Causes of involutional ectropion and entropion--age-related tarsal changes are the key.

PURPOSE: To measure tarsal plates across various age-groups, to determine whether tarsal size changes with increasing age and whether size correlates with involutional ectropion and entropion.
METHODS: Comparative, observational, case-control study design. Data were obtained for length and height of tarsus in each of the four eyelids. The data were constructed to determine: (I) right-to-left-side comparison data, (II) sex difference data, (III) age normal data, (IV) involutional entropion data, (V) involutional ectropion data.
RESULTS: (I) There is no difference in tarsal dimensions between right and left sides; (II) males have larger tarsal dimensions than females; (III) tarsal plates are on average smaller in older age ranges; (IV) patients with entropion have smaller than average age-normal tarsal plates; (V) patients with ectropion have larger than average age-normal tarsal plates.
CONCLUSIONS: (I) Right and left tarsal plates have equal dimensions, and involutional changes likely occur on both right and left sides equally frequently; (II) males have larger tarsal plates than females and entropion is more frequent in females and ectropion in males; (III) tarsal plates may have a general tendency to atrophy or shrink with age; this may explain why some eyelids develop entropion and others ectropion; (IV) entropion results from the mechanical effect of an atrophied or smaller than age-normal, partially or fully disinserted, tarsal plate being overcome by the normal or increased tone of the preseptal/pretarsal orbicularis muscle; (V) ectropion results from an age-normal or larger than normal tarsal plate mechanically overcoming the normal or decreased tone of the preseptal/pretarsal orbicularis muscle in combination with medial/lateral canthal tendon laxity.