The rate of disuse osteopenia in admitted, non-weight-bearing patients

Disuse osteopenia is a well-recognized consequence of prolonged physical inactivity, but its rate after orthopaedic injuries necessitating non-weight-bearing is not well studied. The purpose of this study was to estimate the rate of disuse osteopenia at the lumbar spine and proximal femur in patients with lower extremity trauma admitted to the hospital. We performed a retrospective chart review of patients with lower extremity trauma with a period of strict non-weight-bearing between completion of two computed tomography (CT) scans. The radiodensity of the proximal femur or lumbar vertebrae was measured from the earliest and latest available CT scans within the non-weight-bearing timeframe. The change in estimated bone mineral density (eBMD) was calculated as a proxy for disuse osteopenia. A total of 189,111 patients were screened, with 17 patients in the proximal femur group and 15 patients in the lumbar spine group meeting inclusion and exclusion criteria. The average rate of change in eBMD of the proximal femur was a decrease of 7.54 HU/day, 95% confidence interval (CI) [3.65, 11.43]. The average rate of change in eBMD of the lumbar spine was an increase of 1.45 HU/day, 95% CI [-3.15, 6.06]. In admitted, non-weight-bearing orthopaedic trauma patients, our novel study suggests that the proximal femur experiences disuse osteopenia during periods of non-weight-bearing, although this finding was not observed at the lumbar spine. The clinical significance of this data underscores the important consideration of disuse osteopenia by all physicians when caring for patients that may require non-weight-bearing restrictions.

Osteoporosis does not affect bone mineral density change in the proximal humerus or the functional outcome after open reduction and internal fixation of unilateral displaced 3- or 4-part fractures at 12-month follow-up

BACKGROUND

The aim of this prospective study was to investigate bone mineral density (BMD) changes in the proximal humerus of the shoulder during a healing period of 12 months after displaced 3- or 4-part proximal humerus fractures treated with open reduction and internal fixation (ORIF) with an anatomic angular stable locking plate and the influence on fracture healing and functional outcomes.

METHODS

In a prospective multicenter study, 36 patients (29F and 7M, age range: 38-83) with unilateral displaced 3- or 4-part proximal humerus fractures were included for ORIF. Dual-energy x-ray absorptiometry for osteoporosis status was employed. Postoperative and 6-week, 3-, 6-, and 12-month shoulder radiographs and dual-energy x-ray absorptiometry of the shoulder with BMD measures in 4 templated regions of interest (ROIs) were performed. Functional outcomes, Western Ontario Osteoarthritis of the Shoulder index, Constant score, visual analog scale pain (VAS), and 36-Item Short Form Survey, were collected.

RESULTS

A total of 17 of 36 patients had osteoporosis. We found no differences in BMD changes, functional outcomes, radiology, or need for revision surgery between the osteoporosis and nonosteoporosis groups. The BMD values gradually declined from baseline to 3-month follow-up in all 4 ROIs of the operated shoulders. All 4 ROIs in the operated shoulder presented with a reduction in BMD at 3, 6, and 12 months compared with baseline, whereas no significant BMD changes were seen in the healthy shoulder during the study period. The functional outcomes displayed an increase in Constant score from 3 to 12 months, but a decrease in domains of the 36-Item Short Form Survey from preinjury to 12 months (physical functioning, general health, and bodily pain). Preinjury and 12-month Western Ontario Osteoarthritis of the Shoulder index, VAS pain at rest, and VAS pain at activity were comparable.

CONCLUSION

BMD changes appeared swiftly in the proximal humerus, after the treatment of displaced 3- or 4-part fractures with ORIF, particularly affecting the proximal diaphysis of the humerus. Shoulder function was restored to preinjury levels for most of the patients. Osteoporosis may not be regarded as a contraindication for the treatment of displaced 3- or 4-part fractures with ORIF.

