Risk and Epidemiology of Periprosthetic Knee Fractures After Primary Total Knee Arthroplasty: A Nationwide Cohort Study

BACKGROUND

Periprosthetic knee fractures (PPKF) following total knee arthroplasty (TKA) are uncommon, but potentially serious injuries. We analyze the risk and risk factors for a PPKF in standard primary TKA patients who have osteoarthritis (OA) and a minimally (cruciate-retaining TKAs without a femoral box cut) or posterior-stabilized TKA. In addition, we report the risk for patients who have other underlying knee disorders and/or a higher level of TKA constraint.

METHODS

All primary TKAs were identified from the Danish National Patient Register and the Danish Knee Arthroplasty Register using data between 1997 and 2022. Subsequent fractures were identified through the International Classifications of Disease (ICD) diagnosis code, Nordic Medico-Statistical Committee (NOMESCO) procedure code, or indication for revision TKA.

RESULTS

We included 120,642 standard primary TKA patients who had 1,434 PPKFs. The cumulated proportions were 0.3% (95% confidence interval (CI) 0.3 to 0.3) at 2 years 0.7% (0.6 to 0.7) at 5 years. At 10 years, the cumulated proportion was 1.5% (1.4 to 1.6), with 1.2% in the femur, 0.2% in the patella, and 0.1% in the tibia. Significant risk factors were (hazard ratio (HR) (95% CI)); ipsilateral hip arthroplasty (2.5 (2.1 to 2.8)); women (2.0 (1.8 to 2.3)), osteoporosis (1.4 (1.1 to 1.7)); age 80+ (1.4 (1.2 to 1.6)), and Charlson Comorbidity Index (CCI) score 3+ (1.4 (1.0 to 1.8)). An additional 22,624 primary TKA patients who had other underlying knee disorders and/or a higher level of implant constraint were included with 485 PPKFs. The 10-year cumulated proportions were 5.5% (95% CI 4.4 to 6.9) when the underlying disorder was a previous fracture, 2.3% (1.7 to 2.9) for rheumatic disorders, and 3.5% (1.4 to 8.8) for osteonecrosis. In patients who had condylar constrained knees, it was 4.3% (2.9 to 6.3), and 7.3% (4.1 to 13.1) for hinges.

CONCLUSION

In standard primary TKA patients, the 10-year cumulated proportion of PPKFs was 1.5%, and ipsilateral hip arthroplasty, women, osteoporosis, advanced age, and higher CCI increased the risk. Higher risks were observed in non-OA patients and/or patients who had a higher level of TKA constraint.

A literature review and expert consensus statement on diagnostics in suspected metal implant allergy

BACKGROUND

Although rare, allergic reactions to metal implants represent a diagnostic challenge in view of missing guidelines.

OBJECTIVES

To develop an European expert consensus on characteristics of metal allergy reactions and the utility of various diagnostic tools in suspected metal implant allergy.

METHODS

A nominal group technique (NGT) was applied to develop consensus statements. Initially an online literature database was created on a secure server to enable a comprehensive information. Twenty-three statements were formulated on potential aspects of metal implant allergy with a focus on diagnostics and grouped into five domains. For the consensus development, the panel of 12 experts initially did refine and reformulate those statements that were ambiguous or had unclear wording. By face-to-face (9/12) or virtual participation (3/12), an anonymous online voting was performed.

RESULTS

Consensus (≥80% of agreement) was reached in 20/23 statements. The panel agreed that implant allergy despite being rare should be considered in case of persistent unexplained symptoms. It was, however, recommended to allow adequate time for resolution of symptoms associated with healing and integration of an implant. Obtaining questionnaire-aided standardized medical history and standardized scoring of patient outcomes was also considered an important step by all experts There was broad consensus regarding the utility/performance of patch testing with additional late reading. It was recognized that the lymphocyte transformation test (LTT) has to many limitations to be generally recommended. Prior to orthopaedic implant, allergy screening of patients without a history of potential allergy to implant components was not recommended.

CONCLUSIONS

Using an expert consensus process, statements concerning allergy diagnostics in suspected metal implant allergy were created. Areas of nonconsensus were identified, stressing uncertainty among the experts around topics such as preoperative testing in assumed allergy, histological correlate of periimplant allergy and in vitro testing, which underscores the need for further research.

Mismatch 'never events' in hip and knee arthroplasty: a cohort and intervention study

AIMS

The aim of this study was to give estimates of the incidence of component incompatibility in hip and knee arthroplasty and to test the effect of an online, real-time compatibility check.

MATERIALS AND METHODS

Intraoperative barcode registration of arthroplasty implants was introduced in Denmark in 2013. We developed a compatibility database and, from May 2017, real-time compatibility checking was implemented and became part of the registration. We defined four classes of component incompatibility: A-I, A-II, B-I, and B-II, depending on an assessment of the level of risk to the patient (A/B), and on whether incompatibility was knowingly accepted (I/II).

RESULTS

A total of 26 524 arthroplasties were analyzed. From 12 307 procedures that were undertaken before implementation of the compatibility check, 21 class A incompatibilities were identified (real- or high-risk combinations; 0.17%; 95% confidence interval (CI) 0.11 to 0.26). From 5692 hip and 6615 knee procedures prior to implementation of the compatibility check, we found rates of class A-I incompatibility (real- or high-risk combinations unknowingly inserted) of 0.14% (95% CI 0.06 to 0.28) and 0.17% (95% CI 0.08 to 0.30), respectively. From 14 217 procedures after the introduction of compatibility checking (7187 hips and 7030 knees), eight class A incompatibilities (0.06%; 95% CI 0.02 to 0.11) were identified. This difference was statistically significant (p = 0.008).

CONCLUSION

Our data presents validated estimates of the baseline incidence of incompatibility events for hip and knee arthroplasty procedures and shows that a significant reduction in class A incompatibility events is possible using a web-based recording system. Cite this article: Bone Joint J 2019;101-B:960-969.

