The two factors that can affect the mechanical properties of a tendon are the type of tendon and the sterilization methods. Tendon grafts show to be promising for transplants. It is important to sterilize the tendons to ensure no bacteria is transmitted that could cause infections. We reviewed peroneus longus, peroneus brevis, tibialis anterior, tibialis posterior, patellar tendon, human flexor digitorum superficial, Achilles, bone-patellar tendon-bone, semitendinosus, semitendinosus + gracilis, fascia latta types of tendon and gamma irradiation, SCCO2, ethylene oxide, BioCleanse, E-Beam, PE-R, hydrogen peroxide, chlorhexidine sterilization methods.
Gamma radiation is the most common method of sterilization (Table 3). The 14.8–28.5 kGy (1.48–2.85 Mrad) radiation proved best because this radiation rate killed bacteria. It also kept the failure load (e.g., failure load of Achilles sterilized at18-24 kGy is 3572.54 N vs. failure load of Achilles sterilized at 38–46 kGy is 392.01 N, tibialis anterior sterilized at 18–24 kGy: 3107.76N, failure load of tibialis anterior sterilized at 38–46 kGy: 2678.96N. [39–40]. Comparing with Sect. 3.1.1 (non-sterilized tendon), the results showed that failure load improved (Achilles: 3032 N, tibialis anterior: 2427.3 N). It can be observed that the failure load’s tendency of quadriceps was different (18–24 kGy: 3184.32 N, 38–46 kGy: 3464.35 N). At 38–46 kGy (3.8–4.6 Mrad), the radiation killed viruses as well, but the biomechanical properties deteriorate [Tables 2 and 3.]. Viruses can be tested for in labs prior to a transplant [28, 31–41].
The supercritical carbon dioxide (SCCO2) sterilization method has been discussed only by Baldini et al. [27]. Baldini et al. discussed anterior or posterior tibial tendon treated with SCCO2. Based on the results, there was a negligible improvement in the failure load and Young’s modulus of elasticity of the tendons (non-sterilized: 2427.3 N, 75.4 MPa vs. SCCO2: 2450.3 N, 91.9 MPa) [27; Table 2. and 3.]. The origin of samples and measurement methods are different; the result could not be compared to the results of different sterilization methods. Therefore, we cannot accurately conclude.
The Ethylene oxide procedure does not yield better mechanical properties than other methods [42; Table 5.]. Tendons sterilized with this method generate connective tissue growth. During this tissue growth, cancerous cells can develop. Bechtold et al. [46] examined the patellar tendon and its Young’s modulus of elasticity. After sterilization, Young’s modulus of the tendon deteriorated slightly (non-sterilized: 158.0 MPa, sterilized: 143.0 MPa). This sterilization method is also not comparable to other sterilization methods, but the method impaired Young's modulus of the patellar tendon [42].
The BioCleanse wash poses the same issues as the hydrogen peroxide and chlorhexidine method [43–45, 54; Table 6.]. Since this is a wash, only the outside of the tendon is sterilized. Dangerous bacteria might still be found in the tendon. The BioCleanse solution is not as effective as gamma irradiation. The sterilization method is addressed in three studies [Table 6.]. Conrad et al. [8] examined the failure load and Young’s modulus of the Achilles. Non-sterilized BioCleanse and gamma irradiation sterilization methods are investigated. In the case of failure load, BioCleanse shows worse results than non-sterilized, but even worse are the results of the gamma irradiation sterilization method (non-sterilized: 3032 N, BioCleanse: 2472 N, gamma irradiation: 1972 N). The values of the modulus of elasticity showed a similar tendency (292.04-154.06-129.48 MPa) [8]. Colaco et al. [43] and Schimizzi et al. [44], the tibialis anterior was examined. Colaco et al. [43] compared different BioCleanse mixtures. However, no comparison was made with other sterilization methods. Schimizzi et al. [44] compared the method used BioCleanse to non-sterilized and gamma irradiation methods. Based on the results, BioCleanse did not achieve significantly worse results (failure load and Young’s modulus) compared to the other two groups (non-sterilized: 1665 N and 19.9 MPa, gamma-irradiated: 1671.9 N and 22.6 MPa, BioCleanse: 1651.6 N and 19.7 MPa) [43–45].
