Initial stability of an uncemented femoral stem with modular necks. An experimental study in human cadaver femurs

Background

Uncemented implants are dependent upon initial postoperative stability to gain bone ingrowth and secondary stability. The possibility to vary femoral offset and neck angles using modular necks in total hip arthroplasty increases the flexibility in the reconstruction of the geometry of the hip joint. The purpose of this study was to investigate and evaluate initial stability of an uncemented stem coupled to four different modular necks.

Methods

A cementless femoral stem was implanted in twelve human cadaver femurs and tested in a hip simulator with patient specific load for each patient corresponding to single leg stance and stair climbing activity. The stems were tested with four different modular necks; long, short, retro and varus. The long neck was used as reference in statistical comparisons. A micromotion jig was used to measure bone-implant movements, at two predefined levels.

Findings

A femoral stem coupled to a varus neck had the highest value of micromotion measured for stair climbing at the distal measurement level (60 μm). The micromotions measured with varus and retro necks were significantly larger than motions observed with the reference modular neck, P < 0.001.

Interpretation

The femoral stem evaluated in this study showed acceptable micromotion values for the investigated loading conditions when coupled to modular necks with different lengths, versions and neck-shaft angles.     

Changes in strain distribution of loaded proximal femora caused by different types of cementless femoral stems

BACKGROUND: A number of clinically used total hip femoral implants are claiming a more or less physiologic load transfer, mostly without providing experimental data. To compare three clinically cementless total hip stems of fundamentally different design, the strain distribution before and after insertion in human cadaver femora was measured in vitro. METHODS: A conventional straight stem based on a distal anchorage concept, a so-called "anatomic" stem designed to have a proximal force transmission and a "stemless" femoral neck prosthesis were evaluated under similar loading conditions. Strain distribution was measured with tri-axial strain-gauge rosettes before and after implantation of the stems. The same bending moment was used in all femora tested to compare magnitude and direction of the resulting strains. FINDINGS: The straight and the "anatomic" stem both led to a decrease of the longitudinal strains in the proximal femur, while the femoral neck implant mainly led to an increase of measured strains on the lateral side of the greater trochanter. The observed medial strains were closer to physiological values in the "stemless" prosthesis than those of the two full-stem prosthesis. INTERPRETATION: The decrease in strains seen in the proximal region of the femora with implanted conventional hip prosthesis corresponds well to the decrease of bone density in this region noted in clinical follow-up studies. The more physiological strain at the inferior base of the neck seen in "stemless implant" may induce a remodelling process that better retains bone stock in that area. However, the increase of strains noted after implantation of this prosthesis require further investigation to assess the risk they may pose to bone failure.     

Bone remodelling inside a cemented resurfaced femoral head

BACKGROUND: Although the short-term performance of modern resurfacing hip arthroplasty is impressive, the long-term performance is still unknown. It is hypothesised that bone remodelling and the resulting changes in stress/strain distribution within the resurfaced femur influence the risk of fixation failure. METHOD: Three-dimensional finite element models and adaptive bone remodelling algorithms have been used to predict long-term changes in bone density following cemented femoral head resurfacing. Applied loading conditions include normal walking and stair climbing. The remodelling simulation was validated by comparing the results of an analysis of a proximal femur implanted with a Charnley femoral component with known clinical data in terms of bone density adaptations. FINDINGS: Resurfacing caused a reduction of strain of 20-70% in the bone underlying the implant as compared to the intact femur, immediately post operative. Elevated strains, ranging between 0.50 and 0.80% strain, were generated post-operatively around the proximal femoral neck regions, indicating a potential risk of neck fracture. However, this strain concentration was considerably reduced after bone remodelling. After remodelling, bone resorption of 60-90% was observed in the bone underlying the implant. Reduction in bone density of 5-47% occurred in the lateral femoral head. Bone apposition was observed in the proximal-medial cortex, around the inferior edge of the implant. Hardly any changes in bone density occurred in the distal neck or the femoral diaphysis. INTERPRETATION: Although resurfacing has produced encouraging clinical results, bone remodelling within the femoral head might be a concern for long-term fixation. Regions of strain concentration at the head-neck junction, which may increase the initial risk of femoral neck fracture, are reduced with bone remodelling. In order to reduce this risk of femoral neck fracture, patients should avoid activities which induce high loading of the hip during the early rehabilitation period after surgery.     

