Recovery potential after postnatal shoulder paralysis. An animal model of neonatal brachial plexus palsy

Transient neonatal shoulder paralysis causes early osteoarthritis in a mouse model

Neonatal brachial plexus palsy (NBPP) occurs in approximately 1.5 of every 1,000 live births. The majority of children with NBPP recover function of the shoulder. However, the long-term risk of osteoarthritis (OA) in this population is unknown. The purpose of this study was to investigate the development of OA in a mouse model of transient neonatal shoulder paralysis. Neonatal mice were injected twice per week for 4 weeks with saline in the right supraspinatus muscle (Saline, control) and botulinum toxin A (BtxA, transient paralysis) in the left supraspinatus muscle, and then allowed to recover for 20 or 36 weeks. Control mice received no injections, and all mice were sacrificed at 24 or 40 weeks. BtxA mice exhibited abnormalities in gait compared to controls through 10 weeks of age, but these differences did not persist into adulthood. BtxA shoulders had decreased bone volume (-9%) and abnormal trabecular microstructure compared to controls. Histomorphometry analysis demonstrated that BtxA shoulders had higher murine shoulder arthritis scale scores (+30%), and therefore more shoulder OA compared to controls. Articular cartilage of BtxA shoulders demonstrated stiffening of the tissue. Compared with controls, articular cartilage from BtxA shoulders had 2-fold and 10-fold decreases in Dkk1 and BMP2 expression, respectively, and 3-fold and 14-fold increases in Col10A1 and BGLAP expression, respectively, consistent with established models of OA. In summary, a brief period of paralysis of the neonatal mouse shoulder was sufficient to generate early signs of OA in adult cartilage and bone.

The effect of mechanical stress on enthesis homeostasis in a rat Achilles enthesis organ culture model

Tendons and ligaments are jointed to bones via an enthesis that is essential to the proper function of the muscular and skeletal structures. The aim of the study is to investigate the effect of mechanical stress on the enthesis. We used ex vivo models in organ cultures of rat Achilles tendons with calcaneus including the enthesis. The organ was attached to a mechanical stretching apparatus that can conduct cyclic tensile strain. We made the models of 1-mm elongation (0.5 Hz, 3% elongation), 2-mm elongation (0.5 Hz, 5% elongation), and no stress. Histological evaluation by Safranin O staining and Toluidin Blue and Picro Sirius red staining was conducted. Expression of sex-determining region Y-box 9 (Sox9), scleraxis (Scx), Runt-related transcription factor 2 (Runx2), and matrix metalloproteinase 13 (Mmp13) were examined by real-time polymerase chain reaction and immunocytochemistry. Terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate biotin nick end-labeling and live/dead staining and was conducted for evaluation of the apoptosis and cell viability. The structure of the enthesis was most maintained in the model of 1-mm elongation. The electronic microscope showed that the enthesis of the no stress model had ill-defined borders between fibrocartilage and mineralized fibrocartilage, and that calcification of mineralized fibrocartilage occurred in the model of 2-mm elongation. Sox9 and Scx was upregulated by 1-mm elongation, whereas Runx2 and Mmp13 were upregulated by 2-mm elongation. Apoptosis was inhibited by low stress. The results of this study suggested that 1-mm elongation can maintain the structure of the enthesis, while 2-mm elongation promotes degenerative changes.

Experimental nerve transfer model in the neonatal rat

Clinically, peripheral nerve reconstructions in neonates are most frequently applied in brachial plexus birth injuries. Most surgical concepts, however, have investigated nerve reconstructions in adult animal models. The immature neuromuscular system reacts differently to the effects of nerve lesion and surgery and is poorly investigated due to the lack of reliable experimental models. Here, we describe an experimental forelimb model in the neonatal rat, to study these effects on both the peripheral and central nervous systems. Within 24 hours after birth, three groups were prepared: In the nerve transfer group, a lesion of the musculocutaneous nerve was reconstructed by selectively transferring the ulnar nerve. In the negative control group, the musculocutaneous nerve was divided and not reconstructed and in the positive control group, a sham surgery was performed. The animal´s ability to adapt to nerve lesions and progressive improvement over time were depict by the Bertelli test, which observes the development of grooming. Twelve weeks postoperatively, animals were fully matured and the nerve transfer successfully reinnervated their target muscles, which was indicated by muscle force, muscle weight, and cross sectional area evaluation. On the contrary, no spontaneous regeneration was found in the negative control group. In the positive control group, reference values were established. Retrograde labeling indicated that the motoneuron pool of the ulnar nerve was reduced following nerve transfer. Due to this post-axotomy motoneuron death, a diminished amount of motoneurons reinnervated the biceps muscle in the nerve transfer group, when compared to the native motoneuron pool of the musculocutaneous nerve. These findings indicate that the immature neuromuscular system behaves profoundly different than similar lesions in adult rats and explains reduced muscle force. Ultimately, pathophysiologic adaptations are inevitable. The maturing neuromuscular system, however, utilizes neonatal capacity of regeneration and seizes a variety of compensation mechanism to restore a functional extremity. The above described neonatal rat model demonstrates a constant anatomy, suitable for nerve transfers and allows all standard neuromuscular analyses. Hence, detailed investigations on the pathophysiological changes and subsequent effects of trauma on the various levels within the neuromuscular system as well as neural reorganization of the neonatal rat may be elucidated. This study was approved by the Ethics Committee of the Medical University of Vienna and the Austrian Ministry for Research and Science (BMWF-66.009/0187-WF/V/3b/2015) on March 20, 2015.

