Comparison of the trabecular architecture and the isostatic stress flow in the human calcaneus

Conventional vs Augmented Reality-Guided Lateral Calcaneal Lengthening Simulated in a Foot Bone Model

BACKGROUND

Acquired adult flatfoot deformity (AAFD) results in a loss of the medial longitudinal arch of the foot and dysfunction of the posteromedial soft tissues. Hintermann osteotomy (H-O) is often used to treat stage II AAFD. The procedure is challenging because of variations in the subtalar facets and limited intraoperative visibility. We aimed to assess the impact of augmented reality (AR) guidance on surgical accuracy and the facet violation rate.

METHODS

Sixty AR-guided and 60 conventional osteotomies were performed on foot bone models. For AR osteotomies, the ideal osteotomy plane was uploaded to a Microsoft HoloLens 1 headset and carried out in strict accordance with the superimposed holographic plane. The conventional osteotomies were performed relying solely on the anatomy of the calcaneal lateral column. The rate and severity of facet joint violation was measured, as well as accuracy of entry and exit points. The results were compared across AR-guided and conventional osteotomies, and between experienced and inexperienced surgeons.

RESULTS

Experienced surgeons showed significantly greater accuracy for the osteotomy entry point using AR, with the mean deviation of 1.6 ± 0.9 mm (95% CI 1.26, 1.93) compared to 2.3 ± 1.3 mm (95% CI 1.87, 2.79) in the conventional method (P = .035). The inexperienced had improved accuracy, although not statistically significant (P = .064), with the mean deviation of 2.0 ± 1.5 mm (95% CI 1.47, 2.55) using AR compared with 2.7 ± 1.6 mm (95% CI 2.18, 3.32) in the conventional method. AR helped the experienced surgeons avoid full violation of the posterior facet (P = .011). Inexperienced surgeons had a higher rate of middle and posterior facet injury with both methods (P = .005 and .021).

CONCLUSION

Application of AR guidance during H-O was associated with improved accuracy for experienced surgeons, demonstrated by a better accuracy of the osteotomy entry point. More crucially, AR guidance prevented full violation of the posterior facet in the experienced group. Further research is needed to address limitations and test this technology on cadaver feet. Ultimately, the use of AR in surgery has the potential to improve patient and surgeon safety while minimizing radiation exposure.

CLINICAL RELEVANCE

Subtalar facet injury during lateral column lengthening osteotomy represents a real problem in clinical orthopaedic practice. Because of limited intraoperative visibility and variable anatomy, it is hard to resolve this issue with conventional means. This study suggests the potential of augmented reality to improve the osteotomy accuracy.

Trabecular bone variation in the gorilla calcaneus

OBJECTIVES

Calcaneal external shape differs among nonhuman primates relative to locomotion. Such relationships between whole-bone calcaneal trabecular structure and locomotion, however, have yet to be studied. Here we analyze calcaneal trabecular architecture in Gorilla gorilla gorilla, Gorilla beringei beringei, and G. b. graueri to investigate general trends and fine-grained differences among gorilla taxa relative to locomotion.

MATERIALS AND METHODS

Calcanei were micro-CT scanned. A three-dimensional geometric morphometric sliding semilandmark analysis was carried out and the final landmark configurations used to position 156 volumes of interest. Trabecular thickness (Tb.Th), trabecular spacing (Tb.Sp), and bone volume fraction (BV/TV) were calculated using the BoneJ plugin for ImageJ and MATLAB. Non-parametric MANOVAs were run to test for significant differences among taxa in parameter raw values and z-scores. Parameter distributions were visualized using color maps and summarized using principal components analysis.

RESULTS

There are no significant differences in raw BV/TV or Tb.Th among gorillas, however G. b. beringei significantly differs in z-scores for both parameters (p = <0.0271). All three taxa exhibit relatively lower BV/TV and Tb.Th in the posterior half of the calcaneus. This gradation is exacerbated in G. b. beringei. G. b. graueri significantly differs from other taxa in Tb.Sp z-scores (p < 0.001) indicating a different spacing distribution.

DISCUSSION

Relatively higher Tb.Th and BV/TV in the anterior calcaneus among gorillas likely reflects higher forces associated with body mass (transmitted through the subtalar joint) relative to forces transferred through the posterior calcaneus. The different Tb.Sp pattern in G. b. graueri may reflect proposed differences in foot positioning during locomotion.

Benign incidental lesion of the calcaneus: the calcaneal vascular remnant

BACKGROUND

The calcaneal vascular remnant, first described by Fleming et al. in 2005, is a benign intramedullary lesion of the calcaneus with a vascular origin.

PURPOSE

To determine the prevalence and magnetic resonance imaging (MRI) characteristics of incidental calcaneal vascular remnant on routine ankle MRI.

MATERIAL AND METHODS

We retrospectively reviewed 457 ankle MRI scans for the presence of calcaneal vascular remnant. MRI was considered positive when a focal cyst-like area was seen on a T2-weighted sequence, and a low signal intensity was identified on a T1-weighted image beneath the calcaneal sulcus. Patients with calcaneal vascular remnants were further evaluated for age, gender, right or left foot location, size, and lesion characteristics.

RESULTS

The prevalence of incidental calcaneal vascular remnant was 21.7% on our consecutive ankle MR examinations. The average lesion size was 5.5 mm. No statistically significant difference was noted in the frequency of lesion detection between gender, age, and side of the lesions (P > 0.05). Multilobulated lesions were detected predominantly in women (P = 0.013) and classic type lesions were detected predominantly in men (P = 0.036).

CONCLUSION

This report is the first to determine the prevalence and MRI characteristics of calcaneal vascular remnants. Detecting and reporting this lesion on routine MRI is essential to avoid confusion with other pathologic entities.

Biomechanical Analysis of a Novel Double-Point Fixation Method for Displaced Intra-Articular Calcaneal Fractures

The development of minimally invasive procedures and implant materials has improved the fixation strength of implants and is less traumatic in surgery. The purpose of this study was to propose a novel “double-point fixation” for calcaneal fractures and compare its biomechanical stability with the traditional “three-point fixation.” A three-dimensional finite element foot model with a Sanders type IIIAB calcaneal fracture was developed based on clinical images comprising bones, plantar fascia, ligaments, and encapsulated soft tissue. Double-point and three-point fixation resembled the surgical procedure with a volar distal radius plate and calcaneal locking plate, respectively. The stress distribution, fracture displacement, and change of the Böhler angle and Gissane’s angle were estimated by a walking simulation using the model, and the predictions between the double-point and three-point fixation were compared at heel-strike, midstance, and push-off instants. Double-point fixation demonstrated lower bone stress (103.3 vs. 199.4 MPa), but higher implant stress (1,084.0 vs. 577.9 MPa). The model displacement of double-point fixation was higher than that of three-point fixation (3.68 vs. 2.53 mm). The displacement of the posterior joint facet (0.127 vs. 0.150 mm) and the changes of the Böhler angle (0.9° vs. 1.4°) and Gissane’s angle (0.7° vs. 0.9°) in double-point fixation were comparably lower. Double-point fixation by volar distal radius plates demonstrated sufficient and favorable fixation stability and a lower risk of postoperative stress fracture, which may potentially serve as a new fixation modality for the treatment of displaced intra-articular calcaneal fractures.

Percutaneous fixation of intraarticular joint-depression calcaneal fractures with different screw configurations - a biomechanical human cadaveric analysis

PURPOSE

The aim of this study was to assess the biomechanical performance of different screw configurations for fixation of Sanders type II B joint-depression calcaneal fractures.

