Achilles Tendon Injury and Repair
The Achilles tendon is the confluence of the independent tendons of the gastrocnemius and soleus, which fuse to become the Achilles tendon approximately 5 to 6 cm proximal to its insertion on the posterior surface of the calcaneus. The gastrocnemius and soleus muscles, via the Achilles tendon, function as the chief plantarflexors of the ankle joint. This musculotendinous unit provides the primary propulsive force for walking, running, and jumping. The normal Achilles tendon can withstand repetitive loads near its ultimate tensile strength, which approach 6 to 8 times body weight .
Complete Achilles tendon ruptures occur most commonly at the mid-substance, but also distally at the insertion site or proximally at the myotendinous junction. These can be traumatic and devastating injuries, resulting in significant pain, disability, and healthcare cost. As many as 2.5 million individuals sustain Achilles tendon ruptures each year and the incidence is rising . This trend is due, in part, to an increase in athletic participation across individuals of all ages.
We recently received an R01 grant from the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS) of the National Institutes of Health (NIH) entitled “Challenging Treatment Paradigms for Achilles Tendon Ruptures in an Animal Model.” As mentioned previously, Achilles tendon ruptures are common and devastating injuries. Unfortunately, rigorous evidence supporting a “preferred” clinical approach is lacking. Therefore, addressing this question from a fundamental, basic science mechanistic perspective, by rigorously evaluating the properties of healing Achilles tendons subjected to commonly-used treatment scenarios, is necessary. This includes assessments of Achilles tendon mechanical, structural, functional, and biological properties in various clinical treatment paradigms. In particular, previous basic research has focused on tendon failure properties. However, fatigue properties may represent a more pertinent measure of Achilles tendon recovery due to the repetitive nature of Achilles loading during rehabilitation and ambulation. The typical response of tendon to fatigue loading is marked by changes in stiffness and deformation that consists of three phases (Figure 1A). Specifically, tendon stiffness increases initially, reaches a maximum, and then gradually decreases. This gradual decrease in stiffness is attributed to sub-rupture damage, the accumulation of which ultimately leads to the dramatic increase in peak deformation, and decrease in stiffness just prior to failure. Following injury, the number of cycles to failure decreases dramatically (Figure 1B).
Since previous studies from our group have demonstrated the close relationship between tendon structure and function, knowledge of structural properties is important. In particular, tendon collagen alignment is an important measure of tendon organization that evaluates the progression of tendon healing. Since many techniques to assess collagen alignment (e.g., polarized light imaging, scanning electron microscopy, and second-harmonic generation imaging) are destructive and are difficult to perform outside of the laboratory, noninvasive methods such as ultrasound are attractive a potentially translatable in vivo technique to study tendon healing. Current work is investigating the use of high frequency ultrasound to directly measure collagen alignment (Figure 2).
For more information see:
- Freedman BR, Sarver JJ, Buckley MR, Voleti PB, Soslowsky LJ. Biomechanical and structural response of healing Achilles tendon to fatigue loading following acute injury. J Biomechanics 2013.
 Allenmark, C. (1992). "Partial Achilles tendon tears." Clinics in sports medicine 11(4): 759-769.
 Suchak, A. A., G. Bostick, et al. (2005). "The incidence of Achilles tendon ruptures in Edmonton, Canada." Foot & ankle international / American Orthopaedic Foot and Ankle Society [and] Swiss Foot and Ankle Society26(11): 932-936.