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ID naloge: 42 Letnik: 1998 Predmet: ortopedija
Ocenjevanje displazije kolka pri odraslih
Avtor: Blaž Mavcic
Mentor: doc. dr. Vane Antolic, dr. med
Somentor: doc. dr. Aleš Iglic, dipl. ing. fizike
Izhodišca: V kolcnem sklepu pogosto pride do degeneracije sklepnega hrustanca idiopatsko (primarna artroza). V primerih z znano patogenezo (sekundarna artroza) pa do degeneracije sklepnega hrustanca najpogosteje pride zaradi mehanicnih deformacij kosti, ki sodelujejo v sklepu. Takšne mehanicne deformacije oznacujemo kot displazijo kolka. Pri odlocanju o nacinu zdravljenja se upoštevajo klinicne, anatomske in biomehanske razmere v kolku, za ocenjevanje slednjih se je kot pomemben parameter v klinicni praksi uveljavil Wibergov kot lateralnega pokritja kolcne glavice. V tem delu predlagamo, da je za displasticne kolke znacilna neugodna porazdelitev tlacne napetosti v kolcni plasti. Z matematicnim modelom ugotavljamo, kateri geometrijski parametri okolcja poleg Wibergovega kota tudi vplivajo na porazdelitev tlacne napetosti po obremenjeni površini kolka. Na tej osnovi predlagamo metodo za ocenjevanje omenjenih parametrov iz standardnih antero-posteriornih rentgenogramov. Predlagano metodo preizkusimo na ilustrativnih primerih preiskovancev z majhnim kotom lateralnega pokritja kolcne glavice, ki kljub temu nimajo klinicnih težav.
Metode: Analizirali smo matematicni model za dolocanje tlacnih napetosti v kolcni plasti pri stoji na eni nogi. V okviru tega modela smo dolocili parametre, ki vplivajo na vrednosti tlacnih napetosti v kolcni plasti pri razlicnih oblikah stegnenice in okolcja. Preiskali smo 411 standardnih antero-posteriornih rentgenogramov preiskovancev z zdravimi kolki in izmed teh izbrali preiskovance, ki so imeli Wibergov kot v zdravem kolku manjši od 30°. Za vsakega od izbranih preiskovancev smo z matematicnim modelom izracunali vrednosti tlacnih napetosti v kolcnem sklepu.
Rezultati: Pri ocenjevanju kolcne displazije so pomembni: polmer krogle r, ki se prilega kolcni sklepni površini, velikost rezultantne kolcne sklepne sile R ter parameter funkcionalni kot pokritja kolcne glavice, ki ga v tem delu prvic predlagamo in je enak vsoti Wibergovega kota in polarnega kota nagiba kolcne sile R glede na navpicnico. Nadalje smo ugotovili, da v populaciji ljudi z zdravimi kolki relativno redko najdemo kolke z majhnim Wibergovim kotom. Od preiskanih 411 je 9 preiskovancev imelo kot manjši od 30°, od teh je bil pri 4 preiskovancih manjši ali enak 20°. Za 5 izmed omenjenih 9 preiskovancev smo lahko izracunali, da so pri njih vrednosti tlacnih napetosti v kolcni plasti primerljive z in vivo izmerjeno referencno vrednostjo, kar pripisujemo vplivu ugodne geometrije okolcja in nizke telesne teže.
Zakljucki: Funkcionalni kot pokritja kolcne glavice, polmer kolcne sklepne površine r in velikost kolcne sile R so parametri, ki jih je z relativno enostavno metodo mogoce izmeriti iz standardnega antero-posteriornega rentgenograma in zato lahko služijo kot pomembni dejavnik pri ocenjevanju kolcne displazije v klinicni praksi. Njihova vloga je še posebej pomembna pri mejnih vrednostih Wibergovega kota, kjer lahko vecji nagib kolcne sile R, velik polmer kolcne sklepne površine in majhna telesna teža kompenzirajo neugodno lateralno pokritje.
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[Abstract / English version]
Assessment of hip dysplasia in adults
Author: Blaž Mavcic
Mentor: doc. dr. Vane Antolic, dr. med
Co-mentor: doc. dr. Aleš Iglic, dipl. ing. fizike
Outline and objectives: In the hip joint, degeneration of the joint cartilage and osteoarthritis most often occur as idiopathic disease (primary osteoarthritis). In cases with known pathogenesis (secondary osteoarthritis) the most common cause for osteoarthritis are the mechanical deformations of bones, designated by the term hip dysplasia. When deciding upon medical treatment, clinical, anatomical and biomechanical status of the hip are taken into account. The estimation of the biomechanical status is often based on the Wiberg centre-edge angle of lateral covering of the femoral head. In this work we propose that the dysplastic hips are characterized by an unfavorable stress distribution over the weight bearing area. A mathematical model is used to find out which geometrical parameters in addition to the Wiberg centre-edge angle also influence the distribution of stresses over the weight bearing surface. Then, a procedure is proposed for assessing these parameters from a standard antero-posterior rentgenogram. The proposed procedure is demonstrated on ilustrative examples of healthy subjects with small centre-edge angles, who are nevertheless void of clinical problems.
Methods: We analyzed the mathematical model for determination of stresses over the weight bearing surface in the hip in one-legged stance. Within this model, the parameters that influence the distribution of stresses in the hip joint for different hip and pelvis configurations were determined. Standard antero-posterior rentgenograms of 411 subjects were examined and the subjects with centre-edge angles smaller than 30° were selected. The mathematical model was used to determine the values of radial stress for the selected subjects.
Results: The relevant parameters for assessment of the hip dysplasia were found radius of the sphere that corresponds to the hip weight bearing surface r, the magnitude of the force R and the parameter functional angle of the femoral head covering, that is proposed in our work for the first time and equals the sum of the centre-edge angle and the angle of the vertical inclination of the resultant hip force R. Further, it was found that the subjects with small centre-edge angles are relatively rare in the healthy population. Out of 411 examined subjects only 9 had the angle smaller than 30° while in four of those the angle was smaller or equal to 20°. The values of peak stress in 5 out of the 9 subjects were estimated to be comparable with the reference measurements, performed in vivo. This is attributed to the compensating effects of the favorable hip and pelvis geometry and low body weight.
Conclusions: The functional angle of the femoral head covering, the femoral head radius and the size of the force R are the parameters that can be determined from a standard AP rentgenogram by using a relatively simple method and can therefore represent an important factor in the hip dysplasia estimation procedure. They are particularly important in the borderline values of the centre-edge angle, where larger inclination of the force R, large femoral head radius and small body weight can compensate unfavorable lateral covering.
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