OrthoLoad

Hip Joint III

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

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

Fermur system

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

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) (http://www.ncbi.nlm.nih.gov/pubmed/11410170?dopt=Abstract).

Patients

H1L	H2R	H3L	H4L	H5L
H6R	H7R	H8L	H9L	H10R

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

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:

Ground reactions forces

Ground reaction forces were measured using two 6 degrees of freedom force plates (AMTI, Watertown, MA). The coordinate system is a right-handed system.

Raw data:

For the calculation of the center of pressure the following offsets were used. The true origins of the coordinate systems are located below the top surfaces, with a distance (zo).

Processed data

(see Comprehensive Data Sample (http://orthoload.com/comprehensive-data-sample/)):

grf2f.eof data:

(Downsampled Ground Reaction Forces)

Bending Moment

The resultant bending moment Mbend , acts in the middle of the femoral neck and perpendicular to the neck axis, and is calculated with the following formula:

with

The forces (F_x’, F_y’, F_z’) and moments (M_x’, M_y’, M_z’) are measured in the “implant coordinate system” x’, y’, z’ centered in the middle of the implant head. The force component F_x’ acts laterally, F_y’ anteriorly, and –F_z’ distally along the femur axis. The measured moment components M_x’, M_y’, and M_z’ turn right around the x’, y’, and z’ axes. N is the distance between the head center and the middle of the femoral neck and is equal to ½ L.

M_tne acts around the neck axis of the femur and represents the torsional loading of the neck. It is calculated by α = -45° rotation of the “bone coordinate system” around the y-axis.

Instrumented Crutches

F_cont – Contralateral Crutch Force

F_ipsi – Ipsilateral Crutch Force

2kN Force Transducer (KM 30z)

Hip joint

Two different hip implants (Hip I and Hip II) monitor the three force components acting on the ceramic head of the hip joint.

Instrumented implant

Hip I with one 4-channel transmitter

The implant is made of a titanium stem and a ceramic head. A compartment, 32 mm deep and 9.5 mm wide, houses the electronic instrumentation inside the neck of the prosthesis. Three semiconductor strain gauges were applied at the lower end of the inner wall and connected to the 4-channel transmitter. Two electrical feed-throughs, welded in the top plate by electron beam, form the transmitter antenna inside the ceramic ball. After the instrumentation, the top plate is welded by laser onto the prosthetic neck, thus sealing the inner space in such a manner that it is absolutely safe against the body.

Since 1988 four instrumented hip joints (Hip I) were implanted in three patients (EBL/EBR, JBR, IBL).

Hip II with two 8-channel transmitters

To get more information about a potential temperature increase of hip implant after longer walking distances, an implant with a hollow shaft was instrumented with two 8-channel telemetry transmitters. A common coil in the middle of the shaft supplies power to both telemetry circuits. Inside, eight temperature sensors are arranged along the whole neck and shaft. Three strain gauges placed inside the prosthetic neck monitor the three force components which act at the centre of the ceramic ball. A fourth strain gauge measures the strain of the stem.

One telemetry transmitter is placed inside the prosthetic neck; the second device is fixed inside the hollow shaft of the implant. A 4-lead feed through is welded by laser in the top plate of the neck and forms two single loop antennas for the signal transmission.

Since 1997 five instrumented hollow shaft hip joints (Hip II) were implanted in four patients (KWL/KWR, HSR, PFL, RHR).

Coordinate system

Femur system

All forces are reported in a right-handed coordinate system of the left femur (different from hip joint type: Hip III). The load components are reported as -Fx, -Fy, -Fz with negative signs. Positive values therefore indicate components acting toward the femoral head.

In many previously produced >OrthoLoad videos from the hip joint the minus signs are lacking!

The femur system is fixed at the centre of the femoral head. The femoral midline (dotted black/white) intersects with the axis of the neck in point P1. This midline leaves the femur distally at point P2. 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 (marked in red). Perpendicular to z and parallel to a plane through the most dorsal parts of the condyles, the x axis is defined (green) and points medially. The y axis (blue) is perpendicular to x and z and points ventrally.

Implant system

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, two angles are required: angle S between the z axis of the bone and the shaft axis of the implant, and the anteversion angle AV of the implant. This data is provided in a table.

Because the angle S is always small, transformation of the force components can be performed with sufficient accuracy by

• Turning the system by +AV around +z axis 
• Turning the system by +S around the +x azis

More details about this transformation are given here and in Bergmann et al. (2001)(http://www.ncbi.nlm.nih.gov/pubmed/11410170?dopt=Abstract).

