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.

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
HBf8562Compression fracture (old) L1
LGf4847Compression fracture L1
AGf6854Compression fracture T12