Simulated Single-leg Stance Research Articles

Footfall Deflection of Antifatigue Flooring During Simulated Human Stance

This study quantified the effect of compression load and duration on the deflection of five separate antifatigue flooring surfaces. Following standardized measurement of A Shore hardness, each sample underwent simulated single-leg stance indentation procedures that differed by compression load (45.3 kg, 90.7 kg, 136.1 kg) and duration (initial = 2.5 s, intermediate = 6.25 s, final = 12.5 s). Vertical deflection was compared across conditions, and the relationship between A Shore hardness and deflection was characterized. When compressed with 45.3 kg, deflection was not influenced by duration, but at 136 kg, deflection differed between durations by up to 15%. The relationship between A Shore hardness and deflection was characterized by a third-order polynomial function ( R2 > 0.991).

Mechanical stability of custom and anatomical femoral stems: an experimental study in human femora.

In this study we have compared the mechanical stability of custom (n=8) and anatomical (n=8) uncemented femoral components, following insertion into human cadaveric femurs, during simulated single leg stance and stair climbing. In the custom group two specimens were excluded from the study due to detachment of the greater trochanter during cyclical loading. As a consequence of their mechanical behaviour both types of stems could be divided into subgroups of "unstable" and "stable" implants. In the course of one thousand loading cycles three anatomical stems and one custom stem migrated more than 1 mm, which was interpreted as mechanical loosening. This difference in rate of mechanical loosening was not significant. However, the majority of the stems were remarkably stable and showed micromotion of less than 18 m and migration of less than 35 m at the proximal implant-bone interface. The corresponding figures for the tip of the stems were 243 m and 170 m, respectively. During torsional loading the custom stems showed less rotatory motion than the anatomical stem (p<0.05). There were no significant differences in the magnitude of cyclical micromotion or migration for the two types of femoral stems. (Hip International 2002; 12: 263-73).

Biomechanical analysis of impending femoral neck fractures: The role of percutaneous cement augmentation for osteolytic lesions

BackgroundManagement of impending pathologic femoral neck fractures includes internal fixation, arthroplasty and megaprostheses. The study aim was to determine the augmentative effect of cement injection for minimally invasive treatment of femoral neck lesions. MethodsTwenty-seven cadaveric femora received a simulated osteolytic lesion previously shown to decrease the femur's failure load by 50%. Specimens were allocated to three groups of nine and loaded to failure in simulated single-leg stance: (1) percutaneous cementation + internal fixation (PCIF); (2) percutaneous cementation (PC); and (3) internal fixation (IF). Lesion-only and augmented finite element models were virtually loaded and stresses were queried adjacent to the lesion. FindingsPCIF resulted in the largest failure load though the increase was not significantly greater than the PC or IF groups. Inspection of the PC and PCIF specimens indicated that the generation of a cement column that spanned the superior and inferior cortices of the femoral neck increased failure loads significantly. Finite element analysis indicated that IF and PCIF constructs decreased the stress adjacent to the lesion to intact femur levels. Cementation without superior-to-inferior femoral neck cortical contact did not restore proximal femoral stress toward the intact condition. InterpretationInternal fixation alone and internal fixation with or without cementation produce similar levels of mechanical augmentation in femora containing a high-risk lesion of impending fracture. A cement injection technique that produces a cement column contacting the superior and inferior femoral neck cortices confers the highest degree of biomechanical stability, should percutaneous cementation alone be performed.

Characterization of the protective capacity of flooring systems using force-deflection profiling

‘Safety floors’ aim to decrease the risk of fall-related injuries by absorbing impact energy during falls. Ironically, excessive floor deflection during walking or standing may increase fall risk. In this study we used a materials testing system to characterize the ability of a range of floors to absorb energy during simulated head and hip impacts while resisting deflection during simulated single-leg stance. We found that energy absorption for all safety floors (mean (SD)=14.8 (4.9)J) and bedside mats (25.1 (9.3)J) was 3.2- to 5.4-fold greater than the control condition (commercial carpet). While footfall deflections were not significantly different between safety floors (1.8 (0.7)mm) and the control carpet (3.7 (0.6)mm), they were significantly higher for two bedside mats. Finally, all of the safety floors, and two bedside mats, displayed 3–10 times the energy-absorption-to-deflection ratios observed for the baseline carpet. Overall, these results suggest that the safety floors we tested effectively addressed two competing demands required to reduce fall-related injury risk; namely the ability to absorb substantial impact energy without increasing footfall deflections. This study contributes to the literature suggesting that safety floors are a promising intervention for reducing fall-related injury risk in older adults.

