A novel low-profile external skeletal fixator for type IIIB open tibial fractures: A biomechanical and clinical pilot study.

BACKGROUND: Although external fixator is standard for managing staged treatment of open tibial fracture, the main disadvantage of this device is too bulky to be tolerated by most patients for longtime use. The purposes of this pilot study were to compare the biomechanical properties of a novel low-profile external fixator (LP-ESF) with a traditional ESF and also to evaluate its performance in patients with Gustilo type IIIb tibial open fractures. METHODS: A prospective clinical pilot study started from January 2015 to December 2017, and 18 patients with Gustilo type IIIb open tibial fractures underwent the fixation with a novel LP-ESF system. The biomechanical properties of the LP-ESF were compared with the Synthes External Fixation System according to the standard ASTM F1541-02. These patients were divided into two groups according to the size of bony defect. The postoperative clinical outcomes were subsequently collected. RESULTS: The biomechanical properties of the LP-ESF were comparable with those of Synthes External Fixation System and had an improved the axial/torsional stiffness and ultimate strength. In the clinical study, all patients with LP-ESF had fracture union. The duration of application of LP-ESF was 3.5 to 18 months until fracture union. In 10 of 18 patients, their fractures were immobilized with the LP-ESF until bone union, and no pin tract infection and no chronic osteomyelitis were recorded. The 36-Item Short Form Health Survey life quality and health survey were good to excellent in these patients. Notably, the LP-ESF allowed a patient with severe bone and soft-tissue defects to preserve the leg and joints function. CONCLUSION: In this study, we found that the novel LP-ESFs had improved clinical outcomes. The long-term LP-ESF application seems to be tolerable in our patients. This novel approach permits better controls in deep infection and faster healing of fractures, and thus may provide a viable alternative treatment for Gustilo type IIIb open tibial fractures.

Influence of gap distance between bone and plate on structural stiffness and parallel interfragmental movement in far-cortical locking technique - a biomechanical study.

Sufficient interfragmental movement is the key to successful fracture healing in the theory of secondary bone healing. The far-cortical locking technique enables both stiffness reduction and parallel motion for ideal callus formation and fracture healing, but the influence of plate-bone gap on the performance of far-cortical locking technique remains unclear. The current study conducted a series of finite element analyses with mechanical validation to clarify this issue. Plate-bone gaps were assigned by 1, 2, 3, and 4 mm in a simulated mid-shaft fracture model fixed with locking plate and six semi-rigid locking screws. Axial compressive load to 500 N was applied to the fixation structure to evaluate the structural stiffness, pattern of interfragmental movement (parallel motion), and stresses on the screws. Results revealed the increased plate-bone gaps reduced the structural in order (315.3, 288.8, 264.9, and 243.4 N/mm). Tilting angles for determining parallel interfragmental movement (1.58°-1.85°) and stresses on semi-rigid screws for evaluating implant safety were not severely altered. Greater stresses were found on the screws adjacent to the fracture site in all simulated models. The current study suggested that 1 mm gap between the locking plate and the bone shall be ideal in view of parallel motion achieved balanced callus formation in far-cortical locking technique. Issue of reducing structural stiffness with limited plate-bone gap distance should be further investigated.

Gender differences in femoral trochlea morphology.