Re-admissions, re-operations and length of stay in hospital after aseptic revision knee replacement in Denmark: a two-year nationwide study

We present detailed information about early morbidity after aseptic revision knee replacement from a nationwide study. All aseptic revision knee replacements undertaken between 1st October 2009 and 30th September 2011 were analysed using the Danish National Patient Registry with additional information from the Danish Knee Arthroplasty Registry. The 1218 revisions involving 1165 patients were subdivided into total revisions, large partial revisions, partial revisions and revisions of unicondylar replacements (UKR revisions). The mean age was 65.0 years (27 to 94) and the median length of hospital stay was four days (interquartile range: 3 to 5), with a 90 days re-admission rate of 9.9%, re-operation rate of 3.5% and mortality rate of 0.2%. The age ranges of 51 to 55 years (p = 0.018), 76 to 80 years (p < 0.001) and ≥ 81 years (p < 0.001) were related to an increased risk of re-admission. The age ranges of 76 to 80 years (p = 0.018) and the large partial revision subgroup (p = 0.073) were related to an increased risk of re-operation. The ages from 76 to 80 years (p < 0.001), age ≥ 81 years (p < 0.001) and surgical time > 120 min (p < 0.001) were related to increased length of hospital stay, whereas the use of a tourniquet (p = 0.008) and surgery in a low volume centre (p = 0.013) were related to shorter length of stay. In conclusion, we found a similar incidence of early post-operative morbidity after aseptic knee revisions as has been reported after primary procedures. This suggests that a length of hospital stay ≤ four days and discharge home at that time is safe following aseptic knee revision surgery in Denmark.

Translation, cross-cultural adaptation and validation of the Danish version of the Oxford hip score: Assessed against generic and disease-specific questionnaires

OBJECTIVES

The Oxford hip score (OHS) is a 12-item questionnaire designed and developed to assess function and pain from the perspective of patients who are undergoing total hip replacement (THR). The OHS has been shown to be consistent, reliable, valid and sensitive to clinical change following THR. It has been translated into different languages, but no adequately translated, adapted and validated Danish language version exists.

METHODS

The OHS was translated and cross-culturally adapted into Danish from the original English version, using methods based on best-practice guidelines. The translation was tested for psychometric quality in patients drawn from a cohort from the Danish Hip Arthroplasty Register (DHR).

RESULTS

The Danish OHS had a response rate of 87.4%, no floor effect and a 19.9% ceiling effect (as expected in post-operative patients). Only 1.2% of patients had too many items missing to calculate a sum score. Construct validity was adequate and 80% of our predefined hypotheses regarding the correlation between scores on the Danish OHS and the other questionnaires were confirmed. The intraclass correlation (ICC) of the different items ranged from 0.80 to 0.95 and the average limits of agreement (LOA) ranged from -0.05 to 0.06. The Danish OHS had a high internal consistency with a Cronbach's alpha of 0.99 and an average inter-item correlation of 0.88.

CONCLUSIONS

This Danish version of the OHS is a valid and reliable patient-reported outcome measurement instrument (PROM) with similar qualities to the original English language version.

A comparison of structural and mechanical properties in cancellous bone from the femoral head and acetabulum

Mechanical interlock obtained by penetration of bone cement into cancellous bone is critical to the success of cemented total hip replacement (THR). Although acetabular component loosening is an important mode of THR failure, the properties of acetabular cancellous bone relevant to cement penetration are not well characterized. Bone biopsies (9mm diameter, 10mm long) were taken from the articular surfaces of the acetabulum and femoral head during total hip replacement. After mechanical and chemical defatting the two groups of bone specimens were characterized using flow measurement, mechanical testing and finally serial sectioning and three-dimensional computer reconstruction. The mean permeabilities of the acetabular group (1.064 × 10−10 m2) and femoral group (1.155x 10−10m2) were calculated from the flow measurements, which used saline solution and a static pressure of 9.8 kPa. The mean Young's modulus, measured non-destructively, was 47.4 MPa for the femoral group and 116.4MPa for the acetabular group. Three-dimensional computer reconstruction of the specimens showed no significant differences in connectivity and porosity between the groups. Results obtained using femoral head cancellous bone to investigate bone cement penetration and fixation are directly relevant to fixation in the acetabulum.

Changes in the three-dimensional microstructure of human tibial cancellous bone in early osteoarthritis

We obtained medial and lateral subchondral cancellous bone specimens from ten human postmortem proximal tibiae with early osteoarthritis (OA) and ten normal age- and gender-matched proximal tibiae. The specimens were scanned by micro-CT and the three-dimensional microstructural properties were quantified.

Medial OA cancellous bone was significantly thicker and markedly plate-like, but lower in mechanical properties than normal bone. Similar microstructural changes were also observed for the lateral specimens from OA bone, although there had been no sign of cartilage damage. The increased trabecular thickness and density, but relatively decreased connectivity suggest a mechanism of bone remodelling in early OA as a process of filling trabecular cavities. This process leads to a progressive change of trabeculae from rod-like to plate-like, the opposite to that of normal ageing. The decreased mechanical properties of subchondral cancellous bone in OA, which are due to deterioration in architecture and density, indicate poor bone quality.

Changes in the three-dimensional microstructure of human tibial cancellous bone in early osteoarthritis

We obtained medial and lateral subchondral cancellous bone specimens from ten human post-mortem proximal tibiae with early osteoarthritis (OA) and ten normal age- and gender-matched proximal tibiae. The specimens were scanned by micro-CT and the three-dimensional microstructural properties were quantified. Medial OA cancellous bone was significantly thicker and markedly plate-like, but lower in mechanical properties than normal bone. Similar microstructural changes were also observed for the lateral specimens from OA bone, although there had been no sign of cartilage damage. The increased trabecular thickness and density, but relatively decreased connectivity suggest a mechanism of bone remodelling in early OA as a process of filling trabecular cavities. This process leads to a progressive change of trabeculae from rod-like to plate-like, the opposite to that of normal ageing. The decreased mechanical properties of subchondral cancellous bone in OA, which are due to deterioration in architecture and density, indicate poor bone quality.

Mutual associations among microstructural, physical and mechanical properties of human cancellous bone

Previous studies have shown that low-density, rod-like trabecular structures develop in regions of low stress, whereas high-density, plate-like trabecular structures are found in regions of high stress. This phenomenon suggests that there may be a close relationship between the type of trabecular structure and mechanical properties.

