Orthopedics

Feature Article 

Mechanical Augmentation With Absorbable Alginate Sheet Enhances Healing of the Rotator Cuff

Dong Hyun Kim, MD; Seung Gi Min, MD; Jong Pil Yoon, MD; Ga Young Park, PhD; Jin Hyun Choi, PhD; Jae Wook Jung, MD; Hyun Joo Lee, MD; Hee June Kim, MD; Seok Won Chung, MD, PhD; Joon Yub Kim, MD, PhD

Abstract

For anatomical restoration of a repaired rotator cuff, mechanical augmentation of the repaired structure is essential. Using histological and biomechanical evaluation in a rat model, the authors sought to determine the efficacy of an absorbable alginate sheet at the supraspinatus tendon-to-bone repair site for healing of the rotator cuff tear. Forty adult (12 weeks old) male Sprague– Dawley wild-type rats were used in this study. The animals were randomly separated into 2 groups: group 1, conventional supraspinatus repair with acute repair; or group 2, supraspinatus repair with absorbable alginate sheet. Biomechanical and histological analyses were performed at 6 and 12 weeks after index rotator cuff surgery. Compared with group 1, group 2 exhibited a significantly greater mean ultimate failure load (group 1, 23.70±3.87 N; group 2, 61.44±43.67 N; P=.023) and mean ultimate stress (group 1, 2.83±0.50 MPa; group 2, 7.36±2.87 MPa; P=.020). However, 6-week outcomes were not significantly different. On histological scoring, compared with group 1, group 2 exhibited a significantly greater mean 6-week score (group 1, 4.10±1.72 points; group 2, 7.80±1.47 points; P<.001) and mean 12-week score (group 1, 3.50±1.00 points; group 2, 5.25±2.62 points; P=.020). Mechanical augmentation with absorbable alginate may improve tendon healing after surgical repair of the rotator cuff. [Orthopedics. 2019; 42(1):e104–e110.]

Abstract

For anatomical restoration of a repaired rotator cuff, mechanical augmentation of the repaired structure is essential. Using histological and biomechanical evaluation in a rat model, the authors sought to determine the efficacy of an absorbable alginate sheet at the supraspinatus tendon-to-bone repair site for healing of the rotator cuff tear. Forty adult (12 weeks old) male Sprague– Dawley wild-type rats were used in this study. The animals were randomly separated into 2 groups: group 1, conventional supraspinatus repair with acute repair; or group 2, supraspinatus repair with absorbable alginate sheet. Biomechanical and histological analyses were performed at 6 and 12 weeks after index rotator cuff surgery. Compared with group 1, group 2 exhibited a significantly greater mean ultimate failure load (group 1, 23.70±3.87 N; group 2, 61.44±43.67 N; P=.023) and mean ultimate stress (group 1, 2.83±0.50 MPa; group 2, 7.36±2.87 MPa; P=.020). However, 6-week outcomes were not significantly different. On histological scoring, compared with group 1, group 2 exhibited a significantly greater mean 6-week score (group 1, 4.10±1.72 points; group 2, 7.80±1.47 points; P<.001) and mean 12-week score (group 1, 3.50±1.00 points; group 2, 5.25±2.62 points; P=.020). Mechanical augmentation with absorbable alginate may improve tendon healing after surgical repair of the rotator cuff. [Orthopedics. 2019; 42(1):e104–e110.]

One study1 found a high incidence (greater than 50%) of rotator cuff tearing in patients older than 50 years after trauma to the shoulder girdle, and 42% of these rotator cuff tears were large to massive. Three studies2–4 reported that healing of traumatic tears was consistent, with 65% to 69% having total healing. A systematic review1 reported failure rates of 7% in tears smaller than 1 cm and approximately 69% in large to massive rotator cuff tears. The composition and the anatomy of the repaired tissue are not identical to those of undamaged healthy tissue. Most uninjured areas contain fibro-cartilage transitions between the tendon and bone5,6; however, this is not observed in repaired tendons, which instead contain disordered fibrovascular connective tissue with mechanical properties that are inferior to those of undamaged tissue. Recent studies have sought to improve healing of tendon-to-bone by regulating the mechanical or biological environment of the repair site.7,8

