lumbar spine has 5 vertebrae

1. BIOMECHANICS
OF
LUMBAR SPINE

2. Structure of the Lumbar Region
2
● The first four lumbar L1-L4 vertebrae are similar in
structure.
● The fifth lumbar L5 vertebra has structural
adaptations for articulation with the sacrum.

3. Typical Lumbar Vertebrae
3
Body
● Transverse diameter is greater than the anterior
diameter and height.
● The size and shape reflect the need to support great
compressive loads caused by body weight, ground
reaction forces, and muscle contraction.

4. 4

5. 5

6. 6
Arches
● Pedicles.
○ short and thick
○ project posterolaterally
● Laminae
○ short and broad

7. 7
Arches
● Zygapophyseal Articular Processes (facets)
○ Both the superior and inferior zygapophyseal facets vary
considerably in shape and orientation
○ Mamillary processes
• appear as small bump, are located on the posterior edge of each
superior zygapophyseal facet
• serve as attachment sites for the multifidus and medial
intertransverse muscles.
○ The inferior zygapophyseal facets are vertical and convex and
face slightly anteriorly and laterally

8. 8

9. 9
Arches
● Transverse Process.
○ Long and slender and extends horizontally.
○ Accessory processes:
• small and irregular bony prominences,
• located on the posterior surface of the transverse process near its
attachment to the pedicle (Fig. C).
• serve as attachment sites for the multifidus and medial
intertransverse muscles.

10. 10
Arches
● Spinous Process
○ broad and thick
○ extends horizontally
● Vertebral Foramen
○ Triangular
○ Larger than the thoracic vertebral foramen
○ Smaller than the cervical vertebral foramen.

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12. 12

13. 13

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15. 15Arches
Fifth lumbar vertebra (L5)
● It is a transitional vertebra and differs from the rest of the
lumbar vertebrae
○ L5 has a wedge-shaped body
• anterior portion of the body is of greater height than the posterior portion
● The L5/S1 lumbosacral disk also is wedge shaped.
● The superior diskal surface area of L5 is about 5%
greater than the areas of disks at L3 and L4.
● The inferior diskal surface area of L5 is smaller than the
diskal surface area at other lumbar levels.
● The spinous process of L5 is smaller than other lumbar
spinous processes

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17. 17

18. 18
● Lumbosacral articulation is
formed by the fifth lumbar
vertebra L5 and first sacral
segment S1.
● The first sacral segment, which
is inclined slightly anteriorly and
inferiorly, forms an angle with
the horizontal called the
Lumbosacral Angle Lumbosacral Angle is
determined by measuring the
angle formed by a line drawn
parallel to the superior aspect of
the sacrum and a horizontal line.

19. 19
Lumbosacral Angle
● The size of the angle
○ varies with the position of the pelvis
○ affects the superimposed lumbar curvature
● An increase in this angle will
○ result in an increase in lordosis of the lumbar curve
○ increase the amount of shearing stress at the lumbosacral
joint

20. Intervertebral Disks
20
● Specific regional variations occur in the intervertebral disks
of the lumbar region
• which differ from the disks of the cervical region
○ Collagen fibers of the anulus fibrosus are arranged in sheets called
LAMELLAE
● The lamellae are arranged in concentric rings that surround
the nucleus.
● Collagen fibers in adjacent rings are oriented in opposite
directions at 120o to each other.

21. Intervertebral Disks
21
● Advantage of the varying fiber orientation by layer:
○ Anulus fibrosus is able to resist tensile forces in nearly all directions
● The lumbar intervertebral disks are the largest in the body
○ as are the vertebral bodies
● The shape of each disk is not purely elliptical but concave
posteriorly.
● This provides a greater cross-sectional area of anulus fibrosus
posteriorly and increased ability to resist the tension (posterior
stretch) that occurs in flexion

22. Articulations
22
Interbody Joints
● The interbody joints of the lumbar region are
capable of translations and tilts in all directions.

23. Articulations
23
Zygapophyseal Joints
● The zygapophyseal joints of the lumbar region
● like all others
○ are true synovial joints

24. Articulations
24
Zygapophyseal Joints
● The joint capsules are
○ more lax than in the thoracic region
○ but more taut than those of the cervical region.
● The dorsal capsule has been demonstrated to be
fibrocartilaginous in nature,
○ so this portion of the capsule is subject to compressive as
well as tensile forces

25. Articulations
25
Zygapophyseal Joints
● In a newborn,
○ Zygapophyseal joints in the lumbar region lie predominantly
in the frontal plane in the presence of lumbar kyphosis.
● As the child develops and assumes an upright
posture,
○ Curve of the lumbar region changes to lordosis, and the
orientation of the zygapophyseal joints change as well.

26. Articulations
26
Zygapophyseal Joints
● The orientations of the adult lumbar
zygapophyseal joints
○ display great variability both between individuals and within
individuals;
○ the majority of them have a curved structure that is biplanar
in orientation.

