Biomechanics of Nested Transforaminal Lumbar Interbody Cages

Minimally Invasive Transforaminal Lumbar Interbody Fusion: Comparison of Grade I Versus Grade II Isthmic Spondylolisthesis

Background

Minimally invasive transforaminal lumbar interbody fusion (MIS-TLIF) is often used to treat low-grade isthmic spondylolisthesis (IS). No studies have compared surgical outcomes for grade I and II IS following MIS-TLIF. Therefore, the objective of the current study was to compare outcomes between patients with grade I and II IS following MIS-TLIF.

Methods

A retrospective cohort analysis was performed on a prospectively maintained database of patients who underwent a primary 1-level MIS-TLIF for treatment of IS between 2007 and 2015. Grade I patients underwent a unilateral tubular approach with a single interbody cage and bilateral pedicle screw instrumentation. Grade II patients underwent a bilateral tubular approach with bilateral interbody cage and pedicle screw placement. Baseline patient demographics and characteristics were compared using Student t test and χ² analysis. Differences in peri- and postoperative outcomes were assessed using Poisson regression with robust error variance or linear regression adjusted for perioperative variables.

Results

A total of 58 patients with IS underwent MIS-TLIF; 21 (36.2%) were grade I and 37 (63.8%) were grade II. The grade I cohort was younger (42.2 versus 50.6 years, P = .029); no other differences in preoperative variables were observed. No significant differences in operative time, estimated blood loss, length of hospital stay, postoperative visual analogue scale scores, or complication and revision rates were demonstrated between cohorts. Arthrodesis rate was lower in the grade I cohort, though not statistically significant.

Conclusions

Despite the grade I cohort being younger with less-severe diagnoses, the grade II cohort experienced similar outcomes. This finding may be due to the grade II cohort receiving bilateral cages, potentially providing a better fusion environment.

Clinical Relevance

These results suggest that MIS-TLIF provides sufficient stabilization and fusion for treatment of grade II IS despite increased vertebral body displacement. In addition, MIS-TLIF with bilateral approach and interbody cage placement should be examined for treatment of high-grade IS cases.

Design and material evaluation for a novel lumbar disc replacement implanted via unilateral transforaminal approach

The degeneration of the intervertebral disc is one of the principal causes of low back pain. Total disc replacement is a surgical treatment that aims to replace the degenerated disc with a dynamic implant to restore spine biomechanics. This paper proposes the first design of an elastomeric lumbar disc replacement that is implanted as a pair of devices via unilateral transforaminal surgical approach. Furthermore, several biomaterials (Polyurethanes (PU) and Polycarbonate Urethanes (PCU)) are evaluated for the purpose of the implant to mimic the axial compliance of the spine. Bionate II 80A (a pure PCU), Elast Eon 82A E5-325 (a PU with polydimethylsiloxane and polyhexamethylene oxide), Chronosil (a PCU based silicone elastomer) 80A with 5% and 10% of silicone were obtained and injection moulded according to the shape of the implant core, which was defined after a stress distribution analysis with the finite element method. The dimensions for each specimen were: 14.6 × 5.6 × 6.1 mm (length, width and height). Quasistatic compression tests were performed at a displacement rate of 0.02 mm/s. The obtained stiffness for each material at 1 mm displacement was: Bionate II 80A, 448.48 N/mm; Elast Eon 82A E5-325, 216.55 N/mm; Chronosil 80A 5%, 127.73 N/mm; and Chronosil 80A 10%, 126.48 N/mm. Dimensional changes were quantified after two quasi-static compression tests. Plastic deformation was perceived in all cases with a total percentage of height loss of: 4.1 ± 0.5% for Elast Eon 82A E5-325; 3.2 ± 0.5% for Chronosil 80A 10%; 2.7 ± 0.3% for Chronosil 80A 5% and 1.1 ± 0.2% for Bionate II 80A. The mechanical behaviour of these biomaterials is discussed to assess their suitability for the novel disc replacement device proposed.