Study of the stress-strain state of the posterior lumbar fusion models in case of normal indicators of the sagittal balance of the spine and pelvis
Authors: Popsuyshapka K.O. , Kovernyk O.V. , Pidgayska O.O. , Karpinsky M.Yu. , Yaresko O.V. 
Published: September 2023

Background. Patients suffering from hip-spine syndrome with significant changes in the hip joint complain of pain in the lumbar spine in 21.2–49.4 % of cases. After performing lumbar fusion, the mobility of the pelvis decreases, which leads to an increased risk of dislocations and the development of impingement after hip arthroplasty that is the cause for repeated surgical interventions. Goal: to study the stress distribution in the models of posterior lumbar fusion in case of normal values of the sagittal contour of the spine and lumbar lordosis. Materials and methods. A finite-element model has been developed reflecting the condition that occurs in the combined course of degenerative diseases of the lumbar spine and hip joint and is characterized by normal lordosis of 40º and forward body tilt due to flexion contracture in the hip joints. The following options were modeled: 1 — posterior fusion of the L4-L5 vertebrae using a transpedicular structure with 4 screws and an interbody support; 2 — posterior fusion of the L3-L4-L5 vertebrae using a transpedicular construction with 6 screws; 3 — posterior fusion of L1-L5 vertebrae using a transpedicular structure with 10 screws. When conducting the research, the values of stresses in the Th1-L5 vertebrae, on the screws and rods of the transpedicular structure were studied. Results. Posterior fusion with a transpedicular construction on two L4-L5 vertebrae leads to the occurrence of maximum stresses in vertebral bodies of the lumbar spine, especially L4-L5. The lowest stresses in the lumbar vertebral bodies can be obtained when the transpedicular structure is applied to all 5 vertebrae. The use of all options for posterior fusion, except for the 4-screw scheme, allows to reduce the stress in the vertebral arches of the lumbar spine below the level of the normal spine model, except for the L1 vertebra. This leads to an increase in the level of stress from the Th6 to Th12 vertebrae. The construction placed on all 5 vertebrae ensures the lowest level of stress in the arches of thoracic vertebrae. The construction placed on all the vertebrae of the lumbar spine provides a minimum level of stress in the bone tissue around the fixing screws. Reducing the length of fixation leads to a significant increase in stress in these zones. With all types of installation of the transpedicular construction, the values of the stresses on the screws in the L3-L5 vertebrae are comparable. When using the design for 5 vertebrae of the lumbar spine, the locking screws in the L1 and L2 vertebrae will experience significant loads, which, accordingly, will cause significant stress in them. The maximum level of stress in the rods occurs when two L4-L5 vertebrae are instrumented, the minimum is when the structure is placed on all five vertebrae of the lumbar spine. Conclusions. Given the stress distribution, the length of fixation plays an important role: the longer the length of fixation, the lower the stress level, both in the bone elements of the model and in the elements of metal structures.