Porous, absorbent matrices synthesized from synthetic donor polymers are currently being studied as scaffolds for cell and tissue transplantation.9 Previous studies have attempted to deliver cells or sheets for the growth of repaired tissue using biomaterials such as alginates, fibrin clots, collagen-glycosaminoglycan copolymers, collagen gels and sponges, and various absorbent structures. Alginate has been successfully applied in tissue engineering for many years. Alginate is also a natural biocompatible polysaccharide that has been used in cell transplantation, cell encapsulation, and tissue engineering.10,11 Alginate not only is soluble in water at room temperature, but also cross-links with certain divalent cations, such as barium and calcium, to form a stable matrix.12,13 In addition, alginate is an important component used in various pharmaceutical applications, having a low cost compared with other natural materials.9

Alginates have various applications in the fields of engineering and biomedicine owing to their favorable properties, which include biocompatibility and ease of gelation. Because alginate hydrogels can be manipulated to structurally mimic the extracellular matrix of tissues, they can be applied for various purposes, including wound healing, drug delivery, and scaffolding, making them an attractive material for tissue engineering.14 Alginate is an anionic polymer composed of 2 types of uronic acid monomers distributed both in blocks of 1→4 linked α-L-guluronic acid (G) or β-D-mannuronic acid (M) and heteropolymeric mixed sequences (G–M, usually alternating).15 Alginates have more than 50% G-content and are widely applied in biomedical technologies and therapies because they do not cause cytokine production, induce an immune response, or result in reactive oxygen species production.16,17

The purpose of this study was to determine the effect of an absorbable alginate sheet at the supraspinatus (SSP) tendonto-bone repair site on healing of the rotator cuff tear through both biomechanical and histological evaluation in a rat model. The authors hypothesized that absorbable alginate hydrogel sheets applied to tendon-to-bone repair sites of the SSP enhance rotator cuff healing in rats.

Materials and Methods

Experimental Group

The outcomes of biomechanical and histological analyses at 12 weeks were the primary endpoint for the control and physical support groups after rotator cuff surgery. The outcomes of biomechanical and histological analyses at 6 weeks were the secondary endpoint to evaluate short-term outcomes compared with those at 12 weeks.

In another animal model, the sample size required to identify a significant difference in ultimate load leading to failure was reported to be 20.18 For histological analysis, an additional 10 rats were assigned to the groups. Thus, 40 mature male Sprague–Dawley rats (weight, 300 to 350 g; age, 12 weeks) were randomly allocated to 2 groups (20 rats in total—10 for histological analysis and 10 for mechanical analysis): the conventional repair group (group 1—control group) and the mechanical support group with the alginate hydrogel sheet (group 2—experimental group).

Preparation of Alginate-Based Augmentation Sheet

An aqueous solution of alginate (1% weight/volume) was prepared by adding 1.0 g of sodium alginate powder (molecular weight approximately 500,000) to 100 mL of deionized water for 1 hour, after which the alginate solution was poured into a Petri dish and vacuum dehydrated at 20°C for 6 hours. The dried hydrogel was then cross-linked with 2.2% calcium chloride and washed with deionized water. Prior to biomechanical analysis and animal experiments, washed hydrogels were dehydrated until the water was gone (Figure 1).19

Photographs showing 1.0 g of sodium alginate powder (A) prepared via vacuum dehydration (B) to create the alginate sheet (C).

Figure 1:

Photographs showing 1.0 g of sodium alginate powder (A) prepared via vacuum dehydration (B) to create the alginate sheet (C).

Analysis of Tensile Strength

Alginate hydrogel sheets were prepared in 50×20-mm2 sections. They were processed for analysis of tensile strength using a Universal Test Machine (OTT-03; Oriental TM, Siheung-si, South Korea) with a 196.13-N load cell at a 10-mm/min tensile speed until failure. The ultimate elongation and ultimate tensile strength were measured from the stress-strain analysis.19

