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6 Biom ech anics of the Spine and Spinal In strum entat ion
Ta b l e 6 . 1 Bio m e c h a n i c a l m e a s u r e m e n t s
Pat h o lo g y Me as ure m e nt Abno rm a l No te s
Occiput–C1
inst a b ilit y
Dist a n ce fro m t he t ip
of the dens to basion
of the occiput
Powers rat io use d t o
determine anterior
atlanto-occipital
dislocation
Gre a t e r t ha n 1 m m
translation on exion–
extension is abnormal
A r a t io > 1 s ig n i e s
anterior atlantooccipital instability
Dist an ce b e t we en t he
basion (midpoint of
the anterior margin
of the foramen
magnum) and the
spinolaminar line of
C1 d ivid e d b y t he
distance between the
posterior margin of
the foramen magnum
Ba s ila r
invag in a t io n
McGre g or’s lin e Gre at e r t han 4 .5 m m
odontoid projection
above the foramen
magnum
Ra n a w a t ’s C1 – C2
ind e x
Re d l u n d – Jo h n e l l O–
C2 in d e x
Le s s t h a n 1 3 m m i s
abnormal
Le s s t h a n 3 4 m m
(men)
Le s s t h a n 2 9 m m
(opisthion) and the
posterior margin of
the anterior arch of C1
At l a n t o a x ia l
(C1–C2)
inst a b ilit y
At l a n t o d e n s i n t e r v a l
(ADI)
Spa ce ava ilab le fo r
the cord (SAC)
(women)
ADI > 3 m m in d ic a t e s
rupture of the
transverse ligament
Gre at er t han 5 m m of
ADI in d ic a t e s ru p t u r e
of the transverse and
alar ligament
Great e r t h an 4 .5 m m
is abn o rm al in child ren
SAC < 1 4 m m
im ping e s on t he
Tr a n s v e r s e l i g a m e n t
is e sse nt ial fo r
stabilit y
At l a s f r a c t u r e w i t h
> 6 .9 m m lat e ra l
displacement indicates
rupture of the
transverse ligament
spinal cord
6 Biom echanics of t he Spine and Spinal Inst rum e nt ation 87
Fi g . 6 . 2 Illu s t r a t io n o f t yp e II h a n g m a n ’s f ra c t u r e .
b
a
c
Fi g . 6 . 3 ( a – c ) St ar t in g p oin t s a nd t ra je ct o r y fo r a C1 la t e ra l m a ss scre w (Ha rm s t e ch niq ue ) a nd a C2
pedicle screw.
88
6 Biom ech anics of the Spine and Spinal In strum entat ion
ab
Fi g . 6 . 4 ( a ) Anteroposterior and (b) lateral radiographs demonstrating a C1–C2 posterior cervical
fusion accom plished via a C1 lateral mass scre w and C2 pedicle screw.
ab
Fig . 6 . 5 ( a ) Open m o ut h a n d (b) la t era l rad iogra p h s d e m on st ra t ing a n t erio r o d o n t oid scre w xat ion
for a C2 de ns frac t u re.
Ta b l e 6 . 2 White and Panjabi checklist for clinical inst abilit y
1. A total of 5 points or more is considered unstable.
a. Disruption of anterior elements: 2
b. Disruption of posterior elements: 2
c. Relative sagit tal plane translation > 3.5 mm: 2
d. Relative sagittal plane rotation > 11°: 2
e. Positive stretch test: 2
f. Cord d a m age : 2
g. Root damage: 1
h. Abnormal disk narrowing: 1
I. Da ng e ro u s lo a d in g a n t ic ip at e d: 1
6 Biom echanics of t he Spine and Spinal Inst rum e nt ation 89
a
Fi g . 6 . 6 ( a , b ) Li g a m e n t o u s d i s r u p t i o n o f > 3 . 5 m m o r 1 1 ° i n d i c a t i v e o f i n s t a b i l i t y .
b
(a) Lam inectomy or facetectomy.
i. Lam inectom y.
• Eigh tee n percen t loss of stabilit y.
• Lam in ectom y at C2 or C7 cau ses greater in stabilit y.
ii. Lam inectomy plus facetectomy.
