Quantitative CT with finite element analysis: towards a predictive tool for bone remodelling around an uncemented tapered stem

Stress-Strain State Investigation and Ultimate Load on Femoral Implants Based on S-Type Ti6Al4V Titanium Alloy

Hip replacement is a widespread surgical procedure that eliminates pain and restores motor functions of the pathologically altered hip joint. The issue lies in the lack of pre-operative strength calculations for implant shapes. So, they tend to break after surgery or damage the bone due to the complex stress-strain state. In the present paper, we studied the stress-strain state and ultimate load of S-type canine femoral implants based on titanium alloy Ti6Al4V using finite element analysis for static and cyclic loads. X-ray computed micro tomography data were used to construct the models. Re-engineering and restoration of the 3D geometry of the product were conducted. Strength analysis was performed in the finite element analysis software package Ansys Mechanical was used for various types of implant support. Locations with stress concentrators were identified, and ultimate loads on the implant were obtained. The influence of the rigidity of the support on the prosthesis stem was also studied. For the case of rigid support, the stress-strain state of the prosthesis was studied and the ultimate load was found to be 30.1 kg.

Stress shielding effect after total hip arthroplasty varies between combinations of stem design and stiffness-a comparing biomechanical finite element analysis

PURPOSE

Total hip arthroplasty (THA) has become a highly frequent orthopaedic procedure. Multiple approaches have been made to design the femoral component for THA with a mechanical behaviour as close as possible to a natural femur. The aim of this study was to compare different combinations of design and biomechanical properties of THA prostheses and their impact on stress shielding of the periprosthetic bone.

METHODS

Virtual implantation of different stem designs (straight standard stem, straight short stem, anatomical short stem) by finite element analysis based on in vivo data from computer tomography was performed. For each stem, three grades of stiffness were generated, followed by a strain analysis.

RESULTS

Reduction of stem stiffness led to less stress shielding. Implantation of an anatomical short-stem prosthesis with low stiffness provided the most physiological strain-loading effect (p < 0.001).

CONCLUSION

A combination of a short and an anatomically designed stem with a low stiffness might provide a more physiological strain transfer during THA. Biomechanical properties of the femoral component for THA should be considered as a multifactorial function of dimensions, design, and stiffness.

Proximal femur lag screw placement based on bone mineral density determined by quantitative computed tomography

Following internal fixations for intertrochanteric fractures in elderly patients, lag screws or screw blades frequently cut the femoral head, leading to surgical failure. The bone mineral density (BMD) at various parts of the proximal femur is significantly correlated with the holding force of the lag screw, which in turn is closely associated with the stability of the fixation. However, the appropriate placement of the lag screw has been controversial. As a novel detection method for BMD, quantitative computed tomography (QCT) may provide relatively accurate measurements of three-dimensional structures and may provide an easy way to determine the appropriate lag screw placement. A total of 50 elderly patients with intertrochanteric fractures were selected for the present study. The BMD of the proximal femur on the healthy side, including the femoral intertrochanter, neck and head, was measured using QCT. For testing, the femoral head was divided into medial, central and lateral sections. The BMD of the femoral head was determined to be the highest, while the BMD of the femoral neck was the lowest. In the femoral head, the central section had the highest BMD, while the lateral section had the lowest BMD. The present study used QCT to detect differences in the BMD at various regions of the proximal femur and provided a novel theoretical reference for the placement of lag screws. To obtain maximum holding power, the lag screw must be placed in the central section of the femoral head.

Cortical Atrophy Related to Tumor Prosthesis in Skeletally Immature Osteosarcoma Patients

BACKGROUND

Cortical atrophy is commonly observed after prosthetic reconstruction for bone sarcomas. However, relevant literature regarding this phenomenon in skeletally immature patients is limited. Therefore, in this study, we evaluated the incidence and patterns of cortical atrophy. We then assessed its predisposing factors. Finally, we analyzed whether cortical atrophy was associated with poor prosthesis survival.

METHODS

We retrospectively reviewed 31 stems in 19 skeletally immature osteosarcoma patients who were treated with resection and tumor prosthesis surgery. We measured the cortical thickness using plain radiographs annually. The mean patient age was 11 years (range, 6 to 14 y) and the mean follow-up period was 10 years (range, 2 to 14 y).

