Published online May 18, 2021. doi: 10.5312/wjo.v12.i5.310
Peer-review started: January 10, 2021
First decision: February 15, 2021
Revised: February 15, 2021
Accepted: March 28, 2021
Article in press: March 28, 2021
Published online: May 18, 2021
Because of aging populations worldwide the number of spinal instrumentations performed annually is increasing dramatically. The most frequently encountered comorbidity in elderly adults is osteoporosis, which is a main reason for development of pedicle screw instability. Strategies to increase pedicle screw fixation stability include changes in pedicle screw design, but evaluations of pedicle screw loosening are controversial because of differences in the criteria used as indicators for implant loosening and heterogeneity of study groups.
It has be assumed that pedicle screw design influences the stability of the bone-screw interface and it has been extensively studied. The weak points of those studies is in the extrapolation of mechanical data and the results of pullout tests that to clinical practice. It is clear that pullout forces cause minority of cases of pedicle screw instability and that and bone characteristics influence the performance of screws. The strong point of our study is that the influence of screw parameters were tested.
The study aim was to test the impact of a screw design on loosening rate and to assess how screw parameters interfere with patient- and surgery-related factors.
This study was a prospective evaluation of 175 patients with degenerative bone diseases and instability of lumbar spine segments. Participants underwent spinal instrumentation employing pedicle screws with posterior only or transforaminal interbody fusion. Follow-up was for 18 mo. Patients with signs of pedicle screw loosening signs on CT were registered; logistic regression analysis was used to identify the factors associated with loosening, including screw parameters, bone characteristics, and surgery-related factors.
Parameters included in the analysis were screw geometry, thread type, external and internal screw diameter and helical pitch, bone density in HUs, the number of levels fused, instrumentation without anterior support, laminectomy, and unilateral and bilateral total facet joint resection. The rate of screw loosening decreased with an increase in outer diameter, decrease in core diameter, and helical pitch. The rate of screw loosening was positively correlated with the number of fused levels and with decreasing bone density. Bilateral facet joint removal significantly favored pedicle screw loosening. The influence of other factors was not significant. The overall contribution of screw design to loosening rate was comparable to that of patient and surgery-related factors.
Screw parameters had a significant impact on loosening rate along with bone quality characteristics, the number of levels fused, and the extent of decompression. The significance of the influence of screw parameters was comparable to that of patient- and surgery-related factors. The pedicle screw loosening rate was influenced by helical pitch, inner, and outer screw diameter. The influence of screw geometry and thread type was not significant.
We assume that the rational application of implants of various designs can result in a decrease in implant-related complications, but the design characteristics may interfere with patient- and surgery-related factors. To identify additional effects, including those of higher order, and to address the potential bias relevant to collinearity, a cohort study with enrollment of more patients is required. Multivariate analysis can help in the design of randomized case-control trials that can address potential sources of study bias.