PURPOSE: This study aimed to analyze the morphology of the anterior femoral condyle using a quantitative three-dimensional reconstruction method. The morphological data were compared between genders. METHODS: Computed tomography scans of femurs were taken from 90 healthy subjects and then reconstructed in 3D modeling software. Coaxial cutting planes were created at 10° increments to measure the lateral and medial anterior condylar heights (LACH and MACH, respectively), lateral and medial trochlear groove widths (LTW and MTW, respectively), and for trochlear groove tracking. The absolute values and normalized data were compared between male and female subjects. The sulcus angle and deepest point of the trochlear groove at each cross-section were also analyzed to determine the differences in the depth of the trochlear groove. RESULTS: The absolute dimensions of LACH, MACH, LTW, and MTW were significantly smaller in the female subjects, by 10.5%, 36.9%, 10.3%, and 11.0%, respectively, than in the males (p < 0.05). After normalization, no significant difference was found in the condylar height between the genders. However, the female subjects had a significantly larger value of approximately 7.9% for the normalized trochlear width. CONCLUSION: Male subjects had greater condylar heights and widths than the female subjects. Although the trajectory of the trochlear groove varied greatly among the subjects, the trochlear groove appeared to be wider and shallower in the female subjects than in the male subjects. These results provide important information for the design of femoral trochlea to fit Asian female patients. LEVEL OF EVIDENCE: III.

Importance of a moderate plate-to-bone distance for the functioning of the far cortical locking system.

The far cortical locking (FCL) system, a novel bridge-plating technique, aims to deliver controlled and symmetric interfragmentary motion for a potential uniform callus distribution. However, clinical data for the practical use of this system are limited. The current study investigated the biomechanical effect of a locking plate/far cortical locking construct on a simulated comminuted diaphyseal fracture of the synthetic bones at different distance between the plate and the bone. Biomechanical in vitro experiments were performed using composite sawbones as bone models. A 10-mm osteotomy gap was created and bridged with FCL constructs to determine the construct stiffness, strength, and interfragmentary movement under axial compression, which comprised one of three methods: locking plates applied flush to bone, at 2 mm, or at 4 mm from the bone. The plate applied flush to the bone exhibited higher stiffness than those at 2 mm and 4 mm plate elevation. A homogeneous interfragmentary motion at the near and far cortices was observed for the plate at 2 mm, whereas a relatively large movement was observed at the far cortex for the plate applied at 4 mm. A plate-to-bone distance of 2 mm had the advantages of reducing axial stiffness and providing nearly parallel interfragmentary motion. The plate flush to the bone prohibits the dynamic function of the far cortical locking mechanism, and the 4-mm offset was too unstable for fracture healing.

Partially threaded headless screw may benefit adequate interfragmentary compression and reduced driving torque for small bone fixation.

Headless compression screws (HCSs) are commonly used to fixate small bones and articular fractures. Understanding the biomechanical efficacy of different HCS designs can help surgeons make proper interfragmentary compression when a specific implant is chosen. HCSs with three different central shaft designs (unthreaded, fully threaded, and partially threaded) were studied: the Herbert-Whipple, Mini-Acutrak 2, and headless reduction (HLR). Polyurethane foam blocks were machined with a simulated fracture gap of 0.5 mm and set onto a custom-made jig to simultaneously measure compression force and driving torque during screw insertion. The maximal achievable compression forces and driving torques recorded were 47.4 ± 0.9 N and 145.11 ± 1.65 N mm for the HLR, 50.98 ± 1.29 N and 152.62 ± 2.83 N mm for the Mini-Acutrak 2, and 19.33 ± 1.0 N and 33.4 ± 2.2 N mm for the Herbert-Whipple. Overall, the compression force of the Mini-Acutrak 2 and HLR increased with the torque. Unlike the other screws, the Herbert-Whipple's driving torque increased while the compression force decreased after peak compression force was achieved. The partially threaded shaft design (HLR) demonstrated equivalent biomechanical advantage with the Mini-Acutrak 2 in interfragmentary compression. The HCSs with cone-shaped proximal ends (HLR and Mini-Acutrak 2) maintained their compression force during over-fastening, whereas the unthreaded central shaft of the Herbert-Whipple screw caused it to lose compression force.

Morphometric Analysis of the Clavicles in Chinese Population.

