Mechanical and Microstructural Properties of Pediatric Anterior Cruciate Ligaments and Autograft Tendons Used for Reconstruction

Repair of tendons treated with peracetic acid-ethanol and gamma irradiation by EDC combined with NHS: a morphological, biochemical and biomechanical study in vitro

The purpose of this study was to investigate whether 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) combined with n-hydroxysuccinimide (NHS) can repair tendon damage caused by peracetic acid-ethanol and gamma irradiation sterilization. The semitendinosus tendons of 15 New Zealand white rabbits were selected as experimental materials, and the tendons were sterilized in a solution containing 1% (v/w) peracetic acid and 24% (v/w) ethanol. After 15 kGy gamma irradiation sterilization, the tendons were randomly divided into three groups (n = 10). The tendons were repaired with EDCs of 0, 2.5 and 5 mM combined with 5 mM NHS for 6 h, the tendons were temporarily stored at - 80 ± °C. The arrangement and spatial structure of collagen fibers were observed by light microscopy and scanning electron microscopy, the collagen type and collagen crimp period were observed under a polarizing microscope, and the collagen fibril diameter and its distribution were measured by transmission electron microscopy, from which the collagen fibril index and mass average diameter were calculated. The resistance of collagen to enzymolysis was detected by the free hydroxyproline test, and tensile fracture and cyclic loading tests of each group of tendons were carried out, from which the elastic modulus, maximum stress, maximum strain, strain energy density and cyclic creep strain were calculated. The obtained results showed that the gap between loose collagen fibers in the 0 mM control group was wider, the parallel arrangement of tendons in the 2.5 and 5 mM groups was more uniform and regular and the fiber space decreased, the crimp period in the 5 mM group was lower than that in the 0 mM group (P < 0.05), and the concentration of hydroxyproline in the 5 mM group (711.64 ± 77.95 μg/g) was better than that in the control group (1150.57 ± 158.75 μg/g). The elastic modulus of the 5 mM group (424.73 ± 150.96 MPa) was better than that of the 0 mM group (179.09 ± 37.14 MPa). Our results show that EDC combined with NHS can repair damaged tendons after peracetic acid-ethanol and gamma radiation treatment, and 5 mM EDC has better morphological performance, anti-enzymolysis ability and biomechanical properties than 2.5 mM EDC.

Effects of various load magnitudes on ACL: an in vitro study using adolescent porcine stifle joints

INTRODUCTION

The escalating incidence of anterior cruciate ligament (ACL) injuries, particularly among adolescents, is a pressing concern. The study of ACL biomechanics in this demographic presents challenges due to the scarcity of cadaveric specimens. This research endeavors to validate the adolescent porcine stifle joint as a fitting model for ACL studies.

METHODS

We conducted experiments on 30 fresh porcine stifle knee joints. (Breed: Yorkshire, Weight: avg 90 lbs, Age Range: 2-4 months). They were stored at - 22 °C and a subsequent 24-h thaw at room temperature before being prepared for the experiment. These joints were randomly assigned to three groups. The first group served as a control and underwent only the load-to-failure test. The remaining two groups were subjected to 100 cycles, with forces of 300N and 520N, respectively. The load values of 300N and 520N correspond to three and five times the body weight (BW) of our juvenile porcine, respectively.

RESULT

The 520N force demonstrated a higher strain than the 300N, indicating a direct correlation between ACL strain and augmented loads. A significant difference in load-to-failure (p = 0.014) was observed between non-cyclically loaded ACLs and those subjected to 100 cycles at 520N. Three of the ten samples in the 520N group failed before completing 100 cycles. The ruptured ACLs from these tests closely resembled adolescent ACL injuries in detachment patterns. ACL stiffness was also measured post-cyclical loading by applying force and pulling the ACL at a rate of 1 mm per sec. Moreover, ACL stiffness measurements decreased from 152.46 N/mm in the control group to 129.42 N/mm after 100 cycles at 300N and a more significant drop to 86.90 N/mm after 100 cycles at 520N. A one-way analysis of variance (ANOVA) and t-test were chosen for statistical analysis.

CONCLUSIONS

The porcine stifle joint is an appropriate model for understanding ACL biomechanics in the skeletally immature demographic. The results emphasize the ligament's susceptibility to injury under high-impact loads pertinent to sports activities. The study advocates for further research into different loading scenarios and the protective role of muscle co-activation in ACL injury prevention.

Differences in the microstructural and mechanical qualities of semitendinosus tendon grafts between skeletally immature and mature patients in anterior cruciate ligament reconstruction

BACKGROUND

This study aimed to investigate the microstructural and mechanical properties of semitendinosus tendon graft tissues during anterior cruciate ligament reconstruction and the clinical outcomes in skeletally immature and mature patients.

METHODS

Twenty-two patients who underwent primary anterior cruciate ligament reconstruction using a hamstring tendon graft were analyzed and divided into skeletally immature (n = 7) and mature groups (n = 15) based on magnetic resonance imaging findings of the epiphyseal plate of the distal femur. Tissue samples were collected from the mid-portion of the semitendinosus tendon. The collagen fibril diameter, maximum stress, and strain at maximum stress point in the semitendinosus tendon tissues were calculated for comparison of the microstructural and mechanical properties between the two groups. Postoperative outcomes were also assessed between the two groups.

