Discoid lateral meniscus instability in children: part I. A new grading system of instability to clarify natural history

ObjectiveIn young patients, the discoid lateral meniscus (DLM) usually becomes symptomatic at the time of peripheral rim instability of a complete discoid form. However, little is known about the natural history of meniscal instability. The aim was to detect hidden forms of presentation of instability using the history and clinical mechanical symptoms of instability.MethodsRetrospective study of 114 paediatric knees treated for DLM. A new clinical grading system for instability (stable, locked, snapping, blocked and unblocked knees) was defined and studied in combination with MRI and arthroscopic findings.ResultsOne hundred seven DLMs (94%) showed instability due to edge detachment. Fifteen (13%) knees were stable (grade 0), 2 (2%) were grade 1, 57 (50%) were grade 2, 35 (31%) were grade 3 and 5 (4%) were grade 4. The cause of grade 3 was an extension deficit (26/35) or flexion deficit (9/35). Grade 1 or 2 correlated with limited detachment, unlike grade 3 or 4. The grade of instability never decreased, but rather increased in many knees during the natural history of DLM.ConclusionA new presentation of DLM is presented: episodic locking phenomenon and blocked knee with flexion deficit. Clinicians should be wary of pseudo-improvement with the recovery of mobility after a period of the locked knee, which may be due to a progression towards the ultimate degree of instability. Clinical grading of instability has clarified the natural history of DLM-associated tearing.