Surgical restabilization reduces the progression of post-traumatic osteoarthritis initiated by ACL rupture in mice.

OBJECTIVE: People who sustain joint injuries such as anterior cruciate ligament (ACL) rupture often develop post-traumatic osteoarthritis (PTOA). In human patients, ACL injuries are often treated with ACL reconstruction. However, it is still unclear how effective joint restabilization is for reducing the progression of PTOA. The goal of this study was to determine how surgical restabilization of a mouse knee joint following non-invasive ACL injury affects PTOA progression. DESIGN: In this study, 187 mice were subjected to non-invasive ACL injury or no injury. After injury, mice underwent restabilization surgery, sham surgery, or no surgery. Mice were then euthanized on day 14 or day 49 after injury/surgery. Functional analyses were performed at multiple time points to assess voluntary movement, gait, and pain. Knees were analyzed ex vivo with micro-computed tomography, RT-PCR, and whole-joint histology to assess articular cartilage degeneration, synovitis, and osteophyte formation. RESULTS: Both ACL injury and surgery resulted in loss of epiphyseal trabecular bone (-27-32%) and reduced voluntary movement at early time points. Joint restabilization successfully lowered OA score (-78% relative to injured at day 14, p < 0.0001), and synovitis scores (-37% relative to injured at day 14, p = 0.042), and diminished the formation of chondrophytes/osteophytes (-97% relative to injured at day 14, p < 0.001, -78% at day 49, p < 0.001). CONCLUSIONS: This study confirmed that surgical knee restabilization was effective at reducing articular cartilage degeneration and diminishing chondrophyte/osteophyte formation after ACL injury in mice, suggesting that these processes are largely driven by joint instability in this mouse model. However, restabilization was not able to mitigate the early inflammatory response and the loss of epiphyseal trabecular bone, indicating that these processes are independent of joint instability.

The heart-bone connection: relationships between myocardial infarction and osteoporotic fracture.

Myocardial infarction (MI) and osteoporotic fracture (Fx) are two of the leading causes of mortality and morbidity worldwide. Although these traumatic injuries are treated as if they are independent, there is epidemiological evidence linking the incidence of Fx and MI, thus raising the question of whether each of these events can actively influence the risk of the other. Atherosclerotic cardiovascular disease and osteoporosis, the chronic conditions leading to MI and Fx, are known to have shared pathoetiology. Furthermore, sustained systemic inflammation after traumas such as MI and Fx has been shown to exacerbate both underlying chronic conditions. However, the effects of MI and Fx outside their own system have not been well studied. The sympathetic nervous system (SNS) and the complement system initiate a systemic response after MI that could lead to subsequent changes in bone remodeling through osteoclasts. Similarly, SNS and complement system activation following fracture could lead to heart tissue damage and exacerbate atherosclerosis. To determine whether damaging bone-heart cross talk may be important comorbidity following Fx or MI, this review details the current understanding of bone loss after MI, cardiovascular damage after Fx, and possible shared underlying mechanisms of these processes.

Sex differences in systemic bone and muscle loss following femur fracture in mice.

Fracture induces systemic bone loss in mice and humans, and a first (index) fracture increases the risk of future fracture at any skeletal site more in men than women. The etiology of this sex difference is unknown, but fracture may induces a greater systemic bone loss response in men. Also sex differences in systemic muscle loss after fracture have not been examined. We investigated sex differences in systemic bone and muscle loss after transverse femur fracture in 3-month-old male and female C57BL/6 J mice. Whole-body and regional bone mineral content and density (BMC and BMD), trabecular and cortical bone microstructure, muscle contractile force, muscle mass, and muscle fiber size were quantified at multiple time points postfracture. Serum concentrations of inflammatory cytokines (IL-1ß, IL-6, and TNF-α) were measured 1-day postfracture. One day postfracture, IL-6 and Il-1B were elevated in fracture mice of both sexes, but TNF-α was only elevated in male fracture mice. Fracture reduced BMC, BMD, and trabecular bone microstructural properties in both sexes 2 weeks postfracture, but declines were greater in males. Muscle contractile force, mass, and fiber size decreased primarily in the fractured limb at 2 weeks postfracture and females showed a trend toward greater muscle loss. Bone and muscle properties recovered by 6 weeks postfracture. Overall, postfracture systemic bone loss is greater in men, which may contribute to sex differences in subsequent fracture risk. In both sexes, muscle loss is primarily confined to the injured limb and fracture may induce greater inflammation in males.

Systemic bone loss following myocardial infarction in mice.

