Comparisons Between Proximal and Distal Layers

The experiments described in the previous sections looked at the comparison between layers for both proximal and distal regions of the patella tendon at every 10% force level interval. The measurements taken were converted to a strain percentage and showed that the proximal anterior and distal posterior regions gave the largest strains. In this section, there is a comparison of overall mean strains between 50% and 100% force. Table 6.1 shows the instantaneous strain values at 50% and 100% with the relative difference and corresponding mean forces (%) for all layers at both proximal and distal regions of the patella tendon as shown in Equation 6.1:

d=x100−x50 (6.1)

where d is the difference between two means and xn is the relative force level of 50% or 100%. The standard error of the difference between means is shown in equation 6.2:

σ100−50 = σ100

2 +σ

50

2 _(6.2)

The overall mean strain was greater at 100% force with a mean strain value of 9.77±0.75% compared to a mean strain value of 5.33±0.73% at 50% force. The proximal strain showed a lower mean strain difference between 50 and 100% force with the posterior proximal giving the least difference at 2.66±024%, followed by anterior proximal with a difference of 4.62±0.03. The mid section gave the highest percentage difference of 5.61±0.21%. The distal strain results showed higher strain differences for each layer, as the anterior distal were seen to be 4.35±0.18% different between 50% and 100%. The mid section gave highest difference at 5.28±0.13%, and the posterior distal gave the smallest difference of 4.12±0.25%.

Table 6.1: Instantaneous mean strain for all layers at both proximal and distal regions of the patella tendon at 50% and 100% force levels.

Figure 6.3 shows that all force levels and regions showed no interaction towards each other (p>0.05) and mean values (i.e. average of 50 & 100% force levels) for strain indicated that the proximal anterior strain (*) and the distal posterior strain (^) were greater than all other regions (p<0.05). The proximal mid section strain was shown to be significantly different (p=0.02) to the proximal posterior strain (~). These huge differences in strain between 50% and 100% force could indicate shear between the tendon boundary layers causing differential longitudinal movement between layers (shear force). In time, this relative difference in strain at different levels of force may be a factor in the development of cumulative tendon injury.

Figure 6.3: Instantaneous strain values for all layers at both proximal and distal Force

(%)

Strain (%)

Proximal Distal Total

Mean Strain

(%) Anterior Mid

Section Posterior Anterior

Mid Section Posterior 50 7.24 ±0.52 4.69±0.94 3.32±0.64 4.61±0.73 4.27±0.67 7.84±0.90 5.33±0.73 100 11.86±0.55 10.30±0.73 5.98±0.88 8.96±0.91 9.55±0.80 11.96±0.65 9.77±0.75 Difference 4.62±0.76 5.61±1.19 2.66±1.09 4.35±1.17 5.28±1.04 4.12±1.11 4.44±1.05 0 2 4 6 8 10 12 14 16

Proximal Anterior Proximal Mid Proximal Post Distal Anterior Distal Mid Distal Post

Str ai n (% ) Tendon Region 50% Force 100% Force ~     ^ ~   * ^

The experiments showed that multi-layer tracking was able to estimate frame- to-frame displacements using multiple-ROI where 2-ROI end points were placed at each layer. Previous studies have validated the use of localized tracking of tendon using block-matching techniques similar to that used here (Haraldsson, et al., 2005, Pearson, et al., 2007, Kim, et al., 2011) with a small margin of errors. A recent study (Couppé, et al., 2008) reported that the tracking of tendon movement using 2-ROI end points can be achieved during twitch contraction, electrically stimulating the muscle. However, the forces in the tendon were only moderate (up to 50% of maximum), which is a significant difference to the approach in this study, where high forces were elicited and thus larger tendon deformations would be expected making tracking more difficult. In addition, previous work has indicated that contraction time can affect the amount of excursion seen in the tendon (Pearson, et al., 2007), which can be explained due to the viscoelastic nature of the tendon. It could then be speculated that the composition of the tendon at different regions may be proportionally different in terms of the viscous and elastic components, which would affect the time-course of extension under load to different degrees.

The findings showed that the patella tendon for a group of healthy young subjects, the greatest strains (100% force) during isometric ramped contractions were seen in the anterior layer at the proximal end (11.86±0.55%) and posterior layer at the distal end (11.96±0.65 %) with significant differences (p<0.05) between proximal and distal tendon. The strain calculated here were within the range of those reported for this structure in young males (Onambele, et al., 2007, Child, et al., 2010, Hansen, et al., 2010) where the previous references showing a range of 6 – 10.6% strain. It can also be concluded from the results that the proximal mid section and posterior tendon strain were at 10.30±0.73% and 5.98±0.88% respectively. The distal anterior and mid section showed 8.96±0.91 and 9.55±0.80% tendon strain respectively, which are considerably larger than those reported by others (Basso, et al., 2002).

The reason for such larger values may be due to differences in the level of load, application of load and also that here the strain was determined in the proximal and distal sections of the tendon compared to the mid-third utilised by the other study (Basso, et al., 2002). It may be that the tendon is not homogenous throughout its length and could be structurally different in terms of the collagen content, type and

extracellular matrix density. Another explanation might be related to the samples used in other study were taken from a cadaver, compared to the live specimen used in this study (Basso, et al., 2002).

The method of using 2-ROI at multiple layers of the tendon has the potential to improve clinical knowledge relating to the tendon mechanical properties. It is clear that the strain throughout the tendon structure is not equal for a given external force, lending itself to tissue shear and hence to potential for increase injury risk in specific regions of the tendon. The information generated by the tracking algorithm could help to determine how and why these differences in strain may affect the aetiology of disease and effects of training rehabilitation. These studies will give further insight into the aetiology of tendon injury, repair, response to various training interventions and the time course of tissue adaptation with disease.

6.5 Summary

The experiments described in this Chapter examined and compared localised proximal strain at both the anterior and posterior regions of the patella tendon using 2- ROI tracking on multiple layers of anterior, posterior and mid section in-vivo. The measurements obtained from the tracking were converted into mean strains, which were used to investigate the interactions between the layers of the tendon. The experimental setup for the experiments was described in Chapter 3. The algorithm used for the tracking was Normalized Cross Correlation (NCC), which was selected based on the analysis conducted in Chapter 4 and the optimal tracking settings, was based on the analysis carried out in Chapter 5.

The experiment showed that the anterior layer at the proximal end gave the highest mean strain with 11.86±0.55% with mean difference between 50% and 100% force was seen to be at 4.62±0.76% and posterior layer at distal end gave the highest mean strain with 11.96±0.65% with mean difference between 50% and 100% force to be 4.12±1.11%. Significant differences were also seen (p<0.05) between proximal and distal tendon. These different patterns of strain between the tendon layers at the sites measured could indicate shear of the tendon structure. It is clear that the strain

throughout the tendon structure is not equal for a given external force, lending itself to tissue shear and hence to potential for increased injury risk in specific regions of the tendon. The huge differences in strain between 50% and 100% force could also indicate shear between the tendon boundaries, which over time may develop into cumulative tendon injury. The use of multiple layer image tracking could generate valuable information that can be used to describe in detail how the tendon works at the different layers and regions by measuring the strain during ramped isometric voluntary contractions to improve clinical knowledge relating to the tendon mechanical properties to give further insight of the cause of tendon injury.

Chapter 7