Introduction: The hip joint is usually considered a ball-in-socket. However, there have been few studies evaluating normal hip kinematics and the contribution coming from soft tissues. Capsular laxity is at the basis of injury to the acetabular labrum (most common pathological lesion seen during hip arthroscopy). The objectives of this study were to (1) assess hip kinematics with all the soft tissues intact using a surgical navigation system, (2) assess the relative contributions of the soft tissues to hip stability and (3) assess the relative contributions of periarticular soft tissues to hip range of motion.

Materials and Methods: We used 4 normal hemicorpse specimens for a total of 8 hips. A navigation system (KLEE, Orthokey) was used to acquire the kinematic data. The anatomical reference system was identified through the palpation of landmarks: (1) anterior superior iliac spines (ASIS) and (2) pelvic tubercles for the pelvis, (3) femoral head center and (4) epicondyles for femur. There were 12 passive kinematic tests repeated 3 times in 3 different limb conditions (‘intact’, ‘no-skin-muscle’, ‘labral tear’) to explore the whole kinematic range. We analysed the differences in flexion/extension, abduction/adduction, internal/external rotation ranges (Wilcoxon’s Signed Ranks Test).

Results: The kinematic analysis applied on the limbs highlighted the following range of motion: (1) the F/E was 115.7 ± 2.4° (12.9 ± 1.0° in extension/101.7 ± 3.0° in flexion) in ‘intact’ limb, 139.2 ± 10.8° (14.7 ± 2.7° in extension/120.7 ± 8.6° in flexion) in ‘no-skin no-muscle’ condition, and 174.3 ± 34.1° (25.3 ± 0.5° in extension/147.4 ± 35.4° in flexion) in ‘capsule cut’ condition; all the ranges were statistically different (p < 0.05); (2) the A/A was 44.5 ± 13.7° (35.4 ± 1.5° in abduction/10.1 ± 13.4° in adduction) in ‘intact’ limb, 59.2 ± 1.8° (38.5 ± 3.2° in abduction/21.7 ± 0.7° in adduction) in ‘no-skin no-muscle’ condition, and 82.0 ± 4.6° (57.4 ± 2.5° in abduction/25.6 ± 6.8° in adduction) in ‘capsule cut’ condition; all the ranges were statistically different (p < 0.05); (3) the IR/ER was 52.2 ± 10.5° (32.0 ± 11.9° in IR/21.5 ± 1.0° in ER) in ‘intact’ limb, 59.2 ± 1.8° (36.1 ± 14.1° in IR/26.5 ± 1.2° in ER) in ‘no-skin no-muscle’ condition, and 116.4 ± 54.4° (58.2 ± 16.1° in IR/55.6 ± 36.3° in ER) in the ‘capsule cut’ condition; all the ranges were statistically different (p < 0.05), except the ranges of ‘intact’ condition and ‘no-skin no-muscles’ one (p = 0.37).

Discussion: The study of the 3 different conditions highlighted the critical role of the soft tissues in hip stability and kinematics; the soft tissues do provide stability mainly in limiting hip range of motion. This study’s findings are a preliminary contribution in the understanding of the contribution of periarticular muscles, joint capsule and ligaments to hip kinematics.