Design of an Expandable In-Pedicle Anchor for Spinal Fusion Surgery

Abstract

Loosening of pedicle screws after spinal fusion surgery can lead to serious complications and may prevent fusion between vertebrae. This problem, which has increased in recent years, is a more frequent problem in patients with osteoporosis. In this study, we explore the possibility to use an expanding in‐pedicle anchor with the goal of increasing the contact area of the in‐pedicle anchor with the pedicle cortex to reduce toggling. Toggling is the rocking motion that can occur when an in‐pedicle anchor is subjected to lateral forces that can occur during the patient’s daily activities. A stainless steel scaled‐up 2D proof‐of‐principe prototype was developed and manufactured consisting of a central bolt with flattened sides, a nut, and ten sliding wedges. The wedges expand by applying a compression force by tightening the nut. In order for the prototype to reach the pedicle cortex, it needs to compress the surrounding cancellous bone first. It was shown in experiments that the proof‐of‐principle prototype was able to compress 5 and 10 PCF Sawbones solid foam, which corresponds to osteoporotic human cancellous bone. The proof‐of‐principle prototype was able to make contact with the top and bottom of a custom‐made 2D pedicle model where a conventional screw would have been limited to contact on the two flanks. The proof‐op‐principle prototype showed a better resistance
to lateral loads than an unexpanded model with the same dimensions. The proposed in‐pedicle anchor shows potential for improved resistance to caudocranial toggling by increasing the number of contact points with the pedicle cortex. The use of an in‐pedicle expansion to prevent toggling holds a promising future for possible clinical applications.