A strut is a structural piece designed to resist pressure in the direction of its length. On typical "MacPherson Strut" use, the shock absorber is built into the strut. Most shock absorber struts are hydraulic units. Some MacPherson systems used on Ford vehicles are equipped with low-pressure, gas-filled shock struts. They are nonadjustable and nonrefillable. Like the hydraulic shock struts, faulty units must be replaced as an assembly. Another similar front suspension system is called the "hydraulic shock strut." This strut serves as a shock absorber and replaces the upper control arm. The coil spring, however, is located between the lower control arm and the body structure instead of being mounted directly on the strut.