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Biomechanics of posterior dynamic stabilizing device (DIAM) after facetectomy and discectomy
Frank M Phillips1, Leonard I Voronov, Ioannis N Gaitanis
1Rush University Medical Center, 1725 W. Harrison Street, Chicago, IL 60612, USA.
Background Context:
Lumbar fusion has been associated with inconsistent clinical outcomes and significant complications. Posterior dynamic devices have been developed to stabilize painful diseased lumbar motion segments while avoiding fusion. The Device for Intervertebral Assisted Motion (DIAM) is a silicone interspinous process "bumper" that is being clinically implanted for varied indications.
Purpose:
We analyzed the effects of the DIAM device on the biomechanical response of the lumbar spine in flexion-extension, lateral bending, and axial rotation after partial facetectomy and discectomy; the clinical situations in which its use might be considered.
Study Design/Setting:
A biomechanical study was performed using whole lumbar spine specimens (L1-sacrum). Surgical interventions were simulated at the L4-L5 level, and motions were measured at the operated and adjacent segments.
Patient Sample:
Six fresh human lumbar spine specimens were used.
Methods:
The lumbar spines were subjected to moments in flexion-extension (+/-6 Nm), lateral bending (+/-5 Nm), and axial rotation (+/-4 Nm). The specimens were tested under the following conditions: 1) intact; 2) after unilateral hemifacetectomy at L4-L5; 3) #2 and discectomy; and 4) #3 with DIAM. The angular motion values at the operated and adjacent segments were analyzed using analysis of variance and multiple comparisons with Bonferroni correction.
Results:
Unilateral hemifacetectomy did not increase angular motion. Subsequent discectomy increased L4-L5 angular motion (degrees) from 9.2+/-1.6 to 11.7+/-2.0 in flexion-extension (p=.01), from 6.7+/-1.1 to 8.5+/-1.5 in lateral bending (p=.01), and from 2.6+/-0.7 to 3.8+/-0.8 in axial rotation (p=.00). Insertion of the DIAM device after discectomy restored the angular motion to below the level of the intact segment in flexion-extension (6.7+/-0.7 vs. 9.2+/-1.6, p=.02). In lateral bending, DIAM reduced the increased motion induced by discectomy (7.8+/-1.0 vs. 8.5+/-1.5, p<.05), but not to the intact level (7.8+/-1.0 vs. 6.7+/-1.1, p=.05). DIAM insertion did not reduce the increased axial rotation induced by discectomy, and the axial rotation remained larger than the intact value (4.1+/-0.6 vs. 2.6+/-0.7, p=.00).
Conclusions:
The DIAM device is effective in stabilizing the unstable segment, reducing the increased segmental flexion-extension and lateral bending motions observed after discectomy. In flexion-extension the DIAM restored postdiscectomy motion to below the intact values (p<.05). Interestingly, the DIAM device did not reduce the increased axial rotation motion observed after discectomy. These biomechanical effects must be considered when evaluating the clinical applications of the DIAM.
