Since the first report in 1953, lumbar posterolateral intertransverse fusion has been most commonly performed for patients with lumbar instability and has been reported to achieve successful union and clinical outcomes [1–5]. Recently, hydroxyapatite (HA) has been used as a substitute for autologous bone graft, and there have been no detailed reports of failure of intertransverse fusion with HA [6, 7].
There have been few reports of long-term follow-up in cases of intertransverse fusion with granules of HA. It is reported that macrophages responding to HA particles are capable of osteoclast differentiation, and new bone formation occurs between and within HA granules when bone union is successful . In the present case, histological findings showed that formation of osteoclasts did not continue and that activity of these cells ceased within seven years. This was accompanied by insufficient bone formation from failure of coupled remodeling. On the other hand, in some parts of the marginal layer of the cavity, multinuclear giant cells and lymphocytes were observed, partially forming a foreign body granuloma. These cells are usually observed with HA, but few inflammatory reaction was observed in the whole tissue which was resected, indicating patient’s pain was not induced by inflammation of the transplanted tissue, but might be induced by another unstable structures including the intervertebral discs or facet joints [9–11]. Motomiya et al . reported the bony bridge which is achieved within autologous bone graft may be formed more easily and earlier than bone ingrowth to HA, but no obvious bony bridge which regenerated from autologous bone was seen in the present case. The exact pathomechanism for nonunion is unclear, but for spinal fusion, factors in the host (e.g., nutrition, age, smoking, mechanical stress), in biology (type or quality of bone graft, fusion site preparation, internal fixation), or in postoperative care (activity level, external immobilization) are reported to affect the union success rate in both human and animal studies [13, 14]. These complex factors might influence bone formation and may have induced failure of the intertransverse fusion.
Also, preparation of fusion site might be insufficient in this case. More careful preparation of the soft tissue bed or bone surface onto which the graft material is placed might be necessary, the importance of which is indicated in previous reports [15, 16].
Recently, many factors (e.g., growth factor, bone morphogenetic protein) that stimulate bone formation have been investigated as a substitute for autologous bone graft, but these kinds of bone substitutes are still prohibited for spine surgery in Japan [17, 18]. So, most bone substitutes that are used in Japan are HA and β-tricalcium phosphate, which can provide a scaffold for new bone formation. Because fusion status is critical to achieve good post-operative results, research in failed cases is very important . Further understanding of the causes and mechanisms of nonunion are important to achieve higher union rates and to avoid poor clinical results in patients after surgery.