1Department of Physical Medicine and Rehabilitation, Tarlac Medical Center, Tarlac, Philippines
2Research Center for Health Sciences, Faculty of Medicine and Surgery, University of Santo Tomas, Manila, Philippines
3Department of Physical Medicine and Rehabilitation, Our Lady of Lourdes Hospital, Manila, Philippines
4Department of Physical Medicine and Rehabilitation, SPC Medical Center, San Pablo, Philippines
Knee osteoarthritis (OA) has a global joint involvement where cartilage, ligament, menisci, and synovium
are affected. However, few case studies have reported the involvement of the fi bular nerve as the possible pain
generator in knee OA. We report a case of a 62-year-old female who has had chronic right knee OA with acute
pain exacerbation of 4 months duration. Ultrasound of the right knee showed the following results: knee OA,
medical meniscal protrusion, suprapatellar effusion, and fi bular neuropathy. Aspiration of synovial fl uid from
the suprapatellar area and hydrodissection or perineural deep injection of the fi bular nerve were performed,
resulting in signifi cant pain reduction. Fibular neuropathy should be considered a pain generator in knee OA,
especially that pain is felt at the lateral aspect of the knee.
A 62-year-old female was initially evaluated because of right knee pain. The patient has had bilateral knee osteoarthritis (OA) since 2010. The pain was bearable, intermittent, and diffused with a grade of 2–4/10. However, 1 month before consultation, there was an increase in the intensity of right knee pain. Two intraarticular steroid injections were done at 4 weeks interval, which did not afford pain relief. She was referred to rehabilitation medicine. Lifestyle: The patient is a dentist. She does household chores, mainly cleaning the fl oor using a mop.
On consultation, the right knee pain had a visual analogue scale score of 9/10. Pain was dull, diffused, and aggravated by prolonged standing and walking. She was ambulatory with an antalgic gait. On physical examination of the right knee, there was warmth with suprapatellar swelling. Genu valgus deformity with pronation of the foot was observed. Range of motion was limited to 20–90°. Muscle strength was normal, On consultation, the right knee pain had a visual analogue scale score of 9/10. Pain was dull, diffused, and aggravated by prolonged standing and walking. She was ambulatory with an antalgic gait. On physical examination of the right knee, there was warmth with suprapatellar swelling. Genu valgus deformity with pronation of the foot was observed. Range of motion was limited to 20–90°. Muscle strength was normal,and no sensory deficit was present on the right lower extremity. Atrophy of the right quadriceps was evi-
X-ray of both knees 1 month before consultation showed degenerative osseous changes with narrowing of the knee joint spaces, more on the right knee.
Ultrasound of the right knee using Sonosite Edge with a linear probe (8–15 Mhz) showed suprapatellar effusion measuring 2.25 cm in longitudinal and 0.90 cm as widest anteroposterior diameters in the sagittal scan, medial meniscus protrusion of 0.40 cm from medial joint space, decreased clarity of the cartilage at the trochlear area, and an increase in the cross-sectional area of the fi bular nerve at the level of the fi bular head with a cross-sectional area of 20 mm2 in the transverse scan and anteroposterior diameter of 28 mm in the sagittal scan. There was pain elicited at the course of the fi bular nerve on sonopalpation.
Ultrasound-guided intervention aspiration of the joint fluid from the suprapatellar area was performed where 10 mL clear, straw-colored fluid was aspirated, and hydrodissection of the right fibular nerve at the level of the knee using 1 mL of 2% lidocaine and 5 mL normal saline solution (Figure 1).
After the procedure, the patient felt instant relief from pain and was able to ambulate without any gait deviation.
Follow-up after 1 week showed a decrease in pain to 5/10. The right knee was not warm and not tender to touch. There was a limited range of motion of the right knee, but no pain was felt during flexion and extension.
The patient was initiated to physical therapy to decrease pain further and improve range of motion. Medications given were pregabilin, collagen hydrolysate, and vitamin B complex. She had physical therapy for 1 month.
A follow-up was done after 1 month, showing that the patient only complained of minimal stiffness in the morning. Physical examination of the right knee was essentially normal except for the limited range of motion of the right knee. She was discharged from formal physical therapy and was advised to do a home exercise program. Collagen hydrolysate and vitamin B were continued for 1 more month.
The patient was seen 15 months after the initial consult. There was no complaint of pain, but she felt minimal stiffness of the right knee in the morning. There was still a limited range of motion at 20–98°.
A repeat ultrasound showed minimal anechoic fluid collection with synovial proliferation on the suprapatellar area and a decrease in the fibular nerve dimension with a cross-sectional area of 11 mm2 and 5 mm2 at the level of the popliteal fossa and fibular head, respectively.
