Categorization of Different Clamp Types Used For The Endurance Test of Human Grafts – A Systematic Review


 BackgroundThe 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. Five major scientific literature databases (Web of Science, Science Direct, Scopus, PloS ONE, Hindawi) and additional sources were used. ResultsThe objective of this systematic literature review is to investigate and categorize existing clamps used in the determination of the biomechanical properties (maximum load, maximum strength, modulus of elasticity, ultimate strain, stiffness) of tendons. Studies had to use an endurance test of tendons and clamps in detail. The database search and additional sources resulted in 274 records. 216 records eliminated during the screening for various reasons. The number of articles used in the final synthesis was 58. Search for publications dated between 1991 and Oct 31st, 2020. A variety of clamps for use during the endurance test of tendons were identified and categorized according to the temperature used during the measurement. The clamps are divided into three groups: room temperature, cooled or heated clamps.ConclusionsOn 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 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.


Background
The use of tendon allografts for orthopedic repair has gained wide acceptance in recent years, most notably in anterior cruciate tendon reconstruction [1]- [3]. Multiple studies support the use of tendon allografts and the bene ts of its use are well accepted and understood [2]; [4]- [7]. Speci cally, these bene ts include decreased surgical time, decreased surgical morbidity and unaltered mechanics secondary to harvesting. Furthermore, animal and human studies have shown that soft tissue allografts are statistically similar to autografts on a histological and biomechanical basis [8]- [10].
Anterior cruciate ligament (ACL) reconstruction is a common procedure in orthopedic practice. One of the most important decisions for the surgeon to make is the right choice of graft. Although autografts have proven to be capable and showed good clinical outcomes, graft harvest can cause persistent pain at the harvest site and a limited range of motion [11]- [14]. Therefore, allograft use has signi cantly increased in the last decades. Since it eliminates donor-site morbidity, and albeit its use is associated with higher costs, it remains a viable option, especially in revision cases. In order to ensure that there is a minimal biomechanical difference between the ACL and the graft, the biomechanical properties need to be tested so that we can choose which tendons can be good substitutes [7], [15]. The tendons are subjected to tensile testing, which can be static or dynamic. From these we get a force-elongation diagram, which can be calculated based on, for example the Young's modulus of elasticity [16]- [18].
The purpose of a clamp is a proper xation technique for allograft endurance tests, and adapt it to be compatible for the loading machine [10], [19]. The main problem with tendon clamps is that it is hard to maintain the high pressure needed to provide enough friction force between the tendon and the clamp to resist a large tensile load, and at the same time to reduce the cutting effect of the clamp (Shi D, Wang D, Wang C, Liu A: A novel, inexpensive and easy to use tendon clamp for in vitro biomechanical testing. Med. Eng. Phys. 34, 516-520 (2012)) [7], [20]- [23].
Various clamps have been developed for the assessment of the endurance test. These clamps are usually speci c for measurement methods, thus, the results of the measurement methods are di cult to compare [1], [8], [11]- [15], [24]- [25].

Aim of study
The literature of the effect of the sterilization method on the material properties of the tendon is well researched and discussed [72][73][74][75][76]. Nevertheless, there are no systematic reviews on the subject that would provide guidance on the clamps used for the measurements. The objective of this study is to investigate and categorize existing clamps used in the determination of the biomechanical properties. conjunction included: tendon, allograft tendon, biomechanical pull-out testing, mechanical properties, and additional synonyms of these terms. Search terms were modi ed according to the syntax requirements of each database. (Table 1) As an example, we highlight the electronic search for the Science Direct database, which is shown below. These terms were added into the Advanced search option, using the 'All elds' option: (

Screening materials
After removing the duplicates, the identi ed publications were screened based on their title and their abstracts. Publications of exclusively theoretical work or Materials of purely theoretical work or with topics deviating from the aim of study were excluded.

Eligibility check of materials
In order to con rm eligibility for the study, the reviewers de ned the inclusion and exclusion criterias. The publications had to meet each inclusion criterion to be incorporated in the nal synthesis (Table 1). If a study failed to meet any inclusion criteria, or met an exclusion criterion, it was excluded. The criteria were carefully chosen to ensure a quality assessment of the material to a certain extent, i.e., the methods used had to be well communicated and the evaluation of measurement results had to be objective.

