Tibiofemoral instability is usually due to an acute or chronic injury of the ACL, PCL, medial knee structures or posterolateral complex of the knee. True tibiofemoral joint instability results in the knee subluxing, sliding front to back or having a side-to-side instability, with activities. These activities can include twisting, turning, pivoting or other activities.

It is important to determine true instability from those episodes of giving way. When a patient describes they are ambulating and have a sudden giving way of the knee, this is most commonly due to quadriceps inhabitation and patellofemoral dysfunction rather than true knee instability.

Most knee ligament instabilities are graded based on the amount of tearing. In general, most grade I (partial) or grade II (partial tears but not complete with an endpoint) can be initially treated without surgeries but most grade III (complete tears) of a knee ligament (other than an isolated grade III MCL tear) usually need a further workup with an MRI, stress radiographs or other means to determine if surgery is necessary.

Patients with an ACL tear usually describe instability in the knee with twisting, turning, or pivoting activities. In general, patients with an ACL tear do not have trouble walking or stair use. Patients with a PCL tear usually describe problems with deceleration, going down stairs, or going down inclines. Patients with a posterolateral corner injury usually describe a side-to-side instability with normal walking while patients with a medial knee structure injury also describe side-to-side instability and may have difficulty with planting and pivoting.