From diagnosis to rehabilitation of trigger finger: a narrative review

Abstract

Trigger finger (TF), also known as stenosing flexor tenosynovitis, is a common pathology of the fingers causing functional deficit of the hand. In recent years, new therapeutic approaches such as extracorporeal shock wave therapy (ESWT) and ultrasound-guided (USG) procedures have joined the most traditional conservative treatments as the adaptation of daily activities involving the affected hand and the orthosis. Likewise, the ultrasound (US) examination of the affected finger using modern high-frequency probes has progressively become part of the comprehensive assessment of patients with TF coupled with the medical history, the physical examination, and the functional scales. In this sense, considering the technological advances in both diagnostic and therapeutic fields, the non-surgical strategies have progressively grown defining a rehabilitation panel more complex than in the past. The present manuscript aims to provide an updated practical guide for clinicians and surgeons reviewing the state-of-art of both the assessment and the treatments of patients with TF to plan tailored rehabilitation management taking advantage of the matching of traditional and novel techniques.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-024-08192-5.

Introduction

Recently, novel approaches such as extracorporeal shock wave therapy (ESWT) and ultrasound-guided (USG) procedures have joined the most traditional conservative treatments as the adaptation of daily activities involving the affected hand and the orthosis in the management of the trigger finger (TF). Moreover, high-resolution ultrasound (US) imaging has progressively merged with the medical history, physical examination, and functional scales defining a more comprehensive assessment of this frequent musculoskeletal disorder.

In this sense, due to technological advances a wide rehabilitation panel is progressively mounting in recent years to optimize the management of TF patients in diagnostic and therapeutic fields. The present manuscript aims to provide an updated practical guide for clinicians and surgeons reviewing the state-of-art of both the assessment and the treatments of patients with TF to plan tailored rehabilitation management taking advantage of the matching of traditional and novel techniques.

Anatomy

Trigger finger (TF), also known as stenosing flexor tenosynovitis, is a common hand pathology causing significant loss of function in the upper limb. It is characterized by locking phenomena, palpable nodule formation, and thickening localized to the flexor tendons or its sheath [1, 2]. Patients may complain of locking of the digit during either flexion or extension, but the extension is usually more painful and limited compared to flexion. The smooth movements of the flexor tendons of the hand are normally guaranteed by a stabilizing mechanism consisting of an inner synovial layer and an outer retinacular layer (i.e., the pulley system) [1, 2]. The latter consists of bands of fibrous tissue that prevent the detachment of the tendon from the underlying cortical bone during flexion of the finger (Fig. 1). The reticular system presents five annular pulleys (A1-A5) and four cruciform pulleys (C1-C4) [1, 2].

Fig. 1.

Removing the skin and subcutaneous tissue from the cadaveric sample (A), the superficial (S) and deep (D) flexor tendons of the finger can be accurately observed (B). Interestingly, in this cadaver, the A1 and A2 pulleys are fused generating a unique stabilizing structure (B). By further removing the pulley system (C), the two slips (white arrowheads) of the superficial flexor tendon (S) can be visualized wrapping the deep one

Pathology

Epidemiologically, the TF is more common in the ring finger and thumb but can occur on any finger [3]. The A1 pulley at the metacarpophalangeal (MCP) joint is the most commonly involved; instead, the A2 pulley at the proximal interphalangeal (PIP) joint and the A3 pulley at the distal interphalangeal (DIP) joint are more rarely affected [3]. The prevalence is about 2% in the general population and is most common in women in the fifth or sixth decade of life [4].

Molecular studies have shown the upregulation of specific types of collagen fibers along with the downregulation of two variants of metalloprotease-3; however, a hereditary link has not been established [5]. Systemic pathologies predisposing to develop the TF include diabetes mellitus, rheumatoid arthritis, amyloidosis, gout, and thyroid disease [4].

The thickening and narrowing of the A1 pulley are coupled with progressive degeneration of its inner fibrocartilaginous sliding surface and excessive friction with the underlying tendons. Likewise, chronic impingement at this level may lead to the development of nodular thickenings of the flexor tendons [1, 2]. The tendon mainly affected by the rubbing and locking is the flexor digitorum superficialis (FDS) which lies just underneath the A1 pulley, but also the flexor digitorum profundus (FDP) may be involved [3]. Usually, the patient complains of the feeling of snap when the tendons abruptly travel through the narrowed osteo-fibrous channel. Interestingly, at the thumb, the flexor pollicis longus tendon slips into the A1 pulley with a greater angle compared to the FDP and FDS tendons in the other fingers of the hand. This biomechanical condition may be related to the highest incidence of stenosing tenosynovitis at this level [5].

Hueston and Wilson demonstrated that the repetitive friction between the tendon and the overlying fibrous pulley may progressively lead to the development of a nodular thickening of the tendon [6]. Moreover, the hypovascular segment of flexor tendons beneath the A1-A2 pulley system (the watershed area) is more prone to develop degenerative changes [7].

