Some effects of gamma irradiation on patellar tendon allografts

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

Objective

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

Methods

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

Results

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

Conclusion

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

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Different sterilization and disinfection methods used for human tendons - a systematic review using mechanical properties to evaluate tendon allografts

Background

It is important to know the biomechanical properties of an allograft. This is because when looking to do a transplant of a tendon, the tendon must have very similar biomechanical properties to the original tendon. To use tendon allografts, it is critical to properly sterilize the tendon before implantation. In past decades, several sterilization procedures have been used. This study aimed to systematically evaluate the existing literature to compare the values of failure load/ultimate strength and Young’s modulus of elasticity of different sterilization methods on commonly used tendon allografts. Five major scientific literature databases (Web of Science, Science Direct, Scopus, PLOS ONE, Hindawi) and additional sources were used.

Results

Studies used had to show a particular sterilization method. Studies were identified to meet the following inclusion criteria: is a controlled laboratory study, gamma irradiation (dose reported), and other sterilization methods. Search for publications dated between 1991 and March 31st, 2020. The database search and additional sources resulted in 284 records. Two hundred thirty records eliminated during the screening for various reasons. The number of articles used in the final synthesis was 54.

Conclusions

Identified sterilization methods (gamma irradiation, ethylene oxid, supercritical carbon dioxide (SCCO2), BioCleanse, Electron Beam) are offered as a catalog of potential methods.

As a result of the broadness of the present research, it provides an overview of sterilization methods and their effect on the mechanical properties (failure load and Young’s modulus of elasticity) of tendons. It does not stand for the state-of-the-art of any single process. Based on a systematic literature review, we recommend freezing and gamma irradiation or electron beam at 14.8–28.5 kGy. These methods are effective at keeping or improving the mechanical properties, while fully sterilizing the inside and the outside of the tendon. Other sterilization method (ethylene oxide, supercritical carbon dioxide (SCCO2), BioCleanse) deteriorated the mechanical properties. These methods are not recommended.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-021-04296-4.

Rebuilding Tendons: A Concise Review on the Potential of Dermal Fibroblasts

Tendons are vital to joint movement by connecting muscles to bones. Along with an increasing incidence of tendon injuries, tendon disorders can burden the quality of life of patients or the career of athletes. Current treatments involve surgical reconstruction and conservative therapy. Especially in the elderly population, tendon recovery requires lengthy periods and it may result in unsatisfactory outcome. Cell-mediated tendon engineering is a rapidly progressing experimental and pre-clinical field, which holds great potential for an alternative approach to established medical treatments. The selection of an appropriate cell source is critical and remains under investigation. Dermal fibroblasts exhibit multiple similarities to tendon cells, suggesting they may be a promising cell source for tendon engineering. Hence, the purpose of this review article was in brief, to compare tendon to dermis tissues, and summarize in vitro studies on tenogenic differentiation of dermal fibroblasts. Furthermore, analysis of an open source Gene Expression Omnibus (GEO) data repository was carried out, revealing great overlap in the molecular profiles of both cell types. Lastly, a summary of in vivo studies employing dermal fibroblasts in tendon repair as well as pilot clinical studies in this area is included. Altogether, dermal fibroblasts hold therapeutic potential and are attractive cells for rebuilding injured tendons.

Irradiation sterilization used for allogenetic tendon: a literature review of current concept

Tendon injury is a very common type of sports trauma, and its incidence has increased over the past decades. Surgical reconstruction with tendon allograft has been increasingly used to restore the motor function and stability of the injured site. However, the risk of disease transmission caused by allogeneic tendon transplantation has been a major problem for tissue bank researchers and clinicians. In order to eliminate the risk of disease transmission, a process of terminal sterilization is necessary. Ionizing irradiation, including gamma irradiation and electron beam irradiation is the most commonly used method for the terminal sterilization, which has been widely proved to be able to effectively inactivate the contained pathogens. Nevertheless, some accompanying damage to the mechanical and histological properties of collagen fibers in tendons will be caused. Therefore, more and more studies have begun to pay attention to the protective effect of radiation protection agents, including the radical scavengers and cross-linking agents, in the irradiation sterilization of allogeneic tendons.

Carbodiimide cross-linking counteracts the detrimental effects of gamma irradiation on the physical properties of collagen-hyaluronan sponges

Collagen-based scaffolds are extensively used in biomaterials and tissue engineering applications. These scaffolds have shown great biocompatibility and versatility, but their relatively low mechanical properties may limit use in orthopaedic load-bearing applications. Moreover, terminal sterilization with gamma irradiation, as is commonly performed with commercial devices, presents concerns over structural integrity and enzymatic stability. Therefore, the goal of this study was to test the hypothesis that EDC/NHS cross-linking (10 mM/5 mM) can protect collagen-hyaluronan sponges from the damaging effects of gamma irradiation. Specifically, we evaluated compressive and tensile mechanical properties, enzymatic stability, porosity and pore size, and swelling ratio. Ultimate tensile strength and elastic modulus exhibited increases (168.5 and 245.8%, respectively) following irradiation, and exhibited over tenfold increases (1049.2 and 1270.6%, respectively) following cross-linking. Irradiation affected pore size (38.4% decrease), but cross-linking prior to irradiation resulted in only a 17.8% decrease. Cross-linking also showed an offsetting effect on the equilibrium modulus, enzymatic stability, and swelling ratio of sponges. These results suggest that carbodiimide cross-linking of collagen-hyaluronan sponges can mitigate the structural damage typically experienced during gamma irradiation, warranting their use in tissue engineering applications.

Allograft tissue irradiation and failure rate after anterior cruciate ligament reconstruction: A systematic review

AIM: To evaluate whether anterior cruciate ligament (ACL) allograft irradiation is effective for sterility without compromising graft integrity and increasing failure rate.

