Quantitative comparison of three rat models of Achilles tendon injury: A multidisciplinary approach

Preclinical tendon and ligament models: Beyond the 3Rs (replacement, reduction, and refinement) to 5W1H (why, who, what, where, when, how)

Several tendon and ligament animal models were presented at the 2022 Orthopaedic Research Society Tendon Section Conference held at the University of Pennsylvania, May 5 to 7, 2022. A key objective of the breakout sessions at this meeting was to develop guidelines for the field, including for preclinical tendon and ligament animal models. This review summarizes the perspectives of experts for eight surgical small and large animal models of rotator cuff tear, flexor tendon transection, anterior cruciate ligament tear, and Achilles tendon injury using the framework: "Why, Who, What, Where, When, and How" (5W1H). A notable conclusion is that the perfect tendon model does not exist; there is no single gold standard animal model that represents the totality of tendon and ligament disease. Each model has advantages and disadvantages and should be carefully considered in light of the specific research question. There are also circumstances when an animal model is not the best approach. The wide variety of tendon and ligament pathologies necessitates choices between small and large animal models, different anatomic sites, and a range of factors associated with each model during the planning phase. Attendees agreed on some guiding principles including: providing clear justification for the model selected, providing animal model details at publication, encouraging sharing of protocols and expertise, improving training of research personnel, and considering greater collaboration with veterinarians. A clear path for translating from animal models to clinical practice was also considered as a critical next step for accelerating progress in the tendon and ligament field.

Plantar flexor deficits following Achilles tendon rupture: A novel small animal dynamometer and detailed instructions

Plantar flexor functional deficits measured using joint dynamometry are associated with poor outcomes in patients following Achilles tendon rupture. In this study, we developed a small animal dynamometer to quantify functional deficits in a rat Achilles tendon rupture model. Like our reported plantar flexor deficits in patients recovering from Achilles tendon ruptures, we found in our small animal model functional deficits across the ankle range of motion, resulting in an average 34% less positive work being done compared to the uninjured contralateral limb. These functional deficits are similar to 38% less plantar flexor work done by patients who were treated non-surgically in our prior research. Further, these torque deficits were greater in plantar flexion than dorsiflexion, which agree with clinical complaints of limited function during tasks like jumping and hiking. These findings serve as compelling evidence that our Sprague Dawley rat model of an Achilles tendon rupture recapitulates the functional deficits we observed in patients treated nonsurgically. We provide thorough documentation for other groups to build their own dynamometers, which can be modified to meet unique experimental criteria.

Quantitative analysis of tendon histopathology using digital pathology in rat models with Achilles tendon injury

Although digital image analysis methods that quantify histopathologic features have emerged, no validated quantitative methods are available to evaluate tendon injury. This study aimed to propose and validate a quantitative analysis method to identify the histopathologic features of tendon injuries. The histopathologic features of two Achilles tendon injury models (a partial full-thickness defect model and a collagenase injection model) using Sprague-Dawley rats were evaluated by semiquantitative grading and a quantitative analysis method using a digital pathology software at weeks 1 and 4 after tendon injury (six tendons per group at each time point). The outcome variables between tendon injury models and between time points were compared, and the correlation between semiquantitative scores and the results of the quantitative analysis was investigated. The proposed analysis method quantified the severity of the histopathological features after tendon injury. Quantitative analysis differentiated the cell morphology between tendon injury models and time points better than semiquantitative scoring. The results from quantitative measurements correlated significantly with the semiquantitative scores. The proposed quantitative method can be effective in evaluating the histopathology of tendon injuries.

Hybrid Nanofibrous Composites with Anisotropic Mechanics and Architecture for Tendon/Ligament Repair and Regeneration

Rupture of tendons and ligaments (T/L) is a major clinical challenge due to T/L possess anisotropic mechanical properties and hierarchical structures. Here, to imitate these characteristics, an approach is presented by fabricating hybrid nanofibrous composites. First, hybrid fiber-reinforced yarns are fabricated via successively electrospinning poly(L-lactide-co-ε-caprolactone) (PLCL) and gelatin (Ge) nanofibers onto polyethylene terephthalate (PET) fibers to improve biodurability and biocompatibility. Then, by comparing different manufacturing methods, the knitted structure succeeds in simulating anisotropic mechanical properties, even being stronger than natural ligaments, and possessing comfort compliance superior to clinically used ligament advanced reinforcement system (LARS) ligament. Moreover, after inoculation with tendon-derived stem cells and transplantation in vivo, hybrid nanofibrous composites are integrated with native tendons to guide surrounding tissue ingrowth due to the highly interconnected and porous structure. The knitted hybrid nanofibrous composites are also ligamentized and remodeled in vivo to promote tendon regeneration. Specifically, after the use of optimized anisotropic hybrid nanofibrous composites to repair tendon, the deposition of tendon-associated extracellular matrix proteins is more significant. Thus, this study indicates a strategy of manufacturing anisotropic hybrid nanofibrous composites with superior mechanical properties and good histocompatibility for clinical reconstruction.

