Biomechanics of one-level anterior cervical discectomy and plating using two screws versus four screws

Background context: Most one-level anterior cervical plates use two screws per vertebra (four screws in total). No study has addressed whether a simplified plate using one screw per vertebra is adequate for one-level fixation.

Purpose: To compare stability achieved by four-screw and two-screw plates after discectomy and placement of interbody spacer.

Study design: Nondestructive multidirectional flexibility tests were performed in three independent groups of cadaveric spines to assess spinal stability after instrumentation.

Methods: Human cadaveric C4-C7 specimens were tested intact and after discectomy followed by placement of a polyetheretherketone interbody graft and an anterior plate. Rigid two-screw (n=8), semiconstrained four-screw (n=8), and rigid four-screw (n=8) plates were compared. Nonconstraining pure moments were applied under load control (maximum 1.5 Nm) to induce flexion, extension, lateral bending, and axial rotation, whereas vertebral motion was measured optoelectronically. Mean range of motion (ROM) was compared among groups.

Results: All three plates significantly reduced ROM relative to normal in all directions of loading (p<.003). Mean ROMs±standard deviation (and corresponding intergroup p value) for rigid two-screw, semiconstrained four-screw, and rigid four-screw plates, respectively, were as follows: flexion: 2.6±2.0°, 1.8±1.1°, 1.8±0.8° (p=.46); extension: 2.5±2.6°, 2.1±1.3°, 1.4±1.3° (p=.45); lateral bending: 1.8±1.0°, 1.3±1.0°, 1.1±0.5° (p=.29); axial rotation: 2.9±1.9°, 1.6±0.9°, 1.5±0.7° (p=.08). Despite a tendency for the rigid two-screw plate to allow more motion than the four-screw plates, there was no significant difference among groups during any loading mode.

Conclusions: In terms of immediate postoperative cervical stability after one-level discectomy and placement of an interbody spacer, the rigid two-screw plate performed comparably to conventional rigid four-screw and semiconstrained four-screw plates. Further research on relative fatigue endurance of the different plate types is also needed.