Mucoid degeneration of the ACL was first described by Caan in 1924 in his anatomopathological studies for ACL tears. It remains a rare pathology, often misinterpreted as a rupture of the ACL on MRI in current practice. It is mostly a pathology of young adults, responsible for posterior knee pain and restriction in the range of motion [2].

Many theories have been put forth to explain the pathological disorganization in the collagen fibers of the Anterior Cruciate Ligament [3]. Some suggest that it may be caused by the accumulation of synovial fluid inside the substance of ACL in a herniated pouch of synovium, known as the synovial theory. The traumatic theory postulates the accumulation of the mucin substance secreted by fibroblasts within the ACL, post-injury; or that multiple micro traumas result in the formation of microcysts which eventually merge into large mucoid formations. The degenerative theory suggests that mucoid degeneration could simply occur due to aging. Finally, the ectopic theory stipulates that ectopic synovial tissue could exist inside ACL substance leading to microcyst formation [3].

The most common and consistent symptom is an insidious onset chronic knee pain, mostly posterior. Symptoms may also include discomfort during mobilization, a meniscus pain point, or limitation in the range of motion, directing the diagnosis to another intra-articular lesion such as a meniscus lesion, cartilage lesion, or a foreign body [4]. The duration of symptoms may vary from weeks to months. Most patients have no history of trauma, and when present, it is usually trivial [2]. Symptoms such as pain and limitation in the range of motion are hypothetically due to an increase in tension and volume of the ACL, mechanical impingement on the PCL and posterior capsule, causing bone erosion, or, as mentioned in some studies, due to disturbance in nociceptive properties [2,3].

The Lachman test, anterior drawer test, and pivot shift test are most of the time negative. Joint line tenderness and limitation of movement during clinical examination might be the only findings. Indeed, all our patients complained of posterior knee pain increasing with knee flexion, and none of them reported knee instability or extension block.

These clinical features are not pathognomonic and do not respond to treatment with NSAIDs and physiotherapy. However, they should raise suspicion and the need for further evaluation with MRI [2].

Conventional x-rays might reveal osteoarthritic changes but are not helpful for the diagnosis, whereas the gold standard imaging is MRI [4].

A few features found on MRIs are suggestive of ACL mucoid degeneration [4,5]:

A thickened, ill-defined, and bulky ACL maintaining its normal orientation and the continuity of its fibers.

An increased intra-ligamentous signal on all sequences as it shows an intermediate signal on T1-weighted images and high signal on T2-weighted images and proton density-weighted images.

The celery stalk sign consisting of subtle linear low signal intensity fibers parallel to the long axis of the otherwise hyperintense ligament (celery stalk sign).

ACL Mucoid degeneration might coexist with an ACL ganglion cyst and is many times associated with meniscal tears, chondral damage, or intraosseous cysts [5].

The most frequent error is mistaking mucoid degeneration for a tear of the ACL, which is the main differential diagnosis. But clinical correlation is crucial and can help rule out a ligamentous tear when patients neither have a history of trauma or instability, nor do they present a secondary stigma of a ruptured ACL such as bone bruises or anterior tibial subluxation [5].

Arthroscopic debridement of the mucoid degeneration of the ACL is the main treatment and is considered safe and effective [6].