DISTRACTION OSTEOGENESIS

Q .What is craniofacial distraction or distraction osteogenesis?

A. It is a technique that applies gradual and incremental traction force onto surgically separated bony segments to produce additional bone

(Distraction Histiogenesis – distraction of skeleton also causes enlargement of overlying and surrounding soft tissues)

 

History

Codivilla – 20th Century – Femur elongation following osteotomy

Abbot – 1927- Femur elongation following osteotomy

Ilizarov – popularized the distraction osteogenesis in long (endochondral) bone of extremities for limb lengthening and closure of bony defects

McCarthy – 1989 – Craniofacial distraction

 

Principles

Terminology – 

Distraction Zone: The location of bony separation

Latency period: Duration of reparative callus formation in distraction zone

Activation period: Duration during which distraction forces are applied to callus (for elongation)

Distraction degenerate: The newly formed bone following activation

Consolidation period: Period for which external fixation is maintained in position to allow newly formed bone to consolidate

Rhythm: The rate of activation

0.25 mm four times a day or   0.5 mm two times a day

i.e. Total 1 mm /day

Vector: Direction along which forces are applied

 

Sequence

  1. Bony separation in two segments (osteotomy or corticotomy)
  2. Latency period (5-7 days)
  3. Activation period
  4. Consolidation period (8 weeks)

Process of distraction starts with –

Performing osteotomy or corticotomy.

After which the distraction device will be applied depending on which direction the bone lengthening is desired (vector of distraction).

Following osteotomy a latency period (usually of 5-7 days) is given for callus formation, before activation of the device.

Following latency period, gradual distraction force is applied to separate the segments and thus elongate the intersegmentary callus – this is the activation period.

After, the desired length of bone has been achieved, the activation is stopped and the distraction device in maintained in position to allow consolidation of the newly formed bone- the consolidation period.

 

Osteotomy – Full thickness bony separation

Corticotomy – Spares the endosteum or marrow space

The most usually done distraction is  – transosteotomy (or transcorticotomy) distraction.

Distraction can be done across a open suture (such as in young patient) – trans-sutural distraction.

 

Three types of Distraction

  • Unifocal
  • Bifocal
  • Trifocal

Bifocal and Trifocal is across a skeletal defect

Unifocal: Single osteotomy

Distraction forces on either side of osteotomy

Bifocal: Single osteotomy

Transport segment (of bone) spanned across the defect using single distraction device

Trifocal: Two osteotomies used to fill skeletal defect in bidirectional manner

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When distracting across a skeletal defect the transport segment has a fibro-cartilagenous

cap which needs to be removed after final “docking” and replaced with bone graft.

 

Histological Analysis

  • Latency period: Hematoma formation

Migration of inflammatory cells into osseous gaps (PMNs)

  • Activation Period: Presence of tapered cells similar to fibroblasts

New blood vessels (endothelial cells)

New fibrovascular matrix (Type 1 collagen)

At Day 14 –      Osteoid synthesis and mineralization

At day 21 –       Calcification of linearly oriented collagen bundle

Appearance of osteoblasts

Formation of bony spicules

(Linear orientation is parallel to distraction vector)

Four Temporal Zones –

  1. Fibrous central zone – mesenchymal proliferation
  2. Transition zone – osteoid formation
  3. Remodeling zone – osteoclasts
  4. Mature bone zone

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Biomolecular analysis

Marked increase in TGF-B1 level

TGF-B1:         Activates VEGF & bFGF

Increase collagen deposition and non-collagen ECM proteins

Leading to mineralization and remodeling of bone

Osteoclast migration, differentiation and bone remodeling

 

Biomechanics

Tensile force applied to developing callus causes elongation of callus.

The mechanical forces are converted to cellular signals – termed as mechanical transduction.

This mechanical transduction is mediated by – Integrin mediated signal transduction

 

Tensile strain: Defined as amount of elongation as a fraction of original bone length

Eg.     1mm elongation in 1mm osteotomy defect- tensile strain 1/1 = 100%

By day 10 the bone gap will be 10mm

So, tensile strain will be 1/10 = 10%

Maximum tensile strain for bone is 1-2%

So, bone formation does not occur in distraction zone until approx. 4 weeks of activation

i.e. 1/30 = 3%

 

Mechanical environment in distraction zone depends upon –

  1. Stability of distraction device
  2. Applied distraction force
  3. Inherent physiological loading (muscle action)
  4. Properties of all the local soft tissue

 

For formation of successful (stable) regenerate –

  1. The distraction device must be stable
  2. Latency period should be adequate (not too short or too long)
  3. Distraction should be gradual
  4. Sufficient time should be given for consolidation

 

Patient factors that can affect regenerate formation –

Age –

In younger patient – better and faster distraction can be achieved. Latency period in children can be as low as 2-3 day (due to rapid callus formation).

Blood supply – adequate neovascularization should occur to support newly forming bone.

Radiation or chemotherapy – patient receiving RT or CT has poor blood supply and impairs osteogenesis.

 

[Credits : Dr. Anoop S. (Mch, department of burn and plastic surgery, VMMC & Safdarjung Hospital New Delhi)]

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