## 2 Materials and Methods  
### Experimental Study  
Experimental investigations were performed using specimens sourced from eight domestic swine, aged between 6 and 10 months and weighing between 90 and 110 kg. Isolated specimens of the thoracolumbar spine (Th7-L5) were meticulously prepared by removing the surrounding soft tissues while retaining the intact intervertebral discs (IVDs), ligaments, articular joints, and, to some extent, ribs and muscles. To evaluate the influence of transverse connectors in posterior spinal fixation system designs on selected mechanical parameters, five distinct experimental groups underwent testing (Figure 1). First, the physiological specimen (P) was subjected to testing. Subsequently, a compression fracture of the Th12 vertebra (F) was simulated in the physiological specimens by making a precise incision in the vertebral body. This incision was designed to mimic the formation and morphology of such a fracture, representing a type A fracture, which is statistically the most common in the thoracolumbar junction [14; 15; 16]. Following the creation of the compression fracture, transpedicular fixation was performed in each of the affected thoracolumbar segments using three different configurations: four-segment fixation without transverse connectors (long-segment posterior fixation system, LSF4--2 levels above and 2 levels below the compression fracture), four-segment fixation with one transverse connector (LSF4 TC1), and four-segment fixation with two transverse connectors (LSF4 TC2). The experiment utilized the SOCORE transpendicular fixation system manufactured by NovaSpine (Domaslaw, Poland). For the vertebral bodies of the thoracic spine, polyaxial screws with a diameter of 5 mm and a length of 35 mm were implanted, while for the vertebral bodies of the lumbar spine, screws with a diameter of 6 mm and a length of 40 mm were used. These pedicle screws were then connected with 5.5 mm diameter rods. Transverse connectors, measuring 55 mm in length, were employed. To confirm the correct implantation of pedicle screws in the vertebral bodies (Th10, Th11, L1, and L2), each specimen of the thoracolumbar spine underwent diagnostic X-ray imaging in both sagittal and coronal planes. The experimental tests were conducted in a manner similar to the test procedure presented in a previous study that analyzed the impact of the length of fixated segments on the damaged thoracolumbar spine [1]. Loading was performed using an MTS 858 Mini Bionix testing machine (MTS Systems, Eden Prairie, MN, USA), and the specimens were subjected to forces simulating typical activities of daily life. The prepared specimens were secured in a specially designed test rig equipped with upper and lower grips, where the vertebral bodies Th7 and L5 were clamped using eight cylindrically arranged screws. An analysis of all considered configurations was carried out for loading conditions simulating flexion. Previous experimental research [1] demonstrated that compression did not significantly affect spinal stability. It was noted that spinal stability is influenced by both flexion and extension. However, during extension, the primary load is transmitted through articular processes, resulting in a lesser impact of vertebral fractures on load transmission in the examined spinal segment. Flexion represents the most complex loading scenario among the types analyzed, as it involves the greatest range of motion in the sagittal plane. Therefore, the experiment was conducted with a load-simulating flexion in the angular range of 0\({}^{\circ}\) to 4\({}^{\circ}\)[17]. Each study group was subjected to 20 load cycles at a frequency of 1 Hz, corresponding to the frequency of human gait. It was also assumed that the first 4 cycles served as conditioning cycles due to the hyperelastic properties of soft tissues such as intervertebral discs (IVDs) and ligaments, requiring preloading to stabilize the test system.