Symptomatic ACL mucoid degeneration in middle-age athletes

Gonzalo Samitier Solís; Chaitanya Waghchoure; Gustavo Vinagre

doi:10.1016/j.jor.2022.04.004

. 2022 Apr 11;31:67–71. doi: 10.1016/j.jor.2022.04.004

Symptomatic ACL mucoid degeneration in middle-age athletes

Gonzalo Samitier Solís ^a, Chaitanya Waghchoure ^b, Gustavo Vinagre ^c,^d,^∗

PMCID: PMC9038521 PMID: 35496356

Abstract

Background

Mucoid degeneration of the anterior cruciate ligament (ACL) is an uncommon non-traumatic cause of knee pain and motion restriction, typically seen in a middle-aged population. Primarily, the management consists of partial arthroscopic debridement and notchplasty, which has proven satisfactory clinical and functional outcomes.

Study objectives

This review aims to highlight key clinical, radiological and arthroscopic findings of mucoid ACL degeneration, and also to provide an approach to manage a symptomatic middle-aged athlete.

Rationale

Due to the paucity of literature on ACL mucoid degeneration, symptomatic presentation in a middle-aged athlete can be challenging to manage. Diffuse central pain, motion restriction in extension or flexion, absence of trauma, and an intact enlarged ACL on Magnetic resonance imaging (MRI) should raise suspicion for mucoid degeneration in middle-aged athletes. Specific radiological and arthroscopic findings can help to confirm the diagnosis. In this review article, we have also described a new clinical test to mimic the pain due to anterior impingement in the presence of an enlarged ACL.

Conclusion

In symptomatic middle-aged athletes, knowledge of characteristic findings can help in the timely diagnosis of mucoid degeneration of ACL. Treatment options include arthroscopic debridement, notchplasty, ACL augmentation, and ACL reconstruction. The presence of associated injuries can influence return-to-sports prognosis.

Keywords: Anterior cruciate ligament, Mucoid degeneration, Athletes, Arthroscopy

1. Introduction

Mucoid degeneration of the anterior cruciate ligament (ACL) is a rare cause of knee pain and was first described by Kumar.¹ The incidence of mucoid degeneration is reported to be between 1.0 and 5.3%, with a mean age ranging from 31 to 78 years.2, 3, 4 Typically, ACL mucoid degeneration is characterized by thickening of the ACL due to the deposition of mucinous material, which impinges on the notch or into the lateral compartment, causing deep knee pain and movement restriction. Data from the past two decades shows that mucoid degeneration of the ACL is an under-diagnosed condition. It is a frequently unrecognized source of pain, leading to a delayed diagnosis.5, 6, 7, 8

Although mucoid degeneration is a common condition in pre-arthritic aged knees, treating symptomatic ACL mucoid degeneration in middle-aged athletes is challenging, due to a lack of studies. This review aims to discuss the etiology and pathogenesis of ACL mucoid degeneration, its clinical features, diagnosis, and treatment with a focus on the management of this condition in symptomatic middle-aged athletes.

2. Etiology and pathogenesis

The underlying pathogenic cause of mucoid ACL degeneration is unknown. However, several theories have been postulated: traumatic, synovial, degenerative, and presence of ectopic synovial tissue.²^,⁷^,9, 10, 11 The 'traumatic theory' suggests that trauma or repetitive activities can disrupt ligament fibers with inadequate repair, followed by the formation of mucoid material from an organized interstitial or hemorrhagic fluid.¹² This phenomenon could explain the presence of mucoid deposition on the ACL seen in young individuals/athletes. Traumatic events to the synovium may also contribute to this condition. A protective effect of the synovial sheath around the ACL has been extensively studied.¹³^,¹⁴ Injury to the synovium exposes the ACL to synovial fluid and hemarthrosis, increasing the degradative collagenase activity leading to cell necrosis.¹³ Lancaster et al.¹⁵ believed that this injury to the synovium (microtrauma) form microcysts, which eventually coalesce to form large mucoid deposits. Further, traumatic theories suggest that cellular response to trauma releases mucin substance hyaluronic acid, which dissects the ligament and gets interspersed within the ligament fibers (as the joint is constantly in motion), leading to fusiform dilation of the ligament.¹⁶

The 'synovial theory' postulates that herniation of synovial tissue through a defect in the tendon, or the capsule creates a ganglion cyst.¹⁷^,¹⁸ The 'degenerative theory' considers mucoid degeneration as a continuum of the ageing process due to a high incidence of associated degenerative meniscal/chondral lesions.²^,⁶^,⁹^,¹⁹ The 'ectopic synovial theory' describes the presence of congenitally displaced synovial tissue inside the ACL substance forming microcysts.¹⁰^,¹²

