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Suspensory Ligament Ultrasound Using Oblique Incidence for Identification of Ligament Anatomy With Comparison to MRI

Natasha M. Werpy, D.V.M., DACVR; Jean-Marie Denoix*, D.V.M., Ph.D., agrégé; C. Wayne McIlwraith, BVSc, Ph.D., DSc, FRCVS, DACVS

Take Home Message

Combining the standard ultrasound examination of the suspensory ligament with oblique incidence technique improves identification of the normal anatomic features of the suspensory ligament (SL).
 
Injury to the SL is a common condition affecting horses of different ages and disciplines.1-2 Lameness is often localized to the SL region using local infiltration of anesthetic solution around the ligament or perineural analgesia.3 Once injury of the SL is suspected based on the clinical examination and response to analgesia, diagnostic imaging of this region is often performed. Radiography can be used to evaluate the proximal palmar aspect (face) of the third metacarpal bone for evidence of sclerosis, lysis, proliferation or avulsion fracture at the attachment of the SL.4-5 However, examination of the ligament requires a modality that allows visualization of the soft tissue structures. Ultrasound has traditionally been the imaging modality of choice for diagnosis of SL injury. The technique has been describes the ultrasound probe being placed at the palmar surface of the limb with the beam oriented perpendicular to the longitudinal axis of the fibers.6 This technique has several limitations. The traditional technique for ultrasound of the SL creates an image which causes to the SL to appear as a rectangular shaped echogenic structure (Fig. 1).
 
However, the proximal aspect of the SL is a bilobed structure surrounded by connective tissue. The muscle and adipose tissue are most prominent in the central aspect of the each lobe and they are surrounded primarily by ligament fibers. Using the oblique incidence technique the SL is first imaged with the beam perpendicular to the long axis of the ligament creating an echogenic appearance (Fig. 2).7 The ultrasound beam is then angled obliquely relative to the long axis of the SL (Fig. 2). This causes the margins of the SL to become evident. The surround connective tissue remains bright. The ligament fibers become dark. The adipose tissue remains bright. The muscle becomes dark, but not as dark as the ligament fibers. Preliminary work suggested this technique produced images that provided anatomic information about the fiber versus adipose tissue/muscle distribution similar to MRI images and gross sections. This information was vastly different than what is obtained with the traditional technique (Fig. 2). We hypothesized that the combination of the standard ultrasound examination with additional imaging of the limb while flexed with the probe oriented oblique and perpendicular to the longitudinal axis of the fibers would provide the greatest amount of anatomic information about the SL. Furthermore this ultrasound technique would provide similar information about the distribution of ligament fibers versus adipose tissue/muscle distribution as can be determined with MR imaging.

Materials and Methods

Ultrasound examination, both with perpendicular and oblique beam orientation as well as MRI was performed on the forelimbs of 10 horses (age 2-5 years). Ultrasound examination was performed beginning at level of the carpometacarpal joint and ending at the level of the SL branches using a GE Logiq E (Carlsbad, Calif.) ultrasound machine with a 10 MHz linear probe. Transverse and longitudinal images were obtained with the ultrasound beam placed perpendicular to the ligament fibers with the limb in a weight-bearing position. Ultrasound and MR images were obtained at 2, 3, 4, 6, and 8 cm distal to the carpometacarpal joint. MR images (3 mm) were obtained post-mortem using proton density, T2-weighted fast spin echo and STIR sequences on a 1.0 Tesla OrthoOne ONI (Wilmington, Mass.).
 
Following MR imaging, the SL were harvested and sectioned. Gross evaluation of the sections and Masson's trichrome stain was used to identify areas of ligament, muscle, and adipose. The circumferential area of the SL of the SL was measured on all modalities. Flexed oblique incidence ultrasound and MR images of the right and left SLs were compared at each level and subjective evaluation for asymmetry of lobe size and shape as well as the fat and muscle distribution was performed.

Results

The standard ultrasound examination with probe placed perpendicular to the longitudinal axis of the SL did not allow accurate identification of the ligament margins relative to the surrounding connective tissue. In addition, this examination method did not allow visualization of the entire SL, specifically excluding the medial and lateral extent of the ligament. Examination with the limb in a non-weight bearing position with the carpus mildly flexed and the ultrasound beam perpendicular to the ligament fibers did allow visualization of the entire ligament. This was characterized by consistent identification of the axial margins of the second and fourth metacarpal bones. However, it did not consistently allow identification of the ligament margins and it did not allow differentiation between the ligament fibers versus areas of muscle and adipose tissue. Examination with the limb in a non-weight bearing position with the carpus mildly flexed and the ultrasound beam oblique to the longitudinal axis of the ligament allowed visualization of the ligament margins as well as identification of ligament fibers versus areas of adipose tissue and muscle.
 
In contrast to the MR images, which allowed areas of adipose tissue versus muscle to be easily distinguished, US images allowed identification of regions of adipose tissue and muscle but differentiation between the two tissue types was not reliably achieved (Fig. 3). Differences in the shape of the ligament margins or lobe size were well detected on the US images (Fig. 4). However, small areas of adipose tissue or muscle tracking through the ligament fibers were not well detected on US images.
 
