Effects of complete stress-shielding on the mechanical properties and histology of in situ frozen patellar tendon

Plunge-Freezing Cryopreservation of Tendons

Allograft transplantation is an important method for tendon reconstruction after injury, and its clinical success highly relies on the storage and transportation of the grafts. Cryopreservation is a promising strategy for tendon storage. In this study, we report a novel cryopreservation agent (CPA) formulation with a high biocompatibility for tendon cryopreservation. Mainly composed of natural zwitterionic betaine and the biocompatible polymer poly(vinylpyrrolidone) (PVP), it exhibited ideal abilities to depress the freezing point and inhibit ice growth and recrystallization. Notably, after cryopreservation via plunge-freezing for 1 month, Young's modulus (144 MPa, 98% of fresh tendons) and ultimate stress (46.7 MPa, 99% of fresh tendons) remained stable, and the cross-linking of collagen microfibers, protein structures, and glycosaminoglycan (GAG) contents changed slightly. These results indicate that the formulation (5 wt % betaine and 5 wt % PVP in phosphate-buffered saline, PBS solution) effectively maintains the biomechanical properties and tissue structure. This work offers a novel cryopreservation method for tendons and may also provide insights into the long-term preservation of various other tissues.

Regional healing trajectory of the patellar tendon after bone-patellar tendon-bone autograft harvest for anterior cruciate ligament reconstruction

Graft site morbidities after bone-patellar tendon-bone (BPTB) autograft harvest for anterior cruciate ligament reconstruction (ACLR) negatively impacts rehabilitation. The purpose of this study was to establish tendon structural properties 1-month after BPTB autograft harvest compared to the uninvolved patellar tendon, and subsequently to quantify the healing trajectory of the patellar tendon over the course of rehabilitation. Patellar tendon morphology (ultrasound) and mechanical properties (continuous shear wave elastography) from 3 regions of the tendon (medial, lateral, central) were measured in 34 participants at 1 month, 3-4 months, and 6-9 months after ACLR. Mixed models were used to compare tendon structure between limbs at 1 month, and quantify healing over 3 timepoints. The involved patellar tendon had increased cross-sectional area and thickness in all regions 1-month after ACLR. Thickness reduced uniformly over time. Possible tendon elongation was observed and remained stable over time. Tendon viscosity was uniform across the three regions in the involved limb while the medial region had higher viscosity in the uninvolved limb, and shear modulus was elevated in all three regions at 1 month. Viscosity and shear modulus in only the central region reduced over time. Statement of Clinical Significance: The entire patellar tendon, and not just the central third, is altered after graft harvest. Tendon structure starts to normalize over time, but alterations remain especially in the central third at the time athletes are returning to sport. Early rehabilitation consisting of tendon loading protocols may be necessary to optimize biologic healing at the graft site tendon.

Zonal variation in primary cilia elongation correlates with localized biomechanical degradation in stress deprived tendon

Tenocytes express primary cilia, which elongate when tendon is maintained in the absence of biomechanical load. Previous work indicates differences in the morphology and metabolism of the tenocytes in the tendon fascicular matrix (FM) and the inter-fascicular matrix (IFM). This study tests the hypothesis that primary cilia in these two regions respond differently to stress deprivation and that this is associated with differences in the biomechanical degradation of the extracellular matrix. Rat tail tendon fascicles were examined over a 7-day period of either stress deprivation or static load. Seven days of stress deprivation induced cilia elongation in both regions. However, elongation was greater in the IFM compared to the FM. Stress deprivation also induced a loss of biomechanical integrity, primarily in the IFM. Static loading reduced both the biomechanical degradation and cilia elongation. The different responses to stress deprivation in the two tendon regions are likely to be important for the aetiology of tendinopathy. Furthermore, these data suggest that primary cilia elongate in response to biomechanical degradation rather than simply the removal of load. This response to degradation is likely to have important consequences for cilia signalling in tendon and as well as in other connective tissues. © 2016 The Authors. Journal of Orthopaedic Research Published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society. J Orthop Res 34:2146-2153, 2016.

Influence of mechanical unloading on histological changes of the patellar tendon insertion in rabbits

BACKGROUND

The purpose of this study was to clarify the influence of mechanical unloading on histological changes of the patellar tendon (PT) insertion in rabbits.

MATERIALS AND METHODS

The PT was completely released from stress by drawing the patella toward the tibial tubercle with a stainless steel wire installed between the patella and tibial tubercle (mechanical unloading group, n=28). The animals of the sham group underwent the same surgical procedure; however, the wire was not tightened (n=28). The average thickness of the Safranin O-stained glycosaminoglycan (GAG) area, chondrocyte apoptosis rate and chondrocyte proliferation rate of the cartilage layer at the insertion were measured at one, two, four, and six weeks.

RESULTS

The chondrocyte apoptosis rate in the mechanical unloading group was significantly higher than that in the sham group at one and four weeks (p<0.05). The chondrocyte proliferation rate in the mechanical unloading group was significantly lower than that in the sham group at four and six weeks (p<0.05). The average thickness of the GAG-stained area in the mechanical unloading group was significantly lower than that in the sham group at six weeks (p<0.05).

CONCLUSION

Mechanical unloading significantly affected the increase in the chondrocyte apoptosis rate, decrease in the chondrocyte proliferation rate, and decrease in the GAG layer thickness at the PT insertion for up to six weeks in rabbits.

CLINICAL RELEVANCE

We suggest that more than 6 weeks of mechanical unloading should be avoided to prevent degeneration at the PT insertion.

Intra-articular remodelling of hamstring tendon grafts after anterior cruciate ligament reconstruction

PURPOSE

A summary is provided on the existing knowledge about the specific healing phases of the intra-articular hamstring tendon graft used for ACL reconstruction. Differences between human and animal in vivo studies are explained, and implications for the postoperative time period are laid out.

METHODS

A systematic review of the existing literature was performed on the topic of tendon remodelling of hamstring grafts in ACL reconstruction using Medline database. Publications between 1982 and 2012 were included. Special focus was directed on in vivo human and animal studies analysing intra-articular free tendon graft remodelling.

RESULTS

Animal and human in vitro and vivo researches have demonstrated three characteristic stages of graft healing after ACL reconstruction: an early graft healing phase with central graft necrosis and hypocellularity and no detectable revascularization of the graft tissue, followed by a phase of proliferation, the time of most intensive remodelling and revascularization and finally, a ligamentization phase with characteristic restructuring of the graft towards the properties of the intact ACL. However, a full restoration of either the biological or biomechanical properties of the intact ACL is not achieved.

CONCLUSION

Significant knowledge on human cruciate ligament remodelling has been added in the understanding of the processes during the course of graft healing. Most importantly, the remodelling process in humans is prolonged compared to animal studies. While today´s rehabilitation protocols are often extrapolated from findings of animal in vivo healing studies, current findings of human in vivo healing studies might require new post-operative regimens following hamstring ACL reconstruction.

The native cell population does not contribute to central-third graft healing at 6, 12, or 26 weeks in the rabbit patellar tendon

Investigators do not yet understand the role of intrinsic tendon cells in healing at the tendon-to-bone enthesis. Therefore, our first objective was to understand how the native cell population influences tendon autograft incorporation in the central-third patellar tendon (PT) defect site. To do this, we contrasted the histochemical and biomechanical properties of de-cellularized patellar tendon autograft (dcPTA) and patellar tendon autograft (PTA) repairs in the skeletally mature New Zealand white rabbit. Recognizing that soft tissues in many animal models require up to 26 weeks to incorporate into bone, our second objective was to investigate how recovery time affects enthesis formation and graft tissue biomechanical properties. Thus, we examined graft structure and mechanics at 6, 12, and 26 weeks post-surgery. Our results showed that maintaining the native cell population produced no histochemical or biomechanical benefit at 6, 12, or 26 weeks. These findings suggest that PTA healing is mediated more by extrinsic rather than intrinsic cellular mechanisms. Moreover, while repair tissue biomechanical properties generally increased from 6 to 12 weeks after surgery, no further improvements were noted up to 26 weeks.

