Introduction

Every movement you make, from gripping onto a coffee cup or even just wiggling your toes, would not be possible without a system of strong ropes and slippery tunnels that work hand in hand behind the scenes! These ropes are your tendons, dense connective tissue structures that bridge muscles to bones for force transmission to facilitate movement.¹ Their high tensile strength and ability to resist high forces, despite being relatively inelastic, allows them to function the way they do, but tendons do not work alone! They are surrounded by special tunnels known as synovial sheaths that provide built-in lubrication. The sheath reduces friction and wear by lining the inner surfaces of the joint capsules, tendon sheaths and bursae.² 

Just like a bicycle cable needs a smooth casing, our bodies have a similar system to keep our tendons gliding effortlessly. However, tendonitis, tenosynovitis, and ruptured tendons can arise from overuse of the joints or tendon injuries (tendinopathy). The restricted blood flow complicates the tendon recovery process, so rest, physical therapy and anti-inflammatory medications are the current main solutions to combat tendon-related deterioration issues for patients living with immense discomfort.¹ The best way to preserve our tendons is to engage in strength training, make ergonomic adjustments, and do appropriate warm-ups before exercising.

Tendons: structure and function

Tendons are known for their tough and fibrous characteristics that serve as connective tissues to stabilise and allow movement to the body. This is accomplished by the tendon’s intricate structure, which ensures the tendon’s durability to bear high tensile forces whilst retaining a degree of flexibility to avoid overstretching. Tendons are made up of the basic building blocks of collagen molecules that are arranged in a hierarchical pattern to form fibrils, fibres, and ultimately collagen bundles.³ 

Tendons are primarily composed of:¹ 

• Type I collagen, which provides strength and flexibility
• Type III, V, and Xll collagens are found in smaller amounts 
• Elastin, glycoproteins, and proteoglycans
• Specialised cells:³
  • Immature tenoblasts that give rise to tenocytes 
  • Tenocytes help to maintain and repair the tendon 

A tendon consists of three main layers¹ 

• Endotenon: the inner layer that surrounds individual collagen, which helps it function smoothly.
• Peritenon: the layer that surrounds groups of fibres that enables them to move within the tendon 
• Epitenon: The outermost layer that reduces friction and provides lubrication 

The reason why tendons heal a lot slower than other parts of the body is that they have less blood supply than other parts of the body.¹ The blood vessels enter through the peritoneum and the myotendinous junction, where the tendon meets the muscle.

Additionally, the outer layer of the tendon contains nerves that are responsible for pain perception and proprioception.¹ Overall, this brilliantly designed structure allows tendons to efficiently transfer forces from muscles to bones whilst keeping their structure under stress for smooth movement and durability.

Synovial sheaths

Synovial sheaths are specialised connective tissue structures that surround certain tendons in the body that pass through narrow spaces and experience frequent movement.² They act as protective sleeves to minimise friction for tendons to glide smoothly - think of a synovial sheath like a straw with a smoothie inside and a string moving through it. The string is like the protected tendon, moving back and forth.

The two main layers of the synovial sheath are: the outer fibrous parietal layer that provides structural support, and the inner synovial membrane. The synovial membrane is further divided into two divisions² 

• The intimal layer that has specialised cells such as fibroblasts and macrophages
• The subintimal layer is made up of loose connective tissue

The synovial membrane produces synovial fluid, which is a viscous ‘egg-white consistency fluid’. This fluid lubricates and nourishes the fibrous sheath; preventing wear and tear to ensure gliding joint movements during repetitive motion.¹ 

The synovial sheaths are most commonly found in the hands and fingers, where the sheaths surround the flexor and extensor tendons that allow smooth finger movement for any type of motor skill, including writing and typing.^4/5 They are also found in the feet and toes providing support to tendons such as those of the tibialis posterior and flexor hallucis longus muscles when walking or running.

These sheaths also cover tendons in the wrists and ankles, where multiple tendons cross over the joints. Joints that are subjected to frequent bending and stretching also exhibit this: the shoulders and elbows support arm movement, and the knees assist with bending and weight-bearing movements.

