Aseptic Tenosynovitis of the Digital Flexor Tendon Sheath, Fetlock and Pastern Annular Ligament Constriction

Aseptic Tenosynovitis of the Digital Flexor Tendon Sheath, Fetlock and Pastern Annular Ligament Constriction

TENDON AND LIGAMENT INJURIES: PART II 0749-0739/ 95 $0.00 + .20 ASEPTIC TENOSYNOVITIS OF THE DIGITAL FLEXOR TENDON SHEATH, FETLOCK AND PASTERN ANNUL...

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TENDON AND LIGAMENT INJURIES: PART II

0749-0739/ 95 $0.00 + .20

ASEPTIC TENOSYNOVITIS OF THE DIGITAL FLEXOR TENDON SHEATH, FETLOCK AND PASTERN ANNULAR LIGAMENT CONSTRICTION Kees J. Dik, DVM, PhD, Sue J. Dyson, MA, VetMB, PhD, DEO, FRCVS, and Tim B. Vail, DVM

The digital sheath envelops the superficial (SDFT) and deep digital flexor tendons (DDFT) palmar (plantar) to the fetlock joint and the pastern region. The sheath originates at the bifurcation of the suspensory ligament (SL), slightly proximal to the palmaroproximal pouch of the fetlock joint, is 14 to 20 cm long, and extends distad to the palmaroproximal extremity of the middle phalanx. 10, 13 Palmar to the fetlock joint the sheath passes through the inelastic fetlock canal created by the palmar (plantar) annular ligament of the fetlock, the palmar (plantar) surface of the proximal sesamoid bones, and the intersesamoidean ligament. Further distally, the proximal and distal digital annular ligaments also bind the flexor tendons and digital flexor tendon sheath to the palmar (plantar) aspect of the pastern. The digital annular ligaments are less well defined than the palmar annular ligament. Proximal to the palmar (plantar) annular ligament of the fetlock the

From the Department of Radiology, Universiteit Utrecht (KJD), Utrecht, the Netherlands; Equine Clinical Unit, Animal Health Trust (SJD), Suffolk, England; and Desert Equine Veterinary Hospital (TBV), La Quinta, California

VETERINARY CLINICS OF NORTH AMERICA: EQUINE PRACTICE VOLUME 11 • NUMBER 2 • AUGUST 1995

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proximal portion of the sheath is irregularly shaped as a result of folding and sacculation.13 Distal to the fetlock joint, a small rounded sheath pouch is found medial and lateral to the SDFT, in the space between the annular ligament of the fetlock and the proximal digital annular ligament. Between the proximal and distal attachments of the quadrilateral proximal digital annular ligament, a slightly irregular outpouching of the digital sheath is found superficial to the medial and lateral aspects of the distal sesamoidean ligaments. Distal to the proximal digital annular ligament, a rounded outpouching of the distal portion of the digital sheath protrudes on the palmar midline, superficial to the DDFT. The distal digital annular ligament covers the terminal portion of the digital flexor tendon sheath deep to the proximal portion of the digital cushion. 9 On the palmar aspect of the fetlock and proximal pastern, the SDFT is adherent sagittally to the palmar aspect of the sheath wall; therefore, ultrasonographically, it may appear contiguous with the annular ligaments, even if the sheath is distended. Within the proximal part of the digital sheath the DDFT is attached to the sheath wall medially and laterally by a mesotendon (Fig. lA). These can be seen ultrasonographically in transverse images as echodense bands, extending medially and laterally (Fig. lB). They are most easily seen if the tendon sheath is distended (Kent A, Smith R, personal communication, 1993) and should not be misinterpreted as adhesions. The palmar annular ligament in normal horses is thin, but it may be discernible ultrasonographically as an echodense band palmar to the SDFT, especially in large horses. The digital annular ligaments, however, are too thin to detect. Distension of a digital flexor tendon (a windgall) is a common incidental finding not associated with lameness, especially in hindlimbs. The degree of distension may vary depending on the environmental temperature and the amount and intensity of daily exercise. It is a particularly common finding in the hindlimbs of dressage horses. The sheath wall may feel thickened, and there is sometimes loculation of the sheath.

