The use of, and interest in, total knee replacements (TKR) has been growing over the last few
de... more The use of, and interest in, total knee replacements (TKR) has been growing over the last few
decades. Loosening and migration of tibial components have been identified as one of the
primary causes of failure in the proximal tibia. Clinical studies show the use of metal
implants as one of the primary methods for the treatment of knee joints and associated bone
defects. Alignment and fixation techniques play an important role in achieving high success
rates. Defective bone stock requires the use of augments to stabilise the tibial plate. In these
cases, current clinical practice is to use an extended implant stem to ensure stability. The
problem with this is that it reduces the potential for future knee revision
In this research Finite Element Analysis (FEA) has been used to undertake virtual in-vivo
assessment of various configurations of augmented and non-augmented TKR that can be used
for the treatment of tibial defects. These configurations are based on a standard tibial insert,
namely a fixed bearing revision tibial tray. This has provided insight and information that can
be used to improve surgical decision making when dealing with defective bone stock.
The 3D FE models of a non-defect TKR with a fixed bearing tibial insert showed a stable
construct with stresses lying within an allowable threshold. The use of a stem extension
generally showed a reduction in stress levels in the cancellous bone contributing to an
increase in stress shielding and thus it is recommended that these are not used unless there is
some other overriding clinical requirement.
Further, the analysis demonstrated that, contrary to some clinical opinion, wedge
augmentation (rather than block augmentation) may provide a better approach to treat the
defect. This was largely due to improved cement stress distribution caused by a mechanism
termed “reverse-shear”.
The use of a cement augment was found to provide a more favourable stress distribution in
the cancellous bone. However, metal augments have been recommended as the cement
augment was shown to operate too close to its fatigue endurance limits.
The use of, and interest in, total knee replacements (TKR) has been growing over the last few
de... more The use of, and interest in, total knee replacements (TKR) has been growing over the last few
decades. Loosening and migration of tibial components have been identified as one of the
primary causes of failure in the proximal tibia. Clinical studies show the use of metal
implants as one of the primary methods for the treatment of knee joints and associated bone
defects. Alignment and fixation techniques play an important role in achieving high success
rates. Defective bone stock requires the use of augments to stabilise the tibial plate. In these
cases, current clinical practice is to use an extended implant stem to ensure stability. The
problem with this is that it reduces the potential for future knee revision
In this research Finite Element Analysis (FEA) has been used to undertake virtual in-vivo
assessment of various configurations of augmented and non-augmented TKR that can be used
for the treatment of tibial defects. These configurations are based on a standard tibial insert,
namely a fixed bearing revision tibial tray. This has provided insight and information that can
be used to improve surgical decision making when dealing with defective bone stock.
The 3D FE models of a non-defect TKR with a fixed bearing tibial insert showed a stable
construct with stresses lying within an allowable threshold. The use of a stem extension
generally showed a reduction in stress levels in the cancellous bone contributing to an
increase in stress shielding and thus it is recommended that these are not used unless there is
some other overriding clinical requirement.
Further, the analysis demonstrated that, contrary to some clinical opinion, wedge
augmentation (rather than block augmentation) may provide a better approach to treat the
defect. This was largely due to improved cement stress distribution caused by a mechanism
termed “reverse-shear”.
The use of a cement augment was found to provide a more favourable stress distribution in
the cancellous bone. However, metal augments have been recommended as the cement
augment was shown to operate too close to its fatigue endurance limits.
Total knee replacements (TKR) have been of great interest for over three decades. Loosening of ti... more Total knee replacements (TKR) have been of great interest for over three decades. Loosening of tibial components has been identified as one of the primary causes of failures in early knee replacement. Alignment and fixation techniques play an important role in achieving high success rates. About 27,000 knee replacements are carried annually in UK alone and revision knee surgery accounts for about 10-12% worldwide. Defective bone stock requires the use of augments to stabilise the tibial plate. In these cases, current clinical practice is to use an extended implant stem to ensure stability. The problem with this is that it reduces the potential for future knee revisions. Biomechanical analysis of this procedure is being undertaken in order to determine best practice. The research will also include an analysis to compare the use of cement to fill the defected region rather than augments. The report outlines the clinical studies; discussing various implants and different approaches to the modelling work. As part of the research, some modelling software packages were assessed. Detailed studies were performed to select a suitable package that allows the replication of clinically observed environments in static simulations. The report discusses the final modelling approach, and boundary conditions used to perform finite element analysis. Detailed stress and strain distributions have been obtained and the results show that the solution is a clinically acceptable working model. This model can be modified to include varying material properties and densities to allow for realistic simulation. Future work will involve modifying the model to include the augments, different lengths of stem and a range of other clinically relevant parameters.
The aim of this FE study was to analyse the comparative behaviour of cement and metal based augme... more The aim of this FE study was to analyse the comparative behaviour of cement and metal based augments in TKR and quantify the stresses within these different augments and underlying cancellous bone.Materials and methods A three-dimensional FE model was constructed from a CT scan of the proximal tibia using SIMPLEWARE v3.2 image processing software. The tibial component of a TKR was implanted with either a block or wedge-shaped augment made of either metal or cement. The model was axially loaded with a force of 3600N and testing was conducted with both evenly and eccentrically distributed loads.Results Upon loading the FE model, the von-Mises stresses in the cancellous bone underneath the augments was higher with cement based augments in comparison their metal counterparts. When evenly loaded the maximum recorded compressive stresses within the metal augments were 5 times less than the endurance limit of the material, whilst the stresses within cement augments were only half the endur...
