置钉密度与Lenke1型青少年特发性脊柱侧凸矫正率的相关性

[目的]回顾性分析全椎弓根螺钉矫形治疗Lenke 1型青少年特发性脊柱侧凸矫正率与置钉密度间的相关性.[方法]2007～2009年,56例行后路全椎弓根螺钉矫形内固定的Lenke 1型青少年特发性脊柱侧凸患者纳入回顾性研究.测量术前、术后影像学资料,分析侧凸矫正率与置钉密度的相关性.[结果]所有患者无手术相关的严重并发症发生.主胸弯角度由术前平均53.4°±11.7°矫正为18.4°±10.5°(P<0.05),冠状面平均矫正率为66.8%.胸椎后凸由术前31.8°±4.39°降至22.3°±3.13°(P<0.05).平均置钉密度为1.56 (1.0～2.0).矢状面胸椎后凸的减小与冠状面Cobb角的矫正度数和矫正率明显相关(P<0.05).置钉密度与侧凸矫正率无相关性.[结论]椎弓根螺钉系统治疗胸椎特发性侧凸可以获得良好的冠状面矫形,但会造成矢状面胸椎后凸减小.置钉密度与侧凸的矫正率无相关性.     

全椎弓根螺钉系统矫正特发性脊柱侧凸

[目的]探讨胸椎椎弓根螺钉的植入方法,总结钉棒系统矫正脊柱侧凸的效果.[方法]咬除进钉点骨皮质,以据术前测量的深度和旋转的程度,分别先后用1.5 mm、2.5 mm克氏针沿椎弓根方向钻孔,如阻力加大、克氏针弯曲,说明遇到骨皮质,调整进针方向.达到测定的深度停止进针,球形探子探查无误后改用锤子将导锥顺着制造的钉道小心缓慢击入,深度一致后,再次用球形探子探查,植入螺钉.[结果]胸椎椎弓根螺钉一次性植入成功率胸段97%(600/619),腰段99%(733/740).术后未出现脊髓损伤和神经功能障碍,无切口感染.术后冠状面平均矫正率73%.矢状面后凸Cobb's角(T1～T12)6°～30°,平均23°.旋转畸形矫正Ⅰ～Ⅱ度.103例平均随访4.9年,躯干平衡良好,无平背畸形,植骨融合良好,末次随访冠状面角度丢失率平均为3.7%,迟发性感染1例,螺钉断裂2例,均行内固定取出.[结论]克氏针制备螺钉钉道,是胸椎椎弓根螺钉植入的较好方法.钉棒结构具有良好的三维矫正控制力.全椎弓根螺钉系统矫正特发性脊柱侧凸效果良好.     

胸椎椎弓根螺钉在脊柱侧凸矫形术中的应用效果评价

[目的]评价胸椎椎弓根螺钉在脊柱侧凸矫形术中的应用效果。[方法]2008～2010年采用后路胸椎椎弓根螺钉技术治疗特发性和先天性脊柱侧凸患者26例。根据术后临床表现、矫正率和术后X线片判定胸椎椎弓根螺钉置钉情况,对胸椎椎弓根螺钉在脊柱侧凸矫形术中的应用效果进行评价。[结果]本组病例术后平均矫正率为63.81%,术后患者脊柱长度平均增加6.2 cm;术后X线片判定置入的308枚胸椎椎弓根螺钉中,置钉不良率为16.9%;所有患者术后均无胸部脏器及神经系统损伤表现。[结论]在脊柱侧凸临床治疗中,采用胸椎椎弓根螺钉进行侧凸矫形是有效、安全的方式。     

