3 - Statistical Shape Modelling and Analysis of Spinopelvic Bone and Muscle Relationships

Poster Board Number: 3
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Abstract Body : INTRODUCTION: Spinopelvic stability is attributed to bone morphology, muscles, and ligaments. Although muscles play a crucial role in both static and dynamic stability, the relationship between spinopelvic muscle and bone morphology and its variations are not well understood. Statistical shape modelling (SSM) is useful in addressing this gap, as it can uncover the shape variations within a group. The purpose of this study was to assess the anatomical parameters and relationships between spinopelvic muscle and bone, using a hybrid approach involving medical imaging, cadaveric dissection, and SSM.

METHODS: A mixed cohort of 14 non-pathological, fresh-frozen cadaveric (Science Care, USA; age = 70±5 years, BMI = 22±4 kg/m^2, m:f = 6:8) and 4 live participant (age = 23±0 years, BMI = 25±1 kg/m^2, m:f = 1:3) hips were included (approved ethics # 121404). Hips underwent computed tomography (CT; Siemens), and 3T magnetic resonance imaging (MRI; Siemens 3T Prisma), where bone morphologies (femur, pelvis, sacrum) were measured for multiple anatomical parameters (alpha angle, acetabular version, femoral neck-shaft angle (NSA), lateral centre-edge angle); and the physiological cross-sectional areas (PCSA) were obtained for muscle at the level of the femoral head centre. The following muscles were investigated: iliacus, psoas major, iliocapsularis, tensor fascia lata (TFL), sartorius, gluteus medius, gluteus minimus, piriformis, superior gemellus, obturator internus, inferior gemellus, and quadratus femoris. Associations between bone and muscle values were determined using Pearson’s correlation coefficient (strong > 0.8, moderate > 0.6, weak > 0.4). Structures were then reconstructed using segmentation software (3D Slicer; The Slicer Community), to allow for subsequent SSM analysis (ShapeWorks; University of Utah). Full dissection of cadaveric specimens followed, to confirm imaging observations and observe any muscle variants in situ.

RESULTS: In the cadaveric hips, more than 95% morphological variation was captured in 4 and 6 modes of variation of the femur and pelvis groups, respectively. The shape maps showed a difference in femoral NSA (115-131 degrees) and anterior acetabular coverage (3.3mm difference). Femoral alpha angles were correlated highly with TFL (0.96) and sartorius (0.89), and moderately with iliacus (0.77), psoas major (0.70), rectus femoris (0.73), and iliocapsularis (0.76) PCSAs. Lateral centre-edge angle inversely correlated highly with sartorius (-0.81), and moderately with iliacus (-0.77) and TFL (-0.62) PCSAs. There was moderate correlation of NSA with iliacus (0.68) and sartorius (0.64) PCSAs, and acetabular version with psoas major (0.71) PCSA. Interestingly, the dissected specimens revealed four separate variants of piriformis morphology: three bifid variants and one muscular fusion between piriformis and gluteus medius, confirmed on MRI.

CONCLUSION/SIGNIFICANCE: The most important finding was that spinopelvic muscle and bone parameters showed strong to moderate correlations. This indicates the importance for further investigation into the muscle-bone relationship, as changes in bone morphology may indicate alterations in muscle morphology to maintain stability. Understanding this relationship can provide a baseline reference to support diagnostics of pathological hips, the effect of hip osteoarthritis progression on muscle characteristics, and the value of preserving muscle during surgical procedures.