# The Influence of Bone Bruises on Bone Tunnel Enlargement Regarding ACL Rupture
## 1 Introduction  
Anterior cruciate ligament (ACL) ruptures resulting from an indirect mechanism of injury are associated with sport participation and general military and physical activity. The common surgical procedure, ACL reconstruction (ACLR), has demonstrated good clinical outcomes resulting in the restoration knee stability but is associated with complications, e.g., bone tunnel enlargement (BTE). The cause of BTE remains unclear, and two main types of factors are suspected of being involved--mechanical and biological factors [1, 2, 3, 4, 5]. Mechanical factors such as tunnel placement, the number of tunnels, the fixation method, the drilling technique used following bone necrosis, and the aggressiveness of the rehabilitation process may be involved [2, 6, 7, 8, 9, 10, 11, 12]. Biological factors are related to the graft healing process in the emerged bone tunnel, which is always influenced by the presence of synovial fluid and inflammatory state mediators [1, 11]. The clinical importance of BTE is not yet well understood. Despite research having been conducted on BTE, there is a lackof studies on the correlation between BTE and surgical success [13; 14; 15; 16; 17; 18; 19]. It is possible that BTE is an early sign of graft failure resulting in a future renewed lesion [9], as evaluated by radiographs and arthrometry. BTE after six months may suggest poor graft healing, which can lead to screw loosening and joint instability [18]. Nevertheless, BTE occurrence should always be considered seriously in regard to revision surgeries, as the probability of surgery failure increases in the presence of massive BTE [20; 21]. Currently, magnetic resonance imaging (MRI) is crucial in detecting ACL rupture and determining the mechanism of injury, despite its static nature. In addition to complete or partial ACL rupture, concomitant injuries such as tears of the meniscus or other ligaments or muscle and bone damage have been described [22]. ACL tears are linked with large external loads in combination with ligament injury susceptibility in the presence of unfavorable weight distribution, leading to rapid impact to the tibia and femur, and the generated forces transfer through the cartilage to the bone itself. This process leads to visible cancellous bone edema in diagnostic imaging [23] (Figure 1). The main cause of bone bruises (BBs) is direct contact of the articular surfaces influenced by tangent tightening fusion generated during a ligament tear [24]. Bone confusion leads to a large spectrum of types of bone degeneration, including bleeding, fractures, and trabecular bone microcompression fractures [25; 26; 27]. MRI scans performed after acute ACL tears in patients allow us to detect tibial and femoral BBs, which occur in more than 80% of ACL rupture cases [23; 28; 29; 30; 31; 32]. Subchondral injuries, as well as bone marrow edema, are widely recognizable in MR images and are often considered BBs. Nevertheless, their size and the potential effects of their occurrence, such as graft healing in bone tunnels after ACLR, have not been studied. Multiple studies in the scientific literature describing the influence of subchondral and cancellous bone injuries on the knee joint (its structure and function) are limited to degenerative aspects (osteoarthritis) [33; 34]. Until now, researchers have not studied extensively the possible effect of BBs on the surgical success of ACLR; hence, there is a lack of imaging studies that can be used to explicitly define whether and if so, in what range the bone tunnels drilled during ACLR pass through the areas of the most common cancellous bone injuries or close to them. Recent findings indicate that the presence of bone marrow edema correlates  
Figure 1: Magnetic resonance image of an extensive bone bruise in femur associated with anterior cruciate ligament injury. with the prognosis at mid- and long-term follow-up [35].