VALIDATION OF A SIT-STAND-SIT TEST AS AN ORTHOPAEDIC OUTCOME MEASURE USING INERTIA BASED MOTION ANALYSIS

Abstract

In orthopaedics new objective functional outcome tools are required to validate the benefits of new surgical techniques or implants for which classic scores such as the KSS, HHS or Womac have been shown not to be discriminative enough. Inertia based motion analysis (IMA) is a cheap, fast and simple technique which requires no gait lab or specialist personnel and thus is suitable for routine clinical outcome assessment. IMA on gait has been validated for total knee replacement (TKR) but normal gait was considered not demanding enough for certain orthopaedic differences. Sit-stand-Sit is a more demanding task of daily activity which can be assessed quickly during consultation. This study investigates whether an IMA assessed sit-stand-sit test can differentiate healthy subjects from pre-op TKR patients.

Rising (sit-to-stand) from a chair and sitting down (stand-to-sit) at comfortable, self-selected speed was measured three time using a triaxial accelerometer (range: +/−2g, f=100Hz, 64×62×13mm, m=54g) taped to the sacrum. The chair (no armrests) was height adjustable (legs at 90deg flexion) to level the effort for different body heights.

70 healthy volunteers (f/m=48/22, age range: 17–81yrs) were compared to a pathological group of 20 patients with knee osteoarthritis indicated for unilateral TKR (Biomet Vanguard) measured at 1–10 days pre-op (f/m=11/9; mean age: 65.6yrs, range: 45–79; KSS: 43.5, range: 5–65). The healthy group was split into two subgroups, an age-matched “Old” group (> 50yrs: n1=28, mean age: 65.2yrs) and a “Young” group (< 50yrs: n2=32, mean age: 28.0yrs).

Motion parameters derived were the time to stand up (Tup), time to sit down (Tdwn), the time difference between rising and sitting down (Tu−d) and the combined time of rising and sitting down (tu+d) as mean values and per individual repetition.

All motion parameters were sign. slower with higher variance for the pre-TKR versus the healthy subjects, even when compared to the age-matched subgroup (except Tu−d). Threshold values could be defined to delineate healthy from pathological performance, e.g. Tup> 220ms (6/70=9% vs 17/20=85%, p< 0.01) or Tdwn> 240ms (4/70=6% vs 18/20=90%, p< 0.01) producing high test sensitivity (90%, C.I. 72–98) and specificity (94%, C.I. 89–97). In some false positives (3/6) originally unknown orthopaedic problems were identified in retrospect.

The simple IMA assessed sit-stand-sit test produced motion parameters comparable to values reported for smaller subject groups using methods unsuitable for routine clinical application (e.g. electrogoniometry). Healthy and pathological motion could be distinguished with high sensitivity and specificity even versus age matched controls supporting the validity to use the IMA assessed sit-stand-sit test to complement classic outcome scores with an objective functional component.