In vitro biomechanical evaluation of a strutted intradiscal spacer for lumbar discectomy

Background: Discectomy plus implantation of a strutted intradiscal spacer has been shown to reduce reoperations and reherniations versus discectomy alone following lumbar disc herniation. This study explored the underlying biomechanics of the intradiscal spacer. Methods: Six fresh-frozen cadaveric lumbar spine specimens (L2 to L5) from three donors were used. Following box annulotomy to simulate disc herniation, a discectomy was performed. One segment from each donor was randomly assigned to either an untreated control group or the test group where an intradiscal spacer was implanted. A six degree of freedom universal spine tester assessed range of motion (RoM) in flexion/extension, lateral bending and axial rotation in the native state in load controlled [f7.5 Nm] and intervals up to 60,000 cycles. Disc height was measured on fluoroscopy for multiple timepoints. The segments were also analyzed to detect possible reherniation during the cycling loading. Findings: Following 60,000 cycles, the mean percentage RoM increase versus the intact state was less for discectomy plus the intradiscal spacer versus discectomy alone for lateral bending (170.7 f 10.0 vs. 222.5 f 33.3 %), flexion/extension (178.5 f 6.1 vs. 204.6 f 44.3 %) and axial rotation (284.4 f 127.2 vs. 362.3 % f 240.4 %). Mean overall disc height loss versus the annulotomy state was also less with the intradiscal spacer versus discectomy alone (-19.3 f 3.7 vs. -29.1 f 6.1 %). There was no evidence of device subsidence or migration. Interpretation: This study helps to explain the clinical observation that insertion of a strutted intradiscal spacer following discectomy reduces reherniation rate by mechanically limiting the increase in RoM and disc height loss following lumbar discectomy.