Insulin-Functionalized Bioactive Fiber Matrices with Bone Marrow-Derived Stem Cells in Rat Achilles Tendon Regeneration

Approximately half of annual musculoskeletal injuries in the US involve tendon tears. The naturally hypocellular and hypovascular tendon environment makes tendons injury-prone and heal slowly. Tendon tissue engineering strategies often use biomimetic scaffolds combined with bioactive factors and/or cells to enhance healing. FDA-approved growth factors to promote tendon healing are lacking, which highlights the need for safe and effective bioactive factors. Our previous work evaluated insulin as a bioactive factor and identified an optimal dose to promote in vitro mesenchymal stem cell survival, division, and tenogenesis. The present work evaluates the ability of insulin-functionalized electrospun nanofiber matrices with or without mesenchymal stem cells to enhance tendon repair in a rat Achilles injury model. Electrospun nanofiber matrices were functionalized with insulin, cultured with or without mesenchymal stem cells, and sutured to transected Achilles tendons in rats. We analyzed rat tendons 4 and 8 weeks after surgery for the tendon morphology, collagen production, and mechanical properties. Bioactive insulin-functionalized fiber matrices with mesenchymal stem cells resulted in significantly increased collagen I and III at 4 and 8 weeks postsurgery. Additionally, these matrices supported highly aligned collagen fibrils in the regenerated tendon tissue at 8 weeks. However, treatment- and control-regenerated tissues had similar tensile properties at 8 weeks, which were less than that of the native Achilles tendon. Our preliminary results establish the benefits of insulin-functionalized fiber matrices in promoting higher levels of collagen synthesis and alignment needed for functional recovery of tendon repair.

Achilles Tendon Ruptures in Middle-Aged Rats Heal Poorly Compared With Those in Young and Old Rats [Formula: see text]

BACKGROUND

Achilles tendon ruptures are painful and debilitating injuries and are most common in middle-aged patients. There is a lack of understanding of the underlying causes for increased rupture rates in middle-aged patients and how healing outcomes after a rupture might be affected by patient age. Therefore, the objective of this study was to define age-specific Achilles tendon healing by assessing ankle functional outcomes and Achilles tendon mechanical and histological properties after a rupture using a rat model.

HYPOTHESIS

Rats representing the middle-aged patient population would demonstrate reduced healing capability after an Achilles tendon rupture, as demonstrated by a slower return to baseline ankle functional properties and inferior biomechanical and histological tendon properties.

STUDY DESIGN

Controlled laboratory study.

METHODS

Fischer 344 rats were categorized by age to represent young, middle-aged, and old patients, and Achilles tendon ruptures were induced in the right hindlimb. Animals were allowed to heal and were euthanized at 3 or 6 weeks after the injury. In vivo functional assays and ultrasound imaging were performed throughout the healing period, and ex vivo tendon mechanical and histological properties were assessed after euthanasia.

RESULTS

Rats representing middle-aged patients displayed reduced healing potential compared with the other age groups, as they demonstrated decreased recovery of in vivo functional and ultrasound assessment parameters and inferior mechanical and histological properties after an Achilles tendon rupture.

CONCLUSION

These findings may help explain the increased rupture rate observed clinically in middle-aged patients by suggesting that there may be altered tendon responses to daily trauma.

CLINICAL RELEVANCE

The results provide novel data on age-specific healing outcomes after an Achilles tendon rupture, which underscores the importance of considering a patient's age during treatment and expectations for outcomes.