In this study, 160 cancellous bone specimens were produced from 40 normal human tibiae aged from 16 to 85 years at post-mortem. The specimens underwent micro-CT and the microstructural properties were calculated using unbiased three-dimensional methods. The specimens were tested to determine the mechanical properties and the physical/compositional properties were evaluated.

The type of structure together with anisotropy correlated well with Young’s modulus of human tibial cancellous bone. The plate-like structure reflected high mechanical stress and the rod-like structure low mechanical stress. There was a strong correlation between the type of trabecular structure and the bone-volume fraction. The most effective microstructural properties for predicting the mechanical properties of cancellous bone seem to differ with age.

Parallel plate model for trabecular bone exhibits volume fraction-dependent bias

Unbiased stereological methods were used in conjunction with microcomputed tomographic (micro-CT) scans of human and animal bone to investigate errors created when the parallel plate model was used to calculate morphometric parameters. Bone samples were obtained from the human proximal tibia, canine distal femur, rat tail, and pig spine and scanned in a micro-CT scanner. Trabecular thickness, trabecular spacing, and trabecular number were calculated using the parallel plate model. Direct thickness, and spacing and connectivity density were calculated using unbiased three-dimensional methods. Both thickness and spacing calculated using the plate model were well correlated to the direct three-dimensional measures (r(2) = 0. 77-0.92). The correlation between trabecular number and connectivity density varied greatly (r(2) = 0.41-0.94). Whereas trabecular thickness was consistently underestimated using the plate model, trabecular spacing was underestimated at low volume fractions and overestimated at high volume fractions. Use of the plate model resulted in a volume-dependent bias in measures of thickness and spacing (p < 0.001). This was a result of the fact that samples of low volume fraction were much more "rod-like" than those of the higher volume fraction. Our findings indicate that the plate model provides biased results, especially when populations with different volume fractions are compared. Therefore, we recommend direct thickness measures when three-dimensional data sets are available.

Properties of growing trabecular ovine bone. Part II: architectural and mechanical properties

We aimed to highlight the relationship between age and the architectural properties of trabecular bone, to outline the patterns in which the variations in these properties take place, and to investigate the influence of the architecture on the mechanical properties of trabecular bone in growing animals. We studied 30 lambs in three age groups and 20 sheep in two age groups. Cubes of subchondral bone were cut from the proximal tibia according to a standardised protocol. They were serially sectioned and their architectural properties were determined. Similar cubes were obtained from the identical anatomical position of the contralateral tibia and their compressive mechanical properties measured. The values obtained from the skeletally immature and mature individuals were compared. Multiple regression analyses were performed between the architectural and the mechanical properties. The bone volume fraction, the mean trabecular volume, the architectural and the mechanical anisotropy, the elastic modulus, the bone strength, the energy absorption to failure, and the elastic energy correlated positively with increasing age whereas the connectivity density, the bone surface density, the ultimate strain, the absorption of viscoelastic energy and the relative loss of energy correlated inversely. The values of all variables were significantly different in the skeletally mature and immature groups. We determined the patterns in which the variations took place. The bone volume fraction of the trabecular bone tissue was found to be the major predictor of its compressive mechanical properties. Together with the mean trabecular volume and the bone surface density, it explained 81% of the variations in the compressive elastic modulus of specimens obtained from the contralateral tibiae.

Properties of growing trabecular ovine bone

We aimed to highlight the relationship between age and the architectural properties of trabecular bone, to outline the patterns in which the variations in these properties take place, and to investigate the influence of the architecture on the mechanical properties of trabecular bone in growing animals. We studied 30 lambs in three age groups and 20 sheep in two age groups. Cubes of subchondral bone were cut from the proximal tibia according to a standardised protocol. They were serially sectioned and their architectural properties were determined. Similar cubes were obtained from the identical anatomical position of the contralateral tibia and their compressive mechanical properties measured. The values obtained from the skeletally immature and mature individuals were compared. Multiple regression analyses were performed between the architectural and the mechanical properties.

The bone volume fraction, the mean trabecular volume, the architectural and the mechanical anisotropy, the elastic modulus, the bone strength, the energy absorption to failure, and the elastic energy correlated positively with increasing age whereas the connectivity density, the bone surface density, the ultimate strain, the absorption of viscoelastic energy and the relative loss of energy correlated inversely. The values of all variables were significantly different in the skeletally mature and immature groups. We determined the patterns in which the variations took place. The bone volume fraction of the trabecular bone tissue was found to be the major predictor of its compressive mechanical properties. Together with the mean trabecular volume and the bone surface density, it explained 81% of the variations in the compressive elastic modulus of specimens obtained from the contralateral tibiae.

Constitutive relationships of fabric, density, and elastic properties in cancellous bone architecture

The hypothesis that trabecular morphology can predict the elastic properties of cancellous bone has only partly been verified and no predictive analytical model is currently available. Such models are becoming increasingly relevant as the resolution levels of three-dimensional scanning techniques approach the size of trabeculae. This study took advantage of micro-finite-element methods and tested the aforementioned hypothesis in normal cancellous bone material collected at six anatomical locations from 56 individuals. Numerical analysis was based on high-resolution three-dimensional computer reconstructions of cancellous bone specimens from which the complete elastic characteristics and trabecular morphology, represented by three different fabric measures (the mean intercept length and two volume-based ones), were calculated. Each fabric measure was analyzed individually using the tensorial relationships derived by Cowin (Mech Mater 4:137-147; 1985). Models for both stiffness and compliance entries were developed. The models based on stiffness entries could explain 93.4%-95.6% of the variance, whereas those based on compliance entries could explain 89.2%-89.4%. When using the former model, the MIL (mean intercept length measure) performed slightly better than the two volume-based measures, VO (volume orientation) and SVD (star volume distribution), with 23% less remaining variance. The high correlations found strongly support the hypothesis and increase the hope that, on the basis of information on trabecular morphology, it will be possible to obtain considerably better estimates of bone quality in vivo compared with the rough two-dimensional density measurements used today.

