Study design, setting and equipment
This was a laboratory-based accuracy study conducted in February 2018 [24]. Experiments were designed and conducted following the International Organization for Standardization (ISO 5725-1) for the accuracy of measurement methods and results [24].
VerteTrack overview
The VerteTrack frame (width 1080 mm × height 1090 mm × length 1, 510 mm) suspends an aluminium gantry that supports movement of a rolling indenter head (RIH) in three axes: X-axis (longitudinal, superior-inferior), Y-axis (transverse, left-right) and Z-axis (vertical, posterior-anterior) via stepper motors (resolution = 0.007 mm, www.stepperonline.com, China) (Fig. 3). A string potentiometer (resolution = 0.020 mm, accuracy ±0.010 mm, TE Connectivity, USA) is used to record Z-axis displacement. A vertically-oriented laser assists the operator to align the RIH upon pre-determined anatomical landmarks (GLX Laser Site, Barska). During spinal stiffness assessment, the VerteTrack applies discrete loads via addition of weighted plates (“plates”) with a nominal mass of 1 kg each (RIH + k plates; k = 0, 6). These loads were selected as they represent loads that have previously been used in VerteTrack studies [22, 23, 26] and are comparable to loads applied in other mechanical indentation studies [14, 20]. Plates were numbered and always added in the same order for each indentation cycle. For more detail about the VerteTrack see Brown et al. 2017 [22].
Methods of indentation
The VerteTrack can perform two modes of indentation testing: single-level and multiple-level continuous indentation. Single-level indentation assesses a single spinal level and requires the operator to position the RIH directly above the target tissue. Loads are then applied incrementally to the spine in a posterior to anterior direction with the resulting deformation of the spinal tissues recorded (Z-axis displacement). Multiple-level continuous indentation requires the operator to first identify the spinal trajectory that the RIH will travel within the horizontal (X-Y) plane. This is achieved by manually aligning each spinous process (determined by palpation or ultrasonography) with the RIH using the embedded laser pointer. The laser points are memorised by the device and then replayed to move the RIH continuously along the same pre-defined trajectory for each successive load. The resolution of the RIH is identical to the resolution of stepper motors (0.007 mm).
Load and displacement precision
Load precision (random error) of the VerteTrack was estimated by the coefficient of variation (CV = SD / load mean) over 10 repetitions for each load. The RIH was measured using recently calibrated digital scales (OHAUS, model TS4KD: Resolution 0.1 g, accuracy ±0.07 g) (Fig. 4, panel a). Each plate was added to the RIH, then repeated up to a total of 5 plates. Loads were converted to Newtons (N) using mass (kg) x gravity (9.81 m/s2). Displacement precision (z-axis, depth) of the VerteTrack was also estimated using coefficient of variation over 10 repetitions at each of 6 discrete levels of the RIH on a custom-engineered wooden wedge to simulate tracking of a spinal sagittal curve (Fig. 4, panel b).
Load and displacement bias
Load bias (systematic error) was estimated by comparing each load delivered through the VerteTrack against the same load externally. Mean load bias was estimated by calculating the differences between reference loads and loads measured by the VerteTrack, and the 95% confidence interval of the difference [25]. Reference loads were calculated by the addition of successive plates placed directly upon the digital scale (i.e. not through the VerteTrack RIH) plus the load measured through the RIH alone. Each reference load (k plates; k = 1, 5) was measured ten times. Displacement bias was also estimated using the same method employed to determine load bias. Mean displacement bias was determined over 10 repetitions at each of 6 discrete levels as reported by the VerteTrack, compared to an external digital calliper (Wixey, WR200: Resolution = 0.05 mm, accuracy ±0.025 mm) (Fig. 4, panel b).
Comparison of single-level and multiple-level continuous operation
A method-comparison experiment was conducted to evaluate the performance of VerteTrack for measurement of stiffness during multiple-level continuous and single-level (reference) modes of operation. Terminal stiffness values (i.e. the ratio of the maximum load to the maximum displacement) [26] were used in our analysis. The stiffness of a deformable foam test medium (AIREX® balance beam, Switzerland) was measured during both single-level and multiple-level continuous modes of operation to simulate measurement at a single vertebral level and across multiple vertebral levels respectively. The test medium was chosen to emulate the physiological stiffness encountered for the in vivo adult lumbar spine (range: 2–10 N/mm) [12, 20, 26]. Five equidistant locations (5 cm apart) were marked on the foam medium along a straight line (RIH landing, L1, L2, L3 and RIH lift-off) for stiffness assessment (Fig. 4, panel c).
Precision during single-level and multiple-level continuous indentation
Precision of the VerteTrack during measurement of stiffness on the test medium was estimated by the coefficient of variation (CV = SD / stiffness mean) over 300 trials for both single-level and multiple-level continuous indentation. Stiffness was measured during multiple-level continuous indentation (Stiffnessmultiple) and single-level indentation (Stiffnesssingle) at three discrete locations (L1, L2, L3) on the medium. Incremental loads (plates) were added to the RIH in a predefined sequence (RIH + k; k = 1, 6). Between each trial, 90 s elapsed to allow for any residual deformation to resolve. Between each cycle (six trials of increasing load), an additional 5 min elapsed to allow any residual deformation to resolve after the maximum load was applied to the medium. A total of ten cycles were performed.
Single-level versus multiple-level continuous indentation
Each trial for Stiffnessmultiple was compared to Stiffnesssingle, to quantify bias between indentation methodologies we calculated the stiffness differences and 95% confidence intervals of the difference. Bias calculation, and a plot of raw stiffness data were conducted to assist interpretation. In addition, Lin’s Concordance Correlation Coefficient (LinCCC, Rc) was reported for load and displacement. LinCCC tests both agreement and linearity [27]. The strength of agreement was graded as “almost perfect” (Rc > 0.99), “substantial” (Rc > 0.95–0.99), “moderate” (Rc > 0.90–0.95), or “poor” (Rc < 0.90) [28]. Alpha was set at 0.05 for all statistical significance tests of agreement.