Modular neck for prevention of prosthetic impingement in cases with excessively anteverted femur

Background

It is important to adjust stem anteversion in cases of excessive femoral version to avoid prosthetic impingement-related complications in total hip arthroplasty. Although modular necks are considered an effective solution, their application in cases with wide variations in femoral anteversion remains to be elucidated. This study aimed to simulate the effects of different modular necks on prevention of prosthetic impingement due to excessive femoral anteversion.

Methods

We investigated range of motion without prosthetic impingement by collision detection using implant computer-aided design models of the ANCA-Fit system.

Findings

Modular necks could provide an adequate range of motion in cases with up to 60° of femoral anteversion. However, few alternative necks were available in cases with excessive femoral anteversion, while many options could be used for femoral offset and version control in cases with average amounts of femoral anteversion without prosthetic impingement.

Interpretation

We conclude that modular necks might provide a marginal advantage over other options such as cemented, conical or modular stems for cases with an excessively anteverted femur, although they could help to maintain the femoral offset in some cases with average femoral anteversion.     

Locking plates and their effects on healing conditions and stress distribution: A femoral neck fracture study in cadavers

Background

Implants are used to stabilize femoral neck fractures to achieve successful fracture healing, but there is still a high rate of fracture non-unions. We compared micromotions in femurs with fractured femoral necks stabilized with three screws with or without a locking plate. We also investigated whether osteoporosis was associated with micromotion magnitudes, and explored the influence of implants on load distribution in the upper femur.

Methods

Twelve pairs of human cadaver femurs with femoral neck fractures (AO/OTA 31-B1) were allocated to fracture fixation by three locked screws or three individual screws. All femurs underwent dual energy X-ray absorptiometry. Physiological subject-specific axial load and torque was applied for 10,000 cycles. Micromotion of the head fragment was measured every 100 cycles with high-resolution optical motion detection. Load distribution was measured with strain-gauge rosettes attached to the lateral and medial proximal diaphysis.

Findings

The locking plate group showed reduced micromotion about the femoral neck axis (P = 0.035, effect size = 0.62). No differences were found in valgus–varus or antegrade–retrograde rotations, or in the three translations. Micromotion magnitudes were not associated with osteoporosis. The overall micromotions of the upper femur and the load distribution in the proximal diaphysis were not influenced by fixation type.

Interpretation

The locking plate group showed increased resistance to shear forces compared with the screw group. This effect was not associated with a diagnosis of osteoporosis. The locking plate did not affect the load distribution in the proximal femur.     

绝经后女性股骨颈容积性定量CT测量研究

目的应用vQCT技术测量绝经后女性股骨颈的容积性骨密度（BMD）和几何参数,评价各参数对骨质疏松性骨折的预测能力。方法选取绝经后妇女47例,其中正常组26例,骨质疏松组14例,骨质疏松伴腰椎骨折组7例。应用多层螺旋CT对三组患者左股骨颈进行容积扫描并薄层重建,利用Osteo CAD软件对多平面重组图像进行测量,计算出股骨颈的皮质骨、小梁骨和整体骨的容积BMD,股骨颈轴长和最小横截面积,并进行统计学分析。结果正常组与骨质疏松组、正常组与骨折组之间vQCT及DXA各BMD差异均有统计学意义,在调节年龄、身高和体重因素后差异仍存在;骨质疏松组与骨折组之间只有小梁BMD及总体BMD差异有统计学意义,且小梁BMD下降幅度较大,达32.4%。结论股骨近端vQCT测量比DXA能更敏感地反映绝经后女性股骨颈BMD的变化情况,为早期预测骨质疏松性骨折提供科学依据。     

Evaluation of femoral strains with cementless proximal-fill femoral implants of varied stem length

Background

The design intent of proximally-filling lateral flare femoral stems is to load the endosteal surface of the proximal femur both laterally and medially, to achieve normal bone strains. However, the long stem can contact the femoral cortex and may offload the proximal region to some extent. Therefore, in this study, we sought to determine if reducing the stem length, would result in physiologic strain patterns.

