Examining risk factors for posterior migration of fusion cages following transforaminal lumbar interbody fusion: a possible limitation of unilateral pedicle screw fixation.

OBJECT:Because the authors encountered 4 cases of hardware migration following transforaminal lumbar interbody fusion, a retrospective study was conducted to identify factors influencing the posterior migration of fusion cages. METHODS:Patients with lumbar degenerative disc disease (125 individuals; 144 disc levels) were treated using transforaminal lumbar interbody fusion and followed for 12-33 months. Medical records and pre- and postoperative radiographs were reviewed, and factors influencing the incidence of cage migration were analyzed. RESULTS:Postoperative cage migration was found in 4 patients at or before 3 months. Because all the cages that migrated postoperatively were bullet-shaped (Capstone), only these cages were analyzed. The analysis of preoperative radiographs revealed that higher posterior disc height ([PDH] > or = 6 mm) significantly increased the incidence of postoperative cage migration, but percent slippage, translation, range of motion, and Cobb angle did not. The incidence of cage migration in patients with unilateral fixation (3 [8.3%] of 36) was not significantly different from that in patients with bilateral fixation (1 [2.1%] of 48). Patients who had scoliotic curvature with a Cobb angle > 10 degrees when treated with unilateral fixation demonstrated a tendency to have more frequent postoperative cage migration than patients treated with bilateral fixation. To examine the influence of the height of fusion cages, a value obtained by subtracting preoperative anterior disc height (ADH) or PDH from cage height was defined as "Cage height - ADH" (or "Cage height -PDH"). The analysis revealed that the value for "Cage height -ADH" as well as "Cage height -PDH" was significantly lower in migrated levels than in nonmigrated levels, suggesting that the choice of undersized cages may increase the incidence of cage migration. CONCLUSIONS:The results suggest that the use of a bullet-shaped cage, higher PDH, the presence of scoliotic curvature, and undersized fusion cages are possible risk factors for cage migration. One patient with postoperative cage migration following bilateral screw fixation underwent revision surgery, and the pedicle screw fixation was found to be disrupted. Other than in this patient, cage migration occurred only in those treated by unilateral fixation. The potential for postoperative cage migration and limitations of unilateral fixation should be considered by spine surgeons.