J Biomech – Pavlik 2013

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Mechanics of hip dysplasia reductions in infants using the Pavlikharness: A physics-based computational model

Orlando J. Ardila a, Eduardo A. Divo a,b, Faissal A. Moslehy a, George T. Rab c, AlainJ. Kassab a,n, Charles T. Price d,e

a Department of Mechanical and Aerospace Engineering, University of Central Florida, Orlando, FL 32816, United Statesb Mechanical Engineering Department, Embry-Riddle Aeronautical University, Daytona Beach, FL 32114, United Statesc Department of Orthopaedic Surgery, University of California Davis, Sacramento, CA 95817, United Statesd Pediatric Orthopedic Surgery, Arnold Palmer Hospital and International Hip Dysplasia Institute (IHDI), Orlando, FL 32806, United Statese Department of Medical Education, College of Medicine, University of Central Florida, Orlando, FL 32827, United States

a r t i c l e i n f o

Article history:
Accepted 30 March 2013
Biomechanical factors influencing the reduction of dislocated hips with the Pavlik harness in patients ofDevelopmental Dysplasia of the Hip (DDH) were studied using a three-dimensional computer model

Keywords:Pavlik harnessHip dysplasiaDynamical analysisPassive reductionNon-linear muscle model

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esponding author. Tel.: +1 407 823 5778.ail address: [email protected] (A.J. Kassab)

e cite this article as: Ardila, O.J., et autational model. Journal of Biomech

a b s t r a c t

simulating hip reduction dynamics in (1) subluxated and (2) fully dislocated hip joints. Five hip adductormuscles were identified as key mediators of DDH prognosis, and the non-dimensional force contribution ofeach in the direction necessary to achieve concentric hip reductions was determined. Results point to theadductor muscles as mediators of subluxated hip reductions, as their mechanical action is a function of thedegree of hip dislocation. For subluxated hips in abduction and flexion, the Pectineus, Adductor Brevis,Adductor Longus, and proximal Adductor Magnus contribute positively to reduction, while the rest of theAdductor Magnus contributes negatively. In full dislocations all muscles contribute detrimentally toreduction, elucidating the need for traction to reduce Graf IV type dislocations. Reduction of dysplastic hipswas found to occur in two distinct phases: (a) release phase and (b) reduction phase.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Developmental Dysplasia of the Hip (DDH) is an abnormalcondition where hip joint dislocation, instability or mal-alignmentis present. As many as 1/20 infants may have some type ofhip insufficiency, while 6 out of 1000 babies require treatment(Bialik et al., 1998). The Pavlik harness (Fig. 1) is a standard non-surgical DDH treatment method (Weinstein et al., 2003; Ramseyet al., 1976) designed to maintain hips in abduction and flexionsimultaneously, as this position directs the femoral head to theacetabulum (Ramsey et al., 1976).

Treatment success is inversely related to age at onset oftreatment and initial dislocation severity (Graf, 2006). Pavlikharness treatment fails in approximately 15% of cases (Weinsteinet al., 2003; Mubarak et al., 1981), and incidence of failure doubleswhen treatment begins after three weeks of age (Harding et al.,1997). Prolonged Pavlik harness treatment of inadequatelyreduced hips carries deleterious consequences (Mubarak et al.,1981; Rombouts and Kaelin, 1992), hence accurate and timelyreduction is the goal of DDH Pavlik harness treatment. Our study

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l., Mechanics of hip dysplasianics (2013), http://dx.doi.o

utilizes engineering fundamentals and physics-based computersimulations to elucidate the dynamics of DDH reductions with thePavlik harness, to explore harness mechanics, and to determinefactors leading to successful treatment.

2. Methods

A 3D dynamic computer model was developed to simulate treatment of twoseverities of hip dysplasia with the Pavlik harness.

2.1. Model definition

To construct the model, we used CT-scans of a 6 month-old female and themedical segmentation software Mimics (Materialise Inc., Plymouth, MI) to measurethe geometrical features of the hip joint including acetabular and femoral headdiameter, acetabular depth, and geometry of the acetabular labrum. The lowerextremity was modeled by three segments: thigh, leg, and foot. The mass (Table 1)and the location of the center of gravity of each segment were calculated usinganthropometry (Osterkamp, 1995; Clauser et al., 1969; Drillis and Contini, 1966;Dempster, 1955). Calculations were based on the total body mass of a 6-month oldfemale infant at the 50th length-for-age percentile (CDC, 2009).