METHODS

Fifteen human cadaveric lower limbs were amputated and Sanders II B fractures were simulated. The specimens were randomized to three groups for fixation with different screw configurations. The calcanei in Group 1 were treated with two parallel longitudinal screws, entering superiorly the Achilles tendon insertion, and two screws fixing the intraarticular posterior facet fracture line. In Group 2 two screws entered the tuberosity inferiorly to the Achilles tendon insertion and two transverse screws fixed the posterior facet. In Group 3 two screws were inserted along the bone axis, one transverse screw fixed the posterior facet and one oblique screw was inserted from the posteroplantar part of the tuberosity supporting the posterolateral part of the posterior facet. All specimens were biomechanically tested to failure under progressively increasing cyclic loading.

RESULTS

Initial stiffness did not differ significantly between the groups, P = 0.152. Cycles to 2 mm plantar movement were significantly higher in both Group 1 (15,847 ± 5250) and Group 3 (13,323 ± 4363) compared with Group 2 (4875 ± 3480), P ≤ 0.048. No intraarticular displacement was observed in any group during testing.

CONCLUSIONS

From a biomechanical perspective, posterior facet support by means of buttress or superiorly inserted longitudinal screws results in less plantar movement between the calcaneal tuberosity and the anterior fragments. Inferiorly inserted longitudinal screws are associated with bigger interfragmentary movements.

Calcaneal fracture maps and their determinants

Background

Calcaneal fractures are associated with numerous complications and a poor prognosis with significant long-term quality-of-life issues, regardless of treatment. Therefore, in-depth research into the underlying mechanism of calcaneal fracture is still of great interest, with the goal of improving treatment for patients suffering from this condition. This study aimed to investigate the relationship between the distribution of calcaneal fracture lines and their determinants, especially those related to the internal structure of the calcaneus. This goal was achieved by fracture maps created by copying and stacking fracture lines as viewed from six surfaces of the calcaneus.

Methods

A total of 210 consecutive patients with 226 calcaneal fractures were retrospectively analyzed. Fracture lines were copied from a reduced 3D calcaneal fracture model and stacked on calcaneal templates to generate fracture maps. The stacked images of six calcaneus surfaces were also converted into spectrograms with MATLAB to highlight the fracture frequency at specific locations.

Results

There were four concentrated bands of fracture lines and two fracture hot spots on the superior surface. Three dense bands of fractures were observed on the medial surface, and four fracture bands were observed lateral to the calcaneus. Vertical fracture lines dominated the anterior calcaneal fracture map. On the posterior surface, the fracture lines appeared to be centered superiorly. All fracture locations coincided with the interfaces between the trabecular groups.

Conclusions

The fracture maps showed fracture patterns and recurrent fracture zones on all calcaneal surfaces. The shape of the talus and calcaneus and the architecture within the calcaneus, especially the arrangement of the trabeculae, are essential factors for calcaneal fractures.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13018-022-02930-y.

Assessing patterns of variation in BV/TV in the calcaneus and C2 vertebra of Gorilla gorilla, Pan troglodytes, and populations of Homo sapiens from the Pleistocene and Holocene that differ in physical activity levels

OBJECTIVES

Because trabecular bone volume fraction (BV/TV) is influenced by variations in physical activity recent declines in BV/TV in humans are often attributed to modern sedentary lifestyles. This study tests the hypothesis that presumed variations in mechanical loading between groups can predict the observed BV/TV patterns in humans, chimpanzees and gorillas in two bones: the calcaneus which experiences high and well characterized impact forces, and the C2 vertebrae which experiences reduced locomotor forces.

MATERIALS AND METHODS

BV/TV and other structural variables were quantified from high-resolution microCT scans in gorillas, chimpanzees, and four Homo sapiens populations: Pleistocene, semi-sedentary Natufians; Holocene hunter-gatherers from Point Hope, Alaska; Holocene nomadic pastoralists from medieval Europe; and modern, sedentary Americans.

RESULTS

In the calcaneal tuberosity, Natufian BV/TV was 36, 46, and 46% greater than Alaskans (p = .02), Europeans (p = .005) and modern Americans (p = .002), respectively, but not significantly different from apes. BV/TV was not significantly different between modern Americans and Alaskans or Europeans. In the C2, Natufian BV/TV was 53 and 25% greater than in the Alaskan (p = .0001) and European (p = .048) populations.

DISCUSSION

These results suggest that phenomena other than or in addition to variations in physical activity are needed to explain BV/TV patterns observed in H. sapiens, and point to a systemic decline in H. sapiens BV/TV after the Pleistocene.

Investigation of Biomechanical Characteristics of Orthopedic Implants for Tibial Plateau Fractures by Means of Deep Learning and Support Vector Machine Classification

An experimental comparative study of the biomechanical behavior of commonly used orthopedic implants for tibial plateau fractures was carried out. An artificial bone model Synbone1110 was used and a Schatzker V type tibial plateau fracture was created in vitro, then stabilized with three different implant types, classic L plate, Locking Plate System (PLS), and Hybrid External Fixator (HEF). The stiffness of the bone—implant assembly was assessed by means of mechanical testing using an automated testing machine. It was found that the classic L plate type internal implant has a significantly higher value of deformation then the other two implant types. In case of the other implant types, PLS had a better performance than HEF at low and medium values of the applied force. At high values of the applied forces, the difference between deformation values of the two types became gradually smaller. An Artificial Neural Network model was developed to predict the implant deformation as a function of the applied force and implant device type. To establish if a clear-cut distinction exists between mechanical performance of PLS and HEF, a Support Vector Machine classifier was employed. At high values of the applied force, the Support Vector Machine (SVM) classifier predicts that no statistically significant difference exists between the performance of PLS and HEF.

Baby steps towards linking calcaneal trabecular bone ontogeny and the development of bipedal human gait

Trabecular bone structure in adulthood is a product of a process of modelling during ontogeny and remodelling throughout life. Insight into ontogeny is essential to understand the functional significance of trabecular bone structural variation observed in adults. The complex shape and loading of the human calcaneus provides a natural experiment to test the relationship between trabecular morphology and locomotor development. We investigated the relationship between calcaneal trabecular bone structure and predicted changes in loading related to development of gait and body size in growing children. We sampled three main trabecular regions of the calcanei using micro-computed tomography scans of 35 individuals aged between neonate to adult from the Norris Farms #36 site (1300 AD, USA) and from Cambridge (1200-1500 AD, UK). Trabecular properties were calculated in volumes of interest placed beneath the calcaneocuboid joint, plantar ligaments, and posterior talar facet. At birth, thin trabecular struts are arranged in a dense and relatively isotropic structure. Bone volume fraction strongly decreases in the first year of life, whereas anisotropy and mean trabecular thickness increase. Dorsal compressive trabecular bands appear around the onset of bipedal walking, although plantar tensile bands develop prior to predicted propulsive toe-off. Bone volume fraction and anisotropy increase until the age of 8, when gait has largely matured. Connectivity density gradually reduces, whereas trabeculae gradually thicken from birth until adulthood. This study demonstrates that three different regions of the calcaneus develop into distinct adult morphologies through varying developmental trajectories. These results are similar to previous reports of ontogeny in human long bones and are suggestive of a relationship between the mechanical environment and trabecular bone architecture in the human calcaneus during growth. However, controlled experiments combined with more detailed biomechanical models of gait maturation are necessary to establish skeletal markers linking growth to loading. This has the potential to be a novel source of information for understanding loading levels, activity patterns, and perhaps life history in the fossil record.

