371 THE EFFECT OF UTERINE VOLUME, UTERINE PRESSURE AND MATERNAL POSITIONON CERVICAL STRESS: THE BIOMECHANICS OF COMMON STATIC LOADSMICHAEL HOUSE1, ANASTASSIA PASKALEVA2, KRISTIN MYERS2, SABRINACRAIGO1, SIMONA SOCRATE2, 1Tufts University, Obstetrics/Gynecology, Boston,Massachusetts, 2Massachusetts Institute of Technology, Mechanical Engineer-ing, Cambridge, Massachusetts

OBJECTIVE: In the absence of contractions, the cervix is subjected to a rela-tively static mechanical load. This static load arises primarily from hydrostaticforces and intrauterine pressure. Our objective was to use a three-dimensional(3D), solid body, computer-based model of the pelvic organs during pregnancyto demonstrate how intrauterine volume, intrauterine pressure and maternalposition affect the stress distribution within the cervix.

STUDY DESIGN: Mathematical equations that model the mechanicalresponses of the pelvic organs were combined with 3D anatomic geometryin a finite element framework. Representative anatomic geometry was derivedfrom 3D pelvic magnetic resonance images during pregnancy. We assumedthat the cervical stroma is the load bearing tissue within the cervix. Staticloading conditions and uterine growth were simulated using finite elementmethodology. Three simulations were performed. The internal stress distribu-tion within the cervical stroma was measured as a function of 1) intrauterinevolume, 2) intrauterine pressure and 3) maternal position.

RESULTS: As intrauterine volume increased, the cervical stroma wassubjected to increasing cervical stress, which produced a characteristic stressdistribution. The distribution was characterized by peak stress at the internalos, decreasing stress along the axis of the cervical canal and minimum stress atthe external os. Changes in maternal position corresponding to bedrest orTrendelenberg caused a decrease in the magnitude of cervical stress becausethe mechanical load decreased. In contrast, a rise in intrauterine pressureincreased the magnitude of cervical stress. Although the magnitude of thestress distribution changed in response to changes in maternal position andintrauterine pressure, the shape of the stress distribution pattern remained thesame.

CONCLUSION: The internal stress distribution within the cervical stroma isa complex function of intrauterine volume, intrauterine pressure and anatomicposition with respect to gravity.

372 THE BIOMECHANICS OF CERVICAL ANATOMY: THE EFFECT OF CERVICAL LENGTHAND ORIENTATION ON CERVICAL STRESS MICHAEL HOUSE1, ANASTASSIAPASKALEVA2, KRISTIN MYERS2, SABRINA CRAIGO1, SIMONA SOCRATE2, 1TuftsUniversity, Obstetrics/Gynecology, Boston, Massachusetts, 2MassachusettsInstitute of Technology, Mechanical Engineering, Cambridge, Massachusetts

OBJECTIVE: We hypothesize that the biomechanical anatomy of the cervixconsists of the cervical stroma, the fetal membranes and the decidual con-nection between the cervix and membranes. Our objective was to use a three-dimensional (3D), solid body, computer-based model of the pelvic organsduring pregnancy to demonstrate how anatomic changes affect stress distri-bution within the cervix during pregnancy.

STUDY DESIGN: Mathematical equations that model the mechanicalresponses of the pelvic organs were combined with 3D anatomic geometryin a finite element framework. Anatomic geometry was derived from 3D pelvicmagnetic resonance images during pregnancy. We assumed that the cervicalstroma was the load bearing tissue within the cervix. The stroma was modeledas a cylindrical object with a hollow core. Cervical dilation was modeled as thediameter of the hollow core. The decidual connection between membranesand cervix was modeled as adhesive contact. Static loading conditionswere simulated using finite element methodology. Three simulations wereperformed. The internal stress distribution within the cervical stroma wasmeasured as a function of 1) decreasing cervical length, 2) increasing cervicaldilation and 3) the presence or absence of membrane adhesion.

RESULTS: Under a constant load, as the inside diameter of the cervicalstroma increased, the stroma was subjected to increasing mechanical stress. Asthe inside diameter was held constant and the cervical length decreased, themechanical stress increased as well. The increase in stress was most pro-nounced as the cervical length decreased below 2.5 cm in length. The stress wasfurther increased if adhesive contact between membranes and cervix wasdisrupted.

CONCLUSION: Decreased cervical length, increased cervical dilation anddecreased membrane adhesion were associated with increased cervical stress.

