Clinical presentation
50 year old woman with a chronic history of pain in the lower back. A bone SPECT-CT of the lumbar spine was performed.
Key Findings
- Lumbosacral transitional vertebra (LSTV) with fused right transverse process of L5 with the sacrum.
- Low grade uptake in the Castellvi 2a LSTV.
- Intense increased uptake in the left L4-5 facet joint with only mild subchondral sclerosis and osteophyte formation on CT, indicating osteoarthritis.
Diagnosis
Castellvi IIa lumbosacral transitional vertebra with biomechanical stability at the pseudoarthrosis but adjacent segment arthritis in the facet joint.
Lumbosacral transitional vertebrae (LSTV)
Lumbosacral transitional vertebrae are a relatively common normal variant are seen in approximately 25% of the population.
They can either represent lumbarisation of a sacral vertebra or sacralisation of a lumbar vertebra. There are varying degrees of fusion from partial fibrous fusion to complete osseous fusion.
When counting and labelling the lumbar vertebrae it is important to identify the iliolumbar ligament. This is a band of tissue which extends from the tip of the transverse process to the posterior iliac wing or crest and can easily be seen on MRI and CT. The most superior vertebra from which this ligament is seen is regarded as L5.
The Castellvi classification is often used to differentiate different forms of transitional pseudoarthrosis.
- Type 2 prone to pseudoarticulation
- Type 2a and 3a prone to contralateral facet joint degeneration
- Type 3 and 4 usually result in adjacent segment arthritis
In this case, the partial fusion on the right side of the pseudoarthrosis has resulted in increased stress on the spine on the contralateral L4-5 facet joint. The increased uptake in the facet joint is due to increased bone turnover following chronic stress and osteoarthritis. It is likely that the facet joint is the pain generator in this case, rather than the pseudoarthrosis.
A diagnostic facet joint injection or nerve block can be performed to confirm the cause of pain and as treatment for the facet joint arthritis.
