Does the Absence of the Palmaris Longus Affect Grip and Pinch Strength?

INTRODUCTION

The palmaris longus is believed to be phylogenetically degenerating and shows the characteristics of this, namely a short belly with a long tendon (McMinn, 1994). In addition, its absence can easily be determined clinically, it is easy to harvest and it is a long and flat tendon allowing good revascularization (Brand and Hollister, 1992). For these reasons, it is commonly used as a tendon graft by plastic and hand surgeons (Pulvertaft, 1956; Wehbe, 1992). A common question posed by patients in whom the palmaris longus is to be harvested as a tendon graft is “What will happen if my palmaris longus is taken away?” We felt that our answers to this question were mostly anecdotal and not supported by any literature-based evidence.

We, therefore, attempted to assess the functional value of this muscle by measuring the grip and pinch strength of a sample normal population, to see if there was any statistically significant difference in strength between subjects having the tendon and those lacking it. We used grip and pinch strength measurements because these seemed pertinent, are easy to measure and are commonly used measures of evaluating hand function.

MATERIALS AND METHODS

A sample of the normal population was taken from subjects at a health screening programme. The sample population included 418 subjects of Asian descent. Subjects with a history of injury/disease or abnormality of the upper limb which would preclude the examination for the presence of the palmaris longus tendon were excluded from the study. The examination was conducted in two parts.

The first part of the examination assessed the presence of the palmaris longus tendon. This entailed observation of the palmar aspect of the wrist looking for the palmaris longus tendon in its usual anatomical position just ulnar to the flexor carpi radialis tendon. If the tendon was not visible, the patient was asked to do the standard test for assessment of the palmaris longus tendon, viz. oppose the thumb to the little finger while flexing the wrist. If the tendon was not visualized or palpable, it was considered absent. The presence or absence of the palmaris longus tendon was recorded on both sides.

The second part of the study was the measurement of grip and pinch strength. Grip strength was measured using a calibrated Jamar dynamometer at level 2 in a standardized position, as described by the American Association of Hand Therapists. Three readings were taken alternatively for each hand, starting with the dominant hand. The average of the three readings was used for final analysis. Key pinch strength was measured using a calibrated Padgett pinch meter. One reading was taken for each hand, starting with the dominant hand.

RESULTS

Four hundred and eighteen subjects of Asian descent, aged between 7 and 85 years with a mean age of 42 years (SD 16), were examined. They comprised 78% Chinese, 10% Malay, 9% Indian and 3% other races. Seventy per cent of the subjects were women. Ninety-six per cent of them were sedentary workers and 4% were manual workers.

In most subjects (95%), the right hand was dominant. The palmaris longus was absent unilaterally in 17 subjects (4%) (right wrist in 5 subjects/left wrist in 12 subjects), while it was absent bilaterally in 7 subjects (2%). The overall incidence of palmaris longus absence (unilateral or bilateral) was 6% (95% CI 4–8). Of the 24 subjects with an absent palmaris longus (unilateral or bilateral), 16 were women and eight were men. Overall, no significant difference was found between the incidence of the absence of the palmaris longus and the gender of the subject (Table 1).

The mean (SD) grip strength of the right hand was 30 (10) kg (Median: 28; Range: 6–63 kg), while the mean grip strength of the left hand was 28 (10) kg (Median: 26; Range: 5–64 kg). The mean key pinch strength of the right hand was 7 (2) kg (Median: 7; Range: 2–14 kg), while the mean key pinch strength of the left hand was 7 (2) kg (Median: 7; Range: 2–14 kg).

There was no significant difference in the grip and pinch strengths between subjects with or without palmaris longus for both hands. No significant difference was found in the grip and pinch strengths between subjects with or without the palmaris longus, even after adjusting for age, gender, hand dominance and occupation.

DISCUSSION

The palmaris longus is inserted into the palmar aponeurosis and fans out at its insertion. It is a weak flexor of the wrist and assists in cupping the palm through its extension into the longitudinal fibres of the palmar fascia (Brand and Hollister, 1992). It is also believed to play a role in thumb abduction through an extension onto the thenar eminence (Fahrer, 1973; Fahrer and Tubiana, 1976; Kaplan, 1953).

