Development of a variable stiffness external fixation system for stabilization of segmental defects of the tibia

The mechanical properties of a variable stiffness external fixation system were explored. Initial testing of a unilateral fixator configuration demonstrated that system rigidity could be increased by maximizing pin separation distance in the fracture component and the number of pins used while minimizing pin separation distance across the fracture site and the sidebar offset distance from bone. A triangulated system composed of half pin frames mounted anteriorly and medially on the tibial aspects and linked by crossbars was devised. Progressive disassembly of the frame was shown to result in progressive decreases in fixator rigidity in all planes.     

Experimental analysis of effects of pin pretensioning on external fixator rigidity

The effects of same-fragment pin pretensioning on the rigidity of a simple fixator system were investigated. An external fixator device constructed to apply same-fragment pin pretensioning of a defined amount was applied to a bone model made of pedilen cylinders. System rigidity was tested for varying degrees of pin pretensioning in axial compression and AP bending. Pin pretensioning, bringing transfixing pins together, tended to weaken the system in both modalities tested. Tensioning by bringing the pins apart increased the rigidity of the system in AP bending and axial compression.     

Experimental analysis of effects of pin pretensioning on external fixator rigidity

Summary The effects of same-fragment pin pretensioning on the rigidity of a simple fixator system were investigated. An external fixator device constructed to apply same-fragment pin pretensioning of a defined amount was applied to a bone model made of pedilen cylinders. System rigidity was tested for varying degrees of pin pretensioning in axial compression and AP bending. Pin pretensioning, bringing transfixing pins together, tended to weaken the system in both modalities tested. Tensioning by bringing the pins apart increased the rigidity of the system in AP bending and axial compression.     

A biomechanical analysis of the Ilizarov external fixator

Five configurations of the Ilizarov fixator were analyzed in vitro. The overall stiffness, shear stiffness, and axial motion of the fracture site were determined. The data were compared with the results of eight conventional one-half frame fixators previously tested in the same manner. The Ilizarov fixator allowed significantly more axial motion at the fracture site during axial compression than the other fixators tested. The overall stiffness and shear rigidity of the Ilizarov external fixator were similar to those of the one-half pin fixators in bending and torsion. The stability of the Ilizarov fixator was a function of bone position within the fixator rings and fixation wire tension. The use of olive stop wires increased the shear resistance of the Ilizarov system.     

肱骨干骨折髓内外固定的生物力学研究

目的 通过对单一加压钢板螺钉加髓内针、交锁髓内钉和微创技术简单有限内固定加单臂外支架3种不同固定方法治疗肱骨干复杂骨折的生物力学性能进行对比研究,为临床应用提供可靠的生物力学依据.方法 取自愿捐赠的18个新鲜湿润肱骨标本,制备肱骨干复杂骨折模型,根据不同固定方式随机分为3组,每组6个.钢板组:采用单一加压钢板螺钉加髓内针固定;髓内钉组:采用交锁髓内钉固定;外支架组:采用微创技术简单有限内固定加单臂外支架固定.分别进行轴向压缩实验和水平扭转实验.结果 轴向压缩实验:各组载荷-位移曲线呈线性到非线性变化.钢板组及髓内钉组最大载荷值分别为(6162.09±521.06)N和(6738.32±525.89)N,两组比较差异无统计学意义(P>0.05);外支架组最大载荷值为(2753.57±185.59)N,与其余两组比较,差异均有统计学意义(P<0.05).钢板组及外支架组刚度值分别为(171.69±6.49)N/mm和(132.59±2.93)N/mm,两组比较差异无统计学意义(P>0.05);髓内钉组的刚度值为(333.04±36.85)N/mm,与其余两组比较差异有统计学意义(P<0.05).水平扭转实验:各组扭矩-扭角曲线呈线性到非线性变化.髓内钉组和外支架组的最大扭矩分别为(17.12±5.73)Nm和(20.26±6.42)Nm,两组比较差异无统计学意义(P>0.05);钢板组的最大扭矩为(38.24±7.08)Nm,与其余两组比较差异有统计学意义(P<0.05).钢板组及外支架组刚度值分别为(16.36±2.07)Ncm/°和(18.79±2.62)Ncm/°,差异无统计学意义(P>0.05);髓内钉组的刚度值为(11.45±0.22)Ncm/°,与其余两组比较差异有统计学意义(P<0.05).结论 钢板组压缩和扭转强度均较强,旋转刚度较强而压缩刚度较弱;髓内钉组压缩强度和压缩刚度较强,而扭转强度和扭转刚度较弱;外支架组仅在扭转刚度上与钢板组相当,而在其他3项指标上均较弱.     