Characteristics of donor and host cells in the early remodeling process after transplant of Achilles tendon with and without live cells for the treatment of rotator cuff defect--what is the ideal graft for the treatment of massive rotator cuff defects?

PURPOSE

We examined the characteristics of donor and host cells in the early remodeling process after transplant of Achilles tendon with and without live cells to repair rotator cuff defects. We also clarified which graft with or without live cells was superior in the early remodeling process.

MATERIALS AND METHODS

Sprague-Dawley (SD) rats and green fluorescent protein (GFP) rats were used; they were divided into 3 groups: in group SD, the Achilles tendons of GFP rats were transplanted into the defects of SD rats; in group GFP, the Achilles tendons of SD rats were transplanted into GFP rats; in group GFP-Fr, frozen Achilles tendons of SD rats were transplanted into GFP rats. At 3 and 7 days after surgery, these sections were examined histologically and immunohistochemically with anti-heat shock protein (HSP) 47 and anti-macrophage antibodies.

RESULTS

Donor cells gradually decreased, but HSP47-positive donor cells were detected at 3 days in group SD. Host cells infiltrated into the graft from the surrounding tissue, and their numbers in groups SD and GFP gradually increased more significantly than in group GFP-Fr. Macrophages derived from the donor tissue were absent in all groups. The remodeling process of the frozen graft was slower than that in the case of the graft that was not frozen.

CONCLUSION

These results demonstrate that live donor cells have a positive effect on the remodeling process. Therefore, autografts with live cells considered to be preferred to frozen allografts or synthetic materials without live cells for transplant for rotator cuff defects.

Graft remodeling and ligamentization after cruciate ligament reconstruction

After reconstruction of the cruciate ligaments, replacement grafts have to undergo several phases of healing in the intra-articular graft region and at the site of graft-to-bone incorporation. The changes in the biological and mechanical properties of the healing graft in its intra-articular region are described as the ligamentization process. Significant knowledge has been added in the understanding of the several processes during the course of graft healing and is summarized in this article. The understanding of the spatial and time-dependent changes as well as the differences between the different models of graft healing are of significant importance to develop strategies of improved treatment options in cruciate ligament surgery, so that full restoration of function and mechanical strength of the intact cruciate ligaments will be achieved.

Callus formation during healing of the repaired tendon-bone junction. A rat experimental model

This study was undertaken to elucidate the mechanism of biological repair at the tendon-bone junction in a rat model. The stump of the toe flexor tendon was sutured to a drilled hole in the tibia (tendon suture group, n = 23) to investigate healing of the tendon-bone junction both radiologically and histologically. Radiological and histological findings were compared with those observed in a sham control group where the bone alone was drilled (n = 19). The biomechanical strength of the repaired junction was confirmed by pull-out testing six weeks after surgery in four rats in the tendon suture group. Callus formation was observed at the site of repair in the tendon suture group, whereas in the sham group callus formation was minimal. During the pull-out test, the repaired tendon-bone junction did not fail because the musculotendinous junction always disrupted first. In order to understand the factors that influenced callus formation at the site of repair, four further groups were evaluated. The nature of the sutured tendon itself was investigated by analysing healing of a tendon stump after necrosis had been induced with liquid nitrogen in 16 cases. A proximal suture group (n = 16) and a partial tenotomy group (n = 16) were prepared to investigate the effects of biomechanical loading on the site of repair. Finally, a group where the periosteum had been excised at the site of repair (n = 16) was examined to study the role of the periosteum. These four groups showed less callus formation radiologically and histologically than did the tendon suture group. In conclusion, the sutured tendon-bone junction healed and achieved mechanical strength at six weeks after suturing, showing good local callus formation. The viability of the tendon stump, mechanical loading and intact periosteum were all found to be important factors for better callus formation at a repaired tendon-bone junction.

The effects of freezing on the tensile properties of repaired porcine flexor tendon

PURPOSE

When conducting complex testing of tendon repairs, it is essential that the samples are adequately preserved to prevent degradation. Freezing of samples is the most convenient method of preservation; however, there is no evidence in the literature to prove that freezing tendon before or after repair is acceptable. We aimed to prove that freezing tendons does not significantly alter the results of linear load-to-failure testing of tendon repairs.

METHODS

After a power study, 150 tendons were harvested from porcine forelimbs and randomized into 5 groups of 30 tendons. After division, tendons were repaired using a Pennington modified core technique with a Silfverskiöld peripheral cross-stitch. Tendons in group 1 were divided, repaired, and tested within 3 hours postmortem. Tendons in group 2 were refrigerated at 4 degrees C for 24 hours prior to repair and testing. Tendons in group 3 were frozen at -25 degrees C for 3 months prior to repair and testing. Tendons in group 4 were frozen at -25 degrees C for 6 months prior to repair and testing. Tendons in group 5 were frozen at -25 degrees C for 6 months, repaired, refrozen for 1 month, and then tested. All repairs were linear load tested to ascertain the ultimate strength and force to produce 3-mm gap in the repair.

RESULTS

Analysis of variance analysis of the results did not demonstrate any significant differences between groups.

CONCLUSIONS

Freezing tendons both before and after suture repair is an acceptable method of preservation when investigating the force to produce 3-mm gap and ultimate strength of tendon repairs.

Local administration of interleukin-1 receptor antagonist inhibits deterioration of mechanical properties of the stress-shielded patellar tendon

We previously found that interleukin (IL)-1beta is over-expressed in the fibroblasts of the stress-shielded patellar tendon using a stress-shielding model [Uchida, H., Tohyama, H., Nagashima, K., Ohba, Y., Matsumoto, H., Toyama, Y., Yasuda, K., 2005. Stress deprivation simultaneously induces over-expression of interleukin-1beta, tumor necrosis factor-alpha, and transforming growth factor-beta in fibroblasts and mechanical deterioration of the tissue in the patellar tendon. Journal of Biomechanics 38(4), 791-798.]. Therefore, IL-1beta may play a role in tendon deterioration in response to stress deprivation. This study was conducted to clarify the effects of local administration of interleukin-1 receptor antagonist (IL-1ra) on the mechanical properties of the stress-shielded patellar tendon as well as the tendon fascicles harvested from it. Twenty-six mature rabbits were equally divided into Groups IL-1ra and PBS after the right patellar tendon underwent the stress-shielding treatment, which completely released the patellar tendon from tension by stretching the flexible wire installed between the patella and the tibial tubercle. In Group IL-1ra, IL-1ra was injected between the patellar tendon and the infra-patellar fat pad. In Group PBS, phosphate-buffered saline was injected in the same manner as IL-1ra. All rabbits were evaluated at 3 weeks after the stress-shielding procedure. The tangent modulus and the tensile strength of the patellar tendons were significantly greater in Group IL-1ra than in Group PBS, while there was no significant difference in the strain at failure between Groups IL-1ra and PBS. Concerning the mechanical properties of the fascicles harvested from the patellar tendon, however, we could not detect any significant differences in the tangent modulus, tensile strength, or strain at failure between Groups IL-1ra and PBS. The present study suggested that IL-1 plays an important role in the deterioration of the mechanical properties of the patellar tendon in response to stress shielding and that IL-1 does not affect the fascicles themselves.

Morphologic and histologic comparison between the patella and hamstring tendons grafts: a descriptive and anatomic study

PURPOSE

Morphologic and histologic comparison of patella and hamstring tendon grafts.

METHODS

Hamstring tendons (semitendinosus and gracilis) and patellar tendons were taken from 20 cadaveric knees and were investigated by using light and electron microscopy, immunohistochemistry, and morphometry. The thickness of collagen fibrils, fibril/interstitum ratio, density of blood vessels, density of fibroblasts, and distribution of the collagen fibrils were analyzed.

RESULTS

The semitendinosus and gracilis tendons provide 20% and 30% more fibril/interstitum ratio compared with the patella tendon (P = .0056 and .0028). Also, the density of fibroblasts was 50% and 35% more (P = .0061 and .0050). No differences regarding the thickness of the collagen fibrils, density of blood vessels, and distribution of the fibrils were found.

