# Review of the use of transrectal ultrasonography for evaluation of the caudal lumbar – including lumbosacral – intervertebral discs and symphyses: Normal and abnormal ultrasonographic appearance

G. Vautravers, V. Coudry, Jean-Marie Denoix

<figure><img src="image_1.png" /></figure>

## To cite this version:

G. Vautravers, V. Coudry, Jean-Marie Denoix. Review of the use of transrectal ultrasonography for evaluation of the caudal lumbar – including lumbosacral – intervertebral discs and symphyses: Normal and abnormal ultrasonographic appearance. Equine Veterinary Education, 2021, 33 (6), pp.310-319. 10.1111/eve.13313. hal-03989313

HAL Id: hal-03989313

https://hal.inrae.fr/hal-03989313

Submitted on 14 Feb 2023

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

<figure><img src="image_2.png" /></figure>
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

## Review Article

# Review of the use of transrectal ultrasonography for evaluation of the caudal lumbar – including lumbosacral – intervertebral discs and symphyses: Normal and abnormal ultrasonographic appearance

G. Vautravers* and J.-M. Denoix

CIRALE, INRA USC 957 BPLC, ENVA-UPEC, Normandie Equine Vallée, Goustranville, France

*Corresponding author email: guillaume.vautravers.vet@gmail.com

**Keywords:** horse; lumbosacral disc; symphysis; transrectal; ultrasonography

## Summary

Transrectal ultrasonography of the lumbosacroiliac area is routinely performed for the diagnosis of the cause of low back pain and poor hindlimb impulsion and engagement. The ventral aspect of the caudal lumbar intervertebral discs and vertebrae can be easily assessed transrectally with ultrasound while other imaging modalities have limitations for diagnosis of intervertebral disc injuries. This paper is a review of the ultrasonographic procedure for imaging the ventral aspect of the caudal lumbar and lumbosacral symphyses, especially the intervertebral discs. Normal images, findings and lesions of the three caudalmost lumbar intervertebral discs are described (between the 4th lumbar and the 1st sacral vertebrae).

## Introduction

Lumbosacroiliac pain in horses is a common cause of poor performance and lack of power in the hindlimbs (Jeffcott et al. 1985; Denoix 1998; Dyson 2010; Peters 2012; Barstow and Dyson 2015; Boado et al. 2020). As in humans (Dansie and Turk 2013) objective assessment of low back and pelvic pain, and clear identification of its cause(s) are not easy in horses (Denoix et al. 2005; Denoix and Dyson 2010; Dyson 2010; Haussler 2010; Denoix 2017). The diagnosis of lumbar, lumbosacral and sacroiliac injuries requires a complete clinical examination with special consideration of physical and dynamic abnormalities (Denoix 1999; Denoix and Dyson 2010; Dyson 2010; Boado et al. 2020).

Local analgesia by infiltration of local anaesthetic solution close to the lumbosacroiliac area has been described (Dyson and Murray 2003; Haussler 2010; Boado et al. 2020; Nagy and Dyson 2020). Nevertheless, this diagnostic approach has been questioned as it may lack sensitivity and specificity, especially for lumbosacral intervertebral disc (L6D) disease due to its ventral position (Nagy et al. 2010), and its use is limited in practice because of the risk of complications such as sciatic or femoral nerve paresis (Haussler 2010; Nagy and Dyson 2020; J.-M. Denoix, personal communication). Therefore, localisation of the source of pain remains challenging (Denoix 1998; Bergman et al. 2013).

Transrectal ultrasonographic examination of the sacroiliac joints and the intervertebral discs between the fourth lumbar vertebra (L4) and the first sacral vertebra (S1) was developed in the late 90s (Denoix 1998, 1999) and is now routinely used in the diagnosis and documentation of lumbosacroiliac

injuries (Denoix and Coudry 2005; Denoix et al. 2005; Denoix et al. 2006; Denoix and Jacquet 2008; Bergman et al. 2013). The technique described for the evaluation of the sacroiliac joints (Denoix 1998; Bergman et al. 2013; Tallaj et al. 2019) is also used for evaluation of the caudal lumbar intervertebral and lumbosacral symphyses (Denoix 1999; Nagy et al. 2010; Bergman et al. 2013; Boado et al. 2020). Lesions have also been documented in clinical cases and confirmed post-mortem in experimental studies (Denoix 1999; Haussler et al. 1999; Krueger et al. 2016; Boado et al. 2020).

The general objective of this paper is to provide a literature review of the transrectal ultrasound technique to evaluate the caudalmost lumbar intervertebral symphyses of the horse.

After a short reminder of basic anatomy, the aims are to present a review of (i) the technique and procedure to examine these discs, (ii) their normal ultrasonographic appearance and (iii) the ultrasonographic findings reported.

## Review of basic anatomy

The lumbosacral junction between the sixth lumbar vertebra (L6V) and the first sacral vertebra (S1V) is composed of six different joints (Barone 2000; Denoix 2019). In the median plane, a symphysis is made of the caudalmost intervertebral disc of the back (L6D), between the L6V fossa and the S1V head (Fig 1). The craniocentral margin of S1V is usually prominent forming the promontorium. Two small synovial joints between the caudal articular processes of L6V and the cranial articular processes of S1V are located dorsal to the vertebral canal on the left and right sides. Dorsally, the L6 and S1 spinous processes are connected by the supraspinous ligament and two interspinous ligaments. Finally, the transverse processes of L6V and S1V articulate on each side through a synovial lumbosacral intertransverse joint that is covered by a dorsal and a ventral intertransverse ligaments (Barone 2000; Denoix 2019).

