Hypoplastic meniscus is an extremely rare abnormality. Some of the reported cases are associated with other congenital abnormalities such as thrombocytopenia-absent radius syndrome or anterior cruciate ligament hypoplasia.1,2 Other cases involve no other congenital abnormalities.3–7 In most of the previous cases, hypoplastic meniscus was observed either medially or laterally but not both. To the best of the authors' knowledge, there have been only 2 reports describing hypoplasia of both medial and lateral menisci to date.2,7 The current authors report the first case of bilateral hypoplasia of both medial and lateral menisci with a partial fusion of meniscus and cartilage surface of the tibia.
A 22-year-old man presented to the authors' hospital with pain and unsteadiness in the left knee. At the age of 10 years, he presented to a nearby hospital after falling and was diagnosed with a left patellar dislocation. He was treated conservatively, but the left knee instability has remained.
A physical examination found that the left patella was laterally dislocated at all ranges of motion. No effusion, joint laxities, or meniscal signs were observed. Radiographs showed a mild valgus alignment with hypoplasia of the lateral femoral condyle and patella baja in both knees. The left patella was laterally dislocated with trochlear dysplasia. The patient was diagnosed with a chronic patellar dislocation, and surgery was scheduled. Preoperative magnetic resonance imaging of the left knee incidentally revealed hypoplasia of both medial and lateral menisci (Figure 1).
T2-star–weighted magnetic resonance images of the left knee. Coronal views from anterior to posterior (A–F) show that the medial meniscus (MM; arrowhead) is totally diminutive, the lateral meniscus (LM; arrow) gradually becomes smaller as it goes from posterior to anterior, and the anterior horn of the LM is absent. Note that the posterior horn of the MM (arrowhead with asterisk) and the middle body of the LM (arrow with asterisk) appear to be fused with the cartilage surface of the tibia. Sagittal view of the medial compartment (G) shows that the anterior horn and the posterior horn of the MM are diminutive, and the posterior horn of the MM appears to be fused with the cartilage surface of the tibia. Sagittal view of the lateral compartment (H) shows that the posterior horn of the LM exists, but the anterior horn is absent.
The authors performed a medial patellofemoral ligament reconstruction and tibial tubercle transfer for the treatment of patellar dislocation. During arthroscopic observation, the authors confirmed hypoplasia of both medial and lateral menisci. The medial meniscus was totally diminutive, and its posterior horn was fused to the cartilage surface of the tibia (Figures 2A–B, Video 1). The posterior horn of the lateral meniscus existed, but the anterior horn and middle body were absent. The degenerative horizontal tear was seen at the posterior horn of the lateral meniscus, and its body was terminated and fused to the cartilage surface of the tibia in front of the popliteal hiatus (Figure 2C, Video 2). There were no meniscal symptoms; therefore, the authors did not perform any arthroscopic procedures.
The medial meniscus is totally diminutive without tear (A), and its posterior horn fuses to the cartilage surface of the tibia (arrowheads) (A–B). Anterior horn and middle body of the lateral meniscus are absent (C). The degenerative horizontal tear is seen at the posterior horn of the lateral meniscus, and its body terminates and fuses to the cartilage surface of the tibia in front of the popliteal hiatus (arrowheads) (C). Abbreviations: LFC, lateral femoral condyle; LTP, lateral tibial plateau; MFC, medial femoral condyle; MTP, medial tibial plateau.
Magnetic resonance images of the contralateral knee showed meniscal anomalies similar to those of the left knee (Figure 3). These findings confirmed the presence of bilateral hypoplasia of both medial and lateral menisci. The postoperative course was uneventful. At the 1-year follow-up, the patient had no reports of knee problems.
T2-star–weighted magnetic resonance images of the right knee. Coronal view of the right knee (A), and sagittal view of the medial (B) and lateral (C) compartments. A contralateral magnetic resonance image showed meniscal abnormalities similar to those of the left knee: hypoplastic medial and lateral menisci with partial fusion with tibial cartilage (arrow). Abbreviations: LM, lateral meniscus; MM, medial meniscus.
