Optimizing Biomechanics of Surgical Correction for Lumbar Flatback Deformity

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Purpose: The long-term goal of this research is to improve the outcomes of surgical correction for lumbar flatback deformity, which is a complex disorder that can affect a large segment of the veteran population. In flatback deformity the torso pitches forward and affects the patient’s ability to stand upright and walk due to muscle fatigue and worsening back pain, leading to functional loss. The goal of surgery is to correct the sagittal malalignment to reduce muscle fatigue and allow patients to stand upright and ambulate, thus improving function. Surgeons who treat older veterans want to know: (1) how much lordosis restoration is sufficient to reduce the burden on muscles to maintain an upright posture; and (2) can this be achieved using a less invasive surgical technique to minimize complications and morbidity. Hypothesis: Our central hypothesis is that restoring lumbar lordosis using lordotic interbody fusion cages implanted in multiple disc spaces will minimize the forces needed to maintain an upright posture in specimens with loss of lordosis across L4 to sacrum. Specific aims: (1) We will use human cadaveric thoracolumbar (T10-sacrum) spine specimens with a range of PI values (35°-70°; the average adult PI is 52°) to quantify the shear force and moment required at the cephalad and caudal boundaries of the specimen to maintain an upright standing posture under the following conditions: (1) hypolordosis across L4-S1, creating a PI-LL mismatch (degenerative or iatrogenic), and (2) improvement of L1-S1 lordosis using lordotic interbody fusion cages placed at one or more segments of the lumbar spine, first without and then with posterior column (facet) osteotomies (PCO). (2) We will analyze the data collected in aim 1 to: (1) define a relationship between the extent of PI-LL mismatch and forces needed to maintain an upright standing posture for the reconstructed spines, and (2) quantify the influence of the following study variables on the ability of lordotic interbody cages to improve standing lumbar lordosis: (a) number of reconstructed disc spaces, (b) specimen’s PI value and flexibility (that is, segmental mobility in flexion and extension), (c) influence of PCO to yield additional correction. Methods: The experimental design consists of two arms: degenerative, and iatrogenic. In the iatrogenic arm, a hypolordotic fusion across L4-S1 will be created by applying distraction forces across pedicle screws inserted in L4, L5 and S1 pedicles; whereas, in the degenerative arm we will select specimens in which there is sufficient loss of disc heights at the L5-S1 and L4-L5 disc spaces. In the iatrogenic arm, surgical restoration of L1-S1 lordosis will be performed by implanting lordotic interbody cages at L3-L4 and L2-L3 disc spaces cephalad to the hypolordotic L4-S1 fusion; whereas, in the degenerative arm surgical restoration of segmental lordosis will be performed at L5-S1 and L4-L5 disc spaces. In both arms of the study, we will aim to eliminate the PI-LL mismatch (PI-LL ≤5°) by adding additional segments where lordotic interbody cages are implanted and, if necessary, performing posterior column (facet) osteotomies to augment the correction potential of interbody cages. In each state the specimens will undergo a battery of biomechanical tests to quantify the shear force and moment required at the cephalad and caudal boundaries of the specimen to maintain an upright posture. Statistical analysis will include repeated-measures ANOVA and multivariate regression analyses. Clinical Impact: The overall percentage of living Veterans expected to be affected by flatback deformity is anticipated to rise as the population ages and lives longer. Thus, the contribution of the proposed research to the knowledge of VA providers who care for our Veterans is likely significant. If the biomechanical data supports the use of segmental lordosis restoration using interbody fusion cages, the less invasive technique will add a valuable option for restoring lumbar lordosis in older patients. The research results are readily applicable for quick translation to veteran- and civilian patients if found to be beneficial.