Variability of in vivo Sarcomere Length Measures in the Upper Limb Obtained With Second Harmonic Generation Microendoscopy.

The lengths of a muscle's sarcomeres are a primary determinant of its ability to contract and produce force. In addition, sarcomere length is a critical parameter that is required to make meaningful comparisons of both the force-generating and excursion capacities of different muscles. Until recently, in vivo sarcomere length data have been limited to invasive or intraoperative measurement techniques. With the advent of second harmonic generation microendoscopy, minimally invasive measures of sarcomere length can be made for the first time. This imaging technique expands our ability to study muscle adaptation due to changes in stimulus, use, or disease. However, due to past inability to measure sarcomeres outside of surgery or biopsy, little is known about the natural, anatomical variability in sarcomere length in living human subjects. To develop robust experimental protocols that ensure data provide accurate representations of a muscle's sarcomere lengths, we sought to quantify experimental uncertainty associated with in vivo measures of sarcomere lengths. Specifically, we assessed the variability in sarcomere length measured (1) within a single image, along a muscle fiber, (2) across images captured within a single trial, across trials, and across days, as well as (3) across locations in the muscle using second harmonic generation in two upper limb muscles with different muscle architectures, functions, and sizes. Across all of our measures of variability we estimate that the magnitude of the uncertainty for in vivo sarcomere length is on the order of ∼0.25 µm. In the two upper limb muscles studied we found larger variability in sarcomere lengths within a single insertion than across locations. We also developed custom code to make measures of sarcomere length variability across a single fiber and determined that this codes' accuracy is an order of magnitude smaller than our measurement uncertainty due to sarcomere variability. Together, our findings provide guidance for the development of robust experimental design and analysis of in vivo sarcomere lengths in the upper limb.

The Biomechanical Basis of the Claw Finger Deformity: A Computational Simulation Study.

PURPOSE: Claw finger deformity occurs during attempted finger extension in patients whose intrinsic finger muscles are weakened or paralyzed by neural impairments. The deformity is generally not acutely present after intrinsic muscle palsy. The delayed onset, with severity progressing over time, suggests soft tissue changes that affect the passive biomechanics of the hand exacerbate and advance the deformity. Clinical interventions may be more effective if such secondary biomechanical changes are effectively addressed. Using a computational model, we simulated these altered soft tissue biomechanical properties to quantify their effects on coordinated finger extension. METHODS: To evaluate the effects of maladaptive changes in soft tissue biomechanical properties on the development and progression of the claw finger deformity after intrinsic muscle palsy, we completed 45 biomechanical simulations of cyclic index finger flexion and extension, varying the muscle excitation level, clinically relevant biomechanical factors, and wrist position. We evaluated to what extent (1) increased joint laxity, (2) decreased mechanical advantage of the extensors about the proximal interphalangeal joint, and (3) shortening of the flexor muscles contributed to the development of claw finger deformity in an intrinsic-minus hand model. RESULTS: Of the mechanisms studied, shortening (or contracture) of the extrinsic finger flexors was the factor most associated with the development of claw finger deformity in simulation. CONCLUSIONS: These simulations suggest that adaptive shortening of the extrinsic finger flexors is required for the development of claw finger deformity. Increased joint laxity and decreased extensor mechanical advantage only contributed to the severity of the deformity in simulations when shortening of the flexor muscles was present. CLINICAL RELEVANCE: In both the acute and chronic stages of intrinsic finger paralysis, maintaining extrinsic finger flexor length should be an area of focus in rehabilitation to prevent formation of the claw finger deformity and achieve optimal outcomes after surgical interventions.

Effect of biceps-to-triceps transfer on rotator cuff stress during upper limb weight-bearing lift in tetraplegia: A modeling and simulation analysis.

