The Anatomy and Basic Biomechanics of the Wrist Joint

Transcription

1 ( SCIENTIFIC/CLINICAL ARTICLES J The Anatomy and Basic Biomechanics of the Wrist Joint Richard A. Berger, MD, PhD Associate Professor and Consultant, Surgery of the Hand, Departments of Anatomy and Orthopedic Surgery, Mayo Clinic and Mayo Foundation, Rochester, Minnesota T he human wrist joint is an elegant mechanism that normally allows the hand to be positioned in space on the stable platform of the forearm. The wrist allows not only for the classic, or cardinal, motions of flexion, extension, radial deviation, and ulnar deviation, but also for limited longitudinal rotation, combined motions, and independent forearm rotation. Each of these motions normally enjoys a substantial magnitude of range, making the challenge of maintaining stability an even more difficult anatomic problem. Considerable effort is currently under way to increase our understanding of normal anatomy and normal biomechanics of the wrist, which is prerequisite to understanding pathomechanics and developing efficacious treatment plans. This article reviews certain basic features of wrist anatomy and applies them to a fundamental review of wrist joint biomechanics. Because this paper is not a comprehensive review, the reader is encouraged to explore these issues in greater detail with a more complete reading of the original references. OVERVIEW OF WRIST JOINT ANATOMY The wrist is conveniently divided into 3 major joint regions: distal radioulnar, radiocarpal, and miacarpal. The carpometacarpal joints form the distal border of the wrist, and are not discussed in detail in this article. It should be noted, however, that the second through the fifth carpometacarpal joints are in communication with the midcarpal joint, via the interosseous articulations between the Correspondence and reprint requests to Richard A. Berger, MD, PhD, Associate Professor and Consultant, Surgery of the Hand, Departments of Anatomy and Orthopedic Surgery, Mayo Clinic and Mayo Foundation, Rochester, MN JOURNAL OF HAND THERAPY bones of the distal row. The synovial space of the first carpometacarpal joint, forming the basal joint of the thumb, is normally isolated from the remainder of the carpus. The bones comprising the wrist include the distal radius, the ulna, the carpal bones, and the bases of the metacarpals (Fig. 1). The carpal bones are conveniently divided into proximal and distal rows, which defines an anatomic and functional relationship between the bones of the 2 rows. The proximal carpal row is composed of (from radial to ulnar) the scaphoid, lunate, and triquetrum. The pisiform has historically been included with the carpus as a member of the proximal carpal row; however, its role as a carpal bone versus that of a sesamoid bone at the insertion of the tendon of flexor carpi ulnaris has placed its inclusion in the carpus in question. The distal carpal row is comprised of (from radial to ulnar) the trapezium, trapezoid, capitate, and hamate. The metacarpals are named by convention numerically, beginning with the thumb metacarpal, referred to as the first metacarpal. The distal radioulnar joint (DRUJ) is composed of the head of the ulna, the sigmoid notch region of the distal radius, the DRUJ capsule, and the triangular fibrocartilage complex (TFCC). The region of the TFCC related to the DRUJ is the articular disk and the distal radioulnar ligaments (Fig. 2).16 The articular disk is a fibrocartilagenous triangle, oriented in the transverse plane, with its apex near the styloid process of the ulna and its base along the distal edge of the radius, just distal to the sigmoid notch. The DRUJ is very closely modeled as a "cylinder" joint, whereby the radius rotates longitudinally about the head of the ulna. Normally, the DRUJ is isolated from the radiocarpal joint, but defects in the articular disk may be present as a result of trauma or age-related degeneration, which will allow a direct communication between the 2 joints.