The L6D is usually the thickest disc of the vertebral column between the first thoracic vertebra and the sacrum (Fig 2) and is very deformable (Townsend et al. 1983; Townsend and Leach 1984; Denoix et al. 2005; Stubbs et al. 2006). Made of peripheral circumferential dense fibres with a softer central part (Jeffcott and Dalin 1980; Townsend and Leach 1984; Barone 2000; Bergmann et al. 2018), it adheres to the hyaline cartilage covering the L6V fossa and the S1V head. As the L6V fossa is not concave but flat or slightly convex, the L6D has a
<figure><img src="image_5.png" /><figcaption>Fig 1: Cranioventrolateral aspect of the lumbosacroiliac area of a bone specimen (cranial is to the left). Note the normal intertransverse synostosis between the 5th and the 6th lumbar vertebrae (asterisks). 1: lumbosacral intervertebral disc space; 2: L5-6 intervertebral disc space; 3: L4-5 intervertebral disc space; 4: ventral aspect of the 1st sacral vertebra; 5: ventral aspect of the 6th lumbar vertebra; 6: lumbosacral promontorium; 7: lumbosacral intervertebral foramens; 8: ventral aspect of the 5th lumbar vertebra; 9: ventral aspect of the 4th lumbar vertebra.</figcaption></figure>

specific biconcave shape (Barone 1999; Denoix 1999). It is crossed ventrally by a thin ventral longitudinal ligament and dorsally by a thin dorsal longitudinal ligament (Barone 2000).

The fifth lumbar intervertebral disc between L5V and L6V (L5D) is usually much thinner than the L6D (Denoix 2019), and usually, the transverse processes of the L5V and L6V are fused. The fourth intervertebral disc between L4V and L5V (L4D) is generally slightly thicker than the L5D (Denoix 1999; Barone 2000).

## Review of equipment and technique

As for any rectal examination, safety measures have to be taken before any transrectal ultrasonographic examination on

<figure><img src="image_6.png" /><figcaption>Fig 2: Median anatomical section of the lumbosacral junction (cranial is to the left, dorsal is to the top). 1: lumbosacral intervertebral disc; 2: L5-6 intervertebral disc; 3a: body of the 1st sacral vertebra; 3b: promontorium; 4: body of the 6th lumbar vertebra; 5: body of the 5th lumbar vertebra; 6: ventral longitudinal ligament; 7: dorsal longitudinal ligament; 8: spinal cord (medullary cone); 9a: dura mater: ventral wall; 9b: dura mater: dorsal wall; 10: epidural fat.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig 3: Schematic drawing of the ventral aspect of the lumbosacroiliac area (cranial is to the top). Depending on the transducer position, different parts of the ventral aspect of the area can be imaged (A, A', B, C). 1: lumbosacral intervertebral disc; 2: L5-6 intervertebral disc; 3: L4-5 intervertebral disc; 4: 1st sacral vertebra; 5: 6th lumbar vertebra; 6: 5th lumbar vertebra; 7: 4th lumbar vertebra; 8: ventral ramus of the left L6 segmental nerve; 9: left lumbosacral truncus; 10: left femoral nerve; 11a: left minor psoas muscle body; 11b: left minor psoas tendon.</figcaption></figure>

the standing horse (Bergman et al. 2013; Tallaj et al. 2019). Ideally, the horse should be placed in stocks. The first concern is to avoid any traumatic injury of the rectal wall. Sedation and anti-spasmodic agents can be used if the horse is nervous to make the procedure safer and to reduce rectal contractions. Faeces are removed from the rectum with all the necessary precautions to preserve its mucosa with adequate lubrication. As reported by Tallaj et al. (2019), the practitioner should take care to handle the transducer properly without putting too much pressure on the rectum wall and to adapt his movements to the horse's reactions. One finger should be placed at the cranial aspect of the transducer to avoid any direct contact between the transducer and the rectum wall (J.-M. Denoix, personal communication).

Ultrasonography of the equine caudal lumbar intervertebral discs is performed with a 5-7.5 MHz curvilinear rectal transducer or with a 5-10 MHz small linear rectal probe (e.g. 5 cm long) which is more often available in field practice (Denoix 1998, 1999; Tallaj et al. 2019). A stand-off pad is not required but can be occasionally used to improve visualisation of the superficial (ventral) aspect of the S1V to L4V bodies and the corresponding intervertebral discs (Denoix 1999). To match the anatomical orientation and to facilitate interpretation, the ultrasound images can be rotated 180° so that the ventral aspect of the vertebrae is to the bottom of the image displayed on the screen (Nagy et al. 2010; Bergman et al. 2013), as it was done for the figures of this paper.

