- Email: [email protected]
Tactile Sensibility on the Fingernail
SCIENTIFIC ARTICLE
Tactile Sensibility on the Fingernail Benjamin Zhi Qiang Seah, Clement Chun Ho Wu, Sandeep Jacob Sebastin, MD, Amitabha Lahiri, MD
Purpose To measure tactile discrimination (static and moving 2-point discrimination) and threshold levels (Weinstein enhanced sensory test) over the nail plate in normal subjects and compare these values with those at the corresponding finger pulps. Methods Tactile discrimination and threshold values over the nail plates and finger pulps were measured on 300 digits in 30 healthy subjects with a mean age of 23 years. Subjects with cosmetic nail modifications, injuries, neurological deficits, dermatological conditions, or history of upper limb surgery were excluded. Equivalence testing was conducted to look for clinical equivalence between values obtained at both sites. Results The mean static 2-point discrimination, moving 2-point discrimination, and threshold value over the human nail were 6.7 mm, 2.4 mm, and 0.06 g, respectively. The corresponding values for the finger pulp were 2.4 mm, 2.2 mm, and 0.01 g, respectively. The static 2-point discrimination and threshold values were superior for the finger pulp, whereas moving 2-point discrimination values at both sites were clinically equivalent. Conclusions Our study suggests that tactile discrimination and threshold levels can be measured over the nail plate and that moving 2-point discrimination values are clinically equivalent to those obtained on the corresponding pulp. This highlights the importance of the nail plate in the sensory function of the fingertip. The normative data from this study may be useful in establishing the impact of nail injury and the contribution of the nail to the overall function of the hand. (J Hand Surg 2013;38A:2159e2163. Copyright Ó 2013 by the American Society for Surgery of the Hand. All rights reserved.) Type of study/level of evidence Diagnostic II. Key words Pulp sensation, nail sensation, normative values, tactile sensibility.
T
HE FINGER PULP has a high concentration of sensory receptors, and its sensibility has been likened to the eye as an organ of sensory exploration. The sensibility of the finger pulp has been well studied, and tests evaluating it are used in routine clinical practice. The fingernail has largely
From the Yong Loo Lin School of Medicine, National University of Singapore; and the Department of Hand and Reconstructive Microsurgery, National University Hospital, Singapore. Received for publication February 19, 2013; accepted in revised form August 19, 2013. No benefits in any form have been received or will be received related directly or indirectly to the subject of this article. Corresponding author: Sandeep Jacob Sebastin, MD, Department of Hand and Reconstructive Microsurgery, Level 11, National University Health System, 1E Kent Ridge Rd., Singapore 119228; e-mail: [email protected]. 0363-5023/13/38A11-0010$36.00/0 http://dx.doi.org/10.1016/j.jhsa.2013.08.112
been considered as a passive support that enhances sensory perception of the pulp. Our observations suggest that the nail plate with its intimate relationship with the nailbed and nailfolds may act as an agent for the transmission of haptic sensation. Common examples of such function are seen in daily life, such as running the edge of the nail plate on surfaces, which allows one to perceive the roughness and hardness of the object. In this study, we evaluated the sensibility of the nail plate for tactile discrimination (static and moving 2-point discrimination [s-2PD and m-2PD]), and threshold levels (Weinstein enhanced sensory test [WEST]) in young healthy volunteers and compared these against values obtained on the corresponding finger pulp. We hypothesized that the sensibility of the nail can be clinically quantified and that it will not be as discriminatory as the finger pulp. The normative s-2PD,
Ó 2013 ASSH
r
Published by Elsevier, Inc. All rights reserved.
r
2159
2160
SENSIBILITY OF THE NAIL
m-2PD, and WEST values obtained through this study may aid in the evaluation of the extent and the monitoring of recovery in injuries involving the nail. SUBJECTS AND METHODS Healthy young adult volunteers who consented to participation were recruited. Individuals who displayed evidence of cosmetic modifications to the nail, gave history of injury or surgery to the upper limb, or had neurological or dermatological conditions that might affect measurement of sensibility were excluded. The institutional review board approved the study, and informed consent was obtained from all volunteers. The study cohort comprised 30 subjects. Sixteen were men. The mean age of the study cohort was 23 years (range, 21e26 y). Tactile discrimination (s-2PD and m-2PD) was evaluated using a commercially available 2-point discrimination tool (Dellon-McKinnon Disk-Criminator, North Coast Medical, Inc., Morgan Hill, CA). Threshold levels (WEST) were evaluated using a set of commercially available nylon monofilaments of varying diameter, each capable of producing a calibrated target force measured in grams (Touch-Test Sensory Evaluators, North Coast Medical, Inc., Morgan Hill, CA).1 Tactile discrimination and threshold levels were measured on both the finger pulp and the nail plate for each digit. All measurements were done with the subject blinded, the forearm and hand resting on a table, and the palmar or dorsal surface of the hand facing upward depending on whether the finger pulp or the nail plate was being tested. Measurement of s-2PD The test instrument was applied first to the finger pulps of all 10 digits, beginning with the right thumb and moving medially, followed by the nail plates (Fig. 1) in a similar fashion. In accordance with Moberg’s protocol,2 the interpoint distance was increased or decreased at 1-mm intervals until the minimum distance was obtained following a consistency of 7 out of 10 successive accurate responses. Measurement of m-2PD We slid the test instrument in a proximal-to-distal fashion along the longitudinal axis of the finger pulp and nail plate, beginning from just distal to the cuticle and ending just proximal to the site on the nail plate where the hyponychium was located anteriorly. Measurement of threshold levels An adapted form of the rapid threshold procedure described by Schulz et al3 was used to conduct the WEST. The monofilaments were applied perpendicular JHS
r
FIGURE 1: s-2PD test over the nail plate. The test instrument was applied perpendicular to the nail plate until blanching of the nailbed was evident.
