Mucoid degeneration of the anterior cruciate ligament corrected with autograft reconstruction

Abstract

The anterior cruciate ligament (ACL) is the most frequently injured knee ligament, typically in non-contact athletic injuries in young adults. Mucoid degeneration of the ACL (ACL-MD) is an uncommon ACL pathology in which glycosaminoglycans deposition within the collagenous bundles leads to hypertrophy, loss of full knee flexion and knee pain without instability. ACL-MD usually presents in individuals over 40 years with sudden-onset knee pain after minimal trauma or as an incidental MRI finding. ACL-MD is rarely described in young adults. We present a case of ACL-MD in a previously healthy adult in his early 20s who presented with 3-month recalcitrant dull left knee pain and limited range of motion after ‘stepping funny’ with slight twisting. This case highlights the need to critically reflect on the anatomical structures when presented with musculoskeletal pathologies and to consider the unique presentation of musculoskeletal disease in atypical age groups.

Background

The anterior cruciate ligament (ACL) provides stability to the knee joint through restraining anterior tibial translation relative to the femur and controlling tibial rotation. It comprises the majority of knee ligament injuries, affecting over 200 000 people in the USA annually.¹ Forty per cent of ACL injuries are secondary to non-contact athletic injuries and are most commonly seen in young adults. A non-traumatic pathology of the ACL known as mucoid degeneration of the ACL (ACL-MD) has been described in the literature with a prevalence on MRI between 0.42% and 4.30% with the median age ranging in the 40s and 50s.^{2 3} ACL-MD manifests as a result of collagenous glycosaminoglycan (GAG) deposition, though the exact aetiology is unknown. It is thought to arise from age-related myxoid degeneration of the ACL, but may also be due to repeated microtrauma in younger individuals. A cursory review of the literature revealed only three reported cases of ACL-MD in patients younger than 30.^{4 5}

Case presentation

A previously healthy adult in his early 20s with no significant medical history presented to the orthopaedic clinic with 3 months of recalcitrant left knee pain and decreased range of motion (ROM) after an episode of twisting his knee after a minor stumble. He denied any history of major trauma, involvement in any sporting activities and family history of musculoskeletal disorders. The patient was on no medications except for sporadic use of acetaminophen for knee pain. The patient described the knee pain as dull and located primarily in the popliteal fossa and on the lateral aspect of his left knee, though he stated that with increased activity his knee pain would become more generalised and increased in intensity. Examination of his left knee was significant for loss of terminal 4–5 degrees of extension and terminal 10–12 degrees of flexion with soft endpoint elicited on Lachman’s test. Left knee effusion was moderate. Other physical examination manoeuvres including McMurray’s testing of the left knee were negative. Right knee examination was normal. The left knee examination findings indicated an ACL pathology, so further investigation was pursued.

Investigations

X-ray of the left knee was negative for any abnormalities with normal bony anatomy and joint space. Fluid aspiration of the left knee produced 6 mL of hazy straw-coloured fluid. Synovial fluid analysis was, however, unremarkable with no white cells, crystals or organisms seen. MRI of the left knee performed 1 month later revealed thickened ‘celery stalk’ appearance of the ACL consistent with ACL-MD (figure 1).

Figure 1.

T2 sagittal section of the left knee showing thickened anterior cruciate ligament with classic ‘celery stalk’ sign on MRI.

Differential diagnosis

Initial examination of the left knee on presentation to the orthopaedic clinic demonstrated decreased knee flexion and soft endpoint on Lachman’s test, indicating an ACL pathology. Other special knee manoeuvres including McMurray’s testing were negative indicating no associated meniscal tearing, which made the likelihood of the ‘unhappy triad’ of injury to the ACL, medial meniscus and medial collateral ligament associated with sports injuries very unlikely.

ACL injuries are typically classified by severity of into grade I sprains, grade II partial tears and grade III complete tears.⁶ Mild injury of the ACL results in grade I sprains which typically present with mild pain and discomfort when walking but is not associated with joint instability and decreased ROM as was seen in this patient. Grade III complete tears are the most common type of ACL injury, typically due to ligament rupture secondary to more major trauma as seen in sports injuries. The patient did not have significant instability, pain or joint effusion and because his history was not consistent with major trauma, the working diagnosis at the time was a grade II ACL injury or partial tear of the ACL.