The clavicle has a complex geometry that makes plate fixation technically difficult. The current study aims to measure the anatomical parameters of Chinese clavicles as reference for plate design. One hundred clavicles were analyzed. The clavicle bone model was reconstructed by using computed tomography images. The length, diameters, and curvatures of the clavicle were then measured. The female clavicle was shorter, more slender, and less curved in lateral part than the male clavicle. There was a positive relationship between height and clavicle parameters except lateral curve and depth. The measurements of Chinese clavicles were generally smaller than Caucasians. The clavicle curves were correlated with the bone length; thus consideration of the curve variations may be necessary as designing size distribution of clavicle plate.

Larger screw diameter may not guarantee greater pullout strength for headless screws - a biomechanical study.

Headless compression screws (HCSs) are commonly utilized devices for small bone fracture fixation. The Mini-Acutrak 2 and headless reduction (HLR) screws are the newer version types, in which both have fully threaded and variable pitch design. Specifically, the HLR is characterized by two thread runouts to facilitate implantation. With the thread runouts, the holding strength of the screw may be compromised. To the best of our knowledge, no study has examined the pullout force of the global sizes of a HCS. We sought to determine the pullout strength of the HLR and compare the strength of this screw with that of the Mini-Acutrak 2. Synthetic bone blocks with simulated transverse fractures were used to conduct the tests. Four commonly used sizes of the HLR were examined, and one Mini-Acutrak 2 was employed for comparison. Five screws of each size were tested. The pullout force of all screws that were tested in this study ranged from 45.23 to 233.22 N. The results revealed that the pullout force increased as the screw diameter increased. Interestingly, we found that one small screw outperformed the Mini-Acutrak 2, which has a larger diameter. This study provided extensive knowledge regarding the pullout strength of fully threaded HCSs of different sizes. An unexpected finding is that a small screw has higher holding power than a large one because of its increased number of threads. Therefore, we suggest that the thread number should be a critical consideration for the design of size distribution of HCSs.

Conceptual finite element study for comparison among superior, anterior, and spiral clavicle plate fixations for midshaft clavicle fracture.

Open reduction internal fixation technique has been generally accepted for treatment of midshaft clavicle fractures. Both superior and anterior clavicle plates have been reported in clinical or biomechanical researches, while presently the spiral clavicle plate design has been introduced improved biomechanical behavior over conventional designs. In order to objectively realize the multi-directional biomechanical performances among the three geometries for clavicle plate designs, a current conceptual finite element study has been conducted with identical cross-sectional features for clavicle plates. The conceptual superior, anterior, and spiral clavicle plate models were constructed for virtual reduction and fixation to an OTA 15-B1.3 midshaft transverse fracture of clavicle. Mechanical load cases including cantilever bending, axial compression, inferior bending, and axial torsion have been applied for confirming the multi-directional structural stability and implant safety in biomechanical perspective. Results revealed that the anterior clavicle plate model represented lowest plate stress under all loading cases. The superior clavicle plate model showed greater axial compressive stiffness, while the anterior clavicle plate model performed greater rigidity under cantilever bending load. Three model represented similar structural stiffness under axial torsion. Played as a transition structure between superior and anterior clavicle plate, the spiral clavicle plate model revealed comparable results with acceptable multi-directional biomechanical behavior. The concept of spiral clavicle plate design is worth considering in practical application in clinics. Implant safety should be further investigated by evidences in future mechanical tests and clinical observations.

Biomechanical effect of the configuration of screw hole style on locking plate fixation in proximal humerus fracture with a simulated gap: A finite element analysis.

