Mechanical loading of bone is anabolic, while aseptic loosening of implants is catabolic. In a rat model of mechanically induced aseptic loosening, osteoclast differentiation is increased dramatically but the underlying mechanism is unknown. The objective was to profile molecular pathways in peri-implant bone resorption. Microarrays on cortical bone samples exposed to pressurized fluid flow were performed 3, 6, 12, 24 and 36 hrs, using time 0 as controls. Of a total of 4093 genes that underwent a 1.25-fold change (p<0.05) due to fluid flow only 21 were common for all time points. Signals linked to inflammation and apoptosis were regulated in a biphasic manner at 3 and 12 and/or 24 hrs. The acute response at 3 hrs was associated with increases in the cytokines IL-6, IL-11, LIF and STAT3. Levels of the pro-apoptotic factor TWEAK were higher while those of BOK, a second pro-survival molecule, were lower. There is an early and late rise in RIPK3, which stimulates a form of programmed necrosis. Osteoblast-related genes were clearly suppressed (osteocalcin, Col1a, PTHr1), while those regulating macrophage and osteoclast differentiation (CSF-1, Bach1, HO-1, RANKL, RANK, OPG) were enhanced. These data suggest that mechanical loading of cortical bone stimulates time-dependent expression of genes regulating the survival, necrosis and differentiation of both the myeloid and mesenchymal cell lineages, resulting in an integrated response leading to a rapid increase in osteoclast numbers.

Improving periprosthetic bone is essential for implant fixation and reducing peri-implant fracture risk. This studied examined the individual and combined effects of iPTH and mechanical loading at the cellular, molecular, and tissue level for periprosthetic cancellous bone. Adult rabbits had a porous titanium implant inserted bilaterally on the cancellous bone beneath a mechanical loading device on the distal lateral femur. The right femur was loaded daily, the left femur received a sham loading device, and half of the rabbits received daily PTH. Periprosthetic bone was processed up to 28 days for qPCR, histology, and uCT analysis. We observed an increase in cellular and molecular markers of osteoblast activity and decrease in adipocytic markers for both treatments, with small additional effects in the combined group. Loading and iPTH led to a decrease and increase, respectively, in osteoclast number, acting through changes in RANKL/OPG expression. Changes in SOST and beta-catenin mRNA levels suggested an integral role for the Wnt pathway. We observed strong singular effects on BV/TV of both loading (1.53 fold) and iPTH (1.54 fold). Combined treatment showed a small additive effect on bone volume. In conclusion, loading and iPTH act through a pro-osteoblastic/anti-adipocytic response and through control of bone turnover via changes in the RANKL/OPG pathway. These changes led to a small additional, but not synergistic, increase in bone volume with the combined therapy.

Introduction

Traumatized musculoskeletal tissue often exhibits prolonged time to healing, mostly due to low blood flow and innervation. Intermittent Pneumatic Compression (IPC) increases blood flow and decreases thromboembolic event after orthopedic surgery,[1] however little is known about healing effects.[2] We hypothesized that IPC could stimulate tissue repair: 1.) blood flow 2.) nerve ingrowth 3.) tissue proliferation and during immobilisation enhance 4.) biomechanical tissue properties.

Healing of tendons is sensitive to mechanical loading, and the callus strength is reduced by ¾ after 14 days, if loading is prevented. Exogenous GDFs stimulate tendon healing. This response is influenced by loading: without loading, cartilage and bone formation is initiated. This suggests that BMP signalling is crucial during tendon healing, and that it is influenced by mechanical loading. We investigated if mechanical loading influences BMP signalling in intact and healing tendons, and how BMP gene expression changes during healing.

The Achilles tendon was transected in rats and left to heal. Half of the rats had one Achilles tendon unloaded by injection of Botox in the calf muscles. Ten tendons were analyzed before transection and for each of four time points. Gene expression for OP-1, GDF-5, -6, -7, Follistatin, Noggin, BMP-receptor 1b and BMP-receptor 2 were analysed with real-time PCR.