In all 10 horses the lobes of the SL were asymmetrical in shape and size at least one level when comparing the right and left limbs (Fig. 4). In eight horses the ligament lobes joined to form at oval shape at approximately 6 cm distal to the carpometacarpal joint. In one horse this occurred at 5 cm in both forelimbs. This occurred in one horse at 5 cm in the left forelimb and 6 cm in the right forelimb.
 
The adipose tissue and muscle distribution was asymmetrical to varying degrees in all 10 horses at all levels when comparing the right and left limbs. The degree to which this occurred was variable, some horses only had slight differences when comparing the right and left forelimbs while others had marked differences.

There was a significant difference (P< 0.05) between the circumferential area of the different tissue types in the SL averaged over all measurement levels (Fig. 5). The ligament is composed primarily of ligament fibers, followed muscle tissue and then adipose tissue. The circumferential area of the ligament when measured on oblique incidence ultrasound images at 2 and 3 cm distal to the carpometacarpal joint was significantly different (P< 0.05) than when measured at the same levels on the MR images. The remaining levels were not significantly different when comparing oblique incidence ultrasound images and MR images.
 
There was no statistically significant difference between the cross sectional area of the SL in right and left forelimbs when comparing the measurement performed while the horse is standing with the ultrasound beam perpendicular to the ligament fibers to the flexed oblique incidence technique when averaged over the measurement distances. There was no statistically significant difference between the cross sectional area of the SL or the cross sectional area of fibers, muscle, or adipose tissue in the ligament when comparing the right and left forelimbs.

Discussion

The combination of multiple ultrasound techniques provides the most comprehensive examination of the SL. It will allow visualization of the entire ligament and a more accurate representation of the normal anatomy, both essential requirements for diagnosis of pathologic change. Although MR examination will still be required to diagnosis certain types or degrees of injury in the SL and will provide additional information, such as demonstrating the presence of fluid in the third metacarpal bone at the ligament insertion, the more comprehensive examination will likely allow diagnosis of abnormalities not previous possible with US. This technique could facilitate diagnosis or monitoring of SL injury in multiple circumstances. It could allow a diagnosis to be obtained without MR examination. This is beneficial for owners who would like to avoid general anesthesia or for those who cannot afford the cost of advanced imaging. In addition it could allow monitoring of lesions over time following MR examination without additional general anesthesia.
 
Oblique incidence ultrasound provides a reliable method for measuring the circumferential area of the SL creating a detectable peripheral margin. The proximal extent of the ligament measured larger on MR images compared to ultrasound. This difference may originate from fibers extending from the fourth metacarpal bone as well as fibers in the palmar aspect of the medial and lateral interosseous spaces. These fibers are readily identified on MR images but are difficult to visualize on ultrasound images (Fig. 6). A convex probe would allow a beam angle that would make these fibers more echogenic and therefore better defined. However, this could potentially obscure other ligament margins still making it difficult to identify all the ligament fibers and measure the complete circumferential area in one image.
 
Although the circumferential area between the right and left front SLs is not significantly different, there was detectable difference in either lobe shape or fat and muscle distribution in all horses in the study (Fig. 4). These differences could be confusing when trying to defect injury using comparison to the opposite limb. However, more work showing the scope of normal anatomic variation and comparison to cases with confirmed pathologic change will provide criteria to allow more accurate identification of normal anatomic variation.

Conclusions

Although this comprehensive ultrasound technique has limitations, it provides additional and essential information not available with the traditional technique. It is currently part of our routine examination when assessing the SL for injury using ultrasound.

References

  1. Marks D., Mackay-Smith M., Leslie A., et al. Lameness resulting from high suspensory disease (HSD) in the horse. Proc Am Assoc Equine Pract 1981;24:493-497
  2.  
  3. Personett L., McAllister S., Mansmann R. Proximal suspensory desmitis. Mod Vet Prac 1983:64:541-545
  4.  
  5. Ford T., Ross M., Orsini P. A comparison of methods for proximal metacarpal anaesthesia in horses. Vet Surg 1988;18:146-150
  6.  
  7. Pleasant R., Baker G., Muhlbauer M., et al. Stress reactions and stress fractures of the proximal palmar aspect of the third metacarpal bone in horses: 58 cases (1980-1990). J Am Vet Med Assoc 1992;201:1918-1923
  8.  
  9. Bramlage L., Gabel A., Hackett R. Avulsion fracture of the origin of the SL in the horse. J Am Vet Med Assoc 1980;176:1004-1010
  10.  
  11. Pharr J., Nyland T. Sonography of the equine palmar metacarpal soft tissues of the horse. Equine Vet 1993;25:30-35
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  13. Sisson S. Equine Syndesmology. In: Getty R, ed. Sisson and Grossman's anatomy of domestic animals. 5th ed. Philadelphia: WB Saunders, 1975;1:349-375
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  15. Denoix J.M., Coudry V., Jacquet S. Ultrasonographic procedure for a complete examination of the proximal third interosseous muscle in the equine forelimbs. Equine Veterinary Education, 2008;20:148-153

Acknowledgement

This study was funded by the College Research Council at Colorado State University. Sincere appreciation to Eric Garcia, D.V.M. for his assistance with this project.
 