to the test site in descending order in terms of the achieved endforce, starting at an easily detectable level. They were pressed against the test site steadily until it bent, held on-site for 1 second, and then gradually lifted off. If slippage of the monofilament occurred, the area was retested. The finger pulps of all 10 digits were tested successively, beginning with the right thumb and moving medially, followed by the nail plates in a similar fashion (Fig. 2). Subjects were instructed from the start of the test at each new site to indicate immediately the presence of any stimulus detected. Response soon after monofilament application ensured true detection. Delayed or hesitant responses were dealt with by retesting with the same monofilament. No response after monofilament application was supplemented by an auditory prompt in case the subject forgot to respond. The same auditory prompt without pressure application (catch trial) was inserted at random at least once at each site to assess authenticity of responses. The subject was reinstructed and the test restarted if false-positive responses were elicited. Statistical analysis The mean difference between the s-2PD and m-2PD and threshold values at the finger pulp and nail
Vol 38A, November 2013
SENSIBILITY OF THE NAIL
2161
although tactile sensibility through the nail was inferior to that through the finger pulp. Moving 2-point discrimination The mean m-2PD values over the finger pulp and nail plate were 2.2 mm (95% CI, 2.2 mm and 2.3 mm; range, 2e3 mm) and 2.4 mm (95% CI, 2.3 mm and 2.5 mm; range, 2e9 mm), respectively. The mean difference of the m-2PD values between the nail plate and the finger pulp was 0.2 mm (SD, 0.86; n, 300; 95% CI, 0.1 mm and 0.3 mm). Declaring a clinical equivalence range of 1.0 mm, our study suggested that m-2PD can be perceived through the nail plate and that the sensibility of the human nail was comparable with the finger pulp.
FIGURE 2: WEST over the nail plate. The test instrument was applied perpendicular to the nail plate until the monofilament bent, held on-site for 1 second, and then gradually lifted off.
plate was tabulated and subjected to equivalence testing with a clinical equivalence range of 1 mm (the smallest interval increase in interpoint distance measurable by the 2-point discrimination tool) for tactile discrimination and 0.01 g (the smallest interval increase in force measurable by the monofilaments) for threshold. Clinical equivalence was established when the 95% confidence interval (CI) of the mean difference fell within the defined clinical equivalence range. RESULTS Static 2-point discrimination The mean s-2PD values over the finger pulp and nail plate were 2.4 mm (95% CI, 2.4 mm and 2.5 mm; range, 2e5 mm) and 6.7 mm (95% CI, 6.5 mm and 6.9 mm; range, 2e9 mm), respectively. The mean difference of the s-2PD values between the nail plate and the finger pulp was 4.2 mm (SD, 1.7; n, 300; 95% CI, 4.0 mm and 4.4 mm). Declaring a clinical equivalence range of 1.0 mm, our study suggested that s-2PD can be observed through the nail plate, JHS
r
Weinstein enhanced sensory test The mean threshold values over the finger pulp and nail plate were 0.01 g (95% CI, 0.011 g and 0.012 g; range, 0.01e0.30 g) and 0.06 g (95% CI, 0.05 g and 0.06 g; range, 0.01e0.28 g). The mean difference of the threshold values between the nail plate and the finger pulp was 0.04 g (SD, 0.043; n, 300; 95% CI, 0.039 g and 0.049 g). Declaring a clinical equivalence range of 0.01 g, our observations suggest that pressure sensation can be perceived through the nail plate, although threshold values over the human nail were significantly higher than that over the finger pulp. DISCUSSION The nail is generally not considered to be a primary sensory structure. It has been thought to play a passive role in increasing sensory perception at the pulp by providing counterpressure necessary for the compression of sensory end organs between the palmar skin and the nail. This provides for efficient and accurate prehension of small objects.4 Results from this study demonstrate the presence of transungual sensation and highlight the role of the nail plate in the transmission of mechanical stimuli from the environment to receptors located in the nailbed and the nailfolds. The nail plate not only transmits pressure sensation with a measurable threshold level but is also able to transmit measurable discriminative sensation. S-2PD values and cutaneous pressure thresholds over the nail plate suggest that it is significantly less sensitive than the finger pulp, which is the primary sensory structure. However, m-2PD over the nail plate was comparable with that over the finger pulp. The nail plate is a hard but deformable structure and is intimately bound to the nail matrix through
