A 42-year-old woman from endemic TB region presents with subacute onset back pain, constitutional symptoms, and imaging findings consistent with spinal tuberculosis. Which is the most common initial site of vertebral involvement within the affected vertebra in Pott disease?

## Intra-vertebral Location of TB in Pott Disease **Key Point:** Tuberculosis of the spine characteristically begins in the **anterior vertebral body near the endplate** (metaphyseal region). This is the most common initial site of infection in ~90% of cases. ### Pathophysiology of Anterior Vertebral Body Involvement 1. **Hematogenous seeding** — tubercle bacilli reach the vertebra via the nutrient arteries that supply the metaphyseal region. 2. **Metaphyseal predilection** — the metaphysis has a rich, slow-flowing vascular network that favors bacterial lodgement and multiplication. 3. **Proximity to endplate** — the infection typically starts just beneath the vertebral endplate (subchondral region), in the cancellous bone. 4. **Anterior location** — the anterior vertebral body receives more blood supply than posterior elements, making it a preferential site for hematogenous seeding. ### Progression Pattern from Initial Site ```mermaid flowchart TD A[TB bacilli reach vertebra<br/>via nutrient arteries]:::outcome --> B[Seed in anterior metaphysis<br/>near endplate]:::action B --> C[Caseous necrosis<br/>and inflammation]:::action C --> D{Progression}:::decision D -->|Anteriorly| E[Anterior vertebral body<br/>destruction and collapse]:::action D -->|Posteriorly| F[Epidural abscess<br/>and cord compression]:::urgent D -->|Laterally| G[Paravertebral abscess<br/>formation]:::action E --> H[Kyphotic deformity<br/>and instability]:::outcome ``` ### Clinical Pearl **High-Yield:** The classic radiological finding is **anterior vertebral body erosion with preservation of the posterior vertebral body and neural arch** in early disease. This distinguishes TB from pyogenic spondylitis, which often affects the disc space first. ### Distinguishing Features: TB vs. Pyogenic Spondylitis | Feature | TB (Pott Disease) | Pyogenic Spondylitis | |---------|-------------------|---------------------| | **Initial site** | Anterior vertebral body (metaphysis) | Disc space (endplate) | | **Disc involvement** | Late (disc preserved early) | Early and prominent | | **Vertebral arch** | Spared initially | Often involved | | **Paravertebral abscess** | Common, cold abscess | Uncommon | | **Number of vertebrae** | Often multilevel (skip lesions) | Usually 2 adjacent vertebrae | ### Why Anterior Body? - **Vascular anatomy:** The anterior vertebral body metaphysis receives branches from the segmental arteries that enter through the anterior vertebral foramen. These vessels have a slow-flowing, low-pressure system ideal for bacterial seeding. - **Biomechanical stress:** The anterior vertebral body bears the most load, which may facilitate bacterial multiplication in areas of relative hypoxia. - **Endplate proximity:** The subchondral bone beneath the endplate is a common site for hematogenous seeding in many chronic infections (TB of long bones, TB of hip). **Warning:** Do not confuse the **initial site** (anterior metaphysis) with the **extent of disease** (which may eventually involve the entire vertebra, posterior elements, and adjacent structures). [cite:Robbins and Cotran Pathologic Basis of Disease 10e Ch 8]