Comparison of Acetabular Measurements Between 2 Validated Software Programs Used in Hip Preservation Surgery.

BACKGROUND: Validated software tools (Clinical Graphics [CG] and Hip2Norm) permit measurement of the percentage of femoral head coverage (%FHC), which aids in morphological classification and prediction of outcome after hip preservation surgery. PURPOSE: (1) To assess whether acetabular parameter measurements determined from 2 commonly used software systems are comparable. (2) To determine which parameters influence the correlation or differences between software outputs and measurements. STUDY DESIGN: Cohort study (diagnosis); Level of evidence, 2. METHODS: The study included 69 patients (90 hips) who underwent periacetabular osteotomy and had comprehensive preoperative imaging available. Lateral center-edge angle (LCEA), acetabular index (AI), and %FHC were determined using 3-dimensional computed tomography (CT) measurements by CG and Hip2Norm software. Images of 18 pelvises were segmented to determine spinopelvic parameters and subtended acetabular angles. Between-group measurements were compared using correlation coefficients and Bland-Altman analyses. The difference in the outputs of the 2 programs was defined as delta (Δ). Radiographic parameters were tested to assess whether they were responsible for differences in %FHC between software programs. RESULTS: Strong correlations between LCEA (ρ = 0.862) and AI (ρ = 0.825) measurements were seen between the Hip2Norm and CG programs. However, weak correlation was seen in the estimate of %FHC (ρ = 0.358), with the presence of a systematic error. Hip2Norm consistently produced lower anterior, posterior, and total %FHC values than CG. The %FHC determined by CG, but not Hip2Norm, correlated with acetabular subtended angles (P < .05). Pelvic tilt measured on CT did not correlate with pelvic tilt estimated by Hip2Norm (P = .56), and ΔPelvicTilt strongly correlated with the difference in %FHC by the 2 software programs (ρ = 0.63; P = .005), pelvic incidence (ρ = 0.73; P < .001), and pelvic tilt (ρ = -0.91; P < .001) as per CT. CONCLUSION: The correlation of %FHC between Hip2Norm and CG was weak (ρ = 0.358). The difference in measurements of %FHC correlated with ΔPelvicTilt. The %FHC determined by CG strongly correlated with the segmented acetabular subtended angles and thus more likely reflected true values. Hip preservation surgeons should be aware of these measurement differences because %FHC is important in the diagnosis and prognosis of acetabular dysplasia.

Acetabular Morphology and Spinopelvic Characteristics: What Predominantly Determines Functional Acetabular Version?

BACKGROUND: In addition to the relative size of the acetabular rim and how the pelvis is positioned in space, the plane in which the acetabular version is calculated also affects its measurement. PURPOSE: To determine the relative contribution of pelvic and acetabular characteristics on morphological version (measured relative to the anterior pelvic plane angle [APPA]) and functional version (measured relative to the horizontal table). STUDY DESIGN: Cross-sectional study; Level of evidence, 3. METHODS: Included were 50 acetabular dysplasia patients and 109 asymptomatic controls. Using image analysis software, morphological parameters of the pelvis and acetabulum were determined from 2-dimensional computed topography: pelvic incidence, pelvic tilt angle, sacral slope, APPA, morphological and functional acetabular versions, and subtended angles (measure of acetabular rim prominence relative to the femoral head center) around the acetabular clockface in 30° increments. Correlation and multivariable regression analyses were performed with morphological and functional version as dependent variables and spinopelvic and acetabular parameters as independent variables. RESULTS: Morphological version was moderately associated with differences between anterior and posterior subtended angles (R = 0.68 [P < .001] and R = 0.57 [P < .001] for differences at 165° and 15° and 135° and 45°, respectively). Functional version was moderately associated with pelvic tilt angle (R = 0.56; P <.001) and the difference in subtended angles between anterior and posterior rims (R = 0.61 [P < .001] and R = 0.50 [P < .001] for differences at 165° and 15° and 135° and 45°, respectively). Multivariate analysis revealed a good model for predicting morphological version (R 2 = 0.44; P < .01) and functional version (R 2 = 0.58; P < .01). Subtended angle difference between 165° and 15° (B = 0.36 [95% CI, 0.24-0.49]; P < .001) was most strongly related to morphological version, and pelvic tilt angle (B = 0.57 [95% CI, 0.46-0.68]; P < .001) was most strongly related to functional version. CONCLUSION: Functional acetabular version was influenced most strongly by pelvic tilt angle rather than the relative prominence of the acetabular rims. Before determining surgical management for version abnormalities, it would be prudent to assess pelvic mobility and characteristics in different functional positions. In patients with minimal pelvic tilt change dynamically, corrective osteotomy would be the treatment of choice to improve functional version.

