[Mechanical loading of the human femoral neck].
PURPOSE OF THE STUDY: The aim of the study is to explain what is the actual loading of human proximal femur. MATERIAL: Measuring of stress is based on 5 femurs of normal shape (bones obtained from dissection and ossuary). In order to determine the direction of the resultant force the authors used 30 ossuary bones of different age but of analogical shape. METHODS: By means of their own graphic method the authors identified the direction of appositional growth of femoral neck which contains with the longitudinal diaphyseal axis the angle of 26 degrees (alpha angle). The value of the angle is identical with the direction of the resultant force acting on the femoral head in vivo. Tensometric method was used for the measuring of stress on the medial and lateral surfaces of femoral neck during loading in direction of the determined resultant force. Linear strain gauges of Hottinger type were used for the measuring and Instron apparatus was applied for biomechanical analysis in which femoral heads were loaded by a standard force of 50 kg, first in the direction of femoral diaphyseal axis and then gradually (always by 5 degrees) in the divergence from the diaphyseal axis up to 50 degrees. RESULTS: Appositional growth of femoral neck with regard to the longitudinal diaphyseal axis is progressing under the alpha angle (alpha = 26 degrees), which corresponds to the direction of the resultant force. The value of the compression stress on the medial surface of the neck ranged between--165 and -300 microstrain (mean value -239 microstrain). On the lateral surface there is a tension stress ranging between +20 up to +140 microstrain (main value +64 microstrain). The ratio of stress on both sides (surfaces) was 3.8:1. The point of zero stress is located in the lateral part of the neck, i.e. in the area of lateral trajectorial system of cancellous bone. DISCUSSION: The method of measuring of the direction of the resultant force on the basis of the bone development (on the basis of superposition of successive growth stages) is despite certain inaccuracies a correct procedure. The alpha angle (alpha = 26 degrees) identified by the authors corresponds with Pauwels' calculation (24 degrees) and Bergman's findings (22 degrees up to 27 degrees). The identified values of stress show that the opinion on a purely axial loading of the neck is wrong and has to be corrected. Femur is not exposed in the frontal plane to the action of the resultant force only in one direction as supposed e.g. by Pauwels but the direction of the resultant force oscillates within a limited extent from different directions. It means that the lateral wall of femoral neck is exposed to alternating both tensile and compression loading. CONCLUSION: The presented work allows to specify the opinions on the way of loading of human proximal femur. Our experiments have shown that apart from axial pressure the femoral neck is exposed also to bending forces. This way of loading also corresponds with the oval shape and thickened medial wall of the neck.