Biomechanics of the Thorax

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Top Contributors - Sonal Joshi and Manisha Shrestha  

This article is currently under review and may not be up to date. Please come back soon to see the finished work! (15/12/2021)

Introduction

Biomechanics is an area of science that uses principles of physics to measure and study, how forces interact with and affect living body. These forces could be Internal i.e. within the body or External i.e. outside the body.

To understand the biomechanics of thorax we will need to study the basic anatomy, joint articulations, muscles associated in the thorax along with kinematics of the rib cage.

The thorax is formed by the thoracic vertebrae ,the ribs and the sternum.

Parts of Thorax

It has two main functions,

• Providing anchor for muscle attachments
• Major role of ventilation

Structure of Thorax

The main three structures mentioned above work in a coordinated motion in thorax to produce respiration.

Anatomy

• The rib cage is a system of various bones and muscle. Bones involved are sternum, 12 pairs of Ribs, 12 Thoracic vertebrae.

It is like a compartment sealed by various structures from each side.

• 
  
  They can be described as,

Side of compartment	Contents
Posterior-laterally	- Thoracic vertebrae - Ribs - Intercoastal muscles and membrane
Anteriorly	- Coastal cartilages - Sternum -Intercoastal muscles and membrane
Superiorly	- Upper ribs and clavicle - Cervical fascia surrounding esophagus and trachea
Inferiorly	-Diaphragm muscle

• The sternum consists of manubrium, body and xiphoid process. Its details can be referred to on page : sternum.
• The ribs are from T1 to T12 region
• Refer the Thoracic Anatomy for further details of Thoracic vertebrae & the joint which they form.

Articulations (IMAGE)

	Joint	Bones involved in the joint articulation	Special features
1.	Manubriosternal	Manubrium and superior part of body of Sternum	- Synchondrosis type of joint - Also called Angle of Louis
2.	Xiphisternal	Xiphoid process and inferior part of body of Sternum	- Synchondrosis type of joint - Ossifies by 40 to 50 yrs. of age
3.	Typical Costovertebral (CV)	Head of ribs 2 to 9, two adjacent vertebral bodies and interposed intervertebral disk	- Synovial type of joint
4.	Atypical Costovertebral	Head of ribs 1, 10 to 12 and corresponding vertebral body	- More mobile than typical CV joint
5.	Costotransverse	Costal tubercle of rib 1 to 10 and costal facet on transverse process of corresponding vertebrae	- Synovial type of joint
6.	Costochondral	1 to 10th rib with costal cartilage	- Synchondrosis type of joint - Have no ligamentous support
7.	Costosternal	Costal cartilage of ribs 1 to 7 with sternum	-Joints with 1, 6 and 7 ribs are synchondroses type. -Joints with 2 to 5 ribs are synovial type.
8.	Interchondral	7 to 10 costal cartilage with cartilage above it	- Synovial type of joint

Function of Thorax

Kinematics

The motion of the ribs in conjunction with sternum and thoracic vertebrae helps produce the movements of respiration. These consist of inspiration and expiration. Thoracic kinematics consists of understanding the changes in intrathoracic volume during ventilation. These occur due to complex, synchronized activity of the ribs along with sternum and primary muscles of respiration. It is determined by

• Type & angles of articulations
• movement of manubrium sternum
• elasticity of costal cartilage

The kinematics involved in the thorax can be further classified into,

• Changes in vertical diameter
• Changes in anterior-posterior diameter (Pump handle motion)
• Changes in transverse diameter (Bucket handle motion)

Changes in Vertical diameter

• This is mainly due to excursion of Diaphragm muscle.
• During inspiration : Diaphragm contracts -> lowering of the dome of diaphragm -> increase in vertical diameter of thorax. (VIDEO)
• During expiration : Diaphragm relaxes -> dome recoils upward to resting position -> decrease in vertical diameter of thorax.

Changes in A-P and transverse diameter

• There is a single axis of motion for 1 to 10 ribs through the center of their costovertebral(CV) and costotransverse(CT) joints. This influences the motion of these ribs during the process of respiration
• The two main motions occurring in thorax during ventilation can be explained as below,

(VIDEO)

	Changes in Anterior-Posterior diameter	Changes in Transverse diameter
Mainly motion of	Upper ribs	Lower ribs
common Axis of motion oriented	nearly in frontal plane	nearly in sagittal plane
Thoracic motion occurs	in sagittal plane	in frontal plane
During Inspiration	Upper ribs elevate Motion is of anterior part of the ribs It pushes sternum ventrally and superiorly Total motion of sternum and upper ribs together increases in anterior-posterior diameter of thorax Motion of sternum resembles pump handle movement, hence the name.	Lower ribs elevate Motion is more in the lateral part of ribs due to its more angled shape and indirect attachment of the ribs Total motion of lower ribs together increases the transverse diameter of thorax Motion of the lower ribs resembles bucket handle movement, hence the name.

Muscles involved in respiration

• To bring about the intrathoracic volume changes during respiration, muscles of respiration play a crucial role. The muscles can be divided into primary and accessory depending on type of breathing i.e. quite or forced, in which they are used.
• The detailed function of each muscle can be accessed by clicking on the muscle name mentioned below. Each has been linked to independent physiopedia pages dedicated to explaining them.

Primary Muscles

• Diaphragm
• Intercoastal muscles
• Scalene

Accessory Muscles

• Sternocleidomastoid
• Trapezius
• Pectoralis major
• Pectoralis minor
• Subclavius
• Levatores costarum
• Serratus posterior superior
• Serratus posterior inferior
• Abdominal muscles
• Transverse thoracis

Developmental Differences with Age

In Newborn

• The chest wall is cartilaginous and therefore more compliant to external pressures applied during birth.
• The muscles act more as stabilizers vs mobilizers in adults.
• The complete ossification of ribs does not happen several months after birth.
• There is more horizontal alignment of ribs as compared to adults.
• There is very little motion of ribcage during tidal breathing.
• The fibers of diaphragm are 20% fatigue-resistant vs 50% in adult.
• Accessory muscle use gradually increases as infant gains head control.
• The anterior part of rib cage becomes oblique as toddler is able to sit and stand. This promotes development of bucket handle motion.

In Elderly

• Articulations of chest wall undergo fibrosis, leading to reduced thoracic mobility.
• Narrowing of airways, decrease in elastic recoil and increase in lung compliance.
• Increased kyphosis causes reduced mobility of ribcage.
• Muscles of ventilation have fewer muscle fibers, lower oxidative capacity and decrease in number of fast twitch type II fibers. Therefore, ventilation is more energy consuming with age.
• The resting position of diaphragm is less domed due to reduced abdominal tone.

References

Original Editor - [[User:Manisha Shrestha|Manisha Shrestha] Top Contributors - Sonal Joshi and Manisha Shrestha