Craig Cunningham, ... Sue Black, in Developmental Juvenile Osteology (Second Edition), 2016
Carpals
These small, irregularly shaped bones are arranged in two transverse rows with four bones in each and they form a broad base for the firm support of the hand. Two bones of the proximal row (scaphoid and lunate) articulate with the radius to form the radiocarpal (wrist) joint, there being no direct articulation with the ulna. The distal row articulates with the metacarpals that form the substance of the palm of the hand. The proximal and distal rows articulate with each other at the transverse midcarpal joints. The carpal bones are wedged together and tightly bound by interosseous ligaments to form a deeply concave palmar arch and a more gently convex morphology to the dorsum of the hand. This palmar/dorsal-shaped incongruity is partly brought about by the considerably smaller palmar surface area of each carpal compared to its relatively larger dorsal surface area. The shape and position of the scaphoid and trapezium bones in particular, further deepen the lateral (radial) side of the palmar concavity as they are turned forwards and as a result carry the thumb in front of the plane of the rest of the digits. The medial (ulnar) side is deepened by the presence of the hook of the hamate and the elevated position of the pisiform bone.
The pillars of the palmar carpal arch are bound together by the flexor retinaculum (a thickening of the deep fascia), which attaches to the tubercles of the scaphoid and trapezium laterally and the pisiform, hook of the hamate and pisohamate ligament medially. The areas of the scaphoid and trapezium that lie lateral to the flexor retinaculum, and the area of the hamate that lies medial to it, are occupied by the sites of attachment of the muscles of the thenar and hypothenar eminences, respectively (Frazer, 1908; Eyler and Markee, 1954). The flexor retinaculum acts as a roof, converting the carpal arch into a carpal tunnel through which pass the median nerve, the eight tendons of the long flexor muscles of the fingers and those of the flexor carpi radialis and flexor pollicis longus muscles. Any lesion that reduces the size of the carpal tunnel significantly, may result in compression of the median nerve and lead to carpal tunnel syndrome. Most commonly, this can arise from tenosynovitis of the tendon sheaths, dislocation of the lunate bone or arthritic change and it is most prevalent in females between the ages of 40 and 60 years. Symptoms are often more common at night and may present as a tingling (paraesthesia), absence of tactile sensation (anaesthesia) or diminished sensation (hypaesthesia) in the lateral 3.5 fingers. There is often a progressive loss of co-ordination and strength in the thumb, which can result in difficulty performing fine movements. In severe cases of compression, there may be wasting or atrophy of the thenar muscles and a procedure called carpal tunnel release may be necessary, which involves either partial or complete division of the flexor retinaculum (Ellis, 1992).
The first reported naming of the carpal bones was by Lyser in 1653 (McMurrich, 1914), but subsequent renaming has introduced some confusion and, as a result, the literature is not always consistent. From lateral to medial, the proximal row of carpal bones comprises the scaphoid (os scaphoideum or navicular), lunate (os lunatum or semilunar), triquetral (os triquetrum or cuneiform) and pisiform (os pisiforme), while the distal row comprises the trapezium (os trapezium or multangulum major), trapezoid (os trapezoideum or multangulum minor), capitate (os capitatum or os magnum) and hamate (os hamatum or unciform) (Holden, 1882; Turner, 1934). It has been remarked upon (Wood–Jones, 1941) that the human carpals are remarkably primitive (constant) in form and comparative anatomy reveals more profound changes in function than in structure (Napier, 1980; Kanagasuntheram et al., 1987; Aiello and Dean, 1990). It is worth bearing in mind that the carpal bones can vary significantly in their appearance so that they may prove to be of some value in the forensic evaluation of personal identity (Greulich, 1960).