Cytotoxicity and Viability of Alginate Hydrogel Sheets

To evaluate the cytotoxicity and metabolic activity of human fibroblasts on the alginate hydrogel sheets, the MTS (G5421; Promega, Madison, Wisconsin) assay was performed according to the instructions provided by the manufacturer.20 In brief, 0.03 g of sponge sterilized with ultraviolet light for 1 hour was incubated in 50 mL of tenocyte growth medium (TEN-1; Zen-Bio, Research Triangle Park, North Carolina) at 37°C for 24 hours with agitation. Simultaneously, human tenocytes (TEN-F; Zen-Bio) were seeded in 96-well culture plates and incubated. After 24 hours, extracts of the alginate hydrogel sheet were filtered to eliminate soluble residues and sterilized via passage through a 0.2-mm filter. The growth medium on the 96-well plates was replaced by either fresh medium or alginate hydrogel sheet. After 24 hours of incubation, the old medium was replaced with 100 µL of growth medium and MTS solution and then the plate was incubated for 4 hours. Cell viability can be calculated by measuring the degree of mitochondrial reduction of MTS to formazan by succinic acid dehydrogenase. The absorbance was measured at 490 nm using a microplate reader (SpectraMax 190; Molecular Devices, Sunnyvale, California). Cell viability (percentage) was measured as the ratio of absorbance of the sample to that of the control.21,22

Generation of the Animal Model and Surgical Procedure

All animal experiments were performed after approval was obtained from the animal studies committee at the authors' institution. The rats were anesthetized by injecting 5 mg/kg of xylazine hydrochloride and 15 mg/kg of zolazepam intramuscularly. The right shoulder of the rat was shaved and sterilized for aseptic conditions. A 3-cm longitudinal incision was made along the scapular spine. The deltoid muscle was dissected bluntly, and the SSP tendon was confirmed at the greater tuberosity. The SSP tendon was isolated with Metzenbaum scissors, and the end of the tendon insertion site was cut with a blade. Two bone tunnels were made using a drill at the greater tuberosity of the humeral head. The SSP tendon was repaired with a single row through the bone tunnel with 2.0 Ethibond (Johnson & Johnson, Malmo, Sweden).

The control group (group 1) was subjected exclusively to repair. In the experimental group (group 2), an alginate hydrogel sheet was placed on the detached tendon and repaired simultaneously (Figure 2). Antibiotics were injected 5 days after surgery, and dressing of the surgical site was performed at 3-day intervals. Rats in both groups were killed at either 6 or 12 weeks postoperatively via inhalation of carbon dioxide gas.

After rotator cuff repair, the control group was subjected exclusively to repair (A, B). In the experimental group, an alginate sheet was placed on the detached tendon (C) and repaired simultaneously (D).

Figure 2:

After rotator cuff repair, the control group was subjected exclusively to repair (A, B). In the experimental group, an alginate sheet was placed on the detached tendon (C) and repaired simultaneously (D).

Geometrical and Biomechanical Analysis

The SSP tendons and humeral heads were harvested together from both shoulders of each rat. The biomechanical ability of the healed tendon-to-bone was evaluated using the Universal Test Machine. The specimen tensile strength was measured to failure at a rate of 10 mm/s with a 3-N load cell, using a custom clamping system. The system was used to measure the tensile strength of the SSP tendon. It was composed of a cryogenic tendon fixation unit and a humeral head fixation unit. The SSP tendon was anchored to this system in the anatomical direction to make a right angle with the tendon-to-bone interface and tensile loading (Figure 3). The tensile load-to-failure data were automatically collected using a data acquisition system on a personal computer.

The cross-sectional area of the supraspinatus tendon (A). Measurements of the mode of failure, load to failure, and ultimate stress (B).

Figure 3:

The cross-sectional area of the supraspinatus tendon (A). Measurements of the mode of failure, load to failure, and ultimate stress (B).

Histological Analysis

Histological analysis was performed on all tissues collected from each group. For pathological analysis, the tendon-to-bone samples were placed in sterile 10% concentrated buffered formalin solution in a universal container and sent to the Pathology Department. After fixation with 10% concentrated buffered formalin solution, samples were dehydrated. The samples were then soaked in paraffin and cut to a thickness of 4 mm.23 Next, Mayer's hematoxylin was used to stain the collected samples for 15 minutes. This was followed by washing them in running water for 20 minutes. Counterstaining was performed for 15 seconds to 2 minutes with eosin according to the age of the eosin and desired counterstain depth. The tissues were dropped several times before being placed in the eosin for the desired time for homogeneous staining. Next, they were dehydrated in 95% alcohol until the remaining eosin was cleared. Finally, the tissues were washed with xylene and mounted on slides. Basophilic nuclei, calcium, and bacteria appeared blue with hematoxylin. Eosinophilic cytoplasm and the remaining tissues counterstained red with eosin. Hematoxylin-eosin staining is often used to identify inflammation or to confirm the integrity of the tissue. Masson's trichrome staining is also used to confirm inflammation or to confirm tissue integrity, such as hematoxylin-eosin staining. Masson's trichrome staining is a 3-color staining protocol for histological analysis. The formulations evolved from the original formulation for specific applications; however, both hematoxylin-eosin staining and Masson's trichrome staining are suited for distinguishing cells from the surrounding connective tissue. Most formulations result in blue or green collagen and bone, red keratin and muscle fibers, dark brown to black cell nuclei, and light red or pink cytoplasm.