Methods

Using the PhotoStress® method we analyzed 13 femurs intact and with three different stem length implants: stemless, ultra-short and short. The test rig loaded the femoral head by simulating the mid-stance single leg support phase of gait with the ilio-tibial band and the hip abductor forces. The strain distribution with each stem length implant was then compared to the intact strain distribution to determine which was most similar.

Findings

As the stem length increased the femurs exhibited a typical pattern of reduced proximal strain and increased distal strain. However, there was some variation in this pattern indicating that the exact stem position and the location of its interaction with the endosteal surface of bone was not the same in each femur.

Interpretation

The stemless design provided the best match compared to the native femur and therefore has the greatest potential to address the shortcomings of a stemmed femoral implant. However, the ultra-short implant also exhibited a strain distribution that closely emulated the intact femur, and may represent the best option as there are still several questions pertaining to stability and alignment of a stemless implant.     

Influence of changes in stem positioning on femoral loading after THR using a short-stemmed hip implant

BACKGROUND: Short-stemmed hip implants were introduced to conserve proximal bone mass and may facilitate the use of minimally invasive surgery, in which smaller incisions limit access to the joint. This limited access may increase the risk of surgical mal-positioning of the implant, however the sensitivity of femoral loading to such mal-positioning of a short-stemmed implant has not been studied. METHODS: Finite element models were developed of a femur and a short-stemmed implant positioned to reproduce the intact hip centre, as well as with the implant placed in increased anteversion or offset. The effect of these surgical variables on femoral loading was examined for walking and stair climbing using loads from a validated musculoskeletal model. Results of the implanted models were compared with an intact model to evaluate stress shielding. FINDINGS: Implant position had little influence on cortical strains along the length of the diaphysis, although strains decreased by up to 95% at the neck resection level compared to the intact femur. In the proximal Gruen zones I and VII strain energy density among the implanted models varied by up to 0.4 kJ/m(3) (28%) and 0.6 kJ/m(3) (24%) under walking and stair climbing, respectively. All implanted models showed characteristic proximal stress shielding, indicated by a decrease in strain energy density of up to 5.4 kJ/m(3) (69%) compared to the intact femur. INTERPRETATION: Small changes in stem placement would likely have little influence on the internal loading of the femur after bone ingrowth has been achieved, however a reduction in strain energy density and therefore stress shielding was seen even for a short-stemmed implant, which may have consequences for longer-term bone remodelling.     

Micromotion and subsidence of a cementless conical fluted stem depending on femoral defect size – A human cadaveric study

BackgroundCementless modular endoprostheses with tapered fluted stems cover a wide spectrum of femoral defects in reconstructive surgery of the hip. Nevertheless, for these hip stems the recommendations concerning the minimum diaphyseal anchorage distance differ widely. The present experimental study investigated the primary stability of a conical fluted revision stem depending on different types of femoral bone defects.MethodsUsing six fresh frozen human femora, the relative movement of a bi-modular revision stem within the implant-bone interface was examined under cyclic loading conditions. Implant subsidence as well as micromotions at the bone-implant interface were captured with linear variable differential transformers for the intact femora and three different defects ranging from Paprosky type II to type IIIB.FindingsCompared to the intact femur, the infliction of a Paprosky type IIIB defect (3 cm of intact diaphysis) notably increased mean stem subsidence (13–389 μm per 500 load cycles; P = 0.116) but the mean interface micromotion vector sum remained unchanged (50 μm vs. 53 μm). In Paprosky IIIB defects the subsidence component resulting from rotation (horizontal plane) was significantly higher than with the intact femur and a Paprosky II defect (P ≤ 0.041).InterpretationWith optimal bone quality and ideal femur preparation a 3 cm conical fixation was sufficient to meet the set criteria of bony ingrowth in vitro. A conical fixation of 7 cm should be recommended to limit rotational subsidence, especially in case of impaired diaphyseal bone quality or expected difficulties with partial weight-bearing.     

