The TFI-musculofasciocutaneous flap has become a standard procedure in recent years. Donor-site problems of this method, especially the biomechanical changes of the hip, the proximal femur and the knee joint are evaluated, employing biomechanical techniques. Harvesting of a TFL-flap leads to functional loss of the M. tensor fasciae latae and the iliotibial tract and band. This might lead to a weakening of active and passive stabilising structures of the hip and the knee joint. Loss of the iliotibial tract causes a loss of the lateral tension-band of the proximal femur and leads to a significant increase of shear forces on the proximal femoral shaft. The functionally important structures of the iliotibial tract for knee biomechanics are not influenced by harvesting of the TFL flap. Loss of the M. tensor fasciae latae leads to a mild reduction of hip flexion and internal rotation. The indication for a TFL-flap should consider the patient's activity to prevent overload damage of the proximal femoral shaft. In childhood, harvesting of the TFL flap is likely to influence the growing axial skeleton due to the loss of the iliotibial tract.