The wrist is comprised of several articulations, the anatomy and biomechanics of which are complex.
Biomechanical interpretation has undergone some modification recently and continues to evolve.
Anatomy
The wrist is comprised of (Fig. 12.1):
1. The radiocarpal joint
At this joint the distal radius articulates with the scaphoid and lunate bones. This joint carries 80 per cent of the axial load of the forearm.
2. The midcarpal joint
The proximal carpal row, i.e. the scaphoid, lunate and triquetrum, articulates with the distal carpal row, i.e. the trapezium, trapezoid, capitate and hamate bones. The pisiform is a sesamoid bone that enhances the mechanical advantage of the most powerful motor in the wrist, i.e. flexor carpi ulnaris. The pisiform articulates with the triquetrum.
3. The distal radioulnar joint (DRUJ) The head of the ulna articulates with the shallow concavity of the sigmoid notch at the distal radius.
Twenty per cent of the axial load of the forearm is carried through the ulnar carpus via the triangular fibrocartilage complex (TFCC).
Radial tilt and inclination
In the sagittal plane, the radius has a palmar tilt of approximately 11 degrees. In the frontal plane, the ulnar inclination of the distal radial articular surface is approximately 23 degrees (Fig. 12.2(a)).
Ulnar variance
Ulnar variance (also known as Hulten’s variance) refers to the length of the ulna relative to the radius. In ulna neutral (or zero) variance, the distal margin of the ulnar head articular surface is level with the medial corner of the radius. In ulna plus (or positive) variance, the ulna is 1–5 mm longer than the radius; in ulna minus (or negative) variance the ulna is 1–6 mm shorter than the Figure 12.1. Schematic drawing of the volar aspect of
the wrist showing the radiocarpal joint, the midcarpal joint, the eight carpal bones and the distal radioulnar joint.
(a)
Ulnar inclination of distal radius
Ulna neutral variance
(b)
RCL
ECU sheath UT UL TFC
TFCC RC
RT
RSL
T C
radius. Ulna positive variance is associated with degenerative changes in the ulnolunate joint.
Kienboeck’s disease is more common in people with an ulna minus variance (Fig. 12.2(b)).
The carpus
The carpus does not move as a solid block. It is a labyrinth of small bones and is flexible and compliant. Movement occurs not only between the carpal rows but also between the individual bones.
There is more play between the bones of the proximal row than the distal, this row being a relatively immobile unit that articulates with the metacarpals to form the carpometacarpal joints.
Range and direction of movement of the carpal bones is determined by their articular arrangements and ligamentous attachments. The scaphoid bone spans both carpal rows anatomically and function-ally, acting as a restraining link that contributes to carpal stability. In transverse section, the carpus forms an arc which is the floor of the carpal tunnel.
Carpal ligaments
The relative importance of the carpal ligament system becomes apparent when it is remembered that all wrist tendons, with the exception of flexor carpi ulnaris, insert beyond the carpus onto the metacarpals, thereby depriving the wrist joint of their stabilizing influence.
Carpal stability is provided by a complex intrinsic and extrinsic ligament system. Intrinsic carpal ligaments connect the carpal bones to each other. Clinically, the most important of these is the Figure 12.2. (a) Palmar tilt of the radius. (b) Ulna
neutral (or zero) variance and ulnar inclination of distal radius.
Figure 12.3. Volar wrist ligaments: RCL, radial collateral ligament; RC, radiocapitate, RT, radiotriquetral; C, capitate;
T, triquetrum; RSL, radioschapholunate; ECU, extensor carpi ulnaris; UT, ulnotriquetral; UL, ulnolunate; TFC, triangular fibrocartilage; TFCC, triangular fibrocartilage complex. (Copyright, Elizabeth Roselius, 1999 with permission. Reproduced from Fernandez, D. L. and Palmer, A. K. 1999. Fractures of the distal radius. In Green’s Operative Hand Surgery (D. P. Green, R. N. Hotchkiss and W. C. Pederson, eds) p. 932, Churchill Livingstone).
The wrist 145
scapholunate (S-L) ligament. Extrinsic ligaments connect the carpal bones to the radius and ulna proximally and the metacarpals distally.
The palmar wrist ligaments are thicker and more plentiful than the thinner dorsal ones. It would appear that a strong palmar system is necessary to stabilize against wrist extension while less support is needed dorsally to stabilize against wrist flexion (Figs. 12.3 and 12.4).