27. Articulations
27
Zygapophyseal Joints
● The anterior aspect of each joint remains in the
frontal plane
● The posterior aspect lies close to or in the sagittal
plane
● The degrees to which this happens vary.

28. 28

29. Articulations
29
Zygapophyseal Joints
● The frontal plane orientation
○ Provides resistance to the anterior shear
• that naturally is present in the lordotic lumbar region.
● The sagittal plane orientation
○ allows the great range of flexion and extension ROM
○ provides resistance to rotation.

30. Ligaments and Fascia
30
● The supraspinous ligament is well developed only
in the upper lumbar region and may terminate at L3,
although the most common termination site appears
to be at L4.
● The ligament is almost always absent at L5/S1.
● The deep layer of the supraspinous ligament is
reinforced by tendinous fibers of the multifidus
muscle.
● The middle fibers of the supraspinous ligament blend
with the dorsal layer of the thoracolumbar fascia.

31. Ligaments and Fascia
31
● The intertransverse ligaments are not true ligaments
in the lumbar area and are replaced by the iliolumbar
ligament at L4.
● The PLL is only a thin ribbon in the lumbar region,
whereas the ligamentum flavum is thickened here.
● In a study of 132 lumbar spine ligaments,
○ The interspinous ligament had the least overall stiffness and
the joint capsules the highest.
○ The anterior longitudinal ligament is strong and well developed
in this region

32. Ligaments and Fascia
32
Iliolumbar Ligaments
● Consist of a series of bands that extend from the tips and
borders of the transverse processes of L4 and L5 to attach
bilaterally on the iliac crests of the pelvis

33. Ligaments and Fascia
33
Iliolumbar Ligaments
● The iliolumbar ligaments as a whole are very strong
and play a significant role in stabilizing the fifth
lumbar vertebra (preventing the vertebra from
anterior displacement) and in resisting flexion,
extension, axial rotation, and lateral bending of L5 on
S1.

34. 34

35. Ligaments and Fascia
35
Thoracolumbar Fascia
● Also called the Lumbodorsal Fascia
● Consists of three layers:
○ Posterior,
○ Middle,
○ Anterior

36. 36

37. Ligaments and Fascia
37
Thoracolumbar Fascia
Posterior layer
● Large, thick, and fibrous
● Arises from the spinous processes and supraspinous
ligaments of the thoracic, lumbar, and sacral spines.
● Gives rise to the latissimus dorsi cranially, travels
caudally to the sacrum and ilium, and blends with the
fascia of the contralateral gluteus maximus.

38. 38

39. Ligaments and Fascia
39
Thoracolumbar Fascia
Middle Layer
● Posterior layer becomes the middle layer
● Travels medially again along the anterior surface
of the erector spinae
● Attaches back to the transverse processes and
intertransverse ligaments of the lumbar spine.
These two layers completely surround the lumbar
extensor muscle group.

40. Ligaments and Fascia
40
Thoracolumbar Fascia
Anterior Layer
● Derived from the fascia of the quadratus lumborum
muscle, where it joins the middle layer, inserts into
the transverse processes of the lumbar spine, and
blends with the intertransverse ligaments

41. Ligaments and Fascia
41
Thoracolumbar Fascia
● Stabilizing corset that forms a ring around the
abdomen along the abdominal muscles and their
fascia
● Anterior layer of the thoracolumbar fascia as the
“passive part”
● Posterior layer as the “active part.”
● Passive part serves to transmit tension produced by
a contraction of the hip extensors to the spinous
processes.

42. Ligaments and Fascia
42
Thoracolumbar Fascia
● Passive part serves to transmit tension produced by
a contraction of the hip extensors to the spinous
processes.
● Active part is activated by a contraction of the
transversus abdominis muscle, which tightens the
fascia.
● The fascia transmits tension longitudinally to the tips
of the spinous processes of L1/L4 and may help the
spinal extensor muscles to resist an applied load

43. Ligaments and Fascia
43
Thoracolumbar Fascia
● Both the gluteus maximus and contralateral
latissimus dorsi tensed the superficial layer and
provided a pathway for the mechanical transmission
of forces between the pelvis and the trunk

44. Function of the Lumbar Region
44
● Kinematics
● Kinetics

45. Kinematics
45
● The lumbar region is capable of movement in flexion,
extension, lateral flexion, and rotation.
● The lumbar zygapophyseal facets favor flexion and
extension, because of the predominant sagittal plane
orientation

46. Kinematics
46
● Flexion of the lumbar spine is more limited than
extension
● Normally, it is not possible to flex the lumbar region
to form a kyphotic curve
● The amount of flexion varies at each interspace of
the lumbar vertebrae,
but most of the flexion takes place at the
lumbosacral joint.
● During flexion and extension,
○ The greatest mobility of the spine occurs between L4 and S1,
which is also the area that must support the most weight.

