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Corresponding author: Nicholas P. Iannuzzi, MD, Orthopaedics Section S-112-ORT, VA Puget Sound Health Care System, 1660 South Columbian Way, Seattle, WA 98108.
Department of Orthopaedics and Sports Medicine, University of Washington, Seattle, WAOrthopedics Section S-112-ORT, VA Puget Sound Health Care System, Seattle, WA
Despite their relatively low prevalence in the population, anomalous muscles of the forearm may be encountered by nearly all hand and wrist surgeons over the course of their careers. We discuss 6 of the more common anomalous muscles encountered by hand surgeons: the aberrant palmaris longus, anconeus epitrochlearis, palmaris profundus, flexor carpi radialis brevis, accessory head of the flexor pollicis longus, and the anomalous radial wrist extensors. We describe the epidemiology, anatomy, presentation, diagnosis, and treatment of patients presenting with an anomalous muscle. Each muscle often has multiple variations or subtypes. The presence of most anomalous muscles is difficult to diagnose based on patient history and examination alone, given that symptoms may overlap with more common pathologies. Definitive diagnosis typically requires soft tissue imaging or surgical exploration. When an anomalous muscle is present and symptomatic, it often requires surgical excision for symptom resolution.
Several anomalous forearm muscles may be encountered during imaging or surgical exposure of the forearm. Although the prevalence of these muscles in the general population is low, many upper extremity surgeons can expect to encounter them during their careers. Anomalous muscles may be noted incidentally during surgery, or may cause symptoms such as a compressive neuropathy for which the patient seeks care. In this review, we describe 6 anomalous muscles: the aberrant palmaris longus (PL), anconeus epitrochlearis (AE), palmaris profundus (PP), flexor carpi radialis brevis (FCRB), accessory head of the flexor pollicis longus (AHFPL), and the extensor carpi intermedius. We review the anatomy and epidemiology of these muscles, and discuss the presentation, diagnosis, and treatment of patients presenting with symptoms attributable to these variants.
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Disclosures for this Article
Editors
David Netscher, MD, has no relevant conflicts of interest to disclose.
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All authors of this journal-based CME activity have no relevant conflicts of interest to disclose. In the printed or PDF version of this article, author affiliations can be found at the bottom of the first page.
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David Netscher, MD, has no relevant conflicts of interest to disclose. The editorial and education staff involved with this journal-based CME activity has no relevant conflicts of interest to disclose.
Learning Objectives
Upon completion of this CME activity, the learner should achieve an understanding of:
•
The anatomy of the more common anomalous and accessory muscles in the forearm
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Familiarity with the potential symptoms these muscles may cause, ranging from asymptomatic, “unexplained” mass, to pain from tendinopathy or nerve entrapment
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The prevalence of anomalous forearm muscles, which may range from very common to quite unusual
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The clinical significance of each of these six described anomalous forearm muscles
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The normal PL originates from the medial epicondyle with a proximal muscle belly and long distal tendon that inserts into the palmar fascia. A substantial amount of variation has been noted, however, with variations reported as duplicate, reversed, centrally located, bifid, divided with an ulnar slip, or hypertrophic (Fig. 1A–F ).
Effort-related compression of median and ulnar nerves as a result of reversed three-headed and hypertrophied palmaris longus muscle with extension of Guyon's canal.
Scand J Plast Reconstr Surg Hand Surg.2007; 41: 45-47
The PL can originate from the lacertus fibrosis, flexor digitorum superficialis, flexor carpi radialis, and flexor carpi ulnaris and may insert into the antebrachial fascia, thenar eminence, flexor carpi ulnaris, or into the carpal bones. An accessory tendon may also exist at the ulnar aspect of the main tendon distally (Fig. 1F).
Effort-related compression of median and ulnar nerves as a result of reversed three-headed and hypertrophied palmaris longus muscle with extension of Guyon's canal.
