Background: The incidence of anterior cruciate ligament (ACL) injuries in professional and recreational athletes has increased in recent years. The main function of the ACL is to connect the back of the thigh to the front of the tibia, providing stability to the knee joint and limiting rotation during movement. A tear of the anterior cruciate ligament affects the biomechanics of the entire knee joint, increases the risk of secondary injury and osteoarthritis, and limits the patient's quality of life, especially in professional athletes. In most cases where the ACL is significantly damaged, surgery is the only treatment that allows patients to return to their daily activities with good results. Currently, two surgical treatments can be performed in medical practice when an ACL tear is diagnosed. The first treatment, which is now considered the gold standard, is ACL reconstruction, and the second is ACL primary repair. ACL reconstruction involves reconstructing the ligament with a graft of healthy donor tissue, while ACL repair involves fusing the torn remnants of the ligament together to take advantage of the healing potential. Both techniques have certain biomechanical disadvantages. However, the ACL repair technique has more significant biomechanical disadvantages and biological defects in the ligament itself that prevent this healing technique from becoming a new gold standard for the medical treatment of proximal ACL rupture. Despite the biomechanical disadvantages, the primary ACL repair technique has great potential in terms of ligament healing properties. Namely, the union of ligamentous remnants after proximal rupture allows healing and preservation of natural tissue as well as preservation of proprioceptive sensory fibers. Therefore, this topic was selected as a research problem to be addressed in this thesis. The proposed research objective is to develop a biomechanical support that improves the healing of the ACL using a minimally invasive surgical procedure. In order to achieve the proposed goal, three currently available ACL repair techniques must be studied experimentally "in vitro" and numerically to identify the biomechanical shortcomings of the existing solutions and to avoid these shortcomings in the development of a new biomechanical support. The research is based on the following hypothesis: it is possible to develop a biomechanical support for the healing of the anterior cruciate ligament of the knee joint with good mechanical properties, ensuring the stability of the knee and the installation of the support without destructive effects on the surrounding tissues. Materials: Experimental studies were performed on fresh knee joint sheep carcasses. The use of animal by-products was approved by the Veterinary and Food Safety Directorate of the Ministry of Agriculture, Zagreb, Croatia, under class: UP / I-322-01 / 20-01 / 32. The specimens were supplied in one piece; the hind legs with all the supporting tissue. There were 35 specimens in total, divided into five groups of seven specimens each. The specimens were prepared for testing in predefined groups before freezing to prevent deterioration of the material properties of the ACL. All soft tissues were removed from the specimens; the only connective tissue remaining in the joint was the ACL. The fibula was also removed from the specimens, leaving only the femur and tibia as bone tissue. Finally, a femur - intact ACL - tibia (FATC) was obtained. After the speciman preparations, it was necessary to adjust the length of the bones for the experimental tests. Therefore, the femur and tibia were cut 20 cm from the joint capsule and drilled with a Φ 6 drill to fit into the mechanical holders constructed according to the available literature to perform experimental tests on the FATC. After the initial preparation of the FATC, techniques to repair the ACL were incorporated into the complex. Rupture of the ACL on the specimens for the ACL repair groups was performed at the proximal level. Five groups of specimens were formed: - Group 0 - FATC with intact ACL; - Group 1 - FATC with proximally dissected ACL repaired using an end-to-end suture ACL repair technique; - Group 2 - FATC with proximally dissected ACL repaired with a primary ACL repair using an Internal Brace bridging technique [1]; - Group 3 - FATC with proximally dissected ACL repaired with a non-absorbable suture using a bridging technique with a distally placed spring - "dynamic bridge". - Group 4 - FATC with proximally dissected ACL repaired with a new biomechanical support. The first test group 0 served as a control group. In group 1, the ACL was repaired using the end-to-end ACL repair technique with 0.2 mm thick Krakow-style medical sutures at the ACL stumps. The proximally dissected ACL was sutured with three self-locking sutures longitudinally on the side of the ligament to its midpoint, then the suture was moved to the other side of the ligament and three more sutures were placed proximally. In group 2, the repair technique was performed with a braided suture tape installed in the FATC by passing a 2.5-mm-thick polyethylene suture through a drilled tibial tunnel, the dissected ACL, and the drilled femoral tunnel and fastening it to the lateral cortical part of the femur and the medial metaphyseal part of the tibia with four-hole buttons. The four-hole buttons, 12 mm in length and 2 mm in diameter, were made of steel and cut with a laser. In group 3, the ACL was repaired with non-absorbable suture in a bridging fashion with a distally placed spring - "dynamic bridge", where the spring mechanism is uniquely designed according to the literature data. Self-locking is provided by a bearing ball to prevent damage to the suture material. The installation of the dynamic bridge repair solution required the creation of two tunnels in which