The transmission of load through the human hip joint

Running-in behavior of dual-mobility cup during the gait cycle: A finite element analysis

The running-in process is considered an essential aspect of the comprehensive wear process. The phenomenon of running-in occurs during the initial stages of wear in the prosthetic hip joint. Within the field of tribology, the running-in phenomenon of the hip joint pertains to the mechanism by which the contact surfaces of the artificial hip joint components are adjusted and a suitable lubricating film is formed. During the process of hip joint running-in, there is an interaction between the metal surface of the ball and the joint cup, which results in adjustments being made until a steady state is achieved. The achievement of desirable wear existence and reliable performance of artificial hip joint components are reliant upon the tribological running-in of the hip joint. Despite the establishment of current modeling approaches, there remains a significant lack of understanding concerning running-in wear, particularly the metal-on-polyethylene (MoP) articulations in dual-mobility cups (DMC). An essential aspect to consider is the running-in phase of the dual mobility component. The present study employed finite element analysis to investigate the running-in behavior of dual mobility cups, wherein femoral head components were matched with polyethylene liners of varying thicknesses. The analysis of the running-in phase was conducted during the normal gait cycle. The results of this investigation may be utilized to design a dual-mobility prosthetic hip joint that exhibits minimal running-in wear.

Biomechanical analysis of fixation methods in acetabular fractures: a systematic review of test setups

Purpose

Optimal anatomical reduction and stable fixation of acetabular fractures are important in avoiding secondary dislocation and osteoarthritis. Biomechanical studies of treatment options of acetabular fractures aim to evaluate the biomechanical properties of different fixation methods. As the setup of the biomechanical test can influence the experimental results, this review aimed to analyze the characteristics, comparability and clinical implications of studies on biomechanical test setups and finite element analyses in the fixation of acetabular fractures.

Methods

A systematic literature research was conducted according to the PRISMA guidelines, using the PubMed/MEDLINE and Web of Science databases. 44 studies conducting biomechanical analyses of fixation of acetabular fractures were identified, which met the predefined inclusion and exclusion criteria and which were published in English between 2000 and April 16, 2021. The studies were analyzed with respect to distinct parameters, including fracture type, material of pelvis model, investigated fixation construct, loading direction, loading protocol, maximum loading force, outcome parameter and measurement method.

Results

In summary, there was no standardized test setup within the studies on fixation constructs for acetabular fractures. It is therefore difficult to compare the studies directly, as they employ a variety of different test parameters. Furthermore, the clinical implications of the biomechanical studies should be scrutinized, since several test parameters were not based on observations of the human physiology.

Conclusion

The limited comparability and restricted clinical implications should be kept in mind when interpreting the results of biomechanical studies and when designing test setups to evaluate fixation methods for acetabular fractures.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00068-022-01936-9.

Assessing the Weight-Bearing Surface in Dysplastic Acetabulae: The Sourcil Index

Background

Although a variety of standardized measurements have been described to evaluate acetabular dysplasia, no single measurement is without limitations. We describe the Sourcil Index (SI), a novel measure of the weight-bearing surface of the acetabulum on anteroposterior pelvis films. The SI is the angle formed by the medial and lateral margins of the sourcil and the center of rotation of the femoral head.

Methods

Anteroposterior pelvis radiographs of skeletally mature patients from 2015 were reviewed. Studies with fractures or implants were excluded. Films were read by 2 orthopedic surgeons and a radiologist 3 times each, 8 weeks apart. The SI, Sharp's Angle (SA), and lateral center edge angle (LCEA) were recorded. Pearson intraclass correlation coefficients with 95% confidence intervals were calculated. The SI was then compared to the SA and LCEA to preliminarily assess diagnostic accuracy.

Results

Five hundred thirty-five hips in 292 patients met inclusion. Intraobserver reliability is as follows: SI = 0.95 (0.93-0.98), LCEA = 0.89 (0.82 -0.96), and SA = 0.90 (0.85-0.96). Interobserver reliability is as follows: SI = 0.90 (0.84-0.94), SA = 0.78 (0.64-0.86), and LCEA = 0.73 (0.56-0.82). There were 51 dysplastic hips within this cohort.

Conclusion

The SI is a reproducible measurement on plain radiographs. The SI is a two-dimensional representation of the size of the weight-bearing surface of the acetabulum and could provide an estimation of joint contact pressures. Used with existing measures, the SI may provide a more nuanced understanding of acetabular morphology.

Hip preservation surgery and the acetabular fossa

As our understanding of hip function and disease improves, it is evident that the acetabular fossa has received little attention, despite it comprising over half of the acetabulum’s surface area and showing the first signs of degeneration. The fossa’s function is expected to be more than augmenting static stability with the ligamentum teres and being a templating landmark in arthroplasty. Indeed, the fossa, which is almost mature at 16 weeks of intrauterine development, plays a key role in hip development, enabling its nutrition through vascularization and synovial fluid, as well as the influx of chondrogenic stem/progenitor cells that build articular cartilage. The pulvinar, a fibrofatty tissue in the fossa, has the same developmental origin as the synovium and articular cartilage and is a biologically active area. Its unique anatomy allows for homogeneous distribution of the axial loads into the joint. It is composed of intra-articular adipose tissue (IAAT), which has adipocytes, fibroblasts, leucocytes, and abundant mast cells, which participate in the inflammatory cascade after an insult to the joint. Hence, the fossa and pulvinar should be considered in decision-making and surgical outcomes in hip preservation surgery, not only for their size, shape, and extent, but also for their biological capacity as a source of cytokines, immune cells, and chondrogenic stem cells.

Cite this article: Bone Joint Res 2020;9(12):857–869.

Evaluation of Sagittal Spine-Pelvis-Lower Limb Alignment in Elderly Women with Pelvic Retroversion while Standing and Walking Using a Three-Dimensional Musculoskeletal Model

Study Design

In vivo biomechanical study using a three-dimensional (3D) musculoskeletal model for elderly individuals with or without pelvic retroversion.

