Anna Fahlgren

Fluid pressure and flow as a cause of bone resorption

Background Unstable implants in bone become surrounded by an osteolytic zone. This is seen around loose screws, for example, but may also contribute to prosthetic loosening. Previous animal studies have shown that such zones can be induced by fluctuations in fluid pressure or flow, caused by implant instability. Method To understand the roles of pressure and flow, we describe the 3-dimensional distribution of osteolytic lesions in response to fluid pressure and flow in a previously reported rat model of aseptic loosening. 50 rats had a piston inserted in the proximal tibia, designed to produce 20 local spikes in fluid pressure of a clinically relevant magnitude (700 mmHg) twice a day. The spikes lasted for about 0.3 seconds. After 2 weeks, the pressure was measured in vivo, and the osteolytic lesions induced were studied using micro-CT scans. Results Most bone resorption occurred at pre-existing cavities within the bone in the periphery around the pressurized region, and not under the piston. This region is likely to have a higher fluid flow and less pressure than the area just beneath the piston. The velocity of fluid flow was estimated to be very high (roughly 20 mm/s). Interpretation The localization of the resorptive lesions suggests that high-velocity fluid flow is important for bone resorption induced by instability.

Radiographic Joint Space Narrowing and Histologic Changes in a Rabbit Meniscectomy Model of Early Knee Osteoarthrosis

The purpose of this study was to compare weightbearing radiographs with histologic cartilage evaluation in a rabbit meniscectomy model of the early stage of osteoarthrosis. Fifteen rabbits had a medial meniscectomy performed in one knee and a sham operation in the other knee. Five rabbits each were sacrificed at 13, 25, and 40 weeks after surgery. Radiographic joint space width and histologic cartilage changes of the medial knee compartment were quantified. Five non-operated knees and five knees in which the meniscus had been removed immediately before the evaluations served as control specimens. Overall, the joint space of the peripheral part of the medial knee compartment was narrower in knees operated on for meniscus removal than in sham-operated knees (P < 0.003). In the knees with the meniscus removed, more cartilage changes were seen at the joint surface area of contact on radiographs than in the sham-operated knees (P < 0.0015). Indeed, the area of contact had cartilage changes similar to those in the whole medial compartment. However, there was no correlation between the degree of histologic cartilage change and the corresponding joint space measurements. Joint space width as measured on weightbearing radiographs is reduced after meniscectomy in the rabbit, but it does not reflect the degree of cartilage damage of the loaded joint surfaces in early stages of osteoarthrosis.

Additive effects of PTH and bisphosphonates on the bone healing response to metaphyseal implants in rats

Background When PTH is used to increase the amount of bone in osteoporotic patients, combination with bisphosphonates is known to attenuate the response. This might be explained by the reduced number of remodeling sites after bisphosphonate treatment, which reduces the number of cells able to respond to PTH. However, in a repair situation after trauma, a large number of osteoblasts reside in the wound site. If their activity is no longer coupled to osteoclasts, decreased resorption by bisphosphonates and stimulation of osteoblastic activity by PTH should both (independently) increase bone formation. Thus, we hypothesized that in contrast to the case in osteoporosis treatment, PTH and bisphosphonates have an additive effect in situations involving bone regeneration. Material and methods Stainless steel screws, either coated with biphosphonates or uncoated, were inserted in 46 rat tibias. This normally elicits a bone repair response, leading to a gradual increase in the strength of screw fixation. Half of the rats also received daily injections of teriparatide (PTH). Thus, there were 4 groups: control, bisphosphonate, PTH, and bisphosphonate plus PTH. Pull-out force and energy were measured after 2 weeks. Results The combined treatment had the strongest effect. It doubled the pull-out force and tripled the pull-out energy, compared to untreated controls. Also, bisphosphonate or PTH alone increased the pull-out force and energy, although less. No treatment cross-dependency was observed. Interpretation Because bisphosphonates mainly influence osteoclasts, and intermittent administration of PTH mainly influences osteoblasts, our findings indicate that to a large extent these cells work without coupling in this model. It appears that bisphosphonates are unlikely to attenuate the response to PTH during the formation of new bone.