RESULTS

The mean and 60th and 80th percentiles of fibril diameters in the skeletally immature group were significantly smaller than those in the mature group (65.9 ± 13.0, 73.5 ± 19.3, and 91.3 ± 27.4 nm in the skeletally immature group; and 90.3 ± 14.7, 94.0 ± 18.4, and 125.3 ± 19.9 nm in the skeletally immature group; p = 0.001, 0.024, and 0.004, respectively). Additionally, the strain at maximum stress was higher in the skeletally immature group (237.2 ± 102.4% vs. 121.5 ± 51.9%, p = 0.024). However, there was no difference in maximum stress between the skeletally immature and mature groups (19.9 ± 14.3 MPa vs. 24.5 ± 23.4 MPa, p = 0.578). Strain was negatively correlated with the mean fibril diameter and the 60th and 80th percentiles of fibril diameters, whereas stress was positively correlated with the mean fibril diameter. The skeletally immature group had a higher pivot shift test-positive rate than the mature group at the last follow-up (p = 0.023).

CONCLUSION

Semitendinosus tendon graft tissues differed microstructurally and mechanically between skeletally immature and mature patients.

LEVEL OF EVIDENCE

Level Ⅳ.

Clinical outcomes of different autografts used for all-epiphyseal, partial epiphyseal or transphyseal anterior cruciate ligament reconstruction in skeletally immature patients - a systematic review

BACKGROUND

Different types of grafts can be used for anterior cruciate ligament reconstruction (ACLR). There is little published data regarding skeletally immature patients. The purpose of this systematic review was to assess the clinical outcomes and complications for different autograft types used in all-epiphyseal, transphyseal and partial epiphyseal/hybrid ACLR in skeletally immature children and adolescents.

METHODS

PubMed, Embase and Cochrane databases were systematically searched for literature regarding ACLR using hamstrings, quadriceps or bone-patellar-tendon-bone (BPTB) autografts in skeletally immature patients. Studies were included if they examined at least one of the following outcomes: graft failure, return to sport(s), growth disturbance, arthrofibrosis or patient reported outcomes and had a minimum follow-up of 1 year. Case reports, conference abstracts and studies examining allografts and extra-articular or over-the-top ACL reconstruction techniques were excluded. Graft failure rates were pooled for each graft type using the quality effects model of MetaXL. A qualitative synthesis of secondary outcomes was performed.

RESULTS

The database search identified 242 studies. In total 31 studies were included in this review, comprising of 1358 patients. Most patients (81%) were treated using hamstring autograft. The most common used surgical technique was transphyseal. The weighted, pooled failure rate for each graft type was 12% for hamstring tendon autografts, 8% for quadriceps tendon autografts and 6% for BPTB autografts. Confidence intervals were overlapping. The variability in time to graft failure was high. The qualitative analysis of the secondary outcomes showed similar results with good clinical outcomes and low complication rates across all graft types.

CONCLUSIONS

Based on this review it is not possible to determine a superior graft type for ACLR in skeletally immature. Of the included studies, the most common graft type used was the hamstring tendon. Overall, graft failure rates are low, and most studies show good clinical outcomes with high return to sports rates.

Characterization of the Temporomandibular Joint Disc Complex in the Yucatan Minipig

The temporomandibular joint (TMJ) disc complex (i.e., the TMJ disc and its six attachments) is crucial to everyday functions such as mastication and speaking. The TMJ can be afflicted by many conditions, including disc displacement and defects. Pathologies of the TMJ disc complex most commonly present first as anterior disc displacement, which the field hypothesizes may implicate the two posterior attachments. As a result of anterior disc displacement, defects may develop in the lateral disc complex. Tissue engineering is poised to improve treatment paradigms for these indications of the TMJ disc complex by engineering biomimetic implants, but, first, gold-standard design criteria for such implants should be established through characterization studies. This study's objective was to characterize the structural, mechanical, biochemical, and crosslinking differences among the two posterior attachments and the lateral disc in the Yucatan minipig, a well-accepted TMJ animal model. In tension, it was found that the posterior inferior attachment (PIA) was significantly stiffer and stronger by 2.13 and 2.30 times, respectively, than the posterior superior attachment (PSA). It was found that collagen in both attachments was primarily aligned mediolaterally; however, the lateral disc was much more aligned and anisotropic than either attachment. Among the three locations, the PSA exhibited the greatest degree of heterogeneity and highest proportion of fat vacuoles. The PIA and lateral disc were 1.93 and 1.91 times more collagenous, respectively, by dry weight (DW) than the PSA. The PIA also exhibited 1.78 times higher crosslinking per DW than the PSA. Glycosaminoglycan per DW was significantly higher in the lateral disc by 1.48 and 5.39 times than the PIA and PSA, respectively. Together, these results establish design criteria for tissue-engineering of the TMJ disc complex and indicate that the attachments are less fibrocartilaginous than the disc, while still significantly contributing to the mechanical stability of the TMJ disc complex during articulation. These results also support the biomechanical function of the PIA and PSA, suggesting that the stiffer PIA anchors the disc to the mandibular condyle during articulation, while the softer PSA serves to allow translation over the articular eminence. Impact Statement Characterization of the temporomandibular joint (TMJ) disc complex (i.e., the disc and its attachments) has important implications for those aiming to tissue-engineer functional replacements and can help elucidate its biomechanical function. For example, the findings shown here suggest that the stiffer posterior inferior attachment anchors the disc during articulation, while the softer posterior superior attachment allows translation over the articular eminence.

