Effects of frozen storage temperature on the elasticity of tendons from a small murine model

Differences in the mechanical properties of intestinal tissue based on preservation freezing duration and temperature

In this study, tissue samples were stress tested to determine if freezing duration and temperature alter their mechanical properties. Tissue samples taken from the small intestine of pigs were assigned to 5 groups: fresh tissue, -28.9 °C for 7 days, -62.2 °C for 7 days, -28.9 °C for 30 days, and -62.2 °C for 30 days. Tissue was stored in PBS for the assigned temperature and duration until testing occurred with the exception of fresh tissue which was tested at sample collection. Before testing, samples were thawed in a room temperature bath, and the thickness was measured. Samples were then mounted in a biaxial test system using four anchoring rakes. Each sample was pulled to a strain of 0.2 with the corresponding forces recorded. This cycle of relaxation to 0.2 strain was repeated 5 times per sample. The thickness and force values were used to find the first Piola-Kirchhoff stress experienced at 0.2 strain and the strain energy. The average stress values in the circumferential direction were: fresh tissue: 22.3 ± 9.85 kPa; -28.9 °C for 7 days: 37.8 ± 14.1 kPa; -62.2 °C for 7 days: 46.5 ± 19.0 kPa; -28.9 °C for 30 days: 46.4 ± 22.7 kPa; -62.2 °C for 30 days: 40.1 ± 19.5 kPa. The stress and strain energy values of frozen tissue were statistically higher than the fresh tissue, although no statistical difference was found by varying duration or temperature. Based on this result, we determined that freezing tissue at any of the tested temperatures or durations increases the stiffness of the thawed tissue. This possibly occurs due to the directional formation of ice, which increases ion concentrations and glycosaminoglycan (GAG) interactions near collagen fibrils.

Age-dependent mechanical properties of tail tendons in wild-type and mimecan gene-knockout mice - A preliminary study

Mimecan, or osteoglycin, belongs to the family of small leucine-rich proteoglycans. In connective tissues mimecan is implicated in the development and maintenance of normal collagen fibrillar organization. Since collagen fibrils are responsible for tissue reinforcement, the absence of mimecan could lead to abnormal tissue mechanical properties. Here, we carried out a preliminary investigation of possible changes in the mechanical properties of tendons in mice lacking a functional mimecan gene, as a function of age. Tail tendons were dissected from mimecan gene knockout (KO) and wild type (WT) mice at ages 1, 4 and 8 months and mechanical properties evaluated using a microtensile testing equipment. Mimecan gene knockout resulted in changes in tendon elasticity- and fracture-related properties. While tendons of WT mice exhibited enhanced mechanical properties with increasing age, this trend was notably attenuated in mimecan KO tendons, with the exception of fracture strain. When genotype and age were considered as cross factors, the diminution in the mechanical properties of mimecan KO tendons was significant for yield strength, modulus and fracture strength. This effect appeared to affect the mice at 4 month old. These preliminary results suggest that mimecan may have a role in regulating age-dependent mechanical function in mouse tail tendon.

Age-related changes in mechanical properties of semitendinosus tendon used for anterior cruciate ligament reconstruction

Background

Hamstring tendons are a popular choice for autografts in anterior cruciate ligament (ACL) reconstruction. However, there is increasing evidence that hamstring tendon autografts carry a high risk of revision and residual instability in young patients. To elucidate the reasons for the inferior outcome of the reconstructed ACL with hamstring tendon autografts in young patients, we investigated the Young’s modulus and the extent of cyclic loading-induced slackening of the semitendinosus tendon used for ACL reconstruction across a broad range of ages.

Methods

Twenty-six male patients (aged 17–53 years), who were scheduled for ACL reconstruction surgery using the semitendinosus tendon autograft, participated in this study. The distal portion of the harvested semitendinosus tendon, which was not used to construct the autograft, was used for cyclic tensile testing to calculate the Young’s modulus and the extent of slackening (i.e., increase in slack length).

Results

Spearman correlation analysis revealed that the Young’s modulus of the semitendinosus tendon was positively correlated with the patient’s age (ρ = 0.559, P = 0.003). In contrast, the extent of tendon slackening did not correlate with the patient’s age.

Conclusions

We demonstrated that the Young’s modulus of the semitendinosus tendon increases with age, indicating that the semitendinosus tendon used for ACL reconstruction is compliant in young patients.

Ex vivo evaluation of novel core tenorrhaphy patterns in dogs

OBJECTIVE

To compare the biomechanical properties and gapping characteristics of four novel tenorrhaphy patterns in a canine flexor tendon model.

STUDY DESIGN

Ex vivo, randomized, biomechanical study.

SAMPLE POPULATION

Superficial digital flexor tendons of 60 forelimbs (30 dogs).

METHODS

Each tendon was transected 25 mm distal to its musculotendinous junction prior to tenorrhaphy with 2-0 polypropylene. Repair patterns included the three-loop pulley (3LP, control), exposed double-cross-lock (ExDCrL), embedded double-cross-lock (EmDCrL), triple-circle-lock (TCiL), and Modified-Tang patterns (MTang) were randomly assigned to each experimental group (n = 12/group). Yield, peak, and failure loads, gap formation and failure modes were compared.

RESULTS

Tendons repaired with ExDCrL (p < .0001), EmDCrL (p < .0001), and MTang (p < .0001) sustained yield, peak, and failure loads ~2.2x, ~2.0x, and ~1.9x, respectively, greater than those repaired with 3LP. Loads to 1 and 3 mm gapping were also higher for ExDCrL (p < .0001), EmDCrL (p < .0004), and MTang constructs (p < .0017) compared to 3LP. Although TCiL constructs sustained higher loads, their resistance to gap formation did not differ from that of 3LP repairs. Failure mode differed between groups (p < .0001), EmDCrL, ExDCrL, MTang, and TCiL constructs failing predominantly by suture breakage compared to 3LP repairs that failed by suture pull-through.

CONCLUSION

Use of novel patterns ExDCrL, EmDCrL, and MTang improved resistance to loads and gap formation and were biomechanically superior compared to 3LP in healthy canine tendon repairs.

CLINICAL SIGNIFICANCE

These results justify in vivo evaluation of ExDCrL, EmDCrL, or MTang pattern for tenorrhaphy in dogs.

