Genicular nerves: ultrasound anatomy, scanning techniques, and clinical implications

Sensory innervation of knee

The genicular nerves form a complex network of sensory fibers responsible for innervating the knee joint capsule, ligaments, and surrounding structures. These nerves play a crucial role in transmitting pain signals from the knee to the central nervous system. However, variability persists in the reported number and origin of the nerve branches supplying the anterior knee capsule⁽⁵⁾. It is well established that significant contributions arise from the femoral, sciatic, obturator, common peroneal, and saphenous nerves. These nerves are both motor and sensory in nature; however, the terminal or distal-most branches supplying the muscles that cross the knee joint are purely sensory and primarily innervate the joint itself^(6,7) (Fig. 1).

Fig. 1.

According to Tran et al., up to eleven nerves and their terminal branches may supply the anterior knee capsule⁽⁸⁾. Multiple cadaveric studies have identified six consistent articular branches – namely, the superomedial, superolateral, inferomedial, inferolateral, recurrent peroneal genicular nerves (RPGN), and the infrapatellar branch of the saphenous nerve (IPBSN) – with relatively predictable anatomical locations. Although the proximal trajectories of these nerve branches may vary, they tend to have consistent distal contact points on the femur and tibia, making them suitable targets for nerve blocks or ablative procedures^(6,9).

Each genicular nerve supplies a distinct region of the knee joint, allowing for precise, targeted interventions based on the specific location of pain⁽⁵⁾. The superomedial genicular nerve (SMGN) innervates the medial patellar retinaculum and the medial aspect of the knee joint capsule. The IPBSN contributes to sensory innervation of the medial knee joint, along with the SMGN. The superolateral genicular nerve (SLGN) supplies the superolateral portion of the tibiofemoral joint capsule. The inferomedial genicular nerve (IMGN) innervates the medial, anterior, and inferior regions of the knee joint capsule. The inferolateral genicular nerve (ILGN) is responsible for innervation of the anterolateral tibiofemoral joint and the proximal tibiofibular joint. Lastly, the RPGN supplies the inferolateral and anterior aspects of the knee joint⁽¹⁰⁾.

The genicular nerves, often difficult to visualize on ultrasound because of their small size, travel alongside the genicular arteries. This relationship makes the arteries valuable sonographic landmarks when attempting to locate the nerves. Similar to the nerves, the arteries demonstrate variability in their origins but have consistent distal courses that correspond to the trajectories of the genicular nerves⁽¹¹⁾. The knee joint is supplied by a network of six genicular arteries. The descending genicular artery originates from the femoral artery, while the popliteal artery gives rise to the superomedial genicular arteries (SMGA), superolateral genicular arteries (SLGA), inferomedial genicular arteries (IMGA), inferolateral genicular arteries (ILGA), and the middle genicular arteries. The recurrent anterior tibial artery (RATA) branches off from the anterior tibial artery⁽²⁾.

Understanding these specific nerve territories enables clinicians to tailor interventions more effectively, targeting the precise areas of joint pain. For effective pain management, it is essential to target these sensory branches while sparing the motor fibers to avoid unwanted weakness or motor deficits. Therefore, performing distal nerve blocks or ablations that focus specifically on the sensory components – known as motor-sparing nerve blocks – is crucial. This approach ensures adequate pain relief or neuromodulation while minimizing the risk of motor impairment and associated complications⁽⁹⁾.

Ultrasound scanning technique for genicular nerves

Ultrasound imaging is an invaluable tool for visualizing genicular nerves and guiding interventional procedures. High-resolution ultrasound facilitates direct visualization of nerves as well as the surrounding soft-tissue and bony landmarks.

For optimal imaging, a very high frequency (12–15 MHz or higher) transducer with a small footprint, such as a hockey-stick probe, is recommended for superficial areas, whereas a medium-frequency (6–15 MHz) linear transducer with a larger footprint is suitable for deeper regions or patients with larger body habitus. The high-frequency transducer provides the best resolution for the small-caliber genicular nerves (0.5–1.5 mm in diameter), which can be challenging to visualize directly. A larger-footprint probe may initially be used to locate the genicular vessels, after which a high-frequency probe is advisable for identifying the nerves. Utilizing color Doppler or power Doppler is essential for identifying accompanying genicular vessels, which serve as crucial landmarks for nerve localization⁽¹⁰⁾.