Quadriceps tendon autograft for pediatric anterior cruciate ligament reconstruction results in promising postoperative function and rates of return to sports: A systematic review

PURPOSE

To assess the performance of the quadriceps tendon (QT) autograft in pediatric anterior cruciate ligament reconstruction (ACLR).

METHODS

A systematic search of MEDLINE, PubMed, and EMBASE was conducted on March 1, 2021. Studies of all levels of evidence reporting outcomes and/or complications after QT autograft ACLR in pediatric patients (≤ 18 years old) were eligible for inclusion. Study demographics, patient demographics, reported outcomes, and complications were abstracted. Screening and data abstraction were designed in accordance with PRISMA and R-AMSTAR guidelines. Descriptive statistics were presented when applicable, with data for heterogeneous outcomes presented in narrative summary fashion.

RESULTS

A total of 14 studies examining 596 patients (46.3% female), mean age 15.4 years, were included in this systematic review. Mean postoperative Lysholm scores ranged from 94.0 to 99.5. Mean postoperative IKDC subjective scores ranged from 75.9 to 94.0. Limb symmetry index ranged from 96.8 ± 10.4 to 100.4 ± 7.6% across multiple hop tests. Return to sports (RTS) rates ranged from 88.9 to 91.7%. Eleven studies reported postoperative complications, whereby 16 patients (4.8%) experienced contralateral complications and injuries. Forty-six patients (9.4%) experienced ipsilateral complications, including ten graft failures (2.5%) and two growth disturbances (0.6%).

CONCLUSIONS

QT autograft ACLR in the pediatric population retains the potential of regaining a preinjury level of knee stability, and yields promising postoperative function and rates of RTS, yielding comparable outcomes relative to HT autograft and the reference-standard BPTB ACLR that have previously been described in the literature. Moreover, use of the QT autograft is associated with low rates of postoperative complications, including graft failure and growth disturbances in this active and high-risk patient population in observational studies to date. Therefore, clinical equipoise exists to further appraise the influence of QT autograft on postoperative outcomes compared to aforementioned autograft options in a randomized control trial fashion.

LEVEL OF EVIDENCE

IV.

Effects of Chemical Sterilization and Gamma Irradiation on the Biochemical and Biomechanical Properties of Human Tendon Allografts In Vitro Study

Objective

Pre‐implantation sterilization procedures for tendons are important measures to reduce the risk of disease transmission, however these procedures may compromise tendon microarchitecture and biomechanical properties to varying degrees. We explore the effects of different sterilization procedures on the micro‐histology, biomechanical strength and biochemical properties of human tendon allografts in vitro study.

Methods

The tendon allografts were harvested from cadaveric donors after the donors were serologically screened by antibody or nucleic acid testing of infectious agents. All samples were divided into five groups, which were fresh‐frozen group (control group), 15 kGy gamma irradiation group, 25 kGy gamma irradiation group, 70% ethanol group, and peracetic acid‐ethanol group. Each group included 10 tendons for testing. Histological staining and transmission electron microscopy were applied to observe the internal structure and arrangement of tendon collagen fibers, while the machine learning classifier was trained to distinguish the darker cross‐sections of collagen fibers and brighter backgrounds of the electron micrograph to detect the distribution of diameters of tendon collagen fibers. The viscoelasticity, mechanical properties and material properties of tendon allografts were examined to detect the influence of different intervention factors on the biomechanical properties of tendons.

Results

Histological staining and transmission electron microscopy showed that the structure of fresh‐frozen tendons was similar to the structures of other experimental groups, and no obvious fiber disorder or delamination was observed. In the uniaxial cyclic test, the cyclic creep of 25 kGy irradiation group (1.5%) and peracetic acid‐ethanol group (1.5%) were significantly lower than that of the control group (3.6%, F = 1.52, P = 0.039) while in the load‐to‐failure test, the maximum elongation and maximum strain of the peracetic acid‐ethanol group were significantly higher than those of the control group (F = 4.60, P = 0.010), and there was no significant difference in other biomechanical indicators. According to the experimental results of denatured collagen, it could be seen that no matter which disinfection procedure was used, the denaturation of the tendon sample would be promoted (F = 1.97, P = 0.186), and high‐dose irradiation seemed to cause more damage to collagen fibers than the other two disinfection procedures (296.2 vs 171.1 vs 212.9 μg/g).