Cancellous bone and theropod dinosaur locomotion. Part I-an examination of cancellous bone architecture in the hindlimb bones of theropods

This paper is the first of a three-part series that investigates the architecture of cancellous ('spongy') bone in the main hindlimb bones of theropod dinosaurs, and uses cancellous bone architectural patterns to infer locomotor biomechanics in extinct non-avian species. Cancellous bone is widely known to be highly sensitive to its mechanical environment, and has previously been used to infer locomotor biomechanics in extinct tetrapod vertebrates, especially primates. Despite great promise, cancellous bone architecture has remained little utilized for investigating locomotion in many other extinct vertebrate groups, such as dinosaurs. Documentation and quantification of architectural patterns across a whole bone, and across multiple bones, can provide much information on cancellous bone architectural patterns and variation across species. Additionally, this also lends itself to analysis of the musculoskeletal biomechanical factors involved in a direct, mechanistic fashion. On this premise, computed tomographic and image analysis techniques were used to describe and analyse the three-dimensional architecture of cancellous bone in the main hindlimb bones of theropod dinosaurs for the first time. A comprehensive survey across many extant and extinct species is produced, identifying several patterns of similarity and contrast between groups. For instance, more stemward non-avian theropods (e.g. ceratosaurs and tyrannosaurids) exhibit cancellous bone architectures more comparable to that present in humans, whereas species more closely related to birds (e.g. paravians) exhibit architectural patterns bearing greater similarity to those of extant birds. Many of the observed patterns may be linked to particular aspects of locomotor biomechanics, such as the degree of hip or knee flexion during stance and gait. A further important observation is the abundance of markedly oblique trabeculae in the diaphyses of the femur and tibia of birds, which in large species produces spiralling patterns along the endosteal surface. Not only do these observations provide new insight into theropod anatomy and behaviour, they also provide the foundation for mechanistic testing of locomotor hypotheses via musculoskeletal biomechanical modelling.

Cancellous bone and theropod dinosaur locomotion. Part II-a new approach to inferring posture and locomotor biomechanics in extinct tetrapod vertebrates

This paper is the second of a three-part series that investigates the architecture of cancellous bone in the main hindlimb bones of theropod dinosaurs, and uses cancellous bone architectural patterns to infer locomotor biomechanics in extinct non-avian species. Cancellous bone is widely known to be highly sensitive to its mechanical environment, and therefore has the potential to provide insight into locomotor biomechanics in extinct tetrapod vertebrates such as dinosaurs. Here in Part II, a new biomechanical modelling approach is outlined, one which mechanistically links cancellous bone architectural patterns with three-dimensional musculoskeletal and finite element modelling of the hindlimb. In particular, the architecture of cancellous bone is used to derive a single 'characteristic posture' for a given species-one in which bone continuum-level principal stresses best align with cancellous bone fabric-and thereby clarify hindlimb locomotor biomechanics. The quasi-static approach was validated for an extant theropod, the chicken, and is shown to provide a good estimate of limb posture at around mid-stance. It also provides reasonable predictions of bone loading mechanics, especially for the proximal hindlimb, and also provides a broadly accurate assessment of muscle recruitment insofar as limb stabilization is concerned. In addition to being useful for better understanding locomotor biomechanics in extant species, the approach hence provides a new avenue by which to analyse, test and refine palaeobiomechanical hypotheses, not just for extinct theropods, but potentially many other extinct tetrapod groups as well.

Mechanical Stress Redistribution in the First Metatarsal Bone After Autologous Bone Harvesting

BACKGROUND

The first metatarsal bone is a viable source for autologous bone grafting in foot and ankle surgery and may serve as another convenient graft site to correct a flail toe deformity. We aimed to determine how progressive bone removal from the first metatarsal affects the mechanical redistribution of the foot and whether this bone removal increases the risk of fracture.

METHODS

A three-dimensional finite element model developed from computed tomographic images obtained from a healthy man were used to evaluate traction stresses on the first metatarsal bone as a function of applied loads on the talus and Achilles tendon at two phases of the gait cycle (and according to the depth of bone removal).

RESULTS

Simulations indicated that when maximum load was applied to the Achilles tendon, tensile stress increased from 2.049 MPa in the intact foot to 5.941 MPa in the area of maximum bone harvest during the stance phase. Furthermore, as the volume of bone extracted from the first metatarsal increased, there was a redistribution of stress that differed significantly from that of the intact foot.

CONCLUSIONS

Although the maximum stress on the first metatarsal was not significantly affected by increasing the volume of bone harvested, the ankle should be splinted in plantarflexion during the postoperative period to eliminate the stance phase of gait and reduce the risk of metatarsal fracture.

Trabecular mapping: Leveraging geometric morphometrics for analyses of trabecular structure

OBJECTIVES

Trabecular microstructure of limb bone epiphyses has been used to elucidate the relationship between skeletal form and behavior among mammals. Such studies have often relied on the analysis of a single volume of interest (VOI). Here we present a method for evaluating variation in bone microstructure across articular surfaces by leveraging sliding semilandmarks.

METHODS

Two samples were used to demonstrate the proposed methodology and test the hypothesis that microstructural variables are homogeneously distributed: tali from two ape genera (Pan and Pongo, n = 9) and modern human distal femora (n = 10). Sliding semilandmarks were distributed across articular surfaces and used to locate the position of multiple VOIs immediately deep to the cortical shell. Trabecular bone properties were quantified using the BoneJ plugin for ImageJ. Nonparametric MANOVA tests were used to make group comparisons and differences were explored using principal components analysis and visualized using color maps.

RESULTS

Tests reveal that trabecular parameters are not distributed homogeneously and identify differences between chimpanzee and orangutan tali with regards to trabecular spacing and degree of anisotropy, with chimpanzee tali being more anisotropic and having more uniformly spaced trabeculae. Human males and females differed in the pattern of trabecular spacing with males having more uniform trabecular spacing across the joint surface.

CONCLUSIONS

The proposed procedure quantifies variation in trabecular bone parameters across joint surfaces and allows for meaningful statistical comparisons between groups of interest. Consequently it holds promise to help elucidate links between trabecular bone structure and animal behavior.

A prospective follow up of age related changes in the subchondral bone density of the talus of healthy Labrador Retrievers

Background

During growth, the skeletal structures adapt to the increased loading conditions and mature to a fully-grown skeleton. Subchondral bone density reflects the effect of long-term joint loading and it is expected to change over time. The aim of this study was to describe the long-term changes in the density distribution of the subchondral bone of the talus of healthy Labrador Retrievers in a prospective study.

Results

The subchondral bone density distribution was evaluated using computed tomographic osteoabsorptiometry (CTOAM). Visually, all joints showed very similar density distribution patterns. No significant differences in the topography of the density maxima were found between t₁ and t₂. The mean density, maximum density, and maximum area ratio (MAR) were significantly increased with increasing age.

Conclusions

The subchondral bone density of the talus of healthy Labrador Retrievers increases with increasing age. It is likely an adaptive response of the subchondral bone due to increased joint loading during growth.

Biomechanical analysis of a novel hemipelvic endoprosthesis during ascending and descending stairs

In this study, the biomechanical characteristic of a newly developed adjustable hemipelvic prosthesis under dynamic loading conditions was investigated using explicit finite element method. Both intact and reconstructed pelvis models, including pelvis, femur and soft tissues, were established referring to human anatomic data using a solid geometry of a human pelvic bone. Hip contact forces during ascending stairs and descending stairs were imposed on pelvic models. Results showed that maximum von Mises stresses in reconstructed pelvis were 421.85 MPa for prostheses and 109.12 MPa for cortical bone, which were still within a low and elastic range below the yielding strength of Ti-6Al-4V and cortical bone, respectively. Besides, no significant difference of load transferring paths along pelvic rings was observed between the reconstructed pelvis and natural pelvis models. And good agreement was found between the overall distribution of maximum principal stresses in trabecular bones of reconstructed pelvis and natural pelvis, while at limited stances, principal stresses in trabecular bone of reconstructed pelvis were slightly lower than natural pelvis. The results indicated that the load transferring function of pelvis could be restored by this adjustable hemipelvic prosthesis. Moreover, the prosthesis was predicted to have a reliable short- and long-term performance. However, due to the occurrence of slightly lower principal stresses at a few stances, a porous structure applied on the interface between the prosthesis and bone would be studied in future work to obtain better long-term stability.

