Artrocinemática de la Muñeca

ARTHROKINEMATICS
Diferentes tecnologias han sido usadas para el estudio cinemático de la muñeca, incluyendo tecnicas  in vitro y técnicas in vivo recientemente. 

Incluyen las siguientes:
• Radiografia 
• Cineradiografia
• Disección anatomica 
• Colocación de sensores en los huesos 
•Imagenología computalizada 3D
• Digitalización sonica
• Roentgen-estereofotografía 
• Sistemas optoelectricos
• Dispositivos de rastreo electromagnetico 
• Tomografía computalizada 3D 
• Electromechanical linkage systems 

Even with these sophisticated techniques, the resulting data describing the kinematics across the regions of the wrist are inconsistent. Precise and repeatable descriptions of the kinematics are hampered by the complexity of the anatomy and the movement (up to eight small bones experiencing multiplanar rotations and translations) and by natural human variation. Although much has been learned over the last two decades, the study of carpal kinematics continues to evolve.* Perhaps the most fundamental and accepted premise of carpal kinematics is that the wrist is a double-joint system, with movement occurring simultaneously at both the radiocarpal and midcarpal joints. The following discussion on arthrokinematics focuses on the dynamic relationship between these two joints.

Wrist Extension and Flexion 

The essential kinematics of sagittal plane motion at the wrist can be appreciated by visualizing the wrist as an articulated central column, formed by the linkages between the distal radius, lunate, capitate, and third metacarpal (Figure 7-14). Within this column, the radiocarpal joint is represented by the articulation between the radius and lunate, and the medial compartment of the midcarpal joint is represented by the articulation between the lunate and capitate. The carpometacarpal joint is a semirigid articulation formed between the capitate and the base of the third metacarpal.

Dynamic Interaction within the Joints  of the Central Column of the Wrist 

The arthrokinematics of extension and flexion are based on synchronous convex-on-concave rotations at both the radiocarpal and the midcarpal joints. At the radiocarpal joint depicted in red in Figure 7-15, extension occurs as the convex surface of the lunate rolls dorsally on the radius and simultaneously slides palmarly. The rolling motion directs the lunate’s distal surface dorsally, toward the direction of extension. At the midcarpal joint, illustrated in white in Figure following, the head of the capitate rolls dorsally on the lunate and simultaneously slides in a palmar direction. 



Combining the arthrokinematics over both joints produces full wrist extension. This two-joint system has the advantage of yielding a significant total range of motion by requiring only moderate amounts of rotation at the individual joints. Mechanically, therefore, each joint moves within a relatively limited—and therefore more stable—arc of motion. Full wrist extension elongates the palmar radiocarpal ligaments and all muscles that cross on the palmar side of the wrist. Tension within these stretched structures helps stabilize the wrist in its close-packed position of full extension.43,44 Stability in full wrist extension is useful when weight is borne through the upper extremity during activities such as crawling on the hands and knees and transferring one’s own body from a wheelchair to a bed. The arthrokinematics of wrist flexion are similar to those described for extension but occur in a reverse fashion (see Figure 7-15). 

Studies quantifying the individual angular contributions of the radiocarpal and midcarpal joints to the total sagittal plane motion of the wrist cite inconsistent data.* With few exceptions, however, most studies report synchronous and roughly equal—or at least significant—contributions from both joints. Using the simplified central column model to describe flexion and extension of the wrist offers an excellent conceptualization of a rather complex event. A limitation of the model, however, is that it does not account for all the carpal bones that participate in the motion. For instance, the model ignores the kinematics of the scaphoid bone at the radiocarpal joint. In brief, the arthrokinematics of the scaphoid on the radius are similar to those of the lunate during flexion and extension, except for one feature. Based on the different size and curvature of the two bones, the scaphoid rolls on the radius at a different speed than the lunate.66 This difference causes a slight displacement between the scaphoid and lunate by the end of full motion. Normally, in the healthy wrist, the amount of displacement is minimized by the restraining action of ligaments, especially the scapholunate ligament (see Figure 7-8, A). Rupture of this important ligament occurs relatively frequently and can significantly alter the arthrokinematics and transfer of force within the proximal row of carpal bones.81,91 Damage to this ligament can occur through trauma, chronic synovitis from rheumatoid arthritis,5 or even surgical removal of a ganglion cyst.


