Compensatory mechanisms contributing to keep the sagittal balance of the spine

Abstract

Introduction: Aging spine is characterized by facet joints arthritis, degenerative disc disease, bone remodeling and atrophy of extensor muscles resulting in a progressive kyphosis of the lumbar spine.

Objective: The aim of this paper is to describe the different compensatory mechanisms for patients with severe degenerative lumbar spine.

Material and methods: According to the severity of the imbalance, three stages are observed: balanced, balanced with compensatory mechanisms and imbalanced. For the two last stages, the compensatory mechanisms permit to limit the consequences of loss of lumbar lordosis on global sagittal alignment and therefore contribute to keep the sagittal balance of the spine.

Results: The basic concept is to extend adjacent segments of the kyphotic spine allowing for compensation of the sagittal unbalance but potentially inducing adverse effects.

Conclusion: Finally, we propose a three-step algorithm to analyze the global balance status and take into consideration the presence of the compensatory mechanisms in the spinal, pelvic and lower limb areas.

Fig. 1

Evaluation of global sagittal alignment using the spino-sacral angle (a) and the C7/SFD ratio (b). The SSA is defined as the angle between the sacral plate and the line connecting the centroid of C7 vertebral body and the midpoint of the sacral plate [23]. Sacro-femoral distance (SFD) is the horizontal distance between the vertical bicoxo-femoral axis and the vertical line passing through the posterior corner of the sacrum. The horizontal distance between C7 PL and the posterior corner of the sacrum (that is SC7 D) was also measured. Then we calculated the C7/SFD ratio corresponding to the ratio between SC7 distance and SF distance [1]

Fig. 2

Classification of global sagittal alignment in three stages with respect to the severity of the imbalance. In stage 3 (unbalanced) C7PL/SFD ratio is superior to 0.5 signifying that C7 plumbline lies closer to femoral heads than to sacral plate

Fig. 3

Sagittal imbalance and the different compensatory mechanisms in the spine, pelvis and lower limb areas

Fig. 4

Patient with lumbar kyphosis and severe multilevel stenosis from L2–L3 to L4–L5: full spine radiographs (a), sagittal T2-weighted (b) and transverse T2-weighted (c) MRI sequences. The patient is still balanced (C7PL/SFD is 0.25) but balance is compensated by three main mechanisms: pelvis back tilt (curved arrow), multilevel retrolisthesis (red circle) and reduction of thoracic kyphosis (calculated to 25°). PI was measured to 47°, PT was 34° and SS was 13°. Compared to group control from normal and asymptomatic population, we should expect value of PT around 10°. On MRI axial slices, retrolisthesis at L3–L4 and L4–L5 are associated with fluid collection in facet joints (straight arrows)

Fig. 5

Patient with lumbar stenosis from L2–L3 to L4–L5 and thoraco-lumbar kyphosis: full spine radiographs (a), X-rays focused on lumbo-pelvic zone (b) and sagittal T2-weighted MRI sequence (c). The patient is well balanced (C7PL/SFD is −0.3) however some compensatory mechanisms are present in the lumbar area. Hyperextension is observed at L5–S1 (curved black arrow) (local lordosis was measured to 24°) and there are multilevel retrolisthesis at L2–L3 (red circle) and L4–L5 (large arrow). The pelvis tilt was quite normal as it was calculated to 22° and the PI to 46°

Fig. 6

Classification of degenerative disc diseases into aging discopathy and compensatory discopathy

Fig. 7

Pelvis back tilt mechanism. Increase of pelvis tilt results in posterior placement of sacrum related to the coxo-femoral heads thus increasing the sacro-femoral distance (red line)

Fig. 8

Illustrative case with lumbar kyphosis and compensatory knee flessum. After surgical correction of the kyphosis (TPO procedure at L4) the knee flessum significantly reduced postoperatively suggesting the improvement of the global sagittal balance. Return to a more physiologic thoracic curve was also observed