Incongruent surfaces in the human hip joint

Impact of Patient-Specific Hip Joint Geometry on the Morphology of Acetabular Fractures

Background: Acetabular fractures continue to pose a major challenge in clinical practice, not least because of the growing geriatric population. While the influence of the force vectors on fracture formation is well established, the impact of anatomical factors on fracture morphology remains poorly understood. The aim of this study was to investigate patient-specific hip joint geometry, identify structural risk factors and correlate these with the resulting fracture patterns. Methods: This retrospective cohort analysis included 226 patients (Mdn age = 58 yrs.) with acetabular fracture categorized by Judet/Letournel and the AO/OTA classification. Computed tomography (CT) datasets of the injured and contralateral sides were analyzed using multiplanar reconstruction. Parameters included modified center-edge (CE) angle (Wiberg), rotation angles (Ullmann and Anda), acetabular sector angle (Anda), true caput-collum-diaphyseal (CCD) angle, femoral head diameter and volume, as well as femoral neck length, circumference, and diameter. In addition, intrarater reliability within a subcohort was assessed for the metric measurements and inter-rater analysis for the classification of the entire sample. Results: The primary analysis showed direct effects of femoral head diameter, femoral neck length and femoral head size on the fracture type according to AO/OTA (type A/B/C), whereby this effect was particularly seen between type A and type C fractures (p = 0.001). Ordinal regression identified femoral head diameter as the only significant predictor (p = 0.02), with a 25% increased likelihood of complex fractures per unit of change. Low-energy trauma doubled the risk of severe fractures. Specific findings include a higher acetabular anteversion in anterior column fractures. Age correlated positively with the cause of injury and fracture type. The inter-rater reliability for fracture classification was excellent, as was the intrarater reliability of the measurements. Conclusions: This study suggests that anatomical factors, particularly proximal femoral geometry, have an impact on acetabular fracture morphology-in addition to factors such as trauma type and patient demographics.

Age-related differences in morphology and kinematics of the native hip

Our objectives were to clarify morphology of the hip as well as infinitesimal femoral head movement in specific positions in young and elderly volunteers without joint degeneration. Both hips of 20 young and 20 elderly healthy volunteers were examined. Magnetic resonance imaging was performed at four different positions for each hip: neutral, 45° flexion, 15° extension, and the Patrick position. Femoral and pelvic bone images were separately extracted when in the neutral position and superimposed over the images of each different position by using voxel-based registration. The distance between the acetabular center and the femoral head center (FHC) at the neutral position was defined as 3D-migration. The distance between FHCs at neutral position and that at each different position was defined as 3D-translation. The x-, y-, and z-axes pointed in the anterior, cranial, and lateral directions, respectively. 3D-migration-y in the elderly was more caudal than that in the young (P < .001). 3D-translation of 45° flexion in the elderly was larger than that in the young with statistical significance (P = .001), while 3D-translation of the Patrick position in the elderly was smaller than that in the young (P = .012). Age was significantly correlated with 3D-translation in 45° flexion (r = .431; P < .001) and that in Patrick (r = -.296; P = .008). These results can be used as a basis for the natural course with aging of morphometry and kinematics of the hip, as well as for potential disease progression in osteoarthritis of the hip.

Cam Osteochondroplasty for Femoroacetabular Impingement Increases Microinstability in Deep Flexion: A Cadaveric Study

PURPOSE

The purpose of this in vitro cadaveric study was to examine the contributions of each surgical stage during cam femoroacetabular impingement (FAI) surgery (i.e., intact-cam hip, T-capsulotomy, cam resection, and capsular repair) toward hip range of motion, translation, and microinstability.

METHODS

Twelve cadaveric cam hips were denuded to the capsule and mounted onto a robotic tester. The hips were positioned in several flexion positions-full extension, neutral (0°), 30° of flexion, and 90° of flexion-and performed internal-external rotations to 5 Nm of torque in each position. The hips underwent a series of surgical stages (T-capsulotomy, cam resection, and capsular repair) and were retested after each stage. Changes in range of motion, translation, and microinstability (overall translation normalized by femoral head radius) were measured after each stage.

RESULTS

Regarding range of motion, cam resection increased internal rotation at 90° of flexion (change in internal rotation = +6°, P = .001) but did not affect external rotation. Capsular repair restrained external rotation compared with the cam resection stage (change in external rotation = -8° to -4°, P ≤ .04). In terms of translation, the hip translated after cam resection at 90° of flexion in the medial-lateral plane (change in translation = +1.9 mm, P = .04) relative to the intact and capsulotomy stages. Regarding microinstability, capsulotomy increased microinstability in 30° of flexion (change in microinstability [Δ_M] = +0.05, P = .003), but microinstability did not further increase after cam resection. At 90° of flexion, microinstability did not increase after capsulotomy (Δ_M = +0.03, P = .2) but substantially increased after cam resection (Δ_M = +0.08, P = .03), accounting for a 31% change with respect to the intact stage.

CONCLUSIONS

Cam resection increased microinstability by 31% during deep hip flexion relative to the intact hip. This finding suggests that iatrogenic microinstability may be due to separation of the labral seal and resected contour of the femoral head.

CLINICAL RELEVANCE

Our in vitro study showed that, at time zero and prior to postoperative recovery, excessive motion after cam resection could disrupt the labral seal. Complete cam resection should be performed cautiously to avoid disruption of the labral seal and postoperative microinstability.

A comparative radiographic morphometric analysis to assess the normal radiological morphology of the adult hip in indian population

OBJECTIVE

Our objective is to analyze the normal radiological morphologic parameters of the adult hip joint of the Indian population and compare it with standard measurements and with other populations to assess the variations.