Measurement of cartilage sub-component distributions through the surface by Raman spectroscopy-based multivariate analysis.

Articular cartilage posesses unique material properties due to a complex depth-dependent composition of sub-components. Raman spectroscopy has proven valuable in quantifying this composition through cartilage cross-sections. However, cross-sectioning requires tissue destruction and is not practical in situ. In this work, Raman spectroscopy-based multivariate curve resolution (MCR) was employed in porcine cartilage samples (n = 12) to measure collagen, glycosaminoglycan, and water distributions through the surface for the first time; these were compared against cross-section standards. Through the surface Raman measurements proved reliable in predicting composition distribution up to a depth of approximately 0.5 mm. A fructose-based optical clearing agent (OCA) was also used in an attempt to further improve depth of resolution of this measurement method. However, it did not; mainly due to a high-spectral overlap with the Raman spectra of main cartilage sub-components. This measurement technique potentially could be used in situ, to better understand the etiology of joint diseases such as osteoarthritis (OA).

CT texture analysis of acetabular subchondral bone can discriminate between normal and cam-positive hips.

OBJECTIVES: The purpose of this study was to determine if the CT texture profile of acetabular subchondral bone differs between normal, asymptomatic cam-positive, and symptomatic cam-FAI hips. In addition, the utility of texture analysis to discriminate between the three hip statuses was explored using a machine learning approach. METHODS: IRB-approved, case-control study analyzing CT images in subjects with and without cam morphology from August 2010 to December 2013. Sixty-eight subjects were included: 19 normal controls, 26 asymptomatic cam, and 23 symptomatic cam-FAI. Acetabular subchondral bone was contoured on the sagittal oblique CT images using ImageJ ®. 3D histogram texture features (mean, variance, skewness, kurtosis, and percentiles) were evaluated using MaZda software. Groupwise differences were investigated using Kruskal-Wallis tests and Mann-Whitney U tests. Gradient-boosted decision trees were created and trained to discriminate between control and cam-positive hips. RESULTS: Both asymptomatic and symptomatic cam-FAI hips demonstrated significantly higher values of texture variance (p = 0.0007, p < 0.0001), 90th percentile (p = 0.007, p = 0.006), and 99th percentile (p = 0.009, p = 0.009), but significantly lower values of skewness (p = 0.0001, p = 0.0013) and kurtosis (p = 0.0001, p = 0.0001) compared to normal controls. There were no differences in texture profile between asymptomatic cam and symptomatic cam-FAI hips. Machine learning models demonstrated high classification accuracy for discriminating control hips from asymptomatic cam-positive (82%) and symptomatic cam-FAI (86%) hips. CONCLUSIONS: Texture analysis can discriminate between normal and cam-positive hips using conventional descriptive statistics, regression modeling, and machine learning algorithms. It has the potential to become an important tool in compositional analysis of hip subchondral trabecular bone in the context of FAI, and possibly serve as a biomarker of joint degeneration. KEY POINTS: • The CT texture profile of acetabular subchondral bone is significantly different between normal and cam-positive hips. • Texture analysis can detect changes in subchondral bone in asymptomatic cam-positive hips that are equal to that of symptomatic cam-FAI hips. • Texture analysis has the potential to become an important tool in compositional analysis of hip subchondral bone in the context of FAI and may serve as a biomarker in the study of joint physiology and biomechanics.

Higher patient activity level and subchondral stiffening in asymptomatic cam femoroacetabular impingement subjects.

