异体脱钙骨基质明胶骨粒/骨水泥复合材料的生物力学性能研究

目的　研究不同质量比的部分脱钙骨基质明胶骨粒 /骨水泥复合材料的结构特征及生物力学性能 ,初步分析与复合比例的关系 ,为临床应用提供依据。方法　按 Urist等的方法制备异体部分脱钙骨基质明胶骨粒 ,再与骨水泥按不同比例混和制成含骨粒质量比分别为 0、4 0 0、5 0 0及 6 0 0 m g/ g的部分脱钙骨基质明胶骨粒 /骨水泥复合材料 ,对其进行扫描电镜观察和抗压极限强度、抗弯极限强度测定。结果　不同复合比例材料中骨粒与骨水泥均匀混合分布 ,呈多点面状接触 ,无序排列 ,其中骨水泥相互延续构成材料的骨架 ,不同大小的骨粒分布其间 ;材料间存在较多 10 0～ 4 0 0 μm不规则相互连通的自然裂隙 ,随材料中骨粒所占比例增加 ,骨水泥含量减少 ,自然裂隙增多。含部分脱钙骨基质明胶骨粒质量比为 0、4 0 0、5 0 0及 6 0 0 mg/ g的复合材料的抗压极限强度分别为 (71.1± 2 .0 ) MPa,(46 .9± 3.3) MPa,(39.8± 4 .1) MPa和 (32 .2± 3.4 ) MPa;抗弯极限强度分别为 (6 5 .0± 3.4 )MPa,(38.2± 4 .0 ) MPa,(33.1± 4 .3) MPa和 (2 5 .3± 4 .6 ) MPa。结论　部分脱钙骨基质明胶骨粒 /骨水泥复合材料具有良好的生物力学性能 ,且制备简便 ,易于塑形 ,其中以 10 0～ 4 0 0 μm自然裂隙有利于宿主骨的长入 ,是很好的     

多孔明胶微球/磷酸钙骨水泥的制备及其性能研究

[目的]制备多孔明胶微球/磷酸钙骨水泥,并于体内体外研究其各种性能.[方法]双相乳化冷凝聚合法制备明胶微球,以不同比例与磷酸钙骨水泥复合(0%,2.5%,5%),制备多孔磷酸钙骨水泥,测定材料孔径率及抗压强度,筛选出最佳比例.消化法培养成骨细胞接种于常规及多孔磷酸钙骨水泥支架上,扫描电镜观察细胞形态;不同材料浸提液(0%,2.5%明胶微球/磷酸钙骨水泥及聚苯乙烯)分别与成骨细胞共培养,以MTT法测定细胞增殖率,试剂盒检测碱性磷酸酶水平.将磷酸钙骨水泥及2.5%明胶微球/磷酸钙骨水泥分别植入山羊椎体内,6个月后收集标本,分别进行X线影像学及组织学观察,评估其降解情况.[结果]不同比例明胶微球/磷酸钙骨水泥的总孔径率、大孔率及抗压强度分别为:38.7%、0%、12.1 MPa(0%);67.5%、40.6%、8.0 MPa(2.5%);72.2%、45.6%、5.0 MPa(5%).成骨细胞在明胶微球/磷酸钙骨水泥上生长良好,细胞增殖及碱性磷酸酶水平均明显高于单纯磷酸钙骨水泥组,与聚苯乙烯组未见明显差异.多孔明胶微球/磷酸钙骨水泥6个月后在体内大部分已降解,而磷酸钙骨水泥未见明显降解.[结论]复合明胶微球可显著提高磷酸钙骨水泥的孔径率促进其降解,增加其生物活性,这种多孔磷酸钙骨水泥可作为非负重部位的骨替代物.     