Tallaj et al. (2019) described the promontorium (prominent ventral margin of the head of S1V at the lumbosacral
<figure><img src="image_8.png" /><figcaption>Fig 4: Ultrasound scans of the ventral aspect of a normal lumbosacral intervertebral disc. (a,c,d: Cranial is to the left, and dorsal is to the top; b: Left side is to the left and dorsal is to the top). Images acquired with a small curvilinear transducer. a) Median ultrasound scan performed on a horse in neutral lumbosacral position. b) Transverse ultrasound scan. c) Median ultrasound scan performed on a horse examined in lumbosacral flexion. d) Median ultrasound scan performed on the same horse examined in lumbosacral extension. 1: lumbosacral intervertebral disc; 2: ventral bone surface of the first sacral vertebra; 3: ventral bone surface of the 6th lumbar vertebra; 4: ventral longitudinal ligament; 5: dorsal longitudinal ligament; 6a: dura mater: ventral wall; 6b: dura mater: dorsal wall; 7: spinal cord (medullary cone).</figcaption></figure>

angulation) as a landmark during transrectal ultrasonography. The probe facing up is placed horizontally in the median plane at the level of the promontorium to visualise the L6D between the L6V and the S1V (position A, **Fig 3**). By slightly moving the probe alternatively to the left and to the right sides of the vertebral column, maintaining a paramedian orientation of the probe, the whole ventral part of the disc is scanned (Bergman et al. 2013). When moving the transducer more laterally, the lumbosacral intervertebral foramen is imaged with the echogenic ventral ramus of the L6 segmental nerve that contributes to the lumbosacral truncus

(Espinosa et al. 2017). A real-time dynamic examination of the lumbosacral junction can be achieved by inducing alternative movements of lumbosacral flexion and extension, with cutaneous reflex stimulation near the tuber sacrale (extension) and then near the tuber ischiaticum (flexion) (Denoix 1999; Werpy 2007). In sedated horses, this mobilisation can be difficult because of the low response to cutaneous stimulation (Denoix 1999). Bergman et al. (2013) described the possibility of performing transverse sections of the L6D with a small curvilinear transducer by rotating the probe perpendicular to the median plane (position A', **Fig 3**). Due to the risk of rectal laceration, this technique should not be done with long linear rectal probes (Denoix 1999).

Imaging of the ventral aspect of the L6V and then the L5D (position B, **Fig 3**) is performed by moving the probe cranially in the median plane. When moving the probe more cranially, the terminal part of the aorta is imaged ventrally to the L5V body or the L5D. More cranially, the ventral part of the L4D can be imaged in relaxed horses (position C, **Fig 3**).

Normal ultrasonographic images

Denoix was the first to describe the normal appearance of the caudal lumbar intervertebral discs in 1999. With the development of the technique, more consideration has been given to the L6D in the literature in the past two decades (Denoix and Coudry 2005; Denoix et al. 2005, 2006; Werpy 2007; Denoix and Jacquet 2008; Denoix 2017) in particular with studies describing the ultrasonographic appearance of the L6D in horses without clinical signs of back pain or hindlimb lameness (Nagy et al. 2010) or in horses with lumbosacroiliac region pain (Bergman et al. 2013; Boado et al. 2020).

On normal ultrasound scans performed in the median plane (position A, **Fig 3**), the ventral bone surfaces of L6V and S1V are imaged as regular and smooth shadowing hyperechogenic curved lines. In an average standing horse, the lumbosacral joint dorsal angle ranges from approximately 140°-150° (Bergman et al. 2013); more precisely in 2010, Nagy et al. reported that the mean angle of the lumbosacral joint was 147° (range: 118-165°, median: 150°) in a study on 43 horses. It should be noted that this angulation changes according to the horse positioning and lumbosacral flexion- extension movements. Between the L6V fossa and the S1V head, the L6D is imaged as a uniformly echogenic or mildly heterogeneous echogenic structure with different portions (**Fig 4a**) (Denoix 1999; Nagy et al. 2010). The ventral part is triangular with a convex ventral border limited by the ventral longitudinal ligament; in the median plane, some parallel horizontal curved fibres can be identified. The central portion of the disc is a 2- to 6-mm-thick homogenous poorly echogenic structure with less apparent architectural organisation. Dorsally, the disc is separated from the vertebral canal by the dorsal longitudinal ligament. In cases of thick L6D, the vertebral canal and its content can be seen through the acoustic window made by the disc: the epidural and subdural (subarachnoidean) spaces are anechogenic and separated by the ventral and dorsal walls of the dura mater, which can be imaged on a median section as thick echogenic lines. Inside the epidural space, echogenic linear structures represent the cauda equina.

As firstly reported by Denoix in 1999, dynamic examination of the lumbosacral disc is useful to assess the structures and
<figure><img src="image_9.png" /><figcaption>Fig 5: Median ultrasound scans showing the normal appearance of the ventral aspect of the caudalmost lumbar vertebrae and lumbosacral junction (cranial is to the left, and dorsal is to the top). Images acquired with a short rectal linear transducer. 1: lumbosacral intervertebral disc; 2: L5-6 intervertebral disc; 3: L4-5 intervertebral disc; 4: ventral bone surface of the 1st sacral vertebra; 5: ventral bone surface of the 6th lumbar vertebra; 6: ventral bone surface of the 5th lumbar vertebra; 7: ventral bone surface of the 4th lumbar vertebra; 8: ventral longitudinal ligament; 9: aorta; 10: caudal vena cava.</figcaption></figure>

mobility of the lumbosacral symphysis. During lumbosacral flexion, the L6D bulges ventrally and its ventral thickness decreases as the lumbosacral joint angulation decreases (Fig 4c). During lumbosacral extension, the ventral aspect of the intervertebral disc straightens due to increased tension and the disc ventral thickness increases, as the lumbosacral joint angulation increases (Fig 4d) (Denoix 1999; Werpy 2007). The total amount of the *in vivo* lumbosacral joint mobility in flexion and extension can reach 10°–15° depending on the cooperation and flexibility of the horse (Denoix 1999).