Vol 38A, November 2013
2162
SENSIBILITY OF THE NAIL
interdigitating ridges. It is also in close contact with the pulp at the dorsal and lateral nailfolds and at the hyponychium. Histology has demonstrated that the nail plate is composed of compacted, dead keratinized epithelial cells. We were unable to find any studies that have specifically looked for sensory receptors in the nail plate. Sensation in the nail plate is likely to be perceived through close contact with tactile receptors in the nailbed and perionychium. We were able to quantify the pressure threshold and discriminatory sensation perceived through the nail plate. The measurement of sensibility directly on the sterile matrix will allow us to determine the role of the nail plate in transmitting sensation. This will require a group of patients who have sustained an avulsion of the nail plate without injuring the nailbed. The mechanical forces applied to the nail are transmitted to the various mechanoreceptors present in the surrounding cutaneous structures. The nature of perception will be affected by the distribution of these receptors in the matrix and the nailfold. The primary mechanoreceptors in the skin are Merkel discs in the basal layer of the epidermis and Ruffini corpuscles in the deep dermis and hypodermis (slowly adapting mechanoreceptors). They mediate perception of constant touch and pressure (s-2PD and WEST). Rapidly adapting Meissner and Pacinian corpuscles are present in the dermis and mediate perception of moving touch (m-2PD) and vibratory stimuli.5 Few studies have detailed the receptor distribution in the nail matrix and the nailfold. Lacour et al6 published data on Merkel cell density at 44 different sites in 6 human cadavers. Three different zones were defined according to Merkel cell density. According to this study, the pulps of the digits, proximal nailfolds, and palms were classified as Merkel cellerich zones containing greater than 50 Merkel cells/mm2. Moll and Moll7 studied Merkel cell distribution in the human nail matrix by examining fingernails obtained from 8 normal human fetuses and 3 adults. They found that, in specimens at 12 to 15 weeks of gestation, Merkel cells were absent from the epithelium of the ventral matrix (surface-near portion), lunula, and nailbed. Even in the epithelium of the proximal nailfold and the dorsal and ventral side of the apex region of the nail anlage, for which Merkel cells were abundant during the same gestational period, they were found less frequently by week 22 and were rare in the adult nail matrix (in about 20 sections investigated, only a single Merkel cell located within the ventral matrix epithelium near the apex was detected). These results were challenged by Cameli et al,8 who studied nail matrix specimens obtained JHS
r
from 8 patients (mean age, 19 y) with ingrowing toenails and 2 autopsy fingernail matrix specimens using similar immunohistochemical methods. They reported regular detection of Merkel cells among the nail matrix epithelium of adults, and the mean number of 21 Merkel cells for nail matrix sections was similar to that reported in the fetal nail by Moll and Moll.7 Ruffini-like spray endings have been described in the nailbed in both monkeys9 and humans.10 These structures are believed to play a role in proprioception given their directional sensitivity to skin stretch.11 The presence of Merkel endings and Meissner and Pacinian corpuscles in the nailbed were not reported in these studies, although their presence in glabrous finger pads was well documented. Slowly adapting type II afferents, which are thought to innervate Ruffini-like nerve endings, have also been described to be present in the half-moonelike area close to the lateral or proximal borders of the nails.12,13 The extent to which they contribute to sensory perception over the nail plate is unknown, although they have been shown to contain important information about fingertip forces such as direction in a 3-dimensional plane necessary for haptic perception.14 Given the correlation between sensibility and histology highlighted by Dellon and Munger5 and our study findings of clinically equivalent m-2PD values over the finger pulp and the nail plate, we postulate that rapidly adapting mechanoreceptors (eg, Meissner corpuscles) are present in equal abundance in cutaneous structures surrounding the nail and in the finger pulp. The m-2PD test developed by Dellon15 was shown to best predict hand function, defined as the ability to identify objects with the fingertips.16 One of the problems with the 2-point discrimination tests is the lack of standardization of applied pressure.17 More pressure will cause more deformity on the skin and bring more receptors into the field of stimulation.18,19 The amount of pressure applied (ie, the cutaneous pressure threshold achieved) was demonstrated to have an inverse relationship with s-2PD values in a study conducted by Aszmann and Dellon.20 In this study, standardization of the applied pressure was attempted by application of the test instrument until blanching around the area underlying the points, both on the finger pulp and on the nailbed, was first perceptible. It has been recommended that the measurement of 2-point discrimination in critical studies of sensibility should be conducted with the PressureSpecified Sensory Device, a neurosensory testing device developed to solve the problem of cutaneous pressure threshold inconsistencies.21