METHODS: A literature search was conducted using PubMed, Cochrane, and Google. The following search terms were used: “Gamma irradiation AND anterior cruciate ligament AND allograft” with a return of 30 items. Filters used included: English language, years 1990-2015. There were 6 hits that were not reviewed, as there were only abstracts available. Another 5 hits were discarded, as they did not pertain to the topic of interest. There were 9 more articles that were excluded: Three studies were performed on animals and 6 studies were meta-analyses. Therefore, a total of 10 articles were applicable to review.

RESULTS: There is a delicate dosing crossover where gamma irradiation is both effective for sterility without catastrophically compromising the structural integrity of the graft. Of note, low dose irradiation is considered less than 2.0 Mrad, moderate dose is between 2.1-2.4 Mrad, and high dose is greater than or equal to 2.5 Mrad. Based upon the results of the literature search, the optimal threshold for sterilization was found to be sterilization at less than 2.2 Mrad of gamma irradiation with the important caveat of being performed at low temperatures. The graft selection process also must include thorough donor screening and testing as well as harvesting the tissue in a sterile fashion. Utilization of higher dose (≥ 2.5 Mrad) of irradiation causes greater allograft tissue laxity that results in greater graft failure rate clinically in patients after ACL reconstruction.

CONCLUSION: Allograft ACL graft gamma irradiated with less than 2.2 Mrad appears to be a reasonable alternative to autograft for patients above 25 years of age.

Progress in cell-based therapies for tendon repair

The last decade has seen significant developments in cell therapies, based on permanently differentiated, reprogrammed or engineered stem cells, for tendon injuries and degenerative conditions. In vitro studies assess the influence of biophysical, biochemical and biological signals on tenogenic phenotype maintenance and/or differentiation towards tenogenic lineage. However, the ideal culture environment has yet to be identified due to the lack of standardised experimental setup and readout system. Bone marrow mesenchymal stem cells and tenocytes/dermal fibroblasts appear to be the cell populations of choice for clinical translation in equine and human patients respectively based on circumstantial, rather than on hard evidence. Collaborative, inter- and multi-disciplinary efforts are expected to provide clinically relevant and commercially viable cell-based therapies for tendon repair and regeneration in the years to come.

Analysis of irradiation on the clinical effectiveness of allogenic tissue when used for primary anterior cruciate ligament reconstruction

BACKGROUND

It is unclear whether the use of low-dose irradiation or other tissue-processing methods, such as preservation by fresh-frozen (FF), freeze-drying (FD), or cryopreservation (CP) methods, affects the clinical outcomes of primary anterior cruciate ligament reconstruction (ACLR) using allograft.

HYPOTHESIS

Low-dose gamma irradiation (<2.5 Mrad) and method of allograft preservation do not affect subjective and objective clinical outcomes after primary ACLR in studies reviewed between November 2010 and September 2012.

STUDY DESIGN

Systematic review; Level of evidence, 3.

METHODS

A computerized search of multiple electronic databases was conducted from November 2010 to September 2012 for prospective and retrospective studies involving primary allograft ACLR. Inclusion criteria were English-language publications with a minimum average of 2 years' follow-up. Studies were excluded if they involved revision surgery, open surgery, multiple ligament procedures, autograft, xenograft, meniscal allograft, skeletally immature patients, or grafts treated with ethylene oxide, Tutoplast, or irradiation>2.5 Mrad or if the tissue-processing methods were not specified. Clinical outcomes were evaluated using the Lysholm score, Tegner score, International Knee Documentation Committee (IKDC) score, KT-1000/2000 arthrometer score, Lachman test, and pivot-shift test, as well as by assessing complications related to graft rupture, revision surgery, and infections.

RESULTS

A total of 21 publications met the criteria, involving a total of 1453 patients, with 415 irradiated and 1038 nonirradiated allografts. Mean follow-up was 49.8 months (range, 12-170 months). Mean age of the patients was 32.2 years. Knees with nonirradiated allografts had higher mean Lysholm scores (89.8 vs 84.4; P<.05), and a higher proportion of <5-mm difference on KT-1000/2000 arthrometer (0.97 vs 0.84; P<.0001), grade 0 and 1 pivot-shift (0.99 vs 0.94; P<.0001), and grade 0 and 1 Lachman (0.94 vs 0.89; P<.01) than those with irradiated grafts. Knees with irradiated allografts had a higher proportion of grade A and B IKDC outcomes (0.91 vs 0.86; P<.05) and revision surgery (0.0250 vs 0.0022; P<.001) compared with those with nonirradiated allografts. The lack of data for FD and CP allografts meant no statistical analysis could be made comparing FF versus FD versus CP allografts. The effect of irradiation was similar within FF allografts. The effect of graft type and surgical technique could not be determined because of insufficient data.

CONCLUSION

These results suggest that primary ACLRs using nonirradiated allografts may provide superior clinical outcomes than those using low-dose (<2.5 Mrad) irradiated grafts.

Influence of sterilisation methods on collagen-based devices stability and properties

Sterilisation is essential for any implantable medical device in order to prevent infection in patients. The selection of the most appropriate sterilisation method depends on the nature and the physical state of the material to be sterilised; the influence of the sterilisation method on the properties of the device; and the type of the potential contaminant. In this context, herein we review the influence of ethylene oxide, γ-irradiation, e-beam irradiation, gas plasma, peracetic acid and ethanol on structural, biomechanical, biochemical and biological properties of collagen-based devices. Data to-date demonstrate that chemical approaches are associated with cytotoxicity, whilst physical methods are associated with degradation, subject to the device physical characteristics. Thus, the sterilisation method of choice is device dependent.

The biomechanical effects of 1.0 to 1.2 Mrad of γ irradiation on human bone-patellar tendon-bone allografts

BACKGROUND

Recent data suggest that anterior cruciate ligament (ACL) reconstruction with irradiated allograft tissue may lead to increased failure rates.

HYPOTHESIS

Low-dose (1.0-1.2 Mrad) gamma irradiation does not significantly alter the preimplantation biomechanical properties of bone-patellar tendon-bone (BTB) allografts.

STUDY DESIGN

Controlled laboratory study.

