Thierry's spatulas are obstetric instruments used for assisted vaginal births, known for their propulsion and guidance mechanism that minimizes traction on the fetal head. Over time, multiple models with design variations have been commercialized under the same name. The biomechanical implications of these variations remain unclear. This study aims to compare different models of Thierry's spatulas to evaluate biomechanical differences.

This was a simulation-based, experimental, prospective, randomized, three-arm study. Six obstetricians with varying levels of experience performed 108 spatula-assisted births using a birthing simulator. Three different spatula models were evaluated. Traction force on the fetal head, perineal pressure, operative time, subjective difficulty, and hand pain were recorded.

Significant differences were observed in the force exerted on the fetal head, with the original model applying the least force (15 (8; 21.5) N, P < 0.001). Obstetricians with more than 15 years of experience applied significantly less traction force compared to those with less than 5 years of experience.

After adjusting for the spatula model and the obstetrician's experience using multiple linear regression, the force applied to the infant was found to be independently associated with both factors.

Thierry's spatulas models exhibit biomechanical differences, with the original design applying the least traction force. Experience influences instrument performance, highlighting the importance of proper training. Clinicians should be aware of model-specific differences when selecting Thierry's spatulas for operative births.