The radiocarpal joint (wrist) is commonly described as being ellipsoidal in shape, with the concavity being formed proximally by the distal articular surface of the radius and the convexity by the scaphoid and lunate distally (Frazer, 1948; Lewis et al., 1970; Last, 1973; Williams et al., 1995). Collateral, radiocarpal and ulnocarpal ligaments (Mayfield et al., 1976) strengthen the articular capsule of the joint. It is difficult to isolate the movements that are possible at this joint from those that occur at the midcarpal and intercarpal sites of articulation. The active movements that can occur at the radiocarpal complex are flexion (approx. 85 degrees), extension (approx. 85 degrees), adduction or ulnar deviation (approx. 45 degrees), abduction or radial deviation (approx. 15 degrees) and circumduction (MacConaill, 1941; MacConaill and Basmajian, 1977; Williams et al., 1995).
The midcarpal (transverse carpal) joint is a compound sinuous articulation between the proximal and distal carpal rows. Both the radiocarpal and midcarpal joints are directly involved in flexion, with the greatest degree of movement occurring in the latter joint. Conversely, in extension at the wrist, a greater proportion of movement occurs at the radiocarpal joint and this is witnessed by the fact that the articular surfaces extend further onto the dorsal surface of the proximal carpal row than they do onto the palmar surface. The greater degree of movement in adduction compared to abduction is explained by the relative shortness of the styloid process of the ulna, which does not tend to impair movement as much as that of the radial styloid. Adduction occurs at the radiocarpal joint, whereas abduction takes place almost entirely at the midcarpal joint. The intercarpal articulations occur between adjacent carpal bones within each carpal row and are generally described as planar joints, which are tightly bound by an extensive network of intercarpal ligaments (Kauer, 1974; Voorhees et al., 1985).
Each carpal bone can be considered to have six surfaces, at least two of which (palmar and dorsal) will be non-articular. The scaphoid is the largest of the carpal bones in the proximal row and it articulates with five bones – the radius proximally; the lunate medially and the trapezium, trapezoid and capitate distally.5 The distal part of its palmar surface bears a rounded tubercle, which is directed somewhat anterolaterally and is the site of attachment for the lateral aspect of the flexor retinaculum and some fibres of the abductor pollicis brevis muscle. At this point, it is crossed by the tendon of the flexor carpi radialis muscle, which can leave a poorly defined groove on the bone, lateral to the site of attachment of the retinaculum. The tubercle is palpable in the living and can be detected in the ‘anatomical snuffbox’, which is that area on the lateral aspect of the wrist that is defined within the boundaries of the tendons of extensor pollicis longus medially and extensor pollicis brevis and abductor pollicis longus laterally. The styloid process of the radius, the scaphoid, trapezium and base of the first metacarpal all lie in the floor of the snuffbox, which is crossed by the radial artery. At this point, the artery gives rise to the dorsal carpal branch, which supplies the posterior aspect of the carpal bones. The narrow dorsal surface of the scaphoid is rough, grooved and pierced by numerous vascular foramina.
The articular facet for the radius encroaches upon the dorsal surface and so it is narrowed to little more than a strip, to which are attached the dorsal ligaments of the wrist. The radial collateral ligament attaches to the slightly roughened lateral surface. The radial articular surface is convex and directed somewhat proximally and laterally and extends for some distance onto the dorsal aspect of the bone. The articular facet for the lunate is relatively flat, semilunar in shape, narrow and directed medially. A well-defined ridge separates it from the deeply concave facet that faces medially and distally for articulation with the capitate. A small roughened area is present between the radial and lunate facets and this is the site of attachment for the strong interosseous ligament that binds the scaphoid to the lunate. The articular surface for the trapezium and trapezoid bones tend to be continuous, convex and directed distally, although a poorly defined ridge can sometimes be found separating the two areas of articulation. The interosseous ligament that binds the scaphoid to the capitate attaches to the area of bone between the trapezoid and capitate articular surfaces.
The scaphoid is one of the most common sites of carpal fracture and it tends to occur following a fall onto the outstretched hand (Barr et al., 1953). Non-union of the fracture is common and avascular necrosis is a frequent complication following an interrupted blood supply to the proximal pole of the bone as the vascular supply tends to enter from the distal extremity (Obletz and Halbstein, 1938; Gasser, 1965; Gelberman and Menon, 1980; Crock et al., 1981; Panagis et al., 1983; Crock, 1996; Oehmke et al., 2009). Fracture of the scaphoid can be confused with a congenital bipartite bone. In this situation, there is no evidence of a fracture line, as the contiguous surfaces are smooth and the division will normally follow an oblique line across the waist of the bone (Waugh and Sullivan, 1950). A full appreciation of the normal anatomical variants of the scaphoid is therefore critical to a successful clinical diagnosis of injury or anomaly (Waterman, 1998; Heinzelmann et al., 2007).
