Thoracolumbar Deformity Arthrodesis to L5 in Adult Scoliotics: The Fate of the L5-S1 Disk: Is it Protected by a Deep-Seated L5?

Purpose: For long deformity fusions to L5 the advantage of preserving the L5-S1 motion segment must be weighted against the potential for accelerated L5-S1 disk degeneration (DDD). Unfortunately, the incidence of subsequent L5-S1 DDD and the protective role of a deep-seated L5 have not been well defined. The purpose of this study is to define the incidence and sequella of subsequent L5-S1 DDD, and determine if a deep-seated L5 provides a protective role.

Methods: 34 consecutive adult deformity pts fused from the thoracic spine to L5 at a single institution underwent radiographic, clinical and SRS-24 evaluation. Excluded were pts with previous lumbar spine procedures at levels distal to L4. For each pt., the L5-S1 disk was preoperatively assessed by x-ray, MRI and/or discography as being free of significant degeneration. Subsequent advanced 5-1 DDD was defined by the L5-S1 disc height >50%, disk lordosis >50% or >3mm new listhesis. L5 seating was characterized by the distance from the midpoint of the L5 pedicles to the intercrestal line (ICL). A ‘deep seated’ L5 was defined by the L5 pedicles >10mm below the ICL. Mean f/u was 5.5 yrs. (2.0-12.8 yrs).

Results: For the 34 pts (mean age: 47; 20-61), an average of 10.5 levels (5-15) were fused. By latest f/u, DDD developed in 23 of 34 pts (66%), disk height >50% (n=13,38%), disk lordosis >50% at L5-S1(n=17,50%) or >3 mm new listhesis(n=4,12%). 8 pts with L5-S1 subsequent DDD (35%) had >5 cm anterior shift of the C7 plumb from postop to latest f/u compared to only one pt among those with preserved 5-1 discs (10%)(p=0.15). Subsequent DDD resulted in arthrodesis extension to the sacrum for 4 pts at 3.5 yrs avg. L5 was seated on avg. 6mm below the ICL (15 to-35mm). Pts with a deep seated L5 had significantly less subsequent DDD at final f/u (43% vs. 75%), (p=0.03). Smokers tended to have incidence of DDD(87%) vs. nonsmokers (47%), (p=0.18). The mean total SRS-24 and subset analyses tended to be superior for pts without subsequent DDD (p>0.05): total SRS-24 (noDDD=90 vs +DDD=81), pain (30vs25), self image (21vs20), function (26vs24) and satisfaction (13vs12) at 4.5 yr mean f/u.

Conclusion: Subsequent 5-1 DDD developed in 66% of pts after long adult fusions to L5. Relative but not absolute protection against progressive DDD is provided by a deep-seated L5 (p=0.03). SRS-24 total score showed a trend toward superior outcome when subsequent L5-S1 DDD did not develop.