Histologically, Drossos using longitudinal dissections of the A1 pulley described 3 different layers [8]. The outermost layer is of the areolar type, well-vascularized, and in a histological continuum with the synovial sheath of the flexor tendons. The intermediate layer is thicker and consists of chondrocyte-like cells and collagen fibers oriented perpendicularly to the tendon’s longitudinal axis. The deepest layer is either unicellular or bicellular and is in contact with the flexor tendon. A similar histological architecture was demonstrated by Ellis et al. on the A2 pulley [9]; and, by Katzman et al. on the A5 pulley [10]. Drossos also compared 104 pathological A1 pulleys obtained from patients with TF and 55 normal A1 pulleys obtained from fresh-frozen cadaveric specimens [8]. Interestingly, a progressive thinning and fissuring of the avascular fibrocartilaginous gliding surface replaced by a vascular network hyperplasia originating from the outer layer and progressively invading the synovial space of the tendon sheath have been observed in pathological pulleys (Fig. 2).

Fig. 2.

Normally (A), the pulley presents an inner avascular unicellular or bicellular gliding layer containing cartilage-like cells (green), an intermediate avascular layer composed of spindle-shaped fibroblasts (yellow), and an outer richly vascularized layer (orange). In pathological conditions (B), a progressive thinner and disruption of the inner (green) and intermediate (yellow) layer, are coupled with vascular hyperplasia (red lines) originating from the outer layer (orange) and invading the synovial sheath (S) of the flexor tendons

Clinical practice

The diagnosis of TF is mainly based on the medical history and clinical findings. Patients usually complain of pain and/or clicking at the MCP joint causing functional limitations in grasping and holding objects [1, 2]. Sometimes, a nodular thickening can be identified by accurate palpation of the palmar aspect of the hand; and direct compression of the A1 pulley can reproduce the pain [1, 2]. In severe cases, the finger may lock in flexion requiring passive and painful manipulation to reach the extended position. In the chronic phase, a capsular contracture at the MCP and/or PIP joint may develop secondary to the global hypomobility of the finger [3, 4].

Ultrasound examination can be performed in clinical practice to accurately evaluate the site of impingement and the pathological changes of the flexor tendons, the synovial sheath, and the pulley. Thickening of the A1 pulley, effusion and hypertrophy of the synovial sheath, and nodular thickening of the flexor tendons are the most common sonographic findings [11]. Dynamic sonographic assessment can also per performed to promptly confirm in real-time the snapping phenomena between tendons and pulley system [12]. Lastly, the US is also a ready-to-use, cost-effective, and safe guidance to perform accurate interventions ranging from injections to the perforation of release of soft tissues [13, 14].

The present manuscript aims to provide an extensive and updated narrative review mainly focused on the clinical rather than the anatomical, histological, and biomechanical aspects of the TF to produce a practical guide for clinicians and surgeons. Indeed, compared to the past, the cross-talk between physical examination and US imaging currently allows us to plan a tailored rehabilitation approach taking advantage of both traditional and more recent non-surgical treatments. For instance, in patients with hypertrophic tenosynovitis (Video 1), a corticosteroid injection (CSI) under US guidance targeting the synovial sheath can be sufficient alone to manage the clinical scenario; likewise, in case of severe tendinosis of the flexor tendons causing mechanical impingement under the A1 pulley a combination of MCP joint blocking orthosis and EWST can be considered a more suitable approach compared to a blind injection with corticosteroid.

Materials and methods

PubMed, Cochrane Library Databases, Google Scholar, PEDro, and Web of Science were used as scientific databases to perform the research of the present narrative review. The workflow has been organized in two different phases.

During the first phase, the keywords inserted in the databases were: “trigger finger”, “diagnosis”, “score”, “grading”, “assessment”, “ultrasound”, and “sonography”. Only articles written in English and published in relevant journals were included. Abstracts were not included in this narrative review. This phase was considered the “assessment” phase and based on the results two different sections of the manuscript have been structured: functional assessment and sonographic assessment.

During the second phase, the keywords inserted in the databases were: “conservative treatment”, “conservative management”, “physiotherapy”, “physical therapies”, “ultrasound-guided”, “procedure”, “injection”, and “intervention”. Once again, only articles written in English and published in relevant journals were included; and, abstracts were not included in this narrative review. This phase was considered the “treatment” phase and based on the results two different sections of the manuscript have been structured: conservative treatments and interventional treatments. Of note, the interventional treatments do not include the open and endoscopic surgical procedures that have not been the specific object of this narrative review.

Assessment

Functional assessment

In clinical practice, in addition to the medical history and physical examination, the functional scales (Table 1) are widely used as useful tools for the diagnosis, prognosis, and follow-up in patients with symptoms and signs of TF.

Table 1.