METHODS

A prospective analysis of the normal pelvis X-rays of 800 persons (1600 hips) was done. We have calculated the acetabular inclination, acetabular index, lateral center edge angle (LCEA) and neck-shaft angle (NSA), sphericity of the femoral head, congruity of the joint, version of the acetabulum, depth of acetabulum, and lateralization of the femoral head in normal X-rays of the pelvis in adult persons. We used RadiAnt DICOM viewer version 4.6.5.18450 (64bit) for measurement. Statistical analysis and mean values were calculated using SPSS software.

RESULTS

There were 978 X-rays of the male hip and 622 female. The acetabular inclination varied from 1 to 9. The mean acetabular index was 26.5. The LCEA was between 20 and 50. The mean neck-shaft angle was 133. There were 35, hips with an aspherical head. 94.2% X-rays the hip joint was congruent. There was 2.9% of the retroversion of acetabulum, 3.3% lateralization.

CONCLUSION

Most of the parameters were comparable to standard values the acetabular index was lower. LCEA and NSA were higher. The acetabular angle was lower. There were femoral head asphericity, joint incongruity, and lateralization of the femoral head in a small proportion of our general population.

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.

Joint Space of the Human Knee and Hip Joint under a Static Load

In order to study the joint contact and joint space of statically loaded human knee and hip joints, observations of serial slices of joint specimens which were frozen during the application of a load were carried out. In the intact joints, the articular cartilage surfaces did not come into direct contact with each other even under a load of more than twice that of the body weight. The minimum distance between cartilage surfaces in each specimen ranged from 0.2 to 0.6 mm. The Indian ink which was injected into the joints before the load application was squeezed out of some areas of the remaining joint space, but a dye-free fluid apparently remained in this space.

Based on our findings, the definition of joint contact and the lubrication mechanism in the intact human joint have been discussed.

An accurate method of radiological assessment of acetabular volume and orientation in computed tomography spatial reconstruction

Background

Two-dimensional (2D) measurements of acetabular morphology and orientation are well known; there is less information on these acetabular characteristics in three dimensions. One important reason is the lack of standardized reference planes for the pelvis, especially in relation to the spinopelvic unit; another is that no method precisely assesses the acetabulum in three-dimensional (3D) orientation based on its axis rather than on the directions of the edges of the acetabular rim. We present an objective, highly reliable and accurate, axis-based approach to acetabular anthropometry in the measurement of acetabular volume and spatial orientation in both normal and pathologic hips. This was done using reference planes based on the sacral base (SB) and true acetabular axis in 3D computed tomography (CT) pelvic reconstruction.

Methods

Radiological examinations of 30 physiologic pelves (60 acetabula) were included in the study. Reliability and accuracy of the method were verified by comparing acetabular angles in 2D pelvic scans with 3D reconstructions. We also applied the method to two pathologic acetabula.

Results

Comparison of axis position in the horizontal plane revealed significant positive correlations between 2D angle measurements (acetabular anteversion angle [AAA] and anterior acetabular index [AAI]) and 3D measurement of anteversion angle (p < 0.001 and p = 0.012, respectively). In the frontal plane, there was no difference between abduction angle, measured on topogram, and inclination angle, obtained from a 3D model (p = 0.517). In the sagittal plane, there was a significant negative correlation between AAA and acetabular tilt (p < 0.001). Inter- and intra-observer reproducibility was excellent for determination of the sacral-base plane and assessment of volume, with Fleiss κ coefficients of 0.850 and 0.783, respectively, and intraclass correlation coefficients of 0.900 and 0.950, respectively. Inter-observer reproducibility for evaluation of acetabular axis ranged from 0.783 to 0.883, and intra-rater reliability ranged from 0.850 to 0.900 for all 3D angles.

Conclusions

Our method is a new, reliable diagnostic tool for assessing the acetabula in both normal and pathologic hip joints. The sacral-base plane can be used as a stable reference that takes the relationship of the acetabulum to the spinopelvic unit into consideration.

Electronic supplementary material

The online version of this article (doi:10.1186/s12891-015-0503-8) contains supplementary material, which is available to authorized users.

Morphological analysis of the acetabular cartilage surface in elderly subjects

PURPOSE

The geometry of acetabular cartilage surface plays an important role in hip joint biomechanics. The aim of this study was to analyze the morphology of acetabular articular cartilage surface in elderly donated bodies to science using a 3D-digitizer.

METHODS

Twenty hemipelves from 12 subjects (mean ages 85 years) were scanned with 3D-digitizer. Each acetabular surface model was divided into four regions: anterosuperior (AS), anteroinferior (AI), posterosuperior (PS), and posteroinferior (PI). In the global acetabulum and each region, the acetabular sphere radius and the standard deviation (SD) of the distance from the acetabular sphere center to the acetabular cartilage surface were calculated. In the global acetabulum, the distance between the acetabular surface model and the maximum sphere which did not penetrate over the acetabular surface model was calculated as the inferred femoral head, and then the distribution was mapped at intervals of 0.5 mm.

RESULTS

The SD in AS was significantly larger than that in AI (p = 0.006) and PI (p = 0.001). The SD in PS was significantly larger than that in PI (p = 0.005). The closest region (0-0.5 mm) tended to be distributed at anterior or posterosuperior acetabular edge.

CONCLUSIONS

The contact between the femoral head and acetabulum might start at the periphery of the lunate surface, especially in the anterior or posterosuperior region. From viewpoint of acetabular morphology, the acetabular articular cartilage in the anterior or posterosuperior edge could be more vulnerable due to direct contact mechanism.

