Cooperstein et al (42) had a palpator place radiopaque markers on the thoracic SPs palpated to correspond with the inferior tip of the scapula, and also the 2 SPs judged to be 3 spinal levels inferior and superior.
According to the strict definition, a palpator would place a soft-tissue marker within the 8mm wide field of the C1 TVP; whereas according to the more clinically relevant expansive definition, the palpator would place the marker closer to the C1 TVP than to either the mastoid process above or the C2 TVP below.
Only when the palpator error exceeded 4.0mm (42.9% of cases) could it be inferred there had been any palpatory error, since the C1 TVP is a surface with a vertical height rather than constituting a point.
Finding S2 in this manner, a palpator would then identify the most immediately cephalad sacral tubercle as S1, and then an equal distance cephalad to this would be the soft depression corresponding to the hypoplastic L5 spinous process, followed by the L4 spinous process (which will likely be caudal to the palpated iliac crests), followed by the L3 spinous process, and so on.
Since the authors of two palpation studies (17,36) stated that their results might have been biased since the palpators were aware of prior studies that consistently showed the spinal site of the palpated TL cephalad to the imaged TL, we attempted meta-analysis with and without these studies included.
Previous studies have shown low accuracy for palpators in numerating lumbar levels and the mistakes tended to be cephalad (7,13-17).