Causes of back pain

This lecture given as a core knowledge module in year 4 of the NUS medical curriculum. It is a 1 and a half hour module given to a 75 strong group of students. It occurred to me that this module would be useful for the patient and generalist as well and allows one to approach pain management in a logical fashion.

The problem I have identified with the students is that while they are schooled to determine differential diagnoses in other specialties based on pathology (eg. epigastric pain secondary to peptic ulcer disease, pancreatitis, cancer, etc), somehow when they come to orthopedics their diagnostic mind-map changes and they confuse syndromes (eg. spinal stenosis) with diagnoses (eg. prolapsed intervetebral disc).

Hence I devised this lecture which essentially looks at back pain from the point of view of anatomical pain generators and pathology.

I will usually start with an actual spine case and get the students to be attuned to where they think the pain comes from. By doing this I am attempting to get them to be more analytical and interactive. After the first part and getting them to identify the flaws in their thinking process, the lecture proper begins. I will draw a segmental level through the spine and the differential diagnosis of back pain next to it. Having done this, I will call out the diagnosis and show where this pain is generated. The drawing stays on the board at the end of the lecture and I welcome insightful questions.

Typical symptoms and their anatomical basis

The first step in the process is breaking own the spine into a functional unit and then subdividing this into clinically relevant anatomical areas (Figure 1). Each of these areas are known to be responsible for a specific syndrome or pain complex.


Figure 1. Functional unit of the human spine. Each of these numbered structures are responsible for a specific type of pain.
  1. Nucleus pulposus
    • The jelly-like centre of the disc should remain in the disc. If the annulus fibrosus tears this substance makes its way to the surface causing irritation and pain on the way (a ruptured disc).
    • Pain is worse on flexion
  2. Annulus fibrosus
    • Tears of the annulus are themselves painful in the acute setting but by permitting the herniation of the nucleus pulposus, these tears are further aggravated. The herniated material is called a prolapsed (or colloquially 'slipped') disc.
    • Pain is worse on flexion
  3. Facet joint
    • Like any other joint this joint can become arthritic or subluxed
    • Pain is worse on extension
  4. Epidural veins
    • If the canal is already tight from spinal stenosis, walking causes these veins to engorge and cause neurogenic claudication
  5. Ligamentum flavum
    • When the spine becomes degenerate, this structure thickens making the space available narrow and worsening spinal stenosis
  6. Spinal cord
    • The spinal cord itself does not feel anything. Any injuries to it are manifested as paralyses or sensory problems.
  7. Pars inter-articularis
    • This structure can be painful if there is a crack.
  8. Body
    • This area is usually painful only with a crack or tumor.
  9. Nerve or nerve root
    • Injury or compression here causes sciatica.

Differential diagnosis of low back pain

By considering the structures affected by the respective diagnosis one can understand the symptom complex (eg. a prolapsed disc would be expected to cause disc pain and nerve pain but not neurogenic claudication as shown in Figure 2).


Figure 2. Having understood the anatomy of the area one can better understand the causes of pain and the diagnoses attributed to these.

1. Congenital

  • Spinal stenosis
    • Spondylolisthetic (1)
    • Spinal canal dysplasia (2)
  • Spondylolysis (3)

2. Traumatic (4)

3. Inflammatory

  • Infective (5)
  • Inflammatory
    • Ankylosing spondylitis (6)
    • Others (7)

4. Neoplastic

  • With cord compression (8)
  • Without cord compression (9)

5. Degenerative

  • Prolapsed intervertebral disc (10)
  • Spinal stenosis (11)
    • Facet joint osteoarthritis (12)
    • Ligamentum flavum hypertrophy (13)
    • Spondylolisthesis (14)
      • Subtypes

6. Differential diagnosis

  • Extraspinal disease
    • Neuritic pain (15)
    • Piriformis syndrome (15)