Modeling the work of pelvic girdle muscles after hip replacement in different size of global femoral offset
Background. Diseases of the hip joint are often accompanied by structural muscular changes, which are one of the reasons that make it difficult to select the components of the endoprosthesis, since it is impossible to take into account the elasticity of the muscle before the surgery. This leads to the fact that the global femoral offset, and it is determine the length of the abductor shoulder after endoprosthetics, does not always correspond to the length of the arm of force of the abductors from the opposite side, and often is less. This leads to disruption of the abduction mechanism and worsens the results of arthroplasty. The purpose of this study is to analyze the work of the muscles responsible for maintaining the postural balance at a different value of the global femoral offset. Materials and methods. A horizontal equilibrium model of the pelvis was used for the calculations. The model is constructed from the calculation of the patient’s weight of 70 kg. The absolute values of the forces developed by each muscle are used, the angles of attachment of muscles and the magnitude of the the arm of force of each muscle are taken into account. The calculations are based on the principle of equality of forces and moments of the forces of abductor muscles and moment of gravity, which provide a horizontal balance of the pelvis. Simulation implied an analysis of the forces that should develop the muscles to maintain horizontal equilibrium of the pelvis under different conditions: a decrease in the global femoral offset, an increase in the weight of the patient, a decrease in the absolute strength of the muscles. Results. As a result of the simulation, it was found that a decrease in the global femoral offset after arthroplasty to 5 mm does not have a critical effect on the efficiency of the abductor mechanism, but the more significant difference between the length of the arm of abductors’ force of the healthy femur and the value of the global femoral offset after arthroplasty significantly worsens the working conditions of the muscles. A very important factor negatively affecting the preservation of the postural balance after arthroplasty is the functional insufficiency of abductor muscles. Excess body weight of the patient is a factor that complicates the work of the abductor mechanism. And in conditions of functional deficiency of muscles, this factor has a critical effect on the process of maintaining the postural balance. Conclusions. The most significant factors negatively affecting the functioning of the abduction mechanism after hip arthroplasty are a decrease in the global femoral offset by more than 5 mm, an excess body weight of the patient, a preoperative decrease in the muscle strength responsible for maintaining the postural balance.
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