Biomechanical properties of canine staphylectomies closed with barbed or smooth suture

OBJECTIVE

To compare the duration of closure and biomechanical properties of staphylectomies closed with absorbable bidirectional barbed suture or smooth monofilament suture in a simple continuous or interrupted pattern STUDY DESIGN: Ex vivo study SAMPLE POPULATION: Soft palates (n = 60) harvested from mesaticephalic canine cadavers METHODS: One centimeter of tissue was excised from the caudal border of each soft palate, and the oral and nasopharyngeal mucosal surfaces were apposed with 2-0 bidirectional Quill Monoderm knotless closure device barbed suture (Q), 3-0 Monocryl in a simple continuous (MC) pattern, or 3-0 Monocryl in a simple interrupted (MI) pattern (n = 20 per group). Duration of closure was compared between groups. Tissues were tested under tension to failure, and mode of failure data were collected by video capture.

RESULTS

Closure time was longer for MI closures than for Q and MC closures, with means of 259.9, 215.4, and 196.7 seconds, respectively (P < .0001). No difference was detected in yield force, force to first tissue rupture, maximum force, and energy required for yield and maximum force between groups. Energy to yield was 190.0, 167.8, and 188.95 N-mm for MI, Q, and MC closures, respectively.

CONCLUSION

Biomechanical properties of staphylectomies closed with barbed or smooth sutures did not differ in this cadaveric model.

CLINICAL SIGNIFICANCE

Barbed suture can be considered as an alternative for closure of canine staphylectomies. These results provide evidence to justify additional research to evaluate clinical outcomes in dogs undergoing staphylectomy.

Influence of barbed epitendinous sutures combined with a core locking-loop suture to repair experimental flexor tendon lacerations

OBJECTIVE

To determine the influence of barbed epitendinous sutures (ES) on the biomechanical properties and gap formation of repaired canine tendons.

STUDY DESIGN

Ex vivo, experimental study.

SAMPLE POPULATION

Eighty (n = 16/group) canine superficial digital flexor tendons (SDFT).

METHODS

After transection, SDFT were repaired with a locking-loop (LL) pattern alone (group 1), an LL + smooth ES with monofilament suture (group 2), an LL + V-loc-ES (group 3), an LL + Quill-ES (group 4), or an LL + Stratafix-ES (group 5). All core LL repairs were performed with 0 USP polypropylene, and all ES were placed with 2-0 USP equivalent. Constructs were preloaded and tested to failure. Yield, peak, and failure loads; occurrence of gap formation; and failure modes were compared.

RESULTS

Yield loads were greater for groups 2 and 5 (P < .0001). Peak and failure loads were greater when an ES was used (P < .005), especially for groups 2 and 5 (P < .0001). Groups with an ES required higher loads to generate 1- and 3-mm gaps compared with specimens without an ES (P < .002). Force to create 1- and 3-mm gaps was greater for group 5 (P < .0001) and groups 2 and 5 (P < .0001), respectively. Failure mechanism did not differ (P = .092) between ES groups, consisting of suture breakage in 51 of 64 constructs compared with pull-through in seven of 16 group 1 constructs.

CONCLUSION

Epitendinous suture placement improved the biomechanical properties of repaired tendons. Stratafix barbed suture performed better as an ES compared with other barbed sutures and similarly to monofilament suture.

CLINICAL SIGNIFICANCE

Stratafix barbed suture eliminates the requirement for knot tying and seems to be equivalent to smooth monofilament suture when used as an ES in this pattern.

Material properties of human patellar-ligament grafts from the elderly population

The aim of the presented study was to estimate the material properties of human patellar ligaments from the elderly population by means of tensile tests. The experimental part was conducted on a custom tensile-testing device, with a built-in enclosure to simulate in-vivo conditions, using 25 (15 female, 10 male) bone-ligament-bone samples from elderly (age 83 (8)) human donors. During the tensile tests, the resultant force and displacement of the sample attachments were recorded. With this data and the values of the initial length and the initial cross-sectional area of the samples, the engineering stress and strain, the Young's modulus and the toughness at rupture were calculated for each sample. The results were then averaged and presented for all the samples together and for the female and male populations separately. The measured Young's modulus and the failure stress values were found to be significantly higher for the female samples compared to the male (p < 0.05). All the other measured properties did not show a significant difference. The toe region's material properties for the patellar ligament were also presented as valuable information for the anterior cruciate ligament reconstruction. The tensile-test results were compared to other research carried on human patellar ligaments using samples from younger donors. The comparison showed that the samples from the elderly population exhibit lower values of strain at the end of the toe region and have a lower failure strain for the patellar ligament. The Young's modulus and the failure stress of the samples in this study were in the range of other research conducted on patellar ligaments.

Effect of presurgical storage conditions on leakage pressures of enterotomy sites closed with unidirectional barbed suture material in fresh, chilled, and frozen-thawed cadaveric canine jejunal specimens

OBJECTIVE

To evaluate the effect of presurgical storage conditions on leakage pressures of enterotomy sites closed with unidirectional barbed suture material in fresh, chilled, and frozen-thawed cadaveric canine jejunal specimens.

SAMPLE

36 grossly normal jejunal segments obtained from 4 dog cadavers.

PROCEDURES

9 jejunal segments were harvested immediately from each euthanized dog and randomly assigned to be tested within 4 hours after collection (fresh segments), stored at 4°C for 24 hours before testing (chilled segments), or stored at -20°C for 7 days and thawed at 21°C for 6 hours before testing (frozen-thawed segments). For leakage pressure testing, a 3-cm-long antimesenteric enterotomy was performed and repaired with 3-0 unidirectional barbed suture material in a simple continuous pattern in each segment. Time to complete the enterotomy, initial leakage pressure, maximum intraluminal pressure, and leakage location were recorded for each segment.

RESULTS

Mean ± SD initial leakage pressure for fresh, chilled, and frozen-thawed segments was 52.8 ± 14.9 mm Hg, 51.8 ± 11.9 mm Hg, and 33.3 ± 7.7 mm Hg, respectively. Frozen-thawed segments had significantly lower mean initial leakage pressure, compared with findings for fresh or chilled segments. Time to complete the enterotomy, maximum intraluminal pressure, and leakage location did not differ among groups.

CONCLUSIONS AND CLINICAL RELEVANCE

Leak pressure testing of cadaveric jejunal segments that are fresh or chilled at 4°C for 24 hours is recommended for enterotomy studies involving barbed suture material in dogs. Freezing and thawing of cadaveric jejunal tissues prior to investigative use is not recommended because leak pressure data may be falsely low.

Ex vivo comparison of the effect of storage temperature on canine intestinal leakage pressures

OBJECTIVE

To determine the effect of storage temperature on cadaveric small intestinal leakage pressures after enterotomy.

STUDY DESIGN

Experimental ex vivo study.

ANIMALS

Grossly normal jejunal segments from four canine cadavers.