3. Diagnosis

3.1. Clinical features

Patients with mucoid degeneration of the ACL frequently present insidious onset of knee pain exacerbated by activities and a possible range of motion restriction, without a significant history of trauma or instability. In athletes, an ACL tear should always be considered a differential diagnosis - in the presence of a history of trauma.¹¹

Posterior knee pain in flexion and deep central knee pain is the most consistent finding in symptomatic patients with ACL mucoid degeneration.⁹^,¹¹^,¹⁹^,²⁰ Pain is usually aggravated by staircase activities or full knee extension. Anterior knee pain - due to the impingement - can be elicited by a very simple maneuver; this new clinical test is titled 'passive hyperextension test' (Fig. 1): in the supine position, the knee is passively hyperextended, reproducing an intense anterior and deep central knee pain due to ACL impingement. This test is mainly positive in patients with diffuse mucoid hypertrophy of the ACL. Rarely, anterior knee pain is the unique complaint - in patients with mucoid-degenerated ACLs.²¹

Fig. 1 — Passive hyperextension test: in the supine position, the knee is passively hyperextended, reproducing intense anterior and deep central knee pain (due to anterior impingement of the hypertrophied ACL against the notch or the lateral compartment).

Patients with ACL mucoid degeneration often have pain associated with motion restriction, either in last degrees of flexion or/and extension, depending on the involvement of anteromedial (AM) or posterolateral (PL) bundle.⁵^,⁷^,⁹^,¹⁹^,²⁰^,²² Mucoid hypertrophy of the AM bundle can lead to pain in flexion or cause flexion restriction by impinging against the lateral compartment or the PCL. Similarly, involvement of the PL bundle may lead to pain in extension and extension deficit by occupying the whole notch or impinging on the lateral wall. Furthermore, extrusion of the hypertrophied ACL into the posterolateral compartment may reproduce 'meniscus-like' symptoms.¹^,²³ Diffuse ACL hypertrophy may limit both movements: flexion and extension.²⁰ Symptoms can be related but not always limited to mechanical impingement. The lack of inflammatory response around the ACL may suggest the existence of intra-tendinous nociceptors, similarly to chronic tendinopathies.²² Moreover, progressive flexion contracture, swelling, peri-ACL associated cysts, posterior capsule distension, or meniscal or chondral injuries can be related factors.²⁴^,²⁵

There are no complaints of instability in most cases, and Lachman's test reveals a firm endpoint and an intact enlarged ACL. Mild joint effusion might be present. Meniscal tests and joint line tenderness can also be positive.

3.2. Imaging

Standard radiographs in patients with ACL mucoid degeneration are relatively normal, except for some early arthritic changes or narrow intercondylar notch.

Magnetic resonance imaging (MRI) is the gold standard imaging modality in diagnosing ACL mucoid degeneration, showing characteristic features described by McIntyre (Table 1).⁶ Classically, mucoid degenerated ACL appears ill-defined and is thicker than usual, with increased signal intensity on all sequences. Ligament fibers are seen better on T2-weighted sequences (T2W) compared to T1-weighted (T1W) or proton density sequences (PD) (Fig. 2A). The ligament is high-intense and oriented usually, parallel to the intercondylar roof or Blumensaat's line. Although an increased signal intensity of a mucoid degenerated ACL is not as bright as the synovial fluid signal, helping to further differentiate from partial tears to intraligamentous cysts.²⁶ A "celery stalk" appearance of the intact fibers on T2 sagittal MRI is the main characteristic of mucoid degeneration of the ACL, with diffuse high signal thickened ligament and subtle low-signal intensity linear fibers parallel to the long axis of the ACL (Fig. 2B). Associated intraligamentous or intraosseous cysts are not uncommon; however, the common origin between peri-ACL cysts and ACL mucoid degeneration remains unproven.²^,⁴^,²⁴ Bony erosions of lateral femoral condyle or near the tibial footprint of ACL are frequently seen. In the occurrence of these bony erosions, a role of direct or indirect osteoclast induction by mucoid degeneration and by the presence of chronic ganglion cysts have been proposed.¹⁰^,²⁷ In addition, meniscal and chondral injuries may be seen. A strong association between mucoid degeneration and medial tibiofemoral chondral lesions in patients above 50 years of age has also been reported on MRI.²⁸

Table 1.

Diagnostic criteria for mucoid degeneration of ACL.⁶

MRI findings	Arthroscopic findings
• Ill-defined ACL • Increased thickness (mucoid-ACL) compared to normal • High-intensity signal in all sequences; fibers are better seen on T2W (compared to T1W or PD sequences) • Normal orientation of the ACL - parallel to Blumensaat's line • "Celery stalk" appearance (T2W sequence)	• Intact ACL • Normal to thickened appearance • Mucoid material expressed with probing • Lack of smooth synovial lining

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Fig. 2 — Knee MRI of a mucoid degenerated ACL. (A) T2W sagittal view showing an ill-defined, intact and thickened ACL. Normal orientation (parallel to intercondylar notch) and high-signal intensity. An associated ganglion cyst is visualized posteriorly. (B) PD sagittal view: "Celery stalk" appearance - diffuse high signal thickened ligament with subtle low-signal intensity linear fibers parallel to the long axis of the ACL.