The role of an effective isotropic tissue modulus in the elastic properties of cancellous bone

Conceptually, the elastic characteristics of cancellous bone could be predicted directly from the trabecular morphology--or architecture--and by the elastic properties of the tissue itself. Although hardly any experimental evidence exists, it is often implicitly assumed that tissue anisotropy has a negligible effect on the apparent elastic properties of cancellous bone. The question addressed in this paper is whether this is actually true. If it is, then micromechanical finite element analysis (micro-FEA) models, representing trabecular architecture, using an 'effective isotropic tissue modulus' should be able to predict apparent elastic properties of cancellous bone. To test this, accurate multi-axial compressive mechanical tests of 29 whale bone specimens were simulated with specimen-specific micro-FEA computer models built from true three-dimensional reconstructions. By scaling the micro-FEA predictions by a constant tissue modulus, 92% of the variation of Young's moduli determined experimentally could be explained. The correlation even increased to 95% when the micro-FEA moduli were scaled to the isotropic tissue moduli of individual specimens. Excellent agreement was also found in the elastic symmetry axes and anisotropy ratios. The prediction of Poisson's ratios was somewhat less precise at 85% correlation. The results support the hypothesis; for practical purposes, the concept of an 'effective isotropic tissue modulus' concept is a viable one. They also suggest that the value of such a modulus for individual cases might be inferred from the average tissue density, hence the degree of mineralization. Future studies must clarify how specific the tissue modulus should be for different types of bone if adequate predictions of elastic behavior are to be made in this way.

Accuracy of cancellous bone volume fraction measured by micro-CT scanning

Volume fraction, the single most important parameter in describing trabecular microstructure, can easily be calculated from three-dimensional reconstructions of micro-CT images. This study sought to quantify the accuracy of this measurement. One hundred and sixty human cancellous bone specimens which covered a large range of volume fraction (9.8-39.8%) were produced. The specimens were micro-CT scanned, and the volume fraction based on Archimedes' principle was determined as a reference. After scanning, all micro-CT data were segmented using individual thresholds determined by the scanner supplied algorithm (method I). A significant deviation of volume fraction from method I was found: both the y-intercept and the slope of the regression line were significantly different from those of the Archimedes-based volume fraction (p < 0.001). New individual thresholds were determined based on a calibration of volume fraction to the Archimedes-based volume fractions (method II). The mean thresholds of the two methods were applied to segment 20 randomly selected specimens. The results showed that volume fraction using the mean threshold of method I was underestimated by 4% (p = 0.001), whereas the mean threshold of method II yielded accurate values. The precision of the measurement was excellent. Our data show that care must be taken when applying thresholds in generating 3-D data, and that a fixed threshold may be used to obtain reliable volume fraction data. This fixed threshold may be determined from the Archimedes-based volume fraction of a subgroup of specimens. The threshold may vary between different materials, and so it should be determined whenever a study series is performed.

Connectivity and the elastic properties of cancellous bone

This study addresses the possible significance of trabecular connectivity for the mechanical quality of cancellous bone. A total of 141 cubic trabecular bone specimens collected from autopsy material from 56 individuals without any known bone or metastatic diseases were used. Age variation was in the range of 14-91 years and a wide range of trabecular architecture was found. Each specimen was three-dimensionally reconstructed with a voxel size of either 20 or 25 microm. Using the detailed three-dimensional reconstructions as input for microstructural finite-element models, the complete elastic properties of the trabecular architecture were obtained and maximum and mean stiffness could be calculated. Volume fraction and true three-dimensional architectural measurements of connectivity density and surface density were determined. Connectivity density was determined in an unbiased manner by the Euler number, which is a topological property. Using multiple regression analysis it was found that volume fraction explained by far the greatest part (84%-94%) of the variation in both mean and maximum stiffness. When connectivity density and surface density were included, the correlations increased marginally to 89%-95%. Noticeably negative regression coefficients were found for connectivity density. The results suggest that, in normal cancellous bone, the connectivity density has very limited value for assessment of elastic properties by morphological variables, but if a relation exists then stiffness decreases with increasing connectivity.

Star length distribution: a volume-based concept for the characterization of structural anisotropy

Determination and quantification of anisotropy is of great interest in research fields dealing with physical structures or surface textures. In this paper, a volume-based method is presented, which essentially determines the mean object length in a certain direction for a typical point within a structure or texture. The mean object lengths for all orientations together form the so-called star length distribution (SLD). The validity and the accuracy of the SLD method are investigated, and illustrated by applying it to trabecular bone. By using a line sampling algorithm, the relation with other anisotropy measures could be studied analytically. Preliminary tests suggest that with SLD a more exact description of the mechanical properties of porous structures may be obtained than with other anisotropy measures. However, due to possible secondary orientations that become apparent with SLD, a fabric tensor must be of rank higher than two in order to properly describe an orthogonal structure mathematically.

The anisotropic Hooke's law for cancellous bone and wood

A method of data analysis for a set of elastic constant measurements is applied to data bases for wood and cancellous bone. For these materials the identification of the type of elastic symmetry is complicated by the variable composition of the material. The data analysis method permits the identification of the type of elastic symmetry to be accomplished independent of the examination of the variable composition. This method of analysis may be applied to any set of elastic constant measurements, but is illustrated here by application to hardwoods and softwoods, and to an extraordinary data base of cancellous bone elastic constants. The solid volume fraction or bulk density is the compositional variable for the elastic constants of these natural materials. The final results are the solid volume fraction dependent orthotropic Hooke's law for cancellous bone and a bulk density dependent one for hardwoods and softwoods.

Glenoid bone architecture

This article describes regional variations in trabecular bone architecture in terms of density and orientation within six glenoid specimens. The mean donor age was 56 years and ranged from 31 to 72 years. An automated imaging technique based on 3-dimensional serial sectioning was used for the direct examination of the glenoid cancellous bone structures. Subchondral plate thickness was on average 1.9 mm and ranged from 1.2 mm to 2.9 mm. The volume fraction of trabecular bone varied from 11% to 45% with peak values at the posterior glenoid vault. On graphic 3-dimensional reconstructions, the glenoid appeared as platelike trabeculae, radially oriented perpendicular to the subchondral plate and interconnected by thin rods. These views also displayed regional variations throughout the glenoid, reflecting differences in the macroscopic appearance. Quantitative structural analysis revealed different degrees of anisotropy at the glenoid cancellous region, predominantly transverse isotropy. Resemblance to direct weight-bearing cancellous bone such as the proximal tibia was evident.

