Chapter 10 Anatomy Biomechanics Lumbar Spine · 2011. 1. 4. · The disc surrounds the intervertebral foramen anteriorly, the pedicle inferiorly and superiorly, and the zygapophyseal
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The anterior wall of the vertebral canal is flattened, and the discs demonstrate no
propensity of bulging into the spinal canal. The anterior wall of the vertebral canal is formed by the
posterior surfaces of the lumbar vertebrae, and the posterior wall is formed by the lamina and
ligamentum flava of the same vertebrae (1). The disc surrounds the intervertebral foramen
anteriorly, the pedicle inferiorly and superiorly, and the zygapophyseal joints posteriorly (1).
Figure 10.1: The Intervertebral Foramen
Table 10.2: Joints of the Lumbar Spine. Joint
Information
Intervertebral disc • The intervertebral disc (IVD) connects the bodies of adjacent vertebrae together. It is classified as a symphysis or amphiarthrosis
• 3 components: 1) Annulus: collagen makes up 50–70% of its weight. Fibers are arranged in concentric rings around the nucleus. These concentric sheaths are called lamellae. They are orientated at an angle of 65–70° from the vertica,l with adjacent layers running in opposite 65° orientations from the vertical (i.e., they criss-cross). The outer portion of the annulus is the only innervated portion of the disc. 2) nucleus: comprises the central portion of the disc. There is no clear boundary between the nucleus and the annulus. The nucleus is 70–90% water. 3) vertebral end plate: is 0.6–1 mm thick
Zygapophyseal (facet) joints
• The facets are oval in shape, are slightly curved or biplanar, and are oriented parallel to the frontal plane. The facet orientation angle changes with respect to the mid-sagittal plane (Bogduk):
• Zygapophyseal joints take ~20% of the spinal weight-bearing load. With flexion, you increase the weight bearing on the disc and decrease the weight bearing on the facet joints
The Zygapophyseal Joints
The zygapophyseal joints, also known as facets or apophysial joints, are enclosed in a
fibrous capsule that contains menisci. The menisci are invaginations of the joint capsule and may
occasionally project into joint space (2). Facets do not have “free” motion as does the disc and are
limited both structurally and by the capsule. Movement is generally restricted to large sagittal
motions guided by the shape of the zygapophyseal joints.
The facets flatten anteroposterior and run slightly dorsally and upward (1). The
zygapophyseal joints and the surrounding structure represent attachment sites for several
intertransverse ligaments and muscles. The intertransverse ligaments attach to each transverse
process and limit side flexion to the opposite side. The transverse process of L5 attaches to the
medial portion of the iliac crest by several strong strands of the iliolumbar ligament, which tends to
ossify at older ages. The anterior portions of the lumbar facets orient coronally (promote side-bend
forces). The posterior facets face sagittal and resist rotation and side-bend forces (2).
Figure 10.2: Zygapophyseal Joints of the Lumbar Spine
and inserts on the hip. This lever mechanism is too inefficient to produce lumbar movement (23).
Many clinicians misperceive the role of the psoas as a significant contributor to low back pain.
Bogduk states, “The isometric morphology of the psoas indicates that the muscle is designed
exclusively to act on the hip” (23). Although a maximum contraction can increase intradiscal loads
(23), this muscle may not contribute to stabilize the spine.
Summary
• Much of low back movement is controlled and guided by the intervertebral disc. • The shape of the lumbar vertebral body and disc promotes a natural lordosis. • The orientation of the facets limits lumbar rotation and aids in stability during rotation. • Several muscles of the lumbar spine are the primary source of dynamic stability,
functioning as prime movers and stabilizers.
BIOMECHANICS OF THE LUMBAR SPINE
The lumbar spine has six degrees of freedom and is generally described by movements
associated with flexion, extension, rotation, and side flexion (Table 10.5).
Table 10.5: Specific Biomechanics and Movement of the Lumbar Spine Region
Biomechanics and Movement
Flexion
• The lower lumbar segments rotate forward from a backward tilted position (reducing umbar lordosis). The lumbar lordosis typically will only reach the neutral position and rarely achieves a kyphosis.
• The superior vertebra rotates anteriorly in the sagittal plane on the inferior vertebra, raising the inferior articular process of the superior vertebra upward and slightly backward, and opening a small gap between the superior and inferior articular facets. Anterior sagittal plane translation then occurs (as a result of gravity or muscular contraction), closing this gap. Impaction of the inferior facet against the anteromedial portion of the superior facet restricts anterior sagittal plane translation. Tension of the articular capsule also limits flexion
• The sacrum nutates the forward nodding movement of the sacrum between the hip bones (innominates) with the sacral base moving anterior and inferior and the sacral apex moving posterior and superior
Extension
• Consists of a posterior sagittal plane rotation combined with a small posterior sagittal plane translation
• The facet joints have a limited role in restricting extension • The ALL and annulus restrict extension along with SP approximation • The canal spinal and the intervertebral foramen diameter decrease
Side flexion • The ipsilateral superior facet slides down the adjacent inferior facet, while the contralateral superior facet slides upward in relation to the contralateral inferior facet
• Can be thought of as unilateral flexion on the contralateral side and unilateral extension on the ipsilateral side
Rotation • Impaction of the contralateral articular facets limit axial extension • The joint space is very narrow; therefore, ROM permitted is small
Many manual therapy disciplines base specific mobilization and manipulation techniques
on selected theories of lumbar coupling direction, theories that are often inconsistently reported
(25). Biomechanical analysis including investigation of coupled motion is often reported as an
essential concept to low back evaluation (26–30). The two principle components of lumbar coupling
are quantity of motion, used in detection of hypo and hypermobility, and direction of coupling
behavior. The most controversial of the two assessment methods is the theory of directional lumbar
coupling, a theory based on the invalidated premise that a “normal” lumbar coupling pattern exists
in nonpathological individuals (31–33). It has been suggested that the link between pathology of the
lumbar spine may be best represented by addressing the pattern or direction of coupling behavior
(30,34–39).
Coupled motion is the rotation or translation of a vertebral body about or along one axis
that is consistently associated with the main rotation or translation about another axis (39). During
movement, translation occurs when movement is such that all particles in the body at a given time
have the same direction of motion relative to a fixed coordinate system (39). With movement,
rotation occurs as a spinning or angular displacement of the vertebral body around some axis.
Historic, foundational works on coupling mechanics used observation or controversial two-
dimensional (2-D) radiographic imagery (30). Past 2-D studies involved cadaveric tissue, X-rays of
live subjects, or single X-rays of segments, and used a small sample of subjects (30,40). Prior to
1969, only 2-D studies were executed for spinal coupling, signifying that any study performed prior
to 1969 encompassed these errant methods (30). 2-D imagery leads to magnification errors,
projection of translations as rotations, and misleading results (30,40). Theories such as Fryette’s
with others who have reported similar numbers and similar declines with advancing age (63,64).
Rotation and side flexion have also been studied extensively.
Summary
• Little to no evidence exists to support the use of directional lumbar coupling biomechanics for manual therapy techniques.
• The use of coupling assessment as a basis for treatment is neither scientific nor evidence based, and therefore may be inappropriate for certain patients.
• The majority of segmental range of the motion in the lumbar spine occurs within a sagittal plane, followed by coronal, and lastly, transverse.
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