skinparam linetype ortho
class Forum
class Topic
Forum *--|> Topic
There is 2 problems:
1) With this simple sample, the direction is inverted with skinparam linetype ortho
2) Missing arrow part with or without skinparam linetype ortho

http://www.plantuml.com/plantuml/png/SoWkIImgAStDuIhEpimhI2nAp5N8oSnBBQaiI5N8Boh9oETApaaiBbPmooygpOMoG_8BCfEvm28AMhgwDNPG4OVKl1IWKG00