Introduction

The anatomical structures composing the spinal column provide a delicate balance between stability and flexibility. Congenital, neuromusculoskeletal, traumatic, neoplastic, infectious, degenerative as well as iatrogenic changes to any of the anatomical structures involved can disturb this balance and cause segmental instability. Different diagnostic approaches and different definitions have been proposed to distinguish pathologic instability from physiological flexibility [1, 2]. While segmental instability can occur in all motion planes, translational instability in the sagittal plane is of special interest [1,2,3,4,5], as it can result in compression of neural structures, has been associated with disc degeneration [3], facet joint osteoarthritis [3], Modic changes [6] as well as clinical symptoms [5]. Translational instability has further been used as a criterion to decide whether to perform spinal fusion additionally to decompression surgery [7]. While functional radiography is a useful tool to assess segmental instability, several limitations have to be considered: No clear cut-off values are defined to distinguish pathologic from physiological motion [1, 8], no consensus on the optimal measurement methods exists [1], reproducibility of the measurements is limited [9] and the execution can be affected by symptoms such as pain [10]. Real-time mechanical measurement of the anteroposterior translation in the surgical situation could eliminate some of these problems such as the effect of muscle tension due to pain or the limited resolution and the projection error in functional radiographs. Furthermore, the availability of such a method could provide novel insights about the extent of anteroposterior translation in the native segment as well as after interventions such as decompression surgery or in specific clinical situations such as vertebral fractures. This information could help to better define the concept of segmental instability.

The aim of this study was to develop an instrument to measure the anteroposterior translation motion of a spinal segments in a simulated surgical situation (with the interfering surrounding tissue) and in the first step, to validate the obtained measurements by well-defined reference measurements of the isolated spinal segment in an advanced biomechanical test setup.

Materials and methods

Simulated surgical situation

Ethical approval for the cadaveric experiments was obtained by the local authorities (BASEC Nr.: 2021-00,207, Kantonale Ethikkomission, Kanton Zürich, Switzerland). To simulate the surgical situation, five fresh-frozen torsi of human cadavers (Table 1) were thawed to room temperature and bedded in a prone position.

Table 1 Specimen overview
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A dorsal midline approach to the lumbar and lower thoracic spine was performed and the interspinous and supraspinous ligaments were resected. No further decompression steps were performed. The vertebral bodies T11 – L5 were instrumented with commercially available polyaxial pedicle screws (M.U.S.T, Medacta International, Castel San Pietro, Switzerland) following the traditional trajectory. A commercially available and FDA-approved reposition tool (Expedium SI Pedicle screw system, one handed rod approximator, 279,712,510, DePuy Synthes Spine, Raynham, MA, USA) was modified with two 5.5 mm steel rods to be compatible with the tulip design of the used pedicle screws. Further, a load sensor (FL25-100 kg, Nonrepeatability ± 0.2 of R.O, Nonlinearity ± 0.5% of R.O, Forsentek Co., Limited, Longgang Dist. Shenzhen, China) and a displacement transducer (S-Series, OP12.5, DS/30, Linearity < 0.2% FSO, Resolution < 0.2 µm, Solartron Metrology Limited, West Sussex, UK) were added to record the load–deflection curves with a specifically developed application in Labview (LabVIEW 2017, National Instruments, Austin, TX, USA). Therewith, real-time feedback on the applied loads was visualized. The modified reposition tool (Repo-Tool) allowed to generate anterior and posterior shear loading by applying compressive or tensile load on the handles, while the setup design limited the motion of the vertebra to the translational motion in the sagittal axis.

After fixation of the Repo-Tool to the pedicle screws with two set screws, 5 loading cycles of compression and tension resulting in 100 N anterior and 100 N posterior shear loading where employed by two readers to evaluate inter-operator reliability of the method. The limit of ± 100 N was chosen as good tradeoff between not risking harm to the patients (as the setup is designed to be used intraoperatively) [11] and still reaching the steep exponential zone of the load displacement curve. An acoustic signal indicated, when the ± 100 N was reached. The measurements were performed on either side of every instrumented spinal level totaling in 12 measurements per reader per specimen. Between testing, the specimens were frequently strayed with phosphate buffered saline (PBS) to prevent dehydration. Testing was performed at room temperature. For data evaluation, the displacement at ± 100 N was determined by taking the passing point at the sloping part of the load–displacement curve (Fig. 2A). To evaluate the necessity of multiple loading cycles, data analysis was repeated by considering only one, two, three and four of the performed loading cycles. Average displacement values between the considered loading cycles were taken for data evaluation.

