Lower-limb internal loading and potential consequences for fracture healing.

Introduction: Mechanical loading is known to determine the course of bone fracture healing. We hypothesise that lower limb long bone loading differs with knee flexion angle during walking and frontal knee alignment, which affects fracture healing success. Materials and methods: Using our musculoskeletal in silico modelling constrained against in vivo data from patients with instrumented knee implants allowed us to assess internal loads in femur and tibia. These internal forces were associated with the clinical outcome of fracture healing in a relevant cohort of 178 extra-articular femur and tibia fractures in patients using a retrospective approach. Results: Mean peak forces differed with femoral compression (1,330-1,936 N at mid-shaft) amounting to about half of tibial compression (2,299-5,224 N). Mean peak bending moments in the frontal plane were greater in the femur (71-130 Nm) than in the tibia (from 26 to 43 Nm), each increasing proximally. Bending in the sagittal plane showed smaller mean peak bending moments in the femur (-38 to 43 Nm) reaching substantially higher values in the tibia (-63 to -175 Nm) with a peak proximally. Peak torsional moments had opposite directions for the femur (-13 to -40 Nm) versus tibia (15-48 Nm) with an increase towards the proximal end in both. Femoral fractures showed significantly lower scores in the modified Radiological Union Scale for Tibia (mRUST) at last follow-up (p < 0.001) compared to tibial fractures. Specifically, compression (r = 0.304), sagittal bending (r = 0.259), and frontal bending (r = -0.318) showed strong associations (p < 0.001) to mRUST at last follow-up. This was not the case for age, body weight, or localisation alone. Discussion: This study showed that moments in femur and tibia tend to decrease towards their distal ends. Tibial load components were influenced by knee flexion angle, especially at push-off, while static frontal alignment played a smaller role. Our results indicate that femur and tibia are loaded differently and thus require adapted fracture fixation considering load components rather than just overall load level.

Is there a Difference in Interfragmentary Compression Strength Between Fully or Partially Threaded Screws? Results of an Experimental Biomechanical Pilot Study.

Objective This study assessed differences between fully- and partially-threaded screws in the initial interfragmentary compression strength. Our hypothesis was that there would be an increased loss in initial compression strength with the partially-threaded screw. Methods A 45-degree oblique fracture line was created in artificial bone samples. The first group (FULL, n = 6) was fixed using a 3.5-mm fully-threaded lag screw, while the second group (PARTIAL, n = 6) used a 3.5-mm partially-threaded lag screw. Torsional stiffness for both rotational directions were evaluated. The groups were compared based on biomechanical parameters: angle-moment-stiffness, time-moment-stiffness, maximal torsional moment (failure load), and calibrated compression force based on pressure sensor measurement. Results After loss of one PARTIAL sample, no statistically significant differences in calibrated compression force measurement were observed between both groups: [median (interquartile range)] FULL: 112.6 (10.5) N versus PARTIAL: 106.9 (7.1) N, Mann-Whitney U-test: p = 0.8). In addition, after exclusion of 3 samples for mechanical testing (FULL n = 5, PARTIAL n = 4), no statistically significant differences were observed between FULL and PARTIAL constructs in angle-moment-stiffness, time-moment-stiffness, nor maximum torsional moment (failure load). Conclusion There is no apparent difference in the initial compression strength (compression force or construct stiffness or failure load) achieved using either fully- or partially-threaded screws in this biomechanical model in high-density artificial bone. Fully-threaded screws could, therefore, be more useful in diaphyseal fracture treatment. Further research on the impact in softer osteoporotic, or metaphyseal bone models, and to evaluate the clinical significance is required.

