Practice Essentials
One element in the differential diagnosis of knee pathology is a discoid meniscus. [1] Discoid meniscus can manifest itself as an abnormal band, [2] as discoid medial and lateral menisci in the same knee, [3] as bilateral discoid medial menisci, [4] or, more commonly, as a discoid lateral meniscus (DLM). [5, 6]
DLMs were first described in the late 1800s. The normal configuration of a meniscus is that of a matured crescent moon, whereas that of a discoid meniscus generally is a thickened, very early crescent moon. Variations of this general shape occur relatively rarely, and occasionally, the lunar appearance is also found in the medial meniscus. The discoid shape results in a membrane barrier that prevents normal contact between the articular surfaces of the knee and has a high incidence of mechanical deformation.
Patients present with any combination of pain, giving way, effusion, and clicking or snapping knee. [7]
The widened and thickened discoid meniscus may be demonstrated on routine radiography of the knee. Magnetic resonance imaging (MRI) has been the modality of choice for evaluating a discoid meniscus before surgery. [8, 9]
Abnormalities of knee function, pain, and effusion are indications for surgical treatment. Surgical treatment varies according to the type of DLM present. An otherwise asymptomatic knee with the incidental finding of discoid meniscus is a contraindication for surgical treatment.
Pathophysiology
Two distinct types of DLM exist:
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Hypermobile, or Wrisberg, lateral meniscus
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Misshapen or discoid form of an otherwise normal lateral meniscus
Each type presents its own unique pathophysiologic problems.
The Wrisberg type lacks an attachment to stabilize the posterior horn to the tibia. [10] It may also be of normal shape rather than discoid. The only attachment of the posterior horn is to the Wrisberg or meniscofemoral ligament. The general configuration produces an unstable or hypermobile lateral meniscus.
The commonly used Watanabe classification defined three types of discoid meniscus, as follows:
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Type I - Complete
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Type II - Incomplete
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Type III - Wrisberg
On the grounds that the Watanabe classification did not capture the full spectrum of discoid meniscus pathology, the Pediatric Research in Sports Medicine (PRiSM) Meniscus Research Interest Group subsequently developed an arthroscopic classification system for DLM that was based on the following four primary characteristics [11] :
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Meniscal width/surface area - Incomplete; near complete/complete
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Meniscal height - Normal; abnormal
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Instability - Normal stability; abnormal stability in (a) anterior meniscus, (b) posterior meniscus, or (c) anterior and posterior meniscus
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Meniscal tear type - No tear or tear in central portion/saucerization zone; horizontal; degenerative/complex/radial
A DLM results from a developmental anomaly before birth. [12] After birth, no sudden change occurs in meniscal development.
Epidemiology
DLMs have been reported to occur at a rate of 1.5-3% in the general population, whereas discoid medial menisci have been reported to occur at thea rate of 0.1-0.3%. [13]
Symptomatic DLMs appear to be most common in adolescent males. [14]
The Asian population has a slightly higher rate of occurrence; Tokyo's Teishin Hospital reported that 16.6% of all knees examined arthroscopically had a DLM. [15] A study using data from the US Pediatric Health Information System found that of children undergoing surgical treatment of discoid meniscus, Hispanic/Latino and Asian patients made up a significantly larger percentage than White patients did. [16]
Prognosis
In a retrospective study of 40 children with symptomatic DLM, miniarthrotomy and arthroscopy were compared. [17] Miniarthrotomy was found to provide slightly better results than arthroscopic resection with regard to functional outcome. The authors recommended miniarthrotomy for the resection of DLM, particularly in young children with narrow joint spaces and for surgeons who are not familiar with arthroscopy in small joints.
Ahn et al evaluated the long-term clinical and radiographic results of arthroscopic reshaping, with or without peripheral meniscus repair, in 38 children (48 knees; mean age, 9.9 y) with symptomatic DLM. [18] The surgical procedure performed was arthroscopic partial meniscectomy alone (22 knees), partial meniscectomy with repair (18 knees), or subtotal meniscectomy (8 knees). At follow-up (mean, 10.1 y), progression of degenerative changes was significantly greater in the group treated with subtotal meniscectomy than in either of the partial meniscectomy groups.
Lee et al carried out a systematic review (11 studies; follow-up >5 y) in which the clinical and radiologic outcomes of surgical treatment of DLM (open or arthroscopic partial central meniscectomy, subtotal meniscectomy, total meniscectomy, or partial meniscectomy with repair) were evaluated. [19] Most of the studies showed good clinical results. There was minimal progression of degenerative change and no findings of moderate or advanced degenerative changes. Possible risk factors for degenerative changes included greater age at the time of surgery, longer follow-up period, and subtotal or total meniscectomy.
In a meta-analysis aimed at comparing clinical and radiographic results between partial and total meniscectomy in patients with symptomatic DLM, Lee et al found that radiographic outcomes were better with partial meniscectomy with or without repair than with total meniscectomy but that clinical outcomes were comparable for the two procedures. [20] These findings suggest that meniscal preservation may be a better option than total meniscectomy for symptomatic DLM.
In a study (N = 128; median age, 24 y; age range, 16-31 y) performed to evaluate long-term (median follow-up, 126.2 mo; range, 113-140) clinical outcomes after arthroscopic management of DLM, Lu et al reported significant postoperative improvements in the visual analogue scale (VAS), International Knee Documentation Committee (IKDC), and Lysholm scores. [21]
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Radiograph of 8-year-old child with bilateral discoid menisci, diagnosis confirmed by MRI. Patient is symptomatic only on left side. Patient underwent arthroscopy and partial meniscectomy and is now asymptomatic. Image courtesy of Dennis P Grogan, MD.
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MRI scan of typical discoid meniscus. Image courtesy of William Morrison, MD.
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MRI scan of typical discoid meniscus. Image courtesy of William Morrison, MD.
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Coronal MRI scan demonstrating complete discoid meniscus (arrow). Image courtesy of Robert D Bronstein, MD.
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Knee radiograph of 17-year-old athlete with discoid lateral meniscus. Lateral joint space is widened, and tibial plateau has flattened appearance. Image courtesy of Robert D Bronstein, MD.
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Arthroscopic appearance of complete discoid lateral meniscus. Probe is showing medial extent of lateral meniscus, which completely covers lateral tibial plateau. Image courtesy of Robert D Bronstein, MD.
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Arthroscopic photograph following saucerization of discoid lateral meniscus. Edge of horizontal tear that traversed meniscus can be observed. Image courtesy of Robert D Bronstein, MD.
