**3. Biomechanical basis of the BDSF-method**

This method's innovation is laying of the three screws in two planes, which allows for the entry points of two of the implants to be placed much more distally, in the solid cortex of the proximal diaphysis, and also to lean onto the femoral neck distal cortex. Thus we establish two points of support. The solid distal cortex of the femoral neck acts as a medial supporting point for the screws, which works under pressure - *supporting point A*. The entry points of two of the screws (the distal and the middle one) in the thick cortex of the diaphysis, ensure a second solid supporting point for the screws – a lateral one, which works under tension (or pressure in proximal direction) - *supporting point B*. The position of the distal screw as well as of the middle screw, which are achieved by the method, in terms of the *statics*, turns them into *a simple beam with an overhanging end, loaded by a vertical force.* This beam with an overhanging end successfully supports the head fragment, bearing the body weight and transferring it to the diaphysis. Furthermore, due to the biplane placement, enough space for a third screw is provided, unlike the classic authors' models, where just one or maximum two implants are placed at an obtuse angle [7,11]. Another advantage of the method is that due to the increase in the distance between the two supporting points, the weight borne by the bone is significantly reduced *(see the static analysis)*. An advantage of the BDSF-method is that the entry points of the screws are positioned wide apart from each other, which ensures that upon weight bearing, the tensile forces spread over a greater surface of the lateral cortex and thus the risk of its fracturing decreases significantly. Another advantage with the BDSF is that the screw, placed at a highly increased angle, works in a direction close to the direction of the loading force, which guarantees better results for the screw in its role as a beam because the influence of its sagging decreases.

Biomechanics of the Fractured Femoral Neck –

*<sup>a</sup>* ;

The New BDSF-Method of Positioning the Implant as a Simple Beam with an Overhanging End 87

accuracy, with the BDSF-method, the static model is considered to be *a simple beam with an* 

**Figure 3.** Static model of the new BDSF-method of fixation – the implant acts like a simple beam with

Applying the well-known equilibrium equations for a beam, we obtain the forces acting on

an overhanging end. *F = load; L = length of beam; a = distance between points A and B* 

the smaller the weight at each of the two supporting points.

The load acting at point **A** is pressure in a distal direction and it equals to *FL <sup>A</sup>*

The load acting at point **B** is pressure in a proximal direction and it equals to *B* **=** *A – F*.

At the BDSF-method, due to the increase in the distance between the two supporting points, the weight borne by the bone is reduced. If we look at two cases of equal vertical weight but different distances between the supporting points, we will see that the greater the distance,

The average anatomical distance from the tip of the screw to the distal femoral neck cortex

With *conventional methods* **(case 1.)** the average distance from **point A** to the entry point of the screws in the lateral cortex (**point B**) is 5.5 cm (a = 5.5 cm). In order to make a comparison with the BDSF, when body weight of 100 kg is given, with conventional methods the load acting on the curve of the femoral neck distal cortex (if the screws lean on

the cortex at supporting points **A** and **B**.

curve (**point A**) is 3.5 cm (Fig. 4.).

this support at all) is estimated as

*overhanging end* (Fig.3). This beam is supported at points **A** and **B** only.
