The preferred type of pin used in teeth that have undergone root canal treatment is Passive cemented pins. This is because root canal-treated teeth are more brittle and susceptible to fracture, so pins that do not exert additional mechanical stress on dentin are ideal. Passive cemented pins rely on cement retention without active threading or self-tapping, minimizing the risk of root or dentin damage. In contrast, threaded or self-tapping pins can induce microcracks or stress concentrations, leading to root fracture. Therefore, passive pins provide adequate retention while preserving tooth integrity in endodontically treated teeth.

Reference: Sturdevant's Art and Science of Operative Dentistry, 7th Edition.