M23.5Non-Billable (Header)Chronic instability of kneeChild CodesM23.50Chronic instability of knee, unspecified kneeM23.51Chronic instability of knee, right kneeM23.52Chronic instability of knee, left kneeCode Hierarchy└M23Internal derangement of knee└M23.5Chronic instability of knee└M23.5Chronic instability of knee