373 CONSTITUTIVE MODEL DEVELOPMENT OF FETAL MEMBRANE MECHANICS:MECHANICAL TESTING AND NUMERICAL SIMULATION MICHAEL HOUSE1,THIBAULT PREVOST2, MING DAO2, HIDEMI KATO2, MICHELLE OYEN3,SABRINA CRAIGO1, SIMONA SOCRATE4, SUBRA SURESH2, 1Tufts University,Obstetrics/Gynecology, Boston, Massachusetts, 2Massachusetts Institute ofTechnology, Department of Materials Science and Engineering, Cambridge,Massachusetts, 3University of Minnesota, Biophysical Sciences & MedicalPhysics, Minneapolis, Minnesota, 4Massachusetts Institute of Technology,Mechanical Engineering, Cambridge, Massachusetts

OBJECTIVE: The fetal membranes are part of the biomechanical environ-ment of the cervix and uterus. Our objective was to develop a quantitativemodel of membrane mechanical behavior as part of a modeling study ofobstetrical biomechanics.

STUDY DESIGN: Fetal membrane samples were obtained from unlaboredwomen at term who were undergoing cesarean section. Two samples wereobtained from each patient. A sample of chorioamniotic membrane was takenfrom a location midway between the placenta and rupture site. A sample ofamnion was obtained by separating the amnion from the chorion on thesurface of the placental disc. Membranes were rinsed in normal saline andstored at �80(C for subsequent biomechanical testing. At the time of testing,membrane samples were unfrozen and fastened to fixtures that were designedto apply uniaxial and biaxial loading. The mechanical responses to differentloading conditions were measured. In a parallel effort, a constitutive (math-ematical) model was developed. The goal of the constitutive model was tocapture the viscoelastic behavior of the fetal membranes under simulatedbiomechanical test conditions. The model was implemented on ABAQUS, afinite element analysis software program. The loading conditions and geom-etry of the biomechanical tests were simulated using finite element methodol-ogy. The model parameters were fit to the mechanical response of themembranes.

RESULTS: Membranes samples from 20 patients were collected for thestudy. Characteristic stress-strain responses – including stress relaxation andcreep – were obtained for all samples. Finite element simulation showed goodagreement between the observed response in the lab and simulated responsewith our constitutive model.

CONCLUSION: The fetal membranes display nonlinear viscoelastic behav-ior. A constitutive model of the fetal membranes was developed that can beincorporated into a biomechanical model of the cervix and uterus duringpregnancy.

374 PREGNANCY-INDUCED ARTHRALGIA AND ARTHRITIS ON SMALL JOINTS OFHANDS JONGKWAN JUN1, HYO JIN CHOI2, YEONG-WOOK SONG2, SOON-SUPSHIM3, EUNBONG LEE2, JOONG SHIN PARK3, B. O. YOON1, 1Seoul NationalUniversity, Department of Obstetrics and Gynecology, Seoul, South Korea,2Seoul National University College of Medicine, Internal Medicine, Divisionof Rheumatology, Seoul, South Korea, 3Seoul National University Collegeof Medicine, Obstetrics and Gynecology, Seoul, South Korea

OBJECTIVE: Discomfort on small joints of hands is a common complaint ofpregnant women. However, a few reports of joint discomfort during pregnancywere published and most of them focused on spine and joints of pelvic girdle.We underwent this study to show clinical features of arthralgia-arthritisinvolved in small joints of hands in otherwise healthy pregnant women.

STUDY DESIGN: We prospectively investigated 157 pregnant women withno previous history of rheumatic diseases. Medical interview, joint examina-tion and laboratory tests were performed. Arthralgia was defined by subjectivepain on joints and arthritis was defined when at least one of the following signs- joint pain on motion, joint swelling, joint tenderness, and erythema on joint -was objectively confirmed

RESULTS: Arthralgia was found in 22 pregnant women (14.0%) andarthritis in 14 (8.9%). Arthralgia or arthritis developed in 3rd trimester exceptone case. Among 22 pregnant women with arthralgia, involved joints were asfollows; 17 cases of proximal interphalangeal (PIP) joint only, 3 cases of PIPand metacarpophalangeal (MCP) joints, 1 case of PIP, MCP and distalinterphalangeal (DIP) joints, and 1 case of MCP joint only. Among 14 preg-nant women with arthritis, involved joints were as follows; 8 cases of PIP jointonly, 3 cases of PIP and MCP joints, 2 cases of PIP and DIP joints, and 1 caseof MCP joint only. PIP joint is the most commonly affected (21/22 inarthralgia and 13/14 in arthritis) and arthralgia worsened in the morning andafter nap. However, rheumatoid factor and antinuclear antibody were negativein all pregnant women with arthritis. All cases with arthralgia or arthritis onsmall joints of hand resolved spontaneously immediately after delivery.

CONCLUSION: Pregnancy-induced arthralgia and arthritis on small jointsof hands looks similar to rheumatoid arthritis, but may be a different diseaseentity.

S112 SMFM Abstracts