Thoracolumbar imbalance analysis for osteotomy planification using a new method: FBI technique

Abstract

Introduction: Treatment of spine imbalance by posterior osteotomy is a valuable technique. Several surgical techniques have been developed and proposed to redress the vertebral column in harmonious kyphosis in order to recreate correct sagittal alignment. Although surgical techniques proved to be adequate, preoperative planning still is mediocre. Multiple suggestions have been proposed, from cutting tracing paper to ingenious mathematical formulas and computerised models. The analysis of the pelvic parameters to try to recover the initial shape of the spine before the spine imbalance occurred is very important to avoid mistakes during the osteotomy planification.

Material and method: The authors proposed their method for the osteotomy planning paying attention to the pelvic, and spine parameters and in accordance with Roussouly's classification. The pre operative planning is based on a full-body X-ray including the spine from C1 to the femoral head and the first 10 cm of the femur shaft. Using all the balance parameters provided, a formula name FBI is proposed. Calculation of the osteotomy is basic goniometry, the midpoint of the C7 inferior plateau (point a) is transposed horizontally on the projected future C7 plumb line (point b) crossing posterior S1 plateau on a sagittal X-ray. These are the first two reference points. A third reference point is made on the anterior wall of the selected vertebra for osteotomy at mid height of the pedicle (point c) mainly L4 vertebra. These three points form a triangle with the tip being the third reference point. The angle represented by this triangle is the theoretical angle of the osteotomy. Two more angles should be measured and eventually added. The femur angulation measured as the inclination of the femoral axis to the vertical. And a third angle named the compensatory pelvic tilt to integrate the type of pelvis. If the pelvic tilt is between 15 and 25° or is higher than 25° you must add 5 or 10°, respectively. This compensatory tilt is based on a clinical analysis of operated patients.

Results: This planification was applied in a retrospective study of 18 patients and showed why in some cases improper correction was performed and prospectively in 8 cases with good clinical outcomes and correct spinal alignment. Sometimes it is necessary to find an acceptable compromise when rebalancing the spine paying attention to the general parameters of the patients like: age, osteoporosis, systemic disease etc.

Conclusion: This FBI technique can be used even for small lordosis restoration: it gave a good evaluation of the amount of correction needed and then the surgeon had the choice to use the appropriate technique to obtain a good balance.

Fig. 1

a Typical position with femur–knee flexion attempting to counteract kyphosis. b OST L4 corrects lack of lumbar lordosis but is insufficient because femur is still in flexion. c If femur is vertical the correction is insufficient. Pelvic incidence is equal for a, b and c

Fig. 2

4 types of spine according to Roussouly’s classification

Fig. 3

Posterior transpedicular osteotomy: subtraction osteotomy

Fig. 4

Symptomatic lumbar canal stenosis with back pain. Type 1 spine according to Roussouly’s classification. Low pelvic incidence: 43°, preop: PT: 18°, C7 plumb line in front of S1 plateau and just in front of femoral head, it is compensated balance, femur are straight: Oswestry: 54%. FBI pre OP: C7TA (8°) + PTC (5°) + FOA (0°) = 13° Post op: one TLIF with Smith Petersen L5/S1, decompression 3 levels and fusion. PT: 11°, C7 plumb line through S1 plateau and just behind femoral head, it is a well balanced spine, femur are straight; Oswestry 18% at 1 year follow-up

Fig. 5

Patient type 4 spine because incidence is high: 78° Left: Preoperative (EOS) X-ray showing typical attitude with sagittal imbalance without femur obliquity, failed back surgery due to insufficient correction with the ALIF L5S1 and 3 previous posterior surgery performed without respect of balance Incidence: 78°, SS 46°. Oswestry: 62%. FBI pre OP: C7TA (20°) + PTC (10°) + FOA (0°) = 30° Right: Postoperative (EOS) X-ray after one level PSO of 30° at L4 showed corrected sagittal balance. C7 plumb line is through the S1 plateau, lumbar lordosis is 80°, OSW: 16% at 6 months’ follow-up

Fig. 6

Patient no. 13. Left: Preoperative (EOS) X-ray showing typical attitude with sagittal imbalance and femur obliquity: PRE OP FBI calculation: C7TA (20°) + PT C (10°) + FOA (18°) = 48°. Right: Postoperative (EOS) X-ray after one PSO at L4 with corrected sagittal balance, C7 plumb line at the level of S1 plateau and but femur still have some degrees of remaining obliquity and PT still high at 30°: conclusion the balance is restored but not perfect. Applying the FBI technique post op shows that correction obtained is 36° so ideally 12° more correction would be better

Fig. 7

Preoperative planning. C7 translation angle: C7TA. Midpoint of C7 inferior plateau (a) is translated on the plumb line ascending from the mid part of the S1 plateau (b). Point c is on the anterior cortex of the selected vertebra for osteotomy, which is mainly L4 vertebra. Femur obliquity angle: FOA. Femur flexion is measured as the angle between the femoral axis and the plumb line (d). Pelvis compensation angle: PTA. Pelvic tilt is measured as usual: line between center femoral head to mid part of S1 plateau and vertical line. If PT between 15 and 25: add 5°. If PT superior 25° add 10°

Fig. 8

a and b are triangles with the same angle and thus the same quantity of OSTEOTOMY as explained in Fig. 6, it is clear that the translation with the Osteotomy is less important when performed at a higher level because for the same angle the C7 plumb line translation stays more anterior

Fig. 9

Ideal balance with C7 plumb line at the posterior part of S1 plateau