阿霉素异体脱钙骨基质骨粒骨水泥缓释体的制备及相关研究

目的：研制阿霉素异体脱钙骨基质骨粒骨水泥缓释体，分析其缓释性能及对骨肉瘤细胞OS－9901的抑制能力。方法：按Urist法制备异体脱钙骨基质骨粒，经冻干、真空吸附等处理，载入阿霉素与骨水泥按1：1复合，制得阿霉素异体脱钙骨基质骨粒骨水泥缓释体。对该缓释体行体内外药物释放及其浸出液的体外抑瘤试验。结果：缓释体体外第1d释放量为总的19．23％，其后在较低水平维持相对稳定的缓慢释放，持续释放70d以上；其第1、20、40、70d的浸出液对骨肉瘤细胞OS－9901的抑制率分别为64．27％，41．68％、28．71％及24．32％。体内释药时，局部骨组织浓度高于血浆中浓度；局部骨组织早期浓度高，以后为稳定的低浓度释放。结论：该缓释体具有良好的缓释功能，在70d内对骨肉瘤细胞OS－9901维持良好有效的抑制率。     

同种异体骨基质明胶治疗鞍鼻畸形的疗效观察

目的：研究同种异体骨基质明胶治疗鞍鼻畸形的可行性。方法：无菌下取人体四肢长骨骨皮质，液氮冷冻10min。骨皮质经氯仿、甲醇脱脂，盐酸脱矿，氯化钙、乙二胺四乙酸、氯化锂处理制备成骨基质明胶。23例患者接受了骨基质明胶植入鼻成形术，其中先天性鞍鼻畸形16例，外伤性鞍鼻畸形7例。结果：经0．5-6年的随访，骨基质明胶植入物无变形或免疫排斥反应，虽有轻微吸收，但外形满意。结论：骨基质明胶植入治疗鞍鼻畸形有临床应用价值。     

硫酸钙/聚氨基酸复合三联抗痨药人工缓释材料的制备及物理性能测定

[目的]通过压缩强度的测定和扫描电镜的观察,评价本研究制备的载三联抗痨药HRZ硫酸钙/聚氨基酸人工缓释材料的物理性能,从而制定构成缓释材料各成分的优化配比方案。[方法]实验组分为A1、A2、A3、A4、A5五个亚组,避光环境下分别根据五种不同比例的配比方案,称取适量硫酸钙/聚氨基酸、异烟肼(INH,H)、利福平(RFP,R)、吡嗪酰胺(PZA,Z)制备成直径9 mm、高6 mm圆柱状载药人工缓释材料,每一亚组制备8块,干燥。对照组B组为不含任何药物的人工缓释材料,以同法制备8块。将制备好的人工缓释材料在电子力学万能材料试验机上测定压缩强度;并在扫描电子显微镜下观察显微结构。[结果](1)实验组各亚组药占比重量A19.09%、A212.28%、A313.79%、A416.67%、A528.57%时,压缩强度分别依次为(11.63±0.18)MPa、(10.74±0.25)MPa、(10.11±0.24)MPa、(3.48±0.22)MPa、(1.18±0.18)MPa;对照组不含药物,压缩强度为(18.62±0.16)MPa;6组数据分别进行组间比较,存在差异,具有统计学意义(F=7 141.068,P=0.000);(2)硫酸钙/聚氨基酸人工缓释材料晶体之间存在大量的空隙,为承载药物提供了丰富的空间。[结论]载三联抗痨药HRZ硫酸钙/聚氨基酸人工缓释材料随着抗痨药物药占比的增加,压缩强度逐渐降低;但在一定药物含量范围内,人工材料的强度变化并不明显;硫酸钙晶体间的空隙是该人工材料承载抗痨药物的空间。     

可降解复合人工骨修复骨缺损的实验研究

[目的]探讨硫酸钙（CS）／骨基质明胶（BMG）复合人工骨修复节段性骨缺损的能力。[方法]分别制备CS、BMG，按一定比例复合，植入兔尺骨15mm骨缺损，并随机分为3组，CS／BMG组植入复合人工骨、CS组单纯植入硫酸钙、空白对照组缺损区旷置。标本于术后4、8、12周分批取材，经大体观察、影像学、组织学及四环素荧光示踪检测，观察修复骨缺损的效果。[结果]术后切口均一期愈合，植入区周围未见炎性细胞浸润。CS／BMG组植入4周，缺损区两端及中央均可见岛状新骨生长，8周时材料已完全降解，新骨生长活跃，并逐渐由编织骨重塑为板层骨，12周缺损区被新生骨替代，骨结构逐渐成熟，与宿主骨间形成骨性桥接，髓腔再通，完成骨性修复。CS组4周两端也见新骨生长，但较CS／BMG组成骨量少，中央部位新骨出现时间晚。8周时材料完全降解吸收，植入区可见大量骨痂生长，两端出现板层样新骨，12周缺损区得到基本修复，但骨髓腔轮廓不清。空白组术后形成骨不连。[结论]。CS／BMG复合人工骨具有良好的生物相容性和可降解性，能有效地修复兔尺骨骨缺损，是一种较为理想的骨移植替代材料。     

