A constraining prosthesis to reduce the possibility of a dislocation of the prosthesis after implantation is taught. A constraining liner is taught to include a constraining passage smaller than an internal diameter of the liner. A head can include a portion that is small enough to be positioned into the liner. The head can include a second dimension that is greater than the opening to constrain the modular head within the liner. A kit is also taught for trialing prosthesis to provide and determine a customized and personal fit of the prosthesis for each patient.