An assessment of the effects of variations in the Palmaris longus tendon and the fifth superficial flexor digitorum on pinch and grip strength

1. Introduction

Grip power is the force exerted by the hand to press subjects or to hold them, an important part of hand strength and one of the most important parameters determining overall hand function [1]. What is more, it has been shown that grip power may be a predictor of myocardial or cerebrovascular accidents [2], a general health indicator, as well as an indicator for cardiovascular disorders and mortality [3], diabetes [4], and even elderly mortality [5]. So it is important to define the “normal value” of grip power; several studies have been undertaken to assess this normal index (e.g. [6, 7, 8, 9, 10]). Nevertheless, when defining the normal values for this variable, mitigating factors should be considered, such as age and sex [11, 12] and the normal variations in the presence or absence of Palmaris longus [13, 14] and the fifth superficial flexor [15, 16]. Taking into consideration that controversial conclusions were reached in recent studies [15, 16] and also the fact that, to the best of our knowledge, no study has considered these variables together and the different variations of the fifth flexor digitorum superficialis (FDS), we undertook the current study.

2. Methods

This cross-sectional study was performed between September 2014 and April 2015 on 523 volunteers. The inclusion criteria were as follows: participants were males and females between the ages of 18 and 30 years; participants granted their informed consent. Exclusion criteria were: 1. Any pain or impairment in upper extremity function; 2. Any history of surgery or fracture in the upper limb; 3. Any disease that could theoretically affect upper extremity force (arthrosis, cervical disc herniation, rheumatoid arthritis, etc.). Other exclusion criteria consisted of: myocardial infarction in the last 6 months; any congenital anomaly of the hand, and/or myopathies and neuropathies.

After offering the participant a complete explanation in order to satisfy the requirements of informed consent, the examiner registered his or her demographic characteristics and then assessed both hands for the variations of Palmaris longus and the fifth superficial flexor, registered as absent or present for Palmaris longus and as present, absent, or dependent for the flexor. For Palmaris the standard test of Schaeffer was utilized, and in the case of a negative result the absence of the tendon was confirmed by another four tests (Thompson, Mishra I and II, and Pushpakumar) [18, 19, 20]. The participant was registered as “Palmaris absent” if the result was negative for all tests. Contradictory or suspicious cases were excluded after thanking the participant for his or her cooperation.

The preliminary procedure for determining the presence or absence of the flexor was as follows: the participant was asked to bend the fifth finger as the examiner was holding the other three digits fully extended. If the proximal interphalangeal joint was flexed to 90 degrees or more, the flexor was present. If s/he could not perform this manoeuver, the second and third fingers were released and the examination was repeated. If the proximal interphalangeal joint was flexed to 90 degrees or more this time, the status was recorded as a dependent FDS; ultimately, if the proximal interphalangeal joint was flexed to 90 degrees, then the flexor was absent [21]. After this, grip and pinch powers were determined, at first in the dominant hand and then in the opposite hand with position 2 of the Jamar dynamometer (distance from the handle to the fixed part was 2.48 cm), which seemed the most appropriate setting [22]. One attempt was made for each hand, due to the limitations of the participants’ and examiners’ time and patience; and as we evaluated the results, we found that this probably did not affect our conclusions, since at least one study found no differences between one and multiple determinations of grip power [23]. The participant was asked to sit in a chair, so the arm being evaluated would place the shoulder at zero adduction and rotation, the elbow at 90 degrees of flexion and neutral rotation, and the wrist at 30 degrees of dorsiflexion and 10 degrees of ulnar deviation. He or she was asked to make fist with the greatest force possible and no further feedback was given. The results were analyzed using SPSS 2. As the results for the right and left hands are not independent, a multiple linear mixed model was used, which is a regression model in which the dependence of the observations is taken into account.

3. Results

Ultimately, 523 enrolled individuals completed the study, 294 men and 229 women. Of these, 490 were right-handed (93.7%). The Palmaris longus was found in 65.5% of the hands (682 hands). In 608 hands (56.1%) the fifth flexor was functional; in 361 (33.3%) it was dependent on the fourth flexor; it was absent in 74 hands (6.8%). Table 1 shows this data in detail.