应用椎板开窗法置入胸椎椎弓根螺钉的精确性和安全性评估

目的 分析应用椎板开窗法行胸椎椎弓根螺钉置入治疗重度脊柱侧后凸患者的精确性和安全性. 方法 1996年6月至2007年12月,应用椎板开窗法行胸椎椎弓根螺钉置入治疗23例重度脊柱侧后凸患者(A组),其中男性9例,女性14例;年龄13～23岁,平均17.8岁;术前主胸弯冠状面Cobb角平均97.3°,平均后凸角67.4°.作为对照,同期应用非开放法置钉治疗重度脊柱侧后凸患者22例(B组),其中男性7例,女性15例;年龄14～21岁,平均17.2岁;术前主胸弯冠状面Cobb角平均为96.6°,平均后凸角62.1°.两组患者术后均行CT扫描,统计螺钉置入并发症,对螺钉穿透椎弓根皮质骨的CT扫描图像进行联机测量并统计分析.结果 A组和B组各置入胸椎椎弓根螺钉209和201枚,术中发生椎弓根骨折5例和16例,发生硬膜撕裂4例和7例,螺钉错置18枚和45枚.B组螺钉错置率高于A组,差异具有统计学意义(P<0.05).A组上、中胸椎与下胸椎之间、凸侧与凹侧之间,螺钉错置率差异均具有统计学意义(P<0.05).两组均无脊髓及大血管损伤. A和B组经平均3.2年、3.4年随访,术后冠状面和矢状面平均矫正度未见明显丢失.结论 重度脊柱侧后凸胸椎椎弓根螺钉置入技术难度较高,应用椎板开窗法可有效增加螺钉置入精确性和安全性.     

多节段椎弓根螺钉内固定系统矫正胸椎侧凸畸形的有效性和安全性评价

目的:评价多节段椎弓根螺钉内固定系统矫正胸椎侧凸畸形的有效性和安全性。方法:回顾我院1994年3月～2002年3月应用椎弓根钉-棒系统矫治的118例胸椎侧凸畸形患者的临床资料,分析评价其手术并发症、侧凸矫正率及长期随访结果。结果:胸椎椎弓根总计置入螺钉916枚,术中及术后螺钉松动16枚;螺钉位置不良12枚;椎弓根骨折7例;脑脊液漏3例;1例术后螺钉松动压迫脊髓。术后平均随访5年,脊柱侧凸畸形平均矫正率为75%,平均矫正度丢失率1.2%,1例出现交界性后凸再次手术治疗。结论:多节段椎弓根螺钉内固定系统是矫正脊柱侧凸畸形一种较安全、有效的三维内固定方式。     

非影像监视下行脊柱侧凸胸椎椎弓根螺钉置入的临床应用

目的:探讨脊柱侧凸胸椎椎弓根螺钉非影像监视下徒手置入的方法及可行性。方法:57例脊柱侧凸患者行后路椎弓根螺钉系统矫形手术,徒手法置入胸椎椎弓根螺钉。术后常规拍摄脊柱全长X线片,随机选取10例患者行CT扫描观察,了解螺钉置入的准确性。结果:共置入胸椎椎弓根螺钉362枚。术后X线片观察到10枚螺钉偏外,4枚螺钉偏下,其中2枚螺钉引起轻微肋间神经痛,3周后完全缓解。CT观察47枚螺钉有2枚螺钉导致椎弓根内壁膨胀内移,没有相应神经症状。主弯Cobb角术前平均60.4°(32°～121°),术后平均18.3°(1°～70°),平均矫正率71.9%(38.1%～98.0%)。结论:徒手法置入脊柱侧凸胸椎椎弓根螺钉是可行的。     

TSRH器械在特发性脊柱侧凸中的应用

[目的]观察TSRH三维矫形系统矫正特发性脊柱侧凸冠状面、矢状面和轴状面的疗效,评价应用钉棒系统和钉钩棒系统随访时的效果。[方法]分析采用TSRH系统治疗特发性脊柱侧凸79例,男48例,女31例,平均15.5岁。全部应用椎弓根螺钉系统56例,胸椎应用椎弓根钩或椎板钩,腰椎应用椎弓根螺钉23例。对照分析2组的矫正效果。[结果]术后冠状位矫正为18~31°,矫正率为56%~87%,平均为68%,矢状位矫正为21~33°,平均25°,旋转矫正Ⅰ度。身高平均增高3 cm。63例平均随访2.5 a,2组冠状面和矢状面的矫正率以及冠状面矫正度的丢失有明显的差异。[结论]TSRH矫形系统治疗脊柱侧凸可得到满意的治疗效果,应用椎弓根螺钉效果更佳。     

