Unilateral arch hypertrophy with spinous process tilt: a sign of arch deficiency.

Unilateral arch hypertrophy (UAH) may be generated by a contralateral deficiency which is most commonly a pars defect (43 out of 50 patients studied). Such a defect can also cause rotational instability. When present, both can be easily detected on simple anteroposterior views through the resulting pedicular and pars asymmetry and spinous process tilt. From simple anteroposterior views, one can predict that at least one arch defect does exist, that it is unstable, and not of recent origin.