The cancellous bone shows higher and more widespread stress near the bone cement interface on the... more The cancellous bone shows higher and more widespread stress near the bone cement interface on the augmented side in the CBA compared to the TKR and MBA models. A similar pattern was found with the Cement (CWA) and Metal (MWA) wedge augmented models. Whilst it could be argued that this higher stress in the cement augments might promote bone growth it would seem from clinical observations that the stresses may be too high. Taking the TKR model as a reference it can be concluded that both forms of augmentation produce conditions that could be less favourable for implant stability.
Stress concentrations close to the augment/bone junction are likely to occur in TKR conical augme... more Stress concentrations close to the augment/bone junction are likely to occur in TKR conical augment surgery due to the geometry of the conical augment. The use of a stem extension may always be necessary when a conical augment is used to treat proximal defects to increase the longevity of the implant. The investigation will extend to larger conical augments in future work.
The novel dynamic locking screw (DLS) was developed to improve bone healing with locked-plate ost... more The novel dynamic locking screw (DLS) was developed to improve bone healing with locked-plate osteosynthesis by equalising construct stiffness at both cortices. Due to a theoretical damping effect, this modulated stiffness could be beneficial for fracture fixation in osteoporotic bone. Therefore, the mechanical behaviour of the DLS at the screw-bone interface was investigated in an artificial osteoporotic bone model and compared with conventional locking screws (LHS). Osteoporotic surrogate bones were plated with either a DLS or a LHS construct consisting of two screws and cyclically axially loaded (8,500 cycles, amplitude 420 N, increase 2 mN/cycle). Construct stiffness, relative movement, axial screw migration, proximal (P) and distal (D) screw pullout force and loosening at the bone interface were determined and statistically evaluated. DLS constructs exhibited a higher screw pullout force of P 85 N [standard deviation (SD) 21] and D 93 N (SD 12) compared with LHS (P 62 N, SD 28,...
Introduction In complex primary and revision total knee replacement (TKR) the operating surgeon m... more Introduction In complex primary and revision total knee replacement (TKR) the operating surgeon may encounter proximal tibial bone defects. The correct management of such defects is fundamental to both the initial stability and long-term survival of the prosthesis. Block or wedge-shaped metal augments are used to address some such type II unconstrained defects.Aim The aim of this finite element (FE) study was to assess the effects of block and wedge-shaped metal augments upon the shear stresses in the cement mantle at the bone-implant interface of an augmented TKR.Materials and methods A three-dimensional FE model was constructed from a computer tomography scan of the proximal tibia using SIMPLEWARE v3.2 image processing software. The tibial component of a TKR was implanted with either a block or wedge-shaped metal augment in-situ. The model was axially loaded with a force of 3600N, equating to four times the body weight of a 90kg patient, and the load evenly distributed between the...
Cementless surface replacement arthroplasty (CSRA) of the shoulder was designed to preserve the i... more Cementless surface replacement arthroplasty (CSRA) of the shoulder was designed to preserve the individual anatomy and humeral bone stock. A matter of concern in resurfacing implants remains the stress shielding and bone remodeling processes. The bone remodeling processes of two different CSRA fixation designs, conical-crown (Epoca RH) and central-stem (Copeland), were studied by three-dimensional (3-D) finite element analysis (FEA) as well as evaluation of contact radiographs from human CSRA retrievals. FEA included one native humerus model with a normal and one with a reduced bone stock quality. Compressive strains were evaluated before and after virtual CSRA implantation and the results were then compared to the bone remodeling and stress-shielding pattern of eight human CSRA retrievals (Epoca RH n=4 and Copeland n=4). FEA revealed for both bone stock models increased compressive strains at the stem and outer implant rim for both CSRA designs indicating an increased bone formation at those locations. Unloading of the bone was seen for both designs under the central implant shell (conical-crown 50-85%, central-stem 31-93%) indicating high bone resorption. Those effects appeared more pronounced for the reduced than for the normal bone stock model. The assumptions of the FEA were confirmed in the CSRA retrieval analysis which showed bone apposition at the outer implant rim and stems with highly reduced bone stock below the central implant shell. Overall, clear signs of stress shielding were observed for both CSRAs designs in the in vitro FEA and human retrieval analysis. Especially in the central part of both implant designs the bone stock was highly resorbed. The impact of these bone remodeling processes on the clinical outcome as well as long-term stability requires further evaluation.
Computer Methods in Biomechanics and Biomedical Engineering, 2014
Loosening and migration of tibial prostheses have been identified as causes of early total knee r... more Loosening and migration of tibial prostheses have been identified as causes of early total knee replacement (TKR) failure. The problem is made more complex when defects occur in the proximal tibia compromising fixation and alignment. Clinical studies using metal augments have shown these to be an alternative to other means of defect treatment. Finite element (FE) analysis can be used to identify regions that may be prone to loosening and migration. In the current work, 3D FE models of TKR uncontained type-2 defects treated with block augments have been constructed and analysed. It has been shown that a metal augment is the most suitable. The use of bone cement (PMMA) to fill proximal defects is not considered suitable as stresses carried by the cement block exceed those of the fatigue limit of bone cement. It has been shown that the stresses in the proximal cancellous bone of block-augmented models are significantly below levels likely to cause damage due to overloading. Furthermore, the use of stem extensions has been shown to reduce the cancellous bone stresses in the proximal region thus increasing the likelihood of bone resorption. Given this, it is recommended that stem extensions are not required unless necessary to mitigate some other problem.
