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Hip joint III

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Revision as of 15:14, 13 November 2015 by Frypress (Talk | contribs)

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The actual hip implant (Hip III) monitor the three force components and the three moment components acting on the ceramic head of the hip joint.


Instrumented implant

Hip III with one 9-channel transmitter

Implant Hip III
Implant Hip III

This new design of a instrumented hip implant was developed to measure contact forces and the friction at the joint in vivo. A clinical proven hip implant ('Spotorno' design) was modified in the neck area. The stem is build by TiAl6V4 and Al2O3-Ceramic was choosen for the implant head material. The neck was widened and enhanced with a 6.2 and 10mm hole. In the hollow neck are housed six semiconductor strain gauges, an internal induction coil and the telemetry. The six strain gauges are applied at the lower part on the inner wall (10mm hole) and connected to the 9-channel transmitter. The antenna, placed under the implant head, is connected by electronically feed-through to the internal telemetry. The feed-through is welded by a laser beam into the top plate. The hollowed neck is closed by the top plate and welded with an electron beam. Therefore the internal space is hermetically closed against the body fluids.

With this implant three contact forces acting onto the implant head center and three friction moments acting between the gliding partners can be measured in vivo.

Since April 2010 ten instrumented hip joints (Hip III)  were implanted in ten patients (H1L, H2R, H3L, H4L, H5L, H6R, H7R, H8L, H9L and H10R) to monitor forces and moments. No further implantations are planned.

Coordinate system

Femur system

Coordinate System at right Femur

All forces are reported in a right-handed coordinate system of the right femur (different from hip joint type I and II). The load components are reported as Fx, Fy, Fz. The femur system is fixed at the centre of the femoral head. The femoral midline (dotted black) intersects the axis of the neck in point P1. Point P2 is defined as the deepest point of the fossa intercondylaris at the distal end of the femur. The straight connection between P1 and P2 defines the z axis of the femur. The z axis of the coordinate system is parallel to the z axis of the femur. The x axis of the coordinate system is defined perpendicular to z and parallel to a plane through the most dorsal parts of the condyles and points laterally. The y axis of the coordinate system is perpendicular to x and z and points ventrally.


Implant system

Implant System Patient

In order to test fatigue or strength of the implant itself, it may sometimes be required to know the force components in an implant-based coordinate system. Axis zi of this system coincides with the shaft axis of the implant. The xi axis lies in the neck-shaft-plane. For the transformation of forces from the femur- to the implant system, three angles are required: angle Sx between the z axis of the bone and the shaft axis of the implant, angle Sy between the z axis of the bone and the shaft axis of the implant and furthermore the anteversion angle AV of the implant. These data are provided by the table in the video ("Info Patient").

  1. Turning the system by + Sx around the - x axis
  2. Turning the system by + Sy around the - y axis
  3. Turning the system by - AV around the + z axis

More details about this transformation are given here and in Bergmann et al. (2001).

Patients

Pat-H1L big.png Pat-H2R big.png Pat-H3L big.png Pat-H4L big.png Pat-H5L big.png
H1L H2R H3L H4L H5L
Pat-H6R big.png Pat-H7R big.png Pat-H8L big.png Pat-H9L big.png Pat-H10R big.png
H6R H7R H8L H9L H10R

Table with basic information about the patients with Hip III implants:

Patient Side Gender Weight
[kg]
Height
[cm]
Age at
Implantation
[years]
Indication
H1L left m 73 178 55 Coxarthrosis
H2R right m 75 172 61 Coxarthrosis
H3L left m 92 168 59 Coxarthrosis
H4L left m 85 178 50 Coxarthrosis
H5L left f 87 168 62 Coxarthrosis
H6R right m 84 176 68 Coxarthrosis
H7R right m 95 179 52 Coxarthrosis
H8L left m 80 178 55 Coxarthrosis
H9L left m 118 181 54 Coxarthrosis
H10R right f 98 162 53 Coxarthrosis

For the hip joint III, the forces and moments in an implant-based coordinate system are of especial interest. The torque around the shaft axis, for example, is one of the most important parameters for the stability of implant fixation. To transform the forces measured relative to the bone, as delivered by OrthoLoad, to the loads acting in the implant system, the anteversion angle AV of the implant, the CCD angle and the neck length L are required. This data is listed in the following table:

Patient Anteversion
Angle AV
[degree]
CCD Angle
[degree]
Neck Length
L
[mm]
Shaft Angle
Sx
[degree]
Shaft Angle
Sy
[degree]
H1L -15.0 135 55.6 2.3 -2.3
H2R -13.8 135 59.3 4.1 0.6
H3L -13.8 135 55.6 4.0 -3.0
H4L -18.9 135 63.3 7.5 -1.7
H5L -2.3 135 55.6 4.0 -2.3
H6R -31.0 135 55.6 5.8 -1.7
H7R -2.4 135 63.3 6.3 -1.7
H8L -15.5 135 59.3 4.6 -1.7
H9L -2,3 135 59.3 4.6 0.6
H10R -9,7 135 59.6 1.7 -1.2