METHODS

Thirty-six jejunal segments (n = 12 segments/group) were harvested immediately after euthanasia and assigned to a fresh group (tested within 4 hours), chilled group (stored for 24 hours at 4°C before testing), or freeze-thaw group (frozen at -20°C for 7 days and thawed at 21°C for 6 hours before testing). A 2-cm antimesenteric enterotomy was performed and repaired with 4-0 monofilament suture in a simple-continuous pattern. Initial leakage pressure (ILP), maximal intraluminal pressure (MIP), and leakage location were recorded, with testing performed at room temperature.

RESULTS

Mean ± SD ILP for fresh, chilled, and frozen-thawed specimens was 52.9 ± 8.4, 51.8 ± 11.9 and 29.8 ± 4.4 mm Hg, respectively. There was a difference in ILP among groups (P < .003), with freeze-thaw samples demonstrating lower ILP compared with other groups. There was no difference in MIP between groups (P = .186) There was a difference in leakage location among groups (P = .004), with the majority of chilled and freeze-thaw samples leaking at the suture holes compared with the incisional line in fresh samples.

CONCLUSION

Freezing and subsequent thawing prior to specimen testing reduced ILP compared with use of fresh and chilled specimens but did not affect MIP among experimental groups.

CLINICAL SIGNIFICANCE

Cadaveric canine intestinal specimens tested immediately after collection or after chilling for 24 hours should be recommended for ex vivo burst pressure assessment in dogs. Additional studies to evaluate loss in testing viability of chilled intestinal specimens are warranted to help govern experimental methodologies.

Effect of a continuous epitendinous suture as adjunct to three-loop pulley and locking-loop patterns for flexor tendon repair in a canine model

OBJECTIVE

To evaluate the effect of combining a continuous epitendinous suture with three-loop pulley (3LP) and locking-loop (LL) core patterns for flexor tendon repair.

STUDY DESIGN

Ex vivo biomechanical study.

SAMPLE POPULATION

Seventy-two cadaveric superficial digital flexor musculotendon (SDFT) units.

METHODS

Tendons were divided into four groups (n = 18/group). After sharp transection, SDFT were repaired with 3LP, LL, 3LP + epitendinous (E), or LL + E suture patterns. After preloading, repaired constructs were tested to failure. Video data acquisition allowed evaluation of failure mode and quantitation of gap formation. Yield, peak, and failure force were measured from force-displacement data. Significance was set at P < .05.

RESULTS

Mode of failure did not differ between repairs with or without an epitendinous suture (P = .255). Gap formation was best prevented with 3LP compared with LL when used alone (P = .001). Mean yield force for 3LP, LL, 3LP + E, and LL + E were 91.4 N ± 25.4, 61.3 N ± 18.4, 195.2 N ± 66.0, 165.3 N ± 46.8, respectively. Tenorrhaphies combined with an epitendinous suture achieved higher yield (P < .0001), peak (P < .0001), and failure forces (P < .0001), without gapping between tendon ends.

CONCLUSION

Addition of an epitendinous suture eliminated gapping between tendon ends until failure and increased resistance to loads tolerated at the repair site.

CLINICAL SIGNIFICANCE

The addition of an epitendinous suture may increase the strength of tendon repairs and resistance to gap formation over core suture use alone. The influence of epitendinous suture placement on tendinous healing and blood supply warrants in-vivo testing.

Biomechanical comparison of three epitendinous suture patterns as adjuncts to a core locking loop suture for repair of canine flexor tendon injuries

OBJECTIVE

To determine the effects of different epitendinous sutures (ES) in addition to core locking-loop (LL) sutures on the mechanical properties and gap formation in a canine cadaveric tendon model.

STUDY DESIGN

Experimental, ex vivo, biomechanical study.

SAMPLE POPULATION

Seventy-two cadaveric superficial digital flexor tendon specimens.

METHODS

Superficial digital flexor tendon specimens were divided into four groups (n = 18): sharply transected and repaired with LL, LL + simple continuous ES, LL + Silfverskiöld cross-stitch ES, and LL + interlocking horizontal mattress ES. Constructs were loaded to monotonic failure. Failure modes, gapping, yield, peak, and failure forces were analyzed. Significance was set at P < .05.

RESULTS

Yield, peak, and failure forces increased by 2.5-fold, two-fold, and twofold, respectively when ES groups were compared with core LL suture patterns alone (P < .0001). Resistance to 1- and 3-mm gap formation was greater in ES groups compared with core LL constructs alone (P < .0001). No differences in yield, peak, failure force, or gapping were observed among ES patterns (P > .827).

CONCLUSION

Adding an ES reduced gap formation and increased yield, peak, and failure forces of tenorrhaphies. No difference was detected between the epitendinous patterns tested in this study.

CLINICAL SIGNIFICANCE

The addition of an ES seems more relevant than the specific type of pattern to improve the biomechanical properties of flexor tendon repairs. In vivo studies are warranted to determine the biological implications of the patterns tested here.

Cryopreservation of tendon tissue using dimethyl sulfoxide combines conserved cell vitality with maintained biomechanical features

Biomechanical research on tendon tissue evaluating new treatment strategies to frequently occurring clinical problems regarding tendon degeneration or trauma is of expanding scientific interest. In this context, storing tendon tissue deep-frozen is common practice to collect tissue and analyze it under equal conditions. The commonly used freezing medium, phosphate buffered saline, is known to damage cells and extracellular matrix in frozen state. Dimethyl sulfoxide, however, which is used for deep-frozen storage of cells in cell culture preserves cell vitality and reduces damage to the extracellular matrix during freezing. In our study, Achilles tendons of 26 male C57/Bl6 mice were randomized in five groups. Tendons were deep frozen in dimethyl sulfoxide or saline undergoing one or four freeze-thaw-cycles and compared to an unfrozen control group analyzing biomechanical properties, cell viability and collagenous structure. In electron microscopy, collagen fibrils of tendons frozen in saline appeared more irregular in shape, while dimethyl sulfoxide preserved the collagenous structure during freezing. In addition, treatment with dimethyl sulfoxide preserved cell viability visualized with an MTT-Assay, while tendons frozen in saline showed no remaining metabolic activity, indicating total destruction of cells during freezing. The biomechanical results revealed no differences between tendons frozen once in saline or dimethyl sulfoxide. However, tendons frozen four times in saline showed a significantly higher Young’s modulus over all strain rates compared to unfrozen tendons. In conclusion, dimethyl sulfoxide preserves the vitality of tendon resident cells and protects the collagenous superstructure during the freezing process resulting in maintained biomechanical properties of the tendon.

The Effect of Mechanical Overloading on Surface Roughness of the Coronary Arteries

Background

Surface roughness can be used to identify disease within biological tissues. Quantifying surface roughness in the coronary arteries aids in developing treatments for coronary heart disease. This study investigates the effect of extreme physiological loading on surface roughness, for example, due to a rupture of an artery.