Patients

EBL/EBR (Hip I)	IBL (Hip I)	JBR (Hip I)
HSR (Hip II)	KWL/KWR (Hip II)	PFL (Hip II)	RHR (Hip II)

Table with basic information about the patients with Hip I and Hip II implants:

For the hip joint, the forces and moments in an implant-based coordinate system are of special 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 are listed in the following table:

Transformation of loads

The loads (forces and moments) were measured in the implant-base coordinate system x’ y’ z’ (IBS). A force vector in the IBS is F’ = (Fx’, Fy’, Fz’), a moment vector is M’ = (Mx’, My’, Mz’). For the implant “HIP JOINT” the IBS was fixed to the left bone, for “HIP JOINT III”, “KNEE” and “SHOULDER” to the right bone. For subjects with implants at the opposite joint, the loads were first mirrored to the other side. Then they were mostly transformed to the bone-based system x y z (BBS). A force vector in the BBS is F = (Fx, Fy, Fz), a moment vector is M = (Mx, My, Mz). Only for the knee joint and the spinal implants they were left in the implant-based system x’ y’ z’.

The example shows the coordinate systems IBS and BBS of an implant of type “HIP JOINT III”. Definitions of the coordinate systems of the other implants are described in the manual under the caption “Implants”.

Relative to the BBS the implant is rotated three times in the order 1, 2, 3. The three rotation angles α1, α2, α3 are stated relative to the axes of the BBS! Their order and values are shown in the window “Info Patient” of the OrthoLoad videos:

In example 1 (implant type “HIP JOINT III”, right-sided implant) the three rotations and their order 1 2 3 are:

1. clockwise by angle α₁ = α_x = +2° around axis +x
2. clockwise by angle α₂ = α_y = -2° around axis +y
3. clockwise by angle α₃ = α_z = -15° around axis +z

For this right-sided implant “HIP JOINT III”, a negative angle α_z indicates an anteversion of the implant neck.

In example 2 (implant type “HIP JOINT III”, right-sided implant) the three rotations and their order 1 2 3 are:

1. clockwise by angle α₁ = α_x = +17° around axis +z
2. clockwise by angle α₂ = α_y = +8° around axis +x
3. clockwise by angle α₃ = α_z = 0° around axis +y

For this left-sided implant an anteversion is indicated by a positive angle α_z.

For transforming a force from the IBS x’ y’ z’ to the BBS x y z, three transformations have to be performed, using the transformation matrices T₁ (α₁), T₂(α₂), T₃(α₃). The complete transformation matrix T’ is then:

T‘ = T₃ (α₃) * T₂(α₂) * T₁(α₁) for F = T‘ * F’ (note the inverse order 3 2 1 of matrices!)

For transforming the loads from the BBS x y z to the IBS x’ y’ z’, this calculation has to be performed in the reverse order and with negative angles:

T = T₁ (-α₁) * T₂(-α₂) * T₃(-α₃) for F’ = T * F

Rotations around the axes x y z are performed by these matrices:

For example 1 the complete transformation matrices T’, T are therefore:

T‘ = T_z(-15°) * T_y (-2°) * T_x(2°)        for  F = T‘ *F’

T = T_x (-2°) * T_y(+2°) * T_z(+15°)      for  F’ = T *F

For example 2 they are:

T’ = T_y (0°) * T_x(8°) * T_z(17°)         for F = T‘ * F‘

T = T_z (-17°) * T_x(-8°) * T_y(0°)       for F‘ = T * F

Transformations of the moments M and M’ are performed separately in an analogue way.

Measuring Units % Body Weight and Newton

In the OrthoLoad videos, the loads are mostly reported in %BW (percent of body weight) for the forces and %BW*m for the moments, except for the spine implants were they are stated in Newton. The subject’s body weight in Newton is stated in the window “Info Patient” (examples see above). To transform loads from %BW/ %BW*m to N / Nm, the forces / moments have to be multiplied by 1% of the body weight, in example 1 by 7.809 and in example 2 by 6.9.