Rotator cuff stress during upper limb weight-bearing lifts presumably contribute to rotator cuff disease, which is the most common cause of shoulder pain in individuals with tetraplegia. Elbow extension strength appears to be a key determinant of rotator cuff stress during upper limb weight-bearing lifts since individuals with paraplegia who generate greater elbow extensor moments experience lower rotator cuff stress relative to individuals with tetraplegia. Biceps-to-triceps transfer surgery can increase elbow extension strength in individuals with tetraplegia. The purpose of this study was to determine whether active elbow extension via biceps transfer decreases rotator cuff stress during weight-bearing lifts in individuals with tetraplegia. A forward dynamics computational framework was used to estimate muscle stress during the lift; stress was computed as muscle force divided by the peak isometric muscle force. We hypothesized that rotator cuff stresses would be lower in simulated lifting with biceps transfer relative to simulated lifting without biceps transfer. We found that limited elbow extension strength in individuals with tetraplegia, regardless of whether elbow strength is enabled via biceps transfer or is residual after spinal cord injury, results in muscle stresses exceeding 85% of the peak isometric muscle stress in the supraspinatus, infraspinatus, and teres minor. The rotator cuff stresses we estimated suggest that performance of weight-bearing activities should be minimized or assisted in order to reduce the risk for shoulder pain. Our results also indicate that biceps transfer is unlikely to decrease rotator cuff stress during weight-bearing lifts in individuals with tetraplegia.

Demonstration of extended field-of-view ultrasound's potential to increase the pool of muscles for which in vivo fascicle length is measurable.

Static, B-mode ultrasound is the most common method of measuring fascicle length in vivo. However, most forearm muscles have fascicles that are longer than the field-of-view of traditional ultrasound (T-US). As such, little work has been done to quantify in vivo forearm muscle architecture. The extended field-of-view ultrasound (EFOV-US) method, which fits together a sequence of B-mode images taken from a continuous ultrasound scan, facilitates direct measurements of longer, curved fascicles. Here, we test the validity and reliability of the EFOV-US method for obtaining fascicle lengths in the extensor carpi ulnaris (ECU). Fascicle lengths from images of the ECU captured in vivo with EFOV-US were compared to lengths from a well-established method, T-US. Images were collected in a joint posture that shortens the ECU such that entire fascicle lengths were captured within a single T-US image. Resulting measurements were not significantly different (p=0.18); a Bland-Altman test demonstrated their agreement. A novice sonographer implemented EFOV-US in a phantom and in vivo on the ECU. The novice sonographer's measurements from the ultrasound phantom indicate that the combined imaging and analysis method is valid (average error=2.2±1.3mm) and the in vivo fascicle length measurements demonstrate excellent reliability (ICC=0.97). To our knowledge, this is the first study to quantify in vivo fascicle lengths of the ECU using any method. The ability to define a muscle's architecture in vivo using EFOV-US could lead to improvements in diagnosis, model development, surgery guidance, and rehabilitation techniques.

Connecting the wrist to the hand: A simulation study exploring changes in thumb-tip endpoint force following wrist surgery.

The wrist is essential for hand function. Yet, due to the complexity of the wrist and hand, studies often examine their biomechanical features in isolation. This approach is insufficient for understanding links between orthopaedic surgery at the wrist and concomitant functional impairments at the hand. We hypothesize that clinical reports of reduced force production by the hand following wrist surgeries can be explained by the surgically-induced, biomechanical changes to the system, even when those changes are isolated to the wrist. This study develops dynamic simulations of lateral pinch force following two common surgeries for wrist osteoarthritis: scaphoid-excision four-corner fusion (SE4CF) and proximal row carpectomy (PRC). Simulations of lateral pinch force production in the nonimpaired, SE4CF, and PRC conditions were developed by adapting published models of the nonimpaired wrist and thumb. Our simulations and biomechanical analyses demonstrate how the increased torque-generating requirements at the wrist imposed by the orthopaedic surgeries influence force production to such an extent that changes in motor control strategy are required to generate well-directed thumb-tip end-point forces. The novel implications of our work include identifying the need for surgeries that optimize the configuration of wrist axes of rotation, rehabilitation strategies that improve post-operative wrist strength, and scientific evaluation of motor control strategies following surgery. Our simulations of SE4CF and PRC replicate surgically-imposed decreases in pinch strength, and also identify the wrist's torque-generating capacity and the adaptability of muscle coordination patterns as key research areas to improve post-operative hand function.

