For intra-tendinous ganglia of the ACL, MRI identified the lesion site, although it was not entirely diagnostic. Arthroscopy and biopsy was necessary to rule out an early neoplastic process. Debridement of the abnormal mucoid tissue relieved symptoms effectively, which has been described previously in the literature [16, 3, 7, 17, 13, 18, 19]. However, we have not found as thorough documentation of clinical findings, MRI, histology, arthroscopy findings and outcomes as in the present case. Gradually decreased range of movement and stiffness of the knee joint in a young athlete without preceding trauma should therefore lead to this suspicion and an MRI and arthroscopy should be undertaken [3, 11–13, 20].
In these cases there is usually no preceding major trauma [8, 7, 10, 12] or instability of the joint [5, 16, 17, 12, 13]. Common MRI findings are high signal on T2-weighted MRI images thickening the ACL with a 'celery-stalk' appearance [16, 11, 6, 17, 12, 21], erosion of cortical bone [22, 11, 10] and intraosseous cyst formation [5, 10]. Arthroscopically ligament fibres are interspersed with a yellow-brown substance and the ACL displaces anteriorly and posteriorly [3, 11, 12, 21]. All of these features were seen in this case. Mucoid degeneration and ganglia of the anterior cruciate ligament are uncommon [7, 9, 13, 3]. Further more so is their coexistence. Bergin et al. reported the prevalence of this to be 0.62% on MRI .
The aetiology of ganglion cysts and mucoid degeneration is unclear [9, 11, 10]. One theory is that mucoid degeneration leads to ganglia formation . This relationship is commonly theorised in the literature but its existence is unproven. Bergin et al. reported that ACL ganglia and mucoid degeneration commonly coexist and gave some evidence to suggest these two entities may share a similar pathogenesis . Another theory suggests that herniation of synovial tissue through a defect in the tendon sheath causes ganglia formation . A third describes displacement of synovial tissue during embryogenesis . The relationship to trauma is unknown. One theory involves the cellular response to trauma that liberates a mucin substance, hyaluronic acid. This is interspersed with the fibres of the ligament, causing its fusiform dilatation. With joint and tissue motion, the mucin substance dissects the ligament fibers and may be found at the ligament attachments or in the intercondylar notch of the knee . Many cases in the literature describe ganglia formation in the absence of trauma. However, excessive training or repetitive minor trauma such as rugby tackles could well be a triggering factor [8, 7, 10, 12]. Although repetitive trauma from rugby may be a contributing factor, the aetiology of the current case is not known and there are no known hereditary factors in the history.
There are no reported cases of ACL rupture following pathogenesis of this type. The literature shows that arthroscopic debridement of the abnormal tissue effectively relieves symptoms [16, 3, 6, 17, 13, 18, 19]. However, this inevitably results in a thinned ACL, which could compromise joint stability. Cases in the literature report no instability in day-to-day activities following debridement [16, 11, 17, 12]. However, none of these patients played sport. Reporting on five cases, Narvekar et al concluded that because none of the patients participated in any type of sporting activity, the thinned ACL mass probably sufficed to provide the requisite stability for day-to-day activities . Nishimori et al concluded that if their patients had participated in any type of sport, they might have had to consider augmentation or reconstruction of the ACL after resection of the lesion .
Only one previous case of an athlete is reported; Fealy et al describe a successful return to sport following arthroscopic debridement of the ACL of a volleyball player .
This rare diagnosis and treatment option should be considered when a young athlete presents with reduced ROM of the knee without preceding trauma.
Arthroscopic debridement of the abnormal tissue effectively relieves symptoms.
Augmentation or reconstruction of the ACL may end up being the definitive treatment if the patient returns to a sport demanding high levels of stability.
This report may also offer some support to the argument that mucoid degeneration and ganglion in the ACL and intraosseous cyst formation share a similar pathogenesis.