2 The dorsal and palmar radioulnar ligaments originate from the dorsal and palmar corners of the radius at the distal margin of the sigmoid notch, and converge along the dorsal and palmar margins of the articular disk toward the ulnar styloid process. Here, they insert into the head of the ulna at the base of the styloid process, as well as into the styloid process itself. They provide an anchor for a multitude of ligaments, including the ulnolunate, ulnocapitate, ulnotriquetral, and extensor carpi ulnaris subsheath. The radiocarpal joint is found between the distal radius-tfcc surface and the proximal surface of the proximal carpal row. Under normal circumstances, the radiocarpal joint is isolated from the DRUJ by the TFCC and from the midcarpal joint by the interosseous ligaments between the bones of the proximal carpal row. There are normal features within this joint that may be misinterpreted as defects in the capsule, including the prestyloid recess. This structure is variable in size but constant in presence, located near the tip of the ulnar styloid process. In many wrists, the prestyloid recess actually allows limited contact with the ulnar styloid process. It is found in the medial wall of the radiocarpal joint in the substance of the TFCC, just distal to the junction of the dorsal and palmar radioulnar ligaments. It is generously invested with a rich complex of synovial villi. In approximately 80% of normal adults, a connection between the radiocarpal joint and the pisotriquetral joint is found, and is called the pisotriquetral orifice. It is located in the ulnotriquetral ligament, just palmar and distal to the prestyloid recess. Some confusion regarding the use of the term "radiocarpal" when describing the ulnar half of the joint may exist. It is by convention that the term "radiocarpal joint" is employed when discussing any aspect of the joint, including the ulnotriquetral and radioscaphoid articulations. OSSEOUS ANATOMY The bones of the wrist region include the radius, the ulna, the carpal bones, and the first through the fifth metacarpals (Fig. 1). The distal articular surface of the radius is concave in both the sagittal and the coronal planes. It is normally inclined degrees palmarly, and degrees ulnarly. There are 2 fossae or facets that articulate with the proximal surfaces of the scaphoid and lunate. The radial fossa, or the scaphoid fossa, is triangular, with the apex directed radially. The lunate fossa is quadrangular. They are separated by a sagittally oriented ridge, usually composed of fibrocartilage, termed the "interfacet prominence.,,3 The scaphoid, also historically called the "navicular," is the second largest carpal bone and was so named because of its remote resemblance to a boat. It is shaped somewhat like a kidney bean, and virtually spans the midcarpal joint. It has named regions, including the proximal pole, the distal pole FIGURE 1. Drawing of the wrist region from a palmar perspective, showing the underlying bony elements and the principal palmar ligaments. Bones: R == radius, U == ulna, S == scaphoid, L == lunate, P == pisiform, Tm == trapezium, Td == trapezoid, C == capitate, H == hamate, I == metacarpal I, V == metacarpal V. Ligaments: RSC == radioscaphocapitate, LRL == long radiolunate, SRL == short radiolunate, UC == ulnocarpal (ulnolunate, ulnotriquetral, and ulnocapitate), PLT == palmar lunotriquetral, TC == triquetrocapitate, TH == triquetrohamate, SC == scaphocapitate, ST == scaphotrapezium-trapezoid, PCTd == palmar trapeziocapitate, PCH == palmar capitohamate. Reproduced with permission from: Berger RA: Anatomy and basic biomechanics of the wrist. In Manske PR (ed): Hand Surgery Update. Englewood, CO, American Society for Surgery of the Hand, 1994, p 6-6. (tubercle), and the waist, which separates the 2 poles. There is a dorsal ridge that spirals through the waist region, which serves as a capsular attachment and has numerous foramina for nutrient blood vessels. It has a broadly curved proximalradial, articular surface, a slightly curved distal articular surface, a small, flat articular surface on the medial (ulnar) surface of the proximal pole for articulation with the lunate, and a surface that articulates with the head of the capitate. The ligaments that attach to the scaphoid include the radioscaphocapitate, the scapholunate interosseous, the dorsal and palmar scaphotriquetral, the scaphocapitate and scaphotrapezium-trapezoid, and the flexor retinaculum. The lunate, so named because of its crescent shape, is wedged between the scaphoid and the triquetrum. It is divided into a dorsal pole and a palmar pole. The dorsal P?le is much smaller than the palmar, giving the lunate a 3-dimensional wedge appearance. The lateral and medial surfaces are small and flat surfaces for articulation with the scaphoid and triquetrum, respectively. The distal surface is concave in both the coronal and the sagittal planes, and articulates with the capitate and variably with the hamate. Proximally, the articular surface is broadly convex in both the sagittal and the coronal planes. The ligaments attaching the lunate include the long and short radiolunate, the ulnolunate, the scapholunate and lunotriquetrat' interosseous, and the dorsal radiocarpal ligaments. The triquetrum is a small, irregularly shaped bone substantially covered in ligaments. The proximal articular surface may not be present at all. The lateral (radial) surface is flat and articulates with the lunate. Distally, the triquetrum has a spiral or a "helicoid" shape, for articulation with the hamate. Palmarly, there is a flat, somewhat elevated oval facet that articulates with the pisiform. Liga- April-June

