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Racial Differences in the Palmaris Longus

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There are many physical differences between racial/ethnic groups. Some of these differences are obvious to the naked eye, others much less so. One racial difference that exists is variation in having a tendon called the palmaris longus (PL). This muscle rests between the flexor carpi radialis and the flexor carpi urinalis. The radius and ulna are bones in the forearm. The radius supports the lateral (thumb) side of the hand whereas the ulna supports the medial (pinky) side; they enable wrist rotation. To see if you have the tendon, take your pinky and touch it to your thumb. If you have the tendon you should see it poking out of your wrist. Fourteen percent of the population lacks the PL, but there is considerable variation by race. These differences, of course, have should be taken into account when doing a tendon graph operation.

The action that the PL performs is flexing the wrist; the origin is the medial epicondyle of the humerus; it inserts in the palmar aponeurosis and flexor rentinaculum of the hand. The antagonist muscles are extensor carpi radialis brevis, extensor carpi radialis longus, and extensor carpi urinalis.

There are unilateral (affecting one side of the body) differences in the variation of this tendon along with bilateral (affecting both sides of the body) differences. There is differential absence of the tendon depending on which hand is dominant (Eric et al, 2011). The tendon also has been found to contribute to the strength of thumb abuction (Gangata, Ndou, and Louw, 2013). However, it has been shown that whether or not one has the PL or not does not contribute to grip/pinch strength (Sebastin et al, 2005).

Soltani et al (2012) followed patients at hand surgery clinics LA county +University of Southern California Medical Center and Keck Medical Center of the University of Southern California. Their objective was to observe the variation in the PL in regard to race, sex and ethnicity (indeed, they placed people into races based on the US Census designations; see Spencer, 2014, they excluded mixed-race people from the ethnicity part of the analysis). They wanted to see the extent that the PL was missing and whether or not it was bilateral or unilateral. They evaluated a group of 516 multi-ethnic individuals while age, race, ethnicity and sex was accounted for. They then administered the Schaeffer test:

a-Schaeffers-test-opposition-of-thumb-to-little-finger-with-flexion-at-wrist-Arrow

schaeffer1

Obviously, this only works with lean individuals. So obese patients needed to get an ultrasound to ascertain whether or not they had the tendon.

In their sample of 516, 415 were Caucasian, 55 were African American, 35 were Asian and 11 were mixed race. The age range was 12-94, while an even number of men and women were tested (288, 288). Soltani et al (2012) write:

There were no differences in the absence of the PL based on laterality. The right side was absent in 11.8% and left 12.0% of the time (see Table 1). Further, there were no differences in the absence of the PL based on gender,  value 0.369 (see Table 2). Ethnically, there was no difference in the absence of the PL between White (non-Hispanic) and White (Hispanic) patients, with prevalence of 14.9% and 13.1%, respectively. However, African American (4.5%) and Asian (2.9%) patients had significantly fewer absences of the PL than the Hispanic reference group.

They then write in the discussion that this has implications for plastic surgery—this anatomic variation between the races has implications for surgery:

This is information that should be taken into account preoperatively when planning surgical algorithms in treating tendon injuries or palsy. The PL is one such option as a tendon transfer for opponensplasty in restoring intrinsic function in cases of recurrent median nerve injury. If the PL is absent on the affected side, it is important to know preoperatively to plan using another donor muscle such as the extensor indicis proprius. In our study, the African American population had a statistically significantly lower rate of absent PL (4.5%), which is radically different than previously published reports from Nigeria, where the absence rates were much higher (31%). This could be due to the ethnic heterogeneity of the African American population of the United States compared to the Nigerian population. Nevertheless, the PL is present in high likelihood in this particular ethnic group which bodes well for using the PL in a surgical scenario. The PL is used quite frequently in cases of secondary tendon reconstruction, and it is useful for the surgeon to be aware of that issue preoperatively for surgical safety and efficiency in harvesting the tendon graft. The patient needs to be aware of the location of possible surgical incisions for tendon harvesting. Further, the surgeon should examine all possible tendon donors preoperatively, and one’s suspicion might be heightened by knowing the patient’s ethnicity. This is particularly important for the White population which in our study had the highest rates of absence, in both the Hispanic and non-Hispanic subsets. The surgeon must be aware in these patients that it is more likely that the PL might be absent. Thus, in Caucasian patients, it is particularly important to have a thorough examination of possible tendon donor sites.

Touching on the point of there being no difference between having or not having a PL and grip/pinch strength: we should not expect that, since whites dominate strength competitions (and the Chinese powerlifters are no slouches either, look at some videos of them). So, just because whites have a higher prevalence of not having the tendon does not mean that they do not have a strong hand grip. Indeed, senior blacks did have a stronger hand grip than whites (Araujo et al, 2010), but the PL is not the cause of the slightly higher hand grip strength (this was seen in a Turkish study; Setin et al, 2013). However, tennis players need a strong grip and the PL is more likely to be found in elite tennis players over the recreational athletes (Vercruyssen, Scafoglieri, and Cattrysse, 2016). But Nekkanti et al (2018) conclude that “There was no statistically significant correlation between agenesis of PL and the mean hand grip. Right-handed dominant individuals had a higher incidence of PL.”

In sum, this is tendon varies by race/ethnicity/sex/handedness like many other traits. understanding these differences will lead to a better understanding of plastic surgery in regard to the tendon. There is a racial difference, but it is of no functional relevance in regard to hand grip/power/strength sports.

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