Effect of Carpal Tunnel Syndrome on Grip and Pinch Strength Compared With Sex- and Age-Matched Normative Data

Authors


  • ClinicalTrials.gov identifier: NCT00803257.

Abstract

Objective

To compare grip and pinch strength of individuals with carpal tunnel syndrome (CTS) to normative values before and after a combined splint/stretching intervention.

Methods

Data collected on grip and pinch strength on 124 subjects with CTS were compared to age- and sex-matched normative data.

Results

In general, our sample had significantly lower strength than the normative sample at baseline. Although there were significant improvements in strength after 4 weeks of splinting/exercise, subjects continued to have significant deficits in comparison to the normative data.

Conclusion

Patients with CTS have moderate to large deficits in grip and pinch strength in comparison to normative data. Splinting/stretching may reduce these deficits; however, CTS patients are often left with residual problems at 4 weeks.

Introduction

The clinical manifestations of carpal tunnel syndrome (CTS) include sensory loss and hand weakness ([1, 2]). The lumbricals, one of the intrinsic muscle groups of the hand, play an important role in both these manifestations. The lumbricals originate on the tendons of the flexor digitorum profundus (FDP) rather than on a bone. When the FDP flexes the fingers, it retracts through the carpal tunnel and pulls the lumbricals proximally ([3]). The lumbricals enter the carpal tunnel with as little as 50% of finger flexion ([4]), reducing the volume of the carpal tunnel, increasing carpal tunnel pressure ([4]), and contributing to the symptoms of CTS.

CTS may cause reduced hand strength for a variety of reasons. Muscle tension–related grip and pinch strength loss in patients with CTS may be a result of the loss of metacarpophalangeal (MCP) joint flexion power from de-innervation of the first and second lumbricals, as the extrinsic flexors are innervated higher in the forearm ([5]). The intrinsic muscles (palmar interossei, dorsal interossei, and lumbrical) are antagonists of proximal interphalangeal (PIP) and distal interphalangeal (DIP) joint flexion of the digits and aid in precision grip and pinch by flexing the MCP joints while extending the IP joints. Patients with CTS often have tightness of the lumbrical muscles, which causes increased resistance/drag into extension, preventing the DIP joints and PIP joints from achieving strong flexion. An additional cause of strength reduction may include sensory disruption common in CTS, which may reduce the ability to accurately regulate force production.

Grip and pinch strength (lateral and palmar) are commonly used to assess motor outcomes after CTS surgery ([6]). However, no study has compared the results to normative data. Examining hand strength of patients with CTS in comparison to normative data allows us to understand the magnitude of the effect of CTS on hand strength and provides an opportunity to observe the extent to which patients with CTS recover their strength in comparison to normative data.

This study compared the hand strength of subjects with CTS to age- and sex-matched normative values before and after a combined splint/stretching intervention. We had 3 hypotheses as follows: 1) baseline grip and pinch strength scores would be significantly lower than age- and sex-matched norms, 2) followup scores of subjects with CTS would show significant improvements in grip and pinch strength compared to baseline scores, and 3) grip and pinch strength scores of subjects with CTS would remain significantly lower, but would trend towards age- and sex-matched norms at followup.

Box 1. Significance & Innovations

  • Patients with carpal tunnel syndrome demonstrated significant deficits in hand strength in comparison to normative data at both baseline and after a 4-week conservative treatment.
  • A splinting/exercise combination improves hand strength over time, but it may take more than 4 weeks to achieve typical strength.

Subjects and methods

Subjects

This is a secondary analysis on data collected during a randomized clinical trial that examined the effectiveness of an intensive lumbrical splint and stretching regimen to less intensive lumbrical treatments for people with mild to moderate CTS ([7]). Subjects were referred by two hand surgeons, one at the University of Pittsburgh Medical Center and one at Alleghany General Hospital. To be eligible for the study, subjects had to be age ≥18 years, have clinical symptoms of CTS (numbness, tingling, and/or pain), have a positive examination for CTS including a positive result for Phalen's sign, Tinel's sign, or Durkin's test, and have an absence of thenar atrophy as observed by the physician. Exclusion criteria included prior CTS surgery, patients needing immediate surgery as determined by the hand surgeon, compressive neuropathy in ipsilateral arm or peripheral polyneuropathy, diabetes mellitus, pregnancy, non–English speaking, and unable to be compliant with a home therapeutic regimen.

Subjects were recruited at 2 outpatient hand clinics from March 2008 to May 2010. Approvals for conducting this study were obtained from the Institutional Review Boards at both sites (University of Pittsburgh and Alleghany General Hospital).

