Mechanics of the hand

Mechanics of the hand and its prosthesis

MUSCLES that control the hand

INSPIRATION

Brain surgery. This is the thought that lead me on to choosing my research area for the term paper. We all know that brain surgery is highly aided by technology and other mechanical devices. However, no matter how much help there is, it success of the surgery all boils down to the surgeon and his hands. His steady hands.

So what does it take for our hands to be steady and what helps it to do this? In my research paper, I will be looking into the muscles that control the hand, which enable us to do such fine and delicate movements. Some people might argue and say that it is the brain that controls such movement, but looking at it from my point of view, without the muscles, there's nothing the brain can do.

Muscles aside, I will also be looking into prosthetic hands and fingers, and how it is possible to have a normal life, or at least carry out simple tasks if one looses a finger or more. For instances, mountain climbers have a higher risk of losing their fingers as the occurrence of frostbite is present, which, unfortunately, makes them likely candidates for this application.

INTRODUCTION

The movement of our hands is mainly controlled by our muscles within the wrist and some of those found in the forearm, and likewise, our bones support the muscles and enable it to function as intended. This relationship, therefore, does not allow these 2 very important components to work independently of the other. It'll be like fishing without a rod to support the string or opening window blinds without a rope.

The natural human hand is made of many bones of different shapes and sizes meant to serve different purposes. These movements range from the simple clenching of the fist to the more complex movements like the opposition of the fingers and the seemingly simple pronation and supination. I referred to our hand as the natural hand as, first, it is the hand a large majority of humans are born with, and secondly, due to advances in biomechanics, a partially functional prosthetic hand is attainable. From now on, the natural hand will be referred to as the hand.

OUR HAND

The skeleton (bones)

As mentioned earlier, our natural human hand is made of many muscles that control movement, and bones which help to form essential joints. In actual fact, from wrist down, 27 bones are involved in creating the hand, namely the carpals (total: 8 bones), metacarpals (total: 5 bones) and phalanges, also known as the fingers (total: 14 bones). A brief introduction on the anatomy of the hands' skeleton is as follow:

The carpus (wrist) is the proximal region of the hand and consists of 8 small bones, the carpus, joined to one another by ligaments (not shown in figure 1 above). Articulations among carpal bones are called intercarpal joints. The carpals are arranged in 2 transverse rows of 4 bones each, where their names reflect their shape. The carpals found in the proximal row, going from lateral to medial, are the scaphoid (boat-like), lunate (moon shaped), triquetrum (3 cornered) and pisiform (pea-shaped). The proximal row of carpals articulates with the distal ends of the ulnar and radius to form the wrist joint. The carpals in the distal row, from lateral to medial, are the trapezium (4 sided with no sides parallel), trapezoid (4 sided with 2 sides parallel), capitate (head-shaped) and hamate (hooked).

The capitate is the largest carpal bone, and its' rounded projection (the head) articulates with the lunate, and the other side articulates with the scaphoid. In other words, the capitates articulates with both bones that form the wrist joint. The lunate articulates with the ulnar to form one half of the wrist joint. The scaphoid articulates with the radius to form the other half of the wrist joint. In about 70% of carpal fractures, only the scaphoid is broken. This is due to the fact that the force of a fall on an outstretched hand is transmitted from the capitate through the scaphoid and then to the radius.

The metacarpus, or palm, is the intermediate region of the hand and consists of a group of five bones called the metacarpals. Each metacarpal bones are named based on their location relative to the body's medial line. They are commonly called the proximal base, an intermediate shaft, and the distal head. The metacarpal bones are numbered I to V (1-5 in roman numerals), starting with the thumb, from lateral to medial. The bases articulate with the distal row of carpal bones to form the carpometacarpal joints. The heads articulate with the proximal phalanges to form the metacaarpophalangeal joints. The heads of the metacarpals are commonly referred to as knuckles and are readily visible in a clenched fist.

Finally, the phalanges, or bones of the digits, make up the distal part of the hand. There are a total of 14 phalanges spread out across the 5 metacarpals to form the 5 digits of each hand. Like the metacarpals, the digits are numbered I to V, starting with the thumb, from lateral to medial. A single bone of a digit is referred to as the phalanx. Each phalanx consists of a proximal base, an intermediate shaft and a distal head, much like the metacarpals. The thumb has 2 phalanges and the other 4 digits consist of 3 phalanges each. In order from the thumb, hese other 4 digits are commonly referred to as the index finger, middle finger, ring finger, and little finger. The first row od phalanes (the proximal row) articulates with both the bones of the metacarpals and the phalanges belonging to the second row. The second row of phalanges (the middle row) articulates with the proximal row and the third row (the distal row). The thumb lacks a middle phalanx. The joints between the phalanges are called the interphalangeal joints.