Posture-Dependent Corticomotor Excitability Differs Between the Transferred Biceps in Individuals With Tetraplegia and the Biceps of Nonimpaired Individuals.

BACKGROUND: Following biceps transfer to enable elbow extension in individuals with tetraplegia, motor re-education may be facilitated by greater corticomotor excitability. Arm posture modulates corticomotor excitability of the nonimpaired biceps. If arm posture also modulates excitability of the transferred biceps, posture may aid in motor re-education. OBJECTIVE: Our objective was to determine whether multi-joint arm posture affects corticomotor excitability of the transferred biceps similar to the nonimpaired biceps. We also aimed to determine whether corticomotor excitability of the transferred biceps is related to elbow extension strength and muscle length. METHODS: Corticomotor excitability was assessed in 7 arms of individuals with tetraplegia and biceps transfer using transcranial magnetic stimulation and compared to biceps excitability of nonimpaired individuals. Single-pulse transcranial magnetic stimulation was delivered to the motor cortex with the arm in functional postures at rest. Motor-evoked potential amplitude was recorded via surface electromyography. Elbow moment was recorded during maximum isometric extension trials, and muscle length was estimated using a biomechanical model. RESULTS: Arm posture modulated corticomotor excitability of the transferred biceps differently than the nonimpaired biceps. Elbow extension strength was positively related and muscle length was unrelated, respectively, to motor-evoked potential amplitude across the arms with biceps transfer. CONCLUSIONS: Corticomotor excitability of the transferred biceps is modulated by arm posture and may contribute to strength outcomes after tendon transfer. Future work should determine whether modulating corticomotor excitability via posture promotes motor re-education during the rehabilitative period following surgery.

Surgical Simulations Based on Limited Quantitative Data: Understanding How Musculoskeletal Models Can Be Used to Predict Moment Arms and Guide Experimental Design.

The utility of biomechanical models and simulations to examine clinical problems is currently limited by the need for extensive amounts of experimental data describing how a given procedure or disease affects the musculoskeletal system. Methods capable of predicting how individual biomechanical parameters are altered by surgery are necessary for the efficient development of surgical simulations. In this study, we evaluate to what extent models based on limited amounts of quantitative data can be used to predict how surgery influences muscle moment arms, a critical parameter that defines how muscle force is transformed into joint torque. We specifically examine proximal row carpectomy and scaphoid-excision four-corner fusion, two common surgeries to treat wrist osteoarthritis. Using models of these surgeries, which are based on limited data and many assumptions, we perform simulations to formulate a hypothesis regarding how these wrist surgeries influence muscle moment arms. Importantly, the hypothesis is based on analysis of only the primary wrist muscles. We then test the simulation-based hypothesis using a cadaveric experiment that measures moment arms of both the primary wrist and extrinsic thumb muscles. The measured moment arms of the primary wrist muscles are used to verify the hypothesis, while those of the extrinsic thumb muscles are used as cross-validation to test whether the hypothesis is generalizable. The moment arms estimated by the models and measured in the cadaveric experiment both indicate that a critical difference between the surgeries is how they alter radial-ulnar deviation versus flexion-extension moment arms at the wrist. Thus, our results demonstrate that models based on limited quantitative data can provide novel insights. This work also highlights that synergistically utilizing simulation and experimental methods can aid the design of experiments and make it possible to test the predictive limits of current computer simulation techniques.

Multicenter Survey of the Effects of Rehabilitation Practices on Pinch Force Strength After Tendon Transfer to Restore Pinch in Tetraplegia.