A cam deformity is proposed as a cause of idiopathic osteoarthritis. Increased subchondral bone mineral density (BMD) is associated with this degenerative process of osteoarthritis, and the patient's activity level may contribute to it. Therefore, the correlation between activity level and subchondral BMD in subjects with cam deformity FAI was studied. In this study, 26 asymptomatic cam deformity subjects (Bump) were compared with 18 subjects with a normal alpha angle (Control). Anterosuperior subchondral femoral neck and acetabular rim BMD were measured using quantitative computed tomography. Activity level was determined using the UCLA activity score. The correlation between BMD and UCLA activity were analysed. The result was a significantly higher BMD for Bump subjects in almost all measured sections. The UCLA score of the Bump versus Control subjects were comparable (8.96 versus 8.77, P = 0.740). While the controls showed no correlation between UCLA and BMD, a positive correlation was found for the Bump subjects on several femoral and acetabular impingement locations. These results support the conclusion that mechanical loading causes subchondral stiffening at the anterosuperior head-neck junction of the femur and anterosuperior acetabular rim. The absence of a correlation between BMD versus UCLA in the Controls supports the hypothesis that activity level may serve as a predictor for higher subchondral BMD in a cam deformity hip joint.

Acetabular and spino-pelvic morphologies are different in subjects with symptomatic cam femoro-acetabular impingement.

Acetabular and spino-pelvic (SP) morphological parameters are important determinants of hip joint dynamics. This prospective study aimed to determine whether acetabular and SP morphological differences exist between hips with and without cam morphology and between symptomatic and asymptomatic hips with cam morphology. A cohort of 67 patients/hips was studied. Hips were either asymptomatic with no cam (Controls, n = 18), symptomatic with cam (n = 26) or asymptomatic with cam (n = 23). CT-based quantitative assessments of femoral, acetabular, pelvic, and spino-pelvic parameters were performed. Measurements were compared between controls and those with a cam deformity, as well as between the three groups. Morphological parameters that were independent predictors of a symptomatic cam were determined using a regression analysis. Hips with cam deformity had slightly smaller subtended angles superior-anteriorly (87° vs. 84°, p = 0.04) and greater pelvic incidence (53° vs. 48°, p = 0.003) compared to controls. Symptomatic cams had greater acetabular version (p < 0.01), greater subtended angles superiorly and superior-posteriorly (p = 0.01), higher pelvic incidence (p = 0.02), greater alpha angles and lower femoral neck-shaft angles compared to asymptomatic cams (p < 0.01) and controls (p < 0.01). The four predictors of symptomatic cam included antero-superior alpha angle, femoral neck-shaft angle, acetabular depth, and pelvic incidence. In conclusion, this study illustrates that symptomatic hips had a greater amount of supero-posterior coverage; which would be the contact area between a radial cam and the acetabulum, when the hip is flexed to 90°. Furthermore, individuals with symptomatic cam morphology had greater PI. Acetabular- and SP parameters should be part of the radiological assessment of femoro-acetabular impingement. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1840-1848, 2018.

Surgical Correction of Cam Deformity in Association with Femoroacetabular Impingement and Its Impact on the Degenerative Process within the Hip Joint.

BACKGROUND: Cam morphology in association with femoroacetabular impingement (FAI) is a recognized cause of hip pain and cartilage damage and proposed as a leading cause of arthritis. The purpose of this study was to analyze the functional and biomechanical effects of the surgical correction of the cam deformity on the degenerative process associated with FAI. METHODS: Ten male patients with a mean age of 34.3 years (range, 23.1 to 46.5 years) and a mean body mass index (and standard deviation) of 26.66 ± 4.79 kg/m underwent corrective surgery for cam deformity in association with FAI. Each patient underwent a computed tomography (CT) scan to assess acetabular bone mineral density (BMD), high-resolution T1ρ magnetic resonance imaging (MRI) of the hips to assess proteoglycan content, and squatting motion analysis as well as completed self-administered functional questionnaires (Hip disability and Osteoarthritis Outcome Score [HOOS]) both preoperatively and 2 years postoperatively. RESULTS: At a mean follow-up of 24.5 months, improvements in functional scores and squat performance were seen. Regarding the zone of impingement in the anterosuperior quadrant of the acetabular rim, the mean change in BMD at the time of follow-up was -31.8 mg/cc (95% confidence interval [CI], -11 to -53 mg/cc) (p = 0.008), representing a 5% decrease in BMD. The anterosuperior quadrant also demonstrated a significant decrease in T1ρ values, reflecting a stabilization of the cartilage degeneration. Significant correlations were noted between changes in clinical functional scores and changes in T1ρ values (r = -0.86; p = 0.003) as well as between the BMD and maximum vertical force (r = 0.878; p = 0.021). CONCLUSIONS: Surgical correction of a cam deformity in patients with symptomatic FAI not only improved clinical function but was also associated with decreases in T1ρ values and BMD. These findings are the first, to our knowledge, to show that alteration of the hip biomechanics through surgical intervention improves the overall health of the hip joint. LEVEL OF EVIDENCE: Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.