Predicting the effect of reduced load level and cell infiltration on spatio-temporal Achilles tendon healing

Mechanobiology plays an important role in tendon healing. However, the relationship between mechanical loading and spatial and temporal evolution of tendon properties during healing is not well understood. This study builds on a recently presented mechanoregulatory computational framework that couples mechanobiological tendon healing to tissue production and collagen orientation. In this study, we investigated how different magnitudes of mechanical stimulation (principal strain) affect the spatio-temporal evolution of tissue production and the temporal evolution of elastic and viscoelastic mechanical parameters. Specifically, we examined the effect of cell infiltration (mimicking migration and proliferation) in the callus on the resulting tissue production by modeling production to depend on local cell density. The model predictions were carefully compared with experimental data from Achilles tendons in rats, at 1, 2 and 4 weeks of healing. In the experiments, the rat tendons had been subjected to free cage activity or reduced load levels through intramuscular botox injections. The simulations that included cell infiltration and strain-regulated collagen production predicted spatio-temporal tissue distributions and mechanical properties similarly to that observed experimentally. In addition, lack of matrix-producing cells in the tendon core during early healing may result in reduced collagen content, regardless of the daily load level. This framework is the first to computationally investigate mechanobiological mechanisms underlying spatial and temporal variations during tendon healing for various magnitudes of loading. This framework will allow further characterization of biomechanical, biological, or mechanobiological processes underlying tendon healing.

Liquid Poly-N-acetyl Glucosamine (sNAG) Improves Achilles Tendon Healing in a Rat Model

The Achilles tendon, while the strongest and largest tendon in the body, is frequently injured. Even after surgical repair, patients risk re-rupture and long-term deficits in function. Poly-N-acetyl glucosamine (sNAG) polymer has been shown to increase the rate of healing of venous leg ulcers, and use of this material improved tendon-to-bone healing in a rat model of rotator cuff injury. Therefore, the purpose of this study was to investigate the healing properties of liquid sNAG polymer suspension in a rat partial Achilles tear model. We hypothesized that repeated sNAG injections throughout healing would improve Achilles tendon healing as measured by improved mechanical properties and cellular morphology compared to controls. Results demonstrate that sNAG has a positive effect on rat Achilles tendon healing at three weeks after a full thickness, partial width injury. sNAG treatment led to increased quasistatic tendon stiffness, and increased tangent and secant stiffness throughout fatigue cycling protocols. Increased dynamic modulus also suggests improved viscoelastic properties with sNAG treatment. No differences were identified in histological properties. Importantly, use of this material did not have any negative effects on any measured parameter. These results support further study of this material as a minimally invasive treatment modality for tendon healing.

Diminishing effects of mechanical loading over time during rat Achilles tendon healing

Mechanical loading affects tendon healing and recovery. However, our understanding about how physical loading affects recovery of viscoelastic functions, collagen production and tissue organisation is limited. The objective of this study was to investigate how different magnitudes of loading affects biomechanical and collagen properties of healing Achilles tendons over time. Achilles tendon from female Sprague Dawley rats were cut transversely and divided into two groups; normal loading (control) and reduced loading by Botox (unloading). The rats were sacrificed at 1, 2- and 4-weeks post-injury and mechanical testing (creep test and load to failure), small angle x-ray scattering (SAXS) and histological analysis were performed. The effect of unloading was primarily seen at the early time points, with inferior mechanical and collagen properties (SAXS), and reduced histological maturation of the tissue in unloaded compared to loaded tendons. However, by 4 weeks no differences remained. SAXS and histology revealed heterogeneous tissue maturation with more mature tissue at the peripheral region compared to the center of the callus. Thus, mechanical loading advances Achilles tendon biomechanical and collagen properties earlier compared to unloaded tendons, and the spatial variation in tissue maturation and collagen organization across the callus suggests important regional (mechano-) biological activities that require more investigation.

Minimum 3.5-year outcomes of operative treatment for Achilles tendon partial tears in the midportion and retrocalcaneal area

BACKGROUND

Achilles tendon partial tears are not easy to diagnose and to manage. Most frequently, they are located in the midportion and insertional area. These entities result from different pathologic pathways, and different treatment strategies are applied. The outcome is rarely investigated.

METHODS

This study includes patients who underwent surgery for partial tears in the midportion or retrocalcaneal Achilles tendon area between the years 2009 and 2015 by a single surgeon. Patients were prospectively assessed preoperatively and 3, 6, and 12 months postoperatively, using the VISA-A-G questionnaire. The final retrospective follow-up was performed after a minimum of 3.5 years postoperatively. Forty-eight Achilles tendon partial tears at the level of the retrocalcaneal bursa (impingement lesions) and 27 midportion Achilles tendon partial tears were identified. After applying rigorous exclusion criteria, 21 and 16 cases, respectively, remained for the final follow-up. Results were analysed by inferential and descriptive statistics.