Loading had no detectable effects on intact tendons. During repair, loading decreased follistatin by more than half (p=0.0001), and increased GDF-5 (p=0.02). All genes showed changes during repair (p=0.0001), but the time sequences differed. GDF-5 and GDF-7 were generally more expressed than OP-1 and GDF-6. GDF-5 and GDF-7 were more expressed in normal tendons than during repair. Noggin was never detected.

Our results suggest that GDF-5 is specific for the mature tendon, and not much involved in repair. This contrasts to GDF-7, which is involved in both. OP-1 and GDF-6 seem to be involved in early healing. There was less expression of follistatin in loaded tendons during healing. The mechanosensitivity is likely of most importance at day 14 and 21 since the difference in strength between loaded and unloaded tendons is huge. An Anova with only these time points reveals effects of loading on GDF-5 and follistatin (p=0.0001 for both) and significant differences between the days for most variables.

Introduction: Many claim that an inflammatory reaction to wear debris particles is the main cause for prosthetic loosening. We have rat model in which bone resorption can be induced both by fluid pressure and particles. We compared the bone resorptive potency of particles and fluid pressure.

Materials and Methods: The rats received a titanium plate at the proximal tibia. A central plug was inserted. After 5 weeks of osseointegration, the central plug was changed to a piston or a hollow plug with 20mg titanium particles. Commercially pure titanium particles with 90 % of particles lesser than 3,6 microns were used. The pressure piston was subjected to a transcutanous force of 5N. Each episode of pressure comprised 20 pressure cycles at 0.17 Hz, applied twice a day. 60 rats were randomized to 6 groups for particle treatment. One group had particle implantation both at day 1 and 7. Additionally 15 rats were randomized into 3 groups with for pressure treatment. All rats were killed after 5 or 14 days. Bone resorption underneath the piston was evaluated blindedly in hematoxlyin/eosine sections and scored as 0 – 5. Differences between groups were analyzed by Kruskal Wallis and Mann-Whitney U-tests.

Results: Many specimens with titanium particles showed no visible resorption at al, and a few showed dramatic osteolysis. After 14 days, the osteolytic effect was significant. Partice refill made no difference. Titanium particles over 10 μm had minor effect. Fluid pressure always caused bone resorption, and significantly more so than particles both after 5 and 14 days.

Discussion: There was lesser variability in fluid pressure induced osteolysis, which might be due to a different signalling pathway. Titanium particles have an osteolytic effect in this model, but in spite of the massive amount of particles, the effect was less predictable than with pressure.

Introduction: In a rat model, fluid pressure causes more bone resorption than particles. Does pressure also cause more inflammation?

Materials and Methods: Rats received a titanium plate at the proximal tibia. A central plug was inserted. After 5 weeks of osseointegration, the central plug was changed to either a piston or a hollow plug with titanium particles. Commercially pure titanium particles with 90% of particles lesser than 3,6 microns were used. The pressure piston was subjected to a transcutanous force of 8N. Each episode of pressure comprised 20 pressure cycles at 0.17 Hz, applied twice a day. 39 rats were randomized to 3 groups: Titanium particles (n=13), fluid pressure (n=13) and controls with neither particles nor fluid pressure (n=13). The rats were killed after 3 days. 6 rats in each group were used for histology and the others for gene expression. Extraction of total RNA was performed using the TRIspin method. Primers for cat K, RANK, RANKL, OPG IL-1, IL-b, TNF-a, iNOS and COX-2 were used. Each sample was normalized to 18S rRNA. Histology was evaluated qualitatively. Differences between the groups were analyzed by Kruskal Wallis and Mann-Whitney U-test.

Results: Both particles and fluid pressure increased the expression of osteoclastic genes. Particles induced an elevated expression of IL-6 and RANK compared to both controls and fluid pressure. There was a tendency that particles induced more expression of other inflammatory genes compared to fluid pressure.

Histology: The controls showed only few osteoclasts at the bone surface. The particle group showed osteoclasts at the surface towards the particles. In contrast, the pressure group showed resorption cavities spread out inside the bone.

Discussion: Although there was more resorption in the pressure group, there was a lesser inflammatory response. This suggests that pressure-induced resorption is mediated via different pathways.