Methods

The porcine left anterior descending (LAD) coronary arteries were dissected ex vivo. Mechanical overloading was applied to the arteries in the longitudinal direction to simulate extreme physiological loading. Surface roughness was calculated from three-dimensional reconstructed images. Surface roughness was measured before and after damage and after chemical processing to dehydrate tissue specimens.

Results

Control specimens confirmed that dehydration alone results in an increase of surface roughness in the circumferential direction only. No variation was noted between the hydrated healthy and damaged specimens, in both the longitudinal (0.91 ± 0.26 and 1.05 ± 0.25 μm) and circumferential (1.46 ± 0.38 and 1.47 ± 0.39 μm) directions. After dehydration, an increase in surface roughness was noted for damaged specimens in both the longitudinal (1.28 ± 0.33 μm) and circumferential (1.95 ± 0.56 μm) directions.

Conclusions

Mechanical overloading applied in the longitudinal direction did not significantly affect surface roughness. However, when combined with chemical processing, a significant increase in surface roughness was noted in both the circumferential and longitudinal directions. Mechanical overloading causes damage to the internal constituents of the arteries, which is significantly noticeable after dehydration of tissue.

Age-related dataset on the mechanical properties and collagen fibril structure of tendons from a murine model

Connective tissues such as tendon, ligament and skin are biological fibre composites comprising collagen fibrils reinforcing the weak proteoglycan-rich ground substance in extracellular matrix (ECM). One of the hallmarks of ageing of connective tissues is the progressive and irreversible change in the tissue mechanical properties; this is often attributed to the underlying changes to the collagen fibril structure. This dataset represents a comprehensive screen of the mechanical properties and collagen fibril structure of tendon from the tails of young to old (i.e. 1.6-35.3 month-old) C57BL6/B mice. The mechanical portion consists of the load-displacement data, as well as the derived tensile properties; the structure data consists of transmission electron micrographs of collagen fibril cross section, as well as the derived cross-sectional parameters. This dataset will allow other researchers to develop and demonstrate the utility of innovative multiscale models and approaches of the extra-cellular and physiological events of ageing of current interest to ageing research, by reducing the current reliance on conducting new mammalian experiments.

Differences Between Men and Women in Balance and Tremor in Relation to Plantar Fascia Laxity During the Menstrual Cycle

CONTEXT

  Although much attention has been paid to the effect of estrogen on the knee ligaments, little has been done to examine the ligaments in the foot, such as the plantar fascia, and how they may be altered during the menstrual cycle.

OBJECTIVE

  To (1) examine sex differences in plantar fascia thickness and laxity and postural sway and (2) identify any menstrual cycle effects on plantar fascia laxity, postural sway, and neuromuscular tremor between menstruation and the ovulation phase.

DESIGN

  Case-control study.

SETTING

  Research laboratory.

PATIENTS OR OTHER PARTICIPANTS

  Fifteen healthy women (age = 25.9 ± 1.8 years) and 15 healthy men (age = 27.3 ± 2.0 years) volunteered to participate in this study.

INTERVENTION(S)

  We asked participants to perform 8 balance tasks on a force platform while we assessed postural sway and tremor.

MAIN OUTCOME MEASURE(S)

  Plantar fascia length and thickness unloaded and loaded with body weight were measured via ultrasound. Postural sway and tremor were measured using a force platform.

RESULTS

  Plantar fascia length and thickness with pressure were greater in ovulating women compared with men ( P < .001), but no differences were found between women during menstruation and men. Postural sway and tremor were greater at ovulation than during menstruation ( P < .05), and men had less sway than ovulating women on the 3 most difficult balance tasks ( P < .01).

CONCLUSIONS

  Plantar fascia laxity was increased and postural sway and tremor were decreased at ovulation compared with menstruation in women. Postural sway and tremor in men were the same as in women during menstruation. These findings support the need to be aware of the effect of sex hormones on balance to prevent lower extremity injuries during sport activities.

Ex vivo biomechanical comparison of barbed suture and standard polypropylene suture for acute tendon laceration in a canine model

Objectives: Evaluate performance and resistance to gap formation of a non-absorbable, barbed, monofilament suture, in comparison with a non-absorbable, smooth, monofilament polypropylene suture, in two different suture patterns: three-loop pulley (3LP) and modified Bunnell-Mayer (BM).

Sample size: Seventy-two medium-sized cadaveric superficial digital flexor muscle tendon units.

Methods: After manual transection and suture repair, individual specimens were placed in an electromechanical tensile testing machine and tested to monotonic failure using tensile ramp loading. Video data acquisition allowed evaluation of failure mode and quantification of gap formation.

Results: Incidence of gap formation between tendon ends was significantly greater in tenorrhaphies repaired with barbed suture compared to those repaired with smooth polypropylene. Use of a 3LP suture pattern caused significantly less gapping between tendon ends when compared to the BM pattern.

Conclusion: Smooth polypropylene suture was consistently superior in load performance than a unidirectional barbed suture. The 3LP pattern was more resistant than a BM pattern at preventing gap formation.

Clinical significance: Smooth polypropylene should be recommended over barbed unidirectional suture for use in canine tendinous repair to provide increased resistance to gap formation. The 3LP is superior to the BM suture pattern, requiring significantly more force to cause tenorrhaphy gap formation and failure, which may translate to increased accrual of repair site strength and tendinous healing in clinical situations.

Freezing does not alter multiscale tendon mechanics and damage mechanisms in tension

It is common in biomechanics to use previously frozen tissues, where it is assumed that the freeze-thaw process does not cause consequential mechanical or structural changes. We have recently quantified multiscale tendon mechanics and damage mechanisms using previously frozen tissue, where damage was defined as an irreversible change in the microstructure that alters the macroscopic mechanical parameters. Because freezing has been shown to alter tendon microstructures, the objective of this study was to determine if freezing alters tendon multiscale mechanics and damage mechanisms. Multiscale testing using a protocol that was designed to evaluate tendon damage (tensile stress-relaxation followed by unloaded recovery) was performed on fresh and previously frozen rat tail tendon fascicles. At both the fascicle and fibril levels, there was no difference between the fresh and frozen groups for any of the parameters, suggesting that there is no effect of freezing on tendon mechanics. After unloading, the microscale fibril strain fully recovered, and interfibrillar sliding only partially recovered, suggesting that the tendon damage is localized to the interfibrillar structures and that mechanisms of damage are the same in both fresh and previously frozen tendons.

Clinical Utility of Ultrasound Measurements of Plantar Fascia Width and Cross-Sectional AreaA Novel Technique

BACKGROUND

We sought to develop a standardized protocol for ultrasound (US) measurements of plantar fascia (PF) width and cross-sectional area (CSA), which may serve as additional outcome variables during US examinations of both healthy asymptomatic PF and in plantar fasciopathy and determine its interrater and intrarater reliability.

