Discoid lateral meniscus (DLM) is a common anatomical variant of the knee. The frequency of DLM is low in Europe but relatively high in Asia. Smillie et al. completed 10,000 cases of meniscectomy, 467 cases of which were DLM (4.6 %) [21]. Dickason et al. evaluated 14,731 menisci and found 102 discoid lateral menisci in 6,691 lateral menisci (1.5 %) [22].
Kim et al. reported that between 1990 and 1992, 77 of 534 arthroscopy cases (14 %) were diagnosed with DLM [23]. Over a 20-year period in Japan, Ikeuchi noted that the frequency of DLM was 16.6 % on arthroscopic examination [6]. In India, 95 in 1643 knees (5.8 %) were diagnosed with DLM [9]. However, these reports are results of arthroscopy in symptomatic patients, whereas studies of the asymptomatic population are almost absent. To determine the prevalence of asymptomatic DLM in general population, Fukuta et al. examined l15 Japanese asymptomatic volunteers (aged from 13 to 76) using MRI, and reported the frequency of DLM to be 13 %; their study also showed that, at least in Japan, asymptomatic DLM could occur in any age group [24].
To our best knowledge, only a few studies have reported the frequency of bilateral DLM. Bae et al. checked arthroscopic features of the lateral meniscus in asymptomatic contralateral knees in 52 DLM patients who presented with symptomatic DLMs [3]. They reported a frequency of bilateral DLM of 79 % (41 of 52 contralateral knees) and 65 % of patients (34 pairs of knees) had the same DLM types [3]. Ahn et al. examined the contralateral knees of 33 Korean male soldiers with single-knee DLM surgery during the period from 2006 to 2008 [13]. They found that bilateral DLM occurred in 97 % of the patients [13]. Our study may be the first to report its prevalence in the Han Chinese patients.
In clinical practice, we occasionally observed asymptomatic DLM in contralateral knees of symptomatic DLM patients, and their bilateral DLM often were of the same type; studies have also supported this finding. Kato et al. evaluated 279 Japanese cadavers to study the shape of the menisci, and found that 91 % of DLMs were of the same morphology [12]. Therefore, when a patient presents with DLM in one knee, it is important to carefully examine the other knee. From the surgeon’s point of view, preoperative imaging with MRI to assess patients with symptomatic DLM, as well as detecting DLM and other subclinical lesions in the contralateral knee, can be important for preoperative planning. In some cases, it is necessary to confirm the presence of DLM by diagnostic arthroscopy to prevent potential injury. But given that MRI is recognized as a reliable alternative to arthroscopic diagnosis [14–16], patients are now less willing to undergo invasive arthroscopic examination for asymptomatic knees; for this reason, we adopted the noninvasive MRI to examine contralateral knees in Han Chinese patients who underwent arthroscopic surgery. We found bilateral DLMs in 72.7 % (80 of 110 DLM patients) of contralateral knees, only slightly fewer than the 79 % reported by Bae et al. [3]. Minor difference is expected as there appears to be difference in the prevalence of DLM among different populations [5–9].
For symptomatic DLM, most authors recommended arthroscopic meniscoplasty to preserve the meniscus [10, 25–28], while DLM complicated by meniscal tear often leads to partial meniscectomy or repair. It has been suggested that abnormal pressure on bone and cartilage caused by DLM could induce osteochondritis dissecans of the lateral femoral condyle [29]. For asymptomatic discoid meniscus, follow-up surveillance would suffice and there is no need for further treatment unless symptoms appear, as the knee joint may have adapted to the anatomic configuration and can maintain normal function. But the knee with DLM is still at risk of tearing and the DLM may cause abnormal conduction of loads at the knee, so that even without history of trauma, symptoms may still eventually develop in knees with asymptomatic DLM. But it remains unclear whether prophylactic meniscoplasty can reduce these risks with few or no complication [6, 30], as subtotal meniscectomy of asymptomatic DLM after injury has been shown to increase the risk of arthritis or degenerative changes [31, 32]. It’s for this reason that our 24-month follow-up was an attempt to approximate when an asymptomatic discoid meniscus began to exhibit symptoms, so meniscoplasty could be carried out to avoid subtotal meniscectomy.