RESULTS

Cortical atrophy developed in 13 stems, all within 3 years. There were 3 gross types of cortical atrophy: hourglass in 5, sharpening in 4, and shortening atrophy in 4. On multivariate analysis, stem-cortex diameter ratio of ≥0.5 and age of less than 10 years were significantly associated with cortical atrophy (P=0.002 and P=0.019, respectively). Cortical atrophy was significantly associated with eventual prosthesis failure (9/13 in the cortical atrophy group vs. 4/18 in the nonatrophy group, P=0.035).

CONCLUSIONS

Cortical atrophy is common in the skeletally immature patients with tumor prosthesis and is associated with prosthesis failure. Patients with cortical atrophy should be closely monitored for prosthesis failure and appropriately managed to prevent failures.

LEVEL OF EVIDENCE

Level IV-therapeutic study.

Quantitating the effect of prosthesis design on femoral remodeling using high-resolution region-free densitometric analysis (DXA-RFA)

Dual energy X-ray absorptiometry (DXA) is the reference standard method used to study bone mineral density (BMD) after total hip arthroplasty (THA). However, the subtle, spatially complex changes in bone mass due to strain-adaptive bone remodeling relevant to different prosthesis designs are not readily resolved using conventional DXA analysis. DXA region free analysis (DXA RFA) is a novel computational image analysis technique that provides a high-resolution quantitation of periprosthetic BMD. Here, we applied the technique to quantitate the magnitude and areal size of periprosthetic BMD changes using scans acquired during two previous randomized clinical trials (2004 to 2009); one comparing three cemented prosthesis design geometries, and the other comparing a hip resurfacing versus a conventional cementless prosthesis. DXA RFA resolved subtle differences in magnitude and area of bone remodeling between prosthesis designs not previously identified in conventional DXA analyses. A mean bone loss of 10.3%, 12.1%, and 11.1% occurred for the three cemented prostheses within a bone area fraction of 14.8%, 14.4%, and 6.2%, mostly within the lesser trochanter (p < 0.001). For the cementless prosthesis, a diffuse pattern of bone loss (-14.3%) was observed at the shaft of femur in a small area fraction of 0.6% versus no significant bone loss for the hip resurfacing prosthesis (p < 0.001). BMD increases were observed consistently at the greater trochanter for all prostheses except the hip-resurfacing prosthesis, where BMD increase was widespread across the metaphysis (p < 0.001). DXA RFA provides high-resolution insights into the effect of prosthesis design on the local strain environment in bone. © 2017 The Authors Journal of Orthopaedic Research published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society. J Orthop Res 35:2203-2210, 2017.

Preoperative QCT assessment of femoral head for assessment of femoral head bone loss

We quantified differences in trabecular volumetric bone mineral density (BMD) of the femoral head between patients with proximal femoral fractures and healthy subjects in the control group by using quantitative computed tomography (QCT) with the purpose of providing guidance for the choice of head screw in the intramedullary nail fixation. Participants suffering from intertrochanteric fractures (n=536 patients) were recruited. In addition, 497 fracture-free, age-matched cases were considered as the control group. The volumetric BMD of different regions of interest (ROI) in the proximal femur scanned by QCT were analyzed between the fracture and control groups. BMD of proximal femur in the fracture group was markedly lower than that in the control group. There were significant differences at distinct regions for male and female between the two groups. Furthermore, the trend of BMD changes among the femoral head, femoral neck and intertrochanter were not parallels in terms of their average value. In conclusion, osteoporosis has been demonstrated to be a main risk factor of the proximal femoral fracture. BMD value of proximal femur was often inconsistent with that of femoral head. Given this, preoperative QCT assessment plays an important role in choosing proper head screw in the intramedullary nail fixation.

