BIOMECHANICAL CHARACTERIZATION OF SEMI-RIGID CONSTRUCTS AND THEIR POTENTIAL EFFECT ON PROXIMAL JUNCTIONAL KYPHOSIS

Long posterior instrumentation and fusion is the mainstay of treatment for adult spinal deformity (ASD); however, it is not without complications. Among the complications, proximal junctional kyphosis (PJK) is paramount, as it can lead to neurological deficits and revision surgery. Abrupt transition in relative motion between the most superior instrumented vertebrae (UIV) and the vertebrae immediately cephalad (UIV +1) is thought to play a role in the development of PJK; thus, various semi-rigid constructs were developed to help create a smoother transition between the long rigid fusion and the non-instrumented spine. The purpose of this study is to investigate if there is a biomechanical advantage between several commonly used semi-rigid constructs in human cadaveric thoracic spines. Nine fresh-frozen human cadaveric spine segments (T1–T12) were subjected to ex-vivo pure moment loading, up to 5 Nm, in flexion-extension, lateral bending, and axial rotation. Each specimen was initially tested in the native condition and then instrumented with a standard pedicle screw and rod (APS) construct from T6–T9. Semi-rigid constructs were sequentially instrumented at T5 using the following: sublaminar bands (SB), supralaminar hooks (SLH), transverse process hooks (TPH), and short pedicle screws (SS). Three testing cycles were conducted for each condition. All data normalized to the native condition. Statistical significance was established at p < .05. Mean normalized range of motion (ROM) is reported as a percentage of the native state ROM. In all directions of motion, there were no significant differences between APS and SB. In contrast, SLH, TPH, and SS significantly reduced ROM at the level of T5 – T6 as compared to APS in all directions of motion. Furthermore, SLH reduced motion significantly two levels above the fusion construct at the level of T4 – T5. However, a large change of motion was seen with SLH at the level of T3 – T4, jumping from 30% ROM to 100% ROM. Both SLH and SS were found to have a high degree of stiffness. In all planes of motion, TPH was found to have the most linear change in motion on a linear regression analysis. This biomechanical study supports previous research that semi-rigid constructs can decrease ROM at adjacent levels and possibly provide a more gradual change in motion. Although SLH substantially decreases ROM, it may lead to increased stress at superior levels as an abrupt change in motion was seen over one level, similar to APS. In contrast, a more gradual change in motion was found with TPH, in keeping with results of previous studies. Overall, this study provides spine surgeons with a direct biomechanical comparison of commonly used semi-rigid constructs and suggests that TPH is best able to provide a smooth transition in motion. Therefore, TPH should be considered over other constructs to top-off long rigid spinal fusions to help decrease the risk of PJK.