Does the Wilson frame assist with optimizing surgical exposure for minimally invasive lumbar fusions?
OBJECT
Minimally invasive lumbar spine surgery has dramatically evolved over the last decade. Minimally invasive techniques and transforaminal lumbar interbody fusion (TLIF) often require a steep learning curve. Surgical techniques require pre-positioning the patient in maximal kyphosis to optimize visualization of the disc space and prevent unnecessary retraction of neural structures. The authors describe their experience in validating the surgical technique recommendation of Wilson frame-induced kyphosis.


METHODS
Over the past 6 months, data obtained in 20 consecutive patients (40 total levels) undergoing minimally invasive TLIF were reviewed. In each patient, preincision intraoperative radiographs were reviewed at L4-5 and L5-S1 with the patient on a Wilson frame in maximal lordosis and then in maximal kyphosis. The change in disc space angle at L4-5 and L5-S1 after changing from maximal lordosis to maximal kyphosis was reviewed. Descriptive statistics were calculated for sagittal plane angular measures at L4-5 and L5-S1 in lordosis and kyphosis, including absolute differences and percentage of change between positions. Inferential statistics were calculated using paired t-tests with alpha= 0.05.


RESULTS
Twenty patients underwent single- or multilevel minimally invasive TLIF. Inducing kyphosis with the Wilson frame aided in optimizing exposure and decreasing the need for neural structure retraction. Both L4-5 and L5-S1 showed statistically significant (p < 0.001) and clinically meaningful changes with increased segmental flexion in the kyphotic position. At L4-5 the mean increase in flexion was 4.5 degrees (95% CI 2.9-6.0 degrees), representing an average 47% change. The mean increase in flexion at L5-S1 was 3.2 degrees (95% CI 2.3-4.2 degrees), representing an average 20.8% change. In lordosis the mean angle at L4-5 was 10.6 +/- 4.4 degrees and at L5-S1 was 17 +/- 7.0 degrees. In kyphosis the mean angle at L4-5 was 6.1 +/- 4.5 degrees and at L5-S1 was 13.8 +/- 6.5 degrees. Additionally, there was a statistically significant difference (p < 0.05) in percentage of change between the 2 levels, with L4-5 showing a greater change (27% more flexion) between positions, but the absolute mean difference between the levels was small (1.3 degrees).


CONCLUSIONS
Minimally invasive TLIF is challenging and requires a significant learning curve. The recommended surgical technique of inducing kyphosis with the Wilson frame prior to incision significantly optimizes exposure. The authors' experience demonstrates that this technique is essential when performing minimally invasive lumbar spinal fusions.