A sound and efficient measure of joint congruence

In the medical world, the term “congruence” is used to describe by visual inspection how the articular surfaces mate each other, evaluating the joint capability to distribute an applied load from a purely geometrical perspective. Congruence is commonly employed for assessing articular physiology and for the comparison between normal and pathological states. A measure of it would thus represent a valuable clinical tool. Several approaches for the quantification of joint congruence have been proposed in the biomechanical literature, differing on how the articular contact is modeled. This makes it difficult to compare different measures. In particular, in previous articles a congruence measure has been presented which proved to be efficient and suitable for the clinical practice, but it was still empirically defined. This article aims at providing a sound theoretical support to this congruence measure by means of the Winkler elastic foundation contact model which, with respect to others, has the advantage to hold also for highly conforming surfaces as most of the human articulations are. First, the geometrical relation between the applied load and the resulting peak of pressure is analytically derived from the elastic foundation contact model, providing a theoretically sound approach to the definition of a congruence measure. Then, the capability of congruence measure to capture the same geometrical relation is shown. Finally, the reliability of congruence measure is discussed.

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.

In vivo hip joint contact distribution and bony impingement in normal and dysplastic human hips

Our objectives were to clarify the 3D articular contact areas of the in vivo normal hip joint and acetabular dysplasia during specific positions using magnetic resonance imaging (MRI), voxel-based registration, and proximity mapping. Forty-two normal and 24 dysplastic hips were examined. MRI was performed at four positions: neutral; 45° flexion; 15° extension; and the Patrick position. Femur and pelvis bone models were reconstructed at the neutral position and superimposed over the images of each different position using voxel-based registration. The inferred cartilage contact and bony impingement were investigated using proximity mapping. The femoral head translated in the anterior or posteroinferior, anterosuperior, and posteroinferior direction from neutral to 45° flexion, 15° extension, and the Patrick position, respectively. Multiple regression analyses showed age, femoral head sphericity, and acetabular sphericity to be associated with higher hip instability. The present technique using subject-specific models revealed the in vivo hip joint contact area in a population of healthy individuals and dysplastic patients without radioactive exposure. These results can be used for analyzing disease progression in the dysplastic hip and pathogenesis of acetabular labral tear.

Contributions of non-spherical hip joint cartilage surface to hip joint contact stress

The natural non-spherical incongruent hip joint cartilage surface is normally assumed as spherical in shape, which has been extensively applied in orthopedic clinic, hip joint simulation studies and hip joint prosthesis design. The aim of the study was to investigate the contributions of non-spherical incongruent hip joint cartilage surface to the hip joint contact stress, and to assess the effect of simplified spherical assumption on the predicted contact stress. Based on our previous anatomic studies that the acetabular cartilage surface was demonstrated as rotational ellipsoid in shape, three finite element (FE) models involving the natural hip joint cartilage shape, the hip joint cartilage shape replaced by the rotational ellipsoid and the sphere, respectively, were developed using the computed tomography (CT) image data of healthy volunteers. The FE predictions of contact stress on the replaced hip joint cartilage surface were compared with that on the natural hip joint cartilage surface. The result showed that the non-spherical hip joint cartilage surface contributed to the optimal contact stress magnitude and distribution. The replaced fitting spherical surface led to the increased contact stress of hip joint and the uneven distributed patterns of contact stress, whereas the replaced fitting rotational ellipsoid surface was comparatively more consistent with the natural results than the sphere one. The surface fitting error of the replaced rotational ellipsoid was fewer than that of the replaced sphere. These results indicate that the simplified spherical assumption will lead to misestimating the contact mechanics of hip joint, and the rotational ellipsoid model rather than the sphere model may represent the hip joint contact surface applied in the hip joint simulation study and the hip joint prosthesis design.

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 shape of the acetabular cartilage surface and its role in hip joint contact stress

The acetabular cartilage is normally represented as a spherical shape in orthopedic clinic and related researches. The aim of the study was to present a new mathematic representation with better fit to the acetabular cartilage surface and to investigate the role of its shape on the hip joint contact stress.

A clinically relevant review of hip biomechanics

The hip is a complex anatomic structure composed of osseous, ligamentous, and muscular structures responsible for transferring the weight of the body from the axial skeleton into the lower extremities. This must be accomplished while allowing for dynamic loading during activities such as gait and balance. The evaluation of hip pain and periarticular pathology can be challenging because of the complex local anatomy and broad differential diagnosis. Recent advancements in the evaluation and surgical treatment of hip pathology have led to a renewed interest in the management of these disorders. An understanding of the basic biomechanical and kinematic function of the hip and the consequences of associated pathology can greatly assist the orthopaedic surgeon in appropriately diagnosing and treating these problems. In this review we discuss the basic biomechanical concepts of the native hip and surrounding structures and the changes experienced as a result of various pathologies including dysplasia, femoroacetabular impingement, labral injury, capsular laxity, hip instability, and articular cartilage injury. We will also discuss the clinical implications and surgical management of these pathologies and their role in restoring or preserving the native function of the hip joint.

A review of the differences between normal and osteoarthritis articular cartilage in human knee and ankle joints

BACKGROUND

Osteoarthritis (OA) is the most common joint disease yet its pathophysiology is still poorly understood. It is more prevalent in some lower limb joints than others; in particular the knee is more commonly affected than the ankle. Research into articular cartilage and OA has primarily focussed on using animal models. However, it is apparent that articular cartilage differs between species, so more research is concentrating on human cartilage.

OBJECTIVE

This paper reviews recent studies that have been undertaken to elucidate the reasons for this, and to discover if the findings would alter the conception that articular cartilage is not capable of repair.

METHOD

Primary research papers into human knee and ankle cartilage published since 1997 have been reviewed.

RESULTS

Differences in the structure, metabolism, physical properties and response to trauma have been found, implying that ankle cartilage may be more resistant to damage.

CONCLUSIONS

More research is needed before definitive conclusions can be reached, but the findings so far suggest that OA should not be accepted as the inevitable outcome of joint injury and individuals and practitioners, such as podiatrists, may be able to use simple measures to prevent or delay its onset.

