S U S P E N S O R Y DESMITIS: D I A G N O S I S USING

REAL-TIME U L T R A S O U N D I M A G I N G

Michael L. Hause r , D V M , MS1; N o r m a n W. R a n t a n e n , D V M , MS1;

and R o n a l d L. Genovese , V M D 2

S U M M A R Y

The textural and dimensional characteristics of the appearance of the normal suspensory ligament lends itself well to diagnosis of clinical disease of this structure. Following injury to the suspensory ligament there are changes that invoNe inflammation, and structural compromise. These changes may be manifest in a spectrum of severity, and documentation may be p rov ided using rea l - t ime u l t r a s o u n d imaging . Ultrasound imaging of the soft tissues in the equine extremity is an important aid; in diagnosis of lameness, prognosis, monitoring of healing, and retrospectively, in determining the more efficacious therapeutic regimens. To illustrate pathologic change of the suspensory ligament, selected cases of suspensory desmitis with varying degrees of severity are discussed.

I N T R O D U C T I O N

The suspensory ligament originates from the distal row of carpal / tarsal bones and the proximal palmar /p lantar surface of the third metacarpal /metatarsal bone. Its insertion is on the proximal sesamoid bones after b i f u r c a t i n g a t t h e d i s t a l t h i r d o f t h e metacarpal /metatarsal bone. Medial and lateral branches of the ligament course distally and dorsally to the dorsal surface of the first phalanx, where they join the common digital extensor tendon. ~ 4 The correlation between the gross anatomy and ultrasound appearance of the soft tissue of the palmar equine metacarpus has been reported. 3 7

The ultrasonographic appearance of the suspensory ligament is related to its histological appearance. Tissues in the suspensory ligament include: striated muscle, adipose tissue, nerves, vessels, loose areolar tissue, and

Author's Address: ~Echo Affiliates Inc., 376 South Broadway, Lexington, Kentucky 40508; 2Randall Veterinary Hospital, 20600 Miles Parkway, Warrensville Heights, Ohio 44128

Acknowledgments: The authors would like to acknowledge the dedication toward excellence of the technical support team at Randall Veterinary Hospital. 258

the dense white connective tissue of the fibers. This diversity of tissue types give the suspensory ligament a unique ultrasonographic appearance relative to adjacent soft tissue structures. 3

Injury to the suspensory ligament may occur at its origin, mid-body, bifurcation, as well as either or both of its branches. The ligament is susceptible to injury from hyperextension of the carpus or extreme dorsoflexion of the fetlock. Predisposition to injury may occur with upright conformation or improper foot trimming. This type of injury usually occurs with the first contact of the foot with the ground (anterior stride phase).

Clinical signs of suspensory disease vary depending on the location of the lesion, the severity of the injury, and the degree of pain tolerance of the individual. Diagnosis is aided by radiography, conventional lameness examination, careful palpation and strategic regional anesthesia. Ultrasound imaging provides further documentation of the lesion, and confirmation of the diagnosis.

Treatment includes supportive care, physical therapy, and appropriate time off from challenging exercise. Prognosis must take into account the location, severity and degree of structural damage incurred, as well as the degree and type of healing, and the athletic level of the horse.

M A T E R I A L S A N D M E T H O D S

A post mortem specimen of a normal suspensory ligament was obtained from a 6-year-old Thoroughbred gelding. It was fixed and prepared by conventional methods for histological examination. Sections were stained with Massons' trichrome and photographed at low power (100x).

Case material was obtained from referral of horses from race track practitioners. After preliminary lameness examinations, and in some cases diagnostic nerve blocks and radiography, the horses were prepared for ultrasound examination. Most limbs were clipped over the palmar surface with a number 40 clipper, and shaved

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with a disposible razor. Aqueous contact gel" was used to couple the skin surface to the transducer. A commercially available sector scanner was used in all examinations. ~ The 7.5 MHz crystal o f a multifrequency rotating sector scanhead was used throughout. Images were recorded on polaroid film. ~

Ultrasound images were obtained in both sagittal and cross-sectional planes. Power output and variable gain settings were adjusted to image the soft tissues of the palmar metacarpus.

R E S U L T S A N D D I S C U S S I O N

A photomicrograph of a longitudinal section of a normal suspensory ligament is shown in Figure 1. The microanatomy of tendons and ligaments is an important corollary to interpretation of the ultrasonographic appearance since each basic tissue type has a different

acoustic impedance. Striated muscle, adipose,nervous, and vascular tissue all appear less eehogenic compared to dense connective tissue of the fibrous portion. Therefore, the appearance of the normal suspensory ligament (Figure 2a and b) may be termed regularly irregular. Soft tissue structures superficial to the third metacarpal bone are identified in Figure 2a in cross section. The digital flexor tendons, in fe r io r cheek ligament, suspensory ligament and bone surface of the third metacarpus can be identified from their ultrasonographic appearance ) 7

The axial alignment of fibers in the sagittal plane are identified in Figure 2b. Both of these scans were made at the mid-body region at the same skin contact point.

