Neuritis of the Infrapatellar Branch of the Saphenous Nerve: A Case Study of Chronic, Mis-Labelled Medial Knee Pain

Why this case?

Chronic medial knee pain is one of the most common presentations in a musculoskeletal clinic and is often attributed to intra-articular pathology, pes anserine bursitis, or medial collateral ligament (MCL) strain. 

Neuropathic contributors are frequently overlooked, particularly the infrapatellar branch of the saphenous nerve (IPBSN), which runs through precisely the region where clinicians most often palpate for “pes anserinus tenderness”. 

When IPBSN neuritis is missed, patients can spend years cycling through injections, physiotherapy and imaging without ever targeting the true pain generator.

This case highlights how an active young woman with 3.5 years of bilateral posteromedial knee pain – previously labelled as reactive arthritis, medial meniscal pain, and refractory pes anserine bursitis – was ultimately found to have bilateral IPBSN neuritis, confirmed by a diagnostic ultrasound-guided saphenous nerve block.

Key Takeaways & Learning Points

• Think neuropathic in chronic, “refractory” medial knee pain. Pain that behaves unpredictably, flares with minimal activity, radiates along a nerve distribution rather than a joint-line distribution.
• The IPBSN runs directly through the pes anserinus. A temporary response to pes anserine bursal injection does not confirm bursitis – it may simply reflect inadvertent peri-neural local anaesthetic effect.
• Anatomy-driven palpation is diagnostic. Very focal tenderness at the point where the IPBSN emerges from beneath the sartorius, with symptom reproduction and a Tinel-like response, is a highly specific examination finding.
• Ultrasound is invaluable. High-resolution ultrasound can visualise the saphenous nerve in the adductor canal and often the IPBSN itself, guide diagnostic/therapeutic injection, and simultaneously assess peri-meniscal structures, bursae and tendons.
• Structural and neuropathic drivers can coexist. A medial meniscal tear with a parameniscal cyst can sensitise the adjacent IPBSN; targeting both is sometimes necessary.
• Extracorporeal shockwave therapy (ESWT) has a dual role. It treats chronic tendinopathic/bursal changes around the pes anserinus and also modulates peripheral pain pathways and raises nociceptor thresholds – useful in sensitised patients.
• A diagnostic block is the reference standard. Numbness in the saphenous and IPBSN distribution after an ultrasound-guided adductor canal block, mapping onto the patient’s pain, confirms the nerve’s role in symptom generation and guides definitive management.
• Central and peripheral sensitisation after years of pain is real. It influences response to any single intervention and often requires a multi-modal, stepwise treatment plan.

Clinical Presentation

History

A 33-year-old otherwise well, highly active woman presented with a 3.5-year history of bilateral posteromedial knee pain.

• Onset: An acute polyarthritic episode in 2022 – acute bilateral knee swelling and pain progressing within 48 hours to involve wrists, ankles, fingers and shoulders after an episode of illness. 

Lyme serology and rheumatological screens were undertaken.

• Initial rheumatology course: Provisional diagnosis of reactive arthritis, escalated to “rheumatoid arthritis” (sulfasalazine and hydroxychloroquine) in 2022, then revised to seronegative arthritis in January 2023 with withdrawal of disease-modifying therapy. 

Widespread joint symptoms resolved, but bilateral knee pain persisted.

• Character and distribution: Pain localised to the posteromedial aspect of both knees, radiating down the medial side into pes anserine. Episodes of the knee flaring on gentle inclines, triggering several days of pain. 

Pain was unpredictable – occasionally provoked by trivial tasks (stairs, dancing) and occasionally absent during full lower-limb gym sessions.

• Previous interventions:
  • Intra-articular steroid to the right knee (December 2024) – no benefit; severe cortisol flare.
  • Bilateral pes anserine bursal steroid injections (May/June 2025) – five weeks of near-complete relief, then return of pain worse than baseline.
  • Second course of bilateral pes anserine bursal injections (September 2025) – minimal benefit; only slight reduction of inflammation on follow-up imaging.
• Functional impact: Pain on most days, often at rest, waking from sleep with knee pain, and marked unpredictability preventing confident planning of daily activities.