Side plate fixation vs. intramedullary nailing in an unstable medial femoral neck fracture model: A comparative biomechanical study

Background

The objective of this study was to investigate primary stability of the proximal femoral nailing antirotation for the indication of unstable medial femoral neck fractures. The device was compared to the dynamic hip screw blade, which is a “gold standard” in the treatment of proximal femoral fractures.

Methods

Six pairs of human cadaver femurs were tested in a cyclic loading model with loads up to 200 N, 400 N, 600 N, 800 N, and 1000 N, respectively. Iliotibial tract was simulated by a chain that applied forces on the greater trochanter during loading. In vitro combined axial and bending loads were applied. Angular displacements during loading were recorded in all directions, and loads to failure were recorded.

Findings

For the cyclic loading test no statistically significant differences between the two groups could be detected. Specimens fixed with the dynamic hip screw blade showed higher displacements in the varus direction at 400 N and 600 N, in the external rotation at 200 N, 400 N and 600 N, and in the anterior direction at 400 N. Load to failure revealed no statistical difference between the two implants.

Interpretation

The proximal femoral nailing antirotation achieves primary stability comparable to the dynamic hip screw blade. The proximal femoral nailing antirotation combines the biomechanical favorable concept of intramedullary fixation with a minimally invasive surgical technique, which theoretically may be advantageous in clinical use. Further biomechanical studies are required to clarify to what extent the results of the present study can be transferred to the clinical situation.     

A fatigue loading model for investigation of iatrogenic subtrochanteric fractures of the femur

BackgroundBiomechanics of iatrogenic subtrochanteric femur fractures have been examined. Previously-described loading models employed monotonic loading on the femoral head, which is limited in emulating physiological features. We hypothesize that cyclic loading combined with the engagement of abductor forces will reliably cause iatrogenic subtrochanteric fractures.MethodsFinite element analysis determined the effects of adding the abductor muscle forces to the hip contact force around holes located in the lateral femoral cortex. Finite element analysis predictions were validated by strain gage measurements using Sawbones^TM femurs (Pacific Research Laboratories, Inc., Vashon, Washington, USA) with or without abductor muscle forces. The newly developed physiologically-relevant loading model was tested on cadaveric femurs (N = 8) under cyclic loading until failure.FindingsFinite element analysis showed the addition of the abductor muscle forces increased the maximum surface cortical strain by 107% and the strain energy density by 332% at the lateral femoral cortex. Strain gages detected a 72.9% increase in lateral cortical strain using the combined loading model. The cyclic, combined loading led to subtrochanteric fractures through the drill hole in all cadaveric femurs.InterpretationFinite element analysis simulations, strain gage measurements, and cyclic loading of fresh-frozen femurs indicate the inclusion of abductor forces increases the stress and strain at the proximal-lateral femoral cortex. Furthermore, a cyclic loading model that incorporates a hip contact force and abductor muscles force creates the clinically encountered subtrochanteric fractures in vitro. This physiologically-relevant loading model may be used to further study iatrogenic subtrochanteric femur fractures.     

Effect of transtrochanteric rotational osteotomy on impingement and contact state of a femoral implant in conversion total hip arthroplasty — Retrospective simulation study

BackgroundThe transtrochanteric rotational osteotomy is a common osteotomy for osteonecrosis of the femoral head, although the results of conversion total hip arthroplasty after the osteotomy are controversial. We evaluated how deformities in the proximal femur after the osteotomy affect hip impingement and the contact state of the stem in total hip arthroplasty.MethodsWe retrospectively reviewed 35 hips (24 men) that had undergone the transtrochanteric rotational osteotomy for osteonecrosis of the femoral head (TRO-group) and compared them with 31 contralateral, unoperated hips (Primary group). The distance between the anterior and posterior greater trochanter at the cutting point of the femur, defined as the greater trochanter width and the contact area of the femoral implant surface with cortical bone were measured by CT-based three-dimensional templating software. We also calculated the hip range of motion in conversion total hip arthroplasty and analyzed the correlations between the greater trochanter width and the range of motion.FindingsThe number of bony-impingement cases was significantly greater, and the range of motion in flexion, internal rotation and external rotation was significantly less in TRO-group. There was a significant negative correlation between the greater trochanter width and range of motion of internal rotation. There were no significant differences between two groups in percentages of femoral-implant contact area.InterpretationsThe femoral implant fixation appears to be satisfactorily fixed in all zones in conversion total hip arthroplasty. However, the greater attention should be paid to minimizing bony impingement, especially on the anterior side, in conversion total hip arthroplasty.     