Injury to the suspensory ligament produces tearing of the striated muscle bundles as well as the fibrous portion. Rupture of small blood vessels causes hemorrhage into the ligament. Inf lammatory change includes edematous infiltrate to the site. The ultrasonographic findings of suspensory desmitis, are echolucency which represent fluid, and increases or decreases in the echogenicity of the

Figure 2a. A sagittal ultrasound scan of the proximal palmar metacarpus representing the normal dimensional and textural characteristics of the suspensory ligament. Superficial digital flexor tendon (SDF), deep digital flexor tendon (DDF), inferior check ligament (ICL), suspensory ligament (SUS), and palmar surface of the third metacarpal (MC liD.

Figure 1. Longitudinal tissue section of the suspensory ligament from a 6-year-old Thoroughbred gelding. Basic tissue types of the suspensory ligament are striated muscle (Sin), adipose tissues, areolar tissue, nervous (N) and vascular (V). The fibers of the suspensory ligament (F) are in parallel bundles.

a Aquasonic 100. Parker Laboratories Inc. Orange, New Jersey 07050. b ATL 4600, Advanced Technology Laboratories, Inc., Pie Data

Medical, 2655 N. Ocean Dr., Singer Island, Florida 33404. c Polaroid Corp., Cambridge, Mass. 02139.

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Figure 2b. A cross sectional ultrasound scan at the same level as in Figure 2a. See 2a for legend.

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Figure 3a. A sagittal ultrasound scan immediately distal to the carpo-metacarpal joint. There is a 1.0 cm echolucency within the suspensory ligament at its origin (OS). This echolucent region represents hemorrhage , edema, and disruption of the ligamentous fibers.

Figure 4a. A cross sectional ultrasound scan at the mid- metacarpus level. Changes in the suspensory ligament (between cursors) include a more diffusely hypoechoic pattern as well as small (3-5mm) anechoic foci representing fiber bundle disruption and edema or hemorrhagic infiltrate.

Figure 3b. A cross sectional ultrasound scan at the same level as Figure 3a.

structural components. Increases in echogenicity would represent fibrosis. Acute change is represented by echolucency (fluid accumulation and fiber disruption). Also the axial (or loss of) alignment of fiber bundles can be evaluated. The normal palmar-dorsal dimension of the suspensory ligament is approximately 0.9 cm. Each branch is approximately 0.9 cm, this can be an important factor in determining the severity of a lesion, in acute cases.

Injury to the supensory ligament may occur at its origin on the proximal third metacarpus, 2 6 which may include small avulsion fractures. 6 This lesion appears echolucent owing to the fluid infiltrate and loss of structural echogenicity due to tissue disruption. Rarely the contour surface of an avulsion fragment can be imaged.

Mid-body injuries to the suspensory ligament vary in severity depending on the amount of hemorrhage and edema and the deg;ee of structural damage. Again, these lesions are documented as both textural and dimensional

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Figure 4b. A cross sectional ultrasound scan at the same level as 4a. This lesion is more severe than that in 4a. The ligament is more hypoechoic and larger in cross section. The larger size of focal echolucencies (arrows) represent a greater portion of disrupted fiber bundles hence greater structural damage.

change of the ligament. Injury may occur to one or both of the branches of the

suspensory ligament. Changes can be documented in the branch without concomitant change in other parts of the ligament.

Case 1 Figures 3a and 3b are cross sectional and sagittal scans

made immediately distal to the earpo-metacarpaljoint of a 2-year-old Standardbred pacer that became lame 2 days after racing. Figure 3a is a sagittal plane demonstrating echolucency and fiber disruption of the origin of the suspensory. Figure 3b, a cross sectional plane at the same level, demonstrates echolucency and lack of normal structural architecture seen in the sagittal plane. These changes were compatible with structural damage and fluid accumulation, (hemorrhage, edema) within the ligament.

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Case 2 Figure 4a is a cross section at the mid-metacarpal level

of a 6-year-old S tandardbred pacer that had been lame off and on for a year, the horse was second degree lame at the t ime of the examinat ion . The u l t rasound diagnosis was chronic suspensory desmitis. Changes included thickening (1.4 cm), and increased echogenicity that most likely represented chronic fibrosis. Acute changes are recognized as focal echolucencies.

