Podcast: Spine: radiology
Indications for radiography
Radiography allows assessment of osseous and soft tissue structures of the spine Spine: radiography .
- Neurological lesions localized to the cervical, thoracolumbar, lumbar or sacral vertebral column.
- Pain/pathology localized to the cervical, thoracolumbar, lumbar or sacral vertebral column (based on clinical examination, scintigraphy, etc).
- Acute or chronic trauma, including suspected fracture.
- Investigation of conformational abnormalities, eg changes in the shape of the withers associated with fracture of the thoracolumbar vertebral spines.
Radiographic technique and evaluation
- For technique and views see Spine: radiography Spine: radiography .
- There is significant radiation scatter Radiography: scattered radiation and grids - good technique and safety procedures are essential.
- High exposures are required in the adult horse, and therefore a fast film/screen combination is needed. This reduces image quality. Rare-earth or ultra-violet film/screen combinations (or digital radiography Digit: radiography where available), allow a lower exposure to be used, and improve image resolution -poor radiographic technique can impact on film and radiology, especially in this area.
- Use of a grid and collimating down on regions of interest, are essential factors to improve image quality Radiography: image quality and improve radiation safety.
- The size of the adult spine and the superimposition of bony structures and soft tissues mean that radiology has a low sensitivity in detecting bony pathology. Lesions on the periphery of the bone are most easily recognized.
- In the adult ventrodorsal views are only possible in the neck, and further diagnostic tests such as myelography Myelography or nuclear scintigraphy Bone: scintigraphy are required to fully evaluate lesions.
- The need for general anesthesia Anesthesia: general - overview , eg for myelography, should be carefully balanced against the risk to the horse (neurological status, stability of fractures/luxations) and personnel. The final decision should be based on the impact radiology will have on the horses treatment and prognosis.
Assessment of the vertebral canal
- The sagittal ratio should be evaluated in cases where stenosis of the vertebral canal is suspected, eg cervical stenotic myelopathy Spine: cervical vertebral malformation (CVM) . The sagittal ratio is more accurate than the minimum sagittal diameter, as it allows for any magnification.
- A precise lateromedial radiograph is required - obliquity will affect measurement.
- The minimum sagittal diameter (MSD) is the smallest distance between the dorsal and ventral margins of the vertebral canal (usually at the cranial aspect of the vertebral foramina).
- The sagittal ratio is the MSD divided by the vertebral body width. The vertebral body width is measured perpendicular to the vertebral canal at the widest region of the cranial vertebral body.
- The minimum sagittal diameter and the sagittal ratio vary depending on the cervical vertebra, and measurements should be compared to published data. In general, a sagittal ratio of <0.5 affecting C4-C7 has a high probability of being abnormal.
- Myelography Myelography may be required to confirm spinal cord compression.
Anatomy of the vertebral column
- The vertebral column in the horses consists of:
- The anatomy of the vertebra varies significantly depending on the region of the spine and its function.
- The anatomy of a typical vertebra is:
- A bony arch sitting above a solid, cylindrical vertebral body.
- Together the arch and body surround and form the vertebral foramina an opening that encloses and protects the spinal cord.
- The term vertebral canal refers to the foramina of multiple vertebrae, lying adjacent and creating a tunnel for the spinal cord and its accessory structures.
- A typical vertebra has a number of bony processes:
- A single dorsal spinous process on the dorsal aspect of the arch.
- Cranial and caudal articular processes on both the left and right aspects of the vertebral arches.
- Transverse processes projecting from each side of the vertebral body.
- There are two main types of joints within the spinal column:
- Symphyses between the vertebral bodies, formed by the intervertebral disks.
- Synovial joints between the articular processes of the vertebral arches.
Cervical vertebrae (C1-7)
- Atlas (C1):
- The atlas consists of two halves or wings it has no vertebral body or articular processes.
- Each wing extends medially as a dorsal and ventral arch. These arches join in the midline forming the vertebral foramen.
- The remainder of the cervical vertebrae have a more typical structure, although there is some variation.
- Features of the axis (C2) are a large dorsal spinous process, and a dens (or odontoid process), the bony protuberance, which articulates with the atlas.
- On C3-C7, the dorsal spinous process is less prominent, but the transverse processes are well developed.
- The sixth cervical vertebra (C6) is shorter, with a ventral process (which can be useful for identification on radiology).