椎弓根螺钉间隔置钉治疗特发性脊柱侧凸5年随访疗效分析

[目的]评估对具有良好柔韧性的特发性脊柱侧凸患者采用椎弓根螺钉间隔置钉三维矫形术后随访5年以上手术疗效。[方法]对51例侧凸角度<75°,柔韧度>40%的特发性脊柱侧凸患者,采用椎弓根螺钉间隔置钉三维矫形,分析术前、术后即刻及末次随访时主弯侧凸Cobb角、顶椎旋转、冠状面平衡、矢状面平衡及手术时间、术中失血量、并发症等。[结果]主胸弯侧凸Cobb角由术前(53.6±6.6)°矫正为(15.8±7.4)°,末次随访为(17.7±6.1)°;顶椎旋转由术前(21.4±7.1)°矫正为(9.6±3.0)°,末次随访时为(11.0±3.3)°;冠状面C7铅垂线与骶正中线距离术前、术后即刻及末次随访分别为(-6.3±11.2)mm、(-3.7±8.0)mm、(-3.9±5.5)mm;矢状面C7铅垂线与S1椎体后上缘距离术前、术后即刻及末次随访分别为(-13.3±10.7)mm、(-2.1±5.2)mm、(-2.9±5.0)mm。患者侧凸Cobb角、顶椎旋转、冠状面及矢状面平衡术前、术后比较差异均有统计学意义(P<0.05),末次随访与术后相比差异无统计学意义(P>0.05)。术后2例内固定失败,3例出现远端延长现象,1例发生伤口表浅感染。术中平均失血量为825 ml,手术时间为155 min。与常规手术方式相比,本组患者椎弓根螺钉置钉数量减少52%。[结论]对于良好柔韧性的特发性脊柱侧凸患者,采用椎弓根螺钉间隔置钉可获得良好的矫形效果,并具有缩短手术时间、减少术中失血、节省手术费用等优点。     

特发性脊柱侧凸患者胸椎椎弓根的CT测量及其临床意义

目的：测量特发性脊柱侧凸患者胸椎椎弓根的有关数据，探讨其临床应用价值。方法：在30例特发性脊柱侧凸患者术前CT扫描片上测量胸椎椎弓根的宽度、深度、角度、椎体旋转角度等数据，根据所得数据选定置入螺钉的直径、长度．确定置入方向和深度。术后对置入螺钉的胸椎椎弓根节段行CT扫描，判断置钉位置。结果：CT测量的各项数据显示胸椎椎弓根适合椎弓根螺钉的置入。以此为依据术中置入胸椎弓根螺钉共245枚，228枚(93％)置入无误，6枚穿破椎弓根外壁，9枚穿破椎弓根下壁，2枚穿破椎弓根内壁，无神经系统并发症。结论：术前CT扫描测量特发性脊柱侧凸患者的胸椎椎弓根的有关数据可为选择适当长度和直径的螺钉并将其准确置入胸椎椎弓根内提供参考。从而保证螺钉安全置入。     

应用AIIMS分型评价脊柱侧后凸畸形胸椎椎弓根螺钉置入的研究

目的:探讨AIIMS(All India Institute of Medical Sciences)分型在评价脊柱侧后凸胸椎椎弓根螺钉置人中的应用价值.方法:1996年6月～2008年1月治疗脊柱侧后凸畸形患者73例,从中选取有完整术后CT资料者25例.其中男10例,女15例,年龄13～25岁,平均17.9岁.特发性脊柱侧后凸14例,先天性脊柱侧后凸5例,神经纤维瘤病伴脊柱侧后凸3例,脊髓空洞伴脊柱侧后凸2例,成骨不全性脊柱侧后凸1例.术前主胸弯冠状面Cobb角75°～1420°,平均93.6°;主弯后凸角50.5°～86.2°,平均65.1°.手术方式采用椎板间开窗法行胸椎椎弓根螺钉置入,后路主弯区附件松解和/或顶椎全脊椎切除,三维矫形和360°植骨融合术.术后根据AIIMS分型统计螺钉置人位置及相关并发症.结果:共置入218枚胸椎椎弓根螺钉,15例出现91枚螺钉偏置(41.74%).AIIMS分型中Ⅰ型(螺钉位置可接受型)占96.33%(210/218),Ⅱ型(螺钉位置不可接受型)占3.67%(8/218),无Ⅲ型(并发症型).术中在凹侧建立椎弓根通道时造成椎弓根骨折5例,未予置钉;术中硬膜撕裂4例,其中2例术后出现脑脊液漏,对症治疗5d后愈合;均无脊髓或大血管损伤.术后平均主胸弯冠状面Cobb角39.7°,平均矫正率57.6%,术后平均主弯后凸角35.5°,平均矫正率45.5%.随访1～5年,平均3.1年,冠状面和矢状面平均矫正度未见明显丢失.结论:AIIMS分型能较全面地评价脊柱侧后凸畸形胸椎椎弓根螺钉置入后的位置和并发症情况,有一定临床应用价值.     

