gms | German Medical Science

Objective: Disc degeneration causes decreased pressures, leading to overload and injury of the annulus fibrosus, which can increase the risk of disc herniation. Discectomies can cause further loss of pressures and increased degeneration. New annular closure devices (ACDs) that reinforce the compromised annulus are emerging and may reduce reherniation rates while also preventing the need for excessive nuclear material removal during discectomy. Theoretically, this could curtail the progression of degeneration after discectomy by maintaining preoperative disc height and pressure. The purpose of this in vitro study was to investigate whether implantation of an ACD during discectomy could restore nucleus pressures to preoperative levels.

Method: Nine fresh-frozen human cadaver lumbar discs were potted in liquid plastic and tested in a custom apparatus which applied unconstrained loads and moments. A pressure transducer was introduced from the anterior and monitored intradiscal pressure. Combined flexion and lateral bending was simulated by applying 2.7 kN of compressive load and 24 Nm of moment. Three rounds of testing were performed on each specimen: 1) baseline (intact); 2) immediately following posterior anulotomy (6 mm tall by 10 mm wide postero-lateral defect); and 3) immediately following implantation of the ACD.

Results: Average intact intradiscal pressure was 16.02 atm. After anulotomy, average pressure decreased significantly to 9.96 atm under identical loading conditions (p=0.0026, paired Student’s t-test). After implantation of the ACD, average intradiscal pressure was restored to 15.80 atm, insignificantly different from baseline (p=0.8515). Normalizing by intact values, the average intradiscal pressure after anulotomy and after Barricaid implantation was 63.7% and 101.3%, respectively. There was no evidence of nuclear extrusions in any of the specimens after intact testing. After a anulotomy was simulated, five of the nine specimens extruded nucleus. The average volume of herniated material was 0.36 cc (range 0.2 to 0.6 cc). No extrusions of nucleus were observed after ACD implantation.

Conclusions: Closure of annular defects with an ACD at the time of anulotomy has the potential to restore intradiscal pressures. Normal physiologic pressures help regulate many key cellular processes within the disc and may therefore promote improved disc health post- anulotomy. Further studies are needed to determine whether this has any clinical effect on lumbar degeneration.