# Ultrasonographic diagnosis of equine thoracolumbar articular process joint lesions

A. Beaumont, L. Bertoni, Jean-Marie Denoix

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## To cite this version:

A. Beaumont, L. Bertoni, Jean-Marie Denoix. Ultrasonographic diagnosis of equine thoracolumbar articular process joint lesions. Equine Veterinary Education, 2022, 34 (11), pp.592-599. 10.1111/eve.13551. hal-03989009

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Submitted on 14 Feb 2023

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EQUINE VETERINARY EDUCATION
Equine vet. Educ. (2022) 34 (11) 592-599
doi: 10.1111/eve.13551

Review Article

Ultrasonographic diagnosis of equine thoracolumbar articular
process joint lesions

A. Beaumont†,‡,∗ ID, L. Bertoni†,‡ ID and J. M. Denoix†

†Ecole Nationale Vétérinaire d'Alfort, CIRALE, Maisons-Alfort, France; and ‡INRAE, Ecole Nationale Vétérinaire d'Alfort, BPLC, Maisons-Alfort, France

*Corresponding author email: audrey.beaumont@vet-alfort.fr

**Keywords:** horse; ultrasonography; back; imaging; facet joints; articular process

Summary

Thoracolumbar lesions are common in horses, but their
identification and location remain a challenge in field
practice. A precise diagnosis improves therapeutic strategy
and, thus, the prognosis of affected horses. Although
radiography remains the preferred diagnostic imaging
modality used for the back, ultrasound is a useful
complementary technique that offers valuable information
and can be performed as a stand-alone procedure when
high-output radiographic equipment is not available. The aim
of this paper is to describe thoracolumbar articular process
joint pathology that can be visualised with ultrasound.

Introduction

Back pathology is a major cause of poor performance and
gait abnormalities in sport and racehorses (Henson, 2009;
Jeffcott, 1981). Thoracolumbar articular process joint (APJ)
injuries are described as a common cause of back pain
(Denoix, 1998a) and are mainly seen at the level of the
thoracolumbar and cranial lumbar regions, where one or
several intervertebral joints can be affected (Girodroux
et al., 2009). The anamnesis and clinical (physical and
dynamic) examination often make it possible to reveal or
suspect functional abnormalities of the vertebral column

(Denoix, 1992; Jeffcott, 1981; Riccio et al., 2018). Nevertheless, the definitive diagnosis of the cause of pain remains difficult to establish for the equine practitioner in field practice. This is especially true when lesions involve the low mobile APJs or 'facet joints' made of several small articular surfaces covered by a thick layer of muscles. Radiography is the first-choice imaging modality (Audigie et al., 2013) to explore the vertebral column from T1 to L4, but, because of the large muscle mass, complete radiographic assessment of the back is not easy in routine practice with a portable machine on a mature horse. High-output x-ray equipment, a focussed grid and computed or digital radiographic equipment are required, resulting in a high level of radiation exposure. Ultrasound examination is a safe and non-invasive procedure that can easily be performed with practice and could then be considered as a routine procedure in the field. Rapid advances in ultrasound technology, with the development of compact portable machines with high image quality, have led to the growing interest in this technique that is now widely available for most equine practitioners.

The aims of this paper are to describe the indications and
current technique used to image the APJs, show normal
ultrasonographic images and present the main abnormal
findings.

<figure><img src="image_3.png" /><figcaption>Fig 1: Up (top image) – Lateral aspect of the axial skeleton. Frames focussing on three areas. Cranial is to the left. Down (bottom image) – Dorsolateral aspect of three thoracolumbar areas showing regional anatomical variation of the articular processes (APs).</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Fig 2: (a) Transverse section of the thoracolumbar area between the last thoracic vertebra and the first lumbar vertebra; (b) Transverse section of the lumbar area between the second and third lumbar vertebrae (Reproduced with permission from Essentials of Clinical Anatomy of the Equine Locomotor System. Denoix, 2019. CRC Press). 1- Caudal articular process; 2- Spinous process; 3- Cranial articular process, 3a- Mammillary process; 4- Vertebral head; 5- Vertebral canal; 6- Transverse process; 7- Rib; 8- Supraspinous ligament; 9- Interspinous ligament; 10- Articular process joint; 11- Multifidus muscle, 11a- multifidus aponeurosus, 11b- multifidus fascia; 12- Erector spinae muscle, 12a- intramuscular aponeurosus, 12b- superficial aponeurosis; 13- Gluteus medius muscle (lumbar part).</figcaption></figure>

Indications

Ultrasonographic examination of the equine thoracolumbar APJs is indicated when there is a history of back pain such as unwillingness to work, poor performance or loss of gait amplitude. Other nonspecific problems such as behavioural problems, saddle intolerance and/or reluctance to jump (Riccio et al., 2018) are further indications. In addition, ultrasonography is indicated to explore physical examination abnormalities such as paravertebral muscle atrophy, heat or

induration identified by palpation, sensitivity and/or reduced range of motion during back mobilisation tests (Denoix, 1998a; Martin & Klide, 1999; Riccio et al., 2018). Finally, the ultrasonographic examination can be used in combination with radiography and bone scintigraphy to document and lateralise abnormal findings (Audigié et al., 2013). When the diagnosis is achieved, ultrasonography is useful to accurately guide periarticular injections (Fuglbjerg et al., 2009).