Conclusion

Biomechanical experiments and collagen denaturation tests showed that 15 kGy gamma irradiation and 70% ethanol can preserve the biomechanical strength and biochemical properties of tendons to the greatest extent, and these two sterilization methods are worthy of further promotion.

Sex-specific biomechanics and morphology of the anterior cruciate ligament during skeletal growth in a porcine model

Pediatric anterior cruciate ligament (ACL) injuries are on the rise, and females experience higher ACL injury risk than males during adolescence. Studies in skeletally immature patients indicate differences in ACL size and joint laxity between males and females after the onset of adolescence. However, functional data regarding the ACL and its anteromedial and posterolateral bundles in the pediatric population remain rare. Therefore, this study uses a porcine model to investigate the sex-specific morphology and biomechanics of the ACL and its bundles throughout skeletal growth. Hind limbs from male and female Yorkshire pigs aged early youth to late adolescence were imaged using magnetic resonance imaging to measure the size and orientation of the ACL and its bundles, then biomechanically tested under anterior-posterior drawer using a robotic testing system. Joint laxity decreased (p < 0.001) while joint stiffness increased (p < 0.001) throughout skeletal growth in both sexes. The ACL was the primary stabilizer against anterior tibial loading, while the functional role of the anteromedial bundle increased with age (p < 0.001), with an earlier increase in males. ACL and posterolateral bundle cross-sectional area and ACL and anteromedial bundle length were larger in males than females during adolescence (p < 0.01 for all), while ACL and bundle sagittal angle remained similar between sexes. Additionally, in situ ACL stiffness versus cross-sectional area regressions were significant across skeletal growth (r²  = 0.75, p < 0.001 in males and r²  = 0.64, p < 0.001 in females), but not within age groups. This study has implications for age and sex-specific surgical intervention strategies and suggests the need for human studies.

Returning Athletes to Sports Following Anterior Cruciate Ligament Tears

PURPOSE OF REVIEW

The purpose of this review is to discuss treatment options, rehabilitation protocols, return-to-play criteria, and expected outcomes after non-operative and operative treatment of anterior cruciate ligament (ACL) tears among an athletic population.

RECENT FINDINGS

Non-operative treatment may be a viable option for some athletes with an ACL tears but can be difficult to predict "copers," and those that resume to sports return at lower performance level and/or less intense activities. Most studies assessing function after ACL reconstruction demonstrate favorable outcomes using patient-reported outcome studies. However, return-to-play and graft re-rupture rates vary substantially based on patient characteristics and level and type of athletic activity. Grafts used to reconstruct ACL produce similar objective outcomes and favorable patient-reported outcomes but have variable re-rupture rates depending on study and differ largely on morbidity associated with graft harvest. Various treatment methods including non-operative and operative techniques have been demonstrated to be efficacious in returning athletes to athletic activity depending on patient age and level of activity. Adherence to fundamental rehabilitation principles and accepted return-to-play guidelines can optimize outcomes and limit re-injury to the injured or contralateral limb.

Categorize the existing clamps used for tensile test of human graft- a systematic review

Background

The use of tendon allografts for orthopedic repair has gained wide acceptance in recent years, most notably in anterior cruciate tendon reconstruction. Multiple studies support the use of tendon allografts and the benefits of its use are well accepted and understood. One of the important criteria of the use of tendon allografts is statistically similar histological and biomechanical properties to autographs. The aim of this systematic literature review is to investigate and categorize existing clamps used in the determination of the biomechanical properties of tendons such as maximum load, maximum strength, modulus of elasticity, ultimate strain, and stiffness. A variety of clamps for use during the endurance test of tendons were categorized according to the temperature used during the measurement. The clamps are divided into three groups: room temperature, cooled and heated clamps. The second goal of our review is to overview of clamps on the following aspects: name of clamp, author and date, type of clamps, type of endurance test (static or dynamic), type preloading (dynamic or static), type of tendon and measured and calculated parameters, and summarize in Table 3, as a comprehensive catalogue.

Methods

This systematic review was carried out in keeping with the PRISMA 2020 E&E and the PRISMA-S guidelines and checklists. A search was conducted for publications dating between 1991 and February 28th 2022 through three electronic databases (Web of Science, Scopus, and PubMed). We used Critical Appraisal Skills Program checklist to check the quality of included articles.

Results

The database search and additional sources resulted in 1725 records. 1635 records eliminated during the screening for various reasons (case report, other languages, book chapter, unavailable text/conference abstract, unrelated topic). The number of articles used in the final synthesis was 90. A variety of clamps for use during the endurance test of tendons were identified and categorized according to the temperature used during the measurement. Based on this, the clamps are divided into three groups: room temperature, cooled or heated clamps.

Conclusions

On the basis of the systematic literature review, mechanical parameters determined by usage with cooled clamps proved to be more reliable than with those at room temperature and with heated clamps. The collected information from the articles included name of clamp, author and date, type of clamps, type of endurance test (static or dynamic), type preloading (dynamic or static), type of tendon and measured and calculated parameters given in Table 3. summarized. The main advantage of the cooled clamps is that there is no limit to the type and length of the tendon. This study provides an overview of clamps and does not represent the modernity of any method.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-022-05650-w.