Relationships between bone morphology and bone elastic properties can be accurately quantified using high-resolution computer reconstructions

It would be advantageous if the mechanical properties of trabecular bone could be directly inferred from stereomorphometric parameters. For that purpose, apparent density and mean intercept length, as measures of bone mass and directionality (fabric), are commonly correlated with the elastic characteristics of bone samples, as determined in compression tests. However, complete and accurate relationships have not yet been established in this way. This may be due not only to the occurrence of artifacts in both the stereomorphometric and the mechanical assessments but also to an inherent inadequacy of mean intercept length in characterizing the full mechanical significance of bone architecture or nonhomogeneities in trabecular tissue properties not accounted for in stereomorphometry. In this study, we introduce a computer modeling approach allowing these biases to be eliminated. With use of high-resolution three-dimensional computer reconstructions of trabecular bone specimens for stereomorphometry and for microstructural finite element models to simulate mechanical tests, unbiased comparisons become feasible. The purpose was to investigate if accurate and complete relationships can be established in this way. Four different fabric measures were considered: mean intercept length and three volume-based ones. Compliance matrices were calculated from fabric tensors, with use of the mathematical relationship proposed by Cowin for 29 vertebral whale-bone specimens. These were correlated with the compliance constants determined directly from the microstructural finite element model simulation. The nine orthotropic elastic constants of all 29 specimens were well predicted from their stereomorphometric fabric and volume fraction values, with correlation coefficients ranging from R2adj = 0.9934 to 0.9963. When individual compliance components were considered (1/Ei, 1/Gij, or -v[ij]/Ei), correlation coefficients ranged from R2adj = 0.924 to 0.982. All four fabric measures performed equally well. It is concluded that volume fraction and fabric measures correlate highly with the apparent elastic properties of bone samples, provided that anisotropy and nonhomogeneity in the elastic properties of the trabecular tissue itself have negligible effects on the apparent properties. Whether this is true for bone in general remains to be seen, as only a subset was analyzed here. These methods, however, can be valuable in similar assessments of other subsets.

Structure and function of vertebral trabecular bone

STUDY DESIGN

A combined morphologic and finite-element study on vertebral trabecular bone.

OBJECTIVE

To relate the form and function of vertebral trabecular bone, in an attempt to better understand the mechanical function of a lumbar vertebra.

SUMMARY OF BACKGROUND DATA

The architecture of bone is closely related to its mechanical function (Wolff's Law). In the human spine, vertebrae are subjected to a large variety of loads. Yet, these bones show a typical architecture, which means that they carry typical loads.

METHODS

Five trabecular bone cubes from specific sites of a lumbar vertebra were 3D-reconstructed for computerized analysis. The architecture of the specimens was quantified by the bone volume fraction and a measure of anisotropy, the mean bone length. A finite element model was used to calculate internal stresses within a homogeneous vertebral body under basic loads. For each load case, bone volume fraction of the specimens was compared with the equivalent von Mises stress, and mean bone length was compared with the principal stress directions.

RESULTS

Bone volume fraction poorly related to the von Mises stress in the physiologic load case of axial compression. However, high bone volume fractions exist at locations where multiple load situations occur (e.g., near the pedicles and endplates). Remarkably, these sites also show finer architectures. Comparison of mean bone length with principal stresses revealed that the vertebral trabecular bone architecture particularly, but not entirely, corresponds to the stress field under axial compression. The horizontal struts near the end-plates were found to be due to the function of the healthy intervertebral disc, and facetal joint loads introduce stress components that relate well with the bone structures near the pedicle bases.

CONCLUSIONS

The trabecular bone architecture and the vertical orientation of the facet joints suggest that walking may be the main activity that determines the lumbar vertebral bone architecture.

Fabric and elastic principal directions of cancellous bone are closely related

Cancellous bone architecture and mechanics are intimately related. The trabecular architecture of cancellous bone is considered determined by its mechanical environment (Wolff's law), and the mechanical properties of cancellous bone are inversely determined by the trabecular architecture and material properties. Much effort has been spent in expressing these relations, but the techniques and variables necessary for this have not been fully identified. It is obvious, however, that some measure of architectural anisotropy (fabric) is needed. Within the last few years, volume-based measures of fabric have been introduced as alternatives to the mean intercept length method, which has some theoretical problems. This paper seeks to answer which of four different fabric measures best predicts finite element calculated mechanical anisotropy directions. Twenty-nine cancellous bone specimens were three-dimensionally reconstructed using the automated serial sectioning technique. A series of large-scale finite-element analyses were performed on each of the three-dimensional reconstructions to calculate the compliance matrix for each specimen, from which the mechanical principal directions were derived. The architectural anisotropy was determined in three-dimensional space for each specimen using mean intercept length (MIL), volume orientation (VO), star volume distribution (SVD) and star length distribution (SLD). Each of the architectural anisotropy results were expressed by a fabric tensor. Architectural main directions were determined from the fabric tensors and compared with the FE-calculated mechanical anisotropy directions. All architectural measures predicted the mechanical main directions rather well, which supports the assumption that mechanical anisotropy directions are aligned with fabric directions. MIL showed a significant, though very small (1.4 degrees), deviation from the primary mechanical direction. VO had difficulty in determining secondary and tertiary mechanical directions; its mean deviation was 8.9 degrees. SVD and SLD provided marginally better predictors of mechanical anisotropy directions than MIL and VO.

Three-dimensional methods for quantification of cancellous bone architecture

Recent development in three-dimensional (3-D) imaging of cancellous bone has made possible true 3-D quantification of trabecular architecture. This provides a significant improvement of the tools available for studying and understanding the mechanical functions of cancellous bone. This article reviews the different techniques for 3-D imaging, which include serial sectioning, X-ray tomographic methods, and NMR scanning. Basic architectural features of cancellous bone are discussed, and it is argued that connectivity and architectural anisotropy (fabric) are of special interest in mechanics-architecture relations. A full characterization of elastic mechanical properties is, with traditional mechanical testing, virtually impossible, but 3-D reconstruction in combination with newly developed methods for large-scale finite element analysis allow calculations of all elastic properties at the cancellous bone continuum level. Connectivity has traditionally been approached by various 2-D methods, but none of these methods have any known relation to 3-D connectivity. A topological approach allows unbiased quantification of connectivity, and this further allows expressions of the mean size of individual trabeculae, which has previously also been approached by a number of uncertain 2-D methods. Anisotropy may be quantified by fundamentally different methods. The well-known mean intercept length method is an interface-based method, whereas the volume orientation method is representative of volume-based methods. Recent studies indicate that volume-based methods are at least as good as interface-based methods in predicting mechanical anisotropy. Any other architectural property may be quantified from 3-D reconstructions of cancellous bone specimens as long as an explicit definition of the property can be given. This challenges intuitive and vaguely defined architectural properties and forces bone scientists toward 3-D thinking.

