(1)
Faculty of Medicine of Montpellier, Montpellier, France
Abstract
Prehension is a vital function according to its important role in serving the oral pole—for feeding—and the genital pole—for grooming and sexual activities. Everything is designed to give a maximum mobility to the terminal organ, the hand, which is in humans, unlike in monkeys, a multi-grasp unspecialised organ. Three different functional systems have to be described. The wrist/hand complex shows a very intelligent organisation of polyarticulated finger chains with an opposable thumb and all corresponding muscles allowing a great quantity of grasp and manipulation procedures. Then, the humero-radioulnar complex can regulate the length of the limb and put the hand in a supine or prone position thanks to the special trochoid radioulnar joint. Finally is the cleido-scapulohumeral complex that allows the upper limb to get 5 degrees of freedom (DF) for its spatial orientation resulting from only one junction to the axial skeleton: the sternoclavicular joint. Full responsibility lies then in the very powerful shoulder muscles for stabilising and moving the upper limb.
4.1 Introduction
This specific function of the upper limb will impose a number of technical requirements [1].
(a)
A high mobility, allowing a wide spatial excursion of the grasping terminal organ. This is particularly evident when looking at the shoulder in comparison to the hip. The clavicle has only one articular junction with the axial skeleton (sternoclavicular joint) which gives 5 DF for the spatial orientation of the upper limb: 2DF for clavicle/sternum and 3 DF for scapulohumeral.
(b)
An unspecialised multi-grasp end-organ able to use any tools, allowing the development of all forms of technology and with some specific features: opposable thumb, mesaxonic symmetry with two dorsal interossei attached to the IIIrd finger, defining the functional axis according to Vialleton and Anthony. This is not the case in monkeys who have two types of specialised autopods: the hook of brachiators monkeys (hylobatidae, pongids, new world monkeys, semnopithecids) with reduced thumb and paraxonic symmetry (between IIIrd and IVth finger) and the hand of arboreal grasping monkeys like lemurians, for example, with their opposable thumb, palmar tactile balls, reduced index and ectaxonic symmetry (IVth finger).
In general, each animal must have a gripper, essential for nutritional survival, which can take four different shapes:
the clamp, like the multiform beak of some birds (sharp, curved, in spatula) or like the clamp jaw for plant grinder of ruminants or like carnivore jaw with a 1 DF temporomandibular joint and protruding canines to catch mobile provender or like pliers of shellfish;
the winding that can take the form of pseudopods in amoeba, spine in snakes or trunk in elephants;
the suspension by gravity, by a hook formed by the fingers or a prehensile tail or a plane cutaneous surface;
the adhesion, using a mucus with adhesive properties as in the chameleon when it throws its tongue onto the prey.
In total, the human hand has the ability to operate on demand all these forms of grasping and thereby is not specialised [2–5] (Fig. 4.1).
Fig. 4.1
The different types of grip. (a) Tridigital grip (very stable due to the fixed three first metacarpal bones). (b) Bidigital grip (very variable from nail grip to pulp grip). (c) Grip by hand seizing (typically French). (d) Lateral grip between pulp of thumb and lateral side of index
(c)
Grasping and manipulating objects
The hand has an easy-to-use segment length (arm/forearm) to serve the oral pole (feeding) and the genital pole (grooming, sexual activity). A rich innervation with an area of wide central control projection sensorimotor cortex to give to the hand a major palpatory role (haptic sense) (Fig. 4.2) and a clear strategy for taking and handling objects in three major phases:
Fig. 4.2
Recognition test of nature and spatial orientation of a test-object (cube) with no vision. (a) Instrumentation: 1. Recording of the contact transducers for the four sides and the twelve borders of the cube; 2. Cube-test with contact transducers to analyse the palpation strategy. (b) Recognition test with extreme profiles of subjects, from up to bottom (subjects no. 5, 9, 14 out of 40 subjects): 1. Palpation only of one face and one border with potential risk of mistake; 2. Optimal recognition by palpation of two faces and seven borders; 3. Redundant palpation of faces and borders. (c) Orientation test with extreme profiles of subjects, from up to bottom (subjects no. 8, 16, 4 out of 40 subjects): 1. Palpation of one face; 2. Palpation of one face and two borders; 3. Redundant palpation of faces and borders
approach: visual, tactile or on order;
grasping in various ways;
grasp releasing (Fig. 4.3).