Incidence and risk factors of allograft bone failure after calcaneal lengthening

BACKGROUND

Calcaneal lengthening with allograft is frequently used for the treatment of patients with symptomatic planovalgus deformity; however, the behavior of allograft bone after calcaneal lengthening and the risk factors for graft failure are not well documented.

QUESTIONS/PURPOSES

(1) What proportion of the patients treated with allograft bone had radiographic evidence of graft failure and what further procedures were performed? (2) What are the risk factors for radiographic graft failure after calcaneal lengthening? (3) What patient factors are associated with the magnitude of correction achieved after calcaneal lengthening?

METHODS

Between May 2003 and January 2014, we performed 341 calcaneal lengthenings on 202 patients for planovalgus deformity, the etiology of which included idiopathic, cerebral palsy, and other neuromuscular disease. Of these, 176 patients (87%) had adequate followup for graft evaluation, defined as lateral radiographs taken before and at least 6 months after the index procedure (mean, 18 months; range, 6-100 months) and 117 patients (58%) had adequate followup for the assessment of the extent of correction, defined as weightbearing anteroposterior and lateral radiographs taken before and at least 1 year after the index procedure (mean, 24 months; range, 12-96 months). These patients' results were evaluated retrospectively. The Goldberg scoring system was chosen for demonstration of allograft behavior. A score lower than 6 at 6 months after surgery was defined as radiographic graft failure; the highest possible score was 7 points, and this represented graft incorporation with excellent reorganization of the graft and no loss of height. The patient age, sex, diagnosis, graft material, ambulatory status, and use of antiseizure medication were evaluated as possible risk factors, and we controlled for the interaction of potentially confounding variables using multivariate analysis. Additionally, six radiographic indices were analyzed for their effects on the extent of correction.

RESULTS

The mean estimated Goldberg score was 6 (SD, 1.14) at 6 months after calcaneal lengthening with 11 feet (4%) classified as radiographic graft failure (Goldberg score < 6). Of these, four feet (1%) underwent reoperation using an iliac autograft bone resulting from pain and loss of correction. Multivariate analysis showed that the tricortical iliac crest allograft was superior to the patellar allograft (odds ratio [OR], 3.2; 95% confidence interval [CI], 1.1-9.8; p = 0.038) and the possibility of radiographic graft failure was found to increase along with age (OR, 1.2; 95% CI, 1.0-1.3; p = 0.006). Radiographically, the extent of correction was found to decrease with patient age, as observed at the anteroposterior talus-first metatarsal angle (p < 0.001), lateral talocalcaneal angle (p < 0.001), lateral talus-first metatarsal angle (p < 0.001), and relative calcaneal length (p = 0.041).

CONCLUSIONS

Graft failure can occur after calcaneal lengthening using allograft. Our study showed that the tricortical iliac allograft was superior to the patellar allograft, and further studies are warranted to further elucidate the effects of age on radiographic graft failure.

LEVEL OF EVIDENCE

Level III, therapeutic study.

Trabecular orientation in the human femur and tibia and the relationship with lower-limb alignment for patients with osteoarthritis of the knee

Wolff׳s Law suggests that the orientation of trabeculae in human bone changes in response to altered loading patterns. The aim of this study was to investigate trabecular orientation in both the femur and tibia and to compare this with the mechanical axis of the leg. The study involved analysis of radiographs from patients with osteoarthritis of the knee (n=91). For each patient, the trabecular orientation in both the distal femur and proximal tibia was measured from a standard anteroposterior radiograph of the knee and the mechanical axis of the leg was calculated from a long leg view taken while weight bearing. There was a significant correlation between the mechanical axis and the trabecular orientation in each of the regions considered in the femur (r=-0.41, -0.30, 0.52, and 0.23) and tibia (r=-0.27 and 0.31). Multiple regression analysis, with mechanical axis as the dependent variable, produced an R(2) of 0.62. Greater trabecular anisotropy (i.e. greater alignment) was observed in the medial femur and tibia compared to the lateral side (p<0.01). The results give an insight into the trabecular changes that may take place during development of osteoarthritis and following surgery. In particular, we propose that the orientation of the trabeculae in both the distal femur and proximal tibia will reflect the angle of mechanical loading through the knee.

Bionic plate design for calcaneal fracture treatment. A biomechanical analysis and first clinical results

PURPOSE

Calcaneal fracture treatment is challenging. Implant failure is one problem encountered with plate osteosynthesis. Therefore a new "bionic" plate was developed, which imitates the trabecular orientation of the human calcaneus. The aim of this study was to biomechanically test this new plate in comparison to a "standard" calcaneal locking plate and present the first clinical results.

METHODS

Six "bionic" and six "standard" calcaneal plates were biomechanically tested for stability and fatigue using synthetic calcanei. Between 4/2012 and 04/2013 the first ten consecutive patients meeting the inclusion criteria were treated with the novel implant and followed-up clinically and radiologically. The 12-month follow-up results are reported.

RESULTS

The "bionic" plate design showed significantly higher fatigue life (68%), load to failure (100%) and reduced displacement under load (90%) if compared to a "standard" locking plate. No major complications were seen; most notably there was no implant failure and no loss of reduction. Mean AOFAS/hindfoot score was 79 (69-86).

CONCLUSIONS

The novel plate architecture offers higher stability and load tolerance while being more resistant to fatigue. The preliminary clinical results are promising. These findings will have to be proved by larger clinical trials.

Principal trabecular structural orientation predicted by quantitative ultrasound is strongly correlated with μFEA determined anisotropic apparent stiffness

The microarchitecture and alignment of trabecular bone adapts to the particular mechanical milieu applied to it. Due to this anisotropic mechanical property, measurement orientation has to be taken into consideration when assessing trabecular bone quality and fracture risk prediction. Quantitative ultrasound (QUS) has demonstrated the ability in predicting the principal structural orientation (PSO) of trabecular bone. Although the QUS prediction for PSO is very close to that of μCT, certain angle differences still exist. It remains unknown whether this angle difference can induce significant differences in mechanical properties or not. The objective of this study was to evaluate the mechanical properties in different PSOs predicted using different methods, QUS and μCT, thus to investigate the ability of QUS as a means to predict the PSO of trabecular bone noninvasively. By validating the ability of QUS to predict the PSO of trabecular bone, it is beneficial for future QUS applications because QUS measurements in the PSO can provide information more correlated with the mechanical properties than with other orientations. In this study, seven trabecular bone balls from distal bovine femurs were used to generate finite element models based on the 3-dimensional μCT images. Uniaxial compressive loading was performed on the bone ball models in the finite element analysis (FEA) in six different orientations (three anatomical orientations, two PSOs predicted by QUS and the longest vector of mean intercept length (MIL) tensor calculated by μCT). The stiffness was calculated based on the reaction force of the bone balls under loading, and the von Mises stress results showed that both the mechanical properties in the PSOs predicted by QUS are significantly higher than the anatomical orientations and comparatively close to the longest vector of MIL tensor. The stiffness in the PSOs predicted by QUS is also highly correlated with the stiffness in the MIL tensor orientation (ATTmax vs. MIL, R(2) = 0.98, p < 001; UVmax vs. MIL, R(2) = 0.92, p < 001). These results were validated by in vitro mechanical testing on the bone ball samples. This study demonstrates that the PSO of trabecular bone predicted by QUS has an equally strong apparent stiffness with the orientation predicted by μCT.