For the musculoskeletal system to function properly, the tendons and synovial sheaths must function correctly in unison.⁷ Tendons deteriorate quickly without these protective sheaths, and inflammation of the synovial sheath (tenosynovitis) may cause pain and limited movement.⁶

Injuries and disorders

Repetitive movements and trauma cause tendon and synovial sheath injuries. This makes athletes more susceptible to tennis elbow, achilles tendonitis, and rotator cuff tendinitis.⁶ Besides, individuals with medical diseases like diabetes and rheumatoid arthritis are at a threefold risk of vulnerability.⁹

While tendinosis refers to the progressive, chronic degradation of a tendon without inflammation, tendonitis is the inflammation that arises from stressing a tendon.¹⁰ Excessive force can also lead to a complete loss of function, as it ruptures the tendon partially or completely in severe circumstances.

Tenosynovitis, often triggered by repetitive hand or wrist movements, is a synovial sheath disorder that is the inflammation of the protective sheath.¹¹ A specific type is De Quervain’s tenosynovitis, which is the thickening of the tendon sheath that painfully entraps and causes swelling in two tendons at the base of the thumb.¹²

 Women have been identified to be at higher risk because pregnancy has been linked as a major risk factor. This is due to the repeated lifting of newborns that elevates the frictional forces in the thumb and wrists, leading to irritation.¹²

Healing and treatment of tendon and synovial Injury 

Inflammation, repair, and remodelling are the three overlapping stages to reduce strain and restore movement in an injured tendon.¹³

The first phase lasts for around 48 hours, during which the immune cells clean the injury site.¹⁴ The repair phase lasts for 1 to 3 weeks and involves the synthesis of type III collagen by tendon fibroblasts to replace the dying tissue. The final stage can extend beyond a year, where type III collagen undergoes replacement with type I collagen, and this is also marked by tissue maturation and formation of scar tissue.¹⁵

Anti-inflammatory drugs and steroid injections may be administered to tackle swelling and pain management, but in extreme cases, surgery may be necessary.¹⁶ The healed tendon is still biomechanically weaker than the healthy tendon, so treatment typically consists of a combination of rest to reduce strain and physical therapy exercises to promote healing and lessen excessive scar tissue development.¹³

FAQs

How common is tendinopathy?

The risk of tendinopathy increases with age and is highlighted as an issue among athletes and active individuals. The incidence rate is 1.85 per 1000 people in the general population for Achilles tendinopathy, but the overall prevalence is about 6% for those who engage in physical activity. Patellar tendinopathy affects almost 25% of volleyball players. This emphasises the need for prevention interventions for those at higher risk.

What are some common signs of a tendon injury?

Common signs of a tendon injury are sharp pain, swelling, stiffness, weakness, reduced range of motion, popping, snapping, muscle cramping, tenderness to the touch, and loss of function in the affected area.

How can my tendons stay healthy?

By doing regular stretching and strength training, it helps to prepare and build both the muscles and tendons, which lowers the chance of injury. Repetitive stress can be reduced by implementing ergonomic changes, by adjusting your posture while sitting and during exercise. You should adopt a well-balanced diet with supplements, if needed, to keep your tendons strong and resilient.

Can diet help with tendon recovery? 

Yes, a diet high in nutrients plays a key role in tendon healing. Vitamin C is beneficial because it supports the body in collagen production, which is the primary component of our tendons. It also reduces inflammation, which speeds up tendon recovery. Other nutrients like zinc, magnesium, and omega-3 fatty acids also contribute to stronger and more resilient tendons.

Summary

The musculoskeletal system is the key mechanism that guarantees our every movement, even actions that come naturally to us, like second nature, originate from our tendons and synovial sheaths working in collaboration.

The synovial sheath acts as a layer of lubrication to reduce friction; however, overuse and sudden trauma can compromise the system, leading to discomfort and tendon-related issues in such areas. 

Consequently, understanding the anatomy of the tendon and synovial sheath is crucial that allows clinicians to diagnose and implement effective interventions to restore tendon function.

Care and maintenance of these tissues should be made more aware to prevent these disorders and ensure optimal long-term musculoskeletal health. If we begin to take proper precautions, we can preserve the health of our tendons for years to come.