47. Kinematics
47
● Rotation in this region, is more limited because of
the shape of the zygapophyseal joints
● The effectiveness of the zygapophyseal joints in
resisting axial rotation depends on the extent that the
superior facets face medially (in the sagittal plane).
● The greater the medial orientation of the joint
surfaces, the greater the resistance to axial rotation.

48. Kinematics
48
● In the lumbar region,
○ Pure flexion and extension can occur
○ But coupled motions always occur with lateral flexion and axial
rotation.
● With lateral flexion,
○ pronounced flexion and slight ipsilateral rotation occurs.
● With axial rotation,
○ substantial lateral flexion in a contralateral direction occurs,
○ but only a slight amount of flexion occurs.
● Lateral flexion and rotation
○ are most free in the upper lumbar region
○ progressively diminish in the lower region.

49. Kinematics
49
● The largest lateral flexion ROM and axial rotation
○ occurs between L2 and L3
● Little or no lateral flexion or rotation is possible at the
lumbosacral joint
○ because of the most common orientation of the zygapophyseal
joints
● Considerable amount of variation in the degree of axial
rotation of lumbar vertebrae
● Amount of rotation available at each vertebral level is
affected by
○ facet orientation
○ position of the lumbar spine

50. Kinematics
50
● When the lumbar spine is flexed, ROM in rotation is
less than when the lumbar spine is in the neutral
position.
● The posterior anulus fibrosus and the PLL seem to
play an important role in limiting axial rotation when
the spine is flexed.
● The zygapophyseal joint capsules limit rotation in
both the neutral and extended positions of the spine.

51. Kinematics
51
Lumbar-Pelvic Rhythm
● It is a specific instance of coordinated, simultaneous activity of
lumbar flexion and anterior tilting of the pelvis in the sagittal plane
during trunk flexion and extension. This combined lumbar and pelvic
motion lumbar-pelvic rhythm.
● The activity of bending over to touch one’s toes with knees straight
depends on lumbar-pelvic rhythm.
● First part of bending forward consists of lumbar flexion, followed
next by anterior tilting of the pelvis at the hip joints
● A return to the erect posture is initiated by posterior tilting of the
pelvis at the hips, followed by extension of the lumbar spine.
● The initial pelvic motion delays lumbar extension until the trunk is
raised far enough to shorten the moment arm of the external load,
thus reducing the load on the erector spinae.

52. Kinematics
52
● The integration of motion of the pelvis about the hip
joints with motion of the vertebral column
○ increases the ROM available to the total column, and also
○ reduces the amount of flexibility required of the lumbar region.
● Hip motion may eliminate the need for full lumbar
flexion,
○ which would serve a protective function by protecting the
anulus fibrosus and posterior ligaments from being fully
lengthened.

53. Kinetics
53
Compression
● One of the primary functions of the lumbar region is
to provide support for the weight of the upper part of
the body in static as well as in dynamic situations.
● The increased size of the lumbar vertebral bodies
and disks
• in comparison with their counterparts in the other regions
○ helps the lumbar structures support the additional weight.
● The lumbar region must also withstand the
tremendous compressive loads produced by muscle
contraction

54. Kinetics
54
Compression
● Experimental testing of 10 cadaver spines subjected to
1000N compressive loading demonstrated that the
lumbar interbody joints shared 80% of the load, and the
zygapophyseal facet joints in axial compression shared
20% of the total load.
● This percentage can change with altered mechanics:
○ with increased extension or lordosis, the zygapophyseal joints will
assume more of the compressive load.
● Also, with degeneration of the intervertebral disk, the
zygapophyseal joints will assume increased compressive
load.

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● Khoo and colleagues lumbosacral loads (ground
reaction forces and accelerations plus forces
generated by erector spinae and rectus abdominis
muscle groups) at the center of the L5/S1 joint in
static versus dynamic situations in 10 men.
● Lumbosacral loads in the erect standing posture
were in the range of 0.82 to 1.18 times body weight,
whereas lumbosacral loads during level walking
were in the range of 1.41 to 2.07 times body weight
(an increase of 56.3%)

56. 56
● Changes in position of the body will change the
location of the body’s line of gravity and thus change
the forces acting on the lumbar spine.

57. Kinetics
57
Shear
● In the upright standing position,
● the lumbar segments are subjected to anterior shear
forces cause by the lordotic position, the body
weight, and ground reaction forces
● This anterior shear or translation of the vertebra is
resisted by direct impaction of the inferior
zygapophyseal facets of the superior vertebra
against the superior zygapophyseal facets of the
adjacent vertebra below

58. Kinetics
58
Shear
● The effectiveness of the zygapophyseal joint in providing
resistance to anterior translation during flexion depends
on the extent to which the inferior vertebra’s superior
facets lie in the frontal plane and face posteriorly.
● The more that the superior zygapophyseal facets of an
adjacent inferior vertebra face posteriorly, the greater the
resistance they are able to provide to forward
displacement because the posteriorly facing facets lock
against the inferior facets of the adjacent superior
vertebra.