The use of prolonged halo stabilisation in a child is increasingly indicated for trauma and congenital instability of the cervical spine, but complications of pin fixation in this age group are frequent. We have analysed four aspects of the mechanics of the halo pin: the forces applied by each of six surgeons was shown to vary widely, penetration of the inner table occurred relatively easily, friction at the pin-halo interface influenced forces, and the skull thickness measured by CT scan varied from 1.1 mm to 4.3 mm in children under six years of age. We recommend CT scanning of the skull before elective halo application in young children to ascertain the safest pin sites.

The term "floating knee" describes the flail joint resulting from fractures of the shaft or adjacent metaphyseal region of the ipsilateral femur and tibia; we have reviewed 15 children with this combined injury seen at the Winnipeg Children's Hospital. All had been involved in motor vehicle accidents, eight having been struck while cycling; the biomechanics of such an injury have also been studied. Treatment is difficult and the regimes varied. Results were poor when both fractures were treated nonoperatively and we recommend that at least one of the fractures should be rigidly fixed.