The direct effect of intraorbital pressure on orbital growth in the anophthalmic piglet

Tissue expanders placed within the orbit can have a positive effect on orbi tal and ipsilateral midfacial growth. To date, there is no precise method f or controlling and monitoring expansion to induce normal growth in the deve loping facial skeleton. The present study was undertaken to determine the o ptimal physiologic pressure required to stimulate normal orbital growth and to determine whether above-normal growth could be achieved with higher int raorbital pressures. Using a neonatal swine model, an accurate method of mo nitoring intraorbital pressure, precisely controlling intraorbital expansio n, and achieving normal orbital growth was explored. Sixteen male, 3-week-old Yorkshire piglets were randomly divided into three surgical groups. In each group, the left orbit was the experimental side, and the contralateral right orbit served as an untreated control. Group 1 ( n = 6) underwent enucleation only. Group 2 (n = 5) underwent enucleation an d orbital expansion at a near-normal physiologic pressure of 20 mmHg. Group 3 (n = 5) underwent enucleation and orbital expansion at a supernormal pre ssure of 60 mmHg. Spherical tissue expanders (10 cc) with a separate inject ion port were utilized as the orbital expanders. Pressure was monitored by an electronic manometer that was calibrated daily. Morphology of the orbits was documented by photography, the dimensions of the orbits were quantitat ed by three-dimensional mechanical digitization, and orbital volumes were c alculated. In the unexpanded, anophthalmic control group, a significant reduction in r adial growth after evisceration was seen. In group 2, the orbit stimulated with a consistent pressure of 20 mmHg, just above the physiologic normal pr essure of 17 mmHg, showed an increase in radial dimension of 8 percent comp ared with the unoperated side. In the high-pressure group of 60 mmHg, an in crease of 16 percent in the radius was observed over the Ii-week period. Th is led to a corresponding increase in orbital volumes with increased pressu re. Utilizing a paired t test, these differences in the radial and volumetr ic growth of the orbit were statistically significant (p < 0.005). The resu lts obtained demonstrated a direct relationship between intraorbital pressu re and the growth of the bony orbit in the radial dimension. On the basis o f this study, we concluded that orbital expansion maintained at normal phys iologic pressure can stimulate normal orbital growth in the neonatal facial skeleton. In addition, application of above-normal pressures for expansion can induce accelerated orbital growth.