Background
Implant loosening is frequently detected after fixation of open book injuries. Thou... more Background
Implant loosening is frequently detected after fixation of open book injuries. Though many authors do not see this as a complication, it is often the reason for hardware removal or reinstrumentation in the case of remaining instability. We hypothesized that the flexibility of the implant has an influence on loosening and thus on failure of the construct.
Methods
We used 6 fresh-frozen pelvic specimens and tested them with our recently introduced test setup for two-leg alternate loading. We subjected them to a non-destructive quasi-static test in the intact condition followed by a non-destructive cyclic test under axial sinusoidal loading with progressive amplitude. Afterwards we simulated an open book injury and performed fixation with three different configurations of a modular fixation system (1-, 2- or 4-rod configuration) in randomized order. Subsequently, the specimens were subjected to 3 cyclic tests with the same loading protocol as previously defined. Finally, each construct was cyclically tested to failure keeping the final rod configuration.
Findings
We detected significantly greater mobility after 1-rod-fixation and no significant differences after 2-rod or 4-rod-fixation compared to the intact symphysis condition.
In the destructive test series the 4-rod-fixation failed first followed by the 1-rod-fixation. The 2-rod-fixation sustained almost 3 times as many load cycles prior to failure as the 4-rod-fixation, whereas the 1-rod-fixation sustained twice as many cycles as the 4-rod-fixation.
Interpretation
In conclusion, flexible fixation of the ruptured pubic symphysis in human specimens shows superior behavior with respect to load bearing capacity and ability to withstand cyclic loading compared to stiff constructs.
The dorsal component of the pelvic ring is considered to be the most essential element for the st... more The dorsal component of the pelvic ring is considered to be the most essential element for the stability of the pelvic ring. None of the current biomechanical set-ups include the effect of shear stresses by alternating loads that the pelvic ring has to withstand during walking. We hypothesize that a biomechanical test set-up with two-leg alternate loading will lead to stress imitation at the pubic symphysis that are more similar to existing strains than other test set-ups, and would, therefore, be more adequate for biomechanical testing of fixation methods.
A new biomechanical two-leg standing test set-up with an alternate pelvic loading was constructed and was validated with six human pelvises from fresh frozen cadavers. Three-dimensional motion tracking was performed. The specimens were subjected to a non-destructive quasi-static test and a non-destructive cyclic test with progressive load amplitude from 170 N to 340 N over 1000 cycles.
The initial rotational 'range of motion' and 'mean displacement' around the vertical axis for a pre-load of 170 N was about 0.3° and 0.2°, respectively, increasing by 0.1–0.2° at a load of 340 N. The rotation around the vertical axis and the translation along the frontal horizontal axis confirmed the stability of the pubic symphysis. The rate of ascend of displacements decreased, once the rotation reached 1° or the translation reached 1 mm.
The current biomechanical test set-up was compared with previous clinical findings, and the method was found valid for measuring inter-segmentary movements at the pubic symphysis.
reatment of proximal tibial defects is important to the survival of tibial prosthesis after total... more reatment of proximal tibial defects is important to the survival of tibial prosthesis after total knee replacement. The objective of this finite element study was to determine a better understanding of the stresses produced by different treatment options for moderate uncontained type-2 defects. Methods analysed were the use of metal wedges, metal blocks, cement wedges, and cement blocks for the two defect angles 15° and 30°. The effect of a stem extension on the stress profiles was also analysed for each defect treatment and angle to establish the necessity of these extensions and consequent bone removal on the stability of the augments. Equivalent stresses in two regions of interest (ROIs) adjacent to the augments and shear stresses along the bone–cement interface of the defect were investigated. The lowest equivalent stresses were found in the metal block augment for both defect angles and ROIs. The highest equivalent stress in the ROIs and shear stress values along the bone–cement interface of the defect were found in the cement wedge augment model for both defect angles. Stem extensions were shown to increase equivalent stresses in the bone closer to the tibial stem but to decrease equivalent stresses closer to the cortical bone. The use of a stem extension significantly increased the shear stresses in the cement in all cases except in the metal block model. It is recommended that metal block augments are used without a stem extension in small-defect (i.e. peripheral defect angle of 15°) total knee replacement procedures.
The use of, and interest in, total knee replacements (TKR) has been growing over the last few
de... more The use of, and interest in, total knee replacements (TKR) has been growing over the last few
decades. Loosening and migration of tibial components have been identified as one of the
primary causes of failure in the proximal tibia. Clinical studies show the use of metal
implants as one of the primary methods for the treatment of knee joints and associated bone
defects. Alignment and fixation techniques play an important role in achieving high success
rates. Defective bone stock requires the use of augments to stabilise the tibial plate. In these
cases, current clinical practice is to use an extended implant stem to ensure stability. The
problem with this is that it reduces the potential for future knee revision
In this research Finite Element Analysis (FEA) has been used to undertake virtual in-vivo
assessment of various configurations of augmented and non-augmented TKR that can be used
for the treatment of tibial defects. These configurations are based on a standard tibial insert,
namely a fixed bearing revision tibial tray. This has provided insight and information that can
be used to improve surgical decision making when dealing with defective bone stock.