Past Medical History

• Venous malformation of the left calf (sclerotherapy and HIFU over 10 years earlier), with resultant reduced left ankle dorsiflexion.
• Small medial meniscal tear aged 12 of the right knee

Drug History

• Vitamin D, whey protein, turmeric/curcumin
• Previously: sulfasalazine, hydroxychloroquine, naproxen, co-codamol – all discontinued.

Exercise History 

Prior to pain starting:

• Strength training 3-4 times per week with a personal trainer.
• Walking 4-5 times weekly.
• Regular yoga; previously boxing.
• Modified programme with reduced quad-dominant and knee-bearing work; wide-stance and heel-raised squatting to accommodate restricted left ankle dorsiflexion.

Clinical Examination

• Gait and functional testing: Double-leg squat tolerated without significant discomfort. Single leg control poor bilaterally. Straight leg raise mildly weak bilaterally.
• Tenderness: Significant tenderness, hyperaesthesia running from proximal to the medial joint line to the pes anserinus bilaterally, right greater than left, disproportionate to imaging findings (MRI below).
• Provocation tests: McMurray’s positive for medial joint line tenderness bilaterally; pain on resisted internal rotation testing of the tibia.
• Collateral ligaments: Valgus and varus stress testing intact bilaterally.
• Hamstrings: Resisted hamstring testing not-painful, normal strength. Some tenderness on palpation of semimembranosus insertion.
• On repeat examination (week 4): Very focal points of tenderness along the medial aspect of both knees, corresponding with the anatomical location at which the IPBSN branches from the main saphenous nerve into the pes anserine – confirmed on ultrasound.

Imaging and Investigations

Ultrasound (initial presentation)

• Right knee: No effusion. Mildly extruded medial meniscus with a parameniscal cyst (1.8 × 0.7 cm) over the anteroproximal region. Small pes anserine bursal fluid. Semimembranosus insertion mildly oedematous. Pes anserinus tendons intact, no Doppler signal.
• Left knee: Intact superficial and deep MCL. Medial meniscus normal. Small pes anserine bursal fluid, intact tendons. Similar semimembranosus oedema.

MRI (previous scans, Whittington, 2023–2024, reviewed)

• Confirmation of a medial meniscal tear of the right knee – source of mechanical right knee symptoms.
• Parameniscal cyst of right medial meniscus corresponding to the ultrasound findings.
• Bilateral “minor pes anserine bursitis” on report – however arguably within normal limits
• No other intra-articular pathology accounting for the magnitude of symptoms.

Dynamic ultrasound (week 4 review)

• Visualisation of the saphenous nerve in the adductor canal and tracking of the IPBSN as it pierces the sartorius and crosses towards the medial knee. Focal tenderness reproduced directly over the IPBSN bilaterally.

Differential Diagnosis and Clinical Reasoning

Pes anserine bursitis / tendinopathy

The initial diagnosis, supported by medial tenderness, semimembranosus insertion oedema and short-lived response to bursal injection. 

However, the magnitude of tenderness was disproportionate to the imaging findings, and the second course of steroid injections produced minimal benefit – unusual if bursitis were the dominant driver.

Medial meniscal pathology / parameniscal cyst

The right-sided parameniscal cyst and confirmed meniscal tear are credible contributors to mechanical right-sided symptoms (“blocking out” on inclines) and local irritation. 

This does not, however, explain the bilateral and highly neuropathic-sounding nature of the pain.

Medial collateral ligament injury

Excluded on examination (stable to valgus stress) and imaging.

Inflammatory arthropathy (reactive / seronegative / RA)

Relevant historically but polyarthropathy resolved off DMARDs with no other joint involvement. Serology and follow-up scans do not support active arthropathy as the current driver of bilateral knee pain.

Referred pain from hip or lumbar spine

Hip examination and imaging were unremarkable; pain distribution did not follow an L3/L4 dermatomal pattern.

IPBSN neuritis / saphenous neuropathy

Emerged as the leading diagnosis on repeat examination. Supporting features included:

• Exquisite, reproducible focal tenderness along the known anatomical course of the IPBSN, maximal where the nerve emerges from the sartorius next to vastus medialis.
• Pain character – sharp, unpredictable, episodic, with disproportionate flares – more neuropathic than inflammatory.
• Transient response to peri-anserine injections explained by local anaesthetic effect on the adjacent nerve rather than cure of a bursal pathology.
• Bilateral pain distribution fitting the IPBSN territory (anteromedial knee, peripatellar, distal medial leg).
• Ultrasound visualisation of tenderness directly over the nerve.