Scand J Plast Reconstr Surg Hand Surg.2007; 41: 45-47
Figure 1Anomalous palmaris longus. A Regular, B duplicate, C distally based muscle belly, D centrally based muscle belly, E bifid muscle, and F divided tendon with an ulnar slip.
published their seminal study in which they examined 1,600 upper extremities and revealed the absence of the PL in 12.8% of specimens. The authors then evaluated 530 of the 1,600 specimens and found that 46 of those 530 (9%) exhibited variations with respect to form, origin, or insertion. Females were 10% more likely to have an absent PL, and absence was more likely in the left upper extremity.
found that the absence of the PL was highest in Caucasian (26%) and American Indian (18%) subjects. Lower rates of absence were noted in subjects of Asian (12%) and African (6%) descent.
Patient presentation
An anomalous PL may be encountered incidentally, or may present insidiously with symptoms similar to carpal tunnel syndrome, exertional compartment syndrome, or compression of the ulnar nerve near Guyon’s canal.
Effort-related compression of median and ulnar nerves as a result of reversed three-headed and hypertrophied palmaris longus muscle with extension of Guyon's canal.
Scand J Plast Reconstr Surg Hand Surg.2007; 41: 45-47
Effort-related compression of median and ulnar nerves as a result of reversed three-headed and hypertrophied palmaris longus muscle with extension of Guyon's canal.
Scand J Plast Reconstr Surg Hand Surg.2007; 41: 45-47
Effort-related compression of median and ulnar nerves as a result of reversed three-headed and hypertrophied palmaris longus muscle with extension of Guyon's canal.
Scand J Plast Reconstr Surg Hand Surg.2007; 41: 45-47
described 1 anomalous PL that presented with bluish swelling over the volar wrist that changed consistency on active flexion or extension of the wrist.
Anconeus Epitrochlearis
Anatomy
The AE is a muscular variation of the Osborne ligament at the medial elbow, and is most commonly encountered incidentally while performing a cubital tunnel release. The AE originates from the medial epicondyle and courses superficial to the ulnar nerve before inserting onto the medial olecranon (Fig. 2).
Symptomatic patients generally present with signs and symptoms associated with cubital tunnel syndrome, including paresthesias in the ulnar nerve distribution, intrinsic muscle atrophy, and a positive Tinel sign at the elbow. In addition to the more common symptoms of ulnar neuropathy, at least 1 case report described baseball pitchers with worsening pitching performance, aching over the elbow, and numbness in the ulnar distribution.
These baseball pitchers continued to have symptoms exacerbated by throwing overhand despite 3 months of rest, physical therapy, and anti-inflammatory medications. An AE was found in each patient on exploration of the medial elbow, and each patient returned to his or her preinjury level of play after the release of the AE.
The FCRB commonly originates from the volar aspect of the mid- to distal-third of the radius and courses superficial to the pronator quadratus outside of the carpal tunnel (Fig. 3). The FCRB has been noted to insert into the second, third, or fourth metacarpals,
described a variant of the FCRB that originated from the site of the pronator quadratus’ insertion on the radius. Innervation of this FCRB was from the anterior interosseous nerve, and an underdeveloped pronator quadratus was present, attributed to compression of the pronator quadratus by the FCRB.
It is unclear if the presence of the FCRB is influenced by patient sex or laterality.
Patient presentation
The FCRB is rarely associated with symptoms, and in many case studies, the FCRB is encountered when performing a volar approach for fixation of a distal radius fracture.
a patient noted an enlarging, painful mass crossing the flexion crease on the radial aspect of his volar forearm. The patient also noted carpal tunnel symptoms, with positive Tinel and Phalen tests over the median nerve. In another case study, Kosiyatrakul et al
reported on a patient who also noted a painful mass over the radial aspect of the volar wrist. In this case, symptoms were exacerbated by resisted wrist flexion or passive hyperextension, although no carpal tunnel symptoms were noted.
Palmaris Profundus
Anatomy
The PP was first described by Frohse and Frankel in 1908, and has been called the “musculus comitans nervi median” by Sahinoglu et al because of its intimate relationship with the median nerve.
