Vertebral body replacement

Instrumented implant

Severe compression fractures of a vertebral body or a tumour in the region of the spine sometimes require the replacement of a vertebral body by an implant. The loads on such an implant are not well known. In order to measure these loads, the commercially available vertebral body replacement ‘SYNEX’ was modified. It allows the in vivo measurement of three force components and three moments acting on the implant. The 9-channel telemetry transmitter developed in our biomechanics laboratory was placed into the cylinder of the implant together with 6 load sensors and a coil for the inductive power supply. Usually, the spine is in addition stabilized dorsally by an internal spinal fixation device implanted from the back side.


Implant: vertebral body replacement

Coordinate system

The bone-based coordinate system was chosen according to ISO 2631. The x- axis in the median plane points anteriorly, the y-axis in the frontal plane to the left side, and the z-axis cranially.

The forces and moments are presented in the measuring units N and Nm.

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Coordinate system Vertebral body replacement



Table with basic information about those patients who had vertebral body replacements:

Patient Gender Weight [kg] Age at Implantation [years] Indication
WP1 m 66 62 Fracture L1
WP2 m 72 71 Fracture L1
WP3 f 64 69 Fracture L1
WP4 m 60 64 Fracture L1
WP5 m 63 67 Fracture L3


Internal Spinal Fixator

Instrumented implant

Little was known about the loads acting on internal spinal fixators. In order to measure the loads, a commercially available implant was modified. A measuring cartridge was integrated into the longitudinal rod containing six load sensors, an 8-channel telemetry transmitter, and the secondary coil for the inductive power supply.

Both telemeterized fixators transmit their load values as a radio frequency pulse train outside the body. For the measurements, a flat power coil, fixed to the patient’s back, supplies the energy needed by both fixators. The power coil has an integrated antenna which delivers the signals to the external components of the telemetry system.

Implant: Internal spinal fixator

Coordinate system

The internal fixators were implanted pairwise. All reported data came from the left implant and are reported in a right-handed coordinate system.

The measured load components act at the centre of the cylindrical part of the implant. The z-axis is

the long axis of the fixator and points upwards. The y-axis is parallel to the axis of the Schanz screw and points ventrally. The x-axis is perpendicular to both others and is directed to the right side. All force components Fx, Fy, Fz act in axis directions while the moment components Mx, My, Mz turn clockwise around the axes.

Due to the anatomical conditions at the implantation site this coordinate system does not coincide exactly with the sagittal and frontal plane of the upper body. The forces and moments are presented in the measuring units N and Nm.



Table with basic information about those patients who had instrumented spinal fixators:

PatientGenderWeight [kg]Age at Implantation [years]Indication
MSf7559Degenerative instability L3
NFm9034Compression fracture (old)
HSf6654Compression frature L3
FJm8072Degenerative instability L4
JTm7536Compression fracture T11
BBm8142Degenerative instability L4
JWf5346Compression fracture (old) T12
HBf8562 Compression fracture (old) L1
LGf4847 Compression fracture L1
AGf6854 Compression fracture T12


Knee joint

Instrumented implant

In order to obtain realistic loading data, a knee implant with a 9-channel telemetry transmitter was developed which enables six-component load measurements in a primary total knee replacement. Both forces in axial, medio-lateral and anterio- posterior direction and flexion-extension, varus-valgus and internal-external moments can be measured.The instrumented knee joint is a modification of the INNEXTM System, Type FIXUC (Zimmer GmbH, Winterthur, Switzerland). The standard femur component and tibial insert are used. Only the tibial component was modified to enable the integration of the electronic devices. During modification of the tibial component, the patients’ safety was deemed to be especially important.

Coordinate system

The coordinate system of the instrumented knee implant is a

a right- handed coordinate system fixed at the right tibial implant (not at the bone!). If forces and moments are measured in a left knee, they are transformed to the right side. The coordinate system is located at the height of the lowest part of the polyethylene insert. The z-axis is aligned with the stem axis of the implant.

The force components +Fx, +Fy and +Fz act in lateral, anterior and superior direction on the tibial tray. The moment Mx acts in the sagittal plane of the tibial component and turns clockwise around the +x-axis. The moment My acts in the frontal plane and turns clockwise around the +y-axis and the moment Mz turns clockwise

around +z-axis in the transverse plane. A positive moment Mz acts if the tibial implant component (or the femur) rotates inwards and/or if the tibia bone rotates outwards. The OrthoLoad videos show the load components relative to the tibial tray. The stem axis z of the tibial implant component is rotated backwards in the sagittal plane by about 7 degree relative to the long axis of the tibia bone. This slope of the implant varies inter-individually.

