Flexor tendon injuries are common and are one of the most challenging injuries for hand surgeons.1 For this reason, hand surgeons have devoted substantial amounts of time and effort to improve functional outcomes.2,3 Surgeons have tried to determine the best method of flexor tendon repair since the 1930s, when the term “no man's land” was coined for zone II flexor tendon injuries.3 Surgeons have tested various repair parameters, including ideal timing, suture, and suture method.4–10
In addition, postoperative hand therapy has been found to be critical to optimal recovery.11,12 Nonetheless, despite excellent surgical technique and the best hand therapy, many patients need prolonged rehabilitation, require multiple procedures, and experience permanent loss of function.13,14 Improving outcomes and shortening rehabilitation after flexor tendon injury are top priorities in the field of hand surgery.
One potential barrier to functional return after injury is the development of awkward compensatory movements of the fingers. For example, patients with weakness in finger extension will quickly learn to use their tenodesis to open their fingers. Although this may be a normal response during recovery, if these compensatory movements are prolonged, they can result in dysfunctional hand use and limit recovery.
In this study, the authors have focused on the complex function of grasp. They wanted to establish norms and assess the compensatory movements that develop after injury to the extrinsic finger flexors. Flexor tendons are important during composite fist formation. When extrinsic finger flexors are dysfunctional, patients will often attempt an alternate method of finger flexion. Specifically, they will utilize intrinsic muscles to flex their metacarpophalangeal joints. However, this paradoxically extends the proximal and distal interphalangeal joints, resulting in a poorly formed fist, incapable of grasping many objects.
Although hand therapists have techniques intended to inhibit these movements, the tendency to fall back on intrinsic muscle use can delay rehabilitation. The overuse of intrinsics and bypassing of extrinsic flexors decreases the gliding of the extrinsic flexors, potentially increasing adhesions. In the end, these compensatory movements can become pathologic, resulting in long-term inability to form a composite fist. The authors believe this type of movement pattern is an important barrier to obtaining full function after extrinsic flexor tendon injuries.
The authors hypothesize that inappropriate use of the intrinsic muscles inhibits gliding of the extrinsic flexors and thus interventions to block this pattern may improve hand function after trauma. Better understanding of these compensatory movements through the use of state-of-the-art time-locked digital video synchronized electromyography (EMG) analyzing composite grip is the first step to understanding this process. Improved understanding will allow for adjustment of post-treatment interventions to potentially improve outcomes.
The current study focuses on the dynamic function of the extrinsic and intrinsic muscles during grip in both normal volunteers and patients who sustained trauma to the finger flexors. The authors' goal was to elucidate the normal grip pattern and the derangement in finger flexors that results in limited grasp after injury. They hypothesized that there would be abnormal activation of intrinsic muscles in patients with flexor tendon injuries.
The authors' specific aims of the study were 3-fold: (1) to obtain time-locked digital video synchronized EMG of the intrinsic hand muscles and extrinsic digital flexors in healthy volunteers to determine the patterns and timing of muscle recruitment during grip; (2) to obtain time-locked digital video synchronized EMG of the intrinsic hand muscles and extrinsic digital flexors in volunteers who have sustained a hand injury to determine the patterns and timing of muscle recruitment during grip; and (3) to compare the timing and pattern of muscle recruitment during grip between healthy volunteers and volunteers who have sustained a hand injury.
With institutional review board approval, 10 subjects were recruited for this pilot prospective cohort study. Five subjects were uninjured, and 5 subjects had sustained hand injuries between 6 and 18 months prior to testing. Subjects were recruited via flyers posted in a hand surgery and hand therapy clinic, as well as through word of mouth. Subjects younger than 18 years or those who could not follow brief instructions in English were excluded from the study.
Uninjured volunteers needed to have no history of hand trauma. Injured volunteers were required to have had hand trauma requiring surgery greater than 6 months but not greater than 2 years prior. Evaluation was performed in a neurology procedure room standardly used for EMG testing.
Subjects responded to a brief questionnaire to obtain their age, handedness, hand function, profession, and information on any prior hand trauma or hand surgery. Electromyography was recorded using a Cascade Elite system (Cadwell, Kennewick, Washington) via pairs of 28-gauge needle electrodes placed intramuscularly into the flexor digitorum profundus (FDP) and flexor digitorum superficialis (FDS) muscles. Intramuscular placement was verified using free running EMG during motions specific to each group of muscles to ensure the correct muscle was being recorded (as opposed to, for example, the nearby wrist flexors).
The hand intrinsic muscles were recorded using a pair of sticky surface electrodes placed over the lumbricals of the hand, with 1 electrode between the first and second lumbricals, and the other between the third and fourth. Electromyography setup and recordings were performed by a neurologist with board-certifications in neurology and clinical neurophysiology (V.N.).
The subjects were then asked to make a “light fist” followed by a “tight fist” motion. Free running EMG and time-locked digital video were recorded during testing. Testing was performed bilaterally on all subjects. Three independent observers (P.M.F., J.D.O., C.M.C.) then analyzed the video-EMG synchronized data to determine the pattern of muscle activation during composite fist formation. The neurologist (V.N.) who performed the testing was excluded from analysis to limit potential bias. Subjects received a $25 gift card as compensation at the completion of testing.
Three patterns of muscle recruitment during grasp formation could be determined from EMG data of healthy volunteers. First, FDP was involved in the initiation of light and tight fist formation in 4 of 5 patients. In 1 unique patient, the intrinsics initiated the fist-forming motion with FDP recruited less 200 ms later.