Geometrical measurements and mass calculations were used to develop adynamic computer model utilizing SolidWorks (Dassault Systèmes Simulia Corp.,Providence, RI) consisting of the pubis, ischium, acetabulum with labrum andfemoral head, neck, and shaft (Fig. 2), capable of simulating dislocated as well asreduced hips in abduction and flexion (Fig. 3a–c). Only the right hip was modeled

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due to symmetry. The origin of the inertial Cartesian reference frame was fixed at thecenter of the right acetabulum (Fig. 4). The femoral head was constrained to move inthe X–Z plane. Hip rotations were constrained about the Z and X axes to account for the

Fig. 1. The Pavlik harness (Weinstein et al., 2003).

Table 1Masses of the segments of the lower extremity (Drillis and Contini, 1966).

Masses of segments of the lower extremity

Total body mass (BM) 7.26 kg

Segment %BM Mass (kg)

Thigh 9.46 0.69Leg 4.2 0.30Foot 1.35 0.10

Full leg 15 1.09

Fig. 2. Three-dimensional dynamic computer model for simulations of hip dysplasia(middle). (b) Hip and right leg assembly viewed axially, displaying modeled musculatu

Please cite this article as: Ardila, O.J., et al., Mechanics of hip dysplasicomputational model. Journal of Biomechanics (2013), http://dx.doi.o

analogous constraints imposed by the anterior (flexor) stirrup strap (Fig. 1) of thePavlik harness, and to account for reaction moments imposed by possible anatomicalstructures not included to reduce complexity. Contact between the femoral head andthe acetabulum was modeled as frictionless solid body contact.

Locations of origins and insertions of muscles were assigned by scaling adultmale data (Dostal and Andrews, 1981) isotropically to match the proportions of the6-month old infant, using the distances between acetabular centers as scalingparameters. The resulting scaling factor was 0.39 and the scaled muscle origins andinsertions (Table 2) matched the expected model landmarks accurately.

2.2. Muscle modeling

Five anatomical muscles were considered to affect DDH reduction with the Pavlikharness: Pectineus, Adductor Brevis, Adductor Longus, Adductor Magnus, and Gracilis.The Adductor Magnus is a large triangular muscle with extensive femoral insertion;consequently, in the computer model, this muscle was represented by three effectivecomponents following (Dostal and Andrews, 1981): Adductor Minimus, AdductorMiddle, and Adductor Posterior, corresponding to the proximal, middle, and distalfemoral insertions of the muscle, respectively. Consequently, although five anatomicalmuscles were considered, the computational model results in seven distinct muscleentities: Pectineus, Adductor Brevis, Adductor Longus, Gracilis, Adductor Minimus,Adductor Middle, and Adductor Posterior.

According to Suzuki (1994) and Iwasaki (1983) reductions of DDH with the Pavlikharness occur passively with muscle relaxation in deep sleep. The passive response ofmuscles to elongation is exponential (Magid and Law, 1985; Hill, 1949, 1952; Sten-Knudsen, 1953). We adopted the model by Magid and Law in Eq. (1), with constants inTable 3, to simulate the response of adductor muscles to elongation.

s¼ E0αðeαðλ−1Þ−1Þ ð1Þ

where λ¼ L=Lois the stretch, L is the deformed length and Lo is the initial length.To obtain muscle tensions (T), we multiplied Eq. (1) by an effective cross-sectionalarea, A¼41 mm2, approximated by dividing the volume of the Adductor Brevismuscle by its length determined from MRI data. The magnitude of this area is anintermediate value between that of the Pectineus and Adductor Longus. Given thatthe Adductor Magnus was discretized into three effective component muscles, thisarea was used as a representative value for all modeled muscles.

The model was then calibrated by calculating a stiffening factor C that allowed themodel to precisely replicate clinical observations in which healthy (non-dysplastic)

reductions. (a) Hip and right leg assembly viewed laterally (topmost) and axiallyre.





Fig. 3. Hip dislocation cases modeled: (a) full dislocation (Graf IV) with femoralhead located posterior to acetabulum; (b) subluxated hip (Graf III) with the femoralhead located over the posterior acetabular labrum; and (c) reduced hip.

Fig. 4. Depiction of reconstructed anatomy and computer model: (a) CT-basedthree-dimensional hip reconstruction; (b) simplified solid model with musculature.Y-axis of the right-handed coordinate system points normal to the page. Bothmodels viewed in the infero-superior direction, on the transverse plane.

