Loading Characteristics of Femoral Ring Allografts in Anterior Lumbar Fusion

· (b-Medtronic Sofamor Danek)

PURPOSE: A biomechanical study of femoral ring allografts when used for anterior lumbar interbody fusion to better understand the pattern of loading, effect of graft surface, orientation, and endplate preparation.

BACKGROUND: Lumbar interbody fusion with femoral rings is becoming increasingly popular. Unfortunately, graft-related problems are among the most common complications in this operation. Good mechanical purchase between allograft and vertebrae requires endplate preparation. This was shown by the small irregular contact area on grafts without endplate preparation. Even the most meticulous manual technique does not compare with instrumented enplate preparation. Our results lead to the conclusions that the strongest portion of the graft should be placed anteriorly, more of the graft should be present anteriorly, and it is desirable to take off less endplate bone anteriorly. Because of the corrugated surfaces of the allografts, the increased area of contact also means greater frictional purchase, and thus greater mechanical stability.

METHODS: Ten human cadaver spines were harvested from L1 to sacrum and constrained to allow only axial loading along its natural lordosis. The L4-L5 disc was excised and the endplates prepared using two different methods. A novel procedure for visualizing contact area and stress on the graft was performed using pressure-sensitive film. A digital image analyzer was used to accurately interpret the images on film. Two types of graft were used, and the spine was loaded in various positions.

RESULTS: Graft loading is irregular even with meticulous manual endplate preparation with curettes, resulting in higher peak stresses. It was consistently more regular, and peak stresses lower when endplates were prepared using specialized tools. The endplate surfaces were smoother, pressure distribution was wider, and thus lower stresses. Greater purchase between graft and vertebrae was noted when contact area was greater. Graft loading proceeded from mostly posterior in extension to anterior in flexion. Highest loads were seen in flexion, in the anterior part of the graft, and the posterior sees the lowest forces throughout ranging.

CONCLUSIONS: Our findings provide a general guideline to optimize the biomechanics of lumbar interbody femoral ring allograft placement.

· If noted, the author indicates something of value received. The codes are identified as: a-research or institutional support; b-miscellaneous funding; c-royalties; d-stock options.