29.2.1 Extensor Retinaculum

This structure is discussed first, as it is integral to discussion of all of the wrist and long digital extensors. The extensor retinaculum is a strong fibrous band extending obliquely across the back of the wrist. Its purpose is to prevent the extensor tendons bowstringing away from the skeleton or displacing excessively in a radial or ulnar direction on activation of their muscles proximally (▶Fig. 29.1). ^{20 , 21 , 22 , 23} It is a strengthened part of the fascia of the forearm (▶Fig. 29.2) that attaches radially to the anterior border of the radius bone (▶Fig. 29.3) and to the triquetral and pisiform bones at its ulnar end (▶Fig. 29.4). Along its course, it attaches to a series of ridges on the dorsum of the radius and ulna (▶Fig. 29.5), creating six tunnels through which the extensor tendons pass (▶Fig. 29.6), with the tendons lying between grooves on the dorsal surfaces of the distal parts of the forearm bones and the overlying retinaculum, each tendon being enclosed in a synovial sheath where it passes through its respective tunnel. ^{4 , 5 , 23}

29.2.2 Extensors of the Wrist

These comprise three musculotendinous units, the extensor carpi radialis longus (ECRL), the extensor carpi radialis brevis (ECRB; ▶Fig. 29.7), and the extensor carpi ulnaris (ECU; ▶Fig. 29.8). ²⁴ Many variants of these muscles are described ^{22 , 25 – 29} and only the most common variants are given hereafter. ²⁴

The radial nerve, from nerve roots C6/7 and C7/8, innervates ECRL and ECRB respectively, and the posterior interosseous nerve, from nerve roots C7/8, innervates ECU. The innervation of the radial wrist extensors more proximally by the main radial nerve gives rise to the difference of muscle palsies seen between a proximal injury of the radial nerve, in which all of the wrist and digital extensors are denervated, and injuries more distally in which the radial wrist extensors remain functional and wrist extension remains, albeit with radial deviation.

The ECRL muscle arises mainly from the lower third of the lateral supracondylar ridge of the humerus and from the anterior aspect of the lateral intermuscular septum. The muscle is overlapped proximally, on its radial border, by the brachioradialis muscle as it runs distally along the radial aspect of the extensor compartment, overlying the proximal radioulnar joint and the proximal third of the radius. The muscle gives rise to its tendon at the junction of the proximal and middle thirds of the forearm. This flat, wide tendon continues along the ulnar border of the radius, adjacent to the brachioradialis tendon, to pass under the abductor pollicis longus (APL) and extensor pollicis brevis (EPB) tendons. It then passes through the second compartment of the extensor retinaculum, which lies between the overlying retinaculum and a wide groove on the dorsum of the radius, immediately lateral to the first compartment, containing the APL and EPB tendons (see hereafter). As it emerges from the extensor retinaculum, the extensor pollicis longus (EPL) tendon crosses it before the ECRL tendon inserts into the dorsum of the second metacarpal bone (▶Fig. 29.9). The ECRB muscle arises from the common extensor origin attachment to the lateral epicondyle of the humerus, from the radial collateral ligament of the elbow joint, and from the intermuscular septum separating it from the adjacent extensor digitorum communis (EDC) muscle in the proximal forearm extensor compartment. The muscle lies adjacent to the ECRL muscle, on its ulnar side, giving rise to a flat, wide tendon in the middle third of the forearm. This runs adjacent to the ECRL tendon under the APB and EPB muscles, through the second extensor retinacular compartment and under the EPL tendon, to attach to the base of the third metacarpal and the adjacent parts of the second and, sometimes, fourth metacarpal bases. Attachment to the third metacarpal base (▶Fig. 29.7 and ▶Fig. 29.9) is the basis for the “piano-key” sign, in which the extended middle finger is subjected to forced passive flexion to activate the ECRB muscle and cause proximal forearm pain, identifying posterior interosseous nerve compressions in which this nerve lies against a sharp fascial edge of the ECRB muscle in the proximal forearm. The attachments of the ECRB tendon to the second and fourth metacarpals allow for complete removal of the third metacarpal base in ray amputations of the middle finger, to achieve better alignment of the index and ring fingers, without defunctioning of the ECRB. ³⁰ While debate continues as to which of the two units should be used, the fact that there are two radial wrist extensors has allowed for one or other to be used for tendon transfers with no seeming loss of wrist extension or radial deviation. Their very wide and flat contour also makes them a convenient source of free tendon, as an alternative to palmaris longus, when operating on the dorsum of the wrist, hand, and fingers. The ECU muscle arises from the lateral epicondyle of the humerus, as the most ulnar component of the common extensor origin, and from a septum attaching to the ulnar border of the ulna. This septal attachment extends as far distally as the junction of the middle and distal thirds of the ulna. This septum also gives rise, on its flexor aspect, to the flexor carpi ulnaris (FCU) and flexor digitorum profundus (FDP) muscles. The ECU muscle gives rise to its tendon in the distal third of the extensor compartment of the forearm, with the muscle reaching distally to a variable extent. The tendon then runs distally through the sixth extensor retinacular compartment, immediately ulnar to the ulnar styloid, between the retinaculum and a groove on the dorsoulnar aspect of the ulnar head, to attach to the tubercle on the dorsoulnar side of the base of the fifth, or little finger, metacarpal (▶Fig. 29.8). In addition to acting as wrist extensors, these three musculotendinous units are partly responsible, in conjunction with their flexor equivalents, for radial and ulnar wrist movements.