• Sixt y p e rce n t loss of stabilit y.
• Par tial b ilatera l facetectom y (> 50%) causes in stabilit y.
D. Ce r vical fusion .
1. Anterior cervical fusion.
a. Disruption of all anterior ligaments reduces strength by 52%.
(1) Anterior interbody fusion restores stability to 100% of normal in
exion.
(2) Fifty- ve percent restoration in extension (Sm ith–Robinson–type graft).
(3) Bone mineral density a ects the compressive strength of the graft.
(4) Anterior interbody fusion plus plating adds strength in extension.
2. Posterior cervical instrumentation.
a. Interspinous wiring.
(1) Thirty-three percent of normal stability in exion.
(a) Strength varies with di erent techniques.
b. Posterior lateral mass screw–rod (Fig. 6.7 and Fig. 6.8).
(1) Strongest in both exion (92%) and extension (60%).
c. Pedicle screw.
(1) Greatest level of rigidity in all planes of m otion.
(2) High incidence of m edial wall violation.
(3) Done w ith either uoroscopic imaging or lam inoforam inotom ies or both.
E. Ce r vic a l o r th o s is (Table 6.3) (Fig. 6.9).
90
6 Biom ech anics of the Spine and Spinal In strum entat ion
abc
Fi g . 6 . 7 Co m p a riso n s ch em a t ic of (a) Magerl, (b) An d e r s o n , a n d (c) An m e t h o d s o f l at e r a l m a s s
screw orientation.
Fi g . 6 . 8 Clo se -u p d iag ra m of t h e
An t e c h n iq u e o f la t e r a l m a s s s cr e w
placement.
6 Biom echanics of t he Spine and Spinal Inst rum e nt ation 91
Ta b l e 6 . 3 Ce rvica l o rt ho sis
Ty p e N o t e s
Soft colla r – Use d o n ly fo r com fort
– Does not maintain stability
Philadelphia (Mia m i J) collars
(Fig. 6.9)
– Thirty percent of normal exion/extension
allowed
– Ine ective in controlling rotation and lateral
bending
Fo u r - p o s t e r - t y p e o r t h o s is – Good for controlling exion/extension at the
midcervical level (20% of normal motion is
allowed)
Ce r vico t h ora c ic-st yle o rt ho sis
St e r n o - o cc ip it al-m a n d ib ula r
im m ob ilized (SOMI) b ra ce
Rig id ce r vic o t h o r a cic b r a ce s
(Yale type)
– Good for controlling upper cervical spine exion
(C2–C5)
– Does not restrict extension e ectively
– Good for controlling exion/extension
– Controls rotation slightly
– Controls bending by only 50%
Halo d e vice s – Best fo r re st rict ion of a ll p lane s o f m ot ion ,
particularly for the upper cervical spine
– Cannot maintain distractive force (Fig. 6.10)
Fig . 6 . 9 Im m o b iliz a t io n o f t h e c e r vic al sp in e.
92
6 Biom ech anics of the Spine and Spinal In strum entat ion
a b
c d
Fi g . 6 . 1 0 ( a – d ) Halo a p p licat io n . The sa fe zo ne is shown for ant e rior halo p in p la ce m e nt . An t erio r
pins should be placed below the equator of t he skull, lat eral to t he supraorbital nerve. The patient’s
eyes should be closed during halo placem ent to allow the pat ient t o blink norm ally once the halo is
placed.
F. Th o r a c i c a n d t h o r a c o l u m b a r s p i n e i n s t r u m e n t a t i o n .
1. Supporting structures:
a. Anterior.
(1) Anterior and posterior longitudinal ligam ents.
(2) Intervertebral disk.
(3) Vertebral body.
b. Posterior.
(1) Ligam entum avum .
(2) Facet joints.
(3) Pedicle.
(4) Costovertebral–transverse complex.
6 Biom echanics of t he Spine and Spinal Inst rum e nt ation 93
2. The thoracic spine is mechanically sti er and less mobile than the lumbar
spine.
a. Stress concentration due to change in sti ness at the thoracolumbar
jun ction .
3. Thoracic or thoracolumbar trauma:
a. Thoracolumbar injury classi cation and severity (TLICS) score (Table 6.4).