METHODS

Cyclic and failure mechanical properties were evaluated for 20 paired central-third human BTB allografts, with and without 1.0 to 1.2 Mrad of gamma irradiation. Testing included cyclic loading at 0.5 Hz for 100 cycles from 50 to 200 N and failure testing at a strain rate of 10% per second.

RESULTS

Cyclic elongation did not change significantly (P = .151) with irradiation, increasing from a mean ± SD of 9.4 ± 2.1 mm to 11.3 ± 3.4 mm. Cyclic creep strain approached a significant increase with irradiation (1.3% ± 0.8% to 2.6% ± 1.5%; P = .076). Failure testing was not affected with irradiation with regard to maximum load (1680 ± 417 mm to 1494 ± 435 mm), maximum stress (40.8 ± 10.6 MPa to 37.5 ± 15.7 MPa), elongation (7.85 ± 1.35 mm to 8.67 ± 2.05 mm), or strain at maximum stress (0.158 ± 0.03 to 0.175 ± 0.03). Graft stiffness significantly decreased by 20% with irradiation (278 ± 67 N/mm to 221 ± 50 N/mm; P = .035).

CONCLUSION

Low-dose (1.0-1.2 Mrad) gamma irradiation decreases BTB graft stiffness by 20%, but it does not affect other failure or cyclic parameters.

CLINICAL RELEVANCE

Aside from graft stiffness during load to failure testing, low-dose (1.0-1.2 Mrad) gamma irradiation of central-third human BTB allografts is not deleterious to preimplantation biomechanical properties.

Production of a sterilised decellularised tendon allograft for clinical use

Application of a high-level decontamination or sterilisation procedure and cell removal technique to tendon allograft can reduce the concerns of disease transmission, immune reaction, and may improve remodelling of the graft after implantation. The decellularised matrix can also be used as a matrix for tendon tissue engineering. One such sterilisation factor, Peracetic acid (PAA) has the advantage of not producing harmful reaction residues. The aim of this study was to evaluate the effects of PAA treatment and a cell removal procedure on the production of tendon matrix. Human patellar tendons, thawed from frozen were treated respectively as: Group 1, control with no treatment; Group 2, sterilised with PAA (0.1 % (w/v) PAA for 3 h) Group 3, decellularised (incubation successively in hypotonic buffer, 0.1 % (w/v) sodium dodecyl sulphate, and a nuclease solution); Group 4, decellularised and PAA sterilised. Histological analysis showed that no cells were visible after the decellularisation treatment. The integrity of tendon structure was maintained after decellularisation and PAA sterilisation, however, the collagen waveform was slightly loosened. No contact cytotoxicity was found in any of the groups. Determination of de-natured collagen showed no significant increase when compared with the control. This suggested that the decellularisation and sterilisation processing procedures did not compromise the major properties of the tendon. The sterilised, decellularised tendon could be suitable for clinical use.

The effect of sterilization on mechanical properties of soft tissue allografts

One major concern regarding soft tissue allograft use in surgical procedures is the risk of disease transmission. Current techniques of tissue sterilization, such as irradiation have been shown to adversely affect the mechanical properties of soft tissues. Grafts processed using Biocleanse processing (a proprietary technique developed by Regeneration Technologies to sterilize human tissues) will have better biomechanical characteristics than tissues that have been irradiated. Fifteen pairs of cadaveric Achilles tendon allografts were obtained and separated into three groups of 10 each. Three treatment groups were: Biocleanse, Irradiated, and Control (untreated). Each specimen was tested to determine the biomechanical properties of the tissue. Specimens were cyclically preloaded and then loaded to failure in tension. During testing, load, displacement, and optical strain data were captured. Following testing, the cross sectional area of the tendons was determined. Tendons in the control group were found to have a higher extrinsic stiffness (slope of the load-deformation curve, p = .005), have a higher ultimate stress (force/cross sectional area, p = .006) and higher ultimate failure load (p = .003) than irradiated grafts. Biocleanse grafts were also found to be stiffer than irradiated grafts (p = .014) yet were not found to be statistically different from either irradiated or non-irradiated grafts in terms of load to failure. Biocleanse processing seems to be a viable alternative to irradiation for Achilles tendon allografts sterilization in terms of their biomechanical properties.

Sterilization effects on the mechanical properties of human bone-patellar tendon-bone allografts

Novel allograft processing methods are available from tissue banks to decrease disease transmission. This study evaluated the effects of 3 of these techniques on the initial mechanical properties of bone-patellar tendon-bone (BPTB) allografts: (1) aseptic harvest with low-dose radiation processing, (2) BioCleanse Tissue Processing System, and (3) Clearant Process. Ten-mm BPTB allografts were potted in an MTS 858 machine (MTS Systems Corp, Eden Prairie, Minnesota), cycled, and loaded to failure at a strain rate of 100%/s. Data were critically analyzed for graft dimensions and age and sex of donor. The 10th cycle and last cycle stiffness after 1000 cycles were measured at the toe region and at all points. The 2% yield stress (MPa), Young's modulus (MPa), elongation failure (mm), strain fracture (%), ultimate stress (MPa), and toughness (kJ) were measured. Forty-two tendons were tested (15 control, 11 BioCleanse, and 16 Clearant). No statistically significant differences were detected between the groups at their 10th cycle and last cycle stiffness (P>.05). Yield stress ranged from 19 to 28.8 MPa without a statistically significant difference (P>.05). Young's modulus ranged from 178.3 to 213.8 MPa without a statistically significant difference (P>.05). Similarly, elongation to failure, strain to failure, ultimate stress, and toughness showed no statistically significant differences among the 3 groups (P>.05). These processing techniques did not affect the time zero mechanical properties of the BPTB allograft tendons under these testing conditions. Clinical use of allografts should proceed with caution for selected patients.

