Introduction: The likelihood of mechanical failure of posterior fixation in the lumbar region of the spine is suspected to be higher when sagittal balance is not adequately restored [1]. Although failures at the proximal instrumented extremity are well documented, the lumbar distal junctional pathology has received less attention in literature [2] and it is unclear whether targeting spino-pelvic parameters is suitable to prevent failure.
This study aimed to investigate the mechanical failure of the caudal end of lumbar posterior fixation in terms of pre-operative and post-operative spinopelvic parameters, correction performed, demographics and clinical data.
Material and Methods: The only lumbar, short thoraco-lumbar, and long thoraco-lumbar posterior fixations performed with pedicle screws and rods between 2017 and 2019 at Rizzoli Orthopaedic Institute were retrospectively analysed. The revision surgeries were collected in the junctional group if the failure in the caudal end was caused by (i) pullout of the screws and/or (ii) breakage of rods or screws and/or (iii) vertebral fracture and/or (iv) degenerative disc disease. Posterior fixations which have not required a revision were collected in the control group. As 81% failures occurred within 4 years, an observational period of 4 years was chosen for both groups. The software Surgimap (Nemaris) was used to measure the main spino-pelvic parameters (sagittal vertical axis (SVA), pelvic incidence (PI), pelvic tilt (PT), sacral slope (SS), thoracic kyphosis (TK), lumbar lordosis (LL), PI-LL, T1 pelvic angle (TPA) and T1 spinopelvic inclination (T1SPi)) for each patient, on standing sagittal X-rays. Demographic and clinical data were extracted for both groups.
Results: 457 patients met the inclusion criteria. The junctional group included 101 patients, who required a revision surgery due to mechanical failure in the caudal end within 4 years. The control group collected 356 primary fixations which had not failed within 4 years. The two most common causes of revision surgery were screws pullout (57 cases) and rod breakage (53 cases). Patients older than 40 years had a high probability of developing distal junctional pathology, as showed by the multivariate logistic regression model. SVA, PT, LL, PI-LL and TPA differed significantly between the two groups (p=0.021 for LL, p<0.0001 for the other parameters, repeated-measures mixed effect models). PT, SS, LL, TK, PI-LL and TPA showed a significant correlation between the two groups and the pre-operative and post-operative conditions (p<0.005, repeated-measures mixed effect models). Both the short and long thoraco-lumbar fixation showed a higher probability of requiring a revision, compared to the only lumbar fixation. Sex, BMI and number of cages did not affect the risk of failure.
Conclusion: This study showed that mechanical failure is more likely to happen in patients over the age of 40 with a thoraco-lumbar fixation where PT, PI-LL and TPA were not properly restored. In addition, the SVA alone should be carefully evaluated in relation to the PT and TPA, so as to take into account all the pelvis compensatory mechanisms.