Can Ankylosis of L5 Transverse Process to Sacral Ala Cause Radiating Leg Pain?
Yes, ankylosis of the left L5 transverse process with the left sacral ala can absolutely cause pain radiating down the leg—this is a recognized pain generator known as Bertolotti syndrome, which causes both axial back pain and radicular symptoms through pseudoarthrosis formation and potential L5 nerve root compression.
Understanding the Mechanism
The fusion or articulation between an enlarged L5 transverse process and the sacral ala creates two distinct pain mechanisms:
1. Pseudoarthrosis Pain
The abnormal joint (pseudoarthrosis) between the transverse process and sacrum becomes a direct source of pain that can radiate to the buttock and down the leg 1, 2, 3. This pain is typically:
- Deep rather than superficial
- Located at the zone of the abnormal articulation
- Radiating unilaterally to the buttock and lower extremity
- Intensity averaging 5.4 points on visual analog scale for leg pain 4
2. Nerve Root Compression
The enlarged transverse process can compress the L4 exiting nerve root or cause L5 nerve entrapment in the far-lateral zone (far-out syndrome) 3, 5, 6. This produces true radiculopathy with dermatomal distribution of pain.
Clinical Distinction from Other Causes
Important caveat: The guidelines distinguish this from nonspecific low back pain. While 7 define sciatica as "pain radiating down the leg below the knee in the distribution of the sciatic nerve, suggesting nerve root compromise," and radiculopathy as "dysfunction of a nerve root associated with pain, sensory impairment, weakness, or diminished deep tendon reflexes in a nerve root distribution," Bertolotti syndrome represents a specific structural cause rather than nonspecific low back pain.
The 8 plexopathy guidelines clarify that "pain that radiates in a dermatomal distribution with or without accompanying sensory loss or motor loss and reflecting a spinal nerve root innervation would be considered clinical evidence of radiculopathy"—which is exactly what can occur with this anatomic variant.
Diagnostic Confirmation
To confirm this finding as the pain generator:
- CT imaging demonstrates the bony ankylosis between L5 transverse process and sacral ala 1, 3
- SPECT-CT shows metabolically active uptake at the pseudoarthrotic joint, confirming it as the pain source 1
- Diagnostic nerve blocks into the pseudoarticulation or at the L4/L5 nerve root provide temporary pain relief, confirming the diagnosis 2, 3, 6
Treatment Implications
The condition responds to:
- Conservative management: Selective nerve root blocks with analgesics and glucocorticoids can provide relief lasting months to a year 4
- Surgical intervention: When conservative treatment fails, minimally invasive L5 transverse processectomy achieves satisfactory results in 86.89% of cases, with complete pain resolution in many patients 1, 2, 3
Key Clinical Pitfall
Do not dismiss this as nonspecific low back pain. While the 9 guidelines on axial spondyloarthritis discuss ankylosis in the context of inflammatory sacroiliitis, the ankylosis you describe is a congenital anatomic variant (lumbosacral transitional vertebra), not an inflammatory condition. This affects 4-8% of the general population and is a frequently missed cause of intractable back and leg pain 2.
The pain can be debilitating, with a sluggish, undulating course lasting months with periodic exacerbations triggered by spinal loading, trauma, excess weight, or pregnancy 4. The unilateral tethering can also encourage coronal deformity and early degenerative changes 2.