Eric R. Hentzen

Rotator cuff muscle architecture: Implications for glenohumeral stability

We examined the architectural properties of the rotator cuff muscles in 10 cadaveric specimens to understand their functional design. Based on our data and previously published joint angle-muscle excursion data, sarcomere length operating ranges were modeled through all permutations in 75º medial and lateral rotation and 75º abduction at the glenohumeral joint. Based on physiologic cross-sectional area, the subscapularis would have the greatest force-producing capacity, followed by the infraspinatus, supraspinatus, and teres minor. Based on fiber length, the supraspinatus would operate over the widest range of sarcomere lengths. The supraspinatus and infraspinatus had relatively long sarcomere lengths in the anatomic position, and were under relatively high passive tensions at rest, indicating they are responsible for glenohumeral resting stability. However, the subscapularis contributed passive tension at maximum abduction and lateral rotation, indicating it plays a critical role in glenohumeral stability in the position of apprehension. These data illustrate the exquisite coupling of muscle architecture and joint mechanics, which allows the rotator cuff to produce near maximal active tensions in the midrange and produce passive tensions in the various end-range positions. During surgery relatively small changes to rotator cuff muscle length may result in relatively large changes in shoulder function.

Stress-dependent and -independent expression of the myogenic regulatory factors and the MARP genes after eccentric contractions in rats

The relationship between muscle mechanical conditions and gene expression was investigated by varying both stress and contraction mode imposed upon rat dorsiflexors (n= 25), activating them at high or low frequencies (150 Hz or 40 Hz) either eccentrically or isometrically. Muscle physiological, immunohistochemical and gene expression changes were then measured 24 h after the exercise bout. Peak stress was the best predictor of muscle injury, independent of contraction mode (i.e. eccentric or isometric). When peak stresses were matched, no physiological or immunohistochemical differences were detected between isometric and eccentric contractions. The expression of certain myogenic regulatory and muscle ankyrin repeat protein (MARP) genes (myoD, myogenin, MLP and CARP) depended both on peak muscle stress achieved during contraction and contraction mode. In contrast, Arpp/Ankrd2 was dramatically upregulated only by eccentric contractions, but not by isometric contractions, even though the stress level of the eccentric contractions varied over a three‐fold range and overlapped with that of the isometric group. The role that Arpp/Ankrd2 upregulation plays in the biological response to eccentric contraction remains to be determined, as does the control mechanism whereby the expression of certain genes (such as myoD, myogenin, MLP and CARP) is sensitive to muscle stress while another (Arpp/Ankrd2) is sensitive only to contraction mode.

Mechanical Strength of the Side-to-Side Versus Pulvertaft Weave Tendon Repair

PURPOSE The side-to-side (SS) tendon suture technique was designed to function as a repair that permits immediate postoperative activation and mobilization of a transferred muscle. This study was designed to test the strength and stiffness of the SS technique against a variation of the Pulvertaft (PT) repair technique. METHODS Flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP) tendons were harvested from 4 fresh cadavers and used as a model system. Seven SS and 6 PT repairs were performed, using the FDS as the donor and the FDP as the recipient tendon. For SS repairs, the FDS was woven through one incision in the FDP and was joined with 4 cross-stitch running sutures down both sides and one double-loop suture at each tendon free end. For PT repairs, the FDS was woven through 3 incisions in the FDP and joined with a double-loop suture at both ends of the overlap and 4 evenly spaced mattress sutures between the ends. Tendon repairs were placed in a tensile testing machine, preconditioned, and tested to failure. RESULTS There were no statistically significant differences in cross-sectional area (p = .99) or initial length (p = .93) between SS and PT repairs. Therefore, all comparisons between methods were made using measures of loads and deformations, rather than stresses and strains. All failures occurred in the repair region, rather than at the clamps. However, failure mechanisms were different between the 2 techniques-PT repairs failed by the suture knots either slipping or pulling through the tendon material, followed by the FDS tendon pulling through the FDP tendon; SS repairs failed by shearing of fibers within the FDS. Load at first failure, ultimate load, and repair stiffness were all significantly different between SS and PT techniques; in all cases, the mean value for SS was higher than for PT. CONCLUSIONS The SS repair using a cross-stitch suture technique was significantly stronger and stiffer than the PT repair using a mattress suture technique. This suggests that using SS repairs could enable patients to load the repair soon after surgery. Ultimately, this should reduce the risk of developing adhesions and result in improved functional outcome and fewer complications in the acute postoperative period. Future work will address the specific mechanisms (eg, suture-throw technique and tendon-weave technique) that underlie the improved strength and stiffness of the SS repair.