METHODS

Ten healthy individuals (20 feet) were enrolled. Participants were assessed twice by two raters each to determine intrarater and interrater reliability. For each foot, three transverse scans of the central bundle of the PF were taken at its insertion at the medial calcaneal tubercle, identified in real time on the plantar surface of the foot, using a fine wire technique. Reliability was determined using intraclass correlation coefficients (ICC), standard errors of measurement (SEM), and limits of agreement (LOA) expressed as percentages of the mean. Reliability of PF width and CSA measurements was determined using PF width and CSA measurements from one sonogram measured once and the mean of three measurements from three sonograms each measured once.

RESULTS

Ultrasound measurements of PF width and CSA showed a mean of 18.6 ± 2.0 mm and 69.20 ± 13.6 mm² respectively. Intra-reliability within both raters showed an ICC > 0.84 for width and ICC > 0.92 for CSA as well as a SEM% and LOA% < 10% for both width and CSA. Inter-rater reliability showed an ICC of 0.82 for width and 0.87 for CSA as well as a SEM% and LOA% < 10% for width and a SEM% < 10% and LOA% < 20% for CSA. Relative and absolute reliability within and between raters were higher when using the mean of three sonographs compared to one sonograph.

CONCLUSIONS

Using this novel technique, PF CSA and width may be determined reliably using measurements from one sonogram or the mean of three sonograms. Measurement of PF CSA and width in addition to already established thickness and echogenicity measurements provides additional information on structural properties of the PF for clinicians and researchers in healthy and pathologic PF.

Viscoelastic properties of mitral valve leaflets: An analysis of regional variation and frequency-dependency

The aim of this study was to determine the regional variation in viscoelastic properties of mitral valve leaflets over a range of physiological and patho-physiological frequencies. This included comparisons to be made between anterior and posterior leaflets, anterior leaflet clear and rough zones, and radial and circumferential leaflet orientation. Dynamic mechanical analysis was used to determine frequency-dependent viscoelastic properties. The valve leaflets were dissected from eight porcine hearts. The leaflets were loaded under a sinusoidal tensile displacement, with a mean dynamic peak to trough strain of 11%, applied to all leaflet samples at nine different frequencies, ranging from 0.5 to 10 Hz. The anterior leaflet has higher storage and loss stiffness than the posterior leaflet. The storage stiffness of circumferential tissue is greater than that of radially oriented valve tissue (2.0 ± 1.6 N/mm cf. 1.7 ± 0.9 N/mm; p < 0.05); however, the loss stiffness is greater for radial tissue (0.15 ± 0.07 cf. 0.14 ± 0.09 N/mm; p < 0.05). Likewise, the storage stiffness of the anterior leaflet clear zone is greater than that of the rough zone (2.4 ± 1.6 cf. 2.1 ± 1.2; p < 0.05), but the loss stiffness is greater for the rough zone (0.17 ± 0.09 N/mm cf. 0.14 ± 0.08 N/mm; p < 0.05). In conclusion, the viscoelastic properties of porcine mitral valve leaflets have regional variations, with dynamic stiffness being dependent on circumferential or radial orientation and on location at a clear or rough zones.

Effects of freezing, fixation and dehydration on surface roughness properties of porcine left anterior descending coronary arteries

BACKGROUND

To allow measurements of surface roughness to be made of coronary arteries using various imaging techniques, chemical processing, such as fixation and dehydration, is commonly used. Standard protocols suggest storing fresh biological tissue at -40°C. The aim of this study was to quantify the changes caused by freezing and chemical processing to the surface roughness measurements of coronary arteries, and to determine whether correction factors are needed for surface roughness measurements of coronary arteries following chemical processes typically used before imaging these arteries.

METHODS

Porcine left anterior descending coronary arteries were dissected ex vivo. Surface roughness was then calculated following three-dimensional reconstruction of surface images obtained using an optical microscope. Surface roughness was measured before and after a freeze cycle to assess changes during freezing, after chemical fixation, and again after dehydration, to determine changes during these steps of chemical processing.

RESULTS

No significant difference was caused due to the freeze cycle (p>0.05). There was no significant difference in the longitudinally measured surface roughness (Ra_L=0.99±0.39μm; p>0.05) of coronary arteries following fixation and dehydration either. However, the circumferentially measured surface roughness increased significantly following a combined method of processing (Ra_C=1.36±0.40, compared 1.98±0.27μm, respectively; p<0.05). A correction factor can compensate for the change Ra_Cβ=Ra_C1+0.46in Ra_C due to processing of tissue, Where Ra_Cβ, the corrected Ra_C, had a mean of 1.31±0.21μm.

CONCLUSIONS

Independently, freezing, fixation and dehydration do not alter the surface roughness of coronary arteries. Combined, however, fixation and dehydration significantly increase the circumferential, but not longitudinal, surface roughness of coronary arteries.

Collagenous Extracellular Matrix Biomaterials for Tissue Engineering: Lessons from the Common Sea Urchin Tissue

Scaffolds for tissue engineering application may be made from a collagenous extracellular matrix (ECM) of connective tissues because the ECM can mimic the functions of the target tissue. The primary sources of collagenous ECM material are calf skin and bone. However, these sources are associated with the risk of having bovine spongiform encephalopathy or transmissible spongiform encephalopathy. Alternative sources for collagenous ECM materials may be derived from livestock, e.g., pigs, and from marine animals, e.g., sea urchins. Collagenous ECM of the sea urchin possesses structural features and mechanical properties that are similar to those of mammalian ones. However, even more intriguing is that some tissues such as the ligamentous catch apparatus can exhibit mutability, namely rapid reversible changes in the tissue mechanical properties. These tissues are known as mutable collagenous tissues (MCTs). The mutability of these tissues has been the subject of on-going investigations, covering the biochemistry, structural biology and mechanical properties of the collagenous components. Recent studies point to a nerve-control system for regulating the ECM macromolecules that are involved in the sliding action of collagen fibrils in the MCT. This review discusses the key attributes of the structure and function of the ECM of the sea urchin ligaments that are related to the fibril-fibril sliding action-the focus is on the respective components within the hierarchical architecture of the tissue. In this context, structure refers to size, shape and separation distance of the ECM components while function is associated with mechanical properties e.g., strength and stiffness. For simplicity, the components that address the different length scale from the largest to the smallest are as follows: collagen fibres, collagen fibrils, interfibrillar matrix and collagen molecules. Application of recent theories of stress transfer and fracture mechanisms in fibre reinforced composites to a wide variety of collagen reinforcing (non-mutable) connective tissue, has allowed us to draw general conclusions concerning the mechanical response of the MCT at specific mechanical states, namely the stiff and complaint states. The intent of this review is to provide the latest insights, as well as identify technical challenges and opportunities, that may be useful for developing methods for effective mechanical support when adapting decellularised connective tissues from the sea urchin for tissue engineering or for the design of a synthetic analogue.

