The saphenous nerve is a sensory branch of the femoral nerve, originating from the L3 and L4 spinal nerves. It is the longest sensory nerve in the human body and primarily provides sensory innervation to the medial aspect of the leg, ankle, and foot. It does not have any motor function, as its role is purely sensory.
Location
The saphenous nerve originates in the thigh, emerging from the femoral nerve. It runs alongside the femoral artery in the adductor canal, a passage in the middle of the thigh, and then exits through the adductor hiatus. The nerve descends along the medial side of the knee, continuing down the medial aspect of the leg to the foot. It travels near the great saphenous vein, providing sensory innervation along its course.
Structure and Anatomy
Origin
The saphenous nerve is a terminal cutaneous branch of the femoral nerve, which arises from the L3 and L4 spinal nerve roots. The femoral nerve itself originates in the lumbar plexus and descends into the thigh, where it divides into several branches, including the saphenous nerve.
Course
The course of the saphenous nerve can be divided into several segments as it travels from the thigh to the foot.
Thigh (Proximal Segment)
The saphenous nerve originates in the femoral triangle of the upper thigh. Here, it is located lateral to the femoral artery and deep to the inguinal ligament. As it descends, it enters the adductor canal (also known as Hunter’s canal), a passageway in the middle of the thigh formed by the vastus medialis muscle, adductor longus, and adductor magnus muscles. The saphenous nerve travels alongside the femoral artery and femoral vein in this canal.
Adductor Canal
The saphenous nerve continues through the adductor canal, a critical segment of its course. As it travels through the canal, the saphenous nerve remains superficial to the vastoadductor membrane, which covers the canal. In the lower part of the canal, it gives off an infrapatellar branch that innervates the area around the knee joint.
Knee (Infrapatellar Segment)
Upon emerging from the adductor canal, the saphenous nerve pierces the deep fascia near the medial side of the knee. It then runs superficially, traveling along the medial side of the patella. At this point, the nerve gives rise to the infrapatellar branch, which provides innervation to the skin over the patellar region and medial knee. The saphenous nerve continues its descent, remaining close to the great saphenous vein.
Leg (Distal Segment)
After crossing the knee, the saphenous nerve continues down the medial side of the lower leg. It follows the medial border of the tibia and runs subcutaneously along with the great saphenous vein. As it travels toward the ankle, it remains superficial, lying between the skin and the deep fascia, making it relatively vulnerable to injury or trauma in this region.
Ankle and Foot
In the final part of its course, the saphenous nerve reaches the medial side of the ankle and the foot. It terminates by innervating the skin over the medial malleolus and the medial side of the foot, but it does not extend into the toes. The saphenous nerve stops at the medial arch of the foot, providing sensory innervation to the area.
Branches
The saphenous nerve gives off two main branches during its course:
- Infrapatellar Branch: This branch arises near the knee and supplies the skin over the anterior and medial aspects of the knee joint, particularly around the patella.
- Medial Crural Cutaneous Branch: This branch continues the nerve’s sensory role along the medial side of the leg and foot, extending down to the medial malleolus and foot arch.
Relations
- Femoral Artery and Vein: In the thigh, the saphenous nerve runs alongside the femoral artery and vein within the adductor canal. This relationship is important during surgical procedures involving the femoral vessels.
- Great Saphenous Vein: In the lower leg, the nerve runs parallel to the great saphenous vein, a large superficial vein used frequently in coronary bypass surgeries.
Anatomical Variations
While the typical course of the saphenous nerve is as described, there are variations in its distribution and branching patterns:
- Infrapatellar Branch Variation: The infrapatellar branch may be absent in some individuals, or its path may differ slightly, affecting the area it innervates around the knee.
- Distribution: The length and termination of the saphenous nerve in the foot can vary, with some individuals having a more extensive distribution than others.
Vascular Supply
The saphenous nerve receives blood supply from adjacent arteries, primarily the femoral artery in the thigh and branches of the posterior tibial artery in the leg. These vessels ensure adequate blood supply along its course, particularly in the adductor canal and leg.
Termination
The saphenous nerve terminates near the medial side of the ankle and foot. It supplies sensory innervation to the skin over the medial malleolus and the medial arch of the foot, but does not extend into the toes. The nerve ends by innervating the superficial skin in these areas.
Function
The saphenous nerve is primarily a sensory nerve that provides innervation to the medial aspect of the lower limb, particularly the skin over the knee, lower leg, ankle, and foot. It does not have a motor function, as its role is purely sensory, transmitting signals related to touch, temperature, pain, and proprioception from the skin of these regions to the central nervous system.
Sensory Innervation of the Thigh
Although the saphenous nerve arises in the thigh, its sensory function starts as it travels through the adductor canal. In this region, it does not contribute much to the sensation of the upper thigh itself, but as it exits the adductor canal, it begins its significant sensory role.
