The biomechanical impact of cement volume and filling pattern for augmented pedicle screws using various density testing blocks

Abstract

Background context: Patients with osteoporosis experience a higher risk of pedicle screw loosening and failure, making PMMA bone cement augmentation a common recommendation to increase stability. However, there is ongoing debate about the ideal cement volume, filling pattern, and measurable increase in stability that cement provides.

Purpose: This aim of this study is to clarify the impact of cement volume and filling pattern on the stability of pedicle screws in synthetic bone blocks of varying density.

Study design: We examined the effects of different volumes of PMMA cement on screw stability by using synthetic bone blocks with densities of 7.5 pcf, 15 pcf, and 30 pcf to simulate human vertebral cancellous bone with osteoporosis, osteopenia, and normal bone density. Two filling patterns were compared: a tip cement cloud and a surrounding cement cloud.

Methods: We tested solid screws, cannulated screws without side holes, and cannulated screws with 4 side holes. Cement volumes of 2 cc, 3 cc, and 4 cc were injected, and the pullout strength was measured using an Instron testing machine. The samples were categorized into the control (no cement) group, S group (solid screws with prefilling), C1 group (cannulated screws without side holes), and C5 group (cannulated screws with 4 side holes). Among these, the C1 group exhibited a cement cloud at the tip, while the S and C5 groups showed a surrounding cement cloud.

Results: Adding an extra 1 cc of cement (from 2 cc to 3 cc, or 3 cc to 4 cc) significantly increased the pullout strength by 10% in 7.5 pcf bone, 47% in 15 pcf bone, and 34% in 30 pcf bone and doubling the cement volume resulted in even greater increases. Regardless of the injected volume, cement augmentation substantially increased the pullout strength in osteoporotic bone compared with noncemented screws in osteopenic bone. For a given cement volume, the anchorage power was greater for the surrounding cement cloud pattern than for the tip cement cloud pattern.

Conclusion: Although the addition of more cement increases the pullout strength, excessive augmentation in osteoporotic bone is unnecessary. The filling pattern is crucial; a larger contact area in the surrounding cement cloud enhances screw stability more effectively than the tip cement cloud does.

Clinical significance: While adding more cement increases pullout strength, excessive augmentation in osteoporotic bone is unnecessary. The filling pattern rather than the screw design is a crucial determinant of screw stability.

Keywords: Bone density; Cannulated screw; Cement cloud; PMMA bone cement; Pedicle screw; Screw pullout test.