The 3D FE models of a non-defect TKR with a fixed bearing tibial insert showed a stable
construct with stresses lying within an allowable threshold. The use of a stem extension
generally showed a reduction in stress levels in the cancellous bone contributing to an
increase in stress shielding and thus it is recommended that these are not used unless there is
some other overriding clinical requirement.
Further, the analysis demonstrated that, contrary to some clinical opinion, wedge
augmentation (rather than block augmentation) may provide a better approach to treat the
defect. This was largely due to improved cement stress distribution caused by a mechanism
termed “reverse-shear”.
The use of a cement augment was found to provide a more favourable stress distribution in
the cancellous bone. However, metal augments have been recommended as the cement
augment was shown to operate too close to its fatigue endurance limits.
The use of, and interest in, total knee replacements (TKR) has been growing over the last few
de... more The use of, and interest in, total knee replacements (TKR) has been growing over the last few
decades. Loosening and migration of tibial components have been identified as one of the
primary causes of failure in the proximal tibia. Clinical studies show the use of metal
implants as one of the primary methods for the treatment of knee joints and associated bone
defects. Alignment and fixation techniques play an important role in achieving high success
rates. Defective bone stock requires the use of augments to stabilise the tibial plate. In these
cases, current clinical practice is to use an extended implant stem to ensure stability. The
problem with this is that it reduces the potential for future knee revision
In this research Finite Element Analysis (FEA) has been used to undertake virtual in-vivo
assessment of various configurations of augmented and non-augmented TKR that can be used
for the treatment of tibial defects. These configurations are based on a standard tibial insert,
namely a fixed bearing revision tibial tray. This has provided insight and information that can
be used to improve surgical decision making when dealing with defective bone stock.
The 3D FE models of a non-defect TKR with a fixed bearing tibial insert showed a stable
construct with stresses lying within an allowable threshold. The use of a stem extension
generally showed a reduction in stress levels in the cancellous bone contributing to an
increase in stress shielding and thus it is recommended that these are not used unless there is
some other overriding clinical requirement.
Further, the analysis demonstrated that, contrary to some clinical opinion, wedge
augmentation (rather than block augmentation) may provide a better approach to treat the
defect. This was largely due to improved cement stress distribution caused by a mechanism
termed “reverse-shear”.
The use of a cement augment was found to provide a more favourable stress distribution in
the cancellous bone. However, metal augments have been recommended as the cement
augment was shown to operate too close to its fatigue endurance limits.
Total knee replacements (TKR) have been of great interest for over three decades. Loosening of ti... more Total knee replacements (TKR) have been of great interest for over three decades. Loosening of tibial components has been identified as one of the primary causes of failures in early knee replacement. Alignment and fixation techniques play an important role in achieving high success rates. About 27,000 knee replacements are carried annually in UK alone and revision knee surgery accounts for about 10-12% worldwide. Defective bone stock requires the use of augments to stabilise the tibial plate. In these cases, current clinical practice is to use an extended implant stem to ensure stability. The problem with this is that it reduces the potential for future knee revisions. Biomechanical analysis of this procedure is being undertaken in order to determine best practice. The research will also include an analysis to compare the use of cement to fill the defected region rather than augments. The report outlines the clinical studies; discussing various implants and different approaches to the modelling work. As part of the research, some modelling software packages were assessed. Detailed studies were performed to select a suitable package that allows the replication of clinically observed environments in static simulations. The report discusses the final modelling approach, and boundary conditions used to perform finite element analysis. Detailed stress and strain distributions have been obtained and the results show that the solution is a clinically acceptable working model. This model can be modified to include varying material properties and densities to allow for realistic simulation. Future work will involve modifying the model to include the augments, different lengths of stem and a range of other clinically relevant parameters.
The aim of this FE study was to analyse the comparative behaviour of cement and metal based augme... more The aim of this FE study was to analyse the comparative behaviour of cement and metal based augments in TKR and quantify the stresses within these different augments and underlying cancellous bone.Materials and methods A three-dimensional FE model was constructed from a CT scan of the proximal tibia using SIMPLEWARE v3.2 image processing software. The tibial component of a TKR was implanted with either a block or wedge-shaped augment made of either metal or cement. The model was axially loaded with a force of 3600N and testing was conducted with both evenly and eccentrically distributed loads.Results Upon loading the FE model, the von-Mises stresses in the cancellous bone underneath the augments was higher with cement based augments in comparison their metal counterparts. When evenly loaded the maximum recorded compressive stresses within the metal augments were 5 times less than the endurance limit of the material, whilst the stresses within cement augments were only half the endur...
The cancellous bone shows higher and more widespread stress near the bone cement interface on the... more The cancellous bone shows higher and more widespread stress near the bone cement interface on the augmented side in the CBA compared to the TKR and MBA models. A similar pattern was found with the Cement (CWA) and Metal (MWA) wedge augmented models. Whilst it could be argued that this higher stress in the cement augments might promote bone growth it would seem from clinical observations that the stresses may be too high. Taking the TKR model as a reference it can be concluded that both forms of augmentation produce conditions that could be less favourable for implant stability.
Stress concentrations close to the augment/bone junction are likely to occur in TKR conical augme... more Stress concentrations close to the augment/bone junction are likely to occur in TKR conical augment surgery due to the geometry of the conical augment. The use of a stem extension may always be necessary when a conical augment is used to treat proximal defects to increase the longevity of the implant. The investigation will extend to larger conical augments in future work.
