Differentiating Medial Knee Pain: Infrapatellar Saphenous vs. Inferior Medial Genicular Nerves

Medial knee pain is common in patients with osteoarthritis, ligamentous instability, and postoperative or overuse syndromes. Two frequent—but often conflated—pain generators live in the same neighborhood: the infrapatellar branch of the saphenous nerve (IPS) and the inferior medial genicular nerve (IMGN). Understanding how to find and treat each one can significantly improve outcomes.

Quick Anatomy Review

• Saphenous nerve & IPS branch: The saphenous nerve originates from the femoral nerve and travels through Hunter’s (adductor) canal, providing cutaneous sensation along the medial knee, calf, and ankle. The infrapatellar branch is a small, recurrent sensory branch that innervates the anteromedial infrapatellar region—superficial, within subcutaneous fascial planes above the pes anserine tendons and superficial to the MCL.
• Inferior medial genicular nerve (IMGN): A capsular branch accompanying the inferior medial genicular artery, curving around the medial tibial flare to innervate the inferomedial joint capsule. It sits deep to the MCL, adjacent to the tibial cortex.

Why They’re Easy to Confuse

Patients often report focal tenderness over the medial tibial plateau/infrapatellar area, where both IPS (superficial, cutaneous) and IMGN (deep, capsular) converge clinically. Palpation alone can be inconclusive; you may elicit tenderness over the pes anserine region, MCL, or along the saphenous track to the medial malleolus without confidently assigning the driver.

Ultrasound Roadmap

1. Landmarks: Place the probe over the medial tibial plateau. Identify the tibial cortex as a bright hyperechoic line (the tibial flare). Superficial to cortex, you’ll visualize the MCL with linear fibrous architecture; superficial to the MCL are the pes anserine tendons.
2. Find the IMGN (via its artery): Activate power Doppler and look for the inferior medial genicular artery at the tibial flare, just deep to the MCL. Adjust Doppler gain high enough to catch small-vessel flashes (too low and you’ll miss it; too high and you’ll get speckle). The nerve tracks with the artery—you may not always visualize the nerve, but the artery is your beacon.
3. Locate the IPS branch: Scan superficial subcutaneous fascial planes over the anteromedial infrapatellar region, above pes anserine and the MCL. The IPS lies in these planes as small hypoechoic fascicles within the fascia.

Treatment Strategy: Layer by Layer

• Superficial (IPS): For cutaneous, burning, or pinpoint medial infrapatellar tenderness, perform perineural hydrodissection of the IPS within the subcutaneous fascial planes. D5W (5–10 mL) is commonly used to separate fascial layers and down-regulate the irritated branch.
• Deep (IMGN): For capsular, “inside the joint” ache with focal tenderness at the tibial flare, target the IMGN deep to the MCL, again using hydrodissection (≈5 mL D5W) around the artery-nerve bundle. This can reduce intra-articular–type pain and relieve entrapment at the capsular margin.
• Adjuncts: Track tenderness along the saphenous route (Hunter’s canal to medial malleolus) to identify broader saphenous involvement. Combine with standard OA and instability care plans as indicated.

Clinical Takeaway

Think in layers: superficial cutaneous pain suggests IPS; deep capsular pain points to IMGN. Use ultrasound landmarks (tibial cortex → MCL → pes anserine) and power Doppler to confidently identify the IMGN via its artery, and treat each plane with targeted hydrodissection. Precise diagnosis plus minimally invasive perineural techniques can meaningfully improve medial knee outcomes.