壳聚糖与Ⅰ型胶原复合制作新型人工神经支架材料及其性能研究

目的 探讨以壳聚糖复合Ⅰ型胶原蛋白结合冷冻干燥技术制备新型人工神经支架材料.方法 将壳聚糖与Ⅰ型胶原蛋白分别溶于0.05 mol/L的醋酸溶液中,利用冷冻干燥技术制备新型人工神经支架材料,并以紫外线照射的方法使其交联.经扫描电镜观察内部结构的排列规律及走行,比较其与周围神经的异同,测量其孔径大小、计算孔隙率等指标.观察在0.01 mol/L磷酸盐缓冲液中30 d体外降解率.并对材料进行力学及细胞毒性实验.结果 构建的材料为均匀圆柱状,内部为孔径均匀且平行排列的微观结构,其微孔直径为60-130μm,孔隙率为83.30%,体外降解率为16.95%,具有较好的力学性能与周阡司组织无毒性反应.结论 使用壳聚糖复合Ⅰ型胶原蛋白结合冷冻干燥技术,制备出具有良好三维空间构型的新型人工神经支架材料,其牛物相容性良好,具有应用潜力.     

同种异体骨基质明胶与部分脱蛋白异种骨复合移植的放射性核素观察

目的 :用99mTc -MDP放射性核素观察同种异体骨基质明胶与部分脱蛋白异种骨复合移植修复兔下颌骨缺损。方法 :用连续化学处理制备骨基质明胶和部分脱蛋白骨。取 5只兔在双侧下颌骨制成 16mm× 8mm全层矩形缺损 ,左侧植入复合骨 ,右侧植入部分脱蛋白骨。移植术后 4周 ,用99mTc -MDP做颌骨扫描和γ计数定量分析。结果 :复合骨移植与部分脱蛋白异种骨移植相比较有明显的放射性核素聚集 ,γ计数定量分析差别非常显著 (P <0 .0 1)。结论 :在诱导成骨方面 ,复合骨移植优于部分脱蛋白异种骨移植。99mTc -MDP做颌骨扫描和γ计数定量分析是观察早期诱导性成骨的敏感方法。     

三维多孔复合人工骨的制备及性能检测

[目的]研制新型三维多孔复合人工骨材料并评价其相关性能.[方法]将海洋贝壳牡蛎粉、消旋聚乳酸按一定比例复合,采用热致相分离法制备多孔复合人工骨(CAB)材料,检测其孔隙率、孔径、生物力学强度;并将CAB和纯PLLA薄片浸泡于37℃平衡液中,观测不同时间点CAB和纯PLLA体外降解变化参数,对其结果进行统计学比较.[结果]研制的CAB材料平均孔隙率为85.1%;电镜下孔径测量大小为100～300 μm,孔隙之间的连通较好,孔隙形态、取向规则有序;压缩强度为2.12 MPa;在观测周期内,随着浸泡时间的延长,CAB和纯PLLA的质量损失率、浸泡液pH值呈现出规律性变化,两组各时期的各项指标比较具有显著性差异(P﹤0.05). [结论] 该法制得的CAB材料的孔隙率、孔径、生物力学强度、体外降解性能表明能满足骨替代材料的要求.     

不同处理的同种异体骨对体外淋巴细胞增殖的影响

未经体内致敏或经体内致敏的ＢＡＬＢ／ｃ小鼠脾淋巴细胞在体外培养状态下，分别加入相应的Ｃ５７ＢＬ／６小鼠新鲜骨、自消化抗原去除骨、骨基质明胶、脱钙骨基质匀浆上清。结果表明：未经体内致敏的情况下，新鲜骨对淋巴细胞增殖的影响不明显，而自消化抗原去除骨、骨基质明胶、脱钙骨基质则抑制淋巴细胞增殖，以骨基质明胶的抑制作用最明显；经体内致敏后，新鲜骨刺激体外淋巴细胞增殖，而其它三种具有骨诱导活性的植入物仍表现抑制体外淋巴细胞增殖。     

深度冷冻大段同种骨的制备及应用

目的：建立现代骨组织库，观察深度冷冻大段同种骨移植的临床效果。方法：采用深度冷冻、物理脱脂、真空包装和辐照灭菌等现代技术处理人类大段骨组织，临床应用24例，包括创伤性肢体骨重建、半侧骨盆重建、半侧骶骨移植、四肢大段骨或骨关节移植等，观察近期临床结果。结果：骨移植材料的无菌检测合格，生物力学强度无明显下降，可远程运输和短期保存．全部病例无明显的免疫排斥反应，成功率为100％.结论：深度冷冻大段同种骨临床使用安全可靠，保存和运输方便.     