ACUTE TENOSYNOVITIS

Proposed causes of acute aseptic tenosynovitis are direct trauma to the digital flexor tendon sheath, or strain of a flexor tendon, SL, or palmar (plantar) annular ligament.10, is, 19 The proximal portion of the sheath is not protected by the annular ligaments of the fetlock. Consequently, direct blunt trauma may cause sheath contusion and subsequent hemorrhage or herniation. In more complicated cases SDFT or DDFT or SL injury may cause swelling of these structures, leading to relative

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Figure 1. A, The proximal digital sheath opened to demonstrate the mesotendon (arrows) of the DDFT. B, Transverse ultrasonogram of the digital flexor tendon sheath. Note the echodense bands medial and lateral to the DDFT, the mesotendon. These should not be confused with adhesions.

stenosis of the fetlock canal and subsequent contusion of the digital flexor sheath. Overstretching or compression of the digital sheath may also cause friction between opposing parietal and visceral layers of the synovial sheath,15 between the normal flexor tendon/ digital sheath attachments,19 or in the intimate attachments between the superficial flexor tendon, digital sheath, and palmar (plantar) or proximal digital annular ligaments. Acute aseptic tenosynovitis usually is characterized by a sudden onset of mild-to-severe lameness, associated with warm, painful effusion of the digital sheath. The distension is most prominent in the proximal

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portion of the digital sheath, but is also palpable in the palmarodistal aspect of the pastern where the distal portion of the sheath protrudes between the proximal and distal digital annular ligaments. Although the clinical signs are obvious, an ultrasonographic examination is important. Tendon lesions, particularly focal lesions of the DDFT, may not be associated with any palpable abnormality (Dyson S, unpublished data; Barr A, personal communication, 1993). It should be remembered that in this region it can be difficult to image both the SDFT and DDFT in focus simultaneously; therefore, hypoechoic artefacts should not be confused with lesions. The presence of hypoechoic instead of anechoic synovial fluid suggests intrathecal hemorrhage. Plain radiography only outlines the soft tissue swelling of the palmar (plantar) aspect of the fetlock and the pastern and generally is not useful. Prominent asymmetric distension of the proximal end of the digital sheath suggests sheath herniation, and contrast radiography is the best imaging technique for confirmation. 3, 5 Initial treatment of acute aseptic tenosynovitis uncomplicated by tendon injuries includes rest and cold water therapy or ice packs, counterpressure wraps or poultices, and systemic nonsteroidal anti-inflammatory medication. Therapeutic ultrasound may be beneficial. Aspiration of fluid and intrathecal injection of corticosteroids or sodium hyaluronate generally should be reserved for cases that do not respond within 1 week. 15 Lameness usually resolves within 7 to 14 days and is associated with reduced distension of the tendon sheath. Longer-term pressure bandaging, however, may achieve a better long-term cosmetic result. The duration of rest required is proportional to the speed of lameness resolution. The prognosis for acute, uncomplicated aseptic tenosynovitis is favorable if treatment begins immediately. Intrathecal hemorrhage may be treated by puncture aspiration and pressure bandaging, but this treatment introduces a risk of iatrogenic septic tenosynovitis. Thorough lavage under arthroscopic control may be safer than needle aspiration if drainage is indicated, but consideration should be given to delaying the procedure for 5 to 7 days to ensure hemostasis. Focal lesions of the SDFT restricted to the digital sheath unassociated with local trauma are unusual; however, lesions of the DDFT do occur in both forelimbs and hindlimbs (Barr A, personal communication, 1993; Dyson S, unpublished data). The initial therapy is similar to that for acute aseptic tenosynovitis, although the intrathecal administration of corticosteroids may be contraindicated. Lameness generally is much slower to resolve. Healing of the lesions should be monitored ultrasonographically. Experience based on a small number of cases indicates that although favorable results have been achieved with small focal lesions,

