Instrumented Crutches

Instrumentes Crutches

Fcont – Contralateral Crutch Force

Fipsi – Ipsilateral Crutch Force

2kN Force Transducer (KM 30z)


Instrumentes Crutches
Instrumentes Crutches
Technical data:
Bridge resistance:350 Ohm
Linearity error:0.1%
Hysteresis error:0.1%
Measurement amplifierBA660
Bridge Voltage:5V
Output Voltage:+/-5V
Linearity error:0.02 %
fc output filter:250Hz


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

Implant Hip I

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

Implant Hip II

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.

Femur System Loads
Coordinate system at left Femur

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)(


EBL/EBR (Hip I)IBL (Hip I)JBR (Hip I)

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

PatientImplantSideGenderWeight [kg]Height [cm]Age at Implantation [years]Indication
EBLHip Ileft m6216883Osteoarthritis
EBRHip I rightm6216883Osteoarthritis
IBLHip I leftf8417076Osteoarthritis
JBRHip I right f4716069Femoral head necrosis
HSRHip II right m8217455Osteoarthritis
KWRHip II right m7216561Osteoarthritis
KWLHip II leftm7216561Osteoarthritis
PFLHip II leftm9817549Osteoarthritis
RHRHip II right f60N/A63Osteoarthritis

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:

Patient Anteversion Angle AV [degree]CCD Angle [degree] Neck Length L [mm] Shaft Angle S [degree]
EBL5 135 6010
EBR5 135 6010
IBL14 135 609
JBR10 135 6010
HSR4 135 6210
KWR-2 135 629
KWL17 135 628
PFL23 135 627


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 = αx = +2° around axis +x
  2. clockwise by angle α2 = α= -2° around axis +y
  3. clockwise by angle α3 = α= -15° around axis +z

For this right-sided implant “HIP JOINT III”, a negative angle αz 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 = αx = +17° around axis +z
  2. clockwise by angle α2 = α= +8° around axis +x
  3. clockwise by angle α3 = α= 0° around axis +y

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


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 T11), T22), T33). The complete transformation matrix T’ is then:

T‘ = T33) * T22) * T11) 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‘ = Tz(-15°) * Ty (-2°) * Tx(2°)        for  F = T‘ *F

T = Tx (-2°) * Ty(+2°) * Tz(+15°)      for  F’ = T *F


For example 2 they are:

T’ = Ty (0°) * Tx(8°) * Tz(17°)         for F = T‘ * F

T = Tz (-17°) * Tx(-8°) * Ty(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]

Measuring Units

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)

4-channel telemetry transmitter

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)

8-channel telemetry transmitter


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)

9-channel telemetry programed
9-channel telemetry transmitter



OrthoLoad Videos

Video format

The older OrthoLoad videos have a frame rate of 25 fps (time resolution = 0.04 s). Newer videos run at a speed of 50 fps (time resolution = 0.02 s).

Video size

All OrthoLoad videos have a size of 1024×768 pixels. In order to see them in best quality in a media player, choose Zoom = 100%. On a notebook with a resolution of 1024×768 pixels you should play the videos in the ‘Full Screen’ modus.

Video codec

We encode all files using the Windows Media Encoder 9 which produces files in the wmv-format (Windows Media Video). The quality and file size are at least as good as those of avi-files using the DivX or Xvid codec. The wmv- codec is supplied with each Windows installation from Windows XP onwards. In nearly all cases the OrthoLoad videos therefore run on Windows computers without any difficulties. This is an advantage as no additional codec must be installed on your PC, and the videos almost always run in PowerPoint presentations. If the wmv-codec is not installed on your PC, you can download it. Or you can download the newest version of the Windows Media Player which includes the wmv codec (see comments on the Windows Media Player).

Furthermore, we convert all OrthoLoad videos in the mp4-video format(H.264) using ffmpeg codec library. You can play mp4-videos on Windows, Mac OS X and Linux computers with VLC player. We suggest VLC Player 2.0 or higher.

Conversion of wmv video format

If you want to convert an OrthoLoad video to another format, for example to an avi-file with the Xvid-codec, we suggest that you use the free conversion program ‘Super’.

Media player

These are some popular free video players:

Media Player Classic

This simple player must not be installed. It plays 25 fps and 50 fps videos at a correct speed and you can study movements in detail frame by frame by using the arrow key button (forwards only !). The time counter has a resolution of only 1s, but the precise time is shown in the small table ‘Video’ & ‘Time Now’ in the OrthoLoad videos. You can open several instances of the program at the same time, for example if you would like to compare different exercises.

FairPlay Lite

This player is very small, easy to handle and for free. Installation is not required. 50 fps videos are shown at correct speed and you can proceed with the arrow keys. The time counter has a resolution of 0.02s (50 fps). Several instances of the player can be opened.