Fig. 4.3
Measure of grasping force during prehension tests (INSERM, Unit 103). (a) Palmar grip of the test-object: 1. Force transducer equipped with strain gauges bridges; 2. Base with strain gauges accelerometers. (b) Instrumentation: 1. Electronic measuring and recording system; 2. Cover of the test-object with a hole to change the weight by filling it with lead powder. (c) The test object: 1. Removable cover; 2. Metallic internal cylinder for filling; 3. Accelerometers; 4. Force transducer. (d) The different profiles of grip force regulation in relation with the growing weight of the test-object: superior line: force level and inferior line: weight of the object. 1. First type: Proportional regulation of grasping force; 2. Second type: regulation with threshold; 3. Third type: regulation in stages; 4. Fourth type: complete lack of regulation of grasping force in relation with the weight of the test-object
Therefore, the upper limb is tailored to its non-specialised terminal organ, capable of taking and manipulating (the etymological meaning of the term) any form of objects in a three-dimensional space and also bear or cling to external structures by mobilising the entire body. This programme requires a basic quality: mobility with a major functional requirement to serve the holes of biological survival (mouth, genitals).
(d)
Robotics inputs
The robotics that built more or less complicated multi-DF manipulators (2–6 DF and more) have taught us a lot about command and control problems of these polyarticulated tools technically identical to the human upper limb [6]. Controlling the motion of the terminal organ can be done either by an open loop without possible regulation or by a closed loop with sensors (touch, force, temperature, …) in order to get the feedback from the effector to the control system. We can create automatic regulation and servomechanisms allowing some form of adaptive intelligence to scheduled tasks and environmental changes. Some scientists reserve the term of “robot” for these intelligent machines (Fig. 4.4).
Fig. 4.4
Anthropomorphic robotics in Japan. (a) Professor Kato lab: Professor Rabischong visiting the laboratory of Professor Ichiro Kato from Waseda University in Tokyo. (b) Professor Kato’s humanoid robot playing on organ: (two hands, one foot) and reading the score of a Concerto of Haendel during the opening ceremony of the International Fair of Tsukuba in 1985
However, to write algorithms and control equations for these robots, two information are essential: the state of the motors measured by the level of electrical power or hydraulic pressure and the angles of the segments measured by potentiometers, either linear or angular.
It is the same technical problem with the human upper limb. The motor muscles are viscoelastic, nonreversible and non-linear. Here is their originality which makes them work without any noise, which is not the case of industrial motors. The central nervous system (CNS), in order to coordinate them in a 3D-space, needs also two relevant information: the state of the muscular actuators and the angles of the joint segments.
Striated muscles are able to change their module of elasticity by the shortcut of their dark disc of contractile proteins without exceeding one-third of the length of the striated fibres, which is an important parameter fixing the construction of muscle plans. Regarding their three possible states: relaxed, contracted or stretched—data which have to be known by the CNS for their control—they can be measured by a mechanical stiffness transducer.
Regarding the measurement of the joint angles, muscles or ligaments cannot do it but the skin itself can; it is the goniometer for all the body segments as we saw previously and as Moberg indicated [7–9].
The upper limb innervation is provided by the brachial plexus located in the neck and formed by the roots C5/T1. It passes between the scalene muscles to enter into the limb and is divided into primary and secondary trunks providing all nerves. The plexus is not done by anastomoses of fibres—which is only possible for vessels—but instead by nervous fibres juxtapositions as in a marshalling yard, according to the multi-root innervations of most of the muscles [10–15].
We counted the number of nerve fibres in the roots and different trunks of the brachial plexus in 20 subjects [16]. There are an average of about 110,000 nerve fibres to innervate the entire upper limb, but we were amazed by the large individual variations of these fibres ranging from one to three. This is largely in favour of inequality of individuals with regard to the size of the nervous potential and, consequently, the level of skills.
The upper limb is in fact a high-tech manipulator [17] with 9 DF up to the hand, which has 20 DF and the carpal joint 10. Its construction is organised starting from the hand in three interactive sets: the carpo-metacarpal-phalangeal complex, then, the humero-radioulnar complex and finally, the cleido-scapulohumeral complex, hanging all the upper limb at the axial skeleton.