Biomechanical properties of common graft choices for anterior cruciate ligament reconstruction: A systematic review

BACKGROUND

This systematic review explores the differences in the intrinsic biomechanical properties of different graft sources used in anterior cruciate ligament (ACL) reconstruction as tested in a laboratory setting.

METHODS

Following Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, two authors conducted a systematic review exploring the biomechanical properties of ACL graft sources (querying PubMed, Cochrane, and Embase databases). Using the keywords "anterior cruciate ligament graft," "biomechanics," and "biomechanical testing," relevant articles of any level of evidence were identified as eligible and included if they reported on the biomechanical properties of skeletally immature or mature ACL grafts solely and if the grafts were studied in vitro, in isolation, and under similar testing conditions. Studies were excluded if performed on both skeletally immature and mature or non-human grafts, or if the grafts were tested after fixation in a cadaveric knee. For each graft, failure load, stiffness, Young's modulus, maximum stress, and maximum strain were recorded.

FINDINGS

Twenty-six articles were included. Most studies reported equal or increased biomechanical failure load and stiffness of their tested bone-patellar tendon-bone, hamstring, quadriceps, peroneus longus, tibialis anterior and posterior, Achilles, tensor fascia lata, and iliotibial band grafts compared to the native ACL. All recorded biomechanical properties had similar values between graft types.

INTERPRETATION

Most grafts used for ACL reconstruction are biomechanically superior to the native ACL. Utilizing a proper graft, combined with a standard surgical technique and a rigorous rehabilitation before and after surgery, will improve outcomes of ACL reconstruction.

Full thickness quadriceps tendon grafts with bone had similar material properties to bone-patellar tendon-bone and a four-strand semitendinosus grafts: a biomechanical study

PURPOSE

Despite increasing interest in utilizing quadriceps tendon (QT) grafts in anterior cruciate ligament reconstruction (ACLR), data on the optimal quadriceps graft thickness are limited. The purpose of this study was to characterize the mechanical properties for the quadriceps tendon, comparing full-thickness (FT) QT grafts with and without bone to a partial-thickness (PT) QT graft, and comparing the three QT grafts to four-stranded semitendinosus (4-SST) and bone-patellar tendon-bone (BTB) grafts and one experimental graft, the two-stranded rectus femoris (RF).

METHODS

Forty-eight (n = 48) young cadaveric grafts (mean age 32 ± 6 years) were utilized for testing with N = 8 specimens in each of the following groups; (1) FT QT with bone, (2) FT QT without bone, (3) PT QT without bone, (4) BTB, (5) RF, and (6) 4-SST. Each specimen was harvested and rigidly fixed in custom clamps to a dynamic tensile testing machine for biomechanical evaluation. Graft ultimate load and stiffness were recorded. Independent groups one-factor ANOVAs and Tukey's pairwise comparisons were performed for statistical analyses.

RESULTS

FT QT with bone and 4-SST grafts demonstrated similar ultimate loads to BTB grafts (both n.s), whereas PT QT demonstrate statistically significantly lower ultimate loads to BTB grafts (n.s) and 4-SST grafts (n.s). Furthermore, no statistically significant differences were observed between the ultimate loads of FT QT vs. PT QT grafts without bone (n.s) or between FT QT with vs. without bone (n.s). FT QT grafts with bone did not demonstrate statistically significantly greater ultimate loads than PT QT grafts without bone (n.s). The RF graft demonstrated statistically significantly lower ultimate loads to BTB grafts (p < 0.005) and 4-SST grafts (p < 0.014).

CONCLUSIONS

Full thickness QT grafts with bone had similar material properties to BTB and a 4-SST grafts, while Partial thickness QT graft without bone had significantly lower material properties than BTB and 4-SST, in a biomechanical setting.

Differences in cellular and microstructural properties of the semitendinosus muscle tendon between young and adult patients

BACKGROUND

Poor outcomes associated with anterior cruciate ligament reconstruction in paediatric patients are a major concern. The tendon structure and its cellular characteristics are key factors that affect the mechanical properties of tendons. This study aimed to evaluate the effects of growth on the cellular and microstructural properties of the tendon of the semitendinosus muscle in humans.

METHODS

Semitendinosus muscle tendon samples from 76 patients who underwent ligament reconstruction were examined and divided into three groups: immature (10.8 ± 2.7 years old), young (16.5 ± 1.8 years old), and adult (35.2 ± 8.6 years old), based on age and the state of the epiphyseal plate in the distal femur. The number of tendon cells per unit area was assessed, and the major-to-minor-length ratio of the tendon cell nuclei was calculated to evaluate the shape of the nuclei using haematoxylin and eosin staining. The collagen fibril diameter and distribution were determined using electron microscopy.