弹性进钉法制备胸椎椎弓根钉道在胸椎侧凸中的应用

目的：探讨弹性进钉法制备胸椎椎弓根钉道植入螺钉的方法，总结临床应用效果。方法：咬除进钉点骨皮质，以据术前测量的深度和旋转的程度，分别先后用1．5mm、2．5mm克氏针沿椎弓根方向钻孔。达到测定的深度停止进针，球形探子探查无误后改用导锥顺着制造的钉道小心缓慢进入，深度一致后，再次用球形探子探查，植入螺钉。结果：胸椎椎弓根螺钉一次性植入成功为96．2％，其中18枚术中发现，偏内侧5枚，偏下方3枚，偏上方4枚，偏外方6枚，其中偏外侧的4枚螺钉把持力坚强外，另2枚重新阅读影像资料，用2．5mm克氏针，改变锥入方向，5枚偏内侧的螺钉中3枚同时切除了该同侧的椎板，行术中探测，钉道均成功得以置备。钉道制备后增加螺钉直径0．5mm，进行补救。其余的14枚螺钉都是术后通过CT或者X线片发现的，且均偏外侧。1例术中脑脊液漏者骨蜡封闭后术后未发生脑脊液漏，本组无脊髓损伤。平均术后身高增加4．6cm，术后冠状面Cobb角平均21°，平均矫正率73％；矢状面Cobb角平均230°旋转畸形矫正Ⅰ～Ⅱ度。平均随访3．5年。最终随访矫正度丢失率为1．8％。迟发性感染1例，螺钉断裂4例，1例螺钉向椎弓根外侧偏移加重。躯干平衡良好、无平背。结论：弹性进钉法制备胸椎椎弓根钉道准确率高、并发症少。     

Effects of surgical errors on small fragment screw fixation

OBJECTIVES: The purpose of this study is to determine the effects of technical errors that occur during the application of small fragment screw fixation and to assess which screw holes can be salvaged. INTERVENTION: Testing of screw pullout from a bone substitute model on a universal testing instrument (Instron Corp., Canton, MA). OUTCOME MEASUREMENTS: Testing was performed on 9 sets of 12 small fragment screws applied to a bone substitute model using the instruments available in a small fragment set (Synthes, Paoli, Pa). In the first 2 sets, 3.5-mm cortical screws and 4.0-mm cancellous screws were placed using the proper instrumentation according to recommended AO/ASIF techniques. The other 7 sets were inserted using "incorrect" methods: a single step was altered intentionally to assess its influence on fixation strength. The third set of screws included 3.5-mm cortical screws placed after drilling the pilot hole with a 3.5-mm drill. For the fourth set, the 2.5-mm drill was used, but the hole was tapped using the 4.0-mm cancellous tap before insertion of a 3.5-mm cortical screw. In set five, 4.0-mm cancellous screws were placed after tapping the hole with a 3.5-mm cortical tap. Set 6 included cancellous screws placed without tapping. The seventh set included 3.5-mm cortical screws that were placed according to recommended methods, and then removed and replaced into the screw hole. Set number 8 included 3.5-mm cortical screws, which were inserted correctly and then stripped by overtightening. The ninth set included 3.5-mm cortical screws that were stripped as those in set 8; the stripped screws were removed, the holes were packed with bone material, and the screws were replaced. All screws were inserted to a thread depth of 32 mm. RESULTS: Drilling a 3.5-mm pilot hole for a 3.5-mm cortical screw and "stripping" the screw by overtightening resulted in 76% and 82% less pullout strength, respectively, than when the proper technique was used (P<0.01). Use of the wrong tap before placement of a 3.5-mm cortical or 4.0-mm cancellous screw decreased pullout strength by 12% and 11%, respectively (P<0.01). Exchanging screws of similar geometry had no significant effect on screw pullout strength (P>0.1). Inserting a 4.0-mm cancellous screw without tapping actually increased pullout strength by 4% (P<0.01). CONCLUSIONS: Alterations from recommended techniques for the placement of orthopedic screws had varying effects on screw fixation, as assessed by the pullout strength. Clinically, these findings indicate that, in some cases, a screw hole that was not initially placed according to the optimal technique may be salvaged. Finally, the authors recommend that careful vigilance be maintained at all times in surgery and that fixation be applied according to sound principles in an effort to avoid some of these problems.     