On transverse sections described by Bergman et al. (2013) (position A', **Fig 3**), the whole disc is imaged as a circular echogenic structure, limited ventrally by the ventral longitudinal ligament and dorsally by the dorsal longitudinal

ligament. The latter separates the disc from the vertebral canal imaged as an anechogenic circular area centred on the echogenic dural cone (**Fig 4b**).

Progressing cranially in the median plane (positions B and C, **Fig 3**), the ventral part of L5D and L4D can be identified as triangular echogenic structures between the hyperechogenic ventral bone surfaces of L6V, L5V and L4V (**Fig 5**). In normal horses, the L4D and L5D spaces are thinner than the L6D space. Only the ventral triangular portion is consistently imaged as an echogenic material covered by the ventral longitudinal ligament, which is always distinct in the median plane at the level of the L4V but gets thinner caudally (Denoix 1999; Denoix *et al.* 2005).

On normal ultrasound scans performed in a paramedian plane, the ventrolateral aspect of the vertebral bodies appears more concave. As the probe is moved laterally at the level of the L6D, the intervertebral foramen is identified as a smoothly demarcated concavity in the bone surface containing the echogenic ventral ramus of the L6 segmental nerve (Fig 6) (Espinosa et al. 2017).

## Ultrasonographic findings and lesions

As the L6D can be easily and completely examined in the horse, it may serve as a model for diagnostic interpretation for ultrasound examination of the other intervertebral discs of the caudal lumbar area. To simplify the presentation, description of ultrasonographic findings and lesions is focused on the L6D examination and further information about the L5D is given when necessary. As no finding has yet been reported on the L4D at transrectal ultrasonographic examination (Denoix 1999; Werpy 2007), no specific data are given for this disc.

## Intervertebral alignment, size and position of the disc

With reference to the type 1 (normal) appearance described in the previous paragraph (Denoix 1999; Denoix *et al.* 2005, 2006; Denoix and Jacquet 2008; Nagy *et al.* 2010; Bergman *et al.* 2013; Denoix 2017), four types of abnormal L6D shape have been described (Denoix 1999) and observed, depending on the severity and extent of the disc thinning

<figure><img src="image_10.png" /><figcaption>Fig 6: Paramedian ultrasound scan in the left side of the normal lumbosacral symphysis imaging the lumbosacral intervertebral foramen (cranial is to the left, and dorsal is to the top). Image acquired with a small curvilinear transducer. 1: ventral ramus of the left L6 segmental nerve; 2: left lumbosacral intervertebral foramen; 3: ventral bone surface of the 1st sacral vertebra; 4: ventral bone surface of the 6th lumbar vertebra; 5: left caudal gluteal artery.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Fig 7:** Schematic drawing showing 5 types of intervertebral disc shape that can be encountered at the lumbosacral junction. Type 1: normal appearance; Type 2: dorsal thinning; Type 3: dorsal and ventral (overall) thinning; Type 4: dorsal (partial) lumbosacral synostosis; Type 5: complete lumbosacral synostosis (Modified, from Denoix 1999). 1: 1st sacral vertebra; 2: 6th lumbar vertebra; 3: lumbosacral intervertebral disc.</figcaption></figure>

(Fig 7). On median section, the L6D can be observed with only its dorsal portion thinner than 2 to 6 mm (type 2), or with global thinning of both the dorsal and ventral portions of the disc (type 3). In other cases, only the ventral part of the disc is imaged because of dorsal and incomplete lumbosacral intervertebral synostosis (type 4) (Fig 8a), or no disc material is seen when the intervertebral synostosis is complete (type 5)

(Denoix 1999; Denoix et al. 2006; Werpy 2007; Bergman et al. 2013).

In case of lumbosacral synostosis, the L5D has been frequently found thicker than normal (Fig 8b) (Denoix 1999; Werpy 2007; Boado et al. 2020). Complete L5-L6 synostosis with no disc material has also been described (Fig 9) (Denoix 1999; Denoix et al. 2005, 2006). In a recent study on horses

<figure><img src="image_12.png" /><figcaption>**Fig 8:** Median ultrasound scans of the ventral aspect of the lumbosacral junction from the 5th lumbar to 1st sacral vertebrae (cranial is to the left, and dorsal is to the top). Images acquired with a small curvilinear transducer. a) Partial lumbosacral synostosis (type 4) with a thin lumbosacral disc and a thick L5-6 intervertebral disc. b) complete lumbosacral intervertebral synostosis (type 5, sacralisation of L6) and thick L5-6 intervertebral disc. 1: lumbosacral intervertebral disc; 2: L5-6 intervertebral disc; 3: ventral bone surface of the 1st sacral vertebra; 4: ventral bone surface of the 6th lumbar vertebral; 5: ventral bone surface of the 5th lumbar vertebra; 6: ventral longitudinal ligament; *: lumbosacral synostosis.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>Fig 9: Median ultrasound scans of the ventral aspect of the caudalmost lumbar vertebrae and lumbosacral symphysis on a horse showing L5-L6 synostosis (cranial is to the left and dorsal is to the top). Images acquired with a small curvilinear transducer. 1: lumbosacral intervertebral disc; 2: L4-5 intervertebral disc; 3: ventral bone surface of the 1st sacral vertebra; 4: ventral bone surface of the 6th lumbar vertebra; 5: ventral bone surface of the 5th lumbar vertebra; 6: ventral bone surface of the 4th lumbar vertebra; 7: ventral longitudinal ligament; 8: aorta; 9: caudal vena cava; *: L5-L6 synostosis.</figcaption></figure>

with clinical signs indicative of lumbosacroiliac pain (Boado et al. 2020), 25% of horses had congenital variants of the L6D and 6.3% had fusion of the L5-L6 symphysis.