Skeletal stabilization with a multiplane external fixation device. Biomechanical evaluation and finite element model

The general question of the influence of fracture stability on bone healing remains unanswered and has important bearing on fracture stabilization by external fixation. The stiffness of an external fixator is dependent on pin placement and frame orientation. These parameters are under the surgeon's control, within limits set by soft tissue injury and fixator design. Fixator configuration parameters include common fragment pin separation, common fragment pin angle, common fragment pin number, effective pin length, use of transfixing pins versus half-pins, and use of two versus three connecting rods. Each configuration parameter was varied independently under compressive, bending, and torsional load to determine the influence of that parameter on fracture stability under such loads. The interaction between these configuration parameters is complex. In general terms, fracture site stability may be increased by increasing common fragment pin separation, placing common fragment pins orthogonal to one another, increasing common fragment pin number, decreasing effective pin length, using transfixing rather than half-pins, and using as many connecting rods as possible. The difference between fracture site stability attainable with transfixing pins and with half-pins may be minimal if implementation of the other parameters combined with half-pins is optimized.     

Behavior of an external fixation frame incorporating an angular separation of the fixator pins. A finite element approach

A finite element model has been developed to simulate the deformation that occurs at the fracture site of an externally fixed bone as a result of applied bending, compression, and torsional loads. The pin configuration in this model is constructed to allow an angular separation of the fixator pins. The mechanical effect of this angular separation and of the distribution of the pins along the fixator bar is examined. The model shows that an angular separation of the pins provides a more symmetric deformation of the fracture site when a bending load is applied in different directions to the bone and thereby protects a fracture from excessive movement in any direction. The torsional stability of an external fixation frame is considerably increased by incorporating an angular separation of the pins. The model also shows that the most stable configuration for the fixator uses a wide separation of the pins along the fixator bar.     

Mechanical evaluation of external fixators used in limb lengthening

Four external fixator systems (five configurations) used for limb lengthening were tested to determine the fixator stiffness and the fracture gap rigidity. There was a statistical difference between fixators in all modes of loading with respect to stiffness, shear, and axial motion. The fixators were graded to determine their relative stiffness, shear rigidity, and axial rigidity. The EBI Orthofix proved to be the most rigid fixator relative to the configurations tested with minimal shear or axial motion at the fracture site. The Ilizarov tibial configuration was the least rigid, demonstrating more shear and axial motion at the fracture gap. The Ilizarov femoral system combined excellent stability and shear resistance with preservation of axial dynamization. Fixators with a high stiffness provide less motion at the fracture site, which may cause stress shielding of the osteotomy. Fixators that provide more motion at the fracture gap are less stable. These data may be useful in determining which fixator may be ideal for a particular clinical situation.     

The unilateral axial dynamic fixator study of its biomechanical property and clinical application

The home made UADF after Bastiani's pattern was mechanically tested in our lab on fracture of cadaveric femurs. For comparison, the same test was also carried out with conventional semicircular external fixator. The results showed that the rigidity and stability of fixation closely related with the diameter of the bone-pin used. The diameter of the treated pins used in UADF was one time larger than that of the round pin used in conventional semicircular external fixator. The compression rigidity of the former frame was 3.5 times stronger than that of the latter one; the extent of displacement of the fragments happened in the former frame was much less than that occurred in the latter one. Clinical Application of UADF. on fracture of tibia and fibula in 31, of femur in 8, knee arthrodesis and osteotomy of tibia and fibula one in each, brought bony healing on successfully. It is apparent that the UADF is multifunctional, universal and adaptable in use, leading to success.     