Acetabular morphology: implications for joint-preserving surgery

Appropriate anatomic concepts for surgery to treat femoroacetabular impingement require a precise appreciation of the native acetabular anatomy. We therefore determined (1) the spatial acetabular rim profile, (2) the topography of the articular lunate surface, and (3) the 3-D relationships of the acetabular opening plane comparing 66 bony acetabula from 33 pelves in female and male pelves. The acetabular rim profile had a constant and regular wave-like outline without gender differences. Three prominences anterosuperiorly, anteroinferiorly and posteroinferiorly extended just above hemispheric level. Two depressions were below hemispheric level, of 9 degrees at the anterior wall and of 21 degrees along the posterosuperior wall. In 94% of all acetabula, the deepest extent of the articular surface was within 30 degrees of the anterosuperior acetabular sector. In 99% of men and in 91% of women, the depth of the articular surface was at least 55 degrees along almost half of the upper acetabular cup. The articular surface was smaller in women than in men. The acetabular opening plane was orientated in 21 degrees +/- 5 degrees for version, 48 degrees +/- 4 degrees for inclination and 19 degrees +/- 6 degrees for acetabular tilt with no gender differences. We defined tilt as forward rotation of the entire acetabular cup around its central axis; because of interindividual variability of acetabular tilt, descriptions of acetabular lesions during surgery, CT scanning and MRI should be defined and recorded in relation to the acetabular notch. Acetabular tilt and pelvic tilt should be separately identified. We believe this information important for surgeons performing rim trimming in FAI surgery or performing acetabular osteotomies.

Neuromuscular Hip Biomechanics and Pathology in the Athlete

Although hip arthroscopic techniques have been developed and evolved over the last 5 to 10 years to help active athletes, the mechanisms of athletic hip injuries across various sports are not well understood. The purpose of this article is to review the literature related to the osseous and ligamentous support as well as the neuromuscular control strategies associated with hip joint mechanics. The neuromuscular contributions to hip stability and mobility with respect to gait will be provided because this data represents the largest body of knowledge regarding hip function. Further, this article will present and describe probable mechanisms of injury in sporting activities most often associated with hip injury in the young athlete.

Does Dega osteotomy increase acetabular volume in developmental dysplasia of the hip?

Whether acetabular volume increases or decreases after acetabular Dega osteotomy is not known. The purpose of this study is to determine the effect of Dega osteotomy on the volume of the acetabulum in patients with developmental dysplasia of the hip. Nine hips of seven patients with developmental dysplasia of the hip that have undergone Dega osteotomy were included in the study. The acetabular index, acetabular depth, and acetabular volume of each hip were calculated before and after surgery. Magnetic resonance imaging was used for the measurement of the acetabular volume. The difference between the preoperative and postoperative values of acetabular index, acetabular depth, and acetabular volume was statistically significant. We conclude that Dega acetabular osteotomy increases the volume of the acetabulum.

Acetabular labral tears with underlying chondromalacia: a possible association with high-level running

PURPOSE

The use of hip arthroscopy has helped delineate intra-articular pathology and has enabled clinicians to further elucidate the factors responsible for injuries, such as running. The subtle development of degenerative changes may be a result of repetitive impact loading associated with this sport. This study presents a population of runners with common pathologic acetabular changes.

TYPE OF STUDY

Case series.

METHODS

Eight high-level runners with an average age of 36 years (range, 19 to 45 years) were seen for complaints of increasing hip pain with running without any history of macrotrauma. All of the patients had either run several marathons (4), were triathletes (1), Olympic middle distance runners (1), or had run more than 10 miles per week for longer than 5 years (2). Plain radiographic analysis revealed no degenerative changes and an average center-edge (CE) angle of 36.7 degrees (range, 28 degrees to 44 degrees).

RESULTS

All patients underwent hip arthroscopy with labral debridement. In 6 patients (75%), a chondral injury of the acetabular cartilage underlying the labral tear was noted. In addition, 3 patients had ligamentum teres disruptions.

CONCLUSIONS

It is possible that the development of these tears is the result of subtle instability, which may be exacerbated by running, eventually leading to labral tearing and possible ligamentum teres disruption. While perhaps concurrently, subtle acetabular dysplasia may play a role. Although this study does not confirm an association between running and the development of labral tears or chondral lesions in the hip, it certainly questions whether there is an injury pattern common to this population, a "runner's hip."

LEVEL OF EVIDENCE

Level IV.

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.

Surface size, curvature analysis, and assessment of knee joint incongruity with MRI in vivo

The purpose of this study was to develop an MR-based technique for quantitative analysis of joint surface size, surface curvature, and joint incongruity and to assess its reproducibility under in vivo imaging conditions. The surface areas were determined after 3D reconstruction of the joint by triangulation and the incongruity by Gaussian curvature analysis. The precision was tested by analyzing four replicated MRI datasets of human knees in 14 individuals. The algorithms were shown to produce accurate data in geometric test objects. The interscan precision was <4% (CV%) for surface area, 2.9-5.7 m(-1) (SD) for the mean principal curvature, and 4.1-7.4 m(-1) for congruence indices. Incongruity was highest in the femoropatellar joint (79.7 m(-1)) and lowest in the medial femorotibial joint (28.6 m(-1)). This technique will permit identification of the specific role of surface size, curvature, and incongruity as potential risk factors for osteoarthritis.