Primary ACL reconstruction using allograft tissue

While bone-patellar tendon-bone (BPTB) autograft continues to be the "gold standard" and most popular graft choice for primary anterior cruciate ligament (ACL) reconstructions, the use of allograft tissues in ACL reconstruction has steadily increased over the last 2 decades. Advantages of allograft include a lack of donor-site morbidity, unlimited available sizes, shorter operative times, availability of larger grafts, smaller incisions, improved cosmesis, lower incidence of postoperative arthrofibrosis, faster immediate postoperative recovery, and less postoperative pain. Disadvantages include the potential for disease transmission and prolonged graft healing. Presented in this article are 2 techniques used at the authors' institution for primary ACL reconstruction with allograft. With the proper indications, knowledge of graft preparation and handling, and technique, allograft tissues in ACL reconstructions can provide the surgeon with clinical results equal to those of autograft reconstructions.

Calorimetric study of extracellular tissue matrix degradation and instability after gamma irradiation

Native extracellular tissue matrix (ECM) is increasingly used for tissue repair and regeneration. The kinetics of gamma irradiation damage on human dermis ECM was studied by differential scanning calorimetry (DSC). Dermis ECM was irradiated at a low-dose rate of 0.23 kGy h(-1) in order to study the progression of ECM damage as the gamma dose increased from 0 to 32 kGy. The study showed that the effect of gamma irradiation above 2 kGy was predominantly peptide chain scission. As the gamma dose increased, the stability of irradiated ECM decreased further, and multiple ECM domains of different stability were detected. Even a moderate gamma dose (7-12 kGy) could decrease the onset denaturation temperature of ECM to below body temperature. DSC analysis also showed partial and spontaneous protein denaturation in gamma-irradiated, rehydrated ECM at 37 degrees C. In vitro rehydration tests confirmed that a significant fraction of the irradiated ECM disintegrated into minute ECM fragments at 37 degrees C, although the irradiated ECM appeared to be normal at 4 degrees C and room temperature. DSC data were correlated well to effects of gamma irradiation on ECM microstructure, mechanical property and in vitro cell response reported earlier by us. A model was presented to describe the kinetics of gamma-irradiation-induced alterations of tissue ECM properties.

Clinical outcomes of allograft versus autograft in anterior cruciate ligament reconstruction

Anterior cruciate ligament (ACL) injuries are the most common complete ligamentous injury to the knee. The optimal graft should be able to reproduce the anatomy and biomechanics of the ACL, be incorporated rapidly with strong initial fixation, and cause low graft-site morbidity. This article reviews the literature comparing the clinical outcomes following allograft and autograft ACL reconstruction and examines current issues regarding graft choice.

Nonirradiated versus irradiated Achilles allograft: in vivo failure comparison

BACKGROUND

Many studies suggest that gamma irradiation decreases allograft strength in a dose-dependent manner. No study has demonstrated that this decrease in strength translates into higher clinical failures.

HYPOTHESIS

Irradiation of allograft tissue will lead to higher early clinical failure in anterior cruciate ligament (ACL) reconstruction.

STUDY DESIGN

Cohort study; Level of evidence, 3.

METHODS

Medical records were reviewed for 90 consecutive patients who had received Achilles allograft reconstruction for unilateral primary ACL injuries at one institution between July 2001 and June 2002. One half of patients received nonirradiated Achilles allograft and the other half received irradiated Achilles allograft at a dose range of 2.0 to 2.5 Mrad. The ACL allograft reconstructions were performed using the same surgical technique. The rehabilitation program was identical for both groups. All clinical failures were recorded.

RESULTS

At least 6 months' follow-up was available on 42 subjects in the nonirradiated group and 33 subjects in the irradiated group. A significant difference was noted in early failure rates between the groups (P <.01). The nonirradiated group had 1 in 42 (2.4%) catastrophic failure. In the irradiated group, 11 of 33 (33%) Achilles tendon grafts failed.

CONCLUSIONS

Less than satisfactory results led the senior authors to discontinue the use of irradiated allografts in ACL surgery. Continued research into alternatives to gamma irradiation is needed.

Effects of a novel sterilization process on soft tissue mechanical properties for anterior cruciate ligament allografts

BACKGROUND

Allograft anterior cruciate ligament reconstruction provides benefits such as earlier return to activities and less pain, but concerns remain regarding potential infection and biomechanical stability.

HYPOTHESIS

There is no difference in biomechanical properties of soft tissue allografts treated with the Biocleanse tissue sterilization process compared with irradiated and fresh-frozen allografts.

STUDY DESIGN

Controlled laboratory study.

METHODS

Thirty-six tibialis anterior allografts were equally divided between Biocleanse, irradiated, and fresh-frozen groups. Grafts were measured for cross-sectional area and looped over a smooth rod with the free sutured ends of the graft fixed in custom clamps. Specimens were tensioned to 10 N for 2 minutes and then loaded between 50 and 300 N for 1000 cycles followed by a failure test. Data for creep (mm); stiffness (N/mm) at cycles 1, 10, 100, and 1000; failure load (N); and failure stress (MPa) were compared with a one-way analysis of variance (P < .05).

RESULTS

There were no statistically significant differences in creep between groups. Sterilized groups (irradiated = 144.7 +/- 17.7 N/mm and Biocleanse = 146.5 +/- 28.2N/mm) were significantly stiffer during the first cycle than the fresh-frozen group (117.8 +/- 15.7 N/mm, P < .005) without statistically significant differences for subsequent cycles. There were no differences between groups for either failure load (fresh-frozen = 1665 +/- 291.3 N, irradiated = 1671.9 +/- 290.2 N, Biocleanse = 1651.6 +/- 377.4 N) or failure stress.

CONCLUSION

Data for "time-zero" physiologic stiffness and failure loads indicate that the Biocleanse process does not adversely affect the biomechanical properties of the allograft material.

CLINICAL RELEVANCE

This novel sterilization technique may provide surgeons with potential allograft material with similar biomechanical properties to native tissue.

Does irradiation affect the clinical outcome of patellar tendon allograft ACL reconstruction?

