The Ilizarov apparatus is a type of external fixation apparatus used in orthopedic surgery to lengthen or to reshape the damaged bones of an arm or a leg; used as a limb-sparing technique for treating complex fractures and open bone fractures; and used to treat an infected non-union of bones, which cannot be surgically resolved. The Ilizarov apparatus corrects angular deformity in a leg, corrects differences in the lengths of the legs of the patient, and resolves osteopathic non-unions; further developments of the Ilizarov apparatus progressed to the development of the Taylor Spatial Frame.

Gavriil Abramovich Ilizarov developed the Ilizarov apparatus as a limb-sparing surgical remedy for the treatment of the osteopathic non-unions of patients with unhealed broken limbs. Consequent to a patient lengthening, rather than shortening, the adjustable-rod frame of his external-fixation apparatus, Ilizarov observed the formation of a fibrocartilage callus at and around the site of the bone fracture, and so discovered the phenomenon of distraction osteogenesis, the regeneration of bone and soft tissues that culminates in the creation of new bone.

In 1987, the Ilizarov apparatus and Ilizarov's surgical techniques for repairing the broken bones of damaged limbs were introduced to U.S. medicine. The mechanical functions of the Ilizarov apparatus derive from the mechanics of the shaft bow harness for a horse.

The Ilizarov apparatus is a specialized external fixator of modular construction, composed of rings (stainless steel, titanium) that are transfixed to healthy bone with Kirschner wires and pins of heavy-gauge stainless steel, and immobilised in place with additional rings and threaded rods that are attached with and through adjustable nuts. The circular construction of the apparatus, the rods, and the controlled tautness of the Kirschner wires immobilises the damaged limb to allow healing.

The mechanical functions of the Ilizarov apparatus are based upon the principles of tension (pulling force), wherein the controlled application of mechanical tension to the damaged limb immobilises the broken bones, and so facilitates the biological process of distraction osteogenesis (the regeneration of bone and soft tissue) in a reliable and reproducible manner. Moreover, external fixation with the apparatus allows the damaged limb to bear weight early in the medical treatment.

Once emplaced onto the limb, the top rings of the apparatus transfer mechanical force to the bottom ring through the rods, and so by-pass the site of the fractured bone, thus the Ilizarov apparatus immobilizes the damaged limb and relieves mechanical stresses from the wound, which then allows the patient to move the entire limb. The middle rings stiffen the support rods and hold the bone fragments in place, whilst supporting the immobilised limb. In by-passing the site of the bone fracture, the top and bottom rings bear the critical load by transferring mechanical force from the area of healthy bone above the fracture to the area of healthy bone below the fracture.

The Ilizarov surgical method of distraction osteogenesis (regeneration of bone and soft tissues) for repairing complex fractures of the bones of the limbs is the preferred treatment for cases featuring a high risk of bacterial infection; and for cases wherein the extent and severity of the fracture precludes using internal fixators to immobilise the damaged bone for proper repair.

In 1968, Ilizarov successfully treated the non-union osteopathy of Valeriy Brumel, a Soviet Olympic gold medalist in the high jump who had broken his right ankle, tibia, and fibula in a 1965 motorcycle accident. After more than twenty failed surgeries, not only had his bones not healed but the injured leg remained shorter than before the breaks. Employing distraction osteogenesis enabled by an external-fixation apparatus, Brumel's osteopathic non-union grew new leg bone, extending it 3.5 cm (1.4 in) to its normal length.