Strain-mode-specific mechanical testing and the interpretation of bone adaptation in the deer calcaneus.

The artiodactyl (deer and sheep) calcaneus is a model that helps in understanding how many bones achieve anatomical optimization and functional adaptation. We consider how the dorsal and plantar cortices of these bones are optimized in quasi-isolation (the conventional view) versus in the context of load sharing along the calcaneal shaft by "tension members" (the plantar ligament and superficial digital flexor tendon). This load-sharing concept replaces the conventional view, as we have argued in a recent publication that employs an advanced analytical model of habitual loading and fracture risk factors of the deer calcaneus. Like deer and sheep calcanei, many mammalian limb bones also experience prevalent bending, which seems problematic because the bone is weaker and less fatigue-resistant in tension than compression. To understand how bones adapt to bending loads and counteract deleterious consequences of tension, it is important to examine both strain-mode-specific (S-M-S) testing (compression testing of bone habitually loaded in compression; tension testing of bone habitually loaded in tension) and non-S-M-S testing. Mechanical testing was performed on individually machined specimens from the dorsal "compression cortex" and plantar "tension cortex" of adult deer calcanei and were independently tested to failure in one of these two strain modes. We hypothesized that the mechanical properties of each cortex region would be optimized for its habitual strain mode when these regions are considered independently. Consistent with this hypothesis, energy absorption parameters were approximately three times greater in S-M-S compression testing in the dorsal/compression cortex when compared to non-S-M-S tension testing of the dorsal cortex. However, inconsistent with this hypothesis, S-M-S tension testing of the plantar/tension cortex did not show greater energy absorption compared to non-S-M-S compression testing of the plantar cortex. When compared to the dorsal cortex, the plantar cortex only had a higher elastic modulus (in S-M-S testing of both regions). Therefore, the greater strength and capacity for energy absorption of the dorsal cortex might "protect" the weaker plantar cortex during functional loading. However, this conventional interpretation (i.e., considering adaptation of each cortex in isolation) is rejected when critically considering the load-sharing influences of the ligament and tendon that course along the plantar cortex. This important finding/interpretation has general implications for a better understanding of how other similarly loaded bones achieve anatomical optimization and functional adaptation.

Exploration of the synergistic role of cortical thickness asymmetry ("Trabecular Eccentricity" concept) in reducing fracture risk in the human femoral neck and a control bone (Artiodactyl Calcaneus).

The mechanobiology of the human femoral neck is a focus of research for many reasons including studies that aim to curb age-related bone loss that contributes to a near-exponential rate of hip fractures. Many believe that the femoral neck is often loaded in rather simple bending, which causes net tension stress in the upper (superior) femoral neck and net compression stress in its inferior aspect ("T/C paradigm"). This T/C loading regime lacks in vivo proof. The "C/C paradigm" is a plausible alternative simplified load history that is characterized by a gradient of net compression across the entire femoral neck; action of the gluteus medius and external rotators of the hip are important in this context. It is unclear which paradigm is at play in natural loading due to lack of in vivo bone strain data and deficiencies in understanding mechanisms and manifestations of bone adaptation in tension vs. compression. For these reasons, studies of the femoral neck would benefit from being compared to a 'control bone' that has been proven, by strain data, to be habitually loaded in bending. The artiodactyl (sheep and deer) calcaneus model has been shown to be a very suitable control in this context. However, the application of this control in understanding the load history of the femoral neck has only been attempted in two prior studies, which did not examine the interplay between cortical and trabecular bone, or potential load-sharing influences of tendons and ligaments. Our first goal is to compare fracture risk factors of the femoral neck in both paradigms. Our second goal is to compare and contrast the deer calcaneus to the human femoral neck in terms of fracture risk factors in the T/C paradigm (the C/C paradigm is not applicable in the artiodactyl calcaneus due to its highly constrained loading). Our third goal explores interplay between dorsal/compression and plantar/tension regions of the deer calcaneus and the load-sharing roles of a nearby ligament and tendon, with insights for translation to the femoral neck. These goals were achieved by employing the analytical model of Fox and Keaveny (J. Theoretical Biology 2001, 2003) that estimates fracture risk factors of the femoral neck. This model focuses on biomechanical advantages of the asymmetric distribution of cortical bone in the direction of habitual loading. The cortical thickness asymmetry of the femoral neck (thin superior cortex, thick inferior cortex) reflects the superior-inferior placement of trabecular bone (i.e., "trabecular eccentricity," TE). TE helps the femoral neck adapt to typical stresses and strains through load-sharing between superior and inferior cortices. Our goals were evaluated in the context of TE. Results showed the C/C paradigm has lower risk factors for the superior cortex and for the overall femoral neck, which is clinically relevant. TE analyses of the deer calcaneus revealed important synergism in load-sharing between the plantar/tension cortex and adjacent ligament/tendon, which challenges conventional understanding of how this control bone achieves functional adaptation. Comparisons with the control bone also exposed important deficiencies in current understanding of human femoral neck loading and its potential histocompositional adaptations.