Bone strength and material properties of the glenoid

The quality of the glenoid bone is important to a successful total shoulder replacement. Finite element models have been used to model the response of the glenoid bone to an implanted prosthesis. Because very little is known about the bone strength and the material properties at the glenoid, these models were all based on assumptions that the material properties of the glenoid were similar to those of the tibial plateau. The osteopenetrometer was used to assess the topographic strength distribution at the glenoid. Strength at the proximal subchondral level of the glenoid averaged 66.9 MPa. Higher peak values were measured posteriorly, superiorly, and anteriorly to the area of maximum concavity of the glenoid joint surface known as the bare area. One millimeter underneath the subchondral plate, average strength decreased by 25%, and at the 2 mm level strength decreased by 70%. The contribution of the cortical bone to the total glenoid strength was assessed by compression tests of pristine and cancellous-free glenoid specimens. Strength decreased by an average of 31% after the cancellous bone was removed. The material properties of the glenoid cancellous bone were determined by axial compression tests of bone specimens harvested from the central part of the glenoid subchondral area. The elastic modulus varied from approximately 100 MPa at the glenoid bare area to 400 MPa at the superior part of the glenoid. With the elastic constants used a predictor of the mechanical anisotropy, the average anisotropy ratio was 5.2, indicating strong anisotropy. The apparent density was an average 0.35 gr. cm-3, and the Poisson ratio averaged 0.263. According to our findings the anisotropy of the glenoid cancellous bone, details concerning the strength distribution, and the load-bearing function of the cortical shell should be considered in future finite element models of the glenoid.

Simultaneous measurement of stiffness and energy absorptive properties of articular cartilage and subchondral trabecular bone

The purpose of this study was to develop a method for simultaneous measurement of the mechanical properties of articular cartilage and underlying trabecular bone. Cylindrical bone-cartilage specimens from human cadaver knees were tested under non-destructive axial compression. The specimens wer mounted in a cage by three screws fixed to the subchondral bone plate to create a reference plane between the cartilage and the bone. Each test was carried out as a single test cycle between a 0.12 MPa (4 N) pre-stress and 0.5 per cent bone strain. Twenty conditioning cycles were performed prior to each test cycle. In order to determine the reproducibility of the test method, the test procedure was repeated after turning the cage through 90 degrees. The specimens were then kept at +5 degrees C for 24 hours, and the procedure was repeated. The normalized stiffness and energy absorptive properties were calculated from each test cycle. No significant difference was found between the results obtained on the first and on the second day. Both days the stiffness of cartilage was significantly larger during the second test, indicating that 20 conditioning cycles may not provide a viscoelastic steady state.

Direct mechanics assessment of elastic symmetries and properties of trabecular bone architecture

A method is presented to find orthotropic elastic symmetries and constants directly from the elastic coefficients in the overall stiffness matrix of trabecular bone test specimens. Contrary to earlier developed techniques, this method does not require pure orthotropic behavior or additional fabric measurements. The method uses high-resolution computer reconstructions of trabecular bone specimens as input for large-scale FE-analyses to determine all the 21 elastic coefficients in the overall stiffness matrix of the specimen, using a direct mechanics approach. An optimization procedure is then used to find the coordinate transformation that yields the best orthotropic representation of this matrix. The method is illustrated here relative to two trabecular bone specimens. The techniques developed here can be used to obtain a complete characterization of the mechanical properties of trabecular architecture. With the development of in vivo reconstruction techniques, even in vivo measurements will be possible.

Identification of oestrogen receptors and oestrogen receptor mRNA in human adipose tissue

The distribution of adipose tissue has a major impact on the morbidity and mortality associated with obesity. Furthermore, the distribution of adipose tissue seems to be regulated by sex hormones. Controversy exists over whether the effects of sex hormones (oestrogen and testosterone) on human adipose tissue are an indirect or a direct effect as contradictory results have been obtained when investigating the existence of these receptors in human adipose tissue. In the present study the authors reinvestigated the possible existence of oestrogen receptors (ERs) in human adipose tissue. Human adipocytes from both genders were found to contain specific oestrogen binding sites determined by ligand-binding techniques. The binding protein had a molecular weight of 65 kD (which is similar to that of the ER found elsewhere) and it was found that adipocytes contained mRNA encoding the ER. Moreover, human preadipocytes had no oestrogen-binding capacity and did not possess mRNA encoding the ER. Finally, the authors detected regional differences in receptor density. Women had an equal oestrogen-binding capacity in adipose tissue from the subcutaneous abdominal and the visceral depot, whereas men had twice as high oestrogen-binding capacity in subcutaneous adipose tissue compared with adipose tissue in the visceral fat depot. These findings indicate that mature human adipocytes possess ERs and thus, might be an oestrogen-responsive tissue and that oestrogen may be acting directly in mature adipocytes via its specific receptor. Human preadipocytes, however, seemed not to be an oestrogen-responsive tissue. Finally, preliminary data suggest that there might be differences in ER densities in different fat depots.

A new method to determine trabecular bone elastic properties and loading using micromechanical finite-element models

The apparent mechanical behavior of trabecular bone depends on properties at the tissue or trabecular level. Many investigators have attempted to determine trabecular tissue properties and loading. However, accuracy and applicability of all methods reported are limited. The small size of the trabeculae and a possible size effect are complicating factors when using traditional testing methods on single trabeculae. Other methods reported, using models that describe the trabecular structure, are of limited value because they consider bone as a repetitive structure in order to describe a reasonably large region of bone. The present study introduces a new finite-element method strategy that enables analysis of reasonably large regions of trabecular bone in full detail. The method uses three-dimensional serial reconstruction techniques to construct a large-scale FE model, by directly converting voxels to elements. A 5 mm cube of trabecular bone was modeled in this way, resulting in a FE model that consists of 296,679 elements. Special strategies were developed to solve the set of equations that results from the FE approach. Using this model in combination with experimental apparent data taken from the literature, the upper and lower boundaries for the tissue modulus were calculated to be 10.1 and 2.23 GPa, respectively. From the local stress and strain distributions it was concluded that the deformation mode of the trabeculae in the present cube was predominantly in bending. It was concluded that the method developed offers new perspectives for the study of trabecular bone.

