Date Published: March 2, 2017
Publisher: Public Library of Science
Author(s): Carrie L. Peterson, Michael S. Bednar, Anne M. Bryden, Michael W. Keith, Eric J. Perreault, Wendy M. Murray, John Leicester Williams.
The biceps or the posterior deltoid can be transferred to improve elbow extension function for many individuals with C5 or C6 quadriplegia. Maximum strength after elbow reconstruction is variable; the patient’s ability to voluntarily activate the transferred muscle to extend the elbow may contribute to the variability. We compared voluntary activation during maximum isometric elbow extension following biceps transfer (n = 5) and deltoid transfer (n = 6) in three functional postures. Voluntary activation was computed as the elbow extension moment generated during maximum voluntary effort divided by the moment generated with full activation, which was estimated via electrical stimulation. Voluntary activation was on average 96% after biceps transfer and not affected by posture. Individuals with deltoid transfer demonstrated deficits in voluntary activation, which differed by posture (80% in horizontal plane, 69% in overhead reach, and 70% in weight-relief), suggesting inadequate motor re-education after deltoid transfer. Overall, individuals with a biceps transfer better activated their transferred muscle than those with a deltoid transfer. This difference in neural control augmented the greater force-generating capacity of the biceps leading to increased elbow extension strength after biceps transfer (average 9.37 N-m across postures) relative to deltoid transfer (average 2.76 N-m across postures) in our study cohort.
Active elbow extension is lost or impaired in individuals who sustain a cervical spinal cord injury (SCI) at or above the C7 spinal level due to complete or partial paralysis of the triceps. Elbow extension function is necessary to perform self-care tasks such as eating, reaching overhead, and assisting with pressure relief. One approach to improve elbow extension function for individuals with C5 or C6 quadriplegia is tendon transfer: a surgical procedure that reassigns a donor muscle primarily innervated above the level of injury to the insertion of the paralyzed triceps. The biceps or posterior deltoid may be donor muscles to improve elbow extension function [1–4]. For some patients, surgical prerequisites (summarized previously [1, 5]) determine whether the biceps or the deltoid is transferred. For example, active brachialis and supinator muscles with sufficient strength are required for the biceps to be transferred, and adequate shoulder stability is a requirement for the posterior deltoid to be transferred. For many patients, both the biceps and posterior deltoid are candidate donor muscles for transfer. In such cases, the decision to undergo either the biceps-to-triceps or the posterior deltoid-to-triceps transfer (referred to as biceps and deltoid transfers hereafter) is influenced by the surgeon’s experience and preference . Maximum elbow extension strength is an important outcome measure after tendon transfer because strength enables individuals to perform additional activities of daily living . Whether the biceps or the deltoid transfer results in greater elbow extension strength post-surgery remains unclear because strength is variable across patients and studies [3, 8–15]. Understanding factors that affect elbow extension strength in arms with biceps transfer and arms with deltoid transfer would better inform donor muscle selection and rehabilitation.
Maximum voluntary activation during elbow extension was greater in arms with biceps transfer relative to arms with deltoid transfer. The main effect of transfer type (F1, 9 = 7.1, p = .03), and the interaction of transfer type and posture (F2, 74 = 5.0, p = .01) were significant in the linear mixed-effect model and ANOVA of voluntary activation. Post-hoc comparisons demonstrated that voluntary activation was greater in the arms with biceps transfer relative to arms with deltoid transfer in the overhead reach (t80 = 3.0, p = .004) and pressure relief postures (t80 = 3.1, p = .002), but the difference was not significant in the horizontal plane posture (Fig 5). Voluntary activation was near complete (0.96 ± 0.02, mean ± one standard error) in the biceps transfer group, and did not change significantly across postures. In contrast, the voluntary activation of individuals with deltoid transfer exhibited a substantial dependence on posture. Within the deltoid transfer group, post-hoc comparisons demonstrated that voluntary activation was greater in the horizontal plane (0.80 ± 0.04) relative to the overhead reach (0.69 ± 0.06, t80 = 2.9, p = .004) and pressure relief postures (0.70 ± 0.06, t80 = 2.5, p = .01) (Fig 5).
We used electrical stimulation techniques to test our hypothesis that voluntary activation during maximum isometric elbow extension would be greater in arms with deltoid transfer relative to arms with biceps transfer. Our hypothesis was not supported. Individuals with a biceps transfer were better able to activate their transferred muscle than those with a posterior deltoid transfer. This difference in neural control augmented the greater force-generating capacity of the biceps leading to increased post-surgery elbow extension strength in our study cohort. Our results demonstrate that deficits in voluntary activation can contribute to the efficacy of tendon transfer surgeries, and that these deficits influenced deltoid transfers more than biceps transfers in our study cohort.
Individuals with a biceps transfer were better able to activate their transferred muscle than those with a posterior deltoid transfer. Thus, motor re-education of the transferred biceps to extend the elbow was better achieved than re-education of the transferred deltoid. This difference in neural control augmented the greater force-generating capacity of the biceps relative to the posterior deltoid leading to increased post-surgery elbow extension strength in arms with biceps transfer relative to arms with deltoid transfer in our cohort.