Data extraction
In accordance with the focus of this review, the nal synthesis of the collected types of clamps included extracted relevant information on the evaluation of mechanical properties. The collected information from the articles included a) author and date, b) name of clamp, c) type preloading (dynamic and static) d) type of endurance test, e) type of tendon, f) type of clamps, and g) measured and calculated parameters. Studies which only included a tendon measurement method without any type of clamp.

Description of tendon and endurance test and clamp
Studies with detailed descriptions of the tendon and endurance test and clamp and the experimental process that was followed.
Studies without detail or incomplete descriptions of the clamp and endurance test and the experimental process that was followed.

Assessment of results
Studies with objective result assessment based on measurable parameters.
Studies with subjective scoring/assessment of results, not (entirely) based on measurable parameters.

Results
The search of the database source gave 366 results (Fig. 1). Removing duplications and unavailable abstracts, 274 literatures remained. When screening the titles and the abstracts, an additional 56 records were excluded, due to not tting the scope. The remaining 218 articles have been read in their entirety. Of these studies, 160 were excluded with justi cations of not meeting the eligibility criteria, and 6 publications were literature review articles related to sterilization methods. These review articles had a different scope from our current study. The number of articles included in the nal synthesis was 58 (n = 58).
Articles have been excluded for the following reasons. Many studies included a task that was outside the scope of our study. If multiple studies used equivalent clamp type with the same mechanical test, the latest study was included (63 articles) [77]. Several articles did not describe a detailed measurement setting and type of clamp used and were therefore excluded (84 articles) [77]. The evaluation of few articles was subjective (6 articles), and there were also clinical case (7 articles).

Room temperature clamps
Measuring at room temperature is a quick test because it requires the least amount of preparation as there is no need for dry ice, liquid nitrogen, heating, etc. Su cient force is applied during the measurement to prevent tendon slippage, but no transverse tension is created during the capture of the tissues, which yields invalid results.
One of the room temperature clamps is the U-shaped frame (Fig. 2), which can be used for the measurement of the tendon together with the bones. The bone was secured in custom-designed xation frame with screws. The precision of the drill was ensured by an outer polyethylene mold. [47], [50] In a special case, the bone is inserted into a separately moulded block while the free tendon is pulled by the clamp. The solution allows to investigate the relationship between bone and tendons. (Fig. 3).

[67]
Some researchers used custom-designed clamps, where the bone block was secured with either interface polymethylmethacrylate-PMMA or polyurethane [36] (Fig. 4). A solution can also be applied where the natural tendon is xedby a bone block at one end and by a pneumatic clamp to prevent slippage [42] (Fig.   5). Here, it is particularly important to prevent slippage between the clamp and the tendon, therefore the surface is scratched by sand spraying in several cases.
A special case is when wedge-grip clamp use involves silicone or some kind of arti cial resin at both ends to ensure the connection between clamp and tendon [34], [38], [68] (Fig. 6).
Several articles use polymer-encapsulated aluminum clamps to achieve better adhesion between the tendon and the clamp (Fig. 7). One of the advantages of the system is that it can be expanded by strain gauges [40], [48], [59], [60].

Cooled clamps
A basic condition for an appropriate measurement method is to prevent the tendon from slipping out of the clamp, therefore various methods are applied for establishing an adequate connection. One of the reasons for slippage is that the tendon is damp. Therefore it is expedient to continuously freeze the surroundings of the clamp, which naturally scratches the surface. It is expedient to use dry ice or liquid nitrogen for freezing. A disadvantage is that it is not easy to place the freezing substance in the surroundings of the clamp [27]- [29], [32], However, one of the simplest solutions is that the clams or clamp inserts can be cooled separately before measuring, regardless of the tensile machine. In this case, they should be placed in a deep-freezer for at least 24 hours. The tendon is placed into the cooled clamp; the grips squeezing the tendon can be xed in one or two rows (Fig. 8) [32], [69].
One of the major advantages of cooled clamp use is that factory clamps can be used; it is required to ensure continuous and adequate cooling by placing a chamber of appropriate size to the proper place [28], [65], (Fig. 9). The custom-designed screwed clamp can be made of aluminum plate with a dry ice chamber, where the dry ice can be replaced continuously for ensuring continuous cooling. (Fig. 10) [56].