Instruments

Demographic information

We gathered descriptive information to characterize the sample (Table 1).

Table 1. Demographics*
VariableFemale (n = 88)Male (n = 35)
  1. Values are the number (percentage) unless otherwise indicated. CTS = carpal tunnel syndrome.
  2. aValid percentage for those who answered.
Age, mean ± SD years51.9 ± 11.755.0 ± 13.8
Sex88 (71.5)35 (28.5)
Ethnicity  
Non-Hispanic or Latinoa82 (98.8)33 (100.0)
White77 (89.5)32 (91.4)
African American8 (9.3)2 (5.7)
Employment  
Full time39 (44.8)14 (40.0)
Part time16 (18.4)4 (11.4)
Other (retired/homemaker/ student)28 (32.1)10 (28.6)
Dominant hand  
Right82 (93.2)30 (85.7)
Left5 (5.7)5 (14.3)
Both equally1 (1.1)
CTS side  
Right42 (48.3)14 (40.0)
Left11 (12.6)9 (25.7)
Bilateral34 (39.1)12 (34.3)
Treatment side  
Right42 (48.8)14 (41.2)
Left11 (12.8)9 (26.5)
Bilateral33 (38.4)11 (32.4)
CTS duration  
≤3 months13 (15.7)5 (14.7)
>3 months to <1 year25 (30.1)7 (20.6)
≥1 year45 (54.2)22 (64.7)

Grip strength

Right and left grip strength were measured using a calibrated Jamar hydraulic dynamometer (Asimov Engineering) while the subject was seated with shoulder adducted and neutrally rotated, elbow flexed at 90°, and forearm and wrist in neutral position ([8]).

Lateral and palmar pinch

Right and left lateral and palmar pinch were measured using a calibrated B&L pinch meter (B&L Engineering) while the subject was seated with shoulder adducted and neutrally rotated with the elbow flexed at 90°. The forearm and wrist were positioned in a neutral position for the lateral pinch and were pronated for the palmar pinch ([8]). While the palmar pinch forearm position was not standard, research suggests that this has only a small effect on outcome ([9]).

Procedure

Grip, lateral pinch, and palmar pinch strength assessments (in that order) were obtained at baseline and after a 4-week at-home regimen of nocturnal splinting combined with 6-times daily home stretching. Interventions were either a standard neutral wrist cock-up splint (Gsp) or a custom lumbrical splint (Lsp; wrist cock-up splint in neutral plus extension of the splint over the proximal phalanges to prevent MCP flexion) combined with 1 of 2 stretching programs as follows: 1) composite digit and wrist extension active range of motion followed by composite digit and wrist flexion active range of motion (Gst) or 2) composite digit and wrist extension passive range of motion, intrinsic (lumbrical) muscle stretch followed by intrinsic muscle massage (Lst) ([7]). Subjects were randomly assigned to 1 of 4 combinations: GspGst, GspLst, LspGst, or LspLst. Subjects received training in their respective stretches (including a handout, verbal instruction, and practice) and the appropriate splint. No other treatments were provided during the 4-week study period.

Data processing

The average of 3 trials was calculated separately for right and left sides. We obtained normative data routinely used in the clinic, including means and SDs ([8]). Study scores were separated by sex, side, and age categories to match the normative data. Ages for both sexes were divided into 5-year increments for years 20–75 (e.g., 20–24, 25–29, etc.) to match the normative data.

Each subject was age-, sex-, and right/left-matched to the appropriate normative score. Difference scores were then calculated by subtracting each subject's mean score from the matched normative score.

Data analysis

Our previous studies had found no significant difference between splinting/exercise regimens ([10]) for hand strength, so group data were combined together. Data were analyzed using SPSS, version 19. Mann-Whitney tests were used to examine the difference in mean rank scores between our subjects with CTS and the matched normative data at baseline and at followup. Wilcoxon's signed rank tests were used to test the change in difference scores from baseline to followup.

To determine the degree of change, we compared difference scores at baseline and followup using the effect size r calculated from the Mann-Whitney Z score ([11]). Interpretations for r were as follows: small effect = 0.10–0.23, medium effect = 0.24 to 0.36, large effect = 0.37–70, and very large effect = ≥0.71 ([12]).

Results

A total of 124 subjects were enrolled at baseline. One male subject did not provide his birth year and was removed from analysis. Subjects were predominantly female, white, employed, and right-hand dominant (Table 1). The mean ± SD age for this sample was 53.3 ± 11.8 years. There were no significance differences between sexes for demographic characteristics. Baseline and followup grip and pinch strength means and SDs are in Table 2.

