Sagittal radiographic parameters in the presence of lumbosacral transitional vertebra (LSTV): relationships between measurements using the upper vs lower transitional vertebra.

In patients with LSTV, it is controversial whether the UTV or LTV should be used as the sacral endplate for sagittal measurements. With this mathematical relationship, rather than completing two sets of measurements, the surgeon would only need to measure one set and the additional angle X to determine the resultant PI-LL mismatch for the other transitional vertebra.