Quantitative Computed Tomography (QCT) derived Bone Mineral Density (BMD) in finite element studies: a review of the literature

BACKGROUND

Finite element modeling of human bone provides a powerful tool to evaluate a wide variety of outcomes in a highly repeatable and parametric manner. These models are most often derived from computed tomography data, with mechanical properties related to bone mineral density (BMD) from the x-ray energy attenuation provided from this data. To increase accuracy, many researchers report the use of quantitative computed tomography (QCT), in which a calibration phantom is used during image acquisition to improve the estimation of BMD. Since model accuracy is dependent on the methods used in the calculation of BMD and density-mechanical property relationships, it is important to use relationships developed for the same anatomical location and using the same scanner settings, as these may impact model accuracy. The purpose of this literature review is to report the relationships used in the conversion of QCT equivalent density measures to ash, apparent, and/or tissue densities in recent finite element (FE) studies used in common density-modulus relationships. For studies reporting experimental validation, the validation metrics and results are presented.

RESULTS

Of the studies reviewed, 29% reported the use of a dipotassium phosphate (K2HPO4) phantom, 47% a hydroxyapatite (HA) phantom, 13% did not report phantom type, 7% reported use of both K2HPO4 and HA phantoms, and 4% alternate phantom types. Scanner type and/or settings were omitted or partially reported in 31% of studies. The majority of studies used densitometric and/or density-modulus relationships derived from different anatomical locations scanned in different scanners with different scanner settings. The methods used to derive various densitometric relationships are reported and recommendations are provided toward the standardization of reporting metrics.

CONCLUSIONS

This review assessed the current state of QCT-based FE modeling with use of clinical scanners. It was found that previously developed densitometric relationships vary by anatomical location, scanner type and settings. Reporting of all parameters used when referring to previously developed relationships, or in the development of new relationships, may increase the accuracy and repeatability of future FE models.

Analyzing bone remodeling patterns after total hip arthroplasty using quantitative computed tomography and patient-specific 3D computational models

BACKGROUND

Computational models in the form of finite element analysis technique that incorporates bone remodeling theories along with DEXA scans has been extensively used in predicting bone remodeling patterns around the implant. However, majority of such studies used generic models. Therefore, the aim of this study is to develop patient-specific finite element models of total hip replacement patients using their quantitative computed tomography (QCT) scans and accurately analyse bone remodelling patterns after total hip arthroplasty (THA).

METHODS

Patient-specific finite element models have been generated using the patients' QCT scans from a previous clinical follow-up study. The femur was divided into five regions in proximal-distal direction and then further divided into four quadrants for detailed analysis of bone remodeling patterns. Two types of analysis were performed-inter-patient and intra patient to compare them and then the resulting bone remodeling patterns were quantitatively analyzed.

RESULTS

Our results show that cortical bone density decrease is higher in diaphyseal region over time and the cancellous bone density decreases significantly in metaphyseal region over time. In metaphyseal region, posterior-medial (P-M) quadrant showed high bone loss while diaphyseal regions show high bone loss in anterior-lateral (A-L) quadrant.

CONCLUSIONS

Our study demonstrated that combining QCT with 3D patient-specific models has the ability of monitoring bone density change patterns after THA in much finer details. Future studies include using these findings for the development of a bone remodelling algorithm capable of predicting surgical outcomes for THA patients.

Use of high resolution dual-energy x-ray absorptiometry-region free analysis (DXA-RFA) to detect local periprosthetic bone remodeling events

Dual-energy x-ray absorptiometry (DXA) is the gold standard method for measuring periprosthetic bone remodeling, but relies on a region of interest (ROI) analysis approach. While this addresses issues of anatomic variability, it is insensitive to bone remodeling events at the sub-ROI level. We have validated a high-spatial resolution tool, termed DXA-region free analysis (DXA-RFA) that uses advanced image processing approaches to allow quantitation of bone mineral density (BMD) at the individual pixel (data-point) level. Here we compared the resolution of bone remodeling measurements made around a stemless femoral prosthesis in 18 subjects over 24 months using ROI-based analysis versus that made using DXA-RFA. Using the ROI approach the regional pattern of BMD change varied by region, with greatest loss in ROI5 (20%, p < 0.001), and largest gain in ROI4 (6%, p < 0.05). Analysis using DXA-RFA showed a focal zone of increased BMD localized to the prosthesis-bone interface (30-40%, p < 0.001) that was not resolved using conventional DXA analysis. The 20% bone loss observed in ROI5 with conventional DXA was resolved to a focal area adjacent to the cut surface of the infero-medial femoral neck (up to 40%, p < 0.0001). DXA-RFA enables high resolution analysis of DXA datasets without the limitations incurred using ROI-based approaches.