The lunate derives its name from its broadly crescentic outline. It lies in the middle of the proximal row and is somewhat obliquely placed. Its position can readily be identified, particularly in thin individuals, as in hyperflexion of the wrist joint, the lunate rises above the level of the other carpals. It articulates with five bones – the radius proximally, the scaphoid laterally, the triquetral medially and the capitate and hamate distally. It has a relatively broad, triangular non-articular palmar surface and a much diminished non-articular dorsal surface, due to the dorsal prolongation of the radial articular facet. The lateral surface is narrow and bears a flat semilunar facet for articulation with the scaphoid. Immediately proximal to this facet is a roughened groove for the attachment of the interosseous ligament that binds the two bones together. The proximal surface is convex for articulation with the radius laterally and with the articular disc of the inferior radio-ulnar joint medially. Rarely, a ridge is present on the proximal surface of the lunate, delimiting the two sites of articulation. The medial surface presents a roughly square four-sided facet for articulation with the triquetral and this is separated from the deeply concave distal facet for articulation with the capitate, by a curved semilunar ridge, which represents the site of articulation with the hamate. Although both non-articular surfaces possess vascular foramina, they are particularly abundant on the dorsal surface (Gelberman et al., 1980). Unilateral or bilateral absence of the lunate bone is not common (Kobayashi et al., 1991; Smet, 2005) and neither is a bipartite lunate (Akahane et al., 2002).
The triquetral (meaning three sided) or carpal cuneiform (meaning wedge shaped) is a somewhat pyramidal-shaped bone with a base that articulates with the lunate, an apex that points distally and medially, a lateral surface that articulates with the hamate, a medial surface that is essentially non-articular, and a palmar surface that bears a single oval facet for articulation with the pisiform. The articular facet for the lunate is almost square in shape and is directed proximally and laterally. The concavo-convex articular surface for the hamate is directed somewhat laterally and distally and is broader proximally and narrower distally. The dorsal and medial surfaces are virtually confluent and roughened dorsally for the attachment of the ulnar collateral ligament of the wrist and smoother proximally for the site of articulation with the disc of the inferior radio-ulnar joint.
The pisiform derives its name from its ‘pea-like’ nodular shape and it is generally considered to possess all the attributes of a sesamoid bone as it forms within the tendon of the flexor carpi ulnaris muscle. It carries one articular facet on its dorsal surface for the triquetral bone and its long axis runs distally and laterally in the direction of the hook of the hamate (Robbins, 1917). The flexor carpi ulnaris tendon attaches to its palmar surface and it continues distally as the pisometacarpal and pisohamate ligaments. Indeed, the attachment of the latter ligament may be so well developed that it forms a distinct tubercle and an ossific bridge in this location results in pisohamate fusion (Cockshott, 1963). The flexor retinaculum is attached to the palmar aspect of the lateral surface and the abductor digiti minimi tendon and the extensor retinaculum are attached to the medial and distal aspects, respectively, of this bone. The ulnar artery may leave a faint depression on the medial surface. The pisiform is readily palpable in the living at the base of the hypothenar eminence and, as such, it is vulnerable to fracture following a fall onto the outstretched hand (Fleege et al., 1991). The ulnar nerve, if it comes into contact at all with the pisiform, is confined to the distal portion, where it lies palmar to the ridge for the attachment of the flexor retinaculum (Robbins, 1917).
