Compressive Fatigue Behaviour of a Polymer-Based Osteochondral Scaffold

Y-H. Hsua, C Luptona, J. Tonga, A. Cosseyb, A. Auc

a Mechanical Behaviour of Materials Laboratory, School of Engineering, University of Portsmouth, UK b Queen Alexandra Hospital, Portsmouth, UK c Smith & Nephew, Advanced Surgical Devices, Andover, MA, USA

Abstract

Cyclic loading from daily activities or from intensive exercise can lead to increased risk of fracture. Implants designed for load bearing purposes, such as repair of articular cartilage and underlying subchondral bone in knees must have the necessary resistance to fatigue loadings. To date, virtually no studies have been reported on the fatigue behaviour of osteochondral scaffolds, despite damage due to repeated loading is of considerable interest for tissue repair purposes.

In this study, a polymer-based osteochondral scaffold has been studied under cyclic loading conditions. Multi-step cyclic tests have been carried out with increasing compressive loads in a phosphate buffered saline solution at 37oC. Changes in secant modulus and residual strain accumulation are monitored. Morphological parameters of the scaffold are determined using micro-computed tomography. The secant modulus and the number of cycles to failure of the scaffold are obtained and compared with those of human trabecular bone (Topolinski et al, 2011).

1. Introduction

A variety of biological and synthetic scaffolds have been developed for articular cartilage and subchondral bone repair purposes.1-11 However, scaffolds often lack the physical structure and mechanical properties necessary to sustain long-term applications.12 Although biocompatibility and bio-related issues have been studied widely, evaluation of mechanical properties has been an area of underdevelopment, with testing parameters and conditions varied greatly; and testing under controlled physiological loading conditions similar to those in vivo particularly lacking. Evaluation of the mechanical properties of scaffolds is not always done in aqueous solution at 37°C, although the mechanical properties of hydrated scaffold samples are known to be much lower than those of dry samples.4-6 13 A number of scaffolds and bones were tested in tension,8, 14, 15 which bears little resemblance to the in vivo loading environment primarily in compression. Human bone is subjected to a variety of loading patterns during daily routine activities. A typical loading condition is repeated or cyclic loading as experienced in walking or running. Bone response under such loading conditions may be referred to as fatigue behaviour.16, 17 Fatigue damage is a process of gradual mechanical degradation caused by repeated loading at stress or strain much lower than those required to fracture in a single application of force. As a result, damage due to cyclic loading is of significant interest for design and application of synthetic scaffolds.

In a conventional fatigue test, specimens are subjected to a constant amplitude load till failure to determine the number of cycle to failure at a given load range. A number of studies based on this method have been carried out on the fatigue behaviour of bones.15, 18-30 Some studies have also been carried out for synthetic biopolymers.31, 32 Using this method a great number of samples may be needed to obtain the fatigue behaviour under variable stress amplitudes. Stepwise-testing has since been developed which would allow the generation of fatigue data with only a small numbers of samples.33, 34 In stepwise-tests, samples are subject to loads of a range of amplitudes in each a number of cycles are allowed to elapse, thus providing an accelerated testing regime with increased data generation. One important parameter obtained from such tests is the evolution of secant modulus 21, 33, 35 which is an indication of damage accumulation.30, 36

Polymer-based osteochondral scaffolds are synthetic resorbable implants designed to replace worn-out cartilage surfaces, restoring mobility and relieving joint pain. Although used in some clinical cases,37-39 there is a lack of published data on the biomechanical properties of the implant. Knowledge of its mechanical behaviour is essential in order to obtain accurate prediction of stresses and deformation in clinical applications, where the structural integrity of the implant is particularly important when cyclic loading is considered. The motivation of the study is to evaluate the fatigue behaviour of an osteochondral scaffold using the step-wise testing method and to compare the results with those from human bone in terms of stress-strain response and secant modulus. Morphological parameters of the scaffold were also obtained and compared with those of human bone.

2. Materials and methods

2.1 Specimen

A commercial scaffold TRUFIT (Smith and Nephew) was used in the study. It is composed of polylactide-co-glycolide (PLG) copolymer with calcium-sulfate and polyglycolide (PGA) fibres. The dual-layer design of the implant contains both a cartilage and a bone phase. Cylinders of the two layers scaffolds were sectioned to obtain a single layer of TRUFIT Bone, with a diameter of 10.5mm, a length of 9 mm and an aspect ratio of 0.86. The aspect ratio was selected based on the information obtained previously,40-42 where reported aspect ratios are generally less than 1. 33, 43-45 The aspect ratio and dimensions of the TRUFIT Bones are in a similar range of those of human trabecular bones in Topolinski’s study.33 The samples were soaked in phosphate buffered saline (PBS) solution overnight prior to testing. 2.2 Morphological Parameters

The microstructure of the specimens was examined prior to fatigue testing using micro-focus computed tomography (CT X-Ray Inspection System, Metris X-Tek Systems Ltd). The Micro- CT scanner was operated at a voltage of 51 kV, current of 160 μA and a voxel size of 17-20 μm. Bone volume density (BV/TV), mean thickness of the trabeculae in the specimen (Tb.Th) and the mean distance between trabeculae (Tb.Sp) were obtained by processing the micro-CT images using Microview software.