Heated clamps
Measurements conducted in an environment of room temperature, using room-temperature or sooled clamps, greatly differ from the temperature of the natural surroundings of tendons (37°C). Environment temperature presumably affects mechanical properties: more accurate results are yielded if tests are conducted at body temperature. In order to ensure this, it is expedient to use heated clamps [37], [57], [70], [71]. A disadvantage is that, contrary to cooled clamps, the connection between the clamps and the tendon is not improved, but it is also important that it is not deteriorated, either. In general, it is expedient to use a heated liquid for warming [57], [70]; heat insulation should be provided around both the clamps and the component to be examined (Fig. 11) [37]. The measurement can also be performed in a bath lled with heated liquid, which is continuously monitored. It is a basic requirement that the heated liquid should not deteriorate the properties of the tendon (Fig. 12) [57]. The circulation of the liquid simulates the behavior of the blood. (Fig. 13) [70].

Discussion
The clamp should be designed to prevent the slippage of the tendon from the clamp, but the clamping force should not change the tensile state of the tendon to be examined. The objective 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 data collected from the articles a) author and date, b) name of clamp, c) type preloading (dynamic and static) d) type of endurance test, e) type of tendon, f) type of clamps, and g) measured and calculated parameters. The data are summarized in Table 2.
The metal U-shaped frame (Fig. 2) allows for bone-tendon strength to be studied [47,50]. This clamp also ensures stability of the tendon, not letting it slip out. Because the tendon is clamped tightly, tissue texture can be damaged. In several cases, capture is performed using natural bones (Figs. 2 and 3) or arti cial blocks (bone cement, silicone, arti cial resin) (Fig. 4) [36 , 42]. Natural tendon ends can be captured by custom -generally pneumatic -clamps (Fig. 5, 7), or embedded in arti cial material (Fig. 6)  . A great advantage of frozen clamps is that surfaces are naturally made coarse by freezing, which assists in establishing an appropriate connection between the clamp and the tendon. The solution is relatively simple: the tendon can be fastened by two metal grips xed by screws. The rst type of cooling is freezing the clamp before testing (Fig. 8). This requires a freezer that can freeze at -70ºC to -80ºC. The frozen clamp also has to be attached to the machine. The tendon takes on the clamp's temperature over time.
The clamps shown in Figs. 9 and 10 use a dry ice container for cooling. The dry ice container allows for the tendon and the clamp to be cooled at the same time. Dry ice needs to be added during measurements, as it  (Fig. 13) can also imitate a human body environment (temperature, blood circulation). [70]. Heated clamps have the same disadvantages as room temperature clamps; the tendon can easily slip out, can be damaged by the clamp, or tear at the point of xation [37], [57], [70]. This study provides an overview of clamps and does not represent the modernity of any method.

Conclusion
The objective 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. a) author and date, b) name of clamp, c) type preloading (dynamic and static) d) type of endurance test, e) type of tendon, f) type of clamps, and g) measured and calculated parameters.
On the basis of systematic literature review, the mechanical properties determined for using with cooled clamps proved to be more reliable than room temperature and heated clamps. The main advantage is that there is no limit to the type and length of the tendon. The dry-ice clamp instead of liquid nitrogen is recommended for the clamping of tendons, because dry ice is cheaper to acquire than liquid nitrogen. Liquid nitrogen evaporates faster than dry ice. It is also easier to work with dry ice, permission is not needed for use, and it does not need to be stored in a container. In similar quantities, liquid nitrogen is colder than dry ice, which can harden the whole tendon, not just at the point of xation. The research reported in this paper and carried out at Budapest University of Technology and Economics has been supported by the NRDI Fund TKP2020 NC, (Grant No. BME-NC) based on the charter of bolster issued by the NRDI O ce under the auspices of the Ministry for Innovation and Technology, Hungary.