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Installation and methodology for the study of the stability of implants used in treating femoral neck fractures.

1. INTRODUCTION

In treating femoral neck fractures through the method of internal fixation, one of the most important problems that surgeons must overcome is choosing the right type of implant (Brandt et al., 2006), (Chen et al., 2004), (Heini et al., 2004).

This is the more difficult considering that the current number of implants used in achieving femoral neck osteosynthesis is very large. The ideal implant has to meet two contradictory conditions. On the one hand it has to insure the stability of the fracture focal point, and, on the other hand, it must have a dynamic behaviour that allows the axial compression of the fragments. With this aim in mind, various installations were created, installations which could allow an investigation of the static and dynamic behaviour of the implants used in the femoral neck fracture osteosynthesis. Therefore, the paper (Ehmke et al., 2005) presents a stall that has the purpose to determine the deformation of the screws used in hip implants. The stall offers a two-direction distribution of the forces pressing against the femoral head. The system which allows the simulation of the loadings occurring during walking consists of a biaxial actuator. The variable force is generated by an axial cam with an angle of 23[degrees]. The implant screw is placed in a main body made of polyurethane. The paper (Bonnaire & Weber, 2002) presents a system of determining the static and dynamic stability of implants used in treating femoral neck fracture. Also, the paper proposes an original transducer for determining the displacements in the area of the fracture. The paper (Cionca, 2008) aims at using osteosynthesis in treating femoral neck fractures, presenting, among others, a comparative study concerning the elastic and plastic deformations that occur when using two types of implants. The methodology and experimental installation presented have as a main advantage the high degree of universality. The final structure of the stall resulted after systematic and scientific studies of numerous cases that can be encountered in the medical practice, thus permitting the performance of a great number of experiments involving the stability of implants used in treating femoral neck fracture.

2. THE PROPOSED METHODOLOGY

The methodology proposed for determining the static and dynamic behaviour of implants used in femoral neck fracture osteosynthesis presents the following stages: 1. Establishing the type of fracture tract which the implant is used for; 2. Establishing the type of bone that is going to be used in researches. One can use bones of corpses, animal bones, or synthetic femoral models made of composite materials; 3. Establishing the type of implant that will be subject to experiments; 4. Creating the fracture tracts. This stage can be achieved by cutting, on condition that one removes as little material as possible, so that the shape of the femoral bone is not deformed; 5. Implant placement. Implants should be placed identically into the bone in order to reduce the variability of the studied phenomenon. In the case of screw implants, it is necessary that they are placed aided by a dynamometric key, so that all screws are clamped at the same time as the action force; 6. Establishing the dynamic character of external forces. External forces can simulate walking or they can have an evolution imposed by the experiment requirements. When walking, the loadings are of the type: periodical variable loadings or tiredness loadings which occur when there is a periodical variation of the efforts, repeated several times. The efforts the implant is subjected to have a variation of the pulsating cycle type to which the external force (the loading) varies from 0 to a maximum value. It is thought that the frequency of loading appliance to a normal walk is 1Hz. 7. Establishing the force number that solicits the femoral head. Only one resulting force can be applied--the one which actions vertically, or several other forces can be predicted, in various directions; 8. Establishing the application points of the external forces; 9. Establishing the direction of the external forces. The femoral bone can be placed into ortostatic position or in various other walking positions; 10. Establishing the size of the forces. The forces the implant is subjected to can be of maximum four times the body weight. Thus, these forces can reach 240 ... 300 daN; 11. Establishing the experimenting strategy. This is done according to the number of available models and implants, the chosen variable parameters, the time available for doing the experiments, etc.

3. THE EXPERMENTAL INSTALLATION

To study the deformation state of the implants used in femoral neck fractures, an experimental stall was created which allows the simulation of implant behaviour (fig. 1).The stall is made of a basic disc (1), on which a frame is placed (2). A worm restrictor is placed on the frame (2) and it is actioned by a triphase asynchronous engine (4). The restrictor (3) rotates a cam with variable cam action throw (5).

[FIGURE 1 OMITTED]

The cam clamps on a tappet (6) which has a roller (7) on the upper part. The tappet (6) is connected to a resistive transducer (8).