Abnormal intervertebral alignment of the lumbosacral junction has been described (Denoix et al. 2005). Spondylolisthesis refers to displacement of one vertebra relative to another (Resnick and Niwayama 1988) (listhesis meaning 'sliding'). Lumbosacral spondylolisthesis or (sub) luxation is identified when ventral displacement of the S1V or L6V is observed (Fig 10) (Boado et al. 2020). In those cases, the vertebral malalignment is associated with thinning of the intervertebral disc, especially dorsally.

In advanced cases of degeneration with dorsal narrowing of the disc, an associated ventral protrusion of the disc has been described (Nagy et al. 2010; Boado et al. 2020). The ventral part of the disc is bulging ventrally with a convex shape of its ventral border and of the ventral longitudinal ligament (Fig 11). Nagy et al. (2010) described a method of evaluation of the degree of ventral protrusion of the disc by measuring the distance between the most ventral aspect of the L6D and the line drawn between the most ventral aspects of the opposing surfaces of the L6 and S1 vertebrae. While a mild ventral protrusion of the L6D (≤2 mm) was found in 74.4% of horses without clinical signs of low back pain or

<figure><img src="image_14.png" /><figcaption>Fig 10: Median ultrasound scans of the ventral aspect of the lumbosacral symphysis showing abnormal vertebral alignment (cranial is to the left, and dorsal is to the top). Images were acquired with a small curvilinear transducer. a) Lumbosacral subluxation with dorsal displacement of the 6th lumbar vertebra and ventral displacement of the 1st sacral vertebral head in a 13-year-old showjumping mare. b) Lumbosacral subluxation with ventral displacement of the 6th lumbar vertebral fossa in a 10-year-old showjumper gelding. Note the dorsal narrowing of the lumbosacral intervertebral disc (arrows) in both cases and absence of ultrasound penetration through the intervertebral disc space to the vertebral canal. 1: ventral bone surface of the 1st sacral vertebra; 2: ventral bone surface of the 6th lumbar vertebra; 3: lumbosacral intervertebral disc; 4: ventral longitudinal ligament.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>Fig 11: Median ultrasound scan of the ventral aspect of the lumbosacral symphysis of a 10-year-old showjumping gelding showing a dorsal narrowing (arrows) of the lumbosacral intervertebral disc with ventral prolapsus of the disc and increased echogenicity corresponding to diffuse fibrosis of its ventral part (arrowheads). Image was acquired with a small curvilinear transducer (cranial is to the left and dorsal is to the top). 1: ventral bone surface of the 1st sacral vertebra; 2: ventral bone surface of the 6th lumbar vertebra; 3: lumbosacral intervertebral disc; 4: ventral longitudinal ligament.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>Fig 12: Median ultrasound scan of the ventral aspect of the lumbosacral symphysis (cranial is to the left, and dorsal is to the top) of a 6-year-old showjumping mare showing a large hypoechogenic area in the depth of the intervertebral lumbosacral disc corresponding to a wide dorsal cavitation of the disc (arrowheads). Image was acquired with a small curvilinear transducer. 1: ventral bone surface of the 1st sacral vertebra; 2: ventral bone surface of the 6th lumbar vertebra; 3: lumbosacral intervertebral disc; 4: ventral longitudinal ligament.</figcaption></figure>

hindlimb lameness, significant ventral protrusion was defined when this distance was >5 mm (Nagy et al. 2010; Boado et al. 2020). Using this measure, Boado et al. (2020) found 56% of ventral protrusion of the L6D in a group of horses with clinical signs indicative of lumbosacroiliac pain confirmed by diagnostic anaesthesia of the sacroiliac region. However, it should be kept in mind that the size of the most ventral part of the disc depends on the position of the horse (lumbosacral flexion vs. extension or neutral position) (Denoix 1999; Werpy, 2007).

## Echogenicity of the discs

On median and transverse sections, several variations of the echogenicity of the disc have been described (Denoix 1999; Denoix et al. 2005, 2006; Werpy 2007; Denoix and Jacquet 2008; Bergman et al. 2013). In 43 horses with no clinical signs of back pain or hindlimb lameness, Nagy et al. (2010) reported uniform or mildly heterogeneous echogenicity of the L6D in the majority of cases (79.1%), but also identified mild to moderate variations in echogenicity (hyperechogenic areas with or without acoustic shadowing). One of the main ultrasonographic findings in a population of horses with clinical signs indicative of lumbosacroiliac pain was

heterogeneous echogenicity of the lumbosacral disc (82.1-100%) with either anechogenic areas within the disc or with hyperechogenic foci (Boado et al. 2020).

A diffuse loss of echogenicity is usually suggestive of a degenerative process with loss of fibre architecture or disc fissures. Mild and limited fibre disintegration is imaged as focal hypoechogenic spots, and more extensive hypoechogenic areas are suggestive of disc cavitation with liquid infiltration (Fig 12) (Denoix 1999; Werpy 2007; Nagy et al. 2010; Bergman et al. 2013).