The Pathogenesis of Glenohumeral Deformity and Contracture Formation in Obstetric Brachial Plexus Palsy-A Review

Contractures of the shoulder joint and glenohumeral joint dysplasia are well known complications to obstetrical brachial plexus palsy. Despite extensive description of these sequelae, the exact pathogenesis remains unknown. The prevailing theory to explain the contractures and glenohumeral joint dysplasia states that upper trunk injury leads to nonuniform muscle recovery and thus imbalance between internal and external rotators of the shoulder. More recently, another explanation has been proposed, hypothesizing that denervation leads to reduced growth of developing muscles and that reinnervation might suppress contracture formation. An understanding of the pathogenesis is desirable for development of effective prophylactic treatment. This article aims to describe the current state of knowledge regarding these important complications.

Zebrafish as an Emerging Model for Osteoporosis: A Primary Testing Platform for Screening New Osteo-Active Compounds

Osteoporosis is metabolic bone disease caused by an altered balance between bone anabolism and catabolism. This dysregulated balance is responsible for fragile bones that fracture easily after minor falls. With an aging population, the incidence is rising and as yet pharmaceutical options to restore this imbalance is limited, especially stimulating osteoblast bone-building activity. Excitingly, output from large genetic studies on people with high bone mass (HBM) cases and genome wide association studies (GWAS) on the population, yielded new insights into pathways containing osteo-anabolic players that have potential for drug target development. However, a bottleneck in development of new treatments targeting these putative osteo-anabolic genes is the lack of animal models for rapid and affordable testing to generate functional data and that simultaneously can be used as a compound testing platform. Zebrafish, a small teleost fish, are increasingly used in functional genomics and drug screening assays which resulted in new treatments in the clinic for other diseases. In this review we outline the zebrafish as a powerful model for osteoporosis research to validate potential therapeutic candidates, describe the tools and assays that can be used to study bone homeostasis, and affordable (semi-)high-throughput compound testing.

What influences contracture formation in lower motor neuron disorders, severity of denervation or residual muscle function? An analysis of the elbow contracture in 100 children with unilateral Brachial Plexus Birth Injury

PURPOSE

As in other neuromuscular disorders, both denervation and muscle paresis/imbalance are implicated as aetiological factors for contractures in children with a Brachial Plexus Birth Injury (BPBI). Although both factors are related, it is unclear which factor is dominant. The aim of this study is to assess whether contracture formation in children is predominantly related to denervation or to residual muscle function/imbalance. This might be relevant for understanding contracture formation in other neuromuscular disorders.

METHODS

A total of 100 children (61 boys; mean age 10.4 years, 4 to 18) with unilateral BPBI were included in this cross-sectional study. Severity of the denervation was classified according to Narakas. Muscle function of flexors and extensors of both elbows was measured (in Newtons) using a hand-held dynamometer and flexion contractures were measured with a goniometer. The relation between denervation, muscle function/muscle balance and flexion contracture was assessed using univariate and multivariate analysis.

RESULTS

Of the children, 57 were Narakas class I, 13 class II and 30 class III. Mean flexion contracture was 25° (90° to -5°). At the affected side the forearm flexion force was 47% and extension force was 67% of the force of the unaffected side. Contractures were more severe in children with higher Narakas classifications (p = 0.001), after neurosurgery (Mann-Whitney U test, p = 0.009) and were related to age (Spearman's Rho = -0.3, p = 0.008) and to paresis of the extensors (Rho = 0.4, p = 0.000). Flexor paresis as a percentage of unaffected side (Rho = 0.06, p = 0.6) and muscle balance had no influence.

CONCLUSION

In BPBI, elbow contractures are related to the severity of the neurological lesion, not to residual muscle function.

LEVEL OF EVIDENCE

Level II - prognostic study.

Effect of Suture Absorbability on Rotator Cuff Healing in a Rabbit Rotator Cuff Repair Model

BACKGROUND

Various suture materials can be clinically used for rotator cuff repair (RCR). RCR with high-strength nonabsorbable sutures may not be ideal, because it may cause stress shielding, which may hinder enthesis regeneration and maturation in the tendon-bone interface. RCR with strength-decreasing sutures (ie, absorbable sutures) may be a better choice. However, the effects of suture absorbability on enthesis regeneration and maturation have not been investigated.

HYPOTHESIS

The use of absorbable sutures in RCR would produce a better tendon-bone connection structure, which provides histological and biomechanical advantages over the use of nonabsorbable sutures.

STUDY DESIGN

Controlled laboratory study.

METHODS

A supraspinatus tear was created on the right shoulder in 108 of 120 skeletally mature male rabbits. The animals were randomly divided into 3 groups, with 36 rabbits in each group, to undergo RCR individually with total absorbable, partial absorbable, and nonabsorbable sutures (TAS, PAS, and NAS). Twelve animals in each group were sacrificed at 4, 8, and 12 weeks after surgery, with 6 operated shoulders used for histological evaluation to detect enthesis regeneration and maturation and the other 6 for biomechanical testing. The remaining 12 animals without supraspinatus tear were used as control.

