1 Introduction

Carpal tunnel syndrome (CTS) is a compressive neuropathy due to the mismatch between the contents and the canal, leading to median nerve compression. However, in the majority of cases, a specific cause is often unknown and is termed idiopathic compressive neuropathy of the median nerve. Of the probable risk factors or mechanisms known to cause idiopathic carpal tunnel syndrome, lumbrical incursion has been thought to play a dynamic role.^1,2 Lumbricals are hypermobile as they are the only muscle group in the human body that originates and terminates into a tendon.¹ Cobb et al. did a cadaveric study to discover the change in carpal tunnel pressure due to lumbrical incursion while holding a tool of variable size. This study of five cadavers found a significant drop in tunnel pressure when the lumbricals were removed, and pressure was measured in full extension and flexion.² As a natural phenomenon, these four muscles enter the carpal tunnel during the flexion of the fingers during daily activities, thus inevitably increasing its content and pressure and could be an essential variable for the cause of work-related carpal tunnel syndrome.³

Previous cadaveric study done to understand the role of lumbricals in CTS have demonstrated the incursion pattern of lumbricals to elucidate the possible mechanism of dynamic increase in tunnel content. It was suggested that there was almost a 30 mm incursion of the lumbrical muscle from a fully extended position to a fully flexed finger, causing pressure changes in the tunnel and median nerve compression. This phenomenon could thus explain the "fist test" described by Berger et al. However, the study was limited in considering only the proximal-most unspecified lumbrical for all measurements.³ 75 cadaveric lumbrical muscles were studied by Mehta et al., using calipers to measure the mean distance of all lumbrical origin to the transverse carpal ligament (TCL). He found that the second lumbrical, being the longest, originated inside the carpal tunnel, unlike the other three lumbricals originating distal to TCL. However, these measurements were done in an unspecified finger position, and no incursion was measured.⁴ The Siegel et al. study of 40 cadaveric and 128 human hands with CTS found that the lumbrical origin was much proximal in the carpal tunnel of the patients affected, but neither did they mention finger flexion status in cadaveric hands, nor did they flex the patient hands.⁵

Considering the above studies, the lumbrical incursion in various hand positions used for daily activities, especially full flexion and lumbrical position, and their migration due to incursion with change in finger positions require further evaluation. Hence, we undertook this study with the primary objective of measuring the lumbrical incursion of all fingers proximal to the distal edge of TCL in various finger positions. The secondary objectives were to assess the proportion of hands demonstrating the lumbrical muscle origin proximal to the distal edge of the TCL and to evaluate the relative position of the four lumbricals to each other.

2 Material & methods

Thirty fresh frozen cadavers were chosen with supple hands for the study. The study was done as per standard ethical guidelines. Extremities revealing an injury to the forearm or the hand with deformity, anomalous muscles in the carpal tunnel seen during dissection, or hands with previous scars were excluded from the study. 15 right & 15 left hands have been included in the study. Two observers made independent measurements in ten hands (5 right and five left hands) to determine interobserver variability initially.

Skin was incised in the midline over the wrist's volar aspect and extended into the palm. Palmar aponeurosis was resected along with the skin of the palm. The distal edge of the transverse carpal ligament was defined, and subsequently, the ligament was incised along the midline to expose the carpal tunnel, as seen in Fig. 1.The superficial palmar arch and the median nerve, with branches, were retracted. The lumbrical muscles were identified under the transverse carpal ligament, and the proximal-most point of their origin was marked by indelible ink, as seen in Fig. 2(A)The distance between the TCL's distal edge and the lumbrical's proximal origin marked with ink was measured using the vernier Caliper. The measurements were recorded in millimeters up to the first decimal. Later, all the fingers were flexed, and the proximal glide of lumbricals was seen and measured individually. All the measurements were carried out with the forearm supinated, the wrist in a neutral position, and the fingers in three specific positions, i.e., full extension, lumbrical position & full flexion, as seen in Fig. 2(A, B, C). In the case of the third and the fourth lumbrical muscles with dual origins, the proximal-most origin was considered for the study, as seen in Fig. 3. Measurements of each in all the positions were documented after correcting the lumbrical slack by applying gentle traction to the tendons of FDP (Supplementary figure 1).

