Heupstressverdeling – Voorspeller van dislocatie bij heupartroplastiek. Een retrospectieve studie van 149 artroplastiek

Heupstressverdeling – Voorspeller van dislocatie bij heupartroplastiek. Een retrospectieve studie van 149 artroplastiek

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Open Access


Research Article

  • Matevž Tomaževič, 
  • Tina Kaiba, 
  • Urban Kurent, 
  • Rihard Trebše, 
  • Matej Cimerman, 
  • Veronika Kralj-Iglič



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.

Citation: Tomaževič M, Kaiba T, Kurent U, Trebše R, Cimerman M, Kralj-Iglič V (2019) Hip stress distribution – Predictor of dislocation in hip arthroplasties. A retrospective study of 149 arthroplasties. PLoS ONE 14(11): e0225459. https://doi.org/10.1371/journal.pone.0225459

Editor: Daniel Pérez-Prieto, Consorci Parc de Salut MAR de Barcelona, SPAIN

Received: June 3, 2019; Accepted: November 5, 2019; Published: November 20, 2019

Copyright: © 2019 Tomaževič et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All relevant data are within the manuscript and its Supporting Information files.

Funding: The study was funded by the Slovenian Research Agency grant P3-0388 to VK-I. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.


Based on patient reported outcome measures, hip arthroplasty is the most successful elective surgical procedure [1]. Thanks to efforts to design and produce an optimal prosthesis offering long-term functionality, more than 95% of arthroplasties survive more than 10 years [2,3]. Some total hip arthroplasties (THAs) nevertheless fail and revision surgery is needed. Hip dislocation after THA is the second most common cause of revision surgery [2]. The reported rate of revision due to dislocation after primary THA is 2–4% in the first six months [4] and increases to 6% after 20 years [5]. After revision surgery, the dislocation rate levels at 5.4% in the first year after the operation [6]. Dislocation studies of THA have been performed based on component positioning, with an emphasis on cup orientation [710], the effect of artificial head size [5,11,12] and impingement as causes of a prosthesis head dislocation [1315]. Still more than 50% of dislocations occur when the cup is in the so-called safe zone position [10]. One of the causative factors for instability after THA might be spino-pelvic imbalance [16], decreased mobility of the spine, decreased tilt of the pelvis that might cause impingement [17] and liner wearing [18]. According to latest review soft tissue insufficiency might be the cause for dislocations and not the approach used [19]. A larger femoral head diameter increases the range of motion before the prosthesis neck impinges on the acetabulum liner, which causes the prosthesis to dislocate [5,11,20]. Despite the careful positioning and selection of THA components, dislocations still occur. The question arises as to whether THA changes the geometry of the hip in such a way as to affect the biomechanical parameters in the joint, forcing the hip to dislocate at the edge of the motion range.

The HIPSTRESS method was developed to calculate biomechanical parameters in the hip considering pelvis and femur anatomy [21,22]. Using this method, the hip stress distribution can be calculated in a neutral hip position during a one-legged stance, using a standard X-ray image of the hip and pelvis. It has been validated by clinical studies considering various pathologies in native hips [23,24] and in hips with total hip arthroplasties [2527].

The HIPSTRESS method demonstrates that linear wear occurs in the direction of the stress pole [25,26] and that it is proportional to the peak stress on the weight bearing area [25]. Because of this effect, the volumetric wear on the cup is less for a larger abduction angle of the cup [27], since the head partly migrates out of the socket [27]. On the other hand, it has been shown that this could be unfavorable in terms of dislocation [27]. The aim of this work was to provide an answer to the question posed by the results of previous work [27]: whether arthroplasties that have suffered dislocation have a less favorable stress distribution.

The hypothesis of this work was that biomechanical parameters (higher peak stress, more lateral stress pole and less negative stress gradient index) are predictors for dislocation of a THA. To test this hypothesis, we compared the biomechanical parameters of a population of THA that had suffered dislocation and a population of THA that were successful at least 10 years post-operatively.