RESULTS

At 12 weeks, in the tendon-bone interface, enthesis regeneration was detected in the TAS group but not in the NAS group. A mature enthesis appeared in the TAS group but not in the NAS group. In the PAS group, enthesis regeneration was also observed; however, the fibrocartilage was not abundant and the enthesis maturity not good as compared with the TAS group. Biomechanical testing showed that the rotator cuff-greater tuberosity connection structure in the TAS and PAS groups had greater values of ultimate load to failure, stiffness, and stress than the NAS group at all time points.

CONCLUSION

In RCR in an acute rabbit rotator cuff tear model, the use of sutures with absorbability lead to enthesis regeneration, increased maturity of rotator cuff insertion, and enhanced rotator cuff-greater tuberosity connection.

CLINICAL RELEVANCE

Compared with the use of NAS, the use of TAS or PAS might be a better choice for RCR.

Expression of Signaling Molecules Involved in Embryonic Development of the Insertion Site Is Inadequate for Reformation of the Native Enthesis: Evaluation in a Novel Murine ACL Reconstruction Model

BACKGROUND

Since healing of anterior cruciate ligament (ACL) grafts occurs by formation of a fibrovascular scar-tissue interface rather than by reformation of the native fibrocartilage transition zone, the purpose of our study was to examine expression of various signaling molecules and transcription factors that are known to be involved in embryologic insertion-site development following ACL reconstruction. We also aimed to characterize a murine model of ACL reconstruction to allow future study of the molecular mechanisms of healing.

METHODS

Seventy-nine mice underwent reconstruction of the ACL with autograft. Healing was assessed using histology in 12 mice and quantitative real-time polymerase chain reaction (qRT-PCR) gene-expression analysis in 3 mice at 1 week postoperatively (Group-1 mice) and by biomechanical analysis in 7, histological analysis in 7, immunohistochemical analysis in 5, microcomputed tomography analysis in 5, and qRT-PCR analyses in 8 at 2 weeks (Group-2 mice) and 4 weeks (Group-3 mice) postoperatively. Fifteen additional mice did not undergo surgery and were used for biomechanical (7 mice), qRT-PCR (3 mice), and immunohistochemical (5 mice) analyses to obtain baseline data for the native ACL.

RESULTS

Histological analysis demonstrated healing by formation of fibrovascular tissue at the tendon-bone interface. Immunohistochemical analysis showed a positive expression of proteins in the Indian hedgehog, Wnt, and parathyroid hormone-related protein (PTHrP) pathways. There was minimal Sox-9 expression. Gene-expression analysis showed an initial increase in markers of tissue repair and turnover, followed by a subsequent decline. Mean failure force and stiffness of the native ACL were 5.60 N and 3.44 N/mm, respectively. Mean failure force and stiffness were 1.29 N and 2.28 N/mm, respectively, in Group 2 and were 1.79 N and 2.59 N/mm, respectively, in Group 3, with 12 of 14 failures in these study groups occurring by tunnel pull-out.

CONCLUSIONS

The spatial and temporal pattern of expression of signaling molecules that direct embryologic insertion-site formation was not adequate to restore the structure and composition of the native insertion site.

CLINICAL RELEVANCE

Development of a murine model to study ACL reconstruction will allow the use of transgenic animals to investigate the cellular, molecular, and biomechanical aspects of tendon-to-bone healing following ACL reconstruction, ultimately suggesting methods to improve healing in patients.

Next Generation Tissue Engineering of Orthopedic Soft Tissue-to-Bone Interfaces

Soft tissue-to-bone interfaces are complex structures that consist of gradients of extracellular matrix materials, cell phenotypes, and biochemical signals. These interfaces, called entheses for ligaments, tendons, and the meniscus, are crucial to joint function, transferring mechanical loads and stabilizing orthopedic joints. When injuries occur to connected soft tissue, the enthesis must be re-established to restore function, but due to structural complexity, repair has proven challenging. Tissue engineering offers a promising solution for regenerating these tissues. This prospective review discusses methodologies for tissue engineering the enthesis, outlined in three key design inputs: materials processing methods, cellular contributions, and biochemical factors.

Humeral Retroversion in Children with Shoulder Internal Rotation Contractures Secondary to Upper-Trunk Neonatal Brachial Plexus Palsy

BACKGROUND

The most common sequela of neonatal brachial plexus palsy is an internal rotation contracture of the shoulder that impairs function and leads to skeletal deformation of the glenohumeral joint. Treatment options include release, transfers, and humeral osteotomy, all ultimately striving for better function through increased external rotation. Prior studies have shown that neonatal brachial plexus palsy alters humeral retroversion but with conflicting findings. We studied retroversion in children with internal rotation contractures from neonatal brachial plexus palsy to clarify its effect on version and surgical planning.

METHODS

Bilateral shoulder and elbow magnetic resonance imaging scans of 21 children with neonatal brachial plexus palsy were retrospectively analyzed. Retroversion referenced to the transepicondylar line at the elbow was measured with respect to 2 different proximal reference axes, the longest diameter of an axial cut of the proximal part of the humerus (the skew axis) and the line perpendicular to the articular surface (the humeral center line). Glenoid version and glenohumeral morphology type (concentric glenoid, posterior-concentric glenoid, biconcave, or pseudoglenoid) were also determined. All geometric variables were assessed for correlation with patient age and the severity of the internal rotation contracture.