Is there a Difference in Interfragmentary Compression Strength Between Fully or Partially Threaded Screws? Results of an Experimental Biomechanical Pilot Study / Há diferença na força de compressão interfragmentar entre parafusos com rosca total ou parcial? Resultados de um estudo biomecânico experimental piloto

Abstract Objective This study assessed differences between fully- and partially-threaded screws in the initial interfragmentary compression strength. Our hypothesis was that there would be an increased loss in initial compression strength with the partially-threaded screw. Methods A 45-degree oblique fracture line was created in artificial bone samples. The first group (FULL, n = 6) was fixed using a 3.5-mm fully-threaded lag screw, while the second group (PARTIAL, n = 6) used a 3.5-mm partially-threaded lag screw. Torsional stiffness for both rotational directions were evaluated. The groups were compared based on biomechanical parameters: angle-moment-stiffness, time-moment-stiffness, maximal torsional moment (failure load), and calibrated compression force based on pressure sensor measurement. Results After loss of one PARTIAL sample, no statistically significant differences in calibrated compression force measurement were observed between both groups: [median (interquartile range)] FULL: 112.6 (10.5) N versus PARTIAL: 106.9 (7.1) N, Mann-Whitney U-test: p = 0.8). In addition, after exclusion of 3 samples for mechanical testing (FULL n = 5, PARTIAL n = 4), no statistically significant differences were observed between FULL and PARTIAL constructs in angle-moment-stiffness, time-moment-stiffness, nor maximum torsional moment (failure load). Conclusion There is no apparent difference in the initial compression strength (compression force or construct stiffness or failure load) achieved using either fully-or partially-threaded screws in this biomechanical model in high-density artificial bone. Fully-threaded screws could, therefore, be more useful in diaphyseal fracture treatment. Further research on the impact in softer osteoporotic, or metaphyseal bone models, and to evaluate the clinical significance is required.

Resumo Objetivo Este estudo avaliou diferenças entre parafusos com rosca total ou parcial na resistência à compressão interfragmentar inicial. Nossa hipótese era de que haveria maior perda de resistência à compressão inicial com o parafuso de rosca parcial. Métodos Uma linha de fratura oblíqua de 45 graus foi criada em amostras de osso artificial. O primeiro grupo (TOTAL, n = 6) foi fixado com um parafuso de 3,5 mm de rosca total, enquanto o segundo grupo (PARCIAL, n = 6) usou um parafuso de 3,5 mm de rosca parcial. Avaliamos a rigidez à torção em ambas as direções de rotação. Os grupos foram comparados com base nos seguintes parâmetros biomecânicos: momento de rigidez-ângulo, momento de rigidez-tempo, momento de torção máxima (carga de falha) e força de compressão calibrada com base na medida do sensor de pressão. Resultados Depois da perda de uma amostra PARCIAL, não foram observadas diferenças estatisticamente significativas na força de compressão calibrada entre os 2 grupos [mediana (intervalo interquartil)]: TOTAL: 112,6 (10,5) N e PARCIAL: 106,9 (7,1) N, com p = 0,8 segundo o teste U de Mann-Whitney). Além disso, após a exclusão de 3 amostras para testes mecânicos (TOTAL, n = 5, PARCIAL, n = 4), não foram observadas diferenças estatisticamente significativas entre os construtos TOTAL e PARCIAL quanto ao momento de rigidez-ângulo, momento de rigidez-tempo ou momento de torção máxima (carga de falha). Conclusão Não há diferença aparente na força de compressão inicial (força de compressão ou rigidez do construto ou carga de falha) com o uso de parafusos de rosca total ou parcial neste modelo biomecânico em osso artificial de alta densidade. Parafusos de rosca total podem, portanto, ser mais úteis no tratamento de fraturas diafisárias. Mais pesquisas são necessárias sobre o impacto em modelos ósseos osteoporóticos ou metafisários de menor densidade e avaliação do significado clínico.

A New System for Periprosthetic Fracture Stabilization-A Biomechanical Comparison.