Three-dimensional reconstruction of entire vertebral bodies

This paper describes a technique for 3-D reconstruction of large cancellous bone regions. The output is a 3-D array describing the original cancellous bone region, and the output can be used for any kind of measurement of the bone architecture. The technique was developed as a tool for researchers conducting experimental and clinical studies related to cancellous bone architecture and, ultimately, to cancellous bone quality. A set of new and unbiased methods for quantification of cancellous bone has been a stimulus for the development of the technique, as the quantification methods rely on 3-D information. The technique is based on automated serial sectioning, and all steps from specimen preparation to image segmentation are described in detail. Examples of 3-D reconstructed vertebral bodies are given. By use of the described technique, between 300 and 600 sections can be made and registered per hour, which means that an average vertebral body can be reconstructed in about 2 h. Compared to previous implementations of the general principle of serial sectioning, this is a significant improvement both in resolution and in time efficiency.

The Conneulor: unbiased estimation of connectivity using physical disectors under projection

The Euler number and the connectivity of an arbitrary object is defined, and it is illustrated why the connectivity of an n-dimensional object cannot be estimated in an (n-1)-dimensional section. The disector--principle for 3-D counting of the Euler--events is illustrated in cancellous bone. The correct handling for unbiased counting of events at artificial edges is outlined. A nomogram for predicting the precision of an estimate is provided.

Mechanical and textural properties of pelvic trabecular bone

So far, virtually nothing is known about the mechanical properties of pelvic trabecular bone. In this study, several techniques have been used to establish some insight in these properties. Dual-energy quantitative computer tomography (DEQCT) was used to look at the distribution of bone densities throughout the pelvic bone and nondestructive mechanical testing was used to obtain Young's moduli and Poisson's ratios in three orthogonal directions for cubic specimens of pelvic trabecular bone. The same specimens were then used for stereological measurements to obtain volume fractions and the spatial orientations of the mean intercept lengths. The combined data on the mechanical tests and the stereological measurements made it possible to calculate Young's moduli and Poisson's ratios for the specimens' principal material axes. DEQCT showed that bone densities within a pelvic bone are significantly higher in the superior part of the acetabulum, extending to the sacroiliac joint area and, secondly, in the area of the pubic symphysis. Volume fractions found for the specimens did not exceed 20%. This may be considered rather low when compared to values reported in the literature for trabecular bone of femoral or tibial origin, but the values do lie in the same range as vertebral trabecular bone. With the volume fraction as its primary predictor, values of Young's moduli were also low. For most specimens these values were not higher than 100 MPa, with an occasional peak of 250 MPa. Looking at the ratio of the highest and lowest Young's modulus or at the components of the fabric tensor, it can be concluded that pelvic trabecular bone is not highly anisotropic. On an average, Poisson's ratio was found to be closer to 0.2 rather than 0.3, which is in accordance with other studies on Poisson's ratio of trabecular bone.

Quantification of connectivity in cancellous bone, with special emphasis on 3-D reconstructions

The connectivity of cancellous bone attracts increasing attention, as it has been hypothesized that a primary reason for decreasing strength and stiffness in osteoporosis is caused by a loss of trabecular elements and consequently a loss in connectivity. The Euler characteristic has in a few previous articles been used to express cancellous bone connectivity, but there are severe problems in using the Euler characteristic uncritically. The Euler characteristic of a three-dimensional structure is a topological invariant, which reports the number of particles of a structure plus the number of enclosed cavities minus the connectivity. As such, one must know the number of components and the number of enclosed cavities in order to use the Euler characteristic as an expression of connectivity. Another difficulty of using the Euler characteristic is that due to edge effects the Euler characteristic of an excised specimen provides a biased estimate of the Euler characteristic of the region from which the specimen was taken. In this article the intuitive concept of connectivity is given a precise mathematical definition, and a basic topological method for quantifying the connectivity of cancellous bone is presented. The method uses the Euler characteristic, but the above-mentioned problems are controlled. The development of the method and the practical implementation is based on a set of topological notes. It must be stressed that the method is free from assumptions concerning trabecular architecture, and that the method is unbiased. This is in contrast to previously presented methods. The unbiased and model-free method is used on a series of 3-D reconstructions of cancellous bone specimens, and it is demonstrated that the connectivity of cancellous bone is not simply related to volume fraction (density), and that biased and model-based 2-D methods aimed at determining connectivity do not have any general relationship to connectivity in cancellous bone.

Physical bone changes in carragheenin-induced arthritis evaluated by quantitative computed tomography

Repeated non-invasive measurements were performed in dogs of trabecular bone density (TBD), low density bone area (LDBA), and high density bone area (HDBA) in chronic arthritis using quantitative computed tomography (QCT). Unilateral chronic arthritis of the knee had been induced by weekly instillation of 2 ml carragheenin into the right knee joint for 12 weeks with the left knee serving as a control. CT scanning of the distal femoral condyles was performed in 12 mature dogs with chronic arthritis. Another 6 dogs underwent a longitudinal CT study starting immediately prior to induction of arthritis. During induction of arthritis TBD decreased (P less than 0.01), LDBA increased (P less than 0.05) and HDBA decreased (P less than 0.01) in the arthritic bone. Opposite changes were found on the control side, i.e. TBD increased (P less than 0.01), LDBA decreased (P less than 0.01) and HDBA increased (P less than 0.01). The chronic arthropathic bone showed 20% lower TBD (P less than 0.0001), greater LDBA (P less than 0.0001) and lower HDBA (P less than 0.0001) as compared with the control bone. Reproducibility tests of TBD showed a coefficient of variation of 0.8%. Indentation tests and histomorphometric analyses confirmed the bone density changes as measured by CT.