RESULTS

Retroversion on the involved side was decreased at 6° compared with 19° (p = 0.003), as measured between the skew axis and transepicondylar line. Retroversion referenced to the humeral center line was also decreased at -2° (anteversion) compared with 20° (p < 0.001). Patient age was inversely correlated with retroversion, but was only significant for the skew axis (r = -0.497, p = 0.022), decreasing in linear regression by 2.4° per year (p = 0.038). Humeral retroversion did not correlate with the severity of the internal rotation contracture, glenoid version, or glenoid morphology type.

CONCLUSIONS

Humeral retroversion is likely to be less on the affected side in children with internal rotation contractures from upper trunk neonatal brachial plexus palsy and merits consideration in surgical planning.

LEVEL OF EVIDENCE

Prognostic Level IV. See Instructions for Authors for a complete description of levels of evidence.

Scleraxis is required for the development of a functional tendon enthesis

The attachment of dissimilar materials is a major engineering challenge, yet this challenge is seemingly overcome in biology. This study aimed to determine how the transcription factor Scleraxis (Scx) influences the development and maturation of the tendon-to-bone attachment (enthesis). Mice with conditional knockout (cKO) for Scx (Scx(flx/-), Prx1Cre(+)) and wild-type [(WT) Scx(flx/+) or Scx(flx/flx)] littermates were killed at postnatal days 7-56 (P7-P56). Enthesis morphometry, histology, and collagen alignment were investigated throughout postnatal growth. Enthesis tensile mechanical properties were also assessed. Laser microdissection of distinct musculoskeletal tissues was performed at P7 for WT, cKO, and muscle-unloaded (botulinum toxin A treated) attachments for quantitative PCR. cKO mice were smaller, with altered bone shape and impaired enthesis morphology, morphometry, and organization. Structural alterations led to altered mechanical properties; cKO entheses demonstrated reduced strength and stiffness. In P7 attachments, cKO mice had reduced expression of transforming growth factor (TGF) superfamily genes in fibrocartilage compared with WT mice. In conclusion, deletion of Scx led to impairments in enthesis structure, which translated into impaired functional (i.e., mechanical) outcomes. These changes may be driven by transient signaling cues from mechanical loading and growth factors.

Osteogenic programs during zebrafish fin regeneration

Recent advances in genomic, screening and imaging technologies have provided new opportunities to examine the molecular and cellular landscape underlying human physiology and disease. In the context of skeletal research, technologies for systems genetics, high-throughput screening and high-content imaging can aid an unbiased approach when searching for new biological, pathological or therapeutic pathways. However, these approaches necessitate the use of specialized model systems that rapidly produce a phenotype, are easy to manipulate, and amenable to optical study, all while representing mammalian bone physiologies at the molecular and cellular levels. The emerging use of zebrafish (Danio rerio) for modeling human disease highlights its potential to accelerate therapeutic and pathway discovery in the mammalian skeleton. In this review, we consider the potential value of zebrafish fin ray regeneration (a rapid, genetically tractable and optically transparent model of intramembranous ossification) as a translational model for such studies.

Degree of Contracture Related to Residual Muscle Shoulder Strength in Children with Obstetric Brachial Plexus Lesions

BACKGROUND AND OBJECTIVES

 Little is known about the relation between residual muscle strength and joint contracture formation in neuromuscular disorders. This study aimed to investigate the relation between residual muscle strength and shoulder joint contractures in children with sequelae of obstetric brachial plexus lesion (OBPL). In OBPL a shoulder joint contracture is a frequent finding. We hypothesize that residual internal and external rotator strength and their balance are related to the extent of shoulder joint contracture.

METHODS

 Clinical assessment was performed in 34 children (mean 10.0 years) with unilateral OBPL and Narakas classes I-III. External and internal rotation strengths were measured with the shoulder in neutral position using a handheld dynamometer. Strength on the affected side was given as percentage of the normal side. Contracture was assessed by passive internal and external rotations in degrees (in 0° abduction). Mallet classification was used for active shoulder function.

RESULTS

 External and internal rotation strengths on the affected side were approximately 50% of the normal side and on average both equally affected: 56% (SD 18%) respectively 51% (SD 27%); r = 0.600, p = 0.000. Residual strengths were not related to passive internal or external rotation (p > 0.200). Internal rotation strength (r =  - 0.425, p <0.05) was related to Narakas class. Mallet score was related to external and internal rotation strengths (r = 0.451 and r = 0.515, respectively; p < 0.01).

CONCLUSION

 The intuitive notion that imbalances in residual muscle strength influence contracture formation cannot be confirmed in this study. Our results are of interest for the understanding of contracture formation in OBPL.

Biomechanical Basis of Shoulder Osseous Deformity and Contracture in a Rat Model of Brachial Plexus Birth Palsy

BACKGROUND

The purpose of this study was to investigate the relative contributions of two proposed mechanisms, strength imbalance and impaired longitudinal muscle growth, to osseous and postural deformity in a rat model of brachial plexus birth palsy (BPBP).