Hyperechogenic areas in the disc can be associated with healing tissue and fibrosis (Fig 11) and, in advanced and chronic L6D disease, some hyperechogenic spots casting acoustic shadows may represent dystrophic mineralisation (Fig 13) (Denoix 1999; Werpy 2007; Nagy et al. 2010; Bergman et al. 2013).

Variations of the echogenicity of the L5D has been most often described when the disc is wider than normal (>10 mm) (Boado et al. 2020). In most cases, abnormal L5D is associated with lumbosacral synostosis, which could presumably induce overstress of L5-L6 junction during locomotion and secondary predisposition to L5D injuries (Denoix 1999; Werpy 2007; Boado et al. 2020).
<figure><img src="image_17.png" /><figcaption>Fig 13: Median ultrasound scan of the ventral aspect of the lumbosacral symphysis of a 12-year-old showjumping pony showing focal hyperechogenic area (arrowheads) casting acoustic shadows (arrows) corresponding to a dystrophic mineralisation of the lumbosacral intervertebral disc. Image was acquired with a small curvilinear transducer (cranial is to the left and dorsal is to the top). 1: ventral bone surface of the 1st sacral vertebra; 2: ventral bone surface of the 6th lumbar vertebra; 3: lumbosacral intervertebral disc; 4: ventral longitudinal ligament.</figcaption></figure>

## Bone margins

L6D enthesopathy with or without avulsion fragmentation has been described as bone remodelling of the L6V fossa (Fig 14a,b) or of the S1V head (Fig 15) (Denoix 1999; Nagy et al. 2010; Bergman et al. 2013). In these cases, enthesophytes are imaged as bony proliferation, most often near the ventral margins of the symphysis (Denoix 1999; Boado et al. 2020). Subchondral bone irregularity with osteolysis is also reported when an ultrasound beam penetrates deep to the bone surface.

In rare cases, marked bone remodelling with hyperechogenic material casting an acoustic shadow is indicative of an avulsion fracture of the L6V fossa or S1V head (Denoix 1999).

## Comparative imaging

Diagnostic imaging tools that can be used to identify lesions of the caudal lumbar intervertebral discs in horses are limited (Denoix 1999; Nagy et al. 2010). Although a laterolateral radiographic projection of the lumbosacral area can be performed on the standing horse with a powerful radiographic machine, the presence of the tuber coxeae impedes severely the diagnostic value of this imaging approach. Informative radiographic examination of this region requires a ventrodorsal projection, the horse being placed on dorsal recumbency under general anaesthesia (Denoix et al. 2005; Bergman et al. 2013). However, this view has limited clinical ability to identify soft tissues and especially intervertebral disc degenerative lesions because of superimposition of abdominal viscera. Thus, this procedure is not routinely used (Denoix et al. 2005).

Nuclear scintigraphy has been frequently reported as a useful imaging diagnostic procedure to localise injuries in the thoracolumbar and pelvic region (Townsend et al. 1986;

<figure><img src="image_18.png" /><figcaption>Fig 14: Median ultrasound scans of the ventral aspect of the lumbosacral symphysis (cranial is to the left, and dorsal is to the top). Images were acquired with a small curvilinear transducer. a) Severe enthesopathy of the lumbosacral intervertebral disc of the 6th lumbar (L6) vertebral fossa in a 13-year-old dressage mare. Note the irregularity of the bone surface of L6 vertebral fossa with penetration of ultrasounds in the subchondral bone (arrowheads) separating an avulsion fragment from the fossa (open arrowheads). b) Enthesopathy of the lumbosacral intervertebral disc of L6 vertebral fossa with a small avulsion fracture fragment (arrowheads) casting acoustic shadow (arrows) in a 3-year-old Thoroughbred male. Note the caudal vertebral growth plate of the 6th lumbar vertebra just cranial to its fossa. 1: ventral bone surface of the 1st sacral vertebra; 2: ventral bone surface of the 6th lumbar vertebra; 3: lumbosacral intervertebral disc; 4: ventral longitudinal ligament; *: Growth plate of the 6th lumbar vertebral fossa.</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>Fig 15: Median ultrasound scan of the ventral aspect of the lumbosacral symphysis on a 9-year-old French Standardbred trotter mare showing caudal enthesopathy of the lumbosacral intervertebral disc on the head of the 1st sacral vertebral, with marked remodelling of bone surface (arrowheads). Image was acquired with a small curvilinear transducer (cranial is to the left and dorsal is to the top). 1: ventral bone surface of the 1st sacral vertebral; 2: ventral bone surface of the 6th lumbar vertebra; 3: lumbosacral intervertebral disc; 4: ventral longitudinal ligament.</figcaption></figure>

Tucker et al. 1998; Erichsen et al. 2002; Dyson et al. 2003a,b,c; Coudry et al. 2006; Bergman et al. 2013). It can be valuable in some cases to localise bone pathology in the lumbosacroiliac region, but superimposition of the ilium limits the evaluation of the region of interest. Moreover, due to their deep position at the ventral aspect of the vertebral column, radiation captation from the last lumbar vertebral symphyses is very limited and nuclear scintigraphy does not provide information on the intervertebral discs or other soft tissue structures (Nagy et al. 2010).

Computed tomography can be useful to examine the lumbosacroiliac area of horses (Krueger et al. 2016) but its use is limited by the size of the machine. Currently, it is only available for small and/or young horses.

