biomechanics of the spine - PowerPoint Presentation

Slide1

biomechanics of the spine

Stanford Hospital and Clinics

April 26, 2010

Slide2

Motion segment

VERTEBRAE

Carry 70-90% of static axial load

Vary in cervical, thoracic, lumbar, sacral and

coccygeal

FACET JOINT

Carry 10-20% of static axial load

Highly innervated and can be a source of low back pain

Cervical spine:

facet orientation is roof-tile shaped, coupling lateral bending and axial rotation in an opposite direction (i.e. bending the head to the left results automatically in an axial rotation to the left

Thoracic spine:

coronal plane orientation with a slight inward tilt in the transverse plane, permitting axial rotation movements with the center of rotation projecting into the vertebral body

Lumbar spine:

sagittal

plane orientation of facet joints, effectively blocking axial rotation movements

Slide3

Degree of Facet range of motion

Slide4

Motion segment

INTERVERTEBRAL DISC

Nucleus

pulposus

surrounded by annulus

fibrosus

Covers 30-50% of disc surface area and contains exclusively type II collagen fibers in a aqueous gel rich in

proteoglycans

Sharpey

fibers insert from the annulus

fibrosus

into the

epiphyseal

ring

Endplates are composed of a dense layer of

trabecular

bone further covered with a layer of hyaline cartilage

Vascular channels within the vertebral bodies have been observed to run directly along the endplates, representing the predominant nutrient source for adult disc cells

Cartilaginous end plates undergo progressive calcification with age, which impedes nutrition and contribute to the progressive disc degeneration throughout adulthood

Disc degeneration also affects other elements of the motion segment. Through disc height loss and apparent changes in axial load transmission, the facet joints become incongruent and may become mechanically overloaded. Also, loss of disc height will result in ligament laxity,

hypermobility

and loss of segmental stiffness.

Slide5

Intradiscal pressure for various activities

Slide6

Motion segment

MUSCLES

LIGAMENTS

Slide7

SPINAL MOTION

Six degrees of freedom

Motion rarely involve a single degree of freedom only

Segmental Instability

“loss of ability of the spine under physiological loads to maintain its pattern of displacement so that there is no initial or additional neurological deficit, deformity or incapacitating pain.”

- White & Panjabi

Slide8

Biomechanics of spinal stabilization

Goals of spinal arthrodesis are:

To support the spine when its structural integrity has been severely compromised

To maintain correction following mechanical straightening of the spine.

To prevent progression of deformity.

To alleviate or eliminate pain by stiffening a region of the spine.

Choose a stabilization method which will best achieve these goals. Therefore, the surgeon needs to have an understanding of how load is transferred

throughout

the spine and how this load transfer is altered by a specific injury or pathology.

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Posterior stabilization

PROVIDES STABILIZATION IN EXTENSION AND LATERAL BENDING

Pullout strength of pedicle screws is directly related to bone density

It is possible to achieve an increase in pullout strength with convergent screw trajectories.

Parallel pedicle screw in short segment constructs can result in an unstable “four bar” mechanism.

Diagonal

cross-linking

is preferable to the horizontal configuration in terms of rotational stability.

An increase in rod diameter provides a more stable construct, but at the same time produces higher internal loads in the implant, on the clamping device, and on the pedicle screws; thus, higher risk of screw breakage.

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Posterior stabilization

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Posterior stabilization

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Posterior stabilization

In the thoracic spine, smaller overall pedicles can result in complications. An alternative is to increase pullout strength with extrapedicular screw trajectory.

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Anterior stabilization

PROVIDES STABILIZATION IN FLEXION AND LATERAL BENDING.

Anterior cervical plates function as tension band during extension and buttress plate during flexion.

Constrained cervical systems have a rigid, angle-stable connection between the plate and screws, whereas, unconstrained systems rely on friction generated by compression of the plate against the anterior cortex for stability.

In extended biomechanical testing, constrained systems have shown a greater rigidity over time, whereas, unconstrained systems can lose stability over time.

No significant difference in stability between monocortical and bicortical screws in the cervical spine.

Slide14

INTERVERTEBRAL CAGES

Distraction of facet joints may be associated with instability

Slide15

Adjacent segment effects

Fusing

one or two levels will lead to increased stress at the remaining adjacent levels, the remaining motion segments will have to take up and accommodate the remaining motion of the spine, these adjacent levels will then tend to age and degenerate faster because of the resultant increased load and

stress.

Slide16

Arthroplasty of the spine

Many

spinal arthroplasty surgeons propose that by performing a disc replacement and restoring motion to a diseased spinal segment, this reconstructed spinal motion segment will offload the stress to the adjacent levels and actually lower the risk of onset of accelerated adjacent segment degeneration. The spine is a mobile structure and motion will occur at the places of least resistance and stiffness, if motion can be restored by a disc replacement then this will lead to reduced stresses at other more degenerated parts of the spine

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“Biomechanical study on the effect of cervical spine fusion on adjacent-level intradiscal pressure and segmental motion”

Eck JC, Humphreys SC, Lim TH, Jeong ST, Kim JG, Hodges SD, An HS. Spine 2002.

Intradiscal pressure and segmental motion are increased in adjacent disc segments after fusion. This cumulatively translates into increased stress on the adjacent non-operated discs, which can accelerate the rate of disc degeneration.

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“Radiculopathy and myelopathy at segments adjacent to site of previous anterior cervical arthrodesis”

Hilibrand AS, Carlson GD, Palumbo MA, Jones PK, Bohlman HH. J Bone Joint Surg, 1999.