Vol 38A, November 2013
SENSIBILITY OF THE NAIL
Fingertip injuries are commonly encountered, and nailbed involvement accompanies 15% to 24% of such injuries.22 Patients with nailbed injury or nail ablation may experience diminished tactile sensation and loss of dexterity secondary to losing the sensory contribution of the nail. The residual functional problems following nailbed injury have not been previously quantified. The impact of nail injury can perhaps be established by using normative data from this study to compare with those from individuals with nail injuries or those without nails. Hence, the value of this study is its potential ability to establish the contribution of the nail plate to the overall function of the fingertip. Our study highlights the role of the nail plate as an active agent for sensory perception. In the clinical context, the measurement of transungual sensation may form a component in the evaluation of outcomes of fingertip and nailbed injuries. REFERENCES 1. Radoiu H, Rosson GD, Andonian E, Senatore J, Dellon AL. Comparison of measures of large-fiber nerve function in patients with chronic nerve compression and neuropathy. J Am Podiatr Med Assoc. 2005;95(5):438e445. 2. Moberg E. The unresolved problem—how to test the functional value of hand sensibility. J Hand Ther. 1991;4:105e110. 3. Schulz LA, Bohannon RW, Morgan WJ. Normal digit tip values for the Weinstein Enhanced Sensory Test. J Hand Ther. 1998;11(3):200e205. 4. Zook EG. Anatomy and physiology of the perionychium. Clin Anat. 2003;16(1):1e8. 5. Dellon AL, Munger BL. Correlation of histology and sensibility after nerve repair. J Hand Surg Am. 1983;8(6):871e875. 6. Lacour JP, Dubois D, Pisani A, Ortonne JP. Anatomical mapping of Merkel cells in normal human adult epidermis. Br J Dermatol. 1991;125(6):535e542.
JHS
r
2163
7. Moll I, Moll R. Merkel cells in ontogenesis of human nails. Arch Dermatol Res. 1993;285(6):366e371. 8. Cameli N, Ortonne JP, Picardo M, Peluso AM, Tosti A. Distribution of Merkel cells in adult human nail matrix. Br J Dermatol. 1998;139(3):541. 9. Paré M, Smith AM, Rice FL. Distribution and terminal arborizations of cutaneous mechanoreceptors in the glabrous finger pads of the monkey. J Comp Neurol. 2002;445(4):347e359. 10. Paré M, Behets C, Cornu O. Paucity of presumptive Ruffini corpuscles in the index finger pad of humans. J Comp Neurol. 2003;456(3):260e266. 11. Knibestöl M, Vallbo AB. Single unit analysis of mechanoreceptor activity from the human glabrous skin. Acta Physiol Scand. 1970;80(2):178e195. 12. Knibestöl M. Stimulus-response functions of slowly adapting mechanoreceptors in the human glabrous skin area. J Physiol. 1975;245(1):63e80. 13. Johansson RS. Tactile sensibility in the human hand-receptive field characteristics of mechanoreceptive units in the glabrous skin area. J Physiol. 1978;281:101e125. 14. Birznieks I, Macefield VG, Westling G, Johansson RS. Slowly adapting mechanoreceptors in the borders of the human fingernail encode fingertip forces. J Neurosci. 2009;29(29): 9370e9379. 15. Dellon AL. The moving two-point discrimination test: clinical evaluation of the quickly adapting fiber/receptor system. J Hand Surg Am. 1978;3(5):474e481. 16. Dellon AL, Kallman CH. Evaluation of functional sensation in the hand. J Hand Surg Am. 1983;8(6):865e870. 17. Lundborg G, Rosén B. The two-point discrimination test—time for a re-appraisal? J Hand Surg Br. 2004;29(5):418e422. 18. Moberg E. Reconstructive hand surgery in tetraplegia, stroke and cerebral palsy: some basic concepts in physiology and neurology. J Hand Surg Am. 1976;1(1):29e34. 19. Moberg E. The Upper Limb in Tetraplegia: A New Approach to Surgical Rehabilitation. Stuttgart: Thieme; 1978. 20. Aszmann OC, Dellon AL. Relationship between cutaneous pressure threshold and two-point discrimination. J Reconstr Microsurg. 1998;14(6):417e421. 21. Slutsky DJ. Use of nerve conduction studies and the pressurespecified sensory device in the diagnosis of carpal tunnel syndrome. J Hand Surg Eur Vol. 2009;34(1):60e65. 22. Chang J, Vernadakis AJ, McClellan WT. Fingertip injuries. Clin Occup Environ Med. 2006;5(2):413e422.