The study was designed as a retrospective individual case control study, level of evidence 3B. It was approved by Slovene National Medical Ethics Committee letter No.: 110/04/15 which serves also as an institutional review board approval prior to performing the study. Since the study was retrograde and biomechanical analysis was done on the X-ray images that were coded and already taken before the beginning of the study no informed consent was needed.

Anteroposterior (AP) X-ray images of the hip and pelvic skeleton of patients that had undergone THA were used to measure geometrical parameters relevant for a determination of biomechanical parameters within the HIPSTRESS method. X-ray images of patients that had suffered dislocation of hip arthroplasties were included in the study group. Patients were chosen from the emergency department database on the basis of a diagnosis of hip dislocation, ICD S73.0. Patients admitted to the Emergency Department, University Clinical Center Ljubljana from November 2012 until September 2015 were included. Images were downloaded from the Impax server and coded. Eighty-one patients with a diagnosis of dislocation of the hip joint were gathered. Exclusion criteria were patients that had suffered hip dislocation due to high energy trauma (17 patients), patients with whom dislocation had occurred due to material breakage of the hip prosthesis (2 patients) and patients with whom the contours of the femur or pelvis were not clearly visible on the X-ray image (1 patient). X-ray images of the whole pelvis and hips after reduction of the dislocation were used for analysis. Images were taken in a supine position. The patients were awake when taking the image. After excluding all patients with the exclusion criteria, 61 patients remained in the study group. Forty-one of them were female and 20 male, and the average age was 64.4 years. There were 24 (39%) right hips and 37 (61%) left hips. Twenty-five (41%) of them had already undergone hip arthroplasty on the contralateral hip. The time from the operation to dislocation was on average 402,1 (SD 771,4) days. Dislocation occurred in 47 (77%) cases in less than one year after the THA procedure. The average time to dislocation in this group was 57,2 (SD 79,7) days. Dislocations occurred between 1 and 10 years in 14 (23%) cases after the THA procedure (average 4,27 years with SD 2.53 years). In the study group, the posterior approach was used in 9 patients, the anterolateral approach in 12 patients and the lateral approach in 40 patients.

Patients with partial arthroplasty involving two artificial femoral heads (6 patients) were excluded from the study, since different biomechanics applies due to the double mobility of the partial arthroplasty joint [28].

Patients admitted with partial hip arthroplasties and three patients with THA (altogether 9 patients) were operated at the Clinical Department of Traumatology, University Medical Center Ljubljana. In these patients, the size of the femoral head could be retrieved from the archive. In the remaining 52 patients, the size of the femoral head was not known. Standard X-ray images of the hip and pelvis taken at the Emergency Department, University Medical Center Ljubljana were assumed to have an average magnification of 115% and the size of the prosthesis head was estimated by rounding to 28mm, 32mm or 36 mm using software developed for preoperative planning at the Department of Traumatology, University Medical Center Ljubljana. Twenty-six (47%) hips in the study group with THA had a femoral head diameter of 28 mm, 12 (22%) had a femoral head diameter of 32 mm and 17 (31%) had a femoral head diameter of 36 mm. On average, the radius of the prosthesis head was 15.67 mm ± 1.75 mm. In the study group, there were two cases where elevated liners were seen on the x ray pictures. The data on the inlay and the prosthesis head were available from the archive for three cases only. In these three cases we did not see any elevated liner. Stems were cemented in 26 cases and uncemented in 29 cases. None of the femoral stems were loosened. The acetabulum components were uncemented in 23 cases and cemented in 32 cases. There were signs of loosening of the cemented acetabulum component in one case.

