Anneliese D. Heiner

Structural properties of a new design of composite replicate femurs and tibias

The purpose of this study was to compare the structural properties of a new vs. established design of composite replicate femurs and tibias. The new design has a cortical bone analog consisting of short-glass-fiber-reinforced (SGFR) epoxy, rather than the fiberglass-fabric-reinforced (FFR) epoxy in the currently available design. The hypothesis was that this new cortical bone analog would improve the uniformity of structural properties between specimens, while having mean stiffness values in the range of natural human bones. The composite replicate bones were tested under bending, axial, and torsional loads. In general, the new SGFR bones were significantly less stiff than the FFR bones, although both bone designs reasonably approximated the structural stiffnesses of natural human bones. With the exceptions of the FFR bone axial tests, the highest variability between specimens was 6.1%. The new SGFR bones had similar variability in structural properties when compared to the FFR bones under bending and torsional loading, but had significantly less variability under axial loading. Differences in epiphyseal geometry between the FFR and SGFR bones, and subsequent seating in the testing fixtures, may account for some of the differences in structural properties; axial stiffness was especially dependent on bone alignment. Stiffness variabilities for the composite replicate bones were much smaller than those seen with natural human bones. Axial strain distribution along the proximal-medial SGFR femur had a similar shape to what was observed on natural human femurs by other investigators, but was considerably less stiff in the more proximal locations.

Structural properties of fourth-generation composite femurs and tibias.

The purpose of this study was to measure the structural properties of the latest design (fourth-generation) of composite femurs and tibias from Pacific Research Laboratories, Inc. Fourth-generation composite bones have the same geometries as the third-generation bones, but the cortical bone analogue material was changed to one with increased fracture and fatigue resistance, tensile and compressive properties, thermal stability, and moisture resistance. The stiffnesses of the femurs and tibias were tested under bending, axial, and torsional loading, and the longitudinal strain distribution along the proximal-medial diaphysis of the femur was also determined. The fourth-generation composite bones had average stiffnesses and strains that were for the most part closer to corresponding values measured for natural bones, than was the case for third-generation composite bones; all measurements were taken by the same investigator in separate studies using identical methodology. For the stiffness tests, variability between the specimens was less than 10% for all cases, and setup variability was less than 6%.

Chondrocyte senescence, joint loading and osteoarthritis.

cellular level is not completely understood, but both aging and loading-induced stresses have been shown to undermine cell functions related to the maintenance and restoration of the cartilage matrix. Based on precedents set by studies of other age-related degenerative diseases, we have focused our laboratory work on senescence as the cause of age-dependent decline in chondrocytes and on the impact of excessive mechanical stresses in promoting senescence. We hypothesized that senescent chondrocytes accumulate with age in articular cartilage and we propose that excessive mechanical stress plays a role in this process by promoting oxidative damage in chondrocytes that ultimately causes them to senesce. To test this hypothesis, we measured cell senescence markers (β-galactosidase expression, mitotic activity, and telomere length) in human articular cartilage chondrocytes, and determined the effects of chronic exposure to oxidative stress on chondrocyte growth and senescence. In addition, we measured the effects of abnormally high levels of mechanical shear stress on the release of oxidants in cartilage explants. We found that senescent chondrocytes accumulated with age in articular cartilage. In vitro studies showed that chronic oxidative stress caused by repeated exposure to peroxide, or by growth under superphysiologic oxygen tension caused chondrocyte populations to senesce prematurely, before extensive telomere erosion occurred. Mechanical shear stress applied to cartilage explants considerably increased the production of oxidants. These observations support the hypothesis that senescence accounts for age-related decline in chondrocyte function and indicate that mechanically induced oxidative damage plays a role in this process. This suggests that new efforts to prevent the development and progression of osteoarthritis should include strategies that slow the progression of chondrocyte senescence or replace senescent cells.

