Results of the nucleus replacement unconfined compression tests

Figure 2 shows the load–displacement curve obtained during these tests. The first relevant difference between designs 1 and 2 is the inflexion point for design D2 with a 300- to 350-N compression load. This “jump” is due to the central partition wall in the implant deforming when going over 300 N.

Fig. 2

Unconfined compression load–displacement curve characteristic of each implant design

Average and standard deviation values for both designs and non-sterilized samples regarding implant stiffness and deformation at different load ranges are presented in Fig. 3 as raw data and box and whisker plots. The data were analyzed to find any statistically significant differences between the two designs with an ANOVA followed by a post hoc analysis. The results showed that there were statistically significant differences in the mechanical behavior between both implant designs. D2 was stiffer than D1 under physiological (500–2000 N) and higher compression loads. When looking at the 25 kGy gamma sterilization influence, irradiated implants were stiffer at physiological loads (500–2000 N), while this difference was inverted for higher loads (2000–6000 N), where they showed minor deformation.

Fig. 3

Results from the unconfined compression tests

Results for the artificial annulus confined compression tests

The annulus compression stiffness was 403.06 ± 9.94 (SD) N/mm at 0–300 N, 1116.59 ± 8.03 N/mm at 500–2000 N and 3059.89 ± 82.28 N/mm at 2000–6000 N. Thus, these data remained within the ASTM WK 4863 recommended ranges for the natural annulus under physiological loads (500–2000 N/mm) [10, 18] (Additional File 8: Fig. S8).

Results for the nucleus replacement confined compression tests

Both designs offered a similar behavior load–displacement curve compared with unconfined compression tests. Figure 4 presents the average and standard deviation values for the artificial disc complex (nucleus replacement + silicone annulus) stiffness and deformations at different load ranges for implant designs, represented by box and whisker plots. No statistically significant differences could be seen between the implant designs, apart from the D1 results showing higher dispersion. Furthermore, in the ANOVA analysis, no statistically significant differences were found in any parameters in the confined compression, unlike the unconfined compression.

Fig. 4

Confined compression implant stiffness and deformation under different compression loads

Results for the nucleus replacement confined compression + shear tests

Under this test, both designs had very similar behavior. Average and standard deviation values for the artificial disc complex (nucleus implant + silicone annulus) stiffness and deformation at different load ranges and box and whisker plots are presented in Fig. 5. They show a distance between the stiffness of the two designs in the physiological load range, but no statistically significant differences were seen in the other mechanical parameters. The ANOVA analysis confirmed that both designs had different stiffness in the physiological load range (500–2000 N), but not in any of the analyzed parameters in the confined compression, probably because the silicone artificial annulus minimizes the mechanical differences between the two designs.

Fig. 5

Confined compression + shear test implant stiffness and deformation under different loads

Conclusions from the static strength tests

D1 had a more continuous and softer behavior under lower loads in unconfined compression tests, while D2 showed a “jump effect,” probably due to central partition wall deformation. There were statistically significant differences in stiffness and deformation between D1 and D2 under physiological and higher compression loads. D2 was stiffer than D1, and D2 deformed 1 mm more than D1. The sterilized implants were more rigid under physiological loads and deformed less.

Both designs showed equivalent behavior under confined compression, and when adding shear, D1 was stiffer in the physiological load range. In addition, the silicone annulus minimized the mechanical differences between the two designs.

Results from fatigue tests

The average cumulative wear measured every million cycles during the fatigue test, 4 days after it (measurement points 11–13), and 1 month later (measurement points 14–15) for both implant designs are shown in Fig. 6.

Fig. 6

Evolution of the average cumulative volumetric wear during and after the compression fatigue test for both implant designs

At the end of the 10 million cycles, design D1 had shrunk by an average of 1.47 mm3, equivalent to 0.07% of first implant volume (2144.67 mm3), and design D2 had shrunk by an average of 4.75 mm3, which is equal to 0.19% of first implant volume (2043.72 mm3). A few days after the test, design D1 recovered its size, and rehydrated, resulting in a complete weight rest

Besides wear, the main implant dimensions were periodically measured, looking for cyclic compression-induced permanent deformation. Every million cycles, implant samples were photographed in distinct positions using a magnifying glass and dimensions calculated with image analysis software. Figure 7 shows the evolution of the implant dimensions during and after the fatigue tests for both designs.