the polyethylene suture is placed using the "dynamic bridge" technique. The support is attached to the lateral part of the femur with a button and a slightly larger hole is drilled in the tibia to place the spring system, through which the polyethylene brace is passed and attached to the end of the system with a bearing ball and screw. Group 4 is a group in which a new biomechanical support was applied to a proximally dissected ACL and developed as part of the doctoral research. Numerical testing setup: For FATC three-dimensional (3D) reconstruction, it was necessary to collect the sheep knee CT scans. To obtain CT scans, the sheep knee joint cadaver was processed in the radiology laboratory on a Siemens SOMATOM Definition Edge ultra-computed tomography (Siemens Healthcare GmbH), in Sestre milosrdnice Clinical Hospital Center (Clinical Department of Radiology, Zagreb, Croatia). Data from CT were imported into Mimics (Materialise, Leuven, Belgium), and models were further refined in SolidWorks 2018. For mesh generation, C3D10: A 10-node square tetrahedral finite element type was used. The load applied to the finite element model corresponds to the performed experimental tensile loading testing. For the numerical tests, the femur was loaded with a tensile force of 100 N for all groups except group 1 for which a force of 10 N was applied. The entire tibia was fixed (U1=U2=U3=UR1=UR2=UR3=0) for all groups. Therefore, to simplify the calculations, isotropic, homogeneous, and elastic material properties were assigned to all parts, including the bone. Five test models were created: - Model 0 corresponds to experimental group 0; - Model 1 corresponds to experimental group 1; - Model 2 corresponds to experimental group 2 [1]; - Model 3 corresponds to experimental group 3 - Model 4 corresponds to experimental group 4 Methods: Experimentally, the biomechanical deficits were determined by measuring the forces and displacements on the static and servo-hydraulic testing machine on sheep cadavers and by recording the general condition of the ligament and bones after testing. Experimental testing was divided into three types. The first type was the cyclic loading test, which was performed on a servo-hydraulic fatigue testing machine type LFV-50-HH, Walter+Bai (Switzerland). The purpose of the cyclic loading tests was to determine the mechanical properties, such as the stability of the FATC, under native and repaired ACL and the condition of the tissue itself due to exposure to the same cyclic loading regime. Tests were performed under 2000 cycles at a frequency of 1 Hz under an applied force ranging from 5 N - 100 N for group 0, group 2, group 3, and group 4. Since the same force range could not be obtained for group 1, the group was tested with a force in the range of 5 N - 10 N. In a dynamic cyclic loading test, the tibia was tightened while the femur was loaded under limited displacement conditions. The second type of test was an extension and flexion simulation performed on a specially designed biomechanical device attached to the servo-hydraulic testing machine. Tests were performed under 5000 cycles at a frequency of 1 Hz without any load being applied. During experimental testing, the tibia was tightened while the femur could move within the specified limits of 27 ± 4 degrees. Determining the stability of the joint, as well as the effect of ACL repair technique methods on bone fixation after a specified number of cycles, was the goal of dynamic testing on a biomechanical device. The last group was a uniaxial tensile test on a static testing machine Beta 50-5, Messphysik (Austria) with a maximum loading force of 50 kN. In a uniaxial tensile test, the tibia was clamped while the femur was loaded with tensile force at a speed of 200 mm/s until failure. Within this test, force-displacement curves were compared to determine the maximum load to failure (Ft,max) that a given test group could withstand. The mechanical bone holders were used for the tensile load tests. These holders are designed to allow positioning of the axis of the femur and tibia in accordance with the axis of the ACL to avoid the occurrence of varus / valgus and changes in torsional rotation. The specimens were removed from the freezer 24 hours before testing and were moistened with saline (NaCl) throughout the test period. Numerical tests were performed in the Abaqus software package using three- dimensional models of sheep knees obtained by geometry reconstruction from computed tomography images. The new design of biomechanical support was selected based on computer simulations and in vitro experimental testing of the bone-implant complex, while measurement of displacement was performed using the digital image correlation method. The conditions and type of experimental and numerical testing were the same as for testing the existing ACL repair techniques. The biomechanical support was made of thermoplastic polyethylene, circular in shape with three adhering parts on which are the holes for the screws with which the support is fixed to the bone. Results: Comparing all four techniques based on experimental testing results, the highest stress to failure was in group 0, followed by group 2, group 3, and finally ACL repair with suture (group 1). All groups showed satisfactory stability during dynamic testing, except for group 1. In the controlled displacement group, the test was performed within the set displacement limits. In group 2, shear action and notch effect occurred at the site of button suture fixation, but the ligament remained preserved. Based on the collected results of the von Mises stress and displacement values, it can be concluded that the lowest stress and displacement values are those at the native ACL (Model 0). The increase in the value of von Mises stress in the comparison of other