Acetabular labral tears in the athlete

Pathologic involvement of the acetabular labrum is an increasingly recognized phenomenon. Athletes involved in sports that require repetitive twisting or who suffer trauma to the hip are at risk of injury to the acetabular labrum. Injury mechanisms that include hyperextension, hyperflexion, or extremes of abduction place the labrum at particular risk. Symptoms may be acute in onset or, more commonly, insidious onset with persistence or escalation of symptoms. The orthopaedic surgeon evaluating patients with sports-related hip injuries needs to remain cognizant of intraarticular injuries within the hip and, in particular, injuries to the acetabular labrum. Further investigation is needed to fully define the functional importance of the acetabular labrum. Arthroscopic management has been successful in evaluation and management of acetabular labral tears.

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.

Contact Stresses in the Human Hip: Implications for Disease and Treatment

Contact stresses in the hip articular surfaces relate in some way to normal maintenance as well as destruction of joints. In vivo determinations of cartilage-on-cartilage contact pressure histories have never been reported, and current technology does not allow such measurements without the potential for artifact: all experimental methods require introducing some material between the surfaces, and all numerical methods have yet to be fully validated. Nonetheless, a variety of distinct experimental and numerical approaches lead to estimates of contact stresses and surprisingly, despite the choice of technique, values for peak contact stresses lie within a range of one order of magnitude (i.e. 0.5–5.0 MPa) and usually closer. Pathological conditions increase this to the range of over 5.0 MPa, while surgical procedures designed to reduce peak pressures theoretically can achieve reductions. Two critical unresolved issues are 1.) What aspect of the contact stress history (e.g. contact stress gradients over time) in fact cause the biological responses? 2.) What level of contact stress history is tolerated by the cartilage? Future research will need to address these critical issues.

Acetabular morphology and resurfacing design

The bony surfaces of 18 archaeological hemipelves were scanned using a 3D laser surface scanner and CyDir software on a Silicon Graphics workstation. The acetabular area was selected and point data from the approximately spherical bone surface saved. These data were input to a MATLAB routine that calculated the radius and centre of the best-fit sphere. The goodness of fit was estimated using the mean and standard deviation of the distance of the bone surface points from the sphere surface. Eight points, at approximately equal distances around the acetabular rim, were selected with reference to bony landmarks. A plane containing three of these points served as an orientation reference plane. The vectors joining the eight rim points to the centre of the best-fit sphere were found. The angles between these vectors and the normal to the reference plane were calculated. Paired angles were summed to give the angle subtended by the acetabular rim in four directions. The overall mean angle was 158 degrees (range of mean angles 145 degrees -173 degrees ). The largest individual angles, some exceeding 180 degrees, were in the superior-inferior direction, while the mean angle in the anterior-posterior direction, i.e. that controlling flexion-extension, was 152 degrees. Males had larger subtended angles than females, although the difference was not statistically significant. Simulated reaming increased all angles by approximately 10 degrees. The subtended angles are important parameters in the design of the acetabular component of a hip replacement and particularly important in resurfacing hip replacement when the volume available is tightly constrained.

Radiographic anatomic structure of the arthritic acetabulum and its influence on total hip arthroplasty

Acetabular bone structure is not the same in all patients and can be defined by the radiolucent triangle superior to the acetabulum. Of 132 hips, 81 had an isosceles triangular shape, which was named type A acetabulum. Forty-six hips had an extension of the triangle into the teardrop, which created a thickened medial wall and was named type B. Five hips had a right-angle triangle, which was found only with congenital disease of the hip and was named type C. The density of the superior acetabular bone in the triangle could be normally radiolucent (stage I), have vertical and transverse trabeculae throughout the triangle (stage II), or have the triangle filled with bone and cysts (stage III). The relationship between progressive radiolucent lines and acetabular type showed that type A3 (thin medial wall with dense triangle bone) had the highest incidence of progressive radiolucent lines (P < .05).

The acetabular labrum seal: a poroelastic finite element model

OBJECTIVE

The aim of the study is to investigate the labrum's ability to seal a pressurised layer of synovial fluid within the joint, and to study the influence of this sealing mechanism on cartilage deformation, interstitial fluid pressure and collagen solid matrix stresses.

BACKGROUND

Cartilage degeneration has been observed in conjunction with labrum pathology. However, little is known about the function of the labrum. Experimental observations have been reported which are consistent with a sealing function of the labrum.

METHODS

The model was an axisymmetric geometric approximation of the acetabular and femoral cartilage layers and the surrounding labrum. A poroelastic formulation was used to account for the solid and fluid components of these hydrated tissues. A sensitivity analysis of the labrum material properties was carried out.

RESULTS

With a compressive load of 1200 N applied across the joint model, the labrum could seal a layer of pressurised fluid between the femur and acetabulum, thus preventing contact of the articulating surfaces. With this sealing effect, loads were transferred across the joint predominantly by uniform pressurisation of the interstitial fluid of the cartilage layers. In the absence of this sealing, strains within the solid matrix of the cartilage layers were higher (e.g. 20% vs. 3%).

CONCLUSIONS

The labrum can seal against fluid expression from the joint space. This sealing function protects the cartilage layers of the hip.

RELEVANCE

Current treatments for labrum damage and early arthrosis may compromise the sealing function of the labrum. With continued study of the function and importance of the labrum, new surgical repair strategies can be developed to maintain the overall function of the hip joint.

Biomechanical evaluation of impaction fractures of the femoral head

OBJECTIVES

To measure the effect of an impaction fracture of the femoral head on load transmission in the hip joint.

DESIGN

We measured the contact areas and pressure between the acetabulum and femoral head of cadaveric pelves in four different conditions: intact, with an operatively created one-square-centimeter defect in the superior femoral head, with a two-square-centimeter defect, and with a four-square-centimeter defect. All defects were uniformly three millimeters deep.

SETTING

Hips were loaded in a simulated single-limb stance. Pressure and area measurements were made with Fuji pressure-sensitive film.

SPECIMENS

Seven hip joints in seven whole pelves were tested.