- C7 is shorter again and has a small dorsal spinous process.
Thoracic vertebrae (T1-18)
- The thoracic vertebrae have shortened bodies with facets for articulation with the ribs, and small transverse processes.
- The main features are the large spinous processes that give the contour and shape of the withers.
- T6 or T7 usually has the tallest dorsal spinous process.
- T15 is normally the anticlinal vertebra (where the direction of the dorsal spinous process changes from sloping caudally to sloping cranially).
- The spinous processes are closer together around T15.
Lumbar vertebrae (L1-6)
- The lumbar vertebrae have long bodies, short dorsal spinous processes and long transverse processes.
The sacral and caudal vertebrae
- The sacrum is formed by the fusion of the sacral bones. In the horse, the dorsal processes are not fused but remain distinct, and there is a prominent lateral sacral crest.
- The caudal vertebrae have a prominent body and small vertebral arch. Normal features, such as the dorsal spinous process, transverse and articular processes are present.
Normal radiographic anatomy
- Radiology of the spine can be confusing due to the superimposition of structures.
- A careful evaluation of the normal anatomical features outlining the vertebral canal, articulations and various processes will assist interpretation.
- The spine is predominantly imaged using the laterolateral view - this will result in superimposition of the right and left aspects of the vertebrae, eg left and right articular processes.
- The cervical vertebral canal is wide, and the vertebral column has a slight, gradual S-shaped bend - curved dorsally in the cranial region, and ventrally in the caudal region. A degree of hyperflexion/subluxation may occur normally at C2-3 or C3-4 when the neck is flexed.
- The thoracic vertebral canal should remain the same width throughout.
- Use of radio-opaque markers help with positioning and identification of different vertebrae during radiology.
- Anatomical features that are useful for identification are:
- Shape of the atlas and axis.
- Ventral process and shape of C6.
- Dorsal spinous process and shape of C7.
- Tallest dorsal spinous process (T6 or less commonly T7).
- Anticlinal vertebra (T15).
- Large transverse processes of the lumbar vertebrae.
- There are various centers of ossification, and closure times vary between different regions.
- The atlas originates from two halves (left and right side) in the neonate, a radiolucent line can be seen in the midline on the ventrodorsal view.
- The axis has separate centers of ossification for the dens, head, body and caudal epiphysis. The dens is fused to the head by 7 months of age, and the caudal epiphysis by 5 years.
- The remainder of the cervical vertebra have centers of ossification at the cranial and caudal physes. The cranial physis is closed by 2 years of age, and the caudal by 4-5 years.
- A separate center of ossification can occur in the ventral process of C6.
- The thoracic and lumbar vertebrae have centers of ossification at the cranial and caudal physes. The cranial physis is closed by 6-12 months of age, and the caudal by 2-4 years.
- T2-T8 develop separate centres of ossification within the cartilage component of the dorsal spinous process at 12 months of age. This area may not ossify completely or may ossify with an irregular opacity and outline.
- Cervical vertebrae:
- C2 bony spur may occur in normal horses on the caudal epiphysis, overlying the vertebral canal.
- The articular surface of the cranial articular processes can be curved or flattened congruity with the adjoining caudal articular process, and comparison with adjacent vertebra are important in distinguishing this normal finding from pathology.
- The ventral process of C6 may transpose to C7 or rarely C5. In these cases, the shorter size and presence of a dorsal spinous process on C7 will help distinguish this from C6.
- The dorsal spinous process of C7 may become superimposed on the articulation between C6/C7. This causes an increas in opacity that should be distinguished from pathological new bone.
- Spondylitc new bone on the ventral aspect of the vertebral bodies can be an incidental finding in older horses.
- Thoracic and lumbar vertebrae:
- In horses with high sprung ribs, the ribs may be superimposed over the articular processes of the thoracic vertebrae, interfering with evaluation of the joints.
- The distance between the dorsal spinous processes is variable. It is usually narrowest around the anticlinal vertebra, and the dorsal spinous processes may appose in normal horses.
- Smooth irregular new bone on the cranial and caudal aspects of the dorsal spinous processes of T2-T10 can be an incidental finding.
- The number of lumbar or sacral vertebrae varies (see Normal radiographic anatomy above).