The Skeletal Cellular and Molecular Underpinning of the Murine Hindlimb Unloading Model

Bone adaptation to spaceflight results in bone loss at weight bearing sites following the absence of the stimulus represented by ground force. The rodent hindlimb unloading model was designed to mimic the loss of mechanical loading experienced by astronauts in spaceflight to better understand the mechanisms causing this disuse-induced bone loss. The model has also been largely adopted to study disuse osteopenia and therefore to test drugs for its treatment. Loss of trabecular and cortical bone is observed in long bones of hindlimbs in tail-suspended rodents. Over the years, osteocytes have been shown to play a key role in sensing mechanical stress/stimulus via the ECM-integrin-cytoskeletal axis and to respond to it by regulating different cytokines such as SOST and RANKL. Colder experimental environments (~20-22°C) below thermoneutral temperatures (~28-32°C) exacerbate bone loss. Hence, it is important to consider the role of environmental temperatures on the experimental outcomes. We provide insights into the cellular and molecular pathways that have been shown to play a role in the hindlimb unloading and recommendations to minimize the effects of conditions that we refer to as confounding factors.

Dynamic hydraulic flow stimulation on mitigation of trabecular bone loss in a rat functional disuse model

Bone fluid flow (BFF) has been demonstrated as a critical regulator in mechanotransductive signaling and bone adaptation. Intramedullary pressure (ImP) and matrix strain have been identified as potential generators to regulate BFF. To elevate in vivo oscillatory BFF using ImP, a dynamic hydraulic stimulation (DHS) approach was developed. The objective of this study was to evaluate the effects of DHS on mitigation of bone loss and structural alteration in a rat hindlimb suspension (HLS) functional disuse model. Sixty-one 5-month old female Sprague-Dawley rats were divided into five groups: 1) baseline control, 2) age-matched control, 3) HLS, 4) HLS+static loading, and 5) HLS+DHS. Hydraulic flow stimulation was carried out daily on a "10 min on-5 min off-10 min on" loading regime, 5 days/week, for a total of 4 weeks in the tibial region. The metaphyseal trabecular regions of the proximal tibiae were analyzed using μCT and histomorphometry. Four weeks of HLS resulted in a significant loss of trabecular bone, leading to structural deterioration. HLS with static loading alone was not sufficient to attenuate the bone loss. Bone quantity and microarchitecture were significantly improved by applying DHS loading, resulting increase of 83% in bone volume fraction, 25% in trabecular number and mitigation of 26% in trabecular separation compared to HLS control. Histomorphometry analysis on trabecular mineralization coincided with the μCT analysis, in which DHS loading yielded increases of 34% in histomorphometric BV/TV, 121% in MS/BS, 190% in BFR/BS and 146% in BFR/BV, compared to the HLS control. Overall, the data demonstrated that dynamic hydraulic flow loading has potentials to provide regulatory signals for mitigating bone loss induced by functional disuse. This approach may provide a new alternative mechanical intervention for future clinical treatment for osteoporosis.

Alteration of contraction-to-rest ratio to optimize trabecular bone adaptation induced by dynamic muscle stimulation

Disuse osteopenia has shown to decrease bone mineral density and compromise bone's integrity, i.e., in aging population and long-term functional disuse. The degree of attenuation of trabecular bone loss and deterioration of its microarchitecture is closely dependent on the mechanical loading parameters within the regimen. Dynamic muscle stimulation as a preventive countermeasure for disuse osteopenia has been shown to be effective. The objective of this study is to determine whether the contraction-to-rest ratio is a crucial parameter to affect the skeletal adaptive responses under a functional disuse environment. Fifty-six skeletally matured Sprague-Dawley rats were divided into seven groups for the 4-week experiment: baseline control, age-matched control, hindlimb suspended (HLS), and HLS plus muscle stimulation with a contraction-to-rest ratio of 1/4, 2/8, 4/6, and 2/28 s. Muscle stimulation was carried out for total of 10 min/day, 5 days/week, for 4 weeks. Trabecular bone in the distal femurs was analyzed with microcomputed tomography and histomorphometry. HLS alone for 4-week resulted in a 25-45% trabecular bone loss in the distal femur. Dynamic muscle stimulation, applied at 50 Hz frequency, with a 2/8 s contraction-to-rest ratio demonstrated significant attenuation of trabecular bone loss against the 4-week disuse, with up to +74% in bone volume fraction, +164% in connectivity, +20% in trabecular number, and -18% in spacing (p<0.05). Stimulation with 1/4 and 4/6 also showed similar effects but with lesser percentage differences when comparing to the HLS animals. Similarly, histomorphometric analysis showed partial enhancement in mineralizing surface and mineral apposition rate. The results suggested the potentials of dynamic muscle stimulation in regulating skeletal adaptive responses and illustrated the effects of optimized contraction-to-rest in mitigation of bone loss, in which 2/8 s has shown maximal adaptive response among all tested ratios.