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larger lesions seem to have a more guarded prognosis (Barr A, personal communication, 1993; Dyson S, unpublished data). Acute lesions of the SOFT that extend from proximal to the digital flexor tendon sheath, distad into the sheath, are often associated with distension of the sheath, with or without apparent constriction by the palmar annular ligament. In many cases, the distension and apparent constriction resolve spontaneously with rest. The prognosis depends on the tendon injury. Overreach or tread injuries to the palmar (plantar) aspect of the fetlock may result in acute tenosynovitis and localized injury to the SOFT. These injuries are often associated with persistent or recurrent lameness resulting from adhesion formation.

LONG-TERM TENOSYNOVITIS

Long-term tenosynovitis may follow acute effusion that has not resolv.ed completely, may result from persisting tendon and ligament injury, or can develop from multiple minor trauma, such as recurrent overloading. 11• 15• 19 Continuous irritation of the sheath may lead to diffuse or nodular thickening of the sheath wall, adhesions, reduced sheath capacity and elasticity, or stenosis of the fetlock canal. Unlike acute tenosynovitis, the fluctuant swelling usually is cold and painless in chronic cases. In horses with chronic digital sheath distension but little or no lameness, there usually is no evidence of adhesions or irregularities of the internal contour of the sheath. Slight-to-moderate lameness has been associated with a minor degree of adhesion formation, whereas marked lameness is found in horses with massive adhesions and reduced sheath capacity. 11 Ultrasonography is effective in demonstrating digital sheath synovial proliferation (Fig. 2) and adhesions. This is easier if there is prominent fluid distension of the digital sheath, but even then the extent of adhesion formation may be underestimated. Ultrasonography is invaluable for the assessment of tendon or ligament injury (Fig. 3). Contrast radiography is also useful to evaluate digital sheath abnormalities, and may better demonstrate the extent of adhesion formation. Unlike chronic septic tenosynovitis, productive and/ or destructive bony lesions at the abaxial surface of the proximal sesamoid bones usually are not observed. Arthroscopy is a suitable diagnostic and prognostic tool to assess digital sheath and associated flexor tendon abnormalities. It also provides an effective technique for resection of adhesions and solitary sheath wall proliferations.12 The prognosis of chronic tenosynovitis is guarded. In cases that are unresponsive to fluid aspiration and injection of intrathecal sodium

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B Figure 2. Longitudinal ultrasonogram (A) and corresponding diagram (B) of the severely distended lateral aspect of the upper end of the digital sheath of the right hindlimb of an 8year-old Warmblood, showing a solitary nodular sheath wall proliferation. 1 = Nodular sheath wall proliferation; 2 = anechoic distended sheath lumen.

hyaluronate or corticosteroids, arthroscopic surgery may be used to resect solitary nodular proliferations or minor adhesions. It is unlikely, however, to effectively relieve massive synovial proliferation and/ or adhesions.

STENOSIS OF THE FETLOCK CANAL OR FETLOCK ANNULAR LIGAMENT CONSTRICTION

Stenosing palmar (plantar) ligament desmitis, digital tenosynovitis, or digital flexor tendinitis may be more descriptive of the injury and effect resulting in fetlock canal stenosis than fetlock annular ligament constriction. 17 The principle cause of lameness seems to result from a Palm.

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B Figure 3. Transverse ultrasonogram (A) and corresponding diagram (B) at the palmar aspect of the right front fetlock of a 10-year-old Warmblood, showing enlargement and an ill-defined hypoechoic defect of the SOFT. 1 = SOFT; 2 = OOFT; 3 = flexor surface of the proximal sesamoid bones.