Assessment of hip fracture risk by cross-sectional strain-energy derived from image-based beam model

BackgroundClinicians have been looking for a simple and effective biomechanical tool for the assessment of hip fracture risk. Dual-energy X-ray absorptiometry (DXA) is currently the primary bone imaging modality in clinic, and the engineering beam is the simplest model for a mechanical analysis. Therefore, we developed a DXA-based beam model for the above purpose.MethodsA beam model of the proximal femur was constructed from the subject's hip DXA image and denoted DXA-beam. Femur stiffness was calculated at cross-sections of interest using areal bone-mineral-density profile. Impact force induced in a sideways fall was applied as a critical loading. Fracture risk index at a cross-section was defined as the ratio of strain-energy induced by the impact force to the allowable strain-energy. A clinic cohort was used to study the discriminability of DXA-beam, which was measured by the area under the curve and odds ratio, both with 95% confidential interval.FindingsFracture risk measured by DXA-beam model at the femoral neck [odds ratio 2.23, 95% confidence interval (1.83, 2.57)], inter-trochanter [2.49, (2.14, 3.25)] and sub-trochanter [2.82, (2.38, 3.51)] were strongly associated with hip fracture. The area under the curve by DXA-beam at the femoral neck [0.74, 95% confidence interval (0.70, 0.76)], inter-trochanter [0.77, (0.75, 0.82)] and sub-trochanter [0.76, (0.74, 0.81)] were higher than that by femoral neck bone mineral density [0.71, (0.65, 0.78)].InterpretationThe DXA-beam model is a simple and yet effective mechanical model. It had promising performance in discrimination of fracture cases from controls.     

Numerical evaluation of bone remodelling and adaptation considering different hip prosthesis designs

BackgroundThe change in mechanical properties of femoral cortical bone tissue surrounding the stem of the hip endoprosthesis is one of the causes of implant instability. We present an analysis used to determine the best conditions for long-term functioning of the bone–implant system, which will lead to improvement of treatment results.MethodsIn the present paper, a finite element method coupled with a bone remodelling model is used to evaluate how different three-dimensional prosthesis models influence distribution of the density of bone tissue. The remodelling process begins after the density field is obtained from a computed tomography scan. Then, an isotropic Stanford model is employed to solve the bone remodelling process and verify bone tissue adaptation in relation to different prosthesis models.FindingsThe study results show that the long-stem models tend not to transmit loads to proximal regions of bone, which causes the stress-shielding effect. Short stems or application in the calcar region provide a favourable environment for transfer of loads to the proximal region, which allows for maintenance of bone density and, in some cases, for a positive variation, which causes absence of the aseptic loosening of an implant. In the case of hip resurfacing, bone mineral density changes slightly and is closest to an intact femur.InterpretationInstallation of an implant modifies density distribution and stress field in the bone. Thus, bone tissue is stimulated in a different way than before total hip replacement, which evidences Wolff's law, according to which bone tissue adapts itself to the loads imposed on it. The results suggest that potential stress shielding in the proximal femur and cortical hypertrophy in the distal femur may, in part, be reduced through the use of shorter stems, instead of long ones, provided stem fixation is adequate.     