Functional Assessment of TF

Severity	Quinnel Grading System
Pain	Numeric Pain Rating Scale (NPRS), Visual Analogue Scale (VAS)
Grip Strenght	Jamar® Hydraulic Hand Dynamometer (JD)
Dexterity	Functional Dexterity Test (FDT), Purdue Pegboard Test (PPT)
Disability	Michigan Hand Outcomes Questionnaire (MHQ) Disabilities of the Arm Shoulder and Hand Questionnaire (DASH)

The clinical grading system developed by Quinnell and modified by Green is commonly used in daily practice to classify TF severity. At the onset (grade 1), the clinical scenario is mainly characterized by pain, local edema, and mild deficit in performing active flexion-extension movements of the finger. Progressively, the flexion block appears, which can be actively (grade 2) or passively (stage 3) corrected by the patient himself. In the chronic stages of the disease, the block is no longer reducible with a fixed flexion contracture of the finger (grade 4) [15].

For pain, two scales are commonly used in patients with TF (i) the Numeric Pain Rating Scale (NPRS) and (ii) the Visual Analogue Scale (VAS). Atthakomol et al., in 2023 defined the minimal clinically important difference (MCID) of 2 for VAS to determine clinically significant improvement after conservative treatment of TF [16]. The same authors have also defined the MCID of the Michigan Hand Outcomes Questionnaire (MHQ) of 15 for the non-surgical treatments of TF. The latter consists of 37 items grouped into 6 main categories – function, aesthetics, work, pain, satisfaction, and daily living [16].

The American Society of Hand Therapists has recommended the use of a Jamar^® Hydraulic Hand Dynamometer (JD) for the assessment of grip strength in patients with TF. The grip strength, defined as the force exerted by the hand and fingers, can be considered a measurable ability to apply pressure to objects. Psychometric testing found good interrater reliability and high test-retest reliability [17]. Likewise, dexterity is the ability to use your hands skillfully – i.e., fine, voluntary movements used to manipulate small objects in a specific task [18]. According to Langer et al., dexterity in people with TF can be measured by using the Functional Dexterity Test (FDT) and the Purdue Pegboard Test (PPT) [19]. The same authors have also demonstrated that all three aforementioned tools discriminate between people with and without TF; but, only the PPT has a statistically significant group effect on the clinical grades [19].

The disabilities of the arm shoulder and hand (DASH) questionnaire was developed to describe the disability experienced by people with upper extremity disorders and to monitor changes in symptoms and function over time. This scale especially concerns the limitations in carrying out daily life activities. The score given for each item can be 1 (no restrictions), 2 (slight limitations), 3 (moderate limitations), 4 (severe limitations), 5 (unable). The percentage score can vary from a minimum of 0 to a maximum of 100, as the score increases there will be an increase in the patient’s disability in carrying out daily life activities [20]. The DASH questionnaire has been demonstrated through a cross-sectional study to be a useful tool to distinguish between participants with TF and controls, and between mild and severe clinical grades [21]. Langer et al., in 2017 have also demonstrated that JD, FDT, and PPT all moderately correlate with the DASH score [19].

The functional domains explored by the aforementioned scales and questionnaires are pivotal to quantifying the non-surgical treatment efficacy and the quality of rehabilitation strategy, complex outcomes that cannot be simply “weighted” on the improvement of symptomatology [22].

Sonographic assessment

The most common sonographic finding of the TF is the hypoechoic thickening of the A1 pulley (Table 2) [11, 23].

Table 2.

Sonographic Assessment of TF

Anatomy	Sonography
Pulley	Hypoechoic thickening (global or nodular), hypervascularization #
Flexor Tendons	Focal or diffuse tendinosis, partial tear, intra-tendinous cystic degeneration, longitudinal split, dark tendon sign §, dynamic snapping
Synovial Sheath	Effusion, synovial hypertrophy, synovial hypervascularization, synovial cyst
Volar Plate	Hypoechoic thickening +/- focal fissurations

^# mainly involving the superficial portion of the pulley

^§ anisotropy-induced hypoechogenicity secondary to tendons entrapment and deflection under the pulley

The thickening of the pulley may involve the entire inverted U-shape structure (global thickening) or only a part of it (nodular thickening) [24]. A comparative scanning, using a transverse plane, is the best approach to promptly measure the thickness of the pulley in the affected digit and the normal contralateral finger as suggested by the European Federation of Societies for Ultrasound in Medicine and Biology (EFSUMB) [23, 25]. Spirig et al., in 2016 defined a diagnostic cut-off value of the pulley thickness at 0.62 mm to differentiate a TF from a healthy finger, with a sensitivity and specificity of 85%, independent of age, gender, height, and BMI [26].

Modern high-end US machines and high-frequency probes may depict hypervascularization mainly involving the most superficial portion of the thickened pulley (Fig. 3) [24].

Fig. 3.

