Osteopathic Medical Student OUHCOM galloway, Ohio, United States
Abstract Body : Introduction and Objective:
Extension of the thumb is carried out predominantly by three muscles: extensor pollicis longus (EPL), extensor pollicis brevis, and abductor pollicis longus. However, in 2% of individuals, there is a fourth muscle, termed the extensor pollicis et indicis, that arises from the mid-radial shaft and interosseous membrane to insert onto the DIP of the thumb and index, assisting mainly the EPL in abduction and extension of the thumb, and extension of the index finger. A rare variation of the extensor pollicis et indicis is the extensor pollicis tertius muscle, originating from the ulna. This study aims to explore an unreported variation of the extensor pollicis tertius muscle.
Materials and Methods:
This case study was investigated on a whole body donor at the Ohio University Heritage College of Osteopathic Medicine. Upon dissection of the left posterior forearm guided by Grant’s Dissector, an additional extensor muscle of the thumb was discovered. A detailed literature review was conducted to determine if this was a common variant. A Vernier caliper was used to measure the diameter and length of the tendon, width of the muscle belly, and muscle length. Preliminary analysis presents descriptive statistics. Further analysis will include biomechanical calculations.
Results:
The aberrant muscle, which was present only on the left forearm, originates from the mid-radial shaft and interosseous membrane, passes medially to the dorsal tubercle of the radius to insert on to the distal phalanx (DIP) of the thumb. This muscle had a single tendon with a length of 147.25 mm, with a diameter of 1.70 mm. The tendon length from the DIP to the styloid process of the radius was 71.40 mm. The length of the tendon from the muscle to the radial styloid process was 75.85 mm. The muscle had a fusiform muscle belly with a width of 11.72 mm and a length of 99.85 mm.
Conclusion:
This study reports on a new aberrant extensor muscle of the thumb. Previous literature reports a variant muscle on the right forearm on a male donor with an origin on the ulna, termed extensor pollicis tertius. This study demonstrates another variant with a different origin on the radius and interosseous membrane on a female donor. Increased knowledge of anatomical variants such as the one reported here is important when assessing the relative risk of variants in surgery when in proximity to vital components, such as nerves and vasculature.
Significance/Implication:
Due to its proximity to the radial nerve, artery and vein, this aberrant muscle could be an interest of study for certain pathologies and surgical plans. Additionally, the demographic distribution, biomechanical aspects, and unilateral manifestations of this muscle variant on our donor are areas of interest.
