Loads on a spinal implant measured in vivo during whole-body vibration

Abstract

After spinal surgery, patients often want to know whether driving a car or using public transportation can be dangerous for their spine. In order to answer this question, a clinically proven vertebral body replacement (VBR) has been modified. Six load sensors and a telemetry unit were integrated into the inductively powered implant. The modified implant allows the measurement of six load components. Telemeterized devices were implanted in five patients; four of them agreed to exposure themselves to whole-body vibration. During the measurements, the patients sat on a driver seat fixed to a hexapod. They were exposed to random single-axis vibrations in X, Y, and Z directions as well as in multi-axis XYZ directions with frequencies between 0.3 and 30 Hz. Three intensity levels (unweighted root mean square values of 0.25, 0.5 and 1.0 m/s 2 ) were applied. Three postures were studied: sitting freely, using a vertical backrest, and a backrest declined by an angle of 25°. The patients held their hands on their thighs. As expected, the maximum force on the VBR increased with increasing intensity and the number of axes. For the highest intensity level and multi-axis vibration, the maximum forces increased by 89% compared to sitting relaxed. Leaning at the backrest as well as lower intensity levels markedly decreased the implant loads. Driving a car or using public transportation systems-when the patient leans towards the backrest-leads to lower implant loads than walking, and can therefore be allowed already shortly after surgery.

In vivo measurements of the effect of whole body vibration on spinal loads

“…This finding is in agreement with those of previous load-measuring studies [13,14,21,22]. Sato et al [22], for example, measured the intradiscal pressure in eight healthy volunteers aged between 22 and 29 years and observed upright standing values between 215 and 747 kPa.…”

“…It has been used in several studies, e.g., to determine the loads for various sitting positions, during WBV in a car-driving simulation, while lifting and lowering weights at various levels, and to determine the effect of a brace [13][14][15].…”

How does the way a weight is carried affect spinal loads?

“…These patients have been involved in several other load-measuring studies (Rohlmann et al 2010(Rohlmann et al , 2011(Rohlmann et al , 2013. Overall, the patients performed approximately 1000 different combinations of activities and parameters.…”

Forces on the spine

“…When sitting freely, an increase in the maximum force on the VBR of 89% compared to sitting relaxed was measured. Leaning against the backrest during whole-body vibration reduced the forces to values lower than those for sitting relaxed without any vibration (Rohlmann et al, 2010). Flexion of the upper body and walking upstairs caused implant loads more than twice as high as during standing.…”