冷冻干燥同种骨临床应用的初步报告

目的：建立现代骨组织库，观察系列冷冻干燥同种骨移植的临床效果。方法：采用深度冷冻、真空冷冻干燥和辐照灭菌等现代技术处理人类骨组织，临床应用系列冻干骨94例，包括骨缺损填充、骨不连植骨、脊柱和关节融合、人工关节翻修、髋臼造盖等，观察近期临床结果。结果：冻干同种骨的理化性能、无菌检测和生物性能完全符合体内植入物的相关国家标准，常温下可保存2年。94例中的90例无明显的免疫排斥反应，优良率为95．76％，成功率为97．87％。结论：冷冻干燥同种骨使用安全，保存和运输方便，可替代临床自体骨移植。     

活骨移植治疗四肢骨肿瘤切除后长段骨与关节缺损

目的:探讨四肢骨肿瘤的广泛或边缘切除后,长段骨关节缺损的修复方法及其疗效。方法:四肢骨肿瘤切除术后骨缺损14例,男10例,女4例;年龄4~37岁。骨巨细胞瘤4例,骨化性纤维瘤1例,非骨化性纤维瘤2例,韧带样纤维瘤1例,骨纤维结构不良3例,侵袭性骨母细胞瘤1例,软骨母细胞瘤1例,瘤样病损致骨溶解1例。14例四肢侵袭性良性骨肿瘤行肿瘤广泛或边缘切除,应用吻合血管的腓骨、带腓骨头腓骨近端和带旋髂深血管的髂骨瓣移植修复骨缺损、重建桡腕关节和重建肩关节。移植体行简单内固定加外固定。术后定期复查X线片、多普勒血管超声,其中3例行ECT核素骨扫描,并作关节功能评定。结果:随访3个月~5年,13例均于手术后3个月后达Ⅰ期愈合,其中1例因内固定松动再次手术而愈合。1例肿瘤局部复发,再次行肿瘤切刮与植骨术后愈合;2例畸形愈合行截骨矫形后愈合。关节功能评定:优9例,良3例,差2例。结论:骨肿瘤切除后行骨移植修复骨缺损、重建关节,手术切除彻底,复发率低,植骨愈合可靠,重建后的关节功能良好。     

The comparative effects of dichloromethylene diphosphonate (Cl2MDP) and ethane-1-hydroxy-1,1-diphosphonate (EHDP) on growth and modeling of the rat tibia

Summary Male rats weighing 100 g were assigned to groups and injected daily for 10 days with vehicle (control), 0.4, 2.0, 4.0, 10.0, or 20.0 mg/kg/day of ethane-1-hydroxy-1,1-diphosphonate (EHDP) or dichloromethylene diphosphonate (Cl₂MDP). The proximal tibial metaphysis and epiphysis were assayed for changes in percentage of hard tissue and bone formation parameters. From the data, information about hard tissue resorption was deduced.All doses of Cl₂MDP and doses of 2.0 mg EHDP/kg/day and greater caused significant increases in percentage of hard tissues with Cl₂MDP being more effective than similar doses of EHDP in decreasing bone resorption.Osteoclast population parameters were increased with all doses of both Cl₂MDP and EHDP with Cl₂MDP having a greater effect than similar doses of EHDP. Decreases in the proliferation of the osteoprogenitor pool parallel the decreases in osteoblasts and bone formation parameters. These decreases in osteoprogenitor pool proliferation do not account for the increases with diphosphonates in osteoclast population parameters.     