Myocardial infarction (MI) and osteoporotic fracture are leading causes of morbidity and mortality, and epidemiological evidence linking their incidence suggests possible crosstalk. MI can exacerbate atherosclerosis through the sympathetic nervous system (SNS) activation and ß3 adrenoreceptor-mediated release of hematopoietic stem cells, leading to monocytosis. We hypothesized that this same pathway initiates systemic bone loss following MI, since osteoclasts differentiate from monocytes. In this study, MI was created with left anterior descending artery ligation in 12-week-old male mice (n = 24) randomized to ß3 -adrenergic receptor (AR) antagonist (SR 59230A) treatment or no treatment for 10 days postoperatively. Additional mice (n = 21, treated and untreated) served as unoperated controls. Bone mineral density (BMD), bone mineral content (BMC), and body composition were quantified at baseline and 10 days post-MI using dual-energy x-ray absorptiometry; circulating monocyte levels were quantified and the L5 vertebral body and femur were analyzed with microcomputed tomography 10 days post-MI. We found that MI led to circulating monocyte levels increases, BMD and BMC decreases at the femur and lumbar spine in MI mice (-6.9% femur BMD, -3.5% lumbar BMD), and trabecular bone volume decreases in MI mice compared with control mice. ß3 -AR antagonist treatment appeared to diminish the bone loss response (-5.3% femur BMD, -1.2% lumbar BMD), though these results were somewhat inconsistent. Clinical significance: These results suggest that MI leads to systemic bone loss, but that the SNS may not be a primary modulator of this response; bone loss and increased fracture risk may be important clinical comorbidities following MI or other ischemic injuries.

Comparison of knee injury threshold during tibial compression based on limb orientation in mice.

Our previous studies used tibial compression overload to induce anterior cruciate ligament (ACL) rupture in mice, while others have applied similar or greater compressive magnitudes without injury. The causes of these differences in injury threshold are not known. In this study, we compared knee injury thresholds using a "prone configuration" and a "supine configuration" that differed with respect to hip, knee, and ankle flexion, and utilized different fixtures to stabilize the knee. Right limbs of female and male C57BL/6 mice were loaded using the prone configuration, while left limbs were loaded using the supine configuration. Mice underwent progressive loading from 2 to 20 N, or cyclic loading at 9 N or 14 N (n = 9-11/sex/loading method). Progressive loading with the prone configuration resulted in ACL rupture at an average of 10.2 ±â€¯0.9 N for females and 11.4 ±â€¯0.7 N for males. In contrast, progressive loading with the supine configuration resulted in ACL rupture in only 36% of female mice and 50% of male mice. Cyclic loading with the prone configuration resulted in ACL rupture after 15 ±â€¯8 cycles for females and 24 ±â€¯27 cycles for males at 9 N, and always during the first cycle for both sexes at 14 N. In contrast, cyclic loading with the supine configuration was able to complete 1,200 cycles at 9 N without injury for both sexes, and an average of 45 ±â€¯41 cycles for females and 49 ±â€¯25 cycles for males at 14 N before ACL rupture. These results show that tibial compression configurations can strongly affect knee injury thresholds during loading.

Osteophytes and fracture calluses share developmental milestones and are diminished by unloading.

Osteophytes are a typical radiographic finding during osteoarthritis (OA), but the mechanisms leading to their formation are not well known. Comparatively, fracture calluses have been studied extensively; therefore, drawing comparisons between osteophytes and fracture calluses may lead to a deeper understanding of osteophyte formation. In this study, we compared the time courses of osteophyte and fracture callus formation, and investigated mechanisms contributing to development of these structure. Additionally, we investigated the effect of mechanical unloading on the formation of both fracture calluses and osteophytes. Mice underwent either transverse femoral fracture or non-invasive anterior cruciate ligament rupture. Fracture callus and osteophyte size and ossification were evaluated after 3, 5, 7, 14, 21, or 28 days. Additional mice were subjected to hindlimb unloading after injury for 3, 7, or 14 days. Protease activity and gene expression profiles after injury were evaluated after 3 or 7 days of normal ambulation or hindlimb unloading using in vivo fluorescence reflectance imaging (FRI) and quantitative PCR. We found that fracture callus and osteophyte growth achieved similar developmental milestones, but fracture calluses formed and ossified at earlier time points. Hindlimb unloading ultimately led to a threefold decrease in chondro/osteophyte area, and a twofold decrease in fracture callus area. Unloading was also associated with decreased inflammation and protease activity in injured limbs detected with FRI, particularly following ACL rupture. qPCR analysis revealed disparate cellular responses in fractured femurs and injured joints, suggesting that fracture calluses and osteophytes may form via different inflammatory, anabolic, and catabolic pathways. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:699-710, 2018.