Chapter 74Diseases of the Digital Flexor Tendon Sheath, Palmar Annular Ligament, and Digital Annular Ligaments
Synovial effusion of the digital flexor tendon sheath (DFTS) is common in all types of working horses. Frequently, effusion is idiopathic in origin and affects the DFTS of both hindlimbs without causing lameness. Occasionally synovial effusion is seen in a single limb along with lameness. Before the advent of ultrasonography and tenoscopy, injury to the soft tissue structures of the DFTS often remained unrecognized. The cause of chronic synovial effusion and lameness remained elusive, and a diagnosis of idiopathic tenosynovitis was made readily. Because modern diagnostic techniques have become commonplace in equine lameness practice, specific injuries of the structures of the DFTS have been identified.
The detailed anatomy of the DFTS and its contents was well described1 and is similar in forelimbs and hindlimbs. Reference is made to palmar and metacarpal region throughout, but the terms plantar region and metatarsal region strictly apply to the hindlimb. The DFTS is a thin-walled structure that encompasses the superficial digital flexor tendon (SDFT) and the deep digital flexor tendon (DDFT) from the level of the distal third of the metacarpal region to the T ligament, just proximal to the navicular bursa and the palmar pouch of the distal interphalangeal joint. The DFTS is mesenchymal in cell origin and is composed of two layers: an outer fibrous layer and an inner synovial layer. The fibrous layer provides strength and vascularity to the sheath. The synovial layer provides a smooth, frictionless surface and produces constituents of the synovial fluid. The palmar wall of the sheath incorporates three annular ligaments: the palmar annular ligament (PAL) and the proximal and the distal digital annular ligaments. These annular ligaments bind the digital flexor tendons to the palmar aspect of the digit, are effectively thickenings of the fibrous layer of the sheath wall with a transverse fiber orientation, and measure 2 mm or less in thickness.
The PAL or annular ligament of the fetlock joint inserts on the palmar border of the proximal sesamoid bones (PSBs), is continuous with the palmar (intersesamoidean) ligament of the fetlock joint, and thus converts the proximal scutum into an inelastic canal. The strong, transversely arranged fibers of the PAL bind down the digital flexor tendons into the proximal scutum. Distal to the PAL and immediately under the skin, the deep fascia forms a second, quadrilateral ligament: the proximal digital annular ligament. This ligament is a fibrous sheet that covers and adheres to the palmar surface of the SDFT and attaches laterally and medially by two bands to the proximal phalanx: one to the proximopalmar tuberosity and one that joins the insertion of the distal branch of the SDFT to the distal part of the proximal phalanx. This arrangement firmly binds the SDFT and DDFT, enveloped by the DFTS, in the palmar pastern region. The distal digital annular ligament adheres to the palmar surface of the distal part of the DDFT and binds down the terminal part of this tendon. The ligament is a crescent-shaped fibrous sheet attached by a strong band on either side of the middle of the proximal phalanx, covering the distal branches of the SDFT.
The dorsal wall of the DFTS is formed by the proximal scutum, middle scutum, and distal sesamoidean ligaments. The proximal scutum and the middle scutum are strong fibrocartilaginous pads that allow sliding of the digital flexor tendons along the palmar aspect of the fetlock and pastern regions, respectively. The proximal scutum is composed of the two PSBs and the thick intersesamoidean ligament. The latter is a thick sagittal structure made of transversely aligned collagen fibers. The intersesamoidean ligament covers and is attached strongly to the whole palmar and axial aspect of the PSBs and creates a solid union between these bones. The concave palmar face of the proximal scutum allows sliding of the digital flexor tendons in the palmar fetlock region. The proximal scutum extends proximally to the apex of each PSB between the two distal branches of the suspensory ligament (SL). Distally the proximal scutum gives origin to the distal sesamoidean ligaments, which represent the functional continuation of the SL and consist of the straight, oblique, cruciate, and short sesamoidean ligaments. The straight sesamoidean ligament inserts distally on the middle scutum, together with the distal branches of the SDFT and the palmar ligaments of the proximal interphalangeal joint. The middle scutum is a thick, fibrocartilaginous structure attached to the proximopalmar aspect of the middle phalanx. The middle scutum contacts the palmar aspect of the distal condyles of the proximal phalanx dorsally and the DDFT palmarly.
