Anterior Cruciate Ligament (ACL) Mucoid Degeneration

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Anterior Cruciate Ligament Mucoid Degeneration

Anterior Cruciate Ligament is an important structure of the knee joint. As it is arranged in crossed pattern to the Posterior Cruciate Ligament, it is also known as cruciform ligament. It has two fibers, anterio-medial and postero-lateral, respectively.The cruciate ligament have mechanoreceptors, which are responsible to detect any change in position of the knee, direction, speed and tension. It gets taut at 30 and 90 degrees of knee flexion. The ligament is mainly responsible for restraning anterior translation of knee. Along with that, it also controls the internal rotation. Therefore, Anterior Cruciate Ligament tear is susceptible during any injury to the knee.

What is Anterior Cruciate Ligament Mucoid Degeneration?

This is a rare pathological condition, in which there is increased signal from within the ligament. Mucoid degeneration, is irreversible degeneration of the tissue with the infiltration of mucin within the fibers. Multiple theories have been put forth that explains the pathological disorganisation in the collagen fibers of the Anterior Cruciate Ligament, such as synovial, traumatic, degenerative, ectopic, altered joint mechanics, etc.“Synovial” theory postulates accumulation of synovial fluid inside the substance of ACL in a herniated pouch of synovium.“Traumatic” theory highlights the fact that post injury, ACL fibroblasts secrete glycosaminoglycans that are deposited inside the substance of ACL. Another theory of traumatic origin says that it is the cellular response to trauma that liberates a mucin substance, hyaluronic acid. With joint and tissue motion, the mucin substance dissects the ligament fibers and gets interspersed within the fibers of the ligament, causing its fusiform dilatation. Other theories include “Degenerative” theory which states that MD could occur due to ageing. “Ectopic” theory is an interesting hypothesis wherein ectopic synovial tissue could exist inside ACL substance giving rise to microcyst formation.

Clinical Findings

Painful knee flexion

Stiffness

No extension block

Posterior knee pain

No instability

Causes

Trauma

Ageing

Primary condition- Osteoarthritis

Complications

Impingement

Ganglionic cyst

Tendon tear/ Tendon rupture

Radiological Findings

ULTRASOUND:

Hypoechoic foci are seen,within the fibrillar architecture of tendons or ligaments.

ARTHROSCOPY:

Arthroscopically ACL is intact but fibrillated, yellowish and hypertrophied, with interspersed yellowish mucinous material along the fibers, lack of synovial linning of the ACL and absent ligamentum mucosum.

MRI:

Characteristic:"celery stalk" appearance.

Ligaments and tendons affected by mucoid change might display a thickened and/or ill-defined appearance characterized by nonspecific linear or globular increase in signal intensity.

It is mislead as ACL tear on MRI findings, hence requires probing and histopathology for definitive diagnosis.

Differential Diagnosis

Conditions that resembles the clinical findings are:

• Ligament tear
• Ligament rupture
• Fatty infiltration
• Ligament sprain
• Ganglion cyst