Reference method

To evaluate the accuracy of the measurements with the Repo-Tool, three spinal segments (T12/L1, L2/L3 and L4/L5) of every torso were resected and fixed with 3D printed clamps [12, 13] to be loaded in a spine testing machine (Fig. 1C) [14, 15]. After 5 preconditioning cycles, ± 100 N anteroposterior shear loading was applied with a testing speed of 0.5 mm/s during 5 cycles and the average motion was used for further analysis. To correct for any unintended motion in the test machine or in the fixation of the specimens [12, 13], a 3D motion capture system (Atracsys Fusion Track 500, 10 Hz record frequency, tracking accuracy 0.09 mm) was used to measure the relative motion between the two vertebrae in the sagittal axis. Testing was performed at room temperature, and the specimens were frequently sprayed with PBS to prevent dehydration. For data evaluation, only the peak displacement at ± 100 N was evaluated (Fig. 2B).

Fig. 1
figure 1

The modified reposition tool (Repo-Tool): A functionally illustrated B experimental setting C Reference method (figure adapted from [14])

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Fig. 2
figure 2

Illustration of raw data measured by A the Repo-Tool and B the reference method

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Statistical evaluation

Initial data analysis was performed on the average motion recorded on all 5 loading cycles. To evaluate the inter-operator reliability of the Repo-Tool measurements, an inter-correlation coefficient (ICC) analysis was performed between the measurements of the two operators. The variability between the bilateral measurements was evaluated equally on the averaged values obtained by both operators. The validity of the Repo-Tool (single operator with unilateral measurement, single operator with bilateral measurements and two operators with bilateral measurements) was evaluated by computing the ICC with the corresponding motion data obtained by the reference measurement. The ICC (1,1) according to Shrout and Fleiss [16] was used. The effect of the number of loading cycles with the Repo-Tool was evaluated by repeating the above-mentioned analysis on first 1, 2, 3, 4 and 5 loading cycles. Statistical analysis was performed on Matlab (Matlab R2019a, Mathworks Inc.) and statistical significance was assumed with p < 0.05.

Results

During the experiments, no adverse events such as pedicle fractures, screw loosening or implant breakage occurred. All measured data was available and used for statistical evaluation. The mean translation motion for the reference measurement was 1.53 mm with a standard deviation of 1.14 mm. The mean ap-translation motion for the Repo-Tool was measured at 1.96 mm with a standard deviation of 1.04 mm considering the values recorded at both sides and by both operators. The measured values for each specimen, for both operators and for both sides as well as for the reference system are listed in a table as supplementary material. The ICC for the intra-reader assessment, which arises from the measurements of two different operators, was 0.93 (p < 0.0001) with a 95% confidence interval (CI) of 0.88–0.96 (Fig. 3). The ICC between the measurement on the left and the right side was 0.73 (p < 0.0001, CI = 0.40–0.90) (Fig. 4). The ICC between the averaged values of the Repo-Tool (both operators and both sides) and the reference system was 0.82 (p < 0.0001, CI = 0.56–0.94) (Fig. 5). The ICC was also evaluated with the Repo-Tool data shifted with a 0.44 mm (see “Discussion” section), which resulted in an ICC of 0.9 (p < 0.0001, CI = 0.74–0.97). When performing the same analysis for the two operators independently, the ICC without offset correction was 0.68 for operator one and 0.58 for operator two. With offset correction it was 0.84 and 0.76. For individual side measurements, the ICC value for the right side for operator one was 0.72 without correction and 0.85 with correction. For operator two the ICC was 0.7 and 0.84, respectively. The ICC value for the left side for operator one was 0.56 without correction and 0.74 with correction and for operator two these values were 0.39 and 0.56, respectively.

Fig. 3
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Inter-operator dependency of the Repo-Tool measurements

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Fig. 4
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Side dependency of the Repo-Tool measurements

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Fig. 5
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Comparison of the Repo-Tool and the reference measurement