Management

Phase 1 – Shockwave therapy and structural assessment (weeks 1–6)

A course of extracorporeal shockwave therapy (ESWT) was delivered bilaterally to the pes anserinus region with two aims: to treat the chronic bursal/tendinopathic changes and to exploit ESWT’s capacity to modulate peripheral nociceptor activity and raise pain thresholds.

• Session 1: Bilateral pes anserine and more proximal towards medial joint line. 2,000 impulses at 1.5 bar bilaterally – more painful on the left, tolerated well.
• Session 6: Repeat bilateral pes anserinus and IPBSN region with 2,000 impulses at 2.2 bar.

The patient initially reported several days of significant improvement after each session before slowly returning to baseline, suggesting partial but not durable effect. 

The pattern – clear response followed by recurrence – was consistent with a nerve-dominant pathology for which ESWT alone may be insufficient. Subsequent sessions produced a longer lasting effect and overall symptoms were 40% better at the end of the 6 sessions. 

Activity was maintained at a modified level, avoiding excessive quad-dominant and deep-flexion loading.

Phase 2 – Diagnostic and therapeutic ultrasound-guided intervention

Following completion of the ESWT course, a combined ultrasound-guided procedure was undertaken:

• Right medial parameniscal cyst: aspiration followed by injection of 20 mg Depo-Medrone with 2 mL 1% lidocaine under ultrasound guidance.
• Bilateral saphenous nerve blocks at the adductor canal: perineural injection of 4 mL of 0.25% bupivacaine with 3.3mg of dexamethasone under ultrasound guidance, targeting the saphenous nerve proximal to the take-off of the infrapatellar branch.

Written consent was obtained, and the risks (infection, nerve injury, and potential partial spread to the motor nerve to vastus medialis) were explained.

Outcome

Within 10 minutes of the block, the patient reported numbness throughout the saphenous distribution and the IPBSN territory – mapping precisely onto the regions of her pain. This confirmed the diagnosis of bilateral IPBSN neuritis as the dominant pain generator.

The patient ambulated immediately post-procedure (saphenous is a purely sensory nerve however sometimes vastus medialis may be slightly affected temporarily due to anaesthetic spreading to the femoral nerve branch to vastus medialis) and returned home with planned functional testing – including hill-walking during a planned Christmas trip – as a real-world assessment of durability of response. 

Contact by patient 8 weeks later confirmed durability of response as patient was able to do long, hilly hikes for the first time in several years without significant pain and overall her pain was at least 80% better.

Discussion

Anatomy of the saphenous nerve and the IPBSN

The saphenous nerve is the largest purely sensory branch of the femoral nerve. 

It descends through the femoral triangle and enters the adductor (Hunter’s) canal, travelling alongside the femoral artery under the sartorius. 

Within or just distal to the canal, the saphenous nerve pierces the fascia between the sartorius and gracilis and divides proximal to the medial femoral condyle into:

• The sartorial branch, supplying skin over the medial leg and medial malleolus.
• The infrapatellar branch (IPBSN), which emerges from above, below or from the sartorius and fans out anteromedially across the knee to innervate the anteromedial peripatellar region and the skin overlying the pes anserinus.

Crucially, part of the IPBSN’s course takes it directly across the pes anserinus – the exact site targeted in bursal injections and in many standard medial-knee physiotherapy releases. 

This anatomical fact underpins why IPBSN neuritis can be mislabelled as pes anserine bursitis, and why bursal injections may give transient relief through an unintended peri-neural effect or through less mass effect on the nerve from bursa.

Aetiology of IPBSN neuritis

Reported causes include:

• Iatrogenic injury – arthroscopic and open knee surgery (most commonly medial meniscectomy, ACL reconstruction with hamstring harvest, total knee arthroplasty, medial collateral ligament repair), saphenous vein harvesting for CABG, and varicose vein procedures.
• Direct trauma – blunt blows to the medial knee, dashboard injury, kneeling-type compression (e.g. flooring, plumbing, religious kneeling, yoga).
• Entrapment – between the sartorius and gracilis at the point of nerve emergence, aggravated by repetitive flexion/extension or tight compressive clothing.
• Chronic traction / friction – adjacent tendinopathy or bursitis producing local fibrosis around the nerve.
• Adjacent pathology – medial meniscal tears and parameniscal cysts, as in this case, can create a perineural inflammatory milieu. MCL thickening can also contribute.
• Metabolic / systemic – diabetes mellitus, post-inflammatory states following polyarticular arthritis (plausibly contributory in this patient), and varicose disease.
• Idiopathic – a proportion of cases have no identifiable cause.