A Review of the Collagen Orientation in the Articular Cartilage.

OBJECTIVE: There has been a debate as to the alignment of the collagen fibers. Using a hand lens, Sir William Hunter demonstrated that the collagen fibers ran perpendicular and later aspects were supported by Benninghoff. Despite these 2 historical studies, modern technology has conflicting data on the collagen alignment. DESIGN: Ten mature New Zealand rabbits were used to obtain 40 condyle specimens. The specimens were passed through ascending grades of alcohol, subjected to critical point drying (CPD), and viewed in the scanning electron microscope. Specimens revealed splits from the dehydration process. When observing the fibers exposed within the opening of the splits, parallel fibers were observed to run in a radial direction, normal to the surface of the articular cartilage, radiating from the deep zone and arcading as they approach the surface layer. After these observations, the same samples were mechanically fractured and damaged by scalpel. RESULTS: The splits in the articular surface created deep fissures, exposing parallel bundles of collagen fibers, radiating from the deep zone and arcading as they approach the surface layer. On higher magnification, individual fibers were observed to run parallel to one another, traversing radially toward the surface of the articular cartilage and arcading. Mechanical fracturing and scalpel damage induced on the same specimens with the splits showed randomly oriented fibers. CONCLUSION: Collagen fiber orientation corroborates aspects of Hunter's findings and compliments Benninghoff. Investigators must be aware of the limits of their processing and imaging techniques in order to interpret collagen fiber orientation in cartilage.

Bone ingrowth comparison of irregular titanium and cobalt-chromium coatings in a translational cancellous bone model.

The use of cobalt-chromium (CoCr)-bearing surfaces in total joint replacement (TJR) remains the predominate bearing surface. The conundrum with using this biomaterial has been selecting an ideal porous coating to assure reproducible skeletal attachment. There has been evidence that smooth CoCr beads may be inferior for skeletal attachment compared to identically shaped titanium (Ti) beads. Recent in vitro studies have demonstrated that an increased surface area and roughness favors osteoblast adhesion to metallic biomaterials. Therefore, we hypothesized that an irregular shape CoCr bead with an increased surface texture would help correct the negative bone responses that have been reported with smooth beaded CoCr coatings and thus allowing for bone ingrowth equivalently as an irregular commercially pure Ti porous coating with similar porosity. This investigation employed a weight-bearing translational sheep cancellous bone model to accurately simulate a cancellous bone response as it would be clinically in a human TJR. The data analyses obtained from this investigation revealed similar bone responses between the porous coatings. By 12 weeks the irregular shape CoCr coating was able to achieve similar bone ingrowth with skeletal interlock when compared to a clinically proven Ti porous coating.

Variation in bone response to the placement of percutaneous osseointegrated endoprostheses: A 24-month follow-up in sheep.