The joints

When the bones in the fingers articulate with each other, they form joints. Different bones articulate with their neighbours to form various specialised joints that aid our hand in movement.

Types of diarthrodial joint,

(a) ellipsoidal joint between the metacarpal and phalanx of digit II,

(b) saddle joint between the trapezium of carpus (wrist) and metacarpal of thumb,

(c) planar joint the forms the intercarpal joints (between carpal bones at the wrist);

Credits to: OVRT resources for the humanoid animation working group (http://ovrt.nist.gov/projects/vrml/h-anim/jointInfo.html)

The joints between the proximal and distal row of carpals are referred to as the midcarpal joints, and the joints between the individual joints in each row are called the midcarpal joints. The joints that connect the scaphoid and lunate to the radius and ulnar respectively are known as the condyloid joint. This joint is identified when the convex oval-shaped projection of one bone fits into the oval-shaped depression of another bone. A condyloid joint is biaxial as the movement it permits is around 2 axes (here, an axis is a straight line around which a rotating / revolving bone moves), where flexion-extension and abduction –adduction can occur. Apart from the example mentioned earlier, another example is the metacarpophalangeal joints for digits II to V. Digits II and III are occasionally referred to as the “stable rays” as their carpometacarpal joints have little movement. The last 2 digits, IV and V are occasionally referred to as the “mobile rays” as there is a wider range of movement of the joints there which allow us to cup our hands.

For the metacarpophalangeal joint for the first digit (the thumb), it is a saddle joint instead of the previously mentioned condyloid joint. In a saddle joint, the articular surface of 1 bone is saddle-shaped and the articular surface of the other bone fits into the ‘saddle' as a sitting rider would sit. A saddle joint is a modified condyloid joint in which the movement is somewhat freer. Saddle joints are triaxial (condyloid joints are only biaxial), permitting movements in 3 axes, where flexion-extension, abduction –adduction and rotation can occur.

Last is the planar joint. The articulating surfaces in the planar joint are either flat or slightly curved. They primarily permit back-and-forth and side-to-side movements between the flat surfaces of the bones. Many planar joints are biaxial as they permit movement around 2 axes.

The muscles

The muscles of the hand fall mainly into 2 groups: the extrinsic and the intrinsic muscles of the hand. Extrinsic muscles of the hand move the digits in various ways and generally produce powerful but crude movements of the digits. The intrinsic muscles of the hand (found within the palm) produce the weak but intricate and precise movements of the digits that characterise the human hand. The muscles in this group are named as such as their origins and insertions are within the hand. As the muscles of the hand are very information heavy, i will only represent the information relevant to this topic in the tables below.

For the extrinsic muscles

Muscle

Origin

Insertion

Action

Superficial carpi radialis

Medial condyle of radius

2nd and 3rd metacarpals

Flexes and abducts hand (radial deviation) at wrist joint

Palmaris longus

Medial condyle of humerus

Flexor retinaculum and palmar aponeurosis (fascia in centre of palm)

Weakly flexes hand at wrist joint

Flexor carpi ulnaris

Medial condyle of humerus and superior posterior border of ulnar

Pisiform, hamate, and base of 5th metacarpal

Flexes and abducts hand (ulnar deviation) at wrist joint

Flexor digitorum superficialis

Medial condyle of humerus. Coronoid process of ulnar and a ridge along lateral margin of anterior surface of radius

Middle phalanx of each finger (excludes thumb)

Flexes middle phalanx of each finger ar proximal interphalangeal joint, proximal phalanx of each finger at metacarpophalangeal joint, and hand at wrist joint

Flexor pollicis longus

Anterior surface of radius and interosseous membrabne (sheet of fibrous tissue that holds shafts of ulnar and radius together)

Base of distal phalanx of thumb

Flexes distal phalanx of thumb ar interphalangeal joint

Flexor digitorum profundus

Anterior medial surface of body of ulnar

Base of distal phalanx of each finger

Flexes disral and middle phalanx of each finger at interphalangeal joints, proximal phalanx of each finger at metacarpophalangeal joint, and hand at wrist joint

The above discussions are of importance as it will help in the understanding of the topic which will follow (i.e. the prosthetic hand/ finger).

THE PROSTHETIC FINGER

THE PROSTHETIC HAND

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