**5. The testing methods**

The corpectomy model was used to evaluate the strain measurement system prior to spine testing. This model has similar bending to a four-point bending model therefore the rod strain does not change significantly along the length of the rod. A single metal foil strain gauge reference was attached to the rod adjacent to the housing for comparison to the transducer system. Comparing the metal foil reference gauge to the spinal fusion sensor placed in close proximity is justified. Figure 9 shows the sensor assembly on a housing without a container cover.

Fig. 9. The transducer sensor system on a housing that attaches to a stainless rod before a hermetically sealed container is assembled.

if sufficient DC power is available from the regulator. However at the far end of the region, planar and axial alignment becomes very important. With distance from the reader, inductive coupling is reduced thereby reducing the AC voltage across the LC tank and thus the modulated signal amplitude. The data signal also relies on the same low coupling between the implant and the reader. It is necessary to have a sensitive reader to detect the

The corpectomy model was used to evaluate the strain measurement system prior to spine testing. This model has similar bending to a four-point bending model therefore the rod strain does not change significantly along the length of the rod. A single metal foil strain gauge reference was attached to the rod adjacent to the housing for comparison to the transducer system. Comparing the metal foil reference gauge to the spinal fusion sensor placed in close proximity is justified. Figure 9 shows the sensor assembly on a housing

Fig. 9. The transducer sensor system on a housing that attaches to a stainless rod before a

implant at the far end of the region.

**5. The testing methods** 

without a container cover.

hermetically sealed container is assembled.

Fig. 8. The detection region is within the cone shape.

The strain measurement system was tested using a corpectomy model designed as a simplified mechanical analog of a spine section and then will be tested using a human spine. Figure 10 shows a free body diagram of the forces and bending moments applied to the spinal rods through the pedicle screws due to loading of the hard plastic blocks. Note that when the spinal fusion rods are fastened to the fixtures, often the initial strain is introduced and recalibrated. On the first test, the corpectomy apparatus was assembled inside a clear acrylic water tank without water on the MTS machine for application of the load; the sensor system was oriented facing out to be detected. The MTS machine's dynamic motion only changes 3 mm in the detection distance between the interrogating reader and the strain sensor. The read range was limited to 10 cm due to the reader design used and the sensor coil design constraints. A more optimal reader would increase the range. For other applications where a larger coil diameter is acceptable the range would be increased as well.

Fig. 10. The corpectomy model: front view, top and side view, bottom.

The data acquisition is obtained with LabView software, an interfacing program designed to transfer and record live data between instruments and computer by National Instrument. The strain information is recorded by the commecial metal foil strain gauge through a strain

Inductively Coupled Telemetry In Spinal Fusion Application Using Capacitive Strain Sensors 71

For the next application in water tank test, the sensor is expected to be in a fluidic environment. To protect from the influence of water or water vapor, the system was hermetically sealed in a container made of nylon and painted heavily with silicone and polyethylene. The container protected the device during corpectomy testing where it is submerged in a water tank used to simulate the body and during spine testing where it is in

Caution has been taken for the temperature difference between water and ambient. A temperature dependent test on the sensor shows that the frequency drift from 1752 to 1778 Hz from 0 oC to 22.2 oC and vice versa with no strain at the rate of about 1.8 Hz/ oC. The interrgating reader was not moved at about 10 cm from the sensor system, through water, glass and air. The difference of the readings before the introduction of water and after was unnoticeable. The corpectomy model in water test was successful in showing the repeatability for the cyclic loads. It aslo suggestes that measurements were possible in conditions present in-vivo. The tests of the corpectomy model in water tank were successful

A discectomy was performed on an excised spine from a cadaver and was constrained and loaded in a MTS system to simulate a 113.4 kg (250 lb) patient. The excised spine was potted using a liquid lead/bismuth alloy (Cerro Products, Bellefont, PA) and attached to a MTS Bionix mechanical testing system. A custom-built test apparatus was used to apply anatomic loads to the spine. The intact spine was tested by applying loads ranging from 100 to 200 N in an increasing and decrease manner with a 5-second period of pause at every increment to accommodate the detection speed of the system. The disc between L3 and L4 was surgically removed to simulate an unstable spine prior to fusion surgery, and the test sequence repeated. The comparison of results before and after the disk is removed are shown in

With the same loading steps on the spine from MTS, the spinal fusion rods experienced less strain increment in the spine with the disk than that in the spine with the disk removed.

in showing that measurements were possible in conditions present in-vivo.

Fig. 13. Measured strains during excised spine testing.

**5.1 Water tank test** 

contact with tissue.