Biomechanical analysis of a transiliac internal fixator

We evaluated the biomechanical characteristics of the transiliac internal fixator (TIFI) as compared to two well-established methods of internal posterior pelvic ring fixation. Six freshly frozen human pelves were used for simulated single-leg stance loading of an AO type C injury model (pubic symphysis diastasis and unilateral sacroiliac joint disruption). The symphysis rupture was stabilized with a dynamic compression plate. Afterwards the three internal stabilization systems (TIFI, iliosacral screws and ventral plate osteosynthesis) were analysed. Fragment movement was measured in a contact-free manner with a stereophotometric infrared system. No significant differences in the three-dimensional deformation tolerated by the TIFI as compared to the other internal fixation systems were found. The transiliac internal fixator provides the same biomechanical stability as the other reference implants tested. We suggest the use of this device as a suitable alternative to the other implants.

Use of the Reamer-Irrigator-Aspirator for Bone Graft Harvest: A Mechanical Comparison of Three Starting Points in Cadaveric Femurs

The mechanical behavior of cadaveric femurs after intramedullary reaming using the Reamer-Irrigator-Aspirator (RIA) for autogenous bone graft harvest has not been fully described. We hypothesized that reamed femurs, regardless of starting point, would adequately withstand cyclic loading simulating postoperative single-leg stance. Twenty-one cadaveric pairs were randomly assigned to one of three groups based on starting point: Group 1 (trochanteric), Group 2 (piriformis fossa), and Group 3 (retrograde). Each femur underwent dual-energy x-ray absorptiometry scanning and radiographs. Each test femur was reamed to 15 mm using the RIA with the contralateral femur serving as the control. The specimens were loaded to 1400 N of axial compression with 2 degrees simultaneous torsion for 10,000 cycles. If the femur survived cyclic loading, it was then loaded to failure in axial compression. Comparisons regarding survival of cyclic loading were made using Fisher exact test. No differences were seen between groups regarding age, sex, and T-score. The mean T-score for the femurs was -2.531 +/- 1.372. Overall, 18 of 21 (86%) test femurs and 20 of 21 (95%) control femurs withstood cyclic loading (P = 0.606). Statistical significance was not reached for the three pairwise comparisons between test groups. The femurs failed in patterns consistent with simple pertrochanteric, basicervical, midcervical, or subcapital fractures. Intramedullary reaming for bone graft harvest using the RIA without subsequent intramedullary stabilization did not significantly degrade the mechanical behavior of cadaveric femurs in simulated single-leg stance regardless of reamer starting point. It appears safe to allow single-leg stance weightbearing on a reamed, unstabilized femur after bone graft harvesting using the RIA.

Effect of acetabular labral tears, repair and resection on hip cartilage strain: A 7 T MR study