Your Computer

Computer Requirements

We suggest

• A PC with Microsoft Windows XP/Vista/Win7/Mac OS X/Linux
• A display having XGA resolution 1024×768 pixel or higher
• Mozilla Firefox 2.0 or Microsoft Internet Explorer 7.0 (or higher)
• Microsoft Media Player as standard player for the wmv/video format and/ or VLC Player 2.0 (or higher) as the standard video player for mp4/video format
• The installation of Windows Media Player if the wmv video codec is not installed.

Internet Browser

OrthoLoad was tested using Microsoft Internet Explorer 7.0 , Mozilla Firefox 4.0, Google Chroma 1ß.0 and Safari 5.0 and higher. For playing the OrthoLoad videos in your browser you need the Flash Plug-in.

Measuring units [%BW, %BW*m or Nm]

Forces from the spine have the measuring unit N, moments have the unit Nm. Data from all other implants are given in %BW (percent of body weight) for the forces and %BW*m for the moments. This is done because the results are then more uniform between the subjects. If you replace %BW by one percent of the patient’s body weight, you will then obtain the forces or moments in N or Nm.

Example: If a patient has a body weight of 850 N (86.4kg) you have to multiply the forces or moments, given in %BW or %BW*m, with the factor 8.5 to obtain them in N or Nm. The body weight and multiplication factor of each individual subject is stated in the window ‘Info Patient’ of the OrthoLoad videos, for example 8.5*{%BW, %BWm} –> {N, Nm}.

External equipment

External unit TELEPORT

TELEPORT is built up in a 19-inch case and has all of the external components needed for the telemetric measurements with one multi-channel telemetry transmitter.

The power oscillator generates a low-frequency sinusoidal output voltage for the energy coil. Amplitude and frequency are controlled automatically. The radio- frequency receiver has an input range from about 50 MHz up to 220 MHz and

input facility for active and passive antennas. The signal generator (marker) creates a rectangular mark signal with a frequency of 1 kHz to synchronise time segments with other systems like gait-analysis systems. The microcontroller system (data link) controls the inductive power supply to a constant value, checks and synchronises the pulse-interval-modulated (PIM) data received, creates a time base and transmits all data by an USB cable to a personal computer. Special software shows all data in real-time on the screen. The PIM data – and, when necessary, the mark signal as well – were recorded synchronously with the patient’s activity on the two audio tracks of the digital video camcorder.

TELEPORT is used mainly for load measurements involving our instrumented hip-, knee-, shoulder implants and vertebral body replacements.

Internal telemetry transmitters

Implantable 4-channel telemetry transmitter

The 4-channel telemetry transmitter was manufactured in double-sided thick film hybrid technology with 14 off-the-shelf integrated circuits and 17 passive components. It was used for three-dimensional force measurements with hip endoprostheses (hip I, 1984).

Dimensions: 15 x 7 mm. 8 connection pads for 3 strain sensors, energy coil and antenna.

Download data sheet: 4-channel-transmitter.pdf (330 KB)

Implantable 8-channel telemetry transmitter

The 8-channel telemetry transmitter was manufactured in double-sided thick film hybrid technology with a bipolar transistor array (4.2 x 5.0 mm) and 17 passive components. It was used for load measurements with internal spinal fixators and hip joint force and temperature measurements (hip II, 1991).

Dimensions: 14 x 7 mm. 12 connection pads for 6 external sensors, energy coil and antenna.

Download data sheet: 8-channel-transmitter.pdf (444 KB)

Implantable 9-channel telemetry transmitter

The 9-channel telemetry transmitter is made in double sided thick-film hybrid technology and has the dimensions of 12.5 x 6 mm. Analogous and digital parts are combined on a single custom-made chip in BICMOS technology with a structure of 0.8 µm. The total die size is only 2 x 2.6 mm. This low-power circuit includes a 9-channel multiplexer, a programmable memory, a pulse interval modulator and a radio-frequency transmitter. Channels 1 to 6 are used for semiconductor strain gauges or as temperature sensors. Channel 7 transmits the temperature of the hybrid circuit. Depending on the power information transmitted, the magnetic field is regulated by channel 8. Synchronisation of the pulse train is carried out by the aid of channel 9. A Zener diode PROM module stores all information of the calibration process and thus adapts each channel to its corresponding sensor resistance. After successful programming, this part of the hybrid circuit is cut off and total size is now reduced to 9.5 x 6 mm.

The 9-channel telemetry transmitter has been used since 2004 for the instrumentation of shoulder-, knee- , hip III implants and vertebral body replacements.

Download data Sheet: 9-channel-transmitter.pdf (324 KB)