Knee osteoarthritis (OA) is characterized by wear and tear of the joint cartilage, resulting in pain and deformity. Surgical treatment options include osteotomy, unicompartmental knee arthroplasty (UKA), and total knee arthroplasty (TKA). The choice of procedure depends on factors such as age and degree of deformity. Artificial joint replacement is often indicated in older individuals, and UKA is suitable for cases with mild deformities. UKA preserves the anterior cruciate ligament (ACL). In contrast, TKA is commonly performed in cases of advanced deformity; it typically involves ACL excision. Although TKA typically yields excellent pain relief and long-term outcomes, approximately 19% of patients are dissatisfied [1]. Removing an intact ACL during TKA causes decreased satisfaction due to ACL functional impairment [2]. The ACL is crucial for anterior-posterior stability, rotational stability, and proprioception of the knee joint [3]. Recently, TKA has been performed in younger and more active patients, leading to the development of ACL-preserving TKA, aimed at achieving a more functional knee and improved satisfaction [4].

Although preserving the ACL is desirable, understanding the pathology of ACL mucoid degeneration is crucial [5]. ACL mucoid degeneration can cause pain and limit the range of motion, potentially yielding less favorable outcomes despite a preserved ACL [6]. The characteristic findings of swelling and high signal intensity on magnetic resonance imaging (MRI) resembles a celery stalk, hence referred to as the “Celery stalk sign [7]"; however, a clear definition and reports of quantitative evaluation of mucoid degeneration using MRI are lacking. The prevalence of MRI findings suggesting ACL mucoid degeneration ranges from 1.3% to 13.8%. However, owing to the often-asymptomatic nature of the condition, it may go unnoticed in clinical practice [[8], [9], [10]]. The remaining rate of the ACL during TKA is reported as 43–80% [11], but even cases with continuity may exhibit histological evidence of advanced degeneration [12]. Only a few case studies have revealed a relationship between MRI and histological findings [13]. Cases have been reported in which patients with mild deformity on radiography, despite showing pain and limited range of motion, were diagnosed with ACL mucoid degeneration based on swelling and high ACL signal intensity on MRI [13]. These patients underwent TKA with ACL excision, yielding favorable outcomes. Surgery to preserve the ACL in such cases may result in poor postoperative outcomes. Therefore, if ACL degeneration can be predicted before surgery for knee OA, better patient satisfaction may be obtained by preserving the non-degenerated ACL.

Despite recent developments, a pathological relationship between knee OA progression and mucoid degeneration has been suggested. Differentiated chondrocytes may be present in the degenerated ACL, generating an extracellular matrix that is not found in a normal ACL. However, the mechanism underlying this phenomenon remains unclear [14,15].

This study aimed to investigate whether histological mucoid degeneration of the ACL can be predicted using preoperative MRI images and to examine the histological changes associated with the progression of ACL mucoid degeneration. We hypothesized that MRI features of swelling and high signal intensity correlate with histopathological changes.