Purpose

To evaluate the effect of pelvic retroversion on the sagittal alignment of the spine, pelvis, and lower limb in elderly females while standing and walking.

Overview of Literature

Patients with hip–spine syndrome have concurrent hip-joint and spine diseases. However, the dynamic sagittal alignment between the hip joint and spine has rarely been investigated. We used a 3D musculoskeletal model to evaluate global spinopelvic parameters, including spinal inclination and pelvic tilt (PT).

Methods

A total of 32 ambulant females (mean age=78 years) without assistance were enrolled in the study. On the basis of the radiographic measurement for PT, participants were divided into the pelvic retroversion group (R-group; PT≥20°) and the normal group (N-group; PT<20°). A 3D musculoskeletal motion analysis system was used to analyze the calculated value for the alignment of spine, pelvis, and lower limb, including calculated (C)-PT, sagittal vertical axis (C-SVA), pelvic incidence, lumbar lordosis, T1 pelvic angle (C-TPA), as well as knee and hip flexion angles while standing and walking.

Results

While standing, C-PT and C-TPA in the R-group were significantly larger than those in the N-group. Hip angle was significantly smaller in the R-group than in the N-group, unlike knee angle, which did not show difference. While walking, C-SVA and C-TPA were significantly increased, whereas C-PT decreased compared with those while standing. The maximum hip-flexion angle was significantly smaller in the R-group than in the N-group. There was a significant correlation between the radiographic and calculated parameters.

Conclusions

The 3D musculoskeletal model was useful in evaluating the sagittal alignment of the spine, pelvis, and leg. Spinopelvic sagittal alignment showed deterioration while walking. C-PT was significantly decreased while walking in the R-group, indicating possible compensatory mechanisms attempting to increase coverage of the femoral head. The reduction in the hip flexion angle in the R-group was also considered as a compensatory mechanism.

Translatory hip kinematics measured with optoelectronic surgical navigation

PURPOSE

An optoelectronic surgical navigation system was used to detect small but measurable translational motion of human hip cadavers in high-range passive motions. Kinematic data were also examined to demonstrate the role of soft tissues in constraining hip translation.

METHODS

Twelve cadaver hips were scanned using CT, instrumented for navigation, and passively taken through motion assessment. Center of the femoral head was tracked in the acetabular coordinates. Maximum non-impinging translation of the femoral head for each specimen hip was reported. This was repeated for 5 tissue states: whole, exposed to the capsule, partially or fully incised capsule, resection of the ligamentum teres and labrectomy. Femoral motions were compared to the reported value for ideal ball and socket model.

RESULTS

Whole and exposed hips underwent maximal translations of [Formula: see text] and [Formula: see text] mm, respectively. These translational motions were statistically significantly different from reported value for a purely spherical joint, [Formula: see text]. Further tissue removal almost always significantly increased maximum non-impingement translational motion with [Formula: see text].

CONCLUSION

We found subtle but definite translations in every cadaver hip. There was no consistent pattern of translation. It is possible to use the surgical navigation systems for the assessment of human hip kinematics intra-operatively and improve the treatment of total hip arthroplasty patients by the knowledge of the fact that their hips translate. Better procedure selection and implantation optimization may arise from improved understanding of the motion of this critically important human joint.

A Finite Element Study of the Human Hip Joint

A finite element study of human hip joint is described. The results show that the articular cartilage is well able to distribute the applied load. Values for the compressive radial stress were calculated and compare favourably with published results. Values for compressive hoop stress and shear stress are also given.

A three-dimensional finite element model for biomechanical analysis of the hip

The objective of this study was to construct a three-dimensional (3D) finite element model of the hip. The images of the hip were obtained from Chinese visible human dataset. The hip model includes acetabular bone, cartilage, labrum, and bone. The cartilage of femoral head was constructed using the AutoCAD and Solidworks software. The hip model was imported into ABAQUS analysis system. The contact surface of the hip joint was meshed. To verify the model, the single leg peak force was loaded, and contact area of the cartilage and labrum of the hip and pressure distribution in these structures were observed. The constructed 3D hip model reflected the real hip anatomy. Further, this model reflected biomechanical behavior similar to previous studies. In conclusion, this 3D finite element hip model avoids the disadvantages of other construction methods, such as imprecision of cartilage construction and the absence of labrum. Further, it provides basic data critical for accurately modeling normal and abnormal loads, and the effects of abnormal loads on the hip.

Biphasic investigation of contact mechanics in natural human hips during activities

The aim of this study was to determine the cartilage contact mechanics and the associated fluid pressurisation of the hip joint under eight daily activities, using a three-dimensional finite element hip model with biphasic cartilage layers and generic geometries. Loads with spatial and temporal variations were applied over time and the time-dependent performance of the hip cartilage during walking was also evaluated. It was found that the fluid support ratio was over 90% during the majority of the cycles for all the eight activities. A reduced fluid support ratio was observed for the time at which the contact region slid towards the interior edge of the acetabular cartilage, but these occurred when the absolute level of the peak contact stress was minimal. Over 10 cycles of gait, the peak contact stress and peak fluid pressure remained constant, but a faster process of fluid exudation was observed for the interior edge region of the acetabular cartilage. The results demonstrate the excellent function of the hip cartilage within which the solid matrix is prevented from high levels of stress during activities owing to the load shared by fluid pressurisation. The findings are important in gaining a better understanding of the hip function during daily activities, as well as the pathology of hip degeneration and potential for future interventions. They provide a basis for future subject-specific biphasic investigations of hip performance during activities.

