## 6 Discussion  
In this study, an innovative clamp design based on the metric thread profile was proposed and manufactured using fused deposition modeling (FDM) as a 3D printing technique for the future determination of the biomechanical properties of the human gracilis and quadriceps tendons. Unlike those produced by conventional material processing, the clamps that were 3D printed were very user-friendly in terms of material, time and production cost, supervising, office implementation, reusability and biofriendliness [17]. The idea for the clamping technique to use bolts for clamps connection was taken from [5]. The average 3D printing time was 2 h and 30 min per clamp. The infill was set to 100% with a 0.2 mm layer height. The consumption of PLA material was 13 g (or 4.46 m) for a clamp with a rounded part and 11 g (or 3.36 m) for a clamp with the other part. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**The Type of the Specim**} & \multirow{2}{*}{\(n\)**} & \multirow{2}{*}{**Rejected**} & \multicolumn{2}{c}{**Ultimate Tensile Strength,**} & \multirow{2}{*}{**Stroke Strain, \%**} & \multicolumn{1}{c}{\multirow{2}{*}{**Structural Modulus,**}} \\  & & & & **MPa** \\ \hline Gracilis & 8 & - & 82.57 (8.41) & 21.41 (2.15) & 458.81 (93.89) \\ Quadriceps & 8 & - & 40.71 (7.75) & 14.75 (1.17) & 387.44 (71.88) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The biomechanical properties of the human tendons. Figure 13: Clamping area and specimen failure. The printing temperature was set to 210 \({}^{\circ}\)C and 60 \({}^{\circ}\)C for the printing base. During printing, no issues were found. A good clamping behavior was noticed during tests and no damage to the clamps was found. By printing the clamps for each prepared specimen, the time required for machine setup and calibration was significantly reduced because we had specimens tightened in the clamps in advance, as presented in [17; 18]. Unlike those in other studies [3; 6; 7; 16; 17; 18], this design is based on a well-known part from the field of mechanical engineering. With the lack of standardization in the determination of tissue biomechanical properties, the approach presented in this study could contribute to solving that issue. As much as is possible, the intention of this study was to achieve the same bone-tendon-muscle complex as exists in the human body. We achieved the clamp-tendon-clamp complex with the flaring sides of the tendon tissue (which is similar to the bone-tendon-muscle complex found in the human body and is reported in [17; 18] as well). The most important outcome for us has been to mimic the anatomy of the human body, since our future experiments will be dealing with determining the biomechanical properties of human gracilis and quadriceps tendons and their correlation with the medial patellofemoral ligament (MPFL) to discover which of these tendons can serve as the best MPFL replacement in the case of patella dislocation. By searching the literature and relevant scientific bases, we did not find any such replacement and we are still looking for the best testing protocol and the most suitable equipment. We did not reach any conclusion through statistical analysis. The next important issues to address are tendon slipping and rupturing. We solved these problems by intersecting the metric thread profile and obtained a pattern full of truncated pyramids, very similar to the one reported in [17] with sharp pyramids, but without the associated slipping. The recommendation for conducting these tests on the thin/thick specimen is to use the flat/sharp pyramid patterns [18]. We found that a truncated pyramid pattern was the most suitable. In the case of porcine tendons, the stress concentrations occurred during the clamping procedure (caused by excessive pressure and/or clamp geometry), causing misalignment of the collagen fibers within the tissue towards the direction of the tensile force. Such stress inequalities cause tendon rupturing and slipping (Figure 6).