Extensor apparatus injuries are too often underestimated, especially in the fingers. Although a cursory clinical examination may give the impression of integrity, it is only a few weeks later that deformity in the digital chain (boutonnière, swan neck, etc.) appears, leading to a delayed diagnosis. This results in secondary surgery that can be more difficult to perform and is less likely to yield a satisfactory outcome.
On the other hand, thorough exploration of injuries and early diagnosis enable optimal repair of the extensor apparatus. This allows for early protected mobilization.
Anatomy, Biomechanics and Healing
Characteristics of the Extensor Apparatus
The characteristics of the flexor apparatus cannot be applied to those of the extensor apparatus. With the exception of the zone under the extensor retinaculum, which is surrounded by a synovial sheath, the rest of the extensor apparatus is extrasynovial.
The structure of the extensor tendons consists of longitudinal fibers that hold suture material poorly; the flexor tendons have a more resistant helical structure.
The extensor mechanism is situated directly beneath the skin surface, which makes it vulnerable and in cases of trauma prone to skin and tendon adhesions. These mechanical restraints are, however, compensated for by the mobility of the skin. Extension and flexion deficits of the digital chain may also be compensated for by the tenodesis effect of the wrist.
When the injuries are proximal to the metacarpophalangeal (MCP) joints, the deficit is initially not apparent, owing to the action of the intertendinous junctions and interosseous expansions. It is only later that the extension deficit manifests itself. The extensor apparatus is less powerful than the flexor apparatus; it contracts with less force but faster. It cannot function normally if its anatomy is not accurately restored. If the tendon callus is actively stressed early on, it will extend by a few millimeters, resulting in an extension deficit. By contrast, the slightest shortening will cause a flexion deficit.
In addition, extensor apparatus injuries are rarely isolated. Injuries most often occur when the digital chain is flexed, making the MCP and proximal interphalangeal (PIP) joints more vulnerable. This results in the loss of skin, tendon and sometimes joint surface, for which primary repair is more complex. Treatment of these injuries requires a thorough knowledge of osteosynthesis and skin coverage techniques (see Chapters 7 and 9 ).
Anatomy and Biomechanics of the Fingers
In terms of the digital chain, it makes sense to speak of extensor apparatus and not extensor tendons, because it is through the integration of the extrinsic extensor tendons with the intrinsic apparatus, consisting of the interosseous ligaments and lumbricals, that function is ensured.
The extrinsic tendons have their muscle junction at the level of the lower middle third of the forearm ( Fig. 11.1 ). The tendons of the index and little fingers, the extensor pollicis longus (EPL) and the extensor digitorum communis, become single at the extensor retinaculum so as to glide in the osteofibrous spaces. The tendons are then spread out on the metacarpals and next to the metaphyses while remaining interconnected by bona fide intertendinous junctions, or juncturae tendinum.
The anatomy becomes more complex in the digital chain. Tubiana and Valentin’s terminology best clarifies its organization ( Fig. 11.2 ):
The extensor tendons stabilize the extensor hood through the sagittal bands, which attach to the intermetacarpal ligament.
The insertion of the tendon by a thin slip on the base of the proximal phalanx is variable.
The central slip, stemming from both the trifurcation of the extensor tendon and the intrinsic expansion, attaches to the base of the middle phalanx.
The lateral slips are bound together on the distal phalanx by the Stack triangle.
Extension of the digital chain can only be achieved through perfect coordination of the extrinsic and intrinsic extensor apparatus. Indeed, contraction of the extrinsics alone only generates extension of the proximal phalanx; the other two phalanges remain flexed. These can only be extended by the action of the interossei and lumbricals. The interossei, inserted onto the lateral tubercles of the proximal phalanx, are responsible for MCP joint function. Their superficial insertions contribute to the extensor hood and provide the most distal fibers that reinforce the central slip of the extensor tendon. They also combine with the lumbrical insertions to strengthen the lateral bands.
Overall the intertwining of the extrinsic and intrinsic system constitutes a fascia on the dorsum of the fingers, which is anchored by the transverse and oblique retinacular ligaments (Landsmeer).
In 1867 Duchenne de Boulogne observed that the intrinsic system was essential for ensuring the function of each phalanx but also to stabilize them relative to one another.
