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 reconstructive strategy for improving elbow function in late obstetric brachial plexus palsy.

Children with previously untreated obstetric brachial plexus palsy frequently have abnormal elbow function because of motor recovery with aberrant reinnervation, or because of paresis or paralysis. From 1988 to 1997 (9-year period), 62 children with obstetric brachial plexus palsy with resulting elbow deformity underwent various methods of palliative reconstruction to improve elbow function. For motor recovery with aberrant reinnervation, release of aberrantly reinnervated antagonistic muscles and augmentation of paretic muscles form the basis of surgical intervention. The surgical procedures included triceps-to-biceps transfer, biceps-to-triceps transfer, brachialis-to-triceps transfer, or combined biceps- and brachialis-to-triceps transfer. Choice of procedures was individualized and randomly determined on the basis of the degree and pattern of aberrant reinnervation between elbow flexors and extensors. In patients' motor recovery with paresis or paralysis, persistently weak elbow flexion was salvaged with a functioning free muscle transplantation or Steindler's flexorplasty, or regional shoulder muscle transfer. In addition, patients with aberrant reinnervation between shoulder abductors and elbow flexors underwent anterior deltoid-to-biceps transfer with a fascia lata graft. All patients had a minimum follow-up of 2 years. Results are assessed and discussed and a reconstructive algorithm is recommended. In general, reconstruction of elbow extension should precede that of elbow flexion. Biceps-to-triceps transfer with preservation of an intact brachialis muscle, or brachialis-to-triceps transfer with preservation of an intact biceps, allows 50 percent of these patients to achieve acceptable elbow flexion and extension in a single-stage procedure.     

Surgical strategy for improving forearm and hand function in late obstetric brachial plexus palsy

For the purpose of treatment, obstetric brachial plexus palsy can be subdivided into two distinct phases: initial obstetric brachial plexus palsy, and late obstetric brachial plexus palsy. In the latter, nerve surgery is no longer practical, and treatment often requires palliative surgery to improve function of the shoulder, elbow, forearm, and hand. Late obstetric brachial plexus palsy in the forearm and hand includes weakness or absence of wrist or metacarpophalangeal or interphalangeal joint extension; weakness or absence of finger flexion; forearm supination, or less commonly pronation contracture; ulnar deviation of the wrist; dislocation of the radial or ulnar head; thumb instability; or sensory disturbance of the hand. Palliative reconstruction for these forearm and hand manifestations is more difficult than for the shoulder or elbow because of the lack of powerful regional muscles for transfer. This report reviews the authors' experience performing more than 100 surgical procedures in 54 patients over a 9-year period (between 1988 and 1997) with a minimum of 2 years' follow-up. Surgical treatment is highly individualized, but the optimal age for forearm and hand reconstruction is usually later than for shoulder and elbow reconstruction because of the requirement for a preoperative exercise program. Multiple procedures for forearm and hand function were often performed on any given patient. Frequently, these were done simultaneously with reconstructive procedures for improving shoulder and/or elbow function. Traditional tendon transfer techniques do not provide satisfactory reconstruction for those deformities. Many of the authors' patients required more complex techniques such as nerve transfer and functioning free-muscle transplantation to augment traditional techniques of tendon and/or bone management. Sensory disturbance of the forearm and hand in late obstetric brachial plexus palsy seems a minor problem and further sensory reconstruction is unnecessary.     

Clinical and neuropathological study about the neurotization of the suprascapular nerve in obstetric brachial plexus lesions

Background

The lack of recovery of active external rotation of the shoulder is an important problem in children suffering from brachial plexus lesions involving the suprascapular nerve. The accessory nerve neurotization to the suprascapular nerve is a standard procedure, performed to improve shoulder motion in patients with brachial plexus palsy.

Methods

We operated on 65 patients with obstetric brachial plexus palsy (OBPP), aged 5-35 months (average: 19 months). We assessed the recovery of passive and active external rotation with the arm in abduction and in adduction. We also looked at the influence of the restoration of the muscular balance between the internal and the external rotators on the development of a gleno-humeral joint dysplasia. Intraoperatively, suprascapular nerve samples were taken from 13 patients and were analyzed histologically.