MAIN OUTCOME MEASUREMENTS

Contact area, load, and mean and maximum pressures were measured.

RESULTS

Peripheral loading was seen in the intact acetabulum. This was not disrupted after impaction fractures of any size. A significant increase in mean maximum pressures in the superior acetabulum was seen with two-square-centimeter and four-square-centimeter defects.

CONCLUSIONS

In contrast to prior biomechanical studies of acetabular fractures, our investigation revealed that disruption of the peripheral distribution of load does not occur with impaction fractures of the femoral head. Clinical series indicate that impaction injuries to the femoral head are associated with a poor prognosis. Previous biomechanical data on acetabular fracture patterns associated with a poor prognosis have shown increases in mean and peak pressures in the superior acetabulum. This was seen with two-square-centimeter and four-square-centimeter impaction injuries. Other factors, such as wear of the articular cartilage during joint motion or associated microscopic damage to the remainder of the joint surface at the time of injury, may also contribute to the rapid joint deterioration seen in these injuries. Further study is indicated.

Quantitative determination of joint incongruity and pressure distribution during simulated gait and cartilage thickness in the human hip joint

The objective of this study was to provide quantitative data on hip-joint incongruity and pressure during a simulated walking cycle and on articular-cartilage thickness in the same set of specimens. Using a casting technique in eight specimens of the human hip (age: 18-75 years), we determined the width of the joint space (incongruity) required at minimal load for contact at four phases of the gait cycle. The pressure distribution, measured with pressure-sensitive film, was determined at physiologic load magnitudes on the basis of in vivo measurements of hip-joint forces. Cartilage thickness was assessed with A-mode ultrasound. At minimal loading, the average maximum width of the joint space ranged from 1.1 to 1.5 mm in the acetabular roof, with the contact areas located ventro-superiorly and dorso-inferiorly throughout the gait cycle. At physiological loading, the width decreased and the contact areas covered the complete articular surface during midstance and heel-off but not during heel-strike or toe-off. The pressure distribution was inhomogeneous during all phases, with average maximum pressures of 7.7 +/- 1.95 MPa at midstance. The cartilage thickness varied considerably throughout the joint surfaces; maxima greater than 3 mm were found ventro-superiorly. These data can be used to generate and validate computer models to determine the load-sharing between the interstitial fluid and the solid proteoglycan-collagen matrix of articular cartilage, the latter being relevant for the initiation of mechanically induced cartilage degeneration.

Arthroscopic findings in the initial stages of hip osteoarthritis

Standard radiographic projections have poor diagnostic ability in detecting early degenerative changes of the hip. Comparison of arthroscopic and radiographic findings is the best method to assess the accuracy of conventional radiography. This study reviewed the records and radiographs of 234 hip arthroscopies. One hundred eighty-six of the patients had normal preoperative radiographs; of these, 60 (32.2%) had evidence of osteoarthritis at arthroscopy. The extent and location of the chondral damage were studied, and both of these parameters were correlated to the radiographic appearance of osteoarthritis. Hips with normal radiographs but arthroscopic osteoarthritis were found to have less damage compared with the radiographically osteoarthritic hip. Hips with normal radiographs also were more likely to have only one side (60%) of the joint damaged, either the acetabulum or femoral head. Conversely, when osteoarthritis was evident radiographically, both sides of the joint were usually involved (75.7%). These patients are often young (average age: 36 years), and women are more likely to be affected than men (71% versus 29%).

The role of the acetabular labrum and the transverse acetabular ligament in load transmission in the hip

We performed a biomechanical study of seventeen hip joints in the pelves of nine cadavera in order to assess the role that the acetabular labrum and the transverse acetabular ligament play in load transmission. The distribution of contact area and pressure between the acetabulum and the femoral head was measured with the hip in four different conditions: intact (seventeen hips), after removal of the transverse acetabular ligament (eight hips), after removal of the entire labrum (nine hips), and after removal of both the transverse acetabular ligament and the labrum (seventeen hips). The hip joint was loaded in simulated single-limb stance, and the measurements were made with use of pressure-sensitive film. A peripheral distribution of load was seen in the intact acetabula. This pattern was altered only minimally after removal of the transverse acetabular ligament or the labrum, or both. When both of these structures were removed, the only significant change was a decrease in the maximum pressure in the posterior aspect of the acetabulum (p = 0.02). No significant changes were detected with regard to the contact area, load, mean pressure, or maximum pressure in the anterior or superior aspect of the acetabulum under any testing condition.

Biomechanical consequences of anterior column fracture of the acetabulum

OBJECTIVES

To measure biomechanical consequences of a high anterior column acetabular fracture.

DESIGN

A benchtop biomechanical model using quasi-static loading of the hip joint in a simulated single-leg stance. Pressure-sensitive prescale (Fuji) film was used to determine hip joint loading parameters.

PARTICIPANTS

Six cadaveric hemipelvi with one hip tested in each specimen. Three right and three left hips were tested.

INTERVENTION

Creation of an anterior column fracture with anatomic reduction and fixation, followed by gap malreduction/fixation, and subsequently step malreduction/fixation.

MAIN OUTCOME MEASUREMENTS

Contact pressure, contact area, and load distribution throughout the hip joint in each experimental condition.

RESULTS

There were significant increases in load (p<0.01) and peak pressures (p<0.01) in the superior acetabular region after gap malreduction and in peak contact pressures after step malreduction (p<0.01) as compared with the intact acetabulum. Anatomic reduction was not associated with increased mean or peak contact pressures (in any region).

CONCLUSIONS

Both step and gap malreductions of a high anterior column fracture resulted in significantly increased peak contact pressures in the superior acetabular region. These biomechanical data cannot be directly extrapolated to clinical applications, but these data suggest that anatomic reduction of anterior column fracture affords the best opportunity to restore contact pressures, contact area, and load distribution within the hip to levels similar to those seen in the intact acetabulum.