In recent years, an increase in periprosthetic femur fractures has become apparent due to the increased number of hip replacements. In the case of Vancouver type B1 fractures, locking plate systems offer safe procedures. This study compared the distal lateral femur plate (LOQTEQ®, aap Implantate AG) with a standard L.I.S.S. LCP® (DePuy Synthes) regarding their biomechanical properties in fixation of periprosthetic femur fractures after hip arthroplasty. We hypothesized that the new LOQTEQ system has superior stability and durability in comparison. Eighteen artificial left femurs were randomized in two groups (Group A: LOQTEQ®; Group B: L.I.S.S. LCP®) and tested until failure. Failure was defined as 10° varus deformity and catastrophic implant failure (loosening, breakage, progressive bending). Axial stiffness, loads of failure, cycles of failure, modes of failure were recorded. The axial stiffness in Group A with 73.4 N/mm (SD +/- 3.0) was significantly higher (p = 0.001) than in Group B (40.7 N/mm (SD +/- 2.8)). Group A resists more cycles than Group B until 10° varus deformity. Catastrophic failure mode was plate breakage in Group A and bending in Group B. In conclusion, LOQTEQ® provides higher primary stability and tends to have higher durability.

Unexpected Sex Differences in the Relationship of Sacroiliac Joint and Lumbar Spine Degeneration.

The relationship between degenerative changes of the sacroiliac joints and the lumbar spine on CT has not been studied yet. The aim of this analysis is to determine the nature of their association as well as the influence of fixed anatomical spinopelvic parameters on sacroiliac joint degeneration. For this institutional review-board-approved investigation, imaging datasets as well as electronic medical records of 719 patients without back pain from the clinical routine of our department of radiology were included. Age, sex, weight category (slim, normal, obese), parity in women and indication for imaging were noted for all patients. The presence of degenerative lesions of the lumbar spine (disc degeneration, endplate degeneration, spondylophytes, and facet joint osteoarthritis) was noted separately at each lumbar segment (L1 to L5). Sacroiliac joints were assessed for sclerosis and osteophytes. Fixed anatomical spinopelvic parameters were measured: pelvic radius = PR; pelvic incidence = PI; sacral table angle = STA. Correlation as well as regression analyses were performed; data were analyzed for males and females separately. PI increased significantly with age in both women and men, while STA decreased and PR remained constant; neither of them was associated with SIJ degeneration. SIJ degeneration correlated with disc degeneration (tau = 0.331; p < 0.001), spondylophytes (tau = 0.397; p < 0.001), and facet joint degeneration (tau = 0.310; p < 0.001) in men, but with no parameter of spinal degeneration in women. Lumbar spinal degeneration increased the risk of sacroiliac joint degeneration in men significantly (OR 7.2; 95%CI 2.8-19.0), but it was not a significant covariable in women. Fixed spinopelvic parameters have little impact on sacroiliac joint degeneration. The degeneration of the sacroiliac joints and the lumbar spine appear to be parallel processes in men, but are largely unrelated in women.

Breakage of intramedullary femoral nailing or femoral plating: how to prevent implant failure.

INTRODUCTION: Intramedullary (IM) fixation is the dominant treatment for pertrochanteric and femoral shaft fractures. In comparison to plate osteosynthesis (PO), IM fixation offers greater biomechanical stability and reduced non-union rates. Due to the minimally invasive nature, IM fixations are less prone to approach-associated complications, such as soft-tissue damage, bleeding or postoperative infection, but they are more prone to fat embolism. A rare but serious complication, however, is implant failure. Thus, the aim of this study was to identify possible risk factors for intramedullary fixation (IMF) and plate osteosynthesis (PO) failure. MATERIALS AND METHODS: We searched our trauma surgery database for implant failure, intramedullary and plate osteosynthesis, after proximal-pertrochanteric, subtrochanteric-or femoral shaft fractures between 2011 and 2019. Implant failures in both the IMF and PO groups were included. Demographic data, fracture type, quality of reduction, duration between initial implantation and nail or plate failure, the use of cerclages, intraoperative microbiological samples, sonication, and, if available, histology were collected. RESULTS: A total of 24 femoral implant failures were identified: 11 IMFs and 13 POs. The average age of patients in the IM group was 68.2 ± 13.5 years and in the PO group was 65.6 ± 15.0 years, with men being affected in 63.6% and 39.5% of cases, respectively. A proximal femoral nail (PFN) anti-rotation was used in 7 patients, a PFN in one and a gamma nail in two patients. A total of 6 patients required cerclage wires for additional stability. A combined plate and intramedullary fixation was chosen in one patient. Initially, all intramedullary nails were statically locked. Failures were observed 34.1 weeks after the initial surgery on average. Risk factors for implant failure included the application of cerclage wires at the level of the fracture (n = 5, 21%), infection (n = 2, 8%), and the use of an additional sliding screw alongside the femoral neck screw (n = 3, 13%). In all patients, non-union was diagnosed radiographically and clinically after 6 months (n = 24, 100%). In the event of PO failure, the placement of screws within all screw holes, and interprosthetic fixation were recognised as the major causes of failure. CONCLUSION: Intramedullary or plate osteosynthesis remain safe and reliable procedures in the treatment of proximal femoral fractures (pertrochanteric, subtrochanteric and femoral shaft fractures). Nevertheless, the surgeon needs to be aware of several implant-related limitations causing implant breakage. These may include the application of tension band wiring which can lead to a too rigid fixation, or placement of cerclage wires at the fracture site.