Verdan (1975) has said that a divided palmaris longus is of little importance and need not be repaired. We feel that repair of a divided palmaris longus should always be done, even if it is the only divided tendon, as is common in patients with self-mutilation. A repaired palmaris longus protects the median nerve in this group of patients who are likely to injure themselves again. Repair also restores function and preserves it as a tendon graft/tendon transfer or a free autogenous transplant, for use at a later date (Sarangapani and Brown, 1977).

Some authors feel that there are equally good or better sources for graft material and the palmaris longus should be used as a free graft only if the damage is very localized and unlikely to spread or require revision (Brand and Hollister, 1992). Nevertheless, the palmaris longus tendon is most commonly used for this purpose. Patients requiring a palmaris longus tendon graft can be divided into two main groups depending on the functional status of the donor upper limb. In the first group, the donor upper limb is normal and the palmaris longus is needed for reconstruction elsewhere, e.g. lip augmentation, ptosis correction, management of facial paralysis, tendon reconstruction in the opposite limb, etc. The second group of patients is those in whom the donor upper limb is not normal. This group can be further subdivided into two subgroups, one where the primary pathology does not involve the wrist flexors or thumb abductor muscles, e.g. ulnar collateral ligament reconstruction, interposition tendon grafts and CMC joint arthroplasty. The other subgroup includes those patients where the primary pathology involves the wrist flexors or thumb abductor muscles, e.g. high ulnar and low median nerve palsies.

The functional impact of the loss of the palmaris longus will, therefore, depend on the group to which the patient belongs. However, quantifying the functional loss secondary to the harvest of the palmaris longus is difficult. One problem is the difficulty in measuring the functional contribution of the palmaris longus separate from the other wrist flexors and thumb abductors. Wrist flexion or thumb abduction strength measurements can be used as an indirect indicator of palmaris longus function (Boatright et al., 1997; Liu et al., 2000). However, instrumentation to accurately gauge these measures is not easily available. In addition, these indices do not truly reflect overall hand function. We used grip and pinch strength measurements, which are easily available and have good intra-observer and inter-observer reliability (Jones, 1989; Marx et al., 1999). Although grip and pinch strength measurements may not reflect independent function of the palmaris longus, they are able to give an estimation of the contribution of palmaris longus to overall hand function. In addition, hand function, as reflected by grip and pinch strength measurements, provides us with quantitative evidence that is easily understood by patients.

The ideal sample for this study would be a group of patients with a normal donor upper limb in whom the palmaris longus has been used for reconstruction elsewhere. Only in these patients would an assessment of pre-operative and postoperative grip and pinch strength reflect the functional loss due to harvest of the palmaris longus. However, getting a sufficient number of such patients is extremely difficult. In patients where the donor upper limb is not normal, the measurement of grip and pinch strengths, pre- and postoperatively, might not accurately assess the contribution of the palmaris longus.

Taking into consideration the above practical difficulties, we attempted to assess the functional value of the palmaris longus muscle by seeing whether there was any statistically significant difference in grip and pinch strength of a sample normal population between subjects having the tendon and those lacking it. We were unable to find any significant difference in the grip and pinch strengths. The absence of a difference in strength in the normal population may indicate the gradual phylogenetic degeneration of this muscle, with its functional ability having been taken over by the other forearm flexors in subjects where it is absent. A drawback of this study is that absence may not necessarily be the same as loss, especially in patients with pathology involving the other wrist flexors or thumb abductors. In such patients, the harvest of the palmaris longus may further weaken an already damaged hand. In all other patients, the loss of the palmaris longus is less likely to be functionally significant as its functional ability has been, or will be, taken over by the other wrist flexors.

In conclusion, absence of the palmaris longus is not correlated with decreased grip or pinch strength measurements. We feel that this study can be cited to patients with a normal donor upper limb in explaining that there is unlikely to be any functional loss of grip or pinch strength following the harvest of the palmaris longus.