Volumetric Topological Analysis: A Novel Approach for Trabecular Bone Classification on the Continuum Between Plates and Rods

Trabecular bone (TB) is a complex quasi-random network of interconnected plates and rods. TB constantly remodels to adapt to the stresses to which it is subjected (Wolffs Law). In osteoporosis, this dynamic equilibrium between bone formation and resorption is perturbed, leading to bone loss and structural deterioration. Both bone loss and structural deterioration increase fracture risk. Bones mechanical behavior can only be partially explained by variations in bone mineral density, which led to the notion of bone structural quality. Previously, we developed digital topological analysis (DTA) which classifies plates, rods, profiles, edges, and junctions in a TB skeletal representation. Although the method has become quite popular, a major limitation of DTA is that it provides only hard classifications of different topological entities, failing to distinguish between narrow and wide plates. Here, we present a new method called volumetric topological analysis (VTA) for regional quantification of TB topology. At each TB location, the method uniquely classifies its topology on the continuum between perfect plates and perfect rods, facilitating early detections of TB alterations from plates to rods according to the known etiology of osteoporotic bone loss. Several new ideas, including manifold distance transform, manifold scale, and feature propagation have been introduced here and combined with existing DTA and distance transform methods, leading to the new VTA technology. This method has been applied to multidetector computed tomography (CT) and micro-computed tomography (μCT) images of four cadaveric distal tibia and five distal radius specimens. Both intra- and inter-modality reproducibility of the method has been examined using repeat CT and μCT scans of distal tibia specimens. Also, the methods ability to predict experimental biomechanical properties of TB via CT imaging under in vivo conditions has been quantitatively examined and the results found are very encouraging.

A Biomechanical Analysis of Polyethylene Liner Cementation into a Fixed Metal Acetabular Shell

Background: A common clinical scenario encountered by an orthopaedic surgeon is a patient with a secure cementless acetabular shell and a failed polyethylene liner. One treatment option is to cement a new liner into the fixed shell. The purpose of this study was to evaluate technical variables to improve the mechanical strength of such cemented liner constructs.Methods: The contributions of shell texturing, liner texturing, and cement mantle thickness (between the liner and the shell) were evaluated by comparing torsional strength (among nine groups of constructs) and lever-out strength (among eight groups of constructs).Results: Failure almost always occurred at the cement-liner interface. The two exceptions (failure at the shell-cement interface) occurred with a polished, untextured shell with no screw-holes. This finding indicates that if a shell has existing texturing (such as holes), further intraoperative scoring of the shell is unnecessary, but some sort of texturing is necessary to avoid construct failure at the shell-cement interface. Textured liners had significantly (a = 0.05) greater torsional and lever-out strength than untextured liners. The greatest construct strength occurred when liner grooves were oriented so as to oppose the applied loading. A 4-mm-thick cement mantle resulted in slightly greater torsional strength than a 2-mm-thick cement mantle, and a 2-mm-thick cement mantle resulted in considerably greater lever-out strength than a 4-mm-thick cement mantle, but these differences were not significant.Conclusions: When cementing a liner into a well-fixed shell, a surgeon should ensure that both the shell and the liner are textured, as interdigitation of the cement with the shell and the liner is crucial to the mechanical strength of this construct.

Instability Dependency of Osteoarthritis Development in a Rabbit Model of Graded Anterior Cruciate Ligament Transection

BACKGROUND Joint instability has long been empirically recognized as a leading risk factor for osteoarthritis. However, formal mechanistic linkage of instability to osteoarthritis development has not been established. This study aimed to support a clinically accepted, but heretofore scientifically unproven, concept that the severity and rapidity of osteoarthritis development in unstable joints is dependent on the degree of instability. In a survival rabbit knee model of graded joint instability, the relationship between the magnitude of instability and the intensity of cartilage degeneration was studied at the organ level in vivo. METHODS Sixty New Zealand White rabbits received either complete or partial (medial half) transection of the anterior cruciate ligament or sham surgery (control) on the left knee. At the time that the animals were killed at eight or sixteen weeks postoperatively (ten animals for each treatment and/or test-period combination), the experimental knees were subjected to sagittal plane stability measurement, followed by whole-joint cartilage histological evaluation with use of the Mankin score. RESULTS Sagittal plane instability created in the partial transection group was intermediate between those in the complete transection and sham surgery groups. The partial and complete transection groups exhibited cartilage degeneration on the medial femoral and/or medial tibial surfaces. The average histological score (and standard deviation) for the medial compartment in the partial transection group (2.9 ± 0.9) was again intermediate, significantly higher than for the sham surgery group (1.9 ± 0.8) and significantly lower than for the complete transection group (4.5 ± 2.3). The average histological scores for the medial compartment in the partial transection group correlated significantly with the magnitude of instability, with no threshold effect being evident. The significance level of alpha was set at 0.05 for all tests. CONCLUSIONS The severity of cartilage degeneration increased continuously with the degree of instability in this survival rabbit knee model of graded instability.

