each side from neutral). Patients lay on the table in either the supine or lateral
decubitus position with their hips and pelvis on the lower part of the table whilst
their torso remains stationary. As for the weight-bearing procedure, the standard
range of trunk motion in the coronal plane is 40 degrees left and right but, in the
sagittal plane, the range is 40 degrees extension and 40 degrees flexion. This
accounts for the flattening of the lumbar lordosis that occurs as the patient is
recumbent.
The cervical OSMIA (see figure 3) uses the same bucky as the weight-bearing
lumbar spine OSMIA but with a different attachment guiding the patient’s neck
through flexion and extension. For both recumbent and weight-bearing lumbar
spine OSMIAs, the x-ray beam is centred to the mid-lumbar region (lumbar
vertebrae 3/4) and the fulcrum of the table or disc. For the cervical spine, the
beam is centred to cervical vertebrae 3/4 and collimated accordingly.
Figure 3. Cervical spine OSMIA
The other main feature of OSMIA is
Figure 1. Weight-bearing OSMIA
automatic vertebral tracking, undertaken
following acquisition of the fluoroscopic
loop of spinal motion. The images are
individually extracted and the first image
is obtained. Pixel recognition templates
are manually placed around each vertebra
in the field of view (see figure 4) before
the templates automatically scan every
subsequent image to obtain the best fit
(ie they follow the vertebrae through
the motion sequence). An output of
absolute vertebral angles is produced
which are adjacently subtracted (eg L5
- L4) to produce continuous intervertebral
rotation (see figure 5) and translation
data. Combining rotation and translation
allows the calculation of the instantaneous centre of rotation (ICR), a biomechanical
Figure 4. Automatic vertebral tracking. Pixel
term used to describe a precise point of rotation of a vertebra with respect to its recognition templates automatically follow
neighbour over a given interval of time. Of course, if the vertebra is translating
the vertebrae throughout the fluoroscopic
Figure 2. Recumbent OSMIA
as well as rotating, then the ICR will be located at different points throughout
sequence.
the movement, as demonstrated in cadaveric spines
60
. The location of the ICRs
alter spinal motion, particularly voluntary or involuntary contractions (‘guarding’ or throughout the bend may be another indicator of spinal instability, or abnormal
muscle spasm), leading potentially to false conclusions about the quality of inter- motion but, until recently, it has been impossible to measure these in vivo without
vertebral motion. Recumbent OSMIA overcomes this by measuring passive motion taking multiple radiographs
61
.
only (ie motion not influenced by muscles). For recumbent OSMIA, a specially
designed passive motion table (Atlas Clinical Ltd) fits onto existing fluoroscopy The inter-observer error for OSMIA is 1.86 degrees for rotation
52
and 0.72mm for
tables; the lower half swings through an arc of up to 80 degrees (40 degrees translation (previously unreported). Earlier OSMIA studies of the lumbar spine
2009
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