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

Abstract

Introduction

Several authors propose that a change in sacral slope of ≤10° between the standing and relaxed-seated positions (ΔSS_{standing→relaxed-seated}) identifies a patient with a stiff lumbar spine and have suggested 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_{standing→relaxed-seated} can identify patients with a stiff spine.

Methods

A prospective, multi-centre, consecutive cohort series of 312 patients had standing, relaxed-seated and flexed-seated lateral radiographs prior to THA. ΔSS_{standing→relaxed-seated} 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_{standing→relaxed-seated} for characterising a stiff spine was assessed.

Results

A weak correlation between ΔSS_{standing→relaxed-seated} and LF was identified (r²= 0.13). 86 patients (28%) had ΔSS_{standing→relaxed-seated} ≤10°, and 19 patients (6%) had a stiff spine. Of the 86 patients with ΔSS_{standing→relaxed-seated} ≤10°, 13 had a stiff spine. The positive predictive value of ΔSS_{standing→relaxed-seated} ≤10° for identifying a stiff spine was 15%.

Conclusion

In this cohort, ΔSS_{standing→relaxed-seated} ≤10° was not correlated with a stiff spine. Utilizing 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_{standing→relaxed-seated} ≤10° as being stiff is misleading. The flexed-seated position should be used to effectively assess a patient's spine mobility prior to THA.