29.2.3 Extensors of the Thumb

The extensor tendons of the thumb are the APL, the EPB, and the EPL (▶Fig. 29.3), all of which are innervated by the posterior interosseous nerve, from nerve roots C7 and 8. ²⁴ The APL muscle belly lies just distal to the supinator in the extensor compartment in the mid forearm. It arises from the lateral aspect of the posterior part of the ulna in its middle third, the adjacent interosseous membrane, and the middle third of the posterior surface of the radius. The EPB muscle, arising from the posterior surface of the radius and the adjoining interosseous membrane just distal to the APL origins, lies immediately adjacent and distal to the APL muscle belly. The distal part of both the APL and the EPB muscles as well as their tendons lie side by side obliquely across the radial wrist extensors (ECRL and ECRB). Any friction between the tendons in this anatomical location can result in swelling under the tight investing fascia causing pain and crepitus in a condition known as “intersection syndrome”. The APL and EPB tendons continue distally to the base of the thumb, passing through the first extensor retinacular compartment in a groove in the lateral border of the radius, immediately dorsal to the radial styloid (▶Fig. 29.10). Frequently, they pass under the extensor retinaculum in separate tunnels, and it is necessary to identify all of these when releasing de Quervain’s stenosing tenovaginitis (▶Fig. 29.11). ³¹ Most commonly, the EPB has a single tendon, or sometimes two, while the APL commonly has several tendons which insert mostly into the base of the first metacarpal but sometimes also into the trapezium and muscles of the thenar eminence (▶Fig. 29.12). The APL largely acts to stabilize the base of the thumb. The EPB tendon continues distally along the dorsal surface of the first metacarpal to insert most commonly into the base of the proximal phalanx of the thumb (▶Fig. 29.13). However, the insertion of the EPB is variable and this tendon may extend more distally to attach to the base of the distal phalanx. The EPL muscle, much larger than the EPB, arises from the lateral aspect of the posterior part of the ulna in its middle third, just distal to the APL origin, and from the adjacent interosseous membrane. The muscle belly passes obliquely under the long finger extensor muscles proximal to the extensor retinaculum to form the EPL tendon, which enters the third extensor retinacular compartment. This is a separate tunnel formed between the retinaculum and a narrow groove on the dorsolateral surface of the radius (▶Fig. 29.6), lying immediately radial to the fourth extensor compartment, housing the long finger extensors. The EPL tendon curls around the ulnar, then distal, side of the palpable landmark of Lister’s tubercle on the dorsoulnar aspect of the distal radius (▶Fig. 29.14), where it is prone to attrition rupture during the weeks after radial wrist injury, possibly as a result of interruption of its blood supply. It then crosses over the distal parts of the radial wrist extensor tendons, running obliquely toward the thumb. At the base of the thumb, it lies at a distance from the EPB tendon, with a space between the two when both muscles are activated giving rise to the “anatomical snuffbox” (▶Fig. 29.3). It then passes along the ulnar border of the first metacarpal onto the dorsolateral aspect of the MCP joint, where it lies immediately adjacent to the EPB tendon (▶Fig. 29.13), held in position by transverse aponeurotic fibers having the same functions as the sagittal bands in zone 5 of the finger extensor tendons (see later). Because the EPL tendon approaches the thumb from the dorso-ulnar direction, not only is it an extensor of the MCP and interphalangeal (IP) joints of the thumb but it also has an adduction action, which can give rise to confusion in diagnosis when other thumb base muscles are inactivated. Just distal to the MCP joint, on the dorsum of the proximal phalanx, it is joined on the radial and ulnar sides by tendinous extensions of the APB (radial) and the adductor pollicis and first dorsal interosseous muscles (ulnar), respectively, (▶Fig. 29.15 and ▶Fig. 29.16), before attaching to the base of the distal phalanx. ³² The involvement of these muscles in extension of the IP joint, and the variable insertion of the EPB tendon give rise to a variability of extension of the MCP and IP joints after division of the EPL more proximally than the MCP joint that can be confusing. The distal EPL tendon is a thick structure capable of taking a core suture of the kind used in flexor tendon repair and does not need the postoperative protective splinting necessary after repairing the distal extensors in zones 1 and 2 in the fingers. In fact, such protection may compromise the vital rapid flexion–extension activity of the thumb IP joint essential to fine pinch function. ³³

