Intraligamentous Synovial Chondromatosis of the Anterior Cruciate Ligament

Abstract

Synovial chondromatosis is a rare disease that causes disability and dysfunction of the involved synovial joint. We describe the second case in the literature of intraligamentous synovial chondromatosis involving the anterior cruciate ligament, confirmed by pathology after arthroscopic removal of the chondral bodies. We also describe associated magnetic resonance imaging findings which may be helpful for diagnosis of this very rare entity.

Introduction

Primary synovial chondromatosis (SC) is a rare disease, characterized by proliferation of multiple hyaline cartilaginous nodules, typically in the subsynovial tissue, that leads to dysfunction of the involved synovial joint [1]. Genetic abnormalities have been identified, including FN1 and ACVR2A, and SC is now classified as a neoplasia rather than metaplasia [2]. SC can occur in any synovial-lined structure, most commonly in the knee, and is usually intracapsular and monoarticular [3]. This disease can cause significant pain and disability to the patient, and the diagnosis prior to surgical intervention may be aided by imaging. There have been several case reports of synovial chondromatosis surrounding the cruciate ligaments [4-6], but to date has been only one case report of SC occurring inside the anterior cruciate ligament (ACL) [7]. Mio et al. discovered intraligamentous SC upon arthroscopy and pathological diagnosis, and no specific pre-surgical MRI findings were noted [7]. We present the second known case of intraligamentous SC in the ACL, but with initial magnetic resonance imaging (MRI) findings of irregular, similar-sized lobulations within the course of the ACL, which may be potentially helpful in suggesting the diagnosis in future cases.

Case Report

A 65-year-old male presented for re-evaluation of persistent left knee pain, exacerbated when standing from a seated position. He had continued stiffness and pain despite compliance with physical therapy and anti-inflammatory pills. He was also given an intra-articular knee joint steroid injection which helped the patient with his range of motion (ROM) and pain for 6 weeks, but his symptoms returned. On physical exam, he had a normal gait, full ROM on the right knee, and 2-90° of flexion on the left knee. He had moderate effusion, with some crepitations when the left knee was fully flexed. The anterior drawer, Lachman, and the pivot shift tests were negative bilaterally. There was no opening with varus or valgus stresses bilaterally. McMurray tests were negative both medially and laterally. Radiographs were unremarkable. MRI was obtained, which demonstrated marked enlargement of the ACL with multiple irregular lobulations throughout the whole ligament (Fig. 1). Lobulations were similar in size and hyperintense on fluid-sensitive sequences without hypointense regions to suggest calcifications. Also present was a small radial tear of the posterior horn of the medial meniscus and moderate-grade chondral abnormalities in all three compartments.

Figure 1.

Sagittal oblique (A and B), coronal (C) and axial (D) fluid-sensitive MR images of the left knee show multiple irregular lobulations of similar size within the enlarged ACL (arrows).

Due to the imaging findings and failure of conservative therapy, arthroscopy was performed, which revealed an edematous and erythematous ACL, which was firm on probing (Fig. 2A). Upon incision of the ligament (Fig. 2B), multiple blocks of whitish-blue, hard cartilaginous tissue were removed, with the remaining portions of the ACL left intact (Fig. 2C). Overall, the multiple fragments of excised tissue measured approximately 2 × 1 cm (Fig. 2D). The small radial tear of the posterior horn of the medial meniscus was confirmed and a partial medial meniscectomy was performed. On pathologic examination, characteristic lobules of hypercellular hyaline cartilage surrounded by synovial lining were seen, consistent with synovial chondromatosis (Fig. 3).

Figure 2.

Arthroscopic images through an anterior portal. An enlarged, erythematous ACL was seen (A), which was incised to reveal multiple whitish-blue masses (*, B and C), ranging from 0.6-1 cm in diameter (D).

Figure 3.

Synovium with multiple nodules of hyaline cartilage, demonstrating clusters of chondrocytes consistent with synovial chondromatosis. Hematoxylin and eosin stain, 200X.

After arthroscopic surgery and physical therapy, the patient initially did well and regained pain-free, full ROM for approximately 6 months. Unfortunately, the symptoms recurred after that time and progressively worsened. A repeat MRI and PET/CT approximately 1.5 years after the surgery demonstrated extensive recurrence of disease (Fig. 4). Recurrent SC had extended beyond the ACL and filled the entire joint, including the suprapatellar, subpopliteal, and anterior and posterior recesses of the joint. Several masses demonstrated characteristic chondroid calcifications (Fig. 4). A complex, two-stage, open anterior and posterior synovectomy was required for adequate tumor debulking. Histopathological examination of tissue obtained from both surgeries again demonstrated SC, without evidence of malignant transformation. On short-term follow-up the patient reported that he was doing very well with pain-free, full ROM.

Figure 4.

MRI approximately 1.5 years after Figure 1. Sagittal oblique (A and B) and axial (C) fluid-sensitive MR images of the left knee show extensive recurrence of disease, now characterized by multiple masses distending all recesses (large arrows), including some with central regions of hypotensity corresponding to calcifications (thin arrows). Axial fused PET/CT image (D) confirms the calcifications (arrow) and shows diffuse increased F18-FDG uptake within the synovium measuring up to 3.1 SUV.