Periodontal microstructure change and tooth movement pattern under different force magnitudes in ovariectomized rats: an in-vivo microcomputed tomography study

INTRODUCTION

The purpose of this study was to evaluate the dynamic changes in the periodontal microstructure and the molar displacement pattern during orthodontic tooth movement in ovariectomized rats.

METHODS

Twenty ovariectomized rats received either 100 or 30 g of orthodontic force to induce mesial movement of the maxillary left first molars over 14 days. Ten healthy rats underwent sham operations as controls. Periodontal ligament thickness, alveolar bone microstructural properties, and displacement of the molar were measured with 6 in-vivo microcomputed tomography scans for each sample.

RESULTS

The ovariectomized rats that received 100 g of orthodontic force had obvious changes in periodontal ligament thickness at day 1 and poor periodontal ligament thickness recovery from days 5 through 14. The bone volume fraction increased and the trabecular separation decreased significantly in this group at day 3, and obvious bone loss was observed at day 14. Molar linear and angular movements were also higher in this group than in the other 2 groups.

CONCLUSIONS

Relatively heavier force applications in ovariectomized rats resulted in poor periodontal ligament thickness recovery and local alveolar bone overcompression, and consequently induced undermining resorption and obvious alveolar bone loss; these led to high rates of tooth movement and molar inclination.

Mechanical stress redistribution in the calcaneus after autologous bone harvesting

The calcaneus is a desirable site for harvesting autologous bone for use in foot surgery. However, fracture of the calcaneus is a serious complication associated with bone harvesting from this site. Currently it is unknown how much bone may be safely harvested from the calcaneus without inducing a fracture. The purpose of this study was to investigate the effect of progressive bone removal from the calcaneus onto the mechanical stress redistribution of the foot, and therefore on the increase in fracture risk. Different loads were applied on the talus to evaluate the calcaneus stress distribution at different situations. Because of the potential increase in mechanical stress in the calcaneus, secondary to contraction of the Achilles tendon, we also evaluated the mechanical behavior properties of the foot with increasing traction force in the Achilles tendon. A three-dimensional (3D) finite element (FE) model developed from CT images obtained from a healthy individual was used to compute displacement, tension and compression stresses in six situations, including intact foot, and five depth of the bone block removed, with a maximum depth of 7.5 mm. The results from these simulations indicated that when the maximum load was applied at the Achilles tendon, the tension stress increased from 42.16 MPa in the intact foot to 86.28 MPa with maximum bone harvesting. Furthermore, as the volume of bone extracted from the calcaneus increases, there is a redistribution of stresses that differs significantly from the intact foot. In fact, although the maximum stress was not significantly affected by increasing the volume of bone harvested-except when increasing the Achilles tendon force-, stresses did increase in areas of the calcaneus is vulnerable to injury, leading to an increase in fracture risk.

A Wolff in sheep's clothing: trabecular bone adaptation in response to changes in joint loading orientation

This study tests Wolff's law of trabecular bone adaptation by examining if induced changes in joint loading orientation cause corresponding adjustments in trabecular orientation. Two groups of sheep were exercised at a trot, 15 min/day for 34 days on an inclined (7°) or level (0°) treadmills. Incline trotting caused the sheep to extend their tarsal joints by 3-4.5° during peak loading (P<0.01) but has no effect on carpal joint angle (P=0.984). Additionally, tarsal joint angle in the incline group sheep were maintained more extended throughout the day using elevated platform shoes on their forelimbs. A third "sedentary group" group did not run but wore platform shoes throughout the day. As predicted by Wolff's law, trabecular orientation in the distal tibia (tarsal joint) were more obtuse by 2.7 to 4.3° in the incline group compared to the level group; trabecular orientation was not significantly different in the sedentary and level groups. In addition, trabecular orientations in the distal radius (carpal joint) of the sedentary, level and incline groups did not differ between groups, and were aligned almost parallel to the radius long axis, corresponding to the almost straight carpal joint angle at peak loading. Measurements of other trabecular bone parameters revealed additional responses to loading, including significantly higher bone volume fraction (BV/TV), Trabecular number (Tb.N) and trabecular thickness (Tb.Th), lower trabecular spacing (Tb.Sp), and less rod-shaped trabeculae (higher structure model index, SMI) in the exercised than sedentary sheep. Overall, these results demonstrate that trabecular bone dynamically adjusts and realigns itself in very precise relation to changes in peak loading direction, indicating that Wolff's law is not only accurate but also highly sensitive.

Mechanical and microarchitectural analyses of cancellous bone through experiment and computer simulation

The relationship between microarchitecture to the failure mechanism and mechanical properties can be assessed through experimental and computational methods. In this study, both methods were utilised using bovine cadavers. Twenty four samples of cancellous bone were extracted from fresh bovine and the samples were cleaned from excessive marrow. Uniaxial compression testing was performed with displacement control. After mechanical testing, each specimen was ashed in a furnace. Four of the samples were exemplarily scanned using micro-computed tomography (μCT) and three dimensional models of the cancellous bones were reconstructed for finite element simulation. The mechanical properties and the failure modes obtained from numerical simulations were then compared to the experiments. Correlations between microarchitectural parameters to the mechanical properties and failure modes were then made. The Young's modulus correlates well with the bone volume fraction with R² = 0.615 and P value 0.013. Three different types of failure modes of cancellous bone were observed: oblique fracture (21.7%), perpendicular global fracture (47.8%), and scattered localised fracture (30.4%). However, no correlations were found between the failure modes to the morphological parameters. The percentage of error between computer predictions and the actual experimental test was from 6 to 12%. These mechanical properties and information on failure modes can be used for the development of synthetic cancellous bone.

Ontogenetic changes in the internal and external morphology of the ilium in modern humans

Trabecular architecture forms an important structural component of bone and, depending on the loading conditions encountered during life, is organised in a systematic, bone- and species-specific manner. However, recent studies suggested that gross trabecular arrangement (e.g. density distribution), like overall bone shape, is predetermined and/or affected by factors other than loading and perhaps less plastic than commonly assumed. To explore this issue further, the present cross-sectional ontogenetic study investigated morphological changes in external bone shape in relation to changes in trabecular bundle orientation and anisotropy. Radiographs of 73 modern human ilia were assessed using radiographic and Geometric Morphometric techniques. The study confirmed the apparently strong predetermination of trabecular bundle development, i.e. prior to external loading, although loading clearly also had an effect on overall morphology. For example, the sacro-pubic bundle, which follows the path of load transmission from the auricular surface to the acetabulum, is well defined and shows relatively high levels of anisotropy from early stages of development; the situation for the ischio-iliac strut is similar. However, while the sacro-pubic strut retains a constant relationship with the external landmarks defining the joint surfaces, the ischio-iliac bundle changes its relationship with the external landmarks and becomes aligned with the iliac tubercle only during late adolescence/early adulthood. It is tentatively proposed that the rearrangement of the ischio-iliac strut may reflect a change in locomotor pattern and/or a shift in positional behavior with increasing mass after growth of external bone dimensions has slowed/ceased.

