In clinical practice, patients frequently present with snapping sensations in their fingers, often attributed to the common pathology of trigger finger stemming from issues with the A1 pulley.^1,2 This condition, characterized by the constriction of the flexor tendon within the pulley system, typically manifests as a palpable or audible snap upon finger flexion and extension.^1,2 Consequently, it has become standard practice to predominantly investigate and treat such cases under the assumption of trigger finger, due to its prevalence as the primary etiology for these symptoms.

However, among the commonly diagnosed cases of trigger finger, there exists a subset of patients who exhibit snapping phenomena due to less conventional etiologies. These cases, while rarer, warrant careful consideration and investigation to accurately diagnose and address the underlying pathology. From anomalous tendon insertions to atypical joint mechanics, these uncommon causes of finger snapping challenge clinicians to broaden their differential diagnoses.^3,4 Such instances underscore the necessity for a comprehensive understanding of both common and rare etiologies, ensuring optimal patient care and management. Among these less frequent causes, finger snapping attributed to the finger extensor mechanism stands out as particularly uncommon, often eluding initial recognition due to its rarity and unfamiliar presentation.

This case report explores a rare occurrence: finger snapping attributed to lateral band subluxation, a condition scarcely documented in medical literature. Remarkably, only a handful of cases have been reported thus far, highlighting the novelty and scarcity of this pathology.^5,6 Even more noteworthy is the scarcity of reports utilizing ultrasound (US) for diagnosis, with only one documented instance found in literature.⁷ Thus, the primary objective of this case report is to provide a detailed account, supported by specific US images and videos, clarifying the diagnostic process of lateral band subluxation as the underlying cause of finger snapping. Through a detailed examination and presentation of ultrasound findings, this report aims to enhance the understanding and recognition of this uncommon etiology, thereby facilitating the use of US as a diagnostic method.

A 70-year-old woman presented to the outpatient department of our hospital’s rehabilitation medicine unit, reporting a snapping sensation in her right middle proximal interphalangeal joint (PIPJ) that began approximately 3 months ago. She denied any significant past medical history, history of trauma, or prior hand surgeries. One month prior to her visit, she sought medical attention at a local clinic with a similar complaint and was diagnosed with trigger finger. Despite receiving a corticosteroid injection at the base of the tendon sheath, her symptoms persisted without relief.

Thorough physical examination was conducted. The patient exhibited full active and passive range of motion (ROM) in the affected PIPJ. Notably, snapping was elicited when the PIPJ was flexed between approximately 65 and 75 degrees, which could only be confirmed upon close examination (Supplementary Video 1). Upon visual observation of the dorsal side of the PIPJ, a band-like structure exhibited medial movement when the patient flexed her finger (Fig. 1). It seemed that this structure became caught near the medial proximal phalangeal condyle during finger flexion. Subsequently, when the finger was extended, the structure returned to its original position. While snapping, the patient reported mild pain, scoring between 2 to 3 on the visual analog scale (VAS). The pain and snapping sensation were considered tolerable and did not significantly impair hand grip, grip strength, or activities of daily living.

Figure 1. Visual examination of the right third proximal interphalan-geal joint. (A) Pre-flexion view. (B) Upon finger flexion, a band-like structure (arrow) is observed crossing to the medial side of the proximal phalangeal condyle, becoming trapped. (C) Upon subsequent extension of the right middle finger, the band-like structure returns to its initial position.

Ultrasound (Samsung Medison Co., Ltd, Seoul, Korea) examination using a linear transducer (frequency range of 4.0 MHz to 18.0 MHz) was performed to assess the finger abnormalities. To observe the kinetic changes associated with the structure causing snapping, the patient underwent repeated finger flexion and extension under US imaging. Ultrasound examination, conducted by comparing the symptomatic finger with the asymptomatic contralateral finger, revealed medial-lateral band subluxation at the PIP joint of the right third finger, observed specifically when the finger was flexed between 65 and 75 degrees (Fig. 2, Supplementary Video 2 and Supplementary Video 3). This finding led to the diagnosis of lateral band snapping syndrome.

Figure 2. Ultrasound images depicting the kinetic change of the medial lateral band (red circle). The medial side is denoted by a white star in all images. (B, D) illustrate the consistent position of the lateral band of the left third proximal interphalangeal joint during both extension and flexion. Conversely, in (A, C), the lateral band on the affected side is observed deviating from its normal position during flexion between 65 and 75 degrees, becoming entrapped around the proximal phalangeal condyle. In Supplementary Video 2, snapping due to lateral band subluxation was observed during PIP joint flexion at the 2∼4 second mark on the right side. Conversely, in Supplementary Video 3, no snapping was observed during PIP joint flexion on the left side, which demonstrated a smooth motion.

