Abstract: A tension spring capable of large elastic deformation and having a high resistance to breakage and particularly, but not exclusively, for use as an artificial prosthetic ligament. The spring comprises an elongate elastic core having a low modulus of elasticity and defining opposed ends. A tensionable wrapping of thread having a high resistance to breakage is disposed in contact about the core to compress the core and to cause it to elongate axially upon opposite relative axial displacement of opposed ends of the wrapping secured to traction means.