Lumbosacral Stabilization

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Lumbosacral Stabilization

Noel Fitzpatrick

26.1 Introduction

Degenerative lumbosacral stenosis (DLSS) is a common cause of caudal spinal pain in medium‐ and large‐breed dogs. It is seen less commonly in small dogs and cats. DLSS results in a narrowing of the vertebral canal and intervertebral foramina at the level of the lumbosacral junction. The narrowing results in compressive radiculopathy of the L7 nerve roots and the nerve roots of the cauda equina. Compression of the nerve roots is a multifactorial process. Direct compression of the nerve roots can be caused by prolapse of the annulus fibrosus, hypertrophy of the joint capsule of the L7‐S1 (LS) articular facets and hypertrophy of the interarcuate ligament. Osteophytosis / spondylosis at the level of the intervertebral foramena may also cause direct compression of the L7 nerve roots abaxially. The compressive process may also have a dynamic component whereby compression of the nerve roots is exacerbated or relieved depending on degree of flexion or extension at the LS junction. This is the result of further disc protrusion, motion of the spondylotic new bone on the caudal aspect of L7 and the cranial aspect of S1, ventral migration of the dorsal lamina of the sacrum relative to L7 and relative motion of the inflamed articular facets. It is well established that the intervertebral foramen undergoes narrowing when the LS junction is placed into extension [1, 2].

26.2 Clinical Examination

Though direct consequences of compression of the L7 and cauda equina nerve roots can be observed with neurogenic deficits producing paresis, muscle weakness and urinary issues, the most commonly encountered clinical signs are related to pain, both acute and chronic, and lameness, both profound and subtle. In the author’s experience, pain only or pain and lameness are often the only signs manifested. Because muscle spasms can be associated with persistent or intermittent claudication of the L7 nerve roots, the condition is commonly confused with other scenarios such as iliopsoas muscle strain‐sprain or sacroiliac pain.

In dogs affected by DLSS, pain may often be elicited during physical examination by application of direct pressure over the lumbo‐sacral junction dorsally, but one must be careful to neutralize the coxofemoral joints so as not to confuse hip pain. Simultaneous hyperextension of the LS spine may also produce a painful response. Along the sciatic nerve pathway, pain may be evoked on deep digital pressure application, either externally by placing the thumb in the caudal thigh recess between the biceps femoris and the semitendinosus‐membranosus (Holsworth Test) or per rectum using a gloved index finger where the sciatic nerve can be palpated on the axial aspect of the lesser ischiatic notch (Fitzpatrick Test). Clinical signs are caused by direct compression of the cauda equina and impingement of the ganglia of the spinal nerve roots as they exit through the intervertebral foramena [3].

26.3 Decision‐Making

26.3.1 Radiography

  • Plain and dynamic radiography has been evaluated in the diagnosis of LS disease. However, the author never undertakes surgery in cases affected by DLSS without advanced imaging [4].
  • The effect of intervertebral disc protrusion, spondylosis, and facet inflammation on the cauda equina and L7 nerve roots cannot be determined without cross‐sectional imaging.

26.3.2 CT Scan

  • The author prefers CT scan over MRI scan for evaluation of the osseous compressive elements of DLSS, especially in the neuroforamina on transverse planar imaging and evaluation of positional alignment of the dorsal lamina of the sacrum relative to L7 in neutral and hyperextended positions. CT scan can demonstrate a reduction in the volume of the intervertebral foramena of dogs abaxially (by bone and soft tissue) but unlike MRI, cannot evaluate the neuronal structures traveling through the intervertebral foramena [5].
  • CT scan is also a very important tool for patients undergoing instrumented surgical management of DLSS so that implant sizes can be estimated presurgically, thus facilitating operative planning.

26.3.3 MRI

  • MRI is the gold standard modality for evaluating the anatomic and dynamic features of DLSS. This modality allows accurate assessment of the neuronal structures and the soft tissue components of compression while also demonstrating osseous compressive elements.
  • Dynamic imaging sequences, whereby the dog is scanned in different degrees of extension of the LS junction, are very useful to fully understand how neural impingement changes as the dog moves. This is pertinent to the L7 nerve roots abaxially and the cauda equina when affected by protrusion, spondylosis, facet inflammation and ventral migration of the dorsal lamina of the sacrum relative to L7 (Figure 26.1a).
  • Traditional parasagittal imaging of the neuroforamena has been shown to underestimate neuroforaminal volume. Angled cross‐section imaging of the L7‐S1 neuroforamena using parasagittal oblique sequences allow a more clinically relevant understanding of the compression of the nerve root at the entry, middle, and exit zones, (Figure 26.1b). This imaging is a key tool in the decision‐making process of the author regarding stabilization of the L7‐S1 junction, and we have published this data recently [6].

Image described by caption.