Inhomogeneity of tissue-level strain distributions in individual trabeculae: Mathematical model studies of normal and osteoporosis cases

Little is known about the distributions of mechanical strains and stresses in individual trabeculae of cancellous bone, despite evidence that tissue-level strains affect the metabolism of bone. Recently, micro-finite element (micro-FE) studies have provided the first insights into the mechanical conditions in trabeculae, and suggested that osteoporotic cancellous bone experience higher and substantially less-uniform strains with respect to healthy cancellous bone. We may therefore ask whether the inhomogeneity of bone tissue strains is predominantly a consequence of micro-architectural differences between trabeculae, or is it mostly caused by the curvatures of each individual trabecula. Accordingly, the objectives of the present study were to determine the contribution of the shape of a trabecula to the intra-trabecula strain inhomogeneity, and to determine potential differences in intra-trabecula strain inhomogeneities between normal and thinner, osteoporotic-like trabeculae. We employed our previously reported generic single-trabecula model, which is a mathematical representation of the shape of a trabecula based on statistical analyses of mammalian trabecular dimensions. The single-trabecula model was loaded axially and in bending, and strain distributions were calculated for individual trabeculae as well as for "populations" of trabeculae, formed by assigning different trabecular thickness values in the trabecular model, in order to represent the distributions of trabecular shapes in normal and osteoporotic bones. We found that when subjected to equivalent loads, thinner, osteoporotic-like individual trabeculae and populations of thin trabeculae developed substantially greater strain inhomogeneities compared with normal trabeculae. We conclude that the intra-trabecula strain inhomogeneities are likely to be an important factor contributing to the overall increased strain inhomogeneity in osteoporotic cancellous bone, as previously observed in micro-FE studies.

Fuzzy logic structure analysis of trabecular bone of the calcaneus to estimate proximal femur fracture load and discriminate subjects with and without vertebral fractures using high-resolution magnetic resonance imaging at 1.5 T and 3 T

Newly developed fuzzy logic-derived structural parameters were used to characterize trabecular bone architecture in high-resolution magnetic resonance imaging (HR-MRI) of human cadaver calcaneus specimens. These parameters were compared to standard histomorphological structural measures and analyzed concerning performance in discriminating vertebral fracture status and estimating proximal femur fracture load. Sets of 60 sagittal 1.5 T and 3.0 T HR-MRI images of the calcaneus were obtained in 39 cadavers using a fast gradient recalled echo sequence. Structural parameters equivalent to bone histomorphometry and fuzzy logic-derived parameters were calculated using two chosen regions of interest. Calcaneal, spine, and hip bone mineral density (BMD) measurements were also obtained. Fracture status of the thoracic and lumbar spine was assessed on lateral radiographs. Finally, mechanical strength testing of the proximal femur was performed. Diagnostic performance in discriminating vertebral fracture status and estimating femoral fracture load was calculated using regression analyses, two-tailed t-tests of significance, and receiver operating characteristic (ROC) analyses. Significant correlations were obtained at both field strengths between all structural and fuzzy logic parameters (r up to 0.92). Correlations between histomorphological or fuzzy logic parameters and calcaneal BMD were mostly significant (r up to 0.78). ROC analyses demonstrated that standard structural parameters were able to differentiate persons with and without vertebral fractures (area under the curve [A(Z)] up to 0.73). However, none of the parameters obtained in the 1.5-T images and none of the fuzzy logic parameters discriminated persons with and without vertebral fractures. Significant correlations were found between fuzzy or structural parameters and femoral fracture load. Using multiple regression analysis, none of the structural or fuzzy parameters were found to add discriminative value to BMD alone. In summary significant correlations were obtained at both field strengths between all structural and fuzzy logic parameters. However, fuzzy logic-based calcaneal parameters were not well suited for vertebral fracture discrimination. Although significant correlations were found between fuzzy or structural parameters and femoral fracture load, multiple regression analysis showed limited improvement for estimating femoral failure load in addition to femoral BMD alone. Local femoral measurements are still needed to estimate femoral bone strength. Overall, parameters obtained at 3.0 T performed better than those at 1.5 T.

Clinical significance of musculoskeletal finite element model of the second and the fifth foot ray with metatarsal cavities and calcaneal sinus

AIM

The aim of this study was to establish musculoskeletal finite element (FE) model of the second and the fifth foot ray accounting for metatarsal cavities and calcaneal sinus. The model was then used to predict the effects of metatarsal cavities and calcaneal sinus on internal stresses/strains of plantar longitudinal arches.

MATERIALS AND METHODS

The geometry of foot bones and soft tissues were constructed by CT and MRI images of Virtual Chinese Human "female No. 1". Two types of nonlinear FE models of sagittal foot rays were developed with or without metatarsal cavities and calcaneal sinus using ANSYS software package. The sagittal trabecular architecture of metatarsals and calcaneus were obtained by cutting, defatting and bleaching fresh foot specimen of a cadaver.

RESULTS

The model proposed was able to describe the isostatic stress flows in sagittal planes of plantar longitudinal arches. The size of metatarsal cavity or calcaneal sinus could affect stress/strain distributions on metatarsals and calcaneus, but had almost no effects on stress/strain of other foot bones and plantar soft tissues. During balance standing, the maximum von Mises stresses were predicted at the shaft and the basis of metatarsals, while the maximum strains of bony regions were found around metatarsal cavities. Among plantar soft tissues, relative high tensions were burdened by plantar fascia, followed by long plantar ligament. The minimum tensions occurred in plantar intrinsic muscles.

CONCLUSIONS

The study shows that the tension/compression stress flows are geometrically similar with the tension/compression trabecular architectures in sagittal sections of metatarsal and calcaneus. The FE predictions of stress/strain concentration on metatarsals and fascia are useful in enhancing biomechanical knowledge on metatarsal stress fractures and plantar fasciitis.

Bone surface topology mapping and its role in trabecular bone quality assessment using scanning confocal ultrasound

INTRODUCTION

Quantitative ultrasound (QUS) has been used to assess non-invasively bone quality, in which ultrasound velocity (UV) is a primary acoustic property.

METHODS

While UV calculation requires the tissue thickness in the ultrasound path, a bone surface topology mapping (STM) method was developed in this study for enhancing the accuracy of the UV measurement. STM accuracy was verified by both aluminum and a QUS heel phantom, indicating that the STM can determine the phantom thickness within 0.02 mm thickness error and the aluminum calibration step within 0.1 mm thickness error. STM performance was further evaluated using 25 cadaveric human calcanei samples.

RESULTS

The UV calculations using STM had a significant better correlation to bone mineral density (BMD) (r = 0.75, p < 0.05), volume fraction (r = 0.72, p < 0.05) and modulus (r = 0.69, p < 0.05) than the UV with fixed thickness. The later correlation coefficients were r = 0.64 for BMD, r = 0.65 for volume fraction, and r = 0.58 for modulus. The nBUA value determined using STM was also highly correlated to BMD (r(2) = 0.74) and modulus (r(2) = 0.62). This was comparable to the correlation result for BUA (BMD: r(2) = 0.76; Modulus: r(2) = 0.64).

CONCLUSION

These results suggested that STM technique in scanning ultrasound is capable of determining calcaneus bone thickness and hence enhancing the accuracy of UV measurement.

Incidence and morphologic characteristics of benign calcaneal cystic lesions on MRI

BACKGROUND

Since we have frequently noted osseous cyst-like lesions within the calcaneus on MRI studies, we sought to systematically evaluate this finding to determine the incidence and morphologic characteristics on MRI.

METHODS

Three observers blinded to age evaluated 198 MRIs of the ankle (74 males, 124 females; mean age 47 years, range 13 to 99 years), recording the presence and size of calcaneal cyst-like foci. Statistical analysis was performed to determine if there was an association with age. Additionally, MRI of 12 ankles precontrast and postcontrast were reviewed for the presence of blood vessels in the calcaneus corresponding to the location of the cyst-like lesions and 24 ankle MR arthrograms were reviewed to evaluate communication of the cyst with the adjacent joint.

