Subaxial Cervical Fixation

Techniques

Mohamed Mohi Eldin, MD,Professor of Neurosurgery,

Faculty of Medicine, Cairo University,

Egypt.

Before placement of any implant, a some questions should be answered

1. Is implant indicated?

2. Is a rigid or dynamic implant optimal?

3. Is deformity reduction, correction, or prevention required?

4. Which system is ideal for

– obtaining fusion and

– preventing subsidence?

Subaxial Cervical Fixation Techniques

• Ventral stablization

• Posterior stabilization

• Combined (360) stabilization

Anterior Fixation Techniques

First introduced (1955) by Smith & Robinson

Then popularized by Cloward

1. Anterior distraction (resisting compression),

2. Anterior compression (tension band),

3. Anterior cantilever beam fixation.

Anterior Distraction implants

1. Interbody struts:– Bone,

– Cages,

– Acrylic, or

– Metal implants

2. Screw-plate construct: – Fixed-moment arm,

– Non-fixed-moment arm,

– Applied-moment arm, or

– dynamic mode.

Interbody implants

• Bone Graft– Autograft

– Allograft

• Cages (Graft substitutes)– Carbon fiber reinforced

polymers

– Polyetheretherketone(PEEK)

– Acrylic

– Titanium

Cage design evolution

1. Threaded (Screw cages)

2. Non-threaded

– Box-shaped

– Vertical ring designs

– Anterior plating

Anterior Compression (Tension Band) Fixation

• +/- interbody struts

• Allows the application of compression using a screw-rod construct, thereby

• enabling preloading of bone graft, increasing bone healing.

Anterior screw-plate construct

• Plates Types:

1. Nonconstrained Plates:

First-generation

2. Constrained (rigid) Plates

Second-generation (static)

2. Semiconstrained (semirigid)

Third-generation (dynamic)

First-Generation Plates

• Bohler (1967) first use• Orozco and Houet (1970s) :

– One-third tubular plate– ‘H’ and ‘HH’ plates

• Herrmann (1975) • Caspar ‘trapezoidal’ plate

(1980)

• First anterior cervical plates were unlocked and required bicortical purchase.

First-Generation Plates(abandoned nonrigid)

• Motion at screw-plate interface. • Compressive forces (higher chance of fusion)

• Both unicortical and bicortical screws• High rate of screw backout and breakage.

H-Plates

Second-Generation Plates(Constrained-rigid plates)

• Screw convergence

• Ventral distraction fixation in neutral position.

• Usually with interbody graft

• In extension, resist distraction (tension-band)

Orion Plate

Third-Generation Plates(Dynamic semi-constrained)

• Prevent stress shielding

• Allow subsidence

• Mechanisms of dynamism :

1. screw toggling

2. Allowance of axial settling

Codman Plate

ABC plate

Screw Toggling (permission of axial subsidence)

• Rounded screw head/cup configuration allows the screw to rotate in the sagittal plane with respect to the plate as subsidence occurs.

Allowance of axial settling

• The screws allowed to slide along the long axis of the plate for a limited distance

• Allow subsidence while minimizing the risk of screw cutout.

Subsidence (settling)• Loss of disc height following surgery

• Due to 1. Bone Graft remodeling and resorption (normal, complex

biological process) before being replaced by new living bone

2. Graft collapse, and

3. Pistoning of the graft.

Stress shielding

• Bone heals best under compression (Wolfe’s law).

• Stress shielding is defined as ‘an implant induced reduction of bone healing, enhancing stresses leading to osteoporosis, or nonunion’

Multilevel Fixation

• The caudal end of the construct is the most likely to fail (longer moment arm and increased forces)– screw loosening or – hardware failure

• This can be decreased by 1. maximizing screw purchase at

caudal end2. dynamic fixation3. good bone-grafting techniques4. Postoperative immobilization (rigid

collar in the first few months)

Advantages of Anterior Cervical Plates

• Enhancing solid fusion

• Resisting kyphosis

• Reduce external bracing

• Mobilization of adjacent segments

• Reduce risk of graft extrusion

• Reduce rate of nonunion.

Disadvantages of Anterior Cervical Plates• Increased cost

• Special instruments and training

• Plate-specific complications:

– screw loosening or fracture,

– infection,

– neural injury

Posterior Cervical Fixation Techniques

1. Wiring: (Historical)– Sublaminar

– Facet

– Interspinous

2. Laminar screw fixation,

3. Lateral Mass:– Plate

– Rod

4. Pedicle Screws

Interspinous Wiring

• Intact posterior elements

• Restore posterior tension band

• After soft tissue injury

• Augment other anterior or posterior fixation techniques

Rogers’ interspinous wiring

• Burr hole at the base of the upper and lower spinous processes.