Landsmeer showed that each phalanx is governed by three muscular systems or by two muscular and one ligamentous system, ensuring a tenodesis effect. Thus the proximal phalanx is controlled by the extensor, the flexors and the interossei. The second and third phalanges are controlled by the extensor, the flexors and the retinacular ligaments.
Digital function requires perfect control of the MCP joint. If this joint is fully extended, the interossei extend the last two phalanges through the lateral bands, but if the MCP is flexed to 90 degrees, the interossei only strengthen the flexion of the proximal phalanx. However, if the MCP joint is in the intermediary position, the interosseous ligaments have a dual function: stabilization of the MCP joint and extension of the distal phalanges.
The lumbricals also contribute to extension of the PIP and distal interphalangeal (DIP) joints by opposing the pull from the flexor digitorum profundus through their direct insertion on it and the synergy they have with the common extensor.
The retinacular ligaments are necessary for proper coordinated movements of the last two phalanges; they act by a tenodesis effect. Thus the transverse retinacular ligaments prevent hyperextension of PIP joints through attachments on each of the lateral bands. Complete disruption of these ligaments results in a swan neck deformity.
The oblique retinacular ligament acts by the tenodesis effect and it coordinates movement of the distal phalanx with respect to the middle phalanx. It originates from the flexor sheath at the PIP joint and obliquely inserts into the terminal tendon (see Fig. 11.2 ). Its lengthening leads to a distal phalanx extension deficit. Its retraction, however, creates distal phalanx hyperextension and pulls on the palmar aspect of the lateral bands, which initiates the so-called boutonnière deformity of the finger.
The entire extensor apparatus has a variable excursion depending on the anatomic level. According to Bunnell (1944), for the middle finger, the extensor tendon moves 41 mm at the wrist, 16 mm at the MCP joint, 3 mm at the PIP joint and 0 mm at the DIP joint. Elliot and McGrouther (1985) have shown that flexion of the middle finger generates an extension of 50 mm of the extensor tendon at the wrist and 3 mm at the level of the IP joints.
Anatomy of the Thumb Extensor Apparatus
The extensor apparatus of the thumb is also where the extrinsic system (consisting of the long and short thumb extensors) and the intrinsic system come together. The latter is made up of the abductor pollicis brevis, adductor pollicis and flexor pollicis brevis. This junction occurs at the level of the MCP joint and forms a band that stabilizes the long extensor on the convexity of the head of the first metacarpal (see Fig. 11.1 ).
The intrinsic system participates in extension of the distal phalanx of the thumb, but it is the long extensor that is responsible for its hyperextension. The latter is also the main extensor of the MCP joint.
A neglected EPL rupture or laceration causes a swan neck deformity of the thumb. There is hyperextension of the thumb MCP joint by the concentrated pull of the extensor. Conversely an extensor pollicis brevis injury accompanied by a rupture of the band dislocates the extensor to the palmar side, creating a Z-shaped deformity called boutonnière thumb, characterized by flexion of the MCP and hyperextension of the IP joint.
The classification recognized by the International Federation of Societies for Surgery of the Hand (IFSSH) is derived from that of Verdan ( Fig. 11.3 ). There are eight zones for the extensor mechanism of the fingers. Doyle added a ninth zone that concerns the muscle mass of the forearm.
Four zones are assigned to the extensor apparatus of the thumb and are preceded by the letter T (thumb). The odd numbers correspond to joints, the even numbers to the diaphyseal zones. Zone 1 corresponds to the DIP joint, zone 8 to the musculotendinous junction.
Extensor Tendon Healing
Although flexor tendon healing mechanisms have been studied extensively, there is an odd lack of experimental work regarding extensor tendons. Despite this, it is generally recognized that extensor tendons have a dual extrinsic and intrinsic healing mechanism, but it is difficult to draw a strict parallel with that of the flexor tendons, given the anatomic, histologic, vascular and biomechanical differences.
Mason found that extensor tendon repairs became strong enough for active mobilization after the fifth week, highlighting that most of the extensor apparatus is extrasynovial, which promotes the appearance of numerous skin and periosteal adhesions.
The paratenon plays an important role in the proliferation phase in tendon healing. This is all the more pronounced when the initial trauma involves a crush injury and there is untimely unprotected active mobilization. It is in this context of a scar “storm” that adhesions and tendon blockages occur, resulting in a flexion deficit of the fingers in most cases.