Results

Most patients (71.5%) showed good recovery of the active external rotation in abduction (60°-90°). Better results were obtained for the external rotation with the arm in abduction compared to adduction, and for patients having only undergone the neurotization procedure compared to patients having had complete plexus reconstruction. The neurotization operation has a positive influence on the glenohumeral joint: 7 patients with clinical signs of dysplasia before the reconstructive operation did not show any sign of dysplasia in the postoperative follow-up.

Conclusion

The neurotization procedure helps to recover the active external rotation in the shoulder joint and has a good prevention influence on the dysplasia in our sample. The nerve quality measured using histopathology also seems to have a positive impact on the clinical results.     

Motor grading of elbow flexion – is Medical Research Council grading good enough?

We present a new surgical technique for a pedicled teres major muscle transfer to improve shoulder abduction and flexion in children with sequelae of obstetric brachial plexus palsy. In addition, we provide the clinical outcome in the first 17 operated children.     

Computational analysis of glenohumeral joint growth and morphology following a brachial plexus birth injury

Children affected with brachial plexus birth injury (BPBI) undergo muscle paralysis. About 33% of affected children experience permanent osseous deformities of the glenohumeral joint. Recent evidence suggests that some cases experience restricted muscle longitudinal growth in addition to paralysis and reduced range of motion at the shoulder and elbow. It is unknown whether altered loading due to paralysis, muscle growth restriction and contracture, or static loading due to disuse is the primary driver of joint deformity after BPBI. This study uses a computational framework integrating finite element analysis and musculoskeletal modeling to examine the mechanical factors contributing to changes in bone growth and morphometry following BPBI. Simulations of 8 weeks of glenohumeral growth in a rat model of BPBI predicted that static loading of the joint is primarily responsible for joint deformation consistent with experimental measures of bone morphology, whereas dynamic loads resulted in normal bone growth. Under dynamic loading, glenoid version angle (GVA), glenoid inclination angle (GIA), and glenoid radius of curvature (GRC) (−1.3°, 38.2°, 2.5 mm respectively) were similar to the baseline values (−1.8°, −38°, 2.1 mm respectively). In the static case with unrestricted muscle growth, these measures increased in magnitude (5.2°, −48°, 3.5 mm respectively). More severe joint deformations were observed in GIA and GRC when muscle growth was restricted (GVA: 3.6°, GIA: −55°, GRC: 4.0 mm). Predicted morphology was consistent with literature reports of in vivo glenoid morphology following postganglionic BPBI. This growth model provides a framework for understanding the most influential mechanical factors driving glenohumeral deformity following BPBI.     

The outcome of Erb's palsy when the decision to operate is made at 4 months of age

Since the establishment of the obstetrical brachial plexus clinic in Saudi Arabia, the author has designed a prospective study in which the indication for brachial plexus surgery in infants with Erb's palsy was the lack of active elbow flexion against gravity at 4 months of age. Forty-three infants were included in the study and were distributed among four groups: group A (n = 20) included infants who had active elbow flexion at the initial assessment or at 2 months of age; group B (n = 9) included infants who had active elbow flexion at 3 months of age; group C (n = 11) included infants who had active elbow flexion at 4 months of age; and group D (n = 3) included infants who did not have active elbow flexion at 4 months of age. At the final follow-up, all children in group A demonstrated complete spontaneous recovery of the motor power of the limb. All children in group B also had satisfactory spontaneous recoveries, and none required secondary corrective procedures. Five of the 11 patients in group C had satisfactory spontaneous recoveries. The remaining six children in group C had good recovery of elbow flexion but a poor recovers of shoulder function. Finally, all three patients who did not have elbow flexion at 4 months of age (group D) underwent surgical exploration and reconstruction of the brachial plexus, using nerve grafts. The results of this prospective study are discussed, along with the controversial issue regarding the timing of primary plexus surgery in Erb's palsy.     

Direct cord implantation in brachial plexus avulsions: revised technique using a single stage combined anterior (first) posterior (second) approach and end-to-side side-to-side grafting neurorrhaphy

Background

The superiority of a single stage combined anterior (first) posterior (second) approach and end-to-side side-to-side grafting neurorrhaphy in direct cord implantation was investigated as to providing adequate exposure to both the cervical cord and the brachial plexus, as to causing less tissue damage and as to being more extensible than current surgical approaches.