The novel dynamic locking screw (DLS) was developed to improve bone healing with locked-plate ost... more The novel dynamic locking screw (DLS) was developed to improve bone healing with locked-plate osteosynthesis by equalising construct stiffness at both cortices. Due to a theoretical damping effect, this modulated stiffness could be beneficial for fracture fixation in osteoporotic bone. Therefore, the mechanical behaviour of the DLS at the screw-bone interface was investigated in an artificial osteoporotic bone model and compared with conventional locking screws (LHS). Osteoporotic surrogate bones were plated with either a DLS or a LHS construct consisting of two screws and cyclically axially loaded (8,500 cycles, amplitude 420 N, increase 2 mN/cycle). Construct stiffness, relative movement, axial screw migration, proximal (P) and distal (D) screw pullout force and loosening at the bone interface were determined and statistically evaluated. DLS constructs exhibited a higher screw pullout force of P 85 N [standard deviation (SD) 21] and D 93 N (SD 12) compared with LHS (P 62 N, SD 28,...
Introduction In complex primary and revision total knee replacement (TKR) the operating surgeon m... more Introduction In complex primary and revision total knee replacement (TKR) the operating surgeon may encounter proximal tibial bone defects. The correct management of such defects is fundamental to both the initial stability and long-term survival of the prosthesis. Block or wedge-shaped metal augments are used to address some such type II unconstrained defects.Aim The aim of this finite element (FE) study was to assess the effects of block and wedge-shaped metal augments upon the shear stresses in the cement mantle at the bone-implant interface of an augmented TKR.Materials and methods A three-dimensional FE model was constructed from a computer tomography scan of the proximal tibia using SIMPLEWARE v3.2 image processing software. The tibial component of a TKR was implanted with either a block or wedge-shaped metal augment in-situ. The model was axially loaded with a force of 3600N, equating to four times the body weight of a 90kg patient, and the load evenly distributed between the...
Cementless surface replacement arthroplasty (CSRA) of the shoulder was designed to preserve the i... more Cementless surface replacement arthroplasty (CSRA) of the shoulder was designed to preserve the individual anatomy and humeral bone stock. A matter of concern in resurfacing implants remains the stress shielding and bone remodeling processes. The bone remodeling processes of two different CSRA fixation designs, conical-crown (Epoca RH) and central-stem (Copeland), were studied by three-dimensional (3-D) finite element analysis (FEA) as well as evaluation of contact radiographs from human CSRA retrievals. FEA included one native humerus model with a normal and one with a reduced bone stock quality. Compressive strains were evaluated before and after virtual CSRA implantation and the results were then compared to the bone remodeling and stress-shielding pattern of eight human CSRA retrievals (Epoca RH n=4 and Copeland n=4). FEA revealed for both bone stock models increased compressive strains at the stem and outer implant rim for both CSRA designs indicating an increased bone formation at those locations. Unloading of the bone was seen for both designs under the central implant shell (conical-crown 50-85%, central-stem 31-93%) indicating high bone resorption. Those effects appeared more pronounced for the reduced than for the normal bone stock model. The assumptions of the FEA were confirmed in the CSRA retrieval analysis which showed bone apposition at the outer implant rim and stems with highly reduced bone stock below the central implant shell. Overall, clear signs of stress shielding were observed for both CSRAs designs in the in vitro FEA and human retrieval analysis. Especially in the central part of both implant designs the bone stock was highly resorbed. The impact of these bone remodeling processes on the clinical outcome as well as long-term stability requires further evaluation.
Computer Methods in Biomechanics and Biomedical Engineering, 2014
Loosening and migration of tibial prostheses have been identified as causes of early total knee r... more Loosening and migration of tibial prostheses have been identified as causes of early total knee replacement (TKR) failure. The problem is made more complex when defects occur in the proximal tibia compromising fixation and alignment. Clinical studies using metal augments have shown these to be an alternative to other means of defect treatment. Finite element (FE) analysis can be used to identify regions that may be prone to loosening and migration. In the current work, 3D FE models of TKR uncontained type-2 defects treated with block augments have been constructed and analysed. It has been shown that a metal augment is the most suitable. The use of bone cement (PMMA) to fill proximal defects is not considered suitable as stresses carried by the cement block exceed those of the fatigue limit of bone cement. It has been shown that the stresses in the proximal cancellous bone of block-augmented models are significantly below levels likely to cause damage due to overloading. Furthermore, the use of stem extensions has been shown to reduce the cancellous bone stresses in the proximal region thus increasing the likelihood of bone resorption. Given this, it is recommended that stem extensions are not required unless necessary to mitigate some other problem.
Background
Implant loosening is frequently detected after fixation of open book injuries. Thou... more Background
Implant loosening is frequently detected after fixation of open book injuries. Though many authors do not see this as a complication, it is often the reason for hardware removal or reinstrumentation in the case of remaining instability. We hypothesized that the flexibility of the implant has an influence on loosening and thus on failure of the construct.
Methods
We used 6 fresh-frozen pelvic specimens and tested them with our recently introduced test setup for two-leg alternate loading. We subjected them to a non-destructive quasi-static test in the intact condition followed by a non-destructive cyclic test under axial sinusoidal loading with progressive amplitude. Afterwards we simulated an open book injury and performed fixation with three different configurations of a modular fixation system (1-, 2- or 4-rod configuration) in randomized order. Subsequently, the specimens were subjected to 3 cyclic tests with the same loading protocol as previously defined. Finally, each construct was cyclically tested to failure keeping the final rod configuration.