29.2.4 Extensors of the Finger Metacarpophalangeal Joints

It is convenient to consider the extensor system of the MCP joints of the fingers as a single system, although it is also involved in extending the IP joints of the fingers beyond the MCP joints; this is considered later. This system is subdivided into zones 5, 6, 7, 8, and 9 in the Verdan classification (the zones 1, 2, 3, and 4 are in the fingers) for convenience of description of the anatomical variations within its different parts. ▶Fig. 29.17a and b shows the current modification of Verdan’s subdivision of extensor tendon injuries by Doyle. ^{34 , 35 , 36} Classifying the different parts allows us to consider each part of this system separately with respect to its particular functional and surgical significance. Without this classification, we could not begin to discuss any of the problems, primary and secondary, simple and complex, that befall each part of the extensor system.

It has been recognized for centuries that the extensors of the fingers are very variable between individuals, with respect to both their muscle component in the forearm and the actual tendon patterns crossing the dorsum of the wrist and hand, ^{4 , 5 , 22 , 37 – 56} Only the most common variants are discussed hereafter. ²⁴

This system comprises the extensor indicis proprius (EI) to the index finger only; the EDC to all four fingers; and the extensor quinti, or digiti minimi (EDM), to the little finger only (▶Fig. 29.9 and ▶Fig. 29.14). All of these muscles are innervated by the posterior interosseous nerve, from nerve roots C7 and C8. The EDC and EDMin muscles originate from the lateral epicondyle of the humerus in the common extensor origin, while the EI arises from the middle third of the ulna on its radial border and from the adjacent interosseus membrane, immediately distal to the EPL attachment.

