C7-T1 ventral interbody fusions: Opportunities, nuances and expectations
John Janicek
Brazos Valley Equine Hospitals-Salado, Salado, Texas, USA
Correspondence: John Janicek
Email: john.janicek@bveh.com
Santos et al. (2023) describes an interesting case report of
discospondylitis at the level of C7-T1 in a young horse that was managed
surgically. This is the first report of surgical treatment for
discospondylitis at this level in the horse and the authors should be
commended for their efforts and surgical modifications to provide a
successful outcome to the patient. Caudal cervical vertebrae are
becoming a more recognized source of pathological conditions that
clinically result in pain, lameness, ataxia, inferior performance or any
combination of these clinical signs (Dyson, 2011).
The etiology of discospondylitis in horses remains unclear, but
infection and trauma are often cited as the inciting cause (Moore 1992).
Horses affected with discospondylitis can develop degradation of the
intervertebral disc, collapse of the intervertebral space, dorsal
protrusion of the intervertebral disc into the spinal canal,
intervertebral foramen narrowing, and/or endplate osteolysis of the
affected vertebrate (Figure 1).
Clinical signs of discospondylitis include cervical spine pain, stiff
gait, and varying levels of ataxia that can originate from intertwined
pathological processes. Horses with caudal cervical pathologic lesions
often have uni- or bilateral atrophy of pectoral muscles, atrophy of
muscles where the neck and shoulder muscles tie-in together, and
antebrachial muscle atrophy. Interestingly in this case, cervical spine
and forelimb stiffness were present without any ataxia. Cervical spine
pain results in limited cervical spine range of motion and consequently
a stiff gait. The pain can be severe enough that a facial grimace and/or
“wide-eyed” appearance can be appreciated when the horse moves its
head and cervical spine. Cervical spine pain can be attributable to
instability caused by vertebral endplate osteolysis, and loss of
intervertebral disc and collapse of the intervertebral space; pain can
also be from vertebral endplate cartilage degradation resulting in
subchondral bone and nerve exposure. A stiff gait associated with
discospondylitis not only comes from cervical spine pain but may also
originate from caudal nerve root neuritis and/or nerve root compression.
Caudal cervical vertebral spine (C6-T1) instability secondary to
pathological changes associated with discospondylitis may result in
intervertebral foramen narrowing causing nerve root compression and a
stiff forelimb gait. Ataxia that occurs in cases with discospondylitis
can be multifactorial. One explanation is the instability caused by
vertebral endplate osteolysis and loss of intervertebral disc causing
varying degrees of vertebral kyphosis resulting in cervical vertebral
stenotic myelopathy. A second explanation of ataxia associated with
discospondylitis would be dorsal protrusion of the intervertebral disc
into the spinal canal because of collapse of the intervertebral space.
Horses with C7-T1 pathological lesions can be separated into two
different classes. Class 1 patients present with a varying degree of
neurological deficits that originate from dorsal and/or lateral spinal
cord compression. Class 2 patients usually present with moderate to
severe forelimb lameness that cannot be eliminated with forelimb
peri-neural anesthesia or forelimb intra-articular anesthesia; these
cases are often associated with intervertebral foramen stenosis of the
caudal cervical spine or some level of intervertebral disc disease
(Ricardi and Dyson, 1993).
Imaging the C7-T1 region can be a challenge as this area is covered by
heavy muscle and the scapulohumeral joint often superimposes this
region. Diagnostic modalities used to assess the caudal cervical spine
include cerebral spinal fluid analysis, standing survey radiographs,
nuclear scintigraphy, ultrasonography, cervical myelography, computed
tomography (CT) and magnetic resonance imaging (MRI). In this case
report, serial complete blood cell counts, and serum amyloid A
measurements were used to help decide that an infectious process was
under control. Cerebral spinal fluid culture and sensitivity along with
cytology could have been viable options to help assess the presence of
infection as well. Fan beamed CT myelography was performed in this case
providing great detail about structures at C7-T1 which included
collapsed intervertebral disc space, irregular shaped regions of lysis
on vertebral endplates, ventral periosteal proliferation on vertebral
endplates, and mild to moderate spinal cord compression. Multiplanar CT
reconstruction and three-dimensional imaging using fan-beam CT
technology is consistently attainable at the C7-T1 level with the horse
under general anesthesia allowing for thorough examination of dorsal and
lateral contrast dye columns, assessment of articular facet sizes,
shapes, and associated pathologic lesions, permits for determining
intervertebral foramen stenosis, and detection of disc degeneration. CT
myelography should be considered the current gold standard for caudal
cervical spine diagnostic imaging and pre-surgical planning.
