A rare variation of intrinsic and extrinsic hand muscles represented by a bi-ventered first lumbrical extending into the carpal tunnel combined with bilateral fifth superficial flexor digitorum tendon regression

A rare unilateral variation of the first left lumbrical muscle was discovered in a female Caucasian cadaver dissected during a first year anatomy course at the Paracelsus Medical University - Nuremberg, Germany. The muscle possessed two venters with the first originating near the medial epicondyle of the humerus together with the intramuscular tendon of the superficial flexor digitorum tendon, and the second presenting as a regular first lumbrical muscle with radial palmar origin from first tendon of the deep flexor digitorum muscle. Both muscle bellies were connected by a 1.42mm thick tendon that passed beneath the median nerve and ran through the carpal tunnel. The second belly was enlarged and entered the distal part of the carpal tunnel. Interestingly, the donor revealed further rare variations such as a bilateral regression of the fifth superficial flexor tendons to an obviously non functional connective tissue strand, lack of both palmaris longus muscles, a discoid lateral meniscus in the left knee, and reduction of the plantaris muscle to a fascia-like structure on the right leg. Lumbrical muscle variations extending into the carpal tunnel, especially those associated with auxiliary tendons, have significant clinical relevance due to their association with carpal tunnel syndrome.

Morphological variability of the palmaris longus muscle in human fetuses

Purpose

The palmaris longus (PL) muscle is characterized by high-morphological variability. It is clinically important as it is routinely harvested for the reconstruction of other tendons. The study characterizes the morphology of the PL in human fetuses and creates a new classification based on its variations that would relate to the spectrum of morphological variability in adults.

Methods

Eighty spontaneously aborted human foetuses (44 male, 36 female, 160 upper limbs), aged 18–38 weeks of gestation, were examined.

Results

The palmaris longus muscle was present in 62.5% of fetuses. The absence was bilateral in 26.25%, and unilateral in 22.5%. Nine types of palmaris longus muscles were identified based on the morphology of its insertion (Types I–IX). All types originated on the medial epicondyle of the humerus. The most common type was Type I, which was characterized by insertion to the palmar aponeurosis (52%). The rarest types were Type VII and Type IX (1% each). Type VII was characterized by partial doubling of the muscle belly, which then turned into two separate tendons that inserted together into the palmar aponeurosis. Type IX was characterized by fusion with the flexor carpi ulnaris muscle.

Conclusion

Our findings concerning morphological variability of the PL in fetuses present a new perspective on the understanding nature of the morphological variation of the PL muscle in adults.

List of evidence

Basic Science Study.

Anatomical variations of the palmaris longus muscle including its relation to the median nerve - a proposal for a new classification

BACKGROUND

The palmaris longus (PL) muscle is characterised by high morphological diversity, and its tendon crosses the median nerve (MN) at different levels. Due to the fact that the palmaris longus tendon is routinely harvested for reconstruction of other tendons, knowledge of its morphological variations is clinically important. Therefore, the purpose of the study was to suggest a new morphological classification of the PL muscle and characterise the relationship of its tendon to the median nerve.

METHODS

Standard dissection was performed on 80 randomised and isolated upper limbs (40 left and 40 right) fixed in a 10% formalin solution. Measurements of muscle belly and tendon were obtained. The course and location of tendon insertion, as well as its relationship to the median nerve, were noted.

RESULTS

The palmaris longus muscle was present in 92.5% of specimens. Three types of palmaris longus muscle were identified based on the morphology of its insertion (types I-III) and these were further subdivided into three subgroups (A-C) according to the ratio of the length of the muscle belly and its tendon. The most frequent was type I (78.8%), where the tendon attached to the palmar aponeurosis, and subtype B, where the tendon-to-belly ratio was 1-1.5 (41.1%). The mean distance from the interstyloid line to the crossing between the median nerve and the palmaris longus tendon was 31.6 mm. In addition, two types of palmaris longus were described.

CONCLUSION

The presented classification of palmaris longus muscle types allows a better characterization of its diversity and may be useful in planning tendon grafting.

Congenital absence of the palmaris longus muscle: A meta-analysis comparing cadaveric and functional studies

The aim of our paper was to provide comprehensive data on the prevalence of absence of palmaris longus muscle (PLM) and its anatomical characteristics and conduct two separate meta-analyses comparing cadaveric and functional studies while identifying variation among different ethnic groups. An extensive search was conducted through the major electronic databases to identify eligible articles. Data extracted included prevalence of absence of PLM among subjects, ethnicity, laterality, side, and gender. Our main findings revealed that the absence of PLM is more frequently reported in functional studies. Moreover, functional tests likely overestimated the absence of PLM and recommend future studies to assess the validity of functional tests and use an imaging assessment prior to excluding the use of a palmaris longus tendon graft in patients in whom a function test identified the absence of PLM.

A combined variation of Palmaris longus and Flexor digitorum superficialis: Case report and review of literature

The muscles of the anterior compartment of the forearm often exhibit anatomic variations. During dissection of the upper right limb of a preserved cadaver, morphological variations in the Palmaris longus and Flexor digitorum superficialis muscles were found. The Palmaris longus muscular fibers converged to a tendon that passed beneath the Flexor retinaculum, and inserted at the base of the middle phalanx of the fourth digit, replacing the tendon of Flexor digitorum superficialis. The Flexor digitorum superficialis was divided into two muscular heads: a digastric medial head giving tendons to the second and fifth digits, and a lateral head giving one tendon to the third digit. All these tendons were inserted in the respective digits by two bundles between which were located the tendon of the Flexor digitorum profundus muscle. Variations of flexor muscles must be documented because of their clinical significance and their potential use of these muscles in orthopaedic and reconstructive surgery.

Evolutionary Transformation of the Palmaris Longus Muscle in Flying Squirrels (Pteromyini: Sciuridae): An Anatomical Consideration of the Origin of the Uniquely Specialized Styliform Cartilage

A long-standing issue in squirrel evolution and development is the origin of the styliform cartilage of flying squirrels, which extends laterally from the carpus to support the gliding membrane (patagium). Because the styliform cartilage is one of the uniquely specialized structures permitting gliding locomotion, the knowledge of its origin and surrounding transformation is key for understanding their aerodynamic evolution. The developmental study that would definitely answer this question would be difficult due to the rarity of embryological material. Instead, anatomical examinations have suggested two major hypotheses on the homology of the styliform cartilage: the pisiform bone of other mammals, or an additional carpal structure, such as the ulnar sesamoid of some of the other mammals or the hypothenar cartilage of the non-gliding squirrels. To test these hypotheses, a detailed examination of the anatomy of the carpus of gliding and non-gliding squirrels, and the colugo were undertaken. Based on physical and virtual dissections of the carpus, this study showed that both the pisiform bone and styliform cartilage were present in flying squirrels. This finding is further supported by demonstration that a "true Palmaris longus," with innervation typical for this muscle, inserts on the styliform cartilage. Taken together, our osteological, muscular, and neurological results suggest that the styliform cartilage was transformed in flying squirrels from an initially superficial and ulnar-derived anlagen into its current form. Anat Rec, 300:340-352, 2017. © 2016 Wiley Periodicals, Inc.