Table 2Coordinates of muscle origins and insertions scaled to fit 6-mo old female infant forhip configuration: zero abduction, zero flexion, zero rotation. Unscaled dataobtained from Dostal and Andrews (1981).

Scale: 0.39 Origin on pelvis(mm)

Femoral insertions(mm)

X Y Z X Y Z

Iliopsoas** 11.1 9.5 2 −0.8 −24.2 5.91 Pectineus 17.4 −1.2 −15 −1.6 −45.1 13.92 Adductor Longus 16.2 −12.3 −25.7 2 −80.8 10.33 Adductor Brevis 8.3 −17.8 −26.5 −0.8 −51.9 154a Magnus Adductor Minimus 2.8 −19.4 −24.2 −1.6 −49.1 15.84b Adductor Middle −12.3 −24.2 −17.4 2 −90.3 10.74c Adduct Posterior −19 −23.4 −13.5 0.4 −160 −12.35 Gracilis 4 −19.4 −26.9 −5.5 −171.9 −16.2

nn Effect of the IIiopsoas tendon was studied separately.

Table 3Variables in Eqs. (1) and (2) as defined by Magid and Law (1985).

Variable definitions for Eqs. (1) and (2)

α Empirical constant 4.2870.19E0 Initial elastic modulus 2.670.25 (�103 N/m2)A Cross-sectional area 41 mm2

λ Muscle stretch L/L0

O.J. Ardila et al. / Journal of Biomechanics ∎ (∎∎∎∎) ∎∎∎–∎∎∎ 3

infant hips are maintained in static equilibriumwhen infants are supine and the hips areplaced in abduction and flexion, with slight infero-superior support to maintain flexion.We adopted 801 abduction and 901 flexion (Fig. 4) as the “reference configuration”.

For model calibration, unknown tensions in seven pre-stretched muscles(Pectineus, Gracilis, Adductor Brevis, Longus, Minimus, Middle and Posterior) wererelated as ratios to the tension of the adductor Brevis, which experiences thelargest stretch (Table 4). The Finite Element Method (FEM) software package NXNastran (Siemens PLM Software, Plano, TX, USA) was employed to solve for theunknown tensions. For this FEM solution, we used rigid bar elements (RBAR)to account for the femur, tibia and fibula (modeled as a single component), andfoot. Due to the passive nature of reductions of DDH with the Pavlik harness(Iwasaki, 1983; Suzuki, 1994), the weight of the leg is the only driving force in themodel. The weight of the leg was modeled as a point load of 10.7 N pointed in the−X direction (Fig. 2, Table 1), acting through the center of mass of the leg. Alltranslations and rotations were constrained at the hip joint except rotation aboutthe y-axis. Resolution of the force components along the lines of action of themuscles (Table 2) yielded the approximate muscle tensions necessary for staticequilibrium at the reference configuration. These were then used to calculate acalibration constant resulting in a value of C¼5.5 (Fig. 5), yielding the final musclemodel:

Tcalib ¼ CAE0αðeαðλ−1Þ−1Þ ð2Þ

Stiffening is attributed to a combination of unaccounted leg-supportingstructures, difference in the density of elastic muscle fibers between muscles,and difference in mechanical behavior between human and amphibian muscles. Inthe dynamic model, hip muscles were subsequently simulated using action/


reaction force elements positioned between muscle origin and insertion points,following the non-linear constitutive equation provided in Eq. (2).

2.3. Simulations

Simulations were carried out using the ADAMS dynamics module embedded inthe software package SolidWorks, which numerically solved the set of coupleddifferential equations of motion (MSC, 2012):

M €q þ βϕTq−A TF ðq ; _qÞ ¼ 0 ð3Þ





Table 4Static equilibrium muscle tensions for a 6-month old infant hip at 801 abductionand 901 flexion. Li defines the length of the muscles in the natural position inthe body.

Muscle Initial musclelength (Lo)nn

Stretch (λ) at 801abduction, 901 flexion

Tension(N)

Pectineus 0.6� Li 1.96 8.9Adductor Longus 0.6� Li 2.32 38.2Adductor Brevis 0.6� Li 2.54 97.8Magnus Adductor

Minimus0.675� Li 2.31 21.3

AdductorMiddle

0.675� Li 2.28 23.6

AdductorPosterior

0.675� Li 1.91 6.2

Gracilis 0.675� Li 1.92 26.7

nn Per Hill muscles are 25–40% stretched in their natural position in the body(Hill, 1949).