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Discussion

The object of this study was to develop and validate a surgical device for direct real-time quantification of spinal segmental stability. The newly developed method to measure anteroposterior translation in the simulated surgical situation shows “moderate” to “excellent” reliability [17] with an ICC of 0.82 (0.56–0.94) compared to the reference measurements (Fig. 5), despite the differences in boundary conditions. The reliability was even higher with an ICC of 0.9 (0.74–0.97) when the constant offset in the displacement measurements was corrected for. The offset value of 0.44 mm was determined from the experimental data and is assumed to arise from the slightly different loading conditions between the Repo-Tool and the reference measurements. While the reference technique constrains all rotational degrees of freedom, this is not inherently ensured with the Repo-Tool and the operator has to ensure that no lateral tilting of the Repo-Tool occurs during measurement. While this aspect should not interfere with the measurements under conditions in which all bony and mechanical structures are completely rigid, this is not the case in reality and the torque induced by the Repo-Tool can result in some tilting of the screws in the bone, which can increase the recorded displacement. While this aspect has to be considered, the method shows “good” to excellent” inter-operator reliability with an ICC of 0.93 (0.8–0.96), which indicates that the measurements are not strongly dependent on the executing operator (Fig. 3). An ICC of 0.73 (0.40–0.90) indicating only “poor” to “good” reliability was determined for the bilateral measurement. Interestingly, this ICC is the lowest and we assume it to arise from anatomical left–right asymmetries (Fig. 4). Particularly, the morphology and the degenerative state of the facet joints might cause this finding. While additional loading cycles can potentially further increase the reliability of the Repo-Tool measurements, our findings suggest that the measurements are sufficiently reliable after two loading cycles (Fig. 6). In a potential clinical application of the method, the gain in reliability by averaging the measurements by two operators (ICC of 0.9) instead of one (ICC 0.76–0.84) must be weighed against the increase in measurement time. Similarly, the benefit of measuring both sides must be weighed against a single side measurement. Since single operator, single side measurements were providing an ICC ranging from 0.56 to 0.85, compared to the more robust ICC values when measuring on both sides (0.76–0.84), the authors evaluate the benefit of eliminating potential outliers with both side measurements to outweigh the additional measuring time; however, the additional benefit of a second operator appears less evident. Performing two measurement cycles by one operator on either side appears to be a good compromise between reliability of the results and duration of the measurements.

Fig. 6
figure 6

Cycle dependency of the Repo-Tool measurements

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These findings pave the route for the ethical and regulatory processes necessary to introduce the modified reposition tool into the surgical theater as a diagnostic tool. Accurate intraoperative measurement of the anteroposterior translation motion can potentially benefit the surgically treated patient by augmenting the instrumentation construct in a very unstable situation (e.g., with the addition of an intervertebral anterior support). However, since the method relies on pedicle screws, the decision on whether instrumentation is required cannot be supported as it must be made beforehand. The ability to measure anteroposterior translation in a surgical situation will however open a new way to gain information about the translational stability of spinal segments in specific clinical situations. It will furthermore allow to validate clinically applicable measurements techniques for segmental instability such as functional radiographs or facet joint distance in MRI. As a secondary goal, it could be used to directly measure the effect of surgical interventions such as decompression surgery; however, the analysis whether the sensitivity of the Repo-Tool is sufficient for this purpose lies outside of the scope of this study.

While the results are very promising, certain limitations must be considered: Both the measurement in the simulated surgical situation as well as the reference measurements were performed on fresh frozen cadavers at room temperature. While this aspect can limit the direct transferability of the results into the clinical situation, it allows to create largely equal conditions for the comparison between the two methods. As the interspinous and supraspinous ligaments were resected during the preparation of the torsi, the specimens were not in their native state. However, since this was equal for both measurement techniques, and since the interspinous and supraspinous ligaments provide only about 1% to the stability of the spine in anteroposterior shear loading [14], the effect on the results should be minimal. As it has not been in the scope of this project, the Repo-Tool has not been analyzed on whether the destabilizing effect of surgical interventions such as decompression or nucleotomy can be measured with sufficient accuracy and this aspect must be further investigated. However, based on the obtained results the authors suspect the reliability of the Repo-Tool to be sufficient for this type of analysis. The potential effect of the testing sequence (Repo-Tool measurements first) is evaluated to be of minor importance for the recorded results. Even though both the Repo-Tool measurements as well as the reference measurements were designed to apply only transversal shear loading in the sagittal axis while restricting coupled motion such as flexion, extension or axial rotation, small evasive motions due to bone, implant and test setup deformation cannot be excluded, particularly with the Repo-Tool. While the 3D motion capture device in the reference method helps limit this potential effect, no such correction was performed with the Repo-Tool. Also, the identification of the sagittal axis in the surgical model was purely based on the visible anatomical landmarks and must be assumed to be more variable than the motion plane of the reference measurements, which was planned with three-dimensional CT models of the vertebral bodies in Blender (Blender Foundation, Amsterdam, the Netherlands). Lastly, the large lever arm of the Repo-Tool could result in unintended rotation of the vertebral bodies, which could in turn generate a measurable translational force at the load cell. Despite these mechanical limitations, good correlation between the Repo-Tool measurements and the reference measurements were found.

In conclusion, the novel methodology allows to measure the anteroposterior translation of spinal segments in a simulated surgical situation with “moderate” to “excellent” reliability [17], which opens up the possibility to perform such measurements in an intraoperative situation.