Facetectomy

The 50% Threshold: In minimally invasive procedures like percutaneous endoscopic transforaminal discectomy (PETD), resecting 50% or less of the SAP (whether lateral, medial, or ventral) results in minor increases in range of motion (ROM) and posterior element stress.
Complete Resection Risks:

Removing 100% of the SAP (complete resection) leads to severe segmental instability.
This can cause a >300% increase in ROM and a dramatic 800% increase in intervertebral disc stress during extension. This excessive load can lead to long-term risks such as accelerated disc degeneration and adjacent segment disease.

Flexion: Facetectomy is considered insignificant to stability during flexion as long as the posterior ligaments remain intact, as they are the primary stabilizers in this direction.
Extension: The facet joints are major load-bearing structures in extension, bearing 60–70% of the lumbar axial load. A unilateral complete facetectomy (100%), especially when combined with any resection on the opposite side, markedly alters rotational motion and flexibility.

Some research indicates that resection combinations less than or equal to 75% on both sides may result in only minor changes in flexibility (averaging 2.48%).
In these models, a bilateral complete facetectomy (100% on both sides) was necessary to induce a large change (26.54%) in flexibility.

The risk of instability may vary by spinal level; for instance, the L5-S1 segment may be less sensitive to SAP resection than the L4-L5 segment because it has a larger articular surface area and a more coronal orientation.