Morphological study of anomalous deep extensors of the hand

Abstract

Background Anomalous muscles on the dorsum of the hand include the extensor medii proprius (EMP), extensor indicis et medii communis (EIMC), extensor digitorum brevis manus (EDBM), and anomalous extensor indicis proprius (aEIP). They are commonly seen during routine cadaveric dissections and hand surgeries. There are very few studies on them in the recent past. Hence, this study was conducted with the aim of studying their incidence and morphology. Materials and Methods This study was conducted at the Pushpagiri Institute of Medical Sciences and Research Centre, Thiruvalla, Kerala, over a period of 15 months, on 80 free upper limbs. Deep extensor muscles of the forearm were dissected. The frequency of the anomalous muscles was noted. Length and width of the muscles and tendons were measured using thread, ruler, and vernier calipers. The median and range were calculated. The musculotendinous (MT) junction of the EIP was observed. It was considered to be aEIP if its MT junction was beyond the distal edge of the extensor retinaculum in wrist flexion. Results EMP was seen in 8.75% of the limbs, with average muscle length and width of 4.53 ± 0.7 cm and 3.7 ± 0.8 mm and average tendon length and width of 9.7 ± 1.2 cm and 1.6 ± 0.8 mm, respectively. Frequency of EIMC was 3.75%, its average muscle length and width being 3.67 ± 1.14 cm and 2.3 ± 0.6 mm and its tendon length and width being 9.8 ± 0.61 cm and 2.0 ± 1.7 mm. Incidence of EDBM was 2.5%; its average muscle length and width being 4.5 ± 0.71 cm and 1.25 ± 0.3 mm and tendon length and width being 3.15 ± 0.91 cm and 3.5 ± 0.7 mm. Frequency of aEIP was observed to be 6.25%. Conclusion Anomalous extensors have a frequency of less than 10%. Tendons of the EMP and EIMC are very thin and asymptomatic. The EDBM belly is distal to the retinaculum and does not cause symptoms, unless hypertrophied. aEIP passes through the narrow compartment and is more likely to cause pain.