Background Tears of the acetabular labrum are frequently present in patients with groin pain. While it is clear that the labrum contributes to the surface area articulating with the femoral head, it is not clear whether labral repair yields different load distribution in the hip compared to labral resection. Purpose Determine whether labral repair reduces cartilage strain more effectively than labral resection. Methods Six human cadaveric hips (mean age 37 years) were loaded in a simulated single-leg stance within the bore of a 7 T MR scanner. After cartilage had reached a steady-state thickness distribution, a scan of the cartilage was acquired with a voxel size of 0.1×0.1×0.3 mm. This method was repeated for each of six specimens when the labrum was intact, after a surgically simulated labral tear, after an arthroscopic labral repair and after labral resection. Cartilage thickness and strain in an anterosuperior region of interest were measured from the MR scans. A paired t-test was used to compare mean and maximum cartilage strain when the labrum was intact vs. torn, torn vs. repaired and repaired vs. resected. Three-dimensional patterns of cartilage strain distribution were qualitatively compared for the different labral conditions. Results For the number of specimens tested we found no change in mean and maximum cartilage strain, and little obvious change in the pattern of cartilage strain distribution after a simulated labral tear. Labral repair caused a 2% decrease in mean cartilage strain compared to a torn labrum ( p=0.014). Labral resection caused a 4% and 6% increase in mean and maximum cartilage strain, respectively, compared to labral repair ( p=0.02), and the cartilage strain distribution was elevated throughout the region of interest. Conclusion Based on our ex vivo findings of increased cartilage strain after labral resection when compared to labral repair, we have demonstrated the associated consequences to the mechanical environment of the cartilage following surgical treatment of the labrum.

Mechanical stability of custom and anatomical femoral stems: an experimental study in human femora.

In this study we have compared the mechanical stability of custom (n=8) and anatomical (n=8) uncemented femoral components, following insertion into human cadaveric femurs, during simulated single leg stance and stair climbing. In the custom group two specimens were excluded from the study due to detachment of the greater trochanter during cyclical loading. As a consequence of their mechanical behaviour both types of stems could be divided into subgroups of "unstable" and "stable" implants. In the course of one thousand loading cycles three anatomical stems and one custom stem migrated more than 1 mm, which was interpreted as mechanical loosening. This difference in rate of mechanical loosening was not significant. However, the majority of the stems were remarkably stable and showed micromotion of less than 18 m and migration of less than 35 m at the proximal implant-bone interface. The corresponding figures for the tip of the stems were 243 m and 170 m, respectively. During torsional loading the custom stems showed less rotatory motion than the anatomical stem (p<0.05). There were no significant differences in the magnitude of cyclical micromotion or migration for the two types of femoral stems. (Hip International 2002; 12: 263-73).

THE BIOMECHANICS OF INTRAMEDULLARY NAILING: A PROTOCOL FOR LABORATORY TESTING

In this work, a testing protocol has been set up, in order to quantify the load-sharing imposed near the fracture site by an interlocking-type intramedullary nail. A simulated single-leg stance has been applied to a composite fiberglass femur in three different configurations simulating: an intact femur, a fractured nailed femur and a fully healed nailed femur. The repeatability of the measurements and the influence of ilio-tibial band muscle force have been investigated. The tested nail proved to share with the femoral shaft an amount of the total applied load ranging from 50% immediately after the nailing to 80% at full healing.

Biomechanical consequences of secondary congruence after both-column acetabular fracture.

To create a both-column acetabular fracture model with secondary congruence and to determine the intraarticular loading characteristics present in simulated single-leg stance. The normal contact pressures on the weight-bearing portion of the acetabulum in simulated single-leg stance are different from those present in a both-column fracture model exhibiting secondary congruence. Cadaveric Biomechanical model. Biomechanical testing laboratory. Nine fresh frozen cadaveric hemipelves. Both-column fracture model with secondary congruence of the acetabular articular surface with respect to the femoral head was created and tested using Materials Testing Machine (MTS Systems Corp., Minneapolis, MN) and Fuji pressure-sensitive film (Sensor Products, Inc., East Hanover, NJ). Testing data recorded and analyzed comparing the fractured and unfractured states. With respect to the intact specimen, the contact area, mean pressure, and peak pressure in the dome region all increased (p < 0.003) in the both-column model. The contact area in the anterior articular region decreased (p < 0.02) as did the mean pressure (p < 0.032). The posterior articular region demonstrated a trend toward decreased contact area and increased mean and peak pressures. Descriptively, the stress concentration shifted toward the fracture in all cases with the most anterior and most posterior articular regions having little contact in the fracture model. In the authors' both-column model of secondary congruence, the stress concentration during simulated single-leg stance was increased significantly in the dome of the acetabulum adjacent to the fracture line.