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relative stenosis of the inexpansible fetlock canal, disturbing the function of its contents. Even though the cause of fetlock canal dysfunction can be obscure, the clinical features of this syndrome are characteristic. Restriction of the free movement of the flexor tendons within the fetlock canal clinically is characterised by a long-term, persistent, mild-to-moderate lameness that improves little, if at all, with rest and that worsens with return to exercise or training. Fetlock flexion evinces pain. In most cases, there is distension of the digital sheath. Viewed from the side there usually is a characteristic "notch" in the palmar (plantar) outline of the fetlock. Diagnostic intrathecal anesthesia usually decreases or eliminates lameness. The hindlimbs of Paso fino and Warmblood-type horses are thought to be predisposed (19 times and 5 times, respectively) to injury resulting in stenosis of the fetlock canal. 18 In other breeds the incidence in forelimbs and hindlimbs was similar. The proposed causes for this condition include the following: l. Contraction of the annular. ligament resulting from direct

2. 3. 4. 5.

trauma. 1 Constriction by the annular ligament resulting from swelling (tendinitis) of the digital flexor tendons. 16 Restriction of the free movement of the flexor tendons through the fetlock canal due to primary synovitis of the digital sheath.8 Constriction within the digital sheath resulting from desmitis of the annular ligament caused by external or internal trauma. 19 Pain associated with the accumulation of fluid, or tenosynovial or tendon thickening resulting from sepsis.

A perceived positive relationship between increasing patient age and incidence of injury resulting in fetlock canal stenosis may reflect agerelated weakening or loss of elasticity caused by degenerative changes in the dense connective tissue of the palmar (plantar) annular ligament. Biopsies from the palmar (plantar) annular ligament from older normal horses tended to have fibroblast hypocellularity and chondroid metaplasia, although elastin fibers remained arranged in a fine circumferential network. In contrast, samples from clinical cases had a multifocal pattern of fibrocyte loss and fibroblastic hyperplasia and/ or chondroid metaplasia. There was loss of organization of collagen bundles and elastin fibers, and elastin fiber content was decreased. 18 Although the diagnosis usually rests on the characteristic clinical appearance, various imaging modalities may be used to provide supportive and prognostic information. Air-tendography has been used to demonstrate thickening of the annular ligament but has limited use for evaluation of focal tendon lesions. 19 Entheseous new bone at the site of the attachment of the annular ligament on the proximal sesamoid bones may be detected radiographically and is best seen on a proximodistal

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sesamoid skyline projection. 14 In an ultrasonographic study of this syndrome including 16 cases, four previously proposed causes were identified.4 In nine of these horses, the condition was characterized by thickening and distension of the digital sheath and of the annular ligament (Fig. 4). In three cases, the constriction was dominated by sheath thickening and distension, not associated with tendon or annular ligament injury. In three horses, severe swelling and loss of echogenicity of the superficial flexor tendon and thickening of the annular ligament was observed, with or without marked distension of the digital sheath. An additional constriction type not previously reported was observed in one horse that was thought to result from a thick layer of subcutaneous fibrosis covering the palmar aspect of the digital sheath that extended proximally far beyond the proximal sheath limit. In these affected horses, the characteristic "notch" in the palmar (plantar) outline of the fetlock resulted either from palmar distension of the digital sheath, or from swelling of the SDFT proximal to the palmar (plantar) annular ligament.

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B Figure 4. Transverse ultrasonograms (A) and corresponding diagrams (8) of the palmar aspect of the right front fetlock of a 15-year-old Warmblood. The oblique scan imaging the annular ligament attachment to the abaxial surface of the lateral proximal sesamoid bone (A) shows thickening of the ligament. The normal ligament is a very thin structure that usually is not visible on ultrasonograms. Thickening of the digital sheath is visible on the palmarodorsal scan proximal to the sesamoid bones (8). These findings represent the most common fetlock annular ligament constriction type. 1 = Skin, 2 = thickened annular ligament; 3 = abaxial border lateral proximal sesamoid bone; 4 = DDFT; 5 = SOFT; 6 = thickened digital sheath.