Dynamic Viscoelasticity and Surface Properties of Porcine Left Anterior Descending Coronary Arteries

The aim of this study was, for the first time, to measure and compare quantitatively the viscoelastic properties and surface roughness of coronary arteries. Porcine left anterior descending coronary arteries were dissected ex vivo. Viscoelastic properties were measured longitudinally using dynamic mechanical analysis, for a range of frequencies from 0.5 to 10 Hz. Surface roughness was calculated following three-dimensional reconstructed of surface images obtained using an optical microscope. Storage modulus ranged from 14.47 to 25.82 MPa, and was found to be frequency-dependent, decreasing as the frequency increased. Storage was greater than the loss modulus, with the latter found to be frequency-independent with a mean value of 2.10 ± 0.33 MPa. The circumferential surface roughness was significantly greater (p < 0.05) than the longitudinal surface roughness, ranging from 0.73 to 2.83 and 0.35 to 0.92 µm, respectively. However, if surface roughness values were corrected for shrinkage during processing, circumferential and longitudinal surface roughness were not significantly different (1.04 ± 0.47, 0.89 ± 0.27 µm, respectively; p > 0.05). No correlation was found between the viscoelastic properties and surface roughness. It is feasible to quantitatively measure the viscoelastic properties of coronary arteries and the roughness of their endothelial surface.

Advanced glycation end-products: Mechanics of aged collagen from molecule to tissue

Concurrent with a progressive loss of regenerative capacity, connective tissue aging is characterized by a progressive accumulation of Advanced Glycation End-products (AGEs). Besides being part of the typical aging process, type II diabetics are particularly affected by AGE accumulation due to abnormally high levels of systemic glucose that increases the glycation rate of long-lived proteins such as collagen. Although AGEs are associated with a wide range of clinical disorders, the mechanisms by which AGEs contribute to connective tissue disease in aging and diabetes are still poorly understood. The present study harnesses advanced multiscale imaging techniques to characterize a widely employed in vitro model of ribose induced collagen aging and further benchmarks these data against experiments on native human tissues from donors of different age. These efforts yield unprecedented insight into the mechanical changes in collagen tissues across hierarchical scales from molecular, to fiber, to tissue-levels. We observed a linear increase in molecular spacing (from 1.45nm to 1.5nm) and a decrease in the D-period length (from 67.5nm to 67.1nm) in aged tissues, both using the ribose model of in vitro glycation and in native human probes. Multiscale mechanical analysis of in vitro glycated tendons strongly suggests that AGEs reduce tissue viscoelasticity by severely limiting fiber-fiber and fibril-fibril sliding. This study lays an important foundation for interpreting the functional and biological effects of AGEs in collagen connective tissues, by exploiting experimental models of AGEs crosslinking and benchmarking them for the first time against endogenous AGEs in native tissue.

ALIGNMENT OF CELLULAR FOCAL CONTACTS AND THEIR SHAPES BY SUBSTRATE ANISOTROPY

Cell adhesion to the extracellular matrix is accomplished by the clustering of receptor–ligand bonds into focal contacts on the cell-substrate interface. The contractile forces applied onto these focal contacts lead to elastic deformation of the surrounding, which results into a cellular mechanosensory capability that plays a key role in cell adhesion, spreading, and migration, among many others. The mechanosensitivity can be manipulated by the substrate anisotropy, by which focal contacts may align into certain directions so to minimize the total mechanical potential energy. Using the elastic anisotropic contact analysis, this work systematically analyzes the dependence of the alignment on the elastic anisotropy, and more importantly, the direction of the inclined contractile forces. The contact displacement fields are a complex function of the elastic constants, so simple analysis based on tensile or shear softest direction cannot properly predict the alignment orientation. It is also proved that if these focal contacts are of elongated shape, the major axis will be parallel to the alignment direction.

Consequences of Ultra-Violet Irradiation on the Mechanical Properties of Spider Silk

The outstanding combination of high tensile strength and extensibility of spider silk is believed to contribute to the material’s toughness. Thus, there is great interest in engineering silk for biomedical products such as suture or implants. Additionally, over the years, many studies have also sought to enhance the mechanical properties of spider silk for wider applicability, e.g., by irradiating the material using ultra-violet radiation. However, the limitations surrounding the use of ultra-violet radiation for enhancing the mechanical properties of spider silk are not well-understood. Here, we have analyzed the mechanical properties of spider silk at short ultra-violet irradiation duration. Specimens of spider silk were subjected to ultra-violet irradiation (254-nm wavelength, i.e. UVC) for 10, 20, and 30 min, respectively, followed by tensile test to rupture to determine the strength (maximum stress), extensibility (rupture strain), and toughness (strain energy density to rupture). Controls, i.e., specimens that did not received UVC, were also subjected to tensile test to rupture to determine the respective mechanical properties. One-way analysis of variance reveals that these properties decrease significantly (p < 0.05) with increasing irradiation duration. Among the three mechanical parameters, the strength of the spider silk degrades most rapidly; the extensibility of the spider silk degrades the slowest. Overall, these changes correspond to the observed surface modifications as well as the bond rupture between the peptide chains of the treated silk. Altogether, this simple but comprehensive study provides some key insights into the dependence of the mechanical properties on ultra-violet irradiation duration.

Resolving the viscoelasticity and anisotropy dependence of the mechanical properties of skin from a porcine model

The mechanical response of skin to external loads is influenced by anisotropy and viscoelasticity of the tissue, but the underlying mechanisms remain unclear. Here, we report a study of the main effects of tissue orientation (TO, which is linked to anisotropy) and strain rate (SR, a measure of viscoelasticity), as well as the interaction effects between the two factors, on the tensile properties of skin from a porcine model. Tensile testing to rupture of porcine skin tissue was conducted to evaluate the sensitivity of the tissue modulus of elasticity (E) and fracture-related properties, namely maximum stress (σU) and strain (εU) at σU, to varying SR and TO. Specimens were excised from the abdominal skin in two orientations, namely parallel (P) and right angle (R) to the torso midline. Each TO was investigated at three SR levels, namely 0.007-0.015 s(-1) (low), 0.040 s(-1) (mid) and 0.065 s(-1) (high). Two-factor analysis of variance revealed that the respective parameters responded differently to varying SR and TO. Significant changes in the σU were observed with different TOs but not with SR. The εU decreased significantly with increasing SR, but no significant variation was observed for different TOs. Significant changes in E were observed with different TOs; E increased significantly with increasing SR. More importantly, the respective mechanical parameters were not significantly influenced by interactions between SR and TO. These findings suggest that the trends associated with the changes in the skin mechanical properties may be attributed partly to differences in the anisotropy and viscoelasticity but not through any interaction between viscoelasticity and anisotropy.