[What constitutes a good osteosynthesis?] / Was macht eine gute Osteosynthese aus?

What constitutes a "good osteosynthesis"? Although the question seems trivial, on closer inspection there are manifold influencing factors that affect fracture healing, so that this question is ultimately not that easy to answer. The first steps are already set with taking the patient history and initial diagnostics. An adequate analysis of the fracture with a coherent preoperative concept for stabilization based on the latest scientific findings and a subsequent adequate implementation of the planning in the operating room make the success of an osteosynthesis and thus a "good osteosynthesis". Digital support is playing an increasingly important role in this field. This review article deals with the topic in depth and summarizes the most important elements of the necessary cascade.

Is initial interfragmentary compression made to last? An ovine bone in vitro study.

Interfragmentary compression, a major principle of fracture treatment, is clinically not quantified and might be lost quickly even without functional loads. We designed an experimental study hypothesizing that (1) compression can be controlled using either lag screw or compression plate, and expecting similar initial compression, (2) loss of interfragmentary compression through relaxation within one hour is reduced with neutralization locking plate next to lag screw compared to compression plate. Twelve ovine femora (N=6) and humeri (N=6) were assigned into groups: Group 1 received a 45° oblique osteotomy at mid-diaphysis and was fixated using a 3.5 mm interfragmentary lag screw and locking compression plate (3.5 mm LCP, DePuy Synthes) as neutralization plate. Group 2 received a transverse osteotomy and was fixated with dynamic compression using compression plate (LCP). Interfragmentary pressure and relative bone fragment displacements were recorded over one hour. Median loss of compression over one hour time (relaxation) were 0.52% in Group 1, and 0.17% in Group 2 (p>0.05). Median rotational displacements amounted to 0.46° for Group 1, and 0.31° for Group 2, and axial displacement to a median of -20 µm in Group 1 and 25 µm in Group 2. Ovine bone interfragmentary stress relaxation maintains compression over the first hour for lag screw with neutralization plate for an oblique fracture line or compression plate for a transverse fracture line. Measured compression forces around 100 N could be overcome by physiological tension loading in bending or torsion, necessitating for instance tension band plating, additional lag screws or absolutive stability.

Biomechanical models: key considerations in study design.

This manuscript summarizes presentations of a symposium on key considerations in design of biomechanical models at the 2019 Basic Science Focus Forum of the Orthopaedic Trauma Association. The first section outlines the most important characteristics of a high-quality biomechanical study. The second section considers choices associated with designing experiments using finite element modeling versus synthetic bones versus human specimens. The third section discusses appropriate selection of experimental protocols and finite element analyses. The fourth section considers the pros and cons of use of biomechanical research for implant design. Finally, the fifth section examines how results from biomechanical studies can be used when clinical evidence is lacking or contradictory. When taken together, these presentations emphasize the critical importance of biomechanical research and the need to carefully consider and optimize models when designing a biomechanical study.

Antegrade versus retrograde screw fixation of anterior column acetabular fractures: a biomechanical in vitro study.

