Biomechanical restoration in unilateral facet arthroplasty: a cadaveric evaluation

Facet incompetency can lead to altered spine biomechanics. However, while total posterior arthroplasty techniques in lieu of fusion are of increasing interest, unilateral techniques are yet to be investigated. This study aimed to assess the biomechanical effect of a unilateral facet arthroplasty (UFA) system on motion and loading characteristics at the treated L4-5 level.

Seven cadaveric spines from L3-Sacrum were tested within the multidirectional pure moment bending model between 0 to ± 10 Nm in flexion-extension (FE), flexion-extension with 400 N of compressive follower load (FE-FL), axial rotation (AR), and lateral bending (LB). We measured the L4-L5 angular range of motion, the intradiscal pressure, and the contralateral facet contact force. Each specimen was tested in intact (I), capsulotomy (C), medial facetectomy (MF), total facetectomy (TF), and UFA conditions. We also measured the load transfer at the bone-implant interface in the UFA condition.

UFA significantly restored 93% (FE), 77% (FE-FL), 123% (AR), and 106% (LB) of the native range of motion (p<.05). UFA significantly restored the native intradiscal pressure during compressive follower load (I:173±67KPa vs. TF:103± 82KPa vs. UFA:174±69KPa), and partially during contra-axial rotation (I:32±27KPa vs. TF:81±55KPa vs. UFA:59±46KPa). No significant changes were observed in facet contact force. Contra-axial rotation induced the most substantial loads transferred through the bone-implant interface.

UFA is most effective at restoring native intervertebral disc load transfer without altering facet loading at the treated level. UFA is equivalent to the intact condition in FE, FE-FL, and LB, but larger in AR, even after reverting 55% of induced instability.