3 LRL LT PRUJ FIGURE 2. Drawing of the radiocarpal/ulnocarpal joint surface from a distal perspective. The radius is toward the left, and is composed of the triangular scaphoid fossa (left) and the quadrangular lunate fossa (right), separated by the sagittally oriented interfacet prominence. The triangular fibrocartilage complex (TFCC) is composed of the triangular fibrocartilage (TFC) surrounded by the palmar and dorsal radioulnar ligaments (PRU/ and DRU/, respectively). The ulnotriquetralligament (UT) forms as a convergence of the radioulnar ligaments. The ulnolunate ligament (UL) originates from the palmar radioulnar ligament, while the ulnocapitate ligament (UC) originates from the base of the ulnar styloid process, deep to the prestyloid recess (PR). Other radiocarpal ligaments shown at origin include the short radiolunate (SRL), long radiolunate (LRL), radioscaphocapitate (RSC), and dorsal radiocarpal (DRe). Note that Lister's tubercle (LT) is in the same sagittal plane as the interfacet prominence. Reproduced with permission from: Berger RA: Anatomy and basic biomechanics of the wrist. In Manske PR (ed): Hand Surgery Update. Englewood, CO, American Society for Surgery of the Hand, 1994, p ments attaching to the triquetrum include the lunotriquetral interosseous, the ulnotriquetral, the ulnocapitate, the dorsal radiocarpal, the dorsal intercarpal, the dorsal and palmar scaphotriquetral, the triquetrohamate, and the triquetrocapitate ligaments. The pisiform bone, so named because of its resemblance to a pea, is a sesamoid bone of the flexor carpi ulnaris tendon. This tendon continues distally as the pisohamate ligament. There is a single, elevated, flat oval articular facet on the dorsal surface FIGURE 3. Drawing of the palmer radiocarpal and ulnocarpal ligaments from a palmar perspective. Ligaments: RSC = radioscaphocapitatb, LRL = long radiolunate, SRL = short radiolunate, UL = ulnolunate, UT = ulnotriquetral. Bones: L := lunate, S := scaphoid, C = capitate. Reproduced with permission from: Berger RA, Garcia-Elias M: General anatomy of the wrist. In An K-N, Berger RA, Cooney WP (eds): Biomechanics of the Wrist Joint. New York, Springer-Verlag, 1991, p JOURNAL OF HAND THERAPY of the pisiform for the triquetrum. In addition to the pisohamate ligament, there are attachments of the ulnocapitate ligament and the flexor retinaculum. The trapezium, or greater multangular, is interposed between the scaphoid and the base of the first metacarpal. It has a slightly concave proximal surface for articulation with the scaphoid, a flat facet medially for articulation with the trapezoid, and a saddle configuration distally for articulation with the first metacarpal. It is substantially invested with ligament attachments, including all of the carpometacarpal (anterior and posterior oblique, dorsal-radial, ulnar collateral), dorsal and palmar trapezium-trapezoid, and scaphotrapezium ligaments. The trapezoid, or lesser multangular, is one of the smallest carpal bones, interposed between the trapezium and the capitate. Only the dorsal and palmar surfaces are "nonarticular" for attachment of the palmar and dorsal trapezium-trapezoid and palmar and dorsal trapezoid -capitate ligaments. Additionally, there are trapezoid-metacarpal ligaments attaching both dorsally and palmarly. The articular surfaces for the trapezium, base of the second metacarpal, and scaphoid are all relatively flat. There is a notch-like geometry to the trapezoidcapitate articulation, where the deep trapezoidcapitate ligament is found. The capitate is the largest carpal bone, and is divided into the head, neck, and body regions. The head is hemispherical and entirely covered by articular cartilage with no ligamentous insertions. The neck is covered in periosteum, and represents a narrowed transition between the head and the body. The body is large and covered completely palmarly and dorsally by ligament insertions, including the palmar and dorsal trapezoid -capitate, the palmar and dorsal capitohamate, the scaphocapitate, the radioscaphocapitate, the triquetrocapitate, the ulnocapitate, and the capitometacarpal ligaments. Additionally, the articular surfaces ing the trapezoid and hamate are interrupted by the deep trapezoid/capitate and the capitohamate ligaments. Dist<;lily, the articular surface has 2 slightly angled facets for the base of the third metacarpal. Medially, the articular surface resembles a pan handle, with the handle extending distally and dorsally. The hamate, so named because of the hook (hamulus), is a large bone, forming the ulnar border of the distal carpal row. It is divided into proximal pole, hook, and body regions. The proximal pole is completely covered with the articular cartilage, similar to the capitate head, and the hook is completely covered with ligament attachments, including the flexor retinaculum and the pisohamate ligament. The body has 2 relatively flat distal facets for the bases of the fourth and fifth metacarpals, as well as a pan-handled lateral surface to match the capitate. Ligaments attaching to the body include the palmar and dorsal capitohamate, the deep capitohamate, the palmar and dorsal hamate-metacarpal, and the triquetrohamate ligaments.

