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6 Biom ech anics of the Spine and Spinal In strum entat ion
e. Some evidence suggests that thoracolumbar burst fractures m ay not
require fusion.
(1) Surgical m anagem ent of thoracolum bar fractures involves the
restoration of vertebral height and avoidance of kyphosis.
(2) Often supplem ented with transpedicular screw xation for rigid
stabilization.
(3) In addition, fusion enables early rehabilitation and am bulation.
(4) Recent, albeit weak, evidence suggests that posterior instrum entation
alone provides comparable outcomes to instrumentation and fusion
for t h oracolu m b a r fract u r e s .
5. Spinal deformities in the thoracolumbar spine:
a. Scoliosis.
(1) Lateral exion with rotation of the spinous process toward the
concavit y of the spine.
(2) Hypokyphosis and posterior wedging of the vertebral body are
frequ en tly seen (Fig. 6.13).
(3) Correction.
(a) Thoracic curve: distraction on the concave side corrects coronal
alignment and produces thoracic kyphosis, which is generally
desired.
(b) Lum bar cur ve: com pression on th e convex side corrects coronal
alignment and restores lumbar lordosis.
Fi g . 6 . 1 3 Hypokypho sis an d p o st e rior we dging o f t he ve rteb ral body.
6 Biom echanics of t he Spine and Spinal Inst rum e nt ation 97
(c) Bending and translation:
i. Cantilever bending and segm ental xation correct th e
deformity in the coronal and sagittal plane.
ii. Rotation–derotat ion also corrects the defor m it y in coronal
and sagittal planes by shifting the regions of the spine en bloc.
b. Kyphosis.
(1) The anterior colum n fails with compression.
(2) The posterior colum n fails with tension.
(3) Deform ity increases the mom ent arm , further increasing deform ity.
(4) Eccentric loading a ects cartilaginous grow th.
(a) Com pression decreases grow th anteriorly.
(b) Tension increases grow th posteriorly.
(5) Various instrum entations are used to correct kyphotic deform ities.
(a) Posterior com pression rods can correct mild and exible curves
(Fig. 6.14).
(b) Greater curves should be approached by com bin ed an terior an d
posterior fusion and instrumentation.
i. Posterior instrum entat ion is applied w ith cantilever bending
and compression forces.
Fi g . 6 . 1 4 Post e rio r rod in st rum e nt at ion .
98
6 Biom ech anics of the Spine and Spinal In strum entat ion
G. Lu m b a r an d lu m bosacral sp in e.
1. Stability (Fig. 6.15):
a. Anterior stability:
(1) Anterior longitudinal ligam ent.
(2) Vertebral body.
(3) Annulus brosus.
b. Posterior stability:
(1) Facet joint.
c. The role of the m uscles, including the erector spinae, abdom inal m uscles,
and psoas, is important in overall stability.
a b
c d
Fig . 6 . 1 5 ( a – d ) White an d Panjab i’s criteria for lum bar se gm e nt al inst abilit y.
6 Biom echanics of t he Spine and Spinal Inst rum e nt ation 99
2. Lower lumbar and lumbosacral spine.
a. Degenerative disk and facet diseases.
(1) Intervertebral disk.
(a) Annulus brosus.
i. Collagen bers arranged in an obliqu e direct ion .
ii. Provides axial loading stabilit y and 40 to 50% torsion al
stability.
(b) Nucleus pulposus.
i. Th e gel-like core acts as a ball bearin g, ch an ging the center of
rotation.
(2) Intradiskal pressure.
(a) The load on the disk is about tw ice the body weight w hen sitting.
(b) Thirty percent lower disk pressure w h en standing.
(c) Fifty percent lower disk pressure when lying on the side.
(d) Eight y to 90% lower disk pressure when lying supine compared
with sitting.
(3) Disk degeneration.
(a) Shifts the instant center of rotation posteriorly.
(b) Increases st ress to th e facet joint.
(c) Disk degeneration a ects the motion of the functional spinal
unit.
i. Early disk degeneration w it h radial tears of the annulus
brosus decreases sti ness in exion, lateral bending, and
rotation.
ii. Advanced disk degeneration w ith loss of disk heigh t and
osteophytes increases the sti ness.