Percutaneous osseointegrated (OI) devices for amputees are metallic endoprostheses, that are surgically implanted into the residual stump bone and protrude through the skin, allowing attachment of an exoprosthetic limb. In contrast to standard socket suspension systems, these percutaneous OI devices provide superior attachment platforms for artificial limbs. However, bone adaptation, which includes atrophy and/or hypertrophy along the extent of the host bone-endoprosthetic interface, is seen clinically and depends upon where along the bone the device ultimately transfers loading forces to the skeletal system. The goal of this study was to determine if a percutaneous OI device, designed with a porous coated distal region and an end-loading collar, could promote and maintain stable bone attachment. A total of eight, 18 to 24-month old, mixed-breed sheep were surgically implanted with a percutaneous OI device. For 24-months, the animals were allowed to bear weight as tolerated and were monitored for signs of bone remodelling. At necropsy, the endoprosthesis and the surrounding tissues were harvested, radiographically imaged, and histomorphometrically analyzed to determine the periprosthetic bone adaptation in five animals. Bone growth into the porous coating was achieved in all five animals. Serial radiographic data showed stress-shielding related bone adaptation occurs based on the placement of the endoprosthetic stem. When collar placement and achieved end-bearing against the transected bone, distal bone conservation/hypertrophy was observed. The results supported the use of a distally loading and distally porous coated percutaneous OI device to achieve distal host bone maintenance.

Ability of a wash regimen to remove biofilm from the exposed surface of materials used in osseointegrated implants.

The skin/implant interface of osseointegrated (OI) implants is susceptible to infection, causing excess pain, increased morbidity, and possibly implant removal. Novel distal femoral OI implants with binary nitride coatings have been developed with little physiological modeling to collect microbiological evidence of resistance to bacterial attachment. This in vitro study evaluated a Ti-6Al-4V alloy coated with TiNbN and treated with low plasticity burnishing (LPB) to assess attachment and biofilm formation of methicillin-resistant Staphylococcus aureus (MRSA) under physiologically modeling conditions compared to standard Ti-6Al-4V alloy materials with a polished ("Color Buff") or non-polished finish ("Satin Finish"). Washability of the materials were also assessed and compared. It was hypothesized that the TiNbN/LPB treatments would resist bacterial adhesion and biofilm formation to a greater degree than the other two materials, and have a higher degree of bacterial removal following a clinically relevant wash regimen. Material types were exposed to a constant flow of broth containing MRSA and were analyzed using bacterial quantification, surface coverage analysis, and SEM imaging. Quantification data showed no difference in bacterial attachment among the varying material types both with and without the wash regimen. Surface coverage and SEM analysis confirmed results. The wash regimen led to an approximately 3 log10 reduction in bacteria for all material types. Though the results did not support the hypothesis that a TiNbN coating/LPB treatment might resist bacterial attachment/biofilm formation more than other alloys, or have less bacteria after cleaning, results did support the potential importance of a daily wound-hygiene regimen at the skin/implant interface of OI materials. Published 2018. This article is a U.S. Government work and is in the public domain in the USA.

Advancing the deer calcaneus model for bone adaptation studies: ex vivo strains obtained after transecting the tension members suggest an unrecognized important role for shear strains.