Biomechanical evaluation of a low anterior wall fracture: correlation with the CT subchondral arc

OBJECTIVE

To measure the effect of a simulated low anterior wall fracture of the acetabulum on load transmission in the hip joint.

DESIGN

We measured the contact areas and pressure between the acetabulum and the femoral head of cadaveric pelves in three different conditions: intact, with an operatively created fracture of the anterior wall, and after anatomic reduction and internal fixation of the fracture.

SETTING

Hips were loaded in simulated single-limb stance. Pressure and area measurements were made with Fuji pressure-sensitive film.

SPECIMENS

Seven hip joints in seven whole pelves were tested.

INTERVENTION

Anterior wall fractures were anatomically reduced and fixed.

MAIN OUTCOME MEASUREMENTS

Contact area, load, and mean and maximum pressures were measured.

RESULTS

Anterior wall fractures in our specimens entered the hip joint an average of 9.7 millimeters from the vertex of the acetabulum, corresponding to a 45-degree roof arc measurement. Peripheral loading seen in the intact acetabulum was disrupted after fracture. The loading pattern was not restored to preinjury levels with anatomic reduction and fixation. There was no significant change in the contact area (p = 0.43), force (p = 0.06), or mean (p = 0.57) or maximum (p = 0.20) pressures in the superior aspect of the acetabulum after creation of the anterior wall fracture.

CONCLUSIONS

These results differ from those of previous studies with posterior wall acetabulum fractures, where significant increases in force and mean and maximum pressures were noted in the superior acetabulum after fracture. The lack of significant increases in superior acetabular pressures is discussed in relation to the mean computed tomographic subchondral arc of approximately ten millimeters in our specimens.

Mechanobiological adaptation of subchondral bone as a function of joint incongruity and loading

Computed tomography (CT) has been employed to determine non-invasively the distribution of subchondral bone density in joints and to evaluate their dominant loading pattern. The objective of this study was to investigate the relationship between subchondral bone adaptation, joint incongruity and loading, in order to determine to what extent the loading conditions and/or geometric configuration can be inferred from the distribution of subchondral density. Finite element models of joints with various degrees of incongruity were designed and a current remodeling theory implemented using the node-based approach. Appropriate combinations of joint incongruity and loading yielded subchondral bone density patterns consistent with experimental findings, specifically a bicentric distribution in the humero-ulnar joint and a monocentric distribution in the humero-radial joint. However, other combinations of incongruity and loading produced similar subchondral density patterns. Both the geometric joint configuration and the loading conditions influence the distribution of subchondral density in such a way that one of these factors must be known a priori to estimate the other. Since subchondral density can be assessed by CT and joint geometry by magnetic resonance imaging, the dominant loading pattern of joints may be potentially derived in the living using these non-invasive imaging methods.

Computer simulation of subchondral bone adaptation to mechanical loading in an incongruous joint

BACKGROUND

We hypothesized that the typically bicentric distribution of subchondral bone density (i.e., two maxima) in incongruous joints with deeper sockets could be predicted by a computer simulation employing a concavely incongruous finite-element model and current bone remodeling theory. Additional objectives were to assess the uniqueness of the solution with respect to assumed model parameters and initial conditions and to determine the relationship between contact areas, subchondral bone stress, and subchondral bone density patterns.

METHODS

An idealized model of the humeroulnar joint was constructed with a quantitatively realistic representation of its natural incongruity. A currently accepted remodeling theory was implemented with a finite-element code using a node-based approach.

RESULTS

The simulation predicted a dense subchondral bone plate after application of 3,000 daily load cycles for 300 days. The pronounced bicentric distribution of subchondral mineralization emerged. The solution was virtually independent of the initial density distribution and other assumed model parameters. Furthermore, the model predicted high tensile stresses in the subchondral bone, when the joint socket was spread apart during loading. Therefore, the locations of maximal strain energy density in the subchondral bone did not correspond with areas of joint contact.

CONCLUSIONS

The results of the bone remodeling simulations are consistent with patterns of subchondral bone density determined experimentally. Furthermore, the solutions exhibited a high degree of uniqueness, were rather insensitive to changes in cartilage stiffness, moderately sensitive to number of applied loading cycles, and highly sensitive to loading magnitude. Tensile stresses seem to play a dominant role in subchondral bone remodeling due to bending in the subchondral bone plate. Thus, we conclude that, in the case of an incongruous joints with deeper sockets, the density of the subchondral bone cannot be regarded as a direct measure of the adjacent articular pressure.

Anatomic variation of structural properties of periacetabular bone as a function of age. A quantitative computed tomography study

The shape of the acetabulum, the volume of the periacetabular bone, and its density for 125 patients with a wide age range have been quantified using quantitative computed tomography. The goals were to study the relationship between geometric and densitometric properties and provide normative data for finite-element analysis. Significant correlations were found between acetabular diameter and (1) depth, (2) cancellous periacetabular bone density, and (3) periacetabular total bone volume. Only changes in densitometric properties significantly correlated with age. Sphericity of the acetabulum did not increase with age. Variability in bone morphology and density was found for both male and female groups. Surgeons using purely geometric measures to quantify the integrity of acetabular bone should be aware of their limitations when selecting hardware for total hip arthroplasty.

