Background: Quantitative features of pars plana development in humans are not well characterized. Knowledge of the dimensions of this region is important for our understanding of the anatomy of infant eyes for the purposes of surgery. The purpose of this study was to provide a morphometric description of pars plana growth in relation to 1) postconceptional age and 2) axial length. Methods: We reviewed histologic specimens from 204 human eyes of postconceptional ages ranging from 10 weeks to 5 years. Axial length and temporal pars plana width were measured directly with a reticule system. The relationships among pars plana width, axial length, and postconceptional age were examined. Results: The most rapid phase of pars plana growth occurred between 26 weeks and 35 weeks gestation. A linear relationship between pars plana width and axial length existed once the axial length reached 12 mm (correlation coefficient = 0.918; P = 0.0001). In infants of 38-42 weeks postconceptional age, mean pars plana width was 1.87 mm (range, 0.9-2.8 mm; standard direction 0.48 mm), and mean axial length was 17.7 mm (range, 15-23 mm; standard direction 1.9 mm). After the age of 62 weeks postconception, all eyes had a temporal pars plana width ≤ 3 mm and an axial length ≤ 19 mm. The estimated age at which there was a 95% chance of the pars plana width being ≤ 3 mm was 64.4 weeks (95% fiducial confidence limits, 60.8-86.2 weeks). Conclusions: The pars plana first develops during the second trimester of gestation. A rapid growth phase occurs between 26 weeks and 35 weeks postconception. The dimensions of the pars plana are correlated closely with axial length and postconceptional age. Assuming that a pars plana width of 3 mm is required for surgery to be performed via a pars plana approach, we estimate that pars plana vitreous surgery can be performed when the patient is at least 62 weeks old (postconception), which is a 6- month-old full-term infant.
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