The relationship between femoral periprosthetic cortical bone geometry and porosity after total hip arthroplasty

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.

Finite element analysis of cementless femoral stems based on mid- and long-term radiological evaluation

BACKGROUND

Femoral bone remodeling in response to stress shielding induces periprosthetic bone loss. Computerized finite element analysis (FEA) is employed to demonstrate differences in initial stress distribution. However, FEA is often performed without considering the precise sites at which the stem was fixed. We determined whether FEA reflects mid-term radiological examination exactly as predicted following long-term stress shielding.

METHODS

Femur-stem fixation sites were evaluated radiologically according to the location of spot welds in two anatomical cementless stem designs. Based on mid-term radiological results, four femur-stem bonding site conditions were defined as: (Condition A) no bonding; (Condition B) bonding within the 10 mm area proximal to the distal border of the porous area; (Condition C) bonding of the entire porous area; and (Condition D) bonding of the entire femoral stem, prior to conducting FEA analysis. Furthermore, we radiographically evaluated mid- and long-term stress shielding, and measured bone mineral density of the femur 10 years after total hip arthroplasty.

RESULTS

Spot welds appeared frequently around the border between the porous and smooth areas. FEA showed that, based on mid-term radiological evaluation, von Mises stress was reduced in condition B in the area proximal to the femur-stem bonding sites for both stem designs compared with condition A (no bonding). Conversely, von Mises stress at all areas of the femur-stem bonding sites in conditions C and D was higher than that in condition A. With respect to stress shielding progression, there was no significant difference between the two types of stem designs. However, stress shielding progressed and was significantly higher in the presence of spot welds (p = 0.001). In both stem designs, bone mineral density in zone VII was significantly lower than that in the contralateral hips.

CONCLUSIONS

These results indicate that FEA based on mid-term radiological evaluation may be helpful to predict the influence of long-term stress shielding more precisely.

Mechanical and metabolic interactions in cortical bone development

OBJECTIVES

Anthropological studies of cortical bone often aim to reconstruct either habitual activities or health of past populations. During development, mechanical loading and metabolism simultaneously shape cortical bone structure; yet, few studies have investigated how these factors interact. Understanding their relative morphological effects is essential for assessing human behavior from skeletal samples, as previous studies have suggested that interaction effects may influence the interpretation from cortical structure of physical activity or metabolic status.

MATERIAL AND METHODS

This study assesses cross-sectional geometric and histomorphometric features in bones under different loading regimes (femur, humerus, rib) and compares these properties among individuals under different degrees of metabolic stress. The study sample consists of immature humans from a late medieval Lithuanian cemetery (Alytus, 14th-18th centuries AD). Analyses are based on the hypothesis that metabolic bone loss is distributed within the skeleton in a way that optimizes mechanical competency.

RESULTS

Results suggest mechanical compensation for metabolic bone loss in the cross-sectional properties of all three bones (especially ribs), suggesting a mechanism for conserving adequate bone strength for different loads across the skeleton. Microscopic bone loss is restricted to stronger bones under high loads, which may mitigate fracture risk in areas of the skeleton that are more resistive to loading, although alternative explanations are examined.

DISCUSSION

Distributions of metabolic bone loss and subsequent structural adjustments appear to preserve strength. Nevertheless, both mechanics and metabolism have a detectable influence on morphology, and potential implications for behavioral interpretations in bioculturally stressed samples due to this interaction are explored. Am J Phys Anthropol 160:317-333, 2016. © 2016 Wiley Periodicals, Inc.

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.