Within the DFTS, the SDFT and DDFT are intimately related. A fibrous ring (the manica flexoria) emanates from the lateral and medial borders of the SDFT and encircles the DDFT completely, from the proximal limit of the DFTS to the proximal aspect of the PSBs. The synovial lining of the DFTS adheres to the palmar surface of the SDFT in the sagittal midline proximal to the PAL, along the dorsal surface of the PAL and along the dorsal surface of the proximal digital annular ligament. The synovial lining of the DFTS also adheres to the palmar surface of the DDFT between the proximal digital annular ligament and the distal digital annular ligament, and along the dorsal surface of the distal digital annular ligament. The sagittal adhesion-like mesotendon between the SDFT and PAL is referred to as the vinculum of the SDFT. The DDFT also has a mesotendon that attaches to its palmar surface at the level of the proximal interphalangeal joint (see Figure 70-2, B). These mesotendon attachments contain vascular branches that contribute to the arterial supply of the intrasynovial part of the tendon.
The DFTS facilitates displacement of the digital flexor tendons during flexion and extension of the fetlock and interphalangeal joints. During metacarpophalangeal (metatarsophalangeal) joint movements, the two digital flexor tendons displace together, but during interphalangeal joint movements, displacement of the DDFT is greater than that of the SDFT.
Diagnostic techniques that localize disease to the DFTS include synoviocentesis and synovial fluid analysis and intrathecal or perineural injection of local anesthetic solution. Synoviocentesis of the DFTS can be performed in one of the several recesses of the sheath. Access to the proximal pouch is possible when the sheath is distended with synovial fluid but difficult when it is not. Synoviocentesis can be achieved by introducing a 2.5-cm needle along the dorsal aspect of the DDFT, between the DDFT and the lateral branch of the SL, a few centimeters proximal to the lateral PSB. Easier access can be gained via the distal palmar pouch of the sheath, which extends between the two distal branches of the SDFT and between the two digital annular ligaments, along the palmar surface of the DDFT. One should remember that this pouch is divided sagittally by the mesotendon of the DDFT in its distal part. The needle can be introduced through the skin to one side of the midline and, to avoid iatrogenic damage to the DDFT, gently and slowly advanced at approximately 45 degrees to the skin surface until synovial fluid is seen at the needle hub. To increase distention of the distal palmar recess, it can be useful temporarily to compress the proximal pouch by firm application of an elasticated bandage (see Figure 124-1). Alternatively, the needle can be aimed to access this pouch between the lateral or medial border of the DDFT and the ipsilateral distal branch of the SDFT, which will prevent inadvertent needle penetration of the DDFT, although the palmar pouch of the proximal interphalangeal joint can be entered if the needle is introduced too deeply. The DFTS also can be accessed through its proximal or distal collateral recesses. The proximal collateral recess is situated in the triangular space palmaromedially or palmarolaterally, between the base of a PSB, the proximal insertion of the proximal digital annular ligament, and the dorsal border of the DDFT. The space can be entered 1 cm distal to the base of a PSB and 1 cm palmar or plantar to the neurovascular bundle. The distal collateral recess is located on the lateral (or medial) aspect of the pastern, between the digital flexor tendons and the distal sesamoidean ligaments and between the proximal and distal insertions of the proximal digital annular ligament. A cadaver study showed that synoviocentesis of the DFTS using these techniques was most consistently successful when performed at the level of the proximal lateral recess on a non–weight-bearing limb.2 In addition, a palmar-plantar axial sesamoidean approach was described where the needle was introduced axial to the PSB in the flexed limb. This technique was described as being optimal for synoviocentesis.3 Ten milliliters of local anesthetic solution is injected for adequate analgesia of the DFTS (see Chapter 10). It is important to recognize that intrathecal analgesia of the DFTS is not specific for elimination of pain from the contained structures and can influence pain from the oblique and straight sesamoidean ligaments.5 Moreover, in some horses with DFTS pathology, the response to perineural analgesia is better than to intrathecal analgesia.