Comparative ultrasound scanning shows the hypoechoic thickening of the A1 pulley on the pathological side (A) and; the microvascular imaging (MVI) modality confirms its hypervascularization (B). Using an in-plane technique and distal-to-proximal approach (C, D), the needle’s tip (white arrowhead) can be advanced in between the flexor tendons (FT) and the A1 pulley to inject the synovial sheath. Of note, the flow of the mixture is visible both proximally and distally (yellow arrowheads) to the A1 pulley confirming the correct distension of the synovial sheath (D). Dist: distal, prox: proximal, MC: metacarpal bone, PP: proximal phalanx, vp: volar plate

Interestingly, this sonographic finding accurately matches the aforementioned histopathological description by Drossos of a vascular network hyperplasia that originates from the outer layer of the pulley (Fig. 2) [8]. Intra-pulley vascular signals can be visualized both in longitudinal and transverse planes by reducing the pressure of the transducer over the skin and accurately setting the Doppler box [27]. Guerini et al., in 2008 compared 33 patients with TF and 20 healthy subjects, reporting the hypervascularization of the A1 pulley on power Doppler in 91% of the study group and 0% in the control group [24]. Interestingly, in patients with TF, and Doppler signals detected inside the thickened A1 pulley, some authors suggest persisting with conservative therapies before considering the surgical approach [28]. Indeed, they considered pulley hyperemia as an indirect sonographic sign of local inflammation for which a combination of rest and steroid injection may be a suitable therapeutic intervention.

Another common sonographic sign in patients with TF is the focal thickening of the flexor tendons, fusiform or nodular in shape, that can be located proximally or distally to the pathological pulley (Table 2) [11, 23, 24]. Serafini et al., considered the “swollen” flexor tendons the main cause of TF, suggesting a thickness difference of at least 20% between the normal and pathological sides [29].

In the short-axis view degenerated flexor tendons usually present a more rounded appearance compared to the normal ones are flatter; and, are more hypoechoic [23, 24, 29]. Of note, eventual fake hypoechogenicity of the tendons related to the anisotropy artifact can be promptly avoided during the US assessment by gently tilting the probe [30]. Gruber et al., in 2011 described the “dark tendon sign” as referring to the hypoechogenicity of the flexor tendons secondary to their entrapment and deflection under the overlying thickened pulley – i.e., an anisotropy-induced hypoechogenicity [31]. Sato et al., in 2012 demonstrated on 67 TF that the sonographic thickness of the flexor tendons under the A1 pulley was proportional to the severity of triggering. Moreover, the same authors reported the thickening of the palmar plate (Table 2) as a frequent sonographic finding in patients with persistent triggering probably secondary to abnormal rubbings at the level of the tendon-plate interface [28]. Kim and Lee in 2010 assessing 50 TFs of 41 patients reported loss of normal fibrillar pattern and irregularity or blurring of the tendon margins in 14% and 62% respectively suggesting a combined quantitative (thickness) and qualitative sonographic assessment [32]. Moreover, the same authors noticed an interesting correlation between the clinical and sonographic findings. Specifically, the thickened flexor tendons seem to be linked with the extension deficit of the fingers; instead, the blurred edges of the tendons seem to be associated with the frequent locking phenomena [21]. Progressively, if not properly diagnosed and managed, tendinosis of flexor tendons may be complicated with partial tears, intra-tendinous ganglion cysts, or longitudinal splits [23].

Effusion and hypertrophy of the synovial sheath can be assessed with US (Table 2) [11, 23]. Using a transverse scan and the elevator technique. i.e., widely shifting the probe from proximal to distal and vice versa, the synovial tendon sheath can be fully scrutinized [33]. Guerini et al., comparing 33 patients with TF and 20 healthy subjects reported the presence of tenosynovitis in 55% of patients and 0% in the controls [24]. In the acute phase of tenosynovitis, aberrant vascular signals inside the hypertrophic synovial tissue of the tendon sheath can be accurately identified using color/power Doppler with an accurate setting of the pulse repetition frequency (Video 1) [34]. Breidahl et al., in 1998 also described the diagnostic role of peri-tendinous hypervascularization on power Doppler, mainly related to pathological synovial tissue rather than complex fluid in patients with hypoechoic rim surrounding the tendon suggestive of tenosynovitis [35]. In doubtful cases, dynamic scanning asking the patient for active flexion/extension of the finger can be performed to pump the effusion within the synovial recesses of the sheath optimizing its visibility [12, 36].

Lastly, the US may also reveal secondary causes of TF such as bony exostosis, post-traumatic dysmorphism of the phalanx, degenerative changes of the finger joints, and foreign bodies [23].

Treatment

As previously mentioned in the materials and methods, open and endoscopic surgical treatments have not been investigated in the present narrative review. In this sense, the non-surgical therapeutic approaches to the TF have been classified into (i) conservative treatments and (ii) interventional treatments. Interventional treatments refer to different types of injections and percutaneous procedures such as perforation or release of superficial soft tissues of the finger that can be part of the multimodal rehabilitation management of patients with TF.

Conservative treatments

The first-line conservative treatment of the patients with TF includes modifications of daily activities involving the affected hand with grasping, acute flexion, or repetitive stresses [15]. Manual therapy with massages of the flexor tendons to soften them and forearm muscles to reduce the painful contractures, active exercises of tendon gliding, and passive stretching of the flexor tendons and joint capsules can be considered the basic principles of rehabilitation management (Fig. 4) [37].

Fig. 4.