**5.2 Excise spine test** 

Fig. 13.

indicator (Measurement group model P-3500). The frequency output from the wireless strain sensor system measurement is obtained through the digital universal counter (Agilent 53131A). In normal use, the response time for the strain indicator is 0.5 second, and the universal counter about 0.2 second. However, with the Labview interface the response time for the universal counter changes to 2.3 seconds but does not change for the strain indicator. The lag in time in the acquisition of the frequency data from the sensor system will cause false information when recording a drastic dynamic motion. In order to have a consistent and corresponding readings for the referred strain and the frequency output, a delay or pause in the live measurement is needed. Therefore, the MTS machine is programmed to pause 5 seconds at every increment of load for the frequency output to stabilize. Both the strain and the frequency data then are taken at the same time frame as the Material Test Machine.

The result of the telemetry strain sensor detection in Fig. 11 shows good linearity with R2= 0.99. The gauge factor is calculated to be 249 up to 1000 micro-strain, with R2=0.96 as see in Fig. 12. Similar gauge factor of 252 in the capacitance mode was measured in section 2. The frequency mode also exhibits high gauge factor without any amplification.

Fig. 11. The demodulate frequency responses from the sensor transducer shows a linear response due to bending test of the corpectomy model.

Fig. 12. The plot shows a linear gauge factor 249 in the frequency domain.

#### **5.1 Water tank test**

70 Modern Telemetry

indicator (Measurement group model P-3500). The frequency output from the wireless strain sensor system measurement is obtained through the digital universal counter (Agilent 53131A). In normal use, the response time for the strain indicator is 0.5 second, and the universal counter about 0.2 second. However, with the Labview interface the response time for the universal counter changes to 2.3 seconds but does not change for the strain indicator. The lag in time in the acquisition of the frequency data from the sensor system will cause false information when recording a drastic dynamic motion. In order to have a consistent and corresponding readings for the referred strain and the frequency output, a delay or pause in the live measurement is needed. Therefore, the MTS machine is programmed to pause 5 seconds at every increment of load for the frequency output to stabilize. Both the strain and the

frequency data then are taken at the same time frame as the Material Test Machine.

frequency mode also exhibits high gauge factor without any amplification.

The result of the telemetry strain sensor detection in Fig. 11 shows good linearity with R2= 0.99. The gauge factor is calculated to be 249 up to 1000 micro-strain, with R2=0.96 as see in Fig. 12. Similar gauge factor of 252 in the capacitance mode was measured in section 2. The

Fig. 11. The demodulate frequency responses from the sensor transducer shows a linear

Fig. 12. The plot shows a linear gauge factor 249 in the frequency domain.

response due to bending test of the corpectomy model.

For the next application in water tank test, the sensor is expected to be in a fluidic environment. To protect from the influence of water or water vapor, the system was hermetically sealed in a container made of nylon and painted heavily with silicone and polyethylene. The container protected the device during corpectomy testing where it is submerged in a water tank used to simulate the body and during spine testing where it is in contact with tissue.

Caution has been taken for the temperature difference between water and ambient. A temperature dependent test on the sensor shows that the frequency drift from 1752 to 1778 Hz from 0 oC to 22.2 oC and vice versa with no strain at the rate of about 1.8 Hz/ oC. The interrgating reader was not moved at about 10 cm from the sensor system, through water, glass and air. The difference of the readings before the introduction of water and after was unnoticeable. The corpectomy model in water test was successful in showing the repeatability for the cyclic loads. It aslo suggestes that measurements were possible in conditions present in-vivo. The tests of the corpectomy model in water tank were successful in showing that measurements were possible in conditions present in-vivo.

#### **5.2 Excise spine test**

A discectomy was performed on an excised spine from a cadaver and was constrained and loaded in a MTS system to simulate a 113.4 kg (250 lb) patient. The excised spine was potted using a liquid lead/bismuth alloy (Cerro Products, Bellefont, PA) and attached to a MTS Bionix mechanical testing system. A custom-built test apparatus was used to apply anatomic loads to the spine. The intact spine was tested by applying loads ranging from 100 to 200 N in an increasing and decrease manner with a 5-second period of pause at every increment to accommodate the detection speed of the system. The disc between L3 and L4 was surgically removed to simulate an unstable spine prior to fusion surgery, and the test sequence repeated. The comparison of results before and after the disk is removed are shown in Fig. 13.

Fig. 13. Measured strains during excised spine testing.

With the same loading steps on the spine from MTS, the spinal fusion rods experienced less strain increment in the spine with the disk than that in the spine with the disk removed.

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From the wireless transmision data along with the referred metal foil strain gauge, it is suggested that the spinal fusion rods showed that roughly one third of the load were shared by the intact spine after the spine is fused. The telemetry system clearly shows the rigidity of the intact spine.