Light fist formation primarily involved the FDP with either the FDS or intrinsics (but not both) in all healthy volunteers. In contrast, both the FDS and intrinsics were recruited in all healthy volunteers' tight fist EMGs. However, the sequence of recruitment after fist initiation by FDP differed between healthy volunteers in tight fist formation. A sample video can be seen in the Video.
In the first case of an injured volunteer with a diagnosis of an index finger proximal phalanx fracture treated with closed reduction and percutaneous pinning, EMG analysis showed that the overall grip pattern was maintained compared to the uninjured side. However, FDS recruitment in the injured index finger was decreased during isolated digit testing compared to the contralateral side, as well as the uninjured digits on the same side.
The second case of an injured volunteer with a previous small finger proximal phalanx fracture treated with closed reduction and percutaneous pinning highlighted a unique difference between the 2 injured digits. Electromyography analysis again showed overall grip pattern was maintained between the injured and uninjured hands. However, on isolated FDS testing of the small finger, there was increased intrinsic muscle recruitment and poor proximal interphalangeal joint flexion.
Finally, an injured subject with a complex saw injury to the hand, resulting in a ring finger revascularization with tendon repair and a small finger middle phalanx amputation, revealed the modified interaction of the intrinsics and extrinisics after injury. Electromyography analysis of this patient showed balanced contribution of the FDP, FDS, and intrinsics during light fist formation on the injured side compared to FDP and intrinsic only contribution on the uninjured side.
The first step to treating dysfunctional hand movements is a better understanding of the interactions of intrinsic and extrinsic flexors during grip before and after injury. In the 1960s and 1970s, Long et al15–17 used EMG to describe the roles of intrinsic and extrinsic hand muscles in digit motion. This seminal work created the foundation of knowledge about digit motion and grip. Since then, EMG technology and testing has experienced significant advances. However, these advances have not been applied to improving the authors' understanding of grasp and the ways in which it can become deranged.
The current study has reexamined and built on these early studies. The authors were able to achieve their study objectives of obtaining state-of-the-art time-locked, digital video synchronized EMGs of both healthy and injured volunteers during grip. In addition, they were able to make comparisons between groups to note specific differences that occur after hand trauma and surgery.
Hand therapists continue to make advances in understanding the mechanical aspects of tendon management following hand surgery. However, despite improved suture technique and early motion during postoperative therapy, patient progress remains inconsistent after repair.18 The primary goals of tendon therapy after surgical repair include promoting intrinsic tendon healing and minimizing extrinsic scarring to optimize tendon gliding and functional range of motion.19
Perhaps 1 contributor to poor recovery after tendon repair is that the hand therapy needs to be customized to individual recovery. Currently, the necessary diagnostic and therapeutic modalities are not fully developed to tailor to each patient's treatment. With advances in EMG diagnostic capabilities, this technology can potentially benefit patients in a highly individualized manner to direct therapeutic protocols appropriate to their specific level of functional deficit.
Temporary deactivation of overactive muscles of the upper extremity has been demonstrated through intramuscular injection of botulinum toxin type A.20,21 Substantial evidence has come forth in recent years of the efficacy of injection into muscles with overactivity in reducing resistance to passive movement in joints supplied by the injected muscles.21
In the case of injured intrinsic hand muscles, the paired agonist–antagonist relationship tends to produce an imbalance that impairs the purposeful hand movements.20 In such circumstances, the use of systemic pharmacologic muscular relaxants is not likely to improve active hand function because the treatment would lead to indiscriminate reduction of motor neuron excitability and recruitment in both agonists and antagonists.22
However, local botulinum toxin type A has seen success in controlling symptoms in dystonias and reduction of co-contraction, both abnormal processes of aberrant recruitment of antagonist muscles in voluntary movement.23,24 Although the immediate benefits of these treatments are mechanistically transient, the authors believe botulinum toxin type A injection of intrinsic muscles of the hand could potentially be utilized as a temporary deactivation technique to aid in hand therapy. Given the ease of injection, this treatment option could be helpful in the acute period following injury, administered directly by the neurologist at the time of diagnostic EMG postoperatively. The authors plan to examine this in future studies.
This study had several limitations, one of which was the small sample size of patients, in part due to the prospective voluntary-based recruitment of both healthy and injured patients to participate in a study that required multiple needle placements. There was variability in the type of injury, time since injury, and operative intervention in the injured patient cohort.
Although this heterogeneity of injury types yielded differences in exact EMG tracings, activities of the target muscles were still equally sensitive and relative to the individual subject's hand shape and grip force. Thus, patterns of EMG activity were discernible despite these variations.
Additional studies are warranted to assess longitudinal EMG results, following patients from 1, 3, 6, and 12 months after injury to identify the time line of compensatory muscle recruitment patterns. This study represents the highest level of evidence that compensatory muscle recruitment patterns are less predictable and less consistent than initially reported.15–17
The results of the current study confirm the FDP muscle to be the workhorse in composite fist formation, irrespective of effort. However, the role of FDS and the intrinsic muscles is less consistent across patients. Results suggest the recruitment of either FDS or the intrinsics during light fist formation to be less predictable than initially reported by Long et al.15–17
The authors found the sequence of recruitment of these muscles to vary between patients. They believe this information and limited EMG post-injury can improve patient rehabilitation after flexor tendon repair. Tailored hand therapy regimens can be designed after analysis of patient muscle recruitment thereby expediting recovery.
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