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Biomechanical analysis of a resorbable anterior cervical graft containment plate
Christopher P Ames1, Neil R Crawford, Robert H Chamberlain
1Department of Neurological Surgery, University of California, San Francisco, San Francisco, CA 94122, USA. amesc@neurosurg.ucsf.edu
Study Design:
A series of in vitro experiments were performed in human cadaveric cervical spines in the normal condition, after discectomy and graft, and instrumented with MacroPore resorbable anterior plates (MacroPore, San Diego, CA). Flexibility, graft containment, and load-to-failure were studied.
Objective:
To assess the stability, strength, and resistance to graft extrusion provided by the resorbable plate after discectomy and grafting.
Summary Of Background Data:
Metallic plates are known to improve stability after discectomy and graft. Resorbable plates have not been evaluated regarding stability offered or ability to contain the graft.
Methods:
Specimens were loaded using nonconstraining, nondestructive torques to induce flexion, extension, lateral bending, and axial rotation. One and 2-level specimens were studied: (1) normal, (2) after discectomy and graft, (3) after resorbable plating with 2 screws per vertebra, and (4) after resorbable plating with 3 screws per vertebra. All specimens were loaded to failure after completing nondestructive tests. Additional 1-level specimens with and without resorbable plate were tested for graft containment using anterior shear force directly on the graft.
Results:
The 1-level resorbable plate did not limit motion significantly better than grafted but unplated specimens. However, 2-level resorbable plates allowed significantly less motion than grafted but unplated specimens during all loading modes (P < 0.05). Specimens with resorbable plates resisted graft extrusion significantly better than unplated specimens. With 1-level resorbable plates, 2 or 3 screws per vertebra provided equivalent stability; in 2-level plates, 3 screws provided significantly better stability. Comparison to previous 1-level metallic plate data revealed a significant difference in motion only during flexion.
Conclusions:
The 1-level resorbable plate does not increase stability compared to grafted but unplated specimens and provides less stability than a metal plate, especially during flexion. However, the resorbable plate substantially improves resistance to graft extrusion. The 2-level resorbable plate significantly reduces motion compared to grafted but unplated specimens. When applying a 2-level plate, 3 screws per vertebra are recommended. In a 1-level plate, 2 or 3 screws per vertebra are equivalent.