Functional integrative analysis of the human hip joint: the three-dimensional orientation of the acetabulum and its relation with the orientation of the femoral neck

In humans, the hip joint occupies a central place in the locomotor system, as it plays an important role in body support and the transmission of the forces between the trunk and lower limbs. The study of the three-dimensional biomechanics of this joint has important implications for documenting the morphological changes associated with the acquisition of a habitual bipedal gait in humans. Functional integration at any joint has important implications in joint stability and performance. The aim of the study was to evaluate the functional integration at the human hip joint. Both the level of concordance between the three-dimensional axes of the acetabulum and the femoral neck in a bipedal posture, and patterns of covariation between these two axes were analysed. First, inter-individual variations were quantified and significant differences in the three-dimensional orientations of both the acetabulum and the femoral neck were detected. On a sample of 57 individuals, significant patterns of covariation were identified, however, the level of concordance between the axes of both the acetabulum and the femoral neck in a bipedal posture was lower than could be expected for a key joint such as the hip. Patterns of covariation were explored regarding the complex three-dimensional biomechanics of the full pelvic-femoral complex. Finally, we suggest that the lower degree of concordance observed at the human hip joint in a bipedal posture might be partly due to the phylogenetic history of the human species.

HIP MORPHOLOGY: COMPARATIVE STUDY BETWEEN EGYPTIANS AND JAPANESE ADULTS

The prevalence of osteoarthritis appears to vary widely among ethinic groups. Many studies had been published describing the acetabular morphology of Chinese, Nigerians, Singaporeans and Japanese. No previous studies comparing Egyptian Arabs and Japanese Orientals work available. Purpose: The aim of this study was to determine the ethnic variations of acetabular morphology between Egyptian and Japanese populations. Patients and Methods: A total of 844 Egyptian and 757 Japanese consecutive patients with no clinical evidence of hip osteoarthritis and who underwent pelvic radiography in the supine position for hip trauma or a routine health check were analyzed for the relationships between the center-edge (CE) angle, Sharp angle, acetabular head index, as well as the relationships of the above-mentioned variables with age, gender, body height. Results: Comparing the measurements of those three parameters in both populations showed statistically significant values (all Egyptian versus Japanese, all female versus male are significant). Conclusion: Our data confirm the presence of ethnic variations in acetabular morphology which predispose to various pathologies of the hip joint.

Multi-pelvis characterisation of articular cartilage geometry

The shape of the acetabular cartilage follows the contact stress distribution across the joint. Accurate characterisation of this geometry may be useful for the development of acetabular cup devices that are more biomechanically compliant. In this study, the geometry of the acetabular cartilage was characterised by taking plaster moulds of the acetabulum from 24 dry bone human pelvises and digitising the mould shapes using a three-dimensional laser scanner. The articular bone surface geometry was analysed, and the shape of the acetabulum was approximated by fitting a best-fit sphere. To test the hypothesis that the acetabulum is non-spherical, a best-fit ellipsoid was also fitted to the geometry. In each case, points around the acetabular notch edge that disclosed the articular surface geometry were identified, and vectors were drawn between these and the best-fit sphere or ellipsoid centre. The significantly larger z radii (into the pole) of the ellipsoids indicated that the acetabulum was non-spherical and could imply that the kinematics of the hip joint is more complex than purely rotational motion, and the traditional ball-and-socket replacement may need to be updated to reflect this motion. The acetabular notch edges were observed to be curved, with males exhibiting deeper, wider and shorter notches than females, although the difference was not statistically significant (mean: p = 0.30) and supports the use of non-gender-specific models in anatomical studies.

Labral penetration rate in a consecutive series of 300 hip arthroscopies

BACKGROUND

Intraoperative labral injury during the establishment of the first portal in hip arthroscopy has been reported to be as high as 20%.

PURPOSE

The purpose of the study was to prospectively identify the incidence of acetabular labral injuries that occurred while using a current technique for the establishment of portals during hip arthroscopy.

STUDY DESIGN

Case series; Level of evidence, 4.

METHODS

Between the years 2008 and 2010, data were prospectively collected for all patients undergoing hip arthroscopic surgery. Patients with previous labral resection or Tonnis grade greater than 1 were excluded. Patients were positioned supine, traction was applied, and portals were established. The anterolateral portal was created first by venting the joint with a spinal needle and then re-entering the joint with the same needle with the bevel side facing the labrum. Next, the midanterior portal was created under vision. A thorough examination of the acetabular labrum was conducted arthroscopically through multiple viewing portals, and labral injuries related to the establishment of portals were identified and noted.

RESULTS

A total of 300 patients were included in the study; only 2 patients (0.67%) suffered intraoperative labral injuries at the study period. One injury occurred during revision arthroscopy, while the second involved a hyperplastic labrum in a dysplastic hip. No patient with normal hip morphological characteristics undergoing a hip arthroscopy suffered a labral tear as a result of portal placement.

CONCLUSION

The incidence of iatrogenic labral injury during hip arthroscopy can be as low as 0.67% when using the described technique.

Study of the three-dimensional orientation of the labrum: its relations with the osseous acetabular rim

Understanding the three-dimensional orientation of the coxo-femoral joint remains a challenge as an accurate three-dimensional orientation ensure an efficient bipedal gait and posture. The quantification of the orientation of the acetabulum can be performed using the three-dimensional axis perpendicular to the plane that passes along the edge of the acetabular rim. However, the acetabular rim is not regular as an important indentation in the anterior rim was observed. An innovative cadaver study of the labrum was developed to shed light on the proper quantification of the three-dimensional orientation of the acetabulum. Dissections on 17 non-embalmed corpses were performed. Our results suggest that the acetabular rim is better represented by an anterior plane and a posterior plane rather than a single plane along the entire rim as it is currently assumed. The development of the socket from the Y-shaped cartilage was suggested to explain the different orientations in these anterior and posterior planes. The labrum forms a plane that takes an orientation in between the anterior and posterior parts of the acetabular rim, filling up inequalities of the bony rim. The vectors V(L) , V(A2) and V(P) , representing the three-dimensional orientation of the labrum, the anterior rim and the posterior rim, are situated in a unique plane that appears biomechanically dependent. The three-dimensional orientation of the acetabulum is a fundamental parameter to understand the hip joint mechanism. Important applications for hip surgery and rehabilitation, as well as for physical anthropology, were discussed.

Acetabular fractures following rugby tackles: a case series

INTRODUCTION

Rugby is the third most popular team contact sport in the world and is increasing in popularity. In 1995, rugby in Europe turned professional, and with this has come an increased rate of injury.

