Axial Force Transmission Through Orthosis Straps in a Neonatal Hip Flexion-Abduction Orthosis: An Exploratory In Vivo Study in Infants with Developmental Dysplasia of the Hip.

Background: Developmental dysplasia of the hip (DDH) is the most common congenital musculoskeletal disorder in newborns. Flexion-abduction orthoses are widely used in early treatment; however, in vivo data on their biomechanical load characteristics remain limited. This study aimed to evaluate axial force transmission in a hip flexion-abduction orthosis and to compare load patterns between healthy newborns and infants with DDH. Methods: In this exploratory observational study, 36 newborns (19 healthy, 17 with unilateral DDH) were examined within the first week of life. Axial forces transmitted through a Mittelmeier-Graf hip flexion-abduction orthosis (MGO) were measured using integrated force sensors under symmetrical and asymmetrical adjustment configurations. Intergroup comparisons were performed using non-parametric statistical tests. Results: Mean axial forces were significantly higher in healthy infants than in those with DDH under both symmetrical (4.02 N vs. 2.51 N; p = 0.019) and asymmetrical (3.67 N vs. 1.83 N; p = 0.001) conditions. Relative load corresponded to approximately 11-12% of body weight in healthy infants and 5-7% in the DDH group. No significant intra-individual differences were observed between dysplastic and contralateral hips. Orthosis configuration (symmetrical vs. asymmetrical) did not significantly affect load distribution. Conclusions: This exploratory in vivo study demonstrates that axial load transmission in a hip flexion-abduction orthosis is low and influenced by underlying hip pathology. Infants with DDH generate lower forces than healthy newborns, potentially reflecting altered biomechanics. As no significant differences were observed between orthosis configurations, symmetrical adjustment may be favored in clinical practice due to better usability and compliance. Further studies with larger cohorts are needed to confirm these findings.