Instrumented Spinal Fixation Devices

Unstable spines are often stabilized using an internal spinal fixation device. If, for example, a vertebra is fractured, screws are fixed posteriorly to the adjacent vertebrae left and right from the spinous process. On both sides a longitudinal rod is fixed to the screws.
The spinal load is then partly shared by the paired implanted fixators. In a second operation a cage or/an iliac crest bone graft is inserted into the fractured region from anterior. Usually, the bone graft will fuse with the adjacent vertebrae within a year and form a stable block.

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Instrumented Spinal Fixator

Wirbel-Fixateur interne, Original und instrumentiert, Schnittmodell
Spine model with two instrumented fixation devices

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 needed energy to both fixators. The power coil has an integrated antenna which delivers the signals to the external components of the telemetry system.

Measurements

Messung mit Wirbel-Fixateur interne

A video camera records all movements of the patient and together with the received pulse trains the images are stored on a video tape. Simultaneously the signals are read into a PC, the forces and moments are calculated and displayed in real time on a monitor.

The first instrumented spinal fixators were implanted in 1994. Until now, ten patients received a pair of the instrumented fixators. The loads on the fixators have been measured for many positions and activities of daily life, including lying, sitting, standing, walking and physiotherapy.

Patients

Patients with instrumented Internal Spinal Fixators during load measurements

Publications

2003
ISSLS prize winner: A novel approach to determine trunk muscle forces during flexion and extension: a comparison of data from an in vitro experiment and in vivo measurements.

Wilke H.J., Rohlmann A., Neller S., Graichen F., Claes L., Bergmann G.:
Spine: 28(23) 2585-93, 2003, 2.45 Impact Factor
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2002
Loads on an internal spinal fixation device during physical therapy.

Rohlmann A., Graichen F., Bergmann G.:
Phys Ther: 82(1) 44-52, 2002
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Spinal load bearing during sitting in an office chair with a tilting back
Rohlmann A., Wilke H.J., Graichen F., Bergmann G.:
Biomed Tech: 47(4) 91-6, 2002
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2001
Is it possible to simulate physiologic loading conditions by applying pure moments? A comparison of in vivo and in vitro load components in an internal fixator.

Wilke H.J., Rohlmann A., Neller S., Schultheiss M., Bergmann G., Graichen F., Claes L.E.:
Spine: 26(6) 636-42, 2001, 2.45 Impact Factor
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Loads on an internal spinal fixation device during sitting.
Rohlmann A., Arntz U., Graichen F., Bergmann G.:
J Biomech: 34(8) 989-93, 2001, 2.75 Impact Factor
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Comparison of intradiscal pressures and spinal fixator loads for different body positions and exercises.
Rohlmann A., Claes L.E., Bergmann G., Graichen F., Neef P., Wilke H.J.:
Ergonomics: 44(8) 781-94, 2001
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2000
Changes in the loads on an internal spinal fixator after iliac-crest autograft.

Rohlmann A., Bergmann G., Graichen F., Weber U.:
J Bone Joint Surg Br: 82(3) 445-9, 2000
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Influence of load carrying on loads in internal spinal fixators.
Rohlmann A., Graichen F., Bergmann G.:
J Biomech: 33(9) 1099-104, 2000, 2.75 Impact Factor
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2000 Volvo Award winner in biomechanical studies: Monitoring in vivo implant loads with a telemeterized internal spinal fixation device.
Rohlmann A., Graichen F., Weber U., Bergmann G.:
Spine: 25(23) 2981-6, 2000, 2.45 Impact Factor
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1999
Loads on internal spinal fixators measured in different body positions.

Rohlmann A., Bergmann G., Graichen F.:
Eur Spine J: 8(5) 354-9, 1999
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Braces do not reduce loads on internal spinal fixation devices.
Rohlmann A., Bergmann G., Graichen F., Neff G.:
Clin Biomech: 14(2) 97-102, 1999
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Loading on internal spinal fixation devices
Rohlmann A., Bergmann G., Graichen F., Weber U.:
Orthopade: 28(5) 451-7, 1999
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1998
Placing a bone graft more posteriorly may reduce the risk of pedicle screw breakage: analysis of an unexpected case of pedicle screw breakage.

Rohlmann A., Bergmann G., Graichen F., Mayer H.M.:
J Biomech: 31(8) 763-7, 1998, 2.75 Impact Factor
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Influence of muscle forces on loads in internal spinal fixation devices.
Rohlmann A., Bergmann G., Graichen F., Mayer H.M.:
Spine: 23(5) 537-42, 1998, 2.45 Impact Factor
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1997
Loads on an internal spinal fixation device during walking.

Rohlmann A., Bergmann G., Graichen F.:
J Biomech: 30(1) 41-7, 1997, 2.75 Impact Factor
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Comparison of loads on internal spinal fixation devices measured in vitro and in vivo.
Rohlmann A., Bergmann G., Graichen F., Weber U.:
Med Eng Phys: 19(6) 539-46, 1997
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1995
Telemeterized load measurement using instrumented spinal internal fixators in a patient with degenerative instability.

Rohlmann A., Bergmann G., Graichen F., Mayer H.M.:
Spine: 20(24) 2683-9, 1995, 2.45 Impact Factor
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In vivo measurement of implant loads in a patient with a fractured vertebral body.
Rohlmann A., Bergmann G., Graichen F., Weber U.:
Eur Spine J: 4(6) 347-53, 1995
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In-vitro measurement of loading using an instrumented vertebral internal fixator
Rohlmann A., Eick O., Bergmann G., Graichen F.:
Z Orthop Grenzgeb: 133(2) 136-40, 1995
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1994
A spinal fixation device for in vivo load measurement.

Rohlmann A., Bergmann G., Graichen F.:
J Biomech: 27(7) 961-7, 1994, 2.75 Impact Factor
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1993
Stress measurements with an instrumented internal spinal fixator

Rohlmann A., Eick O., Bergmann G., Graichen F.:
Biomed Tech: 38(10) 255-9, 1993
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Barbara Schiller

Secretariat Prof. Hendrik Schmidt

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