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Evaluation of a lumbar intervertebral spacer with integrated screws as a stand-alone fixation device
Brian P Beaubien1, Andrew L Freeman, Joseph L Turner
1Gustilo Medical Education Center and Midwest Orthopaedic Research Foundation, 700 10th Avenue South, Minneapolis, MN 55415, USA. bpwbeaubien@gmail.com
Study Design:
An in vitro cadaveric biomechanics study.
Objective:
To evaluate the biomechanical contribution of the integration of screws into a polyether-ether-ketone (PEEK) spacer, and to compare the resulting construct to standard anterior lumbar interbody fusion constructs.
Summary Of Background Data:
Pedicular fixation is often performed to increase the rigidity of anterior lumbar interbody fusion constructs but also increases the risk of morbidity. Integration of screws into a PEEK spacer (investigational device, ID) may increase construct rigidity and prevent migration without the drawbacks associated with supplementary fixation.
Methods:
Twenty cadaveric motion segments were potted and tested under pure moments of + or - 7.5 Nm in flexion-extension, lateral bending, and axial torsion. Discectomies were performed and specimens were instrumented with the ID + or - screws, dual threaded cages, structural graft+anterior plate, and graft+pedicle screws. The ID+screws and threaded cage constructs were then pulled out at a constant rate.
Results:
All constructs reduced the mean range of motion (ROM) below the intact level in flexion-extension and lateral bending (P<0.001) and for all devices except cages in torsion (P < or = 0.05). The median flexion-extension/bending/torsion ROM was 9.5/9.4/4.1 degrees for the intact segments, 6.1/5.1/1.8 degrees with the ID+screws, 4.9/5.2/2.4 degrees with threaded cages, 3.3/4.4/1.1 degrees with plates and 1.4/1.6/1.7 degrees with pedicle screws, respectively. The addition of the integrated screws decreased the mean ROM of the ID constructs by 0.9 degrees in flexion-extension, 1.8 degrees in bending, and 2.8 degrees in flexion-extension. The peak pullout force was 962 N for the ID and 337 N for threaded cages.
Conclusions:
The incorporation of screws into a PEEK interbody device was evaluated alongside traditional constructs in terms of construct rigidity and resistance to pullout. The greatest contributions of the integrated screws are in limiting torsional motion and in the containment of the interbody device. Pedicular fixation produces the most rigid constructs, but integrating screws into a PEEK spacer produces a stand-alone construct that is similar to threaded cages and provides more resistance to anterior displacement.
