Diagnostics of infrapatellar saphenous neuralgia—a reversible cause of chronic anteromedial pain following knee surgery

The institutional review board of the local ethics committee approved this fully anonymized retrospective study. All patients at least 18 years old with anteromedial knee pain following open and arthroscopic knee surgery, ultrasound and diagnostic nerve block of the IPBSN, and a clinical follow-up between December 2018 and January 2020 were retrieved from the hospital archive from an IPBSN nerve cohort. Included patients were suspected to have neuroma-specific neuropathic pain, including spontaneous pain, electrical spikes, burning pain, allodynia, and hyperalgesia to touch, pressure, or movement. Clinical examination showed a shooting electrical pain when tapping the injured nerve (positive Hoffman-Tinel sign).

In patients with TKA, instability was defined as abnormal and excessive displacement of the articular elements, which has led to clinical failure of the arthroplasty and is known to be one of the most common causes of aseptic failure following TKA [11]. Instability comprised global instability or instability in flexion or extension. Physical examination included observations of gait and stability was assessed with varus-valgus and anterior-posterior stress tests at 30° or 90° flexion, and full extension attempting to reproduce patient symptoms which were complemented using antero-posterior and varus-valgus stress radiographs. Additionally, implant positioning and limb alignment were estimated radiologically.

All diagnostic ultrasound examinations and perineural injections were performed by three radiologists with musculoskeletal subspecialty training. All diagnostic examinations and procedures were performed with a 14-MHz linear-array transducer on LOGIQ S8 XDclear (GE Healthcare). Patients were examined in the supine position with the knee slightly flexed. Ultrasound of the IPBSN and its branches from proximal to distal followed a standardized assessment protocol to identify any structural and perineural abnormality.

First, the transducer was placed transversally at the mid-medial thigh to locate the saphenous nerve in the distal adductor canal. The canal is bordered by the sartorius muscle superficially, the vastus medialis muscle laterally, and the adductor longus muscle deeply [12, 13]. The saphenous nerve runs with the femoral artery and vein inside the canal and exits the adductor canal together with the descending genicular artery. Near the exit of the adductor canal, the saphenous nerve gives off the infrapatellar branch, emerging through the fascial plane interposed between the vastus medialis and sartorius muscle to the subcutaneous layer (Fig. 1b). Then, the IPBSN was traced distally in its subcutaneous, epifascial location (Fig. 1c) until the branch subdivided into one, two, or three terminal branches. In most of our cases, there was a bifurcation with the superior branch running transversely just inferior to the distal pole of the patella and the inferior branch following the medial border of the patellar tendon to the tibial tubercle. If detectable, all branches were followed until their most distally visible part was reached.

Neuromas were classified as a terminal neuroma or neuroma-in-continuity according to the Seddon and Sunderland classification [14] as a form of ineffective, unregulated nerve regeneration. Basic sonographic findings include focal enlargements with or without the disorganization of the internal fascicular structure indicating a neuroma-in-continuity, or a terminal neuroma in case of nerve transection. One patient with a surgery-proven neuroma-in-continuity of the IPBSN is shown in Fig. 2a and b.

Scar-tethered nerves as defined by Elliot and Sierakowski might cause nerve distortion by shearing, resulting in bursts of pain [15]. In patients with entrapment neuropathy due to overlying scaring, diffuse swelling with decreased echogenicity of the nerve can be sonographically apparent (Fig. 2c, d). The pathophysiological mechanism is thought to be external compression of the nerve causing interference with the intraneural microvasculature, resulting in ischemia or venous congestion, which can lead to epineural edema and increased endoneural fluid [16]. Long-standing scar-tethering with external compression can lead to fibrosis.

Additionally, the nerve size of the IPBSN was measured using diameter (mm) and cross-sectional area (CSA; mm²), which was traced within the hyperechoic epineural rim of the nerve with the transducer perpendicular to the nerve.

When nerve entrapment or a neuroma was identified, the abnormal appearing segment of the nerve was targeted. In cases in which the nerve appeared normal, the injection was directed to the proximal IPBSN (Fig. 3).

Correlation with physical symptoms as tenderness elicited on palpation of the IPBSN (Hoffmann-Tinel sign) or associated paresthesia in the nerve distribution aided in selecting an injection site. Standard sterile techniques were used. Local anesthesia was administered subcutaneously with 1% lidocaine through a 22- or 25-gauge needle, depending on the nerve location depth. In six cases of perineural therapeutic injection, circumferential infiltration around the outer epineurium of the nerve was performed under ultrasound guidance with 1% lidocaine in combination with a water-soluble steroid, dexamethasone (4 mg/mL), in volumes ranging from 1.0 to 5.0 mL.