Biomechanical evaluation of calcium phosphate cement-augmented fixation of unstable intertrochanteric fractures.

The purpose of this study was to evaluate the mechanical effects of using an injectable calcium phosphate cement, Norian Skeletal Replacement System (SRS), which is replaced by the native remodeling process, to augment sliding hip screw fixation of unstable intertrochanteric fractures in a cadaver model. Ten matched pairs of human cadaver femora were used. One randomly selected femur from each pair was designated as experimental, whereas the contralateral femur served as the control. Testing was performed in the Orthopaedic Research Laboratories, The University of Michigan, Ann Arbor, Michigan, U.S.A. Femora with no radiographic evidence of pathology and with below-normal bone mineral density in the neck region were accepted into the study. Three-part, intertrochanteric fractures were repaired by using a sliding hip screw and sideplate, without replacement of the posteromedial fragment. In the experimental side of each pair, SRS was used to grout the hip screw and to fill the posteromedial defect. Femora were subjected to simulated single-leg stance loading to approximately one body weight in a servohydraulic testing machine. Measurements of stiffness, medial bone surface strain, hip screw displacement, and sideplate strain were made in fractured femora. Stiffness and medial bone surface strain baseline data were obtained in the intact bone before fracture creation as well. Augmenting fixation with Norian SRS increased the fracture construct stiffness and minimized sliding hip screw displacement. It also maintained medial bone surface strain closer to the intact state and lowered sideplate strain relative to controls. However, there was no difference in the load to failure between SRS-augmented and control femora. SRS augmentation of unstable, intertrochanteric fractures significantly improved overall stability, facilitated load transfer across the fracture, and decreased both shortening of the proximal femur and stress on the sliding hip screw.

Biomechanical consequences of anterior column fracture of the acetabulum.

To measure biomechanical consequences of a high anterior column acetabular fracture. A benchtop biomechanical model using quasi-static loading of the hip joint in a simulated single-leg stance. Pressure-sensitive prescale (Fuji) film was used to determine hip joint loading parameters. Six cadaveric hemipelvi with one hip tested in each specimen. Three right and three left hips were tested. Creation of an anterior column fracture with anatomic reduction and fixation, followed by gap malreduction/fixation, and subsequently step malreduction/fixation. Contact pressure, contact area, and load distribution throughout the hip joint in each experimental condition. There were significant increases in load (p<0.01) and peak pressures (p<0.01) in the superior acetabular region after gap malreduction and in peak contact pressures after step malreduction (p<0.01) as compared with the intact acetabulum. Anatomic reduction was not associated with increased mean or peak contact pressures (in any region). Both step and gap malreductions of a high anterior column fracture resulted in significantly increased peak contact pressures in the superior acetabular region. These biomechanical data cannot be directly extrapolated to clinical applications, but these data suggest that anatomic reduction of anterior column fracture affords the best opportunity to restore contact pressures, contact area, and load distribution within the hip to levels similar to those seen in the intact acetabulum.

Consequences of transverse acetabular fracture malreduction on load transmission across the hip joint.

To evaluate the biomechanical behavior of gap and step malreductions in a model of transverse acetabular fracture. Cadaver pelvis loading in simulated single-leg stance with intact acetabulum, after transverse acetabular fracture anatomically reduced, and after step and gap malreduction. Five transtectal transverse fractures; five juxtatectal transverse fractures. Quasi-static loading of the hip with simulated abductor mechanism to physiologic loads with pressure-sensitive film interposed in the joint to determine contact area and contact pressure within the hip joint. Hip joint contact parameters: contact area, peak and mean contact pressure, and load distribution. Step malreduction of the transtectal transverse fracture resulted in significantly increased peak contact pressures (20.5 megapascals) in the superior acetabular articular surface as opposed to the intact acetabulum (9.1 megapascals). Gap malreduction of transtectal transverse fracture and step and gap malreduction of juxtatectal fracture did not result in significantly increased contact pressures in the hip. Step malreduction of a transverse acetabular fracture in the superior articular surface results in abnormally high contact forces and may predispose to the development of posttraumatic arthritis.