Findings
We detected significantly greater mobility after 1-rod-fixation and no significant differences after 2-rod or 4-rod-fixation compared to the intact symphysis condition.
In the destructive test series the 4-rod-fixation failed first followed by the 1-rod-fixation. The 2-rod-fixation sustained almost 3 times as many load cycles prior to failure as the 4-rod-fixation, whereas the 1-rod-fixation sustained twice as many cycles as the 4-rod-fixation.
Interpretation
In conclusion, flexible fixation of the ruptured pubic symphysis in human specimens shows superior behavior with respect to load bearing capacity and ability to withstand cyclic loading compared to stiff constructs.
The dorsal component of the pelvic ring is considered to be the most essential element for the st... more The dorsal component of the pelvic ring is considered to be the most essential element for the stability of the pelvic ring. None of the current biomechanical set-ups include the effect of shear stresses by alternating loads that the pelvic ring has to withstand during walking. We hypothesize that a biomechanical test set-up with two-leg alternate loading will lead to stress imitation at the pubic symphysis that are more similar to existing strains than other test set-ups, and would, therefore, be more adequate for biomechanical testing of fixation methods.
A new biomechanical two-leg standing test set-up with an alternate pelvic loading was constructed and was validated with six human pelvises from fresh frozen cadavers. Three-dimensional motion tracking was performed. The specimens were subjected to a non-destructive quasi-static test and a non-destructive cyclic test with progressive load amplitude from 170 N to 340 N over 1000 cycles.
The initial rotational 'range of motion' and 'mean displacement' around the vertical axis for a pre-load of 170 N was about 0.3° and 0.2°, respectively, increasing by 0.1–0.2° at a load of 340 N. The rotation around the vertical axis and the translation along the frontal horizontal axis confirmed the stability of the pubic symphysis. The rate of ascend of displacements decreased, once the rotation reached 1° or the translation reached 1 mm.
The current biomechanical test set-up was compared with previous clinical findings, and the method was found valid for measuring inter-segmentary movements at the pubic symphysis.
reatment of proximal tibial defects is important to the survival of tibial prosthesis after total... more reatment of proximal tibial defects is important to the survival of tibial prosthesis after total knee replacement. The objective of this finite element study was to determine a better understanding of the stresses produced by different treatment options for moderate uncontained type-2 defects. Methods analysed were the use of metal wedges, metal blocks, cement wedges, and cement blocks for the two defect angles 15° and 30°. The effect of a stem extension on the stress profiles was also analysed for each defect treatment and angle to establish the necessity of these extensions and consequent bone removal on the stability of the augments. Equivalent stresses in two regions of interest (ROIs) adjacent to the augments and shear stresses along the bone–cement interface of the defect were investigated. The lowest equivalent stresses were found in the metal block augment for both defect angles and ROIs. The highest equivalent stress in the ROIs and shear stress values along the bone–cement interface of the defect were found in the cement wedge augment model for both defect angles. Stem extensions were shown to increase equivalent stresses in the bone closer to the tibial stem but to decrease equivalent stresses closer to the cortical bone. The use of a stem extension significantly increased the shear stresses in the cement in all cases except in the metal block model. It is recommended that metal block augments are used without a stem extension in small-defect (i.e. peripheral defect angle of 15°) total knee replacement procedures.
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decades. Loosening and migration of tibial components have been identified as one of the
primary causes of failure in the proximal tibia. Clinical studies show the use of metal
implants as one of the primary methods for the treatment of knee joints and associated bone
defects. Alignment and fixation techniques play an important role in achieving high success
rates. Defective bone stock requires the use of augments to stabilise the tibial plate. In these
cases, current clinical practice is to use an extended implant stem to ensure stability. The
problem with this is that it reduces the potential for future knee revision
In this research Finite Element Analysis (FEA) has been used to undertake virtual in-vivo
assessment of various configurations of augmented and non-augmented TKR that can be used
for the treatment of tibial defects. These configurations are based on a standard tibial insert,
namely a fixed bearing revision tibial tray. This has provided insight and information that can
be used to improve surgical decision making when dealing with defective bone stock.
The 3D FE models of a non-defect TKR with a fixed bearing tibial insert showed a stable
construct with stresses lying within an allowable threshold. The use of a stem extension
generally showed a reduction in stress levels in the cancellous bone contributing to an
increase in stress shielding and thus it is recommended that these are not used unless there is
some other overriding clinical requirement.
Further, the analysis demonstrated that, contrary to some clinical opinion, wedge
augmentation (rather than block augmentation) may provide a better approach to treat the
defect. This was largely due to improved cement stress distribution caused by a mechanism
termed “reverse-shear”.
The use of a cement augment was found to provide a more favourable stress distribution in
the cancellous bone. However, metal augments have been recommended as the cement
augment was shown to operate too close to its fatigue endurance limits.
decades. Loosening and migration of tibial components have been identified as one of the
primary causes of failure in the proximal tibia. Clinical studies show the use of metal
implants as one of the primary methods for the treatment of knee joints and associated bone
defects. Alignment and fixation techniques play an important role in achieving high success
rates. Defective bone stock requires the use of augments to stabilise the tibial plate. In these
cases, current clinical practice is to use an extended implant stem to ensure stability. The
problem with this is that it reduces the potential for future knee revision
In this research Finite Element Analysis (FEA) has been used to undertake virtual in-vivo
assessment of various configurations of augmented and non-augmented TKR that can be used
for the treatment of tibial defects. These configurations are based on a standard tibial insert,
namely a fixed bearing revision tibial tray. This has provided insight and information that can
be used to improve surgical decision making when dealing with defective bone stock.