Table 2. Comparison between grip and pinch strength scores at baseline and followup*
VariableMean ± SDMean rankr
  1. For baseline: n = 88 for right and 87 for left for females; n = 35 for males. For followup: n = 74 for females; n = 28 for right and 27 for left for males.
Right grip   
Females baseline42.00 ± 17.6818.80−0.54
Females followup49.69 ± 17.4910.40 
Males baseline81.23 ± 25.1719.10−0.42
Males followup86.91 ± 30.3411.00 
Left grip   
Females baseline41.16 ± 15.5511.50−0.56
Females followup47.47 ± 15.105.30 
Males baseline79.43 ± 25.9410.15−0.43
Males followup82.79 ± 29.197.90 
Right lateral pinch   
Females baseline14.29 ± 3.291.60−0.41
Females followup15.18 ± 3.350.70 
Males baseline22.36 ± 3.961.35−0.13
Males followup23.20 ± 4.321.12 
Left lateral pinch   
Females baseline14.48 ± 3.120.80−0.15
Females followup14.78 ± 3.090.30 
Males baseline20.17 ± 4.243.70−0.09
Males followup20.37 ± 4.142.80 
Right palmar pinch   
Females baseline12.82 ± 3.942.90−0.31
Females followup13.78 ± 4.062.30 
Males baseline19.17 ± 4.923.70−0.28
Males followup20.07 ± 4.932.90 
Left palmar pinch   
Females baseline13.41 ± 3.672.40−0.30
Females followup14.01 ± 3.611.50 
Males baseline18.66 ± 4.564.20−0.01
Males followup18.71 ± 4.202.70 

Our sample had significantly lower strength scores than the normative scores for every assessment at baseline except left grip for males. These differences were moderate to large (Table 3). After 4 weeks of treatment there were significant improvements in hand strength scores (Table 2) for female right and left grips (P < 0.001), right lateral pinch (P < 0.001), and right and left palmar pinches (P = 0.008 and P = 0.01, respectively) and male right and left grips (P = 0.03 and P = 0.02, respectively).

Table 3. CTS baseline and followup data compared to normative data, matched for sex and age*
VariableBaselineFollowup
Mean rankPrMean rankPr
  1. For baseline: n = 88 for right and 87 for left for females; n = 35 for males. For followup: n = 74 for females; n = 28 for right and 27 for left for males. CTS = carpal tunnel syndrome.
Right grip      
CTS females58.56< 0.001−0.5958.57< 0.001−0.37
Female norms118.44  90.43  
CTS males27.26< 0.001−0.4125.160.06−0.20
Male norms43.74  31.84  
Left grip      
CTS females65.24< 0.001−0.4465.830.007−0.20
Female norms109.76  83.17  
CTS males32.500.11−0.1526.500.32−0.06
Male norms38.50  28.50  
Right lateral pinch      
CTS females70.16< 0.001−0.3669.170.06−0.13
Female norms106.84  79.83  
CTS males28.900.004−0.3224.930.05−0.22
Male norms42.10  32.07  
Left lateral pinch      
CTS females75.550.001−0.2472.480.28−0.05
Female norms99.45  76.52  
CTS males26.37< 0.001−0.4520.960.001−0.42
Male norms44.63  34.04  
Right palmar pinch      
CTS females62.15< 0.001−0.5257.40< 0.001−0.40
Female norms114.85  91.60  
CTS males26.03< 0.001−0.4721.770.001−0.41
Male norms44.97  35.23  
Left palmar pinch      
CTS females64.64< 0.001−0.4661.47< 0.001−0.30
Female norms110.39  87.53  
CTS males25.69< 0.001−0.4819.93< 0.001−0.48
Male norms45.31  35.07  

Despite these improvements, significant differences between our subjects and the normative data remained, i.e., female right and left grips (P < 0.001 and P = 0.007, respectively) and right and left palmar pinches (P < 0.001); male right and left lateral pinches (P = 0.05 and P < 0.001, respectively) and palmar pinches (P = 0.001 and P < 0.001, respectively). However, these differences had lessened. Effect size r scores indicated that differences at baseline for grip strength ranged from −0.42 to −0.56, while at followup they ranged from −0.06 to −0.37. Baseline pinch strength r scores ranged from −0.24 to −0.52, while followup effect size r scores ranged from −0.05 to −0.48 (Table 3).

