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

Estudo antropométrico tomográfico do quadril em uma população regional brasileira

Objective  The present study aimed to determine the average hip anthropometry of a regional Brazilian population using measurements based on computerized axial tomography (CAT).

Methods  Retrospective, descriptive analysis of hip measurements from 200 abdominal CATs from patients visiting a medical center. The tests were selected at random to determine 30 previously defined anthropometric measurements. The data were statistically analyzed and compared according to gender and age.

Results  The prevalence of hip dysplasia was 6%. Signs suggesting femoroacetabular impingement were seen in 26% of cases. Patients over 50 years old presented significantly greater measures of horizontal acetabulum sectors, center-edge angle, and acetabular arch, as well as lower extrusion index, cervical-diaphyseal angle and vertical offset. Some measurements were significantly different according to gender: the lateral center-edge angle (µ = 35.5°) and the acetabular arch (µ = 68.7°) were higher in females. Males presented increased extrusion index (µ = 16%), lateral offset (µ = 38.3 mm), depth (µ = 19.5 mm), and neck diameter (µ = 26.4 mm).

Conclusion  The present study characterized the hip anthropometry of a regional Brazilian population. It also demonstrated significant morphological differences per age group and gender.

Osteology of the ilium revised: illuminating the clinical relevance

BACKGROUND

Several studies on anterior and posterior pelvic ring fixation have identified a fragile monocortical area located at the iliac wing. However, there are no current studies regarding this structure's dimensions and relation to known anatomic structures.

METHODS

Eleven human ilia were dissected from 6 specimens. After debulking soft tissue, photoluminescence was used to indicate the fragile area. The size and thickness of the iliac wing were determined and mapped in relation to the anterior superior iliac spine (ASIS) and the posterior superior iliac spine (PSIS).

RESULTS

This photoluminescent unicortical area measured 15.57 cm² with a mean minimal thickness of 1.37 mm at its thinnest part. Its average diameter was 41.15 mm horizontally and 37.45 mm vertically. In all cases, it was located at the middle third of the ilium with a mean distance of 64.58 mm to the AIIS and 62.73 mm to the PSIS. Trajectory angulation above 4.5° from the PSIS lead to violation of this area.

CONCLUSION

This study provides useful anatomical information regarding a thin unicortical area at the iliac wing that is relevant to anterior and posterior pelvic ring fixation and the potential complications that can arise from iatrogenic perforation of this area.

Trabecular architecture of the great ape and human femoral head

Studies of femoral trabecular structure have shown that the orientation and volume of bone are associated with variation in loading and could be informative about individual joint positioning during locomotion. In this study, we analyse for the first time trabecular bone patterns throughout the femoral head using a whole-epiphysis approach to investigate how potential trabecular variation in humans and great apes relates to differences in locomotor modes. Trabecular architecture was analysed using microCT scans of Pan troglodytes (n = 20), Gorilla gorilla (n = 14), Pongo sp. (n = 5) and Homo sapiens (n = 12) in medtool 4.1. Our results revealed differences in bone volume fraction (BV/TV) distribution patterns, as well as overall trabecular parameters of the femoral head between great apes and humans. Pan and Gorilla showed two regions of high BV/TV in the femoral head, consistent with hip posture and loading during two discrete locomotor modes: knuckle-walking and climbing. Most Pongo specimens also displayed two regions of high BV/TV, but these regions were less discrete and there was more variability across the sample. In contrast, Homo showed only one main region of high BV/TV in the femoral head and had the lowest BV/TV, as well as the most anisotropic trabeculae. The Homo trabecular structure is consistent with stereotypical loading with a more extended hip compared with great apes, which is characteristic of modern human bipedalism. Our results suggest that holistic evaluations of femoral head trabecular architecture can reveal previously undetected patterns linked to locomotor behaviour in extant apes and can provide further insight into hip joint loading in fossil hominins and other primates.

What is the tolerated width of periacetabular osteophytes to avoid impingement in cementless THA?: a three-dimensional simulation study

BACKGROUNDS

Impingement is a risk factor for instability and prosthetic failure following total hip arthroplasty (THA). If the periacetabular osteophytes are not removed at surgery, impingement could occur between the osteophytes and the femoral stem following THA. However, excessive removal of the osteophytes could lead to bleeding from the bone. The aim of our study, therefore, was to locate the site of the impingement and to determine the width of tolerable osteophytes, which does not induce impingement during activities of daily living (ADL), using a three-dimensional simulation.

