**4.** *In vitro* **experimental tests of femoral component with cemented prostheses**

A third generation in composite material, long left model 3306, femoral component by Sawbones was used in the study presented in the following text. The cement to fix the implant was CMW3 (polymethylmethacrylate with gentamicin) that is the most usual in cemented prosthesis.

The position of the femural component in the fatigue test to reflect the load on the THA to a certain amount of daily human activity was placed under the "femur coordinate system" defined by Bergmann (Bergmann et al, 2001).

The fatigue equipment was a pressure machine constructed according to the standards by an investigation group of Biomechanical Engineering which is part of the Mechanical Department of Aveiro University, Portugal. In the fatigue test the placement of the prosthesis in the testing machine follows the ISO 7206 standard (Qi, 2000; Ramos et al, 2005). The femur position was 9*<sup>o</sup>* in sagittal plan and 11*<sup>o</sup>* in frontal plan and was done by a base that supported the femoral component in the fatigue machine. The prosthesis fatigue test employed was done in accordance with (Ramos et al, 2005) being considered as the most severe by other authors (Cristofolini et al, 2003; Stolk et al, 2006). The sinusoidal charge cycles had a frequency of 2.5 Hz and 450 N in median amplitude. It was done between 900 N (minimum amplitude) and 1800 N (maximum amplitude). The femoral component under test had been constructed (prostheses collocated) about 3 years ago and it has been already used in a charge of 1 million of cycles and it does not present any visible crack. The test lasted 247343 s which corresponds to 618433 sinusoidal cycles.

Four sensors by Physical Acoustic Corporation (PAC) and Digital Wave Corporation (DAC) were mounted by a cylindrical distribution, as shown by Qi (Qi et al, 2005). The coordinates of the placement of sensors are presented in figure 4. To acquire the signals AE were used the AMSY5, Acoustic Emission System by Vallen.

The interface between sensors and femoral component was made by special synthetic silicon for polymers with excellent properties for the conduction of sound waves. Good collocation and attenuation (verified by Hsu Nielsen principle) essays were implemented to warranty a 10 Will-be-set-by-IN-TECH

**4.** *In vitro* **experimental tests of femoral component with cemented prostheses**

A third generation in composite material, long left model 3306, femoral component by Sawbones was used in the study presented in the following text. The cement to fix the implant was CMW3 (polymethylmethacrylate with gentamicin) that is the most usual in cemented

The position of the femural component in the fatigue test to reflect the load on the THA to a certain amount of daily human activity was placed under the "femur coordinate system"

The fatigue equipment was a pressure machine constructed according to the standards by an investigation group of Biomechanical Engineering which is part of the Mechanical Department of Aveiro University, Portugal. In the fatigue test the placement of the prosthesis in the testing machine follows the ISO 7206 standard (Qi, 2000; Ramos et al, 2005). The femur position was 9*<sup>o</sup>* in sagittal plan and 11*<sup>o</sup>* in frontal plan and was done by a base that supported the femoral component in the fatigue machine. The prosthesis fatigue test employed was done in accordance with (Ramos et al, 2005) being considered as the most severe by other authors (Cristofolini et al, 2003; Stolk et al, 2006). The sinusoidal charge cycles had a frequency of 2.5 Hz and 450 N in median amplitude. It was done between 900 N (minimum amplitude) and 1800 N (maximum amplitude). The femoral component under test had been constructed (prostheses collocated) about 3 years ago and it has been already used in a charge of 1 million of cycles and it does not present any visible crack. The test lasted 247343 s which corresponds

Four sensors by Physical Acoustic Corporation (PAC) and Digital Wave Corporation (DAC) were mounted by a cylindrical distribution, as shown by Qi (Qi et al, 2005). The coordinates of the placement of sensors are presented in figure 4. To acquire the signals AE were used the

The interface between sensors and femoral component was made by special synthetic silicon for polymers with excellent properties for the conduction of sound waves. Good collocation and attenuation (verified by Hsu Nielsen principle) essays were implemented to warranty a

Fig. 3. Axis system and placement sensors (Axinte et al, 2005)

defined by Bergmann (Bergmann et al, 2001).