BACKGROUND: To compare the mechanical strength of antegrade versus retrograde lag screw fixation of anterior column acetabular fractures. METHODS: Standardised anterior column fractures were created in synthetic pelvis models and stabilised by either antegrade (ANTE, n = 4) or retrograde (RETRO, n = 4) anterior column screw fixation. In a validated setup, a cyclic loading protocol was applied with increasing axial force (750 cycles, 250-750 N) followed by load to failure. Construct survival, energy absorbed, construct stiffness, and load to failure were assessed. Descriptive and opto-metric methods were used to describe the mode of failure. RESULTS: All constructs failed with loads below 1500 N. With regard to energy absorbed until failure, the ANTE group resisted to 3.763 × 105 N*cycles (range 3.760 × 105-3.763 × 105) and the RETRO group to 3.762 × 105 N*cycles (range 3.761 × 105-3.765 × 105; p = 1.0). The load to failure was 1254 N (range 977-1299) in the ANTE group and 1234 N (range 1087-1456) in the RETRO group (p = 1.0). Construct stiffness with 250 N was not different between the two groups (ANTE 192 N/mm vs. RETRO 215 N/mm, p = 0.486). In all samples, the mode of failure was a transiliac fracture with screw breakout due to rotation of the pubic fragment around the axis of the screw with a range of rotational motion [ROM] during cyclic testing of 0.96° in one ANTE sample and 0.82° in one RETRO sample for 750 N, and ROM at failure of 2.53° in one ANTE sample and 2.23° in one RETRO sample. There was some plastic deformation of the screws in all cases but no breakage. CONCLUSIONS: In this in vitro mechanical study, antegrade screw fixation of an anterior column acetabular fracture was not different in construct survival, load to failure, stiffness, and mode of failure when compared to retrograde screw fixation.

Locking plate constructs benefit from interfragmentary lag screw fixation with decreased shear movements and more predictable fracture gap motion in simple fracture patterns.

BACKGROUND: A mechanical characterisation of lag screw fixation plus locking plate - although clinically widely used as either "mixed fixation concept" or absolutely stable fixation - is so far missing. This study aimed to evaluate the influence of an interfragmentary lag screw on the resulting motion at the fracture site of locking plate constructs using a simple fracture at the distal femur. METHODS: Human cadaver femora were in vitro loaded in torsion and axial bending-compression with and without lag screw fixation next to a locking plate fixation. In addition, two plate working lengths were tested. Interfragmentary movement was measured optically. FINDINGS: Axial interfragmentary movement is reduced with lag screw (102 mm plate working length, 1000 N, mean): 0.28 mm versus 0.82 mm. With lag screw, the fracture gap stays closed with mean normal interfragmentary movement ≤0.03 mm. Fracture gap tends to open without lag screw: normal interfragmentary movement up to -0.29 mm. Reduction of shear interfragmentary movement was observed throughout all tested loads and groups. Mean true shear remains generally low with lag screw (≤0.42 mm) compared to without lag screw (≤1.46 mm). We also found that interfragmentary movement variance decreases with lag screw, especially for longer plate working length. INTERPRETATION: An interfragmentary lag screw next to locking bridge plating reduces fragment motion in vitro for a simple fracture pattern and provides a sufficient tool to decrease detrimental shear movements. Prospective clinical trials with interfragmentary lag screw fixation should prove these findings in wide clinical use to treat simple fracture patterns.

Anatomic grooved stem mitigates strain shielding compared to established total hip arthroplasty stem designs in finite-element models.

Aseptic loosening remains a major problem for uncemented femoral components in primary total hip arthroplasty (THA). Ideally, bone adaptation after THA manifests minimally and local bone density reduction is widely avoided. Different design features may help to approximate initial, post-THA bone strain to levels pre-THA. Strain-shielding effects of different SP-CL stem design features are systematically analyzed and compared to CLS Spotorno and CORAIL using finite element models and physiological musculoskeletal loading conditions. All designs show substantial proximal strain-shielding: 50% reduced medial surface strain, 40-50% reduction at lateral surface, >120 µm/m root mean square error (RMSE) compared to intact bone in Gruen zone 1 and >60 µm/m RMSE in Gruen zones 2, 6, and 7. Geometrical changes (ribs, grooves, cross sections, stem length, anatomic curvature) have a considerable effect on strain-shielding; up to 20%. Combinations of reduced stem stiffness with larger proximal contact area (anatomically curved, grooves) lead to less strain-shielding compared to clinically established implant designs. We found that only the combination of a structurally flexible stem with anatomical curvature and grooves improves strain-shielding compared to other designs. The clinical implications in vivo of this initial strain-shielding difference are currently under evaluation in an ongoing clinical analysis.

