Critical Evaluation of Biomechanical Principles and Radiographic Indicators for Fusion Assessment in a Novel...

The objectives of this paper were to identify and explain specific design factors for lumbar interbody fusion (IBF) devices that can influence bone exchange and stability at the vertebral endplate interface and to provide supporting evidence of these factors through both laboratory and clinical data. The laboratory study (Part 1) compared the pressure profiles and contact areas for a minimally invasive, expandable, and conformable porous mesh (CPM) IBF device and a rigid monolithic lateral PEEK cage (LPC). Furthermore, to demonstrate how these laboratory results translate clinically, a quantitative and qualitative assessment of subject x-rays and computed tomography (CT) scans from a US Food and Drug Administration (FDA) investigational device exemption (IDE) trial of the CPM was performed (Part 2).

Part 1: Load profile testing. Either CPM or LPC was sandwiched between 2 flat or shaped Grade 15 foam blocks. Each implant type was compressed at a rate of 0.1 mm/s for 3 loads (1100, 2000, or 3000 N). Device and bone graft contact area were analyzed for each test condition, and corresponding load profiles were quantified and mapped using pressure film. Part 2: Radiographic fusion assessment. Two independent radiologists analyzed 12- and 24-month motion studies and CTs for fusion, defined as bridging bone across the intervertebral space. The same CTs were assessed for qualitative biomechanical signs of bone healing.

CPM demonstrated significant direct loading on the bone graft across all tested loading conditions, while the LPC graft registered a negligible amount of pressure at only the extreme load of 3000 N. Contact area was in turn statistically greater (P < .05) for CPM. CPM fusion rates were 97.9% and 99% at 12 and 24 months, respectively. Radiographic signs of bone healing are described in terms of radiating bone struts and regions of greater intensity.

CPM allows for an optimized contact area for bone exchange and graft incorporation. The load profiles demonstrate widespread load sharing across the device. The expandable, compliant, porous mesh provides a unique area for bone exchange, contributing to qualitative biomechanical radiographic evidence of bone healing that ultimately leads to clinically acceptable fusion rates as observed in the FDA IDE trial.