AMSY5, Acoustic Emission System by Vallen.

to 618433 sinusoidal cycles.

prosthesis.

Fig. 4. Schematic placement of sensors and system of forces applied (Gueiral, 2008).

good signal AE acquisition. Usually tape-glue was used to sustain the sensors attached to femoral component.

During the process charge, sensor number 4 has been removed because it does not obtain any signal. After the charge we obtained three sets of AE events, the first appeared close to 7500 s (approximately two hours in fatigue), the second set of events appeared above 20000 s (approximately five and half hours in fatigue) and the third set of events appeared close to 55000 s (approximately fifteen hours in fatigue). Table 1 shows the acoustic energy during the component femural load process, that allows to conclude which of the events and which sensors have a peak stress, so an higher energy release. Analyzing the energy values in table 1,


### Table 1. Acoustic Energy

(a) Crack location in femural component (Gueiral, 2008) (b) Maximum shear stresses (Nabais,

<sup>143</sup> Acoustic Emission Studies in Hip Arthroplasty

modelling of bone-implant failure. The displacement of the implant-cement interface and the failure of the implant-bone interface results in an increase of tensions in the cement mantle

According Nabais (Nabais, 2006), the maximum shear stress simulated by finite elements method are showed in the figure 5b). As one can see is really possible that the AE source detected in this study is located in the region of high stress concentration, figure 5b, when the

The monitoring of acoustic emission in evaluation structures integrity has the advantage to be done when the process is in charge, this does not happen in other non-destructives tests. That advantage is very important when one work with organic structures or organic substitution

The signals obtained by the sensors system have the typical profile of a burst AE which means

The analysis of the acoustic emission results indicate the location coordinates of a crack in the

a good amplitude, duration and sufficient number of crossing the threshold.

structure, which coincides with the crack image observed in the optical microscope.

Fig. 5. AE source location versus finite elements simulation

femoral component is subjected to a fatigue test.

– Peak Stress Impact *In Vitro* Cemented Prosthesis

(Nabais, 2006).

**5. Conclusion**

like in the study developed.

2006)

conclude that the femoral component was subject to maximum peak stress around the 55000 s, the third set of events. The sensor further "punished" is the sensor number two, because it presents a higher acoustic energy released as well as a larger amplitude measured by the sensor as we determine in the following analysis by WT.

To process the AE signal the WT software of Vallen was used to calculate the waves arrival times. That way, one could analyze all transients of acoustic emission signals and obtained really important information not possible with another analysis (Gueiral et al, 2009). Table 2 shows the obtained values for the wavelet coeffcient and arrival times to every set of events and to every sensors. The amplitude values have a permissible maximum error of 0.05 mV


Events Sensor Amplitude(mV) WT Coefficients Arrival Time (*μ*s)

Table 2. Wavelet Transform data (Gueiral, 2008)

and 0.5 s for the arrival time. Observing the values in table 2 it is visible that the three sets of events happened at different hours of the charge cycle, but the arrival time is similar in every set of events. This analysis makes the possibility that a crack has occurred (source AE) in the femoral component. Another fact observed is that the amplitude rises from the second to the third set, this probably means that the crack spread from inside to the surface of the femoral component.

The amplitude in sensor *S*<sup>2</sup> is greater than in sensor *S*<sup>1</sup> and *S*3, which indicates that the position of sensor *S*<sup>2</sup> is near the crack (source AE). The smallest arrival time is in *S*1. So considering the above and the results according to Gueiral (Gueiral et al, 2009) shows that the most probable zone where it happened the crack is between *S*<sup>1</sup> and *S*2.

Making restrictions in sensors positions (Gueiral et al, 2009) according to Axinte (Axinte et al, 2005) the AE sources location methods was made by iterative calculus and the real coordinates of AE source is presented in table 3. The sensor *S*<sup>1</sup> was considered the origin of axis system (figure 4). The femoral component was cut into sections to realize complementary diagnostic by liquid penetrant test and by optical microscopy (200x). The crack was located in the marked zone shown in figure 5a)-b) [with a crack zoom c)].