OBJECTIVE: To identify key components of conventional therapy after brachioradialis (BR) to flexor pollicis longus (FPL) transfer, a common procedure to restore pinch strength, and evaluate whether any of the key components of therapy were associated with pinch strength outcomes. DESIGN: Rehabilitation protocols were surveyed in 7 spinal cord injury (SCI) centers after BR to FPL tendon transfer. Key components of therapy, including duration of immobilization, participation, and date of initiating therapy activities (mobilization, strengthening, muscle reeducation, functional activities, and home exercise), were recorded by the patient's therapist. Pinch outcomes were recorded with identical equipment at 1-year follow-up. SETTING: Seven SCI rehabilitation centers where the BR to FPL surgery is performed on a routine basis. PARTICIPANTS: Thirty-eight arms from individuals with C5-7 level SCI injury who underwent BR to FPL transfer surgery (N=34). INTERVENTION: Conventional therapy according to established protocol in each center. MAIN OUTCOME MEASURES: The frequency of specific activities and their time of initiation (relative to surgery) were expressed as means and 95% confidence intervals. Outcome measures included pinch strength and the Canadian Occupational Performance Measure (COPM). Spearman rank-order correlations determined significant relations between pinch strength and components of therapy. RESULTS: There was similarity in the key components of therapy and in the progression of activities. Early cast removal was associated with pinch force (Spearman ρ=-.40, P=.0269). Pinch force was associated with improved COPM performance (Spearman ρ=.48, P=.0048) and satisfaction (Spearman ρ=.45, P=.0083) scores. CONCLUSIONS: Initiating therapy early after surgery is beneficial after BR to FPL surgery. Postoperative therapy protocols have the potential to significantly influence the outcome of tendon transfers after tetraplegia.

Comparison of wrist and elbow stabilization following pinch reconstruction in tetraplegia.

PURPOSE: Individuals with spinal cord injuries resulting in tetraplegia may receive tendon transfer surgery to restore grasp and pinch function. These procedures often involve rerouting the brachioradialis (Br) and the extensor carpi radialis longus tendons volar to the flexion-extension axis of the wrist, leaving the extensor carpi radialis brevis (ECRB) muscle to provide wrist extension strength. The purpose of this study was to determine whether externally stabilizing the wrist after transfer procedures would improve the ability to activate the transferred Br and resulting pinch force, similar to the effect observed when the elbow is externally stabilized. METHODS: We used a one-way repeated-measures study design to determine the effect of 3 support conditions on muscle activation and lateral pinch force magnitude in 8 individuals with tetraplegia and previous tendon transfer surgeries. Muscle activation was recorded from Br and ECRB with intramuscular electrodes and from biceps and triceps muscles with surface electrodes. We quantified pinch strength with a 6-axis force sensor and custom grip. We recorded measurements in 3 support conditions: with the arm self-stabilized, with elbow stabilization, and with elbow and wrist stabilization. Pairwise differences were tested using Wilcoxon signed-rank tests. RESULTS: Maximum effort pinch force magnitude and Br activation were significantly increased in both supported conditions compared with the self-supported trials. The addition of wrist stabilization had no significant effect compared with elbow stabilization alone. CONCLUSIONS: A strong ECRB has adequate strength to extend the wrist, even after multiple transfers that contribute an additional flexion moment from strong activation of donor muscles. Anatomical and functional differences between the wrist and elbow musculature are important determinants for self-stabilizing joints proximal to the tendon transfer. The ability to increase Br activation and resulting pinch force may be determined, in part, by the individual's ability to develop new coordination strategies.

Muscle moment arms in the first dorsal extensor compartment after radial malunion. A cadaver study.

