Cervical Spinal Fusion and Total Disc Replacement - Computational Biomechanics of the Musculoskeletal System - page 206

flexion and extension. At all the levels considered, the ROM was greater than the average value of
Panjabi et al.'s (2001) study but less than the values of Finn et al.'s (2009) study during axial rotation.
An inverse phenomenon was observed during lateral bending. Although these values were different
than the average value of in vitro studies, they fell into the standard deviation of the experimental
data. Typically, the standard error measurements associated with different studies are high, due to
the inter-subject differences, making validation of gross kinematics difficult. As such, more rigor-
ous and difficult validation is necessary by comparing results that depend on both kinematic and
material considerations such as intervertebral disc pressures.
Intradiscal pressure is considered the main biomechanical factor associated with disc degenera-
tion and has been widely studied in previous in vitro studies. In the study by McNally et al. (1996),
the intradiscal pressure profile was more consistent through the disc cross-section in the posterior
lateral to anterior-posterior direction, with a value of about 1.8 MPa. However, the degenerative
disc had multiple regions of pressure concentrations. In the present study, the maximum intradis-
cal pressure was observed during flexion or lateral bending with values of 2.0 MPa, 1.83 MPa, and
1.83 MPa, at C3-C4, C4-C5, and C5-C6, respectively. Although these values are slightly greater
than McNally's in vitro experimental data, the difference is minute and can be used for compari-
son. Another important consideration was that finite element methods give more discretized field
results than current experimental methods. Intervertebral disc pressure results are generally mea-
sured experimentally using a single needle-type pressure sensor inserted into the nucleus pulposus.
Comparing the pressure results from the current experimental measurements, the finite element
model could provide more discretized field results. The global maximum pressures may potentially
lie outside the region measured by the sensor.

16.3 aPPlICatIon 1: a SImulatIon oF CervICal SPInal arthrodeSIS

16.3.1 m odel d eVelopment
Anterior cervical decompression and fusion is one of the gold standard treatments for degenera-
tive disc diseases. In the present study, a simplified anterior screw/plate fixture system with an
allograft of trabecular bone was used to simulate the spinal fusion surgery. The main dimen-
sion (width × height) of the anterior plate was 12 mm × 25 mm with a thickness of 2 mm,
and the natural intervertebral disc was replaced by allografted cancellous bone (Figure 16.4a).

Plate/screw Graft bone

(a)

Mobi-C

ProDisc-C

C5 C5

(b) (c)

FIgure 16.4 (See color insert.) Surgical details. (a) Anterior cervical decompression and fusion. (b) Total
disc replacement with ProDisc-C. (c) Total disc replacement with Mobi-C.