Insertional torque and pullout strength of reinserted screws for sacroiliac joint fusion: Effects of implant dimensions, bone density, and tapping

Abstract

Background: Sacroiliac joint (SIJ) fusion is a common procedure for SIJ pain and dysfunction. Traditional threaded screws offer limited bony integration, prompting the development of additively manufactured, porous implants aimed at enhancing osseointegration. However, data on their insertional mechanics and pullout strength, especially in revision scenarios, remain sparse. This study aimed to investigate the influence of bone density, tapping, and implant size on re-insertional torque and pullout strength.

Methods: Mechanical testing was conducted in accordance with ASTM F543, using 10 and 20 PCF polyurethane foam blocks to simulate low and normal-density bone, respectively. A 10 × 35 mm screw fusion device served as the control. For subsequent tests, the 10 × 35 mm screw was removed, and various diameters (10, 11.5, 13.5 mm) and lengths (35, 40, 45 mm) were reinserted under 2 conditions: direct and with tapping. The maximum and final insertional torque, as well as the maximum pullout force, were measured. ANOVA techniques with pairwise comparison and Pearson correlation tests were performed (α = 0.05).

Results: Significant effects and interactions were observed for implant size, bone density, and tapping. Larger implant diameter and length were associated with significantly greater insertional torque and pullout strength compared to controls, though excessively large diameters yielded diminishing returns in normal-density bone. Independent of size and tapping, normal-density bone consistently demonstrated greater insertional torque (929.1 ± 6.5 vs. 235.0 ± 6.5N·cm) and pullout strength (1,331.2 ± 14.8 vs. 457.3 ± 14.6 N) than low-density bone (p < .001). Independent of size, tapping improved insertional torque (+33.9%; p < .001) and pullout strength (+108.4%; p < .001) in normal-density bone but reduced pullout strength in low-density bone (-22.4%; p < .001). Final insertional torque strongly correlated with pullout strength (r = 0.946, p < .001).

Conclusions: Our findings provide evidence-based insight for optimizing implant selection and surgical technique in revision SIJ fusion procedures using screw fusion devices. Particularly, it is emphasized that both implant dimensions and patient-specific bone quality must be considered to achieve stable and successful outcomes.

Keywords: Osseointegration; Pelvic screw; Porous fusion/fixation screw; Porous threaded implant; Screw fusion device.

Graphical abstract

Fig. 1

Radiographic images of surgical application. (A) Supine Ferguson view after SIJ fusion using 3 screw fusion devices via a minimally invasive lateral approach. (B) Supine Ferguson view after SIJ fusion where screw fusion devices were used in combination with a triangular titanium implant (TTI). (C) Standing Ferguson view after a multilevel spinal fusion with concomitant sacropelvic fixation/fusion. Specifically, the “bedrock” technique was used by placing bilateral fusion devices in a trajectory cephalad and parallel to sacral-alar-2-iliac pelvic screws. All surgeries were performed by an author.

Fig. 2

Testing set-up and insertion sequence. (A) Multiple bone blocks were compressed together and testing sites were marked. Common surgical practice was followed by first inserting K-wires perpendicular to the surface. (B) A drill bit was placed over the K-wires and respective holes were drilled. (C) Controls (10 × 35 mm) were initially inserted. (D) Depending on testing conditions, a tap of appropriate size was used to prethread and expand the hole after control removal. (E) Screws of varying sizes were reinserted into the block following ASTM F543 standards. (F) K-wires were removed.

Fig. 3

Screw-driver and tested sizes of screw fusion devices. (A) Insertional torque measurements were obtained with a wireless torque reader fabricated to the manufacturer’s ratcheting handle and torx bit driver. (B) A total of 9 screw sizes were tested with 3 different diameters and lengths. The top left (10 × 35 mm) was the control.

Fig. 4

Pullout force testing setup. (A) Overall setup following ASTM F543 guidelines. A threaded rod was screwed into the top of the screws and attached to a compression testing machine. The bone block was securely clamped to the base plate and was verified to have no slippage. (B) An axial force was applied upward as depicted by the red arrow.

Fig. 5

Maximum and final insertional torque for reinserted screws. Mean values for low-density bone (10 PCF) and normal-density bone (20 PCF) for initial insertion of the control (10 × 35) and reinsertion. Implant size (Diameter × Length; 10, 11.5, 13.5 mm × 35, 40, 45 mm) and use/nonuse of a tap were varied. Vertical error bars represent ± standard deviation. Significant differences from the control and the reinserted 10 × 35 screw are indicated by symbols † and *, respectively.

Fig. 6

Maximum pullout force for reinserted screws. Mean values were measured for the initial insertion of the control (10 × 35) and screw reinsertion for all conditions. Implant size (Diameter × Length; 10, 11.5, 13.5 × 35, 40, 45 mm), bone foam density (10, 20 PCF), and use/nonuse of a tap were varied. Vertical error bars represent ± standard deviation. Significant differences from the control and reinserted 10 × 35 screw are indicated by symbols † and *, respectively.

Fig. 7

Linear regression for insertional torque versus pullout strength. (A) Maximum insertional torque (N) correlated to pullout force (N·cm) (r = 0.829; p < .001). (B) Final insertional torque is more strongly correlated to pullout force (r = 0.946; p < .001). Conditions (bone density and use/nonuse of a tap) are separated by shape and grayscale intensity.

Fig. 8

Boney evulsion with large screw diameter. Partially inserted screws (A) 13.5 × 40 mm and (B) 13.5 × 45 mm (largest diameter) were reinserted without tapping into 20 PCF (normal) bone. Bone breakage and evulsion near the point of entry is noted. Similar observational trends were seen while inserting the largest diameter tap.