Orthopedic Biomechanics

沈文哲 台灣 高雄市 博正醫院

Course Objectives • Why are we studying this?

• Terminology • Understanding the condition • Management of fractures • Design and application of implants

Stress Strain Curve Strain

Load Deformation Curve

Creep

Tension band Converts tension forces into compression forces

Which is Strongest?

How many twists are needed?

Which hand do you hold the cane?

Osteonecrosis Rt hip is painful 1) Rt side 2) Left side 3) No difference 4) Depends on the lumbar spine status

The Non-Painful Side

Which leg goes first when climbing stairs?

Rt knee is painful

好腳上天堂 , 壞腳下地獄 Painful knee kept straight.

Which brace to order?

Think of the Forces 3-point fixation

Where to place the knot?

Submandibular position hyperextension distraction “long drop” 83kg – 173 cm 51kg – 244 cm

Deforming Forces

Fracture Patterns AO classification (Muller)

Different Forces

Classification Based on Mechanism of Injury Allen classification of cervical spine injuries

Plate Fixation 6 wks 3 mo 1 yr

A Load Bearing Construct 1 mo 12 mo

How long a plate?

What kind of screw?

Lag screw

What angle?

Where to put the plate?

Placement of Implant

• How long?

• How thick?

• Titanium?

• Solid?

• Cross section?

• Fluted?

• Tapered?

• Bowing?

• Reaming?

• Interlocking?

• Cost!

Removal?

Stress risers

Load Sharing and Stress Shielding Re-fracture

Wolff’s Law Bone deposits along lines of stress transmission

External Fixators Rubber band

Fixator Design

Hip Screws Gamma nail

Choice of Implant

Hip Prosthesis

Choices of Stem

Modularity

Knee Prosthesis

“

軟骨凸出壓迫神經”

Rank the intradiscal pressures • Bending forward at waist • Squatting • Sitting • Standing • Supine

Herniated Disc

How big can you make the hole?

• 1) 75% of lamina • 2) 50% of lamina • 3) 75% of facet • 4) 50% of facet • 5) 25% of facet

flexion Spondylolisthesis Reduction

Missing Anterior Strut

Fatigue Failure of the Screw

Anterior Strut Only Fusion Cage 椎籠 “子彈”

Does Not Resist Extension Forces

40% Failure Rate • Biomechanically, an anterior strut requires posterior support.

• “Supported on two rollers”

Combined Ant/Post Fusion

Stress Transfer 6 wks post-op 2 yrs post-op

13+2 ♀ Menarche: 6 mo Risser II

13+2 ♀ 7

13+2 ♀

3-D Vector Analysis Instrumentation design Translate or Rotate?

13+2 ♀

Thank You!