To define the control group, we examined X-ray images of 311 patients who had undergone total hip arthroplasty (THA) at the Orthopedic Hospital Valdoltra, Slovenia. The first available X-ray images or THA with whole pelvis (in terms of date) were taken from the Impax server at the hospital. Inclusion criteria were an X-ray image of the pelvis and proximal femora after implantation, no event of hip dislocation or septic or aseptic loosening and regular follow-up for at least 10 years. Patients who had undergone any revision procedure during that time were also excluded. Ninety-four hips (54 (63%) right and 35 (37%) left) of 77 patients that met the inclusion criteria (45 female and 32 male) were included in the analysis. Sixty-nine of them also had a prosthesis on the contralateral side. The average age of the patients included at the time of implantation was 59.6 years. There were 79 (84%) THA with a femoral head diameter of 28mm, 2 (2%) of them had a femoral head diameter of 32 mm and 13 (14%) had a femoral head diameter of 36mm. In the control group, acetabulum was cemented in 7 cases and uncemented in 87 cases. Inlay was ceramic–alumina in 20 cases and polyethylene in 60 cases (UHMWPE in 44 cases, crosslinked PE in 14 cases and Eduron PE in 2 cases). Head was ceramic–alumina in 34 cases, CoCr alloy in 30 cases and Fe alloy in 30 cases. Femur stem was cemented in 5 cases and uncemented with diaphyseal fit in 89 cases. In the control group, lateral approach was used in all THA procedures.

For biomechanical evaluation, three-dimensional mathematical models of an adult human hip within the HIPSTRESS method were used [22,24]. The models are described in detail elsewhere (see for example ref. [24]) so only a brief description will be given here. The method consists of two mathematical models: one for determination of the resultant hip force in the representative body position for everyday activities [29], i.e., the one-legged stance [30], and the other for determination of contact hip stress distribution [21]. The model for resultant hip force is based on force and torque equilibrium equations [22,30]. The model describes a system composed of two segments: the loaded leg and the rest of the body. It includes 9 effective muscle forces, the weight of the segments and the intersegment force (the resultant hip force). The reference muscle attachment points are obtained from measurements performed on a cadaver and then re-scaled for the individual hip considered. Since the X-ray image is two-dimensional, data in the third dimension are taken to be equal to the reference values. The model for force uses as input the geometrical parameters of pelvis and proximal femur: pelvic width (C) and height (H), inter-hip distance (l) and the position of the muscle attachment point on the greater trochanter (x,z) (Fig 1). Stress integrated over the load-bearing area yields the resultant hip force R = ʃ p dA, where p is stress and dA is the area element. The calculations and procedures have been explained previously [2325,31].


Fig 1. Geometric and biomechanical parameters within the HIPSTRESS models that are used for calculation of stress distribution in a right artificial hip.

R resultant hip force; pmax peak stress on the load bearing area; θpole angle of the stress pole; ϑabd abduction angle; ϑR angle of the resultant hip force; C horizontal distance between the center of the prosthesis head and the most lateral point on the iliac crest; H vertical distance between the center of the prosthesis head and the highest point on the iliac crest; x vertical distance between the center of the prosthesis head and the point on the greater trochanter in the direction of the femur; z distance between the center of the prosthesis head and the point on the greater trochanter perpendicular to the femur axis; l distance between the centers of the femoral heads.


HIPSTRESS models use hip and pelvis geometric parameters as input data: inter-hip distance (l), height of the pelvis (H), horizontal distance from the prosthesis head center to the lateral edge of the pelvis (C), position of the greater trochanter relative to the prosthesis head center in the coordinate system of the femur (distances z and x) and abduction angle. The geometrical parameters were determined using CorelDRAW Graphics Suite X7, 2015, Ottawa, Canada, by two blinded measurers. HIPSTRESS software [31] was used for calculation of the biomechanical parameters. (Fig 1)

