It is unknown whether activity-based physical therapy (ABPT) modalities that mobilize the paralyzed limbs improve bone integrity at the highly fracture-prone epiphyseal regions of the distal femur and proximal tibia following severe spinal cord injury (SCI). In this study, 4-mo-old skeletally mature littermate-matched male Sprague-Dawley rats received either SHAM surgery or severe contusion SCI. At 1 wk postsurgery, SCI rats were stratified to undergo no-ABPT, two 20-min bouts/day of quadrupedal bodyweight-supported treadmill training (qBWSTT), or hindlimb passive isokinetic bicycle (cycle) training, 5 days/wk for another 3 wk. We assessed locomotor recovery and plantar flexor muscle mass, tracked cancellous and cortical bone microstructure at the distal femoral and proximal tibial epiphyses using in vivo microcomputed tomography (microCT), and evaluated bone turnover at the tibial epiphysis with histomorphometry. All SCI animals displayed persistent hindlimb paralysis and pervasive muscle atrophy. Over the initial 2 wk, which included 1 wk of no exercise and 1 wk of ABPT acclimation, a similar magnitude of bone loss developed in all SCI groups. Thereafter, cancellous bone loss and cortical bone decrements increased in the SCI no-ABPT group. qBWSTT attenuated this trabecular bone loss but did not prevent the ongoing cortical bone deficits. In comparison, twice-daily cycle training increased the number and activity of osteoblasts versus other SCI groups and restored all bone microstructural parameters to SHAM levels at both epiphyseal sites. These data indicate that a novel passive isokinetic cycle training regimen reversed cancellous and cortical bone deterioration at key epiphyseal sites after experimental SCI via osteoblast-mediated bone anabolic mechanisms, independent of locomotor recovery or increased muscle mass. NEW & NOTEWORTHY This study was the first to assess how quadrupedal bodyweight-supported treadmill training or passive isokinetic bicycle (cycle) training impacts bone recovery at the distal femoral and proximal tibial epiphyses in a rat model of severe contusion spinal cord injury. Our results demonstrate that passive isokinetic cycle training completely restored cancellous and cortical bone microstructural parameters at these sites via osteoblast-mediated bone anabolic actions, independent of locomotor recovery or increased plantar flexor muscle mass.

BackgroundHigh-dose testosterone replacement therapy (TRT), paired with finasteride (type II 5α-reductase inhibitor), improves body composition, muscle strength, and bone mineral density (BMD) in older men, without inducing prostate enlargement—a side effect associated with TRT. Men with spinal cord injury (SCI) exhibit neuromuscular impairment, muscle atrophy, bone loss, and increased central adiposity, along with low testosterone. However, sparse evidence supports TRT efficacy after SCI.MethodsThis parallel-group, double-blind, placebo-controlled, and randomized clinical trial (RCT) is a pilot study that enrolled men (N = 12) with low to low–normal testosterone and gait impairments after chronic motor-incomplete SCI. Participants received high-dose intramuscular TRT (testosterone-enanthate, 125 mg/week) with finasteride (5 mg/day) vs. vehicle+placebo for 12 months. Change relative to baseline was determined for body composition, musculoskeletal outcomes, and prostate size, with effect sizes calculated between groups using Hedges’ g. Adverse events and feasibility were assessed.ResultsTRT + finasteride consistently increased testosterone (g = 1.16–3.08) and estradiol (g = 0.43–3.48), while concomitantly reducing dihydrotestosterone (g = 0.31–2.27). Very large effect sizes at both 6 and 12 months suggest TRT + finasteride increased whole-body fat-free (lean) mass (+3–4% vs. baseline, g = 2.12–2.14) and knee extensor (KE) whole-muscle cross-sectional area (+8–11% vs. baseline, g = 2.06–2.53) more than vehicle+placebo. Moderate-to-large effect sizes suggest TRT + finasteride increased KE maximal voluntary isometric torque (+15–40% vs. baseline, g = 0.47–1.01) and femoral neck and distal femur BMD from 6 months onward (g = 0.51–1.13), compared with vehicle+placebo, and reduced fat mass 9–14% within the whole-body, trunk, and android (visceral) regions at 12 months (g = 0.77–1.27). TRT + finasteride also produced small effect sizes favoring lesser prostate growth than vehicle+placebo (g = 0.31–0.43). The participant retention, drug compliance, and incidence and severity of adverse events were similar among the groups.ConclusionThese data provide proof-of-concept and rationale for larger RCTs aimed at discerning the impact of TRT + finasteride on body composition, musculoskeletal health, and physical function in men with SCI, along with effect sizes and variance of responses to assist in planning subsequent trials.Clinical trial registrationClinicalTrials.gov, identifier NCT02248701.