The effect of sterilization by Electron Beam (E-beam) was analyzed in three articles [30, 46–47, Table 7]. Elenes et al.[46] compared failure load of the tibialis anterior with non-sterilized, gamma irradiation, and E-beam sterilization methods. It uses two types of E-beam doses, one low (9.2–12.2 kGy) and one high (17.1–21.0 kGy). Based on the maximum load results [47], the low-dose E-beam performed significantly better compared to the other three categories (non-sterilized: 606.73 N, E-beam low: 876.38 N, E-beam high: 660.24 N, gamma: 597.09 N). In comparison to the values of modulus of elasticity, it can be observed that the values decrease for all three sterilization methods compared to non-sterilized ones. Of the three sterilization methods, the smallest deterioration is shown by E-beam low (non-sterilized: 213.13 MPa, E-beam low: 206.71 MPa, E-beam high: 152.64 MPa, gamma: 179.02 MPa) [46]. Wei et al. [30] compared three types of E-beam doses to the non-sterilized group. Based on the results, all three doses of E-beam performed worse than the non-sterilized human flexor digitorum superficialis (fresh: 402.5 N and 400.5 MPa, 50 kGy: 282.3 N and 291.6 MPa, 50 kGy (Fr.): 360.9 N and 354.3 MPa, 50 kGy (Fr. + As.): 390.6 N and 390.4 MPa) [30]. Hoburg et al. [48] examined the E-beam in the most recent of his studies. Bone-patellar tendon-bone treated with two doses of E-beam radiation (25 and 34 kGy) was compared with two different doses of gamma radiation (25 and 34 kGy) and non-sterilized. Based on the results, all four sterilizations significantly worsened the maximum force values; however, the values of E-beam are better than the values of the same dose of gamma radiation (non-sterilized: 1741 N, gamma 25 kGy: 1009 N, E-beam 25 kGy: 1177 N, gamma 34 kGy: 1073 N, E-beam 34 kGy: 1139 N) [48]. Zhou et al. [50] examined human flexor tendon sterilized with Peracetic acid - ethanol in combination with low-dose gamma irradiation (PE-R). The results showed a worse failure load compared to the non-sterilized tendon (non-sterilized: 360.01 N and 221.55 MPa, PE-R: 306.96 N, and 235.78 MPa). PE-R is a hybrid method consisting of immersing the tendon in a chemical solution and then using gamma irradiation. This method yielded a worse failure load than the Electron beam sterilization and gamma sterilization. [50].
The hydrogen peroxide method is just a wash; the inside of the tendon is not properly sterilized, which can lead to infections. Gardner et al. [51] examined the effect of hydrogen peroxide on several different types of tendons. The ultimate tensile strength was not compared to other sterilization methods or non-sterilized tendons [Table 9.]. Comparing the results of the ultimate tensile strength of previous studies with the results, hydrogen peroxide is not a suitable method for sterilizing tendons [51].
The Chlorhexidine chemical solution is also a wash, the same as the hydrogen peroxide method [52]. While it does pose better results of failure load for the tendon, the inside of the tendon is not properly sterilized. According to the method of Sobel et al. [52] showed that the value of the failure load increases as a result of sterilization (non-sterilized: 1876 N, Chlorhexidine: 2219 N) [Table 10.].
4.1. Limitation
This review analyzed and classified multiple existing sterilization methods used in determining the biomechanical properties of tendons. Since this review looked at different techniques, a meta-analysis could not be performed. Limitations can also surface because only three major scientific databases were utilized in the search for papers. Because of the full range of this research, it only provides an overview of multiple sterilization processes. It does not discuss the advancement of any single process.