Background Context

Different minimally invasive approaches to the lumbar spine have been proposed but they can be associated with increased risk of complications, steep learning curve and longer operative time.

Purpose of the study: Ankylosing spondylarthritis (AS) can progress to stiff thoracolumbar kyphosis which has an invalidating impact. Several publications have reported results with transpedicular osteotomies using horizontal or vertical reference lines. The purpose of our work was to report our experience with a new angle allowing the evaluation of the overall kyphosis (T1-S1) before and after correction by transpedicular osteotomy. The postoperative results were compared with the pelvic incidence (PI).

Material and methods: This was a radiographic study comparing a control group (154 asymptomatic volunteers and a group of patients with AS (n=28) who underwent posterolateral fusion associated with lumbar transpedicular osteotomy. The radiographic protocol was the same for the two groups. A large view including the entire spine was obtained (lateral and AP). We measured the classic pelvic parameters (pelvic incidence and version, sacral slope), C7 tilt and the spinosacral angle (SSA). All measured were made with computer assistance by the same operator. Twelve osteotomies were performed on L4 and 16 on L3.

Results: The PI was greater in the AS group compared with controls (61 vs 51). Seven patients have a PI < 50 (46 on average) and 21 had a PI > 50 (67 on average). For the C7 tile preoperatively, the low incidences had a low sacral slope and low pelvic version and greater global kyphosis than the high incidences (90 vs 98). In the control group, the C7 tilt and the SSA were 95.4 and 135.2 respectively. In the AS group, the C7 tilt increased from 72.6 to 83.1 (p=0.0025). The SSA increased from 96.4 to 13.3 (p=0.003).

Discussion: Pelvi with a low PI have a lower sacral slope than those with a high incidence; thus they can tolerate greater kyphosis before reaching an imbalance. For the high incidences, the pelvis has to retroverse more to obtain a low sacral slope. The minimal extension of the hips can limit this mechanism. After the osteotomy, all of the radiographic parameters were improved, but the SSA remained less than in the control group. The SSA is a good indicator of global kyphosis. Insufficient correction by unique lumbar transpedicular osteotomy explains the persistent retroversion of the pelvis postoperatively. The C7 tilt is useful to assess the improvement in the sagittal balance and the SSA give a better appreciation of the kyphosis correction per se.

This study using digitized radiographs and custom software demonstrates that patients with spondylolysis and low-grade spondylolisthesis have increased Pelvic and L5 Incidence as well as a more vertically oriented L5-S1 intervertebral disc than patients without radiographic abnormality of the spine. We propose that shear across the more vertical L5-S1 disc may underlie the etiology of spondylolysis when Pelvic Incidence is high, while a “nutcracker” mechanism may be involved when Pelvic Incidence is low.

The purpose of this study was to assess whether differences exist in sagittal alignment between normal controls and patients with spondylolysis or low-grade isthmic spondylolisthesis.

Standing PA and lateral spine radiographs from eighty-two consecutive patients with spondylolysis or low-grade spondylolisthesis (Average age nineteen, range 15–44) were retrospectively compared with those from one hundred and sixty normal volunteers. The films were digitized with a VIDAR scanner and key landmarks were determined. Customized software was then used to measure geometric indices. Pelvic Incidence (PI), Sacral Slope (SS), Pelvic Tilt (PT), and L5-S1extension angle were compared between seventy-two patients with high PI (> 45°) versus ten patients with low PI (< 45°). Average high-PI vs. low-PI values were, respectively: PI (67.32° vs. 43.13°), SS (51.08° vs. 38.05°), PT (16.23° vs. 5.08°), and L5-S1ext (−8.69° vs. −9.57°). Furthermore, the range of values for L5-S1extension in the low-PI subgroup was much narrower (−17.81° to 0.93°) than that for the high-PI subgroup (−31.58° to 38.12°).

This study demonstrates that patients with spondylolysis and low-grade spondylolisthesis have increased Pelvic and L5 incidence, a more vertically oriented L5-S1 intervertebral disc, and less segmental extension between L5 and S1 than patients without radiographic abnormality of the spine. We propose that different mechanisms underlie the etiology of spondylolysis depending on the magnitude of the Pelvic Incidence. These data highlight the importance of seeing localized lumbosacral spine disorders in the context of global alignment of the entire spine and pelvis.

Funding: This research was assisted by support from the Spinal Deformity Study Group

This research was funded by an educational/research grant from Medtronic Sofamor Danek