Ulnar and Radial Deviation of the Wrist 
Dynamic Interaction between the Radiocarpal and Midcarpal Joints 

Like flexion and extension, ulnar and radial deviation occurs through synchronous convex-on-concave rotations at both radiocarpal and midcarpal joints. During ulnar deviation, the midcarpal joint and, to a lesser extent, the radiocarpal joint contribute to overall wrist motion (Figure 7-16).35 At the radiocarpal joint shown in red in Figure 7-16, the scaphoid, lunate, and triquetrum roll ulnarly and slide a significant distance radially. The extent of this radial slide is apparent by the final position of the lunate relative to the radius at full ulnar deviation. Ulnar deviation at the midcarpal joint occurs primarily from the capitate rolling ulnarly and sliding slightly radially. Full range of ulnar deviation causes the triquetrum to contact the articular disc. Compression of the hamate against the triquetrum pushes the proximal row of carpal bones against the styloid process of the radius. This compression helps stabilize the wrist for activities that require large gripping forces. Radial deviation at the wrist occurs through similar arthrokinematics as described for ulnar deviation (see Figure 7-16). 

The amount of radial deviation at the radiocarpal joint is limited as the radial side of the carpus impinges against the styloid process of the radius. Consequently, a greater amount of the radial deviation occurs at the midcarpal joint.35 Using magnetic resonance imaging, Moritomo and colleagues specifically measured the three-dimensional movement at the midcarpal joint during radial and ulnar deviation.51 They reported a kinematic association between radial deviation and slight extension, and ulnar deviation and slight flexion. This “dart-throwing” movement pattern observed at the midcarpal joint is similar to that observed during many natural wrist movements.

Additional Arthrokinematics Involving  the Proximal Row of Carpal Bones 

Careful observation of ulnar and radial deviation using cineradiography or serial static radiographs reveals more complicated arthrokinematics than previously described. During these frontal plane movements, the proximal row of carpal bones “rock” slightly into flexion and extension and, to a much lesser extent, “twist.” The rocking motion is most noticeable in the scaphoid and, to a lesser extent, the lunate. During radial deviation the proximal row flexes slightly; during ulnar deviation the proximal row extends slightly.35,37 Note in Figure 7-16, especially on the radiograph, the change in position of the scaphoid tubercle between the extremes of ulnar and radial deviation. According to Moojen and coworkers, at 20 degrees of ulnar deviation the scaphoid is rotated about 20 degrees into extension, relative to the radius.47 The scaphoid appears to “stand up” or to lengthen, which projects its tubercle distally. At 20 degrees of radial deviation, the scaphoid flexes beyond neutral about 15 degrees, taking on a shortened stature with its tubercle having approached the radius. A functional shortening of the scaphoid allows a few more degrees of radial deviation before complete blockage against the styloid process of the radius. The exact mechanism responsible for the flexion and extension of the proximal carpal row during ulnar and radial deviation is not fully understood, but many explanations have been offered.66 Most likely, the mechanism is driven by passive forces in ligaments and compressions between adjacent carpal bones.

Carpal Instability 
An unstable wrist demonstrates malalignment of one or more carpal bones, typically associated with abnormal and painful kinematics. The primary cause of carpal instability is laxity or  rupture of specific ligaments. Although the intrinsic ligaments can tolerate greater relative stretch before rupture than can the extrinsic ligaments, they are more frequently injured. The clinical manifestation of carpal instability depends on the injured ligament (or ligaments) and the severity of the damage. Carpal instability may be static (demonstrated at rest) or dynamic (demonstrated only during free or resisted movement). The following examples describe two of many forms of carpal instability. More detail on this subject is contained in other sources.

ROTATIONAL COLLAPSE OF THE WRIST

Mechanically, the wrist consists of a mobile proximal row of carpal bones intercalated or interposed between two rigid structures: the forearm and the distal row of carpal bones. Like cars of a freight train that are subject to derailment, the proximal row of carpal bones is susceptible to a rotational collapse in a “zigzag” fashion when compressed from both ends (Figure 7-18). The compression forces that cross the wrist arise from muscle activation and contact with the surrounding environment. In most healthy persons the wrist remains stable throughout life. Collapse and subsequent joint dislocation are prevented primarily by resistance from ligaments and from forces in tendons and the shapes of the adjoining carpal bones. 