Origins and insertions may vary, but the PP is characterized by its course through the carpal tunnel adjacent to the median nerve and its insertion into the palmar aponeurosis distally. Possible origins include the radial diaphysis, ulnar diaphysis, flexor digitorum superficialis fascia, PL, flexor pollicis longus, or the medial epicondyle. Insertional variations can include the third metacarpal or the radial carpal bones in some instances (Fig. 4A-E ). It has also been found as either reversed or bitendinous.
In most cases, innervation is derived from the anterior interosseous nerve. The ulnar nerve may innervate the PP if the origin is from the distal ulna.
: A Type 1, with origin from the facia of the flexor digitorum superficialis (palmaris profundus longus muscle); B Type 2, with origin from the proximal or mid-third of the radius (flexor carpi radialis profundus); C Type 3, with origin from the anterior surface of the distal ulna (palmaris profundus ulnaris muscle); D Type 4, with bicipital origin (palmaris profundus radialis)
; and E Palmaris profundus inversus ulnaris proximalis, which originates in the form of a proximal tendon at the ulna shaft and inserts into the dorsal side of the flexor retinaculum and the palmar aponeurosis.
Some early reports described PP as a variant of PL, which may have led to some confusion, but subsequent reports have found PP in the presence of PL, and it is sufficiently different from a developmental and clinical perspective that it has been declared a separate, anomalous muscle.
When endoscopic techniques are attempted, it may not be possible to pass synovial elevators or dilators, which should prompt conversion to an open procedure.
In reported cases, surgical release of the transverse carpal ligament, combined with excision of the distal PP tendon, has been successful in resolving the symptoms.
Accessory Head of the Flexor Pollicis Longus, Gantzer Muscle
Anatomy
The AHFPL, or the Gantzer muscle, refers to an accessory muscle in the forearm that originates from the superficial flexors of the digits and inserts on the deep flexors of the digits.
Most commonly, the muscle originates from the deep surface of the flexor digitorum superficialis, and inserts on the ulnar aspect of the flexor pollicis longus muscle (thus AHFPL), but it may also originate from the coronoid process or the medial epicondyle, and insert on the flexor digitorum profundus (Fig. 5).
The Gantzer muscle has been found to exist in 51% to 71% of the population, and it may be more appropriate to describe it as a normal pattern of anatomy than an anomaly or variation.
The AHFPL has long been debated as a cause of median or anterior interosseous nerve compression due to cadaver anatomical impressions differing amongst investigators. Although symptomatic nerve compressions are rare, Tabib et al
described a patient with anterior interosseous nerve syndrome due to the Gantzer muscle. The patient had isolated paralysis of the FPL with a characteristic weakness of pinch between the thumb and index finger. Pronation of the forearm and extension of the elbow recreated the characteristic pain in the volar proximal third of the forearm. Electrodiagnostic studies revealed moderate slowing of conduction velocity, and an AHFPL was discovered on surgical exploration, with evidence of an hourglass appearance of the anterior interosseous nerve.
Three variants of the radial wrist extensors of the forearm have been described: extensor carpi radialis intermedius (ECRI), extensor carpi radialis accessorius, and extensor carpi radialis tertius. The ECRI is an accessory wrist extensor between the extensor carpi radialis longus (ECRL) and the extensor carpi radialis brevis (ECRB). Most commonly, the ECRI originates between the ECRB and ECRL, but may derive from the radial side of the ECRL or the ulnar side of the ECRB. The muscle courses between the ECRB and ECRL as a thin, slender muscle before inserting on the second and/or third metacarpal bones (Fig. 6A).
The extensor carpi radialis accessorius originates from the fascia of the ECRL or ECRB, or extends as a muscular slip from the ECRL itself. It typically inserts at the base of the thumb, on the first metacarpal, or on the abductor pollicis brevis (Fig. 6B). It may or may not pass under the extensor retinaculum. Extensor carpi radialis tertius was described by Nayak et al
as a third type of radial wrist extensor, which originates on the lateral epicondyle, passes between the ECRL and extensor digitorum communis, under the adductor pollicis longus/extensor digitorum brevis, and inserts on the base of the second and third metacarpals (Fig. 6C). All these anomalous radial wrist extensors are innervated by the posterior interosseous nerve.
