Abstract
Introduction
Our classic outcome scores increasingly fail to distinguish interventions or to reflect rising patient demands. Scores are subjective, have a low ceiling and score pain rather than function. Objective functional assessment tools for routine clinical use are required. This study validates inertial sensor motion analysis (IMA) by differentiating patients with knee versus hip osteoarthritis in a block-step test.
Methods
Step up and down from a block (h=20cm, 3 repetitions) loading the affected (A) and unaffected (UA) leg was measured in n=59 subjects using a small inertial sensor (3D gyro and accelerometer, m=39g) attached onto the sacrum. Patients indicated for either primary unilateral THA (n=20; m/f=4/6, age=69.4yrs ±9.8) or TKA (n=16;m/f=7/9;age=67.8yrs ±8.2) were compared to healthy controls (n=23;m/f=13/10;age=61.7yrs ±6.2) and between each other to validate the test's capacity for diagnostics and as an outcome measure.
The motion parameters derived (semi-) automatically in Matlab for both legs were: front-back (FB-) sway and left-right (LR-) sway (up and down); peak-to-peak accelerations (Acc) during step down. In addition the asymmetry between both legs (ASS) was calculated for each parameter. Group differences were tested (t-test) and the diagnostic value determined by the area under the curve (AUC) of the ROC-curve.
Results
During step-up FB-sway was higher for THA (20.4°±4.9) and TKA (21.7°±5.9) patients than for healthy controls (15.5°±3.4, p<0.001). Also asymmetry was higher (THA=20%, TKA=21%, H=11%, p<0.001). Results were similar during step down except for the affected leg of THA patients where FB-sway (THA=16.2°±3.0) was similar to controls but sign. different to TKA patients (22.2±4.4) producing a high diagnostic power (AUC=0.88) to differentiate THA and TKA.
LR-sway was also indicative for THA patients being the only subjects showing high asymmetry between the legs (A=14.3°±3.7 vs UA=11.9°±3.1, p<0.001). Acceleration during step-down asymmetric in patients, especially in THA (H<TKA<THA;p<0.05; AUC=0.87).
Discussion
The IMA-block-step test could detect pathology specific compensation mechanism: During step-up patients use more FB-sway (+29%) to generate momentum for compensating muscle weakness and decrease joint loading.
During step-down, only THA patients showed less FB-sway with their affected leg avoiding the painful hip flexion. Also in THA the LR-sway was higher in the affected than unaffected side due to the typical abductor weakness and resulting Trendelenburg sign.
The IMA-block-step test could objectify compensation mechanisms used in OA and showed the power to differentiate between H, TKA and THA. It is low cost and fast to perform (<5min) by non-specialist personnel and thus could be used in clinical routine to supplement questionnaire based outcome scores.