The clinical implications of using irradiation to sterilize allograft bone-patellar tendon-bone (BPTB) remains unknown. The purpose of this study was to compare the clinical outcome of anterior cruciate ligament (ACL) reconstruction with irradiated allograft versus autograft BPTB. We hypothesized that patients undergoing ACL reconstruction with irradiated BPTB allograft would have no significant differences in patient-reported and objective parameters compared to those undergoing autograft BPTB reconstruction. Patients who underwent ACL reconstruction with either irradiated allograft or autograft BPTB from 1996 to 2002 were eligible for this study. One hundred and two patients (39 allograft, 63 autograft) met the study criteria and were available for follow-up. The BPTB allografts were obtained from a single tissue bank and were sterilized with 2.5 Mrad of irradiation prior to distribution. Participants completed the International Knee Documentation Committee (IKDC) subjective knee form and returned for physical and radiographic examinations, instrumented measurement of laxity, and functional testing. Patients were evaluated at an average follow-up of 4.2 years (range 1.8-8.4). Those undergoing allograft reconstruction were older (44+/-8.4 vs. 25.3+/-9.3 years, p<0.001) and had a longer median time from injury to surgery (17.1 weeks vs. 9.7 weeks, p=0.04). There was no difference in IKDC Subjective Knee Scores between groups (86.7 allograft vs. 88.0 autograft, p=0.65). The average maximum manual KT-1000 side-to-side difference was 1.3 and 2.2 mm for allograft and autograft, respectively (p=0.04); however, after adjusting for age, this difference was no longer significant. 90.6% of the allograft and 82.8% of the autograft had normal/nearly normal overall IKDC physical examination rating (p=0.37). 66.7% of the allograft and 77.8% of the autograft returned to the same or more strenuous level of sports (p=0.25). Patients undergoing ACL reconstruction with irradiated allograft BPTB had similar clinical outcomes compared to those reconstructed with autograft BPTB. These data suggest that irradiation can be used to sterilize BPTB allograft without adversely affecting clinical outcome.

Meniscal allograft transplantation: long-term clinical results with radiological and magnetic resonance imaging correlations

Long-term data on the clinical outcome and the fate of the meniscus allograft after transplantation are scarce. In this study we present the clinical, radiological and MRI outcome of the meniscus graft and the articular cartilage after 42 meniscus allograft transplantations in 41 patients with a minimum follow-up of 10 years. A total of 27 medial and 15 lateral meniscal allografts were transplanted. Eleven of the medial allograft procedures were associated with a high tibial osteotomy. The patients were evaluated clinically at the time of transplantation and at the final follow-up using the modified HSS scoring system. The knee injury and osteoarthritis outcome score (KOOS) was used as an evaluation tool for patient-related outcome at the final follow-up. Joint space width narrowing and Fairbank changes were radiological outcome parameters, which were available for 32 patients. Femoral and tibial cartilage degeneration, graft extrusion and signal intensity were scored on MRI scans obtained in 17 patients approximately 1 year after transplantation and at the final follow-up (>10 years). For statistical analysis the patients were divided into three groups: lateral meniscal allograft (LMT), medial meniscal allograft transplantation with a high tibial osteotomy (MMT+HTO) and without (MMT). The modified HSS score revealed a significant improvement in pain and function at the final follow-up for all groups. Further analysis also revealed that an MMT+HTO procedure resulted in a greater improvement at the final follow-up when compared to MMT. Nonetheless, the KOOS scores obtained at the final follow-up revealed the presence of substantial disability and symptoms, in addition to a reduced quality of life. Radiographical analysis revealed no further joint space narrowing in 13/32 knees (41%). Fairbank changes remained stable in 9/32 knees (28%). MRI analysis showed no progression of cartilage degeneration in 6/17 knees (35%). An increased signal intensity of the allograft was present, as was partial graft extrusion in the majority of patients at the final follow-up. Seven cases had to be converted to a total knee arthroplasty during the follow-up; the overall failure rate was 18%. Long-term results after viable meniscus allograft transplantation are encouraging in terms of pain relief and improvement of function. Despite this significant improvement, substantial disability and symptoms were present in all investigated subgroups. Progression of further cartilage degeneration or joint space narrowing was absent in a considerable number of cases, indicating a potential chondroprotective effect. Level of evidence is therapeutic study, Level IV and retrospective analysis of prospectively collected data.

Comparison of autologous rectus fascia and cadaveric fascia in pubovaginal sling continence outcomes

OBJECTIVE

The purpose of this study was to compare autologous versus cadaveric grafts in pubovaginal slings.

STUDY DESIGN

Women who had pubovaginal slings from 1994 to 2003 completed history, questionnaires, prolapse staging, and cough stress testing. Failure was defined by recurrent urinary incontinence symptoms and reoperation for stress incontinence. Group differences were evaluated using Student t test or chi-square test. The log-rank test was used to evaluate time to failure between the groups.

RESULTS

Of 303 women enrolled, 153 had autologous and 150 had cadaveric grafts. Regular urine leakage (39.6% vs 28.3%, P = .04) and reoperation for stress incontinence (12.7% vs 3.3%, P = .003) occurred more in the cadaveric versus autologous group, respectively. Adjusting for differing follow-up times, the cadaveric versus autologous group experienced higher rates of urinary incontinence (16 vs 5 per 100 women-years, P < .0001) and higher rates of reoperation (4 vs 1 per 100 women-years, P < .0003).

CONCLUSION

Autologous grafts used in pubovaginal slings have superior continence outcomes compared with cadaveric fascia.

Validating a low dose gamma irradiation process for sterilizing allografts using ISO 11137 method 2B

This paper describes the validation of an allograft sterilization method specifically designed for the processing methods used at AlloSource in Centennial, CO. The methods used for this validation followed ISO Standard 11137, Method 2B. Three hundred allografts, collected from three defined production batches were dosed using a series of five incremental doses, beginning at 1 kGy and increasing by 1 kGy until 5 kGy was achieved. Following sterilization dosing, each allograft test article was analyzed using a sterility test to identify any viable microorganisms. The number of positive sterility samples was used to calculate the verification dose (1.27 kGy), which was then verified by an additional batch of 100 allografts. The results from this validation indicate that sterility (10(-6) SAL) on human allograft tissue using gamma 60Co radiation can be achieved when a dose of at least 9.2 kGy is employed.