2.3 Compression fatigue testing

Fatigue testing was carried out on a Bose ElectroForce 3200 Testing system equipped with an environmental chamber to which PBS solution was filled and controlled at a temperature of 37˚C. Each specimen was placed carefully at the centre of the platens inside the environmental chamber. During the testing, the specimens were unconstrained33 between the platens of the testing machine as fixation of the specimens to the test platens may increase stiffness46 Five TRUFIT Bones were tested under load control26, 27 with a stepwise increase of maximum stress whilst keeping the minimum stress constant, following a testing protocol of Topolinski et al.33

A preload of 5 N was applied first to ensure good end contact and this preload was subsequently kept constant as the minimum load throughout the test. The maximum load started from 10 N with a gain of 10 N at successive steps, as shown in Figure 1. The maximum load was kept constant during a period of 500 cycles and increased to the next level thereafter. The loading scheme was adopted from Topolinski et al33 to allow easy comparison with the results from human trabecular bone.33 The loading waveform was sinusoidal at a frequency of 1 Hz.

Engineering stress and strain were calculated by using the applied load, displacement and the original dimension of the specimens, where lateral deformation under compression was not considered. The changes in the secant modulus and the maximum and minimum strains with fatigue loading were recorded. The change in the secant modulus is considered indicative of damage accumulation in the specimen30, 36 in fatigue studies of bones from the literature.15, 18,

21, 23-29, 33-35, 47 The secant modulus at a given cycle was defined as Δσ/Δ = (σmax-σmin)/(max-min), determined from the stress-strain curve. Figure 2 illustrates this as the slope of the line connecting the lowest and the highest point of a stress-strain loop. The secant modulus and strains were measured throughout the test. The cyclic stress-strain curves were also compared with those obtained under monotonic loading conditions.

3. Results and discussion

Table 1 shows the morphometric values of the TRUFIT Bone and human trabecular bone.33 The average BV/TV of the scaffold (0.343) is well within the range of human trabecular bone (0.076-0.460). The trabecular thickness of the scaffold is slightly lower (0.086mm) than that of human trabecular bone (0.105-0.268mm); whilst the trabecular spacing (0.166mm) is smaller than that of human (0.424-1.829mm). These indicate that the microstructure of the scaffold consists of thinner but more densely packed struts than that of human trabecular bone. Table 1 also includes the maximum stress corresponding to the maximum secant modulus. Samples with higher values of the maximum stress indicate higher fatigue resistance. It is clear that the fatigue resistance of the scaffold at an average of 0.462MPa is much lower than the range (0.859-12.28MPa) from human trabecular bone. A large variation in the results from human bones may be due to the varied sources (e.g., multiple donors with individual characteristics and pathologies).33 In contrast, synthetic TRUFIT bones have more consistent morphometric values. Although a relationship between bone structure and fatigue strength for human bone was reported,33 it is not possible to obtain such a relation for the scaffold due to the lack of variety of the microstructure features.

Typical stress-strain curves of a sample tested under the multi-step loading scheme (LS1 to LS8) are shown in Figure 3. Each block, shown in a different colour, represents the response to a single loading step with 500 loading cycles. A distinctive characteristic at all load levels is the accumulation of residual strain, or cyclic creep, with the increase of loading cycles. Greater strain ratchetting appears to be associated with higher load levels, when non-linear deformation becomes more evident in the larger hysteresis loops under LS7 and LS8. This is consistent with the behaviour observed in trabecular bones.26, 29, 35, 47

The evolution of the secant modulus (as defined in Figure 2) with cycles is shown in Figure 4(a) for the five samples tested. In all cases, it appears that an increase in secant modulus with cycle persisted for most of the loading steps, particularly the early ones. Only towards later at higher load levels reductions in secant modulus become evident. The initial hardening seems to be somewhat unexpected, certainly in contrast to the continuing increase in the residual strain with cycle, as shown in Figure 4(b), where the maximum and the minimum compressive strains of the five samples are recorded. Interestingly samples with lower residual strains (A, B, C) tend to have higher secant modulus (Figure 4(a)). Both maximum and minimum strains increased with the number of cycles during the entire test and the gap between the maximum and the minimum strains is roughly constant for each sample, indicating that the strain range stays constant. In addition, the strain rate increases with the increase of the number of cycles. It is known that residual strain or cyclic creep plays an important role in material failure due to the accumulated plastic deformation.30, 35 Increase in strains with the number of cycles have been generally observed when testing bones under cyclic loading, 27, 34 although increasing strain rate during the entire test was found in bovine trabecular bone.34 Three stages of deformation during compression fatigue tests have been reported for human trabecular bone,27 including a transient behaviour characterised by rapid strain increase within the initial load cycles, saturation of strain and an acceleration towards final catastrophic failure. When secant modulus of the scaffold is compared with those of human trabecular bones,33 as shown in Figure 4(c), much lower secant modulus and total number of cycles to failure were obtained for the scaffold, although the patterns of evolution with cycle appear to be similar between the two. The initial hardening of the bone is also consistent with the results under conventional constant-amplitude fatigue testing. Linde and Hvid51 reported that in human trabecular bones the stiffness increases until a stress level about 50% of ultimate stress is reached followed by decreasing stiffness, although others reported decreasing in modulus from the beginning to the end.18, 22, 24-27, 52, 53 Michel et al35 studied the fatigue behaviour of bovine trabecular bones under load control using a sinusoidal compressive load profile at a frequency of 2 Hz. The results showed that the pattern of modulus change with the number of cycles was associated with the load level. At low cyclic load levels the modulus increased initially followed by a rapid drop in the final stage. At high cyclic loads a continuous decrease in modulus from the beginning was found. The authors35 suggested that both creep and damage accumulation may be responsible for fatigue failure of trabecular bone, i.e. bone fails by creep under high cyclic loads whilst by microcrack damage accumulation at low cyclic loads. A more recent study suggested, however, that creep effects are negligible in fatigue loading cases for trabecular bone.54 An initial increase in modulus has been observed for TRUFIT at most of the load levels except the highest levels, which appears to be consistent with the observation in some of the reports.35 and 51