MOMENT ARMS OF THE HUMAN DIGITAL FLEXORS

For the extrinsic hand flexors (flexor digitorum profundus, FDP; flexor digitorum superficialis, FDS; flexor pollicis longus, FPL), moment arm corresponds to the tendons distance from the center of the metacarpalphalangeal (MP), proximal interphalangeal (PIP), or distal interphalangeal (DIP) joint. The clinical value of establishing accurate moment arms has been highlighted for biomechanical modeling, the development of robotic hands, designing rehabilitation protocols, and repairing flexor tendon pulleys (Brand et al., 1975; An et al., 1983; Thompson and Giurintano, 1989; Deshpande et al., 2010; Wu et al., 2010). In this study, we define the moment arms for all of the extrinsic flexor tendons of the hand across all digital joints for all digits in cadaveric hands.

Anatomical, architectural, and biochemical diversity of the murine forelimb muscles.

We characterized the architecture, fiber type, titin isoform distribution, and collagen content of 27 portions of 22 muscles in the murine forelimb. The mouse forelimb was different from the human arm in that it had the extensor digitorum lateralis muscle and no brachioradialis muscle. Architecturally, the mouse forelimb differed from humans with regard to load bearing, having a much larger contribution from extensors than flexors. In mice, the extensor : flexor PCSA ratio is 2.7, whereas in humans it is only 1.4. When the architectural difference index was calculated, similarities became especially apparent between flexors and extensors of the distal forelimb, as well as pronators. Discriminant analysis revealed that biochemical measures of collagen, titin, and myosin heavy chain were all strong between‐species discriminators. In terms of composition, when compared with similar muscles in humans, mice had, on average, faster muscles with higher collagen content and larger titin isoforms. This report establishes the anatomical and biochemical properties of mouse forelimb muscles. Given the prevalence of this species in biological studies, these data will be invaluable for studying the biological basis of mouse muscle structure and function.

Functional Consequence of Distal Brachioradialis Tendon Release: A Biomechanical Study

PURPOSE Open reduction and internal fixation of distal radius fractures often necessitates release of the brachioradialis from the radial styloid. However, this common procedure has the potential to decrease elbow flexion strength. To determine the potential morbidity associated with brachioradialis release, we measured the change in elbow torque as a function of incremental release of the brachioradialis insertion footprint. METHODS In 5 upper extremity cadaveric specimens, we systematically released the brachioradialis tendon from the radius and measured the resultant effect on brachioradialis elbow flexion torque. We defined release distance as the distance between the release point and the tip of the radial styloid. RESULTS Brachioradialis elbow flexion torque dropped to 95%, 90%, and 86% of its original value at release distances of 27, 46, and 52 mm, respectively. Importantly, brachioradialis torque remained above 80% of its original value at release distances up to 7 cm. CONCLUSIONS Our data demonstrate that release of the brachioradialis tendon from its insertion has minor effects on its ability to transmit force to the distal radius. CLINICAL RELEVANCE These data imply that release of the distal brachioradialis tendon during distal radius open reduction internal fixation can be performed without meaningful functional consequences to elbow flexion torque. Even at large release distances, overall elbow flexion torque loss after brachioradialis release would be expected to be less than 5% because of the much larger contributions of the biceps and brachialis. Use of the brachioradialis as a tendon transfer donor should not be limited by concerns of elbow flexion loss, and the tendon could be considered as an autograft donor.

Regional Ulnar Nerve Strain Following Decompression and Anterior Subcutaneous Transposition in Patients With Cubital Tunnel Syndrome

PURPOSE Simple decompression and anterior subcutaneous transposition are effective surgical interventions for cubital tunnel syndrome and yield similarly favorable outcomes. However, a substantial proportion of patients demonstrate unsatisfactory outcomes for reasons that remain unclear. We compared effects of decompression and transposition on regional ulnar nerve strain to better understand the biomechanical impacts of each strategy. METHODS Patients diagnosed with cubital tunnel syndrome and scheduled for anterior subcutaneous transposition surgery were enrolled. Simple decompression, circumferential decompression, and anterior transposition of the ulnar nerve were performed during the course of the transposition procedure. Regional ulnar nerve strain around the elbow was measured for each surgical intervention based on 4 wrist and elbow joint configurations. RESULTS With elbow extension at 180°, both circumferential decompression and anterior transposition resulted in approximately 68% higher nerve strains than simple decompression. Conversely, with elbow flexion, simple decompression resulted in higher average strains than anterior transposition. Limited regional differences in strain were observed for any surgical intervention with elbow extension. However, with elbow flexion, strains were higher in distal and central regions compared with the proximal region within all surgical groups, and proximal region strain was higher after simple decompression compared with anterior transposition. CONCLUSIONS As predicted by the altered anatomic course, anterior transposition results in lower ulnar nerve strains than simple decompression during elbow flexion and higher nerve strains during elbow extension. Irrespective of anatomic course, circumferential release of paraneurial tissues may also influence nerve strain. Nerve strain varies regionally and is influenced by surgery and joint configuration. CLINICAL RELEVANCE Our data provide insight into how surgery resolves and redistributes traction on the ulnar nerve. These findings may help inform which surgical procedure to perform for a specific patient, guide rehabilitation protocols, and suggest regions of anatomic concern during index and revision surgery.