**4. Static analysis**

With the *conventional methods* of femoral neck fixation - by three cancellous screws, placed parallel to each other and parallel to the femoral neck axis, the entry points of the three screws are placed at the thin, fragile cortex of the greater trochanter or close to it. The screws are often located near the axis of the femoral neck in the soft cancellous bone, without any cortical support. According to my previous investigations [12], with *conventional methods*, due to the lack of two solid supporting points, the implant acts statically like a *beam on an elastic foundation.* The elastic foundation is implemented by the soft cancellous bone (Fig.2).

**Figure 2.** Static model of the conventional methods of fixation – the implant acts statically like a beam on an elastic foundation. *F = load;*

In contrast to the conventional methods, when the *Biplane double-supported screw fixation method* is applied, the implant is additionally supported at points **A** and **B** of the cortex. The interaction between the implant and the cancellous bone is neglected, because of the comparatively small stiffness of the cancellous bone. In this way, with enough practical accuracy, with the BDSF-method, the static model is considered to be *a simple beam with an overhanging end* (Fig.3). This beam is supported at points **A** and **B** only.

86 Injury and Skeletal Biomechanics

**4. Static analysis** 

on an elastic foundation. *F = load;*

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.

With the *conventional methods* of femoral neck fixation - by three cancellous screws, placed parallel to each other and parallel to the femoral neck axis, the entry points of the three screws are placed at the thin, fragile cortex of the greater trochanter or close to it. The screws are often located near the axis of the femoral neck in the soft cancellous bone, without any cortical support. According to my previous investigations [12], with *conventional methods*, due to the lack of two solid supporting points, the implant acts statically like a *beam on an elastic foundation.* The elastic foundation is implemented by the soft cancellous bone (Fig.2).

**Figure 2.** Static model of the conventional methods of fixation – the implant acts statically like a beam

In contrast to the conventional methods, when the *Biplane double-supported screw fixation method* is applied, the implant is additionally supported at points **A** and **B** of the cortex. The interaction between the implant and the cancellous bone is neglected, because of the comparatively small stiffness of the cancellous bone. In this way, with enough practical

**Figure 3.** Static model of the new BDSF-method of fixation – the implant acts like a simple beam with an overhanging end. *F = load; L = length of beam; a = distance between points A and B* 

Applying the well-known equilibrium equations for a beam, we obtain the forces acting on the cortex at supporting points **A** and **B**.

The load acting at point **A** is pressure in a distal direction and it equals to *FL <sup>A</sup> <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 smaller the weight at each of the two supporting points.

The average anatomical distance from the tip of the screw to the distal femoral neck cortex curve (**point A**) is 3.5 cm (Fig. 4.).

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 this support at all) is estimated as

### 88 Injury and Skeletal Biomechanics

**A** equal to 1.63 kN (163.63 kg). The load on the fragile lateral cortex (**point B**) is estimated as **B** equal to 0.63 kN (63.63kg), directed in the opposite direction (proximally).

Biomechanics of the Fractured Femoral Neck –

The New BDSF-Method of Positioning the Implant as a Simple Beam with an Overhanging End 89

lateral cortex over a wide surface and decrease of the fracture risk, contrary to the conventional methods, with which the entry points of the screws are at a distance less than 1

The BDSF-method was introduced in 2007 and it was applied by different surgeons since than. From a series of 178 patients, who underwent surgical treatment, 88 were studied [12]. Inclusion criteria was having late control x-rays and examinations after discharge with questionnaires filled-in. Out of the 88 studied patients, 27(30.68%) were male and 61(69.31%) - female patients; the average age was 76.9 (with the youngest patient at the age of 38 and the eldest at the age of 99). Grouping patients by age: 18 patients (20.45%) were at the age of under 69; 27 patients (30.68%) were at the age of 70 to 79; 37 patients (42.04%) were at the age of 80 to 89; 5 patients (5.68%) were at the age of 90 to 95 ; 1 patient (1.13%) was aged 95 to 100. More than one concomitant disease, which influences the results of Harris Hip Score,

Garden type I: 3 (3.41%); Garden type IІ: 1 (1.14%); Garden type IІІ: 9 (10.23%); Garden type

*Results.* From the studied 88 patients fracture union was registered in 87 patients (98.86%)

*Assessment* according to the Harris Hip Score (modified): Poor results – in 10 patients (11.36%). Fair results – in 20 patients (22.72%). Good results – in 21 patients (23.86%).

Difficult for management are the unstable fractures and the fractures with vertical fracture

*Unstable fractures*. In the elderly patients, at the age above 80, the preoperative reduction is usually achieved easily because of the fact that the fracture occurs upon low-energy trauma and although it seems displaced at a diagnostic X-rays (Garden III and IV), the fracture is usually stable and in the process of reduction there is a good control over the head

In younger and active patients the fracture usually occurs with more severe traumatic influence, for example falling over slippery surface, falling from a greater height (from stairway or in road accidents). In these cases more severely expressed tearing of soft tissues around the fracture occurs frequently and the fracture is severely displaced. Following the

cm from each other and the forces of tension are concentrated over a small surface.

was found in 21 patients (23.86%). The average follow-up period is 8.06 months.

The Garden classification was used for classifying of the fractures as follows:

**5. Clinical results of the BDSF-method** 

IV: 75 (85.02%).

and failure in 1 patient (1.13%).

Excellent results – in 37 patients (42.04%).

**6. Unusual and difficult cases** 

line Pauwels type III.

fragment.

*The average* Harris Hip Score is 84.26 points [13].

With *the BDSF method* **(case 2.)**, with increasing the angle of the implant towards the diaphysis, the distance between points **A** and **B** increases by 4 cm to reach up to 9.5 cm (a = 9.5 cm). That is why, the load on the cortex decreases significantly. Given the same body weight of 100 kg, the load acting on the medial supporting point is estimated as **A** equal to 1.36 kN (136.84 kg) or with *16.38% less than conventional methods*, and on the lateral supporting point the load is estimated as **B** equal to 0.36 kN (36.84 kg) or with *42.11% less than conventional methods.* The distal screw normally applied with the BDSF method has a length of 13 cm.

The lateral cortex stress state around point **B** is complex. It is subjected to compressive stress in a proximal direction, and to horizontal tensile stress as well. In the lower part of the cortex the stress is mainly tensile.

**Figure 4.** Fixation of the femoral neck: a. Conventional method; b. The BDSF-method. [12]

These forces of tension are responsible for the occurrence of subtrochanteric fracture as a complication of the screw fixation. As it was mentioned, these forces of tension are decreased by 42% with the BDSF-method, compared to the conventional methods of fixation. Besides, with the BDSF-method the entry points of the screws are located wide apart from each other (from 2 to 4 cm), which leads to dispersion of the tension stress on the lateral cortex over a wide surface and decrease of the fracture risk, contrary to the conventional methods, with which the entry points of the screws are at a distance less than 1 cm from each other and the forces of tension are concentrated over a small surface.
