Rupture of the anterior cruciate ligament of the knee joint is a common (sports) injury in young adults . Depending on various patient-related factors, such an injury can be treated surgically or conservatively . Arthroscopic and anatomical reconstruction is currently considered the gold standard of surgical treatment. In this procedure, the torn anterior cruciate tendon will be replaced usually with the own semitendinosus ligament, the patellar ligament or the quadriceps tendon by inserting it through bone tunnels in the femur and tibia.

However, the optimal technique for fixing the graft remains unclear. Until the replacement graft has healed, the suspension of the graft remains the weak point of the overall construct . The currently common procedures are fixation by means of interference screw in the bone tunnels and distal fixation using endobuttons. These fixation methods differ only insignificantly in clinical scores, stability, or failure rate .

A frequent complication after reconstruction of the anterior cruciate ligament is secondary tunnel dilatation, which has been described in up to 84% of cases. The cause of this is not yet fully understood. Nakazato et al. cites a longer graft and an increased dorsolateral tibial slope. Taketomi makes the posterior tibial slope particularly responsible for tibial tunnel widening and describes that an osteolysis process occurs due to the penetration of synovial fluid into the space between the tendon and the tunnel . Moon et al. see a negative correlation between the length of the graft insertion and tunnel dilation. The widening of the bone tunnels seem not to induce instability, but complicates a follow-up operation, e.g. a new cruciate ligament plasty in the event of rerupture following new trauma. The often up to twice as large bone tunnels make it difficult to find sufficient bone tissue to create a new tunnel. If the already existing and enlarged tunnel is reused, there is a risk of inadequate fixation and lack of incorporation of the new graft. In the revision, a two-stage procedure is often necessary, especially in the case of pronounced widening of the bone tunnel. The group postulates that with a tunnel width \&amp;amp;gt;14 mm, only a two-stage procedure is possible. In a first operation, the bone tunnel must be filled with autologous or similar, and only in a second session a new reconstruction of the anterior cruciate ligament can be attempted.

the investigators are planning a prospective, randomized, controlled, single-blind and monocentric study with 2 parallel groups to investigate an alternative fixation method in cruciate ligament surgery. This involves a screw made from allogenic bone (Shark Screw ACL®; Surgebright GmbH, A-4040 Lichtenberg, Austria) in the form of an interference screw obtained from donor cortical bone. The insertion of this bone screw is identical to the usual interference screws. This type of osteosynthesis material has already proven in other forms in fracture treatment. The investigators have been using screws made from allogenic bone in a similar form for the treatment of scaphoid fractures for more than 4 years , Calcaneal fractures, corrective osteotomies, and arthrodesis with excellent success in everyday clinical practice. Treatment with an allogenic bone screw is compared with the current standard treatment with resorbable interference screw MectaScrew (composite), Medacta Int. Str. Regina 34, 6874 Castel San Pietro, Switzerland) in a control group. The aim is to include 80 patients who will be included in the study, i.e. 40 per group. Surgical method, treatment plan and follow up treatment do not differ.

The main objective of the planned study is to analyze the dilation of the bone tunnels in the two groups. This will be done by means of magnetic resonance imaging (MRI) and computer tomography (CT) examinations (MRI preoperatively, postoperatively (within 3 days) and at 6, 12 and 24 months after surgery, CT postoperatively within 3 days and at 6 and 24 months after surgery). The bone tunnels are measured and the results of the two groups are compared to determine whether the allograft screw shows less bone tunnel dilation than the current standard treatment. An initial evaluation of the data and publication is planned after 12 months; the final evaluation and publication will take place after 24 months. The secondary objectives are to record the advantages and disadvantages of the allografts and to analyze their safety and effectiveness. In particular, the incorporation of the screws is to be assessed and classified. In order to better assess the outcome of the operation, clinical parameters will be recorded, and the two groups compared with each other. Furthermore, MRI will be analyzed for its informative value with regard to bone tunnel measurement.

The null hypothesis is that the two surgical procedures (bone screw / biocomposite screw) do not differ in outcome. The alternative hypothesis is that the use of the human, allogenic cortical bone screw as a fixation element for anterior cruciate ligament reconstruction results in faster and, above all, better incorporation of the graft, with less widening of the bone tunnel. Based on the experience of the investigators with similar osteosynthesis material made from allogenic bone, it is to be expected that the bone tunnel will heal scare free. The investigators see the advantage of the proposed treatment particularly in its bone-sparing nature. It is to be expected that this will make possible revision easier and that a two-stage procedure will be necessary less often.