An endobutton is a fixation device to secure the upper end of a hamstrings graft during the procedure of cruciate ligament reconstruction.
The endobutton at the top is an example of 'suspensory' cortical fixation, where the device is on the outside of the bone and suspends the ligament, which would be fixed at the lower end with perhaps an interference screw.
The endobutton can be manipulated by the 'strings' so that it can be turned longways to pull it through the tunnel, and then flipped sideways so that it cannot fall back into the tunnel once it is tensioned.
Comparative Study after Hamstring Anterior Cruciate Ligament Reconstruction with Endobutton and Rigidfix: A Clinical Trial Study. Mousavi H, Maleki A and Nobakht A. Adv Biomed Res. 2017; 6: 136.
A Rare Case of Intra-Articular Displacement of EndoButton Following Anterior Cruciate Ligament Reconstruction. Ho SWL and Lee LT. Clin Orthop Surg. 2017 Dec; 9(4): 534–536.