Tendon
Tendon transmits force from muscle to bone.
Structure
- predominantly type I collagen
- collagen organised into fibres and fascicles
- tenocytes maintain the matrix
- crimp in collagen straightens during early loading
- some tendons are surrounded by paratenon; others run within synovial sheaths
Mechanical behaviour
The initial toe region of the stress-strain curve reflects straightening of collagen crimp. With greater load, collagen fibres become increasingly aligned and stiff. Beyond the elastic range, microfailure and then macroscopic rupture occur.
Tendon vascularity
Vascular supply varies with location. Some tendons contain relatively hypovascular regions, which may contribute to impaired healing or degeneration in specific anatomical sites.
Tendon healing
A simplified sequence:
- inflammation
- cellular proliferation and collagen deposition
- remodelling with increasing collagen organisation
Early repair tissue is mechanically weak. Progressive loading can improve matrix alignment, but excessive early loading can disrupt repair.
Healing broadly includes:
- inflammation
- proliferation with fibroblast activity and collagen deposition
- remodelling with maturation and reorganisation
Early repair tissue is mechanically weak. Controlled loading can stimulate matrix organisation and reduce adhesions, but excessive load risks elongation or failure. Rehabilitation therefore tries to balance protection with biologically useful motion.
Intrasynovial flexor tendons are particularly prone to adhesion formation. Modern rehabilitation protocols exploit stronger repair constructs to permit carefully controlled early movement.
Ligament
Ligaments connect bone to bone and guide or restrain joint motion.
Structure
- predominantly type I collagen
- lower proportion of elastin in most ligaments
- fibre bundles are arranged according to the directions of habitual loading
- cells are mainly fibroblast-like ligamentocytes
Tendon versus ligament
Both are collagen-rich, viscoelastic tissues, but:
- tendon is specialised for muscle force transmission
- ligament is specialised for joint stability and guidance
- collagen organisation and insertion anatomy reflect these different tasks
Enthesis
The tendon or ligament insertion may be:
- fibrous
- fibrocartilaginous
A fibrocartilaginous enthesis provides a graded transition from compliant soft tissue to stiff bone, reducing stress concentration.
Viscoelastic behaviour
Tendon and ligament show:
- creep
- stress relaxation
- hysteresis
- rate-dependent stiffness
Repeated subfailure loading can also produce cumulative damage.
Tendons and ligaments demonstrate:
- creep under sustained load
- stress relaxation under constant deformation
- hysteresis during loading-unloading cycles
- rate-dependent stiffness
These characteristics matter in deformity correction, tendon tensioning and interpretation of ligament laxity.
Clinical links
- immobilisation reduces mechanical properties
- graded rehabilitation promotes adaptation
- chronic tendinopathy is not simply an acute inflammatory process; matrix disorganisation and altered cell behaviour are important
- ligament reconstruction must consider graft position, fixation, tension and biological incorporation
Viva framework
Describe composition and hierarchical structure, then explain the toe region, linear region, viscoelasticity, healing and the tendon-ligament functional difference.
Tendon structure and function
Tendon is dominated by longitudinally orientated type I collagen organised into hierarchical bundles. Tenocytes maintain the matrix. The crimp pattern of collagen contributes to the initial non-linear region of the stress-strain curve.
The tendon-bone insertion may be direct, with a graded transition through fibrocartilage and mineralised fibrocartilage, or indirect, with fibres blending into periosteum. This transitional anatomy reduces stress concentration.
Tendon blood supply
Blood supply arises from:
- musculotendinous vessels
- osseous insertion
- surrounding connective tissue or paratenon
- vincular supply in some digital tendons
Areas of relatively limited vascularity are clinically important in structures such as the Achilles tendon and rotator cuff, although tendon failure is multifactorial and not explained by vascularity alone.
Tendinopathy
Chronic tendinopathy is not simply persistent inflammation. Histology often demonstrates collagen disorganisation, altered tenocyte morphology, increased ground substance and neovascularity. Management therefore focuses on load modification and progressive rehabilitation rather than assuming an inflammatory process alone.
Ligament structure
Ligaments are also collagen-rich but their fibre orientation reflects the directions in which they restrain joint motion. Some ligaments contain regions that become taut at different joint positions, allowing restraint across a functional arc.
Ligament proprioceptive input also contributes to dynamic joint control.
Ligament healing
Extra-articular ligaments often have a greater capacity for scar-mediated healing than intra-articular structures. The ACL is the classic example of a ligament with limited spontaneous functional healing after complete rupture in many patients, influenced by synovial environment, gap, motion and tissue biology.
Healing produces scar tissue that may not reproduce the original insertion anatomy or mechanical properties.
Applied surgery
For tendon transfer, consider:
- donor strength and excursion
- synergistic action
- line of pull
- supple joints
- timing relative to nerve recovery
- fixation and postoperative retraining
For ligament reconstruction, success depends on:
- anatomic tunnel or attachment position
- graft properties
- fixation
- avoidance of impingement
- biological incorporation
- rehabilitation
FRCS synthesis
When discussing tendon or ligament, connect collagen architecture → mechanical behaviour → healing strategy. The tissue is not a static rope; it is a living viscoelastic structure whose properties change with loading, injury and remodelling.