Fig. 5. Original muscle model (Magid and Law, 1985) and calibrated model.


subject to the natural constraints:

ϕðq ; tÞ ¼ 0 ð4Þ

whereM is the mass matrix of the system, q is the vector of generalized coordinatesfor the displacements, β is a Lagrange multiplier, ϕ is the set of configuration andapplied motion constraints, AT is the matrix that projects the applied forces in the qdirection, ϕ

qis the gradient of constraints, and F represents the applied forces and

gyroscopic terms of the inertia forces. In the computations, the Gear Stiff (GSTIFF)integrator was used, the Jacobian was updated at each time-step, 25 sub-leveliterations were taken per time-step, and the initial, minimum, and maximumintegrator step sizes were set to 1�10−4, 1�10−7 and 1�10−2.

Two severities of DDH were analyzed and simulated:

(1)
Table 5
Pc

Graf III subluxated hip: center of the femoral head lies on posterior rim of theacetabulum (Fig. 3b).

Comparison of muscle stretch values between reference hip configuration, Graf III,
(2) and Graf IV severities of hip dysplasia.
Muscle stretch (λ)

Muscle Reference configuration Graf III Graf IV

Pectineus 1.96 1.96 1.96Adductor Longus 2.32 2.23 2.21Adductor Brevis 2.54 2.58 2.55Adductor Minimus 2.31 2.4 2.11Adductor Middle 2.28 2.36 2.05Adductor Posterior 1.91 1.98 1.73Gracilis 1.92 1.85 1.68

Graf IV fully dislocated hip: center of the femoral head is located posterior tothe acetabulum and obstructed by the labrum (Fig. 3a).

Magnitudes and directions of forces developed in each muscle were analyzedseparately. First, we compared directional cosines of the lines of action of eachmuscle with those of resultant forces necessary to induce motion towardsreduction. For condition (1) we evaluated the contributive components of muscleforces at 431, 521, 601, 701, and 801 of abduction. For condition (2) we evaluatedcomponents at 521 and 701 of abduction. For each DDH severity studied wereported findings as the percentage contribution of each muscle force in thedirection necessary to affect reduction.

Reduction simulations were carried out by positioning the hips in configura-tions (1) and (2) as initial conditions, and solving the dynamic response of thesystem until equilibrium was reached. We defined successful reductions as

lease cite this article as: Ardila, O.J., et al., Mechanics of hip dysplasiomputational model. Journal of Biomechanics (2013), http://dx.doi.o

conditions in which the femoral head slid into the acetabulum, and concentricreduction was maintained throughout the solution (Fig. 3c) as the leg reachedequilibrium.

2.4. Role of the Iliopsoas tendon

We studied the straight-line path of travel of the Iliopsoas tendon as it departedfrom its last point of contact on the bony pelvis towards its insertion point in thefemur, neglecting tendon volume, to determine possible interference to reductions.The hip configurations analyzed were: (i) zero abduction, zero flexion and zerorotation; (ii) zero flexion, gradual abduction, and (iii) 901 flexion, gradual abductionfrom 01 to 801. This was also studied in a cadaveric dissection.

3. Results

The Pavlik harness maintains hips abducted and flexed, and weapproximated this position as 801 of abduction and 901 of flexion.We found that: (1) the psoas tendon is relaxed and is likely not anobstruction to reduction for any abduction angle while the hips areflexed to 901. This holds true for Graf III and Graf IV dislocations,and the reference configuration; (2) the psoas tendon obstructionto hip reduction only occurs for hips extended beyond approxi-mately 451 as the tendon tightens upon extension, and its straight-line path interferes with the joint capsule.

Inducing a Graf III subluxation, while maintaining the length ofthe Pectineus muscle constant, resulted in a reduction of 281 inabduction angle, which agrees with our clinical observations andthose of (Iwasaki, 1983; Suzuki, 1994), providing a validation of ourmodel. A successful dynamic reduction simulation was carried outindicating that Graf III subluxations can be reduced by the Pavlikharness. To study the mechanisms of reduction we considered(a) the magnitudes and (b) directions of muscle tensions. Tensionmagnitudes are functions of stretch (Fig. 5), however it should benoted that tension values are affected by uncertainty in cross-sectional area, and that muscles are 25–40% pre-stretched fromthe natural length (Li) in the body (Hill, 1949). The initial lengths(Lo) found to best suit the model were 0.60� Li for the Pectineus,Adductor Brevis and Adductor Longus, and 0.675� Li for theGracilis and the components of the Adductor Magnus (AdductorMinimus, Middle, and Posterior defined in Section 2.2).