METHODS

Thirty-two Sprague-Dawley rat pups were divided into four groups on the basis of surgical interventions to induce a strength imbalance, impaired growth, both a strength imbalance and impaired growth (a combined mechanism), and a sham condition in the left forelimb. Maximum passive external shoulder rotation angle (ERmax) was measured bilaterally at four and eight weeks postoperatively. After the rats were killed at eight weeks, the glenohumeral geometry (on microcomputed tomography) and shoulder muscle architecture properties were measured bilaterally.

RESULTS

Bilateral muscle mass and optimal length differences were greatest in the impaired growth and combined mechanism groups, which also exhibited >15° lower ERmax (p < 0.05; four weeks postoperatively), 14° to 18° more glenoid declination (p < 0.10), and 0.76 to 0.94 mm more inferior humeral head translation (p < 0.10) on the affected side. Across all four groups, optimal muscle length was significantly correlated with at least one osseous deformity measure for six of fourteen muscle compartments crossing the shoulder on the affected side (p < 0.05). In the strength imbalance group, the glenoid was 5° more inclined and the humeral head was translated 7.5% more posteriorly on the affected side (p < 0.05).

CONCLUSIONS

Impaired longitudinal muscle growth and shoulder deformity were most pronounced in the impaired growth and combined mechanism groups, which underwent neurectomy. Strength imbalance was associated with osseous deformity to a lesser extent.

CLINICAL RELEVANCE

Treatments to alleviate shoulder deformity should address mechanical effects of both strength imbalance and impaired longitudinal muscle growth, with an emphasis on developing new treatments to promote growth in muscles affected by BPBP.

Tendon-to-bone attachment: from development to maturity

The attachment between tendon and bone occurs across a complex transitional tissue that minimizes stress concentrations and allows for load transfer between muscles and skeleton. This unique tissue cannot be reconstructed following injury, leading to high incidence of recurrent failure and stressing the need for new clinical approaches. This review describes the current understanding of the development and function of the attachment site between tendon and bone. The embryonic attachment unit, namely, the tip of the tendon and the bone eminence into which it is inserted, was recently shown to develop modularly from a unique population of Sox9- and Scx-positive cells, which are distinct from tendon fibroblasts and chondrocytes. The fate and differentiation of these cells is regulated by transforming growth factor beta and bone morphogenetic protein signaling, respectively. Muscle loads are then necessary for the tissue to mature and mineralize. Mineralization of the attachment unit, which occurs postnatally at most sites, is largely controlled by an Indian hedgehog/parathyroid hormone-related protein feedback loop. A number of fundamental questions regarding the development of this remarkable attachment system require further study. These relate to the signaling mechanism that facilitates the formation of an interface with a gradient of cellular and extracellular phenotypes, as well as to the interactions between tendon and bone at the point of attachment.

Botulinum toxin induces muscle paralysis and inhibits bone regeneration in zebrafish

Intramuscular administration of Botulinum toxin (BTx) has been associated with impaired osteogenesis in diverse conditions of bone formation (eg, development, growth, and healing), yet the mechanisms of neuromuscular-bone crosstalk underlying these deficits have yet to be identified. Motivated by the emerging utility of zebrafish (Danio rerio) as a rapid, genetically tractable, and optically transparent model for human pathologies (as well as the potential to interrogate neuromuscular-mediated bone disorders in a simple model that bridges in vitro and more complex in vivo model systems), in this study, we developed a model of BTx-induced muscle paralysis in adult zebrafish, and we examined its effects on intramembranous ossification during tail fin regeneration. BTx administration induced rapid muscle paralysis in adult zebrafish in a manner that was dose-dependent, transient, and focal, mirroring the paralytic phenotype observed in animal and human studies. During fin regeneration, BTx impaired continued bone ray outgrowth, morphology, and patterning, indicating defects in early osteogenesis. Further, BTx significantly decreased mineralizing activity and crystalline mineral accumulation, suggesting delayed late-stage osteoblast differentiation and/or altered secondary bone apposition. Bone ray transection proximal to the amputation site focally inhibited bone outgrowth in the affected ray, implicating intra- and/or inter-ray nerves in this process. Taken together, these studies demonstrate the potential to interrogate pathological features of BTx-induced osteoanabolic dysfunction in the regenerating zebrafish fin, define the technological toolbox for detecting bone growth and mineralization deficits in this process, and suggest that pathways mediating neuromuscular regulation of osteogenesis may be conserved beyond established mammalian models of bone anabolic disorders.

The role of muscle loading on bone (Re)modeling at the developing enthesis

Muscle forces are necessary for the development and maintenance of a mineralized skeleton. Removal of loads leads to malformed bones and impaired musculoskeletal function due to changes in bone (re)modeling. In the current study, the development of a mineralized junction at the interface between muscle and bone was examined under normal and impaired loading conditions. Unilateral mouse rotator cuff muscles were paralyzed using botulinum toxin A at birth. Control groups consisted of contralateral shoulders injected with saline and a separate group of normal mice. It was hypothesized that muscle unloading would suppress bone formation and enhance bone resorption at the enthesis, and that the unloading-induced bony defects could be rescued by suppressing osteoclast activity. In order to modulate osteoclast activity, mice were injected with the bisphosphonate alendronate. Bone formation was measured at the tendon enthesis using alizarin and calcein fluorescent labeling of bone surfaces followed by quantitative histomorphometry of histologic sections. Bone volume and architecture was measured using micro computed tomography. Osteoclast surface was determined via quantitative histomorphometry of tartrate resistant acid phosphatase stained histologic sections. Muscle unloading resulted in delayed initiation of endochondral ossification at the enthesis, but did not impair bone formation rate. Unloading led to severe defects in bone volume and trabecular bone architecture. These defects were partially rescued by suppression of osteoclast activity through alendronate treatment, and the effect of alendronate was dose dependent. Similarly, bone formation rate was increased with increasing alendronate dose across loading groups. The bony defects caused by unloading were therefore likely due to maintained high osteoclast activity, which normally decreases from neonatal through mature timepoints. These results have important implications for the treatment of muscle unloading conditions such as neonatal brachial plexus palsy, which results in shoulder paralysis at birth and subsequent defects in the rotator cuff enthesis and humeral head.