Patient specific glass fiber reinforced composite versus titanium plate: A comparative biomechanical analysis under cyclic dynamic loading.

OBJECTIVES: Free flap fixation with patient specific titanium (TI) plates is commonly performed after oncologic mandible resection, but plate exposure, osseous nonunion and imaging artefacts are associated complications. The aim of this study was to analyze interfragmentary movements and fatigue behaviour of patient specific titanium plates in comparison to a novel glass fiber reinforced composite (GFRC) plate in vitro. METHODS: Two polyurethane fibula segments were fixed to a corresponding mandible (Synbone AG, Malans, CH) with a patient specific 2.0 mm titanium plate (DePuy Synthes, Umkirch, Germany and Materialise, Leuven, Belgium) or one of two patient specific GFRC plates with different glass fiber orientation. Plate fixation to the fibula segments was performed with monocortical non-locking screws in all groups. Plate fixation to the mandible was performed with bicortical locking screws in the titanium group and with bicortical non-locking screws in the GFRC groups. Mastication was simulated via cyclic dynamic loading on the left side at a rate of 1 Hz with increasing peak loading (+0.15 N/cycle, Bionix, MTS, Eden Prairie, USA). A three-dimensional optical measuring system (PONTOS 5 M, GOM, Braunschweig, Germany) was used to determine interfragmentary movements between mandible and fibula segments. RESULTS: Mean plate stiffness of GFRC plates was 431 ±â€¯64 N/mm and 453 ±â€¯70 N/mm versus 560 ±â€¯112 N/mm in the titanium group. No significant differences were found for the number of loading cycles until a vertical displacement of 1.0 mm (p = 0.637) and for vertical displacement over time (p = 0.490). Interosteotomy gap movement differed significantly between titanium and GFRC plates in the right distal (p = 0.001), intermediate (p = 0.006) and left distal gap (p = 0.025). CONCLUSIONS: CAD/CAM titanium plates with locking screws provide increased stiffness and reduced interosteotomy movements in comparison to CAD/CAM glass fiber reinforced composite plates with non-locking titanium screws. Future studies should evaluate the influence of mechanobiologically optimized fixation systems on bone healing in free flap surgery.

Semi-rigid screws provide an auxiliary option to plate working length to control interfragmentary movement in locking plate fixation at the distal femur.

BACKGROUND: Extent and orientation of interfragmentary movement (IFM) are crucially affecting course and quality of fracture healing. The effect of different configurations for implant fixation on successful fracture healing remain unclear. We hypothesize that screw type and configuration of locking plate fixation profoundly influences stiffness and IFM for a given load in a distal femur fracture model. METHODS: Simple analytical models are presented to elucidate the influence of fixation configuration on construct stiffness. Models were refined with a consistent single-patient-data-set to create finite-element femur models. Locking plate fixation of a distal femoral 10mm-osteotomy (comminution model) was fitted with rigid locking screws (rLS) or semi-rigid locking screws (sLS). Systematic variations of screw placements in the proximal fragment were tested. IFM was quantitatively assessed and compared for different screw placements and screw types. RESULTS: Different screw allocations significantly affect IFM in a locking plate construct. LS placement of the first screw proximal to the fracture (plate working length, PWL) has a significant effect on axial IFM (p < 0.001). Replacing rLS with sLS caused an increase (p < 0.001) of IFM under the plate (cis-cortex) between +8.4% and +28.1% for the tested configurations but remained constant medially (<1.1%, trans-cortex). Resultant shear movements markedly increased at fracture level (p < 0.001) to the extent that plate working length increased. The ratio of shear/axial IFM was found to enhance for longer PWL. sLS versus rLS lead to significantly smaller ratios of shear/axial IFM at the cis-cortex for PWL of ≥ 62 mm (p ≤ 0.003). CONCLUSION: Mechanical frame conditions can be significantly influenced by type and placement of the screws in locking plate osteosynthesis of the distal femur. By varying plate working length stiffness and IFM are modulated. Moderate axial and concomitantly low shear IFM could not be achieved through changes in screw placement alone. In the present transverse osteotomy model, ratio of shear/axial IFM with simultaneous moderate axial IFM is optimized by the use of appropriate plate working length of about 42-62 mm. Fixation with sLS demonstrated significantly more axial IFM underneath the plate and may further contribute to compensation of asymmetric straining.