Sheep and deer calcanei are finding increased use as models for studies of bone adaptation, including advancing understanding of how the strain (deformation) environment influences the ontogenetic emergence of biomechanically relevant structural and material variations in cortical and trabecular bone. These artiodactyl calcanei seem ideal for these analyses because they function like simply loaded short-cantilevered beams with net compression and tension strains on the dorsal and plantar cortices, respectively. However, this habitual strain distribution requires more rigorous validation because it has been shown by limited in vivo and ex vivo strain measurements obtained during controlled ambulation (typically walking and trotting). The conception that these calcanei are relatively simply and habitually loaded 'tension/compression bones' could be invalid if infrequent, though biologically relevant, loads substantially change the location of the neutral axis (NA) that separates 'compression' and 'tension' regions. The effect on calcaneus strains of the tension members (plantar ligament and flexor tendon) is also not well understood and measuring strains after transecting them could reveal that they significantly modulate the strain distribution. We tested the hypothesis that the NA location previously described during simulated on-axis loads of deer calcanei would exhibit limited variations even when load perturbations are unusual (e.g. off-axis loads) or extreme (e.g. after transection of the tension members). We also examined regional differences in the predominance of the three strain modes (tension, compression, and shear) in these various load conditions in dorsal, plantar, medial, and lateral cortices. In addition to considering principal strains (tension and compression) and maximum shear strains, we also considered material-axis (M-A) shear strains. M-A shear strains are those that are aligned along the long axis of the bone and are considered to have greater biomechanical relevance than maximum shear strains because failure theories of composite materials and bone are often based on stresses or strains in the principal material directions. We used the same load apparatus from our prior study of mule deer calcanei. Results showed that although the NA rotated up to 8° medially and 15° laterally during these off-axis loads, it did not shift dramatically until after transection of all tension members. When comparing results based on maximum shear strain data vs. M-A shear strain data, the dominant strain mode changed only in the plantar cortex - as expected (in accordance with our a priori view) it was tension when M-A shear strains were considered (shear : tension = 0.2) but changed to dominant shear when maximum shear strain data were considered (shear : tension = 1.3). This difference leads to different conclusions and speculations regarding which specific strain modes and magnitudes most strongly influence the emergence of the marked mineralization and histomorphological differences in the dorsal vs. plantar cortices. Consequently, our prior simplification of the deer calcaneus model as a simply loaded 'tension/compression bone' (i.e. plantar/dorsal) might be incorrect. In vivo and in finite element analyses are needed to determine whether describing it as a 'shear-tension/compression' bone is more accurate. Addressing this question will help to advance the artiodactyl calcaneus as an experimental model for bone adaptation studies.

Sealed osteons in animals and humans: low prevalence and lack of relationship with age.

Sealed osteons are unusual variants of secondary osteons that have received little attention, especially in non-human bones. Sealed osteons are characterized by central canals that are plugged with bone tissue. As with other variants of secondary osteons (e.g. drifting, dumbbell, multi-canal), understanding how and why sealed osteons form can shed light on the mechanisms that regulate normal bone remodeling and how this process can be perturbed with aging and some diseases. In a recent microscopic evaluation of human tibiae obtained after traumatic amputations, 4-5% of the osteons were sealed. It is suggested that this high prevalence reflects occasional localized microscopic ischemia from normal osteonal remodeling; hence sealed osteons are implicated in human skeletal fragility. Therefore, osteon prevalence would be expected to correlate with the bone remodeling seen with aging; for example, showing positive relationships between sealed osteons and the population density of typical secondary osteons (OPD). We evaluated the prevalence of partially sealed (80-99% sealed) and fully sealed osteons with respect to age and variations in OPD in 10 adult human femora (34-71 years) and in various non-human appendicular bones of mature animals that were not of advanced age, including deer calcanei, equine radii and equine third metacarpals. An additional sample of 10 bilateral human femora with unilateral non-cemented total hip replacements (F,+HR) and non-implanted contralateral femora (F,-HR) were evaluated (10 patients; 52-94 years). In non-human bones, sealed + partially sealed osteons were rare (~0.1%) even when having relatively high OPD. When considering sealed + partially sealed osteons in femora from patients without any HR, results showed that 1.6% of the osteons were sealed or partially sealed, which was much lower than anticipated, but this is 10- to 20-fold more than in any of the non-human bones. Additionally, in all bones, sealed + partially sealed osteons were significantly smaller than typical secondary osteons (mean diameters: 125 vs. 272 µm; P < 0.005). In the patients with HR, the percentage of sealed + partially sealed osteons: (i) did not correlate with age, (ii) showed no significant difference between F,-HR and F,+HR (1.9 vs. 2.1%; P = 0.2), and (iii) was positively correlated with OPD (r = 0.67, P = 0.001), which differs from the very weak or lack of correlations in the non-human bones and the other human femur sample. The lack of an age-related relationship, in addition to the very low prevalence of sealed + partially sealed osteons are inconsistent with the idea that they contribute to reduced bone quality seen in aging humans. The small size of sealed and partially sealed osteons, regardless of species affiliation, suggests that they represent closing cones at the termini of some osteons. Available evidence suggests that osteons of primates might have a greater capacity for branching that is associated with closing cones, which might explain the 10-20 times higher prevalence of sealed + partially sealed osteons in the human bones examined in this study.

