Wednesday, September 26, 2018 7:35 AM–9:00 AM ePosters: P9. Potential proximal junctional kyphosis prophylaxis using a novel rib and transverse process band tethering technique in the thoracic spine: a biomechanical study

Abstract

BACKGROUND CONTEXT Proximal junctional kyphosis (PJK) and failure (PJF) are established complications following surgical correction of adult spinal deformity, particularly in patients with low bone density. To mitigate the risk, alternative fixation solutions to traditional pedicle screws (PS) have been proposed. Polyester band tethering at the upper instrumented vertebra (UIV) is a popular treatment option; however, the sub-laminar anchor point requires midline excision of ligamentous structures, potentially augmenting biomechanical risk factors thought to play important roles in the development of PJK. Rib/transverse process (R/TP) band tethering is a novel technique that aims to provide a strong bone-implant interface without compromising adjacent segment stability. PURPOSE The purpose of this study was to investigate the novel use of the rib and TP complex as the anchor point for band fixation at the UIV of a long fusion construct to potentially mitigate the risk of PJK in osteoporotic patients. STUDY DESIGN/SETTING Cadaveric biomechanical study using a six-degrees-of-freedom machine. PATIENT SAMPLE A total of 20 specimens. OUTCOME MEASURES Range of motion. METHODS Twenty thoracic spines (C7-L2) with intact rib cages were divided into four groups matched for bone mineral density (BMD). All specimens were instrumented with bilateral PS and rods from T6-L2. Groups were defined by mode of fixation at the cephalad levels (T4–T5): (1) T4–T5 PS (rigid), (2) T4 sub-laminar bands and T5 PS (SL), (3) T4R/TP bands and T5 PS (R/TP), or (4) T4R/TP bands and T5 sub-laminar bands (hybrid). The starting point of the R/TP tethering technique was 5 mm lateral to the costotransverse joint. The band was passed under both the rib and transverse process and then tensioned and secured posteriorly to the rod with a clamp. Specimens were subject to a pure moment of ±5 Nm in flexion–extension (FE), lateral bending (LB), and axial rotation (AR) both before (intact condition) and after instrumentation. Motion data were normalized to the mean intact condition, and comparisons at the adjacent (T3–T4) and first instrumented (T4–T5) levels were made between groups (p≤.05). RESULTS The BMDs of rigid, SL, R/TP, and hybrid groups were 1.03, 1.06, 1.09, and 1.05 g/m2, respectively (p=.983). At the first instrumented level, each group reduced motion in all 3 bending modes compared to intact. However, when comparisons were made between groups, R/TP demonstrated significantly less motion restriction in FE (vs. all groups) and LB (vs. rigid) (p≤.05). Increases in motion compared to intact at the adjacent level were observed in the hybrid and SL groups (FE: 139.2%, 187.6%, LB: 111.4%, 139.9%, and AR: 97.6%, 114.4%, respectively); statistical significance was seen in FE for the hybrid group (p≤0.05). Whereas, no increases in motion at the adjacent level were observed in the R/TP and rigid groups, with the exception of LB (FE: 92.7%, 97.6%, LB: 83.4%, 123.5%, and AR: 98.3%, 99.7%, respectively). CONCLUSIONS Band tethering at the UIV is an evolving alternative technique in patients with compromised bone and pedicle anatomy. When compared to other tethering techniques, R/TP bands provide comparable stability in LB and AR at the UIV of long fusion constructs. Moreover, it avoids the sacrifice of midline structures, which, as this study demonstrated, leads to increased adjacent level motion. Utilizing the rib and TP complex as an alternative anchor point for band fixation is a viable tethering technique and may mitigate the risk of PJK in osteoporotic patients.