Medication-related osteonecrosis of the jaw (MRONJ) is a potentially severe adverse event in patients treated with antiresorptives. Management of MRONJ is challenging, and no non-antibiotic, established medical treatment exists. Intermittent parathyroid hormone (iPTH) has been used off-label to treat MRONJ with favorable results. However, its medical efficacy has rarely been substantiated in clinical or preclinical experiments. Using a validated rice rat, infection-based model of MRONJ, we evaluated the effects of iPTH on established MRONJ. We hypothesize that iPTH contributes to MRONJ resolution by enhancing alveolar bone turnover and healing oral soft tissues. Eighty-four rice rats began a standard rodent chow diet at age 4 weeks to induce localized periodontitis. Rats were simultaneously randomized to receive saline (vehicle, VEH) or zoledronic acid (ZOL, 80 μg/kg IV) every 4 weeks. Oral exams were conducted bi-weekly to assign a gross quadrant grade (GQG, 0–4) to evaluate any lesion at the lingual aspect of the interdental space between maxillary molar (M2) and M3. 14 of 20 VEH-treated rice rats (70%) developed maxillary localized periodontitis with GQG 2–3 after 30 ± 10 weeks of saline. Additionally, 40 of 64 ZOL-treated rice rats with periodontitis developed MRONJ-like lesions after 30 ± 10 weeks of ZOL treatment. Rice rats with localized periodontitis or MRONJ-like lesions were treated with saline or iPTH (40 μg/kg) subcutaneously (SC) 3 times/week For 6 weeks until euthanasia. We found that iPTH -treated ZOL rats had a lower prevalence of MRONJ ( p < 0.001), with lower severity extent of oral lesions ( p = 0.003) and percentage of empty osteocyte lacunae ( p < 0.001). ZOL rats treated with iPTH displayed a higher osteoblast surface ( p < 0.001), more osteoblasts ( p < 0.001), higher osteoclast surface ( p < 0.001) and more osteoclasts ( p = 0.002) at alveolar bone surfaces than ZOL/VEH rats. Greater gingival epithelial thickness and epithelial cell proliferation rate was found in the oral mucosa and gingiva of ZOL/PTH rats than in ZOL/VEH rats ( p < 0.001). Our data suggest that iPTH is an efficacious non-operative medicinal therapy that accelerates oral healing and enhances the resolution of MRONJ lesions in ZOL-treated rice rats.

Spinal cord injury (SCI) produces paralysis and a unique form of neurogenic disuse osteoporosis that dramatically increases fracture risk at the distal femur and proximal tibia. This bone loss is driven by heightened bone resorption and near-absent bone formation during the acute post-SCI recovery phase and by a more traditional high-turnover osteopenia that emerges more chronically, which is likely influenced by the continual neural impairment and musculoskeletal unloading. These observations have stimulated interest in specialized exercise or activity-based physical therapy (ABPT) modalities (e.g., neuromuscular or functional electrical stimulation cycling, rowing, or resistance training, as well as other standing, walking, or partial weight-bearing interventions) that reload the paralyzed limbs and promote muscle recovery and use-dependent neuroplasticity. However, only sparse and relatively inconsistent evidence supports the ability of these physical rehabilitation regimens to influence bone metabolism or to increase bone mineral density (BMD) at the most fracture-prone sites in persons with severe SCI. This review discusses the pathophysiology and cellular/molecular mechanisms that influence bone loss after SCI, describes studies evaluating bone turnover and BMD responses to ABPTs during acute versus chronic SCI, identifies factors that may impact the bone responses to ABPT, and provides recommendations to optimize ABPTs for bone recovery.