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The "notch" rarely may be absent if the distended sheath does not expand in a palmar direction, or if subcutaneous fibrosis cover the palmar aspect of the sheath. The normal palmar (plantar) annular ligament of the fetlock is difficult to see ultrasonographically, but becomes measurable in cases of annular ligament thickening either on palmaro (plantaro) dorsal scans or oblique scans imaging the annular ligament attachment to the abaxial surface of the proximal sesamoid bones. It is demarcated by easily detectable transducer/skin and palmar (plantar) annular ligament/synovial fluid interfaces. Measurement made ultrasonographically at the level of the apex of the proximal sesamoid bones provides the most reproducible index of annular ligament thickening. 18 In 41 affected limbs, the mean thickness from the skin surface to the dorsal aspect of the annular ligament was 9.1 mm ± 2.3 compared with the measurement in normal equine limbs of 3.6 mm ± 0.7. Ultrasonography is important for outlining thickening of other structures that might also result in stenosis of the fetlock canal. The SDFT and DDFT should be evaluated carefully; particular attention should be paid to their size, margination, and echogenicity. The prognosis is favorable, unless the constriction results from severe injury to the SDFT,16• 18• 20 or DDFT lesions (Dyson S, unpublished data). In a recent review of 49 horses that underwent desmotomy of the palmar (plantar) annular ligament, 24 of 27 horses became fully sound and 3 of 27 were improved when lameness was referable to thickening of the palmar (plantar) annular ligament alone. 18 In contrast, 5 of 13 horses that had concurrent tendinitis in addition to thickening of the palmar annular ligament became fully sound, whereas 7 of 13 were improved and 1 of 13 was unimproved. Although ultrasonographic evaluation of the palmar annular ligament and digital sheath yields important clinical information, assessment of associated tendon injury provides the most important prognostic information for this condition. The presence of intrasynovial adhesion formation also had a negative influence on outcome; this is best assessed by tenoscopy. Regardless of the reason for stenosis of the fetlock canal, the only effective treatment is surgical sectioning of the palmar (plantar) annular ligament. In the past, annular ligament desmotomy has not been recommended in cases of severe tendon injury. 19 However, desmotomy successfully restores function to horses with fetlock canal stenosis, which is caused by annular ligament desmitis alone, and improves the performance of horses with fetlock canal stenosis caused by annular ligament desmitis and digital flexor tendinitis. 18 The closed technique may result in fewer postoperative complications.17 Four to 6 months rest and controlled exercise are recommended when flexor tendon lesions are not demonstrated, and 10 to 12 months of rest are recommended when there

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is injury to the flexor tendons. This is longer than previously documented, but it reflects the experiences of the authors based on the time taken for horses to be sound.

ACUTE DESMITIS OF THE PALMAR ANNULAR LIGAMENT

Acute onset lameness may be seen in association with sudden onset of enlargement of the palmar annular ligament alone. There is localized heat and pain on palpation. Ultrasonographic examination confirms that the palmar annular ligament is enlarged and contains focal hypoechoic areas or is diffusely hypoechoic. Treatment with rest and/ or topical piroxicam has been successful and results in rapid resolution of lameness and swelling. It should also be noted that chronically enlarged palmar annular ligaments that are not associated with lameness occasionally have been seen. Ultrasonographic examination has shown a central hypoechoic area that has contained gelatinous material.