The effect of variable size posterior wall acetabular fractures on contact characteristics of the hip joint.

The indications for open reduction and internal fixation of posterior wall acetabular fractures associated with a clinically stable hip joint are unclear. In previous work a large posterior wall defect (27% articular surface area) resulted in significant alteration of load transmission across the hip; specifically, there was a transition from evenly distributed loading along the acetabular articular surface to loading concentrated mainly in the superior portion of the articular surface during simulated single leg stance. However, the majority of posterior wall fractures involve a smaller amount of the articular surface. Posterior wall acetabular fractures not associated with instability of the hip are commonly treated nonoperatively. This practice does not account for the size of the posterior wall fracture. To study the biomechanical consequences of variably sized articular defects, a laboratory experiment was conducted evaluating three progressively larger posterior wall defects of the acetabulum during simulated single leg stance using superlow Fuji prescale film (Itochu International, New York): (a) 1/3 articular surface width through a 50 degrees arc along the posterior wall of the acetabulum, (b) 2/3, and (c) 3/3 articular width defects through the same 50 degrees arc along the posterior wall of the acetabulum. In the intact acetabulum, 48% of the total articular contact was located in the superior acetabulum. Twenty-eight percent of articular contact was in the anterior wall region of the acetabulum and 24% in the posterior wall region. After the 1/3 width posterior wall defect, 64% of the articular contact was located in the superior acetabulum (p = 0.0011). The 2/3 width posterior wall defect resulted in 71% of articular contact area being located in the superior acetabulum (p = 0.0006). After the 3/3 width posterior wall defect, 77% of articular contact was located in the superior acetabulum, significantly greater than the intact condition (p < 0.0001) and 1/3 width defect (p = 0.0222). The total absolute contact areas for all defect conditions were significantly less than the intact conditions. The results of this study reconfirm the observation that posterior wall fractures of the acetabulum significantly alter the articular contact characteristics in the hip during single leg stance. The relationship between defect size and changes in joint contact showed that the smallest defect resulted in the greatest alteration in joint contact areas, whereas larger defects resulted in minor increments of change in contact area. This finding is of concern because the clinical practice of managing acetabular fractures nonoperatively if the hip joint is stable is based on the supposition that the joint retains enough integrity to function without undue risk of late posttraumatic osteoarthritis. A better understanding of the natural history of stable posterior wall acetabular fractures is needed to ascertain whether some of these fractures merit operative repair.

Monitoring the integrity of the cement—metal interface of total joint components in vitro using acoustic emission and ultrasound

Debonding of the cement—metal interface of cemented femoral components of total hip arthroplasty has been shown from clinical and autopsy material to be a common occurrence. Experimentally, debonding has been shown to increase markedly the strains in the adjacent cement mantle. Studies of autopsy-retrieved specimens demonstrate that debonding of the cement—metal interface is a key initiating event in loosening of cemented femoral components of total hip arthroplasty. However, both the radiographic and autopsy evidence of cement—metal interfacial debonding exist after the fact, that is, after debonding has occurred. The lack of prospective data showing that debonding does indeed occur under physiologic loading and occurs prior to other forms of failure of fixation leaves uncertain the issue of debonding and its role in initiating loosening of cemented femoral components. Knowing when, where, and to what extent the cement—metal interface debonds is critical information in understanding the process of loosening of cemented femoral components. Such information would contribute to improving the durability of stems and improving cementing techniques. In this study, the two nondestructive techniques of acoustic emission and ultrasonic evaluation of the cement—metal interface of cemented femoral stems of total hip arthroplasty were combined to investigate when, where, and to what extent cement—metal debonding occurred in vitro in simulated femurs loaded physiologically in fatigue in simulated single-leg stance. Debonding of the cement—metal interface of a cemented femoral component in this model was both an initiating event and a major mechanism of compromise of the cement—metal interface. Additional acoustic emission signals arose from cracks that developed in the cement.