CONCLUSIONS

Both semitendinous and gracilis tendons provide significantly more density of collagen fibrils as well as density of fibroblasts in comparison with patellar tendons. These findings provide a potential advantage of the hamstrings group on better remodelling and regeneration of the tissue.

CLINICAL RELEVANCE

These grafts have been used as autografts for anterior cruciate ligament reconstruction. Despite the interest on these tendons, their microscopic structure has not been sufficiently investigated yet.

Effects of freezing on the biomechanical and structural properties of human posterior tibial tendons

This work analyzes the effects of storage by fresh-freezing at -80 degrees C on the histological, structural and biomechanical properties of the human posterior tibial tendon (PTT), used for ACL reconstruction. Twenty-two PTTs were harvested from eleven donors. For each donor one tendon was frozen at -80 degrees C and thawed in physiological solution at 37 degrees C, and the other was tested without freezing (control). Transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and biomechanical analysis were performed. We found the following mean changes in frozen-thawed tendons compared to controls: TEM showed an increase in the mean diameter of collagen fibrils and in fibril non-occupation mean ratio, while the mean number of fibrils decreased; DSC showed a decrease in mean denaturation temperature and denaturation enthalpy. Biomechanical analysis showed a decrease in ultimate load and ultimate stress, an increase in stiffness and a decrease in ultimate strain of tendons. In conclusion fresh-freezing brings about significant changes in the biomechanical and structural properties of the human PTT. A high variability exists in the biophysical properties of tendons among individuals and in the effects of storage on tendons. Therefore, when choosing an allograft tendon, particular care is needed to choose a biomechanically suitable graft.

Is the increase in type III collagen of the patellar tendon graft after ligament reconstruction really caused by "ligamentization" of the graft?

To test the hypothesis that extrinsic cells that infiltrate the devitalized patellar tendon (PT) synthesize type III collagen even in the environmental milieu of the native PT, we conducted the present experimental study using the rat in situ frozen-thawed PTs. Tissue culture showed no cell outgrowth from the tendons immediately after the freeze-thaw treatment. Analysis by RT-PCR showed that the expression level of type III procollagen mRNA in the frozen-thawed tendon was significantly higher than that in the sham-operated tendon at 6 and 12 weeks. Immunohistological findings showed positive type III collagen staining around cells that had infiltrated the necrotized tendon at 3, 6, and 12 weeks. In addition, the elastic modulus of the in situ frozen-thawed tendon at 6 weeks was significantly less than that of the sham-operated tendon. The present study indicates that extrinsic cells that had infiltrated the devitalized PT synthesized type III collagen at least for 12 weeks even in the environmental milieu of the native PT. These findings raised the question whether the increase in type III collagen of the PT graft after ACL reconstruction is really caused by "ligamentization," the adaptation of the PT graft to the ACL environment.

The effect of gamma irradiation on anterior cruciate ligament allograft biomechanical and biochemical properties in the caprine model at time zero and at 6 months after surgery

BACKGROUND

High levels of gamma irradiation are required to eliminate the risk of bacterial and viral transmission during implantation of musculoskeletal allografts. The effects of high levels of gamma irradiation on anterior cruciate ligament allograft biomechanics are still not known.

HYPOTHESIS

High-dose gamma irradiation (4 Mrad) adversely affects anterior cruciate ligament allograft biomechanics at surgery and at 6 months after surgery and affects biochemistry at 6 months.

STUDY DESIGN

Controlled laboratory study.

METHODS

Bilateral anterior cruciate ligament reconstructions were performed in 18 adult goats, with one knee receiving an irradiated patellar tendon allograft (4 Mrad) and the other receiving a frozen control allograft (0 Mrad). In 6 recipients (time zero group), graft pairs were tested immediately after sacrifice, and load relaxation of the femur-allograft-tibia preparation was measured during cyclic anterior displacement. Twelve recipients received bilateral anterior cruciate ligament reconstructions, staged 2 months apart, and were sacrificed a mean of 6 months postoperatively. Load relaxation and tensile failure testing were performed, followed by allograft biochemistry assessment.

RESULTS

At time zero, irradiated grafts showed less load relaxation than did contralateral controls, but by 6 months, the trend had reversed because of decreases in control graft relaxation, with no changes in irradiated graft relaxation. By 6 months, irradiated grafts showed lower stiffness and maximum force compared to controls but no differences in modulus, maximum stress, or biochemistry.

CONCLUSION

High levels of gamma irradiation affect anterior cruciate ligament allograft subfailure viscoelastic and structural properties but not material or biochemical properties over time.

CLINICAL RELEVANCE

Although high levels of gamma irradiation may inactivate infectious agents, this treatment is not a feasible clinical option because of altered allograft biomechanics.

Effects of administration of transforming growth factor (TGF)-beta1 and anti-TGF-beta1 antibody on the mechanical properties of the stress-shielded patellar tendon

Previous studies have shown that, in the stress-shielded patellar tendon, the mechanical properties of the tendon were dramatically reduced and TGF-beta was over-expressed in tendon fibroblasts. In the present study, therefore, we tested two supportive hypotheses using 40 rabbits: One was that an application of TGF-beta1 might significantly increase the tensile strength and the tangent modulus of the stress-shielded patellar tendon. The other one was that an administration of anti-TGF-beta1 antibody might significantly reduce the mechanical properties of the stress-shielded patellar tendon. In the results, an application of 4-ng TGF-beta1 significantly increased the tangent modulus of the stress-shielded patellar tendon at 3 weeks (p = 0.019), compared with the sham treatment. Concerning the tensile strength, the 4-ng TGF-beta1 application increased the average value, but a statistical significance was not reached. An application of 50-microg anti-TGF-beta1 antibody significantly reduced the tangent modulus and the tensile strength of the stress-shielded patellar tendon at 3 weeks (p = 0.0068 and p = 0.0355), compared with the sham treatment. Because the stress-shielding treatment used in this study dramatically reduces the tangent modulus and the tensile strength of the patellar tendon, the present study suggested that an administration of TGF-beta1 weakly but significantly inhibited the reduction of the mechanical properties of the stress-shielded patellar tendon, and that inactivation of TGF-beta1 with its antibody significantly enhanced the reduction of the mechanical properties that occurs in the stress-shielded patellar tendon. These results suggested that TGF-beta1 plays an important role in remodeling of the stress-shielded patellar tendon.

Growth kinetics and integrin expression of fibroblasts infiltrating devitalised patellar tendons are different from those of intrinsic fibroblasts

We compared the biological characteristics of extrinsic fibroblasts infiltrating the patellar tendon with those of normal, intrinsic fibroblasts in the normal tendon in vitro. Infiltrative fibroblasts were isolated from the patellar tendons of rabbits six weeks after an in situ freeze-thaw treatment which killed the intrinsic fibroblasts. These intrinsic cells were also isolated from the patellar tendons of rabbits which had not been so treated.

Proliferation and invasive migration into the patellar tendon was significantly slower for infiltrative fibroblasts than for normal tendon fibroblasts. Flow-cytometric analysis indicated that expression of α5β1 integrin at the cell surface was significantly lower in infiltrative fibroblasts than in normal tendon fibroblasts. The findings suggest that cellular proliferation and invasive migration of fibroblasts into the patellar tendon after necrosis are inferior to those of the normal fibroblasts. The inferior intrinsic properties of infiltrative fibroblasts may contribute to a slow remodelling process in the grafted tendon after ligament reconstruction.

Age-related changes in the biomechanics of healing patellar tendon

By 2030, there will be 70 million people in the United States over the age of 65, and by 2050, 22% of the US population will be considered elderly. It is generally believed that injuries in the elderly heal slower and less completely than in adolescents or young adults. To evaluate aging effects on tissue repair a surgical injury was created in the middle third of one patellar tendon in 1- and 4-5-year-old New Zealand White rabbits. The biomechanical properties of the isolated repair tissues and contralateral normal tendon tissues were compared at 6, 12 and 26 weeks post-injury. We hypothesized that repair tissues would exhibit age-related reductions in biomechanical properties at all time intervals of healing, both based on raw data and when normalized to values from contralateral tendons. Repairs from both age groups were similar, with no significant increase in maximum stress, strain at maximum stress, or modulus between 6 and 12 weeks. At 26 weeks, the repairs in the 4-year-old rabbits had higher maximum stress values than repairs in the 1-year-old rabbits (p=0.03). There were no significant differences in the strain at maximum stress or modulus. When repair tissue properties were normalized to values in the contralateral normal tendon, the maximum stress of the patellar tendon repair tissue from the 4 year old was significantly greater than the corresponding value from the 1 year old at the 26 week time point (p=0.04). In conclusion, these findings do not support the presence of age-related declines in the biomechanics of healing tendon.

