Background: Spring-assisted cranioplasty has been demonstrated to correct hypotelorism associated with metopic synostosis. In addition, the fronto-orbital axis rotates toward a more normal orientation. We postulated that spring-induced shear forces and subsequent displacement across the frontobasal sutures are the primary initial mechanisms for change in cranio-orbital morphology.
Methods: Sixteen consecutive patients (32 orbits) with trigonocephaly operated on between 1999 and 2004 were studied retrospectively. After frontal remodeling, a spring was placed across the released metopic suture. Preoperative and 6-week postoperative cephalograms were used to measure the relative translation of the medial orbital wall from the midline at fixed vertical distances above and below the frontoethmoidal suture (FES). The vertical height of the spring above the frontonasal suture and the patient's age were analyzed with respect to the increase in bony interorbital distance (BIOD).
Results: There were significant increases in movement of the medial orbital wall above the FES at each measurement point (P < 0.001). There was no significant postoperative movement of the medial orbital wall below the FES at any measurement point. The relative movements above the suture suggest a degree of plastic deformation adjacent to the suture.There was no significant correlation between the age of patient or the height of the spring above the frontonasal suture and the increase in BIOD.
Conclusion: Tensile spring forces are transmitted directly to the frontoethmoid suture. Most initial increases in BIOD and altered intraorbital morphology are due to distraction of cranial base sutures rather than plastic deformation.