Middle rings (and tensioned wires) act to hold the bone fragments in place and to give greater structural support to the apparatus and limb. However, the critical load bearing rings are the top and bottom rings which transfer the force from the healthy bone down to the healthy bone, bypassing the fracture site. In addition to being used to support a fractured limb, the Ilizarov frame is also commonly used to correct deformity through distraction osteogenesis. The procedure consists of an initial surgery, during which the bone is surgically fractured and the ring apparatus is attached. As the patient recovers, the fractured bone begins to grow together. While the bone is growing, the frame is adjusted by means of turning the nuts, thus increasing the space between two rings. As the rings are connected to opposite sides of the fracture, this adjustment, done four times a day, moves the now-healing fracture apart by approximately one millimeter per day. The incremental daily increases result in a considerable lengthening of the limb over time. Once the lengthening phase is complete, the apparatus stays on the limb for a consolidation period. The patient is able to fully weight bear on the Ilizarov frame, using crutches initially and pain is lessened. Once healing is complete, a second surgery is necessary to remove the ring apparatus. The result is a limb that is significantly longer. Additional surgery may be necessary, in the case of leg lengthening, to lengthen the Achilles tendon to accommodate the longer bone length. The major advantage of this procedure is that because the apparatus provides complete support while the bone is recovering the patient can remain active aiding recovery. A further use is in bone transport, whereby a defect in a long bone can be treated by transporting a segment of bone, whilst simultaneously lengthening regenerating bone to reduce the defect and finally dock with the other segment, producing a single bony unit. While the Ilizarov apparatus is minimally invasive (no large incisions are made), it is not free of complications. Pain is common and can be severe, but is treatable with analgesics. Careful attention to cleaning and hygiene is necessary to prevent pin site infection. Other complications include swelling, muscle transfixion, and joint contractures. Physical therapy is often indicated. The Ilizarov method is widely used to treat complex and/or open bone fractures. This method is preferred over conventional treatment options (such as internal fixator or cast) where there is a high risk of infection or the fracture is of such severity that internal fixators are unworkable.