Congenital meniscal abnormalities are rare. There have been some previously reported cases of various congenital meniscal abnormalities, including discoid meniscus, double-layered meniscus, accessory meniscus, ring-shaped meniscus, and anomalous insertion of the meniscus.8–11 Among others, absent menisci or hypoplastic menisci are extremely rare.12 Tolo1 reported the first case of the complete absence of the medial meniscus and cruciate ligaments in a patient with thrombocytopenia-absent radius syndrome. Ohana et al3 was the first to describe an isolated bilateral hypoplasia of the lateral meniscus without other multiple anomalies. On the other hand, Monllau et al5 reported an isolated bilateral hypoplasia of the medial meniscus. In most of the previous cases, hypoplasia was observed in either the medial or the lateral meniscus alone, and both medial and lateral cases are much rarer, with only 2 cases being previously reported.2,7
In the current case, hypoplasia of both medial and lateral menisci was incidentally found during treatment of the chronic patellar dislocation. Magnetic resonance imaging and arthroscopy revealed that the medial meniscus was diminutive and the anterior horn and middle body of the lateral meniscus were absent. Moreover, the posterior horn of the medial meniscus and middle body of the lateral meniscus were fused with the cartilage surface of the tibia. Although similar hypoplastic menisci have been reported, such partial fusion of the meniscus and tibial cartilage has not been described previously.2,7 Because the same morphological abnormalities of the medial and lateral menisci were found in the contralateral knee by magnetic resonance imaging, the authors diagnosed the patient with bilateral hypoplasia of both medial and lateral menisci that was not acquired secondary to the patellar dislocation but congenitally.
According to the previous reports, the hypoplastic menisci by themselves were asymptomatic, and surgical interventions were done only for treating accompanying pathologies such as meniscal tear,7 anterior cruciate ligament tear,4 patellar dislocation,6 osteochondritis dissecans,13 or chondral lesion.2,3 However, the long-term follow-up studies of hypoplastic meniscus are lacking, and the natural course of hypoplastic meniscus is unknown. A discoid meniscus is known to be more fragile than a normal meniscus, but it is uncertain whether a hypoplastic meniscus is prone to degeneration or tears.14 Considering that the degenerative tear was seen at the posterior horn of the lateral meniscus in the current case without a history of trauma and a similar degenerative tear was also found in a 9-year-old child, a hypoplastic meniscus may also be fragile and increase the risk of future osteoarthritis.7
It has been reported that the meniscus develops from mesenchymal cells within the intermediate layer, called interzone cells, during the 8th to 10th week of conception.15 The articular cartilage also originates from the interzone. Recent evidence indicates that a continuous influx of growth differentiation factor 5–positive cells contributes to joint development.16 Growth differentiation factor 5–positive cells of the interzone give rise to many major cell types and structures of a mature joint, including the ligaments, synovial membrane, menisci, and articular cartilage.17
The etiology of the hypoplastic meniscus is unknown. As mentioned earlier, the fusion of the meniscus and the tibial cartilage has not been previously reported. Although the time the fusion occurs, either during the development of the meniscus or after the maturation, is still unknown, it is more likely that some failures might occur during the process from joint specification to maturation, considering the hypoplasia and the fusion concurrently occurred in the current case.17 One possibility is that some population of meniscal progenitor cells might differentiate into superficial chondrocytes of the tibial plateau incorrectly. Although it is a matter of speculation, the findings in this case support the theory that meniscus and articular cartilage are differentiated from the same mesenchyme. Additionally, they suggest that the meniscal progenitor cells are closer to the tibial articular cartilage lineage than the femoral one. This case may provide some clues to understanding the underlying mechanisms of meniscal development.
The authors reported a case of bilateral hypoplasia of both medial and lateral menisci with a partial fusion of meniscus and tibial cartilage. Although hypoplastic menisci are extremely rare and most of them by themselves are asymptomatic, they should be considered when imaging shows small or no menisci. Additionally, these patients should be carefully followed for the possibility of future osteoarthritis.