BACKGROUND: Locking plate fixation for proximal humeral fractures is a commonly used device. Recently, plate breakages were continuously reported that the implants all have a mixture of holes allowing placement of both locking and non-locking screws (so-called combi plates). In commercialized proximal humeral plates, there still are two screw hole styles included "locking and dynamic holes separated" and "locking hole only" configurations. It is important to understand the biomechanical effect of different screw hole style on the stress distribution in bone plate. METHODS: Finite element method was employed to conduct a computational investigation. Three proximal humeral plate models with different screw hole configurations were reconstructed depended upon an identical commercialized implant. A three-dimensional model of a humerus was created using process of thresholding based on the grayscale values of the CT scanning of an intact humerus. A "virtual" subcapital osteotomy was performed. Simulations were performed under an increasing axial load. The von Mises stresses around the screw holes of the plate shaft, the construct stiffness and the directional displacement within the fracture gap were calculated for comparison. RESULTS: The mean value of the peak von Mises stresses around the screw holes in the plate shaft was the highest for combi hole design while it was smallest for the locking and dynamic holes separated design. The stiffness of the plate-bone construct was 15% higher in the locking screw only design (132.6N/mm) compared with the combi design (115.0N/mm), and it was 4% higher than the combi design for the locking and dynamic holes separated design (119.5N/mm). The displacement within the fracture gap was greatest in the combi hole design, whereas it was smallest for the locking hole only design. CONCLUSIONS: The computed results provide a possible explanation for the breakages of combi plates revealed in clinical reports. The locking and dynamic holes separated design may be a better configuration to reduce the risk of plate fracture.

Biomechanical comparison of conventional and anatomical calcaneal plates for the treatment of intraarticular calcaneal fractures - a finite element study.

Initial stability is essential for open reduction internal fixation of intraarticular calcaneal fractures. Geometrical feature of a calcaneal plate is influential to its endurance under physiological load. It is unclear if conventional and pre-contoured anatomical calcaneal plates may exhibit differently in biomechanical perspective. A Sanders' Type II-B intraarticular calcaneal fracture model was reconstructed to evaluate the effectiveness of calcaneal plates using finite element methods. Incremental vertical joint loads up to 450 N were exerted on the subtalar joint to evaluate the stability and safety of the calcaneal plates and bony structure. Results revealed that the anatomical calcaneal plate model had greater average structural stiffness (585.7 N/mm) and lower von Mises stress on the plate (774.5 MPa) compared to those observed in the conventional calcaneal plate model (stiffness: 430.9 N/mm; stress on plate: 867.1 MPa). Although both maximal compressive and maximal tensile stress and strain were lower in the anatomical calcaneal plate group, greater loads on fixation screws were found (average 172.7 MPa compared to 82.18 MPa in the conventional calcaneal plate). It was noted that high magnitude stress concentrations would occur where the bone plate bridges the fracture line on the lateral side of the calcaneus bone. Sufficient fixation strength at the posterolateral calcaneus bone is important for maintaining subtalar joint load after reduction and fixation of a Sanders' Type II-B calcaneal fracture. In addition, geometrical design of a calcaneal plate should worth considering for the mechanical safety in practical usage.

Biomechanical effect of different femoral neck blade position on the fixation of intertrochanteric fracture: a finite element analysis.

Medial migration or cutout of the neck helical blade has commonly occurred in the treatment of trochanteric fracture of the femur. The position of the helical blade within the femoral head is one of the influencing factors that cause the blade to perforate the intact joint surface; however, the ideal placement of the helical blade is not currently known. A finite element model of a femur/nail construct was utilized to analyze five possible blade positions in the femoral head. Normal strain at the fracture surface, the minimum principal strain in the cancellous bone, and the von Mises stress in the implant itself were calculated and compared between different blade positions. The results showed that a large area of normal compressive strain at the fracture surface was observed in the inferior and posterior blade positions. The volume of cancellous bone strained to yielding in the femoral head and neck was lower for the inferior and posterior positions, whereas it was the highest for the superior position. The inferior and posterior positions had lower von Mises stress in the implant itself. The inferior and posterior positions may be the ideal position for the intramedullary nail with a helical neck blade.

Change in collateral ligament length and tibiofemoral movement following joint line variation in TKA.