Methods

The front and back of the neck, the front and back of the chest up to the midline and the whole affected upper limb were sterilized while the patient was in the lateral position; the patient was next turned into the supine position, the plexus explored anteriorly and the grafts were placed; the patient was then turned again into the lateral position, and a posterior cervical laminectomy was done. The grafts were retrieved posteriorly and side grafted to the anterior cord. Using this approach, 5 patients suffering from complete traumatic brachial plexus palsy, 4 adults and 1 obstetric case were operated upon and followed up for 2 years. 2 were C5,6 ruptures and C7,8T1 avulsions. 3 were C5,6,7,8T1 avulsions. C5,6 ruptures were grafted and all avulsions were cord implanted.

Results

Surgery in complete avulsions led to Grade 4 improvement in shoulder abduction/flexion and elbow flexion. Cocontractions occurred between the lateral deltoid and biceps on active shoulder abduction. No cocontractions occurred after surgery in C5,6 ruptures and C7,8T1 avulsions, muscle power improvement extended into the forearm and hand; pain disappeared.

Limitations include

spontaneous recovery despite MRI appearance of avulsions, fallacies in determining intraoperative avulsions (wrong diagnosis, wrong level); small sample size; no controls rule out superiority of this technique versus other direct cord reimplantation techniques or other neurotization procedures; intra- and interobserver variability in testing muscle power and cocontractions.

Conclusion

Through providing proper exposure to the brachial plexus and to the cervical cord, the single stage combined anterior (first) and posterior (second) approach might stimulate brachial plexus surgeons to go more for direct cord implantation. In this study, it allowed for placing side grafts along an extensive donor recipient area by end-to-side, side-to-side grafting neurorrhaphy and thus improved results.

Level of evidence

Level IV, prospective case series.     

Brachial plexus injury.

A 28-year-old man shot himself in the left posterior triangle of the neck with a shotgun. At the initial operation secondary repair of the resultant brachial plexus injury was decided upon in view of the difficulty in assessing lesions in continuity at this point after injury. The patient had total brachial plexus palsy. Nine weeks after the injury sensory and motor function were returning and the only element of the brachial plexus not showing evidence of nerve fibre continuity was the musculocutaneous nerve. Sural nerve autografts were sutured between the trimmed proximal and distal stumps of this nerve. By 4 months after the injury there was further improvement in both sensory and motor function, and by 18 months there was sensation in the autonomous zones of both median and ulnar nerves and good return of muscle power.     

三通道电刺激仪的研制及对臂丛神经损伤的临床疗效

目的:研制可用于臂丛神经损伤治疗的三通道电刺激仪,并且将之应用于临床臂丛神经损伤患者,观察该仪器治疗臂丛神经损伤的临床效果。方法:由主控模块、显示模块、键盘模块、三个通道的电刺激发生器模块以及电源模块组成系统,可以连续交替释放脉冲刺激,针对不同神经和肌肉,选择不同的刺激位点。将60例臂丛神经损伤术后的患者随机分成试验组(30例)和对照组(30例),试验组术后第三周使用三通道电刺激仪治疗,对照组不做处理,患者术后随访6-12月后,观察患者上肢肩部、肘部功能恢复情况。结果:试验组治疗后上臂丛、全臂丛、下臂丛的肩部、肘部功能均好于治疗前,差异明显,均有统计学意义(P0.05);试验组上臂丛、全臂丛、下臂丛的肩部、肘部治疗效果均显著优于对照组,差异有统计学意义(P0.05)。结论:三通道电刺激仪可以有效地促进臂丛神经损伤后上肢功能的康复,可以对三组神经和肌肉交替进行电刺激,使用方便,并且便于携带,患者较为满意。     

Radial nerve palsy in the newborn: a case series

Background:

Peripheral nerve palsies of the upper extremities presenting at birth can be distressing for families and care providers. It is therefore important to be able to identify patients whose diagnosis is compatible with full recovery so that their families can be reassured.

Methods:

We conducted a retrospective review of all infants presenting with weakness of the upper extremity to our clinic between July 1995 and September 2009. We also conducted a review of the current literature.