Pain relief was recorded following a four-step scale using the visual analog scale (VAS; 0–10; 0 = no pain, 10 = severe pain) before and 40 min following diagnostic nerve block: excellent (completely resolved pain according to an absolute VAS reduction to 0 or 1 point or a VAS decrease of > 75%), good (pain decrease of 50–74%), fair (pain decrease of 25–49%), poor (pain decrease of < 25% or increased pain). Patients with excellent and good responses to a diagnostic nerve block were assigned to the responder group, and patients with fair and poor responses were assigned to the non-responder group.

A retrospective review of the electronic medical records for patient demographic data, history with details of previous and revision knee surgery, interventional pain procedures (IPP), and follow-up symptomatic relief (full/partial/no pain relief and presence/absence of Hoffmann-Tinel sign) as well as post-procedure complications was performed. Following therapeutic nerve treatment, all patients were assigned into two groups according to their pain relief: positive (full pain relief) and negative (partial and no pain relief).

Precise treatment strategy to surgical or interventional procedures of the IPBSN was determined on a case-by-case basis, depending on the presence of structural abnormalities identified with ultrasound and pain reduction after diagnostic nerve block.

Descriptive data are expressed either as the mean with standard deviation (SD) or n with percentages. Univariate analysis using t-tests for interval-scaled continuous variables and chi-square tests for nominal or ordinal variables was applied to compare responders with non-responders to diagnostic treatment and to compare positively versus negatively reacting patients to therapeutic nerve treatment; p ≤ 0.05 was considered statistically significant. All variables with a p value ≤ 0.05 were integrated into a multivariate step-by-step forward logistic regression model. Statistical analysis was conducted by using SPSS Statistical Software Package (IBM).

Mean caliber of the unremarkable IPBSN was 1.1 ± 0.2 mm (mean CSA, 0.9 ± 0.4 mm²), in patients with entrapment 1.4 ± 0.5 mm (mean CSA, 1.8 ± 1.6 mm²) and with neuromas was 3.4 ± 1.3 mm (mean CSA, 10.7 ± 8.2 mm²).

Responders more often showed Hoffmann-Tinel signs (78 vs. 28%) and more likely had an entrapment or a neuroma of the IPBSN (97% vs. 6%) compared to non-responders. The mean pre-interventional VAS score of all patients was 6.4 (range, 2–10) and post-interventional 2.2 (range, 0–8): while this pain did not differ prior to the examination, it was significantly smaller in the responder group afterwards (VAS 0.9 vs. 5.0).

In a stepwise logistic regression model of the univariately significant variables (except sonographic IPBSN pathology and pain), positive Hoffmann-Tinel signs (p = 0.002) and the absence of knee joint instability (p = 0.029) were the only criteria to predict positive results of a diagnostic nerve block. The other factors did not reach statistical significance. Table 2 displays the combination of these two factors: 90% of patients with Hoffmann-Tinel sign and knee joint stability, 14% of patients without Hoffmann-Tinel sign and with knee joint instability, and 53% of patients with only one of those two positive criteria responded to the diagnostic nerve block.

A summary of all therapeutic treatment options is illustrated in Fig. 4. Thirty-four responders (92%) received an IPBSN treatment whereas this was the case for six (33%) of the non-responders due to clinical suspicion of possible infrapatellar saphenous neuralgia despite the negative diagnostic nerve block result, comprising a total of 40 patients. Seven non-responders received another therapy (n = 3 revision TKA; n = 2 arthrolysis and n = 1 anterior cruciate ligament reconstruction). Three responders did not undergo nerve treatment: of these, one patient was diagnosed with a TKA instability causing secondary mechanical stress on the IPBSN, which rather required revision surgery to a constrained TKA. Another two responders reported complete pain resolution after diagnostic nerve block at the time of clinical follow-up.

Of the 40 patients who received a therapeutic nerve treatment (24 women and 16 men with a mean age of 56.6 ± 13.0 years, range 18–78), 28 (70%) sustained full, seven (17.5%) partial, and five (12.5%) no pain relief after treatment (Fig. 4).

For the univariate analysis, patients with negative or partial pain relief were grouped together and compared with those with full pain relief (Table 3). All patients with positive pain relief showed entrapment or neuromas and were responders with lower VAS values following diagnostic nerve block. Additionally, knee joint instability was significantly less frequently evident (4%) in patients with full pain relief as compared to the negative group (25%; p = 0.038).

In a stepwise logistic regression model of the univariately significant variables, the result of sonographic examination (p < 0.001) and absence of knee joint instability (p = 0.050) were the only ones to predict positive results of nerve treatment. Figure 5 shows the detailed influence of response and knee joint instability on pain relief. Table 4 displays the combination of these two factors: 87% of responders with stable knee joints, 33% of responders with knee joint instability, and none of the non-responders reacted positively on IPBSN treatment.