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Skin, muscles, and around nerve roots ( SOFT TISSUE):

Healing happens by connective tissue growth (= fibrosis). The degree of this fibrosis differs from person to person, the nature of the surgery, and its site. 
The scar might be simple or raised and rough ( keloid).

Fibrosis around nerve roots is shown by red arrows.

Healing reaches 60% by the 4th-week post-operation. And reaches nearly 100% after the 3rd month.

Bone healing:

• Takes 1.5 ~ 2 years.
•  The following prolongs this process: smoking, sparkling drinks, and citrus juices.

Estimated Recovery time for simple spine surgeries ( disc surgery) is about 6 months. More complex surgeries ( fusion ) are around a year