Infrapatellar Branch Sensation
The saphenous nerve gives rise to the infrapatellar branch near the knee. This branch innervates the skin over the anterior and medial knee, particularly around the patella. The infrapatellar branch is responsible for transmitting sensory signals from this area, such as:
- Touch: Detecting light and deep touch over the knee area.
- Temperature: Sensation of heat or cold affecting the skin.
- Pain: Pain signals originating from trauma, injury, or irritation around the knee, particularly over the medial and anterior parts of the patella.
Sensory Innervation of the Lower Leg
As the saphenous nerve descends from the knee to the lower leg, it travels along the medial aspect of the leg, providing sensory innervation to the medial side of the leg.
Medial Crural Cutaneous Branch
This branch of the saphenous nerve continues its sensory role down the leg. The skin over the medial tibial region and the surrounding soft tissue receives sensory input from this branch. The saphenous nerve transmits sensory signals like:
- Touch and Vibration: Allowing the body to detect stimuli such as gentle pressure, the texture of materials against the skin, or vibrations from movement or contact.
- Proprioception: Proprioceptive sensory fibers help the body monitor the position and movement of the leg and foot. While this function is less pronounced in cutaneous nerves, it plays a role in helping maintain balance and awareness of limb positioning.
Sensory Innervation of the Ankle and Foot
As the saphenous nerve continues its course toward the foot, it provides sensory innervation to the medial side of the ankle and foot, including the medial malleolus and the arch of the foot. The sensory functions in this region include:
Medial Malleolus Sensation
The skin over the medial malleolus is innervated by the saphenous nerve, transmitting signals related to:
- Light and Deep Touch: Detecting stimuli such as brushing, pressure from footwear, or contact with objects.
- Pain and Temperature: Providing the sensation of discomfort, heat, or cold from the medial side of the ankle, often a common area of irritation due to footwear or injury.
Medial Foot Sensation
The medial side of the foot and the arch are innervated by the saphenous nerve. This area is sensitive to:
- Pressure and Stretch: The saphenous nerve transmits sensations of pressure from the foot’s arch, especially during weight-bearing activities like walking or standing.
- Environmental Awareness: The nerve helps in detecting changes in surface textures, contributing to balance and stability by allowing the foot to sense uneven terrain or obstacles.
Integration with the Central Nervous System
The sensory information gathered by the saphenous nerve is transmitted to the central nervous system (CNS), where it is processed and interpreted. The CNS uses this information to modulate responses, such as reflexes, posture adjustments, and protective actions like withdrawing the leg from painful stimuli.
Coordination of Reflexes
The sensory input from the saphenous nerve contributes to lower limb reflex arcs. For example:
Withdrawal Reflex: In response to painful or harmful stimuli affecting the medial side of the leg or foot, the saphenous nerve transmits signals that trigger protective reflexes, such as retracting the foot from a hot surface or sharp object.
Balance and Stability
While the saphenous nerve is not involved in motor function, its sensory role in detecting the surface of the ground and pressure from the foot helps maintain balance and stability during walking, running, or standing. The nerve’s proprioceptive input also supports the body in adjusting posture and foot positioning.
Contribution to Sensory Integration in the Lower Limb
The saphenous nerve works in coordination with other sensory nerves of the lower limb, such as the sciatic nerve, superficial fibular nerve, and deep fibular nerve. Together, these nerves ensure complete sensory coverage of the leg and foot. The saphenous nerve specifically focuses on the medial aspect, complementing the sensory inputs from nerves that supply the posterior, lateral, and anterior regions.
Clinical Significance
The saphenous nerve plays a crucial role in providing sensory innervation to the medial aspect of the knee, leg, ankle, and foot. While it has no motor function, its injury or entrapment can lead to significant sensory disturbances, often affecting quality of life.
Common Causes of Injury
- Surgical Procedures: The saphenous nerve is at risk during knee surgeries, such as knee replacements or procedures involving the adductor canal, leading to post-operative sensory loss or pain.
- Trauma: Direct trauma to the medial leg, such as fractures or soft tissue injuries, can damage the nerve.
- Entrapment or Compression: The nerve can become compressed in the adductor canal (adductor canal syndrome), causing pain or numbness along its course.
Symptoms of Injury
- Sensory Loss: Damage to the saphenous nerve can cause numbness, tingling, or loss of sensation in the medial knee, leg, and foot.
- Pain: Nerve entrapment can lead to burning or shooting pain along the nerve’s distribution, particularly in the knee and lower leg.
- Hypersensitivity: In some cases, damage can result in heightened sensitivity (allodynia) to light touch or pressure in the affected areas.
Clinical Conditions
- Saphenous Nerve Neuropathy: Characterized by pain, numbness, or paresthesia along the medial leg and foot, often following trauma or surgery.
- Infrapatellar Nerve Injury: Damage to the infrapatellar branch of the saphenous nerve, commonly occurring after knee surgery or trauma, can lead to chronic pain or sensory loss around the knee.