Fig. 7

Average implant dimensions during and after fatigue tests for both designs

Under compression loading, the most affected dimension was the implant’s height, particularly for D1 (1.5 mm reduction), but D1 recovered better after it. The long-term permanent height deformation was 1 mm for D1 and 0.5 mm for D2. The side-to-side width did not significantly change for D1, while it increased 0.5 mm for D2. The anterior-posterior depth was not affected in D1, while D2 spread in that direction during the test and contracted after it. To summarize, D1 suffered a permanent deformation in height but kept its shape in the transverse plane. On the other hand, D2 suffered a minor height loss compared with D1, but it underwent a permanent deformation in the transverse plane.

Conclusions from the fatigue test

Both designs showed excellent mechanical responses to compression fatigue with no breaks, cracks, or delamination. However, at 10 million cycles, D2 showed threefold more extensive wear than D1. After the test, hydration induced weight recovery: complete for D1 but partial for D2 due to permanent wear. Both designs lost height after fatigue compression loading and recovered it partially after the test, once unloaded. The loss was more significant for D1 than for D2, but D1 regained it better after the test. D1 did not show significant changes in depth or width at the end of the trial, while D2 was deformed in the transverse plane (0.5 mm of anterior-posterior reduction and medial-lateral augmentation). After a detailed analysis of the static and fatigue tests results, D1 was chosen for further biomechanical testing because it offered a better and more homogeneous behavior.

D1 design wear performance test results

D1 had a tendency for an almost linear wear increase with the number of test cycles. The nucleus implant’s average volumetric wear rate was 3.22 ± 0.91 mm3 per million cycles (Mcc). After 7 million cycles, the average cumulative wear was 23.58 ± 7.93 mm3; this was 1.09% of the first implant volume, which can be considered an acceptable result (Additional File 9: Fig. S9). These results are within the range of values published for other commercial disc nuclei replacements [19,20,21].

When looking at the effects of the wear test on the main implant dimensions, a specific permanent deformation was seen in design D1. As in the case of compression fatigue, there was a height loss (Fig. 8). The principal height reduction was produced at the beginning of the test (1 mm at 0.5 million cycles) and this was stable until 3 million cycles when there was a partial height recovery of about 0.6 mm until 5 million cycles. Then, the height decreased from 5 to 6 million cycles, with an average value of 0.6 mm. Finally, in the last million cycles, the implant height kept stable. After 7 million wear test cycles, the average implant’s height was 1 mm lower than at the start.

Fig. 8

Evolution of the implant’s height, width, and depth in the wear test (up to 7 million cycles), compared with the controls (underwent the same loadings but without motion)

Dimensions in the transverse plane were also affected by wear test mechanical conditions (Fig. 8). From 0 to 6 million cycles, medial-lateral width increased about 0.75 mm. Width increase was mainly produced at the beginning of the test, after the first 0.5 Mcc, and then it kept relatively stable until 6 million cycles. Then it increased again.

Anterior-posterior (AP) depth was more affected than the side-to-side dimension. From 0 to 3 million cycles, the tendency was to increase the AP depth, with an average of 1.5 mm (Fig. 8). From 3 to 5 million cycles, the AP depth tended to keep stable. From 5 million cycles onwards, the AP dimension suffered fewer oscillations. Under physiological loading conditions, the trend was for the nucleus implant to increase the AP depth 1–2 mm and the medial-lateral width 0.5–1.5 mm.

Conclusions from the D1 design wear test

All the tested nucleus implants showed a good mechanical response to multi-axial loading wear testing as neither breaks, cracks, nor delamination was seen in any sample. After 7 million cycles, wear suffered by the nucleus implant was relatively low, comparable to other commercial disc nuclei replacements [19,20,21]. However, under the wear test loading conditions, the nucleus implant suffered a permanent deformation. After 6 million cycles, the implant’s height decreased by 1 mm and the width and depth increased by 0.75 mm and 1.5 mm, respectively. Under normal physiological loading conditions, the trend is that implant dimensions change mainly initially and then keep relatively stable.