骨基质明胶复合自体红骨髓及同种异体骨修复骨缺损

目的:评价骨基质明胶复合自体红骨髓及同种异体骨联合移植修复骨缺损的疗效。方法:76例良性骨肿瘤和瘤样病损患者,彻底刮除病灶或作肿瘤骨段切除,并对瘤壁作灭活处理,以同种异体骨作支架,周围填充骨基质明胶和自体红骨髓复合物,术后观察机体反应及骨缺损修复情况。结果:术后机体无明显免疫排斥反应,无1例发生感染,所有病例随访时间为5～16个月,X线显示新骨形成时间为术后1.5～4月,完全骨化的时间为术后5～9月,骨缺损骨性愈合74例,并获得较好的关节功能,肿瘤复发2例。结论:骨基质明胶、自体红骨髓、同种异体骨复合物能有效修复骨缺损,有广泛的临床应用前景。     

外固定架及重组合异种骨植骨治疗胫骨骨缺损与骨不连

目的：探讨外固定架和重组合异种骨(RBX)植骨治疗胫骨骨缺损、伴肢体短缩的胫骨骨不连及先天性胫骨假关节的临床疗效。方法：应用外固定架共治疗胫骨骨缺损、伴肢体短缩性骨不连及先天性胫骨假关节20例。胫骨断端清理后短缩长度2—9cm，平均4．8cm。断端应用RBX植骨12例。结果：20例病人随访8个月-7年，平均4年3个月，患肢功能恢复满意。12例应用RBX植骨治疗骨不连的平均愈合时间4．8个月。结论：本手术方法治疗胫骨骨缺损、伴肢体短缩的胫骨骨不连及先天性胫骨假关节，创伤小、操作简单，肢体功能恢复满意；RBX植骨治疗骨不连，安全，对促进骨愈合疗效可靠。     

Treatment of tibial defect and bone nonunion with limb shortening with external fixator and reconstituted bone xenograft

Objective: To explore the effect of external fixator and reconstituted bone xenograft (RBX) in the treatment of tibial bone defect, tibial bone nonunion and congenital pseudarthrosis of the tibia with limb shortening. Methods : Twenty patients ( 13 males and 7 females)with tibial bone defect, tibial bone nonunion or congenital pseudarthrosis of the tibia with limb shortening were treated with external fixation, Two kinds of external fixators were used: a half ring sulcated external fixator used in 13 patients and a combined external fixator in 7 patients.Foot-drop was corrected at the same time with external fixation in 4 patients. The shortened length of the tibia was in the range of 2-9 cm, with an average of 4.8 cm. For bone grafting, RBX was used in 12 patients, autogenous ilium was used in 3 patients and autogenous fibula was implanted as a bone plug into the medullary canal in 1 case,and no bone graft was used in 4 patients. Results: All the 20 patients were followed-up for 8 months to 7 years, averaging 51 months. Satisfactory function of the affected extremities was obtained. All the shortened extremities were lengthened to the expected length. For all the lengthening area and the fracture sites,bone union was obtained at the last. The average healing time of 12 patients treated with RBX was 4.8 months. Conclusions: Both the half ring sulcated external fixator and the combined external fixator have the advantages of small trauma, simple operation, elastic fixation without stress shielding and non-limitation from local soft tissue conditions, and there is satisfactory functional recovery of affected extremities in the treatment of tibial bone defects, tibial bone nonunion and congenital pseudarthrosis of the tibia combined with limb shortening.RBX has good biocompatibility and does not cause immunological rejections. It can also be safely used in treatment of bone nonunion and has reliable effect to promote bone healing.     

一种新型生物活性人工骨的制备及成骨活性的研究

目的：研制CPC/BMP复合人工骨,检测其成骨活性。方法：制备CPC/BMP及CPC骨块,扫描电子显微镜观察表面结构。用小鼠肌袋植入实验观察材料的成骨活性。结果：BMP在CPC中呈微球状均匀分布。CPC植入小鼠肌袋内不能诱导,CPC/BMP植入后1周有软骨细胞出现,2周有编织骨,4周以后小梁骨生成,16周出现成熟的板层骨。同时材料出现降解迹象。有机质含量、碱性磷酸酶浓度在CPC/BMP组出现升高,扫描电镜结果同样证实有新骨形成。结论：CPC/BMP生物活性人工骨可异位诱导成骨,可望成为新型的骨缺损修复材料。     