ABSTRACT Introduction Spinal cord injury (SCI) produces diminished bone perfusion and bone loss in the paralyzed limbs. Activity-based physical therapy (ABPT) modalities that mobilize and/or reload the paralyzed limbs (e.g., bodyweight-supported treadmill training (BWSTT) and passive-isokinetic bicycle training) transiently promote lower-extremity blood flow (BF). However, it remains unknown whether ABPT alter resting-state bone BF or improve skeletal integrity after SCI. Methods Four-month-old male Sprague-Dawley rats received T9 laminectomy alone (SHAM; n = 13) or T9 laminectomy with severe contusion SCI (n = 48). On postsurgery day 7, SCI rats were stratified to undergo 3 wk of no ABPT, quadrupedal (q)BWSTT, or passive-isokinetic hindlimb bicycle training. Both ABPT regimens involved two 20-min bouts per day, performed 5 d·wk−1. We assessed locomotor recovery, bone turnover with serum assays and histomorphometry, distal femur bone microstructure using in vivo microcomputed tomography, and femur and tibia resting-state bone BF after in vivo microsphere infusion. Results All SCI animals displayed immediate hindlimb paralysis. SCI without ABPT exhibited uncoupled bone turnover and progressive cancellous and cortical bone loss. qBWSTT did not prevent these deficits. In comparison, hindlimb bicycle training suppressed surface-level bone resorption indices without suppressing bone formation indices and produced robust cancellous and cortical bone recovery at the distal femur. No bone BF deficits existed 4 wk after SCI, and neither qBWSTT nor bicycle altered resting-state bone perfusion or locomotor recovery. However, proximal tibia BF correlated with several histomorphometry-derived bone formation and resorption indices at this skeletal site across SCI groups. Conclusions These data indicate that passive-isokinetic bicycle training reversed cancellous and cortical bone loss after severe SCI through antiresorptive and/or bone anabolic actions, independent of locomotor recovery or changes in resting-state bone perfusion.

Diminished bone perfusion develops in response to disuse and has been proposed as a mechanism underlying bone loss. Bone blood flow (BF) has not been investigated within the unique context of severe contusion spinal cord injury (SCI), a condition that produces neurogenic bone loss that is precipitated by disuse and other physiological consequences of central nervous system injury. Herein, 4-mo-old male Sprague-Dawley rats received T-9 laminectomy (SHAM) or laminectomy with severe contusion SCI (n = 20/group). Time course assessments of hindlimb bone microstructure and bone perfusion were performed in vivo at 1- and 2-wk postsurgery via microcomputed tomography (microCT) and intracardiac microsphere infusion, respectively, and bone turnover indices were determined via histomorphometry. Both groups exhibited cancellous bone loss beginning in the initial postsurgical week, with cancellous and cortical bone deficits progressing only in SCI thereafter. Trabecular bone deterioration coincided with uncoupled bone turnover after SCI, as indicated by signs of ongoing osteoclast-mediated bone resorption and a near-complete absence of osteoblasts and cancellous bone formation. Bone BF was not different between groups at 1 wk, when both groups displayed bone loss. In comparison, femur and tibia perfusion was 30%-40% lower in SCI versus SHAM at 2 wk, with the most pronounced regional BF deficits occurring at the distal femur. Significant associations existed between distal femur BF and cancellous and cortical bone loss indices. Our data provide the first direct evidence indicating that bone BF deficits develop in response to SCI and temporally coincide with suppressed bone formation and with cancellous and cortical bone deterioration. NEW & NOTEWORTHY We provide the first direct evidence indicating femur and tibia blood flow (BF) deficits exist in conscious (awake) rats after severe contusion spinal cord injury (SCI), with the distal femur displaying the largest BF deficits. Reduced bone perfusion temporally coincided with unopposed bone resorption, as indicated by ongoing osteoclast-mediated bone resorption and a near absence of surface-level bone formation indices, which resulted in severe cancellous and cortical microstructural deterioration after SCI.

