The invention concerns an implantable mechanical device with adjustable geometry comprising, aligned and attached, an input components (1) having a first cylinder (11), an output component (2) having two second (21) and third (22) opposite cylinders, a reference component (3) integral in rotation with part of the organism and having a fourth cylinder (31). A friction spring (7) mounted astride on the first (11) and second cylinders (21) and a second friction spring (8), wound in an opposite direction of the spring (7) mounted astride on the third (22) and fourth (31) cylinders transform the reciprocating rotation applied to the input component (1) from outside the organism through means (931) into a rotation in a direction of the output component (2) which is helically connected (51, 52) to the driven component (4) integral with part of the organism by means (911), which is thereby translated. The device enables in particular manufacture of elongation nails and spiral rods.