To estimate various aspects of the tendon tissue, the slides were analyzed using a scoring system. For all slides, areas of increased vascularity and cellularity, level of maturation of the tendon-to-bone interface structure, and proportion of collagen fibers were measured. The following items were scored: (1) orientation of collagen fibers; (2) continuity of collagen fibers; (3) maturation of the tendon-to-bone interface; (4) collagen fiber density; (5) vascularity; and (6) cellularity. For each item, the histological findings were classified semi-quantitatively into one of four possible scores: 0, 1, 2, or 3 points. A score of 0 points indicated the poorest appearance of the ruptured tendon, a score of 1 point indicated poorer, a score of 2 points indicated better, and a score of 3 points indicated a marked regeneration. Overall, the total score of a tissue slide could range from 0 points (a ruptured tendon) to 18 points (most marked regeneration). To eliminate observer error, a pathologist with at least 10 years of experience examined all slides in a randomized, blinded fashion 3 times at the same position and area of rotator cuff tissue using a microscope (DMIL LED; Leica, Wetzlar, Germany) and image system (LAS V4.8; Leica) at ×50 magnification.23

Statistical Analysis

All statistical analyses were performed using SPSS version 12.0 software (SPSS Inc, Chicago, Illinois), with the level of statistical significance set as P<.05. The Kruskal–Wallis test, followed by a post hoc Mann–Whitney U test, was performed to evaluate the biomechanical and histological differences based on hematoxylineosin staining between groups. Data were presented as mean and standard deviation. Interclass correlation coefficients were used to assess intraobserver and interobserver reliability for histological analysis of the fat-to-muscle ratio.

Results

Validation of Cytotoxicity

The cytotoxicity of the alginate hydrogel sheets was estimated by measuring the mitochondrial metabolic activity of human tenocytes cultured on or in the presence of the biomaterial using a standard MTS assay. Cell viability was calculated by normalizing the absorbance of the sample at 490 nm.6 The viability of cells cultured with alginate hydrogel sheets was 125.77% that of the control (Figure 4). The alginate hydrogel sheets induced no cytotoxicity and enhanced cell viability.

Relative viabilities of untreated human tenocytes (control) and those treated with extracts of alginate hydrogel sheets.

Figure 4:

Relative viabilities of untreated human tenocytes (control) and those treated with extracts of alginate hydrogel sheets.

Gross Inspection and Biomechanical Evaluation

There was no significant recurrence of tearing of the repaired SSP tendon-to-bone sites (P=.531 at 6 weeks and P=.305 at 12 weeks) (Table 1). Biomechanical failure also was not significantly different between the 2 time points (P=.264 at 6 weeks and P=.693 at 12 weeks).

Findings on Gross Inspection

Table 1:

Findings on Gross Inspection

On biomechanical analysis, compared with group 1, group 2 exhibited a significantly greater mean ultimate failure load (group 1, 23.70±3.87 N; group 2, 61.44±43.67 N; P=.023) and mean ultimate stress (group 1, 2.83±0.50 MPa; group 2, 7.36±2.87 MPa; P=.020). However, no significant differences were observed at 6 weeks (Table 2).

Findings on Biomechanical Analysis

Table 2:

Findings on Biomechanical Analysis

Histological Analysis

Results of hematoxylin-eosin staining and Masson's trichrome staining for the SSP tendon repair site at 6 and 12 weeks after repair are shown in Figure 5 and Figure 6, respectively.

Results of hematoxylin-eosin staining (original magnification ×50) of the supraspinatus tendon repair site at 6 and 12 weeks after repair. Supraspinatus repair only at 6 weeks (group 1—control) (A). Supraspinatus repair only at 12 weeks (group 1) (B). Supraspinatus repair with alginate sheet at 6 weeks (group 2) (C). Supraspinatus repair with alginate sheet at 12 weeks (group 2) (D). In group 2, the ground substance and collagen (arrows) were increased compared with those in group 1. In group 2, tendon-to-bone density (arrows) were increased compared with that in group 1.

