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

etween T6 and T9 with a higher degree of kyphosis, thus supporting out hypothesis. In
the study of Briggs et al. [14], two-thirds of the wedge shaped vertebral fractures are
reported to occur between T6 and T9, which is in agreement with our results. Briggs et
al. used a sagittal-plane model to explore the difference in compression and shear
loading between two groups of patients, with the spinal sagittal profile derived from
lateral radiographs. Their study also demonstrated increased compression forces in the
lower thoracic spine, anywhere from 1.1 – 17.1% higher for normal erect standing. The
current musculoskeletal model allows the investigation of more complex activities,
highlighting further the risk for subsequent fractures in the mid- to lower-thoracic spine.
The second goal of this study was to evaluate the influence of sarcopenia (of the erectus
spinae) on local spinal segment loading. Our model showed that higher erector spinae
strength leads to an increase in the compression and shear forces in the lumbar region,
but does not influence the compression and shear forces in the thoracic spine. Therefore
the hypothesis that sarcopenia increases spinal load could not be supported by the
present simulation.
The main limitation of the present model is the assumed spinal rhythm. With the
standard spinal rhythm of the AnyBody model, the total angulation of the spine is
distributed between the individual segments, using predefined, constant ratios. As the
internal disc moment is dependent on the relative angle between two neighbouring VBs,
the predefined motion distribution introduces a degree of uncertainty in the true balance
between internal (disc) and external (muscle) moments.
Validation was performed by comparing our model predictions to the compression
changes reported in the literature for the transition from the upright to the flexed posture
(i.e. normalised to standing). The VBR device is an instrumented spinal implant that
measures the compression force between L1 and L2 [11], the IDP was measured by
Wilke et al. [12] at the L4-L5 spine level , while the muscle activation and subsequent
prediction of the compression forces was taken at L2-L3 [13]. Additionally, these values
are compared with the musculoskeletal model of Han et al. [5]. The normalised
compression force between L1 and L2 reported by Rohlmann A. et al. [11] with the
VBR (233%) is similar to the normalised compression force at this level in our model
with a normal kyphotic curvature (237%). Also Han et al. [5] reported values in the
same range for the same situation (211%). Schultz et al. [13] reported a normalised
compression force between L2 and L3 of 270% based on EMG measurements. For
comparison, our model predicted only 196% for the normalised compression force at
this level, and also Han et al. [5] predicted a lower value (232%). Wilke et al. [12] have
derived a normalised compression force of 220% at L4-L5 with their IDP
measurements. This is again closer to our prediction of 226% and Han et al. [5] reported
values in the same range (240%). Despite the difference between our model and the
force predictions based on EMG measurements (-27%), our model showed good
agreement with both the VBR outcome (+2%) and the IDP results (+3%).
In summary, the present simulation model has demonstrated the importance of sagittal
balance for segmental loading. Increasing kyphosis is associated with higher segmental
loads, and by association with an increased risk of vertebral fracture. The influence of
muscle changes due to sarcopenia is less clear, however model improvements,
specifically in the timing and pattern of the motion of individual segments, may allow a
more definite prediction of the consequences of this poorly understood condition.