4 LIGAMENTOUS ANATOMY The ligaments of the wrist form a complex network of collagen fascicles that virtually ensheath the carpal bones. Numerous theories regarding the function of these structures have been developed; however, it is likely that the ligaments serve several functions, including constraining displacement, guiding motion, and perhaps providing afferent neural input regarding the mechanical status of the joint. On close inspection, one discovers that the ligaments of the wrist are quite varied in their gross and histologic composition, which may be related to the varied functions that have been theorized. Histologic Organization 3 The majority of carpal ligaments are capsular, defined as being found within the layers of the joint capsule. These ligaments are composed of numerous parallel collagen fascicles bound together by perifascicular loose connective tissue, which also transmits longitudinally oriented neurovascular bundles. On the joint surface of these ligaments, a synovial lamina, composed of cuboidal synoviocytes, forms a complete layer, while the superficial surface is covered by a fibrous lamina. The synovial and fibrous laminae together completely encircle the collagen fascicles to form the epiligamentous sheath. Certain ligaments are intra-articular, defined as being found entirely within the joint. These ligaments are surrounded entirely by synovial lamina, but are otherwise similar in composition to the capsular ligaments. Finally, individual ligaments may show tissue-type variances, including the presence of fibrocartilage. 2 Overall Architecture The wrist ligaments can be grouped into distal radioulnar, palmar radiocarpal, dorsal radiocarpal, ulnocarpal, palmar midcarpal, dorsal midcarpal, FIGURE 4. RSC Drawing of the radiocarpal/ulnocarpal joint region from a distal and radial perspective, illustrating the enclosure of the proximal carpal row (removed) by the radiocarpal and ulnocarpal ligaments. Bones: R = radius, U = ulna. Ligaments: RSC = radioscaphocapitate, DRC = dorsal radiocarpal, LRL = long radiolunate, SRL = short radiolunate, UL = ulnolunate. UT = ulnotriquetral, TFCC = triangular fibrocartilage complex, pr = prestyloid recess, is = interligamentous sulcus. and interosseous categories, entirely dependent on the principal location of the fibers of the ligament. 22 Generally, the ligament is named for its most prominent bony connections. Distal Radioulnar Ligaments Two distal radioulnar ligaments surround the triangular fibrocartilage separating the head of the ulna from the radiocarpal joint (Fig. 2)?O The dorsal radioulnar ligament originates from the dorsal sigmoid notch of the radius, has a few fibers inserting into the foveal recess at the base of the ulnar styloid process, and passes over the styloid process where it contributes to the formation of the extensor carpi ulnaris subsheath. The palmar radioulnar ligament originates from the palmar sigmoid notch of the radius and inserts generously into the fovea. The majority of the ulnocarpal ligaments arise from this ligament. SL FIGURE 5. A Drawings of the scapholunate complex and the palmar radiocarpal ligaments. (A) Bones: S = scaphoid, L = lunate. Ligaments: RSC = radioscaphocapitate, LRL = long radiolunate, SRL = short radiolunate, RSL = radioscapholunate, SL = scapholunate interosseous, is = interligamentous sulcus. (B) Bones: L = lunate (scaphoid is removed). Ligaments: LRL = long radiolunate, RSL = radioscapholunate, SRL = short radiolunate, d, m, and p = dorsal, proximal membranous, and palmar regions of the scapholunate interosseous, respectively. Reproduced with permission from: Berger RA, Garcia-Elias M: General anatomy of the wrist. In An K-N, Berger RA, Cooney WP (eds): Biomechanics of the Wrist Joint. New York, Springer-Verlag, 1991, pp 8,14. April-June

5 FIC?URE 6. Palmar Radiocarpal Ligaments Drawing of the Wrist from a dorsal perspective. Bones: R = radius, U = ulna, S = scaphoid, C = capitate, T = triquetrum. Ligaments: DRC = dorsal radiocarpal, DIC = dorsal intercarpal. Reproduced with permission from: Berger RA, Garcia-Elias M: General anatomy of the wrist. In An K-N, Berger RA, Cooney WP (eds): Biomechanics of the Wrist Joint. New York, Springer-Verlag, p 10. There are 4 palmar radiocarpal ligaments (Figs. I, 3, 4, and SA).s,6 The radioscaphocapitate ligament originates from the radial styloid process, crosses the radiocarpal joint, and inserts onto the lateral wall of the waist of the scaphoid and the proximal half of the tubercle. A large proportion of the ligament passes under the waist of the scaphoid and part of the midcarpal joint capsule supportmg the head of the capitate. The long radiolunate ligament originates from the palmar rim of the radius ulnar to the radioscaphocapitate ligament, passes palmar to the proximal pole of the scaphoid, and attaches to the radial edge of the palmar horn of the lunate. These 2 ligaments are separated by a deep sulcus called the "inter ligamentous sulcus" (Figs. 4 and SA). The radioscapholunate ligament emerges into the radiocarpal joint between the long and short radiolunate ligaments and attaches to the scapholunate interosseous ligament. It is largely a neurovascular mesocapsule with minimal collagen contene.s The short radiolunate ligament originates from the radius and passes directly to the palmar horn of the lunate. FIGURE 7. Drawing of the wrist from a palmar perspective, highlighting the palmar interosseous ligaments and the palmar midcarpal ligaments. Bones: L = lunate, S = scaphoid, C = capitate. Ligaments: SL = scapholunate, LT = lunotriquetral, SIT scaphotrapezium - trapezoid, SC = scaphocapitate, TC = triquetrocapitate, TH = triquetrohamate, TT = trapezium-trapezoid, CT = trapezoid-capitate, CH = capitohamate. Reproduced with from: Berger RA, Garcia-Elias M: General anatomy of the Wrist. In An K-N, Berger RA, Cooney WP (eds): Biomechanics of the Wrist Joint. New York, Springer-Verlag, p JOURNAL OF