BIOMECHANICAL STUDY OF RUCKSACK PARALYSIS

Abstract

The rucksack paralysis is currently considered to be caused by the compression or hypertraction of brachial plexus or long thoracic nerve. However, its precise mechanism has not yet been fully clarified. In the present study, we attempted to explain the mechanisms of rucksack paralysis. For this purpose, three sets of studies were performed, i. e., (1) examinations on the exact localization of shoulder straps with the aid of radiographic analysis, (2) measurements of the compression under the straps with load cell, strain gauge and prescale, and (3) anatomical studies on the nerve pathway under the compressed area.
In the experiments with six male and five female subjects, the inside edge of the strap at rest was found to run from area around the center of clavicle to the lateral side of the ribs. Finally, it went down to the inner part of the axilla. However, on tread-mill walking the position of the strap's inside edge shifted to the lateral part of the clavicle and that of the central part moved to both the acromion of the scapula and the head of the humerus. Thus, during the actual walking with rucksack, the strap was considered to move within these areas. In addition, we found that carrying a rucksack displaced the scapulae toward the median.
From measurements of the compression under the strap with six male subjects, the following common findings were obtained: (1) the heaviest load was upon the upper part of the body trunk, i. e., suprascapular region (4 subjects) and clavicular region (2 subjects), and (2) the edge of the strap produced stronger compression than its center did.
Anatomical studies with ten cadavers revealed that the brachial plexus might be strongly compressed in the case of muscular hypertonicity or body surface compression.
The long thoracic nerve arised from the branches of the 5 th, 6 th and 7 th cervical nerve. Joined nerve trunks of the branches of the 5 th and 6 th cervical nerves frequently appeared at the lateral side of the brachial plexus. The branch of the 7 th cervical nerve joined with the nerve trunks running through the middle scalene muscle, although location of this nerve conjoining was somewhat different among various cases, i, e., at the proximal side of the second rib in seven cases and at the area between the 2 nd and 3 rd ribs in three cases. The long thoracic nerve was found to turn downwards at the second rib, and this turning point was located at the tuberosity of the serratus anterior muscle.
From these results, we consider that the paralysis of the brachial plexus is caused by the load of rucksack working as a tractive external force on the nerves between the clavicle and neck, while it acts as a compressive external force on the nerves from coracoid processes to the axillary region. On the other hand, the paralysis of the long thoracic nerve seems to occur due to hypertraction of or compression over the tuberosity of the serratus anterior muscle of the second rib.