"Surprise" Loading in Flexion Increases the Risk of Disc Herniation Due to Annulus-Endplate Junction Failure: A Mechanical and Microstructural Investigation

STUDY DESIGN

Microstructural investigation of compression-induced herniation of the flexed lumbar disc.

OBJECTIVE

To provide a microstructural analysis of the mechanisms of annular wall failure in healthy discs subjected to flexion and a rate of compression comparable with the maximum rate at which the muscles of the spinal column can generate a force.

SUMMARY OF BACKGROUND DATA

Clinical evidence indicates the involvement of the endplate in herniation. It is known that both an elevated rate of compression and a flexed posture are necessary to cause disc failure either within the midspan of the annulus or at the annular-endplate interface. However, the question of what effect a sudden or "surprise" loading might have on the mode of failure is, as yet, unanswered.

METHODS

Twenty-four healthy mature ovine lumbar motion segments were compressed to failure in high physiological flexion (10º). This occurred over approximately 5 mm of crosshead displacement in 0.75 seconds that resulted in a displacement rate of 400 mm/min (defined as a "surprise" rate) and was intended to simulate the maximum rate at which the muscles of the spinal column can generate a force. The damaged discs were then analyzed microstructurally.

RESULTS

Fifty-eight percent of discs suffered annular-endplate junction rupture, 25% suffered midspan annular rupture, and the balance of 17% endplate fracture. Microstructural analysis indicated that annular rupture initiated at the endplate apical ridge in the mid-to-outer region of the annulus in both annular-endplate and midspan annulus rupture.

CONCLUSION

Motion segments subjected to a "surprise" loading rate are likely to fail via some form of annular rupture. Failure under such sudden loading occurs mostly via rupture of the annular-endplate junction and is thought to arise from a rate-induced mechanostructural imbalance between the annulus and the endplate.

LEVEL OF EVIDENCE

N/A.

Influence of maturity on nucleus-endplate integration in the ovine lumbar spine

PURPOSE

Recent investigations using an ovine spine model have established that the disc nucleus contains a highly convoluted fibre network with endplate-to-endplate connectivity, this connectivity being achieved via distinctive nodal attachment points. The purpose of this study was to investigate how this nodal anchoring system might be influenced by maturation.

METHODS

Lumbar motion segments were dissected from newborn, 3, 12 months and fully mature ovine animals, subjected to a novel annular ring-severing procedure to remove the strain-limiting influence of the annulus, then either mechanically tested to destruction or examined microstructurally and ultrastructurally. The morphology of the nodes and their linear density within the relatively thin section planes were analysed to provide a basis for comparison between the four age groups.

RESULTS

Mechanical testing following ring severing revealed that the remaining nuclear material in all samples, irrespective of maturity, had the ability to transmit a substantial load from endplate to endplate. Imaging of the ring-severed samples from all age groups in their stretched, but unruptured state revealed the presence of axially aligned fibrosity in the nucleus region consistent with endplate-to-endplate connectivity. Endplate insertion nodes were observed in all age groups. Ultrastructural examination revealed that the fibrillar architecture of these nodes in the newborn discs was similar to that observed in the nodes of mature discs. However, there was a rapid increase in their linear density between birth and 3 months, after which this remained constant.

CONCLUSIONS

The nodal attachment points identified previously in mature ovine discs are also present in newborn, and 3- and 12-month-old animals with an initial rapid increase in their linear density between birth and 3 months, after which it remained constant. The size and morphology of the attachment points were similar for all ages. Our study suggests that the increase in nodal density in the ovine disc endplate is part of an adaptive response to the loading environment that the disc is exposed to from birth to maturity.

Human growth hormone may be detrimental when used to accelerate recovery from acute tendon-bone interface injuries

BACKGROUND

There have been few scientific studies that have examined usage of human growth hormone to accelerate recovery from injury. The hypothesis of this study was that human growth hormone would accelerate tendon-to-bone healing compared with control animals treated with placebo in a rat model of acute rotator cuff injury repair.

METHODS

Seventy-two rats underwent repair of acute rotator cuff injuries and were randomized into the following postoperative dosing regimens: placebo, and human growth hormone at 0.1, 1, 2, 5, and 10 mg/kg/day, administered subcutaneously once per day for fourteen days (Protocol 1). An additional twenty-four rats were randomized to receive either (1) placebo or (2) human growth hormone at 5 mg/kg, administered subcutaneously twice per day for seven days preoperatively and twenty-eight days postoperatively (Protocol 2). All rats were killed twenty-eight days postoperatively. Mechanical testing was performed. Ultimate stress, ultimate force, stiffness, energy to failure, and ultimate distension were determined.

RESULTS

For Protocol 1, analysis of variance testing showed no significant difference between the groups with regard to ultimate stress, ultimate force, stiffness, energy to failure, or ultimate distension. In Protocol 2, ultimate force to failure was significantly worse in the human growth hormone group compared with the placebo group (21.1 ± 5.85 versus 26.3 ± 5.47 N; p = 0.035). Failure was more likely to occur through the bone than the tendon-bone interface in the human growth hormone group compared with the placebo group (p = 0.001). No significant difference was found for ultimate stress, ultimate force, stiffness, energy to failure, or ultimate distension between the groups in Protocol 2.

CONCLUSIONS

In this rat model of acute tendon-bone injury repair, daily subcutaneous postoperative human growth hormone treatment for fourteen days failed to demonstrate a significant difference in any biomechanical parameter compared with placebo. Furthermore, subcutaneous administration of 5 mg/kg of human growth hormone twice daily from seven days preoperatively until twenty-eight days postoperatively demonstrated lower loads to ultimate failure and a higher risk of bone fracture failure compared with placebo.

Novel biaxial tensile test for studying aortic failure phenomena at a microscopic level

Background

An aortic aneurysm is a local dilation of the aorta, which tends to expand and often results in a fatal rupture. Although larger aneurysms have a greater risk of rupture, some small aneurysms also rupture. Since the mechanism of aortic rupture is not well understood, clarification of the microstructure influencing the failure to rupture is important. Since aortic tissues are stretched biaxially in vivo, we developed a technique to microscopically observe the failure of an aortic rupture during biaxial stretch.