HAND THERAPY Ulnocarpal Ligaments There are 3 ulnocarpal ligaments (Figs. 1, 3, and 4). The ulnocapitate ligament originates from the foveal recess of the ulna, passes palmar to the other and integrates with the palmar lunotrtquetral ligament to attach to the triquetrum. The ulnolunate ligament originates from the palmar radioulnar ligament and parallels the short attaching to the lunate. The lzgament originates from the palmar radioulnar ligament and attaches to the medial surface of the triquetrum. Defects often found as normal features in the course of these ligaments inthe prestyloid recess and the pisotriquetral ortfice. Dorsal Radiocarpal Ligament The dorsal radiocarpal ligament originates from the dorsal rim of the radius ulnar to Lister's tubercle, attaches to the dorsal horn of the lunate, and inserts heavily into the dorsal cortex of the triquetrum (Fig. 6).14 Palmar Midcarpal Ligaments There are 4 palmar midcarpal ligaments (Figs. 1 and 7). The palmar scaphotrapezium-trapezoid ligament originates from the scaphoid tubercle, splits into a "V".shape, and inserts into the trapezium and trapezoid. The scaphocapitate ligament originates from the tubercle of the scaphoid and attaches to the body of the capitate, passing just distal to the fibers of the radioscaphocapitate ligament. The triquetrocapitate ligament originates from the distallateral corner of the triquetrum and attaches to the of the capitate. The triquetrohamate ligament ongmates from the distal edge of the triquetrum and attaches to the body of the hamate. These ligaments are typically separated by a deep sulcus. TI:e ligament interdigitates wlth flbers from the ulnocarpal ligament to form the majority of the midcarpal joint capsule palmar to the head ofthe capitate. Dorsal Midcarpal Ligaments There are 2 dorsal midcarpal ligaments. The dorsal scaphotriquetral ligament forms the dorsal labrum of the midcarpal joint. It attaches to the dorsal surface of the waist of the scaphoid, attaches to the distal edge of the dorsal horn of the lunate, and attaches to the dorsal cortex of the triquetrum. The dorsal intercarpal ligament originates from the dorsal cortex of the triquetrum and inserts into the dorsal cortices of the trapezoid and trapezium (Fig. 6). Proximal-row Interosseous Ligaments There are 2 interosseous ligaments in the proximal carpal row, named the scapholunate and lunotriquetral interosseous ligaments (Fig. 7). Each liga-

6 FIGURE 8. CH CT Drawing of the distal carpal row (transected) from a distal and radial perspective. Bones: H = hamate, C = capitate, T = trapezoid. Ligaments: IT = dorsal trapeziumtrapezoid, CT = dorsal trapezoid-capitate, CH = dorsal cap i- tohamate, DCT = deep trapezoid-capitate, DCH = deep capitohamate. Reproduced with permission from: Berger RA, Garcia-Elias M: General anatomy of the wrist. In An K-N, Berger RA, Cooney WP (eds): Biomechanics of the Wrist Joint. New York, Springer-Verlag, 1991, p 15. ment is "C"-shaped, attaching the dorsal, proximal, and palmar edges of their respective joints (Fig. 5B). Each ligament is composed of a true capsular ligament dorsally and palmarly and a fibrocartilaginous proximal membrane. 2 The radioscapholunate ligament merges with the proximal region of the scapholunate interosseous ligament (Fig. 5A and B)2,3,5 Distal-row Interosseous Ligaments There are 3 interosseous ligaments in the distal carpal row, named the trapezium-trapezoid, trapeziocapitate, and capitohamate interosseous ligaments (Figs. I, 7, and 8). Each ligament has strong capsular dorsal and palmar components. The trapeziocapitate and capitohamate ligaments have deep intra-articular components as well (Fig. 8). NEUROVASCULAR ANATOMY The innervation of the wrist joint corresponds to the innervation of the skin overlying the wrist, following the principles of Hilton's law. Each superficial nerve sends articular branches to the wrist joint capsule that enter the ligamentous or capsular tissue. Little is known about the type of sensory endings within the joint; however, it is postulated that mechanoreceptors of some type will be found within the joint capsule. Nerves identified with the wrist joint capsule include the superficial radial nerve, the dorsal sensory branch of the ulnar nerve, the deep branch of the ulnar nerve, the lateral antebrachial cutaneous nerve, the anterior interosseous nerve, and the posterior interosseous nerve. 8 Denervation procedures for the treatment of chronic wrist pain have been developed based on the specific capsular innervation of these nerves. The blood supply of the wrist is based on branches from the radial artery, the ulnar artery, and the interosseous artery system. 9 There are numerous anastomotic pathways, both longitudinal and transverse, as well as both palmar and dorsapi As a generalization, there are 3 dorsal arches and 3 palmar arches traversing the carpus. Coursing longitudinally between the arches are anastomotic bridges. The arches feed capsular branches that enter the carpal bones through the capsular ligaments. Two carpal bones in particular, the scaphoid and the lunate, have vulnerable blood supplies. 17 The scaphoid gains its blood supply from nutrient vessels entering at the distal pole and the waist of the scaphoid. This implies that the proximal pole is dependent on continuity with the waist of the scaphoid in order to maintain vascular viability. Therefore, the proximal pole of the scaphoid is at risk of osteonecrosis following fractures proximal to the waist of the scaphoid. The lunate generally receives nutrient vessels from the dorsal and palmar ligamentous attachments, but has a variable intraosseous anastomotic pattern. In certain individuals, these patterns are felt to predispose to osteonecrosis of the lunate, called Kienbock's disease. BIOMECHANICS OF THE WRIST l The biomechanics of the wrist can be simplified by discussing material properties, kinematics, and kinetics. Material properties deal largely with the material properties of the carpal ligaments. Kinematics deals with a description of motion without consideration of force, whereas kinetics introduces forces across the wrist joint. Material Properties The material properties of the carpal ligaments have been studied by several investigators, and all show relatively similar results. 