The trapezium is the most lateral of the distal row of carpal bones and articulates with four bones – the first and second metacarpals distally, the scaphoid proximally, and the trapezoid medially.6 The palmar surface of the trapezium is characterized by a well-defined tubercle laterally and a deep groove in the middle of the surface that runs in a somewhat mediodistal direction. The tubercle gives attachment to the superficial layer of the flexor retinaculum, while the deeper layer attaches to the medial lip of the groove that houses the tendon of the flexor carpi radialis muscle. Thus it can be said that the tendon essentially passes through the retinaculum as it splits to accommodate its passage. The tubercle can be detected on deep palpation in the living, but it is generally masked by muscles of the thenar eminence. The superficial head of the flexor pollicis brevis muscle arises from the distal region of the lateral aspect of the tubercle, the abductor pollicis brevis from the proximal region, and the opponens pollicis arises between these two. The dorsal surface is somewhat roughened and elongated in shape and is closely related to the passage of the radial artery. The tubercle on the dorsal surface marks the site of attachment for the first carpometacarpal ligament. The lateral surface of the trapezium is non-articular and roughened for the attachment of the radial collateral ligament and capsular ligament of the first carpometacarpal joint. The proximal surface bears a shallow, hollowed out articular site for the scaphoid and a distinct interarticular ridge separates this from the relatively shallow, concave facet for the trapezoid on the medial surface.
The most distal extremity of the trapezium extends between the bases of the first and second metacarpals and presents a small quadrilateral facet that is directed somewhat distally and medially for articulation with the lateral aspect of the base of the second metacarpal. The distal surface that articulates with the first metacarpal at the sellar (saddle shaped) first carpometacarpal joint has received much attention. It is the unique form and function of the thumb, in terms of its mobility and dexterity, which has resulted in the considerable degree of interest expressed in this joint. The two articular surfaces are not congruent and this facilitates a large degree of joint movement (MacConaill and Basmajian, 1977). Flexion and extension are said to occur in a plane that is parallel to the plane of the hand, while abduction and adduction occur in a plane that is at right angles to that of flexion and extension. Flexion of the thumb brings about a concomitant medial rotation which, when combined with abduction, brings the pad of the thumb into contact with the pads of the other fingers (Bunnell, 1938; Terry, 1943; Haines, 1944; Napier, 1955; Jacobs and Thompson, 1960; Forrest and Basmajian, 1965; Kuczynski, 1974). This action of opposition is unique to the primates and has naturally been widely researched in the evolutionary and comparative literature (Montagu, 1931; Musgrave, 1971; Vlček, 1975; Susman and Creel, 1979; Napier, 1980; Trinkaus, 1983, 1989).
The action of opposition is of paramount importance with regard to the functional integrity of the thumb in the precision grip (Napier, 1956). Osteoarthritic change at the joint or paralysis of the thenar muscles, which can arise following suicide attempts by slashing of the wrists, tend to leave the mobility of the thumb severely impaired and the hand is rendered virtually useless as a precision instrument (Jacobs and Thompson, 1960; Forrest and Basmajian, 1965; Leach and Bolton, 1968). Most clinicians operate under the principle that all attempts should be made to ensure the integrity of the thumb in, for example, industrial accidents involving the hand. Often, however, it cannot be saved and unless some attempt at restitution of a thumb is attempted, the hand will become a tool of limited dextrous value. Pollicization of the index finger is a successful operation, which involves the reorientation of the index finger and fashioning it into a makeshift pollex (Jeffery, 1957; Clark et al., 1998).7
The trapezoid also derives its name from the Greek meaning ‘small table’, but it is further defined by the fact that none of the sides are actually parallel. It is also known in older texts as the multangulum minor because of its irregular outline. It is a small bone, characterized by a dorsal non-articular surface, roughly four times the size of its palmar surface, around which is a virtually continuous cylinder of articular surfaces. The palmar surface continues onto the lateral surface for a short distance, where the slightly convex facet for articulation with the trapezium can be found. This surface is continuous with the proximal slightly convex facet for articulation with the scaphoid, which is in turn continuous only towards the upper surface with the small square facet for articulation with the distal aspect of the lateral surface of the capitate. This surface is then continuous with the distal aspect, which bears the largest facet for articulation with the grooved head of the second metacarpal. The facet is somewhat triangular in outline, convex from medial to lateral, and concave from palmar to dorsal. The palmar surface of the trapezoid gives attachment to the deep head of the flexor pollicis brevis muscle, which arises as a large fasciculus from the adductor pollicis muscle (Day and Napier, 1961).
