Biomedical Engineering Reference
In-Depth Information
Fusion techniques for the spinal column were based on the concept that instability causes pain and
hence restriction of the mobility would offer relief. Clinical trials have shown that spinal fusion
offered favorable outcomes by stabilizing the spine column and providing pain relief and consistent
improvement of neurologic symptoms. The overall rates of improvement in neurological status were
83.6%, 83.2%, and 88.9% at 24, 36, and 60 months after cervical spinal fusion (Burkus et al. 2010).
A prospective study reported a four-year overall success rate of 72.5% for the fusion surgery (Sasso
et al. 2011).
However, approximately 3% of patients with spinal fusion annually suffer from adjacent segment
degeneration (ASD) (Hilibrand and Robbins 2004). Baba et al. (1993) observed that 25% of their 100
patients subsequently developed new spinal canal stenosis above the previously fused segments. Gore
and Sepic (1984) observed new spondylosis in 25% of 121 patients and progression of preexisting
spondylosis in another 25% of 121 patients who had previously undergone anterior cervical fusion.
Bohlman et al. (1993) found that 9% of their 122 patients went on to develop adjacent segment
diseases requiring additional surgery. In addition, Williams, Allen, and Harkess (1968) found that
17% of their 60 patients undergoing anterior cervical decompression and fusion developed adjacent
segment disease and needed additional surgery.
Previous research attributed this complication to fusion-induced abnormal kinematics, hyper-
mobility, and an increase in intradiscal pressure (IDP) at the adjacent level. These symptoms are
directly associated with adjacent disc degeneration (Eck et al. 2002; Dmitriev et al. 2005). Finite-
element models of the cervical spine have been used to investigate the effect of cervical spine fusion
on adjacent levels (Hong-Wan, Ee-Chon, and Qing-Hang 2004). It was reported that increasing the
stiffness of intervertebral graft materials is associated with increased internal stress at adjacent
levels.
16.1.3 p
romiSinG
a
lternatiVe
to
S
pinal
f
uSion
: t
otal
d
iSc
r
eplacement
Non-fusion treatments, including nucleus replacement and total disc replacement, were developed
for preserving movement functions and decreasing physical strain and stress on the adjacent seg-
ments. As an alternative to spinal fusion, total disc replacement aimed to restore segment mobility
and load-carrying capacity and was expected to avoid inducing adjacent segment degeneration.
Total disc replacement surgery involves removing the pain-causing disc and replacing it with a
mechanical device that mimics normal spine kinematics. In the lumbar region, disc replacement is
indicated for patients with disc-related back pain, whereas in the cervical region, wider indications
could be addressed, including myelopathy and radiculopathy.
Clinical studies have shown that total disc replacement offers satisfactory clinical results for the
treatment of disc degeneration diseases. A prospective randomized controlled clinical trial of the
Prestige replacement disc showed that the overall rates of maintenance or improvement in neuro-
logical status in the total disc replacement group were 91.6%, 92.8%, and 95.0%, respectively, at
24, 36, and 60 months (Burkus et al. 2010). Another clinical trial demonstrated a 93.3% success for
Bryan replacement disc arthroplasty with a neck disability index (NDI) score of 51 preoperatively
and 16.7 at 48 months (Garrido, Taha, and Sasso 2010).
In theory, cervical disc replacement should decrease the development of adjacent segment
degeneration by maintaining normal disc kinematics. In cadaveric studies, cervical arthroplasty
has shown the ability to maintain motion and mechanics within physiologic ranges at the operative
segment and decrease stresses on adjacent segments (Dmitriev et al. 2005). Although few clini-
cal studies have specifically aimed to evaluate adjacent segment degeneration after cervical disc
replacement, some reports have shown that total disc arthroplasty did not influence the incidence of
adjacent segment disease in the cervical spine (Yang et al. 2012; Jawahar et al. 2010). Meta-analysis
indicated that there were fewer incidences of adjacent segment disease and adjacent segment sur-
gery in comparison to cervical disc replacement with anterior cervical discectomy and fusion, but
the difference was not statistically significant (Yang et al. 2012). Jawahar et al. (2010) used three
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