RESULTS

The major-to-minor-length ratio of the tendon cell nuclei significantly increased with age (p-value; immature vs. young: 0.018, young vs adult: 0.001, immature vs adult: 0.001). The shape of the tendon cell nuclei was rounder in the immature group and more elongated in the adult group. A significant decrease in the number of tendon cells was observed with age (immature: 565 ± 134/mm², young: 356 ± 105/mm², adult: 272 ± 81/mm²; p-value: immature vs young: 0.001, young vs adult: 0.012, immature vs adult: 0.001). The mean fibril diameter in the immature group was significantly smaller (p-value: immature vs young: 0.018, young vs adult: 0.001, immature vs adult: 0.001). The distribution of the collagen fibrils changed from right skewed in the immature group to flat in the adult group.

CONCLUSIONS

The characteristics of the tendon cells and the microstructure of collagen in muscle tendons significantly changed with age.

MRI Signal Intensity of Quadriceps Tendon Autograft and Hamstring Tendon Autograft 1 Year After Anterior Cruciate Ligament Reconstruction in Adolescent Athletes

BACKGROUND

Hamstring tendon autograft (HTA) is a common graft choice for anterior cruciate ligament (ACL) reconstruction (ACLR) in skeletally immature patients. Recently, the use of quadriceps tendon autograft (QTA) has shown superior preliminary outcomes in this population.

PURPOSE

To evaluate graft maturity by comparing magnetic resonance imaging (MRI) signal intensity of HTA versus QTA used in primary ACLR.

STUDY DESIGN

Cohort study; Level of evidence, 3.

METHODS

All patients under the age of 18 years who underwent a primary ACLR by the senior authors using either an HTA or a QTA were retrospectively reviewed. A total of 70 skeletally immature patients (37 in the HTA group and 33 in the QTA group) with an available MRI at 6 and 12 months postoperatively were included. Signal intensity ratio (SIR) was measured on sagittal MRI by averaging the signal at 3 regions of interest along the ACL graft and dividing by the signal of the tibial footprint of the posterior cruciate ligament. Statistical analysis was performed to determine interrater reliability and differences between time points and groups.

RESULTS

Age, sex, and type of surgery were not associated with any differences in SIR. There was no significant difference in SIR between groups on the 6-month MRI. However, the SIR of the QTA group was significantly less than in the HTA group on the 12-month MRI (2.33 vs 2.72, respectively; P = .028). Within the HTA group, there was no significant difference in SIR at either MRI time point. In the QTA group, there was a significant decrease in SIR between the 6-month and 12-month postoperative MRI (2.70 vs 2.33, respectively; P = .045).

CONCLUSION

These findings suggest improved graft maturation, remodeling, and structural integrity of the QTA compared with the HTA between 6 and 12 months postoperatively. This provides evidence that, at 1 year postoperatively, QTA may have a superior rate of incorporation and synovialization as compared with the HTA.

Peracetic Acid-Ethanol Processed Human Tendon Allograft: A Morphological, Biochemical, and Biomechanical Study In Vitro

OBJECTIVE

To clarify the morphological, biochemical, and biomechanical effects of peracetic acid-ethanol sterilization processing to human hamstring tendon allografts for different time periods.

METHODS

Thirty-two fresh-frozen human hamstring tendon allografts obtained from an allograft supplier were prepared and incubated in peracetic acid-ethanol solution (PES) containing 1% v/v peracetic acid and 24% v/v ethanol. Specimens were randomly classified into four groups according to the PES processing time (untreated as the control group, 30 min as the PES30 group, 120 min as the PES120 group, and 240 min as the PES240group). Light microscopy with hematoxylin-eosin and toluidine blue were performed, along with transmission electron microscopy (TEM) to measure the collagen fibril diameters and their distributions, from which the collagen fibril index (CFI) and mass average diameter (MAD) were calculated. The thermal stability and collagen denaturation were analyzed by differential scanning calorimetry (DSC) and collagen denaturation test by α-chymotrypsin. Cyclic loading and failure testing were applied on five tendons from each group, from which the cyclic creep strain, elastic modulus, maximum stress, maximum strain, and strain energy density were calculated.

RESULTS

Tendons in the control, PES30, PES120 groups showed similar regularly aligned collagen fibers in light microscopy images, while the images from the PES240 group revealed relatively disordered and heterogeneous collagen bundles with larger interfiber spaces. TEM analysis showed that the mean diameter (F = 3.09, P = 0.04) was lower in the PES120 group (87.15 ± 4.76 nm) than it was in the control group (99.39 ± 9.19 nm) but not statistically (P = 0.05). Moreover, the CFI value in the PES30 group (65.37 ± 4.14%) was the lowest among groups (all P ≤ 0.01), while no variance existed in density and MAD among groups (F = 2.09, P = 0.13, and F = 0.27, P = 0.85, respectively). The onset temperature (H = 8.74, P = 0.03) and peak temperature (H = 9.97, P = 0.02) were decreased in the PES30 group compared to the control group (P = 0.02 and P = 0.01, respectively), but there were no differences in enthalpy of denaturation among groups (F = 2.20, P = 0.17). The collagen denaturation test revealed lower hydroxyproline concentrations in PES-treated specimens with no statistical differences among groups (H = 8.86, P = 0.07). The maximum stress showed variance (F = 10.52, P < 0.01) that it was higher in PES30 group (68.29 ± 10.86 MPa) compared to the PES120 and the PES240 group, while it was lower in the PES120 group (19.40 ± 4.94 MPa) compared to the control and the PES30 group (all P < 0.05). The strain energy density (F = 7.34, P < 0.01) was over 4 times higher in the PES30 group (7.39 ± 2.51 MPa) than it was in the PES120 group (1.56 ± 0.64 MPa, P < 0.01).