Case 3 Figure 4b is a mid-metacarpal scan of a 4-year-old

S tandardbred pacer. Palpat ion revealed very slight enlargement of the suspensory. The horse had been "on the left line" for the last three starts. Ul t rasonographic f indings included slight thickening (I. 1 cm) and anechoic foci representing fiber disrupt ion with hemorrhagic infiltrate.

Figure 4c. A cross sectional ultrasound scan at the same level as both 4a and b. Most severe of the three lesions to the suspensory ligament (between bars), this ligament has a central core lesion of the ligament that represents an absence of fibrous structural components replaced by a large central area of hemorrhage.

Figure 5a. A cross sectional ultrasound scan of the medial suspensory branch. The scanning plane is oriented so that it bisects the branch as it courses over the abaxial surface of the medial sesamoid. The normal appearance is a dense compact echo pattern and diameter of approximately 0.9 cm.

Figure 4d. A cross section of the most severe injury of the previous 4a, b, and c. Arrows indicate a large central area of fiber disruption and hemorrhage.

Figure 5b. A cross sectional ultrasound scan of a lateral suspensory branch (between cursors) at the same level as in 5a. Changes noted here include enlargement as well as textural changes indicating intraligamentous fluid accumulation.

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Figure 5c. A cross sectional ultrasound scan at the same level as in 5a and 5b. The changes are more severe and a central area of echolucency (arrow) represents an area of hemorrhage and fiber bundle disruption.

C a s e 4 Figure 4c is an u l t r a sound scan of the mid -me taca rpus

of a 3-year-old S t a n d a r d b r e d pacer . This colt was second degree l ame and was consis tent ly "on the right l ine." This l igament had more p rominen t in f l ammat ion , th ickening and s t ruc tura l damage than in F igure 4b.

Case 5 The most severe suspensory lesion presented here, was

that of a 10-year-old S t a n d a r d b r e d pacer , (F igure 4d). The l o s s o f n o r m a l e c h o e s , as wel l as e x t r e m e en la rgement represent severe s t ruc tura l damage with hemorrhage .

Case 6 F igure 5b is a t ransverse scan of the lateral b ranch o f

the suspensory l igament of a 6-year -o ld T h o r o u g h b r e d gelding. The scan was made over the surface of the sesamoid . This b ranch measured 1.4 cm, and had a centra l core echolucency tha t represented s ignif icant s t ruc tura l a l tera t ion. Figure 5c shows the same horse 10 days la ter after its next race. Not ice the diffuse anechoic a p p e a r a n c e of the suspensory b ranch as well as ex t reme en largement . Compar i son with Figure 5b indicates fur ther damage to the suspensory branch. This horse was unable to cont inue racing.

Case 7 Figure 5d is a scan of a suspensory branch of a 4-year-

old T h o r o u g h b r e d gelding tha t became acutely lame af ter a race. A n echolucent lesion of the medial suspensory

Figure 5d. A cross sectional ultrasound scan at the same level as in 5a, b, and c. A n echolucency can be seen adjacent to an osseous fragment from the abaxial surface of the medial sesam0id. This represents damage to the suspensory branch with avulsion fracture of the abaxial surface of the medial sesamoid.

branch is present ad jacen t to an avulsed bone f ragment f rom the apical abax i a l surface of the media l sesamoid bone.

CONCLUSION

Ult rasound imag ing proved to be of value in the diagnosis and prognos i s in suspensory l igament injury. Fu tu re repor ts will include re t rospect ive review of var ious the rapeu t i c regimens and d e t e r m i n a t i o n of prognosis f rom the u l t r a sound scan of the tendons and l igaments of the horse.

REFERENCES

1. Adams OR: Lameness In Horses. Lea and Febiger, Philadelphia PA, 1979.

2. Bramlage LR; Gabel AA; and Hackett RP: Avulsion fractures of the origin of the suspensory ligament in the horse. JA VMA Vol. 176 No. 10; 1004-1010, 1980.

3. Hauser ML; and Rantanen NW:Uitrasound appearance of the palmar metacarpal soft tissues of the horse. J Eq Vet Sci3:19-22, 1983.

4. Pasquini C; Reddy VK; and Ratzlaff MH: Atlas of Equine Anatomy (2rid ed.) Sudz Pub. Co., 1984.

5. Rantanen NW: The use of diagnostic ultrasound in limb disorders of the horse: A preliminary report. J Eq Vet Sci 2:62-64, 1982.

6. Rantanen NW; Gaines RD: The use of diagnostic ultrasound to detect structural damage to the soft tissues of the extremities of horses. J Eq Vet Sei 3:134-135, 1983.

7. Spautding K: Ultrasonic anatomy of the tendons and ligaments in the distal metacarpal-metatarsal region of the equine limb. Vet Radiol 25 No. 4:155-166, 1984.

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