Extensor Apparatus Testing
Thumb Extensor Apparatus Test
The hand is placed flat. The patient is asked to lift the thumb from the examination surface. Only with an intact EPL can this movement be performed in combination with hyperextension of the IP joint. In this position the outline of the EPL is visible on the ulnar side of the anatomic snuffbox ( Fig. 11.4 ).
The short extensor is tested against the resistance of a pencil applied to the proximal phalanx with a flexed MCP joint ( Fig. 11.5 ).
Test of the Extensor Apparatus of the Fingers
The index and little finger extensors are tested separately. They are independently responsible for full extension. While the middle and ring fingers remain flexed, the ability of the hand to make “horns” (to extend index and little fingers) confirms the integrity of the extensor indicis proprius and the extensor digiti minimi ( Fig. 11.6 ).
The common extensor must be carefully tested because the intertendinous junctions may conceal a more proximal section of one of the common tendons. The patient should be able to extend the finger against dorsal resistance applied to the proximal and then the middle phalanx ( Fig. 11.7a ). Testing of active extension of the distal phalanx must be carried out while keeping the middle phalanx extended. In this position, only intact terminal slips are able to perform this function (see Fig. 11.7b ). Despite rigorous performance of these tests, their accuracy may be compromised by the presence of edema, bruising, fractures and the presence of intertendinous junctions.
Injury Patterns and Examination
The examination must be precise and take the mechanism of trauma into consideration. It should, among other things, distinguish lacerations, subcutaneous ruptures, crush injuries, avulsions and dorsal abrasions of the hand. In theses cases, osteoarticular injuries and skin loss are often present.
Finally, septic wounds and, in particular, bites that frequently combine tendon wounds and joint wounds should be considered as an entity that can lead to suppurative arthritis.
The most frequently observed injury is avulsion of the tendon at the base of the distal phalanx, with or without an associated bone fragment. This injury, referred to as a mallet finger, is rare in the thumb. The EPL is most often injured at the wrist when it changes course in the osteofibrous tunnel formed by the Lister tubercle and the extensor retinaculum. This is a zone of hypovascularization for this tendon. It is often ruptured with fractures of the distal radius by attrition from the fracture or the osteosynthetic device.
The pathology of the punch thrower includes, among other things, ruptures of the extensor hood at the MCP joints, dislocating the tendon into the intermetacarpal valley.
The index and thumb are the fingers most commonly affected. Together they represent 55% of extensor tendon wounds.
Zone 5 opposite the MCP joint is the most common site of injury (42%), followed by the wrist (zone 7, 16%) and the PIP joint (zone 5, 14%). Therefore 72% of injuries are located at the joints, which is easily understood in the context of work-related injuries whereby a grasping, pushing or guiding motion of a hand places the joints in semiflexion and exposes them directly to the tool. This is the case for injuries by circular saws, routers, planers, band saws and other such devices. It is also a lesion typical when someone has delivered a punch.
They are also particularly vulnerable zones in sports. It is well known that these zones (1, 3, 5, 7) are the sites not only of clean-cut wounds but also of complex injuries with bone and soft tissue loss.
The diaphyseal zones 2, 4 and 6 are injured more often by crushing—by presses, hammers and so forth—and combine bone injuries requiring osteosynthesis and crushed skin destined to undergo a degree of necrosis.
Zone 6 is often severely injured. Abrasive discs, sanding machines, butchers’ machines, hot presses, and sideswipe injuries create deep injury with loss of bone, tendon and skin. They may extend to involve zone 5, which raises the issue of restoration of MCP joint function.
Zone 8 is more often affected by severe trauma with fractures of the two forearm bones after crushing by presses, saw lacerations, and sideswipe injuries that affect the musculotendinous junctions.
Doyle’s zone 9 injury is most often the consequence of a crushing avulsion of the extensor muscles of the thumb and fingers, associated with radial nerve lesions.
The evaluation of injuries and injury mechanisms should be meticulous because it will determine the therapeutic strategy. If the wound is simple and the debridement complete, the patient can benefit from a comprehensive emergency repair using surgical techniques compatible with early mobilization. However, in cases where it is impossible to immediately repair injuries, it is advisable to postpone definitive procedures by 48 hours.