CDMP-2 injection improves early tendon healing in a rabbit model for surgical repair.

This study examines the hypothesis that cartilage‐derived morphogenic protein‐2 (CDMP‐2) can improve tendon healing after surgical repair. We have previously found improved tendon healing by applying CDMP‐2 in models for conservative treatment with mechanically loaded Achilles tendon defects in rats and rabbits. In this study, the patellar tendon was unloaded by patello‐ tibial cerclage and cut transversely in 40 rabbits. Two hours post‐operative, the rabbits received a dose of 20 μg of CDMP‐2 or vehicle injected into the hematoma. Specimens were harvested after 14 and 28 days and evaluated by biomechanical testing, radiography and histology. At 14 days, CDMP‐2 caused a 65% increase in force at failure, a 50% increase in ultimate stress and a 57% increase in stiffness, as compared with controls. There was no effect on callus size. At 28 days, no differences between the treatment groups could be demonstrated. No bone or cartilage was found in any tendon or regenerated tissue at any time point. Thus, early tendon repair can be stimulated by CDMP‐2 in an unloaded model. These results suggest that CDMP‐2 might be of interest for clinical use as a complement to surgical treatment of tendon ruptures.

Compression therapy promotes proliferative repair during rat Achilles tendon immobilization

Achilles tendon ruptures are treated with an initial period of immobilization, which obstructs the healing process partly by a reduction of blood circulation. Intermittent pneumatic compression (IPC) has been proposed to enhance tendon repair by stimulation of blood flow. We hypothesized that daily IPC treatment can counteract the deficits caused by 2 weeks of immobilization post tendon rupture. Forty‐eight Sprague‐Dawley SD) rats, all subjected to blunt Achilles tendon transection, were divided in three equal groups. Group A was allowed free cage activity, whereas groups B–C were immobilized at the operated hindleg. Group C received daily IPC treatment. Two weeks postrupture the rats were euthanatized and the tendons analyzed with tensile testing and histological assessments of collagen organization and collagen III‐LI occurrence. Immobilization significantly reduced maximum force, energy uptake, stiffness, tendon length, transverse area, stress, organized collagen diameter and collagen III‐LI occurrence by respectively 80, 75, 77, 22, 47, 65, 49, and 83% compared to free mobilization. IPC treatment improved maximum force 65%, energy 168%, organized collagen diameter 50%, tendon length 25%, and collagen III‐LI occurrence 150% compared to immobilization only. The results confirm that immobilization impairs healing after tendon rupture and furthermore demonstrate that IPC‐treatment can enhance proliferative tendon repair by counteracting biomechanical and morphological deficits caused by immobilization.

Meniscectomy leads to an early increase in subchondral bone plate thickness in the rabbit knee

We evaluated morphological changes in the tibial bone after meniscectomy in a rabbit model. 15 rabbits subjected to a medial meniscectomy in the right knee and a sham-operation in the left. Histomorphometric parameters were evaluated in the subchondral bone plate and the underlying trabecular bone, 13, 25 and 40 weeks after surgery. 5 rabbits were used as unoperated controls.Meniscectomized knees had a thicker subchondral bone plate than sham-operated contralateral ones in 13 of the 15 rabbits (p= 0.01), but the trabecular bone showed no morphological differences. The meniscectomized knees of these rabbits developed mild osteoarthrosis, described elsewhere, which may have been partly due to a change in the mechanical properties of the thickened subchondral bone plate. Our findings suggest that the first bony response after meniscectomy occurs in the subchondral bone plate rather than in the trabecular bone.

Histology of an undisplaced femoral fatigue fracture in association with bisphosphonate treatment. Frozen bone with remodelling at the crack.