- The angulation of the dorsal spinous processes of the lumbar vertebrae varies depending on whether there are five or six vertebrae.
- These include absent, distorted or fused vertebra (Roentgen changes in size, shape, number or position).
- Occipitoatlantoaxial malformation (OAAM) Spine: occipitoatlantoaxial malformation is the most common lesion. It is seen predominantly in Arabs Arab and has a variety of different manifestations.
- Two or more views (laterolateral and ventrodorsal) may be required to fully evaluate lesions.
- Fused vertebrae may be impossible to distinguish from chronic fractures where the fracture has healed and remodeled.
- Subluxation can occur as a result of ligament damage, fracture or hypoplasia of the dens of the axis (with or without OAAM Spine: occipitoatlantoaxial malformation ).
- Radiographic signs include excessive range of motion at the atlantoaxial junction or luxation of the axis ventral to the atlas. Suspected cases should be handled and positioned with care to avoid exacerbating the condition.
- Radiology should include evaluation of possible underlying causes (fracture, hypoplasia, OAAM Spine: occipitoatlantoaxial malformation ).
- Two or more views (laterolateral and ventrodorsal) are required to fully evaluate lesions, particularly when surgical intervention is considered.
Cervical vertebral malformation (CVM) or Wobbler syndrome
- Most common cervical lesion in young horses Spine: cervical vertebral malformation (CVM) .
- Developmental disorder most commonly seen in young, fast growing animals.
- The condition results in spinal cord compression, and this may be static (present regardless of the position of the neck) or dynamic (only present during certain neck positions, usually flexion).
- Horses with dynamic compression may only show radiographic abnormalities on flexed or extended views during myelography Myelography .
- False positives on radiography have also been reported, particularly for the caudal cervical vertebrae, where bony remodeling can occur without causing spinal cord compression.
- Myelography Myelography is therefore important in confirming a diagnosis of CVM.
- Radiographic abnormalities of cervical stenotic myelopathy include:
- Abnormal sagittal diameter (see Section 2 - Radiographic technique and evaluation above) this is the most sensitive and specific abnormality.
- Degenerative joint disease of the synovial joints between the articular processes of the vertebral arches.
- Subluxation or malalignment of adjacent vertebrae.
- Flaring of the caudal epiphysis of the vertebral body.
- Abnormal ossification patterns.
- Caudal extension of the dorsal aspect of the vertebral arch.
- A scoring system assessing each of these parameters aids in the diagnosis of CVM.
- Abnormal sagittal diameter:
- Must be evaluated on a true lateral view (no obliquity).
- Sagittal ratio is more accurate than mean sagittal diameter.
- Single or multiple vertebrae may be involved.
- Caudal cervical vertebrae are most commonly involved (C4-C7).
- Caused by short vertebral pedicles (lateral bone of the vertebral foramen).
- Degenerative joint disease of the synovial joints between the articular processes of the vertebral arches:
- Most common radiographic abnormality.
- Clinical significance depends on any impingement on the spinal cord and associated nerves.
- Radiographic signs include:
- Loss of articular margins.
- Changes in joint shape and congruity.
- Peri-articular new bone formation.
- Areas of radio-lucency.
- Pathological fractures secondary to bony remodeling.
- Subluxation, malalignment or abnormal angulation of adjacent vertebrae:
- Most common site is C3-4.
- Normal S-Shaped bend in the neck should be considered.
- Abnormalities may be exacerbated on flexed lateral views.
- Clinical significance of lesions depends on the width of the vertebral canal and the degree of impingement.
- Flaring of the caudal epiphysis of the vertebral body:
- Usually occurs in horses <1 year of age.
- Radiographic changes are enlargement of the dorsal aspect of the caudal epiphysis (ski-jump appearance) → impingement on the vertebral canal.
- Abnormal ossification of the articular processes:
- Radiographic changes are consistent with osteochondrosis Bone: osteochondrosis .
- Caudal extension of the dorsal aspect of the vertebral arch:
- In the normal horse, the dorsal aspect of the vertebral arch should reach up to, but not beyond, the cranial epiphysis of the body of the adjacent vertebra.
- A line, perpendicular to the vertebral canal, drawn from the caudal aspect of the vertebral arch through the vertebral body, gives an objective assessment of this parameter.