Coordinate system knee joint



Table with basic information about the knee joint patients:

PatientSideGenderWeight [kg]Height [cm]Age at Implantation [years]Tibio-femoral anglePosterior slopeIndication
K1Lleftm100177633.0 – varus5Osteoarthritis
K2Lleftm93171715.0 – varus 11 Osteoarthritis
K3Rrightm95175703.5 – varus 10 Osteoarthritis
K4R right f92170634.5 – valgus3 Osteoarthritis
K5R right m94175601.0 – varus7 Osteoarthritis
K6Lleftf76174654.0 – valgus7 Osteoarthritis
K7L left f70166746.5 – varus7 Osteoarthritis
K8L left m77174704.0 – varus11 Osteoarthritis
K9L left m100166757.0 – varus6 Osteoarthritis


Shoulder joint

Instrumented implant

The picture shows an instrumented shoulder implant capable of measuring forces, moments and, in addition, the temperature acting in the glenohumeral joint. It was developed in the Biomechanics Lab of the Charité and contains a measuring unit with 6 semiconductor strain gauges and a 9-channel telemetry transmitter. Each strain gauge requires one channel of the telemetry while the remaining three channels are used for transmitting the temperature, the current supply voltage and a synchronising signal. At the lower end, an inductive coil ensures the power supply. The measuring signals are led with a pacemaker feed-through to the antenna (protected by a cap of PEEK) which transmits the signals to the external measuring unit.

Implant: Shoulder joint

Coordinate system

Humerus system

All loads are displayed as acting at the humerus. They are based on the ISB- recommended coordinate system (Wu et al., 2005) for the right shoulder joint. In this bone-based shoulder coordinate system, the positive x-Axis points in the anterior, the y-axis in the superior and the z-axis in the lateral direction. The moments Mx, My and Mz turn clockwise around the +x, +y and +z axes.

This system is right-handed for a right shoulder joint. For patient S3L, who obtained her implant on the left side, all values are mirrored to the right side to make it comparable to the other patients.

Coordinate System Shoulder Joint
Implant System Shoulder Joint

Implant system

In the implant-based coordinate system of the shoulder joint, the positive z-axis coincides with the neck of the implant and points in the medial- cranial direction. x- and y-axes are in the plane perpendicular to the implant neck. Axis x points laterally and y is oriented anteriorly. Load components relative to this implant-base system may be used to test fatigue or wear of implants, for example.

Implant System Shoulder Joint

To obtain the forces and moments relative to the implant, the retroversion of the humeral head has to be known, indicated as α in the picture below. It can be measured relative to the anatomical landmarks of the epicondyles at the elbow or related to the orientation of the forearm in 90° elbow flexion as it is chosen during surgery (Hernigou et al., 2002). For some patients in OrthoLoad exact values for the retroversion to the epicondyles are available from a postoperative CT, taken for medical reasons. For the other patients a retroversion angle of 30° relative to the forearm in 90° elbow flexion was assumed as chosen by the surgeon during implantation.

The retroversion value for each patient can be found in the “Info Patient” window in OrthoLoad as the third rotation angle (picture below, right). In this example the given rotation angle of 63° corresponds to a retroversion angle of 27° (90°-63°). The other two angles are determined by the geometry of the implant and are therefore the same for all patients. The vector plot pictures (below, left) are simplified representations for better visualisation. The shown angle α is always the same and differs from the true angle in the patients.

General advice for the transformation of loads from a bone-based to an implant-based system is described here.

Implant System Shoulder Joint

Scapula system

To obtain the loads relative to the scapula, a coordinate transformation would be required, taking into account the relative movement between humerus and scapula. This requires an accurate movement analysis. Such transformations are already planned but are not yet available.




Table with basic information about the shoulder joint patients:

Patient Side Gender Weight [kg] Height [cm] Age at Implantation [years] Indication
S1R right m 101 186 69 Osteoartheritis
S2R right m 85 161 61 Osteoartheritis
S3L left f 72 168 70 Osteoartheritis
S4R right f 50 154 80 Osteoartheritis
S5R right f 103 163 66 Osteoartheritis
S6R right m 135 186 50 Osteoartheritis
S7R right m 89 172 68 Osteoartheritis
S8R right m 83 173 72 Osteoartheritis

Hernigou, P., Duparc, F., Hernigou, A., 2002. Determining humeral retroversion with computed tomography. J

Bone Joint Surg Am 84-A, 1753-1762 ( Wu, G., van der Helm, F.C., Veeger, H.E., Makhsous, M., Van Roy, P., Anglin, C., Nagels, J., Karduna, A.R., McQuade, K., Wang, X., Werner, F.W., Buchholz, B., 2005. ISB recommendation on definitions of joint coordinate systems of various joints for the reporting of human joint motion–Part II: shoulder, elbow, wrist and hand. J Biomech 38, 981-992 (