CASE PRESENTATION

In a six-month period from July to December, two open reduction and internal fixations of acetabular fractures were performed in young Caucasian men (16 and 24 years old) who sustained their injuries after rugby tackles. Both of these cases are described as well as the biomechanical factors contributing to the fracture and the recovery. Acetabular fractures of the hip during sport are rare occurrences.

CONCLUSION

Our recent experience of two cases over a six-month period creates concern that these high-energy injuries may become more frequent as rugby continues to adopt advanced training regimens. Protective equipment is unlikely to reduce the forces imparted across the hip joint; however, limiting 'the tackle' to only two players may well reduce the likelihood of this life-altering injury.

Articular surface remodeling of the hip after periacetabular osteotomy

PURPOSE

Periacetabular osteotomies are a family of surgical procedures used to treat hip dysplasia. In a periacetabular osteotomy, the operating surgeon aims to increase acetabular coverage of the femoral head. The surgical correction has mechanical goals of increasing the stability of the joint and to improving the pressure distribution across the acetabulum. Although it is known that bone will remodel under changing load at the microstructural level, it is unclear whether there is any gross remodeling of the acetabulum or the femoral head in response to the change in loading following a periacetabular osteotomy. This observational study aims to quantify the shape of operative and contralateral hip joint surfaces pre and postoperatively to determine whether there are gross morphological changes in the shape of any of the bony articular surfaces of the joint.

METHODS

Preoperative and postoperative computed tomography (CT) scans were segmented as triangulated meshes. The bony articular surfaces of these meshes were then isolated. The vertices of these surfaces were fit to spheres and to general ellipsoids and, in the case of the acetabulum, examined in anatomical coordinate frames to look for changes between pre and postoperative segmentations.

RESULTS

Spherical fit results were consistent preoperatively and postoperatively, with small changes in the radii of the spheres of best fit for both operative and nonoperative hips. Ellipsoid fitting showed variations between preoperative and postoperative scans in both eccentricity and orientation.

CONCLUSIONS

Because there is no clear evidence of gross articular surface remodeling, periacetabular osteotomy for an adult should be planned with the expectation that the patient's existing articular structure will be preserved.

The labro-acetabular complex

The human hip is subjected to several hundred million loading cycles during a lifetime. Hip instability and femoro-acetabular impingement cause damage to the rim of the acetabulum. The acetabular rim is a highly specialized structure known as the labro-acetabular complex. A unidirectional flow of synovial fluid has been identified in this region. The synovial fluid circulation is driven by the bellows-like movement of the zona orbicularis and depends on hip flexion and extension. Surgical repair of the damaged labro-acetabular complex should satisfy two goals. First, the labrum should be preserved or reconstructed when possible. Second, the precipitating cause of the labro-acetabular damage must be addressed.

Importance of the dome and posterior wall as evidenced by bone density mapping in the acetabulum

BACKGROUND

Characterizing the distribution of bone density in the acetabulum is of importance in better understanding and guiding treatment for both osteoarthritis and trauma of the hip joint. This study aims to develop a highly automated method to quantify the pattern of subchondral bone density in the acetabulum using clinically identifiable regions.

METHODS

Subchondral acetabular bone density distribution maps were created bilaterally from 30 non-pathologic pelvic CT scans. The density maps were aligned orthogonal to the acetabular rim plane and divided into twelve zones. Average bone density was calculated in each of these zones and compared to investigate differences between regions within each acetabulum and between right and left sides in a given patient.

FINDINGS

In all patients, the zones corresponding to the dome and posterior wall of the acetabulum demonstrated significantly higher average bone densities than all other regions (P<0.01). Significant correlations (R=0.4 to 0.76, P<0.05) were found between corresponding regions of the left and right sides in 10 of the 12 zones.

INTERPRETATION

The pattern of subchondral bone density distribution found in this study is consistent with previously observed bone density patterns in the acetabulum. Correspondence of right and left sides suggests that the distribution of loading on the acetabulum is similar on both sides in healthy individuals, though differences may exist in load sharing. Quantifying bone density patterns using zonal density map analysis may lead to a better understanding of the impact of traumatic injuries and progression of pathologic conditions in the hip joint.

[Biomechanical principles for treatment of osteoporotic fractures of the pelvis]

Fractures of the pelvis are of increasing interest, especially in older patients due to the often concomitant osteoporosis. The low bone quality can be a problem in several fixation situations. In this review the present and relevant literature on biomechanical data of unstable pelvic ring injuries and all biomechanical data dealing with osteosynthesis for acetabular fractures are discussed.

Labral base refixation in the hip: rationale and technique for an anatomic approach to labral repair

Recent literature has defined the importance of anatomic repair in shoulder and knee arthroscopy. New advances in hip arthroscopy have created opportunities to apply the principle of anatomic repair to the hip. To address the obstacles in the restoration of labral anatomy, we describe an anatomic approach to labral refixation. We reviewed the literature on biomechanics of the labrum to identify the factors that are essential to the function of the labrum. Existing techniques for arthroscopic labral repair and potential challenges in restoration of labral anatomy were reviewed. A list of criteria for anatomic labral repair was created, and a technique for anatomic labral base refixation was developed. The technique incorporates the understanding of the function and biomechanical role of the labrum and builds on existing techniques to fulfill the criteria for restoration of anatomy. Our purpose was to review the anatomy, biomechanics, and existing repair techniques of the labrum, as well as to describe the rationale and surgical steps for anatomic labral base refixation in the hip.

The effect of tibial lengthening on immature articular cartilage of the ankle joint

Is the articular cartilage of the immature ankle joint damaged by tibial lengthening? Sixteen immature rabbits underwent a 30% diaphyseal lengthening by tibial callotasis. Damage was assessed by scanning electronic microscopy and histomorphometry at the completion of distraction and after an additional 5 weeks. Despite joint contracture, little damage in the articular cartilage was observed in contrast to the knee joint. The findings show that the immature ankle joint is more resilient to stress than the knee and implies that reduced weight bearing and decreased joint movement alone are not sufficient to cause cartilage damage, at least in the ankle.