Effects of lyophilization on the infectivity of enveloped and non-enveloped viruses in bone tissue

Recently reported qualitative experiments proved that retroviral infectivity is not destroyed by lyophilization performed on systemically infected bone and tendon. The now accomplished quantitative determination of residual infectivity for enveloped and non-enveloped viruses allows a validation of the production process regarding viral safety in freeze-dried bone transplants. The lyophilization effect on the infectivity of two non-enveloped viruses (Maus Elberfeld virus, MEV; Porcine parvovirus, PPV) and one enveloped virus (Vesicular Stomatitis virus, VSV) was examined for virus-spiked bone material in comparison to lyophilized viruses, original virus stock, and air-dried viruses. All experiments were carried out with both cell-free and cell-associated virus. Significant differences were observed regarding the reduction of virus titers (TCID50). Infectivity of VSV was reduced by about 3-4 log10 using lyophilization in presence of bone matrix and of MEV by 6-7 log10, while no substantial reduction in virus titers was observed for PPV. Lyophilization of cell-free or cell-associated virus is not sufficient to inactivate viruses completely. However, lyophilization could have an additive effect in line with other production steps used in the manufacturing process.

Transplantation of viable meniscal allograft. Survivorship analysis and clinical outcome of one hundred cases

BACKGROUND

Few medium-term or long-term reports on meniscal allograft transplantations are available. In this study, we present the results of a survival analysis of the clinical outcomes of our first 100 procedures involving transplantation of viable medial and lateral meniscal allografts performed in ninety-six patients.

METHODS

Thirty-nine medial and sixty-one lateral meniscal allografts were evaluated after a mean of 7.2 years. Survival analysis was based on specific clinical end points, with failure of the allograft defined as moderate occasional or persistent pain or as poor function. An additional survival analysis was performed to assess the results of the sixty-nine procedures that involved isolated use of a viable allograft (twenty of the thirty-nine medial allograft procedures and forty-nine of the sixty-one lateral allograft procedures) and of the thirteen viable medial meniscal allografts that were implanted in combination with a high tibial osteotomy in patients with initial varus malalignment of the lower limb.

RESULTS

Overall, eleven (28%) of the thirty-nine medial allografts and ten (16%) of the sixty-one lateral allografts failed. The mean cumulative survival time (11.6 years) was identical for the medial and lateral allografts. The cumulative survival rates for the medial and lateral allografts at ten years were 74.2% and 69.8%, respectively. The mean cumulative survival time and the cumulative survival rate for the medial allografts used in combination with a high tibial osteotomy were 13.0 years and 83.3% at ten years, respectively.

CONCLUSIONS

Transplantation of a viable meniscal allograft can significantly relieve pain and improve function of the knee joint. Survival analysis showed that this beneficial effect remained in approximately 70% of the patients at ten years. This study identified the need for a prospective study comparing patients with similar symptoms and clinical findings treated with and without a meniscal allograft and followed for a longer period with use of clinical evaluation as well as more objective documentation tools regarding the actual fate of the allograft itself and the articular cartilage.

Altered Mechanical Behavior of Epicardium Under Isothermal Biaxial Loading

Most soft tissues that are treated clinically via heating experience multiaxial states of stress and strain in vivo and are subject to complex constraints during treatment. Remarkably, however, there are no prior data on changes in the multiaxial mechanical behavior of a collagenous tissue subjected to isometric constraints during heating. This paper presents the first biaxial stress-stretch data on a collagenous membrane (epicardium) before and after heating while subjected to various biaxial isometric constraints. It was found that isometric heating does not allow the increase in stiffness at low strains that occurs following isotonic heating. Moreover, increasing the degree of stretch prior to heating increased the thermal stability of the tissue consistent with the concept that mechanical loading primarily affects the activation entropy, not the activation energy.

The biomechanical effects of low-dose irradiation on bone-patellar tendon-bone allografts

BACKGROUND

Despite evidence that low-dose irradiation of 2 Mrad (20 kGy) is not virucidal for patellar tendon allografts and reduces tissue strength, many tissue bank protocols include low-dose irradiation.

HYPOTHESIS

Maintaining tissue mechanical integrity may be particularly relevant toward accelerated rehabilitation of the injured knee, where the cyclic function of patellar tendon allografts is critical.

STUDY DESIGN

Controlled laboratory study.

METHODS

The cyclic and failure mechanical properties of paired bone-patellar tendon-bone allografts, with and without current low-dose irradiation of 20 kGy, were evaluated. Specimens were loaded from 50 N to 250 N for 1000 cycles at 0.5 Hz and subsequently loaded to failure at a strain rate of 100% per second.

RESULTS

After 1000 cycles, grafts elongated 27% more when irradiated than when not (4.4 +/- 1.5 mm vs 3.4 +/- 1.0 mm; P = .03). Failure load averaged 1965 +/- 512 N for irradiated grafts and 2457 +/- 647 N for nonirradiated grafts (P = .007).

CONCLUSIONS

The diminished strength of irradiated grafts may contribute to overt anterior cruciate ligament graft failure, and the increase in cyclic elongation may also be detrimental to graft function.

CLINICAL RELEVANCE

These results suggest that one should consider the use of nonirradiated allografts as an alternative to irradiated grafts in anterior cruciate ligament reconstruction.

Lyophilization does not inactivate infectious retrovirus in systemically infected bone and tendon allografts

BACKGROUND

A review of multiple transplantations of human immunodeficiency virus-infected musculoskeletal allografts found that recipients of lyophilized (freeze-dried) bone or tendon from an infected donor all tested negative for human immunodeficiency virus. The finding that 75% of the recipients of fresh-frozen bone from the same donor contracted human immunodeficiency virus has led to speculation that freeze-drying may render retroviral-infected musculoskeletal allografts noninfectious.