The patient is typically positioned supine with the knee slightly flexed and rotated as needed to access the specific nerve being examined. Nerves should be visualized in both short- and long-axis views. The ultrasonographic appearance of normal peripheral nerves reflects their macroscopic anatomy. In the short-axis view, nerves exhibit a “honeycomb” pattern, with hypoechoic fascicles surrounded by hyperechoic perineurium and encased in echogenic epineurium. In the longitudinal view, nerves resemble “train tracks,” with fascicles appearing as linear hypoechoic structures outlined by hyperechoic borders. As nerves progress distally, their cross-sectional area (CSA) typically decreases as branches are given off⁽¹²⁾. Since genicular nerves are terminal branches, they are generally monofascicular, possessing a small CSA and appearing as small, round hypoechoic structures in short-axis views. They may resemble adjacent genicular arteries on grayscale imaging; therefore, color Doppler is employed to differentiate between them. A low pulserepetition frequency, typically between 3 and 4 cm/s, is generally recommended to enhance visualization of low-velocity flow within genicular vessels⁽¹⁰⁾.

Clinical implications

Understanding the anatomy of the genicular nerves and the corresponding ultrasound scanning protocols has several important clinical implications:

Post-intervention nerve injury diagnosis and treatment

A thorough understanding of genicular nerve anatomy is essential for diagnosing nerve injuries following knee surgery, trauma, or other interventions⁽¹⁶⁾. Ultrasound imaging can reveal nerve damage, entrapment, or neuroma formation (Fig. 2). Applying focal compression with the transducer at a site of ultrasonographic abnormality may elicit pain or a tingling sensation – a response known as the ultrasonographic Tinel sign (Fig. 3). This sign helps pinpoint the exact site of neuropathy, thereby enhancing diagnostic accuracy and guiding management⁽¹⁷⁾ (Fig. 4, Fig. 5).

Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.

Genicular nerve blocks

Genicular nerve blocks involve injecting a local anesthetic, often combined with a corticosteroid, near the genicular nerves to temporarily inhibit nerve conduction. This procedure provides immediate pain relief and also serves as a diagnostic tool: a significant reduction in pain following the block suggests that the targeted nerves are contributing to the patient’s symptoms⁽¹⁸⁾.

Radiofrequency ablation (RFA)

RFA may be employed as a longer-lasting solution, particularly for patients who experience substantial pain relief from a genicular nerve block. RFA uses thermal energy to create a lesion on the nerve, disrupting its ability to transmit pain signals and thereby providing sustained pain relief⁽¹⁸⁾.

While the traditional method for genicular nerve pain reduction, as described by Choi et al. with fluoroscopic guidance and later adapted to ultrasound using the genicular arteries, targeted three nerves (SMGN, SLGN, and IMGN), current approaches increasingly utilize a five-nerve target (SMGN, SLGN, IMGN, IPBSN, and RPGN) based on recent cadaveric and clinical studies incorporating modified landmarks for potentially better results^(1,19,20). Possible complications of genicular nerve block or ablation include injury to muscles or blood vessels, unintentional puncture of nearby motor nerves, and, rarely, complex regional pain syndrome following radiofrequency ablation. When targeting the RPGN, there is a risk of foot drop due to common peroneal nerve (CPN) block. The ILGN is usually avoided as a target because of the potential risk of injury to the adjacent CPN⁽²¹⁾.

The detailed anatomy, ultrasound scanning protocols, and approaches for targeting the six genicular nerves for both nerve blocks and RFA are described below, providing clinicians with a comprehensive guide to optimizing pain management interventions (Tab. 1).