Properties of the cartilage layer from the cam-type hip impingement deformity.

Femoro-acetabular impingement (FAI) is associated with significant acetabular cartilage damage and degenerative arthritis. To understand the contact stress and thus biomechanical mechanisms that may contribute to degeneration, the material behaviour of the cartilage layer is required. The objective of this study is to determine the fibril-reinforced poroelastic properties and composition of cartilage from cam deformities and to compare to those of normal cartilage. Patients undergoing surgical treatment of a symptomatic cam FAI deformity were recruited from the clinical practice of one of the authors. Osteochondral specimens were retrieved from the deformity during surgery using a trephine. Control specimens were retrieved from the anterior femoral head bearing surface during autopsy procedures. Indentation stress-relaxation tests were performed to determine the modulus (ES), Poisson's ratio (ν) and permeability (k0) of the poroelastic component, and the strain-independent (E0) and -dependent (Eε) moduli of the fibril-reinforcement using finite element analysis and optimization. Safranin-O staining was used to quantify proteoglycan content. ES and ν were 71% and 37% lower, respectively, in Cam specimens compared to controls, and k0 was approximately triple that of Control specimens (p<0.05). No significant differences were seen in the fibrillar components, E0 and Eε. Proteoglycan content was substantially depleted in Cam specimens, and was correlated with ES, ν and k0. This study showed that cartilage from the cam deformity exhibits severe degeneration in terms of the mechanical behaviour and composition changes, and is consistent with osteoarthritis. This further supports the hypothesis that FAI is a cause of hip osteoarthritis.

Influence of ingrowth regions on bone remodelling around a cementless hip resurfacing femoral implant.

Hip resurfacing arthroplasty is an alternative to traditional hip replacement that can conserve proximal bone stock and has gained popularity but bone resorption may limit implant survival and remains a clinical concern. The goal of this study was to analyze bone remodelling patterns around an uncemented resurfacing implant and the influence of ingrowth regions on resorption. A computed tomography-derived finite element model of a proximal femur with a virtually implanted resurfacing component was simulated under peak walking loads. Bone ingrowth was simulated by six interface conditions: fully bonded; fully friction; bonded cap with friction stem; a small bonded region at the stem-cup intersection with the remaining surface friction; fully frictional, except for a bonded band along the distal end of the cap and superior half of the cap bonded with the rest frictional. Interface condition had a large influence on remodelling patterns. Bone resorption was minimized when no ingrowth occurred at the bone-implant interface. Bonding only the superior half of the cap increased bone resorption slightly but allowed for a large ingrowth region to improve secondary stability.

Stress distribution and consolidation in cartilage constituents is influenced by cyclic loading and osteoarthritic degeneration.

The understanding of load support mechanisms in cartilage has evolved with computational models that better mimic the tissue ultrastructure. Fibril-reinforced poroelastic models can reproduce cartilage behaviour in a variety of test conditions and can be used to model tissue anisotropy as well as assess stress and pressure partitioning to the tissue constituents. The goal of this study was to examine the stress distribution in the fibrillar and non-fibrillar solid phase and pressure in the fluid phase of cartilage in axisymmetric models of a healthy and osteoarthritic hip joint. Material properties, based on values from the literature, were assigned to the fibrillar and poroelastic components of cartilage and cancellous and subchondral compact bone regions. A cyclic load representing walking was applied for 25 cycles. Contact stresses in the fibrillar and non-fibrillar solid phase supported less than 1% of the contact force and increased only minimally with load cycles. Simulated proteoglycan depletion increased stresses in the radial and tangential collagen fibrils, whereas fibrillation of the tangential fibrils resulted in increased compressive stress in the non-fibrillar component and tensile stress in the radial fibrils. However neither had an effect on fluid pressure. Subchondral sclerosis was found to have the largest effect, resulting in increased fluid pressure, non-fibrillar compressive stress, tangential fibril stress and greater cartilage consolidation. Subchondral bone stiffening may play an important role in the degenerative cascade and may adversely affect tissue repair and regeneration treatments.