Clinical anatomy

Each thoracic and lumbar vertebra has two articular processes (APs) on both sides of the base of its spinous process: a caudal AP and a cranial AP. The cranial AP is elongated laterally by a mammillary process (Barone, 2000a; Denoix, 1999; Jeffcott, 2009).

The APJ is located dorsally to the vertebral canal. It consists of the caudal AP of one vertebra, the synovial joint

<figure><img src="image_5.png" /><figcaption>Fig 3: Placement of the transducer and beam orientation to make a transverse section of the articular process joint (APJ) complex.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig 4: Transverse ultrasound scan of the lumbar area between the first and the second lumbar vertebrae (L1-L2), in a 4-year-old French Standardbred Trotter mare. 1- Median plan; 2- Caudal articular processes of the first lumbar vertebra; 3- Cranial articular processes of the second lumbar vertebra, 3a- mammillary process; 4- Transverse process of the second lumbar vertebra; 5- Multifidus muscle, 5a- multifidus fascia; 6- Erector spinae muscle; 7- Skin. White arrowhead: Joint space of the L1-L2 articular process joint (APJ).</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig 5: Transverse ultrasound scan of the right thoracic area between the fifteenth and eighteenth thoracic vertebrae, in a 4-year-old French Standardbred Trotter mare. 1- Fifteenth thoracic spinous process; 2- Articular processes joints; 2a- Joint space; 3- Caudal articular process of seventeenth thoracic vertebra; 4- Cranial articular process of eighteenth thoracic vertebra; 5- Mammillary process; 6- Rib; 7- Multifidus muscle; 8- Erector spinae muscle; 9- Skin.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig 6: Transverse ultrasound scan of the thoracolumbar area between the seventeenth and eighteenth thoracic vertebrae (a), and between the last thoracic vertebra and the first lumbar vertebra (b). 1- Articular processes joint; 2- Eighteenth rib; 3- Transverse process of the first lumbar vertebra; 4- Multifidus muscle; 5- Erector spinae muscle; 6- Skin.</figcaption></figure>

(including articular cartilage, synovial fluid and membrane, articular capsule) located at the base of the interspinous space and the cranial AP of the following vertebra (Denoix, 1999, 2019). Depending on the area, the APJs vary in shape (Fig 1). The articular facets are flat and oriented obliquely in the cranial thoracic area. The joint space becomes more vertical in the mid-thoracic area. In the caudal

thoracolumbar area, the articular facets are more rounded with a convex caudal AP corresponding to a concave cranial AP. Because of the orientation of the spinous processes in the thoracic area, the joint space is located directly ventral to the apex of the spinous process of the vertebra that precedes the joint. In the mid-thoracic region, the joint space is located in the middle of the interspinous
<figure><img src="image_9.png" /><figcaption>Fig 7: Transverse ultrasound scans of the caudal lumbar area, showing the fourth and fifth lumbar vertebrae (a), the fifth and sixth lumbar vertebrae (b) and the sixth lumbar vertebra and fifth sacral vertebra (c) articular processes joints, in a 4-year-old French Standardbred Trotter mare. 1- Articular processes joints; 2- Caudal articular process of the fifth lumbar vertebra; 3- Cranial articular process of sixth; 4- Mammillary process; 5- Multifidus muscle; 6- Erector spinae muscle, 6a- superficial aponeurosis; 7- Gluteus medius muscle; 8- Ilium wing; 9- Skin.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Fig 8: Transverse ultrasound scan of the lumbar area between the first and second lumbar vertebrae. The left articular process joint (APJ) complex (arrowhead) is bigger than the right one.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig 9: Transverse ultrasound scan of the thoracolumbar area of two horses with dorsal periarticular bone proliferation. (a) Transverse ultrasound scan of the T17-T18 articular process joint (APJ) with moderate bone proliferation over the left APJ (arrowhead); (b) Transverse ultrasound scan of the T18-L1 APJ with severe bone proliferation over the right APJ (arrowhead). 1- Mammillary process; 2- Multifidus muscle.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Fig 10:** (a) Lateral radiographic image of the thoracolumbar junction and lumbar area from the eighteenth thoracic vertebra to the third lumbar vertebra, showing a bone bridge over the L1-L2 articular process joints (APJs) (open arrowhead). L1: First lumbar vertebra. (b) Transverse ultrasound scan between the first and second lumbar vertebrae from the same horse. The bone bridge is located over the right L1-L2 APJ (arrowhead).</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Fig 11:** (a) Lateral radiographic image of the thoracolumbar junction and cranial lumbar area from the eighteenth thoracic vertebra to the fourth lumbar vertebra, showing severe alteration of the bone density of the L2-L3 and L3-L4 articular process joints (APJs) with dorsal bone proliferation (open arrowheads). L1: First lumbar vertebra. (b) Transverse ultrasound scan between the second and third lumbar vertebra. There is a marked bone modelling of the right L2-L3 APJ with dorsal and ventral proliferation (arrowheads).</figcaption></figure>