Transcortical or intracondylar? Which model is accurate for predicting biomaterial attachment in total joint replacement?

Despite four decades of research on material and porous coatings intended for cementless fixation in total joint replacement (TJR), aseptic mechanical loosening unrelated to particulate disease remains a concern. One main question asked is how translational are the animal models used to screen material and porous coatings intended for TJR fixation? Another question is how specific are the translational models at targeting the cementless TJR components that have the highest loosening rates? The hypothesis tested was that the bone response would be different between the two bone types-cortical and cancellous-used in translational animal modeling. The osteoblastic jumping distance (OJD), percent ingrowth, and appositional bone response were measured to assess the response between cancellous and cortical bone at two different anatomical locations, within the same limb. With 500 µm inset, titanium porous coated implants and negative control dinosaur (coprolite) implants were investigated. The data demonstrated that cortical bone had 7 times OJD than cancellous bone. The bone ingrowth data demonstrated 16 times higher bone ingrowth than the cancellous bone. Light microscopy showed predominately fibrous tissue attachment (98%) in cancellous bone. Screening of materials intended for TJR require a translational model predictive of the clinical condition. The results demonstrated that the transcortical model rendered false-positive data. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 578-588, 2018.

P2 porous titanium implants improve tendon healing in an acute rat supraspinatus repair model.

BACKGROUND: Current techniques in rotator cuff repair often lack structural integrity. P2 porous titanium-coated constructs (DJO Surgical, Austin, TX, USA) promote osseointegration and soft tissue ingrowth. This study examined the ability of this material to improve the structural integrity of supraspinatus tendon repair in a rat model. We hypothesized that P2 implants placed at the tendon-to-bone interface would improve mechanical and histologic measures of supraspinatus healing. METHODS: Forty rats underwent supraspinatus repairs with P2 implants in 1 shoulder and standard repair in the other. Rats were humanely killed at time 0 (n = 3), 2 weeks (n = 8), 4 weeks (n = 15), and 12 weeks (n = 14). Tendon-to-bone composite specimens were harvested and evaluated mechanically and histologically. RESULTS: Tendon cross-sectional area was decreased in the P2 implant group at 4 weeks, percentage of relaxation was increased at 2 weeks, elastic modulus was increased at 4 weeks, and maximum load and maximum stress were both increased at 2 and 4 weeks. Histologic analysis revealed no foreign body reactions within or around the P2 implant, and healthy viable bone was visible within the P2 implant. CONCLUSION: The results support our hypothesis, specifically in early healing, in this randomized controlled animal study. These data support the use of P2 porous titanium implants to improve tendon-to-bone healing.

Does vitamin E-blended UHMWPE prevent biofilm formation?

BACKGROUND: Biofilm-related periprosthetic infections are catastrophic to patients and clinicians. Data suggest the addition of vitamin E to UHMWPE may have the ability to reduce biofilm formation on the surface of UHMWPE; however, previous studies were performed using stagnant broth solutions that may not have simulated a physiologic environment. In addition, the observed differences in levels of bacterial attachment, though statistically significant, may not be clinically significant. QUESTIONS/PURPOSES: We blended vitamin E with UHMWPE material and tested it for the ability to resist biofilm formation using a clinical isolate of methicillin-resistant Staphylococcus aureus (MRSA). Three additional materials were tested for comparison: highly crosslinked UHMWPE, compression-molded UHMWPE, and polyetheretherketone. We also determined whether the surface roughness of these materials facilitated biofilm formation. METHODS: Using a flow cell system, samples of each material type were placed into separate chambers. A 10% solution of brain-heart infusion broth containing 10(5) colony-forming units (CFUs)/mL was flowed through the flow cell over 48 hours. The number of bacteria that adhered to the surface was quantified and biofilm formation was observed qualitatively using scanning electron microscopy. Optical profilometry was used to determine the surface roughness of each material type. RESULTS: Vitamin E-blended UHMWPE did not reduce biofilm formation of a clinically relevant strain of MRSA compared to materials that did not have vitamin E. More specifically, vitamin E-blended materials had similar amounts of biofilm formation (~ 8 log10 CFUs/cm(2)) compared to materials not containing vitamin E (~ 8.1 log10 CFUs/cm(2)) (p > 0.4). The roughness of vitamin E-blended material surfaces (mean ± SD: 1.85 ± 0.46 µm) compared to that of materials without vitamin E (2.06 ± 1.24 µm) did not appear to influence biofilm formation. CONCLUSIONS: Under physiologically relevant conditions, vitamin E-blended UHMWPE did not have the ability to reduce the formation of biofilms by MRSA. CLINICAL RELEVANCE: These data indicate that the addition of vitamin E to UHMWPE may not reduce clinically relevant rates of biofilm-related periprosthetic infections of total joint arthroplasty devices.