HYPOTHESIS

Lyophilization does not inactivate retrovirus in systemically infected bone and tendon.

STUDY DESIGN

Controlled laboratory study.

METHODS

Tendons and cortical bone segments from cats systemically infected with feline leukemia virus were used in this study. Feline embryonic fibroblast cells were cultured in the presence of fresh-frozen or freeze-dried cortical bone or tendon segments. At each passage, feline leukemia virus p27 antigen was measured in media by enzyme-linked immunosorbent assay, and feline leukemia virus (pro)viral nucleic acids were quantified by real-time quantitative polymerase chain reaction in the DNA extracted from cells.

RESULTS

Enzyme-linked immunosorbent assay results and quantitative polymerase chain reaction results demonstrated retroviral antigen and proviral DNA in all cultured cell replicates after exposure to fresh-frozen or freeze-dried bones or tendons.

CONCLUSION

Freeze-drying (lyophilization) of retroviral-infected cortical bone and tendon does not inactivate retrovirus.

CLINICAL RELEVANCE

These results conclusively demonstrate that freeze-drying should not be relied on to inactivate infectious retrovirus in systemically infected musculoskeletal allografts.

Characterization of the mechanical properties of demineralized bone

Demineralized bone has had limited use as a medical implant, although, recently Jackson et al. (Am J Sports Med 1996;24(4):405-414) examined using demineralized bone as an anterior cruciate ligament (ACL) replacement and found that within 1 year the demineralized bone had remodeled from a haversian system (bone-like) into a ligament-like structure. Little research has been done to characterize the mechanical properties of demineralized bone and examine its potential as a ligament replacement. This research examined the mechanical properties of demineralized bone as a function of acid saturation time and methods of processing and sterilization using specimens from various anatomic locations. Bone specimens were manufactured from the tibia, metatarsus, femur, radius, and humerus of bovine bone. Tensile strength, strain, and modulus of elasticity were examined for various acid saturation times and sterilization treatments. Additionally, viscoelastic properties, creep and stress relaxation, and fatigue properties were examined. The findings indicate that completely demineralized bone has mechanical properties similar to the ACL and, therefore, has increased likelihood for success in ACL reconstruction surgery.

Donor cell survival and repopulation after intraarticular transplantation of tendon and ligament allografts

The specific cells within ligaments and tendons are important to maintain the unique structural and material properties of these tissues. The use of tendon and ligament allografts with living cells for ligament reconstruction would be desirable assuming that these cells would survive after transplantation and continue to function. We assessed the fate of donor cells in fresh allografts of the patellar and anterior cruciate ligaments after transplantation. The cells in these allografts used to reconstruct the anterior cruciate ligament did not survive. This was demonstrated using a DNA probe technique that clearly distinguished donor cells from host cells in the Spanish goat model. The donor cells were replaced by host cells in a rapid manner. The host cells that repopulated the allografts assumed the histologic similarity to the fibroblasts they replace. Simultaneous full-thickness skin transplants in the same animals were not rejected during the interval of rapid loss of donor DNA from the allografts. The absence of rejection of the skin grafts at the one-week interval suggests that no pre-existing antibody associated with an immune reaction was responsible for the rapid loss of DNA in the allografts. The clinical basis for utilizing intra-articular allografts with living donor cells needs further justification to account for their increased expense, more complicated surgical logistics, and higher potential risk of disease transmission.

Intermediate term failure of pubovaginal slings using cadaveric fascia lata: a case series

PURPOSE

The pubovaginal sling procedure using autologous fascia has become the gold standard for treating intrinsic sphincter deficiency and stress urinary incontinence in women. A recent modification has been the use of cadaveric fascia as the sling material. We recently reported similar results for cadaveric fascia lata in 121 women and autologous fascia lata in 46 at a mean followup of 12 months. We have now identified 8 patients who experienced intermediate term failure at 4 to 13 months using cadaveric fascia lata.

MATERIALS AND METHODS

We retrospectively reviewed the records of 8 of 121 patients who received a pubovaginal sling using cadaveric fascia lata between February 1997 and June 1999 and had recurrent stress incontinence after 4 to 13 months. We reviewed the type of fascia, surgical technique, preoperative and postoperative urodynamics, surgical history and medical co-morbidities.

RESULTS

We identified 8 patients who underwent a pubovaginal sling using cadaveric fascia lata and had recurrent stress urinary incontinence at a mean of 6.5 months (range 4 to 13) after the procedure. Fresh frozen fascia from a local tissue bank was used and the surgical technique was identical in all cases. Postoperatively urodynamics confirmed recurrent intrinsic sphincter deficiency. Previous incontinence surgery had been done in 7 of the 8 patients. Patient co-morbidities included neurological disease, diabetes mellitus, previous pelvic irradiation and previous pelvic surgery.

CONCLUSIONS

We identified a higher than expected intermediate term failure rate using fresh frozen cadaveric fascia lata. This risk must be weighed against the advantages of decreased operative time and patient morbidity. Longer followup and larger numbers are necessary to determine how much of a problem exists and what patient characteristics are relevant when selecting cadaveric grafts.

Donor fascia in urogynaecological procedures: a canine model

OBJECTIVE

To explore the in vivo characteristics of donor fascia used in urogynaecological procedures, in a canine model.

MATERIALS AND METHODS

Two experiments were conducted. In the first, donor fascia grafts were obtained from 12 dogs, the grafts freeze-dried and half were irradiated. The grafts were used for sacrocolpopexy and suburethral slings in each of five dogs. The dogs were killed at 2, 6 and 12 weeks after graft implantation, the grafts retrieved and assessed using tensilometry. In the second experiment, unirradiated sacrocolpopexy grafts were implanted in eight dogs; four grafts were placed under no tension and four under moderate tension. At 8 weeks, the grafts were retrieved and assessed by tensilometry. Measures of strength in both experiments included the ultimate tensile strength, ultimate strain and stiffness. All measures were compared using Kruskal-Wallis nonparametric tests in both studies.