Effect of boundary conditions, impact loading and hydraulic stiffening on femoral fracture strength.

Patient specific quantitative CT (QCT) imaging data together with the finite element (FE) method may provide an accurate prediction of a patient's femoral strength and fracture risk. Although numerous FE models investigating femoral fracture strength have been published, there is little consent on the effect of boundary conditions, dynamic loading and hydraulic strengthening due to intra-medullary pressure on the predicted fracture strength. We developed a QCT-derived FE model of a proximal femur that included node-specific modulus assigned based on the local bone density. The effect of three commonly used boundary conditions published in literature were investigated by comparing the resulting strain field due to an applied fracture load. The models were also augmented with viscoelastic material properties and subject to a realistic impact load profile to determine the effect of dynamic loads on the strain field. Finally, the effect of hydraulic strengthening was investigated by including node specific permeability and performing a coupled pore diffusion and stress analysis of the FE model. Results showed that all boundary conditions yield the same strain field patterns, but peak strains were 22% lower and fracture load was 18% higher when loaded at the greater trochanter than when loaded at the femoral head. Comparison of the dynamic models showed that material viscoelasticity was important, but inertial effects (vibration and shock) were not. Finally, pore pressure changes did not cause significant hydraulic strengthening of bone under fall impact loading.

Joint stabilisers or moment actuators: the role of knee joint muscles while weight-bearing.

Previous investigations have identified the roles of knee joint muscles in supporting external loads during non-weight-bearing tasks and found these to depend on moment arm orientation (MAO). However, during weight-bearing tasks ground reaction forces (GRF) are transferred up through the knee, subjecting it to large multi-directional forces and stability is dependent on articular geometry, loading, and muscle activation. The purpose of this study was to investigate activation strategies used by healthy individuals to generate and support highly controlled GRF during weight-bearing. Twenty healthy males (23.9±1.9 yrs) stood with their foot in a boot fixed to a force platform. Subjects controlled an onscreen cursor by modulating normalised GRF and were required to produce 30% of their maximal force in 12 directions of the horizontal plane while maintaining 50% body weight on the test leg. Lower limb electromyography, kinematics and kinetics were recorded for each trial. Mean muscle activation was plotted in polar coordinates based on GRF orientation. Muscle activation symmetry was determined and when applicable, the mean direction of activation and muscle specificity index reported. The measured GRF were comparable to activities of daily living (0.48-0.58±0.17-0.19 N/kg in horizontal plane). Muscle activations were repeatable (ICCs: 0.78-0.98), however, only semitendinosus (ST) activation was indicated by its MAO. Considering the joint moments and activations patterns we therefore classified muscles as: (1) general joint stabilisers (vastus lateralis and medialis), (2) specific joint stabiliser (BF), and (3) moment actuators (ST and rectus femoris). General joint stabilisers were active in all load directions; specific stabilisers were active in directions opposite their MAO; moment actuators had higher specificities and activations corresponding to their MAO. We suggest the stabiliser muscles create a rigid mechanical linkage at the knee which allows the actuators of the hip and knee to modulate GRF.

Reliability of knee joint muscle activity during weight bearing force control.