Histocompositional organization and toughening mechanisms in antler.

Mechanical testing studies by Krauss et al. (2009) and Gupta et al. (2013) suggest that the extraordinary toughness of antler bone is primarily achieved by intrinsic/nanostructural mechanisms instead of extrinsic/microstructural mechanisms. However, this conclusion is based on data from extremely small specimens from one antler loaded only in tension, which impedes discernment of the relative importance of intrinsic vs. extrinsic mechanisms. In the present study we conducted analyses into the microstructural features of antler for details of potential additional microscale toughening characteristics, as suggested by recent mechanical testing studies of bulk specimens. The data are also considered in view of the above-mentioned studies concluding that extrinsic/microstructural toughening mechanisms are less important than nanoscale/intrinsic toughening mechanisms in antler. Mule deer antlers were evaluated using: (1) backscattered electron imaging for micro-mineralization, (2) circularly polarized light for osteonal interfacial complexity and collagen fiber orientation (CFO) heterogeneity, and (3) X-ray 3D micro-computed tomography for osteon/vessel orientation, density, and size. Results showed: (1) hyper-mineralized seams of approximately 3-4 microns thickness within relatively hypermineralized "zones" that course circuitously along osteonal interfaces, (2) highly heterogeneous CFO, including increased oblique-to-transverse CFO near/adjacent to osteon peripheries, and (3) osteons are often highly elongated in 2D. 3D reconstructions show that a considerable percentage of the vascular canals course obliquely with respect to the antler long axis. While results show multiple possible extrinsic-level histological characteristics in antler bone, it remains to be determined if microstructural characteristics become subsidiary to nanostructural characteristics in enhancing toughness during the majority of post-yield behavior of antler bone when loaded in a biologically relevant fashion.

Radiographic evaluation of bone adaptation adjacent to percutaneous osseointegrated prostheses in a sheep model.

BACKGROUND: Percutaneous osseointegrated prostheses (POPs) are being investigated as an alternative to conventional socket suspension and require a radiographic followup in translational studies to confirm that design objectives are being met. QUESTIONS/PURPOSES: In this 12-month animal study, we determined (1) radiographic signs of osseointegration and (2) radiographic signs of periprosthetic bone hypertrophy and resorption (adaptation) and (3) confirmed them with the histologic evidence of host bone osseointegration and adaptation around a novel, distally porous-coated titanium POP with a collar. METHODS: A POP device was designed to fit the right metacarpal bone of sheep. Amputation and implantation surgeries (n = 14) were performed, and plane-film radiographs were collected quarterly for 12 months. Radiographs were assessed for osseointegration (fixation) and bone adaptation (resorption and hypertrophy). The cortical wall and medullary canal widths were used to compute the cortical index and expressed as a percentage. Based on the cortical index changes and histologic evaluations, bone adaptation was quantified. RESULTS: Radiographic data showed signs of osseointegration including those with incomplete seating against the collar attachment. Cortical index data indicated distal cortical wall thinning if the collar was not seated distally. When implants were bound proximally, bone resorbed distally and the diaphyseal cortex hypertrophied. CONCLUSIONS: Histopathologic evidence and cortical index measurements confirmed the radiographic indications of adaptation and osseointegration. Distal bone loading, through collar attachment and porous coating, limited the distal bone resorption. CLINICAL RELEVANCE: Serial radiographic studies, in either animal models or preclinical trials for new POP devices, will help to determine which designs are likely to be safe over time and avoid implant failures.