CONCLUSION

PES treatment for 30 min has no adverse effect on the properties of human hamstring tendon allografts, longer processing time could not promise better properties preservation.

Change in Collagen Fibril Diameter Distribution of Bovine Anterior Cruciate Ligament upon Injury Can Be Mimicked in a Nanostructured Scaffold

More than 200,000 people are suffering from Anterior Cruciate Ligament (ACL) related injuries each year in the US. There is an unmet clinical demand for improving biological attachment between grafts and the host tissue in addition to providing mechanical support. For biological graft integration, it is important to provide a physiologically feasible environment for the host cells to enable them to perform their duties. However, behavior of cells during ACL healing and the mechanism of ACL healing is not fully understood partly due to the absence of appropriate environment to test cell behavior both in vitro and in vivo. This study aims at (i) investigating the change in fibril diameter of bovine ACL tissue upon injury and (ii) fabricating nanofiber-based scaffolds to represent the morphology and structure of healthy and injured ACL tissues. We hypothesized that distribution and mean diameter of ACL fibrils will be altered upon injury. Findings revealed that the collagen fibril diameter distribution of bovine ACL changed from bimodal to unimodal upon injury with subsequent decrease in mean diameter. Polycaprolactone (PCL) scaffold fiber diameter distribution exhibited similar bimodal and unimodal distribution behavior to qualitatively represent the cases of healthy and injured ACL, respectively. The native ACL tissue demonstrated comparable modulus values only with the aligned bimodal PCL scaffolds. There was significant difference between mechanical properties of aligned bimodal and unaligned unimodal PCL scaffolds. We believe that the results obtained from measurements of diameter of collagen fibrils of native bovine ACL tissue can serve as a benchmark for scaffold design.

Altering the Mechanical Load Environment During Growth Does Not Affect Adult Achilles Tendon Properties in an Avian Bipedal Model

Tendon mechanical properties respond to altered load in adults, but how load history during growth affects adult tendon properties remains unclear. To address this question, we adopted an avian model in which we altered the mechanical load environment across the growth span. Animals were divided at 2 weeks of age into three groups: (1) an exercise control group given the opportunity to perform high-acceleration movements (EXE, n = 8); (2) a sedentary group restricted from high-intensity exercise (RES, n = 8); and (3) a sedentary group also restricted from high-intensity exercise and in which the gastrocnemius muscles were partially paralyzed using repeated bouts of botulinum toxin-A injections (RES-BTX, n = 8). Video analysis of bird movement confirmed the restrictions eliminated high-intensity exercise and did not alter time spent walking and sitting between groups. At skeletal maturity (33-35 weeks) animals were sacrificed for analysis, consisting of high-field MRI and material load testing, of both the entire free Achilles tendon and the tendon at the bone-tendon junction. Free tendon stiffness, modulus, and hysteresis were unaffected by variation in load environment. Further, the bone-tendon junction cross-sectional area, stress, and strain were also unaffected by variations in load environment. These results suggest that: (a) a baseline level of low-intensity activity (standing and walking) may be sufficient to maintain tendon growth; and (b) if this lower threshold of tendon load is met, non-mechanical mediated tendon growth may override the load-induced mechanotransduction signal attributed to tendon remodeling in adults of the same species. These results are important for understanding of musculoskeletal function and tendon health in growing individuals.

Anterior Cruciate Ligament Re-tear and Revision Reconstruction in the Skeletally Immature Athlete

PURPOSE OF REVIEW

With an increase in high-demand sporting activity, the rate of pediatric and adolescent anterior cruciate ligament (ACL) reconstruction is increasing. Yet, the failure rates after reconstruction are much higher than the adult population. The purpose of this paper is to understand failure rates, reasons for graft failure, and strategies for successful revision surgery.

RECENT FINDINGS

A complete understanding of the failure etiology is essential for the clinician treating this population prior to revision. This begins with an assessment of post-operative patient compliance and sporting activity. Surgical technique must then be scrutinized for non-anatomic tunnel placement and poor graft size/type. Concurrent bony deformity must also be addressed including lower extremity valgus alignment and tibial slope abnormalities. Meniscus and chondral injury must be aggressively treated. Furthermore, imaging must be examined to look for missed posterolateral corner injury. Lateral extra-articular tenodesis (in the setting of ligamentous laxity or rotational instability) may be also indicated as well. The surgeon can then choose a graft type and surgical technique that optimizes outcome and respects skeletal growth. Prior to surgical intervention, the clinician must also counsel patients in regard to the guarded prognosis and outcomes in this setting. Prolonged rehabilitation protocols/return-to-play timing as well as sporting activity modification in the post-operative period after revision are critical. There is limited literature on revision ACL reconstruction in the skeletally immature athlete. An understanding of all the risk factors for failure is essential in order to achieve treatment success.

