Variability of the pullout strength of cancellous bone screws with cement augmentation

Abstract

Background: Orthopaedic surgeons often face clinical situations where improved screw holding power in cancellous bone is needed. Injectable calcium phosphate cements are one option to enhance fixation. Methods: Paired screw pullout tests were undertaken in which human cadaver bone was augmented with calcium phosphate cement. A finite element model was used to investigate sensitivity to screw positional placement. Findings: Statistical analysis of the data concluded that the pullout strength was generally increased by cement augmentation in the in vitro human cadaver tests. However, when comparing the individual paired samples there were surprising results with lower strength than anticipated after augmentation, in apparent contradiction to the generally expected conclusion. Investigation using the finite element model showed that these strength reductions could be accounted for by small screw positional changes. A change of 0.5 mm might result in predicted pullout force changes of up to 28%. Interpretation: Small changes in screw position might lead to significant changes in pullout strength sufficient to explain the lower than expected individual pullout values in augmented cancellous bone. Consequently whilst the addition of cement at a position of low strength would increase the pullout strength at that point, it might not reach the pullout strength of the un-augmented paired test site. However, the overall effect of cement augmentation produces a significant improvement at whatever point in the bone the screw is placed. The use of polymeric bone-substitute materials for tests may not reveal the natural variation encountered in tests using real bone structures.

Inter-trabecular bone formation: a specific mechanism for healing of cancellous bone

“…Estimations from finite element analyses in combination with screw pullout testing on human cadavers suggest that a change in screw positioning as small as 0.5 mm could change the pullout force by up to a quarter (Procter et al. 2015 ), indicating that pullout force is sensitive to local variation in trabecular geometry.…”

Trabecular deformations during screw pull-out: a micro-CT study of lapine bone

“…the study conducted by Procter (2015), where the augmented screws gave a lower pull-out force than non-augmented screws in 4 out of 21 human femoral bone specimens. This work suggests that there are other factors affecting the pull-out strength, such as positioning of the screw with respect to the bone morphology and that the pull-out force is highly sensitive to initial conditions such as the exact positioning, the insertion depth or the angle of insertion.…”

“…Cements have been used together with screws as a means to improve load transfer (Procter 2015 ; Larsson 2012 ; Choma 2012 ; Fölsch 2012 ; Stadelmann 2010 ; Juvonen 2015 ; Kainz 2014 ), but they do not always improve the pull-out strength, as illustrated by, e.g. the study conducted by Procter ( 2015 ), where the augmented screws gave a lower pull-out force than non-augmented screws in 4 out of 21 human femoral bone specimens. This work suggests that there are other factors affecting the pull-out strength, such as positioning of the screw with respect to the bone morphology and that the pull-out force is highly sensitive to initial conditions such as the exact positioning, the insertion depth or the angle of insertion.…”

Salvaging Pull-Out Strength in a Previously Stripped Screw Site: A Comparison of Three Rescue Techniques

“…One study attempted augmenting the screw site with suture wire but could not prove that such augmentation provided clinically significant improvement over the original screw’s pull-out strength (POS) [ 10 ]. Bone cement is widely used for surgical procedures involving cancellous bone [ 11 ]. However a limit of this technique is the inability to control cement migration to unwanted locations, thus creating potential for neurological damage [ 6 ].…”