Amputation is not preferred by a surgeon because the nature of the surgery is not to recover function, but to eliminate function and humanity. Taylor CL investigated socket rotation based on radioulnar rotation of six amputees in 1954 [14]. Since then, there has been no follow-up trial to confirm their results. Although a surgeon could manage forearm amputation for decades through insightful investigation, residual rotation of forearm amputation has been out-of-interest because residual rotation of forearm amputation is useless with conventional socket prosthesis. However, osseointegration in the transradial amputee can restore the natural forearm rotation now [11]. In the near future, reconstructive surgery with biomimetic hands will become possible. Thus, there is a need to address residual rotation of forearm amputation.
In wrist disarticulation, rotation axis can be preserved through intact distal radioulnar joint. Complete rotation was observed even without hands. In the 18 cm forearm stump, the preservation of pronosupination was 80% compared to residual rotation of wrist disarticulation. Although supinator, biceps brachii, and pronator teres were preserved, loss of distal radioulnar joint derailed the balanced rotation and decreased the residual rotation. In the 10 cm forearm stump, residual rotation fell below half of the residual rotation of wrist disarticulation. Insertions of pronator teres were partially transected in 4 of 5 cadaveric specimen of the 10 cm forearm stump. The majority of interosseous membranes (central band and distal oblique bundle) were also resected in all specimens. The pronation simulation test at the 10 cm stump could be completely finished in only one cadaver. The residual pronation was smaller than the residual supination in the 10 cm forearm stump. Such smaller pronation ability could be partially explained by diverging loads applied to the radius by supinator muscle and the biceps brachii which might have aggravated the unstable rotation without distal radioulnar joint or interosseous membrane [16].
Close proximity of the amputation site to the elbow significantly decreased residual rotation. While Taylor CL investigated the socket rotation of amputees, we measured the rotation of radius around the fixed ulna in fresh-frozen cadavers. Although direct comparison between Taylor’s study and our study was not possible, our finding was consistent with the decreasing tendency of rotation as the level of amputation got shorter. Moritomo H et al. [17] have reported that distal three ligaments of the interosseous membrane are essentially isometric stabilizers of the forearm. Two proximal ligaments (proximal oblique cord and dorsal oblique accessory cord) changed substantially in length, with their attachments out of the course of the axis. In the 25 cm forearm amputation, distal radio-ulnar joint (DRUJ) and all contents of interosseous membrane were intact. In the 18 cm forearm amputation, DRUJ and distal membranous portion including distal oblique bundle were eliminated while proximal membranous portion and middle ligamentous complex remained intact. In the 10 cm forearm amputation, DRUJ, distal membranous portion, and middle ligamentous complex were all eliminated while only fluctuating proximal membranous portion remained. Unstable and short forearm stump resulted in a decrease of residual rotation.
Separate traction of pronator quadratus and pronator teres could make an equal pronation to residual pronation resulting from simultaneous traction of both muscles in specimens of 25 cm amputation. No study has shown that solitary contraction of pronator quadratus can make a pronation of forearm. However, many clinical studies have investigated pronation with or without repair of pronator quadratus in patients treated surgically for distal radial fractures and shown that there is no significant difference in the range of motion between the two groups (pronation with or without repair of pronator quadratus) [18,19,20]. Our results support these findings, showing that independent traction of pronator teres could make an equal pronation to residual pronation resulting from simultaneous traction of both muscles in the specimens of 25 cm amputation.
Another finding of our study was that supination made by independent traction of biceps brachii was affected by flexion of elbow. While biceps brachii functioned primarily as a powerful supinator of the forearm when the elbow partially flexed from 30° to 60°, the biceps brachii did not function primarily in 90° or 120°. Considering that the main action of biceps brachii is supination and flexion of elbow joint, supination of biceps brachii can be synergistic when it is combined with the elbow flexion. Castration of flexion force due to fixation of the ulna in 90° and 120° of elbow might have decreased the supinator effect of biceps brachii. However, traction of supinator made a supination equal to residual supination with simultaneous traction of both muscles. These results were consistent with previous studies, demonstrating the importance of the supinator muscle in forearm supination. In electromyographic studies of forearm supination, the supinator muscle was the most active one in unresisted supination, showing increased biceps activity with heavy loading [21]. Selective denervation of the supinator muscle by peripheral blockade of the radial nerve with preserved biceps function via the musculocutaneous nerve has been shown to decrease the supination strength by 64% [22]. These findings might support previous studies that compared clinical results of biceps tenotomy with that of tenodesis. These studies have shown that there is no significant difference in supination power between the two groups [23, 24].
Our results revealed that the effect of elbow flexion on residual rotation was not statistically significant. Independent traction of pronator teres and pronator quadratus can make full residual pronation at any position of the elbow. Although the attribution of biceps brachii to supination was influenced by the flexion of elbow, supination caused by the traction of supinator was constant.
Our study has several limitations. First, muscle contraction was substituted with traction of muscle. It is an inherent limitation of a cadaveric study. We could not guarantee the load to pull the muscle completely for forearm rotation. Second, maximum loads applied to PQ, PT, and biceps brachii were chosen based on derived ratios. However, these ratios could only provide an estimate of the relative force produced by muscles, not the absolute force. Further study is needed to determine effects of alternative muscle-loading. Third, the accuracy in modeling muscles using a single suture line, especially those muscles with a broad origin (such as PQ, PT, and supinator muscles), might be questionable. Fourth, the effect of myodesis using forearm muscle on radius or ulna was not evaluated. The scar tissue is often present abundantly in the stump. Remained muscles and scarred soft tissue could influence the residual rotation. Finally, our study was conducted with Korean male cadavers. We corrected the level of amputation considering the size of Korean male cadavers. Thus, results of this study could not be applied to the general population due to differences in race and/or gender.