The transducer has a pressure head (9) attached. On the basic disc, a supporter (10) is placed and on it the centring box chuck (11) is placed. The chuck clamps the superior part of the femoral bone that is to be subjected to the deformation state research. The supporter (10) allows the bevelled positioning of the femoral bone in an angle of 16[degrees]. The ensemble electric engine-restrictor-cam allows applying a frequency force of about 1Hz. The electrical engine's rev is about 1300 rot/ min, and the worm restrictor permits a rev reducing gear of about 26 times. This way, a cam rev of 52 rot/ min. (0,86 Hz) is obtained. The cam allows obtaining variable processes and implicitly variable forces, which are applied to the femoral head. These forces are applied through a tensometric transducer of the type Hottinger Baldwin Messtechnik, S9, which permits the measurement of forces of the maximum value of 500 daN. The transducer has an entry resistance of 505,64 ohm and an exit resistance of 350,43 ohm. The transducer is calibrated so that the connection between the measured force and the tension is 1 daN=0,0001 V. The transducer signal is taken up by a signal conditioner of the type SC-2043-SG National Instruments, with 8 entries. The analogical signal is taken up by the signal conditioner and is transmitted to the data acquisition disc of the type 6023E National Instruments, placed on the basic disc of a computer. The acquisition disc turns the analogical signal into a digital signal, having a sampling rate of 200,000 samples per second. The resolution is 12 bit and 16 analogical entries. To operate, visualise and store data, a virtual measurement instrument of the type Continuous Strain Measurement was used and created in the LabView programming medium of the National Instruments trade mark (fig. 2). This instrument allows a periodical visualisation of the force developed with the help of the tensometric transducer.

The displacements that occur at the level of the fracture focal point are measured in two directions, aided by two comparative, digital and mechanic instruments of the precision 0,01 mm. To determine the absolute displacements of the femoral head from the residual femoral neck, the comparative instruments were placed in such a way that they were unitive with the residual femoral neck. The displacements were measured in two directions.

[FIGURE 2 OMITTED]

According to the literature of the field, one displacement measurement focused on the direction perpendicular on the fracture tract, at its superior limit. The other displacement measurement consisted of measuring the displacement of the femoral head on a vertical direction, along the fracture tract.

4. CONCLUSIONS

The stall was used to study the stability of two types of implants: the AO screw system and the Hansson pin hook system. Various comparative studies can be made between the various types of implants that are used for the internal fixation of the femoral neck fracture.

These researches proved the universality, reliability and precision of the proposed installation. Further activities connect to the development of the stall by including a hydraulic installation which could permit enlarging the range of forces to be applied.

5. REFERENCES

Bonnaire, F.A. & Weber A.T. (2002). Analysis of fracture gap changes, dynamic and static stability of different osteosynthetic procedures in the femoral neck. Injury, International Journal of the care Injured, 33, Suppl. 3, pp. S-C24-S-C32.

Brandt, E.; Verdonchot N.; Vugt, A van & Kampen, A van (2006). Biomechanical analisys of the percutaneous compression plate and sliding hip screw in intracapsular hip fractures: Experimental assesment using synthetic and cadaver bones. Injury, International Journal of the care Injured, 37, pp. 979-983.

Chen, W.P.; Tai C. L.; Shih C. H.; Hsieh P. H.; Leou M. C. & Lee M. S. (2004). Selection of fixation devices in proximal femur rotational osteotomy: clinical complications anf finite element analysis. Clinical Biomechanical, 19, pp. 255-262.

Cionca, D., (2008). Contemporaneousness of Osteosynthesis in Treating Femoral Neck Fractures. Doctoral Thesis, "Grigore T. Popa" Medicine and Pharmacy University, Iasi, Romania.

Ehmke, L.W.; Fitzpatrik, D. C.; Krieg, J. C.; Madey, S. M. & Bottlang M. (2005). Lag screws for hip fracture fixation: Evaluation of migration resistance under simulated walking. Journal of Orthopaedic Research, 23, 6, pp. 1329-1335.

Heini, P.F.; Franz, T.; Fankhauser, C.; Gasser B. & Ganz R. (2004). Femoroplasty-augmentation of mechanical properties in the osteoporotic proximal femur: a biomechanical investigation of PMMA reinforcement in cadaver bones. Clinical Biomechanics, 19, pp. 506-512.
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Author:Seghedin, Neculai; Cionca, Dan; Drosescu, Radu
Publication:Annals of DAAAM & Proceedings
Article Type:Report
Date:Jan 1, 2008
Words:1589
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