Tendons are dense, fibrous connective tissues that connect muscle to bone. This connection allows the force generated by muscle contraction to be transmitted across a joint, resulting in movement. When a tendon is injured, the body initiates a complex repair process that often results in the formation of scar tissue, or fibrosis, at the injury site. This fibrotic tissue is the body’s rapid, temporary patch that prioritizes structural integrity over the high-performance elasticity of the original tendon. The presence of this compromised tissue ultimately affects the tendon’s ability to function and withstand normal stresses.
The Biological Process of Tendon Healing
Tendon healing proceeds through three overlapping phases following an injury. The first is the inflammatory phase, which begins immediately and typically lasts about 48 hours. During this initial stage, inflammatory cells migrate to the wound site to clean up debris and release chemical factors that recruit repair cells.
The inflammatory stage transitions into the proliferative phase, which may last up to three weeks. Fibroblasts rapidly produce new extracellular matrix material to bridge the gap in the injured tissue. This quick repair material is predominantly composed of Type III collagen, laid down in a disorganized pattern. This swift deposition forms scar tissue, providing immediate structural stability at the expense of tissue quality.
The third and longest phase is remodeling, which can start around six weeks post-injury and continue for many months. The goal of this phase is to gradually convert the weaker, disorganized Type III collagen into the stronger Type I collagen found in healthy tendons. During remodeling, the collagen fibers attempt to align themselves parallel to the mechanical forces placed on the tendon. However, this process is slow and often incomplete, meaning the repaired tissue remains biomechanically inferior.
How Fibrosis Compromises Tendon Function
The resulting scar tissue fundamentally alters the mechanical properties required for optimal tendon function. Healthy tendon tissue is made up of approximately 95% Type I collagen fibers, which are thick, highly aligned, and provide high tensile strength and stiffness. The disorganized Type III collagen that dominates the early repair site is thinner and significantly less rigid.
When the healed tendon contains a high proportion of poorly aligned Type III collagen, the tissue loses its natural elasticity and becomes stiff. This increased stiffness restricts the normal range of motion and makes the tendon less capable of absorbing and transmitting force efficiently. Because the tissue is weaker and less adaptable, the scarred tendon is at a higher risk for re-injury when subjected to normal mechanical loads.
The fibrotic process can also lead to the formation of adhesions, where scar tissue binds the injured tendon to adjacent structures. In areas like the hand, where tendons must glide smoothly within a sheath, these adhesions severely compromise movement mechanics. This mechanical failure can contribute directly to chronic pain and is a key factor in the development of chronic tendinopathies.
Strategies for Scar Tissue Remodeling
Influencing the remodeling phase is the primary goal of rehabilitation to improve the quality of the scar tissue. One of the most effective non-invasive methods is controlled mechanical loading, particularly through specific exercise protocols. Exercises that emphasize the eccentric, or lengthening, phase of muscle contraction, mechanically signal the fibroblasts within the healing tendon.
This controlled, high-load tension encourages the newly formed collagen fibers to align themselves parallel to the direction of the stress. This alignment is necessary for the fibers to mature and develop the cross-links that give Type I collagen its superior strength and stiffness. Studies suggest that high-intensity eccentric training may be more effective at influencing the material properties compared to lower-load protocols.
In addition to exercise, manual therapy techniques are often used to address the physical quality of the scar tissue. Methods like Instrument-Assisted Soft Tissue Mobilization (IASTM) use specialized tools to apply focused pressure to the affected area. The goal of these techniques is to mechanically disrupt poorly aligned fibers and stimulate a renewed healing response to facilitate the formation of a better-organized extracellular matrix.
The timing of these interventions is important, as introducing controlled movement early helps guide fiber alignment before the scar tissue matures and becomes rigid. Combining manual mobilization with controlled strengthening exercises can help relieve stiffness, improve range of motion, and shorten the overall rehabilitation period.