Reliability of detecting prosthesis/cement interface radiolucencies in total hip arthroplasty

A laboratory study assessed the reliability of detecting radiolucencies at the prosthesis/cement interface in femoral components for total hip replacement. Radiolucencies (thicknesses = 0.1, 0.3, 0.5 and 0.7 mm) were created by randomly masking non‐tip Gruen zones (six per stem) in a group of 72 matte‐finish femoral components. Only half of all Gruen zones were masked. The femoral components were reproducibly implanted in composite fiberglass replicate femurs. Anteroposterior and lateral radiographs taken using conventional techniques were then presented to 10 experienced readers, who scored each of 432 non‐tip Gruen zones for radiolucency presence or absence. The series‐average radiolucency detection rate was 47.1%, with a breakdown of 9.8%, 20.7%, 69.8% and 88.0% for radiolucency thicknesses of 0.1, 0.3, 0.5 and 0.7 mm, respectively. The false positive rate was 7.5%. The findings argue strongly that in many or most instances, initial cement‐prosthesis debonding is not reliably detected from conventional plain film radiographs. Radiolucencies of at least 0.7 mm width are necessary to be accurately detectable by most viewers.

Development and Physical Validation of a Finite Element Model of Total Hip Dislocation

Component-on-component impingement, followed by levering of the femoral head, is a common mode of dislocation in total hip arthroplasty. While there have been many registry-based studies of dislocation incidence, confounding factors and sources of variability in the clinical domain make it difficult to identify specific parameter influences. A three dimensional nonlinear finite element model has been developed for the purpose of studying the dislocation event, to allow determination of how individual factors such as component design and clinical implantation position affect the propensity for dislocation. Also, a laboratory testing apparatus was constructed to provide physical validation of the computational model. The finite element model correctly predicted the range of motion observed in the physical apparatus to within 1%, and predicted the peak resisting moment to within 2.5%. Under even a light joint load of 200 N, the von Mises stresses developed in the polyethylene insert reached 13 MPa, and the contact stresses rose to as high as 30 MPa. These deleterious elevations occurred not only at the site of neck impingement, but also at the site of head egress from the liner.

Replication of chronic abnormal cartilage loading by medial meniscus destabilization for modeling osteoarthritis in the rabbit knee in vivo

Medial meniscus destabilization (MMD) is a surgical insult technique for modeling osteoarthritis (OA) by replicating chronic abnormal cartilage loading in animal joints in vivo. The present study aimed to characterize the immediate biomechanical effects (ex vivo) and short‐term histological consequences (in vivo) of MMD in the rabbit knee. In a compressive loading test, contact stress distribution in the medial compartment was measured in eight cadaver rabbit knees, initially with all major joint structures uninjured (Baseline), after MMD, and finally after total medial meniscectomy (TMM). Similarly, the effects on sagittal joint stability were determined in an anterior–posterior drawer test. These biomechanical (ex vivo) data indicated that both MMD and TMM caused significant (p < 0.001), distinct (>1.5‐fold) elevation of peak local contact stress in the medial compartment, while leaving whole‐joint stability nearly unchanged. Histological consequences in vivo were assessed in a short‐term (8‐week) survival series of MMD or TMM (five animals for each group), and both caused moderate cartilage degeneration in the medial compartment. The MMD insult, which is feasible through posterior arthrotomy alone, is as effective as TMM for modeling injurious‐level chronic abnormal cartilage loading in the rabbit knee medial compartment in vivo, while minimizing potential confounding effects from whole‐joint instability.

Effects of episodic subluxation events on third body ingress and embedment in the THA bearing surface.

In total joint arthroplasty, third body particle access to the articulating surfaces results in accelerated wear. Hip joint subluxation is an under-recognized means by which third body particles could potentially enter the otherwise closely conforming articular bearing space. The present study was designed to test the hypothesis that, other factors being equal, even occasional events of femoral head subluxation greatly increase the number of third body particles that enter the bearing space and become embedded in the acetabular liner, as compared to level-walking cycles alone. Ten metal-on-polyethylene hip joint head-liner pairs were tested in a multi-axis joint motion simulator, with CoCrMo third body particles added to the synovial fluid analog. All component pairs were tested for 2h of level walking; half were also subjected to 20 intermittent subluxation events. The number and location of embedded particles on the acetabular liners were then determined. Subluxation dramatically increased the number of third body particles embedded in the acetabular liners, and it considerably increased the amount of scratch damage on the femoral heads. Since both third body particles and subluxation frequently occur in contemporary total hip arthroplasty, their potent synergy needs to be factored prominently into strategies to minimize wear.