PURPOSE: The primary intent of total knee arthroplasty is the restoration of normal knee kinematics, with ligamentous constraint being a key influential factor. Displacement of the joint line may lead to alterations in ligament attachment sites relative to knee flexion axis and variance of ligamentous constraints on tibiofemoral movement. This study aimed to investigate collaterals strains and tibiofemoral kinematics with different joint line levels. METHODS: A previously validated knee model was employed to analyse the change in length of the collateral ligaments and tibiofemoral motion during knee flexion. The models shifted the joint line by 3 and 5 mm both proximally and distally from the anatomical level. The data were captured from full extension to flexion 135°. RESULTS: The elevated joint line revealed a relative increase in distance between ligament attachments for both collateral ligaments in comparison with the anatomical model. Also, tibiofemoral movement decreased with an elevation in the joint line. Conversely, lowering the joint line led to a significant decrease in distance between ligament attachments, but greater tibiofemoral motion. CONCLUSION: Elevation of the joint line would strengthen the capacity of collateral ligaments for knee motion constraint, whereas a distally shifted joint line might have the advantage of improving tibiofemoral movement by slackening the collaterals. It implies that surgeons can appropriately change the joint line position in accordance with patient's requirement or collateral tensions. A lowered joint line level may improve knee kinematics, whereas joint line elevation could be useful to maintain knee stability. LEVEL OF EVIDENCE: V.

Proximal femoral morphology and the relevance to design of anatomically precontoured plates: a study of the Chinese population.

Adequately shaped femoral plate is critical for the fixation of fracture in the pertrochanteric regions. Lateral aspect of greater trochanter is an important region where the proximal femoral plate anchored. However, little is known regarding the morphology of greater trochanter. The objective of this study was to measure main dimensions of greater trochanter and other regions in the proximal end of the femur to provide an anatomical basis for the design of the proximal femoral plate. Anthropometric data on the proximal femur were performed utilizing three-dimensional computational modeling. Computed tomography images of healthy femurs in 53 women and 47 men were contributed to three-dimensional femur modeling. All data were compared between male and female femora. The results showed that mean values for male group were found to be greater in most of measured femoral dimensions. Oppositely, females demonstrated higher neck-shaft angle on anteroposterior view and femoral anteversion angle. The anthropometric data can be used for the anatomical shape design of femoral plates for osteosynthesis of fractures in the trochanteric regions. A distinct plate design may be necessary to accommodate differences between the genders.

Biomechanical comparisons between a new avascular necrosis of femaral head stem based on Chinese patients with avascular necrosis and two other designs.

BACKGROUND: There is a relatively high failure rate of the femoral component in patients with avascular necrosis at the intermediate-term follow-up. Improving the geometrical fit of the femoral stem against the medullary canal may help to provide long-term survivorship of the hip replacement for patients with avascular necrosis. METHODS: We designed a specific stem, based on morphometric studies of proximal femoral canals in Chinese avascular necrosis patients and evaluated the stem by finite element analyses, comparing the novel stem with two commercially available and commonly used stems. RESULTS: The morphometric data from avascular necrosis patients showed specific geometric differences in the proximal femoral canal, including profile curves in both the sagittal and coronary planes than the patients with femoral neck fracture. The shorter stemmed prostheses (Fitmore(®) and our stem) performed better than the longer stemmed prosthesis (VerSys(®)). CONCLUSIONS: This is the first study to investigate the femoral geometries of Chinese avascular necrosis patients. Our stem provides better stability and is theoretically beneficial to bone ingrowth, which may increase the long-term stability and fixation of the implant.

The effects of different articulate curvature of artificial disc on loading distribution.