Effects of separate application of three growth factors (TGF-beta1, EGF, and PDGF-BB) on mechanical properties of the in situ frozen-thawed anterior cruciate ligament

OBJECTIVE

To clarify effects of a separate application of TGF-beta1, EGF, and PDGF-BB on the material properties of the in situ frozen-thawed anterior cruciate ligament.

DESIGN

Twenty-eight rabbits were divided into four groups after undergoing the in situ freeze-thaw treatment in the right anterior cruciate ligament. In 3 of the 4 groups, 4 ng TGF-beta1, 20 ng EGF, and 4 microg PDGF-BB was applied to the frozen anterior cruciate ligament, respectively. In the remaining sham treatment group, only fibrin sealant as a vehicle was applied. Each animal was sacrificed at 12 weeks after surgery.

BACKGROUND

If the role of growth factors in ligament healing and remodeling is understood, better therapies can be designed for ligament trauma.

METHODS

The freeze-thaw treatment was performed three times using the originally developed cryo-probe. The cross-sectional area of the anterior cruciate ligament was measured by the optical non-contact method. After preconditioning, each specimen was stretched to failure. The ligament strain was determined with a video dimension analyzer.

RESULTS

The tensile strength and the tangent modulus of the anterior cruciate ligament in the TGF-beta1 group was significantly higher than in the sham group, but significantly lower than in the normal control group. There were no significant differences in the strength and the modulus between the EGF group, the PDGF-BB group, and the sham group.

CONCLUSIONS

In this model, an application of 4 ng TGF-beta1 significantly inhibited some of the material deterioration that occurs in the in situ frozen-thawed anterior cruciate ligament, while an application of 20 ng EGF or 4 microg PDGF-BB did not significantly affect the deterioration.

RELEVANCE

This information will be useful in the future to develop a new biological therapy for ligament reconstruction to prevent the graft deterioration after transplantation.

Functional efficacy of tendon repair processes

Despite various attempts to repair and replace injured tendon, an understanding of the repair processes and a systematic approach to achieving functional efficacy remain elusive. In this review the epidemiology of tendon injury and repair is first examined. Using a traditional paradigm for repair assessment, the biology and biomechanics of normal tendon, natural healing, and repair are then explored. New treatment strategies such as functional tissue engineering are discussed, including a functional approach to treatment that involves the development of in vivo functional design parameters to judge the acceptability of a repair outcome. The paper concludes with future directions.

The effect of growth factors on biomechanical properties of the bone-patellar tendon-bone graft after anterior cruciate ligament reconstruction: a canine model study

BACKGROUND

No studies have dealt with the effect of growth factors on the free tendon autograft in anterior cruciate ligament reconstruction.

HYPOTHESIS

Application of exogenous transforming growth factor-beta and epidermal growth factor may affect the structural properties and histology of the bone-patellar tendon-bone autograft after anterior cruciate ligament reconstruction.

STUDY DESIGN

Controlled laboratory study.

METHODS

Twenty dogs underwent anterior cruciate ligament reconstruction with the autogenous bone-patellar tendon-bone graft in bilateral knees. In 10 animals, 12 ng transforming growth factor-beta and 300 ng epidermal growth factor mixed with fibrin sealant of 0.6 mL were applied to the left knee. In the remaining 10 dogs, fibrin sealant alone was applied to the left knee. No additional treatments were applied to the right knee.

RESULTS

The growth factor application increased the stiffness and maximum failure load of the femur-graft-tibia complex at 12 weeks (P =.016 and P =.012, respectively); the sham treatment did not significantly affect them. Histologically, most of the cells in the grafts treated with growth factors had spindle-shaped nuclei; cells in the other grafts had round-shaped nuclei.

CONCLUSIONS

Application of transforming growth factor-beta and epidermal growth factor improves the structural properties of the autograft after anterior cruciate ligament reconstruction in the canine model.

CLINICAL RELEVANCE

Application of growth factors is a possible strategy to prevent graft deterioration in anterior cruciate ligament reconstruction.

Stress shielding of patellar tendon: effect on small-diameter collagen fibrils in a rabbit model

The purpose of this study was to assess the effects of stress shielding on the microstructure and ultrastructure of the patellar tendon using 40 mature female Japanese white rabbits. The patellar tendon was completely released from stress by drawing the patella toward the tibial tubercle with a stainless steel wire installed between them. Microstructurally, stress shielding for 3 and 6 weeks increased the number of cells approximately fivefold, to that of the control tendon. Collagen bundles were less well oriented in the stress-shielded tendon than in the control. Ultrastructurally, small collagen fibrils with a diameter of less than 90 nm increased in the stress-shielded tendon. The median collagen fibril diameter in 6-week stress-shielded tendon was significantly smaller ( P << 0.05) than in the control tendon (58.8% of control). The ratio of the total area of collagen fibrils to the whole visualized area in the stress-shielded patellar tendon was significantly smaller at 3 and 6 weeks than that in the control. This study demonstrated that complete stress shielding significantly affects the microstructure and ultrastructure of the patellar tendon

Reduction of initial tension in the in situ frozen anterior cruciate ligament

The purposes of this study were to develop a new method to measure in situ tension in the anterior cruciate ligament, and to show degradation changes in the tension applied to the in situ frozen-thawed anterior cruciate ligament during a 12-week period postoperatively. Eighteen mature Beagles were used in the current study. Each dog had the in situ freeze and thaw treatment to its anterior cruciate ligament, and an initial tension of 20 N was applied to the anterior cruciate ligament. The in situ tension in the anterior cruciate ligament was measured immediately after surgery (n = 4) and at 6 and 12 weeks (n = 5, respectively) with a method developed specifically for this study. A commercially available force transducer, the utility and the validity of which were established in previous studies, was inserted into the anterior cruciate ligament, and the transducer was calibrated in situ using a tensile tester after sacrifice. The four remaining animals were used for histologic examinations. The average tension in the anterior cruciate ligament decreased from 19.1 N immediately after surgery to 14.2 N and 4.7 N at 6 and 12 weeks, respectively. There were significant differences among the times. At 12 weeks, histologically, degenerative changes were seen with asteroid-shaped cells and vacuoles in the anterior cruciate ligament. The current study showed that the high initial tension applied to the in situ frozen anterior cruciate ligament gradually decreased with time until 12 weeks.

Biomechanical comparisons of three posterior cruciate ligament reconstruction procedures with load-controlled and displacement-controlled cyclic tests

BACKGROUND

Biomechanical behaviors of posterior cruciate ligament reconstructions under cyclic loading have not been sufficiently clarified.

HYPOTHESIS

Biomechanical behaviors of the reconstruction that involves use of flexor tendons and an Endobutton are significantly different under cyclic loading from behaviors of the two standard reconstructions in which bone-patellar tendon-bone graft is used.

STUDY DESIGN

Controlled laboratory study.

METHODS

In a porcine model, the tendon/Endobutton reconstruction, the tendon-bone/interference screw reconstruction, and the tendon-bone/tibial-inlay reconstruction were biomechanically compared by using two cyclic tests. In each group of 15 specimens, 5 knees underwent tensile testing without cyclic loading, and the remaining 10 underwent the same tensile test after 5000 cycles of load-controlled or displacement-controlled loading.

RESULTS

At the 5000th cycle, the peak displacement or the peak load was affected by each type of cyclic loading to a significantly greater degree in the knees with the tendon/Endobutton procedure than in the knees reconstructed with the other two procedures.

CONCLUSIONS

Plastic deformation occurred more easily during cyclic loading in the knees with the tendon/Endobutton reconstruction than in the knees with the tendon-bone reconstructions.

