In vitro biomechanical effects of reconstruction on adjacent motion segment: comparison of aligned/kyphotic posterolateral fusion with aligned posterior lumbar interbody fusion/posterolateral fusion

Abstract

<jats:p content-type="fine-print"><jats:italic>Object.</jats:italic> Posterior lumbar interbody fusion (PLIF) was developed to overcome the limitations of posterolateral fusion in correcting spinal deformity and maintaining lumbar lordosis. In this study the authors compare the biomechanical effects of three different posterior reconstructions on the adjacent motion segment.</jats:p> <jats:p content-type="fine-print"><jats:italic>Methods.</jats:italic> Ten calf spinal (L2—S1) specimens underwent nondestructive flexion—extension testing (± 6 Nm). The specimens were destabilized at the L5—S1 levels after intact testing. This was followed by pedicle screw fixation with and without interbody cages as follows: 1) with straight rods (“aligned” posterolateral fusion); 2) with kyphotically prebent rods (“kyphotic” posterolateral fusion); and 3) with interbody cages combined with straight rods (“aligned” PLIF/posterolateral fusion). The range of motion (ROM) of the operative segments, the intradiscal pressure (IDP), and longitudinal lamina strain in the superior adjacent segment (L4–5) were analyzed.</jats:p> <jats:p content-type="fine-print">The ROM associated with aligned PLIF/posterolateral fusion-treated specimens was significantly less than both the aligned and kyphotic posterolateral fusion-treated procedures in both flexion and extension loading (p &lt; 0.05). The aligned PLIF/posterolateral fusion was associated with greater IDP and the lamina strain compared with the aligned and kyphotic posterolateral fusion groups in flexion loading. Under extension loading, greater IDP and lamina strain were present in the kyphotic posterolateral fusion group than in the aligned posterolateral fusion group. The highest IDP and lamina strain were shown in the aligned PLIF/posterolateral fusion group.</jats:p> <jats:p content-type="fine-print"><jats:italic>Conclusions.</jats:italic> Compared with kyphotic posterolateral fusion, PLIF may lead to even higher load at the superior adjacent level because of the increased stiffness of the fixed segments even if local kyphosis is corrected by PLIF.</jats:p>

Are the spines of calf, pig and sheep suitable models for pre-clinical implant tests?

“…The calf is the species mainly used in vitro. Tests with pedicle screw systems are counted among these experiments [3,4,6,9,11,17,18]. According to the results of the present study, these experiments reveal the right trends but should not be interpreted quantitatively.…”

“…Similar to the calf, the pig is also often used in vitro. Both calf and pig are models used to measure IDP [1,8,[12][13][14]18]. These measurements reveal the implant's general effects on intradiscal pressure expected in healthy human intervertebral discs but may differ from the human in magnitude.…”

Biomechanical study on the effect of five different lumbar reconstruction techniques on adjacent-level intradiscal pressure and lamina strain

“…1,18,20,21 The flexion and extension moments ranged from 0 to 6 N-m, which include the ranges of moments used in previous biomechanical studies using lumbar reconstruction models. 1,9,10,18,21 As pilot studies have previously demonstrated that load-displacement and load-strain or pressure data after the second load-unload cycle were almost identical to the second, the loading procedure involved the application of a three load-unload cycle. The third loading cycle was used for data analysis of all biomechanical parameters.…”

“…A displacement gauge (Model 25C-20, MTS Systems, Inc. Minneapolis, MN) spanning the operative segment was utilized to quantify segmental displacement 18,21 (if two-segment fusion was simulated, the gauge was placed over the whole fusion length of L5 to S1 the lateral side of the L4-5 intervertebral disc into the center of the nucleus pulposus. 6,21 The insertion point and depth of the needle was controlled using both anterior-posterior and lateral radiographs. Uniaxial surface-strain gauges (KFG-02-120-C1-11L3M2R; Kyowa Dengyo Co., Tokyo, Japan) were sagittally mounted on the bilateral L4 lamina just craniad to the L4-L5 facet joint by using cyanoacrylate.…”

“…However, the details of these interactions or the synergistic effects of these factors are not yet fully understood. In our previous biomechanical study, 21 it was demonstrated that comparing posterolateral fusion with residual kyphotic deformity, the PLIF/ posterolateral fusion treatment may lead to an even higher load on the adjacent segments because of the increased stiffness in the fixed segments even if the local kyphosis is corrected by PLIF. It was only simulated, however, a two-segment fixation model and did not investigate the effects of the number of fixed segments.…”