- Tolo VT. Congenital absence of the menisci and cruciate ligaments of the knee: a case report. J Bone Joint Surg Am. 1981;63(6):1022–1024. doi:10.2106/00004623-198163060-00025 [CrossRef]
- Beyzadeoglu T, Gokce A, Bekler H. Osteochondritis dissecans of the medial femoral condyle associated with malformation of the menisci. Orthopedics. 2008;31(5):504. doi:10.3928/01477447-20080501-02 [CrossRef]
- Ohana N, Plotquin D, Atar D. Bilateral hypoplastic lateral meniscus. Arthroscopy. 1995;11(6):740–742. doi:10.1016/0749-8063(95)90121-3 [CrossRef]
- Tetik O, Dorai MN, Atay ÖA, Leblebicioglu G, Türker S. Partial deficiency of the lateral meniscus. Arthroscopy. 2003;19(5):E42. doi:10.1053/jars.2003.50162 [CrossRef]
- Monllau JC, González G, Puig L, Cáceres E. Bilateral hypoplasia of the medial meniscus. Knee Surg Sports Traumatol Arthrosc. 2006;14(2):112–113. doi:10.1007/s00167-005-0654-5 [CrossRef]
- Benedick A, Shiraj S, Williams J, Sundaram M, Subhas N. Congenital medial meniscus hypoplasia. Orthopedics. 2016;39(3):e593–e594. doi:10.3928/01477447-20160404-08 [CrossRef]
- Youm YS, Cho SD, Cho HY, Kang TW. Bilateral hypoplasia of the medial and lateral menisci. Knee Surg Relat Res. 2017;29(2):150–152. doi:10.5792/ksrr.15.070 [CrossRef]
- Takayama K, Kuroda R, Matsumoto T, et al. Bilateral double-layered lateral meniscus: a report of two cases. Knee Surg Sports Traumatol Arthrosc. 2009;17(11):1336–1339. doi:10.1007/s00167-009-0815-z [CrossRef]
- Saygi B, Yildirim Y, Senturk S, Sezgin Ramadan S, Gundes H. Accessory lateral discoid meniscus. Knee Surg Sports Traumatol Arthrosc. 2006;14(12):1278–1280. doi:10.1007/s00167-006-0093-y [CrossRef]
- Atay OA, Aydingöz U, Doral MN, Tetik O, Leblebicioglu G. Symptomatic ring-shaped lateral meniscus: magnetic resonance imaging and arthroscopy. Knee Surg Sports Traumatol Arthrosc. 2002;10(5):280–283. doi:10.1007/s00167-002-0292-0 [CrossRef]
- Wang JH, Wong AK, Romanowski JR II, Fu FH. Meniscus tear developed by pulling of the anomalous insertion of medial meniscus on anterior cruciate ligament. Knee Surg Sports Traumatol Arthrosc. 2011;19(10):1689–1692. doi:10.1007/s00167-011-1464-6 [CrossRef]
- Ali S, Dass C, Shah P, Sewards JM. Normal variants of the meniscus. Appl Radiol. 2013;42(5):14–18.
- Mitsuoka T, Horibe S, Hamada M. Osteochondritis dissecans of the medial femoral condyle associated with congenital hypoplasia of the lateral meniscus and anterior cruciate ligament. Arthroscopy. 1998;14(6):630–633. doi:10.1016/S0749-8063(98)70063-5 [CrossRef]
- Clark CR, Ogden JA. Development of the menisci of the human knee joint: morphological changes and their potential role in childhood meniscal injury. J Bone Joint Surg Am. 1983;65(4):538–547. doi:10.2106/00004623-198365040-00018 [CrossRef]
- Gardner E, O'Rahilly R. The early development of the knee joint in staged human embryos. J Anat. 1968;102(pt 2):289–299.
- Ray A, Singh PN, Sohaskey ML, Harland RM, Bandyopadhyay A. Precise spatial restriction of BMP signaling is essential for articular cartilage differentiation. Development. 2015;142(6):1169–1179. doi:10.1242/dev.110940 [CrossRef]
- Salazar VS, Gamer LW, Rosen V. BMP signalling in skeletal development, disease and repair. Nat Rev Endocrinol. 2016;12(4):203–221. doi:10.1038/nrendo.2016.12 [CrossRef]