Surface Anatomy and Applied Back Anatomy

External occipital protuberance: palpable midline posterior skull prominence, the origin point of the ligamentum nuchae/trapezius.

• C7 spinous process (vertebra prominens); the most prominent, easily palpable cervical spinous process, the standard starting point for counting vertebral levels by palpation.
• Thoracic spinous processes: palpable but, given their steep downward angulation, correspond to a vertebral body level one to two segments below the palpated spinous process tip in the mid-thoracic region.
• Inferior angle of scapula: palpable, corresponds approximately to the T7 spinous process/vertebral level in the standard anatomical (arms-at-sides) position, a useful thoracic-level landmark, though its precise correspondence varies somewhat with arm position and individual scapular anatomy.
• Iliac crest: palpable, its highest point (the intercristal/supracristal line) corresponds approximately to the L4 vertebral level, the key lumbar puncture landmark.
• Posterior superior iliac spine (PSIS): palpable, often marked by a visible skin dimple, corresponds approximately to the S2 vertebral level, and to the sacroiliac joint's midpoint.
• Sacral hiatus: palpable as a small midline depression at the inferior sacrum, just above the coccyx, the caudal epidural anaesthesia access point.
• Coccyx: palpable at the inferior termination of the vertebral column.

• C7 (vertebra prominens): cervicothoracic junction landmark.
• T3 (scapular spine): the scapular spine's medial end corresponds approximately to the T3 spinous process level in the neutral anatomical position.
• T7 (inferior angle of scapula).
• L4 (iliac crest line): the intercristal line, the essential lumbar puncture landmark.

• Trapezius: readily palpable as the most superficial back muscle, particularly its upper fibres along the neck/shoulder line and its lower fibres converging toward the thoracic spinous processes.
• Erector spinae: palpable as a firm, longitudinal muscular mass on either side of the spinous processes, particularly prominent in the lumbar region, becoming more tense/prominent on active spinal extension against resistance.
• Latissimus dorsi: palpable along the posterior axillary fold and its broad lower back origin, particularly prominent on resisted shoulder extension/adduction.

Intervertebral disc prolapse:

• Most commonly posterolateral, most frequent at L4-L5/L5-S1 (lumbar) given the shear/load factors described throughout this content; cervical disc prolapse follows an analogous posterolateral pattern, most frequent at C5-C6 and C6-C7 (the cervical spine's most mobile lower segments), affecting the C6 and C7 nerve roots respectively.

Cervical radiculopathy:

• Nerve root compression in the cervical spine, most commonly from posterolateral disc herniation or uncovertebral joint osteophyte formation narrowing the intervertebral foramen, C6 radiculopathy (biceps/brachioradialis weakness and reflex loss, thumb/lateral forearm sensory loss) and C7 radiculopathy (triceps weakness and reflex loss, middle finger sensory loss) are the two most commonly encountered patterns, directly reflecting the C5-C6/C6-C7 disc levels' predisposition to herniation.

Lumbar radiculopathy:

• L5 and S1 radiculopathy are the two dominant patterns, directly reflecting the L4-L5/L5-S1 disc herniation predilection.

Spinal canal stenosis:

• An analogous process to lumbar stenosis with neurogenic claudication can occur in the cervical spine (cervical spondylotic myelopathy, from chronic canal narrowing directly compressing the spinal cord itself rather than an individual nerve root, given the relatively narrower cervical canal margin for the cord), producing a combined upper and lower motor neuron pattern reflecting cord rather than isolated root compression.

Scoliosis:

• An abnormal lateral curvature of the spine, often combined with vertebral rotation (giving the characteristic rib prominence seen on forward bending, the "rib hump," used in clinical screening), most commonly idiopathic (particularly adolescent idiopathic scoliosis, the most common form overall), though congenital (vertebral malformation), neuromuscular (e.g., secondary to cerebral palsy or muscular dystrophy, reflecting asymmetric muscular control of spinal posture), and degenerative causes also occur.

Kyphosis:

• Exaggeration of the normal thoracic primary curvature (excessive anterior concavity/posterior convexity), causes include postural kyphosis (correctable, muscular in origin), Scheuermann's disease (a structural, vertebral wedging-related adolescent condition), and age-related osteoporotic vertebral compression fractures, the latter being the most common cause of significant kyphosis in older adults.

Lordosis:

• Exaggeration of a normal secondary curvature (cervical or, more commonly discussed clinically, lumbar (excessive anterior convexity)) lumbar hyperlordosis is commonly associated with weak abdominal/gluteal musculature, tight hip flexors, obesity, or pregnancy (reflecting altered pelvic tilt and the compensatory postural adjustment this produces), and can contribute to mechanical low back pain via altered facet joint loading patterns.

Vertebral compression fracture:

• Anterior vertebral body height loss, typically stable (posterior elements intact), commonly osteoporotic.

Burst fracture:

• Involves both anterior and posterior vertebral body cortex, canal fragment retropulsion risk, potentially unstable.

Spondylolysis:

• Pars interarticularis stress fracture, classically L5, athletic/adolescent association.

Spondylolisthesis:

• Anterior vertebral slippage, most commonly secondary to bilateral spondylolysis (younger patients) or degenerative facet disease (older patients), predominantly at L5-S1/L4-L5.

Spinal cord compression:

• Deficits below the compression level, pattern depending on the specific cross-sectional structures involved.

Cauda equina syndrome:

• Bilateral leg involvement, saddle anaesthesia, bladder/bowel dysfunction, a surgical emergency.

Epidural abscess:

• Infection within the epidural space, risk of progressive cord/cauda equina compression if untreated.

Epidural haematoma:

• Bleeding within the epidural space (internal vertebral venous plexus-related), a similar compressive emergency to epidural abscess.

Lumbar puncture:

Epidural anaesthesia:

• Epidural space injection without dural puncture, suited to continuous catheter-based techniques.

Spinal anaesthesia:

• Subarachnoid space injection, dense/rapid-onset block.