New method of fixation of in-bone implanted prosthesis

This article presents results on the effectiveness of a new version of the titanium porous composite skin and bone integrated pylon (SBIP). The SBIP is designed for direct skeletal attachment of limb prostheses and was evaluated in a preclinical study with three rabbits. In accordance with the study protocol, a new version of the pylon (SBIP-3) was implanted into the hind leg residuum of three rabbits. The SBIP-3 has side fins that are designed to improve the bond between the bone and pylon. The fins are positioned inside two slots precut in the bone walls; their length can be adjusted to match the thickness of the bone walls. After 13 (animal 1) or 26 (animals 2 and 3) wk, the animals were sacrificed and samples collected for histopathological analysis. The space between the fins and the bone into which they were fit was filled with fibrovascular tissue and woven bone. No substantial inflammation was found. We suggest that if further studies substantiate the present results, the proposed method can become an alternative to the established technique of implanting prostheses into the medullary canal of the hosting bone.

Design features of implants for direct skeletal attachment of limb prostheses

In direct skeletal attachment (DSA) of limb prostheses, a construct is implanted into an amputee's residuum bone and protrudes out of the residuum's skin. This technology represents an alternative to traditional suspension of prostheses via various socket systems, with clear indications when the sockets cannot be properly fitted. Contemporary DSA was invented in the 1990s, and several implant systems have been introduced since then. The current review is intended to compare the design features of implants for DSA whose use in humans or in animal studies has been reported in the literature.

Cortical bone response to the presence of load-bearing percutaneous osseointegrated prostheses

Although the current percutaneous osseointegrated (OI) prosthetic attachment systems are novel clinical treatments for patients with limb loss, there have only been limited translational studies undertaken to date. To bridge this knowledge gap, from a larger study group of 86 animals that were implanted with a novel percutaneous OI implant construct, 33 sheep were randomly selected from the 0-, 3-, 6-, 9- and 12-month groups for histomorphometric analyses of periprosthetic cortical bone tissue. At necropsy, implanted and nonimplanted limbs were harvested and processed for the evaluation of cortical bone porosity and mineral apposition rate (MAR). The data showed a maximum increase in bone porosity within the first 3 months following implantation and then a progressive reduction in porosity to the baseline steady-state ("Time 0") value by 12 months. The data further verified that the MAR increased during the first 6 months of implantation, reaching a plateau between 6 and 9 months, followed by a progressive decline to the baseline steady state. It was concluded that clinical load bearing and falls precautions, taken during the first 3-6 months following percutaneous OI device implantation surgery, could greatly limit bone fractures during this vulnerable time of increasing cortical bone porosity.

Reorganization of the femoral cortex due to age-, sex-, and endoprosthetic-related effects emphasized by osteonal dimensions and remodeling

Cortical remodeling is of particular importance to femoral strength. However, osteonal morphology and its quantitative changes are still poorly understood. Therefore, osteons as the cortexes basic structural units should serve for histomorphometrical assessment of reorganization mechanisms. Total hip replacement (THR) cases as well as relevant controls were investigated to gain insight into changes within stress shielded as well as age- and sex-associated bone with emphasis on the underlying osseous cell interactions. Control (n = 23) and THR (lifetime Ø14 months, n = 8) femora were prepared to grinding specimen. Osteon density (On.Dn.), osteon area (On.Ar.), haversian canal area (H.Ar.), osteon mean wall thickness (MWT), and cortical porosity (Ct.Po.) were evaluated periosteal and endosteal. Osteonal activity (OA) was determined by mineralization differentiation of active and quiescent osteons. Sex- and age-related differences in osteonal remodeling were evident in controls. Females showed age-dependent increases of H.Ar. during concomitant changes of On.Ar. and MWT resulting in elevated Ct.Po. at the entire cortex. Males showed this occurrence only periosteal. Increased OA and therefore higher remodeling rates were found in females above 70 years. THR led to decreased On.Dn. at the entire cortex. Periosteal increased OA represented increased periprosthetic remodeling resulting in elevated H.Ar., which enhanced Ct.Po.. Massive losses of endosteal circular osteonal structures following THR are reflected in a spongy reorganized cortex leading to increased Ct.Po.. Imbalances in osteoblast-osteoclast interactions whose intensity is related to the respective cortex regions occur in both stress shielded and aging bone and may thus contribute to implant failure and fracture risk.