Comparison of Short-Segment Pedicle Instrumentation with Supplemental Hook Fixation under Axial Compression in Relation to Graft Positioning and Posterior Ligamentous Integrity: A Biomechanical Study on the Calf Spine

Abstract

Study design: Biomechanical study.

Purpose: This study investigates the benefits of supplemental hook fixation (SHF) on short-segment pedicle instrumentation (SSPI) in relation to anterior strut graft positioning. In addition, it seeks to determine whether the integrity of the posterior ligamentous complex (PLC) affects the stability of the spinal construct.

Overview of literature: Implant and/or bone failure with progressive kyphotic deformity after SSPI is common. To prevent this, several approaches are available, including SHF, anterior strut grafting, use of longer spinal constructs, and extension of the fusion to additional adjacent segments.

Methods: A total of eight calf spines were instrumented with SSPI (n=4) and SHF (n=4) with strain gauges on the implants. Strain measurements were performed under axial compression in the following order: intact spine, corpectomy, ventral positioned strut grafting, posterior positioned strut grafting, ventral positioned grafting with resected PLC, and corpectomy with resected PLC.

Results: The SHF group showed slightly lower strain values than SSPI in instrumented corpectomy-only specimens, but there were no statistically significant differences between them (p >0.05). The SHF group was significantly more stable than SSPI when strut grafting is employed, regardless of the location of the grafts (p =0.000). In the SSPI group, ventral positioning of the graft contributed significantly to the stability (p =0.000). There was no statistically significant difference between the ventral or posterior positioning of the graft in the SHF group (p =0.187). In addition, the integrity of the PLC did not affect stability in either group (p >0.005).

Conclusions: Although not statistically significant, our investigation demonstrated that the most stable method was the SHF along with ventral positioned strut graft. However, if the SSPI is the treatment of choice, ventral positioned strut graft support will be useful in minimizing the risk of implant failure and progressive kyphotic deformity.

Keywords: Biomechanics; Bone grafting; Instrumentation; Lumbar vertebrae; Spinal fractures.

Fig. 1.

(A, B) Illustration showing preparation of specimens and strain gauge locations. (C) Test specimen placed on the universal testing device. Exposed areas of specimens were covered with saline soaked gauze during the tests.

Fig. 2.

Comparison of group 1 (short-segment pedicle instrumentation) and 2 (supplemental hook fixation) in test 3 (corpectomy and ventral strut graft) (A), test 4 (corpectomy and posterior strut graft) (B), and test 5 (corpectomy, anterior strut graft, and resected posterior ligamentous complex) (C). Supplemental hook fixation significantly reduces strain ratios on the implant system. * p=0.000.

Fig. 3.

Comparison of test 3 (corpectomy and ventral strut graft) and test 4 (corpectomy and posterior strut graft) in roup 1 (SSPI). Ventral graft positioning significantly reduces strain ratios on the implant system in the SSPI group. SSPI, short-segment pedicle instrumentation. * p=0.000.