RESULTS

Of the 198 ankle MRI examinations, 81 (40%) had hyperintense foci, all within the mid-calcaneal body. The size ranged from 0.01 cm(2) to 2.47 cm(2), with a mean size of 0.36 cm(2) (+/- 0.45 cm(2)). Thirty-seven (46%) were linear or elongated, whereas 44 (54%) were ovoid or round. Seven of the 81 foci (8.6% or 3.5% of the total population) were 1 cm(2) or larger. There was no significant association of patient age and presence (p = 0.49) or size (p = 0.48) of the focus. Location of the cyst-like foci, which often are ovoid or linear, corresponds to penetrating microvessels on precontrast and postcontrast MRI. One MR arthrogram showed communication of a cyst-like focus and the subtalar joint.

CONCLUSIONS

Calcaneal lesions are relatively common incidental findings on MRI. These lesions appear to represent intraosseous ganglion cysts that arise from the anterior margin of the posterior facet of the subtalar joint in the anatomical neighborhood where vessels penetrate the superior calcaneal cortex. The lesions can vary is size; however, there is no correlation between patient age and lesion size and no significant association between age and presence of these foci.

Morphological changes in pedal phalanges through ornithopod dinosaur evolution: a biomechanical approach

The evolution of ornithopod dinosaurs provides a well-documented example of the transition from digitigrady to subunguligrady. During this transition, the ornithopod pes was drastically altered from the plesiomorphic dinosaurian morphology (four digits, claw-shaped unguals, strongly concavo-convex joints, phalanges longer than wide, excavated collateral ligament fossae, presence of sagittal ridge, and prominent processes for the attachment of tendons) to a more derived condition (tridactyly, modification of the unguals into hooves, phalanges wider and thinner than long, lack of collateral ligament fossae, loss of sagittal ridge and tendon attachment processes, relatively flattened articular surfaces). These changes are particularly noteworthy given the overall conservatism in pedal morphology seen across Dinosauria. But what are the functional consequences of these specific morphological transitions? To study them, we examine a wide range of pedal morphologies in four non-avian dinosaurs and two birds. Our analyses of the external morphology, two-dimensional models (using Finite Element Analysis), and internal bone structure demonstrate that this evolutionary shift was accompanied by a loss of digit mobility and flexibility. In addition, pedal posture was modified to better align the pes with the main direction of the ground reaction force, thus becoming well suited to support high loads. These conclusions can be applied to other, parallel evolutionary changes (in both dinosaurs and mammals) that involved similar transitions to a subunguligrade posture.

Intrinsic mechanical properties of trabecular calcaneus determined by finite-element models using 3D synchrotron microtomography

To determine intrinsic mechanical properties (elastic and failure) of trabecular calcaneus bone, chosen as a good predictor of hip fracture, we looked for the influence of image's size on a numerical simulation. One cubic sample of cancellous bone (9 x 9 x 9 mm(3)) was removed from the body of the calcaneus (6 females, 6 males, 79+/-9 yr). These samples were tested under compressive loading. Before compressive testing, these samples were imaged at 10.13 microm resolution using a 3D microcomputed tomography (muCT) (ESRF, France). The muCT images were converted to finite-element models. Depending on the bone density values (BV/TV), we compared two different finite element models: a linear hexahedral and a linear beam finite element models. Apparent experimental Young's modulus (E(app)(exp)) and maximum apparent experimental compressive stress (sigma(max)(exp)) were significantly correlated with bone density obtained by Archimedes's test (E(app)(exp)=236+/-231 MPa [19-742 MPa], sigma(max)(exp)=2.61+/-1.97 MPa [0.28-5.81 MPa], r>0.80, p<0.001). Under threshold at 40 microm, the size of the numerical samples (5.18(3) and 6.68(3)mm(3)) seems to be an important parameter on the accuracy of the results. The numerical trabecular Young's modulus was widely higher (E(trabecular)(num)=34,182+/-22,830 MPa [9700-87,211 MPa]) for the larger numerical samples and high BV/TV than those found classically by other techniques (4700-15,000 MPa). For rod-like bone samples (BV/TV<12%, n=7), Young's modulus, using linear beam element (E(trabecular)(num-skeleton): 10,305+/-5500 MPa), were closer to the Young's modulus found by other techniques. Those results show the limitation of hexahedral finite elements at 40 microm, mostly used, for thin trabecular structures.

Differences in the mechanical properties of calcaneal artificial specimens, fresh frozen specimens, and embalmed specimens in experimental testing

BACKGROUND

Artificial calcanei, fresh-frozen cadaver specimens, and embalmed cadaver specimens were compared in experimental testing under biocompatible loading to clarify the biocompatibility of artificial calcaneal specimens for implant testing.

METHODS

Two different artificial calcaneal bone models (Sawbone, Pacific Research Laboratories, Vashon, WA, and Synbone, Synbone Inc., Davos, Switzerland), embalmed cadaver calcaneal specimens (bone density, 313.1 +/- 40.9 g/cm2; age, 43.8 +/- 7.9 years), and fresh-frozen cadaver calcanei (bone density, 238.5 +/- 30.0 g/cm2; age, 44.4 +/- 8.2 years) were used for testing. Seven specimens of each model or cadaver type were tested. A mechanical testing machine (Zwick Inc., Ulm, Germany) was used for loading and measurements. Cyclic loading (preload 20 N, load was increased every 100 cycles by 100 N from 1,000 to 2,500 N, 0.5 mm/s) and load to failure (0.5 mm/s) were performed. The loads were applied through an artificial talus in a physiological loading direction. The displacement of the posterior facet in the primary loading direction was measured.

RESULTS

The four different specimen groups showed different stability and different displacement in the primary loading direction during cyclic loading. The variation of the maximal displacement in the primary loading direction for the entire cyclic loading was higher in artificial specimens than in the cadaver specimens.

CONCLUSIONS

Artificial calcanei (Sawbone, Synbone) showed different biomechanical characteristics than cadaver bones (embalmed and fresh-frozen) in this experimental setup with biocompatible cyclic loading. These results do not support the use of artificial calcanei for biomechanical implant testing. Fresh-frozen and embalmed specimens seem to be equally adequate for mechanical testing. The low variation of mechanical strength in the unpaired cadaver specimens suggests that the use of PAIRED specimens is not necessary.

Automated method to measure trabecular thickness from microcomputed tomographic scans and its application

Trabeculae form the internal bony mesh work and provide strength to the bone; interconnectivity, overall density, and trabecular thickness are important measures of the integrity of the internal architecture. Such strength is achieved only gradually during ontogeny, whereby an increase in trabecular thickness precedes an increase in mineralization. Loss of bone mass later in life may be compensated for by thickening of the remaining trabeculae. These facts, and the role of trabeculae in mineral homeostasis, highlight the importance of investigating trabecular thickness within and between species. While nondestructive imaging techniques (i.e., muCT and MRI) are becoming increasingly popular, quantification of trabecular thickness using nondestructive techniques has proved difficult owing to limitations imposed by scanning parameters, uniform thresholding, and partial volume averaging. Here we present a computer application, which aims to overcome these problems. Validation is carried out against a phantom and against trabecular thickness measured in corresponding histological sections. Good agreement was found between these measurements. Furthermore, when trabecular thickness is recorded for modern human fetal ilia, a trend toward trabecular thickness increase is found and is in line with reports of ontogenetic morphometric changes using histological sections. However, there are discrepancies. These may in part be due to partial volume effects of obliquely oriented structures. More crucial, however, are problems inherent in histological sections, e.g., shrinkage and distortion, especially where differences in mineralization are concerned; this may affect biological interpretations.