The developing shoulder has a limited capacity to recover after a short duration of neonatal paralysis

Mechanical stimuli are required for the proper development of the musculoskeletal system. Removal of muscle forces during fetal or early post-natal timepoints impairs the formation of bone, tendon, and their attachment (the enthesis). The goal of the current study was to examine the capacity of the shoulder to recover after a short duration of neonatal rotator cuff paralysis, a condition mimicking the clinical condition neonatal brachial plexus palsy. We asked if reapplication of muscle load to a transiently paralyzed muscle would allow for full recovery of tissue properties. CD-1 mice were injected with botulinum toxin A to paralyze the supraspinatus muscle from birth through 2 weeks and subsequently allowed to recover. The biomechanics of the enthesis was determined using tensile testing and the morphology of the shoulder joint was determined using microcomputed tomography and histology. A recovery period of at least 10 weeks was required to achieve control properties, demonstrating a limited capacity of the shoulder to recover after only two weeks of muscle paralysis. Although care must be taken when extrapolating results from an animal model to the human condition, the results of the current study imply that treatment of neonatal brachial plexus palsy should be aggressive, as even short periods of paralysis could lead to long-term deficiencies in enthesis biomechanics and shoulder morphology.

Computational sensitivity analysis to identify muscles that can mechanically contribute to shoulder deformity following brachial plexus birth palsy

PURPOSE

Two mechanisms, strength imbalance or impaired longitudinal muscle growth, potentially cause osseous and postural shoulder deformity in children with brachial plexus birth palsy. Our objective was to determine which muscles, via either deformity mechanism, were mechanically capable of producing forces that could promote shoulder deformity.

METHODS

In an upper limb computational musculoskeletal model, we simulated strength imbalance by allowing each muscle crossing the shoulder to produce 30% of its maximum force. To simulate impaired longitudinal muscle growth, the functional length of each muscle crossing the shoulder was reduced by 30%. We performed a sensitivity analysis to identify muscles that, through either simulated deformity mechanism, increased the posteriorly directed, compressive glenohumeral joint force consistent with osseous deformity or reduced the shoulder external rotation or abduction range of motion consistent with postural deformity.

RESULTS

Most of the increase in the posterior glenohumeral joint force by the strength imbalance mechanism was caused by the subscapularis, latissimus dorsi, and infraspinatus. Posterior glenohumeral joint force increased the most owing to impaired growth of the infraspinatus, subscapularis, and long head of biceps. Through the strength imbalance mechanism, the subscapularis, anterior deltoid, and pectoralis major muscles reduced external shoulder rotation by 28°, 17°, and 10°, respectively. Shoulder motion was reduced by 40° to 56° owing to impaired growth of the anterior deltoid, subscapularis, and long head of triceps.

CONCLUSIONS

The infraspinatus, subscapularis, latissimus dorsi, long head of biceps, anterior deltoid, pectoralis major, and long head of triceps were identified in this computational study as being the most capable of producing shoulder forces that may contribute to shoulder deformity following brachial plexus birth palsy.

CLINICAL RELEVANCE

The muscles mechanically capable of producing deforming shoulder forces should be the focus of experimental studies investigating the musculoskeletal consequences of brachial plexus birth palsy and are potentially critical targets for treating shoulder deformity.

Muscle loading is necessary for the formation of a functional tendon enthesis

Muscle forces are essential for skeletal patterning during development. Eliminating muscle forces, e.g., through paralysis, leads to bone and joint deformities. Botulinum toxin (BtxA)-induced paralysis of mouse rotator cuffs throughout postnatal development closely mimics neonatal brachial plexus palsy, a significant clinical condition in infants. In these mice, the tendon-to-bone attachment (i.e., the tendon enthesis) presents defects in mineral accumulation and fibrocartilage formation, presumably impairing the function of the tissue. The objective of the current study was to investigate the functional consequences of muscle unloading using BtxA on the developing supraspinatus tendon enthesis. We found that the maximum endurable load and stiffness of the supraspinatus tendon attachment decreased after four and eight weeks of post-natal BtxA-muscle unloading relative to controls. Tendon cross-sectional area was not significantly reduced by BtxA-unloading, while, strength, modulus, and toughness were decreased in the BtxA-unloaded group compared to controls, indicating a decrease in tissue quality. Polarized-light microscopy and Raman microprobe analysis were used to determine collagen fiber alignment and mineral characteristics, respectively, in the tendon enthesis that might contribute to the reduced biomechanical performance in BtxA-unloaded shoulders. Collagen fiber alignment was significantly reduced in BtxA-unloaded shoulders. The mineral-to-matrix ratio in mineralized fibrocartilage was not affected by loading. However, the crystallographic atomic order of the hydroxylapatite phase (a measure of crystallinity) was reduced and the amount of carbonate (substituting for phosphate) in the hydroxylapatite crystals was increased. Taken together, these micrometer-scale structural and compositional changes partly explain the observed decreases in the mechanical functionality of the tendon enthesis in the absence of muscle loading.