定量CT评价股骨近段骨皮质厚度及骨密度

目的 初步探讨采用定量CT(QCT)骨结构分析系统(BIT)测量股骨近段骨皮质的信度和可重复性。方法 收集30例髋部低能量骨折患者,正常侧股骨用于测量,采集髋部QCT扫描数据,于QCT分析工作站上BIT自动选定垂直于股骨颈中段长轴最狭窄处横断面,将该横断面分为4个象限,即上前象限(SA),下前象限(IA),下后象限(IP),上后象限(SP),BIT自动估算各象限骨皮质厚度(C.Th),由3名测量者分别进行测量,比较测值的差异及一致性。结果 3名测量者测得的股骨颈最狭窄处横断面平均C.Th及SA、SP、IA象限C.Th和骨密度差异均无统计学意义(P均>0.05),一致性均较好,其中平均C.Th的ICC值最高,为0.883。而3名测量者测得的IP象限C.Th和骨密度差异均有统计学意义(P均<0.05)。结论 BIT能够实现获取股骨近段骨密度结果的同时获得骨结构信息,测得的股骨颈横断面上象限及前下象限C.Th及骨密度具有很好的重复性。     

股骨近段CT扫描与全髋关节置换的术前计划

背景：做好全髋关节置换前假体型号预测的前提是对股骨近段的充分了解和精确测量,但X射线片仅提供一个平面图像,不能了解股骨近段横断面的情况,而股骨上段CT扫描可以提供更多信息。目的：观察股骨近段CT扫描在全髋关节置换前计划中的作用。方法：对61例进行初次全髋关节置换的患者行股骨近端CT扫描,选取股骨小转子最突出处上方2cm股骨颈平面（T20）、股骨小转子最突出点处的股骨转子区平面（T0）和股骨干髓腔最狭窄处平面（N）的横断面CT影像。测量股骨颈平面髓腔长径、宽径、内侧径;小转子平面髓腔长径、内侧径;股骨峡部平面髓腔长径、宽径、皮质厚度。结果与结论：股骨颈T20长径40.8～63.3mm,平均（49.6±5.1）mm;T20宽径13.3～29.1mm,平均（22.4±3.4）mm;T20内侧径7.2～14.6mm,平均（10.6±1.6）mm。股骨转子区T0长径20.5～40.2mm,平均（28.7±4.4）mm;T0内侧径4.3～13.0mm,平均（8.1±1.7）mm。股骨峡部N长径8.2～22.4mm,平均（14.1±3.1）mm;N宽径6.1～17.9mm,平均（10.2±2.9）mm;N皮质厚度2.7～12.7mm,平均（7.5±1.8）mm。提示股骨近端CT扫描可以提供更多的影像信息和更精确的测量数据,对全髋关节置换前计划有所帮助。     

Patterns of stress distribution at the proximal femur after implantation of a modular neck prosthesis. A biomechanical study

Background

Modular total hip arthroplasty incorporating a double taper design is an evolution offering potential advantages compared to single head–neck taper or monolithic designs. Changes in femoral offset, neck length or femoral anteversion are expected to alter the strain distribution.

Methods

We therefore analyzed the strain patterns after usage of all types of necks of a modular neck prosthesis, implanted in composite femurs.

Findings

The load distribution presented a repeatable pattern. Anteverted neck combinations resulted in higher stress at the anterior surface, whereas the retroverted ones at the posterior (e.g. at the middle frontal site, stress is 13.63% higher when we shifted from the long neutral neck to the long 15° anteverted neck and at the middle back site 19.73% higher when we shifted from the long neutral to the long 15° retroverted neck). Compressive stress was larger at the calcar region and exacerbated by the use of the varus neck (e.g. at the frontal 1 site stress increased by 44.01% when we used the long 8° varus neck in comparison to the long neutral neck). Anteverted neck combinations resulted in higher strain at the anterior cortex around the tip of the prosthesis. Short necks exhibited lower stress at the femoral shaft and higher at the trans-trochanteric area.

Interpretation

Anteverted neck combinations could be more prone to anterior thigh pain. Because of the possible risk of adaptive hypertrophy and early mechanical failure due to increased stress, the surgeon should be cautious when using necks with combined characteristics or short necks.     