Pull-out strengths of bone screws at various sites about the pelvis--a preliminary study

The pull-out strength of both cortical and cancellous screws from bone at various sites about cadaveric pelves was examined. No significant differences were seen between cortical or cancellous screws at similar sites with the possible exception of the sacroiliac joint. Pull-out strengths were best correlated to the depth of bone at a particular screw hole. For practical purposes, the strongest sites are in the thick buttress of bone along the iliopubic column.     

The effect of screw type on the biomechanical properties of SCARF and crescentic osteotomies of the first metatarsal

The basilar crescentic osteotomy is a popular method for correcting moderate to severe hallux valgus. However, inadvertent dorsiflexion of the osteotomy can result from intraoperative malposition or from malunion after fixation failure. The mechanical properties of osteotomies are dependent on the nature of the osteotomy and the type of fixation. This study examines the mechanical properties of the SCARF and crescentic osteotomies of the first metatarsal by using a cannulated asymmetric pitched screw or AO cancellous screws. Sixteen human cadaveric first metatarsal specimens were tested in plantar to dorsiflexion cantilever bending by using a mechanical testing machine. The data was compared with our recent work on the mechanical properties of the SCARF and crescentic osteotomies. Ultimate load and stiffness of the SCARF osteotomy were superior to the crescentic osteotomy but were not dependent on screw type. Screw type was a prominent factor in the stiffness but not in the strength of the crescentic osteotomy. The ultimate load and the stiffness of SCARF osteotomy fixed with the cannulated asymmetric pitched screws were not significantly different compared with AO screws (ultimate load, 124.6 N [SD = 56.8] v 105.3 N [SD = 57.0]; stiffness, 52.0 N/mm [SD = 48.0] v 31.8 N/mm [SD = 19.0]). Modes of failure were fracture of the cortical bone bridge between the screw hole and the osteotomy in all crescentic osteotomies and fracture of the proximal dorsal bridge in all SCARF osteotomies. The superior mechanical properties of the SCARF osteotomy, fixed with cannulated asymmetric pitched screws, make this a more secure construct, with less risk of malunion than the crescentic osteotomy. Stiffness is an important mechanical factor that helps distinguish the mechanical performance of different osteotomy techniques.     

垂直不稳定骨盆骨折内固定垂直稳定性的生物力学研究

目的探讨垂直不稳定骨盆骨折空心螺钉和钢板内固定前后环或后环的垂直方向稳定性差异。方法：将6具尸体骨盆随机取3具做压力测试为正常组，然后制成垂直不稳定骨盆骨折模型，分别行后环空心螺钉结合前环空心钉、后环空心螺钉、后环结合前环钢板、后环钢板内固定。结果在垂直方向，后环空心螺钉内固定强于钢板螺丝钉内固定，增加前环内固定可以显著增加前环的稳定性。结论垂直不稳定骨盆骨折空心螺钉内固定具有较好的生物力学稳定性，前后环内固定要优于单纯后环内固定。     

Does the anchorage form and depth influence the pull-out strength of screws from bone cement?

The pull-out strengths of cortical screws inserted into soft, unpolymerised Refobacin Palacos bone cement (procedure S) and into hardened polymerised cement into which a hole had been drilled and tapped (procedure P) were compared. Cortical screws 58 mm in length, outer diameter 4.5 mm and inner diameter 2.95 mm were used. Screws were inserted into cement cylinders at 5 mm incremental depths between 10 and 30 mm. At a screw depth of less than 25 mm, the screws pulled out, and at a depth of greater than 25 mm, the screws broke in both procedures. There was no statistically significant difference in pull-out strength leading to burst or break between the two procedures for screws inserted to comparable depths, but there was a statistically significant difference regarding the screwing depth regardless of the procedure of screw insertion chosen. The average material stability () of the cortical screws used was calculated to be 1191 N/mm², and the elasticity limit was 5137 N. This study demonstrated that the material stability and not the depth of screw insertion was the limiting parameter in screw anchorage in bone cement while static testing. To avoid screw breakage due to fatigue during continuous alternate loading, the screws should not be loaded above this value.     