dorsocranially by a mammillary process, which is higher and more separated from the APJ space in the caudal thoracic area than in the lumbar area.

The APJs are covered by the multifidus muscle, which has multiple aponeuroses. This muscle is separated from the thick erector spinae muscle by an echogenic fascia. In the lumbar region, the lumbar part of the gluteus medius muscle overlaps part of the erector spinae muscle (Fig 2) (Barone, 2000b; Denoix, 2019).

## Technique and normal images

Ultrasonographic examination of the APJs can be performed with a 3 to 6 MHz macroconvex transducer (Fig 3). Hot water

and acoustic gel should be used to improve contact and therefore quality of the image. In winter or in case of decreased image quality, an area ranging from 6 to 8 centimetres wide can be clipped on both sides of the median plane to improve ultrasound penetration.

Transverse ultrasonographic images of the APJs are the most informative. The transducer is placed on a paramedian position perpendicular to the skin and the median plane (Fig 3) to obtain an overall spatial representation of the dorsal profile of the APJ. Thanks to the width of the ultrasound beam, an overview from the spinous processes to the transverse processes or ribs is imaged (Fig 4). The joint space separating the two APs can be identified as a small anechoic gap between the caudal and cranial APs. The caudal AP from one vertebra is adjacent to the median plane and the cranial AP of the following vertebra is located laterally to it (Fig 4). In the thoracic area, the joint space is difficult to image because of the close proximity of the APJs to the tall spinous processes of this region (Fig 5). The bone surface of the APs appears as a smooth, regular hyperechogenic line. The mammillary processes are seen dorsal to the cranial APs on their lateral aspect (Figs 4 and 5).

A complete cross-section of the dorsal profile of the intervertebral joint can be obtained when the left and right sides are displayed symmetrically on the screen of the machine. The purpose of this approach is to compare the caudal and cranial APs, as well as the joint space of each APJ on both sides. Identification of the thoracolumbar junction is an important step to identify precisely which APJs are imaged. Identifying the correct vertebral segment can be accomplished by progressing caudally from the last rib (demonstrates a convex hyperechoic bone surface) to the first transverse process (demonstrates a flat hyperechoic bone surface). This is T18-L1. Each following or preceding joint can be identified by counting either cranially or caudally. Then, precise imaging of the thoracic and lumbar APJs can be performed (Figs 5-7).

## Findings and lesions

Abnormal findings of the APJs identified with ultrasound are associated with different manifestations of osteoarthritis. For the above-mentioned anatomical reasons, their identification is easier in the lumbar spine and include: left to right asymmetry of the APJs with increased size on one side (Fig 8);
<figure><img src="image_14.png" /><figcaption>Fig 12: (a) Radiographic image of the thoracic vertebral segment from the thirteenth to sixteenth thoracic vertebra showing severe bone proliferation extending from the articular process joint (APJ) up to the base of the spinous processes (arrowhead). (b) Transverse ultrasound scan between the T14-T15 intervertebral joint space showing the irregular surface of the APJ (open arrowhead) and the bone proliferation on the side of the spinous processes (arrowhead). 1- Spinous process of T15, 2- APJ; 3- Mammillary process of T15; 4- Fourteenth rib.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Fig 13: (a) Lateral radiographic image of the thoracolumbar junction and lumbar area from the seventeenth thoracic vertebra to the fourth lumbar vertebra. The T18-L1 articular process joint (APJ) complex presents a marked arthropathy and a moderate arthropathy is noticed in L1-L2 APJ complex (open arrowheads). (b) Transverse ultrasound scan between the last thoracic vertebra and the first lumbar vertebra. Marked osteophytes are seen over the left APJ (arrowhead). (c) Transverse ultrasound scan between the first and the second lumbar vertebrae. Moderate osteophytes on the left APJ and more subtle osteophytes on the right APJ are imaged (arrowheads).</figcaption></figure>

periarticular osteophytes and dorsal periarticular bone proliferation (Fig 9), with the pathological bone surface-reaching or exceeding the mammillary process; complete loss of the joint space with bone proliferation making a bridge between two adjacent vertebrae (Fig 10) and periarticular lysis and modelling of the dorsal bone surface (Fig 11). In the thoracic spine, periarticular bone proliferation of the APJs usually extends up to the base of the spinous process (Fig 12). Based on these findings, degenerative joint disease can be graded from mild to marked and the most affected side can be identified (Fig 13).