Clinical features

Patients typically describe:

• Medial or anteromedial knee pain, often burning, stinging or electric in character.
• Allodynia, hyperaesthesia or paraesthesia over the anteromedial knee.
• Focal tenderness reproducible at the point where the IPBSN crosses the sartorius/pes anserinus.
• A positive Tinel sign over the nerve, although does not need to be present.
• Symptom reproduction with knee flexion/extension or with pressure from clothing, sports equipment or kneeling.
• Poor or transient response to intra-articular and peri-bursal injections.

Investigation

Diagnosis is primarily clinical, supported by:

• High-resolution ultrasound – dynamic assessment to identify nerve swelling, scarring, perineural fluid and to reproduce symptoms on direct probe pressure. Ultrasound also excludes bursitis, tendinopathy and cysts.
• MRI – useful to exclude meniscal, ligamentous and cartilage pathology, and occasionally demonstrates perineural oedema.
• Diagnostic local anaesthetic block – the gold standard. An ultrasound-guided perineural injection at the adductor canal or directly around the IPBSN that abolishes the patient’s typical pain within the duration of the anaesthetic confirms the nerve as the pain generator.

Treatment

Management follows a stepwise, multi-modal pathway:

1. Conservative – activity modification (including kneeling avoidance), neuropathic analgesia (amitriptyline, gabapentin, pregabalin, duloxetine), topical lidocaine or capsaicin, desensitisation, and physiotherapy focused on soft-tissue mobilisation, graded exposure and lower-limb biomechanics.
2. Extracorporeal shockwave therapy – increasingly evidenced for peripheral neuropathic pain through effects on nerve conduction, vascularisation and nociceptor modulation, as well as any coexisting tendinopathy.
3. Ultrasound-guided perineural injection or hydrodissection – local anaesthetic with or without corticosteroid; diagnostic and therapeutic.
4. Pulsed radiofrequency (PRF) – non-ablative modulation of the nerve, useful for refractory cases.
5. Cryoneurolysis / chemical neurolysis – reserved for recalcitrant cases with clear diagnostic block response.
6. Surgical neurolysis or neurectomy – a final option when less invasive measures fail; outcomes are generally favourable given the purely sensory role of the IPBSN.

Lessons from this case

• A 3.5-year diagnostic odyssey is not uncommon in IPBSN neuritis. Systemic arthropathy, meniscal pathology and bursitis all share the anatomical territory, and none fully explained this patient’s bilateral, unpredictable and disproportionately tender presentation.
• Peripheral and central sensitisation should be actively considered in any patient with chronic pain disproportionate to imaging. 

This informs both the choice of intervention (for example, favouring ESWT and neuromodulatory techniques) and realistic expectations around treatment response.

• Bilateral IPBSN involvement is uncommon but plausible, particularly in patients with previous bilateral peri-anserine injection exposure, post-inflammatory pathway sensitisation, or bilateral structural drivers.
• Short-lived response to local injection should prompt a rethink of the diagnosis, not simply a repeat of the same intervention.

Take Home Message

In any patient with chronic medial knee pain that is refractory to conventional joint- and bursa-directed treatment, consider neuritis of the infrapatellar branch of the saphenous nerve. 

Anatomy-led palpation, dynamic ultrasound and a well-placed diagnostic block can turn an apparently intractable problem into a specific, treatable diagnosis – often after years of misattribution. 

A stepwise approach combining structural treatment (addressing the meniscal tear or cyst), neuromodulatory therapy (ESWT) and ultrasound-guided peri-neural injection offers a rational, evidence-based route to meaningful and durable relief.

Get in touch

If you are experiencing persistent knee pain that has not responded to conventional treatment, or you are a clinician caring for a patient with a similar presentation, please contact LBSM for a specialist sport and exercise medicine opinion, including dynamic ultrasound assessment and ultrasound-guided interventional treatment.