Schematic drawing shows the hook fist exercise that allows a differential gliding (arrows) of the superficial (red) and deep (yellow) flexor tendon avoiding mechanical overload on the A1 pulley (black)

Among many, the hook fist exercises allow a differential gliding between the superficial and deep flexor tendon (of 10–11 mm) avoiding the overload on the A1 pulley. Of note, a complete extension of the affected finger during the exercise is paramount to avoid the capsular shortening of the PIP joint simultaneously maximizing the tendons glide [37, 38]. Keeping the palm and knuckles straight, the bending of fingers to create a hook promotes synovial fluid circulation within the tendon sheath, the remodeling of scar tissue between the two tendons (FDS and FDP), and the vascular/lymphatic drainage of congested soft tissues [37, 38]. Indeed, inflammation of the synovial sheath often leads to the development of fibrinous adhesions at the two tendons gliding interface (i.e., the sticky interface) that work as a unique anatomical element [38]. Likewise, gentle “place-and-hold” full-fist exercises ensure maintenance of MCP flexion without exacerbating mechanical stress on the A1 pulley [37]. In this sense, the affected finger is manually flexed using the patient’s normal hand reaching a full fist position; so, the flexed position is held for 5–10 s and then the fist is opened once again using the unaffected hand [39]. Salim et al., in 2012 compared the effectiveness of physiotherapy and CSI in 74 patients with mild TF demonstrating the role of manual therapy (massage) and stretching exercises mainly in preventing recurrence [40]. In this sense, the aforementioned specific exercises should be considered a useful therapeutical tool to preserve the function of the affected finger rather than “magic bullets” to manage the acute pain and locking.

Jung et al., in 2012 defined the conservative approach with passive exercises of the hand and fingers as a successful treatment to manage pediatric TF assessing 30 patients (34 thumbs) with a 4-year follow-up [41]. Interestingly, the bilateral cases and the patients who initially had grade 3 severity had significantly more unfavorable results than the other patients [41].

Iordache et al., in 2023 demonstrated the effectiveness of both manipulations of the hand and forearm myofascial chains, and traditional manual therapies with joint mobilization, eccentric stretching, and self-exercises at home in 34 patients with mild to moderate TF reporting improvements in VAS and Quick-DASH at 6 months follow-up [42].

Although there are no international recommendations supporting the use of nonsteroidal anti-inflammatory drugs (NSAIDs), they are commonly used for the initial management of mild to moderate trigger finger symptoms. NSAIDs should be taken for a maximum duration of 2 to 4 weeks considering the well-known gastrointestinal and cardiovascular risks related to long-term use.

Splinting

The splint can isolate and block either the MCP joint, the PIP joint, or the DIP joint (Fig. 5).

Fig. 5.

Schematic drawings show the metacarpophalangeal joint-blocking orthosis (MCPJ-BO) and the proximal interphalangeal joint-blocking orthosis (PIPJ-BO)(A, B). A custom-made splint produced with 3D printing can be used to selectively block the proximal or distal interphalangeal joint of the affected finger (C)

Biomechanically, the orthosis reduces the movements of finger flexor tendons and their repetitive impingement with the overlying pulley. Leong et al., in 2023 published a systematic review demonstrating that all the aforementioned orthoses are effective for non-surgical management of the TF in adults [43]. In clinical practice, selective blocks of different joints of the affected finger are performed to simulate the function of the orthosis and the tolerance by the patient. The blocking orthosis which generates less (painful) snapping can be considered the most suitable for that patient and its grade of disease (Table 3).

Table 3.

Blocking orthosis

Why	↓ flexor tendon movements, ↓ tendon-pulley impingement/rubbing
Where	MCP joint or PIP joint or DIP joint
How	neutral position (0°) or flexed/extended position (10°-15°)
When	6–10 weeks, 24 h per day §
Selection Criteria	selective blocking tests, tolerability, aesthetic, durability

MCP: metacarpophalangeal, PIP: proximal interphalangeal, DIP: distal interphalangeal

§ blocking orthosis can be removed during the day to perform specific exercises (e.g., “place and hold” full fist exercises)

The block of the MCP joint can be performed in a neutral position (0 grades) or with 10–15 grades of flexion or extension using a custom-made splint. Colbourn et al., in 2008 in a series of 28 patients with TF treated with an MCP joint blocking splint at 10 to 15 degrees of flexion for 6 to 10 weeks reported favourable outcomes in 93% of the group [44]. The authors preferred the orthosis in a flexed position to optimize hand function by allowing tip-to-tip prehension. Conversely, a European multidisciplinary consensus guideline published in 2014 recommended the MCP joint blocking orthosis in 0 degrees [45].

Tarbhai et al., in 2012 compared the block of the MCP joint vs. the immobilization of the DIP joint in 30 subjects with TF, reporting better outcomes and a longer period of usage of the MCP joint block orthosis [46]. Likewise, Teo et al., in 2019 assessing 35 patients with TF demonstrated that the block of the PIP joint is more effective than the MCP joint block in pain reduction and functional improvements by using it 24 h per day for more than 8 weeks [47]. As regards the timing of utilization, Lunsford et al., in 2019 based on a systematic review suggested a mean duration of orthotic treatment of 6–10 weeks [48].