Based on our MRI results, 80 (72.73 %) of the 110 symptomatic DLM patients had bilateral DLM and 68 were of homotypes. We found that 12 (18 %) of 66 bilateral DLM patients who were initially asymptomatic in one knee underwent a second arthroscopic surgery within five years after the first surgery as the contralateral knees developed symptoms. 11 of these 12 knees presented as grade III signal at time of operation, and at least two of the 12 knees showed progression from grade II to grade III over the interim. Many DLMs are usually asymptomatic clinically, but due to the lack of the wedge-shaped supportive role of the normal meniscus, flexion and rotation of the knee can cause shear forces on the DLM, resulting in degeneration or potential injury. Therefore, MRI signals of grade II and III can be found not only in symptomatic DLMs, but also in asymptomatic DLMs, as shown by our results. The data proved our initial hypothesis that contralateral asymptomatic DLM of patients who underwent unilateral DLM surgery may be at risk for future injury. Further research with longer follow-up time is needed to ascertain if meniscoplasty on the asymptomatic knees, especially those with MRI grade II or above, should be performed at time of arthroscopic surgery on their first symptomatic knees.
The increase of MRI signals which do not reach the upper and lower margins of the meniscus (grade I to II signal change), indicates either pure degeneration or an intrameniscal tear, whereas high signal extending to the upper and lower margins (grade III signal change) represents a dominant tear. These findings were based on the analysis of non-discoid meniscus, and may or may not be applicable for the discoid meniscus [15, 16]. We feel that MRI signal changes of DLM undergoing degeneration or tear would correlate with disease severity more than those of the non-discoid meniscus. In this study, of the 12 bilateral DLM patients that had undergone previous arthroscopic surgery, the asymptomatic knees of five cases initially examined by MRI showed three cases of grade III signal and two cases of going from grade II to grade III changes (Table 4). Although there were no definite tears, we feel that grade III meniscal signal changes may indicate potential injury.
The need for a second surgery on the contralateral DLM in the 12 bilateral DLM patients suggests that such cases require further long-term follow-up for the following reasons: (1) although no clinical symptoms were observed in the contralateral DLM, grade III signal change indicated the existence of potential injury; (2) because muscle strength and flexibility of the joint decline in the rehabilitation phase after knee surgery, inappropriate exercise or other physical activity of the knee may cause compensatory increase in stress on the contralateral asymptomatic DLM knee, thus the contralateral DLM knee is more prone to injury; (3) if the patients engage in specific sports activities such as dancing, football or basketball which involve emergent stops and frequent rotations, they are more likely to be injured. We will extend follow-up of our patients in order to gather more information concerning the reasons for (and frequency of) change from asymptomatic to symptomatic DLM knees and the optimal timing of surgical intervention. Most agree that the meniscoplasty is the best surgical approach for DLM injury [10, 26–28, 32]. Therefore, if a certain MRI type or signal change can be correlated with the second surgery, for example, complete DLM or grade III signal change indicating potential injury, it may be advisable to perform meniscoplasty as an early surgical intervention (at the same time of surgery for the symptomatic DLM) in order to avoid subtotal meniscectomy and preserve the function of the meniscus. Also, if particular types of sport are identified to aggravate DLM injury, more serious damage may be prevented through adjustment of activities when the patient is present with asymptomatic DLM knee with subclinical changes.
Our study had several limitations. It was limited only to symptomatic DLM patients who underwent arthroscopic surgery, and not asymptomatic volunteers in the general population, so the result does not reflect the prevalence of bilateral DLM in the general population. Furthermore, the age distribution of our subjects did not reflect that of the general Han population of China. We also did not study the incidence of newly developed bilateral DLM in patients with prior diagnosis of unilateral DLM. Nevertheless, this study still provided information regarding the MRI assessment of knees with subclinical DLM that may be important in planning the course of their treatment. In addition, we did not address potential bias in MRI imaging evaluation in our study. We were also unable to ensure that all patients returned for a follow-up examination at predesignated or scheduled times, so for such patients, we could only contact then by telephone, and for the part of questionnaire that required physical examination, we could only gather answers through verbal questioning. Therefore, inaccurate answers were possible.