Methods

A thinly sliced porcine thoracic aortic specimen was adhered to a circular frame and pushed onto a cylinder with a smaller diameter to stretch the specimen biaxially. To induce failure to rupture at the center, the specimen was thinned at the center of the hole as follows: the specimen was frozen while being compressed with metal plates having holes, which were 3 mm in diameter at their centers; the specimen was then sliced at 50-μm intervals and thawed.

Results

The ratio of the thickness at the center to the peripheral area was 99.5% for uncompressed specimens. The ratio decreased with an increase in the compression ratio ε_c and was 47.3% for specimens with ε_c = 40%. All specimens could be stretched until failure to rupture. The probability for crack initiation within the cylinder was <30% and 100% for specimens with ε_c <10% and ε_c >30%, respectively. Among specimens ruptured within the cylinder, 93% of those obtained from the mid-media showed crack initiation at the thin center area.

Conclusions

Aortic tissues were successfully stretched biaxially until failure, and their crack initiation points were successfully observed under a microscope. This could be a very useful and powerful method for clarifying the mechanism of aortic rupture. We are planning to use this technique for a detailed investigation of events occurring at the point of failure when the crack initiates in the aortic aneurysm wall.

Bimodal collagen fibril diameter distributions direct age-related variations in tendon resilience and resistance to rupture

Scaling relationships have been formulated to investigate the influence of collagen fibril diameter (D) on age-related variations in the strain energy density of tendon. Transmission electron microscopy was used to quantify D in tail tendon from 1.7- to 35.3-mo-old (C57BL/6) male mice. Frequency histograms of D for all age groups were modeled as two normally distributed subpopulations with smaller (D(D1)) and larger (D(D2)) mean Ds, respectively. Both D(D1) and D(D2) increase from 1.6 to 4.0 mo but decrease thereafter. From tensile tests to rupture, two strain energy densities were calculated: 1) u(E) [from initial loading until the yield stress (σ(Y))], which contributes primarily to tendon resilience, and 2) u(F) [from σ(Y) through the maximum stress (σ(U)) until rupture], which relates primarily to resistance of the tendons to rupture. As measured by the normalized strain energy densities u(E)/σ(Y) and u(F)/σ(U), both the resilience and resistance to rupture increase with increasing age and peak at 23.0 and 4.0 mo, respectively, before decreasing thereafter. Multiple regression analysis reveals that increases in u(E)/σ(Y) (resilience energy) are associated with decreases in D(D1) and increases in D(D2), whereas u(F)/σ(U) (rupture energy) is associated with increases in D(D1) alone. These findings support a model where age-related variations in tendon resilience and resistance to rupture can be directed by subtle changes in the bimodal distribution of Ds.

Mechanical properties of human patellar tendon at the hierarchical levels of tendon and fibril

Tendons are strong hierarchical structures, but how tensile forces are transmitted between different levels remains incompletely understood. Collagen fibrils are thought to be primary determinants of whole tendon properties, and therefore we hypothesized that the whole human patellar tendon and its distinct collagen fibrils would display similar mechanical properties. Human patellar tendons ( n = 5) were mechanically tested in vivo by ultrasonography. Biopsies were obtained from each tendon, and individual collagen fibrils were dissected and tested mechanically by atomic force microscopy. The Young's modulus was 2.0 ± 0.5 GPa, and the toe region reached 3.3 ± 1.9% strain in whole patellar tendons. Based on dry cross-sectional area, the Young's modulus of isolated collagen fibrils was 2.8 ± 0.3 GPa, and the toe region reached 0.86 ± 0.08% strain. The measured fibril modulus was insufficient to account for the modulus of the tendon in vivo when fibril content in the tendon was accounted for. Thus, our original hypothesis was not supported, although the in vitro fibril modulus corresponded well with reported in vitro tendon values. This correspondence together with the fibril modulus not being greater than that of tendon supports that fibrillar rather than interfibrillar properties govern the subfailure tendon response, making the fibrillar level a meaningful target of intervention. The lower modulus found in vitro suggests a possible adverse effect of removing the tissue from its natural environment. In addition to the primary work comparing the two hierarchical levels, we also verified the existence of viscoelastic behavior in isolated human collagen fibrils.

The effects of multiple freeze-thaw cycles on the biomechanical properties of the human bone-patellar tendon-bone allograft

Soft tissue allografts, such as the bone-patellar tendon-bone (BPTB) graft, have been frequently used for anterior cruciate ligament (ACL) reconstruction. As allografts are subjected to freezing and thawing for multiple cycles, the objective of this study was to measure the changes of the biomechanical properties of the human BPTB allograft after 4 and 8 freeze-thaw cycles in comparison to a single freeze-thaw cycle. Three BPTB specimens were procured from 21 human donors and divided into three groups: 1, 4, or 8 freeze-thaw cycles. Each freeze-thaw cycle consisted of freezing at -20 ± 10°C for more than 6 h and thawing at 22 ± 3°C for at least 6 h. Tensile testing of the BPTB specimens consisted of loading between 50 N and 250 N for 100 cycles and then loading to failure. Cyclic loading revealed a similar amount of creep (∼0.5 mm) among the three freeze-thaw cycles groups (p = 0.38). The stiffness of the BPTB graft for the 1, 4, and 8 freeze-thaw cycle groups were 244 ± 42 N/mm, 235 ± 39 N/mm, and 231 ± 40 N/mm, respectively (p = 0.43). Similar findings were obtained for the ultimate load of the BPTB graft (p = 0.14) and the tangent modulus of the PT substance (p = 0.41). The results of this study suggest that there would be little measurable effect on the structural properties of the BPTB graft or mechanical properties of the PT tissue substance following 8 freeze-thaw cycles. These BPTB allografts could potentially be re-frozen without a loss in their biomechanical properties, given appropriate storage and care.

Elasticity, thermal stability and bioactivity of polyhedral oligomeric silsesquioxanes reinforced chitosan-based microfibres

A wet-spinning approach was used to extrude ribbon-like micrometer-thick fibres comprising chitosan with 1, 3, 5, 7 and 9% (w/w) polyhedral oligomeric silsesquioxanes (POSS). ANOVA reveals significant variations in the maximum stress (σ), stiffness (E), elastic energy storage (u') and fracture toughness (u) of the microfibres with respect to POSS concentration: σ, u' and u peak at 7% (w/w) but POSS concentration has no effect on E. Scanning electron microscopy of the ruptured microfibres reveals fracture and detachment of POSS precipitates from the chitosan matrix. Bioactivity test using simulated body fluids reveals a net gain in mass (by day 4) and grossly distorted morphology caused by apatite deposition on the microfibre surface. Fourier transform infrared spectroscopy reveals that chitin is partially deacetylated into chitosan and it further shows the presence of POSS in the microfibres. Thermogravimetric analysis shows that the microfibres are thermally stable up to 240°C in a nitrogen atmosphere.