Pathomechanics of Bone 1st Lecture part2 Biome II

Dr .Manal Radwan SalimLecturer of Physical Therapy

Pharos UniversityFall 2013-2014

28-9-2013

IV. TREATING THE FRACTURE

Operative Non-

operative

Rehabilitation Rapid Slow

Risk of joint stiffness LowPresent

Risk of malunion Low Present

Risk of non-union Present Present

Speed of healing SlowRapid

Risk of infection Present Low

V. Biomechanics of fracture fixationFracture fixation could be internal or

external fixation depending on 1-type of fracture. 2-severity of fracture

and 3-age of the patient. 4- general health of the patient.

Both techniques influence the mechanical stresses inside the bone and as a result may enhance or delay the healing process.

A)External fixation: 1- immobilization by cast: A plaster or fiberglass cast is the most common

type of fracture treatment, because most broken bones can heal successfully once they have been repositioned and a cast has been applied to keep the broken ends in proper position while they heal.

2- Immobilization by Functional Cast or BraceThe cast or brace allows limited or "controlled"

movement of nearby joints. This treatment is desirable for some, but not all, fractures.

It is better to use weight bearing plaster cast as it facilitate healing of fractures 2 or 3 times faster than the non-weight bearing plasters

3- TractionTraction is usually used to align a bone or bones by a gentle, steady pulling action.

4- Operative External Fixation: In this type of operation, metal pins or screws are placed into the broken bone above and below the fracture site. The pins or screws are connected to a metal bar outside the skin. This device is a stabilizing frame that holds the bones in the proper position while they heal.

B) Internal fixation: the material used must have 1-adequate mechanical strength 2-adequate fatigue resistance.3- long fatigue life.4-must be applied on the tensile side of

the fracture.(most destructive type of stress.

Advantages of Internal Fixation1-No casts

Prevent skin pressure and fracture blisters

No scars

2- No complications of bed restImportant for the elderly

3- Early motionAvoid stiffnessEnhance fracture healingPrevent muscle atrophy

Types of Internal fixation:1- Wires:Best used in treatment of transverse

fractures. For example fracture of olecranon. Correct wiring convert muscle tensile force

into compressive forces produced by tension in the wire.

The reaction force from the fragment is compressive so when the tendon force increases there is an increase in the compressive force across the fracture surface, By bringing the two fragments together.

Another example Fixation pins. Frontal

radiograph of the wrist shows a comminuted intraarticular distal radius fracture transfixed with three Kirschner wires and a standard uniplanar external fixator with Steinman pins in the distal radius and in the second metacarpal bone.

Example:Tension band wire. Frontal (a) and lateral (b) radiographs of the knee show a transverse patellar fracture that is transfixed with a tension band wire (combination of two cancellous screws and two wires).

2- Plate and screws:the bone fragments are first repositioned

(reduced) in their normal alignment, and then held together by attaching metal plates to the outer surface of the bone It should best be applied at tensile surface of the fracture so the fracture will be compressed by muscular force.

if fracture is subjected to bending, plate will resist the tensile forces (so bending is not allowed) and there will be compressive forces across the fracture

pressing the two fragments together.

Example: T he broken bones of the forearm are held in position by plates and screws while they heal.

3 -Dynamic Compression Plate (DCP)

Designed to compress the fracture Offset screws exert

force on specially designed holes in plate

Force between screw and plate moves bone until screw sits properly

Compressive forces are transmitted across the fracture

ttb.eng.wayne.edu/ ~grimm/ME518/L19F3.html

Beginning

End Result

If the plate is not applied on the tensile side, during bending of the plate resistive effect will be cantered at the neutral axis and the fracture will be compressed in the compressive load part only.

Disadvantages of plate and screws:

1- The plate may be thin:So it cant’s withstand stresses placed upon it and fatigue failure may occur.

2- The screw may be not set tightly:

Bending stresses will occur at screws. The plate will bear the bending moment alone

3- the fracture may be comminuted or has a gap:

To overcome this problem two plates and screws must be used one plate on tensile side and the other on the compressive side

So using only one plate and screws may cause fracture to be unstable.

or

Plate and oblique fracture

A: For ONLY torsional loads: 45° to long axis

B: For ONLY bending loads: Parallel to long axis

Realistically: loads in both directions will be applied: Divide angle between long axis and 45°

A

B

4- Screws:

Best used in cases of spiral fractures, but two

screws are needed a transverse screw is

needed to avoid bending loads while an

oblique screw is used to avoid torsional

stresses.

Oblique plate and screws will subject bone to

bending at the fracture site.

As the transverse plate

and screws alone will

cause the bone to be

subjected to torsion

 Photograph shows a variety of screws used in internal fixation: the Schanz screw (A), cannulated cancellous screws (B), partially threaded cortical screw (C), and cortical screws (D) (the first two of which are self tapping and the third is non-self tapping).

Diagram illustrates screw “anatomy.”

Figure 16.  Herbert screw. Frontal radiograph of the foot shows a Herbert

screw transfixing the proximal fifth metatarsal (Jones) fracture .

5- Intramedullary nailThe fragments may also be held together by inserting rods down through the marrow space in the center of the bone.. provides strong fixation for this thigh bone fracture.

  Frontal radiograph of the leg shows a tibial shaft fracture that is transfixed with an antegrade intramedullary nail with two proximal and two distal interlocking screws. A fibular shaft fracture is present at the same level.

  Flexible intramedullary rods. Frontal radiograph of the femur shows two flexible intramedullary nails (Ender) that transfix a proximal femoral shaft fracture.

6-  Flexible intramedullary rods

 Photograph shows a variety of plates used in internal fixation: tibial condylar plate (A), blade plate (B), reconstruction plate (C), calcaneal plate (D), dynamic compression plate (E), and LISS plate (F).

VI. concentration

Application of screws or plates will introduce many changes in the stress distribution in the bones at the fracture site.

- Stresses are normal in the areas away from the plate and screws.

- there is a stress concentration at each end of the plate because this area is the intersection between a normal stress area and a lower stress area.

- stresses decrease at the areas between the screws.

So, re-fracture may occur at areas of lower stress (which is the area between the screws) if the fixation is applied for along time with high loading behaviour ( weight bearing load).

Stresses are represented by number of lines which are proportional to the total load on the segment.

After removal of screws:There will be holes inside the bone.If the segment is loaded (in compression for

example) the total load is the same for any cross section of the bone so; - stresses are concentrated in the holes