The lunate is the most frequently dislocated carpal bone.65 Normally its stability is provided by ligaments and articular contact with adjacent bones of the proximal row, most notably the scaphoid (Figure 7-19, A). By virtue of its two poles, the scaphoid forms an important mechanical link between the lunate and the more stable, distal row of carpal bones. The continuity of this link requires that the scaphoid and adjoining ligaments be intact.46,76,81 Consider, as an example, a fall over an outstretched hand with a resulting fracture in the waist region of the scaphoid, and tearing of the scapholunate ligament (see Figure 7-19, B).

Disruption of the mechanical link between the two bones can result in scapholunate dissociation and subsequent malalignment of either or both bones.39 As shown in Figure 7-19, B, the more unstable lunate most often dislocates, or subluxes, so its distal articular surface faces dorsally. This condition is referred to clinically as dorsal intercalated segment instability (DISI) (Figure 7-20). Injury to other ligaments, such as the lunotriquetral ligament, may cause the lunate to dislocate such that its distal articular surface faces volarly (palmarly). This condition is referred to as volar (palmar) intercalated segment instability (VISI).75 

Regardless of the type of rotational collapse, the consequences can be painful and disabling. Changes in the natural arthrokinematics may create regions of high stress, eventually leading to joint destruction, chronic inflammation, and changes in the shapes of the bones. A painful and unstable wrist may fail to provide a stable platform for the hand. A collapsed wrist may also alter the length-tension relationship and moment arms of the muscles that cross the region.81

ULNAR TRANSLOCATION OF THE CARPUS 

As pointed out earlier, the distal end of the radius is angled from side to side so that its articular surface is sloped ulnarly about 25 degrees (see Figure 7-4, A). This ulnar tilt of the radius creates a natural tendency for the carpus to slide (translate) in an ulnar direction.3 Figure 7-21 shows that a wrist with an ulnar tilt of 25 degrees has an ulnar translation force of 42% of the total compression force that crosses the wrist. This translational force is naturally resisted by passive tension from various extrinsic ligaments, such as palmar radiocarpal ligament. A disease such as rheumatoid arthritis may weaken the ligaments of the wrist. Over time, the carpus may migrate ulnarly. An excessive ulnar translocation can significantly alter the biomechanics of the entire wrist and hand.

MUSCLE AND JOINT INTERACTION 

Innervation of the Wrist Muscles and Joints 

INNERVATION OF MUSCLE

The radial nerve innervates all the muscles that cross the dorsal side of the wrist (see Figure 6-32, B). The primary wrist extensors are the extensor carpi radialis longus, extensor carpi radialis brevis, and extensor carpi ulnaris. The median and ulnar nerves innervate all muscles that cross the palmar side of the wrist, including the primary wrist flexors (see Figure 6-32, C and D). The flexor carpi radialis and palmaris longus are innervated by the median nerve; the flexor carpi ulnaris is innervated by the ulnar nerve. As a reference, the primary spinal nerve roots that supply the muscles of the upper extremity are listed in Appendix II, Part A. In addition, Appendix II, Parts B to D include additional reference items to help guide the clinical assessment of the functional status of the C5 to T1 spinal nerve roots and several major peripheral nerves of the upper limb.

SENSORY INNERVATION OF THE JOINTS 

The radiocarpal and midcarpal joints receive sensory fibers from the C6 and C7 spinal nerve roots carried in the median and radial nerves.19,26,30 (This terminal sensory branch of the radial nerve often develops a painful neuroma within the wrists’ dorsal capsule.) The midcarpal joint is also innervated by sensory nerves traveling to the C8 spinal nerve root via the deep branch of the ulnar nerve.

Function of the Muscles at the Wrist The wrist is controlled by a primary and a secondary set of muscles. The tendons of the muscles within the primary set attach distally within the carpus, or the adjacent proximal end of the metacarpals; these muscles act essentially on the wrist only. The tendons of the muscles within the secondary set cross the carpus as they continue distally to attach to the digits. The secondary muscles therefore act on the wrist and the hand. This chapter focuses more on the muscles of the primary set. 

FUENTE: Neumann, Donald A., Kinesiology of the Musculoskeletal System , 2010