Can Posterior Lumbar Interbody Fusion with Carbon Spacers Restore Lordosis and Disc Height?

Introduction Posterior lumbar interbody fusion can theoretically allow neural decompression directly and by restoration of disc height and appropriate lumbar lordosis. The technique of insertion of a trapezoidal lordotic wedge spacer (ramp) into the disc space before rotating it into position theoretically will obtain both an increase in disc height and allow correction of lordosis. However observations suggest that incongruity between a flat implant and a curved end plate, and possible settling of the implant into the vertebral body may limit the ability of the technique to achieve its full theoretical potential. This paper attempts to establish the capacity of this technique to (1) restore disc height, and (2) alter segmental lordosis. Methods Preand post-operative lateral radiographs were obtained from 34 patients who had undergone posterior lumbar interbody fusion using carbon fibre spacers with a lordotic angle of five degrees. Supplemental pedicle screws were used in all cases. The procedure was performed at l2/3 in one case, at l3/4 in two cases, at l4/5 in 16 cases and l5/1 in 15 cases. Measurements of preand post-operative lordosis, anterior and posterior disc height, slip percentage and anterior and posterior positioning of the prosthesis were made. To allow for comparison of length measurements the raw data were normalised by dividing by the inferior end plate length. Results Stepwise multiple linear regression showed the only variable to be related to final post-operative lordosis was pre-operative lordosis (p = 0.026). There was no relationship between final lordosis and implant placement or slip percentage. The regression line suggested that small pre-operative segmental angles (less than 7.5 degrees) were increased post-operatively while large pre-operative angles (greater than 7.5 degrees) were reduced. This suggests that the segment is attempting to accommodate to the five-degree implant. The regression equation only explains 14% of the total variance (rsquared = 0.144). The mean normalised posterior disc height increased significantly by 55% (0.1195 to 0.1844) (paired t test p < 0.0001) and the mean normalised anterior disc height increased by 18% (0.27151 to 0.32251) (paired t test p < 0.007). Changes in both anterior and posterior disc height were highly correlated with pre-operative disc height (r = -0.6729 p < 0.0001, r = -0.7402 p < 0.0001). Discussion Posterior lumbar interbody fusion using a five degree wedged spacer can lead to significant improvements in anterior and posterior disc height when the disc space is narrowed and maintain disc height when the disc height is normal. The insertion of a wedged implant causes the segment to approximate the lordosis of the implant. The variation is however large. Possible causes for this variation are a mismatch between the flat implant and a curved end plate and end plate subsidence. Having a curved implant end plate and a selection of lordotic angles may possibly reduce the former effect.