Figure 5:

Results of hematoxylin-eosin staining (original magnification ×50) of the supraspinatus tendon repair site at 6 and 12 weeks after repair. Supraspinatus repair only at 6 weeks (group 1—control) (A). Supraspinatus repair only at 12 weeks (group 1) (B). Supraspinatus repair with alginate sheet at 6 weeks (group 2) (C). Supraspinatus repair with alginate sheet at 12 weeks (group 2) (D). In group 2, the ground substance and collagen (arrows) were increased compared with those in group 1. In group 2, tendon-to-bone density (arrows) were increased compared with that in group 1.

Results of Masson's trichrome staining (original magnification ×50) of the supraspinatus tendon repair site at 6 and 12 weeks after repair. Changes in collagen fiber arrangements. Supraspinatus repair only at 6 weeks (group 1—control) (A). Supraspinatus repair only at 12 weeks (group 1) (B). Supraspinatus repair with alginate sheet at 6 weeks (group 2) (C). Supraspinatus repair with alginate sheet at 12 weeks (group 2) (D). In group 1, the collagen fibers (arrows) were poorly organized and fiber continuity with the bone had not yet been established. In group 2, a more organized structure with a higher collagen fiber (arrows) density was observed compared with group 1. Furthermore, more collagen fibers bridged the interface, revealing good tendon-to-bone (arrows) integration, and longitudinally oriented collagen fibers were also visible.

Figure 6:

Results of Masson's trichrome staining (original magnification ×50) of the supraspinatus tendon repair site at 6 and 12 weeks after repair. Changes in collagen fiber arrangements. Supraspinatus repair only at 6 weeks (group 1—control) (A). Supraspinatus repair only at 12 weeks (group 1) (B). Supraspinatus repair with alginate sheet at 6 weeks (group 2) (C). Supraspinatus repair with alginate sheet at 12 weeks (group 2) (D). In group 1, the collagen fibers (arrows) were poorly organized and fiber continuity with the bone had not yet been established. In group 2, a more organized structure with a higher collagen fiber (arrows) density was observed compared with group 1. Furthermore, more collagen fibers bridged the interface, revealing good tendon-to-bone (arrows) integration, and longitudinally oriented collagen fibers were also visible.

On histological analysis, group 2 exhibited a significantly greater mean total score at 6 weeks (P<.001) and 12 weeks (P=.020) than group 1 (Table 3). At the 6-week evaluation, collagen fiber continuity (P<.001), collagen fiber orientation (P=.001), and collagen fiber density (P<.001) were significantly different between groups 1 and 2. However, there were no significant differences in maturation of the tendon-to-bone interface structure (P=.087), vascularity (P=.151), or cellularity (P=.177). Similarly, at the 12-week evaluation, collagen fiber continuity (P=.018), collagen fiber orientation (P=.024), and collagen fiber density (P=.024) were significantly different between groups 1 and 2. However, maturation of the tendon-to-bone interface structure (P=.177), vascularity (P=1.000), and cellularity (P=.441) were not significantly different.

Scoring of Findings on Histological Analysis

Table 3:

Scoring of Findings on Histological Analysis

Discussion

The authors sought to evaluate the effect of an absorbable alginate sheet at the SSP tendon-to-bone repair site on the healing of rotator cuff tear through histological analyses and biomechanical assessments in a rat model of rotator cuff injury and repair. They showed that absorbable alginate hydrogel sheets enhance biomechanical and histological outcomes at both 6 weeks and 12 weeks after surgery.

Rotator cuff tear is a common shoulder injury. However, outcomes of massive rotator cuff repairs have often been either unsatisfactory or unpredictable. Rates of postoperative failure differ due to tear size, patient age, muscle atrophy, fatty degeneration, and chronicity but have approached 94%. To address these issues, an augmentation graft containing patch material was introduced for large to massive rotator cuff tears, irreparable tears, or poor-quality ten-dons.

