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A study was carried out utilizing cryo-clamp and VSA testing methodologies to characterize mechanical changes to tendon with subfailure ramp loading to varying strain limits in order to better characterize the strain thresholds at which matrix damage occurs in tendon. Contrary to our stated hypothesis, the current study was unable to demonstrate a decrease in the strength and stiffness properties (Figure 4 & Figure 5) or an increase in permanent (residual) strain ( ) with ramp loading to strain limits ranging from 1-14%. In addition, no change in failure strain or strain energy density (Table 1) was found with ramp loading to the specified strain limits. These results are in contrast to classic early tendon biomechanical studies that have suggested that strength and stiffness decline with loading beyond a strain limit of 4% (Rigby et al. 1959) and permanent deformation can begin to occur with strains exceeding 4% (Abrahams 1967).

Table 1

It is believed that the difference in results between this study and past studies is largely related to the mechanical clamping methodologies used in past studies that may have exaggerated damage to the tissue due to local stress concentration effects near the grips. While it is possible that these differing results may be due to species specific effects, this would appear to be unlikely due to the similarity in the mechanical properties of avian tendon to human tendon. The ultimate stress (96.7MPa), grip elastic modulus (735.7 MPa) and strain at failure (16.3 %) of avian tendon reported in the current study compare relatively well with human patellar tendon studies that have reported values of 65-111 MPa, 612-660 MPa and 13-26% for the ultimate stress, modulus and failure strain, respectively (Butler et al. 1984; Butler et al. 1986; Johnson et al. 1994). The inference that stress concentrating effects near the grips are the cause of the difference is supported by recent studies of ligament (Panjabi et

al. 1996) that have reported an absence of reductions in strength, deformation at failure, and energy at failure following subfailure stretches of up to 80% of the failure deformation on bone-ligament-bone complexes. In addition, in this ligament study an increase in

deformation was observed at low-level loads (5-50% of failure) in ligament receiving the subfailure stretch. Similarly, in the current study we saw a minor increase (Table 1) in strain at 10MPa loading after the initial ramp loading. However, as this increase in strain was not found to be dependent on the level of the strain-limited ramp loading, it would appear that this strain effect might be a tissue conditioning effect rather than a damaging effect. In addition, due to the small size of this strain change and the more dynamic adaptability of the length of the myotendinous unit, it is questionable if this strain change has physiological significance.

The current study has significance in that it suggests 1) tendon behaves as an elastic material to higher strains (i.e. nearly to failure) then previously reported and 2) a decline in mechanical properties due to matrix damage from a single loading stimulus is less likely to occur than previously thought. Readers are advised however, that this does not imply in vivo

tendons or their matrix cannot be damaged by subfailure stretches. Tendons in vivo wrap around structures such as bones and pulleys, and are therefore exposed to compressive and shear loads that this in vitro study does not take into account. These loads vary not only between species, but also across tendon sites, and would have a definite effect on in vivo

damage. Therefore, the results should be used in analyzing the basic matrix tensile characteristics of tendons in general.

A limitation of the current study is that it focuses on the mechanical changes of tendon alone while damage may also occur at the myotendinous and osteotendinous

junctions. An additional limitation of the study is the form of the loading stimulus: a single ramp loading at a strain rate slower than that which occurs in many injury situations. The selected strain rate was a compromise to achieve a fairly sensitive strain resolution within the dynamic capabilities of the VSA system. Additionally, a preponderance of the literature has been performed at strain rates similar to that selected. While mechanical changes of tendon with cyclical loading is likely of greater significance to the majority of clinical injuries to tendon, it was believed that a fundamental understanding of the mechanical changes as a result of a single ramp load was required to more fully investigate the response to cyclical loading. The limited studies that have examined the effect of cyclical loading on tendon have shown declines in strength and stiffness with increasing load cycles (Wang et al. 1995; Schechtman and Bader 1997).

The decision to utilize cryo-clamps for testing necessitated a compromise be made between how well the environmental conditions could be altered to mimic physiological temperature and hydration conditions. In this study, spray irrigation and wrapping the tissue in saline-soaked gauze was utilized to maintain hydration during the rest period. The

temperature of the tissue near the grip edge was found to be approximately 10° C during testing. While temperatures effects on the viscoelastic properties of the tissue are debatable, most studies have observed no significant differences in the load elongation curve for a single loading for temperature ranges between 0°C & 55°C (Rigby et al. 1959; Hasberry and Pearcy 1986; Lam et al. 1990). In addition, the tendons were frozen twice before testing for practicality purposes. Effects due to this were examined by evaluating the mechanical properties of once versus twice frozen specimens in our lab and no significant differences were found. In addition, past studies (Smith et al. 1996) have found little difference in

properties between samples frozen once and up to five times before testing. A final

consideration in examining the lack of differences in mechanical properties is in reference to the number of specimens tested per strain-limit group. While the power of statistical tests may be improved with additional specimens per group, it is unlikely that this would uncover any physiologically significant differences in the strain-limit groups due to the proximity of the means of the mechanical properties of each group.

Limitations withstanding, these results suggest that a single tensile subfailure

stretching does not directly cause significant matrix damage. These results also suggest that the cellular response to a loading stimulus may not be due to damage of the matrix, but due to tendon cells directly responding to the loading stimulus itself such that their cellular response causes a decline in the mechanical properties of the matrix. The clinical significance of this is that a loading event may not instantaneously cause damage to the matrix, and therefore the clinician may have a greater opportunity to prevent matrix changes by biochemically altering the cellular response to a loading event.