ring - shaped menisci , which are characterized by the presence of an interhorn bridge , are rare . watson - jones first reported a medial ring - shaped meniscus in 1930 , and noble first reported a lateral ring - shaped meniscus in 1975 . watson - jones and basmajiian debated whether the origin of the ring - shaped meniscus is congenital or traumatic . johnson and simmons stated that the ring - shaped meniscus probably represents a locked bucket - handle tear that has been altered by time and pressure . on the other hand , arnold stated that the ring shape is likely to be due to a developmental abnormality rather than a degenerative alteration because ring - shaped menisci are a common feature in primates , especially at the lateral meniscus . we herein report an exceptional case of regeneration of completely ring - shaped meniscus - like tissue after partial resection of a discoid lateral meniscus with acl reconstruction . a 37-year - old female patient twisted her right knee and presented to our institution . the lachman test and pivot shift test were positive , and magnetic resonance imaging ( mri ) confirmed the acl injury and an intrasubstance horizontal tear of the discoid lateral meniscus . in the third month after the injury partial meniscectomy was performed because the preoperative mri showed an intrasubstance horizontal tear ( fig . although the patient developed a large joint hematoma postoperatively , aspiration was not performed . following 10 days of immobilization in a slightly flexed position , partial weight - bearing and full weight - bearing gaits were permitted at weeks 3 and 5 , respectively . because the patient complained of unrelenting pain at the lateral site with deep knee flexion 6 months after the surgery , arthroscopy was performed . the lateral tibial plateau , excluding a portion of the center of the weight - bearing area , was covered in the form of a ring by meniscus - like tissue that was connected from the anterior to posterior horn , forming an interhorn bridge ( fig . a pathological section of the inner portion of the ring around the bridge was evaluated . new tissue in the form of a triangular cross section was very similar to a meniscus . in terms of histologic structure , hematoxylin - eosin ( he ) staining ( fig . 2a ) showed that the majority of the cells comprising the tissue were not fibroblasts , but chondrocyte - like round cells . furthermore , this part was completely stained pale red with safranin o , suggesting that it contained a higher amount of proteoglycan ( fig . immunostaining revealed that it was densely stained for type i collagen ( fig . 2c ) and was not stained for type ii collagen ( fig . however , the fiber orientation was random / amorphous , and tie fibers were not found . one year after the initial surgery , she readily accepted the recommendation to undergo removal of the fixation devices with simultaneous arthroscopy . the re - repeat arthroscopy demonstrated that the lateral meniscus was in good condition without an interhorn bridge ( fig . in the present case , ring - shaped meniscus - like tissues regenerated inward toward the center from the stump formed from the partial meniscectomy of the discoid lateral meniscus and were connected from the anterior to posterior horn , forming an interhorn bridge . histological findings revealed that this regenerated tissue was relatively similar to the normal meniscus , although there was a random / amorphous fiber orientation and absence of tie fibers . one possible cause of such a phenomenon is the intra - articular environment that develops after acl reconstruction . during acl reconstruction , soft tissue debridement triggers cytokine expression , and drilling of bone tunnels leads bone marrow mesenchymal stem cells to the joint . because this patient developed a large joint hematoma , it is likely that clots containing a high abundance of bone marrow mesenchymal stem cells and cytokines formed in the dead space after the meniscectomy . these blood clots may have served as a scaffold for meniscus - like fibrous cartilaginous tissue to emerge as a regenerated meniscus . remnants of the acl left in the interhorn region may have metamorphosed to meniscus - like tissue because cells of the acl have chondrogenic characteristics . most authors reported it as a congenital anomaly because their patients had no symptoms and/or no history of a major traumatic episode . there are only two reports of a ring - shaped lateral meniscus combined with an acl injury . kim et al . incidentally found a ring - shaped lateral meniscus 2 months after an acl injury . he concluded that it was probably caused by a morphological anomaly based on the arthroscopy findings . reported a ring - shaped lateral meniscus that was mistaken for an incomplete discoid meniscus in a case of acl injury . in the first arthroscopy , they found an incomplete discoid meniscus with a concomitant acl tear , and the torn acl stump was debrided ; acl reconstruction was not performed . they concluded that the ring meniscus was overlooked because a re - examination of the lateral meniscus was not performed after debridement in the first arthroscopy . however , it is possible that tissue regeneration occurred between the two horns through the synovial reaction and the blood clots or through remnants of the acl because the torn acl stump was debrided during the first arthroscopy . if the oversight had not occurred , their case may have been identical to ours . kim et al . concluded that for the differential diagnosis of a ring - shaped meniscus , we should take a careful history of any traumatic events , consider the possibility of misinterpretation of the mri findings , and probe carefully during arthroscopic operations . furthermore , it must be kept in mind that the phenomenon found in our case is actually possible . to prevent the occurrence of such phenomenon observed in our case , we should pay attention to curetting unnecessary remnants thoroughly during the surgery , avoiding accumulation of a large amount of hematomas during the perioperative period , and starting the exercise of range of motion immediately after the surgery . we reported a patient who exhibited symptoms due to regeneration of ring - shaped meniscus - like tissue following acl reconstruction and partial meniscectomy of a discoid lateral meniscus . the reason for the regeneration is unknown , but may have been attributed to the specific intra - articular environment that developed after the acl reconstruction . when a ring - shaped lateral meniscus is diagnosed , we must judge carefully whether it is really a congenital anomaly in consideration of our case . written informed consent was obtained from the patient for publication of this case report and accompanying images . k. tabuchi is a doctor in charge of patient care at the time of the second hospitalization . t. inoue is a doctor in charge of patient care at the time of first hospitalization .
introductionthe ring - shaped lateral meniscus is very rare . 
 although it is essentially known as a congenital anomaly , a central tear in an incomplete discoid meniscus or an old bucket - handle tear in a meniscus may be easily mistaken for a ring - shaped meniscus . 
 we experienced a ring - shaped lateral meniscus that regenerated after partial resection of a discoid meniscus together with anterior cruciate ligament ( acl ) reconstruction.presentation of casea 37-year - old female patient still experienced unrelenting knee pain 6 months after acl reconstruction and partial meniscectomy of a discoid lateral meniscus . 
 a repeat arthroscopy was performed . 
 the lateral tibial plateau was covered in the form of a ring by meniscus - like tissue . 
 the meniscus - like tissue appeared to have regenerated inward toward the center from the stump after the partial meniscectomy and was connected from the anterior to posterior horn , forming an interhorn bridge . 
 partial meniscectomy was repeated . 
 histologically , the regenerated tissue was not meniscal , but comprised mature fibrocartilage ; macroscopically ; however , it was very similar to meniscal tissue . 
 two years after the initial operation , the patient had no complaints and experienced full return of function.discussionthe reason for such regeneration is unknown , but may have been attributed to the specific intra - articular environment that developed after the acl reconstruction.conclusionthis is the first report of regenerative development of a ring - shaped lateral meniscus . 
 when a ring - shaped lateral meniscus is diagnosed , we must accurately determine whether it is a true congenital anomaly in consideration of the present case .