Previous studies [3236] have indicated that the peak stress on the load-bearing area pmax is a useful biomechanical parameter. If the stress pole is located inside the load bearing area, pmax is equal to the value of the stress at the pole. If the stress pole lies outside the load-bearing area, contact stress is highest at the point of the load-bearing area that is closest to the pole (Eq 1), (1) where p0 is the value of stress at its pole, ϑabd is the abduction angle of the prosthesis cup and Θpole is the angle of the stress pole (Fig 1). Another indicator of the stress distribution is the gradient of stress, represented by its value at the lateral rim of the cup Gp (Eq 2) [27,37], (2) where r is the radius of the articular surface. If the pole of stress distribution lies outside the load-bearing area (i.e., if Θpole > π/2 –ϑabd), then Gp is positive, stress attains its highest value at the lateral rim and falls off rapidly in the medial direction, while the corresponding weight bearing area is small [38]. Such a distribution represents dysplastic hips [38]. If, however, the pole of stress distribution lies inside the load–bearing area (if Θpole abd), then stress reaches its peak within the weight bearing area, which is consequently larger and Gp is negative [38]. The functional angle of load bearing ϑf is defined as (Eq 3).


In relation to dislocation, high resultant hip force and high peak stress are unfavorable. However, a high gradient index and more laterally positioned pole are expected to represent an even greater risk of dislocation, since the head is pushed more laterally each time the leg is loaded.

The peak stress pmax is proportional to r-2, while the hip stress gradient index Gp is proportional to r-3 [21,38]. Since different sizes of prosthesis heads were involved, the effect of the femoral head size on biomechanical parameters was eliminated by multiplying pmax by r2 and Gp by r3. The resultant hip force, peak stress and stress gradient index are also proportional to body weight Wb. Since the body weight was unknown, its effect was eliminated by normalizing the respective parameters by Wb. The normalized biomechanical parameters R/WB, pmaxr2/Wb, Gpr3/Wb, ϑf and Θpole express the geometry of the pelvis and the proximal femur and the geometry and position of the arthroplasty’s elements (but not the size of the artificial head).

Statistical analysis of the biomechanical parameters between the two groups was done using the Student T—test. If the p value was ≤ 0.05, the difference between these two groups was significant. The statistical power (1-β) of the result was taken as sufficient when the power was >80%. Statistical analysis was done using Microsoft Excel 2010 (14.0.7188.5000, Microsoft Corporation, Santa Rosa, California, USA). The power of the statistics was calculated using a statistics power calculator on the internet: http://clincalc.com/Stats/Power.aspx


Normalized resultant hip force R/Wb and normalized peak stress pmax.r2/Wb were considerably and statistically significantly less favorable in the study group than in the control group, with sufficient statistical power. The position of the stress pole was more lateral (less favorable) in the study group. The difference was statistically significant but with somewhat deficient statistical power (Table 1). The normalized stress gradient index Gp.r3/Wb and functional angle of load bearing showed no statistically significant difference, although it was less negative and smaller (which is unfavorable) in the study group.

There was a considerable and statistically significant difference in parameters x (distance between the center of the prosthesis head and the point on the greater trochanter in the direction of the femur axis) and C (horizontal distance between the center of the prosthesis head and the most lateral point of the iliac crest) between the study and control groups (Table 2). Higher x and C indicate less favorable biomechanical parameters in the study group. The difference between the abduction angles in the study group and control group was minute, which explains the lack of statistical significance of the difference in the functional angle of weight bearing (Table 1).

To evaluate whether there were any significant differences in the geometry of the pelvis and hip biomechanics between the study and control groups, a comparison of the native hips contralateral to the arthroplasties between the study and control groups was performed. Since there were cases in both groups with bilateral arthroplasties, only subjects without contralateral hip arthroplasty were taken for comparison.

There were no statistically significant differences between hips contralateral to arthroplasties that had suffered dislocation and arthroplasties that were functional at least 10 years post-operatively (Tables 3 and 4). This indicates that the geometry of the pelvis and proximal femur of the patient before arthroplasty is not the cause of the difference between the study and control groups and that changes induced by arthroplasty surgery are connected to the difference in the biomechanical parameters of the study and control groups.


Our study supports the hypothesis that contact stress distribution on the prosthesis head is a predictor of arthroplasty dislocation. The hypothesis of this study was formed following previous work by Rijavec et al. [27], in which the effect of cup inclination on predicted contact stress-induced volumetric wear in THA was considered. In that study, it was indicated that a larger abduction angle of the prosthesis cup is more favorable in terms of prosthesis wear, although it can represent a larger risk of its dislocation.