Impingement of the dorsal spinous processes of the thoracic vertebrae (kissing spines or overriding dorsal spinous processes) Spine: spinous processes - overriding
- Radiographic signs may be seen in horses with no clinical signs (present in 30% of normal horses in one study).
- Radiographic changes may not reflect the degree of pathology (acute vs chronic injuries and involvement of soft tissues).
- Most commonly occurs between T10-18, may also occur in the lumbar vertebrae.
- Radiographic changes are:
- Narrowing of the space between adjacent dorsal spinous processes.
- Sclerosis of the cortical margins.
- Changes in shape.
- Areas of osteolysis and periosteal new bone formation on the dorsal spinous processes.
- A grading system has been described based on the severity of the radiographic changes:
- Grade 1: narrowing of the intraspinous space with mild sclerosis of the cortical margins of the processes.
- Grade 2: loss of the intraspinous space with moderate sclerosis of the cortical margins.
- Grade 3: severe sclerosis of the cortical margins, caused in part by transverse thickening of the spinous processes, +/- radiolucent areas.
- Grade 4: severe sclerosis of the cortical margins, osteolysis and changes in the shape of the spinous processes.
- The significance of radiographic changes should be confirmed by regional local anesthesia Anesthesia: local - overview or scintigraphy Bone: scintigraphy .
- Ultrasonography can be useful to evaluate associated soft tissue injuries Ultrasonography: musculoskeletal (lesions of the supraspinous ligament).
Bony pathology associated with soft tissue lesions of the thoracolumbar spine
- Bony remodeling may be seen with pathology of either the supraspinous or interspinous ligaments.
- The supraspinous ligament runs from T10/11 to L5/6 on the dorsal aspect of the dorsal spinous processes.
- Radiographic changes associated with pathology of this ligament include elevation of the periosteum, entheseous new bone formation and separation of a thin fragment (flake) of bone in this location.
- The interspinuous ligament attaches onto the cranial and caudal aspects of the dorsal spinous processes.
- Radiographic changes associated with pathology of this ligament are new bone formation in this location. Smooth irregular new bone may be seed on T1-10 as an incidental finding in normal horses. New bone formation caudal to T10 is usually (but not always) associated with pathology.
Degenerative joint disease (DJD) Musculoskeletal: osteoarthritis (joint disease)
- Common in the cervical region, especially C6-C7 .
- Also occurs in normal horses.
- Radiographic changes of cervical DJD are described under the CVM section above.
- Significance of radiographic findings should be interpreted in the light of physical and neurological examinations and the severity of the radiographic changes.
- DJD can also affect the intervertebral articulations of the thoracolumbar spine, most commonly at the thoracolumbar junction.
- Radiographic changes include peri-articular new bone formation, subchondral bone sclerosis, loss of the joint space and pathological fractures.
Osteomyelitis Spine: osteomyelitis
- Occurs in both foals or adults.
- Affects the vertebral bodies.
- Rhodococcus Rhodococcus equi is a possible cause in the foal.
- Tuberculosis Mycobacterium spp is a possible cause in the adult.
- Radiographic changes are consistent with osteomyelitis Spine: osteomyelitis : irregular areas of radiolucency, with new bone production and surrounding sclerotic regions.
- Fractures may occur as a result of trauma, or be pathological (associated with bony remodeling, osteomyelitis or neoplasia).
- They may present as acute or chronic injuries.
- Chronic healed fractures that originally occurred in foals can be difficult to differentiate from congenital abnormalities.
- Chronic lesions can develop neurological signs as a result of callus formation.
- Full radiological evaluation of vertebral fractures may be difficult as ventrodorsal views are often impossible in the adult, and additional diagnostic tests such as scintigraphy are often necessary.
- The thoracic vertebrae are a potential site for stress fractures in racehorses - scintigraphy is required for diagnosis in most cases.
- Clinical signs and prognosis of fractures depend on the impingement on the spinal cord.
- Several conditions will require further diagnostic tests, as indicated under the individual diseases in Section 4 above.
- Additional studies include:
- Repeat radiography at a later date, eg callus formation in fractures.
- Specific lesion-oriented views collimated down to highlight regions of interest.
- Myelography Myelography to evaluate spinal cord compression.
- Ultrasonography for soft tissue injuries Ultrasonography: musculoskeletal .
- Nuclear scintigraphy Bone: scintigraphy .