(1) Classi cation system based on injury morphology, integrity of the
diskoligamentous complex, and neurological status.
(2) Operative managem ent is w arranted if the TLICS score is ≥ 5.
(a) A score of 4 can be managed with nonoperative or operative
management (dealer’s choice).
(b) A score < 4 sh ou ld be treated non op eratively.
b. Spine stability.
(1) Three-colum n classi cation of Denis (Fig. 6.11) (Table 6.5).
(a) If two or more colum ns are disrupted, the spine is considered to
be unstable.
(b) If the m iddle colum n is disru pted , the spine is regarded as
unstable.
i. Middle-colum n disruption is less important above T8 because
of the stability provided by the rib cage.
c. Compression fractures.
(1) Anterior column failure.
Ta b l e 6 . 4 Th o ra co lu m b ar in ju ry c la s s i cat io n a nd
severity (TLICS) classi cation
Sco re
Mo rp h o lo g y
Co m p re ss io n fra c t u r e
Bu r s t fr a c t u r e
Tr a n s l a t i o n a l / r o t a t i o n a l f r a c t u r e
Distraction
1
2
3
4
Neu rolo g ica l in vo lve men t
In t a c t 0
Ner ve r o ot in volve men t
Co rd , co nu s m e d u lla ris co m p r o m is e
In co m ple t e
Co m p le t e
Cau da e qu in a
Post erio r liga ment o u s co mple x
In t a c t
Susp e ct e d in jury
In ju re d
2
3
2
4
0
2
3
94
6 Biom ech anics of the Spine and Spinal In strum entat ion
Fig . 6 . 1 1 Th re e -co lu m n c la ssi ca tion of Denis.
Ta b l e 6 . 5 Denis classi cation
An t e r io r c o lu m n An t e r io r lo n g it u d in a l lig a m e n t
An t e r io r a n n u lu s b r o s is
An t e r io r h a lf o f t h e ve r t e b r a l b o d y
Mid d le co lu m n Po st e rio r lo n git ud ina l liga m e nt
Post erior annulus b rosis
Post erior half o f t he vert e b ra l bod y
Post erior colum n Pe dicle
Fa c e t j o i n t s
La m i n a
Spin ous p ro ce sse s
In t e r s p in o us a nd su p r a sp in o us lig a m e n t s
(2) End plates are weaker than the intervertebral disks.
(a) Displacem ent of the nucleus pulposus into the vertebral body
may occur.
(3) Bony fractures are more com m on in older osteoporotic patients.
d. Burst fractures.
(1) Anterior and middle colum n failure.
(2) Neurological injuries are comm on secondary to retropulsion of the
middle column.
6 Biom echanics of t he Spine and Spinal Inst rum e nt ation 95
e. Fracture/dislocation.
(1) Three-colum n failure.
(2) Shear/translational, exion/distraction, or exion-rotation injuries.
(3) Posterior stabilization is required.
4. Instrumentation for thoracolumbar fractures:
a. Purpose:
(1) Early m obilization.
(2) Prevent late deform ity and pain.
(3) Indirect decom pression by distraction and extension in traum atic
injuries.
(4) Tem porary stabilization until fusion matures increases healing rates.
b. Transpedicular instrumentation (Fig. 6.12):
(1) Provides the sti est construct with the shortest segm ent fusion.
(2) The gold standard for thoracolum bar fracture management.
c. Anterior fusion:
(1) Primary treatm ent in fractures with neurological de cit.
(2) Adjunct treatm ent after posterior instrumentation.
(3) Anterior plate construct is equal to a construct with an anterior strut
graft plus posterior transpedicular instrum entation.
d. Percutaneous fracture xation:
(1) Uses minimally invasive techniques in an e ort to lessen
int raoperative blood loss, operat ive tim e, postoperative pain, and
risk of infection, and to lessen the e ects on posterior paraspinal
musculature and stability
(2) These bene ts are param ount for polytraum a patients, w ho carry the
greatest risk for complications.
(3) One screw may be adequate clinically.
b
a
Fig . 6 . 1 2 ( a , b ) Tho ra cic p e d icle s cre w s t ar t in g p o in t s.
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