Discussion

To our knowledge, there has only been one other case report of intraligamentous synovial chondromatosis of the ACL. Mio et al. described a 47-year-old male who presented with similar symptoms of pain and limited ROM, with MRI that demonstrated a swollen ACL containing an intraligamentous mass with high signal intensity on fluid-sensitive images [7]. The clinical manifestations included progressive pain, limited ROM, limping gait, and difficulty climbing stairs. The patient had a mild effusion and extension of the knee was limited to −15° with terminal pain [7]. Our patient had a similar history of progressive pain and limited ROM. In the case presented by Mio et al., no distinguishing MRI features were described and arthroscopy was required to make the diagnosis [7].

In general, radiographs may aid in the diagnosis of SC when calcified chondral bodies are identified. This has been reported to occur in 70-95% of cases and the appearance of innumerable bodies that are similar in size and shape has been reported to be pathognomonic [8]. Frequently encountered MRI findings of SC in other joints in the body include similar-sized nodules that demonstrate chondroid signal characteristics of intermediate to low signal on T1-weighted images and high signal on fluid-sensitive images, with or without focal areas of low signal intensity on all pulse sequences (corresponding to calcifications) [8]. In our case, the initial MRI showed multiple irregular lobulations of similar size within the entire course of the enlarged ACL without calcifications. On our initial MRI, differential considerations besides synovial chondromatosis included mucoid degeneration and multilobulated ganglion cysts.

Intraligamentous mucoid degeneration was first described in the scientific literature by Kumar et al. in 1999 [9]. The disease is of disputed theories of origin, which include presence of synovial tissue (either via herniation or ectopia) or mucoid degeneration (either with or without a history of trauma) [10]. It is characterized by infiltration of the ligament by glycoasaminoglycans [10]. When symptomatic, the clinical manifestations may be similar to intraligamentous SC, with central knee pain on terminal extension and associated limited ROM [11]. There is also a strong association of mucoid degeneration with medial meniscal and chondral abnormalities [12. However, mucoid degeneration has well-characterized MRI findings. These include an abnormally thickened and ill-defined ACL with altered intra-ligamentous signal. The signal intensity has been described as intermediate on T1-weighted images and high on fluid-sensitive images. Characteristically the signal alteration extends in between intact fibers with maintenance of normal orientation and continuity of the ACL, creating the so-called “celery stalk” appearance [10]. Histology findings show faint purple-colored mucoid material between collagen fibers [13]. This microscopic pattern correlates with the macroscopic findings of a “celery stalk” appearance on MRI findings. In contrast, our patient’s histological findings of synovium with multiple nodules of lobulated hyaline cartilage demonstrating clusters of chondrocytes is indicative of SC. It is similar to that of SC in other locations [14]. Similarly, this histological pattern correlates with the macroscopic findings of multiple irregular lobulations within the ACL that were seen on our patient’s initial MRI.

Our differential also includes intraligamentous ganglion cysts which commonly coexist with mucoid degeneration, and have also been well-described in the literature [10, 15-18]. MRI findings of ACL ganglia include a mass that may be located in the proximal half, distal half, or throughout the entirety of the ligament [10]. The ganglia may be variably complex, lobulated or multi-lobulated, and demonstrate signal intensity that is slightly hypointense compared with normal muscle on T1-weighted images and homogeneously hyperintense on fluid-sensitive images [10, 16, 18]. Of note, the cases of multi-lobulated ganglia that have been described and shown in the literature appear as variable sized locules and may tend to bulge or extrude either proximally or distally [10, 15, 19] (Fig. 5). In contrast, the initial MRI of our SC case appears as similar sized lobules throughout the ACL. This finding may potentially be useful to suggest intraligamentous SC. On the later exam, the diffuse involvement of the joint may be related to extensive tumor bulk exiting the ligament or prior surgical incision creating an extra-ligamentous pathway. However, the appearance of high signal intensity material filling the joint with enchondral calcifications is highly typical of synovial chondromatosis as previously described [8].

Figure 5.

59-year-old man. Sagittal fluid sensitive MR images (A-C) on initial presentation (A), and 1-year (B) and 3-years (C) after initial presentation show development of concurrent mucoid degeneration (thick arrows) and intraligamentous ganglion cysts (thin arrows) within the ACL. The ganglion cysts demonstrated extra-ligamentous extension in (C). Arthroscopic image (D) demonstrates a hyperemic mass in the ACL (*) which was biopsied, showing synovial tissue with focal myxoid degeneration, consistent with a ganglion cyst.

The ACL is an intra-articular, but extra-synovial structure, and the pathoetiology of intraligamentous SC in our case is unclear. Post-traumatic synovial tissue herniation into the ligament is possible, but our patient did not report a history of trauma. Another possibility is that the lesion arose from the vascular-derived stem cells that are present in the septum between the two distinct ACL bundles, namely the anteromedial and posterolateral bundles. In the human ACL, these vascular-derived stem cells exhibit multi-lineage differentiation potential, including the capacity to undergo chondrogenesis, osteogenesis, adipogenesis, and endotheliogenesis [20]. Indeed, the lesion in our case appeared centered within the ACL between the two bundles, however this was not histologically confirmed since the ACL was not resected.

In summary, we present the second case of intraligamentous synovial chondromatosis in the literature. SC is a rare disease that can be destructive and debilitating, and our patient suffered from significant pain and restricted range of motion. We believe the initial finding of multiple, similar-sized lobulations within the entire ACL should lead the imager to consider this extremely rare diagnosis.