(d) P rrm ann classi cation of disk degeneration (Fig. 6.16):
i. Based on T2-weigh ted MRI ndings.
ii. Provides a standard nom enclature to classify disk
degeneration.
(4) Facet joints.
(a) Provide torsional stability.
(b) Suppor t < 20% of load w ith weight bearing.
b. Spondylolisthesis.
(1) Abrupt change in sti ness across the lum bosacral junction.
(2) The pars interarticularis is strong but susceptible to fatigue fractures,
especially with extension injuries.
(3) Shear stress at the pars interarticularis:
(a) Physiological exion contracture of the hip and secondary
hyperlordosis create a pincerlike e ect from the superior
articular process of S1 and the inferior articular process of L4.
100
6 Biom ech anics of the Spine and Spinal In strum entat ion
Fi g . 6 . 1 6 Algorithm for a grading system and assessm ent of lum bar disk degeneration. (P rrm ann
CWA, Met zdorf A, Zanet ti M, et al. Mag n e t ic reso n a n ce classi cat io n of lum bar int e r ve rt e b ral disc
degeneration. Spine 2001;26(17):1873. Lippincott Williams & Williams, Inc. Used with permission.)
H. Biom echanics of transpedicular instrum ent ation.
1. Anatomy.
a. The pedicle is a cylinder of cortical bone.
(1) The horizontal diam eter from T9 to L5 increases from 7 mm to 1.5 cm
(Fig. 6.17).
(a) The vertical diam eter is ~ 1.5 cm.
(b) The inner diam eter is < 80% of the outer diam eter.
(c) The pedicle screw diam eter should be smaller than the inner
diameter.
(2) The pedicle depth is ~ 45 to 50 mm from the entrance point to the
anterior vertebral margin.
b. Entrance points and directions (Fig. 6.12):
6 Biom echanics of t he Spine and Spinal Inst rum e nt ation 101
Fi g . 6 . 1 7 Pedicle d iam et er (m m ) as a fun c t io n of sp ina l le ve l. (Court esy of Or t h o b u lle t s.)
2. Pedicle screw design and biomechanics.
a. Structural characteristics.
(1) The most signi cant factor for pullout strength is the outer (major)
diameter.
(2) Deeper threads increase the pullout strength.
(a) Bending strength dim inishes due to a smaller minor diam eter
(Fig. 6.18).
(b) Pullout strength is also increased by the depth of pen et rat ion .
(c) Not signi cantly a ected by the shape of thread.
3. Transverse connectors.
a. Important for less rigid systems.
b. Important if triangulation technique is to be used.
c. Im portant in the osteoporotic spine.
d. Improves torsional stability of the construct.
102
6 Biom ech anics of the Spine and Spinal In strum entat ion
Fig . 6 . 1 8 La t e r a l r a d i o g r a p h o f t h e l u m b a r
spine. Note that the sacral screw has broken
because fusion has not occurred. Pedicle
screws will eventually fail with cyclical loading if a so lid arthro d e sis is not obt a in e d .
I. Sacra l/ pelvic xat io n .
1. Types:
a. Galveston technique.
b. Iliosacral screw.
c. Sacral screws.
d. Sacral alar screws.
e. Iliac screws (iliac bolts).
f. Tran silia c b a r.
g. S2 alar iliac screw (S2AI).
h. Intrasacral rods (Jackson).
i. Du nn–McCarthy rods (through S1 foram en).
2. Screws are generally better than hooks in the sacrum.
a. Sacral xation with a single sacral screw has a high failure rate (pullout).
b. S1 screw:
(1) Anterom edial direction toward the sacral prom ontory below the
superior sacral end plate.
(a) Safest and biom echanically acceptable.
c. S2 screw:
(1) Weak but m ay enhance overall stability.
(2 ) The screw is d ire ct ed 30 to 40° laterally.
(a) Avoid penetrance of the anterior cortex if the screw is directed
laterally to avoid injuries to the iliac vein , lum bosacral trunk, and
sigmoid colon.