models is: 22.4% higher in the end - to - end suture technique (Model 1), 49.736% higher in the Internal Brace technique (Model 2), and 24.542% higher in the internal support technique with a distally placed spring (Model 3). The occurrence of significantly higher stress values 458.3 MPa was observed in Model 2 at the contact of the button, and in Model 3 at the same place, and is 400 MPa. The more significant von Mises stress values are those at the suture of the Model 2 and Model 3 and are 547 MPa and 513.7 MPa, respectively. The maximum displacement value at the ACL in Model 1 is 46.226% higher compared to the ACL value in Model 0, 63.548% higher in Model 2 compared to Model 0 and 71.27% in Model 3 compared to Model 0. The appearance of stresses on the femur at the point of contact with the button is indicative of the occurrence of the notch effect and shear action. These stresses may lead to instability of the knee joint during the rehabilitation process and may be the cause of failure for an ACL repair technique with internal brace augmentation. The occurrence of higher stresses on the button and Internal Brace support may cause the suture to pull out during the application of external forces on the knee during rehabilitation. Based on clinical studies, the ACL usually ruptures at the contact points between the bone and the ligament, or these injuries are usually proximal. Consequently, the results of the numerical analysis showed that the distribution of maximum von Mises stress is at these locations. New biomechanical support maintained the stability of the assembly during the dynamic test. Von Mises stress values on the ACL repaired with new biomechanical support are lower than in any of the techniques tested. The design solution showed good mechanical properties compared to existing solutions. The data obtained using the ARAMIS optical system were not applicable as the paint layer peeled off the ligament during the test. Limitations: The entire study was done on an animal model, although the animal model is more acceptable for preclinical studies. The original implant for the DIS technique could not be obtained, so a replica of this ACL repair solution was made. Numerical models are simplified. The ACL is assigned the property of linear elasticity. Since the properties of existing anterior cruciate ligament repair techniques and the impact on surrounding tissues were examined as part of the rehabilitation process where the forces on the knee are of smaller amounts, such a simplification was acceptable. Although the experimental 1DOF ACL tests do not have great clinical significance, the proposed studies can be used for comparison with relevant clinical findings and can be a basis for further identification of biomechanical deficiencies of the tested techniques. Conclusion: Based on the results of the experimental test performed, it is possible to determine the biomechanical flaws of ACL repair techniques through static and dynamic tests. The results of the dynamic test showed that the end-to -end suture repair technique is not a technique that contributes to the stability of the femur-repaired anterior cruciate ligament-tibia complex, while the repair technique Internal Brace and internal support with a spring system provide satisfactory stability of the FATC. The dynamic study showed the occurrence of the notch effect of the femur and tibia in the Internal Brace and DIS technique. This phenomenon is associated with the risk of implant failure in scientific and clinical studies. Fibrous damage in ACL is visible after a dynamic test in all repair techniques. The results of a numerical study of the existing techniques showed that the von Mises stress values at the ACL increase with the repair techniques compared with the native ACL. The critical stress is at the contact between the femur and the button in the Internal Brace technique. Clinical evidence suggests that this site is the cause of potential implant failure. In addition, the occurrence of greater stress at this site is the cause of Stress Shielding, resulting in the weakening of the bone tissue. The design of the new biomechanical support has a lower tensile load compared to other repair techniques. However, based on dynamic testing, the design was found to have satisfactory stability. In addition, the design of the biomechanical support reduces the von Mises stress on the ligament itself and precludes the occurrence of large values of von Mises stress on the femur. The design of the biomechanical support precludes the drilling of a bone tunnel and the placement of implants in the bone marrow, which is the cause of Stress Shielding. In addition, the notch effect on the femur was not noticed. It was found that it is possible to design biomechanical support that has satisfactory mechanical properties in terms of sufficient stability of the knee joint and stress reduction for the ACL itself and, unlike existing solutions, does not have a destructive effect on other surrounding tissues. Future work: The biomechanical support prototype can be improved in various ways. One way is to look like a fishing net. The arrangement of threads within the net can be made by methods of different types of weaving that differ in the different interconnection of vertical and horizontal threads. Weaving threads can be with or without reinforcement. Different designs of the biomechanical support enable the real potential application of the newly constructed solution, which is the goal of such and similar research, and not just a theoretical presentation of a possible solution. Finally, the application of the newly constructed biomechanical support is not limited to the knee joint, but there is the potential to apply it to other tendons and ligaments of the locomotor system to improve the quality of human life.