Sacral Slope Change From Standing to Relaxed-Seated Grossly Overpredicts the Presence of a Stiff Spine.

Background: Several authors propose that a change in sacral slope of ≤10° between the standing and relaxed-seated positions (ΔSS) identifies a patient with a stiff lumbar spine and has suggested the use of dual-mobility bearings for such patients undergoing a total hip arthroplasty (THA). The aim of this study was to assess how accurately ΔSS can identify patients with a stiff spine.

Methods: A prospective, multicentre, consecutive cohort series of 312 patients had standing, relaxed-seated, and flexed-seated lateral radiographs prior to THA. ΔSS was determined by the change in sacral slope between the standing and relaxed-seated positions. Lumbar flexion (LF) was defined as the difference in lumbar lordotic angle between standing and flexed-seated. LF ≤20° was considered a stiff spine. The predictive value of ΔSS for characterizing a stiff spine was assessed.

Results: A weak correlation between ΔSS and LF was identified (r = 0.13). Eighty six patients (28%) had ΔSS ≤10° and 19 patients (6%) had a stiff spine. Of the 86 patients with ΔSS ≤10°, 13 had a stiff spine. The positive predictive value of ΔSS ≤10° for identifying a stiff spine was 15%.

Conclusion: In this cohort, ΔSS ≤10° was not correlated with a stiff spine. Using this simplified approach could lead to a 7-fold overprediction of patients with a stiff lumbar spine and abnormal spinopelvic mobility, unnecessary use of dual-mobility bearings, and incorrect component alignment targets. Referring to patients with ΔSS ≤10° as being stiff is misleading. The flexed-seated position should be used to effectively assess a patient's spine mobility prior to THA.