Some authors have suggested that there may be adequate prevalence, cross-sectional area, and amplitude of such muscles that they should be a consideration when planning tendon transfers, especially in cases such as tetraplegia.
Most ECRI descriptions come from cadaver studies and few reports of symptomatic anomalous radial wrist extensors have been published. In one article, Smith et al
described a 47-year-old woman presenting with symptoms of right second dorsal compartment tenosynovitis. The patient complained of swelling and pain over the dorsal aspect of her right hand for which the provider performed ultrasonography demonstrating an anomalous ECRI; treatment was not discussed.
It is unknown whether the presence of an additional radial wrist extensor might contribute to lateral epicondylitis, intersection syndrome, or other tendinopathy syndromes.
Diagnosis
In symptomatic cases of anomalous muscles of the forearm, typical presentations involve neurological symptoms and/or pain, and physical examination evidence of entrapment neuropathy. In some cases, a mass may be found in the region of compression that enlarges with contraction, but most will require soft tissue imaging such as ultrasound or magnetic resonance imaging, or an awareness of atypical anatomy during surgical exposure.
Most often, anomalous muscles in the forearm are encountered incidentally during surgical exposure. In cases suggestive of entrapment neuropathy, nerve conduction studies are often used to localize the site of nerve compression, but do not confirm or exclude the presence of an anomalous muscle. Magnetic resonance imaging has been used to determine the presence of an anomalous PL, AE, and FCRB.
Although evaluator dependent, knowledge of accessory muscles and variations in the forearm will facilitate diagnosis during ultrasound and/or magnetic resonance imaging evaluation.
Treatment
The most common recommendation for the treatment of persistently symptomatic anomalous muscles that result in entrapment neuropathy or tendinopathy, or of pain that fails to respond to nonoperative treatment, is excision of the muscle.
Effort-related compression of median and ulnar nerves as a result of reversed three-headed and hypertrophied palmaris longus muscle with extension of Guyon's canal.
Scand J Plast Reconstr Surg Hand Surg.2007; 41: 45-47
Surgeons should be careful, however, to avoid erroneous attribution of symptoms to incidental findings. The evidence supporting muscle excision is limited to small case series and individual case reports, and therefore prone to bias.
Reported cases of successfully treated carpal tunnel syndrome typically received both carpal tunnel release and muscle excision, and therefore it is open to debate whether either treatment alone may be sufficient in selected patients.
People often live their entire lives without any awareness of an anomalous muscle, and the presence of an anomalous muscle at the site of pain should not be an automatic causal assumption. Muscular anomalies observed during the surgical approach for fracture or other unrelated indications are generally documented and observed rather than excised (such as FCRB at the time of distal radius fracture fixation). Finally, if excising a so-called anomalous muscle, one must be certain that one is not removing an essential function. This is well recognized, for example, with another as yet unmentioned anomaly, extensor brevis manus that may substitute for the more common normal digital extensors.
Anomalous muscles of the forearm are infrequently present, but awareness of the known types and variations facilitates identification and treatment. Each of the 6 anomalous forearm muscles described in this review may have a variety of subtypes and variations, and familiarity with their unifying and differentiating features prepares surgeons for uncommon situations. Although anatomical descriptions have been made for centuries, ongoing reporting and detailed descriptions of anomalous muscles will facilitate further accurate diagnosis and treatment in future.
Acknowledgment
The authors are obliged to acknowledge Willa Bradshaw for her preparation of the illustrations provided along with this manuscript.
References
Reimann A.F.
Daseler E.H.
Anson B.J.
Beaton L.E.
The palmaris longus muscle and tendon: a study of 1600 extremities.
Effort-related compression of median and ulnar nerves as a result of reversed three-headed and hypertrophied palmaris longus muscle with extension of Guyon's canal.
Scand J Plast Reconstr Surg Hand Surg.2007; 41: 45-47
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