The 3D FE models of a non-defect TKR with a fixed bearing tibial insert showed a stable
construct with stresses lying within an allowable threshold. The use of a stem extension
generally showed a reduction in stress levels in the cancellous bone contributing to an
increase in stress shielding and thus it is recommended that these are not used unless there is
some other overriding clinical requirement.
Further, the analysis demonstrated that, contrary to some clinical opinion, wedge
augmentation (rather than block augmentation) may provide a better approach to treat the
defect. This was largely due to improved cement stress distribution caused by a mechanism
termed “reverse-shear”.
The use of a cement augment was found to provide a more favourable stress distribution in
the cancellous bone. However, metal augments have been recommended as the cement
augment was shown to operate too close to its fatigue endurance limits.
replacement. Alignment and fixation techniques play an important role in achieving high success rates. About 27,000 knee replacements are carried annually in UK alone and revision knee surgery accounts for about 10-12% worldwide.
Defective bone stock requires the use of augments to stabilise the tibial plate. In these cases, current clinical practice is to use an extended implant stem to ensure stability. The problem with this is that it reduces the potential for future knee revisions. Biomechanical analysis of this procedure is being undertaken in order to determine best practice. The research will also include an analysis to compare the use of cement to fill the defected region rather than augments.
The report outlines the clinical studies; discussing various implants and different approaches to the modelling work. As part of the research, some modelling software packages were assessed. Detailed studies were performed to select a suitable package that allows the replication of clinically observed environments in static simulations. The report discusses the final modelling approach, and boundary conditions used to perform finite element analysis. Detailed stress and strain distributions have been obtained and the results show that the solution is a clinically acceptable working model.
This model can be modified to include varying material properties and densities to allow for
realistic simulation. Future work will involve modifying the model to include the augments,
different lengths of stem and a range of other clinically relevant parameters.
Papers by Yash Agarwal
Implant loosening is frequently detected after fixation of open book injuries. Though many authors do not see this as a complication, it is often the reason for hardware removal or reinstrumentation in the case of remaining instability. We hypothesized that the flexibility of the implant has an influence on loosening and thus on failure of the construct.
Methods
We used 6 fresh-frozen pelvic specimens and tested them with our recently introduced test setup for two-leg alternate loading. We subjected them to a non-destructive quasi-static test in the intact condition followed by a non-destructive cyclic test under axial sinusoidal loading with progressive amplitude. Afterwards we simulated an open book injury and performed fixation with three different configurations of a modular fixation system (1-, 2- or 4-rod configuration) in randomized order. Subsequently, the specimens were subjected to 3 cyclic tests with the same loading protocol as previously defined. Finally, each construct was cyclically tested to failure keeping the final rod configuration.
Findings
We detected significantly greater mobility after 1-rod-fixation and no significant differences after 2-rod or 4-rod-fixation compared to the intact symphysis condition.
In the destructive test series the 4-rod-fixation failed first followed by the 1-rod-fixation. The 2-rod-fixation sustained almost 3 times as many load cycles prior to failure as the 4-rod-fixation, whereas the 1-rod-fixation sustained twice as many cycles as the 4-rod-fixation.
Interpretation
In conclusion, flexible fixation of the ruptured pubic symphysis in human specimens shows superior behavior with respect to load bearing capacity and ability to withstand cyclic loading compared to stiff constructs.
A new biomechanical two-leg standing test set-up with an alternate pelvic loading was constructed and was validated with six human pelvises from fresh frozen cadavers. Three-dimensional motion tracking was performed. The specimens were subjected to a non-destructive quasi-static test and a non-destructive cyclic test with progressive load amplitude from 170 N to 340 N over 1000 cycles.
The initial rotational 'range of motion' and 'mean displacement' around the vertical axis for a pre-load of 170 N was about 0.3° and 0.2°, respectively, increasing by 0.1–0.2° at a load of 340 N. The rotation around the vertical axis and the translation along the frontal horizontal axis confirmed the stability of the pubic symphysis. The rate of ascend of displacements decreased, once the rotation reached 1° or the translation reached 1 mm.
The current biomechanical test set-up was compared with previous clinical findings, and the method was found valid for measuring inter-segmentary movements at the pubic symphysis.
decades. Loosening and migration of tibial components have been identified as one of the
primary causes of failure in the proximal tibia. Clinical studies show the use of metal
implants as one of the primary methods for the treatment of knee joints and associated bone
defects. Alignment and fixation techniques play an important role in achieving high success
rates. Defective bone stock requires the use of augments to stabilise the tibial plate. In these
cases, current clinical practice is to use an extended implant stem to ensure stability. The
problem with this is that it reduces the potential for future knee revision
In this research Finite Element Analysis (FEA) has been used to undertake virtual in-vivo
assessment of various configurations of augmented and non-augmented TKR that can be used
for the treatment of tibial defects. These configurations are based on a standard tibial insert,
namely a fixed bearing revision tibial tray. This has provided insight and information that can
be used to improve surgical decision making when dealing with defective bone stock.
The 3D FE models of a non-defect TKR with a fixed bearing tibial insert showed a stable
construct with stresses lying within an allowable threshold. The use of a stem extension
generally showed a reduction in stress levels in the cancellous bone contributing to an
increase in stress shielding and thus it is recommended that these are not used unless there is
some other overriding clinical requirement.