Further insight is gained by comparing the stretch of muscles inhealthy hips with those observed in dysplastic hips of Graf III, andGraf IV (Table 5). The Adductor Brevis has the largest stretch,hence tension, followed by the Adductor Longus and AdductorMinimus, depending on DDH severity.

Examination of directional components of muscle tension uponinducing Graf III subluxation reveals that the Gracilis, AdductorMiddle, and Adductor Posterior contribute negatively to the reduc-tion with the Pavlik harness (Table 6), and that the percentconstructive contribution of the muscles studied increases indirect proportion with abduction angle (Fig. 6) which may explainobservations by different authors of reductions occurring during






loss of muscle tone while in deep sleep (Iwasaki, 1983; Suzuki,1994).

Results in Table 6 indicate that the Pectineus, although lesstense than other muscles in the Pavlik harness position, exhibitsthe highest component of pull in the direction of reduction(towards the center of the acetabulum) based on the line of actionof its force, and thus helps effect and maintain reduction. This waslater confirmed with a dynamic numerical simulation whichsuccessfully reduced the dislocation. Reduction was not achievedupon numerically suppressing the Pectineus.

Inducing a Graf IV type dislocation maintaining constantPectineus muscle length resulted in a decrease in abduction of13.71. Simulation results indicate that reductions from dislocationsof type Graf IV are unlikely to occur by Pavlik harness treatment,since the tensions of all muscles contribute negatively to thedirection of the motion necessary for reduction (Table 7).

Based on origin and insertion points, muscles are shorter in aGraf IV dislocation than in a reduced hip. Since muscles tensewhen their length is increased from equilibrium, energy, hencetraction to overcome muscle tension is necessary to bring thefemoral head over the posterior labrum for reduction. Further-more, for Graf IV dislocations the directional contributions of themuscles also increased in direct proportion with increasing abduc-tion angle (Fig. 7), but, abduction alone is insufficient to turn the

Fig. 6. Percent contribution of muscle tension tow

Table 6Percent directional contributions of muscle tensions in the direction of reduction—Graf III.

Percent directional contributions of muscle tensions—Graf III

Abduction range (deg, measured fromsagittal)

Contributionorder

42.7 52.2 60 70 80

Pectineus 28.1% 38.7% 46.3% 55.5% 63.9% 1Add Longus −3.0% 10.1% 19.7% 31.6% 42.8% 4Add Brevis 10.6% 20.4% 29.1% 38.9% 48.2% 2Add Minimus 4.7% 15.7% 23.9% 34.2% 44.0% 3Add Middle −32.6% −19.3% −9.1% 4.4% 17.8% 5AddPosterior

−44.6% −30.2% −19.0% −3.9% 11.3% 6

Gracilis −50.32% −36.60% −25.18% −10.60% 4.00% 7


detrimental directional cosines of the tensions into beneficialcomponents, hence, reduction did not occur.

4. Discussion

Our findings indicate that the psoas tendon does not obstructreduction when the hip is in flexion, which is the position ofreduction in the Pavlik harness, and that of closed reductions withcast application. However, this tendon tightens and crosses ante-rior to the hip capsule upon hip extension becoming an obstruc-tion to reduction. Releasing this tendon may only be necessarywhen reductions are sought with the hips extended, as in openreductions. Per the medial approach for open reduction of the hip,when the hip is extended, the iliopsoas muscle is taut, andreleased for greater surgical visibility of the capsule. Conversely,the “human position” (flexion and slight abduction) is recom-mended for closed reductions, and we found the Iliopsoas relaxedin this position. Eberhardt et al. (2012) reported arthroscopicreductions in the “human position” without iliopsoas release.

Although the Pectineus is significantly less taut in abductionand flexion, it was found to have a favorable line of action towardsreduction. Conversely, the Gracilis and the portion of the AdductorMagnus of distal femoral insertion (Adductor Middle and AdductorPosterior defined in Section 2.2), develop components of tensionthat oppose the direction desired for reduction. When the femoralhead lies posterior to the acetabulum, these muscles pull the

ards reduction vs. abduction angle—Graf III.

Table 7Percent contribution of muscle tensions in the direction of reduction—Graf IV.

