Ensaio Uniaxial de Tração dos Tendões dos Músculos Grácil e Semitendinoso Humanos Uniaxial Traction Test on Human Gracilis and Semitendinous Tendon

The use of isolated human tendons as well as in associated forms on knee reconstrution has become na usual practice. The literature reveals that the use of implants of patellar tendon as well as the double semitendinous tendon associated to the double gracilis tendon exhibits different post surgical evolution related to the minimal extension loss, anterior-posterior displacement (KT-1000 artrometer), as well as to the sport activities. This research work aim to analyze the mechanical behavior of human tendons (gracilis and semitendinous) subject to uniaxial traction to failure. The mechanical parameters considered includes: stress at failure (MPa), strain energy at failure (N.mm) and strain rate (mm/mm). Results induced to the following conclusions: - tendon of semitendinous muscle is more resistent than the tendon gracilis muscle; exhibits lower strain, stores higher level of strain energy at failure. - as they present distinct mechanical behavior, it would be necessary to acomplish a more detaield analysis to face the time and history dependence wich are the basic characteristics of viscoelastic materials.


SUMMARY
The use of isolated human tendons as well as in associated forms on knee reconstrution has become na usual practice.
The literature reveals that the use of implants of patellar tendon as well as the double semitendinous tendon associated to the double gracilis tendon exhibits different post surgical evolution related to the minimal extension loss, anterior-posterior displacement (KT-1000 artrometer), as well as to the sport activities.
This research work aim to analyze the mechanical behavior of human tendons (gracilis and semitendinous) subject to uniaxial traction to failure.
The mechanical parameters considered includes: stress at failure (MPa), strain energy at failure (N.mm) and strain rate (mm/ mm).
Results induced to the following conclusions: -tendon of semitendinous muscle is more resistent than the tendon gracilis muscle; exhibits lower strain, stores higher level of strain energy at failure.
-as they present distinct mechanical behavior, it would be necessary to acomplish a more detaield analysis to face the time and history dependence wich are the basic characteristics of viscoelastic materials.

INTRODUCTION
The movement of skeletal structures is stabilized and guided by ligaments, that is, bands of loose connective tissue which cross the joint and tie the skeleton: WOO, SMITH, JOHNSON, (1994). Tendons and ligaments are collagenous structures.
Under the evolutive point of view, collagen is defined as highly conserved proteins. It is found from the sponges to the human beings. In man, represents 33% of body weight. Without collagen, there would not be bones, dentin, skin, tendons and a number of other structures as vessels (veins, arteries and capillaries) and the fibrous texture of the organs. (VIDAL, 1990).
Thus, the behavior of the biological materials when submitted to solicitant strains was studied by MASE (1970) using elastic simmetry plans. WOO et al. (1994) state that when samples of the same material are mechanically analyzed, though with distinct surface areas, those with greater section resist more the rupture force. However, considering the rupture strain determined by division of the rupture force by the mean section, the numbers tend to be the same. CABAUD (1983) states that the main factors which determine resistance of the ligaments are type, form and loading speed. SMITH, LIVESAY, WOO (1993) define speed in mechanical tests in three categories: slow (0.003 mm/s), average (0.3 mm/s), and quick (113 mm/s).
According to WOO et al. (1994), the parameter rupture load is the most affected by the "strain ratio" reaching values 30% greater in the higher proportions. AGLIETTI et al. (1994) carried out a prospective study with 60 cases of anterior cruciate ligament reconstruction of the knee. Two options of grafts were randomly and alternatively used: the patellar tendon and the doubled semitendinous associated to the doubled gracilis. The authors concluded that the doubled semitendinous associated to the doubled gracilis must be the choice only in selected cases, while the patellar tendon is a routine option.
The aim of this study was to analyze the mechanical behavior of human tendons (gracilis and semitendinous) when submitted to uniaxial traction until rupture.
Mechanical parameters were analyzed: rupture strain (Mpa), relative deformation, elasticity module (Mpa) and rupture energy (N.mm), all of them as a function of the loading speed.

MATERIAL AND METHODS Material
Tendons of the gracilis and semitendinous muscles from corpses at least 12 hours and at most 36 hours after death.
The period after death was determined considering information provided and rigor mortis, according to ARBENZ (1988) and ZACHARIAS & ZACHARIAS (1991 (4), and polytrauma (5). Thirty-six tendons were studied, 18 from the gracilis muscle and 18 from the semitendinous muscle.

Conservation
The need to maintain the physical and mechanical characteristics of the samples associated to the practical difficulties of sampling demanded storage of human and cattle tendons.
The human tendons were preserved in plastic bags, after identification. The identification card contained the number of the sample, sex, laterality, age, causa mortis.
The tendons were maintained at -20 o C. Defrost was carried out at room temperature (27 o C) emerging the material in saline (S.F. 0.9%).

Determination of linear measures
The non-uniformity of the samples dimension implied the adoption of the "mean section" concept; it is worthwhile to emphasize that this is a characteristic of all biological tendons (human and animal).
Two methods were used to determine the average section: pachymetric and volumetric.

Clamps to anchor the samples
After the preliminary set of tests, clamps composed by 2 metalic plates with inner sinusoidal form associated to longitudinal grooves in its long axle were adopted.
Fixation of the tendons to the clamps was carried out using screws in the 4 corners of the plate.
Uniform strain was determined for all the screws using torque measurement.

The test machine
The

DISCUSSION
Using data from Tables 1 and 2, Figures 1 to 4 were constructed comparing the parameters rupture strain, relative deformation, rupture energy and elasticity module with different loading speeds.
In Figures 1 to 4 where the two strains express the elasticity module and the rupture strain, for speeds above 1 mm/s, the semitendinous muscle endures greater strain.
In Figure 2, for loading speeds above 1 mm/s the gracilis muscle tendon presents greater relative deformations.
In Figure 4, the accumulated rupture energy for the semitendinous muscle tendon is higher (69.48%) than for the gracilis muscle tendon.
The analysis of the figures shows that, under the test conditions, the semitendinous muscle tendon is more resistant; it shows smaller relative deformations and accumulates more rupture energy.
We observed that the semitendinous muscle tendon was 30.23% less deformable than the gracilis muscle tendon, and this can justify the conclusions reached by AGLIETTI et al. (1994) when these tendons were used in ligament reconstruction.
For mean rupture strain, the values ranged from 15 to 22 N/ mm. The semitendinous muscle tendon produced mean values for rupture strain 4.21% higher than the gracilis muscle tendon.
These tendons are used as options in the reconstruction of the anterior cruciate ligament of the knee. Their utilization can be in isolation or combining two doubled tendons, as to provide a greater cross section of the graft, increasing its ability to endure strain.
When carrying out a ligament reconstruction in the knee, the different mechanical behaviors must be considered when the graft option is a doubled semitendinous tendon associated to a doubled gracilis.
Notwithstanding, considering the history and time dependence of the materials, specific tests must be carried out to better evaluate the interactions which occur during surgery and in the subsequent phases of graft integration.

CONCLUSION
The results have shown: -the semitendinous muscle tendon is more resistant than the gracilis muscle tendon; it presents smaller relative deformations and accumulates more rupture energy -the utilization of these tendons as a single graft demands a more detailed mechanical analysis, since they have a distinct mechanical behavior and are history-and time-dependent materials (viscoelastic).