CLINICAL RELEVANCE

When the tendon/Endobutton reconstruction is used, a longer period of postoperative immobilization is necessary.

Timing of administration of transforming growth factor-beta and epidermal growth factor influences the effect on material properties of the in situ frozen-thawed anterior cruciate ligament

One of the future goals in ligament reconstruction is to prevent graft deterioration after transplantation. The aim of this study is to clarify whether an administration of TGF-beta1 and EGF significantly affect biomechanical properties of the in situ frozen-thawed anterior cruciate ligament (ACL), an ACL autograft model, and to elucidate whether the timing of this administration may influence its effect. Rabbits were randomly divided into 4 groups after the freeze-thaw treatment with liquid nitrogen was applied to the right knee. In 2 groups, 4-ng TGF-beta1 and 100-ng EGF mixed with 0.2-ml fibrin sealant were applied around the ACL at 3 and 6 weeks after the treatment, respectively. In the remaining two groups, only 0.2-ml fibrin sealant was applied around the ACL at 3 and 6 weeks, respectively. In each group, all animals were sacrificed at 12 weeks after the freeze-thaw treatment. These growth factors applied at 3 weeks significantly inhibited not only the increase of water content and the cross-sectional area of the ACL but also reduction of the tensile strength and the tangent modulus of the ACL (p<0.0001), which were induced by the freeze-thaw treatment. However, the application at 6 weeks did not significantly affect the changes of these parameters after the treatment. This study demonstrated that the timing of administration of TGF-beta and EGF after the freeze-thaw treatment significantly influences its effect on the biomechanical properties of the frozen-thawed ACL.

Effects of stress deprivation on mechanical properties of the in situ frozen-thawed semitendinosus tendon in rabbits

OBJECTIVE

To clarify the effect of complete stress deprivation on the mechanical properties of the in situ frozen-thawed semitendinosus tendon, an idealized autograft model.

DESIGN

Ninety-six rabbits were divided into three groups. In the frozen group (n=36), we applied the freeze-thaw treatment to the semitendinosus tendon to necrotize fibroblasts in the tendon. In the frozen and stress-shielded group (n=30), after we applied the same freeze-thaw treatment to the tendon, we completely released the tendon from stress. In the sham group (n=30), a sham operation was applied. In each group, 6 rabbits were sacrificed at 0 (only in the frozen group), 1, 2, 3, 6 and 12 weeks after surgery.

BACKGROUND

Previous studies have not clarified remodeling of the semitendinosus autograft in ligament reconstruction or its idealized model.

METHODS

The tendon was frozen with liquid nitrogen. The tendon was released from stress with the originally developed technique using a polyester tape. In each period, 5 out of the 6 rabbits were evaluated with tensile testing, and the remaining rabbit was histologically observed.

RESULTS

Complete stress shielding significantly increased the cross-sectional area of the frozen-thawed tendon at 1 and 2 weeks, while it significantly inhibited the increase of the area due to the freeze-thaw treatment at 3 and 6 weeks. Complete stress shielding significantly reduced material properties of the frozen-thawed tendon after 2 weeks.

CONCLUSIONS

The frozen-thawed semitendinosus tendon has unique remodeling characteristics under a stress-shielded condition, which were not the same as those of the frozen-thawed patellar tendon.

RELEVANCE

Remodeling of the semitendinosus tendon autograft under stress-shielded conditions may be different from that of the patellar tendon autograft.

Hindlimb unloading alters ligament healing

We investigated the hypothesis that hindlimb unloading inhibits healing in fibrous connective tissue such as ligament. Male rats were assigned to 3- and 7-wk treatment groups with three subgroups each: sham control, ambulatory healing, and hindlimb-suspended healing. Ambulatory and suspended animals underwent surgical rupture of their medial collateral ligaments, whereas sham surgeries were performed on control animals. After 3 or 7 wk, mechanical and/or morphological properties were measured in ligament, muscle, and bone. During mechanical testing, most suspended ligaments failed in the scar region, indicating the greatest impairment was to ligament and not to bone-ligament insertion. Ligament testing revealed significant reductions in maximum force, ultimate stress, elastic modulus, and low-load properties in suspended animals. In addition, femoral mineral density, femoral strength, gastrocnemius mass, and tibialis anterior mass were significantly reduced. Microscopy revealed abnormal scar formation and cell distribution in suspended ligaments with extracellular matrix discontinuities and voids between misaligned, but well-formed, collagen fiber bundles. Hence, stress levels from ambulation appear unnecessary for formation of fiber bundles yet required for collagen to form structurally competent continuous fibers. Results support our hypothesis that hindlimb unloading impairs healing of fibrous connective tissue. In addition, this study provides compelling morphological evidence explaining the altered structure-function relationship in load-deprived healing connective tissue.

Effects of combined administration of transforming growth factor-beta1 and epidermal growth factor on properties of the in situ frozen anterior cruciate ligament in rabbits

The mechanical properties of tendon autografts used in reconstruction of the anterior cruciate ligament (ACL) are reduced after surgery. Previous studies showed that growth factors such as transforming growth factor-beta1 (TGF-beta1) and epidermal growth factor (EGF) can stimulate fibroblast proliferation and increase collagen and noncollagenous protein synthesis by these cells. These factors might be useful, therefore, in preventing graft deterioration after transplantation or accelerating mechanical restoration of the deteriorated graft. The purpose of our study, therefore, was to clarify the effects of TGF-beta1 and EGF on biomechanical properties using an in situ freeze-thaw ACL model in the rabbit. A total of 142 rabbits underwent the freeze-thaw treatment in the right ACL and were then divided into four groups. Group I served as a freeze-thaw, but otherwise untreated control. In Group II. a delivery vehicle (fibrin sealant) alone was applied. In Group III, 4-ng TGF-beta1 and 100-ng EGF mixed with the vehicle were applied. In Group IV, higher doses (2-microg TGF-beta1 and 50-microg EGF) of growth factors were mixed with the vehicle. The groups were compared at 6 and 12 weeks on the basis of mechanical properties, water content, and histological and ultrastructural observations. The cross-sectional area of Group III (average, 7.1 mm2) was significantly less than that of Groups I, II, and IV (9.0. 8.2. and 9.4 mm2. respectively) at 12 weeks. The tensile strength of Group lII (62.2 MPa) was significantly greater than that of Groups I, II, and IV (35.6, 43.7, and 36.9 MPa, respectively) at 12 weeks, while the water content of Group III (70.7%) was significantly lower than that of Group I (75.2%). No other significant differences occurred among Groups I, II, and IV. A unimodal distribution of collagen fibril diameters was noted in Groups I and II, while a bimodal pattern was found in Group III. This study demonstrated that low-dose application of TGFbeta1 and EGF significantly inhibited not only the increased water content and cross-sectional area, but also the decreased tensile strength caused by the freeze-thaw treatment, while a high dose of TGF-beta1 and EGF does not have the same beneficial effects.

The progression of anterior translation after anterior cruciate ligament reconstruction in a caprine model

Large post-operative anterior-posterior translations are frequently reported after quadruped anterior cruciate ligament (ACL) reconstructions. To determine when the translation increases occur and the mechanism responsible, we followed the anterior and posterior translation limits in 18 goat knees for six months. Reconstructions were performed using grafts 4 or 7 mm wide placed in initially tight or lax positions. The anterior and posterior translation limits at 50 N were monitored using Roentgen stereophotogrammetric analysis. Graft bone block stability and soft tissue segment lengths were also assessed. Large (> 2 mm) increases in anterior translation were noted in 71% of the subjects at two weeks, and in 88% at eight weeks. The translations in the lax and tight groups were indistinguishable after two weeks. Joints with wide grafts had less anterior translation compared to narrow grafts at all time periods, but were significant different only at 26 weeks. The posterior translation limit moved anteriorly over the 26 weeks. Eight of nine joints had stable graft bone markers and/or increases in graft soft tissue lengths. In conclusion, increased anterior translation occurred soon after ACL reconstruction, was associated with graft soft tissue changes, and appeared to be reduced by larger grafts. A post-surgical decrease in posterior translation limit was also observed.