Hip Osteotomies: A Biomechanical Consideration

Proximal femoral and periacetabular osteotomies for the treatment of osteoarthrosis have produced varying clinical results. Because the underlying pathomechanics of osteoarthrosis are not yet fully understood, it is difficult to predict which type of osteotomy will most improve the biomechanical environment in a given situation. Osteotomies result in relatively small changes in joint load (10% to 20% at most), which probably do not govern tissue responses. However, osteotomies do change the distribution of the load (i.e., stresses) and, perhaps more important, stress gradients. These changes in contact and underlying stresses undoubtedly affect cartilage and bone adaptation. It is likely that the magnitude of stress and strain in cartilage and underlying bone can be altered in such a way that more predictable beneficial remodeling will occur. Emerging imaging and computational technologies may allow patient-specific modeling, which should improve the efficacy and durability of repair.

The hip joint as a conchoid shape

The hip joint is not an exact ball and socket joint. In a meridian section, the mean deviation from a conchoid shape is quite small, so that this shape might better describe the joint's shape. This conclusion was reached by measuring the cartilaginous and osseous shapes of eight normal hip joints (multiorgan donors with average of age 33 yr, range 19-46 yr) using a CNC coordinate measuring machine (CMM). On two additional hip joints, only the osseous shape was determined. A rotational axis was first determined by finding parallels of latitude at the femoral head and acetabulum. At the meridian sections, the best-fitting circle or conchoid was determined from the scanned measuring points, using least-squares regression. Two perpendicular meridians were then measured for each sample and used to evaluate the three-dimensional shape. The medium squared deviation showed a better fit for a conchoid shape compared to a sphere for all samples tested. Furthermore, the equation of the conchoid for the femoral head (r = a + b cos phi) and that for the acetabulum (r' = a' + b' cos phi) were related in that a = b' and b = a' within mean deviation factors of 4%. Their special shape makes the joint less likely to sublux compared to a ball and socket joint. In addition, the rolling and gliding mechanisms between the two shapes may result in less wear.

Dynamic anatomy of the acetabulum: an experimental approach and surgical implications

The deformations and stresses acting on the acetabular rim have not been very precisely documented. The authors present a study based on an experimental simulation of hip loading with anatomic correlations. 122 dissections were performed in order to define the anatomic aspect of the roof (and especially of Byers's "area 17") and the intermediate area between the anterior and posterior acetabular cornua. Ten fresh cadavers were tested on the lines of previous studies on monopodal or bipodal loading. An extensometric study was performed with special attention to the transverse acetabular ligament, supra-acetabular area and obturator foramen. The area 17 of Byers is a transitional zone and the mobility of the posterior cornu is 3 times that of the anterior cornu. Resection of the acetabular ligament modifies the displacement of the posterior cornu under loading but has no influence on deformation of the oburator foramen. The biomechanical behavior of the acetabular roof in the standing position is influenced by the conditions of monopodal or bipodal loading and by femoral rotation, but a tendency to extrusion was constantly noted.

The effect of variable size posterior wall acetabular fractures on contact characteristics of the hip joint

The indications for open reduction and internal fixation of posterior wall acetabular fractures associated with a clinically stable hip joint are unclear. In previous work a large posterior wall defect (27% articular surface area) resulted in significant alteration of load transmission across the hip; specifically, there was a transition from evenly distributed loading along the acetabular articular surface to loading concentrated mainly in the superior portion of the articular surface during simulated single leg stance. However, the majority of posterior wall fractures involve a smaller amount of the articular surface. Posterior wall acetabular fractures not associated with instability of the hip are commonly treated nonoperatively. This practice does not account for the size of the posterior wall fracture. To study the biomechanical consequences of variably sized articular defects, a laboratory experiment was conducted evaluating three progressively larger posterior wall defects of the acetabulum during simulated single leg stance using superlow Fuji prescale film (Itochu International, New York): (a) 1/3 articular surface width through a 50 degrees arc along the posterior wall of the acetabulum, (b) 2/3, and (c) 3/3 articular width defects through the same 50 degrees arc along the posterior wall of the acetabulum. In the intact acetabulum, 48% of the total articular contact was located in the superior acetabulum. Twenty-eight percent of articular contact was in the anterior wall region of the acetabulum and 24% in the posterior wall region. After the 1/3 width posterior wall defect, 64% of the articular contact was located in the superior acetabulum (p = 0.0011). The 2/3 width posterior wall defect resulted in 71% of articular contact area being located in the superior acetabulum (p = 0.0006). After the 3/3 width posterior wall defect, 77% of articular contact was located in the superior acetabulum, significantly greater than the intact condition (p < 0.0001) and 1/3 width defect (p = 0.0222). The total absolute contact areas for all defect conditions were significantly less than the intact conditions. The results of this study reconfirm the observation that posterior wall fractures of the acetabulum significantly alter the articular contact characteristics in the hip during single leg stance. The relationship between defect size and changes in joint contact showed that the smallest defect resulted in the greatest alteration in joint contact areas, whereas larger defects resulted in minor increments of change in contact area. This finding is of concern because the clinical practice of managing acetabular fractures nonoperatively if the hip joint is stable is based on the supposition that the joint retains enough integrity to function without undue risk of late posttraumatic osteoarthritis. A better understanding of the natural history of stable posterior wall acetabular fractures is needed to ascertain whether some of these fractures merit operative repair.

Hip joint congruence: experimental studies

Three-dimensional imaging of hip joint preparations was carried out by means of computed tomography (CT). On the assumption that the head of the femur and acetabulum are incongruent, a special experimental design was used to quantify the extent and direction of the gliding movement of the head and socket. A series of CT images in the supine and prone positions was made from hip preparations, and the cartilage was visualized by injecting a contrast medium. In all the preparations dorsal movement was noted in the supine position, and ventral movement in the prone position. Cryomicrotome slices of the hip preparations were used to verify the results of our CT scans and the measurements correlated with the anatomical examinations. Thus, there is a certain amount of play in the hip joint-it is not an ideal ball-and-socket joint. This knowledge is important for the planing and success of hip adjustment surgery.