4.2 Wrist/Hand Complex
4.2.1 The Human Hand
It is a non-specialised multi-grasps tool, which is not the case in brachiates or grasping arboreal monkeys as we saw before. It comprises five digital rays and one of them, the thumb, lives in the opposition of the others. This allows a series of pollici-digital clips and a secure grip by hand seizing. Each finger is a sensitive probe richly innervated (about 350 mechanoreceptors by sq.mm in the pulp of the index).
4.2.1.1 The Four Middle Fingers
They are formed by a polyarticulated chain with three phalanges: proximal (P1), intermediate (P2) and distal (P3) with three different joints (two 1-DF trochlear interphalangeal type—distal (DIP) and proximal (PIP)—and a 2-DF condylar metacarpophalangeal joint (MP)).
The movements of the chain comprise flexion/extension and lateral bending. The biomechanical functioning principle which justifies the complexity of muscle organisation is the mechanical coupling of the interphalangeal joints and the decoupling of the metacarpophalangeal. In other words, it is not normally possible to flex one interphalangeal joint and expand the other, thanks to the blocking of the linear chain extension by palmar ligaments, powerful for PIP (volar plate) and extensible for DIP, that can be extended in a pulp passive support (which is a fair protection of the article). On the contrary, it is possible to put the MP in flexion with an extension of IP or in extension with flexion of the IP (metacarpophalangeal decoupling).
The Muscular Organisation
This important principle explains the organisation of muscle that we can understand by a few schematic points to remember:
the flexor muscles that close the fingers on an object by wrapping the digital chains are more powerful than extensor which open by releasing the hand grip;
there is one flexor muscle by phalanx: deep flexor for P3, superficial for P2 and interosseous for P1. For maximum efficiency, the long flexor tendons should stay as close as possible to the digital chain. This explains why the deep flexor tendon attached to P3 has to pass through the superficial flexor tendon split into two tongues before its fixation on P2. In addition, it is needed to fix the tendons by highly resistant digital fibrous sheaths having a synovial sliding system in order to allow tendons to slide. The first fibrous sheath is attached to P1 and overflows on the palmar aspect of MP and is lined by distal fibres crossed in X on PIP (Fig. 4.5);
Fig. 4.5
The long flexor tendons of the digital chain. (a) Sagittal section of the metacarpophalangeal joint (MP): 1. Interosseous muscle; 2. Metacarpal bone; 3. MP joint with the extensor apparatus on its dorsal part; 4. Annular fibrous pulley for flexor tendons; 5. P1. (b) Sagittal section of the IP joints: 1. P1; 2. Palmar plate (strong ligament limiting the extension); 3. PIP joint; 4. P2; 5. Profundus flexor tendon; 6. DIP joint. (c) Semi-flexed finger: 1. Superficial flexor tendon inserted on P2 with its vincula (with blood vessels); 2. Profundus flexor tendon inserted on P3 and attached to P2 by its annular pulley; 3. The two bands of the superficial flexor tendon attached to P2. (d) Semi-flexed digital chain with two flexor tendons: 1. MP joint; 2. Fixation of the superficial tendon on the middle of P2; 3. PIP joint; 4. Profundus tendon; 5. DIP joint
the antibrachial extensor muscle tendons are necessarily fixed in the axis of the digital chain by fibrous bands on each side of the MP. This indeed limits their excursion (Fig. 4.6). The extensor tendon attaches to the proximal portion of P2 without bony attachment on P1 except a fibrous adhesion to the articular capsule of MP. Its contraction opens hand incompletely, but sufficiently to release a grip.
Fig. 4.6
Extension of digital chain. (a) Drawing of lateral aspect of the digital chain: 1. First metacarpal; 2. Proximal phalanx (P1); 3. Intermediate phalanx (P2); 4. Distal phalanx (P3); 5. Palmar plate limiting the hyperextension of PIP except in hyperlax subjects; 6. Palmar ligament less strong allowing the hyperextension of P3 in compression. (b) Drawing of the fixation of the extensor tendon on dorsal side of finger (dorsal and lateral view): 1. Collateral ligaments of MP joint centering the tendon traction, limiting its excursion and giving incomplete extension of finger but enough for grasp release; 2. The retinacular ligament of Landsmeer fixing laterally the lateral bands of extensor tendon; 3. Triangular ligament fixing before their fixation on P3 the lateral bands of extensor tendon. (c) Dissection of the third finger extensor system at the level of MP joint: 1. Collateral ligament of MP joint; 2. Median band of the long extensor tendon; 3. Interosseous muscle; 4. Lumbrical muscle; 5. Long flexor tendon
A very clever mechanical system will ensure the full chain extension as well as the flexion of P1:
firstly, the tendinous dorsal lozenge formed by the two lateral bands of the extensor communis placed on either side of the median tongue inserted on P2, on which are attached fibrous expansions on the dorsal interosseous muscles. These lateral bands are maintained in lateral position by the retinacular ligament described by our friend Landsmeer [18–20]. They are also closed distally by transverse fibres (triangular ligament);
secondly, fibres in sling shape come from the interosseous muscles on each side, creating a true flex belt of P1 called the interosseous back plate. They can, by the contraction of the two interosseous, flex P1 and put P2 and P3 in full extension. Contracting at the same time the long extensor tendon allows to get an extension of the entire chain, including the MP, and to play with the flexion/extension system of MP.