Vol 38A, November 2013
Tactile Sensibility on the Fingernail Benjamin Zhi Qiang Seah, Clement Chun Ho Wu, Sandeep Jacob Sebastin, MD, Amitabha Lahiri, MD
Purpose To measure tactile discrimination (static and moving 2-point discrimination) and threshold levels (Weinstein enhanced sensory test) over the nail plate in normal subjects and compare these values with those at the corresponding finger pulps. Methods Tactile discrimination and threshold values over the nail plates and finger pulps were measured on 300 digits in 30 healthy subjects with a mean age of 23 years. Subjects with cosmetic nail modifications, injuries, neurological deficits, dermatological conditions, or history of upper limb surgery were excluded. Equivalence testing was conducted to look for clinical equivalence between values obtained at both sites. Results The mean static 2-point discrimination, moving 2-point discrimination, and threshold value over the human nail were 6.7 mm, 2.4 mm, and 0.06 g, respectively. The corresponding values for the finger pulp were 2.4 mm, 2.2 mm, and 0.01 g, respectively. The static 2-point discrimination and threshold values were superior for the finger pulp, whereas moving 2-point discrimination values at both sites were clinically equivalent. Conclusions Our study suggests that tactile discrimination and threshold levels can be measured over the nail plate and that moving 2-point discrimination values are clinically equivalent to those obtained on the corresponding pulp. This highlights the importance of the nail plate in the sensory function of the fingertip. The normative data from this study may be useful in establishing the impact of nail injury and the contribution of the nail to the overall function of the hand. (J Hand Surg 2013;38A:2159e2163. Copyright Ó 2013 by the American Society for Surgery of the Hand. All rights reserved.) Type of study/level of evidence Diagnostic II. Key words Pulp sensation, nail sensation, normative values, tactile sensibility.
T
HE FINGER PULP has a high concentration of sensory receptors, and its sensibility has been likened to the eye as an organ of sensory exploration. The sensibility of the finger pulp has been well studied, and tests evaluating it are used in routine clinical practice. The fingernail has largely
From the Yong Loo Lin School of Medicine, National University of Singapore; and the Department of Hand and Reconstructive Microsurgery, National University Hospital, Singapore. Received for publication February 19, 2013; accepted in revised form August 19, 2013. No benefits in any form have been received or will be received related directly or indirectly to the subject of this article. Corresponding author: Sandeep Jacob Sebastin, MD, Department of Hand and Reconstructive Microsurgery, Level 11, National University Health System, 1E Kent Ridge Rd., Singapore 119228; e-mail: [email protected]. 0363-5023/13/38A11-0010$36.00/0 http://dx.doi.org/10.1016/j.jhsa.2013.08.112
been considered as a passive support that enhances sensory perception of the pulp. Our observations suggest that the nail plate with its intimate relationship with the nailbed and nailfolds may act as an agent for the transmission of haptic sensation. Common examples of such function are seen in daily life, such as running the edge of the nail plate on surfaces, which allows one to perceive the roughness and hardness of the object. In this study, we evaluated the sensibility of the nail plate for tactile discrimination (static and moving 2-point discrimination [s-2PD and m-2PD]), and threshold levels (Weinstein enhanced sensory test [WEST]) in young healthy volunteers and compared these against values obtained on the corresponding finger pulp. We hypothesized that the sensibility of the nail can be clinically quantified and that it will not be as discriminatory as the finger pulp. The normative s-2PD,
Ó 2013 ASSH
r
Published by Elsevier, Inc. All rights reserved.
r
2159
2160
SENSIBILITY OF THE NAIL
m-2PD, and WEST values obtained through this study may aid in the evaluation of the extent and the monitoring of recovery in injuries involving the nail. SUBJECTS AND METHODS Healthy young adult volunteers who consented to participation were recruited. Individuals who displayed evidence of cosmetic modifications to the nail, gave history of injury or surgery to the upper limb, or had neurological or dermatological conditions that might affect measurement of sensibility were excluded. The institutional review board approved the study, and informed consent was obtained from all volunteers. The study cohort comprised 30 subjects. Sixteen were men. The mean age of the study cohort was 23 years (range, 21e26 y). Tactile discrimination (s-2PD and m-2PD) was evaluated using a commercially available 2-point discrimination tool (Dellon-McKinnon Disk-Criminator, North Coast Medical, Inc., Morgan Hill, CA). Threshold levels (WEST) were evaluated using a set of commercially available nylon monofilaments of varying diameter, each capable of producing a calibrated target force measured in grams (Touch-Test Sensory Evaluators, North Coast Medical, Inc., Morgan Hill, CA).1 Tactile discrimination and threshold levels were measured on both the finger pulp and the nail plate for each digit. All measurements were done with the subject blinded, the forearm and hand resting on a table, and the palmar or dorsal surface of the hand facing upward depending on whether the finger pulp or the nail plate was being tested. Measurement of s-2PD The test instrument was applied first to the finger pulps of all 10 digits, beginning with the right thumb and moving medially, followed by the nail plates (Fig. 1) in a similar fashion. In accordance with Moberg’s protocol,2 the interpoint distance was increased or decreased at 1-mm intervals until the minimum distance was obtained following a consistency of 7 out of 10 successive accurate responses. Measurement of m-2PD We slid the test instrument in a proximal-to-distal fashion along the longitudinal axis of the finger pulp and nail plate, beginning from just distal to the cuticle and ending just proximal to the site on the nail plate where the hyponychium was located anteriorly. Measurement of threshold levels An adapted form of the rapid threshold procedure described by Schulz et al3 was used to conduct the WEST. The monofilaments were applied perpendicular JHS
r
FIGURE 1: s-2PD test over the nail plate. The test instrument was applied perpendicular to the nail plate until blanching of the nailbed was evident.