Despite the superiority of a specific block compared to the others is still a subject of debate, the authors in daily practice often prefer the PIP joint block orthosis considering its tolerability by the patient, aesthetic, and durability. For the thumb, the interphalangeal joint is commonly blocked for 3–12 weeks depending on the severity of the TF [37].

ESWT

The ESWT is based on sound waves that maximize the pressure of the target tissue over a few nanoseconds. Its efficacy has been reported in several tendinopathies, such as calcific tendinopathy of the shoulder, lateral epicondylitis of the elbow, patellar tendinopathy, hamstring tendinopathy, and plantar fasciitis [49]. The mechanism of ESWT is not fully understood, but sound waves directly stimulate the healing processes, soft tissue neovascularization, reabsorption of calcific deposits, and induce neural modulation of local nociceptors [49]. The effectiveness of ESWT depends on several factors, including the point where the pressure is applied, the energy flux density, the total energy, adherence to the principles of shockwave generation, and the device itself.

Ferrara et al., in 2020 performed a systematic review of the biophysical therapies most commonly used for the management of tendinopathies of the wrist and hand in clinical practice, defining the ESWT the most used in patients with TF [50]. Chen et al., in 2021 published a randomized controlled study demonstrating, in patients with grade II TF according to the Quinnell classification, that the pain relief and functional improvement (measured with QuickDASH) after the ESWT are both dose-dependent. Indeed, both pain and QuickDASH questionnaire improved more in the group treated with high energy flux density (0.01 mJ/mm², 5.8 bar, 1500 impulses, once per week for 4wk) compared to the group treated with low energy (energy flux density of 0.006 mJ/mm², 3 bar, 1500 impulses, once per week for 4wk) and to the group with sham therapy [51]. Yildirim et al., in 2016 compared the ESWT and CSI in a randomized controlled study involving 40 patients with actively correctable TF reporting no between-group differences for cure rates, pain, and functional status in the intention-to-treat analyses [52]. The same study group defined the ESWT as an effective non-invasive option, especially in patients who wish to avoid the CSI.

In the authors’ experience, greater efficacy of the ESWT is mainly observed in patients with sonographic findings of tendinosis of flexor tendons rather than in them with effusion and hypertrophy of the synovial tissue (tenosynovitis). So, tendinopathy rather than synovial pathology seems to be the most suitable indication for this therapeutic approach in patients with TF. Likewise, after the US examination, the therapeutical sound waves should be accurately focused along the degenerate segment of the flexor tendons often located beneath the A1-A2 pulley complex [53].

Other biophysical agents

Local application of heat may be useful to improve the elasticity and extensibility of the thickened tendon/pulley. Superficial heat modalities (depths of 2–3 cm) include hot packs, hot wax, and paraffin baths; instead, deep heat modalities (up to 5 cm) include ultrasound therapy and diathermy [54]. A pulse mode of 1w/cm2 over the flexor tendons has been proposed as the technical setting for therapeutic ultrasounds in patients with TF [55]. In patients with acute tenosynovitis, an ice pack on the affected finger can be applied for 10–15 min, 2–4 times per day to reduce the local inflammatory phenomena.

The contrast bath by alternating hot and cold temperatures (30 s in each for 4 min, 3–4 times per day), increases perfusion through the contraction and relaxation of the blood vessels, promotes the drainage of interstitial edema of the affected finger and reduces the stiffness and pain [55].

Interventional treatments

The CSI is suggested for patients whose symptoms have not resolved after 4 to 6 weeks of conservative treatment; indeed, compared to placebo injections with 0.9% NaCl the corticosteroid (triamcinolone acetonide) performs better [56]. Corticosteroids are thought to work by reducing the swelling of soft tissues, allowing the flexor tendons to move freely again. Triamcinolone appears to be more effective than dexamethasone at 6 weeks in a randomized controlled trial published in 2008 with 72 patients with TF, but this advantage is lost by 3 months [57].

Caution is needed in diabetic patients who may have a significant transient elevation in glycaemic levels. Potential complications of blinded CSI for TF include subcutaneous tissue atrophy, bleeding, and skin hypopigmentation. Very rarely, tendon and pulley ruptures, especially in the case of repeated injections of corticosteroids, and deep-site infections have been reported [1, 58, 59].

Kanchanathepsak et al., in 2020 published a randomized controlled trial comparing injection of 1 ml of low-molecular-weight hyaluronic acid (HA) and 1 ml of 10 mg/ml triamcinolone acetate in a cohort of 50 patients with TF. Interestingly, the authors reported better outcomes in early follow-up (1 month) in the group treated with the corticosteroid, but no significant difference between the two groups in the last follow-up (6 months) [60]. Similar findings have also been described by Liu et al., in 2015 in a randomized controlled trial comparing a single injection of HA in one group and corticosteroid in the other both via ultrasound guidance suggesting an interesting role of HA in the long-term outcomes of patients with TF [61]. In a prospective, randomized, double-blinded controlled study published by Shakeel et al., in 2012 a comparison between triamcinolone acetonide (50 patients) and diclofenac sodium (50 patients) was performed once again reporting faster improvement in the group treated with corticosteroid (3 weeks) but no significant difference between the two groups at 3 months [62]. Considering all the aforementioned studies it seems that the main benefit of corticosteroid over the other pharmacological options for the local injection is related to the speed of action rather than to the long-term clinical and functional outcomes.

