Pedicle screw fixation is the most
common fixation technique employed in the posterolateral type of spinal
fusion. This method of fixation of lumbar spine for different diseases
has progressively being more used in the last decades. Zindrick (1991)
in his study described it as the method of choice for stabilization of
the lumbosacral spine. Various pedicle screw systems involve insertion
of screws through the pedicle into the vertebral body from the
posterior aspect. In all types of spinal fusion whether it is
transpedicular screw fixation or some other method, the primary aim is
to immobilize a particular spine level (Lorenz etal., 1993; Stefee 1986; Zindrick). Wires and plates might be secured to pedicle screws. Zindrick et al. (1986)
opined that the success of the transpedicular screw fixation technique
depends upon the ability of the screw to obtain and maintain purchase
within the vertebral body. Factors like the size of the pedicle, its
quality of bone, and whether associated with osteoporosis or any other
demineralising diseases, and finally the choice of the screw determine
the prognosis associated with the success of the pedicle screw
fixation. A pedicle is the strongest part of a lumbar vertebra, which
is made up of entirely cortical bone with a small core of cancellous
bone Roy-Camiller et al. (1986). Strong and large pedicles of
lumbar vertebra as compared with the thoracic and cervical ones make
them ideal for screw instrumentation.
Like any other procedure, this
technique also has some serious drawbacks. Amonoo-Kuofi (1995) reported
complications arising due to oversized screws resulting in displacement
of screws, dural tears, leakage of cerebrospinal fluid and injuries to
nerve roots with neurological deficits (Esses & Sachs 1992; Kag et al. 1986;Matuokae et al. 2002; Weinstein et al. 1992;
Zindrick et al 1986). Detailed knowledge of the anatomy of the spine,
with the clear understanding of the pedicle screw systems
implementation, can reduce the risks of complication (Weinstein et al. ).
Regarding the size of the screw, Zindrick et al.
(1986) reported that larger diameter screws were stronger and gave
better results, hence majority of surgeons continue preferring as large
a screw as possible for any given pedicle. The screw performance is
also influenced by the geometric variables of the design; improvements
in the pullout strength can be achieved by an increase in the major
diameter of the screw Skinner (1990). The choice of the screw diameter
is determined by the minimum (horizontal) diameter of the pedicle,
whereas the pathway of the screw is decided by the transverse (width)
and vertical (height) parameters of the pedicle (Amonoo-Kuofi; Krag et al. 1986; Singel et al. 2004; Weinstein et al; Zindrick et al. 1987).
Chaynes et al. (2001) in their
cadaveric study concluded that of the pedicular measurements only width
limits the diameter of fixation screws. Goel et al. (2005)
opined that the strength of the bone-screw interface, for a given screw
size (or fill within the pedicle cross section) increases with depth of
screw penetration. Weinstein et al. described that
approximately 60% of fixation strength of the thoracic and lumbar
pedicles lies in the pedicle, whereas 20-25% of fixation strength is
derived from the anterior cortex and the rest 15-20%o of strength comes
from the cancellous bone. Brantly et al (1994) reported that besides
the screw diameter the strength of the interface is also determined by
the degree of fill of the screw with respect to the pedicle
cross-section. Increased percent fill increases vertebral fixation
strength in linear manner, especially if the depth of the screw
penetration was at least 80%o. Zdeblick et al. (1993) found an
inverse relationship between the pedicle width and cycles to failure,
for given screw size. McKinley et al. (1997) reported the
effect of pedicle morphometry on pedicle screw loading in a synthetic
model and suggested that screw bending moments within the pedicle
increased incrementally with increasing pedicle length, rising 30%>
as length increased from 8.0 mm to 12.0 mm, whereas changes in pedicle
width did not affect screw loads within the pedicle. They concluded
that in situ pedicle screw loads increased significantly as pedicle
length increased and as pedicle height decreased (McKinley etal). These
biomechanical studies recommend that a surgeon should select a screw
that has a diameter close to the inner pedicle diameter in order to
achieve a good bone screw interface (Goel et al.). In terms of
the screw itself, the biomechanical data supports the use of a screw
that will fill the cancellous bone region of the pedicle and will
penetrate up to eighty percent of the vertebral body along the pedicle
axis. These recommendations are valid for vertebral bodies with normal
bone quality (nonosteoporotic) (Goel et al.)
Rosol et al. (1996) reported
vertebral morphometry derived from digital images using clinical
radiographs measured with vernier calipers and a film phantom, and
claimed it to be accurate, reproducible, and convenient. The
morphometry of the vertebral pedicle has been studied by various
authors (Hasegawa et al., 1996; Kim et al., 1994; Krag et al. 1988;Matuoka&Basile; Olsewskieía/., 1990; Zindrick et al. 1987)
both in cadavers and imagery examinations. Pedicle screw designing
based on pedicle morphology was the most important indication of all
the aforementioned studies. Majority of pedicle morphometry studies
were based on white population in different parts of the world
(Amonoo-Kuofi; Berry et al. 1987; Coles et al. 1988; Krag et al.; Olsewski etal; Roy-Camille etal, 1984; Saillant 1976; Zindrick 1987).
Weinstein et al. reported
anatomical variations regarding the size, shape and angulations of
pedicle even within the same populations. Amonoo-Kuofi, reported in his
study the horizontal and vertical diameters of the pedicles of the
lumbar vertebrae, which were measured from plane anteroposterior
radiographs of the lumbar spines of male and female subjects aged from
10 to 65 years. He concluded with significant age related variations
along with the significant differences between the pedicle diameters of
male and females.
Indian population forms the one-fifth
of the total population of the world and the non-resident Indians are
also distributed widely in many countries whereas only few studies are
available on this population. Secondly there are very limited
literature reported comparing the direct gross morphometry of lumbar
vertebra with the X-rays studies. Hence, the present study was
undertaken. The present work included gross three dimensional
measurements of the minimum horizontal diameter (d), the vertical
height (v), and the anteroposterior length (1) of the lumbar pedicles,
from bones of Indian population. Further horizontal and vertical
diameters of the pedicles were measured from the plain anteroposterior
radiographs obtained from disease or deformity free individuals from
the same place. Final conclusion was made by comparing the observations
of the two separate studies in a scientific manner.
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