Joint laxity varies in response to partial and complete anterior cruciate ligament injuries throughout skeletal growth

Anterior cruciate ligament (ACL) injuries are increasingly common in the skeletally immature population. As such there is a need to increase our understanding of the biomechanical function of the joint following partial and complete ACL injury during skeletal growth. In this work, we aimed to assess changes in knee kinematics and loading of the remaining soft tissues following both partial and complete ACL injury in a porcine model. To do so, we applied anterior-posterior tibial loads and varus-valgus moments to stifle joints of female pigs ranging from early juvenile to late adolescent ages and assessed both kinematics and in-situ loads carried in the bundles of the ACL and other soft tissues including the collateral ligaments and the menisci. Partial ACL injury led to increased anterior tibial translation only in late adolescence and small increases in varus-valgus rotation at all ages. Complete ACL injury led to substantial increases in translation and rotation at all ages. At all ages, the medial collateral ligament and the medial meniscus combined to resist the majority of applied anterior tibial load following complete ACL transection. Across all ages and flexion angles, the contribution of the MCL ranged from 45 to 90% of the anterior load and the contribution of the medial meniscus ranged from 14 to 35% of the anterior load. These findings add to our current understanding of age-specific functional properties of both healthy and injured knees during skeletal growth.

Optical Microscopy and Electron Microscopy for the Morphological Evaluation of Tendons: A Mini Review

The morphological characteristics of tendons have been thoroughly evaluated via microscopy. Optical microscopy and electron microscopy are the most commonly used techniques for tendon tissue observation. According to the principles of both microscopy types, preparation and evaluation methods vary. Simple optical microscopy is commonly used in the observation of cells and extracellular matrix, and many stains, including hematoxylin–eosin, Van Gieson, Prussian blue, Alcian blue, and toluidine blue, are used for evaluating cells, collagen fiber arrangement, and noncollagenous proteins. Histological scoring systems have been used in many studies for semi‐quantification. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are the most commonly used electron microscopy types, and special consideration is needed for the fixation and embedding protocols. Glutaraldehyde followed by osmium is most commonly used in the chemical fixation of tendon tissue, followed by epoxy resin embedment. Longitudinal sections captured in SEM images show the arrangement of collagen fibrils and the cells and lipid drops among them, while cross sections captured in TEM images show the diameter and distribution of collagen fibrils. SEM and TEM are used together for comprehensive evaluations. This mini review is focused on the preparation methodology and related evaluation indexes for the morphological evaluation of tendons.

Mechanical and Microstructural Properties of Native Pediatric Posterior Cruciate and Collateral Ligaments

BACKGROUND

Although anterior cruciate ligament (ACL) tears have received the most attention, the medial collateral ligament (MCL) is thought to be the most commonly injured knee ligament overall. The lateral collateral ligament (LCL) and posterior collateral ligament (PCL) are less frequently compromised but can be involved in severe multiligament injuries. The paucity of information on the native properties of these ligaments in the pediatric population hinders the overall optimization of treatment for these injuries.

PURPOSE

To characterize the mechanical and microstructural properties of pediatric MCLs, LCLs, and PCLs using a rare cadaveric cohort (mean age, 9.2 years).

STUDY DESIGN

Descriptive laboratory study.

METHODS

MCLs, LCLs, and PCLs were harvested from 5 fresh-frozen pediatric knee specimens (3 male, 2 female) and were subjected to a tensile loading protocol. A subset of contralateral tissues from a single donor was analyzed using bright-field, polarized light, and transmission electron microscopy to measure collagen fiber morphology.

RESULTS

The pediatric MCL exhibited values for ultimate stress (11.7 ± 6.7 MPa), ultimate strain (18.2% ± 6.8%), and the Young modulus (93.7 ± 56.5 MPa) that were similar to values for the LCL (11.4 ± 11.5 MPa, 27.7% ± 12.9%, and 64.4 ± 76.6 MPa, respectively). The PCL demonstrated decreased ultimate stress (4.2 ± 1.8 MPa), increased ultimate strain (28.8% ± 11.9%), and a decreased Young modulus (19.8 ± 10.4 MPa) when compared with the MCL and LCL. All 3 ligaments had similar mean crimp wavelengths (MCL, 32.8 ± 3.6 µm; LCL, 27.2 ± 3.5 µm; PCL, 25.8 ± 3.5 µm) and collagen fibril diameters (MCL, 88.0 ± 26.0 nm; LCL, 93.3 ± 34.6 nm; PCL, 90.9 ± 34.0 nm); however, the fibril distribution profiles exhibited different modalities.

CONCLUSION

The pediatric MCL and LCL possessed similar mechanical properties, while the pediatric PCL was weaker but capable of withstanding higher amounts of strain. All 3 of these pediatric structures were weaker than what has been reported in studies with adult cohorts.

CLINICAL RELEVANCE

Results from this study can be considered preliminary mechanical and microstructural data for healthy pediatric collateral and posterior cruciate ligaments that can be used to guide further laboratory and clinical research.