It is shown above (Table 1) that THA that had suffered dislocation had a less favorable distribution of contact stress (given by its peak value and the position of the pole), which pushed the artificial hoofd meer lateraal dan in het geval van prothesen die ten minste 10 jaar na de operatie functioneerden. Een eerdere studie had aangetoond dat het hoofd migreert in de richting van de stresspool [ 25 ]. Dit proces verandert de vorm van de interface tussen het hoofd en de beker en draagt ​​bij aan de ontwikkeling van een hefboom die tot dislocatie leidt.

Vergelijking van native heupen contralateraal met ontwrichte en functionele artroplastiek vertoonde geen statistisch significante verschillen, in beide biomechanische parameters of geometrische parameters (tabellen 3 en 4 ), wat aangeeft dat de initiële geometrie van het bekken en het proximale dijbeen was gemiddeld hetzelfde in de twee groepen. Het is de verandering in de bekkenanatomie na de artroplastiekprocedure die de biomechanica van de heup verandert en deze veranderingen waren minder gunstig in de groep protheses die ontwricht waren.

Dislocatie van het heupgewricht na THA is een van de belangrijkste nevencomplicaties (2% – 4% na primaire totale heupartroplastiek) [ 2 , 39 ] en de oorzaken ervan zijn eerder onderzocht [ 5 , 7 20 , 39 42 ] . Onder patiëntgerelateerde factoren is de aanwezigheid van neuromusculaire aandoeningen, zoals cerebrale parese, spierdystrofie, dementie en de ziekte van Parkinson, een van de belangrijkste risicofactoren [ 39 , 43 ]. Meer dislocaties werden gevonden bij patiënten ouder dan 80 jaar [ 43 ], patiënten met een femur-nekfractuur als primaire diagnose [ 40 , 42 ] en patiënten met een ASA-score (American Society of Anesthesiologists) van 3 of 4 [ 40 ]. Onder aan procedure gerelateerde factoren, de gemeten parameters van componentposities, een bekerhelling buiten het bereik van 40 ° ± 10 °, een bekeranteversie van minder dan 10 ° of meer dan 35 °, een steelanteversie buiten het bereik van 14,8 ° ± 6,01 ° en een hoogte van het middelpunt van de heuprotatie buiten het bereik van 2,16 mm ± 9,11 mm, verhoogde het risico op ontwrichting [ 8 , 9 , 40 ]. De kunstmatige kopgrootte, discrepantie van de beenlengte en acetabulaire inclinatie werden allemaal bestudeerd in twee afzonderlijke studies en er werd gevonden dat dit geen statistisch belangrijke factoren zijn die dislocatie van de femurkop voorspellen [ 7 , 44 ]. Anderzijds bleek in een onderzoek van Berry [ 5 ] een kleinere maat van de prothesekop te zijn gerelateerd aan een hogere dislocatiesnelheid van THA . In een studie van Forde et al. [ 7 ], werd gemeld dat als de femorale offset ten minste 3 mm groter was dan aan de contralaterale zijde, het risico op dislocatie lager was [ 7 ]. Offset wordt in het HIPSTRESS-model uitgedrukt door parameter z. Een grotere z is biomechanisch gunstig, omdat het een lagere resulterende kracht en een grotere hellingshoek impliceert, wat bijgevolg lagere piekspanning, een meer mediale locatie van de paal, een kleinere gradiëntindex en een grotere functionele hoek van gewichtslager betekent [ 24 ]. Onze resultaten zijn daarom in overeenstemming.