In zone 1 we use the Doyle classification, which allows for precise therapeutic options:
Type 1: subcutaneous rupture, with or without a minor displaced fracture
Type 2: wound with tendon laceration
Type 3: wound with loss of skin and tendon
Type 4: mallet finger:
with transepiphyseal fracture of the phalanx (child)
hyperflexion injury with bone fragment affecting 20%–50% of the joint surface
hyperextension injury with bone fragment affecting more than 50% of the joint surface and early or late subluxation of the distal phalanx
Type 1 mallet finger arising from subcutaneous rupture of the distal insertion of the extensor apparatus is treated by immobilization in distal extension using a thermoplastic splint secured by two slips of Tensoplast ( Fig. 11.8 ). This splint is affixed to the distal phalanx and middle phalanges. Full extension is sufficient to reestablish contact between the tendon and its insertion. It is pointless to try immobilization in hyperextension, which would likely stretch the vessels, thereby altering pulp vascularity and creating genuine discomfort. Immobilization of the PIP joint has no merit. The splint is worn continuously for 6–8 weeks, with removal of this device not allowed. The patient must take care to permanently keep the DIP joint in extension. All splint alterations must be carried out by a third person, so as to avoid stretching the tendon or bone callus.
Upon completion of the immobilization, the splint is removed during the day but worn at nighttime for another month so as to avoid elongation of the tendon upon recovery of mobility. Restoration of DIP joint flexion is done by active flexion by the patient, thereby rendering the occupational therapist redundant. Spontaneous forceful active contraction of the flexor leads to an antagonistic relaxation reflex of the extensor, which limits the distraction forces on the healing tendon. Passive DIP joint flexion is to be avoided.
A temporary cessation of nail growth may be observed following the trauma, with the appearance of a transverse groove that progresses to the free edge at a rate of 2 mm per month.
The disinsertion associated with a small bone fragment responds well to this type of immobilization. It requires radiographic follow-up to detect a possible secondary displacement that would require surgery.
Stack’s shoe splint is based on the same principle but requires choosing the perfect size for a good result. The latest generation of shoe splints are hollowed out at the pulp, which allows for the finger to be used ( Fig. 11.9a ). The three-point splint has the advantage of freeing the soft tissues and allowing flexion of the PIP joint, but the rings are not always well tolerated, since their adjustment must be perfect to correct a mallet finger (see Fig. 11.9b ). These two types of splints are applied for a period of 4–6 weeks while leaving the PIP joint free. An ad integrum result is achieved in 98% of cases.
Subcutaneous rupture of the central slip in zone 3 is also within the scope of conservative treatment. Our experience has led us to combine the use of a static orthosis during the day and a dynamic orthosis at night, with the aim of restoring proper functioning of the central slip. The static palm orthosis fulfills the criteria for three pressure points: the MCP joint in 30 degrees of flexion, PIP and DIP joints in full extension ( Fig. 11.10 ). Theoretically it would make sense to leave the DIP joint free so as to mobilize the lateral bands. To address this need the splint would have to stop halfway on P2, which makes keeping the PIP joint in extension impossible. At night the patient wears a dynamic extension orthosis equipped with a Levame blade. Upon changing the orthosis, patients are asked to mobilize the DIP joint while keeping the PIP joint in strict extension. This protocol is applied scrupulously for a period of 6 weeks. Then the static orthosis is removed during the day to mobilize the digital chain, and the dynamic orthosis is worn at night for another 2 weeks. Active flexion must be done with care, and it should be done progressively so as not to induce an extension deficit.
The main repair techniques of the extensor apparatus are simple. The choice ranges between conventional sutures depending on the zone of injury, or reinsertion techniques using Mitek Microfix or Minilok absorbable anchors consistent with protected mobilization protocols. The choice is usually determined by the nature of the associated injuries and skin coverage options. Tissue loss of the extensor apparatus can also be urgently repaired through various plasties.
Approaches ( Fig. 11.11 )
This is a zone that should be approached with caution because there should be no damage to the nail matrix, and excessive undermining is likely to create skin necrosis and should be avoided. The distal extent of the extensor tendon approach is carried out by a transverse incision centered on the IP crease and extended at each of its ends by a V-shaped incision according to Beasley.