The effects of denervation, reinnervation, and muscle imbalance on functional muscle length and elbow flexion contracture following neonatal brachial plexus injury

The pathophysiology of paradoxical elbow flexion contractures following neonatal brachial plexus injury (NBPI) is incompletely understood. The current study tests the hypothesis that this contracture occurs by denervation-induced impairment of elbow flexor muscle growth. Unilateral forelimb paralysis was created in mice in four neonatal (5-day-old) BPI groups (C5-6 excision, C5-6 neurotomy, C5-6 neurotomy/repair, and C5-T1 global excision), one non-neonatal BPI group (28-day-old C5-6 excision), and two neonatal muscle imbalance groups (triceps tenotomy ± C5-6 excision). Four weeks post-operatively, motor function, elbow range of motion, and biceps/brachialis functional lengths were assessed. Musculocutaneous nerve (MCN) denervation and reinnervation were assessed immunohistochemically. Elbow flexion motor recovery and elbow flexion contractures varied inversely among the neonatal BPI groups. Contracture severity correlated with biceps/brachialis shortening and MCN denervation (relative axon loss), with no contractures occurring in mice with MCN reinnervation (presence of growth cones). No contractures or biceps/brachialis shortening occurred following non-neonatal BPI, regardless of denervation or reinnervation. Neonatal triceps tenotomy did not cause contractures or biceps/brachialis shortening, nor did it worsen those following neonatal C5-6 excision. Denervation-induced functional shortening of elbow flexor muscles leads to variable elbow flexion contractures depending on the degree, permanence, and timing of denervation, independent of muscle imbalance.

Could muscle deformity in children with spastic cerebral palsy be related to an impairment of muscle growth and altered adaptation?

Skeletal muscle deformity is common in children with spastic cerebral palsy (CP), but the underlying mechanisms are unclear. This review explores some possible factors which may influence the development of muscle deformity in CP. Normal muscle function and growth appear to depend on the interaction of neuronal, endocrinal, nutritional, and mechanical factors, and also on the development of an appropriate balance between muscle protein synthesis and degradation, and between the development of contractile and non-contractile components. In this context, the changes seen in muscle in children with CP are reviewed and discussed. It is suggested that the development of muscle deformity in children with CP may be related to a multifactorial impairment of muscle growth, on which adaptation of the extracellular matrix due to altered loading may be imposed.

The effect of muscle paralysis using Botox on the healing of tendon to bone in a rat model

HYPOTHESIS

Despite good clinical results after rotator cuff repair, follow-up studies show significant rates of failed healing. This may be because of excessive tension on the repaired tendon due to shoulder motion. We hypothesized that botulinum toxin A injections would result in improved attachment strength and collagen organization at the tendon-bone interface at early time points but may result in decreased mechanical properties at later time points because of the negative effects of stress deprivation.

MATERIALS AND METHODS

We performed division and repair of the supraspinatus tendon in 132 rats: 66 underwent repair alone and 66 received injections of botulinum toxin into the muscle before repair. Rats were killed at 4, 8, and 24 weeks and were evaluated by use of histologic, biomechanical, and micro-computed tomography analyses.

RESULTS

At 4 and 24 weeks, there was no significant difference in load to failure between groups. At 8 weeks, the botulinum group had a significantly lower load to failure compared with controls (27.7 N vs 46.7 N, P < .01). The weight of the supraspinatus muscle was significantly decreased at 4 and 8 weeks in the botulinum group, but it recovered by 24 weeks. Micro-computed tomography analysis showed the botulinum group to have significantly less bone volume, total mineral content, and total mineral density at 8 weeks. Histologic analysis showed formation of a more normal tidemark and increased collagen fiber organization in the botulinum specimens at 4 weeks.

DISCUSSION

Botulinum toxin A-treated specimens had increased collagen fiber organization at 4 weeks and decreased mechanical properties at later time points. The rapid healing of the rat rotator cuff likely makes it difficult to realize benefits from reduction in strain.

Impaired growth of denervated muscle contributes to contracture formation following neonatal brachial plexus injury

BACKGROUND

The etiology of shoulder and elbow contractures following neonatal brachial plexus injury is incompletely understood. With use of a mouse model, the current study tests the novel hypothesis that reduced growth of denervated muscle contributes to contractures following neonatal brachial plexus injury.

METHODS

Unilateral brachial plexus injuries were created in neonatal mice by supraclavicular C5-C6 nerve root excision. Shoulder and elbow range of motion was measured four weeks after injury. Fibrosis, cross-sectional area, and functional length of the biceps, brachialis, and subscapularis muscles were measured over four weeks following injury. Muscle satellite cells were cultured from denervated and control biceps muscles to assess myogenic capability. In a comparison group, shoulder motion and subscapularis length were assessed following surgical excision of external rotator muscles.

