To evaluate the influence of different surgical treatment options for kyphoscoliosis we measure the movement of the SI joint with Radiostereometric analysis (RSA) after lumbar fixation with CT motion analysis (CTMA).

CTMA is a high precision in vivo measuring method for motion. We intend to measure sacroiliac joint (SIJ) movement in long fixations compared to short fixations. The Center for Implant and Radiostereometric Research Oslo (CIRRO) has extensive experience with RSA and established state of the art measuring method of movement in the SIJ. We have compared the methode with CTMA and established a methode for CTMA measurements at CIRRO. We will measure the movement in the SIJ in patients that have long fixations of 3 or more segments down to S1 and compare it to one segment fixation in the level of L4/5. Null hypothesis

• The SI movement will not increase because of long fixation down to sacrum, the SIJ will not work as an adjacent level and not compensatory increase the movement.
• The increased movement will not result in low back pain and /or pseudo radicular pain to the lower limbs CTMA is a high accuracy measuring method comparable to RSA. With a precision of RSA of 1 degree for rotation we need 10 patients in each group to detect a difference of 2 degrees for power of 90% on an alpha = 99% confidence level. Taking drop-outs into account we need a group size of 15 resulting in 30 patients (Two- sided T-test).

Inclusion Criteria:

• Long fixations of 3 or more segments down to S1, (also including not correction surgery patients).
• One segment fixations in the level of L4/5 regardless of olisthesis.

Exclusion Criteria:

• Medical conditions that affect the SI joint (Sacroileitis, Ankylosing spondylitis, psoriasis, autoimmune illness)
• Do not want to participate and not able to answer PROMs