The freedom to heal: nonrigid immobilization by a halo orthosis.

Halo orthoses present a paradox. On the one hand, the nominally rigid immobilization they provide to the head aims to remove loads on the cervical spine following injury or surgery, and the devices are retightened routinely to maintain this. On the other hand, bone growth and remodeling are well known to require mechanical stressing. How are these competing needs balanced? To understand this trade-off in an effective, commercial halo orthosis, the authors quantified the response of a commercial halo orthosis to physiological loading levels, applied symmetrically about the sagittal plane. They showed for the first time that after a few cycles of loading analogous to a few steps taken by a patient, the support presented by a standard commercial halo orthosis becomes nonlinear. When analyzed through straightforward structural modeling, these data revealed that the nonlinearity permits mild head motion while severely restricting larger motion. These observations are useful because they open the possibility that halo orthosis installation could be optimized to transfer mild spinal loads that support healing while blocking pathological loads.