Osteoprotegerin and Bone Mass in Squamous Cell Head and Neck Cancer Patients

Osteoprotegerin (OPG) is considered one of the main regulators of bone remodeling. Various patterns of serum OPG levels have been described in different types of tumors. We undertook this study to determine serum OPG levels in patients with squamous cell head and neck cancer (SCHNC), analyzing their relationship with other metabolic bone parameters and bone mineral density (BMD), as well as the possible influence of chemotherapy. Forty male patients with localized SCHNC were studied, and their results were compared with those of 40 healthy male controls. The type of treatment followed by each patient was noted. Age, weight, height, and lifestyle habits were recorded; and OPG, Ca²⁺, intact parathyroid hormone (iPTH), 25-Hydroxyvitamin D (25OHD) and 1,25-Dihydroxyvitamin D (1,25(OH)₂D), bone alkaline phosphatase, osteocalcin, and serum C-terminal cross-links telopeptide of type I collagen (ICTP) were determined. Dual-energy X-ray absorptiometry BMD at the lumbar spine, femoral neck, and hip was also measured. Serum OPG was higher in patients than in controls (91.7 ± 25.8 vs. 77.2 ± 26.3, P = 0.02). ICTP (a bone resorption marker) was 37% higher in patients (P = 0.007). Bone mass was lower in patients at the lumbar spine, femoral neck, and total hip. Lumbar spine Z-score showed a significant progressive decrease in controls, stage I-III patients, and stage IV patients. Logistic regression analysis showed a significant association between the disease and serum OPG levels, the odds ratio per standard deviation increase of this being 1.9 (95% confidence interval 1.1–3.8, P = 0.04) after adjusting for bone mass and ICTP serum levels, as well as for alcohol and smoking history. Adjustment for alcohol intake and tobacco use did not cancel out BMD differences between patients and controls. Patients with SCHNC show increased OPG serum levels, increased bone resorption, and decreased bone mass. The OPG rise appears to be unrelated to the BMD decrease, and the BMD decrease seems to be, at least in part, independent of smoking and drinking habits. No differences in either OPG or BMD were seen between patients with and without chemotherapy. Further studies are needed to clarify the mechanisms responsible for OPG and BMD changes in SCHNC.     

Parathyroid hormone monotherapy and cotherapy with antiresorptive agents restore vertebral bone mass and strength in aged ovariectomized rats

Previous studies have shown that parathyroid hormone (PTH) monotherapy and cotherapy with estrogen or risedronate augment vertebral bone mass and bone strength in young, ovariectomized (OVX) rats. The current study was designed to determine whether PTH has similar bone anabolic effects in aged OVX rats at a much later stage of estrogen depletion. Female Sprague Dawley rats were subjected to sham surgery or bilateral ovariectomy at three months of age and maintained untreated for one year after surgery to allow for the development of vertebral osteopenia in OVX rats. Groups of baseline control and OVX rats were sacrificed at the end of this pretreatment period. The remaining OVX rats were then treated for ten weeks with vehicle, antiresorptive agents alone (estrogen, risedronate, or calcitonin), or PTH alone. Other groups of OVX rats were treated concurrently with PTH and each of the antiresorptive agents. The first and fourth lumbar vertebral bodies were processed undecalcified for quantitative bone histomorphometry and biomechanical testing, respectively. As expected, bone mass and compressive strength were decreased in the lumbar vertebral body of baseline OVX rats compared to baseline control rats. This bone loss was associated with decreases in trabecular number and width and an increase in trabecular separation. Treatment with estrogen, risedronate, or calcitonin alone failed to reverse the changes in bone mass, structure, and strength induced by ovariectomy. In contrast, treatment of OVX rats with PTH alone restored vertebral cancellous bone volume and ash density to the level of vehicle-treated control rats and increased vertebral maximum load, stress, and normalized load to well above this level. The hormone significantly increased trabecular width, but not number, in the lumbar vertebral body of OVX rats. Concurrent treatments with PTH and the antiresorptive agents did not augment cancellous bone and biomechanical competence to a greater, or lesser, extent than treatment with PTH alone. Compressive strength correlated significantly with bone mass and trabecular width in the lumbar vertebral body. These results indicate that PTH completely restores lost bone mass and improves bone strength in the vertebral body of aged OVX rats with established osteopenia. With our previous study in younger OVX rats, the current study demonstrates that the anabolic effect of PTH is independent of age and the stage of estrogen depletion in the rat skeleton.