Ultrasound scanning protocol

The patient is positioned supine with the knee flexed approximately 30–45 degrees and the leg slightly externally rotated. The ultrasound transducer is aligned in a coronal plane over the medial femorotibial joint line, then moved cephalad and posteriorly to identify the adductor tubercle and the insertion of the adductor magnus tendon. Color Doppler helps identify the adjacent SMGA, located deep to the vastus medialis and directly over the bony cortex of the distal femur. Typically, the SMGN is found immediately adjacent to the SMGA. In the coronal plane, the nerve is visualized in short axis. Once identified, the transducer is rotated 90 degrees to obtain a long-axis image. Key landmarks for identifying the SMGN include the adductor tubercle, the femoral origin of the MCL, and the SMGA. To target the nerve for RFA, after localizing it in the short-axis view, the transducer is rotated to acquire a long-axis image. The RF cannula is then advanced in-plane from anterior to posterior until it reaches the posterior cortex of the femur, positioned either at or just above the adductor tubercle⁽¹⁹⁾ (Fig. 6).

Fig. 6.

Ultrasound scanning protocol

The patient is positioned with the knee flexed approximately 30 degrees and the leg slightly internally rotated. The ultrasound transducer is aligned in a coronal plane over the lateral femorotibial joint line and moved cephalad to locate the junction between the lateral femoral condyle and the femoral shaft. Color Doppler assists in identifying the adjacent SLGA, which lies deep to the myotendinous junction of the vastus lateralis and directly over the bony cortex of the distal femur. In the coronal view, the nerve is visualized in short axis. Once identified, the transducer is rotated 90 degrees to obtain a long-axis view⁽¹⁹⁾.

For RFA targeting, traditional techniques involve positioning the needle at the bony cortex near the artery, around the junction of the femoral shaft and the lateral femoral epicondyle. The RF cannula is advanced in-plane from anterior to posterior, with the tip positioned midway through the femoral depth⁽¹⁾. More recently, alternative approaches have been described, such as targeting the nerve in the superolateral popliteal fossa, medial to the biceps femoris tendon. In this method, the RF cannula is inserted obliquely at approximately 45 degrees to the coronal plane and advanced in-plane until the tip reaches the posterior edge of the lateral femoral cortex. This cortical crest – the bony ridge separating the lateral and posterior cortex – is easily identifiable and serves as a useful landmark for accurate placement^(20,23) (Fig. 7).

Fig. 7.

Ultrasound scanning protocol

For ultrasound visualization, the patient is positioned with the knee flexed between 30 and 45 degrees, with the leg slightly externally rotated. The transducer is initially aligned in a coronal plane over the medial femorotibial joint line and then moved distally to identify the metaphyseal–diaphyseal junction of the tibia and the distal insertion of the MCL. The nerve appears in short axis as a small hypoechoic structure along the tibial bone surface adjacent to the inferomedial genicular vessels. Rotating the transducer 90 degrees provides a long-axis view of the nerve⁽²⁴⁾.

For RFA targeting, the traditional method involves positioning the needle at the bony cortex near the IMGA, around the proximal tibial metaphyseal–diaphyseal junction. The RF cannula is advanced inplane from anterior to posterior, with the tip situated at the mid-point of the tibial width, close to the periosteum⁽¹⁾. Alternatively, the midpoint between the most prominent part of the medial tibial condyle and the tibial insertion of the MCL can be used as a landmark. In this approach, the needle is inserted approximately 1 cm medial to the tibial tuberosity and directed toward the target, with the tip positioned at the medial edge of the medial cortex. This landmark-based technique may be particularly useful when Doppler signals from the arteries are weak, ensuring accurate nerve targeting^(23,25) (Fig. 8).

Fig. 8.

Ultrasound scanning protocol

To visualize the RPGN with ultrasound, the patient is positioned with the knee slightly flexed and the leg rotated slightly inward. The transducer is aligned in a coronal plane over the lateral femorotibial joint line, then moved distally to locate the insertion of the iliotibial tract at Gerdy’s tubercle. The transducer is subsequently rotated 90 degrees and shifted about 1–2 cm distally to visualize the tibialis anterior muscle. In short axis, the nerve appears as a hypoechoic, oval-shaped structure within the subcutaneous tissue, just above the periosteum of the anterior tibia, coursing alongside the RATA. The artery can be confirmed with Doppler imaging⁽¹⁰⁾.