RESULTS

Shoulder internal rotation and elbow flexion contractures developed on the involved side within four weeks following brachial plexus injury. Excision of the biceps and brachialis muscles relieved the elbow flexion contractures. The biceps muscles were histologically fibrotic, whereas fatty infiltration predominated in the brachialis and rotator cuff muscles. The biceps and brachialis muscles displayed reduced cross-sectional and longitudinal growth compared with the contralateral muscles. The upper subscapularis muscle similarly displayed reduced longitudinal growth, with the subscapularis shortening correlating with internal rotation contracture. However, excision of the external rotators without brachial plexus injury caused no contractures or subscapularis shortening. Myogenically capable satellite cells were present in denervated biceps muscles despite impaired muscle growth in vivo.

CONCLUSIONS

Injury of the upper trunk of the brachial plexus leads to impaired growth of the biceps and brachialis muscles, which are responsible for elbow flexion contractures, and impaired growth of the subscapularis muscle, which correlates with internal rotation contracture of the shoulder. Shoulder muscle imbalance alone causes neither subscapularis shortening nor internal rotation contracture. Impaired muscle growth cannot be explained solely by absence of functioning satellite cells.

Effects of botulinum toxin-induced paralysis on postnatal development of the supraspinatus muscle

The mechanical environment plays an important role in musculoskeletal tissue development. The present study characterized changes in supraspinatus muscle due to removal of mechanical cues during postnatal development. An intramuscular injection of botulinum toxin type A (BTX) was used to induce and maintain paralysis in the left shoulders of mice since birth while the right shoulders received saline and served as contralateral controls. A separate group of animals was allowed to develop normally without any injections. Muscles were examined postnatally at various time points. The maximum isometric tetanic force generated by the muscle was significantly reduced in the BTX group compared to saline and normal groups. The paralyzed muscles were smaller and showed significant muscle atrophy and fat accumulation on histologic evaluation. Myogenic genes myogenin, myoD1, myf5, myf6, and fast type II myosin heavy chain (MHC) isoform were significantly upregulated while slow type I MHC isoform was significantly downregulated in the BTX group. Adipogenic genes C/EBPα, PPARγ2, leptin, and lipoprotein lipase were significantly upregulated in the BTX group. Results indicate that reduced muscle loading secondary to BTX-induced paralysis leads to fat accumulation and muscle degeneration in the developing muscle. Understanding the molecular and compositional changes in developing supraspinatus muscles may be useful for identifying and addressing the pathological changes that occur in shoulder injuries such as neonatal brachial plexus palsy.

Musculoskeletal deformities secondary to neurotomy of the superior trunk of the brachial plexus in neonatal mice

The developmental course of musculoskeletal deformities in neonatal brachial plexus palsy (NBPP) has not been studied extensively. The goals of this study were to: (1) evaluate a new animal model of NBPP, (2) characterize the development of musculoskeletal abnormalities in paralyzed shoulders, and (3) investigate the expression of myogenic and adipogenic genes in paralyzed rotator cuff muscles. Neonatal mice were divided into neurotomy and sham groups. The neurotomy group underwent surgical transection of the superior trunk of the brachial plexus within 24 h of birth. The sham group underwent the same surgical exposure, but the brachial plexus was left intact. Musculoskeletal deformities were evaluated with radiological and histological assays at 2, 4, 8, 12, and 30 weeks after birth. The supraspinatus muscles of a separate group of mice were used to examine expression of myogenic and adipogenic genes at 8 weeks. The neurotomized forelimbs developed deformities similar to those seen in human NBPP. The deformities progressed with age. The denervated supraspinatus muscles showed intramuscular fat accumulation and upregulation of both myogenic and adipogenic genes compared to the normal. The current study presents a useful animal model for future research examining musculoskeletal changes secondary to neonatal nerve injury.

Ossification of the proximal humerus in children with residual brachial plexus birth palsy: a magnetic resonance imaging study

BACKGROUND

Children with residual brachial plexus birth palsy may develop deformities of the humeral head and the glenoid. Surgical treatment has been described to maximize function and to lessen deformity by means of glenohumeral remodeling. The timing and technique of surgical intervention have not been resolved. The timing of the appearance on magnetic resonance imaging of the ossific nuclei in patients with brachial plexus birth palsy has not been described. We examined the timing of ossification about the proximal humerus. Our hypothesis was that the timing of ossification of the proximal humerus differs between the side involved in brachial plexus palsy and the uninvolved side.

METHODS

A retrospective study was performed of 117 children with residual brachial plexus birth palsy (aged 5 mo to 10 y) who had magnetic resonance imaging scans of the bilateral shoulders performed at our institution between 2000 and 2007. All axial slices were reviewed by a single observer for evidence of ossification of the humeral head epiphysis, the greater tuberosity, and the lesser tuberosity, as well as for evidence that the 3 were coalesced. Statistical analysis was performed to compare the involved and uninvolved sides.

RESULTS

The appearance of the greater tuberosity ossific nucleus on the involved side was significantly delayed. There was a trend towards delay in the appearance of the lesser tuberosity ossific nucleus and the coalescence of the 3 ossific nuclei on the involved side. The duration during which ossification of the involved brachial plexus side occurred was generally shorter compared with the uninvolved side.

CONCLUSION

There is delay in the ossification of the involved side in brachial plexus birth palsy, and a shorter duration of ossification.