Intraoperative CT navigation for glenoid component fixation in reverse shoulder arthroplasty

CT navigation has been shown to improve component positioning in total shoulder arthroplasty. The technique can be useful in achieving strong initial fixation of the metal backed glenoid in reverse shoulder arthroplasty. We report a 61 years male patient who underwent reverse shoulder arthroplasty for rotator cuff arthropathy. CT navigation was used intraoperatively to identify best possible glenoid bone and to maximize the depth of the fixation screws that anchor the metaglene portion of the metal backed glenoid component. Satisfactory positioning of screws and component was achieved without any perforation or iatrogenic fracture in the scapula. CT navigation can help in maximizing the purchase of the fixation screws that dictate the initial stability of the glenoid component in reverse shoulder arthroplasty. The technique can be extended to improve glenoid component position [version and tilt] with the availability of appropriate software.     

Solid screw insertion for tension band plates: a surgical technique tip

PurposeGrowth modulation with tension band plates (TBPs) and cannulated screws is the current mainstay of treatment for pediatric lower extremity angular deformities. Solid screws have been used as an alternative to cannulated screws to decrease the risk of screw failure, particularly in obese children. The downside of solid screws is the decrease in precision of placement. This study describes a surgical technique to insert solid TBP screws accurately.MethodsTBP insertion starts with the same conventional steps by inserting the guidewires into the epiphysis and metaphysis, straddling the physis. After fluoroscopic confirmation of the position of the guidewires, the cannulated drill bit is used to broach the cortex to a depth of 5 mm in the bone. A standard 4.5-mm cannulated screw from the TBP set is used to tap the screw tract over the guidewires for approximately three-quarters of the planned screw length. After removing the guidewires, the solid screws are then inserted in each hole to follow the tapped tracts.ResultsThis technique was used in five patients including four with Blount disease and one with bilateral genu varum.ConclusionIt is recommended to use solid screws with TBPs in patients with a high body mass index to avoid screw fracture. Our technique describes using a cannulated screw as a tap to create a tract to ease accurate insertion of the solid screws, and prevent the solid screw from deviating outside the desired path.     

The thermal effects of skeletal fixation-pin insertion in bone

In human-cadaver cortical bone, we measured the maximum temperatures and the durations of temperatures in excess of 55 degrees Celsius during experimental insertion of five types of external skeletal-immobilization pins. Drill speed, pin-point design, and predrilling were the variables examined. Drill speed was found to have an effect on the maximum temperature and the duration of temperatures in excess of 55 degrees Celsius only in the immediate vicinity of the hole. Pinpoint design was a more significant factor, and predrilling was highly effective as a method of minimizing temperature elevation.     

Anthropometric Measurement About the Safe Zone for Transacetabular Screw Placement in Total Hip Arthroplasty in Asian Middle-Aged Women: In Vivo Three-Dimensional Model Analysis

BackgroundAlthough the pelvic vascular injury caused by a transacetabular screw is rare, it is a major local complication of total hip arthroplasty. We aimed to obtain anthropometric data about the safe zone for the placement of transacetabular screws by analyzing the three-dimensional (3D) reconstruction model and determine the safe length of transacetabular screws by performing the 3D simulated surgery.MethodsWe reviewed 50 hips of 25 patients who underwent lower extremity angiographic computed tomography scans retrospectively. We reconstructed the 3D models of 50 hips with normal pelvic bone and vascular status using the customized computer software. We measured the central angle and safe depth of the safe zone of the transacetabular screws on the 3D models. We also performed the 3D simulated surgery to confirm the safe length of screws in each hole of the customized cup implant.ResultsThe measured central angle of the posterior-superior area was 79.5°. And we determined a mean safe depth of 49.8 mm in the safe zone, with a central angle of 47.7°. During the 3D simulated surgery, we determined a mean safe length of the transacetabular screw of 43.3 mm when applied to a lateral hole on a line bisecting the posterior-superior area.ConclusionAlthough our study was limited by the use of a virtual computer program, the quantitative measurements obtained can help reduce the incidence of pelvic vascular injury during transacetabular screw fixation in total hip arthroplasty.