BACKGROUND: Functional loss is a common complication of the fractured distal part of the radius. The purpose of the present study was to determine if the moment arms of the first dorsal extensor compartment are altered by distal radial fracture malunion. We hypothesized that the moment arms of the abductor pollicis longus and extensor pollicis brevis are significantly affected by dorsal angulation, radial inclination, and radial shortening, the most common deformities accompanying distal radial malunion. METHODS: Moment arms of the extensor pollicis brevis and abductor pollicis longus were estimated in twelve cadaver wrists with use of the tendon-displacement method, which involves calculating the moment arm as the derivative of tendon displacement with respect to joint angle. Tendon displacement was quantified in different wrist postures before and after a closing-wedge osteotomy simulating a complex malunion of an extra-articular radial fracture. RESULTS: The simulated distal radial malunion resulted in a decrease in the wrist flexion moment arm for both the extensor pollicis brevis (p = 0.0003) and the abductor pollicis longus (p < 0.0001). The wrist flexion moment arms for the extensor pollicis brevis and abductor pollicis longus decreased by a mean (and standard deviation) of 114% +/- 75% and 77% +/- 50%, respectively, after the osteotomy. The wrist radial deviation moment arms for the extensor pollicis brevis and abductor pollicis longus increased by 16% +/- 26% (p = 0.071) and 28% +/- 44% (p = 0.043), respectively, after the osteotomy. Radiographs of the wrist that were made before and after the osteotomy indicated that radial tilt changed from 11.1 degrees of volar angulation to 14.8 degrees of dorsal angulation, radial inclination decreased from 21.8 degrees to 7.7 degrees, and radial height decreased from 11.6 to 4.4 mm. CONCLUSIONS: Distal radial malunion alters the mechanical advantage of the muscles in the first dorsal extensor compartment.

Variability in surgical technique for brachioradialis tendon transfer. Evidence and implications.

BACKGROUND: Transfer of the tendon of the brachioradialis muscle to the tendon of the flexor pollicis longus restores lateral pinch function after cervical spinal cord injury. However, the outcomes of the procedure are unpredictable, and the reasons for this are not understood. The purpose of this study was to document the degree of variability observed in the performance of this tendon transfer. METHODS: The surgical technique used for the brachioradialis tendon transfer was assessed in two ways. First, the surgical attachment length of the brachioradialis was quantified, after transfer to the flexor pollicis longus, with use of intraoperative laser diffraction to measure muscle sarcomere length in eleven individuals (twelve limbs) with tetraplegia. Second, ten surgeons who regularly performed this procedure were surveyed regarding their tensioning preferences. Using a biomechanical model of the upper extremity, we investigated theoretically the effect of different surgical approaches on the active muscle-force-generating capacity of the transferred brachioradialis in functionally relevant elbow, wrist, and hand postures. RESULTS: The average sarcomere length (and standard deviation) of the transferred brachioradialis was 3.5 +/- 0.3 mum. That length was significantly correlated to the in situ sarcomere length (r(2) = 0.53, p < 0.05). Surgical tensioning preferences varied considerably; however, six of the ten surgeons positioned the patient's elbow between full extension (0 degrees of elbow flexion) and 50 degrees of flexion when selecting the attachment length, and six of the ten stated that their goal was to tension the transfer slightly tighter than its resting tension. The computer simulations suggested that a "tighter" brachioradialis transfer would produce its peak active force in an elbow position that is more flexed than the elbow position in which a "looser" transfer would produce its peak active force. CONCLUSIONS: This study provides evidence that experienced surgeons perform this tendon transfer differently from one another. Biomechanical simulations suggested that these differences could result in substantial variability in the active force that the transferred brachioradialis can produce in functionally relevant postures. CLINICAL RELEVANCE: The surgical attachment length and the position of the patient's limb at the time of tendon transfer are both controllable and measurable parameters. Understanding the relationship between surgical technique and postoperative muscle function may provide surgeons with more control of clinical outcomes.

Activation of brachioradialis muscles transferred to restore lateral pinch in tetraplegia.

PURPOSE: Surgical transfers of muscles are used to restore lateral pinch in tetraplegia; however, outcomes are variable. The purpose of this study was to compare activation of the brachioradialis (Br) after transfer to the flexor pollicis longus during maximum effort in its primary function (elbow flexion) with maximum effort in its postoperative function (lateral pinch) and to record Br activation during functional tasks. METHODS: Fine-wire electrodes recorded activation of the Br in 11 arms with tetraplegia. Subjects produced maximum lateral pinch force with and without elbow stabilization and were classified according to elbow strength. The elbow was stabilized by supporting the arm and limiting elbow motion. A force sensor mounted on a custom grip recorded the pinch force. Electromyographic (EMG) signals recorded during lateral pinch were expressed as a percentage of the maximum voluntary contraction recorded during maximum-effort elbow flexion. RESULTS: The EMG activation was significantly lower during lateral pinch compared with resisted elbow flexion. The mean EMG during lateral pinch in the self-supported elbow condition was 34% of the maximum voluntary contraction; with the elbow stabilized the EMG increased to 55% of the maximum voluntary contraction. Postoperative pinch-force magnitude was 14 N with self-support and 20 N with the elbow stabilized. Subjects with weak elbow extension strength produced significantly lower pinch forces compared with subjects with strong elbow extension but had similar ability to activate the Br. The Br activation was higher when the pinch tasks were performed successfully. CONCLUSIONS: These findings suggest a reduced ability to activate the transferred muscle fully in lateral pinch function after surgery, even with the addition of elbow support. The Br activation is linked to successful performance of lateral pinch tasks. The subjects' inability to activate the transferred muscle fully may be affected by postoperative muscle re-education and contribute to postoperative weakness.