Tendon healing in a bone tunnel. Part I: Biomechanical results after biodegradable interference fit fixation in a model of anterior cruciate ligament reconstruction in sheep

PURPOSE

Interference fit fixation of soft-tissue grafts has recently raised strong interest because it allows for anatomic graft fixation that may increase knee stability and graft isometry. Although clinical data show promising results, no data exist on how tendon healing progresses using this fixation. The purpose of the present study was to investigate anterior cruciate ligament (ACL) reconstruction biomechanically using direct tendon-to-bone interference fit fixation with biodegradable interference screws in a sheep model.

TYPE OF STUDY

Animal study.

METHODS

Thirty-five mature sheep underwent ACL reconstruction with an autologous Achilles tendon split graft. Grafts were directly fixed with poly-(D,L-lactide) interference screws. Animals were euthanized after 6, 9, 12, 24, and 52 weeks and standard biomechanical evaluations were performed.

RESULTS

All grafts at time zero failed by pullout from the bone tunnel, whereas grafts at 6 and 9 weeks failed intraligamentously at the screw insertion site. At 24 and 52 weeks, grafts failed by osteocartilaginous avulsion. At 24 weeks, interference screws were macroscopically degraded. At 6 and 9 weeks tensile stress was only 6.8% and 9.6%, respectively, of the graft tissue at time zero. At 52 weeks, tensile stress of the reconstruction equaled 63.8% and 47.3% of the Achilles tendon graft at time zero and the native ACL, respectively. A complete restitution of anterior-posterior drawer displacement was found at 52 weeks compared with the time-zero reconstruction.

CONCLUSIONS

It was found that over the whole healing period the graft fixation proved not to be the weak link of the reconstruction and that direct interference fit fixation withstands loads without motion restriction in the present animal model. The weak link during the early healing stage was the graft at its tunnel entrance site, leading to a critical decrease in mechanical properties. This finding indicates that interference fit fixation of a soft-tissue graft may additionally alter the mechanical properties of the graft in the early remodeling stage because of a possible tissue compromise at the screw insertion site. Although mechanical properties of the graft tissue had not returned to normal at 1 year compared with those at time zero, knee stability had returned to normal at that time. There was no graft pullout after 24 weeks, indicating that screw degradation does not compromise graft fixation.

Biomechanical properties and vascularity of an anterior cruciate ligament graft can be predicted by contrast-enhanced magnetic resonance imaging. A two-year study in sheep

Magnetic resonance imaging has been used to determine graft integrity and study the remodeling process of anterior cruciate ligament grafts morphologically in humans. The goal of the present study was to compare graft signal intensity and morphologic characteristics on magnetic resonance imaging with biomechanical and histologic parameters in a long-term animal model. Thirty sheep underwent anterior cruciate ligament reconstruction with an autologous Achilles tendon split graft and were sacrificed after 6, 12, 24, 52, or 104 weeks. Before sacrifice, all animals underwent plain and contrast-enhanced (gadolinium-diethylenetriamine pentacetic acid) magnetic resonance imaging (1.5 T, proton density weighted, 2-mm sections) of their operated knees. The signal/noise quotient was calculated and data were correlated to the maximum load to failure, tensile strength, and stiffness of the grafts. The vascularity of the grafts was determined immunohistochemically by staining for endothelial cells (factor VIII). We found that high signal intensity on magnetic resonance imaging reflects a decrease of mechanical properties of the graft during early remodeling. Correlation analyses revealed significant negative linear correlations between the signal/noise quotient and the load to failure, stiffness, and tensile strength. In general, correlations for contrast-enhanced measurements of signal intensity were stronger than those for plain magnetic resonance imaging. Immunohistochemistry confirmed that contrast medium enhancement reflects the vascular status of the graft tissue during remodeling. We conclude that quantitatively determined magnetic resonance imaging signal intensity may be a useful tool for following the graft remodeling process in a noninvasive manner.

Comparisons of intraosseous graft healing between the doubled flexor tendon graft and the bone-patellar tendon-bone graft in anterior cruciate ligament reconstruction

PURPOSE

The purpose of this study was to compare intraosseous graft healing between the doubled flexor tendon (FT) graft and the bone-patellar tendon-bone (BPTB) graft in anterior cruciate ligament (ACL) reconstruction.

TYPE OF STUDY

Randomized trial.

METHODS

A biomechanical and histologic study was conducted with 24 adult beagle dogs. Bilateral ACL reconstructions were performed in each animal. Autogenous doubled FT and BPTB grafts were used for the left and right knees, respectively. Each end of the 2 grafts was tethered with a polyester suture to a screw post with a washer. The animals were then allowed unrestricted activities in their cages. Eight animals were killed at 3, 6, and 12 weeks, respectively.

RESULTS

Histologically, the FT graft was anchored to the tunnel wall with newly formed collagen fibers resembling Sharpey's fibers by 12 weeks. These fibers were more abundant in the anterior (ventral) gap than in the posterior (dorsal) gap. In the BPTB graft, the bone plug was anchored with newly formed bone at 3 weeks, although osteocytes in the plug trabeculae were necrotic for 12 weeks. Degeneration of the tendon-bone junction in the plug progressed at 6 weeks. Tensile testing showed that the weakest site was different not only between the 2 grafts but also between the observation periods. In the FT graft, the weakest site was the graft-wall interface at 3 weeks and the intraosseously grafted tendon at 6 weeks. In the BPTB graft, the weakest site was the graft-wall interface at 3 weeks and the proximal site in the bone plug at 6 weeks. The ultimate failure load of the FT graft was significantly inferior (45.8%) to that of the BPTB graft at 3 weeks (P =.021). At 6 weeks, the load of the FT graft was 85% that of the BPTB graft without a significant difference (P =.395).

CONCLUSIONS

As to the clinical relevance, the fixation device chosen for soft-tissue fixation appears to be more important than comparing it to the BPTB graft, although this has yet to be conclusively proven.

Extrinsic cell infiltration and revascularization accelerate mechanical deterioration of the patellar tendon after fibroblast necrosis

This study was performed to determine the contribution of extrinsic cell infiltration and revascularization into the patellar tendon in alteration of the mechanical properties of the patellar tendon after intrinsic fibroblast necrosis using 77 rabbits. In Group I, after the patellar tendon underwent the in situ freeze-thaw treatment, a wrapping treatment was performed to inhibit any extrinsic cell infiltration into the tendon. In Group II, the patellar tendon underwent the freeze-thaw treatment without any of the wrapping treatment. In Group III, the patellar tendon underwent the same wrapping treatment but without any freeze-thaw treatment. The cell culture study demonstrated that the in situ freeze-thaw treatment killed from 97 to 100 percent of the cells in the patellar tendon. Histologically, no cells were found in the midsubstance of the patellar tendon in Group I at 1, 3, and 6 weeks. In Group II, a number of cells and some vessels were found scattered in the tendon at 3 and 6 weeks. Mechanically, the elastic modulus and the tensile strength of the patellar tendon of Group II were significantly lower than those of Groups I and III at 3 and 6 weeks. These facts suggest that extrinsic cell infiltration and revascularization from the surrounding tissues accelerate the deterioration of the mechanical properties of the patellar tendon matrix after intrinsic fibroblast necrosis.

Effects of growth on the response of the rabbit patellar tendon to stress shielding: a biomechanical study

OBJECTIVE

To know the effect of stress deprivation on the dimensions and mechanical properties of the patellar tendon during growth.

DESIGN

The dimensions and tensile properties of stress-shielded patellar tendons were studied in growing rabbits and compared to those in mature animals.

BACKGROUND

Although the effects of stress deprivation on the remodeling of ligaments and tendons have been studied in various animal models, the effect of growth on the remodeling has not been studied well.

METHOD

A stress shielding technique was applied to 1-, 2-, and 3-month-old Japanese white rabbits to completely remove stress in the patellar tendons for 4, 7, and 14 days. Changes in the dimensions and mechanical properties as well as fibroblast density of the tendon were determined.

