Spine Biomechanics

Chronic low back pain is a significant public health problem in industrialized society. The intact spine carries the upper body and external loads, allows motion in a physiological range and protects the spinal cord. These different demands necessitate a high degree of complexity with various sources for disorders and pain. The Julius Wolff Institute conducts research in order to counter pain causes and to further optimize the pain treatment.

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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.

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