The distal radioulnar joint
Forearm rotation involves movement of the radius and hand about the ulnar head. Static stability of the DRUJ is provided by:
1. Congruent articular surfaces.
2. The triangular fibrocartilage complex (TFCC).
3. The interosseous membrane.
4. The dorsal wrist retinaculum.
Dynamic stability is provided by the pronator quadratus, extensor carpi ulnaris and flexor carpi ulnaris.
The triangular fibrocartilage complex (TFCC)
This complex is comprised of:
1. The triangular fibrocartilage proper (TFC).
2. The ulnocarpal complex, i.e. the extrinsic ulnolunate and ulnotriquetral ligaments.
3. The extensor carpi ulnaris sheath.
The TFC proper passes from the sigmoid notch on the radius to the base of the ulnar styloid process. As the name implies, it is triangular in shape. The peripheral margins of the TFC are thick, vascular and structurally adapted to bear tensile loading. The central portion is thin, avas-cular and more suited to bearing compressive loads (Fig. 12.5).
Wrist movements
Wrist movement, excluding motion at the DRUJ, occurs in two planes:
1. Flexion/extension in the sagittal plane.
2. Radial/ulnar deviation in the frontal plane.
Figure 12.4. Dorsal wrist ligaments: DIC, dorsal intercarpal ligaments; RS, radioscaphoid; RT, radiotriquetral; TFCC, triangular fibrocartilage complex. (Copyright, Elizabeth Roselius with permission. Reproduced from Fernandez, D. L. and Palmer, A. K. 1999. Fractures of the distal radius. In Green’s Operative Hand Surgery (D. P. Green, R. N. Hotchkiss and W. C. Pederson, eds) p. 931, Churchill Livingstone.)
Figure 12.5. Triangular fibrocartilage and ulnocarpal V ligament (Copyright, Elizabeth Roselius with permission.
Reproduced from Bowers, W. H. The distal radioulnar joint.
1999. In Green’s Operative Hand Surgery (D. P. Green, R. N. Hotchkiss and W. C. Pederson, eds) p. 991, Churchill Livingstone.)
When these movements are combined, a consider-able range of motion occurs from radial deviation and extension to ulnar deviation and flexion. Range of motion can vary considerably among individuals.
Wrist flexion usually ranges between 75 and 90 degrees, extension between 70 and 80 degrees, radial deviation between 15 and 20 degrees and ulnar deviation between 35 and 40 degrees.
Total range of forearm rotation is 150 to 190 degrees at the DRUJ proper and 260 degrees at the hand. Forearm supination and pronation ranges between 80 to 90 degrees when assessed from the midrange position with the elbow flexed to 90 degrees.
During wrist extension, the first two thirds of movement occur at the radiocarpal joint, the remaining third at the midcarpal joint. During wrist flexion, the first half of motion occurs at the midcarpal joint and the second half at the radio-carpal joint. Radial and ulnar deviation occur primarily at the radiocarpal joint.
Assessment of the wrist
1. History
Assessment is made of the mechanism of injury and the force involved. Was the injury associated with a
‘snapping’ or ‘popping’ sound or sensation? Does the wrist ‘give way’ during activity? Where and when is pain present? What factors aggravate or relieve pain? Note should be taken of the patient’s expectations and physical demands in relation to occupation, sporting and leisure pursuits.
2. Examination
Look for swelling or deformity. Ask the patient to point to the most painful area; this area is palpated last. Compare range of motion and grip strength with the contralateral side. Check for generalized ligamentous laxity and perform specific tests to assess stability, e.g. the Watson scaphoid shift test, where appropriate.
3. Investigations (i) X-rays
Many injuries can be diagnosed by plain X-rays.
Special views may need to be requested, depend-ing on the suspected pathology (see each section for details). If the diagnosis remains unclear after thorough clinical assessment and plain X-ray, other investigations can be useful.
(ii) Bone scan
This is a sensitive although non-specific investiga-tion for suspected bone injury. Two days after injury, plain X-rays can appear normal while a bone scan will be positive, e.g. fracture of the scaphoid.
(iii) Tomography
Tomography can define the anatomy of the injury more accurately than plain X-ray. Bone healing and fracture non-union is more apparent than on plain X-ray, particularly where the scaphoid or hook of hamate are involved.
(iv) Magnetic resonance imaging (MRI) Magnetic resonance imaging can be used to assess certain ligament injuries and is the best investiga-tion for the assessment of bone vascularity, e.g.
Kienboeck’s disease.
(v) Arthroscopy
Arthroscopy has become an increasingly useful tool in recent years for the assessment and treatment of many wrist conditions including ligament injuries, articular cartilage defects and intra-articular