Further, the analysis demonstrated that, contrary to some clinical opinion, wedge
augmentation (rather than block augmentation) may provide a better approach to treat the
defect. This was largely due to improved cement stress distribution caused by a mechanism
termed “reverse-shear”.
The use of a cement augment was found to provide a more favourable stress distribution in
the cancellous bone. However, metal augments have been recommended as the cement
augment was shown to operate too close to its fatigue endurance limits.
decades. Loosening and migration of tibial components have been identified as one of the
primary causes of failure in the proximal tibia. Clinical studies show the use of metal
implants as one of the primary methods for the treatment of knee joints and associated bone
defects. Alignment and fixation techniques play an important role in achieving high success
rates. Defective bone stock requires the use of augments to stabilise the tibial plate. In these
cases, current clinical practice is to use an extended implant stem to ensure stability. The
problem with this is that it reduces the potential for future knee revision
In this research Finite Element Analysis (FEA) has been used to undertake virtual in-vivo
assessment of various configurations of augmented and non-augmented TKR that can be used
for the treatment of tibial defects. These configurations are based on a standard tibial insert,
namely a fixed bearing revision tibial tray. This has provided insight and information that can
be used to improve surgical decision making when dealing with defective bone stock.
The 3D FE models of a non-defect TKR with a fixed bearing tibial insert showed a stable
construct with stresses lying within an allowable threshold. The use of a stem extension
generally showed a reduction in stress levels in the cancellous bone contributing to an
increase in stress shielding and thus it is recommended that these are not used unless there is
some other overriding clinical requirement.
Further, the analysis demonstrated that, contrary to some clinical opinion, wedge
augmentation (rather than block augmentation) may provide a better approach to treat the
defect. This was largely due to improved cement stress distribution caused by a mechanism
termed “reverse-shear”.
The use of a cement augment was found to provide a more favourable stress distribution in
the cancellous bone. However, metal augments have been recommended as the cement
augment was shown to operate too close to its fatigue endurance limits.
replacement. Alignment and fixation techniques play an important role in achieving high success rates. About 27,000 knee replacements are carried annually in UK alone and revision knee surgery accounts for about 10-12% worldwide.
Defective bone stock requires the use of augments to stabilise the tibial plate. In these cases, current clinical practice is to use an extended implant stem to ensure stability. The problem with this is that it reduces the potential for future knee revisions. Biomechanical analysis of this procedure is being undertaken in order to determine best practice. The research will also include an analysis to compare the use of cement to fill the defected region rather than augments.
The report outlines the clinical studies; discussing various implants and different approaches to the modelling work. As part of the research, some modelling software packages were assessed. Detailed studies were performed to select a suitable package that allows the replication of clinically observed environments in static simulations. The report discusses the final modelling approach, and boundary conditions used to perform finite element analysis. Detailed stress and strain distributions have been obtained and the results show that the solution is a clinically acceptable working model.
This model can be modified to include varying material properties and densities to allow for
realistic simulation. Future work will involve modifying the model to include the augments,
different lengths of stem and a range of other clinically relevant parameters.
Implant loosening is frequently detected after fixation of open book injuries. Though many authors do not see this as a complication, it is often the reason for hardware removal or reinstrumentation in the case of remaining instability. We hypothesized that the flexibility of the implant has an influence on loosening and thus on failure of the construct.
Methods
We used 6 fresh-frozen pelvic specimens and tested them with our recently introduced test setup for two-leg alternate loading. We subjected them to a non-destructive quasi-static test in the intact condition followed by a non-destructive cyclic test under axial sinusoidal loading with progressive amplitude. Afterwards we simulated an open book injury and performed fixation with three different configurations of a modular fixation system (1-, 2- or 4-rod configuration) in randomized order. Subsequently, the specimens were subjected to 3 cyclic tests with the same loading protocol as previously defined. Finally, each construct was cyclically tested to failure keeping the final rod configuration.
Findings
We detected significantly greater mobility after 1-rod-fixation and no significant differences after 2-rod or 4-rod-fixation compared to the intact symphysis condition.
In the destructive test series the 4-rod-fixation failed first followed by the 1-rod-fixation. The 2-rod-fixation sustained almost 3 times as many load cycles prior to failure as the 4-rod-fixation, whereas the 1-rod-fixation sustained twice as many cycles as the 4-rod-fixation.
Interpretation
In conclusion, flexible fixation of the ruptured pubic symphysis in human specimens shows superior behavior with respect to load bearing capacity and ability to withstand cyclic loading compared to stiff constructs.
A new biomechanical two-leg standing test set-up with an alternate pelvic loading was constructed and was validated with six human pelvises from fresh frozen cadavers. Three-dimensional motion tracking was performed. The specimens were subjected to a non-destructive quasi-static test and a non-destructive cyclic test with progressive load amplitude from 170 N to 340 N over 1000 cycles.
The initial rotational 'range of motion' and 'mean displacement' around the vertical axis for a pre-load of 170 N was about 0.3° and 0.2°, respectively, increasing by 0.1–0.2° at a load of 340 N. The rotation around the vertical axis and the translation along the frontal horizontal axis confirmed the stability of the pubic symphysis. The rate of ascend of displacements decreased, once the rotation reached 1° or the translation reached 1 mm.
The current biomechanical test set-up was compared with previous clinical findings, and the method was found valid for measuring inter-segmentary movements at the pubic symphysis.