What are the reasons for longitudinal root fractures??

There are two complexes of risk factors:

1. Risk factors that can be controlled by dentists

Risk factors for longitudinal fractures that can generally be controlled by dentists concern fillings, crowns and dentures.

Tooth loss without dentures because the remaining teeth are subjected to higher loads,
not exactly adjusted occlusal surfaces on fillings, crowns and dentures
improper exposure to dentures
super hard functional surfaces made of ceramic or non-precious metal on dentures

Longitudinal root fractures were preceded by root canal treatment in around 90% of cases. Risk factors for root canal treatments and the subsequent treatment of root canal teeth:
(according to Tang W, et al as before)

unnecessary loss of substance and weakening of the tooth when opening the canal access.
inadequate exploration of the fine structures of the channel anatomy.
Ignorance, negligence and haste when preparing the root canal (type of instrument, degree of wear and tear of instruments, exact determination of the preparation length, precision and caution when guiding the instruments, avoiding unnecessary pressure on the canal wall, inadequate removal of drilling chips during preparation and much more.).
Collateral damage when flushing the root canal.
Collateral damage during root filling.
Tilting and jamming of all kinds of instruments in the root canal
many criteria for the selection of pens.
Precision and dimension of the pen preparation.
Material selection and procedure for gluing or cementing root posts.
Planning and precision of the implementation of abutments (anchoring, resilience, bond with the tooth structure).
insufficient coverage of the fragile root by the artificial crown.
Missing a necessary crown extension.

Failure to protect the tooth at risk of breakage in time with a crown.
inadequate static and caudal dynamic planning of the crown preparation.
lack of accuracy of fit of the crown on the edge and on the occlusal surface.
insufficient cleanliness, tightness and stability of the cementation.

2. Despite all care, uncontrollable risk factors remain:

Regionally different chewing and eating habits
Grinding teeth and clenching
accidents

How stable is a tooth after root canal treatment??

The stability depends on the size of the defect and the type of care after root canal treatment. Stress tests on removed teeth give the following values - provided that they are handled properly.