Considering the limitations of other imaging modalities, ultrasonography is the technique of choice to evaluate vertebral soft tissues in routine practice, and, as the caudalmost lumbar intervertebral discs are at the ventral aspect of the vertebral column, a transrectal approach is appropriate.

However, it has some limitations. With this technique, good visualisation of the ventral part of the discs can be obtained but the dorsal part is more difficult to image. It is more difficult for the L4D and L5D that are thinner than the L6D, because of the curved shape of the lumbar vertebral extremities. In contrast to the lumbosacral junction where L6V fossa and S1V head are flat or slightly convex allowing a good penetration of the ultrasound beam into the whole

thickness of the L6D, the fossae of L4V and L5V are concave and the corresponding heads of L5V and L6V are convex (Barone 1999). The curvature of these vertebral extremities obscures the L4D and L5D spaces (Werpy 2007). For the same reasons, it is also difficult to assess completely the bone surface of the vertebral fossa and the head of the concerned vertebrae.

The technique and interest in transrectal ultrasonography in the evaluation of horses showing lack of hindlimb impulsion, lumbosacral stiffness or poor performance is now well documented in the literature. Nevertheless, diagnosis of lumbosacroiliac injury remains challenging and findings seen at transrectal ultrasonography should be interpreted in conjunction with clinical features at work, complete clinical examination and results of diagnostic procedures and other imaging modalities.

## Authors' declaration of interest

No conflicts of interest have been declared.

## Ethical animal research

Not applicable.

## Source of funding

None.

## Authorship

All authors contributed to the preparation of this manuscript.

## References

* Barone, R. (1999) Anatomie comparée des mammifères domestiques, Tome 1: Ostéologie. 4th edn., Vigot, Paris.

* Barone, R. (2000) Anatomie comparée des mammifères domestiques, Tome 2: Arthrologie et myologie, 4th edn., Vigot, Paris.

* Barstow, A. and Dyson, S.J. (2015) Clinical features and diagnosis of sacroiliac joint region pain in 296 horses: 2004–2014. Equine Vet. Educ. 27, 637-647.

* Bergman, E.H.J., Puchalski, S.M. and Denoix, J.M. (2013) How to perform a transrectal ultrasound examination of the lumbosacral and sacroiliac Joints. Proc. Am. Ass. Equine Practnrs. 59, 229-237.

* Bergmann, W., Bergknut, N., Veraa, S., Gröne, A., Vernooij, H., Wijnberg, I.D., Back, W. and Grinwis, G.C.M. (2018) Intervertebral disc degeneration in warmblood horses: morphology, grading, and distribution of lesions. Vet. Pathol. 55, 442-452.

* Boado, A., Nagy, A. and Dyson, S.J. (2020) Ultrasonographic features associated with the lumbosacral or lumbar 5-6 symphyses in 64 horses with lumbosacral-sacroiliac joint region pain (2012–2018). Equine Vet. Educ. 32 Suppl. 10:136–143.

* Coudry, V., Audigié, F., Didierlaurent, D. and Denoix, J.M. (2006) Comparison between nuclear scintigraphy and transrectal ultrasonography in the detection of sacroiliac abnormalities. In: International Radiology Association and American College of Veterinary Radiology 2006 Joint Conference, Vancouver, Canada. p 76.

* Dansie, E.J. and Turk, D.C. (2013) Assessment of patients with chronic pain. Br. J. Anaesth. 111, 19-25.

* Denoix, J.M. (1998) Diagnosis of the cause of back pain in horses. In: Conference on Equine Sports Medicine and Science 1998, Cordoba, Spain. pp 97-110.

* Denoix, J.M. (1999) Ultrasonographic evaluation of back lesions. Vet. Clin. N. Am.: Equine Pract. 15, 131-159.
Denoix, J.M. (2017) Diagnosis of low back (lumbar and lumbosacral) pain in equine athletes. In: *Proceeding 2017 ACVIM Forum Research, National Harbor, Maryland*. pp 305-309.

Denoix, J.M. (2019) *Essentials of Clinical Anatomy of the Equine Locomotor System*, CRC Press, Boca Raton, Florida. pp 171-188.

Denoix, J.M. and Coudry, V. (2005) Diagnosis and treatment of lumbosacral and sacroiliac pain in horses. In: XVI Tagung über Pferdekrankheiten 2005, Essen, Germany. 50-55.

Denoix, J.M. and Dyson, S.J. (2010) Thoracolumbar spine. In: *Diagnosis and Management of Lameness in the Horse*, 2nd edn., Ed: M.W. Ross and S.J. Dyson, W.B. Saunders, Philadelphia. pp 509-521.

Denoix, J.M. and Jacquet, S. (2008) Diagnosis and treatment of lumbosacral and sacroiliac pain in horses. In: *Proceeding of 10th Congress of the World Equine Veterinary Association 2006*, Moscow, Russia. pp 290-297.

Denoix, J.M., Audigié, F. and Coudry, V. (2005) Review of diagnosis and treatment of lumbosacral pain in sport and race horses. *Proc. Am. Ass. Equine Practnrs.* **51**, 366-373.

Denoix, J.M., Audigié, F. and Coudry, V. (2006) Diagnosis imaging of back and pelvis injuries in horses. In: *Proceeding of 9th Congress of the World Equine Veterinary Association 2006*, Marrakech, Morocco. pp 42-50.