We developed a novel approach that requires subjects to produce and finely tune ground reaction forces (GRFs) while standing. The aim of this study was to examine the reliability of electromyographic data recorded during these tasks. Healthy young adults stood with their dominant leg in a boot fixed to a force platform. A target matching protocol required subjects to control both the direction and magnitude of GRF along the horizontal plane while maintaining constant inferior-superior loads of 50% body-weight (BW). Each target matching task was repeated three times in a random order. Subjects were retested with the same protocol 2-3 days later. Normalised electromyography data of eight muscles crossing the knee joint was collected for each successful target match. A random model, single measures intra-class correlation analysed the reliability for both test-retest and intra-day results, in addition to inter-subject reliability. The GRFs required to meet the targets were comparable to a range of activities of daily living, ranging from 0.48 to 0.58 N/kg of BW in the horizontal plane while maintaining 50% BW in the vertical plane. We observed moderate to high ICC values (0.60-0.993) for most muscles in most directions, indicating low within-subject variance. In addition, moderate to high between-subject reliability was observed in all eight muscle activation profiles, indicating subjects used similar neuromuscular control strategies to achieve the desired GRFs. In conclusion, our protocol identifies non-random weight-bearing motor control strategies while generating direction dependent GRFs. These results provide reliable insight into knee joint stabilisation strategies during weight bearing.

The accuracy of the use of functional hip motions on localization of the center of the hip.

BACKGROUND: The hip joint is generally considered a ball-and-socket joint, the center of which is used as an anatomic landmark in functional analyses and by surgical navigation systems. The location of the hip center has been estimated using functional techniques using various limb motions. However, it is not clear which specific motions best predicted the functional center. PURPOSE: This study aims to compare the predicted functional center of the hip evaluated from multiplanar circumduction and star motions, and to compare this functional center with the geometric center. METHODS: Eight hips in four fresh-frozen cadavers were used and verified as morphologically normal in CT scans. Three-dimensional motion of each lower limb was recorded using arrays of reflective markers rigidly attached to the femur and pelvis. Each hip was manipulated to produce circumduction or star motion, i.e., abduction-adduction and flexion extension. The hip was then dissected and the bearing surface traced with a probe, from which a best-fit sphere was calculated. The functional center was calculated from the motion data and compared to the geometric technique. RESULTS: There was no difference between the functional hip center predicted by circumduction or star motions, although this was offset from the geometric hip center by up to 14 mm. For all except two hips, the functional center was less than 6 mm from the geometric hip in each anatomic direction. Test-retest differences were smaller for circumduction than for star motions. CONCLUSIONS: Estimation of the hip center based on motion of the femur relative to the pelvis could localize the geometric center of the joint within 14 mm and circumduction motions were more repeatable. CLINICAL RELEVANCE: Many surgical navigation systems make use of the functional hip center as a landmark for alignment or reconstruction. Errors associated with this would have a very minor influence in lower limb alignment, e.g., for knee reconstruction, but could affect proximal femoral geometry relevant to hip reconstruction.

Computer-assisted correction of cam-type femoroacetabular impingement: a Sawbones study.

BACKGROUND: Assessing the adequacy of bone resection when correcting cam-type femoroacetabular impingement can be difficult when the surgeon is inexperienced or when less-invasive arthroscopic surgical techniques are used. The primary purpose of the present study was to compare, using a Sawbones model, the results of computer-assisted navigated osteochondroplasty of the femoral neck junction with correction with use of femoral head spherometer gauges. The second objective was to compare the results of computer-assisted osteochondroplasty performed by surgeons who had varied experience with the procedure. METHODS: We calculated and compared the post-resection alpha angle in custom-molded Sawbones models with cam-type impingement following both surgical techniques, performed by three surgeons with varied experience with the procedure. The alpha angle was measured at two positions (the three o'clock and one-thirty positions of the femoral head-neck junction) before and after resection. RESULTS: At the three o'clock position, there were no significant differences between the computer-navigation and spherometer groups (p = 0.83). There was undercorrection at the one-thirty position, with the median alpha angle being greater in the navigation group as compared with the spherometer group (71.0 compared with 58.6; p = 0.05). In the navigation group, there were no significant differences in the post-resection mean alpha angle among the three surgeons at either the one-thirty plane or the three o'clock plane. CONCLUSIONS: Navigation enabled the inexperienced surgeon to perform an equivalent amount of bone resection as the more experienced surgeons. However, all surgeons did not sufficiently resect the cam deformity as compared with the gold-standard open technique at the one-thirty position.

Influence of changes in stem positioning on femoral loading after THR using a short-stemmed hip implant.