13,15,20 Parameters studied included yield strength, stress, strain, hysteresis, and creep, in isolated bone-ligament-bone preparations. Overall,/the wrist ligaments have properties similar to those of other joint systems. The palmar capsular ligaments, such as the radioscaphocapitate and the long radiolunate ligaments, are typical viscoelastic structures, generally failing at approximately 100 newtons (N). The intrinsic (interosseous) ligaments between the bones of the proximal carpal row are much stronger, requiring up to 300 N to fail. The strain at failure of these ligaments is much greater than that of the capsular ligaments, exceeding 50% compared with 10-35%, respectively. On the other end of the spectrum is the radioscapholunate ligament, shown histologically to be a neurovascular pedicle with little collagen organized in a ligamentous fashion. Distraction testing has shown it to be quite weak, failing at less than 50 N. The subregions of the scapholunate and lunotriquetral ligaments have undergone material and constraint property testing. The dorsal region of the scapholunate ligament is the strongest, requiring April-June

7 more than 250 N to fail, followed by the palmar region, failing at approximately 125 N. A similar but reversed phenomenon has been observed in the lunotriquetralligament, with the palmar region the strongest, followed by the dorsal region. In both ligament systems, the proximal region is quite weak to distraction testing, failing at approximately 25 N, owing to its fibrocartilaginous composition. Constraint Properties The act of constraining motion has been attributed to ligaments in general, but until recently, little information has been available regarding the specific contributions to constraints of motion by specific wrist ligaments. For example, the most important stabilizer preventing excessive radiocarpal pronation is the radioscaphocapitate ligament. ls Conversely, radiocarpal supination is constrained mainly by the dorsal radiocarpal ligament, with assistance from the ulnolunate ligament. Structures originating from the ulnar aspect of the wrist tend to change in constraint properties with changes in forearm rotation, while those originating from the radius do not. Similar studies have addressed the resistance to ulnar translation of the carpus. 24 While one study demonstrated the global disruption that is necessary to generate clinically significant ulnar translation, another study found that the radioscaphocapitate and dorsal radiocarpal ligaments are the principal constraints against ulnar translation of the carpus. Finally, constraint of the DRUJ has been evaluated using cadaver models in 2 classic studies Unfortunately, controversy has erupted as a result of the interpretation of these studies. Schuind et al. determined with quantitative analysis of intact DRUJs that the dorsal radioulnar ligament becomes taut in pronation, and the palmar radioulnar ligament becomes taut in supination.2o Ekenstam and Hagert reported the results of qualitative observations of DRUJ stability after removing the dorsal and palmar comers of the sigmoid notch of the distal radius, noting that the palmar ligament is important in preventing dorsal dislocation of the ulna during pronation and the dorsal ligament is important in preventing palmar dislocation of the ulna during supination. 7 Although they appear to contradict each other, these studies on close examination actually complement each other, and are describing different measures for the same phenomenon. During pronation, the dorsal radioulnar ligament normally becomes taut. But as the ulna continues to pathologically translate dorsally at the extreme of pronation, the palmar radioulnar ligament is the final impediment, which prevents dislocation. Conversely, during supination, the palmar radioulnar ligament normally becomes taut. But as the ulna continues to pathologically translate palmarly at the extreme of supination, the dorsal radioulnar ligament is the final impediment, which prevents dislocation. Therefore, both ligaments are critical to the normal functional range of motion 90 JOURNAL OF HAND THERAPY (ROM) and prevention of dislocation of the DRU] in both pronation and supination. Kinematics 4,10,12,14,19 From a functional standpoint, the motion of the carpus lends itself to a grouping of the carpal bones into proximal and distal rows, which is convenient from an anatomic perspective. It can be seen with close inspection, however, that carpal bone behaviors vary within each row, particularly within the proximal carpal row. When viewed as an integrated functional unit, the wrist behaves very much as a universal joint, with axes in palmar flexion/ dorsiflexion and radial/ulnar deviation. Combinations of these motions produce circumduction. Additionally, much has been written recently about the dartthrow axis, which represents motion from dorsiflexion-radial deviation to palmar flexion-ulnar deviation, as the principal functional axis of motion of the wrist. Although there is still controversy about the existence of a "center of rotation" of the wrist, most agree that if one exists it will be found in the head of the capitate, for both palmar-dorsiflexion and radial-ulnar deviation. 