Percent directional contribution of tensions—Graf IV

Abduction angle Contribution order

52.31 701

Pectineus −42.9% −29.7% 1Add Longus −64.8% −52.2% 4Addr Brevis −53.5% −42.1% 2Add Minimus −57.3% −45.7% 3Add Middle −77.7% −67.9% 5Add Posterior −86.5% −77.3% 6Gracilis −91.3% −84.0% 7





Fig. 7. Percent contribution of muscle tension towards reduction vs. abduction angle—Graf IV.

Fig. 8. Simplified model depicting the directions of necessary motion to completethe (a) release and (b) reduction phases of the mechanism of reduction of hipdysplasia.


femoral head further posteriorly, trapping it in this location. Thisimplies that traction is necessary to overpower the detrimentalcomponents of tension of these muscles when the hip is fullydislocated (Graf IV). Another avenue to overcome this could be thesurgical release of these muscles, or alternative methods for closedmanagement (Papadimitriou et al., 2007). Hence, novel proceduresthat combine various techniques, in addition to the possibleartificial activation of the Pectineus muscle, may enhance treat-ment and contribute to successful reductions for hips that areirreducible with the Pavlik harness.

Based on our findings, reduction of higher degrees of dysplasiacan be considered to take place as two distinct, consecutive events(Fig. 8), and muscles act distinctively in each phase:

(a)

Plco

Release phase: The femoral head is brought from a regionposterior to the acetabulum to a region over the perimeter ofthe labrum.

(b)
Reduction phase: follows the release phase and the femoralhead is pulled into the acetabulum from a region over theperimeter of the labrum.
In (a) the femoral head is released from external resistancesconstraining its movement such as it being trapped behind theacetabulum, the Piriformis muscle resisting its movement, or otherresistive factors. (b) follows when the Pectineus muscle, due to itsdirectional advantage, and the adductors (primarily the AdductorBrevis), take over and pull the femoral head into the acetabulum.

ease cite this article as: Ardila, O.J., et al., Mechanics of hip dysplasimputational model. Journal of Biomechanics (2013), http://dx.doi.o

Acetabular geometry and the tension in the portion of the AdductorMagnus of distal femoral insertion help maintain concentric reduction.

Suzuki (1994) describes three degrees of hip dysplasia andstates that the greatest severity, when the femoral head center liesposterior to the labrum of the acetabulum, requires traction forreduction. This is supported by our computational model: traction,referred to by Suzuki, is a method to free the femoral head fromconstraints, hence manually executing a release phase. Our find-ings, which correlate with previous observations, lead us toconclude that for hip dysplasia of greater severity (Graf IV), therelease phase is highly unlikely via the Pavlik harness alone.Traction, surgical release of the distal adductors and/or a differenttreatment avenue is therefore required.

5. Conclusions

We created a three-dimensional computer model to simulatehip dysplasia reduction dynamics with the Pavlik harness. Weidentified five anatomical adductor muscles as key mediators inthe prognosis of hip dysplasia (Pectineus, Adductor Longus,Adductor Brevis, Adductor Magnus, Gracilis), and found thatreductions occur in two distinct phases: (a) release phase and(b) reduction phase. Results indicate that the muscles studied actdistinctively in each phase and the mechanical effects of musclesvary with the severity of hip dislocation. For subluxated hips inabduction and flexion, the Pectineus, Adductor Brevis, AdductorMagnus of proximal femoral insertion, and Adductor Longuscontribute positively to reduction, while the portions of theAdductor Magnus with middle and distal femoral insertion con-tribute negatively. Conversely, for fully dislocated hips, all musclescontribute detrimentally to reduction, explaining the need fortraction to reduce Graf IV type dislocations. The Iliopsoas tendonwas not found to affect reductions when hips are flexed, as duringtreatment with the Pavlik harness. Our model is consistent withclinical observations reported in the literature and may provide ameans to determine modifications for treatment that could lead tonew insights for non-surgical management of DDH by passivereduction with the Pavlik harness or similar devices.

Conflict of interest

None.






Acknowledgments

This study was supported in part by the US National ScienceFoundation under Grant number CBET-1160179, Orlando Health,and the International Hip Dysplasia Institute. The authors grate-fully acknowledge the contribution of Mechanical Engineeringstudents Sergio Gomez, Kyle Snethen, and Justin Kingsley.

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Documents

J Biomech – Pavlik 2013