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

Abstract

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, 341 N axial compression was applied. Then, 10 N m 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.

Introduction

Apophyseal ring fracture is one of the important pathologies causing low back pain among the pediatric population (Ikata et al., 1995, Arlet and Fassier, 2001, Sucato, 2001). The pediatric spine is anatomically different from the adult spine (Sairyo et al., 1998, Kajiura et al., 2001, Sairyo et al., 2001), and the spine growth has been divided into three stages based on the radiographical observation of ossification of secondary ossification center (apophyseal ring), Fig. 1, (Sairyo et al., 2001). In the cartilaginous (C) stage, the apophyseal ring is cartilage tissue, and it ossifies in the apophyseal (A) stage. According to the review article by Uraoka and Murase (1991), the apophyseal ring of lumbar spine starts ossification around 9 years old. The start of ossification varies individually; and at 12 years old, half of the children show stage C and other half show stage A. At this age, some of the rings begin to fuse to the vertebral body, indicating maturation of the spine (epiphyseal stage). At 15 years old, no children are in the C stage; mostly they are in the A stage, and some are in the epiphyseal (E) stage. At the age of 18, all are in the E stage.

Apophyseal ring fracture could occur among the immature spines (C and A stage), but not during E stage which is an indicator of maturation of spine. It suggests that this disorder is related to the maturity of the spine. Although in the literature a case of a 5 year old boy (Arlet and Fassier, 2001) or 6 year old boy (Martinez-Lage et al., 2003), whose spines showed C stage has been reported; the majority of the patients have been reported to be at A stage (Lippitt, 1976, Handel et al., 1979, Dake et al., 1985, Martinez-Lage et al., 1998) and whose herniated material includes bony fragments of the apophyseal ring. According to our experience between 1998 and 2003 in our clinic, all 20 pediatric patients with this disorder were in the A stage and bony fragments were confirmed on CT scans. In the literature, CT scans, rather than MRI, are recommended for correct diagnosis (Dietemann et al., 1988, Peh et al., 1998). Arlet and Fassier (2001) stated that MRI only can fail to detect this disorder.

Recently, some reports proposed the mechanism of this disorder to be avulsion fracture due to mechanical weakness of the growth plate surrounding the apophyseal bony ring (Sucato, 2001). This statement could be supported by the several biomechanical studies using fresh immature calf cadaveric spines (Sairyo et al., 1998, Kajiura et al., 2001) showing that the growth plate of the vertebral body is mechanically the weakest-link. Kajiura et al. measured the mechanical strength of the vertebral growth plate from different kinds of growth stages, and found that the growth plate in the cartilaginous stage to be the weakest, rather than the other stage (Kajiura et al., 2001). If the avulsion of the apophyseal ring closely relies on the mechanical strength of the growth plate, then C stage could show the most prevalent incidence. In reality, C staged patients are rarely reported. Thus, we assumed that in addition to the weakness of the growth plate, there should be other factors causing apophyseal ring fractures, especially in the A stage with ossified ring.

We hypothesized that the ossification of the apophyseal ring may play an important role for the prevalence of the fracture. When the cartilaginous ring changes to bony ring (stage A), material properties also change. This change would cause mechanical stress concentration on the bone and cartilage interface. Thus, the purpose of this study was to prove this hypothesis using three-dimensional finite element pediatric spine model.