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Fig. 1 Incised TCL exposing the FDP tendons and the proximal origin of lumbricals underneath extending distally towards the digits.

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Fig. 2 With fingers in (A): full extension, (B) lumbrical position, (C) full flexion position; the proximally migrated origin of first lumbrical is noted with blue indelible ink, and incursion is measured using a vernier caliper from the distal edge of TCL also marked with indelible ink.

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Fig. 3 Dual origin of third lumbrical arising from the middle and ring finger FDP.

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Descriptive statistics were used for frequency and mean estimations. A paired t-test was used to measure the mean change in incursion of each lumbrical from full extension to lumbrical position and from lumbrical to full extension position. ANOVA followed by Tukey's post hoc test was used to measure the mean difference in incursion among the lumbricals at full extension, lumbrical, and full flexion positions and was also used to calculate the most significant incursion amongst the four lumbricals. The Chi-Square Test has been used to demonstrate the statistical significance of the change in the proportion of hands showing lumbrical muscle origin proximal to the distal border of the transverse carpal ligament from finger extension to flexion.

3 Results

Interobserver Variability: A paired t-test was used, and no statistically significant difference was found between measurements taken by the two observers (p > 0.05).

4 Discussion

Carpal tunnel syndrome (CTS) is predominantly idiopathic, accounting for 90 % of nerve compressions.⁶ Historically, it has been attributed to a mismatch between the contents and the size of the carpal tunnel, leading to increased pressure and disruption of the median nerve and its blood flow. However, dynamic forces may play a role in idiopathic CTS.^2,3 Idiopathic CTS is linked to connective tissue degeneration, vascular sclerosis, collagen fragmentation, and synovial edema due to fibrous hypertrophy, all contributing to carpal tunnel enlargement and subsequent median nerve compression. During digit flexion, microdamage to the synovial tissue and median nerve may result from flexor tendons and corresponding muscle belly incursion. Typically, the flexor digitorum profundus (FDP) during flexion generates around 6.4 kgf force, which exerts compressive forces against TCL. This may damage the synovial tissue due to continuous and excessive use or lumbrical hypertrophy in hand-driven occupational workers.^7,8

The lumbrical incursion into the carpal tunnel is a normal phenomenon, but repetitive finger motions can lead to lumbrical hypertrophy, potentially compressing the median nerve and contributing to CTS. Due to their high fiber length/muscle length ratio, lumbricals can significantly encroach into the carpal tunnel during finger flexion, increasing tunnel pressure.^9,10

Cadaveric studies have further explored lumbrical incursion patterns. Cobb et al. analyzed five cadaveric hands to determine the amount of lumbrical incursion in the carpal tunnel. They were compared at full extension, 50 %, 75 %, and 100 % finger flexion and found that lumbrical incursion increased with finger flexion from 7.8 mm distal to TCL in full extension to being displaced by 30 mm into TCL, potentially contributing to median nerve compression. This study also observed significant lumbrical incursion at 75 % and 100 % flexion, suggesting a relationship between proximally originating lumbricals and idiopathic CTS.³

Quantitative measurements, such as the cross-sectional area of the median nerve, flattening ratio, and palmar bowing of the flexor retinaculum, are used to assess these changes via ultrasonography (USG) or MRI. A study by Nadar et al. using USG found a significant increase, p = 0.04, in the carpal tunnel area during full finger flexion, indicating a link between lumbrical incursion and increased tunnel pressure. Another study by Nadar et al., using MRI, observed significant changes with p < 0.001 in the flexor retinaculum's palmar bowing and the median nerve's flattening ratio due to incursion of lumbricals into the carpal tunnel during an isometric grip.^11,12

