The basic meaning given by Jastrow is ‘knot’, but the meaning ‘joint’ clearly follows from some of the contexts; ḳṭr, pl. ḳyṭryn ‘joint (of fingers)’ [DJPA 488] (the first meaning is ‘knot’, also ‘node of a plant’).
PS
*ki/uṯ̣r- - joint (of the ribs, fingers, etc.)
(SED I No. 155)
Though reconstructed as an anatomic term on the PS level, the present root is likely related to, or rather derived from, Sem. *k(ʷ)ṯ̣r ‘to bind, knot, fix’