Hip biomechanics

As the primary link between the trunk and the lower limb, the hip joint plays an important role in the generation and transmission of forces during routine activities of daily living and athletic activities alike. This joint is characterized by an extraordinary amount of inherent bony stability, with differences in osseous anatomy significantly impacting the biomechanical properties of the human hip. These biomechanical principles have important implications relative to the diagnosis and surgical treatment of structural hip abnormalities, and the physical demands placed on the hip joint during athletic activities may predispose to injury or other chronic pathologic processes.

Arthroscopy for labral tears in patients with developmental dysplasia of the hip: a cautionary note

Patients with developmental dysplasia of the hip may present with acetabular rim overloading, labral hypertrophy, and tear. Our hypothesis was that isolated arthroscopic treatment of labral tear is likely to fail in most patients. We investigated 34 patients who underwent at least one arthroscopy of the hip for labral tear. Developmental hip dysplasia or other morphologic abnormalities of the hip were confirmed in all patients. Arthroscopy failed to relieve pain in 24 patients. We observed accelerated arthritis in 14 patients and migration of the femoral head in 13 patients. Sixteen patients underwent further surgery (further surgeries included periacetabular osteotomy [6 patients], femoroacetabular osteoplasty [7 patients], and total hip arthroplasty [3 patients]). At the latest follow-up, all patients but one were pain-free. Patients with evidence of abnormal hip morphologies may not benefit from hip arthroscopy and isolated treatment of the labrum; in fact, the latter may accelerate the process of arthritis in some patients.

Development and Validation of Patient-Specific Finite Element Models of the Hemipelvis Generated From a Sparse CT Data Set

To produce a patient-specific finite element (FE) model of a bone such as the pelvis, a complete computer tomographic (CT) or magnetic resonance imaging (MRI) geometric data set is desirable. However, most patient data are limited to a specific region of interest such as the acetabulum. We have overcome this problem by providing a hybrid method that is capable of generating accurate FE models from sparse patient data sets. In this paper, we have validated our technique with mechanical experiments. Three cadaveric embalmed pelves were strain gauged and used in mechanical experiments. FE models were generated from the CT scans of the pelves. Material properties for cancellous bone were obtained from the CT scans and assigned to the FE mesh using a spatially varying field embedded inside the mesh while other materials used in the model were obtained from the literature. Although our FE meshes have large elements, the spatially varying field allowed them to have location dependent inhomogeneous material properties. For each pelvis, five different FE meshes with a varying number of patient CT slices (8–12) were generated to determine how many patient CT slices are needed for good accuracy. All five mesh types showed good agreement between the model and experimental strains. Meshes generated with incomplete data sets showed very similar stress distributions to those obtained from the FE mesh generated with complete data sets. Our modeling approach provides an important step in advancing the application of FE models from the research environment to the clinical setting.

Pre-clinical evaluation of the Cambridge acetabular cup

It is postulated that the stiffness of current acetabular designs compromises long-term component stability. We present a novel acetabular component design that is horseshoe shaped and has a large diameter bearing. It is made from composite materials and is designed to match the stiffness of subchondral bone. It is intended that stress shielding will be minimised and that the distribution of stress will be improved. The mechanical and biological suitability of the composite has been confirmed. A range of standard and non-standard, pre-clinical, tests have established the robustness and safety of the new component. The efficacy of the new design has been evaluated by clinical trial on 50 patients. Optimal results were obtained using the hydroxyapatite (HA) coated cups. Our results support the new design concept, with the caveat that biological fixation is imperative. Minor design modifications are recommended.

The shape of the acetabular cartilage surface: a geometric morphometric study using three-dimensional scanning

The acetabular cartilage surface plays an important role in hip joint biomechanics, locomotion and lubrication, but few studies has focused on its geometric morphometry. The aim of this study was to present a novel, accurate mathematical representation of the acetabular cartilage surface based on a new method, combined with a reverse engineering technique, surface-fitting algorithms and mathematical curve surface theory. By using a three-dimensional (3D) laser scanner, a 3D triangulated mesh surface approximation of acetabular cartilage was created. Using surface-fitting algorithms and mathematical curve surface theory, two main curvature parameters, Gaussian curvature and mean curvature at each point on the surface of the acetabular cartilage, were calculated. The distribution patterns of both parameters over the curved surface were elucidated and the eigenvalues of the surface were calculated to determine the shape of the acetabular cartilage surface. By statistically analyzing 25 specimens, it was found that the shape of the acetabular cartilage surface was not theoretically spherical but rotational ellipsoidal, which is a novel mathematical description. The surface-fitting error of a rotational ellipsoid shape was significantly smaller than that of a spherical shape for representing the acetabular cartilage surface (p<0.001). The highest surface-fitting error for a spherical shape was seen in the roof area of the acetabular cartilage, where a rotational ellipsoid surface presented a better anatomical fit. The results will not only be helpful in gaining a new anatomical understanding of the acetabular cartilage surface, but will also be usable in the construction of a precise 3D numerical model in simulation studies of the hip joint.

Hip Contact Stress during Normal and Staircase Walking: The Influence of Acetabular Anteversion Angle and Lateral Coverage of the Acetabulum

Hip contact stress is considered to be an important biomechanical factor related to development of coxarthrosis. The effect of the lateral coverage of the acetabulum on the hip contact stress has been demonstrated in several studies of hip dysplasia, whereas the effect of the anterior anteversion remains unclear. Therefore, the joint hip contact stress during normal level walking and staircase walking, in normal and dysplastic hips, for small and large acetabular anteversion angle was computed. For small acetabular anteversion angle, the hip contact stress is slightly increased (less than 15%) in staircase walking when compared with normal walking. In hips with large angle of acetabular anteversion, walking downstairs significantly increases the maximal peak contact stress (70% in normal hips and 115% in dysplastic hips) whereas walking upstairs decreases the peak contact stress (4% in normal hips and 34% in dysplastic hips) in comparison to normal walking. Based on the presented results, we suggest that the acetabular anteversion should be considered in biomechanical evaluation of the hips, especially when the lateral coverage of the acetabulum is small.