• Stainless steel or titanium wire or cable through the burr holes in a figure eight pattern.

• Wire is tightened using a Tensioner.

Abdu’s triple-wiring• As the Rogers’ technique. • 2nd wire through upper burr hole and looped around upper spinous

process. • 3rd wire through lower burr hole and looped around the lower

spinous process. • These two wires are passed through two autologous bone graft

struts, lateral to spinous processes. • Wires are tightened under tension

SUBLAMINAR WIRINGCables are generally safer than wires

SUBLAMINAR WIRING• Pros:

– Simple– Safe– Large surface area for fusion

• Cons:– Wire breakage or cutout– Not suitable if posterior elements deficient– Poor fixation in axial load & rotation

SUBLAMINAR WIRING

Almost never used in the subaxial spine

because the spinal canal is smaller compared to the spinal canal at the C1/C2 levels.

Specially in patients with degenerative or congenital cervical stenosis.

Facet Wires

• Facet could easily get broken

Facet-Spinous Process Wiring

• Not secure & facet could easily get broken

Posterior cervical wiringComplications (rare)

• Wire pullout • Injury (cord or spinal nerves)• Over-tightening (avulsion

fractures). • Loss of fixation (poor bone

quality) • Inadequate postoperative

immobilization. • Nonunion, malunion• Hardware failure • Infection

Posterior cervical screw fixation

1. Laminar screw,

2. Lateral Mass Screw:

– Plate

– Rod

3. Pedicle Screws

LAMINAR SCREWS(Translaminar screw fixation)

• Uncommon in subaxial spine

• C7 has larger laminar size– high unilateral screw placement success

rate:• 100% for 3.5 mm screw,

• 92% for 4.0 mm screw

– moderate bilateral screw placement success rate • 90% for 3.5 mm,

• 68.8% for 4.0 mm.

• At C3-C6, success rates much lower.

Utilized in only selected cases

• Deficient lateral masses

• Failure to place a lateral mass screw

• Requires intact posterior elements, specifically intact laminae

Complications of laminar screw

• laminar cortical breach:

– medial cortex (thecal and cord injury)

• Violation of the facet joint

• Screw loosening

• hardware failure

Lateral mass screw fixation

widely considered the mainstay technique for posterior fixation of the subaxial spine

With high fusion rates, (85-100%)

LATERAL MASS SCREW FIXATION

• Restore posterior column tension band

• Rotational & axial stability

• Greater stability in lateral bending

• Applicable C3 to C7 levels

• No need for intact lamina

LATERAL MASS SCREW (Anatomical Factors)

• Size

• Orientation

• Bone Quality

• Vertebral Artery

• Nerve Root

The relation between the lateral mass and the VA

• At the C7 level, the VA is more laterally located. Thus, at C7 the direction should be calculated carefully.

Lateral Mass Screws

Cervical Pedicle Screws

• To correct deformity esp. Kyphosis

• More risky

• Needs very lateral dissection to allow for 45 degrees angulation

• Higher resistance to pullout than lateral mass screws.

CERVICAL PEDICLE SCREWS

• Pedicular width 3.5–6.5 mm,

• Pedicular height is 5–8 mm

• Pedicular angulation decreases from 50 degrees medially at the C5 to 11 degrees medially at the T5 in the transverse plane.

• The pedicle angulation in the sagittal plane is 3–5 degrees downward with reference to the lower endplate of C7.

C7 Pedicle Screw

• C7 lateral mass is often inadequate (average thickness is about 9 mm)

• A pedicle screw at C7 is preferable.

C7 Pedicle Screw• A small laminotomy

(paplate pedicle)

• Entry point at junction of two lines:– Vertical line (middle of C6–7 facet joint)– Horizontal line 1 mm under middle of

C7 transverse process.

• Direction – 30–35 degrees medially – 5 degrees downward (reference to C7

lower endplate)

• Screw:– length 20- to 22-mm– diameter 3.5-mm

Medial Pedicle Wall Breach(spinal canal violation)

Lateral Pedicle Wall Breach(transverse foramen penetration)

Cervical Pedicle Screws

In certain cases with anterior column failure

• CPS may be an option

360 Stabilization

• Provides very strong construct in severe cervical instability

Posterior column + Anterior columnDisruption

• An anterior standalone bone graft will not be sufficient for fixation…. WHY ?– Graft extrusion,

– Kyphotic deformity

– Significant risk of neural injury.

• To avoid dislocation and graft extrusion : 1. Anterior plating

2. Supplemental posterior fixation,

3. Rigid external orthosis (halo vest)

As a rule

Any stand-alone posterior fixation technique, is insufficient to restore

stability in cases involving the anterior and/or middle

columns.