With respect to cases of synovial chondromatosis originating from the cruciate ligament in the knee, only 2 cases originating from the posterior cruciate ligament have been reported [3
] and no case has been reported of occurrence originating from the anterior cruciate ligament. Synovial chondromatosis is considered to be a disease wherein mesenchymal cells under the overlying cells of the synovial tissue of joints, peritendons, and synovial capsule cause metaplasia to be changed to cartilaginous tissue. According to the stage classification of synovial chondromatosis reported by Miligram [6
], phase 1 is the stage with synovial lesions, phase 2 is the stage with a combination of synovial lesions and free bodies, and phase 3 involves the presence of numerous free bodies.
The theory that the etiology of synovial chondromatosis originates in synovium is the major thrust. However, Coolican et al
] and Kay et al
] suggested that the etiology of chondromatosis is present in free cartilage and it was hypothesized that a cause of this disease is the proliferation of chondrocytes within the articular fluid with free bodies subsequently incorporated into the synovium. In the present case, significant proliferation of the synovium was found only around the ACL. In the pathological findings of the present case, cartilaginous segments were present in the synovium of the ACL. Further, no synovial proliferation was found other than around the synovium of ACL. We did not find any significant chondral lesions at arthroscopy. Therefore, it is reasonable to think that in the present case synovial chondromatosis originated from synovium around the ACL.
Preoperative diagnosis was difficult for this case. Although various diseases causing locking symptoms such as meniscus damage, osteoarthritis, osteochondritis dissecans, osteochondral fractures, discoid meniscus, synovial cysts, pigmented villonodular synovitis were considered for identification, the clinical symptoms did not completely agree with any of them. It was clear from MRI that there were some lesions in the ACL because of abnormal signals in the ACL. However, no report was available on synovial chondromatosis originating from the ACL and it was difficult to make a preoperative diagnosis of synovial chondromatosis at this point. In radiological viewpoint, correct diagnosis was difficult preoperatively, because synovial chondromatosis may have nonspecific imaging findings on MRI as seen in this case. Intraarticular free bodies detected arthroscopically were not clearly visualized on MRI, as they had similar signal intensity to joint fluid. Previously reported case of synovial chondromatosis arising from PCL also had nonspecific imaging findings of intraarticular lesion surrounding PCL without alteration of PCL morphology and signal [4
]. In this case, swelling of ACL with poor visualization of ACL fibers accompanied by adjacent cystic lesions on T2-weighted image are reminiscent of ganglion cyst with coincident mucoid degeneration of ACL [8
]. Complete or incomplete tear of ACL may have similar imaging findings [9
]. Arthroscopy was considered most appropriate for a definite diagnosis of the present case.
In the treatment of synovial chondromatosis it has been reported that arthroscopic discharge of free bodies is considered to be sufficient so synovectomy is not necessary due to problems with arthrotomy in functionality such as postoperative restriction of ROM [1
]. On the other hand, there was a report suggesting combination with synovectomy [11
]. In the present case when numerous cartilaginous segments are present in the synovium, it is technically easier to remove cartilaginous segments along with the synovium. As long as the synovium is considered to be a cause of the expression of the present disease, it seems desirable to remove as much synovium as possible.
Because the observation period after treatment of this case was short, we must observe the outcome very carefully with respect to recurrence of the disease in the future. Because the synovium around the ACL was dissected, the blood flow was temporarily reduced so there is a possibility that loosening of the ACL may increase during the process of remodeling. It is important to continue long-term observation of the outcome regarding the functions of the ACL.