Results:

During the study period, 953 infants presented to our clinic. Of these patients, 25 were identified as having isolated radial nerve palsy (i.e., a radial nerve palsy in isolation with good shoulder function and intact flexion of the elbow). Seventeen infants (68.0%) had a subcutaneous nodule representing fat necrosis in the inferior posterolateral portion of the affected arm. Full recovery occurred in all patients within a range of one week to six months, and 72.0% of the patients (18/25) had fully recovered by the time they were two months old.

Interpretation:

Although the outcome of obstetrical brachial plexus palsy is highly variable, isolated radial nerve palsy in the newborn carries a uniformly favourable prognosis.Peripheral nerve palsies in the newborn are uncommon and usually involve the brachial plexus or facial nerve.¹ Isolated radial nerve palsy in the newborn is a rare phenomenon and may be misdiagnosed as obstetrical brachial plexus palsy. It is important to differentiate between these two conditions, as the management and prognosis of each diagnosis are different. Over the last three decades, multiple reports of small numbers of patients with radial nerve palsy present at birth have been published.^2–6 This paper presents the largest single series of this lesion currently documented in the literature.     

Modeling mechanical strains and stresses in soft tissues of the shoulder during load carriage based on load-bearing open MRI

Shoulder strain is a major limiting factor associated with load carriage. Despite advances in backpack designs, there are still reports of shoulder discomfort, loss of sensorimotor functions, and brachial plexus syndrome. The current study is aimed at characterizing mechanical loading conditions (strains and stresses) that develop within the shoulder's soft tissues when carrying a backpack. Open MRI scans were used for reconstructing a three-dimensional geometrical model of an unloaded shoulder and for measuring the soft tissue deformations caused by a 25-kg backpack; subsequently, a subject-specific finite element (FE) model for nonlinear, large-deformation stress-strain analyses was developed. Skin pressure distributions under the backpack strap were used as reference data and for verifying the numerical solutions. The parameters of the model were adjusted to fit the calculated tissue deformations to those obtained by MRI. The MRI scans revealed significant compression of the soft tissues of the shoulder, with substantial deformations in the area of the subclavian muscle and the brachial plexus. The maximal pressure values exerted by a 25-kg load were substantial and reached ～90 kPa. In the muscle surrounding the brachial plexus, the model predicted maximal compressive strain of 0.14 and maximal tensile strain of 0.13, which might be injurious for the underlying neural tissue. In conclusion, the FE model provided some insights regarding the potential mechanisms underlying brachial plexus injuries related to load carriage. The large tissue deformations and pressure hotspots that were observed are likely to result in tissue damage, which may hamper neural function if sustained for long time exposures.     

Significance of shoulder function in the reconstruction of prehension with double free-muscle transfer after complete paralysis of the brachial plexus

Reconstruction of shoulder stability and movement in cases with complete paralysis of the brachial plexus was performed to improve the outcomes for universal function of prehension after double free-muscle transfer (Doi's procedure). In cases in which the C5 or C6 nerve root was available as a donor, neurotization of the supra-scapular nerve was performed with a nerve graft. If the C5 or C6 nerve root was not available, then the contralateral C7 nerve root was chosen as the donor motor nerve and was transferred to the suprascapular nerve by using a vascularized ulnar nerve graft. Seven cases with ipsilateral C4, C5, or C6 nerve root transfer to the suprascapular nerve and one with contralateral C7 transfer were evaluated, and the functional outcomes for the range of shoulder motion were compared with those for patients who had undergone arthrodesis of the humeroscapular joint or had undergone no procedures for shoulder function reconstruction. The patients who underwent supra-scapular nerve repair demonstrated statistically significantly better ranges of motion for flexion and abduction of the shoulder, compared with the other two groups. Shoulder function is important for achieving prehensile function among patients with complete paralysis of brachial function, when they undergo double free-muscle transfer.     

Prognosis and early management of birth injuries to the brachial plexus.

Twenty-four cases of birth injury to the brachial plexus were seen in 21 infants over 15 years. Obstetric complications were common, and in 11 cases traction was needed to deliver the shoulders. Three out of every four arms fully recovered after exercises. Splints were not needed. Reconstructive procedures were performed on three permanently paralysed arms when the children were aged about 4. There was no way of predicting which patients would recover. The muscles supplied by the suprascapular nerve were paralysed in all patients, and this paralysis persisted in those whose arms failed to recover fully.     