All of these muscles give rise to their respective tendons in the distal third of the forearm before passing under the extensor retinaculum. EI passes through the radial side of the fourth compartment of the extensor retinaculum adjacent to the EDC tendons that occupy the bulk of the available space in this compartment. EDMin passes through a separate compartment of the retinaculum, the fifth, which lies against the distal radioulnar joint (▶Fig. 29.6). Zone 9 of Verdan’s classification includes the most proximal part of this system: the extensor muscles. The attached tendons running distally to the extensor retinaculum on the dorsum of the wrist constitute zone 8. While the details of the muscle proximal attachments are not usually of particular interest to the surgeon, their innervation by the posterior interosseous nerve and knowledge of the course and ramification of this nerve after its passage through the supinator muscle is of importance, as laceration of the muscles in the mid forearm may cut the terminal branches of the nerve, making suture of the muscles a pointless exercise unless the nerve divisions can be reconnected. The main benefit of Doyle’s split of Verdan’s original zone 8 into two zones is a recognition that the new zone 8 is a tendon injury of tendons of significant size and that strong suture of these is possible. Zone 9 is a muscle injury and thus is less safe to mobilize early unless the trauma surgeon has looked carefully within each muscle for the tendon extension and has carried out strong tendon repairs of these intramuscular extensions.

Zone 7 is that part of the finger extensors, passing through compartments four and five of the extensor retinaculum, the purpose of which is to prevent bowstringing of the finger extensor tendons off the back of the wrist—an ugly, if not particularly functionally disabling, defect. Avoiding this complication after lacerating injury at this level is almost always possible by careful consideration of the level of tendon injury, then appropriate resection of enough of the retinaculum to allow both tendon repair(s) and free gliding of the repair(s) while leaving a strap 1 cm in width of the retinaculum in place. While this requires that the tendon surgery be carried out around and under this strap, bowstringing can usually be avoided. This is possible because of the longitudinal dimension of the extensor retinaculum: this is variable but is always several centimeters, allowing a retaining retinacular strap to be placed where appropriate to the repairs. In the rare circumstance in which the extensor retinaculum of compartment four has been destroyed, it must be replaced by a tendon reconstruction, commonly with palmaris longus, or half of the ECRB or ECRL as a free graft or by turnover of the retinaculum from the more ulnar and/or radial parts of this structure. Before the advent of recent drug treatment of rheumatoid arthritis, the hand surgeons most frequently explored zone 7 to carry out synovectomy. This synovium may also be involved in tuberculosis and gout. In rheumatoid arthritis, tendon ruptures at this level are most commonly of the extensor tendons to the ring and little fingers and are more frequently due to attrition on osteophytes on the dorsum of the distal radioulnar joint than synovial tendon injury. However, the middle finger extensor lies radial to these osteophytes, and rupture of this tendon should alert the surgeon to a synovial origin of the problem. In my experience, rupture by osteophytes from the wrist under the tendons in compartment four is very rare. During this surgery, the extensor retinaculum also needs be preserved, except in case when the wrist movement into extension is minimal or absent. Wrist surgeons are aware of the presence of the terminal branches of the posterior interosseous nerve innervating the wrist, lying deep to the radial tendons in the fourth compartment. Division of this nerve is also used to prevent and treat secondary hypersensitivity on the dorsum of the hand after injury and relocation of the superficial radial nerve and, less commonly, the dorsal branch of the ulnar nerve. ⁵⁷

Zone 6 of the finger extensors (▶Fig. 29.18) is that part of their course in which they run across the dorsum of the hand to the MCP joints, at which point they are defined as zone 5. The EI tendon passes to the MCP joint of the index finger on the ulnar side of the EDC tendon to this finger. Over the MCP joint, the two tendons coalesce to become a single long extensor tendon (▶Fig. 29.18). EI is, perhaps, best known to the hand surgeon in its various roles as a tendon transfer. Its loss from the index finger occasionally results in a slight loss of index extension at the MCP joint. Usually, there is no loss of either full extension of this joint or of the ability to point with the extended index finger alone. ^{58 , 59 , 60} It is suggested that this is because the EDC is named inappropriately: although this muscle starts from the common extensor origin on the lateral epicondyle of the humerus as a single muscle, it quickly splits into four muscles whence the four main tendons arise. ⁵⁹ Occasionally, the extensor indicis is absent, giving rise to a need to use another motor for tendon transfers. The ECRB makes a suitable alternative after extension, either using the palmaris longus tendon or using half the wide tendon of ECRB itself as its own extension.