BACKGROUND3D image registration is a technique where in‐vivo microCT scans are collected at different timepoints and regions of interest (ROI) are constructed and aligned to improve the precision of determining bone microstructure. In the rodent spinal cord injury (SCI) model, the rapid bone loss occurring at the distal femur precludes the use of standard 3D registration strategies.PURPOSETo (1) adapt a microCT‐based 3D registration protocol to our rodent SCI model, (2) determine the degree of cancellous bone loss at the distal femoral epiphysis after SCI, and (3) assess the effects of bodyweight‐supported treadmill training (TM) or passive bicycle training (PBT) on bone loss after SCI.METHODS16‐wk old male Sprague‐Dawley rats were stratified to receive: 1) T9 laminectomy (SHAM) (n=9), 2) severe T9 contusion (SCI) (n=10), 3) SCI+TM (n=10), or 4) SCI+PBT (n=14). TM and PBT began 1‐wk post‐surgery (post‐sx, two 20‐min bouts/day, 5‐d/wk for 3‐wks). In‐vivo microCT scans were performed pre‐sx and 2‐ and 4‐wks post‐sx. Images were aligned with a 3D registration protocol. ROIs were developed to assess cancellous bone microstructure at the distal femoral epiphysis using two separate protocols that either included or excluded new bone formed by periosteal bone expansion over the 4‐wk experiment.RESULTSDifferences were noted between the ROI protocols, with the ROI that included periosteal bone growth underestimating SCI‐induced bone loss. As such, the results reported hereafter were derived from the ROI that excluded new bone resulting from periosteal bone expansion. No differences in bone outcomes were present in SHAMs at any timepoint except for a slightly higher trabecular separation (Tb.Sp) at 4‐wks (p<.05). At 2‐wks, SCI displayed 14% lower cancellous bone volume (BV/TV) than pre‐sx (p<.01), characterized by 13% lower trabecular number (Tb.N) (p<.05) and 7% higher Tb.Sp (p<.05). Bone loss was more pronounced at 4‐wks after SCI, evidenced by lower trabecular thickness (Tb.Th) and higher Tb.Sp vs 2‐wks (both p<.01). SCI+TM and SCI+PBT displayed a similar magnitude of bone loss to SCI at 2‐wks (1‐wk after starting exercise). Thereafter, SCI+TM displayed no further bone loss, resulting in 9% less BV/TV loss than SCI (p<.01). In comparison, PBT increased BV/TV 15% from 2‐ to 4‐wks (p<.01), due to 5% higher Tb.Th (p<.01) and 12% higher Tb.N (p<.05), ultimately restoring BV/TV to pre‐sx levels. Structural model index (SMI) and trabecular pattern factor (Tb.Pf) increased in SCI (p<0.05) and SCI+TM at 4‐wks (p<.05 to <.01), signifying transition from rod‐like to weaker plate‐like trabeculae and a less connected trabecular network, respectively. In comparison, SMI (p<.05) and Tb.Pf (p<.01) increased in SCI+PBT at 2‐wks before returning to pre‐sx levels by 4‐wks.CONCLUSIONUsing our 3D registration protocol, we determined that SCI causes severe cancellous bone loss and changes indicative of an overall weakening of bone architecture at the distal femoral epiphysis. TM attenuated bone loss at this skeletal site, while PBT promoted new bone formation and restored BV/TV to pre‐sx levels.Support or Funding InformationThis work was supported by the APS UGSRF.