On imaging, the most important differential diagnosis is an ACL tear. For a complete ACL tear, apart from the typical loss of continuity of ACL fibers and abnormal high-intensity signals, secondary signs can be bone contusions, anterior tibial translation of 5 mm or more, uncovered posterior horn of lateral meniscus, Segond's fracture and effusion.29, 30, 31 In addition, when there is uncertainty regarding diagnosis, the ACL angle, Blumensaat line and PCL can further help to distinguish between an ACL tear and mucoid degeneration.³² A reduced ACL angle (threshold of 45°), an increase in the Blumensaat angle (threshold of 0–9°) and a reduced PCL angle (threshold of 105–115°) are definite of ACL tears on MRI.²⁹^,³²^,³³ Other more rare differential conditions are intra-articular metastasis of lung adenocarcinoma, which can mimic mucoid degeneration seen on the MRI - highlighting the importance of using biopsy during arthroscopy.³⁴

3.3. Arthroscopic findings

In the arthroscopic evaluation, a mucoid degenerated ACL is bulky, homogenous and fills the entire intercondylar notch (Fig. 3A). As a rule, the ACL is intact - in its length - with normal or increased tension. A yellowish degenerative mucoid material can be visualized when probed within both bundles. Although complete involvement of ACL is frequently seen, segmental involvement of the ACL is not uncommon.³^,⁷^,¹⁹^,²² Between 20 and 30% of patients with mucoid degeneration of ACL are likely to have isolated AM or PL bundle involvement.³^,¹⁹ Hypertrophied ACL may be seen bulging over the notch, lateral wall, PCL or extruding in the posterolateral compartment, contributing to the impingement-like symptoms and limiting the range of movement. A prevalent finding of absence of synovial lining (ligamentum mucosum) is seen in a majority of the patients, emphasizing the protective role of the synovium (Fig. 3A).⁵^,⁶^,¹⁹^,²⁴

In brief, McIntyre's typical findings can be considered diagnostic criteria to define mucoid degeneration of ACL on arthroscopic evaluation (Table 1).⁶ Additionally, associated intra-articular conditions such as chondral injuries, meniscal tears, and peri-ACL ganglion cysts are not uncommon.²^,²⁴^,²⁵^,³⁵ A strong association of degenerative meniscal (69%) and chondral injuries (82%) with ACL mucoid degeneration has been recently shown.⁴

3.4. Histology

A biopsy is crucial to confirm the diagnosis of ACL mucoid degeneration and rule out other conditions. Regarding histopathological findings, mucoid degeneration is characterized by increased collagen tissue with multifocal myxomatous degeneration. This has to be with dense granular glycosaminoglycans (GAG) deposition (e.g. hyaluronic acid) in between the thin and loose collagen fibrils of the ligament.⁷^,²²^,³⁶ Alcian blue special staining can mark mucoid tissue specifically and be used for confirmation.¹^,⁵ Melloni et al.¹⁰ reported an absence of proper synovial lining. Unlike mucoid degeneration, biopsies of partial tears reveal neo-vascularization and inflammatory infiltration.¹¹

4. Treatment

In symptomatic middle-aged athletes, the goal of the treatment is to provide symptomatic pain relief and functional improvement without compromising ACL stability. An athlete may present with either pain or range of motion restriction. It is also crucial to assess knee joint stability and identify associated injuries that might influence prognosis.

Firstly, conservative management should be considered using analgesics and physical therapy. Recently, ultrasound-guided percutaneous aspiration, fenestration and steroid injection has been shown to be a viable, safe and minimally-invasive treatment option for cystic, mucoid or mucoid-cystic lesions.³⁷ No differences were found regarding the nature of the lesion (cystic, mucoid or mucoid-cystic) with a subjective satisfaction rate of 65.2% (median follow-up between 9 and 63 months). In the athletic population, conservative options may not lead to complete resolution of symptoms and early return to sports. Therefore, arthroscopic evaluation and treatment should be considered. Furthermore, arthroscopy allows to diagnose and manage associated injuries.¹⁹^,²⁹

In case that conservative management fails, further surgical treatment with arthroscopic evaluation, debridement and decompression of the hypertrophied ACL with or without notchplasty can be effective. In a recent systematic review of 292 patients, Sweed et al.⁴ found that arthroscopic debridement resulted in more significant pain relief (90–95%), increased range of motion and improved functional postoperative knee scores.