RESULTS

The VISA-A-G outcome scores improved significantly from preoperative to 6 months, 12 months, and final postoperative assessment. Preoperatively, the average VISA-A-G score was 42.1 (range, 18-73) for patients operated for Achilles tendon partial tears at the level of the retrocalcaneal bursa and 44.6 (range, 10-73) for the midportion Achilles tendon partial tear group, respectively. At final follow-up 88.8 (range, 15 to 100) and 96.9 (range, 71 to 100) were scored in the respective treatment groups. A repeated measures ANOVA determined that mean performance levels showed a statistically significant difference between measurements (p < 0.001). There was no systematic effect found between groups (p = 0.836).

CONCLUSIONS

In Achilles tendon partial tears recalcitrant to conservative treatment, operative intervention is highly successful in most cases, irrespective of the level of the injury. Results were statistically equal when comparing the midportion and retrocalcaneal Achilles tendon partial tear groups.

TRIAL REGISTRATION

DRKS, DRKS00014266. Registered 06 April 2018. 'Retrospectively registered', https://www.drks.de/drks_web/navigate.do?navigationId=results .

In-Vivo Efficacy of Recombinant Human Hyaluronidase (rHuPH20) Injection for Accelerated Healing of Murine Retrocalcaneal Bursitis and Tendinopathy

The deposition of aggrecan/hyaluronan (HA)-rich matrix within the tendon body and surrounding peritenon impede tendon healing and result in compromised biomechanical properties. Hence, the development of novel strategies to achieve targeted removal of the aggrecan-HA pericellular matrix may be effective in treating tendinopathy. The current study examined the therapeutic potential of a recombinant human hyaluronidase, rHuPH20 (FDA approved for reducing HA accumulation in tumors) for treating murine Achilles tendinopathy. The 12-week-old C57Bl/6 male mice were injected with two doses of rHuTGF-β1 into the retrocalcaneal bursa (RCB) to induce a combined bursitis and tendinopathy. Twenty-four hours following induction of injury, treatment groups were administered rHuPH20 Hyaluronidase (rHuPH20; Halozyme Therapeutics) into the RCB. At either 6 h (acute), 9 days, or 25 days following hyaluronidase treatment, Achilles tendons were analyzed for gene expression, histology and immunohistochemistry, fluorophore-assisted carbohydrate electrophoresis, and biomechanical properties. The rHuPH20 treatment was effective, particularly at the acute and 9-day time points, in (a) removing HA deposits from the Achilles tendon and surrounding tissues, (b) improving biomechanical properties of the healing tendon, and (c) eliciting targeted increases in expression of specific cell fate, extracellular matrix metabolism, and inflammatory genes. The potential of rHuPH20 to effectively clear the pro-inflammatory, HA-rich matrix within the RCB and tendon strongly supports the future refinement of injectable glycosidase preparations as potential treatments to protect or regenerate tendon tissue by reducing inflammation and scarring in the presence of bursitis or other inducers of damage such as mechanical overuse. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:59-69, 2020.

Effects of Pulsed Electromagnetic Field Therapy on Rat Achilles Tendon Healing

The Achilles tendon is frequently injured. Data to support specific treatment strategies for complete and partial tears is inconclusive. Regardless of treatment, patients risk re-rupture and typically have long-term functional deficits. We previously showed that pulsed electromagnetic field (PEMF) therapy improved tendon-to-bone healing in a rat rotator cuff model. This study investigated the effects of PEMF on rat ankle function and Achilles tendon properties after (i) complete Achilles tendon tear and repair with immobilization, (ii) partial Achilles tendon tear without repair and with immobilization, and (iii) partial Achilles tendon tear without repair and without immobilization. We hypothesized that PEMF would improve tendon properties, increase collagen organization, and improve joint function, regardless of injury type. After surgical injury, animals were assigned to a treatment group: (i) no treatment control, (ii) 1 h of PEMF per day, or (iii) 3 h of PEMF per day. Animals were euthanized at 1, 3, and 6 weeks post-injury. Joint mechanics and gait analysis were assessed over time, and fatigue testing and histology were performed at each time point. Results indicate no clear differences in Achilles healing with PEMF treatment. Some decreases in tendon mechanical properties and ankle function suggest PEMF may be detrimental after complete tear. Some early improvements were seen with PEMF after partial tear with immobilization; however, immobilization was found to be a confounding factor. This body of work emphasizes the distinct effects of PEMF on tendon-to-bone healing and supports trialing potential treatment strategies pre-clinically across tendons before applying them clinically. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:70-81, 2020.