In daily practice, injection of the TF can be performed blinded or under US guidance. Several tips and tricks have been described to perform the blinded injection, from moving the injected finger to make sure you have not advanced the needle’s tip inside the tendon, to the use of “bone stop” to make sure you have completely crossed the tendon reaching the cortical bone of the underlying phalanx. Using the US guidance a specific anatomical target can be injected by releasing the mixture inside the synovial sheath of the flexor tendons, within or around the pulley (Table 4).

Table 4.

Ultrasound-Guided Procedures for the TF

Procedure	Technical Notes
Intra-sheath injection	the needle’s tip is advanced between the inner surface of the pulley and the volar surface of the flexor tendons to inject the synovial sheath
Peri-pulley injection	the needle’s tip is advanced between the subcutaneous adipose tissue and the volar surface of the pulley to inject the peri-pulley space
Pulley perforation	multiple back and forward movements with the needle inside the thickened pulley are performed in an attempt to partially disrupt it
Pulley release	a customized knife (or special needle with a blade at the tip) is scrolled along the dorsal surface of the pulley to release it

The main difference between the blinded and USG injection is the ability to easily avoid the intra-tendinous release of the corticosteroid. Lee et al., in 2011 published a cadaveric investigation injecting 20 fingers with a blinded technique and 20 fingers under US guidance using a methylene blue dye. After dissections, the dye was observed only in the synovial sheath (optimal outcome) in 70% of sonographically guided injections, and in only 15% of blind injections. Moreover, the dye was observed inside the tendon tissue (unsafe outcome) in 30% of blind injections and 0% of sonographically guided injections [63].

Considering one of the main purposes of the present narrative review is to provide an update on the different interventional procedures for the management of TF, we have classified them below in (i) USG injection and (ii) USG release of the pulley. Likewise, comparative studies between the blinded and USG injection have been reported.

USG injection

Positioning the probe on the palmar aspect of the affected finger in a longitudinal plane, using an in-plane technique and distal-to-proximal approach the needle’s tip can be advanced in between the thickened pulley and flexor tendons to inject the synovial sheath (Fig. 3) [11]. Slightly rotating the probe in a longitudinal oblique plane the needle can also be inserted via the interdigital wing skin reducing the pain related to the procedure, considering the poor innervation of the skin in this area compared to the palmar of the hand [14]. An out-of-plane technique was described by Bodor et al., in 2008 by targeting the triangular space bordered by the A1 pulley, the flexor tendons, and the volar plate with a 90% successful rate at 1 year for complete resolution of symptoms after a single injection in 24 patients with TF [64].

For all the aforementioned USG procedures, the color/power Doppler allows prompt identification of the neurovascular bundles to plan safe intervention [53]. In the author’s experience, a thin needle of 27–30 Gauge can be visualized using high-frequency linear probes to inject a mixture of corticosteroids and local anesthetics which do not have a high viscosity. When the needle’s tip is located in the tendon-pulley interface, the mixture can be injected, and; a progressive distension of the synovial sheath both proximally and distally to the pulley is considered as the sonographic feedback of a correct injection (Video 2). If the aforementioned distribution is not visualized during the procedure, a post-procedural US assessment in short-axis view is recommended to check the eventual spilling of the mixture out of the synovial sheath in the surrounding subcutaneous tissue (Fig. 6) [65].

Fig. 6.

If the needle’s tip has not been not accurately positioned inside the lumen of the synovial sheath during the ultrasound-guided injection; in the post-procedural sonographic assessment (A, B, C), the mixture (yellow asterisks) usually presents an anechoic inverted U-shape in between the flexor tendons and the subcutaneous tissue (sc). S: superficial flexor tendon, D: deep flexor tendon, PP: proximal phalanx

Interestingly, Mardani-Kivi et al., in 2018 randomized 166 patients with TF to receive either intra-sheath (83 patients) or extra-sheath (83 patients) injection under ultrasound guidance at the level of the first annular pulley [66]. Surprisingly, they found no difference in the rate of TF resolution at one year, re-injection rate, or final Quinnell grade between the two different techniques. Based on the findings of Mardani-Kivi et al., it seems that the local absorption of corticosteroid by the flexor tendons, the pulley, and the synovial sheath may be sufficient to exploit the clinical and functional efficacy. Taras et al., in 1998 using a radiopaque dye and postinjection x-rays suggested that true intra-sheath injection offers no apparent advantage over subcutaneous injection with corticosteroid in the treatment of TF [67]. The authors speculated that the aforementioned results may be related to a high concentration of the steroid around the A1 pulley with the subcutaneous injection compared to a greater dispersion of the mixture with the intra-sheath injection reducing its direct effect on the pulley tissue. In this sense, especially in patients with severe thickening of the pulley, under US guidance a peri-pulley injection (Video 3) rather than an intra-sheath injection can be planned to maximize the contact between the corticosteroid and its outer surface (Table 4). Indeed, we speculate that positive outcomes reported in the pertinent literature after the subcutaneous (extra-sheath) injection could be related to the diffusion of the corticosteroid within the hyperplastic vascular network mainly located in the outer portion of the pathological pulley (Fig. 2) as described by Drossos in the histological investigations [8].