During arthroscopy, important pearls should be considered during ACL resection. Besides the standard anterolateral (AL) and anteromedial (AM) portals, a central low transpatellar portal can be helpful. The transpatellar portal is safe and provides the optimum angle for adequate resection of the mucoid tissue of the ACL and notchplasty to achieve a pain-free range of motion.³⁸ A bulky ACL can be gradually resected using a traditional shaver, starting peripherally and moving towards the center - depending on the involved bundle or tissue impingement (Fig. 3B). Alternatively, radiofrequency ablation is shown to be equally effective for an adequate resection, benefiting from adding radiofrequency coagulation on the ACL synovium bleeding.³⁹^,⁴⁰ In the presence of a narrow stenotic notch or osteophytes, a notchplasty with a high-speed round burr may be additionally performed.⁵^,¹⁹^,²⁵^,⁴¹ However, if meticulous partial resection of hypertrophied ACL is performed, notchplasty is seldom required.⁷^,²⁴^,⁴⁰^,⁴²^,⁴³ Conversely, Lee et al.²⁰ suggested starting with lateral wall notchplasty, which can reduce the need for excessive partial resection of the ACL, thus preventing symptomatic instability. During surgery, passive extension is frequently needed to check for adequate resection and impingement-free motion between the ACL and the notch. As previously emphasized, it is recommended to have a biopsy of the mucinous material and degenerated ligament - not only to confirm the diagnosis but also to rule out metastatic lesions, which may radiologically resemble mucoid degeneration.³⁴

During arthroscopic surgery, all knee compartments should be carefully assessed to evaluate the state of the meniscus and cartilage and treated if necessary. Posterior ACL-associated cysts can be resected using a shaver from the anterior aspect of the knee, going through the interval between the ACL and the PCL. Occasionally, a posterior transeptal portal can clear the central posterior tissue on the femoral attachment and resect larger cysts in the posterior compartment.²⁰^,⁴⁴ Any bulging mucoid tissue in the posterolateral compartment should be resected until the lateral compartment is clear.

Although minimal data has been published on symptomatic ACL mucoid degeneration in athletes, arthroscopic debridement has shown excellent pain relief, functional recovery and early return-to-sports.²²^,³⁵^,⁴⁵ In athletes with segmental lesions, in which one of the bundles is normal, arthroscopic resection and augmentation of the affected bundle is critical.³⁵ Primary ACL reconstruction can be the definitive treatment, especially when a significant amount of the ACL is resected.³⁵^,⁴⁶^,⁴⁷ This tries to avoid the chance of a traumatic rupture of the remaining degenerated ACL fibers and further symptomatic instability.

4.1. Risks

Arthroscopic treatment is not exempt from risks. This includes residual instability (particularly in aggressive ACL resection), incomplete notch resection, residual pain, and non-treated associated injuries in pre-arthritic knees.¹⁹^,²⁰^,²⁴^,⁴⁴ Despite cautious partial resection, Cha et al. found that 6% of patients complain of anterior instability (10 years follow-up) in the absence of trauma, and half show complete resolution of mucoid tissue on the MRI.¹⁹ Residual instability can be seen in patients with partial or total resection of the ACL. However, symptomatic instability is uncommon.⁴ Besides that, anterior instability can be very detrimental in athletes and high-demand individuals. Moreover, a recent study (96 months of follow-up) has shown higher progression rates of cartilage degeneration in patients with traumatic and non-traumatic ACL abnormalities (mucoid degeneration) despite previous knee joint degeneration.⁴⁸ Therefore, signs of wear and cartilage degeneration should be carefully evaluated and discussed in athletes with ACL mucoid degeneration.

5. Rehabilitation

Immediately after an arthroscopic resection, full range of motion, full weight-bearing, and muscle strengthening exercises should be initiated. In ACL augmentation or reconstruction, a complete structured ACL rehabilitation program should be designed. In the absence of associated meniscal-chondral lesions in athletes, a complete resolution of symptoms and early return-to-sports can be expected.

6. Conclusion

In summary, ACL mucoid degeneration should be ruled out in middle-aged athletes with dull aching pain and limited range of motion without trauma or instability. We have described the 'passive hyperextension test’ that can assist in eliciting pain due to anterior impingement of a hypertrophied ACL. In addition, radiological and arthroscopy findings can help to confirm the diagnosis and address associated conditions that might influence the final prognosis. Arthroscopic evaluation and treatment include meticulous partial resection of the ACL and notchpasty. This is an effective surgical technique for early and safe return-to-sports, minimizing the risk of instability. ACL augmentation or reconstruction can be the final and definitive treatment in cases of non-functional/incompetent ACLs after resection.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Contributions

All authors have made substantial contributions to the conception, design, drafting and approval of the manuscript.

Declaration of competing interest

None.

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PERMALINK

Symptomatic ACL mucoid degeneration in middle-age athletes

Gonzalo Samitier Solís

Chaitanya Waghchoure

Gustavo Vinagre