PASTERN ANNULAR LIGAMENT CONSTRICTION

Contrary to the well-documented fetlock annular ligament constriction syndrome, constriction of the proximal digital annular ligament in the pastern has received little attention in veterinary literature. Four horses presenting with this condition demonstrated a longterm, unimproving, unilateral forelimb lameness that was substantially or completely improved by palmar nerve block at the level base of the proximal sesamoid bones, but which was not improved by palmar digital nerve block at the level of the distal pastern. Inspection and/ or palpation showed marked palmar distension of the digital sheath distal to the proximal digital annular ligament. Distension of the digital sheath proximal to the fetlock was subtle. Radiography did not demonstrate bone or joint lesions in any of these horses. Ultrasonographic examination demonstrated marked thickening of the skin/proximal digital annular ligament layer and distension of the digital sheath in all four cases (Fig. 5). Ultrasonograms of the flexor tendons and distal sesamoidean ligaments were normal. Thickening of the distended sheath was noticed in one horse. Desmotomy of the proximal digital annular ligament performed in two horses resulted in complete recovery. Corrective wedge pad shoeing requested by the owner of one horse was not successful. One horse was sold and lost to follow up. 6 Desmitis of the proximal digital annular ligament accompanied by thickening of the digital sheath associated with

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B Figure 5. Longitudinal ultrasonogram (A) and corresponding diagram (B) of the mid-distal pastern region of the left front limb of a 4-year-old Standardbred. Thickening of the skinproximal digital annular ligament layer and slight distension of the digital sheath, associated with pastern annular ligament constriction. 1 = Thickened skin-annular ligament layer; 2 = DDFT; 3 = straight distal sesamoidean ligament; 4 = distended digital sheath; 5 = proximal phalanx.

injury of the SDFT or as a sequel of septic tenosynovitis also has been reported briefly. 2 Constriction by this ligament and/ or subcutaneous fibrosis may be an unusual cause of lameness, but the clinical appearance together with the response to nerve blocks described previously and the absence of bone or joint lesions, provides support for its diagnosis. Ultrasonographic abnormalities in the proximal digital anular of the pastern may provide the definitive diagnosis in these cases.

References 1. Adams OR: Constriction of the palmar (volar) or plantar annular ligament of the

fetlock in the horse. Vet Med Small Anim Clin 69:327-329, 1974 2. Denoix J-M, Crevier N, Azevedo C: Ultrasound examination of the pastern in horses. In Proceedings of the American Association of Equine Practitioners, 1991, pp. 364-380 3. Dik KJ: The efficacy of contrast radiography to demonstrate "false thoroughpins" in five horses. Equine Vet J 22:223-225, 1990 4. Dik KJ: Ultrasonographic evaluation of fetlock annular ligament constriction in the horse. Equine Vet J 23:285-288, 1991 5. Dik KJ: Ultrasonography of the equine tarsus. Vet Radio! Ultrasound 34:36-43, 1993 6. Dik KJ, Buroflka B, Stalk P: Ultrasonographic assessment of the proximal digital annular ligament in the equine forelimb. Equine Vet J 26:59-64, 1994 7. Fricker CH: Zur Struktur des Fesselringbandes beim Pferd. Schweisz Arch Tierheilk 124:315-319, 1982 8. Gerring EL, Webbon PM: Fetlock annular ligament desmotomy: A report of 24 cases. Equine Vet J 16:113-116, 1984 9. Getty R: Sisson and Grossman's The Anatomy of the Domestic Animal, ed 5. Philadelphia, WB Saunders, 1975 10. Haga BED, Vaughan LC: Radiographic anatomy of tendon sheaths and bursae in the horse. Equine Vet J 18:102-106, 1986

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11. Hago BED, Vaughan LC: Use of contrast radiography in the investigation of tenosynovitis and bursitis in horses. Equine Vet J 18:375-382, 1986 12. Mcilwraith CW: Fortschritte in der Arthroscopie beim Pferd. Pferdeheilkunde 8:8594, 1992 13. Spaulding K: Ultrasonic anatomy of the tendons and ligaments in the distal metacarpal-metatarsal region of the equine limb. Vet Radio! 25:155-166, 1984 14. Stanek C, Edinger H: Rontgendiagnostik bei der Striktur des Fesselringbandes bzw.
Kees J. Dik, DVM, PhD Radiology Department Universiteit Utrecht Faculty of Veterinary Medicine Yalelhan 10, NL-358U CM The Netherlands