RESULTS

In the first experiment, a significant minority (23%) of grafts had complete loss of strength. Measures of graft strength did not vary when analysed according to donor dog, host dog, history of graft irradiation, duration of implantation or location of graft. In the second experiment, grafts placed under no tension tended to have lower tensile strength (chi2(1) = 3.125, P = 0.077), lower stiffness (chi2(1) = 3.125, P = 0.077) and lower ultimate strain (chi 2(1) = 3.182, P = 0.074).

CONCLUSION

Graft irradiation as an isolated variable did not predispose grafts to failure in vivo. Biomechanical factors at the implantation site are likely to play a critical role in determining ultimate graft strength.

Histologic analysis of the tibial bone tunnel after anterior cruciate ligament reconstruction using solvent-dried and gamma-irradiated fascia lata allograft

Anterior cruciate ligament (ACL) reconstruction using free tendon graft requires biologic fixation in the bone tunnel. This report describes the intratunnel histology retrieved from a 47-year-old woman who underwent high tibial osteotomy 17 months after ACL reconstruction using a hybrid graft (a solvent-dried and gamma-irradiated fascia lata allograft as a core wrapped with iliotibial autograft). The patient underwent revision because of pain resulting from osteoarthrotic change, and the graft appeared to be taut and healthy on second-look arthroscopy. The sample was taken from the site of the metaphyseal osteotomy site. Histologic examination of the samples was performed with a light microscope (H&E and Masson trichrome stain). Biologic fixation of the graft to the bone was evident from observation of collagen fiber continuities, resembling Sharpey fibers. Integration of the autograft and allograft seemed to have occurred; the junction between the allograft and the autograft could not be determined. However, there was a difference in ligamentization depending on location. In the peripheral region of the graft (the autograft region), the collagen fibers showed a distinct crimped pattern; the fibroblasts were arranged regularly along the major axis of the collagen fiber bundle. In the central part of the graft (the allograft region), remodeling seemed to be delayed. There were acellular, bubbly or myxoid degeneration areas in which the fiber bundles were less oriented and there was increased vascularity.

Functional failure of fascia lata allografts

OBJECTIVES

Fascia lata allografts are commonly used in urogynecologic procedures. Functional failure of several grafts has occurred, and such failure has been recognized as a materials problem in 12 patients.

STUDY DESIGN

Twelve patients with failure of an initial urogynecologic procedure performed with irradiated and freeze-dried donor fascia lata grafts underwent reoperation. Portions of the implanted fascia lata grafts could be retrieved in 7 cases. Graft specimens underwent histologic processing followed by hematoxylin and eosin staining.

RESULTS

Histopathologic analyses of the retrieved material demonstrated several ongoing processes in the failed grafts. A few grafts showed areas of ideal remodeling. Most grafts, however, showed areas of disorganized remodeling and areas of graft degeneration. Evidence of immune reaction to the graft was observed in some cases.

CONCLUSION

The high materials failure rate associated with the use of irradiated and freeze-dried donor fascia lata grafts suggests that such tissue should not be used for urogynecologic procedures.

Mechanical properties of tendons: changes with sterilization and preservation

Tendon allografts are commonly used to replace damaged anterior cruciate ligaments (ACL). Some of the sterilization and preservation techniques used by tissue banks with tendon allografts are thought to impair the mechanical properties of graft tissues. The tensile mechanical properties of porcine toe extensor tendons were measured using a dynamic testing machine following either freezing, freeze-drying, freezing then irradiation at 25 kGy (2.5 MRad), freeze-drying then irradiation, or freeze-drying then ethylene oxide gas sterilization. There was a small but significant difference in Young's modulus between the frozen group (0.88 GPa + 0.09 SD) and both the fresh group (0.98 GPa 1 0.12 SD) and the frozen irradiated group (0.97 GPa 1 0.08 SD). No values of Young's modulus were obtained for the freeze-dried irradiated tendons. The ultimate tensile stress (UTS) of the freeze-dried irradiated group (4.7 MPa 1 4.8 SD) was significantly different from both the fresh and the frozen irradiated groups, being reduced by approximately 90 percent. There were no significant changes in UTS or Young's modulus between any of the other groups. If irradiation is to be used to sterilize a tendon replacement for an ACL it must take place after freeze-drying to maintain mechanical properties.

Dose-dependent response of gamma irradiation on mechanical properties and related biochemical composition of goat bone-patellar tendon-bone allografts

We studied the effects of gamma irradiation on the dimensions, mechanical and material properties, and mature hydroxypyridinium crosslink density of collagen in goat patellar tendon-bone specimens. Left and right patellar tendon-bone units were removed from 10 adult female goats and were bisected longitudinally. Each tendon half was frozen, and then the left halves were exposed to 4, 6, or 8 Mrad (40,000, 60,000, or 80,000 Gy) of gamma irradiation. The contralateral tendon halves served as controls (no irradiation). Each specimen then was loaded to failure in tension, and its soft-tissue midsubstance was processed to measure collagen content and hydroxypyridinium crosslink density. Dose-dependent reductions in the mechanical properties were found, including 46% (p < 0.01) and 18% (p < 0.05) reductions in maximum force and stiffness, respectively, at 4 Mrad. Similar reductions were noted in material properties, including 37% (p < 0.005) and 8% (p > 0.05) reductions in maximum stress and modulus, respectively, at 4 Mrad. These results are consistent with our previous report involving 2 and 3 Mrad (20,000 and 30,000 Gy) of exposure. We also found significant decreases in hydroxypyridinium crosslink density with 6 Mrad of irradiation (p < 0.05). However, since only one biomechanical parameter (modulus) correlated significantly with only one biochemical measure (hydroxypyridinium crosslink density) (p < 0.05), other possible mechanisms also are being explored to more fully explain these dose-dependent changes.