ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

3. IN VIVO MEASURED LOAD COMPONENTS
A direct in vivo measurement of all load components is not yet possible. Intradiscal
pressure has been measured by Nachemson [1, 2], Wilke et al. [3, 4], Sato et al. [5] and
others. It is a measure for the overall spinal load and thus allows a comparison of spinal
loads for different activities. Intradiscal pressure has been measured for various
activities. The magnitude varied between 0.1 MPa for a lying supine position to 2.3
MPa for lifting 20 kg with a rounded back. Standing caused a typical value of 0.5 MPa.
The loads on internal spinal fixation devices have been measured in 10 patients by
Rohlmann et al. [6, 7]. A comparison of the peak intradiscal pressures measured by
Wilke et al. [3] and the peak flexion bending moments in the spinal fixation devices –
related each time to the corresponding value for standing – showed for many activities a
very good agreement [8].
Recently, Rohlmann et al. [9-11] also measured the loads on a vertebral body
replacement in 5 patients. In these patients, the resultant force on the implant during
walking was nearly twice the value of that for standing. This value increased to 225%
when walking down stairs and even to 265% when walking up stairs. They found that
the spinal loads can be strongly reduced by leaning against a backrest while sitting, by
supporting the upper body with the arms, e.g. during rising from a chair, by placing the
hands on the armrest or thighs during sitting, as well as by carrying a weight in a
backpack or laterally instead of in front of the body.
4. ANALYTICAL MODELS FOR SPINAL LOAD ESTIMATION
Spinal loads can also be estimated by using analytical models. Schultz and Andersen
[12] for example used a model that had 10 single-equivalent muscles. Since the number
of unknowns exceeded the number of equations, their model led to a statically
indeterminate problem. Therefore, they used an optimization method to calculate the
forces. They calculated a spinal force of more than 1000 N when a weight of 40 N was
held at a distance of 40 cm from the spine.
A detailed spine model using commercially available software (AnyBody Technology,
Aalborg, DK) was recently introduced by De Zee et al. [13]. Han et al. [14] added short
segmental muscles, lumbar ligaments and disc stiffnesses to that model and performed a
validation study. They predicted the highest muscle forces in the m. longissimus (146
N) for flexion, in the m. rectus abdominis (363 N) for extension, and in the m. psoas
major (144 N and 81 N) for lateral bending and axial rotation.
5. FOLLOWER LOAD
In in vitro experiments, ligamentous spinal specimens are unstable and buckle under
vertical compression forces [15, 16]. A specimen which is loaded in such a way that the
compressive load path passes the centres of rotation is able to carry high loads without
buckling [17-19]. It is assumed that the follower load simulates the upper body weight
and the stabilizing effect of the in vivo acting muscle forces [16]. The follower load path
should be optimized in finite element models. Otherwise, flexion angles of up to 10°