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Internal spinal fixator

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Instrumented implant

Implant: internal spinal fixator
Implant: internal spinal fixator

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.

Coordinate system

Coordinate system Internal Spinal Fixator
Coordinate system Internal Spinal Fixator

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.


Pat WFI MS big.png Pat WFI NF big.png Pat WFI HS big.png Pat WFI FJ big.png Pat WFI JT big.png
Pat WFI BB big.png Pat WFI JW big.png Pat WFI HB big.png Pat WFI LG big.png Pat WFI AG big.png

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

Patient Gender Weight
Age at
MS f 75 59 Degenerative instability L3
NF m 90 34 Compression fracture (old) L4
HS f 66 54 Compression fracture L3
FJ m 80 72 Degenerative instability L4
JT m 75 36 Compression fracture T11
BB m 81 42 Degenerative instability L4
JW f 53 46 Compression fracture (old) T12
HB f 85 62 Compression fracture (old) L1
LG f 48 47 Compression fracture L1
AG f 68 54 Compression fracture T12