Figure 26.1 Typical MRI findings in DLSS. T2 weighted (a–c) and STIR (d) MRI scans of a typical presentation of degenerative lumbosacral stenosis (DLSS). (a) Sagittal plane demonstrating marked compression of the cauda equina dorsal to L7‐S1 intervertebral space. Nucleus pulposus signal is reduced and irregular. (b) Transverse plane through L7‐S1 disc with dramatically reduced nucleus pulposus signal and obliteration of both the spinal canal and the L7 neuroforamina, which normally manifest nerve roots within fat signal. (c) Parasagittal plane manifesting hypointensity of lateralized disc protrusion within the neuroforamen when compared with more cranial segments. (d) Dorsal plane demonstrating near‐complete discontinuation of nerve root and fat signal at junction of conus medullaris and cauda equina.

26.4 Surgical Planning: Patient Positioning and Surgical Approach

The patient is positioned in ventral recumbency and supported in the midline with positioning aids to maintain patient stability. The hind limbs are drawn forward with the metatarsal bones parallel to the top of the surgical table to flex the lumbosacral spine and open the L7‐S1 space. A standard dorsal approach to the L7‐S1 junction is adopted [7] with dorsal laminectomy involving approximately one‐third of the cranial to caudal width of the L7 lamina and two‐thirds of the cranial extent of the sacral lamina. Meticulous hemostasis is important to maintain a clear surgical field during implant positioning, but this can be very challenging because frequently in chronic cases there is significant bleeding associated with chronic inflammation, and bleeding from the ventral venous sinuses can be considerable. Muscular dissection is extended cranially to allow access for screw placement on either side of the L7 vertebral body.

26.5 Surgical Techniques

26.5.1 Decompression Alone – Dorsal Laminectomy and Foraminotomy

Surgical management of LS disease involves either decompression alone, stabilization alone, or distraction‐stabilization, with dorsal facet and /or ventral vertebral body fusion being preferable for any stabilization technique. Decompression without surgical stabilization may provide immediate symptomatic relief for some patients, and this may be maintained indefinitely, although analysis of long‐term outcomes is required to validate this approach [8]. However, laminectomy alone can only be successful for central protrusion or extrusion, and this accounts for a very small proportion of clinical cases of DLSS in the author’s practice. Decompression via dorsal laminectomy alone does not address lateralized neuroforaminal impingement and it can be difficult to achieve adequate decompression of all of the entry, middle, and exit zones by foraminotomy techniques alone. The possibility for iatrogenic damage of the L7 nerve roots must also be considered and impairment of visibility produced by hemorrhage potentially compromises optimal removal of osseous and soft tissue compressive elements.

Dorsal laminectomy and dorsal disc annulectomy destabilize the L7‐S1 spinal segment and increase the range of motion in flexion and extension [911]. Extension of the mobile L7‐S1 junction that ensues may decrease the foraminal aperture and exacerbate the effect of foraminal stenosis on nerve root impingement [12], even when foraminotomy has been performed. Foraminotomy may be a successful intervention for lateralized L7 compression but fails to address the dynamic compressive effect of spinal motion on foraminal occlusion. Ongoing lumbosacral instability can accelerate the progression of lumbosacral disease and may exacerbate clinical signs associated with nerve root occlusion [1, 12]. Additionally, even if satisfactory decompression is achieved at the time of foraminotomy, bone and scar tissue may grow back such that longer‐term success of decompression might not be maintained.

26.5.2 Decompression + Dorsal Fixation Fixation with Facet Screws

Transarticular fixation of the facet joints with screws following decompression seeks to preserve relative vertebral position and to maintain foraminal aperture (Figure 26.2a), but it has been observed that such stabilization may not significantly alter long‐term outcomes versus decompression alone [13, 14]. Furthermore, the author has observed fracture of the L7‐S1 facets following screw stabilization after both dorsal laminectomy and foraminotomy, separately or combined. Additionally, facet screws do not result in distraction and therefore may not adequately resolve clinical signs because of residual static impingement.

Image described by caption.

Figure 26.2 Fixation modalities in lumbosacral fusion. (a) Facet screw fixation. (b) Noncustomized pedicle screw with fixed‐angle connecting rod and clamp. (c) Contoured SOP plate with fixed‐angle pedicle screws. (d) Multi‐angled pins secured with PMMA. Fixation with Screws + Plates or Screws /Pins + Cement

Stabilization of the L7‐S1 junction has been achieved using a variety of methods. Pins or screws and polymethylmethacrylate cement deployed either in neutral or flexed lumbo‐sacral positions have been deployed most commonly (Figure 26.2d). Custom locking plates have purportedly been employed but not yet reported. The string‐of‐pearls (SOP™, Orthomed, Huddersfield, UK) locking plate applied dorsally has been reported (Figure 26.2c) but found to have no biomechanical benefit over pins and PMMA cement [15]. It is challenging to effectively deploy fixed‐angle locking systems in the lumbosacral area as the anatomy makes it difficult to obtain sufficient bone stock and plates can be challenging to contour. Variable angle locking systems may be more suitable for use, however these systems often lack the angular freedom needed for successful deployment.

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Jun 13, 2021 | Posted by in SUGERY, ORTHOPEDICS & ANESTHESIA | Comments Off on Lumbosacral Stabilization
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