"Telescope fingers" occur in

Ahritis mutilans (A feature of psoriatic ahritis) This is a deforming erosive ahritis targeting the fingers and toes; it occurs in 5% of cases of PsA. Prominent cailage and bone destruction results in marked instability. The encasing skin appears invaginated and 'telescoped' ('main en lorgnette') and the finger can be pulled back to its original length. Ref Davidsons 23e p1033