Given the patient’s preference for non-invasive management and mild discomfort, conservative treatment were implemented. To promote immobilization and maintain full extension of the PIPJ, a splint was applied during nighttime. Additionally, the patient was instructed in activity modification techniques, emphasizing the avoidance of repetitive gripping or forceful finger movements throughout the day. Three weeks later, the patient reported consistent splint usage and increased reliance on their left hand for activities. The patient reported an improvement in her pain, with a VAS score of 1, and a reduction in discomfort during daily activities following conservative treatment. The patient still maintained full ROM in the affected PIPJ, and the degree of interference with daily life showed improvement. Unfortunately, we were unable to perform this follow-up as the patient did not return to the hospital after this visit.

The PIP joint’s extensor mechanism comprises intricate anatomical structures crucial for finger extension. Central to this mechanism is the lateral band, which originates from the dorsal hood and merges with the extensor tendon over the PIP joint.^8-10 The lateral band plays a pivotal role in transmitting the force generated by the extensor digitorum communis muscle to facilitate finger extension.^8-10 Additionally, it serves to stabilize the extensor mechanism during finger movement, ensuring efficient and coordinated motion.^8-10 Any disruption or subluxation of the lateral band can result in aberrant finger mechanics, potentially leading to snapping phenomena, as observed in this case. Understanding the anatomy and function of the lateral band is essential for clinicians to accurately diagnose and manage conditions affecting the PIP joint’s extensor mechanism.

Lim et al.⁵ presented a case of 74-year-old male with a traumatic radial lateral band volar subluxation in his left middle finger. He also initially presented with symptoms suggestive of trigger finger. The difference is this patient’s condition was precipitated by a fall, leading to locking of the digit. Physical examination revealed locking upon flexion and an inability to actively extend the digit, resembling a Grade IIIA trigger. However, inspection revealed an anatomic structure moving over the radial proximal phalangeal condyle during flexion, with relocation to a dorsal position upon passive extension. Surgical intervention involved freeing the radial lateral band from scar tissue and relocating it dorsally, followed by splinting and mobilization therapy. Postoperatively, the patient achieved painless, non-locking PIPJ with a satisfactory range of motion.

Lee et al.⁶ presented a case of a 24-year-old man, whose symptoms persisted after misdiagnosis and prior surgical intervention for A1 pulley release. Exploration during surgery revealed injury and attenuation of the retinacular ligament complex, which allowed dislocation of the radial side lateral band volarly at the PIPJ level during flexion. Surgical repair of the joint capsule and retinacular ligament complex, followed by postoperative immobilization and dynamic splinting, resulted in successful resolution of symptoms and restoration of full range of motion.

In contrast to the two cases, the symptoms in this case were mild and, despite the lack of a clear etiology due to the absence of a history of trauma, improvement was achieved through conservative treatments such as splinting and lifestyle modifications. In the cases described, lateral band subluxation, initially misinterpreted as trigger finger, was only definitively diagnosed through surgical exploration. However, our approach differed as we utilized non-invasive ultrasound to make a precise differential diagnosis. Ultrasound offered several advantages in this context. Firstly, it provided real-time visualization of dynamic structural changes, facilitating the identification of characteristic features of lateral band subluxation.^11-13 Secondly, ultrasound is a non-radiating, low-cost modality, ensuring minimal discomfort to the patient and making it suitable for routine clinical examinations.^11-13 Additionally, its flexibility allowed for immediate adjustments in examination position and angle, enhancing diagnostic accuracy.^11-13

In conclusion, this case report underscores the significance of highlighting the differential diagnosis of lateral band subluxation, a relatively uncommon condition, particularly when presenting with a chief complaint of snapping finger. By utilizing ultrasound as a non-invasive diagnostic tool, we were able to accurately identify lateral band subluxation in our patient, avoiding the need for surgical exploration. This not only contributed to a more precise diagnosis but also minimized patient discomfort and healthcare costs. Given the numerous advantages of ultrasound, including real-time visualization and flexibility in examination, its widespread utilization in the differential diagnosis of hand diseases is anticipated by readers. Therefore, this case report serves as a valuable contribution to the literature, emphasizing the importance of considering lateral band subluxation in patients presenting with similar symptoms and advocating for the routine use of ultrasound in clinical practice for enhanced diagnostic accuracy and patient care.

Supplementary materials can be found via https://doi.org/10.35827/cp.2024.23.2.105.

The authors affirm that there are no conflicts of interest related to this study.