Matej Daniel

Mathematical estimation of stress distribution in normal and dysplastic human hips

By using a mathematical model of the adult human hip in the static one‐legged stance position of the body, the forces acting on the hip, peak stress in the hip joint and other relevant radiographic and biomechanical parameters were assessed. The aims were to examine if the peak stress in dysplastic hips is higher than in normal hips and to find out which biomechanical parameters contribute significantly to higher peak stress. The average normalized peak stress in dysplastic hips (7.1 kPa/N) was markedly higher (≈︁100%) than the average normalized peak stress in normal hips (3.5 kPa/N). The characteristic parameters that contributed to higher peak stress in dysplastic hips included the smaller lateral coverage of the femoral head, the larger interhip distance, the wider pelvis, and the medial position of the greater trochanter. These results are consistent with the hypothesis that stress distribution over weight‐bearing surface of the hip joint is the relevant parameter for assessment of the risk for developing coxarthrosis.

The shape of acetabular cartilage optimizes hip contact stress distribution.

The biomechanical role of the horseshoe geometry of the acetabular cartilage is described using a three‐dimensional mathematical model. It is shown that the acetabular fossa contributes to a more uniform articular contact stress distribution and a consequent decrease in the peak contact stress. Based on the results it is suggested that the characteristic horseshoe shape of the articular cartilage in the human acetabulum optimizes the contact stress distribution in the hip joint.

Computer determination of contact stress distribution and size of weight bearing area in the human hip joint.

The mathematical models and the corresponding computer program for determination of the hip joint contact force, the contact stress distribution, and the size of the weight bearing area from a standard anteroposterior radiograph are described. The described method can be applied in clinical practice to predict an optimal stress distribution after different operative interventions in the hip joint and to analyze the short and long term outcome of the treatment of various pathological conditions in the hip. A group of dysplastic hips and a group of normal hips were examined, with respect to the peak contact stress normalized by the body weight, and with respect to the functional angle of the weight bearing area. It is shown that both these parameters can be used in the assessment of hip dysplasia.

Gradient of contact stress in normal and dysplastic human hips

The stress gradient index (G(p)) is introduced for the assessment of dysplasia in human hip joint. The absolute value of G(p) is equal to the magnitude of the gradient of the contact stress at the lateral acetabular rim. The parameter G(p) normalized with respect to the body weight (W(B)) is determined from the standard anteroposterior radiographs of adult human hips and pelvises using the mathematical model. The average value of G(p)/W(B) was determined for the group of dysplastic hips and for the group of normal hips. In the group of normal hips the average value of G(p)/W(B) is smaller (-0.445x10(5) m(-3)) than in the group of dysplastic hips (+1.481x10(5) m(-3)). The difference is statistically significant P<0.001. The average value of G(p)/W(B) changes its sign at the value of the centre-edge angle theta(CE) approximately 20( composite function ) which is usually considered as the boundary value of theta(CE) (lower limit) for the normal hips. Accordingly we suggest a new definition for the hip dysplasia with respect to the size and sign of the normalized stress gradient index G(p)/W(B). The hips with positive G(p)/W(B) are considered to be dysplastic while the hips with negative G(p)/W(B) are considered to be normal. The statistically significant correlation between the value of the Harris hip score, used in the clinical assessment of the hip dysplasia, and the normalized stress gradient index was found.

Morbid Obesity May Increase Dislocation in Total Hip Patients: A Biomechanical Analysis

BackgroundObesity has reached epidemic proportions in the United States. Recently, obesity, especially morbid obesity, has been linked to increased rates of dislocation after THA. The reasons are unclear. Soft tissue engagement caused by increased thigh girth has been proposed as a possible mechanism for decreased joint stability.Questions/PurposesWe asked (1) whether thigh soft tissue impingement could decrease THA stability, and if so, at what level of BMI this effect might become evident; and (2) how THA construct factors (eg, head size, neck offset, cup abduction) might affect stability in the morbidly obese.MethodsThe obesity effect was explored by augmenting a physically validated finite element model of a total hip construct previously comprising just implant hardware and periarticular (capsular) soft tissue. The model augmentation involved using anatomic and anthropometric data to include graded levels of increased thigh girth. Parametric computations were run to assess joint stability for two head sizes (28 and 36 mm), for normal versus high neck offset, and for multiple cup abduction angles.ResultsThigh soft tissue impingement lowered the resistance to dislocation for BMIs of 40 or greater. Dislocation risk increased monotonically above this threshold as a function of cup abduction angle, independent of hardware impingement events. Increased head diameter did not substantially improve joint stability. High-offset necks decreased the dislocation risk.ConclusionsExcessive obesity creates conditions that compromise stability of THAs. Given such conditions, our model suggests reduced cup abduction, high neck offset, and full-cup coverage would reduce the risks of dislocation events.

Biomechanical evaluation of hip joint after salter innominate osteotomy: a long-term follow-up study

Abstract The biomechanical state of the hip after a Salter innominate osteotomy was investigated by using the radiographic data of 38 operated and 21 contralateral nonoperated hips from our archives. The centre-edge angle of Wiberg was determined from the radiographs taken shortly after the operation. From the radiographs of the latest follow-up (7–13 years after the operation), we also determined the peak value of contact hip joint stress normalized by the body weight, and the functional angle of the weight-bearing area. A mathematical model was used. We show that the geometrical parameters aside from the centre-edge angle may considerably influence the contact hip stress distribution. We also show that the functional angle of the weight-bearing area is a more relevant parameter than the normalized peak stress if the exact magnification of the images is not known and if there is considerable variation of the image size within the sample. The development of the centre-edge angle of the operated hips and of the contralateral hips was also studied. We found that the centre-edge angle increases on average during the follow-up time in the operated hips as well as in the contralateral nonoperated hips, but the average increase is smaller in the former. It is shown that an unfavorable stress distribution is connected to the decrease of the centre-edge angle over time. Finally, we found a weak positive correlation between the centre-edge angle shortly after the operation and the functional angle of the weight-bearing area at the of the latest follow-up.

