Direct real-time measurement of in vivo forces in the lumbar spine

Abstract

Background context: Accurate knowledge of the mechanical loads in the lumbar spine is critical to understanding the causes of degenerative disc disease and to developing suitable treatment options and functional disc replacements. To date, only indirect methods have been used to measure the forces developed in the spine in vivo. These methods are fraught with error, and results have never been validated using direct experimental measurements.

Purpose: The major aims of this study were to develop a methodology to directly measure, in real time, the in vivo loading in the lumbar spine, to determine if the forces developed in the lumbar spine are dependent on activity and/or posture and to assess the baboon as an animal model for human lumbar spine research based on in vivo mechanical loading.

Study design: Real-time telemetered data were collected from sensor-imbedded implants that were placed in the interbody space of the lumbar spines of two baboons.

Methods: An interbody spinal implant was designed and instrumented with strain gauges to be used as a load cell. The implant was placed anteriorly in the lumbar spine of the baboon. Strain data were collected in vivo during normal activities and transmitted by means of a telemetry system to a receiver. The forces transmitted through the implant were calculated from the measured strain based on precalibration of the load cell. Measured forces were correlated to videotaped activities to elucidate trends in force level as a function of activity and posture over a 6-week period. The procedure was repeated in a second baboon, and data were recorded for similar activities.

Results: Implants measured in vivo forces developed in the lumbar spine with less than 10% error. Loads in the lumbar spine are dependent on activity and posture. The maximum loads developed in the lumbar spine during normal (baboon) activities exceeded four times body weight and were recorded while animals were sitting flexed. Force data indicate similar trends between the human lumbar spine and the baboon lumbar spine.

Conclusions: It is possible to monitor the real-time forces present in the lumbar spine. Force data correlate well to trends previously reported for in vivo pressure data. Results also indicate that the baboon may be an appropriate animal model for study of the human lumbar spine.