Introduction: An observation was made that when removing self-tapping cortical screws from patients bones, stripping or shearing of the head of the screw occurred more often in screws whose cutting flutes sat in cortical bone compared with screws that had penetrated the distal cortex with flutes exposed. Method: A model was designed to simulate screws with their cutting flutes either in contact with cortical bone or deep to cortical bone. Screws were grouped depending on the location of their cutting flutes and removal torque was measured. Results: Eighteen screws were included in final analysis. There was a statistically significant difference in average initial removal torque and average maximal removal torque with screws with their cutting flutes in substrate requiring significantly more torque to remove. Conclusion: We conclude that self-tapping cortical screws whose cutting flutes sit in cortical substrate require more torque to remove and are therefore more likely to fail. This finding may be used as a guide in pre-operative planning for removal of metalwork from patients.
Metal screws are widely used in orthopaedic operations in both the elective and trauma setting. Most of these screws are machine screws with uniform thread and blunt tips which are placed in a pilot hole [
Self-tapping screws cut their own reciprocal thread as they are advanced obviating the need for a separate tap after drilling. Self-tapping screws are either thread-forming (cancellous) or thread-cutting (cortical). Thread- forming screws have no cutting flutes, produce little debris and compact the receiving bone. Thread-cutting screws have sharp flutes at the tip which cut a channel for the screw and remove debris [
Although removal of metalwork is not a recommended routine procedure [
At our institution along with others [
On the basis of these observations, a model was designed to test whether location of the cutting flute of self-tapping screws in bony substrate affected the removal torque and therefore the chance of screw breakage during removal.
The model was designed to simulate screws with their cutting flutes either in contact with cortical bone or deep to cortical bone. Twenty identical self-tapping, thread-cutting (cortical) screws were tested. Screws were manufactured by Smith and Nephew (Smith and Nephew Inc., 1450 Brooks Road, Memphis, Tennessee, USA) and thread diameter was 4.5 mm. A Teflon tray was constructed with 10 smooth pre-drilled recesses in the base and a metal rack above from which screws could be suspended. The tray was designed to be dismantled so as not to disturb the cast during extraction (
Screws were arranged in two groups of ten screws. Group A were placed into the pre-drilled holes in the Teflon base so cutting flutes were not exposed to cement. Group B were suspended from the metal rack so the flutes were set in cement and the tips rested against the Teflon base. The tray sat on a level surface at standard room temperature. Zimmer low viscosity cement (Zimmer P.O. Box 708, 1800 West Center Street, Warsaw, IN, USA)
was used to represent cortical bone and was poured around the screws to a depth of 0.85 cm and allowed to harden for 3 hours.
After setting the tray was disassembled and the cement cast containing the screws was recovered. A vernier calliper was used to measure cement depth. One screw from group A was removed due to cement leaking into flutes and one non-uniform screw from group B was removed leaving eighteen screws available for testing. A torque-measuring machine was applied to screws which were removed by turning. Initial and maximal torques were recorded. Screws were also inspected after removal.
Statistical analysis was performed using Graph Pad Software (Graph Pad Software Inc.) applying t-test to calculate the exact P value. A P value < 0.05 was regarded as significant.
Results from all tested screws are presented in
Screws in group A and B displayed markedly different patterns in removal torque. In group A screws torque rose rapidly to overcome the resistance of thread in the cement. The rate of increase then slowed until maximal torque was reached later in the revolution. Group B screws demonstrated a different pattern with a rapid rise to maximal torque in the first quarter of a second before decreasing rapidly (
No screws had any sign of head stripping or shearing. No group A screws had cement in the cutting flutes. A significant amount of cement remained in the cutting flutes of group B screws (
The experiment was designed to test the hypothesis that self-tapping screws whose cutting flutes sit in cortical bone are more difficult to remove, and thus are more prone to shearing, than screws that have penetrated the distal cortex. The model showed that the removal torque varied significantly depending on the location of the flutes of self-tapping screws relative to the cement and confirmed the initial observation that screws with cutting flutes in contact with cement are more difficult to remove.
An explanation for the higher torque required for group B screw removal was found on closer examination of the flutes from screws removed. In group B screws, cement was adherent to the cutting flutes and it appeared to have fractured away from the cement mantle. Group A screws were retrieved with no adherent cement. It was noticed at time of testing that group B screws loosened with an audible “crack”, while group A screws turned silently.
A limitation to this experiment was in modelling bone. In vivo when screws are advanced into bone they cause localised necrosis which over time remodels with closely apposed woven bone [
In vivo bone can grow over and cover plates sitting on the cortex and the same remodelling can occur at the periosteum and the tip of a screw that has penetrated the distal cortex of the bone. The bone formed could fill the cutting flutes. This in theory could lead to no benefit in terms of easier removal should a longer screw be utilised. To validate the model further research in animal and human tissue is needed.
There will be risks associated with the application of these findings to clinical practice. It is not always possible to protrude screws through the distal cortex because this risks irritating soft tissues or damaging neuro- vascular structures. Results from this model also suggest that all cutting flutes need to be outside of the bone. To do this in practice requires the length of cutting flute to be known and the depth gauge altered to give the surgeon a screw length that ensures this.
Screw design plays an important role in its mode of failure. Having smaller cutting flutes would mean the possibility of less bony ingrowth and less resistance to removal. The metallurgist should also pay attention to torque strength of the head to the shaft of the screw, as well as ensuring that the head is strong enough to resist stripping.
Removal of metalwork following orthopaedic surgery is generally not required; however, in some cases, particularly in the paediatric population, removal is routine. The findings from this experiment may serve as a guide in pre-operative planning of metalwork removal as to which screws may be more difficult to remove.