Kyphosis Of Thoracolumbar Spine Affect Sagittal Balance Of The Lumbar Spine: A Biomechanics Study And A Clinical Study

Objective:To investigate the changes of the Range of 3D-Motion of lumbar vertebrae secondary to the kyphosis of thoracolumbar spine and to explore the changes of the lumbar lordosis angle and its clinical significance secondary to the kyphosis of thoracolumbar spine.Methods:24 fresh porcine thoracolumbar spine(T10～L6) specimens were prepared for biomechanical testing. At harvest, muscle tissue was removed from the spines, but all intervertebral ligaments and facet capsules were left intact, all structures superior to the T12 end plate and inferior to the L6 end plate were excised and kept frozen in plastic bags at-20℃. At random, specimens were divided into 3 groups(8 in each group):the normal or intact group(groupⅠ); experiment group 1 (groupⅡ) whose kyphosis Cobb's angle was (36.57±5.71)degree; experiment group 2(groupⅢ) whose kyphosis Cobb's angle was (67.98±1.59)degree. In groupⅡ, thoracolumbar kyphosis was created on each T14 or L1 vertebral body by single-level osteotomy and lumbar lordosis was created on each L3 vertebral body by single-level bone grafting. In group III, thoracolumbar kyphosis was created on each T14 and L1 vertebral body by two-level osteotomy and lumbar lordosis was created on each L3 and L4 vertebral body by two-level bone grafting. The 2 end vertebrae (T12 and L6) was tightly fixed in the square polymethylmethacrylate casts when potted with the specimen in an upright neutral posture. A hollow cubic plastic maker about 1×1×1 cm3 was pinned onto the measured vertebra. Pure moments of flexion-extension, left-right axial torsion, and left-right lateral bending were applied to the top vertebra in a flexibility machine, and the ROM(range of motion) and NZ(neutral zone) of the L2-3 and L4-5 vertebral were recorded with a motion measurement system after preconditioning. There were 30 seconds of creep allowed at each load step, and 3 load-unload cycles were used to precondition the specimen and minimize viscoelastic effects. Stereophotographs were taken of the markers on the specimen, only on the third load cycle. A 3-dimensional laser scanning measurement system was used in the flexibility test to track segmental lumbar motion for each testing condition. During the entire tests, special care was taken to keep the specimens moist using a saline solution, and the testing of each specimen was completed within 6 hours. The measued results of ROM & NZ were statistically analyzed by means of one-way ANOVA and snk-test.At the same time,18 patients(13 men/5 women, mean age 31.7 years, range,7 to 59 years) and 22 volunteers(9 men/13 women, mean age 41.55 years, range,14 to 68 years) had the lateral X-ray films of the lumbar and thoracolumbar spine, which were studied by measuring Cobb's angle with E-ruler. And then the results were compared statistically between the normal group and the invalid group.Results:1. Biomechanical study:The average of ROM and NZ of L2-L3's flexion-extension in kyphosis group(groupⅡand groupⅢ) were larger than that in normal group(P<0.05), and the larger kyphosis Cobb's angle, the larger ROM arid NZ of L2～L3's flexion-extension. However it was found that the average of ROM and NZ of L2～L3's left-right axial torsion and left-right lateral bending in kyphosis group(group II and groupⅢ) were not larger than that in normal group(P>0.05). It wasn't found statistical difference that both ROM and NZ of L4～L5's moments of flexion-extension, left-right axial torsion, and left-right lateral bending among three groups(P>0.05). 2. Clinical study:The average of Cobb's angle of the lumbar spine in invalid group were larger than that in normal group(P<0.01), however it was found that the amount of increased lordosis in the higher part(L2～3,L3～4) of the lumbar spine was more than that in the lower part of the lumbar spine(L4～5andL5～S1). The Cobb's angle of segmental lordosis of L2-3 and L3～4 were increased 2.41 times and 1.50 times of that of the correspondent level of the normal spine respectively.Conclusions:Biomechanical study indicates that:over augmentation of the ROM and NZ of upper lumbar's flexion-extension are main compensations secondary to the kyphosis of the thoracolumbar spine, however, ROM and NZ of upper lumbar's left-right axial torsion and left-right lateral bending have no changes. Thoracolumbar kyphosis have no effect on the ROM and NZ of upper lumbar's flexion-extension, left-right axial torsion and left-right lateral bending.Clinical study indicates that:over lordosis of the lumbar spine are main changes secondary to the kyphosis of the thoracolumbar spine. These changes usually maintaine the balance of the whole spine in most conditions, whereas at the same time make great influences on the lumbar spine especially in its high part.