PURPOSE: Deeper insights into the mechanical behavior of lumbar disc prostheses are required. Prior studies on the biomechanical performance of artificial discs were mostly performed with finite element analyses, but this has never been analyzed with altering articulate curvature. This study aimed to ascertain the influence of the geometry of a ball-and-socket disc prosthesis for the lumbar spine. MATERIALS AND METHODS: Three-dimensional finite element model of human L4-L5 was reconstructed. Convex, concave, and elliptic artificial disc models were also established with Computer-Aided-Design software. Simulations included: (1) three articulate types of polyethylene (PE) insert were implanted inferiorly and (2) concave and convex PE inserts were implanted on the superior or inferior sides in flexion/extension, lateral bending, and axial rotation in the lumbar spine. Shear stresses and von Mises stresses on PE insert were assessed for their loading distributions. RESULTS: High shear stresses of all articulate types occurred in flexion, and convex PE insert performed the maximum stress of 23.81 MPa. Under all conditions, stresses on concave PE inserts were distributed more evenly and lower than those on the convex type. Elliptic geometry enabled confining the rotation of the motion unit. The shear force on the convex PE insert on the inferior side could induce transverse crack because the shear stress exceeded yielding shear stress. CONCLUSIONS: The concave PE insert on the inferior side not only decreased loading concentration but had relatively low stress. Such a design may be applicable for artificial discs.

Design and test of hip stem for medullary revascularization.

This study aims at developing a femoral stem which maintains appropriate mechanical properties for clinical use and provides enough medullary space for revascularization. Hollow and drilled stems were designed to gain sufficient medullary space. Three-dimensional finite element models of the hollow stems were built including the hollow parameters of different wall thicknesses, bore depths and transverse hole patterns. The effects of these parameters on stress under load bearing were investigated. The results indicated that the effects of thickness on the stem depended on their location along the stem. The hollow depth should be designed as deep as possible to ensure a smooth change of cross-sectional area and avoid unexpected stress concentrations. The transverse hole pattern must be arranged very carefully to avoid high stress concentrations. Then the proper parameters of hollow stem were determined. Through the above processes, the appropriate shape of femoral stem for medullary revascularization was determined as a hollow structure with gradually changing thickness and a distinct hole pattern. Fatigue tests were conducted on three prototypes, which all three prototypes passed. The finite element models and design processes were therefore proved useful for our purpose.

Comparison of cage application modality in posterior lumbar interbody fusion with posterior instrumentation--a finite element study.

BACKGROUND: Posterior lumbar interbody fusion (PLIF) cages have achieved satisfactory fusion results, but cases of cage migration or mechanical failure were repeatedly reported. Therefore, we conducted a finite element study to determine the impact of cage applications including number, positioning, and adding posterior instrumentation (PI). METHODS: A finite element model of the lumbar spine was generated from Computer Tomography scans and was validated. Partial laminectomy, diskectomy with partial facetectomy, and single or double-cage insertion were simulated onto the model, with or without posterior pedicle screws instrumentation. The maximal von Mises stresses between different loading conditions were compared between modalities to evaluate the risks of migration or mechanical failure. RESULTS: Posterior instrumentation decreases at least half the distortion stress of cage-endplate interface and facet joints, and diminishes the differences between the stresses of inserting one or two cages. Based on one cage insertion, adding posterior instrumentation provides more efficient stability than an additional cage. Furthermore, an obliquely inserted cage with posterior instrumentation produced lower stress than a cage inserted on one side due to better structural symmetry. Conclusively, one oblique anterior cage and bilateral posterior pedicle screws reconstructed the tripod system as the intact disc and facet joints, and provided similar stability as two cages did.

Incidence of hip replacement among national health insurance enrollees in Taiwan.