Dyson, S.J. (2010) Poor performance and lameness. In: *Diagnosis and Management of Lameness in the Horse*, 2nd edn., Ed: M.W. Ross and S.J. Dyson, W.B. Saunders, Philadelphia. pp 828-832.

Dyson, S.J. and Murray, R. (2003) Pain associated with the sacroiliac joint region: a clinical study of 74 horses. *Equine Vet. J.* **35**, 240-245.

Dyson, S.J., Pilsworth, R.C., Twardock, A.R. and Martinelli, M.J. (2003a) *Equine Scintigraphy*, Equine Veterinary Journal Ltd., Newmarket, UK.

Dyson, S.J., Murray, R., Branch, M., Whitton, C., Donovan, T. and Harding, E. (2003b) The sacroiliac joints: evaluation using nuclear scintigraphy. Part 1: the normal horse. *Equine Vef. J.* **35**, 226-232.

Dyson, S.J., Murray, R., Branch, M. and Harding, E. (2003c) The sacroiliac joints: evaluation using nuclear scintigraphy. Part 2: Lame horse. *Equine Vet. J.* **35**, 233-239.

Erichsen, C., Berger, M. and Eksell, P. (2002) The scintigraphic anatomy of the sacroiliac joint. *Vet. Radiol. Ultrasound* **43**, 287-292.

Espinosa, P., Benoit, P., Salazar, I., de la Fuente, J. and Heiles, P. (2017) Transrectal ultrasonography of equine lumbosacral nerves: pilot study in 28 healthy warmblood horses. *Vet. Radiol. Ultrasound* **58**, 228-236.

Haussler, K.K. (2010) Diagnosis and management of sacroiliac joint injuries In: *Diagnosis and Management of Lameness in the Horse*, 2nd edn., Eds: M.W. Ross and S.J. Dyson, W.B. Saunders, Philadelphia. pp 583-591.

Haussler, K.K., Stover, S.M. and Willits, N.H. (1999) Pathology of the lumbosacral spine and pelvis in Thoroughbred racehorses. *Am. J. Vet. Res.* **60**, 143-153.

Jeffcott, L.B. and Dalin, G. (1980) Natural rigidity of the horse's backbone. *Equine Vet. J.* **12**, 101-108.

Jeffcott, L.B., Dalin, G., Ekman, S. and Olsson, S.E. (1985) Sacroiliac lesions as a cause of poor performance in competitive horses. *Equine Vet. J.* **17**, 111-118.

Krueger, C.R., Gold, J., Barrett, M.F. and Aboellail, T.A. (2016) Diagnosis of lumbosacral intervertebral disc disease and protrusion in a horse using ultrasonographic evaluation and computed tomography. *Equine Vet. Educ.* **28**, 685-689.

Nagy, A. and Dyson, S.J. (2020) Recumbency following diagnostic analgesia of the sacroiliac joint regions: 15 horses. *Equine Vet. Educ.* **32** Suppl. **10**:144-150.

Nagy, A., Dyson, S.J. and Barr, A. (2010) Ultrasonographic findings in the lumbosacral joint of 43 horses with no clinical signs of back pain or hindlimb lameness. *Vet. Radiol. Ultrasound* **51**, 533-539.

Peters, D.F. (2012) Thoracolumbar and lumbosacral considerations in hind leg lameness. In: *Proceedings of 2012 AAEP Focus Meeting on Hindlimb Lameness*, Oklahoma City, OK. pp 87-93.

Resnick, D. and Niwayama, G. (1988) Degenerative disease of the spine. In: *Diagnosis of Bone and Joint Disorders*, Vol. **3**, 2nd edn., Eds: D. Resnick, G. Niwayama, WB Saunders Company, Philadelphia. pp 1481-1561.

Stubbs, N.C., Hodges, P.W., Jeffcott, L.B., Cowin, G., Hodgson, D.R. and McGowan, C.M. (2006) Functional anatomy of the caudal thoracolumbar and lumbosacral spine in the horse. *Equine Vet. J.* **38**, Suppl. **36**, 393-399.

Tollaj, A., Coudry, V. and Denoix, J.M. (2019) Transrectal ultrasonographic examination of the sacroiliac joints of the horse: technique and normal images. *Equine Vet. Educ.* **31**, 666-671.

Townsend, H.G.G. and Leach, D.H. (1984) Relationship between intervertebral joint morphology and mobility in the equine thoracolumbar spine. *Equine Vet. J.* **16**, 461-465.

Townsend, H.G.G., Leach, D.H. and Fretz, P.B. (1983) Kinematics of the equine thoracolumbar spine. *Equine Vet. J.* **15**, 117-122.

Townsend, H.G.G., Leach, D.H., Doige, C.E. and Kirkaldy-Willis, W.H. (1986) Relationship between spinal biomechanics and pathological changes in the equine thoracolumbar spine. *Equine Vet. J.* **18**, 107-112.

Tucker, R.L., Schneider, R.K., Sondhof, A.H., Ragle, C.A. and Tyler, J.W. (1998) Bone scintigraphy in the diagnosis of sacroiliac injury in twelve horses. *Equine Vet. J.* **30**, 390-395.

Werpy, N.M. (2007) Imaging of the thoracolumbar region and pelvis: In: *Proceeding of American Association of Equine Practnrs Focus Meeting on Lameness and Imaging*. Fort Collins, Colorado. pp 183-189.