For RF treatment, the target point is approximately 2 cm distal to Gerdy’s tubercle, where the nerve runs anterior to the tibial periosteum and deep to the tibialis anterior muscle, aligned with the RATA, and located near the midpoint of the tibial width. The transducer is positioned in an axial plane, and the RF cannula is inserted just lateral to the tibial tuberosity, advancing toward the bone while maintaining an in-plane approach⁽²³⁾ (Fig. 9).

Fig. 9.

Ultrasound scanning protocol

Fonkoue et al. described the “treatment line” for the IPBSN as a longitudinal path approximately 4 cm medial to the patellar apex, connecting transverse lines passing through the patellar tip and the top of the tibial tuberosity. To visualize the nerve at the knee, the probe is aligned in a coronal plane along this line, identifying the nerve as a hypoechoic, ovoid structure within the subcutaneous tissue overlying the MCL, often accompanied by a small artery. If the nerve is not visible, the probe is shifted posteromedially and cephalad (retrograde exploration) to search for it. Alternatively, the saphenous nerve may be followed from its emergence at mid-femoral level (anterograde exploration) toward the knee⁽²³⁾ (Fig. 10).

Fig. 10.

For RFA, if direct visualization is unsuccessful, an indirect targeting approach involves placing the transducer in the coronal plane along the treatment line. The RF cannula is then inserted near the superior edge of the line and advanced distally, deep into the subcutaneous tissue just above the MCL. The cannula should be positioned as deeply as possible within the subcutaneous tissue to minimize the risk of skin burns caused by the heat generated during RFA⁽²⁰⁾.

Ultrasound scanning protocol

For ultrasound visualization, the patient is placed supine with the knee slightly flexed and the leg mildly internally rotated. The transducer is aligned in a coronal plane over the lateral femorotibial joint line and subsequently moved distally to identify the LCL insertion at the fibular head. The nerve appears in its short-axis view as a small hypoechoic structure along the surface of the lateral tibial plateau, just above the fibular head and beneath the LCL. On color Doppler, it can be identified coursing with the inferolateral genicular vessels. Rotating the transducer 90 degrees provides a long-axis view. Because of the ILGN’s proximity to the CPN, there is a potential risk of accidental CPN blockade, which may result in iatrogenic foot drop if not carefully avoided^(10,29) (Fig. 11).

Fig. 11.

USG versus fluoroscopy guidance

Ultrasound guidance for genicular nerve block and RFA offers several advantages over traditional fluoroscopy. While fluoroscopy-guided procedures rely primarily on bony landmarks to approximate the position of the targeted nerves, ultrasound incorporates both bony and soft-tissue landmarks – including ligaments, muscles, and vessels – for more precise localization⁽²²⁾. Since the vascular patterns accompanying each genicular nerve are well established, arterial blood flow visualized with Doppler can further aid in the accurate identification of the nerves. An additional benefit of real-time ultrasound imaging is that it facilitates dynamic assessment, enabling clinicians to observe needle advancement relative to surrounding structures. As a result, needles can be inserted and advanced without applying direct pressure to the periosteum, reducing patient discomfort and pain during the procedure. Importantly, ultrasound does not expose patients or clinicians to radiation, making it a safer and more desirable option^(12,30).

However, when comparing nerve blocks with RFA, it is important to recognize that RFA requires greater precision because the lesion produced is typically confined to a small, well-defined area. In contrast, for nerve blocks, the injectate tends to diffuse over a broader region, making precise anatomical landmarks less critical. Ultimately, the success of both genicular nerve block and RFA depends on accurately targeting the articular branches innervating the knee joint capsule, highlighting the importance of precise visualization techniques. The growing number of publications on ultrasound-guided genicular nerve blocks and RFA for chronic knee pain reflects increasing clinical interest in these techniques⁽¹⁸⁾.