BACKGROUND: There is no national joint replacement registry in the country of Asia and reports of national outcomes of joint replacement in Asia as yet. Therefore, this study was then to report a national data of the number of hip replacements, incidence rate, demographic characteristics of hip replacement patients, and short-time survival rate after hip replacement of Han Chinese in Taiwan. METHODS: We analyzed 105,688 cases of hip replacements (including primary partial hip replacement, primary total hip replacement and revision of hip replacement) from National Health Insurance research database between 1996 and 2004. The survival rate of primary hip replacement was estimated for each disease by the Kaplan-Meier method. RESULTS: Average annual number of primary partial hip replacement and primary total hip replacement were 4,257 and 6,206 cases, respectively. The most two common diagnosis of primary partial hip replacement were femoral neck fractures (73.6%, 34% men, mean age 76 years) and avascular necrosis (18.0%, 84% men, mean age 48 years). In primary total hip replacements, the most two common diagnosis were avascular necrosis (46.9%, 79% men, mean age 50 years) and osteoarthrosis (41.6%, 43% men, mean age 60 years). Both the number of primary partial hip replacements and primary total hip replacements increased steadily between 1996 and 2004. The cumulative survival of primary partial hip replacements and primary total hip replacements in all patients were 93.97% and 79.47% in 9 years follow-up, respectively. CONCLUSION: Avascular necrosis is the main disease in total hip replacement in Taiwan. The epidemiology of hip diseases was different between Han Chinese (in Taiwan) and Caucasian and the number of hip replacements increased substantially in Taiwan between 1996 and 2004.

Difference in femoral head and neck material properties between osteoarthritis and osteoporosis.

BACKGROUND: Osteoarthritis and osteoporosis are the two most common musculoskeletal diseases found in the aged population. It is of interest to measure and study the material properties of the femoral head and neck of these two groups, and hopefully to offer explanation of the observed phenomenon that most patients suffer from one of the two disorders, not both. METHODS: Seven osteoarthritic and seven osteoporotic femoral heads were used for this study. The principal compressive region of the femoral heads were cut to determine the Young's modulus and yielding stress by a material testing machine. Comparisons between these two groups were conducted by using material properties and the properties normalized by individual patient physical parameters, including body weight, body height and femoral head diameter, respectively. The finite element model of femoral neck cuboid in OA and OP were obtained based on the micro-CT-scan cross-section. The intrinsic material properties were calculated from the solid FE models. FINDINGS: The results showed significant differences in density, modulus and strength between the osteoarthritic and osteoporotic femoral heads as measured, with the former having 2-3 times the values of the latter. Femoral head diameter has stronger influence in mechanical properties than patient's body weight and body height. Regarding to bone volume (BV), bone surface (BS), bone volume fraction (BV/TV), trabecular thickness (Tb.Th), trabecular number (Tb.N), and true trabecular elastic modulus, the intrinsic material properties of femoral neck with OA were higher than OP. INTERPRETATION: It is still unknown why patients do not suffer from both osteoporosis and osteoarthritis at the same time. Many studies aimed to investigate the mechanical property of two groups. However, individual difference of the femoral head and neck is too difficult to obtain a reasonable comparison between these two groups. This study investigated the two groups more quantitatively and further estimated the factors which influence mechanical properties from a biomechanical point of view.

Effects of varying material properties on the load deformation characteristics of heel cushions.

Various insole materials were used in attenuation of heel-strike impact. This study presented a compression test to investigate the deformation characteristics of common heel cushions. There were two materials (thermoplastic elastomer "TPE" and silicone) with three hardness and six thickness being analyzed. They underwent consecutive loading-unloading cycles with a load control mode. The displacement of material thickness was recorded during cyclic compression being applied and released from 0 to 1050 N. The energy input, return and dissipation were evaluated based on the load deformation curves when new and after repeated compression. The TPE recovered more deformed energy and thickness than the silicone after the first loading cycle. The silicone would preserve more strain energy with increasing its hardness for the elastic recovery in the unloading process. The deformed energy was decreased as the original thickness did not completely recover under cyclic tests. The reduction in hysteresis area was gradually converged within 20 cycles. The silicone attenuated more impact energy in the initial cycles, but its energy dissipation was reduced after repeated loading. To increase hardness or thickness should be considered to improve resilience or accommodate persistent compression without flattening. The careful selection of cushion materials is imperative to meet individual functional demands.