Comments: This article provides penetrating insight into the importance of considering foot mechanics when treating knee injuries. Many knee problems can be precipitated or aggravated by foot-related problems such as collapsed or overly high arches. Also very problematic can be outwards-tilted feet (e.g. ankle sticks outwards, hence varus angulation of the ankle) or inwards-tilted feet (i.e. ankle protrudes inwards, hence valgus angulation). Custom-made foot orthotics can work wonders here, but they must be designed and fitted by an experienced orthotist. Keep in mind that rigid orthotics can cause problems with the foot's natural shock-absorption capabilities, which depend partly on flexion of the arches. Rigid orthotics can also impede the shock-absorption capabilities of athletic footwear, and so flexible orthotics are generally preferable. Foot problems can have major implications in the context of knee-ligament injuries, in particular those involving the ACL. In some cases, foot orthotics can be used to help treat the secondary problems that arise as a consequence of chronic ACL deficiency. Or, orthotics can be used to address some preexisting underlying biomechanical issues which, together with newfound ACLlessness, translate into daily-living functional impairments. For people with ACL tears who are contemplating not having surgery, it should be kept in mind that even the best foot mechanics and the strongest leg muscles, albeit helpful, cannot entirely compensate for the unique functions of the knee ligaments. For example, the ACL is responsible for controlling inwards tibial twisting, and helps the MCL control inwards forcing of the knee; also, the ACL limits forwards tibial sliding (also known as anterior drawer). And, the ACL works in concert with the PCL to limit extension of the knee. (Like the ACL, the PCL fulfills multiple roles too; e.g. control of outwards twisting and rearwards tibial sliding.) A knee that is unstable due to ACL deficiency or PCL deficiency can often be helped through the use of orthotics (along with other measures such as muscle strengthening and application of functional bracing). However, especially for patients who wish to return to knee-demanding activities, there are practical limits to how much can be accomplished without pursuing ligament-reconstructive surgery.