Intra-articular compressive stress of the elbow joint in extension: an experimental study using Fuji films

The use of Fuji films is simple but their manipulation and result interpretation seem to be difficult in the framework of medical research. The reliability and reproducibility of Fuji films have been proved by many previous studies. This study was undertaken to know precisely the articular zones of the elbow and to determine the compressive stress these areas undergo during different activities, in order to assess the importance of different articular contact areas. These data indicate the need for better-adapted elbow prosthesis and can be eventually used to design more durable prosthesis for the elbow. The compressive stress on the radial head was less than 25% in extension. The stress on the radial head varied from the neutral position (23% of the stress), to full pronation (11% of the stress) and to full supination (6% of the stress). The Humero-ulnar compartment had the maximum impact. Coronoid process seemed to be a fundamental element of the elbow joint in extension (60% of total compressive stress). The Medial humero-ulnar compartment was less stressed than the lateral compartment. The radial head does not seem to play a major role in the stability of the elbow in extension if the ulnar collateral ligament exists. The ulnar collateral ligament is essential to the elbow joint stability. The lifespan of a non-constrained prosthesis would depend on the existence of the couple: radial head/ulnar collateral ligament; the absence of radial head could compromise the humero-ulnar stability. This work paved the way for the designing of new non-constrained elbow prosthesis with the reconstruction of the radial head.

In vivo contact pressures in total knee arthroplasty

This study compared the in vivo femoropolyethylene contact pressures generated in fixed-bearing total knee arthroplasty (TKAs) with those in mobile-bearing TKAs. In vivo kinematics obtained from a 2-dimensional to 3-dimensional registration technique and soft tissue locations derived from computed tomographic scans were entered into a 3-dimensional inverse dynamics mathematical model to determine the in vivo bearing contact forces. The contact areas were obtained from the assembly of computer-aided design models of the components. The contact pressure was defined as the ratio of the contact forces to the contact areas. The results indicate that the in vivo contact pressures in each TKA are greater for the medial condyle than for the lateral condyle. The ability of the mobile-bearing TKA to rotate maintains higher femoropolyethylene contact, resulting in lesser contact pressures, as compared with the fixed-bearing TKA.

Lateral acetabular rotation improves anterior hip subluxation

Surgical reorientation of the acetabulum is used to improve stability of subluxated or dysplastic hips, but the specific mechanical consequences of reorientation have not been quantified. I used a rigid body spring model of the human hip to study the effects of different acetabular positions on hip stability during single-limb stance. The model predicted subluxation direction and magnitude, and the effective joint contact area, as functions of acetabular position. Frontal plane acetabular orientation varied from 20 degrees medial rotation to 50 degrees lateral rotation, corresponding to center-edge angles from 0 degrees to 70 degrees. Sagittal acetabular orientation varied from 45 degrees anterior rotation to 15 degrees posterior rotation. Center-edge angles less than 20 degrees produced progressive anterolateral subluxation, with dislocation occurring when center-edge angles were less than 0 degrees. Lateral subluxation disappeared when cen-ter-edge angles were 30 degrees or greater. Anteroposterior subluxation was controlled by anterior or posterior rotation of the acetabulum in the presence of low center-edge angles, but there was no specific position of stability that effectively stabilized the femoral head. Anterior subluxation also was controlled by lateral rotation of the acetabulum. Joint contact area increased 1% for every 3 degrees lateral acetabular rotation. The anterolateral subluxation associated with hip dysplasia can be controlled by acetabular reorientation. Joint contact area will increase, thereby reducing peak joint pressure. Anterior and lateral subluxation can be improved by lateral rotation alone, which may reduce the severity of anterior femoroacetabular impingement after periacetabular osteotomy.

The position of the bipolar cup reflects the direction of the hip contact force acting on it

We radiographically measured the bipolar cup position to analyze the direction of joint force acting on the bipolar cup. The abduction angle of the bipolar cup was measured in each radiograph taken immediately and at six 12 weeks and yearly after the operation. Radiographs in patients with weight bearing were also investigated. The results indicated that the abduction angle of the bipolar cup was 24.1 degrees +/- 11.2 degrees immediate postoperatively and was 16.2 degrees +/- 5.1 degrees at 6 weeks, 16.1 degrees +/- 5.1 degrees at 3 months, and 16.2 degrees +/- 5.1 degrees at 1 year. The cup abduction angles with weight bearing were not different from those without weight bearing and were 15.9 degrees +/- 4.9 degrees , 16.2 degrees +/- 4.4 degrees , and 16.1 degrees +/- 4.7 degrees on the supine, double-legged stance, and single-legged stance radiographs, respectively. Because the position of the bipolar cup reflects the direction of loads pivoting on it, the direction of the joint force in the frontal plane acting on the bipolar prosthesis is about 16 degrees to vertical.

Bone remodeling around the Cambridge cup: a DEXA study of 50 hips over 2 years

BACKGROUND

In a prospective 2-year study we have used dual-energy X-ray absorptiometry to measure periprosthetic bone mineral density (BMD) following implantation of a novel, "physiological", acetabular component designed using composite materials.

METHOD

The acetabular components were implanted in hydroxyapatite (HA) and HA-removed options. They were implanted in conjunction with a cemented femoral component in 50 female patients who presented with displaced, subcapital, fractures of the neck of the femur. Regions of interest (ROI) were defined according to De Lee and Charnley. BMD during follow-up was compared with immediate postoperative values for the affected limb.

RESULTS

The mean precision error (CV%) was 1.01%, 2.26% and 1.12%, for ROI I, II and III respectively. The mean change in BMD, for both cups, was analyzed. There was no significant difference between the BMD changes induced with the HA- and non-HA-coated cups.

INTERPRETATION

After an initial fall in BMD in all 3 ROI at 6 months, ROI I and ROI II showed return to baseline BMD by 2 years. ROI III showed no significant decrease in BMD beyond 6 months, but did not return to baseline levels. Statistical analysis revealed no significant decrease in BMD in ROI I and ROI II at 2 years, compared with immediate postoperative values. The changes in BMD reflect a pattern of maximally reduced stress in the non-weight-bearing zone (ROI III), with preservation of bone density in weight bearing zones ROI I and ROI II. These results support the design principles of the Cambridge cup.