Studied 374 patients and found that symptomatic adjacent segment disease occurred at a relatively constant rate of 2.9% during the decade after surgical fusion.

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Indications and contraindications

Failure in medical management

Normal cervical alignment and mobility with radiculopathy cause by:

soft or hard disc herniation

foraminal osteophytes

myelopathy due to soft disc herniation

Arthroplasty is contraindicated in the setting of significant segmental or global

deformity

Radiographic instability should be treated with

arthrodesis

Recent history of infection or osteomyelitis preclude the use of prosthetic device

Relative contraindications are RA, RF, osteoporosis, cancer, chronic

corticosteroid

use

Marked degenerative changes and no segmental motion cannot be expected to regain mobility by implanting a total disc replacement

Outcomes of patients with isolated axial cervical pain who are treated with an arthroplasty have not been well delineated

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DISC ARTHROPLASTY

Artificial

discs

approved

by the FDA for

use:

Charite

by DePuy

for use in the lumbar

spine

ProDisc

by Synthes

for use in the lumbar spine and cervical

spine

Prestige

by Medtronics for

use in the cervical

spine

Bryan Disc

by Medtronics for use in the cervical spine

These are

FDA approved for one-level applications, after clinical trials showed patient improvement in motion and pain equivalent to spinal fusion.

Two

-level disc replacement surgery is considered experimental in the United States, but has been performed in Europe for many years.

S

ome

insurance companies in the United States do not cover the surgery, still classifying it as experimental.

Effective

August 14, 2007, the Centers for Medicare & Medicaid Services (CMS) will not cover Lumbar Artificial Disc Replacement (LADR) for patients over the age of 60, on a national basis. Individual localities regulate the use of the procedure in patients 60 and under.

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Prestige® ST Cervical disc

Became available in 2002

Constructed of stainless steel in a two-piece ball and trough configuration and attaches to the vertebrae with screws

Provides relatively unconstrained motion comparable to that of a normal cervical spinal segment

Angulation between the base and anterior portion matches the normal anatomy of cervical vertebrae

The surfaces of the device contacting the endplates are grit-blasted to promote bone osteointegration.

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Prestige® LP Cervical disc

Manufactured from a unique titanium ceramic composite material that is highly durable and CT/MRI compatible

Ball and trough configuration is identical to Prestige ST

Initial fixation is achieved via four rails, two on each component

A porous titanium plasma-spray coating on the endplate surface facilitates bone ingrowth and long-term fixation

Slide23

Bryan cervical Disc

Conceived in the early 1990s by neurosurgeon Vincent Bryan and first implanted in Belgium in 2000 by Dr. Jan Goffin.

Cervical disc replacement designed to allow for motion similar to the normal cervical spine functional unit.

Consists of two titanium alloy shells with a polyurethane nucleus.

The bone implant interface of each shell has applied porous coating to facilitate ingrowth of bone and promote long-term stability.

The nucleus is surrounded with a polyurethane sheath to establish a closed articulation environment.

Sterile saline is injected into this sheath and functions as an initial lubricant.

Titanium alloy seal plugs allow for retention of the saline lubricant

Small anterior flanges on the shells serve to grasp the device for insertion.

Slide24

Prodisc-C cervical disc

Similar design principles as the Prodisc lumbar prosthesis.

First implantation was in December 2002.

Metal-on-polyethylene articulating device consisting of two cobalt-chromium molybdenum endplates and ultra high molecular weight polyethylene inlay.

The endplates are initially secured to the vertebral body with central keels and have a plasma sprayed titanium coating for long-term fixation stability.

Slide25

Pcm (Porous Coated Motion) Prosthesis

Made by Cervitech Inc, Rockaway, NJ.

Uses metal on polyethylene articulating surface

The cobalt-chromium-molybdenum alloy components are covered with a TiCaP porous coating

Immediate fixation is achieved by inserting the prosthesis in a “press-fit” fashion.

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Kineflex C Prosthetic Device

Developed in Africa and manufactured by Spinal Motion, Inc, Mountain View, CA

Uses a three-piece modular design consisting of two-cobalt-chromium-molybdenum endplates and a mobile core.

Opposing sides of the endplates have a highly polished concave articulating surfaces.

The core is indented at its equator to permit seating within a retention ring which is an integral part of the inferior endplate. This prevents subluxation of the core.

Initial fixation is achieve via central keel and a pyramidal surface.

Endplate are plasma-sprayed.

Slide27

Cervicore disc

Total cervical disc replacement by Stryker Spine, Allendale NJ.

Constructed of cobalt-chromium alloy.

Articulation occurs across a pair of saddle-shaped bearing surfaces.

Two fins containing three fixation spikes provide initial stability.

Bony contact surfaces have titanium plasma-sprayed coating.

Slide28

Future direction

Some companies are working

to develop products and methodologies to replace only the inner portion of the

disc, referred

to as

disc nucleus

replacement

. These

new techniques would allow for posterior insertion and a potentially shorter surgery with fewer complications.

Recently new artificial discs with innovative designs that incorporate an artificial nucleus to allow shock absorption and a woven fiber annulus for graded variable motion resistance in all directions have been approved in Europe. These next generation implants are designed to reduce the risk of implant migration found with earlier designs. They also should protect against impact injury and facet joint wear.

Slide29

Dynamic

stabilization

(

Dynesys

)

Slide30

Interspinous process distraction