Conversely, Tunçez et al., in 2023 in a cohort of 66 patients with persistent symptoms of TF performed injections under ultrasound guidance in 34 patients, and blinded injections in 32 patients reporting shorter recovery time, shorter time to return to work and lower rates of repeated injections in the first of the two groups [68]. The same authors described better results with the USG injections vs. the blind injection in the early period (1–4 weeks), while clinical outcomes were the same in both techniques in the long run (12–24 weeks). In this sense, the ultrasound guidance seems to guarantee a faster clinical and functional recovery rather than an overall superior outcome in the long term.

Considering the controversies of the scientific evidence currently available about the optimal target for the injection in patients with TF, the debate of which procedure is superior to the other (blinded vs. ultrasound-guided) can be considered still open requiring further rigorous studies. In the authors’ experience, the anatomical target of injection can be extremely variable in daily practice ranging from the synovial sheath in a patient with tenosynovitis to the outer surface of the pulley in a patient without tenosynovitis but a remarkable thickening of the pulley system (Table 4). In this sense, the US guidance allows the clinician/surgeon to plan the more suitable intervention for the specific clinical case tailoring the rehabilitation approach [69].

USG release of the pulley

The percutaneous release of the pathological pulley in patients with TF, using clinical landmarks, was first described in 1958 [70]. Considering the potential iatrogenic injuries to the flexor tendons, nearby pulleys, and interdigital neurovascular bundles; a growing interest in the ultrasound guidance to perform this percutaneous procedure has mounted in the pertinent literature.

Zhao et al., in 2014 published a systematic review and meta-analysis on the percutaneous release of the pulley in patients with TF, including 34 studies and 2114 procedures, reporting a total success rate of 94%. The same authors also stated that percutaneous release with sonographic guidance had a significantly higher success rate than non-sonography guidance [71]. The same research team found no significant difference in success rate among different types of needles and knife blades used to perform the procedure – e.g., a hypodermic needle ranging from 18 to 21-gauge (fenestration of the pulley), an 18-gauge needle with a blade at the tip (incision of the pulley), and a customized knife requiring a skin incision (Table 4) [71]. In 2023 Nakagawa et al. included 17 studies involving 749 procedures in a systematic review focused only on the USG release of the A1 pulley reporting an overall success rate of 97% [72]. Minor complications (23 cases) such as hematoma, persistent pain, and transient numbness were reported; instead, no major complications have been described. Of note, the transient numbness involving three thumbs and one middle finger, resolved spontaneously 3 weeks after the procedure.

Conclusion

Nowadays, by matching the clinical/functional scales and the sonographic examination a very comprehensive assessment of patients with TF can be guaranteed in order to accurately tailor the rehabilitation approach. Indeed, modern treatments such as the ESWT and the different procedures under US guidance have joined the traditional therapeutic options such as exercises and orthosis defining a complex and varied panorama of rehabilitation interventions. In this sense, the present narrative review aims to provide a practical guide for clinicians/surgeons to plan a personalized rehabilitation approach in patients with TF.

Electronic Supplementary Material

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Abbreviations

TF

Trigger finger

ESWT

Extracorporeal shockwave therapy

USG

Ultrasound-guided

US

Ultrasound

MCP

Metacarpophalangeal

PIP

Proximal interphalangeal

DIP

Distal interphalangeal

FDS

Flexor digitorum superficialis

FDP

Flexor digitorum profundus

CSI

Corticosteroid injection

NPRS

Numeric Pain Rating Scale

VAS

Visual Analogue Scale

MCID

Minimal clinically important difference

MHQ

Michigan Hand Outcomes Questionnaire

JD

Jamar^® Hydraulic Hand Dynamometer

FDT

Functional Dexterity Test

PPT

Purdue Pegboard Test

DASH

Disabilities of the arm shoulder and hand

NSAIDs

Nonsteroidal anti-inflammatory drugs

HA

Hyaluronic acid

Author contributions

Conceptualization, D.D., V.R.; writing—original draft preparation, D.D., V.R., F.O.; writing—review and editing, D.D., V.R., P.B., F.O., F.V.; supervision, O.N., F.C., L.T. All authors have read and agreed to the published version of the manuscript.

Funding

No funding was received.

Data availability

All data generated or analysed during this study are included in this published article. The data used to support the findings of this study are available in the text and can be acquired from the corresponding author upon request.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.

Declaration of originality

This manuscript is original, has not been previously published and has not been submitted for publication elsewhere while under consideration. Each named author has substantially contributed to conducting the underlying research and drafting this manuscript.