MR imaging of meniscal tears with discoid lateral meniscus

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Abstract

Objective: To study MR findings of meniscal tears with discoid lateral menisci (DLMs) and to evaluate the incidence and accuracy of MRI compared to arthroscopical surgery. Material and methods: MR appearances of surgically proved torn discoid lateral menisci (DLM) were studied in 57 knees (40 patients). They were all performed with a 1.5T MR before surgery. MR sequences included T1- and T2* weighted images on both coronal and sagittal planes and 3D-axial images with a slice thickness of 0.7 mm. Result: Of 57 DLMs, there were 32 complete DLMs and 25 incomplete DLMs. Twenty-five of 32 complete DLMs had tears; including seven with intrasubstance tear, five with radial tear, five with other kinds of tears and eight with severe tears involving whole meniscus. On the other hand, 13 of 25 incomplete DLMs had tears; including two with intrasubstance tear, six with radial tear, four with other kinds of tears and one with severe tears. All DLMs were correctly diagnosed on MR images. Twenty-seven of 38 tears with DLM were correctly identified on conventional 2D MR images. This yielded 71.1% sensitivity, 100% specificity and 80.7% overall accuracy. When adding axial 3D MR images to 2D MR images, 36 of 38 tears were correctly diagnosed. A combination of both techniques yielded a sensitivity of 94.7% and a specificity of 100%. Ten of 11 radial tears with DLM were correctly identified on 3D axial images, where only three of them could be diagnosed on conventional 2D images. Eight of nine intrasubstance tears were correctly identified on 3D axial images, where six of them could be diagnosed on conventional 2D images. Conclusion: DLMs had a much higher incidence of meniscal tears than normally shaped lateral menisci and MR is the only modality of choice to evaluate them before surgery. Especially 3D axial MR images were quite useful in the detection of intrasubstance and radial tears often associated with DLMs.

Introduction

A discoid lateral meniscus (DLM) of the knee described as a congenital and morphologically enlarged meniscus is not an uncommon anomaly 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14. Although a discoid lateral meniscus itself may cause `snapping knee syndrome' or gonalgia 5, 6, 7, clinical symptoms should appear mainly when a DLM is injured. So it is of great importance to know the presence of meniscal tears with DLM in presurgical planning. Lately, MR has become the first choice of modality to evaluate the knee structure 1, 8, 9, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, so we tried to study MR appearances of torn DLM.

Section snippets

Materials and methods

From January 1990 to July 1993, we had examined 1140 knees (969 patients) with MRI. In these knees, surgically proved 57 DLMs (40 patients) were consecutively selected and studied. All associated meniscal tears were confirmed by surgery in our study. Of them, 17 patients underwent arthroscopical surgery in both knees. However, the other 23 patients (all had DLMs on both knees) underwent surgery in one knee because the other knee was not symptomatic. They were 18 men and 22 women, ranging in age

Results

According to Watanabe's classification [11], we arthroscopically classified 57 DLM; 32 as complete type and 25 as incomplete type. Of 32 complete DLMs, meniscal tears were observed in 25 DLMs, whereas only one opposite medial meniscus was injured. As to 25 incomplete DLM, 13 DLMs and six opposite medial menisci were injured. The remaining 19 DLMs (seven complete and 12 incomplete DLMs) had no tears.

Of 38 DLMs with meniscal tears, nine menisci (seven complete and two incomplete DLMs) had

Discussion

Meniscal tears are main clinical pathologic lesions of the menisci 5, 8, 11, 15, 20, 24, 26, 27, 28and DLMs have been said to be more commonly injured than the normally shaped lateral menisci [26]. This is supported by our results that DLM had the much higher incidence of meniscal tears than the opposed medial menisci by 38:6 (Table 1). Generally, a medial meniscus is thought to be at greater risk for tears than a lateral meniscus because the former has greater radius of curvature and tight

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