Characteristics of synovial fluid of the DFTS do not vary from those of the distal limb synovial joints. Normal synovial fluid is clear yellow and has a nucleated cell count of 770 cells/mcL or less and a total protein concentration of 1.0 g/dL or less.4 More sophisticated analysis of the synovial fluid for molecular markers offers the prospect of better preoperative identification of the presence of intrathecal tendon pathology.6
Diagnostic ultrasonography is by far the most commonly used technique for evaluating the DFTS. The DFTS is first encountered at level 3A and continues distally to level P1C and beyond (see Chapter 16).8 The PAL can be demonstrated in normal horses as a thin (1 to 2 mm) echogenic band immediately adjacent to the palmar surface of the SDFT at level 3C. The proximal digital annular ligament and distal digital annular ligament usually cannot be recognized in the palmar midline, unless they are abnormally thickened. The vinculum of the SDFT at the level of the PSB is easily identifiable by ultrasonography, but the distal mesotendon of the DDFT in the phalangeal region is only occasionally visible, usually when distention of the DFTS provides negative contrast (see Figure 70-2, B). A normal synovial reflection or mesotendon joins the lateral and medial borders of the DDFT in the proximal recess of the DFTS, which should not be mistaken for an adhesion (see Figure 70-2, A). The thickness of the DFTS can be assessed at levels 3A and 3B, where the capsule is identifiable as an echogenic band dorsal to the DDFT and the manica flexoria.
Before the widespread use of ultrasonography for examining the soft tissues of the DFTS, negative contrast radiography was described for the assessment of tenosynovitis and annular desmitis.9 To perform this technique, a tourniquet is applied distal to the carpus in a standing or anesthetized horse; 50 to 100 mL of air is injected into the DFTS; and another 200 to 300 mL of air is injected subcutaneously. Sometimes extra air is injected between the SDFT and DDFT at the midmetacarpal level. One postinflation lateromedial radiograph obtained at half the milliampere second value of standard skeletal exposure for this region is normally sufficient to make an accurate diagnosis. Although this technique effectively demonstrates the thickness of the PAL, its use has become superseded by the widespread availability of diagnostic ultrasonography.
Survey radiography of the DFTS is performed to demonstrate evidence of intrathecal air or gas caused by a penetrating wound, the presence of metaplastic mineralization of injured soft tissue structures, or concurrent pathological conditions of the bone. Positive contrast radiography10 may provide the most conclusive evidence of wound communication with the synovial space and may also provide additional information in the diagnosis of synovioceles associated with the DFTS. Although fistulography and filling of the intrathecal space with sterile iodine-based contrast medium is diagnostic of communication between the wound and the DFTS, we prefer to access the DFTS by placing a needle at a site remote from the wound. This minimizes the risk of forcing bacteria or foreign material present in the deeper layers of the wound into the synovial space. Using a remote site further avoids the risk of inadvertently introducing bacteria while passing the needle through an area of cellulitis into the synovial cavity. Infiltration of 10 to 20 mL of sterile contrast medium followed by manipulation of the digit should result in flow of contrast medium from the wound and can be demonstrated radiologically (Figure 74-1).
Fig. 74-1 Lateromedial radiographic image of the fetlock region. A positive-contrast tenogram of the digital flexor tendon sheath, followed by manipulation of the digit, resulted in flow of radiodense contrast medium from a wound (arrow).
Tenoscopy is the ultimate imaging modality for evaluating the internal structures of the DFTS.11 An endoscope is introduced routinely in the proximal collateral recess of the sheath, 1 cm distal to the base of the PSB and 1 cm palmar or plantar to the neurovascular bundle, but access to other synovial recesses is also possible (see Chapter 24). This approach allows for a complete examination of the DFTS and its contents, except for the palmar surface of the SDFT, unless substantial thickening of any of the annular ligaments or extensive subcutaneous fibrosis has occurred. Access in ponies and cob-types with very thick skin can also be difficult. The approach also facilitates therapeutic maneuvers within the DFTS, such as PAL desmotomy, adhesiotomy, synovial mass removal, and debridement of fibrillated or torn areas of the digital flexor tendons, manica flexoria, or intersesamoidean ligament.
Magnetic resonance imaging has superior soft tissue contrast and has been of use to identify occult lesions not seen radiologically or ultrasonographically such as isolated adhesions, lesions of the SDFT or DDFT, and injury of the distal digital annular ligament.