The finite element analysis in the field of orthopaedics has earned a privileged position in the use of numerical computational techniques applied to the evaluation of stresses and displacements in structural components. Some application of the finite element method in the field of orthotics make analysis of the mechanical behavior of bone and joints, as well as


Table 3. Coordinates of the location of the source EA (Gueiral, 2008)

12 Will-be-set-by-IN-TECH

conclude that the femoral component was subject to maximum peak stress around the 55000 s, the third set of events. The sensor further "punished" is the sensor number two, because it presents a higher acoustic energy released as well as a larger amplitude measured by the

To process the AE signal the WT software of Vallen was used to calculate the waves arrival times. That way, one could analyze all transients of acoustic emission signals and obtained really important information not possible with another analysis (Gueiral et al, 2009). Table 2 shows the obtained values for the wavelet coeffcient and arrival times to every set of events and to every sensors. The amplitude values have a permissible maximum error of 0.05 mV Events Sensor Amplitude(mV) WT Coefficients Arrival Time (*μ*s) 1 1.7 0.00210 11.5

1 2 7.0 0.00825 23.5

2 2 6.5 0.00842 24.0

3 2 13.0 0.01452 26.0

3 3.5 0.00394 60.0 1 1.7 0.00226 11.0

3 3.6 0.00401 60.0 1 3.6 0.00426 13.5

3 6.5 0.00755 59.5

and 0.5 s for the arrival time. Observing the values in table 2 it is visible that the three sets of events happened at different hours of the charge cycle, but the arrival time is similar in every set of events. This analysis makes the possibility that a crack has occurred (source AE) in the femoral component. Another fact observed is that the amplitude rises from the second to the third set, this probably means that the crack spread from inside to the surface of the femoral

The amplitude in sensor *S*<sup>2</sup> is greater than in sensor *S*<sup>1</sup> and *S*3, which indicates that the position of sensor *S*<sup>2</sup> is near the crack (source AE). The smallest arrival time is in *S*1. So considering the above and the results according to Gueiral (Gueiral et al, 2009) shows that the

Making restrictions in sensors positions (Gueiral et al, 2009) according to Axinte (Axinte et al, 2005) the AE sources location methods was made by iterative calculus and the real coordinates of AE source is presented in table 3. The sensor *S*<sup>1</sup> was considered the origin of axis system (figure 4). The femoral component was cut into sections to realize complementary diagnostic by liquid penetrant test and by optical microscopy (200x). The crack was located in the marked

The finite element analysis in the field of orthopaedics has earned a privileged position in the use of numerical computational techniques applied to the evaluation of stresses and displacements in structural components. Some application of the finite element method in the field of orthotics make analysis of the mechanical behavior of bone and joints, as well as Events X(cm) Y(cm) Z(cm) 1(7500s) -0.601 - 2.556 0.972 2(20000s) -0.821 - 2.473 1.025 3(55000s) -0.697 - 2.611 0.732

most probable zone where it happened the crack is between *S*<sup>1</sup> and *S*2.

Table 3. Coordinates of the location of the source EA (Gueiral, 2008)

sensor as we determine in the following analysis by WT.

Table 2. Wavelet Transform data (Gueiral, 2008)

zone shown in figure 5a)-b) [with a crack zoom c)].

component.

(a) Crack location in femural component (Gueiral, 2008) (b) Maximum shear stresses (Nabais, 2006)

Fig. 5. AE source location versus finite elements simulation

modelling of bone-implant failure. The displacement of the implant-cement interface and the failure of the implant-bone interface results in an increase of tensions in the cement mantle (Nabais, 2006).

According Nabais (Nabais, 2006), the maximum shear stress simulated by finite elements method are showed in the figure 5b). As one can see is really possible that the AE source detected in this study is located in the region of high stress concentration, figure 5b, when the femoral component is subjected to a fatigue test.