股骨近端几何结构结合骨密度预测髋部骨质疏松性骨折风险的可行性分析

目的:探讨髋部骨质疏松性骨折患者股骨近端几何结构的变化,结合骨密度测定,分析其对髋部骨质疏松性骨折危险性的影响。方法:选取2010年6月—2011年5月南华大学附属第一医院50岁以上受检者233例,其中髋部新鲜骨质疏松性骨折组46例,对照组187例,利用法国Osteocore 3双能X线骨密度仪及其配置的高级骨科专用分析测量工具包,测量股骨近端骨密度和几何结构,包括股骨颈轴长、颈干角、内侧偏距和颈长,进行统计学分析,比较各组间的差异。结果:髋部骨折组与对照组的股骨近端几何结构测量值均符合正态分布分析,与对照组比较,髋部骨折组中股骨颈轴长、内侧偏距和颈长均增大(P<0.05),颈干角减小(P<0.05),与其它各组有显著性差异。结论:髋部骨质疏松性骨折患者股骨颈轴长、颈干角、内侧偏距和颈长等股骨近端几何结构的特点,使骨骼负荷增加,是导致骨折发生的重要因素,结合骨密度测定,可以提高髋部骨质疏松性骨折风险的预测水平。     

The role of lesser trochanter fragment in unstable pertrochanteric A2 proximal femur fractures - is refixation of the lesser trochanter worth the effort?

BackgroundInstability of osteoporotic pertrochanteric fractures is defined by loss of medial/lateral cortical integrity with the posteromedial fragment including the lesser trochanter being pivotal for load distribution. Literature addressing the importance of lesser trochanter refixation is scarce. To clarify the effect of lesser trochanter refixation on primary stability in these fractures, following study was performed.Methods21 femora were match-paired in 3 groups and osteotomized, creating pertrochanteric fractures (AO-31A2). Group 1 was stabilized with a proximal femoral nail, group 2 with a dynamic hip screw and group 3 with an augmented proximal femoral nail. Each femur was tested non-destructively at 200 and 400 N with and without refixation of the lesser trochanter (configuration A/B). The overall stiffness and movement of the femoral neck was recorded.FindingsAt 200 N, refixation reduced movement of the femoral neck and increased overall stiffness significantly in group 1 and 3. At 400 N, refixation decreased movement of the femoral neck not significantly in all groups (1 = 38%, 2 = 36%, 3 = 43%). The augmented proximal femoral nail after refixation showed the highest stability of all constructs.InterpretationRefixation of the lesser trochanter may increase the primary stability of pertrochanteric fracture osteosynthesis as all groups showed a higher primary stability. Therefore, refixation should be considered in unstable, osteoporotic fractures. If additional trauma through refixation appears inappropriate, cement augmentation should be performed as it showed only 9% less stability than a non-augmented proximal femoral nail with refixation of the lesser trochanter.     

Should extramedullary fixations for hip fractures be removed after bone union?

Background

Osteosynthesis implants, which remain in the patient after fracture union to save additional surgery, may affect the strain distribution within the bone. A reduction of strain within the bone is known to result in localized bone loss (“stress shielding”) and increased fracture risk. The purpose of this study was to examine whether extramedullary fixations for femoral neck fractures have to be removed after fracture union to prevent reductions in cortex strains.

Methods

In a biomechanical experiment, six pairs of human cadaver femora (mean age 56 years, range 48 to 64) were supplied with five strain gauges per bone. The bones were equally supplied with a compression hip screw or a femoral neck plate. Before surgery, after surgery and after removal of the implants, axial compression tests were conducted to measure surface strains during loading.

Findings

The compression hip screw reduced the amount of strain at the superior neck by 88% (P = 0.015) and at the lesser trochanter by 51% (P = 0.038). The femoral neck plate reduced the amount of strain at the superior neck by 89% (P = 0.001), and increased the amount of strain at the inferior neck by 58% (P = 0.02) and at the lesser trochanter by 63% (P = 0.005). After implant removal, there was no significant difference in strain compared to pre-fracture levels, except for the compression hip screw with 21% less strain (P = 0.047) at the superior neck.

Interpretation

Removal of osteosynthesis implants after bone union reverts bone strains to pre-fracture levels, and might prevent further bone loss induced by stress shielding.