Finite element contact analysis of the hip joint

The hip joint plays an important role in the musculoskeletal system; however, current knowledge of the mechanics of the hip joint, especially with regard to the distribution of stress, remains limited. In experimental research, difficulties arise during reproduction of physiological conditions of daily activities and practicable measurement of locations inside the hip joint without violating the physiological environment. On the other hand, numerical approaches, such as finite element analysis, have become useful tools in the field of biomechanics. In finite element contact analysis of the hip joint, due to discretization of contact surfaces, computational instability might occur when the contact nodes move near the edges of the contact elements. In this study, to overcome this problem, a contact smoothing approach was introduced by applying Gregory patches. Contact analysis of the hip joint was then performed for three representative daily activities; i.e., walking, rising up from a chair and knee bending. The effectiveness of the adopted smoothing approach was verified by comparing the results with those obtained experimentally. The distribution and history of contact stress, which have heretofore been scarcely reported, were also obtained and the implications associated with osteoarthritis were discussed.

[Femoroacetabular impingement: trigger for the development of coxarthrosis]

Femoroacetabular impingement (FAI) is frequent; the estimated prevalence ranges between 10 and 15%. Our 10-years experience strongly suggests that FAI leads to osteoarthritis. Isolated acetabular or femoral abnormalities are rare, even though in women acetabular and in men femoral abnormalities predominate. Normal radiographs do not exclude the presence of FAI. Symptoms are related to the degree of deformity and occur earlier in the presence of activities requiring high levels of motion. The majority of patients with FAI are under the age of 40 years. In contrast to impingement in total hip replacement, the natural hip is under much higher constraint, not allowing to escape from impingement-induced shear forces by subluxation or complete dislocation. FAI-induced shear forces due to an aspherical femoral head/neck (cam type) are therefore high, causing outside-in damage with cleavage lesions of the acetabular cartilage by forced flexion and internal rotation. The cartilage of the femoral head remains initially intact, which cannot be explained by the classic concept of osteoarthritis. After the femoral head has migrated into the acetabular cartilage defect, vertical forces contribute to the further course of osteoarthritis. Tears between the labrum and cartilage, as seen by MRI, are not avulsions of the labrum from the cartilage but rather outside-in avulsions of the cartilage from the labrum. In acetabular overcoverage (pincer type) the labrum is the first structure to fail and acetabular cartilage damage develops thereafter. The treatment of FAI in patients under the age of 40 years is aimed at joint preservation. The clinical result is worse in the presence of significant cartilage damage. Therefore, early appreciation of FAI and timely therapeutic intervention as well as professional and athletic adjustment are important if osteoarthritis is to be prevented.

Theoretical study of subluxation in early Legg-Calvé-Perthes disease

The relationship between mechanical subluxation and femoral head necrosis geometry in Legg-Calvé-Perthes disease (LCP) was investigated with a three-dimensional rigid body-spring method hip model. Femoral head models with progressively larger regions of necrosis, corresponding to the four Catterall grades, were placed in a spherical acetabular model and studied in static single-limb stance configuration, with variable mechanical rigidity of the necrotic segment. The degree of subluxation was dependent on the geometric region of involvement, mechanical properties of the segment, and direction of loading force. In general, femoral head subluxation was always in the direction of the necrosis, modified by the anatomic and force environment. In the neutral position, the Catterall I models exhibited minimal subluxation. The Catterall II model subluxated anteriorly as collapse occurred, and the Catterall III model subluxated anteriorly and superiorly. Lateral subluxation could be produced by changing the loading force to a more vertical orientation. The Catterall IV model collapsed directly along the line of force application rather than subluxating. Reorientation of models with minor necrosis could improve stability, but reorientation of models with extensive necrosis had minimal effect on subluxation behavior. Higher-grade LCP involvement may lead to early subluxation, particularly anteriorly, which is difficult to visualize radiographically. The appearance of lateral subluxation may signal a change in the mechanical environment of the hip from clinical progression of the disease. Femoral head reorientation (osteotomy) may improve femoral head stability when necrosis is limited but is unlikely to reduce subluxation or collapse when extensive necrosis is present.

Three-dimensional dynamic hip contact area and pressure distribution during activities of daily living

Estimation of the hip joint contact area and pressure distribution during activities of daily living is important in predicting joint degeneration mechanism, prosthetic implant wear, providing biomechanical rationales for preoperative planning and postoperative rehabilitation. These biomechanical data were estimated utilizing a generic hip model, the Discrete Element Analysis technique, and the in vivo hip joint contact force data. The three-dimensional joint potential contact area was obtained from the anteroposterior radiograph of a subject and the actual joint contact area and pressure distribution in eight activities of daily living were calculated. During fast, normal, and slow walking, the peak pressure of moderate magnitude was located at the lateral roof of the acetabulum during mid-stance. In standing up and sitting down, and during knee bending, the peak pressures were located at the edge of the posterior horn and the magnitude of the peak pressure during sitting down was 2.8 times that of normal walking. The peak pressure was found at the lateral roof in climbing up stairs which was higher than that in going down stairs. These results can be used to rationalize rehabilitation protocols, functional restrictions after complex acetabular reconstructions, and prosthetic component wear and fatigue test set up. The same model and analysis can provide further insight to soft tissue loading and pathology such as labral injury. When the pressure distribution on the acetabulum is inverted onto the femoral head, prediction of subchondral bone collapse associated with avascular necrosis can be achieved with improved accuracy.