A model of the upper extremity for simulating musculoskeletal surgery and analyzing neuromuscular control.

Biomechanical models of the musculoskeletal system are frequently used to study neuromuscular control and simulate surgical procedures. To be broadly applicable, a model must be accessible to users, provide accurate representations of muscles and joints, and capture important interactions between joints. We have developed a model of the upper extremity that includes 15 degrees of freedom representing the shoulder, elbow, forearm, wrist, thumb, and index finger, and 50 muscle compartments crossing these joints. The kinematics of each joint and the force-generating parameters for each muscle were derived from experimental data. The model estimates the muscle-tendon lengths and moment arms for each of the muscles over a wide range of postures. Given a pattern of muscle activations, the model also estimates muscle forces and joint moments. The moment arms and maximum moment-generating capacity of each muscle group (e.g., elbow flexors) were compared to experimental data to assess the accuracy of the model. These comparisons showed that moment arms and joint moments estimated using the model captured important features of upper extremity geometry and mechanics. The model also revealed coupling between joints, such as increased passive finger flexion moment with wrist extension. The computer model is available to researchers at http://nmbl.stanford.edu.

Biomechanical properties of the brachioradialis muscle: Implications for surgical tendon transfer.

PURPOSE: To understand the mechanical properties of the brachioradialis (BR) muscle and to use this information to simulate a BR-to-flexor pollicis longus (FPL) tendon transfer for restoration of lateral pinch. METHODS: The BR mechanical properties were measured intraoperatively. Passive elastic properties were measured by elongating BR muscles at constant velocity while they were attached directly to a dual-mode servomotor. Sarcomere length was measured intraoperatively and in situ by laser diffraction with the elbow fully extended. Then both the mechanical and structural properties were programmed into a surgical simulator to test the hand surgeon's decision making when tensioning muscles in a simulated BR-to-FPL tendon transfer. RESULTS: Passive mechanical BR properties were highly nonlinear. Under slack conditions sarcomere length (mean +/- standard deviation) was 2.81 +/- 0.10 microm (n = 4), corresponding to an active force of 93% maximum. Sarcomere length of the BR measured in situ with the elbow fully extended and the forearm in neutral rotation was 3.90 +/- 0.27 microm (n = 8), corresponding to an active force of only 23% maximum. Surgeons, who tensioned the BR for transfer into the FPL using only tactile feedback from the surgical simulator, attached the muscle at a passive tension of 5.87 +/- 0.97 N, which corresponded to a sarcomere length of 3.84 microm and an active muscle force of 27% maximum. Passive BR tension when both tactile and visual information were provided to the surgeon was significantly lower (2.42 +/- 0.72 N), corresponding to a sarcomere length of 3.56 mum and a much higher active muscle force of 45% maximum. CONCLUSIONS: When these data were used to model pretransfer and posttransfer function dramatic differences in predicted function were obtained depending on the tensioning protocol chosen. This emphasizes the point that the decision-making process used during muscle tensioning has a profound effect on the functional outcome of the transfer.

The effect of percutaneous pin fixation of the interphalangeal joint on the thumb-tip force produced by the flexor pollicis longus: a cadaver study.