In vitro efficacy of a novel active-release antimicrobial coating to eradicate biofilms of Pseudomonas aeruginosa.

Implant-related infections are becoming increasingly difficult to treat due to the formation of biofilms on implant surfaces. This study analyzed the in vitro efficacy of a novel antimicrobial coating against biofilms of Pseudomonas aeruginosa, using a flow cell system. Results indicated that P. aeruginosa biofilms were reduced by greater than 8 log10 units in less than 24 h. Data indicated that this active-release coating may be promising for preventing biofilm implant-related infections.

Progression of bone ingrowth and attachment strength for stability of percutaneous osseointegrated prostheses.

BACKGROUND: Percutaneous osseointegrated prosthetic (POP) devices have been used clinically in Europe for decades. Unfortunately, their introduction into the United States has been delayed, in part due to the lack of data documenting the progression of osseointegration and mechanical stability. QUESTIONS/PURPOSES: We determined the progression of bone ingrowth into porous-coated POP devices and established the interrelationship with mechanical stability. METHODS: After amputation, 64 skeletally mature sheep received a custom porous-coated POP device and were then randomized into five time groups, with subsequent measurement of percentage of bone ingrowth into the available pore spaces (n = 32) and the mechanical pullout force (n = 32). RESULTS: Postimplantation, there was an accelerated progression of bone ingrowth (~48% from 0 to 3 months) producing a mean pullout force of 5066 ± 1543 N. Subsequently, there was a slower but continued progression of bone ingrowth (~23% from 3 to 12 months) culminating with a mean pullout force of 13,485 ± 1855 N at 12 months postimplantation. There was a high linear correlation (R = 0.94) between the bone ingrowth and mechanical pullout stability. CONCLUSIONS: This weightbearing model shows an accelerated progression of bone ingrowth into the porous coating; the amount of ingrowth observed at 3 months after surgery within the porous-coated POP devices was sufficient to generate mechanical stability. CLINICAL RELEVANCE: The data document progression of bone ingrowth into porous-coated POP devices and establish a strong interrelationship between ingrowth and pullout strength. Further human data are needed to validate these findings.

Characterization of a novel active release coating to prevent biofilm implant-related infections.

Biofilm implant-related infections cost the US healthcare system billions of dollars each year. For several decades, device coatings have been developed that actively release antimicrobial compounds in an attempt to prevent these infections from developing. To date, few coatings have been put into clinical use. These have shown limited to no efficacy in clinical trials. Recent data have shown the in vitro and in vivo efficacy of a novel active release coating that may address the limitations of coatings that are used clinically. In this study, the novel active release coating was characterized to gain an understanding of the effects of combining an antimicrobial additive, cationic steroid antimicrobial-13 (CSA-13), to a medical grade polydimethylsiloxane (PDMS) material. Results indicated that the addition of CSA-13 did influence the physical properties of the PDMS, but not with adverse effects to the desired material properties. Furthermore, there was no indication of chemical reactivity. It was shown that CSA-13 was uniformly dispersed as small particles throughout the PDMS matrix. These particles were able to dissolve and elute out of the PDMS within a 30-day period. The results of this work suggested that the PDMS with CSA-13 was thermally, chemically and physically stable when used as a device coating to treat local infection and/or prevent biofilm implant-related infections from developing.

Model development for determining the efficacy of a combination coating for the prevention of perioperative device related infections: a pilot study.

