In our study, the participants in the non-inverted group had a significantly lower rate of traumatic history than those in the inverted group. Although there was no significant difference between the groups, the mechanical symptoms and McMurray test tended to be positive in participants in the non-inverted group compared with those in the inverted group. In contrast, the mean duration between the appearance of symptoms and surgery tended to be longer in the inverted group than in the non-inverted group, despite the occurrence of clear trauma and swelling in all inverted-type patients. Patients with a symptomatic DLM with or without tears, especially when young, generally do not have a history of trauma with the onset of pain, and often have mechanical symptoms such as knee locking or catching, as seen on the McMurray test [13, 14].
These results were similar to those of LaMont et al. [9]; they reported that in 19 patients who had an inverted segment, 18 had a clear history of trauma, 11 experienced knee swelling, and all patients had lateral knee pain during exercise. However, they reported that mechanical symptoms and a positive McMurray test were present in only four of the 19 patients. This may be due to the characteristics of inverted-type DLM tears. In the inverted type of DLM tear, the tear occurs only in the anterocentral part of the meniscus, and the torn portion forms a large flap. There were no peripheral tears or instability of the posterior horn. Therefore, once the torn flap of the central discoid body is inverted and moved into the intact posterior horn, its position will be stabilised and it will not move, even during exercises such as extension and flexion of the knee. In fact, in the arthroscopic findings of this study, the torn inverted portion did not move at all during extension and flexion of the knee. Therefore, probing was necessary to expose the torn inverted portion. Because the position of the torn portion was stable, the symptoms were not severe and it was possible for the patient to continue exercising. In addition, mechanical symptoms and the McMurray test, which focus on dynamic motion of the meniscus, were not clear, and the duration of time between the appearance of symptoms and surgery tended to be longer for patients with inverted-type DLM tears than for patients with non-inverted types of DLM tears.
Because the torn flap of the central discoid body is inverted beneath the intact posterior horn, an ‘O’ shape resembling a normal lateral meniscus is observed. Therefore, few features indicate a DLM tear, and it is difficult to diagnose it using MRI. Nine patients had a duplicated or enlarged posterior horn and a blunted inner rim in the sagittal plane, as seen on MRI scans, which we named the inverted sign. This characteristic finding is useful for diagnosing an inverted-type DLM tear. Two of three patients who did not have the characteristic MRI findings of an inverted-type tear but showed mechanical symptoms and had a positive McMurray test were diagnosed as having a DLM tear by MRI. In addition, they had a smaller amount of meniscal inversion than other patients did, as seen on arthroscopy. This partly explains why we could not diagnose these patients with an inverted-type DLM tear. The characteristic physical and MRI findings may be related to the size of the torn inverted portion.
The traditional treatment for a symptomatic DLM tear is total or subtotal meniscectomy [3, 15]. Although some studies have reported good long-term clinical results, radiographically, many studies showed a high rate of early degenerative changes of the lateral compartment of the knee [16, 17]. Therefore, a recently recommended treatment plan for a symptomatic DLM tear is arthroscopic meniscal reshaping using partial meniscectomy, with or without meniscal repair [18, 19]. None of the patients in our study with inverted-type DLM tears had a posterior peripheral rim tear or instability of the posterior horn, but the inverted portion of the central discoid body was torn. Therefore, we performed only resection of the flap portion, which was reduced using a probe, without meniscal repair (Fig. 4a-b). This procedure is called meniscal reshaping or plastic meniscectomy.
The inverted-type DLM tear has two diagnostic points. The first is the patient’s medical history, especially traumatic history. Even if physical findings do not suggest the presence of a meniscal lesion, physicians should suspect a lateral meniscus injury in patients with a clear history of trauma or knee swelling and lateral knee pain during exercise. The second point is to find the inverted sign on the sagittal plane of an MRI scan. It is important to carefully interpret MRI findings while suspecting the presence of an inverted-type tear when the physical examination is poor. There are some benefits to surgeons to knowing these diagnostic points. These diagnostic points are useful in determining the treatment strategy and timing of operation. Because there are few physical symptoms and only partial meniscectomy is often performed during surgery, it is possible to select the surgical timing and infer the timing of return to sports among athletes. These are also useful in preventing missing of meniscus tears during arthroscopy. It is reported that the inverted-type DLM tear looks like a normal lateral meniscus without a meniscus tear at first glance and that probing is necessary to expose the inverted portion [9]. Thus, missing a meniscus tear can be prevented through prior knowledge of an inverted-type DLM tear and its diagnostic points.
One of the strong points of our study is that the inverted-type tear is a rare and hidden tear pattern of the DLM. It is possible to predict the treatment strategy before surgery by knowing the type of DLM tear. In addition, in all patients, we confirmed the tear pattern using arthroscopy. However, this study has some limitations. First, we used a 0.4 T MRI apparatus in this study, which does not have good quality compared with other recent types of MRI apparatuses. It is thus necessary to improve the quality in future studies. Second, the study was retrospective, and bias was generated when we interpreted the MRI scans. Therefore, we measured the data twice in a blinded method regarding the patients’ names and arthroscopic findings. Last, the sample size of this study was small. Inverted-type discoid lateral meniscus tears are not common or well known. Although the characteristics of inverted-type DLM tears could be shown by including a control group in this study, further research on this topic is required.