RESULTS

The tensile strength and tangent modulus of the patellar tendons were markedly decreased by stress shielding, while the cross-sectional area was significantly increased, with the largest changes in 1-month-old rabbits. Fibroblast density also increased; however, the degree of increase was highest in 3-month-old rabbits.

CONCLUSION

The changes in the dimensions and mechanical properties of the patellar tendons induced by stress shielding were greater in younger animals.

RELEVANCE

The biomechanical response of tendons and ligaments to stress deprivation induced by, for example, limb immobilization is greater and occurs earlier in younger subjects, which is important for the surgical treatment and rehabilitation protocol of joint diseases in young subjects.

The effects of stress enhancement on the extracellular matrix and fibroblasts in the patellar tendon

The purpose of the present study is to determine the effect of the stress enhancement and intrinsic fibroblasts on the extracellular matrix of the patellar tendon. Thirty-two female Japanese White rabbits were divided into four groups. In Group 1, the patellar tendon underwent the in situ freeze-thaw treatment to kill intrinsic fibroblasts of the patellar tendon and the patellar tendon underwent the wrapping treatment with nylon membrane filters to inhibit extrinsic cell infiltration. In Group 2, the medial and the lateral portions of the frozen-thawed patellar tendon were resected to enhance the stress, and then the central two-thirds of the patellar tendon underwent the wrapping treatment. In Group 3, the patellar tendon without the freeze/thaw treatment underwent the wrapping treatment. In Group 4, the patellar tendon was narrowed and wrapped in the same manner. All rabbits were killed 6 weeks after surgery. While the elastic modulus and the tensile strength of the patellar tendon in Group 2 were significantly less than those in Group 1, we could not find any significant differences in these parameters between Groups 3 and 4. Histologically, while no fibroblasts were observed in Groups 1 and 2, fibroblasts were found in Groups 3 and 4. This study revealed that stress enhancement decreases the elastic modulus and the tensile strength of the extracellular matrix of the patellar tendon and that intrinsic fibroblasts prevent the detrimental effect of stress enhancement on mechanical properties of the patellar tendon.

Mechanical properties of collagen fascicles from in situ frozen and stress-shielded rabbit patellar tendons

OBJECTIVE

To know the effects of stress shielding on the biomechanical properties of collagen fascicles obtained from in situ frozen patellar tendons (an autograft model).

DESIGN

Collagen fascicles of approximately 300 microm in diameter were obtained from in situ frozen rabbit patellar tendons and also from in situ frozen and stress-shielded ones, and their mechanical properties and fibroblast density were determined.

BACKGROUND

Stress shielding changes the mechanical properties of in situ frozen patellar tendons in which there exist no fibroblasts. The mechanisms of this phenomenon have not been studied well.

METHOD

Patellar tendons of both in situ frozen group and in situ frozen and stress-shielded group were frozen in situ by liquid nitrogen to kill fibroblasts. Then, in the in situ frozen and stress-shielded group, no tension was applied to the tendons for 2, 3, and 6 weeks, while normal tension was applied to the tendons of the in situ frozen group. Tensile properties of the collagen fascicles obtained from these tendons were determined using a microtensile tester, and were compared to the collagen fascicles from non-frozen, stress-shielded patellar tendons.

RESULTS

Tangent modulus and tensile strength of collagen fascicles from the in situ frozen and stress-shielded group progressively decreased with the time of stress shielding; however, these decreases were much smaller than those of the fascicles obtained from non-frozen, stress-shielded tendons. Although there were few fibroblasts in the patellar tendon of the in situ frozen and stress-shielded group at 2 weeks, the modulus and strength of the fascicles from the posterior portion were significantly lower than those in the in situ frozen group. In addition, the reduction of strength caused by stress shielding was much smaller in collagen fascicles than in bulk patellar tendons.

CONCLUSION

The mechanical properties of collagen fascicles in in situ frozen tendons (an autograft model) are affected by stress shielding even under acellular condition. RelevanceThe in situ frozen, stress-shielded patellar tendon is a model of augmented autografts which are clinically used for the reconstruction of injured anterior cruciate ligaments. The sub-macroscopic studies of the tendon are useful to understand the mechanisms of the reduction of graft strength and its gradual recovery observed after reconstruction.

The effect of nonphysiologically high initial tension on the mechanical properties of in situ frozen anterior cruciate ligament in a canine model

An experimental study was performed in 32 adult beagle dogs to clarify the effect of nonphysiologically high initial tension on the mechanical and histologic properties of in situ frozen anterior cruciate ligaments. Both anterior cruciate ligaments in each dog underwent the in situ freeze-thaw treatment. The tibial insertion of the ligament was then made free from the tibia along with a cylindrical bone block. In the right knee, an initial tension of 20 N was applied on the anterior cruciate ligament by translocating the bone block in the distal direction. In the left knee, this bone block was anatomically reduced. Each bone block was firmly fixed with an interference screw. Ten animals were sacrificed at 6 weeks and 10 at 12 weeks. The tensile strength and the tangent modulus in the highly tensioned knee were significantly less than those in the physiologically tensioned knee at 12 weeks. Histologically, cell nuclei appeared to be spindle-shaped in the physiologically tensioned knee, while oval nuclei and focal degenerative changes with a number of vacuoles were occasionally found in the matrix in the highly tensioned knee. This study demonstrated that a nonphysiologically high tension significantly deteriorates the mechanical properties of the in situ frozen anterior cruciate ligament compared with physiologic tension.

Mechanical properties of collagen fascicles from the rabbit patellar tendon

Tensile and viscoelastic properties of collagen fascicles of approximately 300 microns in diameter, which were obtained from rabbit patellar tendons, were studied using a newly designed micro-tensile tester. Their cross-sectional areas were determined with a video dimension analyzer combined with a CCD camera and a low magnification microscope. There were no statistically significant differences in tensile properties among the fascicles obtained from six medial-to-lateral locations of the patellar tendon. Tangent modulus, tensile strength, and strain at failure of the fascicles determined at about 1.5 percent/s strain rate were 216 +/- 68 MPa, 17.2 +/- 4.1 MPa, and 10.9 +/- 1.6 percent (mean +/- S.D.), respectively. These properties were much different from those of bulk patellar tendons; for example, the tensile strength and strain at failure of these fascicles were 42 percent and 179 percent of those of bulk tendons, respectively. Tangent modulus and tensile strength of collagen fascicles determined at 1 percent/s strain rate were 35 percent larger than those at 0.01 percent/s. The strain at failure was independent of strain rate. Relaxation tests showed that the reduction of stress was approximately 25 percent at 300 seconds. These stress relaxation behavior and strain rate effects of collagen fascicles differed greatly from those of bulk tendons. The differences in tensile and viscoelastic properties between fascicles and bulk tendons may be attributable to ground substances, mechanical interaction between fascicles, and the difference of crimp structure of collagen fibrils.

Effects of in situ freezing and stress-shielding on the ultrastructure of rabbit patellar tendons

The effects of in situ freezing and the combination of in situ freezing and stress-shielding on the microstructure and ultrastructure of the patellar tendon were studied with use of 20 mature rabbits. The patellar tendon was frozen in situ with liquid nitrogen to kill fibroblasts and then was completely released from stress by chronically pulling a stainless-steel wire installed between the patella and the tibial tubercle. Microstructurally, the freezing treatment induced separation of collagen fiber bundles and fibroblast necrosis at 3 weeks, although the separation disappeared at 6 weeks. Ultrastructurally, small collagen fibrils with a diameter of less than 90 nm were predominant; at 6 weeks, the area occupied by collagen fibrils had decreased. In the frozen-shielded tendon, numerous large spaces were observed in the matrix at 3 weeks. This treatment increased the number of fibrils with a diameter greater than 360 nm and decreased the number of collagen fibrils per unit of area and the area occupied by collagen fibrils at 3 weeks. This study demonstrated that in situ freezing and the combination of in situ freezing and stress-shielding leads to a smaller volume of collagen fibrils per unit of cross section of the patellar tendon by mechanisms that remain to be defined.