Surgical fusion of the equine cervical spine was first reported by
Wagner et al. in 1979 using the Bagby basket which has been modified
over the past 40 years; currently a titanium fully threaded, or
partially threaded Kerf-Cut Cylinder packed with cancellous bone graft
is available for ventral interbody fusion and globally remains the
mainstay for spinal fusions. Vertebral fusion using a locking
compression plate for cervical vertebral stenotic myelopathy was first
reported in a 3-month-old Warmblood filly (Reardon et al., 2009), which
was followed by a biomechanical study comparing the locking compression
plate to the Kerf-Cut Cylinder (Reardon et al., 2010). Results of that
study indicated that the locking compression plate had higher
biomechanical properties than the Kerf-Cut Cylinder; however, in order
to obtain complete vertebral arthrodesis, a large portion of the
intravertebral disc needs to be removed, which is difficult when
applying a locking compression plate. The use of a polyaxial pedicle
screw and rod construct was first was reported in a proof-of-concept
study (Aldrich et al., 2017), followed by a retrospective study using
the polyaxial pedicle screw and rod construct that included 10 horses
over a 4-year period (Pezzanite et al., 2022). In that study, two horses
were euthanized within the first year. In 6 of 8 horses with ≥1-year
follow-up, ataxia improved by 1-3 grades, with an average improvement of
1.25 grades. In four horses, ataxia improved to grade 0-1. In two horses
the gait was unaffected, but neck comfort improved (Pezzanite et al.,
2022). Recently, an implant system consisting of a 3-D printed plate,
cancellous screws and titanium cage was designed from a 3-D rendering of
the equine cervical spine to contour along the ventral aspect of the
cervical spine. Clinical outcomes of this cervical spine implant system
are favorable (Rossignol, 2022). Excluding the Kerf-Cut Cylinder, all
aforementioned techniques have substantial limitations to arthrodese
C7-T1. Ventral interbody fusion of the C7-T1 articulation using the
Kerf-Cut Cylinder and specialized long-handled instrumentation has been
performed for the past 10 years and is indicated when dorsal dye column
compression is present on myelography and/or CT myelography, lateral
spinal cord compression is present on CT myelography, intervertebral
foramen stenosis is present on CT myelography, or when intervertebral
disc disease is detected on CT myelography. Ventral interbody fusion of
C7-T1 can be successfully performed in mature equine patients and
results of an unpublished study by Grant et al. (2023) suggest that
C7-T1 ventral interbody fusion has a good prognosis to improve comfort
level, resolve lameness, and/or reduce the neurological grade that is
safe for riding or provides a good quality of life (Figure 2).
The success of C7-T1 vertebral arthrodesis in this case using a 4-hole
locking compression plate along with two transvertebral 5.5 mm cortical
bone screws placed in place in lag fashion can be contributed to the
light weight of the horse (weight was not reported, but a 3/9 body
condition is reported) and the lack of an intervertebral disc, and
intervertebral collapse. Although not reported in this case report, the
use of cancellous bone grafting could have been used within the
intravertebral disc space to help form a more solid C7-T1 arthrodesis.
Access to the C7-T1 region requires retraction and securing the
forelimbs caudally before the area is aseptically prepared and draped.
The surgical approach to C7-T1 is no different than when performing
ventral interbody fusion in the remaining cervical regions, but there
are some noteworthy anatomical variables. Gaining access to and
visualization of the C7-T1 area is a challenge because this site is
deeper and partially obstructed by the cranial sternum. The truncus
bicaroticus is the common trunk of the left and right common carotid
arteries. The truncus bicaroticus lies dorsal to the caudal deep
cervical lymph nodes and the bifurcation typically occurs directly over
the C7-T1 articulation. The surgeon should be prepared for variable
positions and length of the truncus bicaroticus. Careful dissection is
necessary to allow for the common carotid trunk to be retracted from the
drill site without damaging the vagosympathetic trunk. C7 does not have
a ventral spinous process but does have two small chevron-shaped bony
tubercles on the ventral midline in the mid-body area. High quality
intra-operative radiographs are necessary during surgery for measurement
purposes and proper implant placement at C7-T1.
The only reported complication after surgery in the report by Santos et
al. (2023) was the development of Horner’s syndrome. This complication
most likely arose from applying too much pressure on the right
vagosympathetic trunk. An anesthetic recovery complication associated
with vagosympathetic trunk damage includes laryngeal spasm or right
recurrent laryngeal nerve paralysis. This complication most likely
occurs from inadvertent pressure on the recurrent laryngeal nerve during
retraction for instrumentation at C7-T1. If the left recurrent laryngeal
nerve was dysfunctional before surgery, then laryngeal spasm or complete
laryngeal collapse can occur and be fatal. This complication can be
reduced by performing a pre-operative endoscopic exam; if the left
arytenoid is not abducting properly, then left recurrent laryngeal
neuropathy is assumed and the surgical approach should be performed on
the left side of the trachea.
It is important for veterinarians to be aware that diagnostics and
treatment of varying pathological processes at the C7-T1 level is
possible with a good prognosis. It is very important to assess C7-T1
when examining the cervical spine in horses. The advancements of CT
myelography can help pre-operative planning and prognosticate for the
owners. The Kerf Cut Cylinder continues to be the most common implant
used for cervical spine fusion; however, innovative surgical options and
new implant systems are imperative to continue to improve the area of
cervical spine surgery. The current case report by Santos et al. (2023)
highlights a unique way to perform ventral interbody fusion for
discospondylitis at C7-T1 and adds another tool in our toolbox for
cervical spine orthopedic surgery.