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Sagittal - R IV Foramina

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1. Multifidus: one of the largest epaxial muscle groups in the lumbar region. Divided into 11 individual portions that orginiate from the articular processes of the sacrum and mammillary processes of T12-L7. Can appear atrophied in dogs with chronic degenerative spinal disease or degenerative myelopathy.

2. Articular process: Synonym: facet. Interdigitating bony prominences that form the articular process joints between adjacent vertebrae. In the lumbar spine, these form the dorsal boundaries of the intervertebral foramina. In dogs that have nerve root compression within the intervertebral foramen, portions of the articular processes often have to be removed.

3. Lamina: forms the dorsal boundary of the vertebral canal. Has small notches on the cranial and caudal margins that form the boundaries of the interlaminar spaces and serve as the attachment sites for the yellow (interarcuate) ligaments. In dogs with idiopathic stenosis, the laminae may appear thickened and sclerotic.

4. Intervertebral vein: branch of the ventral vertebral venous plexus that exits the caudal portion of the intervertebral foramen and empties into the caudal vena cava. Can become congested in dogs with degenerative disc disease and contribute to compression of nerve roots in the intervertebral foramen. Believed to be an important factor in a syndrome called “intermittent claudication”, where lameness arises during exercise and resolves after rest.

5. L7 lamina

6. L7 caudal articular process

7. L6-7 joint space

8. Ventral longitudinal ligament

9. Intervertebral disc: Cartilaginous joint between endplates of adjacent vertebrae. Consists of concentric bands of fibrous tissue (annulus fibrosus) surrounding a central gelatinous portion (nucleus pulposus). Type II disc degeneration is one of the most common causes of nerve root compression in dogs with degenerative lumbosacral disease. With type II disc degeneration, the annulus bulges outwardly in all directions and bone spurs form on the vertebral endplates at the attachment sites for the annulus.

10. L7 nerve roots lead to the cranial gluteal, caudal gluteal, and sciatic peripheral nerves. The cranial and caudal gluteal peripheral nerves innervate the middle gluteal, deep gluteal, and tensor fascia lata muscles, and the superficial gluteal muscles. The sciatic peripheral nerve innervates the biceps femoris, semimembranosus, and semitendinosus muscles, as well as the common peroneal nerve (which innervates the peroneus longus, lateral digital extensor, long digital extensor, and cranial tibial muscles), and the tibial nerve (which innervates the gastrocnemius, popliteus, superficial digital flexor, and deep digital flexor muscles).

11. Body of sacrum: formed by the fusion of the S1-3 vertebral bodies. Ventral subluxation of the sacrum can contribute to compression of nerve roots in dogs with degenerative lumbosacral disease.

12. Sacroiliac joint: partly cartilaginous and partly synovial. Permits rotational and lateral movement of the pelvis relative to the sacrum. Can develop osteoarthritis and become a source of pain in the lower back region. Dogs with congenital lumbosacral vertebral anomalies can also have assymmetrical sacroiliac joints.

13. Sacrocaudalis ventralis lateralis

14. Transverse process: paired bony prominences that project laterally from the region where the pedicle joins the vertebral body. In the lumbar region, they project cranially and ventrally. Serve as attachment sites for paraspinal muscles. Ventral branches of the spinal nerves course ventral to these processes.

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