Development and Testing of a Novel Cortical Allograft Wedge for Posterior Lumbar Interbody Fusion

Charles L. Branch, Jr MD,
Wolfgang Rauschning, MD, PhD (Uppsala, Sweden)
Mingyan Liu, PhD (Paris, France)
Laurence Boyd, MS, MEM (Memphis, TN)

The recent introduction of threaded cylindrical constructs of bone and metal has led to increased interest in the PLIF techniques first developed by Cloward in the early 1940s. However, these threaded constructs often require significant removal of posterior elements and retraction of neural structures. Recently, the authors have developed a precision machined allograft that offers significant benefits over previously available systems.

The cortical wedges allow for disc height restoration, while limiting the amount of bony resection or neural retraction required for their insertion. They are tapered to restore lumbar lordosis and feature raised teeth to assist in endplate engagement and resist retropulsion. The wedges were tested in static and dynamic axial loading. The interface achieved with the preparation and insertion instrumentation was assessed using cryosectioning of a cadaveric lumbar spine.

The average axial compressive strength of the cortical wedges was 17,980 N. This exceeds the average failure load of the vertebral body, which is 9,680 N (Adams, 1982). The cortical wedges endured 5 million cycles of axial loading at a runout value of 5,000 N. This is 1.7 times the average everyday loading values of 3,000 N reported by Nachemson in 1965. Finally, the cryosectioning study demonstrated good endplate contact and engagement and restoration of normal sagittal alignment.

The cortical wedges are sufficiently strong to resist spinal loading and allow for initial restoration of disc height and sagittal alignment. The precise fit achieved ensures load support, correction and stabilization of the spine when combined with supplemental posterior segmental fixation.