Acetabular fracture pattern is altered by pre-injury sacroiliac joint autofusion

Purpose: Acetabular fracture shape is determined by the direction of force applied. We perceive an anecdotally observed connection between pre-existing autofused sacroiliac joints (aSIJ) and high anterior column (HAC) injuries. The purpose of this study was to compare variations in acetabular fracture patterns sustained in patients with and without pre-injury sacroiliac (SI) joint autofusion. Methods: All adult patients receiving unilateral acetabular fixation (level 1 academic trauma; 2008–2018) were reviewed. Injury radiographs and CT scans were reviewed for fracture patterns and pre-existing aSIJ. Fracture types were subgrouped presence of HAC injury (includes anterior column (AC), anterior column posterior hemitransverse (ACPHT), or associated both column (ABC)). Analysis: Logistic regression determined the association between aSIJ and HAC. Results: A total of 371 patients received unilateral acetabular fixation (2008–2018); 61 (16%) demonstrated CT evidence of idiopathic aSIJ. These patients were older (64.1 vs. 47.4, p < 0.01), more likely to be male (95% vs. 71%, p < 0.01), less likely to be smokers (19.0% vs. 44.8%, p < 0.01), and were injured from lower energy mechanisms (21.3% vs. 8.4%, p = 0.01). The most common patterns with autofusion were ACPHT (n = 13, 21%) and ABC (n = 25, 41%). Autofusion was associated with greater odds of patterns involving a high anterior column injury (ABC, ACPHT, or isolated anterior column; OR = 4.97, p < 0.01). After adjusting for age, mechanism, and body mass index, the connection between autofusion and high anterior column injuries remained significant (OR = 2.60, p = 0.01). Conclusions: SI joint autofusion appears to change mode of failure in acetabular injuries; a more rigid posterior ring may precipitate a high anterior column injury. Level of Evidence: Prognostic level III.