to the test site in descending order in terms of the achieved endforce, starting at an easily detectable level. They were pressed against the test site steadily until it bent, held on-site for 1 second, and then gradually lifted off. If slippage of the monofilament occurred, the area was retested. The finger pulps of all 10 digits were tested successively, beginning with the right thumb and moving medially, followed by the nail plates in a similar fashion (Fig. 2). Subjects were instructed from the start of the test at each new site to indicate immediately the presence of any stimulus detected. Response soon after monofilament application ensured true detection. Delayed or hesitant responses were dealt with by retesting with the same monofilament. No response after monofilament application was supplemented by an auditory prompt in case the subject forgot to respond. The same auditory prompt without pressure application (catch trial) was inserted at random at least once at each site to assess authenticity of responses. The subject was reinstructed and the test restarted if false-positive responses were elicited. Statistical analysis The mean difference between the s-2PD and m-2PD and threshold values at the finger pulp and nail
Vol 38A, November 2013
SENSIBILITY OF THE NAIL
2161
although tactile sensibility through the nail was inferior to that through the finger pulp. Moving 2-point discrimination The mean m-2PD values over the finger pulp and nail plate were 2.2 mm (95% CI, 2.2 mm and 2.3 mm; range, 2e3 mm) and 2.4 mm (95% CI, 2.3 mm and 2.5 mm; range, 2e9 mm), respectively. The mean difference of the m-2PD values between the nail plate and the finger pulp was 0.2 mm (SD, 0.86; n, 300; 95% CI, 0.1 mm and 0.3 mm). Declaring a clinical equivalence range of 1.0 mm, our study suggested that m-2PD can be perceived through the nail plate and that the sensibility of the human nail was comparable with the finger pulp.
FIGURE 2: WEST over the nail plate. The test instrument was applied perpendicular to the nail plate until the monofilament bent, held on-site for 1 second, and then gradually lifted off.
plate was tabulated and subjected to equivalence testing with a clinical equivalence range of 1 mm (the smallest interval increase in interpoint distance measurable by the 2-point discrimination tool) for tactile discrimination and 0.01 g (the smallest interval increase in force measurable by the monofilaments) for threshold. Clinical equivalence was established when the 95% confidence interval (CI) of the mean difference fell within the defined clinical equivalence range. RESULTS Static 2-point discrimination The mean s-2PD values over the finger pulp and nail plate were 2.4 mm (95% CI, 2.4 mm and 2.5 mm; range, 2e5 mm) and 6.7 mm (95% CI, 6.5 mm and 6.9 mm; range, 2e9 mm), respectively. The mean difference of the s-2PD values between the nail plate and the finger pulp was 4.2 mm (SD, 1.7; n, 300; 95% CI, 4.0 mm and 4.4 mm). Declaring a clinical equivalence range of 1.0 mm, our study suggested that s-2PD can be observed through the nail plate, JHS
r
Weinstein enhanced sensory test The mean threshold values over the finger pulp and nail plate were 0.01 g (95% CI, 0.011 g and 0.012 g; range, 0.01e0.30 g) and 0.06 g (95% CI, 0.05 g and 0.06 g; range, 0.01e0.28 g). The mean difference of the threshold values between the nail plate and the finger pulp was 0.04 g (SD, 0.043; n, 300; 95% CI, 0.039 g and 0.049 g). Declaring a clinical equivalence range of 0.01 g, our observations suggest that pressure sensation can be perceived through the nail plate, although threshold values over the human nail were significantly higher than that over the finger pulp. DISCUSSION The nail is generally not considered to be a primary sensory structure. It has been thought to play a passive role in increasing sensory perception at the pulp by providing counterpressure necessary for the compression of sensory end organs between the palmar skin and the nail. This provides for efficient and accurate prehension of small objects.4 Results from this study demonstrate the presence of transungual sensation and highlight the role of the nail plate in the transmission of mechanical stimuli from the environment to receptors located in the nailbed and the nailfolds. The nail plate not only transmits pressure sensation with a measurable threshold level but is also able to transmit measurable discriminative sensation. S-2PD values and cutaneous pressure thresholds over the nail plate suggest that it is significantly less sensitive than the finger pulp, which is the primary sensory structure. However, m-2PD over the nail plate was comparable with that over the finger pulp. The nail plate is a hard but deformable structure and is intimately bound to the nail matrix through