New methods for assessing cartilage contact stress after articular fracture

Progress in reducing the incidence and severity of posttraumatic arthritis depends in part on avoiding deleterious stress levels at residual local incongruities. Systematic efforts to elucidate factors adversely influencing cartilage's mechanical environment in turn depend on the availability of suitable modalities to assess intraarticular contact stresses. This has been and remains a challenging biomechanical problem. Technologic approaches used in the past have included mathematical analyses and indwelling physical sensors, each with advantages and limitations. Two emerging, mutually complementary capabilities show promise of dramatically altering the state of the art in this important field. The first of these methodologies, voxel-based contact finite element analysis, provides accurate computational estimates of cartilage stress on a patient-specific basis, and does so while accommodating arbitrarily idiosyncratic patterns of local articular incongruity. The second methodology, instrumentational, involves transient pressure distribution recordings using specially designed piezoresistive array sensors. Operational considerations for both of these new assessment technologies are described, and promising directions for future development are outlined.

Congruency Effects on Load Bearing in Diarthrodial Joints

Modelling load bearing in diarthrodial joints is challenging, due to the complexity of the materials, the boundary and interface conditions and the geometry. The articulating surfaces are covered with cartilage layers that are filled with a fluid that plays a major role in load bearing [Mow, V.C., Holmes, M.H., Lai, W.M. (1984) "Survey article: fluid transport and mechanical properties of articular cartilage: a review", Journal of Biomechanics 17(5), 377-394]. Researchers have tended to approximate joint geometry using axisymmetry [Donzelli, P.S., Spilker, R.L., Ateshian, G.A., Mow, V.C. (1999) "Contact analysis of biphasic transversely isotropic cartilage layers and correlations with tissue failure", Journal of Biomechanics 32, 1037-1047], often with a rounded upper articulating surface, creating a form of Hertz problem [Donzelli, P.S., Spilker, R.L., Ateshian, G.A., Mow, V.C. (1999) "Contact analysis of biphasic transversely isotropic cartilage layers and correlations with tissue failure", Journal of Biomechanics 32, 1037-1047]. However, diarthrodial joints (shoulder, hip and knee) are equipped with peripheral structures (glenoid labrum, acetabular labrum and meniscus, respectively) that tend to deepen the joint contact and thus cause initial contact to be established at the periphery of the joint rather than "centrally". The surface geometries are purposefully incongruent, and the incongruency has a significant effect on the stresses, pressures and pressure gradients inside the tissue. The models show the importance of the peripheral structures and the incongruency from a load-bearing perspective. Joint shapes must provide a compromise between demands for load-bearing, lubrication and the supply of nutrients to the chondrocytes of the cartilage and cells of the peripheral structures. Retention and repair of the functionality of these peripheral structures should be a prime consideration in any surgical treatment of an injured joint.

The role of joint architecture in the etiology of arthritis

Focal degenerative changes occur in some joints very early in life. These changes in the articular cartilage appear to occur in the unloaded, rather than the loaded, areas of the joint. One possible cause for this pattern of degeneration is lack of use or stress in these particular areas of the joint; just as unused bone and unused muscle atrophy, so may unused cartilage. If these unloaded areas were never subjected to mechanical stress, degeneration at these sites perhaps would not be important. However, bones, including their articular ends, are in a constant state of change through the process of remodeling, which continues throughout life. Joint surfaces are not, in general, spherical, and therefore must be incongruent during most of their arc of movement. In the young person, this incongruity maintains physiologic loading and joint nutrition. Studies have shown age-related changes in the remodeling process that lead to increasing joint congruity in old age. These age-related increases in congruity may result in a redistribution of load in the joint such that there is an increased stress on formerly unloaded atrophic cartilage. Arthritis always results in a change in joint shape. It is suggested that a change in shape caused by a disturbance in the remodeling process may itself be an important contributing cause of osteoarthritis.

Normal hip joint contact pressure distribution in single-leg standing--effect of gender and anatomic parameters

A practical and easy-to-use analysis technique that can study the patient's hip joint contact force/pressure distribution would be useful to assess the effect of abnormal biomechanical conditions and anatomical deformities on joint contact stress for treatment planning purpose. This technique can also help to establish the normative database on hip joint contact pressure distribution in men and women in different age groups. Twelve anatomic parameters and seven biomechanical parameters of the hip joint in a normal population (41 females, 15 males) were calculated. The inter-parameter correlations were investigated. The pressure distribution in the hip joint was calculated using a three-dimensional discrete element analysis (DEA) technique. The 3D contact geometry of the hip joint was estimated from a 2D radiograph by assuming that the femoral head and the acetabular surface were spherical in shape. The head-trochanter ratio (HT), femoral head radius, pelvic height, the joint contact area, the normalized peak contact pressure, abductor force, and the joint contact force were significantly different between men and women. The normalized peak contact pressure was correlated both with acetabular coverage and head-trochanter ratio. Change of abductor force direction within normal variation did not affect the joint peak contact pressure. However, in simulated dysplastic conditions when the CE angle is small or negative, abductor muscle direction becomes very sensitive in joint contact pressure estimation. The models and the results presented can be used as the reference base in computer simulation for preoperative planning in pelvic or femoral osteotomy.

Proximal Femoral Density Patterns are Consistent with Bicentric Joint Loads

We developed an alternate method for density-based load estimation and applied it to estimate hip joint load distributions for two femora. Two-dimensional finite element models were constructed from single energy quantitative computed tomography (QCT) data. Load estimation was performed using five loading regions on the femoral head. Within each loading region, individual nodal loads, normal to the local surface, were supplied as input to the load estimation. An optimization procedure independently adjusted individual nodal load magnitudes in each region, and the magnitudes of muscle forces on the greater trochanter, such that the applied tissue stimulus approached the reference stimulus throughout the model. Dominant estimated load resultant directions were generally consistent with published experimental data for loads during gait. The estimated loads also suggested that loads near the extremes of the articulating surface may be important (even required) for development and maintenance of normal bone architecture. Estimated load distributions within nearly all regions predicted bicentric loading patterns, which are consistent with observations of hip joint incongruity. Remodeling simulations with the estimated loads predicted density distributions with features qualitatively similar to the QCT data sets. This study illustrates how applications of density-based bone load estimation can improve understanding of dominant loading patterns in other bones and joints. The prediction of bicentric loading suggests a very fine level of local adaptation to details of joint loading.