Mechanical validation of whole bone composite femur models

Composite synthetic models of the human femur have recently become commercially available as substitutes for cadaveric specimens. Their quick diffusion was justified by the advantages they offer as a substitute for real femurs. The present investigation concentrated on an extensive experimental validation of the mechanical behaviour of the whole bone composite model, compared to human fresh-frozen and dried-rehydrated specimens for different loading conditions. First, the viscoelastic behaviour of the models was investigated under simulated single leg stance loading, showing that the little time dependent phenomena observed tend to extinguish within a few minutes of the load application. The behaviour under axial loading was then studied by comparing the vertical displacement of the head as well as the axial strains, by application of a parametric descriptive model of the strain distribution. Finally, a four point bending test and a torsional test were performed to characterize the whole bone stiffness of the femur. In all these tests, the composite femurs were shown to fall well within the range for cadaveric specimens, with no significant differences being detected between the synthetic femurs and the two groups of cadaveric femurs. Moreover, the interfemur variability for the composite femurs was 20–200 times lower than that for the cadaveric specimens, thus allowing smaller differences to be characterized as significant using the same simple size, if the composite femurs are employed.

A quantitative in vitro assessment of fit and screw fixation on the stability of a cementless hemispherical acetabular component

This investigation quantifies in vitro the effect of component fit, as well as the effect of adjuvant screw fixation, on the initial stability of cementless hemispherical titanium acetabular total hip arthroplasty components and assesses apposition of the acetabular components to bone. Six, fresh human hemipelvi (3 matched pairs) were harvested at autopsy. Titanium alloy acetabular components with a porous surface of commercially pure titanium fiber mesh (Harris Galante Porous acetabular components, Zimmer, Warsaw, IN) were used for implantation. Initially, each acetabulum was underreamed to achieve a 2 mm press-fit with the acetabular component. Pressuresensitive film had been placed along the dome and medial wall at the bone-implant interface to assess the completeness of seating. After the implant was impacted into the acetabular cavity, relative motion between the implant and bone was measured during simulated single leg stance. Adjuvant fixation of the implant was then obtained with the insertion of four 6.5 mm cancellous screws. High-contrast roentgenograms of the specimens in multiple views were obtained after initial cup insertion and again after screw insertion. The stability of each implant under load was measured with four, three, two, one, and no screws in place. Further reaming of the bone was done to create a 1 mm press-fit. The sequence was then repeated. Further reaming was done to create an exact-fit and the sequence was repeated again. Under these conditions, 1 mm press-fit with or without screws provided the optimum combination of fit and stability. With the 2 mm press-fit, incomplete seating of the implant was found in every case, resulting in gaps between the dome of the implant and the bone. This reduced the area of the porous surface in contact with bone, and resulted in an increase in micromotion with the dome of the implant. The addition of supplemental screws did not significantly enhance stability of the implant under press-fit conditions. Exact-fit reaming with the use of two, three, or four screws yielded acetabular component stability at the dome equal to 1 mm press-fit, but at the rim the acetabular component was less stable than 1 mm press-fit. The optimum combination might be a press-fit introduction of the acetabular component, and the advantage of screws must be weighed against possible particulate debris and vascular damage.

In vitro study of initial stability of a conical collared femoral component

This in vitro experimental study compared the initial stability of an uncemented conical collared femoral component to that of the same component with the collar removed. The two configurations examined simulated joint resultant forces encountered in single leg stance and stair climbing. For the simulated single leg stance loads, the data do not allow any inferences about relative component stability. With the exception of one collarless control, all micromotion for single leg stance loading was under 150 μm, measured approximately 1.5 cm below the resection line. For scaled stair-climbing loads, however, the conical collared component group was significantly more stable than the collarless control group in transverse (primarily rotational) micromotion. The overall average measured transverse motion for the collarless control group was more than 3.7 times greater than that of the conical collar group at scaled stair-climbing loads. The two conical collared components loaded to full peak stair-climbing load (2,100 N) exhibited micromotion under 160 μm. The results suggest that the conical collar may improve stability of an uncemented prosthesis under loads that include an out-of-plane (rotational) component.