Acute noninfectious tenosynovitis is a traumatic synovitis/capsulitis of the sheath lining. As for joints, a traumatic synovitis/capsulitis can be caused by accumulative low-grade trauma associated with normal exercise, acute trauma associated with direct impact force (e.g., overreach), or an abnormal force outside the normal range of movement of the fetlock region (e.g., hyperextension). Synovitis/capsulitis is more frequently secondary to damage to the internal or supporting structures of the DFTS, such as disruption of the visceral or parietal synovial layers, tearing of the vincula, tearing of the sheath wall with herniation or synoviocele formation, central or marginal damage to the flexor tendons, tearing of the manica flexoria, and spraining of the PAL or proximal digital annular ligament. Each of these complicating conditions is likely to result in continuous irritation of the sheath and cause chronic tenosynovitis. Chronic tenosynovitis may be associated with villonodular thickening of the sheath lining, especially in the proximal recess; adhesion formation between the visceral and parietal synovial lining; and fibrosis with reduced elasticity of the DFTS capsule. When these conditions are present, a self-perpetuating cycle occurs of improvement with rest, followed by repeated inflammation with exercise. This results in increased inflammation and lameness, further fibrosis, and eventually thickening of the PAL and stenosis of the fetlock canal (see the following discussion of PAL syndrome). Complex tenosynovitis has been defined as tenosynovitis with thickening of the PAL, synovial distention, and adhesions or synovial masses, or both (Figure 74-2).12 Windgalls, especially those occurring bilaterally in the hindlimbs, are another form of low-grade chronic tenosynovitis. Although lameness is not usually a feature of windgalls, the synovial effusion is still likely to reflect the presence of low-grade chronic synovitis of the DFTS, caused by the continuous stress of use-induced overloading.
Acute tenosynovitis is characterized by a sudden onset of mild to severe lameness, accompanied by DFTS distention that can be palpated in the proximolateral and proximomedial pouches of the sheath and in the palmarodistal recess between both branches of the SDFT. The palmar or plantar region of the fetlock has increased skin temperature, and forced flexion of the fetlock is painful and exacerbates lameness. Chronic tenosynovitis produces similar signs, except for the signs of acute inflammation, although repeat injury may cause these signs to be superimposed on an established tenosynovitis. Disproportionate distention and pain in different regions of the DFTS can reflect the site of the primary pathology; pain on palpation of the proximal aspect of the DFTS is present when there is tearing of the digital flexor tendons in this location.
Ultrasonographic examination is essential for identifying any primary pathology responsible for the tenosynovitis, such as adhesions and complicating injuries of the digital flexor tendons (see Figure 69-8, A), the intersesamoidean ligament, or annular ligaments, and to document the staging of the condition.13 DFTS effusion may also accompany injury to the intrathecal part of the SDFT associated with a classic bowed tendon (see Chapter 69). However, intrathecal tendon injuries occur more commonly as focal core lesions in the DDFT (see Chapter 70) or as longitudinal tears of the SDFT, DDFT, or manica flexoria. DDFT tears usually involve the lateral or medial borders of the tendon in the forelimb; hence oblique ultrasonographic views can sometimes identify defects on these borders, although they are easily missed. Oblique images are also required to image the abaxial margins of the SDFT. Manica flexoria tears are more frequently seen in hindlimb tenosynovitis and are also easily missed using transverse ultrasonographic images because the tears are usually incomplete and located at the site of attachment of the manica flexoria to the SDFT. Recently, we have found that a midline longitudinal scan immediately proximal to the PSBs provides the best image for identifying instability and/or thickening of the manica flexoria that accompanies tears (Figure 74-3). However, negative ultrasonographic findings do not preclude a tear in the manica flexoria.
Fig. 74-3 Longitudinal ultrasonographic images from the midline immediately proximal to the metatarsophalangeal joint (distal to the left). The left limb has a normal manica flexoria (arrow). In the right hindlimb, the manica flexoria is displaced from the dorsal surface of the deep digital flexor tendon and is thickened (arrow).
Ultrasonographically, tenosynovitis has three progressive stages.14 Symmetrical distention of the DFTS without evidence of synovial proliferation represents stage 1, or the effusive stage of synovitis. More pronounced, often asymmetrical distention of the proximal pouch, which feels firm on palpation and is accompanied by synovial proliferation, is stage 2. In stage 3 synovitis, extensive synovial proliferation occurs with or without adhesion formation in the sheath. Adhesions are manifest as echogenic material between the tendon and wall of the DFTS and can be more easily seen when fluid from marked distention provides negative contrast. Care must be taken to avoid undue pressure on the ultrasound transducer, which will obliterate this fluid from the field of view. However, the presence of adhesions can be easily overestimated with ultrasonography.