Four decades of finite element analysis of orthopaedic devices: where are we now and what are the opportunities?

Finite element has been used for more than four decades to study and evaluate the mechanical behaviour total joint replacements. In Huiskes seminal paper "Failed innovation in total hip replacement: diagnosis and proposals for a cure", finite element modelling was one of the potential cures to avoid poorly performing designs reaching the market place. The size and sophistication of models has increased significantly since that paper and a range of techniques are available from predicting the initial mechanical environment through to advanced adaptive simulations including bone adaptation, tissue differentiation, damage accumulation and wear. However, are we any closer to FE becoming an effective screening tool for new devices? This review contains a critical analysis of currently available finite element modelling techniques including (i) development of the basic model, the application of appropriate material properties, loading and boundary conditions, (ii) describing the initial mechanical environment of the bone-implant system, (iii) capturing the time dependent behaviour in adaptive simulations, (iv) the design and implementation of computer based experiments and (v) determining suitable performance metrics. The development of the underlying tools and techniques appears to have plateaued and further advances appear to be limited either by a lack of data to populate the models or the need to better understand the fundamentals of the mechanical and biological processes. There has been progress in the design of computer based experiments. Historically, FE has been used in a similar way to in vitro tests, by running only a limited set of analyses, typically of a single bone segment or joint under idealised conditions. The power of finite element is the ability to run multiple simulations and explore the performance of a device under a variety of conditions. There has been increasing usage of design of experiments, probabilistic techniques and more recently population based modelling to account for patient and surgical variability. In order to have effective screening methods, we need to continue to develop these approaches to examine the behaviour and performance of total joint replacements and benchmark them for devices with known clinical performance. Finite element will increasingly be used in the design, development and pre-clinical testing of total joint replacements. However, simulations must include holistic, closely corroborated, multi-domain analyses which account for real world variability.

Predicting Bone Remodeling in Response to Total Hip Arthroplasty: Computational Study Using Mechanobiochemical Model

Periprosthetic bone loss following total hip arthroplasty (THA) is a serious concern leading to the premature failure of prosthetic implant. Therefore, investigating bone remodeling in response to hip arthroplasty is of paramount for the purpose of designing long lasting prostheses. In this study, a thermodynamic-based theory, which considers the coupling between the mechanical loading and biochemical affinity as stimulus for bone formation and resorption, was used to simulate the femoral density change in response to THA. The results of the numerical simulations using 3D finite element analysis revealed that in Gruen zone 7, after remarkable postoperative bone loss, the bone density started recovering and got stabilized after 9% increase. The most significant periprosthetic bone loss was found in Gruen zone 7 (−17.93%) followed by zone 1 (−13.77%). Conversely, in zone 4, bone densification was observed (+4.63%). The results have also shown that the bone density loss in the posterior region of the proximal metaphysis was greater than that in the anterior side. This study provided a quantitative figure for monitoring the distribution variation of density throughout the femoral bone. The predicted bone density distribution before and after THA agree well with the bone morphology and previous results from the literature.

Comparison of mechanical stress and change in bone mineral density between two types of femoral implant using finite element analysis

Stress shielding after total hip arthroplasty (THA) remains an unsolved issue. Various patterns of mechanical stress appear according to the type of femoral stem used. To compare differences in mechanical stress conditions between Zweymuller type and fit-and-fill type stems, finite element analysis (FEA) was performed. Differences in bone mineral density (BMD) changes in the femur were also compared. Maximum stress was confirmed in Gruen zone 4, whereas zone 1 had the minimum amount of stress with both types of implant. The Zweymuller stem group had less mechanical stress and lower BMD in zone 7 than the fit-and-fill stem group. In conclusion, differences in mechanical stress may be related to changes in BMD after THA.