Can a Meniscal Tear Cause Knee Instability?
A meniscal tear itself does not directly cause knee instability, but it can be both a consequence of underlying ligamentous instability (particularly ACL deficiency) and a contributor to subsequent joint damage when instability is present.
Understanding the Relationship Between Meniscal Tears and Instability
Primary vs. Secondary Pathology
Meniscal tears are typically a consequence rather than a cause of knee instability. The primary source of true knee instability is ligamentous injury, particularly to the ACL, which creates abnormal knee kinematics that can lead to secondary meniscal damage 1.
In unstable knees with ACL deficiency, the risk of subsequent meniscal tears is significantly elevated due to altered biomechanics and increased shear forces during pivoting activities 1.
The AAOS guidelines emphasize that instability episodes in ACL-deficient knees lead to meniscal and cartilage damage over time, not the reverse 1.
Meniscal Root Tears: A Special Consideration
Meniscal root tears represent an important exception where the meniscus itself can contribute to functional instability through a different mechanism 2.
Root tears cause meniscal extrusion and loss of circumferential hoop stress transmission, creating a biomechanical state similar to total meniscectomy with dramatically increased tibiofemoral contact pressures 2.
While this doesn't create true ligamentous instability, it produces significant functional impairment and accelerated joint degeneration 2.
Clinical Implications
In ACL-Deficient Knees
Healed meniscal repairs can survive in unstable knees, though the knee remains functionally unstable due to the underlying ligamentous injury 3.
A study following meniscal repairs in ACL-deficient knees for 6-8 years found only 11% rerupture rate despite persistent knee instability (mean +5mm increased laxity), demonstrating that the meniscus is not the source of instability 3.
The most critical period for meniscal repair survival is the immediate healing phase post-surgery, not long-term instability 3, 4.
In Osteoarthritic Knees
Degenerative meniscal tears in middle-aged or older patients often represent early-stage knee osteoarthritis rather than a primary pathology 1, 5.
These tears are frequently asymptomatic incidental findings that result from weakening of meniscal structure due to underlying OA, not a cause of instability 5.
The AAOS recommends against routine arthroscopic partial meniscectomy for degenerative tears in patients with knee OA, as the meniscal tear is a consequence of the degenerative process 1.
Diagnostic Approach
MRI is the gold standard for diagnosing meniscal tears with 96% sensitivity and 97% specificity at 3T, allowing precise characterization of tear patterns 6.
When evaluating a patient with a meniscal tear, always assess for concurrent ligamentous injuries, particularly ACL tears, as these are the true source of instability 1, 6.
MRI facilitates diagnosis of anterolateral ligament injuries and posterolateral corner injuries that frequently associate with ACL injuries and contribute to anterolateral knee instability 1.
Key Clinical Pitfall
The most common error is attributing knee instability to a meniscal tear when the underlying problem is ligamentous injury. If you find a meniscal tear in a patient complaining of instability, always evaluate for ACL, MCL, and posterolateral corner injuries, as these are the actual sources of instability 1. Treating only the meniscal tear while missing the ligamentous injury will result in continued instability and likely meniscal reinjury 1, 3.