PURPOSE: Interphalangeal joint stabilization often is performed concomitantly with tendon transfers that restore key pinch (lateral pinch) to the paralyzed thumb. The goal of this study was to measure the effect of interphalangeal joint stabilization via percutaneous pin fixation on the thumb-tip force produced by the flexor pollicis longus (FPL). METHODS: We applied 10 N of force to the tendon of the FPL in 7 cadaveric specimens and measured the resulting thumb-tip force in the intact thumb and after stabilization of the interphalangeal joint. RESULTS: The nominal thumb-tip force was approximately 6 times less than the applied force and was directed primarily in the thumb's plane of flexion-extension at an oblique angle of 44 degrees relative to the palmar direction (the direction that is perpendicular to the thumb tip in the plane). Joint stabilization increased significantly the nominal force and oriented the force more toward the palmar direction (ie, decreased the obliqueness of the force). CONCLUSIONS: After paralysis and a tendon transfer to the paralyzed FPL the FPL is often the only muscle actuating the thumb. We conclude that the oblique nominal force direction is prone to cause the thumb to slip during pinch. Joint stabilization, however, has the capacity to reduce the tendency for slippage because it rotates the force toward the palmar direction.

Biomechanics of the Steindler flexorplasty surgery: a computer simulation study.

PURPOSE: Our goal was to investigate the capacity of a Steindler flexorplasty to restore elbow flexion to persons with C5-C6 brachial plexus palsy. In this procedure the origin of the flexor-pronator mass is moved proximally onto the humeral shaft. We examined how the choice of the proximal attachment site for the flexor-pronator mass affects elbow flexion restoration, especially considering possible side effects including limited wrist and forearm motion owing to passive restraint from stretched muscles. METHODS: A computer model of the upper extremity was used to simulate the biomechanical consequences of various surgical alterations. Unimpaired, preoperative, and postoperative conditions were simulated. Seven possible transfer locations were used to investigate the effects of choice of transfer location. RESULTS: Each transfer site produced a large increase in elbow flexion strength. Transfer to more proximal attachment sites also produced large increases in passive resistance to wrist extension and forearm supination. CONCLUSIONS: To reduce detrimental side effects while achieving clinical goals our theoretical analysis suggests a transfer to the distal limit of the traditional transfer region.

The influence of elbow position on the range of motion of the wrist following transfer of the brachioradialis to the extensor carpi radialis brevis tendon.

BACKGROUND: In patients who have an injury of the cervical spinal cord, the brachioradialis tendon may be transferred to the extensor carpi radialis brevis tendon to restore voluntary wrist extension. We hypothesized that the active range of motion of the wrist depends on the position of the elbow after this transfer because the brachioradialis changes length substantially during elbow flexion, which implies the maximum force that the muscle can produce varies with elbow position. The objectives of this study were to determine whether the position of the elbow influences the range of motion of the wrist following transfer of the brachioradialis to the extensor carpi radialis brevis tendon and to evaluate the effect of surgical tensioning. METHODS: The range of motion of eight wrists was assessed after brachioradialis transfer. Two positions of the elbow were tested, the passive limit of elbow extension and 120 degrees of flexion. The range of motion of the wrist was also simulated with use of a biomechanical model. Using the model, we compared the active range of motion of the wrist, with the elbow at 0 degrees and 120 degrees of flexion, following three different approaches to surgical tensioning. The simulations were also repeated to evaluate how muscle strength influences outcomes. RESULTS: Wrist extension decreased and passive flexion increased when the elbow was flexed. Maximum wrist extension was significantly correlated with passive flexion in all subjects (r = 0.95 and p < 0.001 when the elbow was extended and r = 0.82 and p < 0.03 when the elbow was flexed). The biomechanical model suggested that tensioning the tendon transfer so that the fibers of the brachioradialis do not become excessively short when the elbow is flexed may improve outcomes. The simulations also revealed that it is more difficult to maintain a consistent wrist position with the elbow in different postures when a weaker muscle is transferred. CONCLUSIONS: The model suggests that altering the surgical tension could improve wrist extension when the elbow is flexed. However, the ultimate result is sensitive to the strength of the brachioradialis.