The effect of stress distribution in artificial hip joint on the dislocation of total hip endoprosthesis

Izpah po vstavitvi TEP kolka je drugi najpogostejši zaplet tega operativnega posega. Namen je raziskati, ali so biomehanski parametri razporeditve tlaka v umetnem kolčnem sklepu pri stoji na eni nogi prediktorji večje verjetnosti za izpah TEP kolka. Zato smo testirali sledeče hipoteze: 1) Pri pacientih, pri katerih se pojavi izpah TEP kolka, je že pred vstavitvijo prisotna predispozicija z manj ugodno porazdelitvijo kolčnega tlaka2) Porazdelitev kolčnega tlaka pri TEP z izpahi je drugačna kot pri TEP, ki so bile asimptomatske več kot 10 let po vstavitvi umetne kolčne proteze3) Najvišji kolčni tlak je višji in lega pola tlaka bolj lateralna pri endoprotezah, pri katerih je prišlo do izpaha kolka. Biomehanske parametre smo izračunali z uporabo 3-dimenzionalnega matematičnega modela v reprezentativnem položaju pri stoji na eni nogi s HIPSTRESS metodo. Geometrijske parametre smo izmerili iz standardnih anteroposteriornih rentgenskih slik medenice z okolčjem. Študijsko skupino petinpetdesetih kolkov, ki so po vstavitvi TEP utrpeli izpah endoproteze, smo primerjali s kontrolno skupino petindevetdesetih TEP kolkov, ki so bili stabilni vsaj 10 let po vstavitvi. Za testiranje predispozicije smo analizirali kontralateralne nativne kolke v obeh skupinah. Rezultantna kolčna sklepna sila je bila 6 % višja (p=0.004), 11 % višji je bil normaliziran najvišji tlak na nosilno površino (p=0.001) in pol tlaka je bil 50 % pomaknjen bolj v lateralno smer (p=0.026) pri TEP kolkov v študijski kot v kontrolni skupini. Vsi biomehanski parametri so bili v povprečju za stabilnost manj ugodni v študijski skupini kot v kontrolni skupini. Med kontralateralnimi nativnimi kolki ni bilo statistično pomembnih razlik v izmerjenih parametrih. 1) Naša raziskava ni potrdila hipoteze, da obstaja biomehanska predispozicija za izpah TEP kolka2) Raziskava je potrdila hipotezo, da je porazdelitev kolčnega tlaka pri TEP kolkov v študijski skupini drugačna kot v kontrolni skupini3) Potrdili smo hipotezo, da sta najvišji kolčni tlak in smer rezultantne kolčne sklepne sile v povprečju drugačna v reprezentativnem položaju – pri stoji na eni nogi. Višji kolčni tlak in bolj lateralna lega pola tlaka sta prediktorja izpaha TEP kolka. Rezultati kažejo, da je oblika medenice in proksimalnih stegnenic po vstavitvi TEP kolka spremenjena in lahko povzroči za stabilnost manj ugodno porazdelitev kolčnega tlaka pri stoji na eni nogi. Spremenjena porazdelitev kolčnega tlaka lahko povzroči izpah TEP kolka. Kljub temu da se izpah ne pojavi med stojo na eni nogi, pa lahko pride do izpaha med drugimi aktivnostmi. Kolčni tlak je pomaknjen bolj na lateralno stran in lahko preoblikuje obliko proteze do te mere, da se poveča verjetnost za pozni izpah kolka.Dislocation after hip arthroplasty is still a major concern. The aim of this work was to study whether biomechanical parameters of contact stress distribution in THA (total hip arthroplasty) in the one-legged stance can indicate a higher possibility of the arthroplasty dislocating. The following hypotheses were tested: 1) Patients who had dislocation of THA had the biomechanical predisposition before the implantation of prosthesis2) Distribution of contact stress with THA’s that dislocated was different than with THA’s that were uneventful3) The peak stress and the position of the stress pole is different in THA’s that dislocated than with THA’s that were uneventful. Biomechanical parameters were determined using 3-dimensional mathematical models of the one-legged stance within the HIPSTRESS method. Geometrical parameters were measured from standard anteroposterior X-ray images of the pelvis and proximal femora. The study group of fifty-five patients subjected to THA that later suffered dislocation of the head and, for comparison, the control group of 95 THA that were functional at least 10 years after the implantation were included in the study. To test the predisposition contralateral native hips were evaluated. Study group arthroplasties had on average a 6 % higher resultant hip force than the control group (p=0.004), 11 % higher peak stress on the load-bearing area (p=0.001) and a 50 % more laterally positioned stress pole (p=0.026), all parameters being less favorable in the study group. There were no significant differences in contralateral native hips between the study and control group measured parameters. Our study did not confirm that there is a biomechanical predisposition for THA dislocation. The study confirmed that THA’s prone to dislocation had less favorable stress distribution than THA’s that were uneventful. The study confirmed that higher stress and more laterally positioned stress pole in the one-legged stance predispose the dislocation of the THA

Hip stress distribution - Predictor of dislocation in hip arthroplasties. A retrospective study of 149 arthroplasties.

Dislocation after hip arthroplasty is still a major concern. Recent study of the volumetric wear of the cup has suggested that stresses studied in a one-legged stance model could predispose arthroplasty dislocation. The aim of this work was to study whether biomechanical parameters of contact stress distribution in total hip arthroplasty during a neutral hip position can predict a higher possibility of the arthroplasty dislocating. Biomechanical parameters were determined using 3-dimensional mathematical models of the one-legged stance within the HIPSTRESS method. Geometrical parameters were measured from standard anteroposterior X-ray images of the pelvis and proximal femora. Fifty-five patients subjected to total hip arthroplasty that later suffered dislocation of the head and, for comparison, ninety-four total hip arthroplasties that were functional at least 10 years after the implantation, were included in the study. Arthroplasties that suffered dislocation had on average a 6% higher resultant hip force than the control group (p = 0.004), 11% higher peak stress on the load-bearing area (p = 0.001) and a 50% more laterally positioned stress pole (p = 0.026), all parameters being less favorable in the group of unstable arthroplasties. There was no statistically significant difference in the gradient index or in the functional angle of the weight bearing. Our study showed that arthroplasties that show a tendency to push the head out of the cup in the representative body position-the one-legged stance-are prone to dislocation. An unfavorable resultant hip force, peak stress on the load bearing and laterally positioned stress pole are predictors of arthroplasty dislocation