External fixation in the treatment of tibial pilon fractures: comparison of two frames in torsion

BACKGROUND: Ankle spanning external fixation has become the initial treatment of choice for complex tibial pilon fractures. Many fixator designs exist, but their biomechanical performance has not been studied extensively for this application. The goal of the present study was to compare the torsional performance of two commercially available frames, the Orthofix XCaliber and the Howmedica Hoffmann II. METHODS: The XCaliber and the Hoffmann II were each applied to six fresh cadaver lower extremities and were loaded in a materials testing machine. Strain gauges were attached to the anteromedial cortex of the distal tibia, and each specimen underwent torsional and axial load testing, with and without the external fixator. A simulated pilon fracture was created, and torsional testing was repeated. RESULTS: Results indicated that the XCaliber was significantly more rigid in internal rotation than the Hoffmann II, before (49%) and after (41%) creation of the pilon fracture. Despite the XCaliber's increased rigidity relative to the Hoffmann II (22% to 31%) in external rotation, statistical significance was not attained. Both fixators reduced strain (25% to 85%) at the anteromedial cortex upon torsional testing, but no significant differences between the two frames were noted. CONCLUSIONS: The present study demonstrates that the XCaliber has mechanical advantages over the Hoffmann II in terms of torsional rigidity for a tibial pilon fracture. CLINICAL RELEVANCE: Increased rigidity of the XCaliber could potentially lead to decreased time to union, and a lower incidence of pin loosening and would prove beneficial in a setting in which the external fixator is used as the definitive fixation.     

External fixation of femoral shaft fractures in children: enhanced stability with the use of an auxiliary pin.

From 1989 through 1994, we used a monolateral external fixator (Orthofix) to treat 39 femur fractures in 37 patients. The average age of the patients was 9.5 years (range, 5+11 to 18+8 years); 38 fractures were closed, and one was a grade I (Gustillo-Anderson classification) open fracture. Twenty-two fractures were treated by using the standard Orthofix pin configuration with two or three pins held in the pin clamps both above and below the fracture. We treated the remaining 16 fractures identically, except for the addition of an auxiliary pin, which was secured to the body of the fixator by using wire and methylmethacrylate. All patients were followed up to union and fixator removal at a mean of 97 days after fixator placement (range, 50-175 days). Thirty-one (84%) patients were followed up for 1 year after injury. Six of 22 femurs without an auxiliary pin required remanipulation for loss of reduction. Only one of 16 femurs treated with an auxiliary pin required remanipulation. Four of 22 femurs without an auxiliary pin went on to malunion. No femur with an auxiliary pin went on to malunion.     

Secondary fractures associated with external fixation in pediatric femur fractures.

Sixty-six femur fractures sustained by children ages 4-14 years and treated with external fixation were reviewed retrospectively to assess factors influencing the incidence of refracture. The total rate of secondary fracture was 12% (eight patients) including five recurrent fractures at the original fracture site and three fractures through the pin sites. After removal of the external fixator, five patients refractured at the original fracture site and one patient fractured through a pin tract. Two patients fractured at pin sites while the fixator was still in place. Multivariate linear-regression analysis showed no correlation between the incidence of refracture and fracture pattern, percentage of bone fragment contact after fixator application, type of external fixator, or dynamization. A statistically significant association (p < 0.05) was found between the number of cortices demonstrating bridging callus [on both anteroposterior (AP) and lateral views] at the time of fixator removal and the rate of refracture. Fractures showing fewer than three cortices of bridging callus had a three (33%) in nine rate of refracture, whereas fractures with three or four cortices of bridging callus had a two (4%) of 57 rate of refracture.     