Selecting boundary conditions in physiological strain analysis of the femur: Balanced loads, inertia relief method and follower load.

Selection of boundary constraints may influence amount and distribution of loads. The purpose of this study is to analyze the potential of inertia relief and follower load to maintain the effects of musculoskeletal loads even under large deflections in patient specific finite element models of intact or fractured bone compared to empiric boundary constraints which have been shown to lead to physiological displacements and surface strains. The goal is to elucidate the use of boundary conditions in strain analyses of bones. Finite element models of the intact femur and a model of clinically relevant fracture stabilization by locking plate fixation were analyzed with normal walking loading conditions for different boundary conditions, specifically re-balanced loading, inertia relief and follower load. Peak principal cortex surface strains for different boundary conditions are consistent (maximum deviation 13.7%) except for inertia relief without force balancing (maximum deviation 108.4%). Influence of follower load on displacements increases with higher deflection in fracture model (from 3% to 7% for force balanced model). For load balanced models, follower load had only minor influence, though the effect increases strongly with higher deflection. Conventional constraints of fixed nodes in space should be carefully reconsidered because their type and position are challenging to justify and for their potential to introduce relevant non-physiological reaction forces. Inertia relief provides an alternative method which yields physiological strain results.

Interfragmentary lag screw fixation in locking plate constructs increases stiffness in simple fracture patterns.

BACKGROUND: The aim of the current biomechanical cadaver study was to quantify the influence of an additional lag screw on construct stiffness in simple fracture models at the distal femur stabilised with a locking plate. METHODS: For biomechanical testing paired fresh frozen human femora of 5 donors (mean age: 71 (SD 9) years) were chosen. Different locking plate configurations either with or without interfragmentary lag screw were tested under torsional load (2/4Nm/deg) or axial compression forces (500/1000N). FINDINGS: Data show that plate constructs with interfragmentary lag screw reveal similar axial and torsional stiffness values compared to intact bone as opposed to bridging plate constructs that showed significantly lower stiffness for both loading conditions. INTERPRETATION: The current biomechanical testing unveils that the insertion of a lag screw combined with a locking plate dominates over a bridging plate construct at the distal femur in terms of axial and torsional stiffness.

Working length of locking plates determines interfragmentary movement in distal femur fractures under physiological loading.

BACKGROUND: This study aimed to investigate the influence of the screw location and plate working length of a locking plate construct at the distal femur on interfragmentary movement under physiological loading. METHODS: To quantitatively analyse the influence of plate working length on interfragmentary movements in a locking plate construct bridging a distal femur fracture, a finite element model based on CT (computed tomography) data was physiologically loaded and fracture gap conditions were calculated. Four working lengths with eight screw variations each were systemically analysed. FINDINGS: Interfragmentary movements for axial (12-19%, p<0.001) and shear movements (-7.4-545%, p<0.001) at all tested nodes increased significantly with longer plate working length, whereas screw variations within the groups revealed no significant influence. The working length (defined by screw location) dominates the biomechanical fracture gap conditions. INTERPRETATION: The current finite element analysis demonstrates that plate working length significantly influences interfragmentary movements, thereby affecting the biomechanical consequences of fracture healing.