What is sacralization of the fifth lumbar vertebra (L5)?

What is Sacralization of L5?

Sacralization of L5 is a congenital spinal anomaly where the fifth lumbar vertebra (L5) becomes partially or completely fused to the sacrum, creating a transitional vertebra at the lumbosacral junction. 1

Anatomical Definition and Classification

Sacralization represents a transitional state at the lumbosacral junction where L5 exhibits structural characteristics of a sacral vertebra rather than maintaining its typical lumbar configuration. 1, 2

Types of Sacralization

• Complete bilateral sacralization: Both transverse processes of L5 are fully fused to the sacrum, occurring in approximately 4.8% of specimens 1
• Unilateral sacralization: Only one transverse process fuses to the sacrum (Castellvi type IIIa variant), occurring in approximately 1.2% of cases 1, 3
• Partial sacralization: Elongated transverse processes form pseudoarthroses with the sacrum without complete bony fusion 4

Structural and Biomechanical Characteristics

Morphological Changes

When L5 becomes sacralized, the resulting sacrum demonstrates several distinctive features compared to normal sacra:

• Increased overall sacral height and width when the fused L5 is included in measurements 1
• Significantly smaller dimensions of the original sacral segments below the fused L5 1
• Auricular surfaces positioned "low-down" spanning from mid-L5 to mid-S2 segments, rather than the typical S1-S3 distribution 1
• Narrower distance between zygapophyseal facets and reduced interauricular distance 1
• Attenuated facet joint areas at the affected level 1

Biomechanical Implications

Sacralization likely represents a structural compensation mechanism for reduced joint interfaces associated with smaller underlying sacra, creating altered load-bearing patterns at the lumbosacral region. 1

Clinical Significance

Prevalence and Association with Symptoms

Lumbosacral transitional vertebrae (LSTV), including sacralization, occur in over 12% of the general population. 2

The relationship between sacralization and low back pain remains debated, though several clinical patterns have emerged:

• Early disc degeneration may occur at the level above the transitional vertebra in younger patients 2
• Symptomatology similar to piriformis syndrome has been documented in cases of unilateral sacralization 3
• Age-related masking: Differences in degenerative changes between sacralized and normal spines become less apparent with aging as other degenerative processes predominate 2

Associated Anomalies

Sacralization may occur in conjunction with other congenital malformations:

• Sacral spina bifida occulta: Incomplete closure of the posterior vertebral arch can coexist with sacralization 4
• Other caudal malformations: Including sacral agenesis, anorectal malformations, and components of syndromes like Currarino triad 5

Critical Clinical Pitfall: Spinal Level Misconception

The most significant clinical implication of sacralization is the high risk of incorrect vertebral counting during surgical planning, which can lead to wrong-level surgery. 6

Frequency of Counting Errors

In surgical populations:

• 54% of patients with LSTV had their vertebrae miscounted at initial evaluation 6
• 15% of surgical plans required modification after recognizing the true spinal anatomy 6
• S1 lumbarization causes more frequent errors than L5 sacralization (28 vs. 10 miscounted cases) 6

Prevention Strategy

A whole-spine radiograph should be obtained before any lumbar spinal surgery to accurately count vertebrae and identify transitional anatomy. 6 This imaging should extend from the cervical spine through the sacrum to establish the true number of vertebral segments and prevent surgical level errors.

Management Considerations

Conservative Approach

For incidental sacralization discovered on imaging:

• Surgery is rarely indicated for asymptomatic sacralization 7
• Manual therapy, spinal manipulation, soft tissue therapies, and stretching exercises may provide partial symptom relief when back pain is present, though complete resolution may not occur 3

Surgical Indications

Surgical intervention should only be considered when specific criteria are met:

• Severe, progressive neurological symptoms with documented nerve compression 7
• Documented instability causing significant functional impairment 7
• Failure of comprehensive conservative management for at least 3-6 months 7

Imaging Recommendations

• MRI is the preferred modality for evaluating neural anatomy and pathologic lesions at the lumbosacral region 7
• CT scans are appropriate for evaluating bony anatomy and the extent of fusion 7
• Follow-up imaging is generally unnecessary unless significant clinical deterioration occurs 7