Alginate hydrogels are broadly used for cell transplantation and cell encapsulation in clinical and experimental studies, and their biocompatibility and low antigenicity have been reported.9,24 Alginate is a natural polysaccharide. Highly purified alginate does not cause an immune response.25 Alginate gels that deliver DNA encoding bone morphogenetic proteins have been used in bone tissue regeneration, and the delivery of multiple factors, either in combination or in sequence, is also being explored in a similar manner for angiogenesis.26,27

Many in vitro and in vivo studies have assessed the biocompatibility of alginate, but the effects of the constituents of alginate remain controversial. Most of the discrepancies are potentially due to differences in alginate purity in many studies. For example, it has been reported that high M-content alginates are more immunogenic than high G-content alginates and are likely to induce an approximately 10-fold increase in cytokine production.28 However, other studies have reported little or no evidence of immune responses caused by alginate implants.29

The immune response might be caused by impurities remaining in the alginate at the injection or implantation sites. Because alginate can be acquired from natural products, it could contain different kinds of impurities (ie, heavy metals, proteins, endotoxins, polyphenolic compounds). However, it was reported that high-purity alginate produced by a multi-step extraction did not cause any notable immune response in vivo.25 Likewise, there was no significant inflammatory response when gels with highly purified alginate were subcutaneously injected into mice.30

This study had limitations. A reliable scoring system to evaluate tendon-to-bone healing does not exist. The histological evaluation was semi-quantitative. An acute rotator cuff tear model in the rat was used. However, chronic degenerative rotator cuff tears are observed in most patients. Therefore, the results of this study cannot be fully applied to the healing of chronic rotator cuff tears. Nevertheless, these findings are relevant to acute tendon injuries and to the human shoulder.

Conclusion

In this rat model, the use of an alginate hydrogel scaffold improved the biomechanical properties of tissue after acute traumatic SSP tendon rupture.

References

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Findings on Gross Inspection

Time and FindingNo.P

Group 1 (n=10)aGroup 2 (n=10)b
6 weeks
  Retear21.531
  Bone-to-tendon failure79.264
  Midsubstance failure31
12 weeks
  Retear10.305
  Bone-to-tendon failure67.693
  Midsubstance failure43

Findings on Biomechanical Analysis

Time and FindingMean±SDP

Group 1aGroup 2b
6 weeks
  Cross-section area, mm26.02±1.767.02±2.36.296
  Ultimate load, N25.53±10.6528.60±17.59.642
  Ultimate stress, MPa4.38±1.894.07±1.69.701
12 weeks
  Cross-section area, mm28.22±1.488.17±3.45.966
  Ultimate load, N23.70±3.8761.44±43.67.023c
  Ultimate stress, MPa2.83±0.507.36±2.87.020c

Scoring of Findings on Histological Analysis

Time and FindingMean±SD Score, pointsaP

Group 1bGroup 2c
6 weeks
  Collagen fiber continuity0.30±0.481.10±0.31<.001d
  Collagen fiber orientation0.50±0.521.40±0.51.001d
  Collagen fiber density0.60±0.511.70±0.48<.001d
  Maturation of the tendon-to-bone inter-face structure1.00±0.471.40±0.51.087
  Vascularity0.00±0.000.20±0.42.151
  Cellularity1.70±0.482.00±0.47.177
  Total score4.10±1.727.80±1.47<.001d
12 weeks
  Collagen fiber continuity1.10±0.311.60±0.51.018d
  Collagen fiber orientation1.20±0.421.70±0.48.024d
  Collagen fiber density1.20±0.421.70±0.48.024d
  Maturation of the tendon-to-bone inter-face structure1.50±0.521.80±0.42.177
  Vascularity0.01±0.310.01±0.311.000
  Cellularity2.10±0.311.90±0.73.441
  Total score3.50±1.005.25±2.62.020d
Authors

The authors are from the Department of Orthopaedic Surgery (DHK, SGM, JPY, JWJ, HJL, HJK), School of Medicine, and the Department of Biofibers and Biomaterials Science (GYP, JHC), College of Agriculture and Life Science, Kyungpook National University, Daegu; the Department of Orthopaedic Surgery (SWC), Konkuk University School of Medicine, Seoul; and the Department of Orthopaedic Surgery (JYK), Myongji Hospital, Hanyang University College of Medicine, Goyang-Si, South Korea.

The authors have no relevant financial relationships to disclose.

This study was supported by a grant from the Biomedical Research Institute, Kyungpook National University Hospital (2018).

Correspondence should be addressed to: Jong Pil Yoon, MD, Department of Orthopaedic Surgery, School of Medicine, Kyungpook National University, 130 Dong-duckro, Jung-Gu, Daegu 700-721, Korea ( jpyoon@knu.ac.kr).

Received: April 02, 2018
Accepted: August 08, 2018
Posted Online: December 13, 2018

10.3928/01477447-20181206-04

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