The influence of the shape of hips and pelvis and the position of hip endoprosthesis on the dislocation of the femoral head and orthotic post-operative care

Uvod: Artroplastika ter sklepna endoprotetika je najpogostejša in zelo uspešna operativna tehnika. Glavni razlogi za to operacijo so osteoartroza, zlom vratu stegnenice pri starostniku ter aseptična nekroza glavice stegnenice. Pooperativni izpah oziroma luksacija kolčne endoproteze je pogosta komplikacija, ki se lahko pojavi po vstavitvi totalne kolčne endoproteze. Verjetnost izpaha kolčne endoproteze je 2% (Dargel et al., 2014) v prvem letu po operaciji, 6% po dvajsetih letih (Berry et al., 2005), v primeru menjave kolčne endoproteze pa verjetnost naraste na 28% (Dargel et al., 2014). Namen: Namen diplomskega dela je ugotoviti, ali imajo kolčne proteze z neugodno porazdelitvijo kolčnega sklepnega tlaka povečano tveganje za izpah. Metode dela: V raziskavi smo primerjali skupino protez, pri katerih je prišlo do izpaha endoproteze (testna skupina) in skupino protez, ki so bile funkcionalne vsaj 10 let (kontrolna skupina). Kontrolna skupina je vsebovala 94 endoprotez 77 pacientov, testno skupino pa je sestavljalo 61 endoprotez 61 pacientov z vstavljenimi kolčnimi endoprotezami. Od tega je bilo šest parcialnih endoprotez in 55 totalnih endoprotez. Uporabili smo metodo HIPSTRESS za določanje porazdelitve kontaktnega kolčnega tlaka pri stoji na eni nogi. Za vstopne podatke smo uporabili geometrijske parametre medenice in okolčja, ki smo jih odčitali iz standardnih anteroposteriornih (AP) rentgenskih slik. S pomočjo geometrijskih parametrov smo v nadaljevanju študije izračunali tudi biomehanske parametre (velikost in smer rezultantne kolčne sklepne sile, lego pola tlaka in največji tlak na obremenjeni površini) z uporabo programa HIPSTRESS. Pri opisu že obstoječih znanj smo uporabili deskriptivno metodo in pregled literature. Rezultati: Pri primerjavi parametrov se pojavijo statistično pomembne razlike med testno in kontrolno skupino v velikosti rezultantne kolčne sile in v največjem kolčnem tlaku, ki sta bila normalizirana s telesno težo. Razlike so bile statistično pomembne, parametri pa manj ugodni v testni skupini. Razprava in zaključek: Ugotovili smo, da je imela populacija protez, pri katerih je prišlo do izpaha, manj ugodno porazdelitev tlaka, ki tudi pri stoji na eni nogi poriva glavico lateralno. Matematični modeli so primerni za načrtovanje optimalne lege proteze.Introduction: Arthroplasty and endoprosthetics is a new and a very successful operative technique. The main indications for arthroplastic surgery are osteoarthritis, fracture of the femoral neck in geriatric patients and aseptic necrosis of the femoral head. Postoperative dislocation of the total hip endoprosthesis is a complication that can occur after the surgery. The possibility for the dislocation is 2% (Dargel et al, 2014) in the first year and 6% (Berry et al, 2005) in the first twenty years. If the hip endoprosthesis has to be replaced, the risk for dislocation increases to 28% (Dargel et al, 2014). Purpose: The purpose of this diploma thesis is to find out whether the hip endoprostheses with poor hip joint pressure distribution are more susceptible to dislocation. Methods: The present research compared a group of unstable hip endoprostheses that underwent dislocation (experimental group) and hip endoprostheses that were functional for at least 10 years (control group). The control group with stable hips consisted of 94 endoprostheses of 77 patients. In the experimental group there were 61 hip endoprostheses from 61 patients. In this group there were 6 partial hip endoprostheses and 55 total hip endoprostheses. The HIPSTRESS method was used to determine the hip stress distribution in one-legged stance. The geometrical parameters of the hip and pelvis were used as input data. These parameters were retrieved from standard anteroposterior (AP) radiograms. The geometrical parameters enabled the calculation of biomechanical parameters (the size and the direction of the resultant hip force, the location of the stress pole, and the peak stress on the weight-bearing area) by using the HIPSTRESS program. The descriptive method with a literature review was used to describe the existing knowledge. Results: The differences between the experimental and the control group occurred in the size of the resultant hip force and in the maximal hip stress which were normalized to body weight. The differences were statistically significant and the parameters were less favourable in the experimental group. Discussion and conclusion: It has been found that the population with the dislocated endoprostheses had a less favourable stress distribution. This means that even in one-legged stance it pushes the femoral head laterally. The mathematical models are suitable for planning the optimal position of the endoprosthesis

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