The A-O external skeletal fixator in the treatment of severe tibia fractures

A series of 33 cases of Grade II and III open tibia fractures were treated with local wound care followed by application of the A-O external fixator. Two transfixing Steinmann pins were usually used above and two below the fracture site. In six cases one Steinmann pin and one anteroposterior Schanz half pin above and below the fracture were combined with a triangulated frame. Additionally, minimal internal fixation with lag screws was used in five cases. Union was achieved in 83% of tibiae in an average time of 9.9 months. Union occurred faster when the fixator was removed in less than 3.5 months but then the incidence of malunion tended to rise. Three patients required early amputation. Eleven tibiae developed deep wound infections. Knee function was well preserved but ankle function was often impaired. The A-O fixator performed as a useful, simple, stable, light weight and versatile system in the care of these Grade II and III fractures. However, many problems intrinsic to the open tibia fracture remain.     

Biomechanical analysis of dynamic external fixation devices for the treatment of distal radial fractures

BACKGROUND: Several dynamic external fixation devices have been introduced to permit early functional treatment of unstable distal radial fractures. METHODS: An intact cadaver wrist was spanned by a dynamic external fixator. Forces between the fixator and the radius were recorded during passive motion using a single, strain-gauged pin. A device with a double ball joint was compared with a new, experimental fixator with 3 degrees of freedom and its center of rotation coincident with that of the wrist. RESULTS: During flexion and extension, the pin load was approximately the same for both fixators. Radioulnar deviation resulted in high pin loads for the fixator with ball joints; the new device showed no considerable pin load. CONCLUSION: The new device has a high degree of kinematic compatibility with the natural wrist joint. When used for the treatment of distal radial fractures, it should provide favorable conditions for the maintenance of fracture reduction compared with a device with an offset ball joint.     

滑动加压股骨粗隆间骨折外固定器的设计和临床应用

目的报告股骨粗隆间骨折滑动加压外固定器的设计和初步临床应用结果。方法在单侧成角外固定器和动力髋螺钉（DHS）的基础上，设计制作滑动加压外固定器。自2003年5月～2005年12月，用此新型外固定器治疗老年股骨粗隆间骨折22例。结果获随访20例，时间2～30个月，平均12个月。骨折全部愈合，愈合时间10～13周，平均12周。术后针孔感染4例，无深部感染发生，无骨折不愈合及髋内翻，髋关节功能恢复良好。结论滑动加压外固定器使用方便、灵活，兼有加压和滑动双重功能，能根据临床需求进行动静力固定转换，使固定更合理，能防止髋内翻及固定针滑脱或穿越股骨头等情况发生，是治疗高龄股骨粗隆间骨折患者较理想方法。     

老年股骨粗隆间骨折外固定术

作者从1994年3月～1999年6月利用解放军304医院研制的单侧成角外固定器治疗老年股骨粗隆间骨折26例,随访6～18个月,骨愈合时间平均11.8周,无脱针、断针,无髋内翻、再手术、深部感染和死亡等。本外固定具有创伤小、操作简单、手术时间短、固定可靠、允许早期负重、功能恢复快、并发症少,通过调节固定器,使骨折部位内翻剪力转变为压应力,促进骨折愈合等优点。对于治疗老年体弱多病,手术耐受力差的股骨粗隆间骨折患者,具有明显优势。     

Can a Modified Anterior External Fixator Provide Posterior Compression of AP Compression Type III Pelvic Injuries?

Background

Current anterior fixators can close a disrupted anterior pelvic ring. However, these anterior constructs cannot create posterior compressive forces across the sacroiliac joint. We explored whether a modified fixator could create such forces.

Questions/purposes

We determined whether (1) an anterior external fixator with a second anterior articulation (X-frame) would provide posterior pelvic compression and (2) full pin insertion would deliver higher posterior compressive forces than half pin insertion.

