Yosef Karo

“ If the leaves grew individually from the beginning, such that there is no tyomet, or if all double leaves are on one side and the other side has no leaves, it is invalid. Rem"a: Some interpret this as meaning that if the central top leave on top of the spine is split across the spine, it is considered a split tyomet and is invalid, and this is our practice (Trumat Hadeshen 97) . However, ab initio it is the preferred way to do the mitzvah to get a lulav whose top leave is not split at all, because there are those who are stringent when even some of it is split. ”

“ Even if the top tyomet is not split such that the lulav would be invalidated because of the split tyomet.
סעיף ז
נסדק אם נתרחקו שני סדקיו זה מזה עד שיראו כשנים פסול: הגה ואפילו לא נחלקה התיומת העליונה בעניין שיפסל הלולב מכח נחלקה התיומת: 645:8 If it has thorny things on its spine, was smashed, shrunk, or bent forwards such that its spine was like the back of a hunchback, it is invalid. Similarly, if one side was bent, it is invalid. But if it is bent backward, it is valid, because this is how it grows. ”