METHODS

On 35 hip models, virtual THA was performed. The acetabular cups were positioned at 45° abduction and 20° anteversion, and the anteversion of femoral stems was 15°. Circular osteophytes with a 30-mm rim were built around the acetabular cup. Fourteen ADL motions were simulated, and the osteophytes were removed until there was no impingement. A clock face was used to map the location and the width of tolerable osteophytes.

RESULTS

The impingement mainly occurred in antero-superior and posterior portions around the acetabular cup. Only 4.2-6.2-mm osteophytes were tolerable at the antero-superior portion (12-3 o'clock) and 6.3-7.2-mm osteophytes at the posterior portion (8-10 o'clock) following a total hip arthroplasty. In antero-inferior and postero-superior portions, over-20-mm osteophytes did not induce any impingement.

CONCLUSION

Osteophytes in the antero-superior and posterior portion of the acetabulum should be excised during a THA to avoid impingement of the femur-stem construct on the acetabular osteophytes during ADLs.

How are dysplastic hips different? A three-dimensional CT study

BACKGROUND

Surgical correction of acetabular dysplasia can postpone or prevent joint degeneration. The specific abnormalities that make up the dysplastic hip are controversial.

QUESTIONS/PURPOSES

(1) What are the relative size, shape, and orientations of the typical nondysplastic hip? (2) How do these variables differ in the developmentally dysplastic hip? (3) Are there version differences between the acetabuli of dysplastic and nondysplastic hips? (4) Are there pairs of variables in which the change in one is always accompanied by a change in the other for both nondysplastic and dysplastic acetabuli?

METHODS

Of 117 consecutive three-dimensional (3-D) CT scans performed for hip dysplasia between March 1988 and October 1995, 48 met criteria of developmentally dysplastic hips by plain radiography. These were retrospectively compared with 55 pelvic 3-D CT scans culled from 81 consecutive scans performed for reasons other than hip dysplasia (ie, hip pain, trauma, infection) that did not affect the hip or pelvic landmarks. The 3-D reconstructions were orientated anatomically for standardization of the measurements to be compared. Representative 3-D volumes of the acetabular space were constructed from which we could measure anatomic positions and dimensional information. One author performed all image orientation and measurements.

RESULTS

Nondysplastic acetabuli are essentially hemispheric with height equal to width and twice the depth. The dysplastic acetabuli were elongated in females (52.4 ± 6.2 mm for dysplastic versus 46.5 ± 4.6 mm for nondysplastic (mean difference, 5.0; 95% confidence interval [CI], 1.9-8.0; p = 0.002) and shallower in both females (18.7 ± 4.9 mm for dysplastic versus 23.6 ± 4.0 mm for nondysplastic; mean difference, 6.5; 95% CI, 4.4-8.5; p < 0.0001) and males (21.1 ± 4.8 mm for dysplastic versus 25.0 ± 4.3 mm for nondysplastic, mean difference, 5.3; 95% CI, 2.6-8.1; p = 0.0002); width was similar to that of nondysplastic hips. Acetabular openings were slightly more vertical than nondysplastic hips in females (5°; 95% CI, 1.9-8.1; p = 0.002) but not in male subjects. The dysplastic acetabuli were smaller in volume (18% in females, p = 0.002, and 19% in males, p = 0.0012) and had less space occupied by the femoral head compared with nondysplastic hips (p < 0.0001 for females, p < 0.0001 for males). Dysplastic hip midacetabulum was 4° more anteverted in females (95% CI, 0.5-6.8; p = 0.022) but not for males (p = 0.538). The upper dysplastic acetabulum was more retroverted in females and males (10.2°; 95% CI, 5.5-15; p < 0.0001, and 7.0°; 95% CI, 0.6-13.4; p = 0.032, respectively). Acetabular volumes in nondysplastic and dysplastic hips were related to acetabular width but not to length.