Methods

We simulated AP compression Type III instability with plastic pelvis models and tested the following conditions: (1) single-pin supraacetabular external fixator (SAEF) using half pin insertion (60 mm); (2) SAEF using full pin insertion (120 mm); (3) modified fixator with X-frame using half pin insertion; (4) modified fixator using full pin insertion; and (5) C-clamp. Standardized fracture compression in the anterior and posterior compartment was performed as in previous studies by Gardner. A force-sensitive sensor was placed in the symphysis and posterior pelvic ring before fracture reduction and the fractures were reduced. The symphyseal and sacroiliac compression loads of each application were measured.

Results

The SAEF exerted mean compressions of 13 N and 14 N to the posterior pelvic ring using half and full pin insertions, respectively. The modified fixator had mean posterior compressions of 174 N and 222 N with half and full pin insertions, respectively. C-clamp application exerted a mean posterior load of 407 N.

Conclusions

Posterior compression on the pelvis was improved using an X-frame as an anterior fixation device in a synthetic pelvic fracture model.

Clinical Relevance

This additive device may improve the initial anterior and posterior stability in the acute management of unstable and life-threatening pelvic ring injuries.     

单边外固定器治疗股骨髁上髁间复杂性骨折

目的探讨应用单边外固定器治疗股骨远端复杂骨折的适应证及治疗效果。方法21例股骨髁上髁间粉碎性骨折患．先采用膝前内侧切口。行股骨髁间骨折复位，解剖复位关节面，于股骨外髁部定位直视下用特制外固定螺纹针加压固定两髁部骨折。透视下闭合牵引复位髁上骨折，并撬拨整复游离骨折片，于骨折近端定位置人外固定针。安装紧固外固定器行骨折固定。结果随访时间8～24个月。平均13．2个月。骨折在4～7个月内全部愈合，并去除外固定器。2例患外固定针道感染，经去除固定针局部换药后愈合。根据Rasumssen评分(总分30分，膝关节活动功能0～6分)评价患膝关节功能恢复情况，优良16例(76．2％)，一般4例(19％)，差1例(4．8％)。结论单边骨外固定器治疗股骨髁上髁间复杂性骨折，具有创伤小。血运破坏少。骨折复位满意，可以早期功能练习等优点。尤其适用于应用内固定难以处理的长节段股骨髁上髁间粉碎性骨折，外固定器可多向、多部位调节，对骨折起到牵引、复位和固定的作用。     

Fixation of trochanteric or subtrochanteric fractures using dynamic external fixators

We present a fixation system for trochanteric or subtrochjanteric fractures using a dynamic external fixator. The Citieffe/CH-N fixator was proposed for elderly patients with a significant operative risk (ASA III or IV). A long dynamic screw measuring 9 mm in diameter is inserted into the neck and the head of the femur on a guide wire through a tube protecting skin and muscles. This long screw is connected to the body of the fixator to enable impaction at the fracture level if compression is needed. The dynamic screw can be released to allow free gliding to achieve a dynamic fixation of the fracture. Distraction with the fixator can also be used for subtrochanteric unstable fractures to correct a varus deformity. This technique is a simple rapid solution with minimal operative risk. Complications are minimal and generally temporary, e.g. pin tract infections.     

Risk of deep infection with intramedullary nailing following the use of external fixators

A retrospective review was carried out to assess the incidence of deep infection occurring when intramedullary nailing was performed following the use of an external fixator. Three groups of patients were identified: group 1 in which the external fixator was used in initial fracture management (ten cases, nine patients), group 2 in which the external fixator was used in the management of established non-union (seven cases, seven patients), and group 3 in which the external fixator was used in limb lengthening procedures (eight cases, eight patients). All the 25 cases reviewed had clinical evidence of pin track infection before removal of the external fixator. After removal of the fixator it is our normal policy to wait for the pin sites to become dry before performing intramedullary nailing. In only one case, where there was a history of preceding recrudescent osteomyelitis, was there evidence of deep infection being reactivated following intramedullary nailing. We therefore conclude that pin track infection does not seem to be a contraindication to the subsequent use of an intramedullary nail, providing that underlying active osteomyelitis is not present. A delay of 7 to 14 days after removal of the fixator is recommended. Some problems experienced during nailing are highlighted, and solutions proposed.