Study of the stress-strain state of a femoral model with a revision stem after extended proximal osteotomy depending on the options for fixing the bone flap with a cerclage wire

Background. One of the problems in revision hip arthroplasty is the removal of the stable femoral component. The use of a cerclage wire for fixing the bone flap after osteotomy raises several unresolved issues: the choice of the optimal number of the osteotomized flap fixation levels, the number of cerclage wire turns, and the degree of postoperative load on the operated lower limb. The purpose was to conduct a comparative analysis of the stress values in the femoral model with a revision stem after extended proximal osteotomy (ETO) depending on the options for fixing the bone flap with a cerclage wire. Materials and methods. A basic finite element model of the femur with a revision endoprosthesis was created. It contained femur, revision endoprosthesis, and bone flap. Three options for fixing the bone flap at one, two, and three levels were modeled. Two turns of a cerclage wire with a diameter of 1.5 mm were applied at each level. The model was loaded with a distributed force of 1100 N. Results. When fixing the bone flap at one level, the maximum stresses in the femur are determined at the distal part of its diaphysis — 48.1 and 43.9 MPa, respectively, from the medial and lateral sides. The minimum stress level of 8.7 MPa was revealed in the bone flap, which is explained by the presence of interfragmentary regenerate between it and the femur, which performs a damping function. Stresses in the endoprosthesis tend to increase from 158.8 MPa in the neck to 213.4 MPa at the proximal end of the stem. The maximum stress level of 834.1 MPa is observed in a cerclage wire, which is 1.6 times higher than the tensile strength of surgical steel and can cause wire breakage. Fixation of the bone flap with a cerclage wire at two levels leads to a decrease in stress values at all control points on the femur. A decrease in stress values by 2 times was determined in a cerclage wire, which is associated with the redistribution of loads to two fixation points. The wire winding at the third level practically didn’t influence the stress distribution in the bone elements of the model. Conclusions. Fixation of the bone flap at two and three levels does not significantly affect stresses in the femur but allows you to halve the magnitude of stress in the wire itself, which with a single-level fixation can exceed the tensile strength of surgical steel. In the absence of technical possibilities to perform fixation at several levels, a single-level fixation can be applied under the conditions of using a wire winding of at least 4 turns and a load on the operated limb of no more than 50 %. The optimal option for fixing the bone flap is a two-level cerclage fixation, which provides a reduction in stresses in the wire below the tensile strength of surgical steel. Although a three-level fixation provides an even greater reduction in stresses in the wire to 326.2 MPa, the difference compared to a two-level fixation is not so significant as to justify additional tissue trauma and extending the operation time. Limiting the load on the operated limb to 50 % allows reducing the tension in a cerclage wire below critical values even with a single-level fixation, which can be recommended for the early postoperative period until a sufficiently strong interfragmentary regenerate is formed.