Which trajectory should we choose? - Defining safe entry-target pathways and underlying femoral morphology for bone marrow injection in osteonecrosis of the femoral head.

Introductions: Bone marrow aspiration concentrate (BMAC) injection is a promising joint-preserving treatment for osteonecrosis of the femoral head (ONFH). However, the procedure remains unstandardized, and iatrogenic complications, including subtrochanteric fractures, have been reported. Despite its growing clinical adoption, no studies have modeled this procedure in a representative cohort to evaluate its technical safety. This study aimed to establish a morphological basis for safer and more standardized needle placement by identifying entry-target combinations that provide the safest trajectories, and determining the proximal femoral parameters influencing the permissible insertion angle. Methods: Forty hips without collapse or deformity from 40 patients with ONFH (20 males and 20 females) were retrospectively analyzed using computed tomography-based three-dimensional reconstructions. Four entry levels (A-D) and four femoral head target points (center-center, center-lateral, anterior-center, anterior-lateral) were defined. Needle trajectories were reconstructed, and the permissible insertion angle was defined as the range within which cortical perforation did not occur. Linear regression analyses were conducted to identify morphological factors associated with the permissible insertion angle. Results: Six entry-target combinations achieved a 100% needle passage rate, representing the safest configurations for BMAC injection. Among these, the anterior-center target was the most accessible, allowing safe insertion at three different entry levels. The widest permissible insertion angle was observed when targeting the center-center region from entry level D (18.1° ± 3.7°). Morphological analysis revealed that a smaller minimum femoral neck diameter and greater femoral anteversion were independently associated with a narrower permissible insertion angle. Female patients demonstrated significantly narrower safe angular ranges than male patients across all entry-target combinations. Conclusion: This study quantitatively identified six safe entry-target combinations and key morphological determinants influencing the permissible insertion angle. These findings provide practical anatomical reference data to support safer, morphology-based planning for ONFH treatment.