1.3 Objetivos de la investigación
1.3.2. Objetivos específicos
The role of axial torque and facet orientation in intervertebral disc injuries and lower back pain is a controversial element of many studies. Previous studies have indicated that disc failure and degeneration can occur only under torsion forces and the facet joints play a protective role (Farfan and Sullivan 1967; Farfan et al. 1970). However, these studies had no final conclusion.
For example, (Adams and Hutton 1980; Adams and Hutton 1981; Kelsey et al. 1984; Shirazi-Adl et al. 1986; Shirazi-Adl 1989) all suggested contrary views, indicating that the torsion alone or without any applied lifting activities cannot cause disc injury. They showed that the apophyseal joint was the first structure that resisted the compression and torsion forces of the lumbar spine.
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In a limited number of single motion segment studies (Ueno and Liu 1987; Shirazi-Adl 1991), the significant role of the facet joint in axial torque has been confirmed. These studies concluded that the facet could endure 10-40 % of the applied torsion forces depending on the gap distance between the superior and inferior articular facets and on which segment that the load was applied. However, (Shirazi-Adl 1994)stated that the gap distance might be affected by the condition of the cartilaginous layer of the articular surface of facet joints.
Similarly, (Criswell 2013) stated that the posterior elements of the spinal segments (the facet joint) has the role in restricting the spinal motion. For example, in the lower thoracic region, axial rotation had an increase in motion of 40 % after posterior elements were removed.
Accordingly, when a tear occurs in capsular ligaments and the volume increases due to hypertrophy, the facets begin to move medially and exhibit tropism. These changes reduce vertebral foramina volume, and ultimately cause anatomic and dynamic foraminal stenosis. The nerve root in each involved foramen becomes compressed by surrounding degenerative tissues.
In this condition, the facets have been identified as a source of pain. With facet degeneration, it is also common to experience disc degeneration at that level. As the disc’s degeneration progresses, the stresses are increased at the facets which propagates the degeneration at the facet location.
Many attempts have been made to replace degenerated facet joints and one of these solutions was the artificial facet (Ozer et al. 2015; Criswell 2013).
Artificial facet replacement is indicated for back and leg pain caused by lumbar stenosis with advanced facet disease requiring destabilizing facetectomy for adequate decompression. Stenosis resulting from grade one degenerative spondylolisthesis represents an ideal indication for artificial facet replacement.
Artificial facets are robust biomechanical devices that are intended to replace the facets and the posterior elements in the lumbar spine. In contradistinction to posterior dynamic stabilisation devices, artificial facets are intended to be a stand-alone, motion- preserving alternative to standard lumbar fusion (Figure 3-11).
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Figure 3-11: ACADIA (Globus Medical) facet replacement system (after Coric 2014)
Generally, many different designs for an artificial facet have been introduced, such as ACADIA (Globus Medical) and TOPS (Premia Spine), the ACADIA facet replacement system is a titanium pedicle screw–based system with bilateral articulating elements that mimic native facets. While, the TOPS artificial facet is a pedicle screw–based system that consists of a single titanium articulating post covered by a cushioning polyurethane layer covered by a polyurethane boot (Coric 2014).
Meanwhile, facet joint replacement devices can be used to replace painful facet joints, restore stability, and/or to retain a failed disc or nucleus prosthesis without losing motion, does not fully restore joint functionality. Therefore, any facet replacement device will require a significant amount of controlled clinical studies before being brought to market. Therefore, in case of artificial facet implants, the artificial facet would have to mate with the natural articular process mechanical properties.
In general, the major factors that lead to success or failure of arthroplasty are the size of the joint surface (stress at the joint surface), the degree and extent of multiplanar motion and/or load transfer through the device or joint, the strength and size of the anchor points, long-term performance (Serhan et al. 2007).
Based on the degree and extent of multiplanar motion and the gapping distance between two articular processes of the normal facet, it has been suggested that the facet angulation can result in a rotational coupling phenomenon in the lumbar spine. In turn, facet orientation is associated with the degree of rotation along the different body axes at each individual motion segment (Kapandji 1974; Ahmed et al. 1990; Van Schaik et al. 1997).
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(Masharawi et al. 2004) mentioned that regardless of which axes in which the lumbar segment motion is involved, a complex multiplanar movement can occur. This could explain the mechanism of rotational movement in the lumbar spine, which depends on the more angulated left facet.
Finite element, cadaveric, MRI and CT models, instrumented spatial linkage methods, modified protractors, micro scribes and three-dimensional apparatus have all been used to evaluate the orientation and the tropism angle of the lumbar facet, depending on the reference lines. The reference lines used can be classified as a mid-sagittal line which passes through the intervertebral disc and divides the spinous into equal halves or as a transverse and a coronal plane. The reference line of the right and left facet was defined as a line connecting two endpoints on the articular facets (Farfan and Sullivan 1967; Noren et al. 1991; Panjabi et al. 1993; Tulsi and Hermanis 1993; Boden et al. 1996; Masharawi et al. 2004; Kozanek et al. 2009). However, (Kozanek et al. 2009) measured the transverse facet angle as the angle between the line of the facet width projected onto the transverse plane of the vertebra and the antero-posterior axis of the vertebrae. However, to reduce the time of the radiation exposure, this study did not evaluate the in vivo instantaneous positions of the lumbar vertebrae during dynamic motion of the body. In addition, the L5 segment was not involved as a result of the limited field of view.
On other hand, based on both techniques, which were defined by (Farfan and Sullivan 1967; Boden et al. 1996), (Chadha et al. 2013;Wang et al. 2015) introduced a different technique to measure the facet tropism angle as being located between the line which is drawn between the two margins of the superior articular facet and a mid-sagittal line, which passes through the intervertebral disc centre and the centre of the base of the spinous process.
A review of the literature reveals that only one study has measured the gaping distance between the superior and inferior articular facets. (Cramer et al. 2000) measured the gaping distance of the articular joint as the linear distance from the centre of the facet joint passing from the inferior to the superior articular processes.
The difference between these previous methods of measurements and the present study is that the present method will present safe, healthy volunteer with natural physiological movements with the consideration of the angle of rotation of the intervertebral disc. It
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will also take differences between the solid compact bone of the articular process and the soft tissues, such as the capsular ligaments, into account. These. As these ligaments tend to prevent the accurate determination of the antero-posterior boarders of the articular processes as a result of the effect of the torsion (Chapter 4).
Three voluntary lower trunk rotational positions will be performed to evaluate the hypothesis that the mean differences between the orientations angle of the right and the left superior articular processes at each individual lower lumbar segment will be the highest at the first rotational position of the lower trunk (mean pelvis angle (90°). It will also be used to test the hypothesis that the cross-sectional area of the opening of the left articular facets at each individual lower lumbar segment will be the highest at the first rotational position of the lower trunk (mean pelvis angle: 90°).
Quantification of the effect of the three voluntary lower trunk rotational positions on the orientation angle of the left and right lower lumbar superior articular processes and on the amount of change in the cross sectional area of the lumbar spine facet opening will be evaluated using 3D T2 MRI images and adjusting the image contrast.
The current study will address the hypothesis that providing highly accurate data of the degree of orientation of the articular processes and the gapping distance between two articular processes of the normal facet during lower trunk rotation will help to successfully design an artificial facet.