Ankylosis is one of the most debilitating diseases of the temporomandibular joint, in which the stunted mandibular growth may affect the oropharyngeal architecture causing moderate to severe airway compromise (obstructive sleep apnoea, OSA) with resulting cardiopulmonary sequelae.
Severe mandibular hypoplasia requires large advancements, including multiplanar movements. This is relatively straightforward in cases of bilateral ankylosis, since the problem is symmetric. However, in cases of unilateral ankylosis, the surgeon confronts a situation in which the amount of distraction on the affected side is limited by the torquing tolerated by the unaffected condyle.
In patients with OSA secondary to a hypoplastic mandible, the amount of advancement needed may be as large as 2–2.5 cm, as seen in our patients. Conventional unilateral mandibular distraction in these cases causes some amount of rotation of the balancing condyle (contralateral side), as this ramus–condyle unit on the side contralateral to the distraction acts as the centre of rotation. Minimal amounts of rotational force on the balancing condyle during unilateral distraction will lead to adaptive changes in the balancing condyle. As the magnitude of distraction increases, the amount of rotational force at the balancing condyle will also increase, leading to a considerable amount of torquing of the balancing condyle, as shown in Fig. 1 .
Extreme amounts of torque and loading of the mandibular condyle has been reported to cause complications including condylar resorption, arthritis, and ankylosis of the joint. Animal studies on excessive loading of the TMJ condyle during distraction have shown destruction of the cartilage of the articular surface, which can lead to arthritic changes or idiopathic resorption of the condyle. It is therefore important to transpose the point of rotation from the balancing joint to the mandibular body on the balancing side (side contralateral to distraction) in order to preserve the condylar architecture on the balancing side.
In consideration of these factors, we advocate the use of bilateral monofocal distraction even for the correction of facial asymmetry in unilateral TMJ ankylosis. In such a case, active advancement is achieved on the side where the corpus length is deficient; the contralateral corpus, in which the osteotomy is performed distal to the dentoalveolar segment, acts as the centre of rotation ( Fig. 2 ). Activation of this balancing side is performed in cases where mandibular corpus lengthening of the normal side may be needed to achieve functional airway correction (with due consideration of the facial profile). Once the required advancement of the balancing side is achieved, dancing distraction is performed on the balancing side wherein the normal side is activated in the morning and deactivated in the evening. This will keep the balancing side in a state of callus, preventing its premature ossification until the contralateral advancement is complete. The distraction regenerate on both sides is then allowed to consolidate after the activation phase for a duration of 6 weeks.