A 57-year-old woman stumbled over her carpet, almost fell, and tried to sit down on the floor, when she heard a crack and sustained a transverse diaphyseal fracture of her left femur. The fracture had been preceded by thigh pain for a couple of months, but this was thought to be caused by spinal stenosis, for which she had been operated twice. She had been diagnosed with seronegative rheumatoid arthritis 10 years before the fracture and had initially undergone different pharmacologcal treatments, all of which except cortisone had been terminated. The diagnosis was repeatedly doubted, but cortisone gave symptomatic relief and was therefore continued at 5 mg/day with intermittent high-dose treatment (25 mg/day) during exacerbations. Apart from prednisolone, no anti-rheumatic drugs had been given during the previous 6 years. The patient had been given alendronate (70 mg/week) in 2001, followed by risedronate (35 mg/week) from 2002 until the fracture in 2009. The patient had also been taking 20–40 mg omeprazol a day since 2000. The fracture had a typical fatigue fracture appearance (Neviaser et al. 2008), and was operated on with intramedullary nailing. Because of a history of pain on weight bearing also in her contralateral thigh, new radiographs were taken and showed a non-displaced stress fracture of the subtrochanteric region (Figure 1). This fracture was also treated with intramedullary nailing. Figure 1. Right femur. Arrow indicates undisplaced fatigue fracture. Surgery (second operation) After the nail had been inserted, the non-displaced fracture was exposed. It could be seen as a dark line the size of a hair on the bone surface, surrounded by a barely visible protrusion of the bone. A 12 × 15-mm specimen including the fracture was excised, with the patients informed consent. The procedure was approved by the Regional Ethics Committee. Both fractures healed uneventfully and mineralized callus was seen at the biopsy site after 5 months.

Bone Resorption Induced by Fluid Flow

A model where bone resorption is driven by stimulus from fluid flow is developed and used as a basis for computer simulations, which are compared with experiments. Models for bone remodeling are usually based on the state of stress, strain, or energy density of the bone tissue as the stimulus for remodeling. We believe that there is experimental support for an additional pathway, where an increase in the amount of osteoclasts, and thus osteolysis, is caused by the time history of fluid flow velocity, fluid pressure, or other parameters related to fluid flow at the bone/soft tissue interface of the porosities in the bone.

Targeting RANKL for reduction of bone loss around unstable implants: OPG-Fc compared to alendronate in a model for mechanically induced loosening

Orthopedic joint prostheses may loosen because of localized bone resorption. Despite initial optimism, there are no reports showing that bisphosphonates can stop the progression of prosthetic loosening once it has begun. This might be due to the strong resorptive stimulus, which continuously recruits new osteoclasts. Therefore, we hypothesized that a treatment targeting osteoclast recruitment would be more efficacious than a treatment reducing osteoclast activity. We used a previously described rat model for instability-induced bone resorption, and compared OPG-Fc with alendronate at a clinically relevant or an extreme dose. A titanium plate was osseointegrated at the rat tibial surface. Instability was simulated by a piston, moving perpendicularly to the bone surface. Piston movement induced bone loss via hydrostatic pressure or fluid flow. Rats were randomized to 5 groups (total n=56), of which 4 were subjected to instability and one was stable. The unstable groups were injected with either high-dose OPG-Fc (10 mg/kg, twice weekly), a high dose of alendronate (20 μg /kg/day), an extreme dose of alendronate (200 μg/kg/day) or saline. Significant protection against resorption could only be shown for OPG-Fc and the extreme alendronate dose. Both alendronate doses reduced serum levels of tartrate-resistant acid phosphatase isoform 5b to a similar extent, demonstrating that the lower dose was able to reduce resorption in the normally remodeling skeleton, although not in the osteolytic lesions caused by instability. Osteoclast numbers in the lesion were increased by the lower bisphosphonate dose and reduced by OPG-Fc. The results suggest the possibility of targeting osteoclast recruitment via the RANKL system in patients with impending prosthetic loosening.

A capsular incision leads to a fast osteoarthritic response, but also elevated levels of activated osteogenic protein-1 in rabbit knee joint cartilage.

We studied whether a small capsular incision alone, or combined with meniscectomy could induce early osteoarthritic changes in the rabbit knee.