Polyaxially-locked plate screws increase stability of fracture fixation in an experimental model of calcaneal fracture

Different calcaneal plates with locked screws were compared in an experimental model of a calcaneal fracture. Four plate models were tested, three with uniaxially-locked screws (Synthes, Newdeal, Darco), and one with polyaxially-locked screws (90° ± 15°) (Rimbus). Synthetic calcanei were osteotomised to create a fracture model and then fixed with the plates and screws. Seven specimens for each plate model were subjected to cyclic loading (preload 20 N, 1000 cycles at 800 N, 0.75 mm/s), and load to failure (0.75 mm/s).

During cyclic loading, the plate with polyaxially-locked screws (Rimbus) showed significantly lower displacement in the primary loading direction than the plates with uniaxially-locked screws (mean values of maximum displacement during cyclic loading: Rimbus, 3.13 mm (sd 0.68); Synthes, 3.46 mm (sd 1.25); Darco, 4.48 mm (sd 3.17); Newdeal, 5.02 mm (sd 3.79); one-way analysis of variance, p < 0.001).

The increased stability of a plate with polyaxially-locked screws demonstrated during cyclic loading compared with plates with uniaxially-locked screws may be beneficial for clinical use.

Trabecular bone in the bird knee responds with high sensitivity to changes in load orientation

Wolff's law of trajectorial orientation proposes that trabecular struts align with the orientation of dominant compressive loads within a joint. Although widely considered in skeletal biology, Wolff's law has never been experimentally tested while controlling for ontogenetic stage, activity level, and species differences, all factors that may affect trabecular bone growth. Here we report an experimental test of Wolff's law using a within-species design in age-matched subjects experiencing physiologically normal levels of bone strain. Two age-matched groups of juvenile guinea fowl Numida meleagris ran on a treadmill set at either 0 degrees (Level group) or 20 degrees (Incline group), for 10 min per day over a 45-day treatment period. Birds running on the 20 degrees inclined treadmill used more-flexed knees than those in the Level group at midstance (the point of peak ground reaction force). This difference in joint posture enabled us to test the sensitivity of trabecular alignment to altered load orientation in the knee. Using a new radon transform-based method for measuring trabecular orientation, our analysis shows that the fine trabecular bone in the distal femur has a high degree of correspondence between changes in joint angle and trabecular orientation. The sensitivity of this response supports the prediction that trabecular bone adapts dynamically to the orientation of peak compressive forces.

A method of quantification of stress shielding in the proximal femur using hierarchical computational modeling

Stress shielding is a biomechanical phenomenon causing adaptive changes in bone strength and stiffness around metallic implants, which potentially lead to implant loosening. Accordingly, there is a need for standard, objective engineering measures of the "stress shielding" performances of an implant that can be employed in the process of computer-aided implant design. To provide and test such measures, we developed hierarchical computational models of adaptation of the trabecular microarchitecture at different sites in the proximal femur, in response to insertion of orthopaedic screws and in response to hypothetical reductions in hip joint and gluteal muscle forces. By identifying similar bone adaptation outcomes from the two scenarios, we were able to quantify the stress shielding caused by screws in terms of analogous hypothetical reductions in hip joint and gluteal muscle forces. Specifically, we developed planar lattice models of trabecular microstructures at five regions of interest (ROI) in the proximal femur. The homeostatic and abnormal loading conditions for the lattices were determined from a finite element model of the femur at the continuum scale and fed to an iterative algorithm simulating the adaptation of each lattice to these loads. When screws were inserted to the femur model, maximal simulated bone loss (17% decrease in apparent density, 10% decrease in thickness of trabeculae) was at the greater trochanter and this effect was equivalent to the effect of 50% reduction in gluteal force and normal hip joint force. We conclude that stress shielding performances can be quantified for different screw designs using model-predicted hypothetical musculoskeletal load fractions that would cause a similar pattern and extent of bone loss to that caused by the implants.

Contribution of muscular weakness to osteoporosis: computational and animal models

BACKGROUND

Chronic weakness of the femoral musculature with old age may result in prolonged exposure of bone to critical understressing and, thus, cause osteoporotic changes. This study aims at quantifying long-term changes in thickness and mechanical properties of trabecular bone at the proximal femur due to muscular weakness.

METHODS

We utilized computational models of typical planar trabecular lattices at the proximal femur for simulating long-term changes in morphological and mechanical properties of trabecular bone. Incorporating cellular communication network with osteocytes as mechanosensors, the models were able to mimic mechanotransduction and consequent thickening and/or thinning of individual trabeculae in response to altered gluteus muscle and hip joint loads. We also studied a rat model (n=14) in which we surgically detached the gluteus muscle, to approximately 50% or completely.

FINDINGS

The computational simulations showed that when the force of the gluteus decreased (with or without simultaneous decrease in hip joint load), the most dramatic degradation in bone density, strength and stiffness occurred at the greater trochanter. Animal studies also demonstrated significant thinning of femoral trabeculae after 19 weeks of adaptation. Specifically, Tukey-Kramer analysis showed that rats subjected to partial surgical detachment of the gluteus had femoral trabeculae that were 22% thinner than controls (P<0.05).

INTERPRETATION

The present study showed that in both the computer and animal models, manipulation of muscle loading produced a significant stimulus for bone to adapt, i.e., a stimulus that is beyond its irresponsive 'lazy zone'. Accordingly, the results obtained herein indicate that muscular weakness may be an important factor contributing to osteoporosis.

A comparison of plates with and without locking screws in a calcaneal fracture model

BACKGROUND

We compared different plates in an experimental calcaneal fracture model under biocompatible loading.

METHODS

Four plates were tested: a plate without locked screws (Synthes), and three different plates with locked screws (Newdeal, Darco, Synthes). Synthetic calcanei (Sawbone) were osteotomized to create a fracture model, and the plates were fixed onto them. Seven specimens for each plate model were subjected to cyclic loading (preload 20 N, 1,000 cycles with 800 N, 0.75 mm/s), and load to failure (0.75 mm/s). Motion, forces, plastic deformation of the plate, and consequent depression of the posterior joint facet were analyzed.

RESULTS

During cyclic loading, all plates with locked screws showed statistically significant lower displacement in the primary loading direction than the plates without locked screws. Mean values (mm) of maximal displacements for each plate during cyclic loading were as follows: Synthes, 3.5; Darco, 4.5; Newdeal, 5.0; Synthes without locked screws, 7.5; (p < 0.001). No statistically significant differences between the plates were found in relation to loads to failure and corresponding displacement.

CONCLUSION

This is the first biomechanical study to assess the stability of different plates currently in use in our practice for the fixation of calcaneal fractures. Our results showed that plates with locked screws provided greater stability during cyclic loading than the plate without locked screws.

Single-trabecula building block for large-scale finite element models of cancellous bone

Recent development of high-resolution imaging of cancellous bone allows finite element (FE) analysis of bone tissue stresses and strains in individual trabeculae. However, specimen-specific stress/strain analyses can include effects of anatomical variations and local damage that can bias the interpretation of the results from individual specimens with respect to large populations. This study developed a standard (generic) 'building-block' of a trabecula for large-scale FE models. Being parametric and based on statistics of dimensions of ovine trabeculae, this building block can be scaled for trabecular thickness and length and be used in commercial or custom-made FE codes to construct generic, large-scale FE models of bone, using less computer power than that currently required to reproduce the accurate micro-architecture of trabecular bone. Orthogonal lattices constructed with this building block, after it was scaled to trabeculae of the human proximal femur, provided apparent elastic moduli of approximately 150 MPa, in good agreement with experimental data for the stiffness of cancellous bone from this site. Likewise, lattices with thinner, osteoporotic-like trabeculae could predict a reduction of approximately 30% in the apparent elastic modulus, as reported in experimental studies of osteoporotic femora. Based on these comparisons, it is concluded that the single-trabecula element developed in the present study is well-suited for representing cancellous bone in large-scale generic FE simulations.