Da Costa Gómez T M, Barrett J G, Sample S J, Radtke C L, Kalscheur V L, Lu Y, Markel M D, Santschi E M, Scollay M C, Muir P
Comparative Orthopaedic Research Laboratory, School of Veterinary Medicine, University of Wisconsin-Madison, 2015 Linden Drive, Madison, WI 53706, USA.
Bone. 2005 Jul;37(1):16-24. doi: 10.1016/j.bone.2004.12.016.
Functional adaptation of bone normally protects the skeleton from fracture during daily activity. Accumulation of microcracking and loss of osteocytes have been implicated in the regulation and initiation of targeted (reparative) remodeling of bone and, in certain situations, the development of fatigue or stress fracture. We performed a histologic study of the dorsal cortex of the mid-diaphysis of the third metacarpal (Mc-III) bone of Thoroughbred racehorses after bones were bulk-stained in basic fuchsin and transverse calcified sections were prepared. The Thoroughbred racehorse is an extreme athlete whose Mc-III bone experiences particularly high cyclic strains during training and racing. A group of non-athletic horses was also included in the experiment. The following variables were quantified: activation frequency (Ac.f); bone formation rate (BFR); resorption space density (Rs.N/T.Ar); microcrack density (Cr.Dn); microcrack mean length (Cr.Le); microcrack surface density (Cr.S.Dn); osteocyte density (Ot.N/T.Ar; Ot.N/B.Ar); and bone volume fraction (B.Ar/T.Ar). Ac.f and BFR were estimated using a mathematical algorithm. Using confocal microscopy, bones were examined for fine microcracks, diffuse matrix injury, and disruption of the osteocyte syncytium. Low values for Cr.Dn (#/mm2) were found in both groups (0.022+/-0.008 and 0.013+/-0.006 for racing Thoroughbreds and non-athletic horses, respectively). There was no significant relationship between Cr.Dn and Ot.N/T.Ar; Ot.N/B.Ar, B.Ar/T.Ar, and Ot.N/T.Ar; Ot.N/B.Ar, and remodeling (Ac.f, Rs.N/T.Ar) and Ot.N/T.Ar; Ot.N/B.Ar. Intense remodeling of the Mc-III dorsal cortex was found in the racing Thoroughbreds (Ac.f 12.8+/-7.4 #/mm2/year; BFR 31.5+/-15.6%; Rs.N/T.Ar 0.19+/-0.09 #/mm2) and was significantly increased compared with non-athletic horses. Overall, remodeling was weakly correlated with Cr.Dn (r2=0.15, P<0.05). Subtle matrix injury, not detectable by bright-field microscopy, was particularly evident adjacent to resorption spaces in Thoroughbred bone. In non-athletic horses, disruption of the dendritic cell processes of osteocytes associated with cement lines and interstitial fragments was more evident. Taken together, these findings suggest that site-specific (targeted) induction of remodeling during functional adaptation of bone in a high-strain skeletal site is not dependent on accumulation of microcracking or loss of osteocytes. We hypothesize that athleticism can directly influence bone turnover in this extreme athlete through pathways that do not involve classical linear microcracks.
骨骼的功能适应性通常可在日常活动中保护骨骼免受骨折。微裂纹的积累和骨细胞的丢失与骨骼靶向（修复性）重塑的调节和启动有关，并且在某些情况下，与疲劳性骨折或应力性骨折的发生有关。我们对纯种赛马第三掌骨（Mc-III）骨干中段背侧皮质进行了组织学研究，将骨骼用碱性品红进行整体染色后制备横向钙化切片。纯种赛马是极限运动员，其Mc-III骨在训练和比赛期间承受特别高的周期性应变。实验中还纳入了一组非运动马匹。对以下变量进行了量化：激活频率（Ac.f）；骨形成率（BFR）；吸收间隙密度（Rs.N/T.Ar）；微裂纹密度（Cr.Dn）；微裂纹平均长度（Cr.Le）；微裂纹表面密度（Cr.S.Dn）；骨细胞密度（Ot.N/T.Ar；Ot.N/B.Ar）；以及骨体积分数（B.Ar/T.Ar）。使用数学算法估算Ac.f和BFR。利用共聚焦显微镜检查骨骼，以观察细微微裂纹、弥漫性基质损伤以及骨细胞合胞体的破坏情况。两组的Cr.Dn（#/mm2）值均较低（纯种赛马和非运动马匹分别为0.022±0.008和0.013±0.006）。Cr.Dn与Ot.N/T.Ar；Ot.N/B.Ar、B.Ar/T.Ar以及Ot.N/T.Ar；Ot.N/B.Ar和重塑（Ac.f，Rs.N/T.Ar）以及Ot.N/T.Ar；Ot.N/B.Ar之间均无显著相关性。在纯种赛马中发现Mc-III背侧皮质有强烈的重塑（Ac.f 12.8±7.4 #/mm2/年；BFR 31.5±15.6%；Rs.N/T.Ar 0.19±0.09 #/mm2），与非运动马匹相比显著增加。总体而言，重塑与Cr.Dn呈弱相关（r2 = 0.15，P<0.05）。在纯种马骨中，在吸收间隙附近特别明显的细微基质损伤，在明场显微镜下无法检测到。在非运动马匹中，与黏合线和间质碎片相关的骨细胞树突状细胞过程的破坏更为明显。综上所述，这些发现表明，在高应变骨骼部位骨骼功能适应性过程中，位点特异性（靶向）重塑诱导并不依赖于微裂纹的积累或骨细胞的丢失。我们推测，运动能力可以通过不涉及经典线性微裂纹的途径直接影响这种极限运动员的骨转换。