The precise pattern of the EDC in zone 6 is inconstant, with the number of tendons running to each finger being very variable. Usually, only one tendon runs to the index finger, but the middle and ring fingers may have two or three EDC tendons. The EDC tendon to the little finger may be an entirely separate tendon. It is frequently absent distally. When absent, it is usually replaced at the metacarpal neck level by a thick juncturae tendinum attaching proximally to the extensor tendon of the ring finger, or its more ulnar tendon if two tendons are present, at the level of the middle part of the little finger metacarpal.

Between the communis tendons are connections called juncturae tendinum that are usually three in number but that are variable both in design and in position between the extensor retinaculum and the MCP joints (▶Fig. 29.19 and ▶Fig. 29.20). ^{5 , 22 , 34 , 49 , 52 , 55 , 58 , 61 , 62 , 63} Usually the juncturae become thicker and more defined as one moves from radial to ulnar, with the flimsy junctura between the communis tendons of the index finger and middle finger being nearest to the wrist (▶Fig. 29.21) and that between the ring and little fingers lying close to the MCP joint. The juncturae move distally during finger flexion. The tension in them increases with this movement, as the transverse metacarpal arch widens distally toward the MCP joints. When in their most distal position, the juncturae contribute to stabilizing the extensor tendons over the heads of the metacarpals during full finger flexion. Brand ⁶⁴ also demonstrated that they have a lateral stabilizing effect on the extended MCP joints. They may give rise to a diagnostic problem when the main tendons of one finger, commonly the middle finger, are divided close to, or over, the MCP joint and the patient can maintain extension of this finger through an intact junctura connecting the distal part of the cut tendon to the intact tendon of the next finger (usually the ring finger). However, extension of the involved finger in this way is usually only partial and/or extension of the MCP joint cannot be entirely achieved actively even if retained once the joint is extended passively. The juncturae are generally divided in realignment or replacement of the MCP joints in rheumatoid disease to prevent ongoing ulnar pull on each joint by the more ulnar extensor tendons after repositioning of the fingers radially. Where the EDC tendon to the little finger is absent and replaced by connection through the juncture to the ring finger extensor, there is a danger of losing EDC function to the little finger if this juncturum tendinum is divided.

The EDMin crosses the dorsum of the hand ulnar to the communis tendon of the little finger to coalesce with this tendon over the MCP joint to become a single long extensor tendon (▶Fig. 29.19). EDMin commonly has two slips, sometimes one, and occasionally more. In the absence of adductor muscle function of the little finger in motor ulnar nerve dysfunction, activity of this muscle is responsible for the abducted position of the little finger. ⁶⁵

It is now realized that many of the extensor tendons divided over the MCP joint, and more proximally, are substantial and capable of taking a core suture of the kind used in flexor tendon repair. In some individuals, particularly more distally in zones 5 and 6, the tendons are flatter and require use of horizontal mattress sutures, rather than flexor core sutures. Even when this is necessary, previous fixed postoperative protective splinting regimes after repairing these tendons are unnecessary and early mobilization is possible. Following slightly behind the changes to more aggressive mobilization of acute flexor tendon injuries, mobilization of extensor repairs in zones 5 to 9 has evolved from immobilization to rubber band traction systems of early mobilization to mobilization without rubber bands, in splints blocking full flexion for the short period of time when the repairs are considerably less than full strength. ^{33 , 66 , 67 , 68 , 69 , 70 , 71 , 72}