Vol 38A, November 2013
2162
SENSIBILITY OF THE NAIL
interdigitating ridges. It is also in close contact with the pulp at the dorsal and lateral nailfolds and at the hyponychium. Histology has demonstrated that the nail plate is composed of compacted, dead keratinized epithelial cells. We were unable to find any studies that have specifically looked for sensory receptors in the nail plate. Sensation in the nail plate is likely to be perceived through close contact with tactile receptors in the nailbed and perionychium. We were able to quantify the pressure threshold and discriminatory sensation perceived through the nail plate. The measurement of sensibility directly on the sterile matrix will allow us to determine the role of the nail plate in transmitting sensation. This will require a group of patients who have sustained an avulsion of the nail plate without injuring the nailbed. The mechanical forces applied to the nail are transmitted to the various mechanoreceptors present in the surrounding cutaneous structures. The nature of perception will be affected by the distribution of these receptors in the matrix and the nailfold. The primary mechanoreceptors in the skin are Merkel discs in the basal layer of the epidermis and Ruffini corpuscles in the deep dermis and hypodermis (slowly adapting mechanoreceptors). They mediate perception of constant touch and pressure (s-2PD and WEST). Rapidly adapting Meissner and Pacinian corpuscles are present in the dermis and mediate perception of moving touch (m-2PD) and vibratory stimuli.5 Few studies have detailed the receptor distribution in the nail matrix and the nailfold. Lacour et al6 published data on Merkel cell density at 44 different sites in 6 human cadavers. Three different zones were defined according to Merkel cell density. According to this study, the pulps of the digits, proximal nailfolds, and palms were classified as Merkel cellerich zones containing greater than 50 Merkel cells/mm2. Moll and Moll7 studied Merkel cell distribution in the human nail matrix by examining fingernails obtained from 8 normal human fetuses and 3 adults. They found that, in specimens at 12 to 15 weeks of gestation, Merkel cells were absent from the epithelium of the ventral matrix (surface-near portion), lunula, and nailbed. Even in the epithelium of the proximal nailfold and the dorsal and ventral side of the apex region of the nail anlage, for which Merkel cells were abundant during the same gestational period, they were found less frequently by week 22 and were rare in the adult nail matrix (in about 20 sections investigated, only a single Merkel cell located within the ventral matrix epithelium near the apex was detected). These results were challenged by Cameli et al,8 who studied nail matrix specimens obtained JHS
r
from 8 patients (mean age, 19 y) with ingrowing toenails and 2 autopsy fingernail matrix specimens using similar immunohistochemical methods. They reported regular detection of Merkel cells among the nail matrix epithelium of adults, and the mean number of 21 Merkel cells for nail matrix sections was similar to that reported in the fetal nail by Moll and Moll.7 Ruffini-like spray endings have been described in the nailbed in both monkeys9 and humans.10 These structures are believed to play a role in proprioception given their directional sensitivity to skin stretch.11 The presence of Merkel endings and Meissner and Pacinian corpuscles in the nailbed were not reported in these studies, although their presence in glabrous finger pads was well documented. Slowly adapting type II afferents, which are thought to innervate Ruffini-like nerve endings, have also been described to be present in the half-moonelike area close to the lateral or proximal borders of the nails.12,13 The extent to which they contribute to sensory perception over the nail plate is unknown, although they have been shown to contain important information about fingertip forces such as direction in a 3-dimensional plane necessary for haptic perception.14 Given the correlation between sensibility and histology highlighted by Dellon and Munger5 and our study findings of clinically equivalent m-2PD values over the finger pulp and the nail plate, we postulate that rapidly adapting mechanoreceptors (eg, Meissner corpuscles) are present in equal abundance in cutaneous structures surrounding the nail and in the finger pulp. The m-2PD test developed by Dellon15 was shown to best predict hand function, defined as the ability to identify objects with the fingertips.16 One of the problems with the 2-point discrimination tests is the lack of standardization of applied pressure.17 More pressure will cause more deformity on the skin and bring more receptors into the field of stimulation.18,19 The amount of pressure applied (ie, the cutaneous pressure threshold achieved) was demonstrated to have an inverse relationship with s-2PD values in a study conducted by Aszmann and Dellon.20 In this study, standardization of the applied pressure was attempted by application of the test instrument until blanching around the area underlying the points, both on the finger pulp and on the nailbed, was first perceptible. It has been recommended that the measurement of 2-point discrimination in critical studies of sensibility should be conducted with the PressureSpecified Sensory Device, a neurosensory testing device developed to solve the problem of cutaneous pressure threshold inconsistencies.21