26 Rotation about the longitudinal axis of the forearm producing pronation and supination of the wrist relative to the forearm is rarely mentioned, but may be critical to the function of the wrist. Normally, there is as little as 10 degrees of rotation between the wrist and the forearm, which is ideal if torque is being applied across the wrist. However, in a relaxed state, as much as 40 degrees of passive rotation can be generated, at both the radiocarpal and the midcarpal joints. s This type of constrained rotation is critical to the function of the wrist in transmitting torque and allowing as much motion as possible in the remaining planes. Derangement of this relationship, either through trauma or through inflammatory conditions such as rheumatoid arthritis, leads to progressive weakness of grip, and may pose to extra-articular processes such as tendon rupture. / The distal carpal-row bones act as a functional unit, behaving essentially like the second and third metacarpals, which define the plane of motion of the hand. This is due to the interlocking of articular surfaces and the dense ligamentous connections between the bones of the distal row and the bases of the metacarpals. Therefore, when the third metacarpal palmar flexes and dorsiflexes, the distal row of carpal bones palmar flex and dorsiflex, respectively. This relationship is maintained not only for the direction of motion, but for the magnitude of motion as well. The proximal carpal-row bones have a unique pattern of motion, which is no doubt responsible in large part for the tremendous ROM and stability enjoyed by the wrist. Overall, the proximal row bones move together, but there is more motion between the individual bones than is allowed between the bones of the distal row. This is reflected in the direction of motion as well as the magnitude of motion between the proximal-row bones. As a

8 FIGURE 9. Schematic of the behavior of the carpal bones during wrist palmar flexion (A) and dorsiflexion (B). c = capitate, s = scaphoid, r = radius, hatch marks = lunate. Note how the proximal row follows the direction of the distal row. Reproduced with permission from: Berger RA: Anatomy and basic biome-, chanics of the wrist. In Manske PR (ed):... Hand Surgery Update. Englewood, CO, American Society for Surgery of the Hand, 1994, p B FIGURE 10. Schematic of the behavior of the carpal bones during radial deviation (A) and ulnar deviation (B). c = capitate, s = scaphoid, r = radius, hatch marks = lunate. Note how the proximal row shows reciprocal motion with the distal row. Reproduced with permission from: Berger RA: Anatomy and basic biomechanics of the wrist. In Manske PR (ed): Hand Surgery Update. Englewood, CO, American Society for Surgery of the Hand, 1994, p generalization, however, the proximal-row bones move with the distal-row bones (and therefore the hand) during flexion and extension of the wrist (Fig. 9A and B). The lunate shows the least ROM, followed by the triquetrum and scaphoid, respectively, but all 3 palmar flex during wrist palmar flexion and dorsiflex during wrist dorsiflexion. As the wrist extends, there is a tendency for the scaphoid to supinate and the lunate to pronate, which effectively separates the palmar surfaces of the 2 bones. The reverse phenomenon occurs during wrist palmar flexion. During wrist radial-ulnar deviation, the proximal-row bones demonstrate a very unique pattern of motion that is best described as reciprocal (Fig. loa and B). As the hand/distal row unit radially deviates, the proximal-row bones principally palmar flex and secondarily and variably counterrotate toward the medial (ulnar) border of the wrist. Conversely, as the hand/distal-row unit ulnarly deviates, the bones of the proximal carpal row principally dorsiflex and secondarily and variably counterrotate toward the lateral (radial) border of the wrist. The same longitudinal rotation (pronation and supination) occurs between the proximal-row bones during radial-ulnar deviation as occurs during wrist palmar flexion and dorsiflexion. the proximal-row bones accommodate wrist radial and ulnar deviation by palmar flexing and dorsiflexing, respectively. The theoretical concerns regarding the actual mechanisms underlying these kinematic behaviors exceed the scope of this article; however, some simplifications may be applicable. Lichtman developed a model of carpal behavior that is based on a "ring" model, in which each row of carpal bones is connected on the radial and ulnar sides through articulations and ligaments, forming a functional ring.l1 For example, during radial deviation, the distal car- pal row and the hand rotate about a center of rotation in the head of the capitate, bringing the trapezium and trapezoid toward the radial styloid process. For this to occur, the scaphoid must "move out of the way" by palmar flexing. This allows the trapezium and trapezoid to slide dorsally on the distal surface of the scaphoid, "pushing" it into palmar flexion. The scaphoid "pulls" the lunate into palmar flexion via the scapholunate interosseous ligament. In addition to this, the dorsal midcarpal joint capsule, including the dorsal intercarpal ligament, "pulls" the triquetrum into palmar flexion, which again leads the lunate into palmar flexion via the lunotriquetral interosseous ligament. The opposite phenomenon can be visualized during ulnar deviation, where the scaphoid is "pulled" into dorsiflexion by the palmar scaphotrapeziumtrapezoid ligaments, the triquetrum is "pushed" into dorsiflexion by the articular contact with the hamate, and the lunate is "pulled" into dorsiflexion by the scapholunate and lunotriquetral interosseous ligaments. Thus, the entire carpus is viewed as a ring, with the intercalated proximal row responding to the "pushing" of articular contact and the "pulling" of ligamentous connections. Kinetics Kinetics is the study of forces as they relate to joint function, namely, motion and transmission of load. Although we tend to think of the wrist joint as principally a motion-generating structure, substantial loads are borne by the wrist, from body weight bearing, such as those forces encountered when arising from a chair, to physiologic loads generated by the extrinsic muscle-tendon units crossing the wrist. The wrist has proven to be B April-June