Degeneration at the insertion weakens the tensile strength of the supraspinatus tendon: a comparative mechanical and histologic study of the bone-tendon complex

The purpose of this investigation was to determine the relationship between the degree of degeneration at the supraspinatus insertion, the tensile strength, and the site of failure of this tendon. Thirty-three fresh cadaveric shoulders (average age: 62 years; range: 39-83 years) were examined. A tensile load to failure was applied at a constant crosshead speed of 25.4 mm/min to a 10 mm wide strip of the supraspinatus tendon that remained attached to the bone. Preexisting degenerative changes at the insertion were assessed and scored histologically and compared with the ultimate tensile stress. Twenty tendons failed at the insertion (the insertion group), and 11 failed in the midsubstance (the midsubstance group). The histologic score of degeneration for the insertion group was significantly higher than that for the midsubstance group (p = 0.0026). There was a negative correlation between the ultimate tensile stress at the insertion and the degeneration score for the insertion group (r = -0.60; p = 0.013). Histologic observations revealed that disruptions of tendon fibers were located mostly in the articular half of the tendon and that they enlarged during mechanical testing in 90% of the specimens of the insertion group. It seems that degenerative changes at the supraspinatus insertion reduce the tensile strength of the tendon and constitute a primary pathogenetic factor of rotator cuff tear.

Effects of initial graft tension on clinical outcome after anterior cruciate ligament reconstruction. Autogenous doubled hamstring tendons connected in series with polyester tapes

We conducted a prospective, randomized, short-term study to clarify the effects of initial graft tension on clinical outcome after arthroscopically assisted anterior cruciate ligament reconstruction with autogenous doubled semitendinosus and gracilis tendons connected in series with polyester tapes. Seventy Japanese patients with chronic, "isolated" anterior cruciate ligament tears were entered in the study. The patients were randomly divided into three groups based on initial graft tension: Group 1 (20 N), Group 2 (40 N), or Group 3 (80 N). No statistical differences were noted among the three groups with regard to their background factors. The patients were observed for 2 years or more after surgery. Postoperatively, the average side-to-side difference in anterior laxity was 2.2 +/- 2.4 mm in Group 1, 1.4 +/- 1.8 mm in Group 2, and 0.6 +/- 1.7 mm in Group 3. Analysis of variance testing showed that the postoperative laxity in Group 3 was significantly less than that in Group 1. Spearman's rank-order correlation analysis also demonstrated significant correlation between the magnitude of initial graft tension and the magnitude of the postoperative laxity. This study demonstrates that relatively high initial tension (up to 80 N) reduces the postoperative anterior laxity of the knee joint after anterior cruciate ligament reconstruction using the doubled autogenous hamstring tendons connected in series with polyester tapes.

Effect of tibial attachment location on the healing of the anterior cruciate ligament freeze model

We studied the healing response of a devitalized anterior cruciate ligament to a treatment of initial anterior-posterior joint translation in goats. Devitalization and devascularization were achieved by five successive freeze-thaw cycles. Anterior-posterior translation was surgically altered by an osteotomy of the tibial attachment of the devitalized ligament and its reattachment either in the anatomical position or in a position 5 mm posterior. Six weeks after the first surgery, the same procedure was performed on the contralateral limb, except that the ligament was reattached in the alternate position. Six months after the initial surgery, femur-anterior cruciate ligament-tibia specimens were tested to determine their structural and mechanical material properties. Anatomic ligament placement resulted in reduced anterior-posterior translation (p < 0.05) and greater anterior joint stiffness (p < 0.05). Maximum load (p < 0.05) and ligament stiffness (p < 0.01) also were greater for the anatomically placed anterior cruciate ligaments. The maximum load for anatomically placed ligaments averaged 1.625 +/- 211 N (SEM). The strength of the posteriorly placed anterior cruciate ligament, 895 +/- 164 N was similar to results of historical anterior cruciate autograft reconstructions. Ligament failure occurred near the tibial insertion in the posteriorly placed ligaments more often than in the anatomically placed ligaments (four of five times compared with one of five times). Ligament failure near the tibial insertion occurred with lower mean maximum load than failure at the midsubstance or by bone avulsion (796 compared with 1.592 N: p < 0.05). These data support the hypothesis that ligament laxity is important to the healing and remodeling of anterior cruciate ligament grafts.

Effects of resumption of loading on stress-shielded autografts after augmentation procedures. An experimental study

An experiment was conducted to verify if resumption of loading (restressing) on autografts after augmentation procedures can recover the mechanical properties of the grafts that were deteriorated by complete stress shielding. One hundred ten Japanese White rabbits were used. After the patellar tendon in each right knee was frozen in situ, tension applied to the tendon was released for an interval in 80 rabbits. These 80 animals were divided into four groups of 20 rabbits. The patellar tendons in these groups were completely shielded from stress for 1, 2, 3, and 6 weeks, respectively. From each group, rabbits were sacrificed immediately before restressing (N = 5), and after 3 (N = 5), 6 (N = 5), and 12 (N = 5) weeks of restressing. The remaining 30 rabbits underwent sham operations. The results showed that complete stress shielding markedly decreased tensile strength of the patellar tendon. However, if restressing was applied within 3 weeks of complete stress shielding, the once-reduced strength gradually increased to the level of the sham-operated tendons by 12 weeks after the onset of restressing. Restressing applied after 6 weeks of complete stress shielding also increased the strength of the tendon; however, it reached only 65% the strength of the sham-operated tendons at 12 weeks after restressing. The timing to apply stress to grafts should be carefully determined for optimal recovery.

Biomechanical and histological changes in the patellar tendon after in situ freezing An experimental study in rabbits

OBJECTIVE: The effects of freezing on the remodelling process of the patellar tendon were examined. DESIGN: An experimental study in rabbits. BACKGROUND: Patellar tendon weakens when grafted as a subsstitute for the anterior cruciate ligament. Fibroblast necrosis is considered to be one of the many factors contributing to this change. Therefore, the effect of freeze-induced necrosis on the patellar tendon has been studied. METHODS: Using a technique for freezing the patellar tendon in situ with liquid nitrogen to kill fibroblasts, we studied the biomechanical and histological changes in the patellar tendon up to 24 weeks after freezing. RESULTS: The cross-sectional area started to increase by week 3, reaching a plateau by week 12. The elastic modulus and tensile strength began decreasing by week 3. Although the maximum load decreased at weeks 12 and 24, the stiffness did not change. Histologically, cells were absent until week 2. Athough cells were apparently normal at week 24, there were none of the dense collagen bundles that are normally seen. CONCLUSIONS: The once-frozen patellar tendon weakens as tissue remodelling occurs. RELEVANCE: The study was designed to ascertain whether the remodelling process of the once-frozen patellar tendon coincides with its mechanical properties in different phases. The weakening of the patellar tendon occurred as new cells proliferated into the tendon and remodelled the tissue.

Biomechanical studies of the remodeling of knee joint tendons and ligaments

Living tissues and organs are dynamic and change their mechanical properties and structure in response to stress alteration as a phenomenon of functional adaptation and optimal operation. This phenomenon is called 'Tissue Remodeling', and Wolff's law on bone remodeling is widely known. Several recent studies have shown that fibrous connective tissues such as tendons and ligaments also have the ability of remodeling. However, relatively little is known about the stress and motion effects on tissue homeostasis in biological soft tissues. This article primarily deals with changes of the biomechanical properties of knee joint tendons and ligaments through a wide variety of treatment modalities, including stress deprivation, recovery after stress deprivation, and stress enhancement. The experimental results indicate that tendons and ligaments have an ability to adapt in response to the change of stress if the extent of stress alteration is within allowable ranges.

Effects of restressing on the mechanical properties of stress-shielded patellar tendons in rabbits

We studied the effects of restressing on the mechanical properties and morphology of stress-shielded rabbit patellar tendons. After completely unloading the patellar tendon for 1 to 3 weeks, tension was again applied to the tendon for subsequent 3 to 12 weeks. Although the stress shielding markedly decreased the tangent modulus and tensile strength of the tendon, restressing significantly increased them. However, the mechanical properties of the tendon were not completely recovered even after a prolonged period of restressing. The microstructure of the tendon was also restored by restressing, although the recovery was incomplete. These results indicate that the mechanical properties and morphology of tendinous tissue change in response to mechanical demands.