Do Radiographic Results of Transforaminal Lumbar Interbody Fusion Vary with Cage Position in Patients with Degenerative Lumbar Diseases?

Abstract

Objective: To investigate whether the radiographic results are affected by cage position in single-level transforaminal lumbar interbody fusion (TLIF).

Method: Between January 2016 and June 2018, 130 patients (62 males and 68 females, average age: 55.28 ± 10.11 years) who underwent single-level TLIF were analyzed retrospectively. Standing lateral radiographs of the lumbar spine were collected and evaluated preoperatively, postoperatively, and at the time of last follow-up. Cage position in the fused segment was recorded using a central point ratio (CPR), which indicated the cage position. CPR is calculated by dividing the distance between the cage center point and the posterior extent of the superior endplate of the inferior vertebra by the length of the superior endplate of the inferior vertebra. Based on cage positions, the patients were divided into three groups: Anterior Group (n = 38); Middle Group (n = 68); and Posterior Group (n = 24). Segmental lumbar lordosis (SLL), foraminal height (FH), posterior disc height (PDH), and anterior disc height (ADH) were evaluated. A subanalysis was also performed on cage height within each group.

Results: The average follow-up time of the patients was 35.20 ± 4.43 months. The mean values of CPR in Anterior Group, Middle Group, and Posterior Group were 0.64, 0.51, and 0.37, respectively. The FH, PDH, and ADH were significantly increased after TLIF in all groups (P < 0.05). There were significant differences in increase of SLL in Anterior Group (4.4°) and Middle Group (3.0°), but not in Posterior Group (0.3°). Furthermore, in the comparison of the three groups, the increase of SLL, FH, and PDH was statistically different (P < 0.05), while not for ADH (P > 0.05). The significant correlations in surgery were: CPR and ΔSLL (r = 0.584, P < 0.001), CPR and ΔFH (r = -0.411, P < 0.001), and CPR and ΔPDH (r = -0.457, P < 0.001). However, ADH had a positive correlation with cage height when the cage was located in anterior and middle of the endplate. Moreover, cage height had a positive correlation with SLL when the cage was located anteriorly and had a negative correlation with SLL when the cage was located posteriorly. FH and PDH both had a positive correlation with cage height in any cage position.

Conclusion: The cage located in different positions has different effects on radiographic results in single-level TLIF. A thicker cage located anteriorly will gain maximum SLL and avoid the reduction of FH and PDH.

Keywords: Cage position; Disc height; Foraminal height; Segmental lumbar lordosis; Transforaminal lumbar interbody fusion.

Fig. 1

(A) Measurement of center point ratio (CPR). The triangle indicates the cage center point. CPR (x/y) is calculated by dividing the distance between the cage center point and the posterior extent of the superior endplate of the inferior vertebra (x) by the length of the superior endplate of the inferior vertebra (y). (B) Segmental lumbar lordosis (SLL) is determined by the angle formed by the lower endplate of the vertebra above the instrumented disc and the upper endplate of the vertebra below the instrumented disc. Foraminal height (FH) is measured as the longest distance between the lower border of the superior pedicle and upper border of the inferior pedicle. Posterior disc height (PDH) is measured as the vertical distance between the posterior end of the inferior and superior endplates, while anterior disc height (ADH) is the distance between the anterior ends.

Fig. 2

Lateral radiographs of the typical cases at preoperative, postoperative, and the final follow‐up: (A) case of posterior cage placement; (B) case of middle cage placement; (C) case of anterior cage placement. The preoperative center point ratio (CRP) values of the typical cases were 0.32, 0.51, and 0.66, respectively. Correspondingly, the changes in segmental lumbar lordosis (SLL) after surgery were −2.8°, 3.2°, and 6.4°, respectively. The changes in SLL at final follow‐up were −2.5°, 3.0°, and 6.1°, respectively. This indicated that the increase of SLL was correlated with the cage position.

Fig. 3

(A) Relationship between center point ratio (CPR) and change in segmental lumbar lordosis (SLL); (B) Relationship between CPR and change in foraminal height (FH); (C) Relationship between CPR and change in posterior disc height (PDH); (D) Relationship between CPR and change in anterior disc height (ADH)

Fig. 4

Relationship between cage height and changes in segmental lumbar lordosis (SLL) with anterior (A), middle (B), and posterior (C) cage placement. Cage height had a positive correlation with SLL when the cage was located anteriorly and had a negative correlation with SLL when the cage was located posteriorly. If the cage was located in the middle, cage height had no significant correlation with SLL.

Fig. 5

Relationship between cage height and changes in foraminal height (FH) with anterior (A), middle (B), and posterior (C) cage placement. FH was positively correlated with cage height in any cage position.

Fig. 6

Relationship between cage height and changes in posterior disc height (PDH) with anterior (A), middle (B), and posterior (C) cage placement. PDH was positively correlated with cage height in any cage position

Fig. 7

Relationship between cage height and changes in anterior disc height (ADH) with anterior (A), middle (B), and posterior (C) cage placement. ADH had a positive correlation with cage height when the cage was located in anterior and middle of the endplate. If the cage was located in the posterior, cage height had no significant correlation with ADH.