BACKGROUND: Short-stemmed hip implants were introduced to conserve proximal bone mass and may facilitate the use of minimally invasive surgery, in which smaller incisions limit access to the joint. This limited access may increase the risk of surgical mal-positioning of the implant, however the sensitivity of femoral loading to such mal-positioning of a short-stemmed implant has not been studied. METHODS: Finite element models were developed of a femur and a short-stemmed implant positioned to reproduce the intact hip centre, as well as with the implant placed in increased anteversion or offset. The effect of these surgical variables on femoral loading was examined for walking and stair climbing using loads from a validated musculoskeletal model. Results of the implanted models were compared with an intact model to evaluate stress shielding. FINDINGS: Implant position had little influence on cortical strains along the length of the diaphysis, although strains decreased by up to 95% at the neck resection level compared to the intact femur. In the proximal Gruen zones I and VII strain energy density among the implanted models varied by up to 0.4 kJ/m(3) (28%) and 0.6 kJ/m(3) (24%) under walking and stair climbing, respectively. All implanted models showed characteristic proximal stress shielding, indicated by a decrease in strain energy density of up to 5.4 kJ/m(3) (69%) compared to the intact femur. INTERPRETATION: Small changes in stem placement would likely have little influence on the internal loading of the femur after bone ingrowth has been achieved, however a reduction in strain energy density and therefore stress shielding was seen even for a short-stemmed implant, which may have consequences for longer-term bone remodelling.

Physiologically based boundary conditions in finite element modelling.

Finite element analysis has been used extensively in the study of bone loading and implant performance, such as in the femur. The boundary conditions applied vary widely, generally producing excessive femoral deformation, and although it has been shown that the muscle forces influence femoral deflections and loading, little consideration has been given to the displacement constraints. It is hypothesised that careful application of physiologically based constraints can produce physiological deformation, and therefore straining, of the femur. Joint contact forces and a complete set of muscle forces were calculated based on the geometry of the Standardised Femur using previously validated musculoskeletal models. Five boundary condition cases were applied to a finite element model of the Standardised Femur: (A) diaphyseally constrained with hip contact and abductor forces; (B) case A plus vasti forces; (C) case A with complete set of muscle forces; (D) distally constrained with all muscle forces; (E) physiological constraints with all muscle forces. It was seen that only the physiological boundary conditions, case E, produced physiological deflections (< 2.0mm) of the femoral head in both the coronal and sagittal planes, which resulted in minimal reaction forces at the constrained nodes. Strains in the mid-diaphysis varied by up to 600 micro-strain under walking loads and 1000 micro-strain under stair climbing loads. The mode of loading, as indicated by the strain profiles on the cortex also varied substantially under these boundary conditions, which has important consequences for studies that examine localised bone loading such as fracture or bone remodelling simulations.

Calcium phosphate cement composites in revision hip arthroplasty.

Loosening of the femoral component in a total hip arthroplasty with concomitant bone loss can pose a problem for revision surgery due to inadequate structure in the remaining femur. While impaction allografting has shown promise, it has also shown serious complications, especially with moderate to severe bone loss. It may be possible to stabilize the graft layer with a bioresorbable cement to improve clinical results. This study examines the mechanical properties of a potential morsellized bone-bioresorbable composite. Morsellized bone was mixed with a commercially available bioresorbable cement (alpha-BSM, Etex Corp.) in compositions of 0%, 25%, 50% and 75% bone. Unconfined compression and diametral tensile and confined compression tests were performed to determine the composite mechanical properties. The composition containing 50% bone tended to exhibit the highest uniaxial strengths, as well as the highest confined compression modulus. The uniaxial compressive strength and stiffness of this composition was in the range of cancellous bone. Uniaxial compressive modulus decreased with increasing bone fraction whereas elongation exhibited the opposite trend. Bone fraction had a significant effect on compressive strength (p < 0.0001), compressive modulus (p < 0.0001), elongation (p < 0.01), tensile strength (p < 0.0001) and confined compressive modulus (p = 0.04). The addition of a bioresorbable cement to the allograft layer may improve the properties of the layer, preventing early subsidence seen in some clinical studies of impaction allografting, and therefore improving the clinical results. Further testing is required to evaluate the in vitro mechanical performance, as well as in vivo remodelling characteristics.