The capitate is the largest of the carpal bones and its name is probably derived from the proximal ‘head-like’ articular region. Being the largest of the carpal bones, it is not surprising that it articulates with the largest number of other bones, seven in total – the second, third and fourth metacarpals distally; the scaphoid and lunate proximally; the trapezoid laterally and the hamate medially. The dorsal surface is roughly triangular in shape and comes to a point that is directed both distally and medially between the bases of the third and fourth metacarpals. Its distal border is obliquely aligned in response to both the styloid process of the third metacarpal and the larger medial projection of the distal articular surface of the second metacarpal. The distal articular surface is roughly triangular in shape, being broader towards the dorsal, and narrower towards the palmar surfaces. The slightly concave facet for the second metacarpal is directed somewhat laterally and the very small facet for articulation with the fourth metacarpal is restricted to a small area on the dorsal rim. The lateral surface displays a concave strip distally for articulation with the trapezoid and a somewhat spherical surface proximally for articulation with the scaphoid. These two areas can be continuous but more often than not, they are separated by a deep depression that houses a strong interosseous ligament. A large articular strip is present on the posterior aspect of the medial surface for articulation with the hamate, which is deeper proximally and narrows towards its distal limit. The area directly anterior to this is roughened for the exceptionally strong interosseous ligament that binds the capitate to the hamate. The medial aspect of the head of the capitate articulates with the deeply concave distal facet of the lunate. The palmar surface of the capitate is small and roughened for ligamentous attachment, although it also gives rise to some of the fibres of the oblique head of the adductor pollicis muscle. The morphology of the capitate is variable and has been described in terms of the different anatomical morphologies present at its articular surfaces (Yazaki et al., 2008).
Both the term hamate and unciform refer to the hook-like appendage of this bone. It is the most medial of the carpals in the distal row and articulates with five bones – the fourth and fifth metacarpals distally, the lunate and triquetral proximally and medially and the capitate laterally. The hamulus (hook) projects from the distal part of the palmar surface and is deeply concave on its lateral aspect. This arises due to its immediate relation to the synovial coverings of the flexor tendons that pass to the little finger. The base of the hamulus can often be traversed by a groove brought about by the presence of the deep terminal branch of the ulnar nerve. The medial aspect of the flexor retinaculum attaches to the tip of the hamulus and lateral to this, the opponens digiti minimi attaches distally, and the flexor digiti minimi proximally. The upper border also gives attachment to the pisohamate ligament, which is considered to be a continuation of the flexor carpi ulnaris tendon. The hamulus can be detected in the living following deep palpation lateral and slightly distal to the position of the pisiform.
The distal surface of the hamate presents as a roughly square facet that is separated by a dorsopalmar ridge into a larger medial facet for articulation with the base of the fifth metacarpal and a smaller lateral facet for articulation with the base of the fourth metacarpal. The true proximal extent of the hamate is thin and wedge shaped and only comes into direct contact with the lunate when the hand is adducted. The articular facet covers the lateral surface for the capitate in all but the palmar and distal angle, which is the site of attachment for the strong capitohamate interosseous ligament. The proximomedial surface is represented by a broad articular strip, which is convex proximally and concave distally for articulation with the triquetral. As with all the other carpal bones, the dorsal surface is roughened for ligamentous attachment and displays numerous vascular foramina.
The prominent position of the hook of the hamate makes it susceptible to injury and although fracture can arise following a fall on the outstretched hand, it more commonly results from traumatic impact during sports activities, particularly those that involve the swinging of a bat or a club (Bray et al., 1985; Foucher et al., 1985; Norman et al., 1985; Stark et al., 1989; Failla, 1993; Wakely and Young, 1995). An untreated hook fracture generally results in weakness of the hand, particularly in the tendons associated with the medial two fingers and this will dramatically affect the strength of the power grip. A fracture of the hook can also lead to numbness of these fingers along with paralysis of the hypothenar muscles.