Our patient presents with a valgus arthritic knee with an associated fibular neuropathy. There are few reported cases of fibular neuropathy in patients with knee OA. All studies presented foot drop as the chief complaint except for the study of Sanchez et al.  in 2011, which had swelling and pain in the posteromedial popliteal fossa. Because pain is the symptom of knee OA, fibular neuropathy is usually not considered a possible pain generator in knee OA until motor weakness becomes noticeable (Table 1) . Fibular neuropathy should be considered a source of knee OA pain if the patient complains of pain in the lateral aspect of the knee. Most patients with knee OA present with generalized knee pain (33%) or medial knee pain (either in isolation or with peripatellar knee pain or lateral knee pain [34%]) 
Studies assessing these changes showed varying involvement of morphologic and inflammatory chang es that produce knee OA pain [3-5]. However, no study included the fibular nerve in sonographic assessment because the nerve is not an integral part of the structures that compose a synovial joint. Our patient has the following morphologic and inflammatory changes that could be the cause of her knee pain: suprapatellar effusion, decreased clarity of joint cartilage, and protrusion of the medial meniscus. However, this could not explain the presence of lateral knee pain.
In our ultrasound evaluation, the sonologist assessed that the fibular nerve’s cross-sectional area was increased to 20 mm2 , representing swelling of the nerve found in patients with fibular neuropathy (normal value: 0.10 ± 0.20 cm2 ) . Furthermore, sonopalpation showed that the course of the pain was similar to the course of the fibular nerve .
Although nerve conduction velocity is used to diagnose the presence of fibular neuropathy, nerve conduction study may be normal in the acute phase, and abnormal findings may take up to 3–4 weeks to fully develop .
Fibular neuropathy in patients with OA had been associated with either nerve traction or compression injury (Table 1) [9-12]. Our patient has the following risk factors that could cause compression and traction injury of the fibular nerve: the presence of genu valgus and her habit of mopping the floor, which causes continual flexion and extension of the knee.
The management of the patient is two-fold. First, there is the presence of suprapatellar effusion that was treated with arthrocentesis. Knee effusion was considered significant when the maximum anteroposterior diameter of the suprapatellar recess was greater than 4 mm . Intraarticular steroid was not injected because the patient recently had two steroid injections. Guidelines recommend that there should be at least a minimum of 4 to 6 weeks before steroid injection should be repeated. Furthermore, steroids should not be repeated if at least 4 weeks of symptomatic relief were not achieved after two injections .
Second, fibular neuropathy was present, which could intensify the pain felt by the patient. According to Baima and Krivickas  in 2008, pain is the earliest manifestation of fibular neuropathy but is the most difficult to treat. Medications and physical therapy modalities could be used for the treatment. In the case studies included in our review, all treatments involved surgical intervention (Table 1).
A novel technique was the hydrodissection of the fibular nerve with 1 mL of 2% lidocaine and 5 mL of normal saline solution. Hydrodissection or perineural deep injection involves injecting volume into scars or fascia to release entrapped nerves. Nerves move smoothly over the fascia in the longitudinal and mediolateral directions. If they are entrapped within the fascia, it could produce pain and autonomic dysfunctions. In doing hydrodissection, saline solution, D5 water, and local anesthetics are used as injectate. Local anesthetics are used to distinguish nerve pain from other pain generators in knee OA. In our patient, pain was instantaneously relieved with hydrodissection, indicating that the pain generator is the fibular nerve. It is performed with ultrasound guidance so that the needle tip is placed perineurally and not intraneurally, which could cause nerve damage . Two systematic reviews on ultrasound-guided nerve hydrodissection showed the effectiveness of the procedure. However, the reviews included studies involving the median, ulnar, saphenous, and lateral femoral cutaneous nerves [17,18]. Further studies are needed to assess its effectiveness in fibular neuropathy.
In summary, in patients with recalcitrant pain in knee OA, fibular neuropathy could be a possible cause of the pain, especially if the pain is felt on the lateral aspect. Ultrasound can be employed in its assessment by measuring its cross-sectional area and by sonopalpation. Nerve hydrodissection is a novel management that could be used in its treatment if conservative treatment fails.
Figure 1. Musculoskeletal Ultrasound (MSUS) of the Right Knee
(A-C) MSUS of the Right Knee (October 13, 2018). (A) Suprapeatellar effusion measuring 2.25 cm in longitudinal diameter and 0.90 cm in an-teroposterior diameter in sagittal scan. (B) Peroneal nerve measuring 20 mm2 in cross-section at level of the fibular head. (C) Peroneal nerve in longitudinal view measuring 28 mm. Procedures done (D-F). (D, E) Aspiration of the joint fluid from right suprapatellar, (F) Hydrodissection of right peroneal nerve; (MSUS of Right knee [January 7, 2020] [G-I]): (G) Suprapatellar effusion measuring 1.66 cm in longitudinal diameter and 0.70 cm in antero-posterior diameter in sagittal scan. (H) Peroneal nerve in cross-section measuring 5 mm2 at the level of fibular head and (I) Pe- roneal nerve in cross-section measuring 11 mm2 at the level of popliteal space.
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