Qualitative dermatoglyphic traits in brachial plexus palsy

It has been considered for many years that the cause of perinatal brachial plexus palsy (PBPP) is excessive lateral traction applied to the fetal head at delivery, in association with anterior shoulder dystocia, but this do not explain all cases of brachial plexus palsy. The incidence found in several family members could be suggestive for inheritance with variable expression. The aim of this study was to prove early found confirmations of genetic predisposition for PBPP In the previous studies, the quantitative dermatoglyphic analysis showed some differences in digito-palmar dermatoglyphs between patients with PBPP and healthy controls. Now this qualitative analysis will try to determine hereditary of those diseases. We analyzed digito-palmar dermatoglyphics from 140 subjects (70 males and 70 females) diagnosed with PBPP and 400 phenotypically healthy adults (200 males and 200 females) from Zagreb area as control group. The results of Chi-square test showed statistically significant differences for frequencies of patterns on fingers in females between the groups observed. Statistically significant differences were found on palms in III and IV interdigital areas in both males and females and in thenar and I interdigital area only in females. As it was found in previous researches on quantitative dermatoglyphic traits, more differences are found between females with PBPP and control group, than between males. The fact, that the main presumed cause of PBPP is obstetrical trauma, it could be associated with congenital variability in formation of brachial plexus.     

Motion Analytics of Trapezius Muscle Activity in an 18-Year-Old Female with Extended Upper Brachial Plexus Birth Palsy

Background  The trapezius muscle is often utilized as a muscle or nerve donor for repairing shoulder function in those with brachial plexus birth palsy (BPBP). To evaluate the native role of the trapezius in the affected limb, we demonstrate use of the Motion Browser, a novel visual analytics system to assess an adolescent with BPBP. Method  An 18-year-old female with extended upper trunk (C5–6–7) BPBP underwent bilateral upper extremity three-dimensional motion analysis with Motion Browser. Surface electromyography (EMG) from eight muscles in each limb which was recorded during six upper extremity movements, distinguishing between upper trapezius (UT) and lower trapezius (LT). The Motion Browser calculated active range of motion (AROM), compiled the EMG data into measures of muscle activity, and displayed the results in charts. Results  All movements, excluding shoulder abduction, had similar AROM in affected and unaffected limbs. In the unaffected limb, LT was more active in proximal movements of shoulder abduction, and shoulder external and internal rotations. In the affected limb, LT was more active in distal movements of forearm pronation and supination; UT was more active in shoulder abduction. Conclusion  In this female with BPBP, Motion Browser demonstrated that the native LT in the affected limb contributed to distal movements. Her results suggest that sacrificing her trapezius as a muscle or nerve donor may affect her distal functionality. Clinicians should exercise caution when considering nerve transfers in children with BPBP and consider individualized assessment of functionality before pursuing surgery.     

斜角肌间臂丛阻滞影响心率变异性的危险因素分析

摘要 目的：本研究旨在探讨斜角肌间臂丛阻滞影响心率变异性的危险因素及Logistic回归分析。方法：选取2021年5月至2022年3月于我院接受肩关节开放手术治疗的患者60例作为研究对象,按照麻醉方式不同随机分为实验组和对照组2组,其中实验组患者给与超声引导下斜角肌间臂丛神经阻滞,对照组患者进行单纯斜角肌间臂丛神经阻滞,统计患者的一般临床资料,对比分析阻滞前后心率变异指标、SpO₂、心率和平均动脉压水平;同时研究危险因素与心率变异性之间的关系。结果：斜角肌间臂丛阻滞影响心率变异性的单因素分析显示,危险因素与性别、平均年龄、BMI、总胆固醇、甘油三酯、高密度脂蛋白、低密度脂蛋白、合并慢性病（糖尿病、高血压）、吸烟史和饮酒史无关（P>0.05）,镇痛维持时间较短、运动恢复时间较长、阻断节数较少的斜角肌间臂丛阻滞患者更易表现出心率变异性（P<0.05）。阻滞后,实验组LF、HF、LF/HF、RMSSD、SDNN和PNN50 %显著低于对照组（P<0.05）,而阻滞后SpO₂、心率和平均动脉压比较无差异（P>0.05）。影响心率变异性危险因素的多因素为镇痛维持时间（OR=6.224,P=0.001<0.05）、运动恢复时间（OR=9.556,P=0.002<0.05）、阻断节数（OR=7.591,P=0.001<0.05）。结论：斜角肌间臂丛阻滞出现心率变异性与镇痛维持时间、运动恢复时间、阻断节数有关。     