In een studie van Rijavec [ 27 ] die het effect van de bekerhelling op de voorspelde door contactstress veroorzaakte volumetrische slijtage in THA in overweging nam, werd spanningsverdeling voorgesteld als een relevante factor die verband hield met de kans op dislocatie van de kunstkop. Onze resultaten laten zien dat genormaliseerde stress inderdaad aanzienlijk en statistisch significant minder gunstig was in de studiegroep. De positie van de paal was statistisch significant minder gunstig in de studiegroep, terwijl het verschil in gradiëntindex Gp . r 3 / W b , hoewel het een minder gunstige configuratie in de studiegroep vertoonde, was statistisch niet significant. Aangezien drie van de parameters gemiddeld minder gunstig waren in de onderzoeksgroep, ondersteunen onze resultaten de voorgestelde hypothese.

De meest beschouwde metingen in prothesen zijn abductiehoek en de laterale offset [ 7 , 44 ]. We vonden gemiddeld geen statistisch significante verschillen tussen abductiehoeken in de studie- en controlegroepen, hoewel de standaarddeviatie iets groter was in de onderzoeksgroep. De gemiddelde waarden van de abductiehoek waren in beide groepen bijna ideaal (45 graden), dus de verschillen in biomechanische parameters konden niet te wijten zijn aan de abductiehoek, die ook werd gevonden in een studie van Forde [ 7 ]. Anderzijds is de offset verbonden met de vorm van de grotere trochanter. We vonden dat er een aanzienlijk en statistisch significant verschil was in de positie van het kenmerkende punt op de grotere trochanter, wat minder gunstig was in heupen die ontwricht waren.

Onze studie- en controlegroepen hadden een andere verdeling van kunstmatige kopgroottes en we concentreerden ons op de effect van de geometrie van het bekken en het proximale dijbeen en de helling van de kunstmatige beker. We kozen daarom voor biomechanische parameters die onafhankelijk waren van de kunstmatige kopgrootte. Het effect van de grootte van de femurkop werd beschouwd als [ 45 ], die rapporteerde dat een grotere kop de kans op een botsing tussen de femurhals en de acetabulaire voering verkleint. Onze analyse was gericht op de eenbenige houding en hield geen rekening met gebeurtenissen die specifiek verband hielden met andere lichaamsposities waardoor de heupkop kon ontwrichten.

Preoperatieve planning wordt geadviseerd vóór implantatie van een kunstheup en wordt meestal gedaan [ 46 ]. Optimalisatie van de keuze en positie van prothese-elementen met behulp van simulatie met een wiskundig model zou kunnen worden opgenomen in preoperatieve planning. Simulatie van postoperatieve biomechanische parameters zou nuttig zijn bij het plannen van de configuratie van prothesen, vooral in demografische groepen waarin heupen kwetsbaarder zijn voor dislocatie, of in revisiezaken. In gevallen waarin preoperatieve planning een artroplastiek voorspelde die vatbaar was voor dislocatie, kon de chirurg een andere implantaatconfiguratie kiezen.

De belangrijkste beperking van het onderzoek is dat biomechanische parameters worden bepaald met behulp van een wiskundig 3D-model op de tweedimensionaal AP-beeld van het bekken en de proximale femora. Vanuit dit beeld zijn spinopelvische uitlijning en anteversie van de beker niet toegankelijk. In de recente studies [ ​​16 , 17 ] werd spinopelvische uitlijning voorgesteld als een belangrijke factor die zou kunnen leiden tot late dislocatie van de totale heup-endoprothese.

Andere beperking is dat de controle-röntgenfoto’s van het bekken en de proximale femora na de dislocatie werden in rugligging genomen en niet in staande positie.



Onze resultaten toonden aan dat de vorm van het bekken en het proximale dijbeen na totale heupprothese invloed had op een minder gunstige spanningsverdeling in de representatieve dagelijkse activiteit, de eenbenige houding, in prothesen die ontwricht waren. Hoewel de heup niet ontwricht tijdens de eenbenige houding, hadden deze heupen hogere stress, lateraal opgehoopt, dan artroplastiek die ten minste 10 jaar functioneerde. De meer laterale positie van de spanningspaal kan het gewricht hermodelleren en dislocatie voor andere activiteiten vatbaar maken.



De auteurs bedanken Martin Cregeen voor de taalbewerking van het papier.


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