## Discussion

Ultrasonography of the APJs is a useful diagnostic modality to complement clinical examination and radiographic findings in horses suspected of back pain. A retrospective survey of the diagnostic approaches used by 168 European veterinarians in 2016 to investigate back pain revealed that radiography and ultrasonography were the preferred imaging modalities (Riccio et al., 2018). Radiography was always used by 45% of veterinarians (50% of them using it sometimes) and ultrasonography was always used by 24% of
veterinarians, with 70% using it sometimes. When a sufficiently powerful generator is unavailable, the use of radiography is frequently restricted to imaging of the spinous processes (Vandeweerd, 2015). Conversely, ultrasonography is a more versatile diagnostic tool that can be used to detect soft tissue lesions (supraspinous ligament desmitis, thoracolumbar fasciitis, myositis), as well as APJ lesions, with a high sensitivity to detect bone remodelling and osteophyte formation (Audigié et al., 2013; Denoix, 1999). In addition, using ultrasonography, APJ lesions can be identified in the caudal lumbar region, an area where APJs are difficult to visualise on radiographs, even with high-output x-ray machines, because of the large muscle mass in this area and the projection of the tuber coxae (Cousty et al., 2010; Denoix, 1999; Denoix & Dyson, 2011). Finally, ultrasonography is the only modality that definitively identifies the most affected side (if one is greater than the other), allowing better correlation with clinical signs and improving the efficacy of directed treatment such as ultrasound-guided injections. Of note, the clinical significance of asymmetry alone is not clear as 83% of euthanised Thoroughbred racehorses had asymmetry of at least one lumbar APJ in one retrospective post-mortem study (Haussler et al., 1997). Such asymmetry may, however, impair the biomechanical behaviour of the intervertebral joints and could predispose to the development of degenerative joint disease (Denoix, 1999).

Limitations of ultrasonography in imaging the APJs are twofold. First, unlike radiography, the ventral part of the joint cannot be visualised and bone density changes cannot be assessed, although densification of the subchondral bone (sclerosis) or its demineralisation (osteolysis) are other osteoarthritis-associated changes. Second, for anatomical reasons, the lumbar APJs (more prominent and closer to the skin surface) are much easier to image than the middle thoracic APJs (superimposed and deeper because of the length of the spinous processes). These limits are relative since a retrospective study (Cousty et al., 2010), revealed that periarticular bone proliferation of the APJ was more prevalent and more severe in a group of Standardbred French Trotters showing signs of back pain than in the control group, which was not the case for subchondral bone sclerosis. This finding suggests that periarticular new bone proliferation, which can be assessed by ultrasound, maybe more correlative with clinical signs of back pain compared to changes in bone density. In addition, APJ osteoarthritis is more frequently located in the thoracolumbar area (Cousty et al., 2010; Denoix, 1999; Girodroux et al., 2009) and cranial lumbar area (Denoix, 1998b, 1999). In the study from Cousty et al. (2010), osteoarthritis-associated lesions of the APJs were most frequently observed between T17 and L2 with the highest frequency in T18-L1 on Standardbreds, while Warmbloods and Thoroughbreds were mostly affected between T16 and L1, with the highest frequency in T16-T17 (Girodroux et al., 2009). The highest mobility of the thoracolumbar region may explain the preferred location of these lesions along the back (Denoix, 1999; Mespoulhes, 2002). Nevertheless, the clinical significance of these lesions seems to be different according to the breed and discipline. Lesions are indeed more severe in racing Thoroughbreds than in racing Standardbred French Trotters for whom mild lesions lead to discomfort and irregular gaits at high speed.

## Conclusion

Ultrasonography is a safe imaging technique that can be successfully performed in the field with regular practice for the diagnosis of APJ lesions in the thoracolumbar area. With sufficient anatomical knowledge and appropriate technique, this procedure provides accurate and real-time diagnostic information in the caudal thoracic and lumbar vertebral areas. Used alone or in combination with other imaging modalities, ultrasonography of the APJs improves the diagnosis and localization of the source of equine back pain, providing valuable information that can improve treatment and management of affected horses.

## Authors' declaration of interest

No conflicts of interest have been declared.

## Ethical animal research

Not applicable to this article.

## Source of funding

The authors thank the Normandy Regional Council and the European Regional Development Funds for their financial support.

## Authorship

The study design was made by A. Beaumont. The study execution and image acquisition were made by all authors. A. Beaumont and L. Bertoni prepared the manuscript. All authors gave their final approval of the manuscript.

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