The surgical treatment of brachial plexus injuries in adults

LEARNING OBJECTIVES: After studying this article, the participant should be able to: 1. Evaluate clinically a patient with brachial plexus paralysis and define the appropriate electrophysiologic and radiographic studies. 2. Differentiate between preganglionic (root) avulsion and postganglionic lesions and identify appropriate motor donors and nerve grafts. 3. Describe various nerve reconstructive strategies and make appropriate selection of secondary procedures for shoulder stability, elbow flexion, and hand reanimation. 4. Anticipate the possible functional outcome.     

Long head of the triceps branch transfer to axillary nerve in C5, C6 brachial plexus injuries: anterior approach

Shoulder abduction is a very complex movement and quite important for upper limb function, as more distal functions depend on a stable shoulder, especially in C5, C6 brachial plexus injuries. Various studies in the literature have emphasized the importance of improved functional outcome and shoulder reanimation with concomitant neurotization of suprascapular nerve and axillary nerve in C5, C6 brachial plexus injuries. A number of approaches to axillary nerve transfer in brachial plexus injuries have been reported. The author describes an innovative anterior deltopectoral approach for axillary nerve transfers in five patients with C5, C6 brachial plexus injuries. The spinal accessory nerve was neurotized with the suprascapular nerve through a transverse supraclavicular incision. The axillary nerve and the long head of the triceps branch were identified through the anterior deltopectoral approach and neurotized at the posterior cord level. This approach gives easy access to other donors such as the medial pectoral, thoracodorsal, and median and ulnar nerves. Oberlin's transfer was also performed for elbow flexion by extending the deltopectoral incision. The regained shoulder active abduction (M5) averaged 120 degrees and active external rotation averaged 65 degrees at the final follow-up of 26 months (average). This anterior deltopectoral approach is an excellent alternative for axillary nerve transfer in brachial plexus injuries and produces results comparable with those of other approaches. All brachial plexus surgeons must understand the anatomy and the relationship of the axillary nerve to the surrounding structures. CLINICAL QUESTION/LEVEL OF EVIDENCE: Therapeutic, V.     

A subject-specific anisotropic visco-hyperelastic finite element model of female pelvic floor stress and strain during the second stage of labor

ObjectivesTo develop an improved model representation of the biomechanics of the levator muscles during the second stage of labor and to use a sensitivity analysis to explore the pathomechanics of levator muscle injury.MethodsA subject-specific finite element model of human pelvic floor and fetal head was developed based on in vivo MRI data of a fetal head and maternal pelvis. An anisotropic visco-hyperelastic constitutive model employed material parameters estimated from biaxial tests on pelvic floor tissues. Boundary conditions reflected both anatomic constraints and the curve of Carus. A short second stage of labor, scaled to 10 min, was then simulated using a single expulsive push made in the absence of levator co-contraction.ResultsLarge levator stresses occurred near the levator hiatus reaching 9 MPa at the pubovisceral muscle enthesis. The dominant principal stresses were located at, and aligned with, the edge of the hiatus. Muscle stretch bordering the levator hiatus was inhomogeneous: the average levator stretch was 3.55 with a high of 4.64 at the pubovisceral muscle enthesis. Decreasing perineal body stiffness by 40%, 50%, and 60% led to reductions in the maximum principal stretch ratio at the pubovisceral muscle enthesis of 8%, 13%, and 18%, respectively.ConclusionsThe pubovisceral muscle enthesis and the muscle near the perineal body are the regions of greatest strain thereby placing them at highest risk for stretch-related injury. Decreasing perineal body tissue stiffness significantly reduced tissue stress and strain, and therefore injury risk, in those regions.