- Morphometry of Vertebral Pedicles: a Comprehensive Anatomical Study in the Lumbar Region

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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