The absence or presence of Palmaris longus and the variations in the fifth flexor had no effect on grip power. The mean grip power was higher in men and on the dominant side. In contrast, statistical analysis showed the effect of the presence of Palmaris longus on pinch strength, which was measured at 25.38 lbs in hands with Palmaris vs 24.43 lbs in hands without Palmaris), but there was no statistically significant effect on pinch strength among variations of the fifth superficial flexor. The relationship between pinch strength and sex and between dominant left or right and hand was analyzed, and a significant difference was found only between males and females (28.89 lbs vs 20.93 lbs in men and women, respectively). These data are shown in more detail in Tables 2 and 3.

In the present study we measured grip in hands with and without the Palmaris longus tendon and with different variations of the fifth superficial flexor, and we reached the conclusion that the absence of the Palmaris longus tendon is associated with a small decrease in pinch strength. Hand surgeons often utilize the Palmaris tendon as their first choice graft for reconstructive surgery [24]. This finding suggests that this practice should be reviewed. However, pinch and grip are only two aspects of hand function and do not represent all functions of the hand, though these are among the most important. Again, though the difference was statistically significant, it was actually very small in functional terms – less than one pound.

The findings of the present study are in complete agreement with the findings of the two other reports that to the best of our knowledge are the only studies performed to date on the effects of Palmaris longus variations on grip and pinch strength [13, 14]. Our conclusion about pinch strength is different, though one of these studies [14] noted that the fourth and fifth fingers’ pinch strength would decrease in the absence of Palmaris longus. We cannot give any explanation for the insight of these researchers, but the fact that there was a common observation about the contrast in Palmaris variations between grip strength, in which no differences were found, and the finding of decreased pinch strength in the absence of Palmaris would seem to warrant further confirmation.

As referred to in our Introduction, previous studies of the role of the fifth flexor (again, the only studies performed, to our knowledge) have reached different conclusions. One of them noted no change in grip power in the absence of the tendon [15], and the other concluded that both the absence of the tendon and the dependence of the tendon on the fourth flexor both decrease grip power [16]. Our method was different from both of these studies, as Puhaindran et al. [15] considered the fifth flexor “present” or “absent”, while we added a “dependent” variant too. On the other hand, Bowman et al. [16] considered each hand as a separate unit and their statistical methods for analysis of the data differed as well.

In the present study, we examined the effect of the fifth flexor on pinch strength, which had not been considered previously. The flexor digitorum superficialis (FDS) begins as a single muscle from the elbow, and in the distal forearm it divides into four strings, each connected to one finger, but there may be many variations in the connection with the fifth finger [25]. Grip strength, as we used to measure it, was affected by pinch strength between the thumb and index finger (forefinger) and this was our rationale for adding this variable, as the variations in the fifth finger could theoretically affect the other fingers.

Because the Palmaris longus and the superficial flexor digitorum have a common origin in the medial epicondyle and they share common innervations, it may be conceivable that their variations could be simultaneous or at least these may be linked, even if not directly, through a mechanism or sequence of events tied to this anatomical factor. This was our rationale for assessing the effect of concomitant variations of the tendons. The only studies published on this topic to date, to the best of our knowledge, the two clinical studies by Thompson and colleagues [26] on the one hand, and by Sebastin and Lim [27] on the other, have not demonstrated such a relationship, and neither noted the variation of fifth flexor dependence on the fourth flexor. The findings of our present study in relation to the effect of Palmaris longus on pinch strength may be a confirmation of the lack of a link between Palmaris absence and grip strength.

We used the Jamar dynamometer, which has been reported to have the best inter- and intrarater reliability among pinch and grip measurement devices [31] and also the a reputation as the most reliable instrument for measuring these parameters [32], which could be a point of strength for the present study.

The most important limitation of our study was the fact that we did not measure all pinch-type strengths and we did not measure each finger separately. We gave up on the task because we assumed that it would be too onerous in terms of the time and patience required of the study participants, as well as for the examiners, to permit these measurements to be performed precisely, but this might a fruitful avenue for further research, if a study could be designed appropriately. Still, the fact that variations in the tendons did not affect grip or pinch strength, other than the absence of Palmaris showing a small effect on pinch strength, makes the possibility of a clear finding unlikely, for a prospective study that would test each finger’s pinch strength individually.

Based upon the findings of the present study, the variation described as the absence of Palmaris longus is associated with a small decrease in pinch strength, but other variations related to Palmaris longus have no effect on grip. What is more, the variations in the fifth superficial flexor have no effect on either pinch or grip strength.