference between the 2 groups in terms of hospital stay (Table 1). Table 1 The demographics of two groups (ISS, injury severity score).  The MPSRF group had a higher Majeed score than the INFIX group at 6 months post-surgery (P < 0.001) and at the time of implant removal (P = 0.012). However, there was no difference in the scores of the two groups at 22 months (Table 2).   [20,21]. Additionally, few studies have investigated whether these 2 fixation techniques lead to sufficiently strong implants and adequate biomechanical reduction.
The FEA results showed that displacement distribution and VM stress were similar in the LFCN irritation is the most common postoperative iatrogenic complication [22]. In a multicentre review, 30% (21/91) of patients had LFCN irritation although in most cases it was self-limiting and improved once the implant was removed [5]. In line with this study, in our investigation the rate of LFCN injury was 33.3% (10/30) in the MPRSF group and 24.2% (4/33) in the INFIX group. A case series of LFCN irritation suggested that screws that are too deeply or insufficiently embedded in the bone and inadequate pre-bending of the rod can lead to irritation of the LFCN and sartorius muscle. To prevent this, a rod-to-bone distance of 20-25 mm (30-40 mm for obese patients) but < 40 mm is recommended [23]. We first locked the screws at bilateral AIIS so that the pullout strength of the screw was mainly concentrated in the supra-acetabular region, where bone density is high. The screw at the pubic tubercle-which has relatively sparse bone-was then locked, thus providing auxiliary support. Furthermore, additional screws should not be placed too close to the lateral pubis to avoid damaging the spermatic cord or round ligament. For unilateral pubic rami fractures, the screw was fixed into the fracture side if the fracture line was far away from the pubic symphysis; otherwise, it was inserted on the uninjured side. For bilateral pubic rami fractures, the screw was fixed into the side with less injury.
Recent studies using the INFIX or MPRSF technique have reported potentially devastating complications, especially femoral nerve palsy. There was one such case in the MPRSF group, but symptoms gradually disappeared once emergency screw adjustment was performed, and there was no permanent nerve damage after implant removal. In a case series of iatrogenic femoral nerve palsy, it was suggested that femoral nerve compression occurs as a result of impingement of the implant on the psoas sheath; meanwhile, delayed palsy may be caused by engorgement of the psoas with blood and/or a change in pressure [24]. The authors noted that this could be avoided by placing the interconnecting rod in such a way that it does not limit the space for the psoas and femoral nerve. Our solution for reducing neurovascular compression was to bend the connecting rod outward in the horizontal direction (Fig. 4). Although follow-up is ongoing, the patient's neurovascular compression symptoms have been significantly alleviated.
Despite the positive clinical outcomes and FEA results, our study had several limitations.
Firstly, this was a single-centre retrospective study with a relatively small sample size; more cases should be examined in a multicentre investigation, with long-term functional assessment. Secondly, the characteristics of the implant materials are not consistently reported; as such, our results depend on input parameters. Additionally, the FEA model was patient-specific and ignored the effects of ligaments and muscles on the stability of the pelvis and implant devices. Finally, the angle of applied force was constant although variations in the angle may have influenced VM stress and displacement, as reported in a previous study [25].

Conclusion
In conclusion, our study demonstrates that both MPRSF and INFIX used as a minimally

Declarations
All procedures were performed in accordance with the Declaration of Helsinki and strictly adhered to institutional guidelines.

Availability of data and materials
The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.     The diagram of the improvement applied to the MPSRF. The diagram of the improvement applied to the MPSRF, bending the connecting rod outward in the