Determination of contact hip stress from nomograms based on mathematical model

Nomograms are presented that enable determination of maximal stress on the hip joint weight bearing area if certain geometrical parameters of the hip and pelvis and the body weight are known. The nomograms are calculated by using previously developed mathematical models. It is demonstrated how the maximal stress on the hip joint weight bearing area is determined from the presented nomograms for a hip for which the geometrical parameters were obtained from a standard anteroposterior rentgenograph. This simple and noninvasive method may give insight into the biomechanical status of the hip which should be considered in routine surgical planning and as a part of the routine examination of the patient without the use of any additional tools.

Boundary cartilage lubrication: review of current concepts

SummaryEffective lubrication of synovial joints is important to prevent cartilage degeneration and to keep the joints healthy. This paper sets out the basics of engineering lubrication with respect to the composition and properties of synovial fluid constituents. Two basic types of boundary lubrication are discussed: the presence of highly hydrophilic proteoglycans that provide a water liquid film, and the existence of multilamellar phospholipids lubricating layers at the surface ofarticular cartilage. Based on current knowledge, we may conclude that no single mechanism of boundary lubrication exists, and that effective boundary lubrication of synovial joints is maintained by the synergic effect of all synovial fluid constituents.ZusammenfassungDie effektive Schmierung der synovialen Gelenke ist wichtig, um der Degeneration des Knorpelgewebes vorzubeugen und die Gelenke gesund zu erhalten. Die vorliegende Arbeit beschreibt die Grundlagen des Schmierungsaufbaus, was die Zusammensetzung und Eigenschaften der Synoviabestandteile betrifft. Zwei grundlegende Arten der Grenzflächenschmierung werden besprochen: das Vorkommen stark hydrophiler Proteoglykane, die einen wasserflüssigen Film bilden, und die Existenz multilamellarer Phospholipide, die auf der Oberfläche des artikulären Knorpels als Schmierschichten dienen. Nach heutigem Wissensstand dürfen wir schließen, dass es sich bei der Grenzflächenschmierung nicht um einen isolierten Mechanismus handelt. Vielmehr wird die effektive Grenzflächenschmierung der synovialen Gelenke durch den synergischen Effekt aller Bestandteile der Synovialflüssigkeit aufrechterhalten.

Role of phospholipid asymmetry in the stability of inverted hexagonal mesoscopic phases.

The role of phospholipid asymmetry in the transition from the lamellar (L(alpha)) to the inverted hexagonal (H(II)) phase upon the temperature increase was considered. The equilibrium configuration of the system was determined by the minimum of the free energy including the contribution of the isotropic and deviatoric bending and the interstitial energy of phospholipid monolayers. The shape and local interactions of a single lipid molecule were taken into account. The minimization with respect to the configuration of the lipid layers was performed by a numerical solution of the system of the Euler-Lagrange differential equations and by the Monte Carlo simulated annealing method. At high enough temperature, the lipid molecules attain a shape exhibiting higher intrinsic mean and deviatoric curvatures, which fits better into the H(II) phase than into the L(alpha) phase. Furthermore, the orientational ordering of lipid molecules in the curvature field expressed as the deviatoric bending provides a considerable negative contribution to the free energy, which stabilizes the nonlamellar H(II) phase. The nucleation configuration for the L(alpha)-H(II) phase transition is tuned by the isotropic and deviatoric bending energies and the interstitial energy.

Shear stress in epiphyseal growth plate is a risk factor for slipped capital femoral epiphysis.

Background: Slipping of the capital femoral epiphysis is an important orthopaedic problem of early adolescence. Many hypotheses about its etiology have been examined, yet the underlying mechanisms have not yet been fully elucidated. We examined elevated shear stress in the epiphyseal growth plate and elevated contact hip stress exerted on the femoral head as risk factors for slipping of the capital femoral epiphysis. Methods: Two groups of hips were compared: a group of 100 hips contralateral to the slipped ones and a group of 70 age- and gender-matched healthy hips. The characteristics of individual hips were incorporated by means of geometrical parameters determined from standard anteroposterior radiographs. Shear stress was calculated by using a mathematical model where the femoral neck was considered to function as an elastic rod. Contact hip stress was calculated by the HIPSTRESS method. Results: Hips contralateral to the slipped ones had higher average shear stress (0.81 vs 0.51 MPa; P < 0.001) and more vertically inclined physeal angle (55.4 vs 63.2 degrees.; P < 0.001) in comparison to healthy hips. Shear stress in the contralateral hips to the slipped ones remained significantly higher even when normalized to the body weight (1400 vs 1060 Pa/N; P < 0.001). There was no significant difference in the average contact hip stress (1.86 vs 1.74 MPa; P = 0.145). Conclusions: Elevated shear stress, but not elevated contact stress, is a risk factor for slipping of the capital femoral epiphysis. Level of evidence: III (prognostic study, case-control study).