Using Dynamic Joint Space During Physiological Loading to Objectively Measure Hip Stability

Abstract

An objective measurement of hip stability during functional loading is needed to improve diagnosis, guide treatment decisions, and evaluate intervention success. This study aimed to develop a reference measurement for stable hips based upon dynamic minimum hip joint space (HJS) in healthy young adults. Synchronized biplane radiographs of the hips of 24 healthy young adults were collected (50 images/s) during treadmill walking and squatting. Subject-specific femur and pelvis models were created from CT scans, and bone motion was determined by a validated volumetric model-based tracking technique. The distance between femur and acetabulum subchondral bone surfaces was calculated during each movement. Range in minimum subchondral bone distance was measured in radial and sagittal regions of the acetabula. Regression analysis identified kinematics and morphologic predictors of range in minimum HJS. Range in minimum HJS during gait in the anterior-inferior (1.8 mm) and posterior-superior regions (1.7 mm) was 31%-38% larger than in the anterior-superior and superior regions (1.3 mm; p ≤ 0.001), and 13%-20% larger than in the posterior-inferior region (1.5 mm; p ≤ 0.001). No differences in minimum HJS were identified in radial regions during squat (range: 0.7-0.9 mm). No sex differences were identified. Femur head translation during gait was a stronger predictor of the range in minimum HJS than changes in femur head morphology. This suggests anterior-inferior to posterior-superior pistoning of the femur may be a mechanical mechanism for commonly observed pathology. These results suggest that gait is a better activity than squatting to assess dynamic hip stability when using this metric.

Keywords: dynamic biplane radiography; dynamic hip joint space; hip stability.

Figure 1

Biplane radiography data collection and processing. (A) Participants walked (shown; 1.1 ± 0.2 m/s) and squatted while (B) synchronized biplane radiographs were collected (50 frames/s, 80 kV, 320 mA, 3.2 ms pulse width). (C) CT scans (0.3 × 0.3 × 0.625 mm) were collected and (D) used to create 3D bone models. (E) Bone motion was tracked using a validated process that matched digitally reconstructed radiographs from CT‐based bone models to the biplane radiographs to determine (F) the range in minimum subchondral bone distance.

Figure 2

Average range in minimum HJS (bold font; standard deviation in parentheses; unit: mm) during gait and squat trials, separated into five evenly spaced radial acetabular regions (1 = anterior‐inferior, 2 = anterior‐superior, 3 = superior, 4 = posterior‐superior, 5 = posterior‐inferior), and three evenly spaced sagittal acetabular regions that progress from in‐to‐out‐of‐the‐page. Colored numbering indicates a significant difference from the corresponding radial region. Asterisks (*) indicate a significant difference between the corresponding sagittal regions.

Figure 3

Average waveforms showing minimum subchondral bone distance during gait within (A) radial and (B) sagittal acetabular regions, as well as (C) anterior‐inferior and posterior‐superior radial regions (isolated from (A) to show their inverse relationship, which suggests a pistoning motion of the femur). Solid lines indicate group mean values, with corresponding shaded regions indicating ± 1 standard deviation. For each hip, the range in minimum HJS within a given region was calculated per trial as the difference between the maximum and minimum values on the corresponding waveform.

Figure 4

The strength of each predictive variable (AP = change in anterior‐posterior translation, SI = change in superior‐inferior translation, FM = change in femur head morphology) in predicting the range in minimum HJS for a given acetabular region during gait (radial regions: anterior‐inferior, anterior‐superior, superior, posterior‐superior, posterior‐inferior; sagittal regions: medial, middle, lateral). Beta values listed are the standardized beta coefficients for each corresponding variable within a given region (higher absolute value indicates a stronger effect on range in minimum HJS; negative value indicates an inverse relationship—from the moment of smallest minimum HJS to the moment of largest minimum HJS—between the predictive variable and the range in minimum HJS). All p values are ≤ 0.001 except: ${\beta }_{\mathrm{FM}}$ in superior region (p = 0.005), ${\beta }_{\mathrm{AP}}$ in posterior superior region (p = 0.003), ${\beta }_{\mathrm{AP}}$ and ${\beta }_{\mathrm{SI}}$ in medial region (p = 0.008 and p = 0.048, respectively), and ${\beta }_{\mathrm{FM}}$ in lateral region (p = 0.016). R ² values listed are for the final model in the corresponding region.