There is also a special extensor tendon doubling the common extensor for the index finger, to show, and to the fifth finger. Finally the total extensor digitorum unit is a large tendinous strip located directly under the dorsal skin and applied on curved back of phalanges like a saddle on back horse (Fig. 4.7).
Fig. 4.7
Transverse sections of P1 (dorsal extensor apparatus and ventral flexor tendons). (a) Proximal section: 1. Extensor apparatus on the dorsal convex shape of phalanx; 2. Annular fibrous pulley of flexor tendons; 3. Non compressible collateral artery during grips; 4. Lateral band of superficial flexor tendon; 5. Profundus flexor tendon; 6. Collateral nerve; 7. Palmar skin thicker than dorsal for contact grip. (b) Distal section: 1. Medial band of extensor tendon; 2. Tendon of the profundus flexor between the two lateral bands of the superficial flexor; 3. Lateral band of the superficial flexor within the annular fibrous pulley; 4. Lateral band of extensor tendon (tendinous fibres from interosseous and lumbrical muscles)
The Physiological Axis of the Hand
It is the third finger on which two dorsal interossei generating abduction of this finger are fixed. There are three palmar interossei, which are attached only on the corresponding metacarpal and not on both for the dorsals. They are making adduction of the IInd, IVth and Vth fingers which have an extreme medial position requiring a specific abductor and P1 flexor associated with the fourth dorsal interosseous.
The Specific Role of Lumbrical Muscles
One can only admire the fantastic possibilities of the digital play so useful for pianists and other musicians. But we miss in our description the need for a viscoelastic tension regulator between flexor and extensor muscles, which is represented by the lumbrical muscles, so named because of their resemblance with earthworms. These are the only muscles of the body which are intertendinous, fixed on the side of the four flexor tendons and on the extensor tendinous lozenge without any bone attachment. They are the richest of all the body in proprioceptive receptors like muscle spindles, simple and complex Golgi organs, Golgi–Mazzoni corpuscles. They are genuine proprioceptive tensiometers between two opposite functional systems and also capable of ensuring a complete extension of the digital chain regardless of the position of the MP (Fig. 4.8).
Fig. 4.8
Dissection of the extensor apparatus of fingers on a left hand (fresh cadaver). (a) Extension of MP joint and flexion of IP joints (decoupled movement): 1. Retinaculum extensorum; 2. Short extensor of thumb; 3. Long extensor of thumb; 4. First dorsal interosseous muscle (DIM); 5. Adductor pollicis; 6. Dorsal tendinous fibres of interosseous muscle; 7. Contracted flexor tendons. (b) Resection of the first DIM: 1. Tendons of the two extensors of index finger; 2. Fibrous lateral metacarpophalangeal strap fixing laterally the extensor tendon (centering traction along the finger axis); 3. Dorsal tendinous fibres of interosseous muscle (active flexion of P1); 4. Tendinous extensor lozenge; 5. Transsected lateral MP strap; 6. First DIM; 7. First lumbrical muscle separated from the flexor tendon with its tendinous fibres attached to the lateral extensor band. (c) Index finger in flexion: 1. Dorsal tendinous fibres of interosseous and lumbrical muscles (flexion of P1 and full extension of IP joints); 2. Median band of extensor fixed on base of P2; 3. Lateral fixation of tendinous lozenge by retinacular ligament (Landsmeer); 4. Lateral bands of extensor both joined dorsally by the triangular ligament and fixed on the base of P3 (distal phalanx); 5. P3. (d) Index finger in semi-flexion: 1. Medial band fixed on P2; 2. Lateral band fixed on P3; 3. Distal fixation on P3
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