Is a Washer a Mandatory Component in Young Trauma Patients with S1-S2 Iliosacral Screw Fixation of Posterior Pelvis Ring Injuries? A Biomechanical Study

Abstract

Background and purpose: Cannulated screws are standard implants for percutaneous fixa-tion of posterior pelvis ring injuries. The choice of whether to use these screws in combination with a washer is still undecided. The aim of this study was to evaluate the biomechanical competence of S1-S2 sacroiliac (SI) screw fixation with and without using a washer across three different screw designs. Material and Methods: Twenty-four composite pelvises were used and an SI joint injury type APC III according to the Young and Burgess classification was simulated. Fixation of the posterior pelvis ring was performed using either partially threaded short screws, fully threaded short screws, or fully threaded long transsacral screws. Biomechanical testing was performed under progressively increasing cyclic loading until failure, with monitoring of the intersegmental and bone-implant movements via motion tracking. Results: The number of cycles to failure and the corresponding load at failure (N) were significantly higher for the fully threaded short screws with a washer (3972 ± 600/398.6 ± 30.0) versus its counterpart without a washer (2993 ± 527/349.7 ± 26.4), p = 0.026. In contrast, these two parameters did not reveal any significant differences when comparing fixations with and without a washer using either partially threaded short of fully threaded long transsacral screws, p ≥ 0.359. Conclusions: From a biomechanical perspective, a washer could be optional when using partially threaded short or fully threaded long transsacral S1-S2 screws for treatment of posterior pelvis ring injuries in young trauma patients. Yet, the omission of the washer in fully threaded short screws could lead to a significant diminished biomechanical stability.

Keywords: S1-S2 sacroiliac screw fixation; biomechanics; motion tracking; posterior pelvis ring injury; synthetic pelvic bone model; transsacral fixation; washer; washerless.

Figure 1

Inlet (uppercase letters) and outlet (lower case letters) x-rays after instrumentation showing specimens from groups TS w/(A,a), TS w/o (B,b), FT w/(C,c), FT w/o (D,d), PT w/(E,e) and PT w/o (F,f).

Figure 2

Setup with a specimen mounted for biomechanical testing. Vertical arrow indicates loading direction.

Figure 3

Cycles to failure (left abscissa) and corresponding load at failure (right abscissa), presented for each separate group in terms of mean value and SD. Star indicates significant differences between the groups.