Windows Media Player

This player is installed together with Windows. It does play 50 fps videos, but it is overloaded with features, and it is difficult to select a single video frame using the mouse. Proceeding frame-by-frame is not possible. The time counter has a resolution of only 1s.

VLC media player

VLC player is a powerfull open source media player für Windows, Mac OS X and Linux. It plays 25 fps and 50 fps videos at a correct speed and you can study movements in detail frame by frame by using the hotkey ‘e’ (see ‘Tools/Options/Hotkeys’).

Other Video Players

Kinovea can be used to play 2 videos synchronously. It doesn’t work properly with wmv-videos, however.

We suggest

Use of the Media Player Classic or FairPlay Lite as the Standard Video Player for the wmv-video format (Windows). Use of the VLC media player as the Standard Video Player for the mp4-video format (Windows, Mac OS X, Linux).



What is OrthoLoad?

OrthoLoad is a public database containing the loads acting in orthopaedic implants. They were measured in vivo using instrumented implants with telemetric data transmission. Currently load data are available for the

Hip joint, shoulder joint, knee joint, vertebral body replacement and internal spinal fixator.

Data is presented as videos, containing the time-dependent forces and moments, force vectors, video images of the patients activity and numerical data. Many results involving the ‘Hip Joint’ are already available in the database. More data are continuously added from the actual ‘Hip Joint III’ where also the three moments were measured. From hip and knee implants, only selected examples are available because access to this data must be detained until it has been published in scientific journals.

From the OrthoLoad database you can select an implant, an activity with parameters, a patient and one or several trials. The time course of the loads is shown in videos (encoded in the wmv- or mp4-format) and can be analyzed in more detail by displaying single video frames. Numerical data files (text files, suffix .akf) can additionally be downloaded and the forces and moments then be used in finite element studies or joint simulators, for example.

It’s recomended to run OrthoLoad on a Windows PCs with Media Player Classic, VLC Player or FairPlay Lite player or on Linux/Mac OS X with VLC Player!


How to use OrthoLoad?

Structure of an ORTHOLOAD

Enter DATABASE and in the top left frame ‘Joint Loads’ you can choose an ‘Implant’ type, an ‘Activity’,  possibly an additional ‘Parameter’, a ‘Patient’ and a trial by ‘File Name’. Then an OrthoLoad video will be shown on the right frame ‘Now Selected’.

Alternatively you may select a trial directly by typing its basic trial number into the upper right frame ‘Select Video by File Name’. The basic trial number is the first part of the file name e.g. ‘s1r_211005_1_80′ of the file ‘s1r_211005_1_80_screen.mp4′

 (s1r = shoulder patient #1 with implant at the right side, _211005 = date: October 21st 2005, _1_80 = internal trial number _screen = video file, _fmax = png-image)

Use the buttons in the top left frame ‘Joint Loads’ to play, show or download videos, data or pictures. You may just play the video in your browser with Flash Plug-in. Or you download and play the video on your computer with your standard video player. We clearly favour the ‘Media Player Classic’ or ‘VLC Player’ for MS Windows and ‘VLC Player’ for Mac OS X and Linux. Videos are encoded in the wmv or mp4(H.264) format.

The vertical marker line inside the force diagram shows the actual measuring time. All other data is displayed synchronously to this time (patient video, vectors, tables …).

The ‘Hip Joint’ was instrumented with a 4-channel telemetry transmitter and only the forces were measured, no moment diagram is shown. All other implants measure forces and moments with measuring units %BW and %BW*m. Using the patients’ body weight (BW), the values in N and Nm can be recalculated.

To show or download a data file *.akf belonging to the video *.wmv or *.mp4 use the buttons ‘Show’ or ‘Download’ in the top left frame ‘Joint Loads’.

Additional Data’ *.eof, *.cof etc. synchronized to the video file is available for some trials


A coil for the inductive power supply is arranged around the implant and the antenna is placed close to it. The data received is at first controlled in regard to transmission errors and then led to a notebook where the forces and moments are calculated and displayed in real time. The images of the patients’ exercises and the synchronous data stream are both recorded on the same video tape. On a monitor or using a video beamer, the forces and moments can be controlled immediately. This allows one to detect unexpected loading situations and immediately modify the way an exercise is performed.



Measuring set-up

During the measurements, a power induction coil and a receiver antenna are fixed near the patient’s implant. The implanted telemetry transmitter is powered inductively by the external magnetic field. The signals measured were transmitted to the external device TELEPORT for on-line force display on a notebook screen. The activities of the patient are videotaped, and the data from the implants are recorded simultaneously on the audio tracks of the same cassette.