CONCLUSIONS

Developmentally dysplastic acetabuli are not deficient in merely a single dimension but are globally deficient. The subluxated femoral head lies in the elongated and retroverted superior acetabulum, which becomes progressively shallower as the acetabulum increases in length. Focally deficient anterior or posterior femoral head coverage is uncommon. Current procedures that redirect the acetabulum, no matter how technically successful, cannot fully compensate for the incongruence of a spherical femoral head within a shallow and elongated acetabulum unless corrected at an early age when acetabular remodeling is possible. Early detection and treatment of acetabular dysplasia should be emphasized.

LEVEL OF EVIDENCE

Level III, prognostic study.

Anatomy of the ilium for bone marrow aspiration: map of sectors and implication for safe trocar placement

PURPOSE

The bony anatomy of the human ilium has been well described from a qualitative perspective; however, there are little quantitative data to help the surgeon to perform bone marrow aspiration from the iliac crest in the thickest part of the ilium. The minimum thickness of the spongiousus bone in an iliac wing (transverse thickness between the two tables) is an important factor in ensuring the safe placement of a trocar between the two tables of the iliac wing. For example, with an 8-gauge (3.26 mm) trocar, one can consider that if the transverse thickness of the spongiousus bone of the iliac wing is <3 mm, it will be difficult to insert the trocar safely between the two tables.

METHODS

For this study, we measured spongiousus bone thickness on 48 iliac wings to map the ilium in six sectors, which were defined by drawing lines from equidistant points spaced along the rim of the iliac crest to the centre of the hip. These sectors can be transposed in the same manner to any patient. To evaluate the risks to reach vascular or neurologic structures, 410 trocars were introduced in the different sectors of 20 iliac bones of ten cadavers.

RESULTS

A map was constructed indicating the thickness of the spongiousus bone in each sector. The thickness data was used to create a map that identifies the sites where bone marrow can be obtained with a trocar of 3-mm diameter according to the thickness of the spongiousus bone. Sectors 2, 3 and 6 appear to be more favourable for accommodating a 3-mm diameter trocar. Sectors 1, 4 and 5 comprise the areas with the thinnest parts of the iliac crest, with some areas being thinner than the trocar diameter. The sector system reliably predicted safe and unsafe areas for trocar placement. In cadavers, dissection demonstrated nine vascular or neurologic lesions created when trocars were introduced into sectors 1, 5 and 6.

CONCLUSION

Using the sector system, trocars can be directed away from neural and vascular structures and towards zones that are likely to contain larger bone marrow stock.

Cross-sectional anatomy of the ilium: implications for acetabular component placement in total hip arthroplasty

BACKGROUND

High hip center reconstructions, used in revision and complex primary THAs, rely on pelvic bone stock at least 35 mm above the anatomic teardrop. However, the technique does not restore normal hip biomechanics and controversy exists regarding acetabular implant survival. Previous reports document a wide range of implant positioning above the teardrop. There is no anatomic guidance in the literature regarding the amount of bone stock available for initial implant stability in this area of the ilium.

QUESTIONS/PURPOSES

We therefore determined the thickness of the human ilium and related it to acetabulum cup coverage in high hip center reconstructions.

METHODS

We sectioned 16 cadaveric hips from the anterior superior iliac spine to the anatomic teardrop in 5-mm increments, then measured the thickness of the ilium for each cross section.

RESULTS

The maximum thickness of 42 ± 9 mm occurred at the dome of the acetabulum 35 ± 3 mm above the teardrop. At a distance of 1 cm above the dome, the ilium was reduced by 24%, to 32 ± 6 mm. At 2 cm above the dome, the ilium thickness was 22 ± 4 mm, a 48% reduction from its maximum.

CONCLUSION

There are substantial anatomic limitations to high hip reconstructions 2 cm above the acetabular dome.

Variations in acetabular anatomy with reference to total hip replacement

Three-dimensional surface models of the normal hemipelvis derived from volumetric CT data on 42 patients were used to determine the radius, depth and orientation of the native acetabulum. A sphere fitted to the lunate surface and a plane matched to the acetabular rim were used to calculate the radius, depth and anatomical orientation of the acetabulum. For the 22 females the mean acetabular abduction, anteversion, radius and normalised depth were 57.1° (50.7° to 66.8°), 24.1° (14.0° to 33.3°), 25 mm (21.7 to 30.3) and 0.79 mm (0.56 to 1.04), respectively. The same parameters for the 20 males were 55.5° (47.7° to 65.9°), 19.3° (8.5° to 32.3°), 26.7 mm (24.5 to 28.7) and 0.85 mm (0.65 to 0.99), respectively.