Vol 38A, November 2013
SENSIBILITY OF THE NAIL
Fingertip injuries are commonly encountered, and nailbed involvement accompanies 15% to 24% of such injuries.22 Patients with nailbed injury or nail ablation may experience diminished tactile sensation and loss of dexterity secondary to losing the sensory contribution of the nail. The residual functional problems following nailbed injury have not been previously quantified. The impact of nail injury can perhaps be established by using normative data from this study to compare with those from individuals with nail injuries or those without nails. Hence, the value of this study is its potential ability to establish the contribution of the nail plate to the overall function of the fingertip. Our study highlights the role of the nail plate as an active agent for sensory perception. In the clinical context, the measurement of transungual sensation may form a component in the evaluation of outcomes of fingertip and nailbed injuries. REFERENCES 1. Radoiu H, Rosson GD, Andonian E, Senatore J, Dellon AL. Comparison of measures of large-fiber nerve function in patients with chronic nerve compression and neuropathy. J Am Podiatr Med Assoc. 2005;95(5):438e445. 2. Moberg E. The unresolved problem—how to test the functional value of hand sensibility. J Hand Ther. 1991;4:105e110. 3. Schulz LA, Bohannon RW, Morgan WJ. Normal digit tip values for the Weinstein Enhanced Sensory Test. J Hand Ther. 1998;11(3):200e205. 4. Zook EG. Anatomy and physiology of the perionychium. Clin Anat. 2003;16(1):1e8. 5. Dellon AL, Munger BL. Correlation of histology and sensibility after nerve repair. J Hand Surg Am. 1983;8(6):871e875. 6. Lacour JP, Dubois D, Pisani A, Ortonne JP. Anatomical mapping of Merkel cells in normal human adult epidermis. Br J Dermatol. 1991;125(6):535e542.
JHS
r
2163
7. Moll I, Moll R. Merkel cells in ontogenesis of human nails. Arch Dermatol Res. 1993;285(6):366e371. 8. Cameli N, Ortonne JP, Picardo M, Peluso AM, Tosti A. Distribution of Merkel cells in adult human nail matrix. Br J Dermatol. 1998;139(3):541. 9. Paré M, Smith AM, Rice FL. Distribution and terminal arborizations of cutaneous mechanoreceptors in the glabrous finger pads of the monkey. J Comp Neurol. 2002;445(4):347e359. 10. Paré M, Behets C, Cornu O. Paucity of presumptive Ruffini corpuscles in the index finger pad of humans. J Comp Neurol. 2003;456(3):260e266. 11. Knibestöl M, Vallbo AB. Single unit analysis of mechanoreceptor activity from the human glabrous skin. Acta Physiol Scand. 1970;80(2):178e195. 12. Knibestöl M. Stimulus-response functions of slowly adapting mechanoreceptors in the human glabrous skin area. J Physiol. 1975;245(1):63e80. 13. Johansson RS. Tactile sensibility in the human hand-receptive field characteristics of mechanoreceptive units in the glabrous skin area. J Physiol. 1978;281:101e125. 14. Birznieks I, Macefield VG, Westling G, Johansson RS. Slowly adapting mechanoreceptors in the borders of the human fingernail encode fingertip forces. J Neurosci. 2009;29(29): 9370e9379. 15. Dellon AL. The moving two-point discrimination test: clinical evaluation of the quickly adapting fiber/receptor system. J Hand Surg Am. 1978;3(5):474e481. 16. Dellon AL, Kallman CH. Evaluation of functional sensation in the hand. J Hand Surg Am. 1983;8(6):865e870. 17. Lundborg G, Rosén B. The two-point discrimination test—time for a re-appraisal? J Hand Surg Br. 2004;29(5):418e422. 18. Moberg E. Reconstructive hand surgery in tetraplegia, stroke and cerebral palsy: some basic concepts in physiology and neurology. J Hand Surg Am. 1976;1(1):29e34. 19. Moberg E. The Upper Limb in Tetraplegia: A New Approach to Surgical Rehabilitation. Stuttgart: Thieme; 1978. 20. Aszmann OC, Dellon AL. Relationship between cutaneous pressure threshold and two-point discrimination. J Reconstr Microsurg. 1998;14(6):417e421. 21. Slutsky DJ. Use of nerve conduction studies and the pressurespecified sensory device in the diagnosis of carpal tunnel syndrome. J Hand Surg Eur Vol. 2009;34(1):60e65. 22. Chang J, Vernadakis AJ, McClellan WT. Fingertip injuries. Clin Occup Environ Med. 2006;5(2):413e422.
Vol 38A, November 2013