As the incursion of FDP to move a joint is based on its moment arm and range of motion, which is consistent across fingers, FDP and corresponding lumbrical incursion should not vary significantly amongst each other. However, our study confirmed that the second lumbrical exhibited the highest mean incursion due to its more proximal origin and not because a longer middle finger would have more incursion of the respective FDP and its corresponding lumbrical.^9,13 A cadaveric study by Mehta and Gardner in 1961 suggested that only the second lumbrical was proximal to the distal edge of TCL, and the other three lumbricals were significantly distal. In our study, the third lumbrical showed the second highest incursion. This differed from Mehta et al.'s finding, where the first lumbrical was second closest to the distal edge of TCL. Their study was limited merely to one unspecified finger position.⁴ The first and fourth lumbricals showed varying incursion patterns, with the fourth lumbrical having the least incursion during full flexion. The first lumbrical displayed the lowest mean incursion in full extension and lumbrical position due to its origin being distal-most than the other three lumbricals. However, from lumbrical to full flexion position, there was significant incursion of the first lumbrical, which reflected the highest percentage increase in the mean incursion on full flexion amongst all four lumbricals. This can be explained due to the independent gliding of the FDP tendon of the index finger.¹⁴

Cartwright et al. conducted a study on laborers indicating that muscle incursion in the carpal tunnel was present in 100 % of cases of CTS and 96.5 % with "possible CTS," suggesting that muscle incursion contributes to compressive changes leading to CTS.¹⁵

The number of hands showing lumbrical origins proximal to the distal edge of the transverse carpal ligament increased significantly from full extension to flexion. This denotes the crowding of the tunnel during activities involving full-finger flexion. In full extension, all four lumbricals were proximal in only nine hands; this increased to 80 % in the lumbrical position and 93 % in full flexion. These findings contrast with Siegel et al.'s study, where, in full extension, the first lumbrical origin proximal to TCL was seen in 22.5 % versus 63.3 % in ours. Similarly, second, third, and fourth lumbrical had 45 %,12.5 %, and 2.6 % in the Siegel study versus 73.3 %, 60 %, and 60 %, respectively, in our study.⁵ The discrepancy may be due to our correction of lumbrical slack and considering the proximal-most origin of dual originating third and fourth lumbricals, which we think is the correct way to measure. These factors could have been overlooked in previous studies. Our findings align with Kaplan's description of lumbrical origins. Kaplan noted that, in extension, the first lumbrical extended to the distal crest of the trapezium, while the second, third, and fourth lumbricals reached the level of the distal end of the pisiform. Finger flexion causes the proximal ends of these origins to move toward the distal end of the radius.¹⁴

With no studies referring to relative positions of the lumbricals, our study found that the origin of the second lumbrical was the most proximal in maximum hands. However, in some hands, the first, third, or even fourth lumbrical has been the proximal-most when compared among the four lumbricals.

5 Limitation and future prospects

Although fresh cadavers were used, the findings may not be fully extrapolated to living subjects. Our study highlights the importance of lumbrical incursion, the proportion of lumbricals proximal to the distal edge of the transverse carpal ligament, and their relative position. Further studies using dynamic ultrasonography in CTS patients and the general population are recommended to correlate these findings.

6 Conclusion

The second lumbrical muscle origin was most proximal in most hands and had the highest mean incursion amongst all four lumbricals. The first lumbrical muscle showed the most significant percentage increase in mean incursion during finger flexion. Flexing the fingers from full extension to the lumbrical position caused the origins of all four lumbrical muscles to move proximally to the distal edge of the transverse carpal ligament in most hands, causing crowding of the tunnel. All these findings suggest that natural incursion or anatomical and pathological variation of these lumbricals increases the contents within the carpal tunnel. Hence, lumbricals are to be considered a dynamic cause for increasing pressure over the median nerve in the tunnel leading to CTS in the backdrop of tunnel morphology, content, and repetitive movements.

CRediT authorship contribution statement

Yogesh A. Kothari: Writing – original draft, Writing – review & editing, Visualization. Raj Kanna: Writing –review, Methodology, Investigation. Amrita Parida: Formal analysis. Anil K. Bhat: Conceptualization, Writing – review & editing, Supervision.

Informed consent (patient/guardian), mandatory only for case reports/clinical image

This was a cadaveric study.

Ethical considerations

This study did not involve human or animal patients. The investigations were conducted strictly within in-vitro parameters, eliminating the need for ethical human or animal experimentation approval. We ensured that the highest ethical standards were maintained throughout the study and that the research design and implementation adhered to established guidelines.

Funding statement

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.