It is known that anterior cervical disectomy and fusion are being used most frequently to treat trauma and diseases in the cervical area [1-4
]. Currently, Smith-Robinson's method [1
] for intervertebral bone graft is being adopted most commonly. The complication rates have been reported to be 9.4-49% across all methods based on bone graft [6
]. The complications include pain, hematoma, infection, lateral femoral cutaneous nerve injury, ilium fracture, peritoneal perforation, hernia, and cosmetic problems [7,8
]. According to Sawin et al. [9
] in patients who experienced postoperative pain at the donor site for autogenous bone graft, 36% of patients continued to experience pain one year postoperatively. Even within four years postoperatively, 31% of patients experience pain at the donor site. In the current study, in the group where the autogenous bone was grafted, patients complained of pain occurring at the donor site. In comparison with patients who received a cage, patients who received autogenous bone grafts took significantly longer to ambulate. To avoid postoperative complications that may occur while bone samples are harvested, methods that entail a cage and alternative bone substitute to bone graft have been introduced.
The cage and bone substitute methods enjoy advantages that include reduced bleeding, operation time and skin scars during harvesting of bone graft materials and there is no need for postoperative orthosis. Thus, early ambulation can be achieved and prompt return to daily lives is possible. At our institution, the use of orthosis is recommended for approximately six weeks in patients who received autogenous bone graft and for up to three days in patients who received a cage.
In the current study, a tricalcium phosphate (TCP) cage was used. TCP has osteogenic potential comparable to that of hydroxyapatite. It has also been reported to produce excellent treatment outcomes in implant denaturation [10
]. According to Xie et al. [11
], TCP has a higher degree of absorptivity compared with hydroxyapatite. It is therefore replaced more promptly by new bones. However, there are also disadvantages, such as lower degree of mechanical rigidity compared with hydroxyapatite. In patients who received a TCP cage, as shown in the current study, the period of bone union was prolonged due to a lack of osteoinductibity. The disadvantages of TCP include rupture and extrusion of cage and cage endplate subsidence. According to Zulkefli et al. [12
] the union rate at 6-month follow-up was 95% in patients who received autogenous bone graft, and 62.1% in patients who received a hydroxyapatite cage. It has been reported, however, that the rate of bone union at final follow-up is close to 100% and the rate of clinical success exceeds 90% [13-15
]. In the current study, the mean period of union was prolonged by approximately 33 weeks in patients who received a cage. In three patients, no union was seen during the follow-up period (at a 12-, 15- and 18-month follow-up, respectively) and there were no notable clinical complications. It is envisaged that continual follow-up would be needed until bone union is achieved.
According to Kettler et al. [16
] and Wilke et al. [17
] subsidence in the cage endplate arises from instability due to mobility of the cervical area following disectomy. Subsidence is variable with cage endplate design. Compared with bone segment, however, subsidence has been reported to occur at higher incidence. Van Jonbergen et al. [18
] reported that subsidence in the endplate occurred at a mean length of 1.06 mm during a 6-month follow-up period. In addition, Schmieder et al. [19
] also reported that it occurred at a mean length of 1.13 mm in the anterior region, and 0.9 mm in the posterior region at 2-year follow-up. Reports suggest that endplate subsidence significantly retains increased intervertebral disc space compared with that preoperatively. It is able to improve the overall sagittal alignment of the cervical vertebrae, and its severity and incidence have no effects on clinical outcomes. In the current study, the mean endplate subsidence was increased by approximately 1 mm in patients who receive a cage compared with patients who received autogenous bone graft. This difference reached statistical significance. Because the period of bone union was prolonged in patients who received a cage, it is assumed that the above results may be related to longer exposure to instability due to cervical movement.
Our results also showed that the curvature of lordosis was increased in both patient groups (8.3° in Group A vs. 4.7° in Group B). There was significant difference between both groups. These results suggest that there is an advantage in forming curvature based on the surgeon's judgment compared with patients in whom a cage was used of curved type by using a wedge shape struct bone for the autogenous bone graft. It is also presumed that the endplate subsidence, presumably occurred as a result of delayed bone union, may be a contributory factor. However, there was no significant difference in clinical symptoms between the two groups. In the current study, however, there was significant difference in endplate subsidence and curvature on radiography performed at final follow-up in 7 Group A patients and 4 Group B patients. It is therefore assumed that continuous follow-up should be performed until there are no changes in these parameters at two consecutive follow-ups in the 11 aforementioned patients.