Mechanical properties of trabecular bone. Dependency on strain rate

The effect of strain rate (epsilon) and apparent density (rho) on stiffness (E), strength (sigma u), and ultimate strain (epsilon u) was studied in 60 human trabecular bone specimens from the proximal tibia. Testing was performed by uniaxial compression to 5% specimen strain. Six different strain rates were used: 0.0001, 0.001, 0.01, 0.1, 1, and 10 s-1. Apparent density ranged between 0.23 and 0.59 g cm-3. Linear and non-linear regression analyses using strength, stiffness and ultimate strain as dependent variables (Y) and strain rate and apparent density as independent variables were performed using the following models: Y = a rho b epsilon c, Y = rho b(a + c epsilon; Y = (a + b rho)epsilon c, Y = a rho 2 epsilon c, E = a rho 3 epsilon c. The variations of strength and stiffness were explained equally well by the linear and the power function relationship to strain rate. The exponent was 0.07 in the power function relationship between strength and strain rate and 0.05 between stiffness and strain rate. The variation of ultimate strain was explained best using a power function relationship to strain rate (exponent = 0.03). The variation of strength and stiffness was explained equally well by the linear, power function and quadratic relationship to apparent density. The cubic relationship between stiffness and apparent density showed a less good fit. Ultimate strain varied independently of apparent density.

Tensile and compressive properties of cancellous bone

The relationship between the mechanical properties of trabecular bone in tension and compression was investigated by non-destructive testing of the same specimens in tension and compression, followed by random allocation to a destructive test in either tension or compression. There was no difference between Young's modulus in tension and compression, and there was a strong positive correlation between the values (R = 0.97). Strength, ultimate strain and work to failure was significantly higher in tensile testing than in compressive testing.

The underestimation of Young's modulus in compressive testing of cancellous bone specimens

In order to determine the accuracy of measurements of Young's modulus of cancellous bone by conventional compression testing, two independent strain measurements were made simultaneously during non-destructive uniaxial compression to 0.8% strain of rectangular specimens (n = 18). Strain was measured by an extensometer attached to the compression anvils close to the specimen and by an optical system covering the central half of the specimens. Mean Young's modulus determined by the extensometer technique was 689 MPa, but was 871 MPa when determined by the optical technique (mean difference = 182 MPa, SED = 50 MPa, p less than 0.002). Uneven strain distribution due to lack of support of cut vertical trabeculae at the anvil-specimen interface is believed to be causing the underestimation of Young's modulus measured by the extensometer technique. The influence of friction at the specimen-anvil interface was studied by performing a finite element analysis. It is concluded that Young's modulus of specimens of the chosen geometry on average is underestimated by about 20% by conventional compressing testing. The underestimation seems not to be dependent upon specimen density.

A direct method for fast three-dimensional serial reconstruction

A method for accurate three-dimensional reconstruction of openly connected porous structures is described. The method is based on embedding of a specimen in a contrast coloured epoxy resin and serial sectioning in a standard hard tissue microtome. A PC-based image processing system is used for direct digitization of the cut surface, and by thresholding two-phase images are obtained. The process is fully automated, and about 170 sections can be produced, digitized, dichotomized, and stored per hour. As an example of its applications, the method is used on trabecular bone, which is an anisotropic porous structure.

Estimation of structural anisotropy based on volume orientation. A new concept

The quantification of anisotropy--its main direction and the degree of dispersion around it--is desirable in numerous research fields dealing with physical structures. Conventional methods are based on the orientation of interface elements. The results of these methods do not always agree with perceived anisotropy, and anisotropic structures do not necessarily turn out to be 'anisotropic' using these methods. In the present paper, we propose an alternative to curve and surface orientation, namely volume orientation. Using trabecular bone as an example of a two-phase anisotropic structure, the new concept is studied in some detail. In particular, a parametric method of estimating volume orientation from sections is presented and discussed.

Density changes at the proximal tibia after medial meniscectomy

Trabecular bone density changes at the proximal tibia were studied after total and partial meniscectomy using quantitative computerized tomography. Thirty-seven medially meniscectomized patients (5- and 10-year follow-up) and 10 control persons were included. In the partially and the totally meniscectomized knees, the area of maximal density in the medial tibial condyle was displaced approximately 4 mm in a posteromedial direction relative to the position in the intact knees. In both partially and totally meniscectomized knees, the density in the region of tibiofemoral contact in the medial tibial condyle was higher compared with the density in the same region in the contralateral intact knees. This difference was of the same order in the 5- and 10-year follow-up groups. The density difference between the meniscectomized and the intact knees in the region of tibiofemoral contact in the medial condyle was significantly higher in totally than in partially meniscectomized patients. It is concluded that not only total but also partial medial meniscectomy is followed by density changes, occurring within the first 5 years after meniscectomy. The results emphasize the importance of the utmost conservatism in surgery on the meniscus.

Compressive axial strain distributions in cancellous bone specimens

The compressive axial strain distribution in cylindrical trabecular bone specimens was studied using digitized images of the specimen surface. Specimens were tested with strain rate 0.00015 s-1. Images were taken at 0, 1, 2, 3, 4, 6, 8 and 10% strain. Using an optical illusion of movement by rapidly changing succeeding images, failures were classified as transverse (33%) or oblique collapses (67%). The location of failure was not determined by the specimen density gradient. Local axial strain in the distal, intermediate and proximal third was measured throughout the compression in the transversely failing specimens, whereas local strain in the obliquely failing specimens was measured in the pre-failure phase only. Axial strain inhomogeneity was observed in the pre-failure as well as in the post-failure phase. In the pre-failure phase the intermediate third was strained significantly less than the thirds near the ends. In the post-failure phase specimen strain occurred solely in the collapsed part. Ultimate strain of the transversely failing specimens was 2.5% and ultimate strain of the failing third was 3.7%. At failure less than 1% strain was observed in the intermediate third and at 10% specimen strain 1.5% local strain was found in the intermediate third. The results indicate unreliability of conventional stiffness and strain measurements in trabecular bone specimens probably due to lack of trabecular constraint at the end surfaces. Conventional measurements tend to underestimate stiffness and, by giving an average value of strain in spite of considerable strain inhomogeneity, to underestimate failure strain.