6 Biom echanics of t he Spine and Spinal Inst rum e nt ation 103
J. In s t r u m e n t a t io n r i g id it y a n d s t i n e ss .
1. A rigid implant construct increases fusion mass.
2. May also cause device-related osteoporosis (stress shielding):
a. Stress shielding by rigid instrumentation is ~ 15%.
b. The bene t of rigid instrumentation outweighs the stress-shielding
phenomenon.
Sugg este d Reading
Izzo R, Gu ar n ieri G, Gu glielm i G, Mu to M. Biom e chanics of t he sp in e. Part I: sp in al st abilit y.
Eu r J Radiol 2013 ;82 (1):118– 126
Izz o R, Gu ar n ie r i G, Gu glielm i G, Mu to M. Biom ech an ics of t h e sp in e . Pa r t II: sp in al in st a-
bilit y. Eur J Radiol 2013 ;82 (1):127–138
Perez- Orribo L, Kalb S, Reyes PM, Chang SW, Craw ford NR. Biom e ch an ics of lum bar cor ti-
cal screw-rod xation versus pedicle screw-rod xation with and w ithout interbody
su p p ort . Spin e 2013;3 8(8):635–64 1
Su r at wa la SJ, Pin t o MR, Gilb e r t TJ, W in te r RB, W ro blew sk i JM. Fun ct io n a l a n d r a d io logical
outcomes of 360 degrees fusion of three or m ore motion levels in the lumbar spine for
degenerative d isc disease. Spin e 200 9;34(10):E3 51–E358
7 Physiology of Bone Healing and Fusion
7.1 General Considerations
I. Bon e cells a n d e x t ra cellu lar m at r ix .
A. Ce lls (Fig. 7.1).
1. Osteoblasts.
a. Responsible for forming the structural bone matrix and regulating
osteoclast activity.
b. Derived from the bone marrow stromal cells and periosteal membrane
cells.
c. Secrete type I collagen.
d. Express parathyroid hormone receptors and alkaline phosphatase.
(1) Critical for regulating bone production.
2. Osteocytes.
a. Active osteoblasts embedded within the mineralized matrix.
b. Do not express alkaline phosphatase.
c. Com municate via canaliculi and regulate bone hom eostasis.
104
Fi g . 7 . 1 Bo n e re s o r p t io n a n d b o n e fo r m a t io n a n d a s s o c ia t e d f a c t o r s .
7 Physiolog y of Bo ne Healing and Fusion 105
3. Osteoclasts.
a. Multinucleated bone resorbing cells.
b. Hematopoietic cells that derive from the monocyte/macrophage family.
c. These cells have a ru ed border that secretes proteases and ions, which
help dissolve the bony matrix (Howship’s lacunae) (Fig. 7.2).
d. Tightly regulated by the receptor activator of nuclear factor kB ligand
(RANKL).
B. Ex t r a ce ll u la r m a t r i x ( ECM ).
1. The ECM consists of 60 to 70% mineral matrix and 20 to 25% organic matrix.
a. Mineral matrix.
(1) Provides the compressive strength of bone.
(2) Calcium (hydroxyapatite) and phosphate (tricalcium phosphate)
make up the majority of the mineral matrix.
b. Organic m atrix.
(1) Com posed prim arily of type I collagen (90%).
(a) Its triple helical conform ation contributes tensile strength to the
ECM.
II. Bon e fo r m at ion .
A. Os s i c a t io n (Fig. 7.3).
1. Intramembranous (e.g., pelvic bones): formation of bone directly from
mesenchymal tissue.
2. Endochondral (e.g., vertebrae): mesenchymal tissue is rst replaced by a
cartilage model, which then undergoes ossi cation.
B. Ke y g r ow t h fa ct or s a n d c y t o k in e s .
1. Bone morphogenetic proteins (BMPs).
2. Transforming growth factor-b (TGF-b).
3. Basic broblast growth factor (bFGF).
4. Insulin growth factor (IGF).
5. Interleukins (ILs).
Fi g . 7 . 2 Micro sco pic h e m a toxylin-eosin image of osteoclasts and Howship’s lacunae.
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