Table 1. Circumferential area of the SL at different levels measured with oblique incidence ultrasound and MRI
Distance (cm)​ ​ Modality​ ​
​Oblique Incidence US ​MRI
​2 ​1.74* (+0.07) ​2.15* (+0.07)
​3 ​1.76* (+0.07) ​2.05 (+0.07)
​4 ​1.57 (+0.07) ​1.68 (+0.07)
​6 ​1.52 (+0.07) ​1.51 (+0.07)
​6 ​1.58 (+0.07) ​1.52 (+0.07)
Values with asterisks in the same row are significantly different (P <0.05)
Distance is centimenters (cm) distal to carpometacarpal joint
 
Figure 1. Transverse ultrasound image made using the current technique with ultrasound probe perpendicular to the ligament fibers in a weight-bearing lime. The suspensory ligament is demarcated by a white rectangle. This ligament appears as a echogenic rectangle shaped structure with hypoechoic areas. The palmar aspect of the third metacarpal bone is the white line just inside the bottom line of the rectangle. This technique causes the suspensory ligament to appear rectangular when in fact it is bilobed with rounded medial and lateral margins at this level.
Figure 2a. Transverse Ultrasound of the Suspensory Ligament
Figure 2b. Transverse Ultrasound of the Suspensory Ligament
Figure 2c. MRI of the Suspensory Ligament
Figure 2d. Gross image of the Suspensory Ligament
 
Figure 2. Transverse ultrasoun (A,B), MRI (C) and gross (D) images of the suspensory ligament (palmar is at the top and medial is to the left). The region of the ligament is demarcated by the white line, it does not exactly outline the ligament margins. Image A is made with the limbs in non-weight bearing position with the probe perpendicular to the ligament. The ligament is echogenic with the probe in this position. Image B is a made at the same level as image A, but the probe is now oriented obliquely in a palmarodistal-dorsoproximal direction to the suspensory ligament. The ligament fibers are now hypoechoic. This horse has primarily muscle in the ligament lobes as opposed to other horses which have a combination of adipose tissue and muscle in the ligament. The muscle is echogenic compared to the ligament fibers with the probe oriented obliquely. The ultrasound image made with an oblique beam angle appears similar to the MR image, with a clear delineation of the ligament margins as well as the ligament fibers in contrast to Figure 1. These two images (B,C) correlate well with the gross image (D) and provide an accurate representation of the anatomy.
Figure 3a. US Image of limb
Figure 3b. MR Image of limb
Figure 3c. Histology Image of limb
 
Figure 3. Corresponding US, MR and histology images of a limb 4 cm distal to the carpometacarpal joint (Dorsal is at the top and medial is to the right). The entire suspensory ligament is visible on the US image, and the size, shape and margins of the ligament are similar to the appearance on the MR images. The lateral lobe of the ligament (arrow) contains primarily adipose tissue (light gray on MR images) with a small area of muscle (dark gray on MR) within the axial aspect of the lobe. The medial lobe of the ligament (arrowhead) contains a mixture of adipose tissue and muscle. The differences between the fat and muscle distribution in the ligament lobes are clearly delineated on the MR image. However, the demarcation between muscle and adipose tissue is more difficult to identify on the US image. The histology slide (Masson's trichrome) confirms the tissue distribution; ligament fibers are blue, adipose tissue is clear and muscle is red.
Figure 4. Paired US and MR images 4 cm distal to the carpometacarpal joint (Dorsal is at the top and medial is to the right). The suspensory ligament lobes of the left fore are symmetrical in size and shape. In contrast, the suspensory ligament lobes in the right fore are asymmetrical with a thinner longer medial lobe and a thicker shorter lateral lobe. The adipose tissue and muscle distribution is different when comparing the two limbs. The asymmetry of the right and left suspensory ligaments is not the result of pathologic change. It is a consequence of normal anatomic variation.
Figure 5. Circumferential area of the ligament and the different tissue types in the ligament averaged over the measurement levels. Circ area, circumferential area; tis,tissue type;1=circumferential area of the suspensory ligament, 2=circumferential area of the muscle,3 =circumferential area of the adipose tissue,4=circumferential area of the ligament fibers.
Figure 6. Paired MR and US image 2 cm distal to the carpometacarpal joint (dorsal is on the top and medial is to the left). There is a focal area of fibers extending from the fourth metacarpal bone (arrow). These fibers are located at the palmar extent of the interosseous space (arrowhead) and are difficult to identify on the US image. On the US image the lateral extent the suspensory ligament appears axial to its actual lateral margin which is apparent on the MR image. Therefore, the circumferential area measure less at this level on the US image compared to the MR image.
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