Though the patient had no risk factors for crystal deposition arthropathies nor any risk factors for septic arthritis of the knee including gonococcal arthritis, synovial fluid analysis was undertaken to rule out possible inflammatory arthritis secondary to an undiagnosed autoimmune disease.

ACL-MD was not considered because of its rarity in young adults until MRI revealed classic signs of the disease. Carcinoma still needed to be excluded with planned biopsy after therapeutic arthroscopy, as even metastatic cancers can present with radiologic findings like ACL-MD on MRI.⁷

Treatment

The patient’s pain improved slightly with oral acetaminophen and non-steroidal anti-inflammatory drugs including ibuprofen and diclofenac; however, it did not improve his knee ROM. Intra-articular triamcinolone administration significantly improved pain; however, ROM remained limited and pain gradually returned a month later.

After MRI established the diagnosis of ACL-MD, left knee arthroscopy was performed with limited synovectomy and planned surgical debulking of the mucoid ACL. Intraoperative arthroscopy demonstrated a thickened, yellowish, rubbery ACL, which degraded into fragments on debulking (figure 2). Further debulking of the myxoid ACL revealed no significant remaining healthy tissue, so the surgeon elected to proceed with ACL autograft reconstruction harvested from the patellar tendon with the patient’s preoperative consent. Intraoperative biopsy of the ACL fragment demonstrated non-specific mucinous degeneration without malignant changes.

Figure 2.

Intraoperative arthroscopic images of the left knee revealing thickened anterior cruciate ligament with yellowish mucinous deposits before surgical debulking (left) and no significant healthy ACL tissue remaining after surgical debulking (right). ACL, anterior cruciate ligament.

Outcome and follow-up

The patient tolerated ACL autograft reconstruction well and there were no complications. Pathological analysis of shaved myxoid tissue samples confirmed diagnosis of ACL-MD while excluding carcinoma. The patient was discharged home with physical therapy and on 4 months follow-up demonstrated full ROM on exam and reported no knee pain.

Discussion

Only three other cases of MD of the ACL-MD in adults aged under 30 years have been described in the literature.^{4 5} These patients similarly presented with difficulties in terminal knee extension and flexion; however, two of three of the cases demonstrated mild instability of the ACL unlike in this case. All patients were treated with resection and autograft reconstruction of the ACL with hamstring tendons, unlike in our patient who had ACL autograft reconstruction with patellar tendon. Recovery after reconstruction was similar in all patients, with return of full knee motion and absence of pain within 3–6 months.

Though there was no particular risk factor or root cause found for these cases, the aetiology of ACL-MD in younger individuals is most likely related to repetitive microtrauma leading to GAG deposition within the collagenous bundles of the ACL, resulting in decreased ability of the ACL to control tibial rotation and restrict forward tibial translation. As a result, these young patients develop issues primarily in terminal flexion of the ACL with some reported terminal extension, leading to pain and even effusion or instability.

The most recommended treatment for ACL-MD is surgical debulking with or without synovectomy, partially resecting the mucoid collagenous bundles of the ACL while leaving a small portion of the extant ACL for use.⁸ In younger adults, because of increased activity and utilisation of the ACL, these partial resections are prone to partial or complete ruptures necessitating reconstruction.⁷ As such, in these rare cases of MD of the ACL in young adults, reconstruction of the ACL should be recommended, especially with autografts from hamstring tendons or patellar tendons to speed recovery.

Patient’s perspective.

I am glad to let physicians learn of the rare knee problem I had and to be part of learning about the disease with the doctors involved. The knee problem I had limited my ability to walk, run, bend, and perform daily chores. Thankfully, I was in my final year of medical school completing final outpatient clinic electives, so I was not running around the wards. I am so thankful to the doctors for learning more about my case and being frank about initially not even considering it. I am grateful that the surgery completely removed my pain and allowed me to start residency without any problems. I hope physicians can use my case as an example of how diseases we typically think of being age-related degenerative pathologies can show up in younger people.

Learning points.

• Clinicians should carefully consider the gross anatomy when presented with musculoskeletal pathologies.

• A thorough examination of the joint and its range of motion can guide diagnosis and selection of imaging studies.

• Imaging studies are a useful aid in visualisation of underlying structures in conjunction with abnormal examination findings.

• Though many orthopaedic diseases typically exhibit in classic age ranges, clinicians should consider unique presentations of disease in atypical age groups so as not to miss crucial diagnosis.

Ethics statements

Patient consent for publication

Consent obtained directly from patient(s).