Discussion

Our subjects had lower grip and pinch strengths compared to normative data with the exception of male left grip strength. Both male and female hand strength increased following 4 weeks of splinting/exercises. However, despite this improvement, our sample did not achieve normative levels for female grip and palmar pinches nor for male lateral and palmar pinches. Although the patients with CTS continued to have significant deficits in strength in comparison to the norm, the differences in their strength was much less and trended towards small to moderate deficits instead of large deficits. These results suggest that while hand strength can improve in patients with CTS, many of these patients do not achieve normal strength at the end of a 4-week intervention.

While previous studies on splinting generally report symptom reduction, not all of them have reported improvements in hand strength ([13, 14]), suggesting that symptom reduction alone is not enough to explain strength improvements. Our study included 2 stretching regimens along with the splinting and found that neither splinting nor stretching alone explained symptom improvement, but that the 2 combined did ([7]). None of our regimens had a strengthening component, which begs the question: if we were not actively strengthening the muscle, how did muscle strength significantly improve, particularly as symptom reduction alone may not be enough to significantly affect strength? One possible explanation relates to our stretching program. These stretches may have reduced tightness in the muscles, reducing the drag produced on PIP and DIP flexion. Reduced tightness/drag would allow for greater overall flexion and have a positive effect on grip and pinch strength. Digit flexors that are strong and tight tend to have their tendons translate proximally in the carpal tunnel. The median nerve can become interposed between these tendons and the transverse carpal ligament, increasing CTS symptoms. Reduced tightness will help relieve these symptoms. Our study did not directly measure whether the stretching reduced lumbrical tightness, so this explanation is highly theoretical and requires further research to confirm or refute it.

Other explanations exist for improvements in strength. As symptoms improved, subjects started using their hand for functional activities, which led to improvements in overall strength. Patients reported a significant decrease in symptoms after the 4-week intervention ([7]), which may have resulted in less sensory disruption and a greater ability to fire motor units. Intrinsic de-innervation resulting in the loss of intrinsic musculature during grip and pinch may have lessened, resulting in more efficient recruitment of the intrinsic muscles during grip. Stretching may improve muscle excursion, power, and endurance.

This study highlights an important characteristic of conservative treatment in CTS. Many subjects continued to have significant deficits in strength after 4 weeks of treatment. More than half our subjects had CTS duration times of 1 year or greater. Significant differences seen at baseline between our CTS sample and the normative data could reflect this chronicity, partially explaining why, despite significant improvement in hand strength over 4 weeks, our CTS scores were still significantly lower than normative adult scores at followup. It may take more than 4 weeks for subjects to achieve hand strength that is within normal limits, particularly for patients with chronic CTS. While lower than the normative data on average, however, followup hand strength was still within functional limits; the majority of subjects were capable of performing most functional hand tasks.

Our study has some limitations. Due to small sample sizes in a few of our age categories, we averaged scores across age for both the CTS and normative data, thus obscuring any differences there may have been between age groups. Future research should assess these differences, matching the age category sample sizes ([8]) for better comparisons. As the purpose of this study was to provide CTS hand strength data compared to normative data, this study did not use a control group. The control group data used in this study were obtained from a study conducted in 1983 ([8]), leading to potential confounders such as differences in occupations, lifestyles, and growth patterns between the 2 groups. We did not have pre-CTS strength measurements; subjects' hand strength could have been lower than the normative data. Future research should assess for these differences prospectively. We did not adjust for severity of symptoms at baseline or at followup. Severity could confound the strength scores; however, we cannot distinguish what is truly related to weak muscles and what is related to reduced sensation or pain. Finally, the subjects could have plateaued in their home program, requiring updating in intensity or frequency of the stretches.

We found, as indicated in other literature, that patients with CTS have moderate to large deficits in grip and pinch strength in comparison to normative data. Nocturnal splinting and stretching appear to reduce these deficits. However, patients with CTS are often left with residual problems at 4 weeks. Future research should consider different types and durations of interventions when conducting comparisons between patients with CTS and matched normative data to better understand the effectiveness of treatments in improving objective outcomes and reducing symptoms.

AUTHOR CONTRIBUTIONS

All authors were involved in drafting the article or revising it critically for important intellectual content, and all authors approved the final version to be submitted for publication. Dr. Baker had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Study conception and design. Baker, Moehling, Desai, Gustafson.

Acquisition of data. Baker, Moehling, Gustafson.

Analysis and interpretation of data. Baker, Moehling, Desai, Gustafson.

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