The orientation of the native acetabulum did not match the safe zone for acetabular component placement described by Lewinnek. During total hip replacement surgeons should be aware that the average abduction angle of the native acetabulum exceeds that of the safe zone angle. If the concept of the safe zone angle is followed, abduction of the acetabular component should be less than the abduction of the native acetabulum by approximately 10°.

Acetabular Cup Geometry and Bone-Implant Interference have More Influence on Initial Periprosthetic Joint Space than Joint Loading and Surgical Cup Insertion

Environmental variations in patient-dependent and surgical factors were modeled using robust optimization with a finite element acetabular cup-pelvis model. A previously developed statistical optimization scheme was used to: (1) determine the cup geometry and the optimal cup-bone interference that maximized bone-implant contact areas and minimized changes in the gap volume between the implant and bone surface during gait loading and unloading; and (2) determine the relative contributions of design, patient-dependent, and surgical factors to variations in bone-implant contact areas and a change in gap volume. The statistical analyses indicated that the design variables, namely the equatorial diameter and eccentricity, explained most of the variations in the performance measures. Further, the hemispherical designs performed better than the nonhemispherical designs. The 58mm hemispherical cup, with 2mm diametral interferences, minimized the change in gap volume and attained 82% and 81% of the maximum predicted total and rim contact areas, respectively. The equatorial diameter and eccentricity, not the patient-dependent and surgical factors, explained most of the variations in the performance measures. Perfect surface apposition was not attained with any of the cup designs.

A three-dimensional finite element model from computed tomography data: a semi-automated method

Three-dimensional finite element analysis is one of the best ways to assess stress and strain distributions in complex bone structures. However, accuracy in the results may be achieved only when accurate input information is given. A semi-automated method to generate a finite element (FE) model using data retrieved from computed tomography (CT) was developed. Due to its complex and irregular shape, the glenoid part of a left embalmed scapula bone was chosen as working material. CT data were retrieved using a standard clinical CT scanner (Siemens Somatom Plus 2, Siemens AG, Germany). This was done to produce a method that could later be utilized to generate a patient-specific FE model. Different methods of converting Hounsfield unit (HU) values to apparent densities and subsequently to Young's moduli were tested. All the models obtained were loaded using three-dimensional loading conditions taken from literature, corresponding to an arm abduction of 90 degrees. Additional models with different amounts of elements were generated to verify convergence. Direct comparison between the models showed that the best method to convert HU values directly to apparent densities was to use different equations for cancellous and cortical bone. In this study, a reliable method of determining both geometrical data and bone properties from patient CT scans for the semi-automated generation of an FE model is presented.

Size and direction of hip joint forces associated with various positions of the acetabulum

When total hip replacement is performed, the position of the acetabular component may affect wear and component survival time. We considered the questions: In what way does displacement of the hip joint center alter (1) the magnitude and (2) the direction of the resultant force? Biomechanical tests were carried out on a human multibody model. After displacement of the hip joint center, the resultant forces were calculated for the single leg stance. With the flexed single leg stance, maximum hip joint forces were observed with lateral, cranial, posterior displacement. The peak forces were affected by the modeling of a gluteus maximus wrapping point at the ischial tuberosity and were overestimated when this was removed. With the straight single leg stance, posterior displacement decreases the total load on the hip joint because of the increased leverage of the rectus femoris. With regard to the direction of the resultant force, medial displacement increases the angles in both planes, cranial displacement increases it in the sagittal plane (cranial, posterior-caudal, anterior), and anterior displacement decreases the angle in the sagittal plane and increases it in the frontal plane (medial, cranial-lateral, caudal). The direction of the force is relatively insensitive to displacement of the hip joint center. The results presented here indicate a marked increase in the force after lateral, cranial, posterior displacement of the center in the flexed single leg stance. To avoid extreme joint loading and to reduce the wear after total hip arthroplasty, the cranial and posterior regions of the acetabulum should be fully reconstructed. A high hip joint center has an adverse effect on the magnitude of the force, although the directions are hardly affected by it.