Antibiotic resistant bacterial infections are a growing problem in patient care. These infections are difficult to treat and severely affect the patient's quality of life. The goal of this translational experiment was to investigate the antimicrobial potential of cationic steroidal antimicrobial-13 (CSA-13) for the prevention of perioperative device-related infections in vivo. It was hypothesized that when incorporated into a polymeric device coating, the release of CSA-13 could prevent perioperative device-related infection without inhibiting skeletal attachment. To test this hypothesis, 12 skeletally mature sheep received a porous coated titanium implant in the right femoral condyle. Group 1 received the titanium implant and an inoculum of 5 × 10(8) CFU of methicillin-resistant Staphylococcus aureus (MRSA). Group 2 received a CSA-13 coated implant and the MRSA inoculum. Group 3 received only the CSA-13 coated implant and Group 4 received only the implant-without the CSA-13 coating or MRSA inoculum. In conclusion, the CSA-13 combination coating demonstrated bactericidal potential without adversely affecting skeletal attachment. The CSA-13 containing groups exhibited no evidence of bacterial infection at the conclusion of the 12 week study and established skeletal attachment consistent with Group 4. In contrast, all of the Group 1 animals became infected and required euthanasia within 6-10 days. The significance of this finding is that this combination coating could be applied to implanted devices to prevent perioperative device-related infections. This method may facilitate significantly reduced incidences of device-related infections as well as a new method to treat and prevent resistant strain bacterial infections.

The significance of calcified fibrocartilage on the cortical endplate of the translational sheep spine model.

To gain an understanding of the vertebral cortical endplate and factors that may affect the ability to achieve skeletal attachment to intervertebral implants and fusion, this study aimed to characterize the hypermineralized tissue on the cortical endplate of the vertebral body on a commonly used animal model. Skeletally mature sheep were injected with tetracycline prior to euthanasia and the C2-C3, T5-T6, and L2-L3 spinal motion segments were excised and prepared. Vertebral tissues were imaged using backscatter electron (BSE) imaging, histology, and tetracycline labeling was used to assess bone remodeling within different tissue layers. It was determined that the hypermineralized tissue layer was calcified fibrocartilage (CFC). No tetracycline labels were identified in the CFC layer, in contrast to single and double labels that were present in the underlying bone, indicating the CFC present on the cortical endplate was not being actively remodeled. The average thickness of the CFC layer was 146.3 ± 70.53 µm in the cervical region, 98.2 ± 40.29 µm in the thoracic region, and 150.89 ± 69.25 µm in the lumbar region. This difference in thickness may be attributed to the regional biomechanical properties of the spine. Results from this investigation indicate the presence of a nonremodeling tissue on the cortical endplate of the vertebral body in sheep spines, which attaches the intervertebral disc to the vertebrae. This tissue, if not removed, would likely prevent successful bony attachment to an intervertebral device in spinal fusion studies and total disc replacement surgeries.

The artiodactyl calcaneus as a potential 'control bone' cautions against simple interpretations of trabecular bone adaptation in the anthropoid femoral neck.

For over a century, the arched trabecular patterns of the human proximal femur have been considered to resemble tension and compression stress trajectories produced by stereotypical bending loads. This reflects conventional modeling of the human femoral neck-head region as a short cantilevered beam. Although this conception is the foundation of many biomechanical, clinical, paleontological, and comparative morphological studies of trabecular bone in various species, attempts have not been made to contrast these data to a bone that could be considered a 'control' for simple/stereotypical bending. We quantified trabecular architectural characteristics in sheep and deer calcanei as a first step in potentially establishing them as 'controls' in this context because they have arched trabecular patterns that resemble tension/compression stress trajectories, and have been shown by strain gauge measurements to be relatively simply loaded in bending. Using micro-computed tomography, calcanei from adult domesticated sheep and wild deer were analyzed where in the dorsal 'compression' and plantar 'tension' trabecular tracts they begin to separate and bending is less complex (mid-shaft), and where trabeculae extensively interconnect and loading is more complex (distal shaft). Of the eight trabecular architectural characteristics evaluated, only one (trabecular number, Tb.N) showed a probable mechanically relevant dorsal/plantar difference. However, this was paradoxically opposite in the sheep calcanei. Aside from Tb.N, the architectural characteristics showed little, if any, evidence of habitual bending. The non-uniformity of the stresses between the trabecular tracts in these bones might be reduced by load-sharing functions of their robust cortices and the nearby ligament and tendon, which might account for the similar morphologies between the tracts. These findings may help to explain why in many cases regional trabecular architectural variations seem to lack sufficient sensitivity and specificity for interpreting habitual bending in other bone regions. This cautions against simple interpretations of trabecular bone adaptation in the anthropoid femoral neck.