Biomechanical rationale of ossification of the secondary ossification center on apophyseal bony ring fracture: a biomechanical study.

Background: Apophyseal ring fracture is one of the important pathologies causing low back pain in children and adolescents. Most of the patients are reported to be in the ossification stage of the ring during growth period rather than early cartilaginous ring stage. There is no previous study clarifying the mechanism of the high prevalence of this disorder in the ossification stage. Thus, in this study, we investigated the effects of ossification of the ring on lumbar spine biomechanics.

Methods: Two three-dimensional finite element pediatric lumbar models were created and analyzed. One model had ossified apophyseal rings and the other one had cartilaginous apophyseal rings. To simulate standing posture, 341N axial compression was applied. Then, 10Nm moment was applied to the model in the six directions of lumbar motion: flexion, extension, lateral bending and axial rotation. Maximum Von Mises stresses in the apophyseal ring were calculated and compared between the two models.

Findings: The maximum stresses were always higher in the bony ring in all lumbar motion at all lumbar levels compared to the cartilaginous ring. The stresses at L4 caudal apophyseal ring in extension were 2.60 and 0.68 (MPa) for bony and cartilaginous rings respectively. In flexion, stresses were 3.95 and 1.49 (MPa), in lateral bending, stresses were 6.75 and 2.66 (MPa), and in axial rotation, stresses were reported to be 3.15 and 1.72 (MPa). Thus, the bony ring was stressed by at least 2-fold more than the cartilaginous ring.

Interpretation: Apophyseal ring has at least two times more stresses in the ossified stage when compared to the cartilaginous stage resulting in frequent fractures at the interface of bone and cartilage.