Stress-strain loops of first cycles at the beginning of each fatigue loading step are presented in Figure 5(a). Each centre of the loops was moved to the origin (0,0) then the points at the top of each loop were linked to form the initial cyclic stress-strain curve. Figure 5(b) shows the final cyclic stress-strain curve which was obtained similarly using the loops of the final cycles at the end of each loading step. These cyclic stress-strain loops at the first and the final cycles do not appear to differ significantly (Figure 6, CSS_1 and CSS_f), suggesting stress softening may not be significant. Figure 6 shows a comparison of cyclic and monotonic stress- strain curves for a typical specimen of TRUFIT. Here LS1_0 to LS5_0 are monotonic stress- strain curves whilst CSS_1 and CSS_f are the first and the final cyclic stress-strain curves. The monotonic curves are the loading parts of a stress-strain curve at the beginning of the each loading step. The slopes of these curves are the modulus of elasticity. A comparison of the mean elastic modulus of the stress-strain curves is shown in Figure 7. The elastic modulus of monotonic stress-strain curves increase with the number of loading steps (a) and the moduli of cyclic curves are generally higher than those of monotonic ones, suggesting stress hardening, consistent with the trend shown in Figure 4(a). However, the elastic modulus of first cyclic stress-strain curve is higher than that of final cyclic stress-strain curve, suggesting damage accumulation due to fatigue. 4. Conclusions

Multi-step cyclic tests have been carried out on TRUFIT Bones to study the fatigue behaviour of the scaffold under increasing compressive cyclic loading conditions. Morphological parameters of TRUFIT scaffold have also been obtained using computed tomography and compared with those of human trabecular bone. The results show an increase in secant modulus and stress hardening during the initial steps of fatigue loading, consistent with that observed in human trabecular bone. The modulus and the number of cycle to failure of the scaffold are, however, significantly lower than those of human trabecular bone. The elastic modulus of monotonic stress-strain curves increases with the number of loading steps and the moduli of cyclic curves are generally higher than those of monotonic curves. Progressive increase in residual strains has been observed in the scaffold for the entire test duration, indicating that residual strain accumulation, or cyclic creep, may be the predominant driving force leading to failure of the scaffold.

Multi-step tests seem to be useful in assessing the essential fatigue behaviour of scaffolds. These tests allow the reduced number of samples for a reliable analysis of fatigue properties of biomaterials.

Acknowledgements

The authors would like to thank Smith & Nephew for the provision of samples and University of Portsmouth for financial support.

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Figure 4. (a) The evolution of secant modulus as a function of cycles for TRUFIT Bone (b) the development of the maximum and minimum strains with cycle and (c) comparison of secant modulus of TRUFIT Bone and human trabecular bone.33 Figure 5. Cyclic stress-strain responses: (a) Loops of first cycles at the beginning of each loading step; (b) loops of the final cycles at the end of each loading step. Figure 6. Comparison of the cyclic and the monotonic stress-strain curves of a typical sample. Figure 7. The elastic modulus of (a) monotonic curves; and (b) cyclic stress-strain curves. Table 1. Morphometric values and maximum stresses of TRUFIT bone and human trabecular bone. 33 Max Stress (MPa) Bone volume density Trabecular thickness Trabecular spacing TRUFIT Bone (corresponding to BV/TV Tb.Th (mm) Tb.Sp (mm) Emax) A 0.359 0.089 0.161 0.547 B 0.340 0.082 0.158 0.328 C 0.353 0.087 0.159 0.547 D 0.328 0.089 0.184 0.337 E 0.333 0.084 0.169 0.552 Human trabecular 0.076 - 0.460 0.105 - 0.268 0.424 - 1.829 0.859 - 12.280 bone [33]