9 a difficult joint to characterize regarding forces, but of those studies published, the salient features are discussed below. Both analytical and experimental studies of force transmission across the wrist, positioned in a neutral position and in neutral forearm rotation, show that approximately 80% of the force is transmitted across the radiocarpal joint and the remaining 20% across the ulnocarpal joint. ll The ulnocarpal force can be further subdivided into that across the ulnolunate joint (14%) and that across the ulnotriquetral joint (8%)?,25 On radiocarpal joint analysis, it is estimated that approximately 45% of the force crosses the radioscaphoid joint and 35% crosses the radiolunate joint. Forces across the midcarpal joint are estimated to be distributed a't 31 % through the scaphotrapezium-trapezoid joint, 19% through the scaphocapitate joint, 29% through the lunocapitate joint, and 21% through the triquetrohamate joint. Forearm rotation and wrist position influence load transmission across the wrist, largely due to changes in distal prominence of the forearm bones and changes in contact patterns, respectively. It has been shown that forearm pronation increases ulnocarpal force transmission (up to 37%, compared with 20% in the neutral position), with a concomitant proportional reduction in radiocarpal joint load transmission. With ulnar deviation of the wrist, ulnocarpal force transmission has been shown to increase to 28%, while radiocarpal forces increase to 87% during radial deviation. Wrist palmar flexion and dorsiflexion exert only modest effects on force transmission distribution across the wrist. Using pressure-sensitive film as a means of defining the area of contact between bones in the wrist, 3 distinct regions of contact have been identified in the radiocarpal joint: radioscaphoid, radiolunate, and ulnolunate. 23,25 Within these contact areas, it has been estimated that only 20% of the total contact area available is actually in contact in any given wrist position. Overall, the scaphoid contact area is 50% greater than that of the lunate. The centers of these contact areas change location with changes in wrist position, as do the areas of contact. For example, palmar flexion of the scaphoid results in a dorsal and radial shift in the center of the contact area, which also decreases in size. This implies a probable increase in contact intensity. Even so, it is thought that because the wrist is not a weightbearing joint under normal circumstances, the peak across the wrist are quite low compared with those of other joints, ranging from 1.4 N/mm2 to 31.4 N/mm2. Using the same techniques, the midcarpal joint has been evaluated, but has been shown to be more difficult to analyze due to its highly curved geometry. It is estimated that less than 40% of the total area available for contact is actually in contact at any given position of the wrist. The relative contributions to the total contact area of the scaphotrapezium -trapezoid, scaphocapitate, lunocapitate, and triquetrohamate joints have been estimated to 92 JOURNAL OF HAND THERAPY be 23%, 28%, 29%, and 20%, respectively. Thus, the head of the capitate is responsible for 50% of the total contact area of the midcarpal joint. CONCLUSION The wrist is an extremely complicated anatomic structure, which evidences equally complicated mechanics to provide a substantial ROM and load transmission from the hand to the forearm. Once the basic anatomy and mechanics of the wrist are understood, it is not difficult to imagine how easily the normal function of the wrist can be impaired by injury or disease. This leads rapidly to substantial dysfunction, which in turn affects not only wrist activity, but the activity of the hand, the entire upper extremity, ami even the entire person. Any hopes to treat disorders of the wrist must be based on a sound understanding of the normal anatomy and mechanics. A substantial effort has been under way over the past two decades to this end, and continues today. As computer capabilities in the laboratory become more accessible, more sophisticated testing of material properties, kinematics, and kinetics will be possible. Additionally, as computer modeling becomes more sophisticated, analytical studies of joint systems of the wrist will enable the researcher to simulate a wide variety of experimental conditions in a controlled fashion. Finally, as the results of these studies are understood by an increasing number of clinicians, the clinical consequences of pathomechanics will be better understood and more effective treatment plans developed. Future basic science work will no doubt concentrate on the integration of material properties, kinematics, and kinetics, while clinical investigations are continuing to attempt to improve our imaging capability, develop alternative treatment techniques, and determine the efficacy of treatments by carrying out carefully constructed clinical trials and reviews with standardized outcome measures. / BIBLIOGRAPHY 1. An K-N, Berger RA, Cooney WP (eds): Biomechanics of the Wrist Joint. New York, Springer-Verlag, Berger RA: The gross and histologic anatomy of the scapholunate interosseous ligament. J Hand Surg [Am] 21A: , Berger RA, Blair WF: The radioscapholunate ligament: A gross and histologic description, Anat Rec 210: , Berger RA, Crowninshield RD, Flatt AE: The three-dimensional rotational behaviors of the carpal bones. Clin Orthop 167: , Berger RA, Kauer JMG, Landsmeer JMF: The radioscapholunate ligament: A gross and histologic study of fetal and adult wrists, J Hand Surg [Am] 16: , Berger RA, Landsmeer JMF: The palmar radiocarpal ligaments: A study of adult and fetal human wrist joints, J Hand Surg [Am] 15: , Ekenstam FA, Hagert CG: The distal radio ulnar joint: The influence of geometry and ligament on simulated Colles' fracture. An experimental study. Scand J Plast Reconstr Surg Hand Surg 19:27-31, 1985.

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